**HYSPEC: A Crystal-Time-of-Flight [Hy]{.underline}brid
[Spec]{.underline}trometer**

**for the Spallation Neutron Source**

![](media/image1.jpeg){width="6.055555555555555in"
height="5.688194444444444in"}

**HYSPEC: A Crystal-Time-of-Flight [Hy]{.underline}brid
[Spec]{.underline}trometer**

**for the Spallation Neutron Source**

**EXECUTIVE SUMMARY:**

The study of phase transitions and novel ordered phases remains at the
forefront of condensed-matter research. In studies of superconductivity,
magnetism, ferroelectricity, colossal magnetoresistance, charge order,
etc., one is interested in determining how each type of order occurs,
including how and why it arises from the disordered state. The energy
scale for excitations that have an impact on ordering is typically on
the order of a few to tens of meV; this is the same energy scale for
excitations that impact transport properties. The intensities and energy
resolutions obtainable with thermal neutrons are ideal for such
inelastic studies. At the same time, one needs to be able to detect and
monitor the relevant order parameter through elastic diffraction
measurements, which frequently involves measurements of superlattice
peaks that may be extremely weak. Often one wishes to study correlations
that have not achieved long-range order, in which case one must measure
diffuse scattering and be able to discriminate elastic from inelastic
contributions. In many cases understanding the order of interest
requires studying how it is modified or induced by an extreme
environment, such as a strong magnetic field, very low temperature, or
high pressure; this is best accomplished by decoupling the sample
environment from the detector system. In order to exploit the full power
of neutron scattering to distinguish magnetic from nuclear scattering,
it is necessary to implement polarization analysis, with polarization
sensitivity in both the incident and scattered beams. Finally, in order
to discover new phenomena, and to not simply characterize phenomena that
have been discovered at other facilities, one has to be able to work
with small crystals, thus requiring that a spectrometer focus as much
flux as possible into a small spot size.

Up to now, the combination of requirements described above has been met
only by triple-axis spectrometers at steady-state sources. Here we
present the case that by combining time-of-flight spectroscopy with
Bragg focusing optics and the enhanced flux of the pulsed beam that will
be available at the SNS, one can build an instrument that not only
satisfies our needs, but which will also significantly exceed the
performance of the best triple-axis spectrometers at the best
steady-state sources. The instrument that we propose, HYSPEC, fills a
niche of dramatic importance to exciting areas of condensed-matter
physics, one that is not competitively covered by any of the other SNS
instruments that have been considered to date.

HYSPEC will be positioned on beam line 15, which looks at a coupled,
supercritical H~2~ moderator. To transport the beam from the moderator
to the sample the instrument will have a straight, 21.8 m long, m=3
supermirror guide that will incorporate T~0~, frame overlap and order
suppressing choppers along its length and, near its downstream end, a
counter rotating chopper pair that will serve to define the neutron
burst width and incident energy. A short distance downstream of the
counter-rotating chopper pair, the monochromatic beam will impinge on
either a pyrolytic graphite or (for polarized beam studies) Heusler
alloy vertical focusing crystal mounted in a rotating drum shield.
Because vertical focusing by crystals is extremely efficient, the 4(w) x
15 (h) cm^2^ beam exiting the guide will be reduced to area as small as
4(w) x 2(h) cm^2^ thus maximizing the flux at the sample position. A
series of collimators and beam definers will be placed immediately
upstream of the sample and a set of radial collimators in the detector
bank just downstream of the sample. In the polarization analysis mode
the radial collimator would be replaced by a set of 19 Fe-Si supermirror
benders that would make it possible to measure simultaneously both
spin-flip and non-spin-flip processes. Detection of the sample scattered
neutrons will be done by 188 ^3^He position-sensitive detectors housed
in a moveable detector bank 4.5 m from the sample that will cover a
horizontal angular range of 60° and a vertical range of 15°.

HYSPEC's flux-on-sample was compared to the other planned inelastic
instruments CNCS, ARCS and HRCS (which have overlapping energy ranges
and resolutions) using the MCSTAS Monte Carlo simulation program. For
HYSPEC, the maximum flux on sample was found to be 1.1 x 10^7^
n/cm^2^-sec at 15 meV with a 3% energy resolution. This is a factor of
two more than the maximum flux of CNCS, which is at 5 meV, and about ten
times more than maximum flux of HRCS and ARCS, which is around 100 meV.
This high sample flux combined with TOF analysis and the wide angular
acceptance of the detector array means that a given scattered neutron
spectrum will be collected in an order of magnitude less time than on
existing single crystal sample instruments.

The easy adaptability of HYSPEC to polarization analysis is unique among
the suite of SNS inelastic instruments. To produce a focused, polarized
beam the PG crystal is simply replaced by a vertical focusing Heusler
alloy crystal. Analysis of the polarization of the scattered neutrons
will be done with a set of 19 broad-spectrum Fe-Si supermirror benders
downstream of the sample: a well-established and extremely reliable
technology. Because the sample-to-detector distance is large, the
spin-flip and non-spin-flip parts of the scattering will fall on
different groups of detectors and can thus be measured simultaneously.

Finally, it should be noted that an SNS Inelastic Neutron Scattering
Workshop held on 11/1/99 at ANL \[SNS Document:
IS-1.1.8.2-8004-MM-A-00\] recommended that a spectrometer with almost
identical characteristics to HYSPEC be viewed as a "potential "day-one"
instrument".

**HYSPEC: A Crystal-Time-of-Flight [Hy]{.underline}brid
[Spec]{.underline}trometer**

**for the Spallation Neutron Source**

1.  Introduction

2.  Scientific Objectives

3.  The HYSPEC Spectrometer

    1.  Primary Spectrometer (Monochromator)

    2.  Sample Stage

    3.  Secondary Spectrometer (Analyzer/Detector)

4.  Polarization Analysis

    1.  The Polarizing Crystal

    2.  The polarization Analyzers

5.  Performance

    1.  Moderator Choice

6.  Additional Advantages of the HYSPEC concept

7.  Future Considerations

8.  Instrument Development Team

    1.  Members of the IDT and their Institutional Affiliations

    2.  Project Current Organization Chart

9.  Estimated Budget and Time Scale

Appendix A. Science Examples

A.1 Complex Phases and Nanomolecules (C. Stassis)

A.1.1 Intermetallic Compounds

A.1.2 Nanosize Magnetic Molecules

A.2 Functional Materials

A.2.1 Nanoscale Features of Functional Materials (V. Kiryukhin)

A.2.2 Anomalous Phonon Behavior (S. Shapiro, G. Shirane)

A.3 Strongly Correlated Electron Systems (G. H. Lander)

A.4 Quantum Critical Points (R. Osborn)

A.5 Correlated Phases in Many-Electron Systems (I. Zaliznyak, J.
Tranquada)

A.6 Quantum Magnetism (A. Zheludev)

A.7 Advanced Polarization Analysis on TAS and TOF Spectrometers (L.-P.
Regnault)

#  INTRODUCTION

This document lays out a proposal by the Instrument Development Team
(IDT) composed of scientists from leading US Universities and National
Laboratories to design and build a conceptually new high-flux inelastic
neutron spectrometer at the pulsed Spallation Neutron Source (SNS) at
Oak Ridge. This instrument is intended to supply users of the SNS, and
scientific community represented by the IDT members in particular, with
a platform for ground-breaking investigations of the low-energy
atomic-scale dynamical properties of crystalline solids. It is also
planned that it will be equipped with a polarization analysis
capability, therefore becoming the first polarized beam spectrometer in
the SNS instrument suite, and the first successful polarized beam
inelastic instrument at a pulsed spallation source in general.

The proposed instrument is designed primarily for inelastic and elastic
neutron spectroscopy of single crystals. In fact, the most informative
neutron scattering studies of the dynamical properties of solids nearly
always require single crystal samples, and they are almost invariably
flux-limited. In addition, in measurements with polarization analysis
the available flux is further reduced through selection of the
particular neutron polarization, which puts even more stringent limits
on the feasibility of a particular experiment. To date, these
investigations have mostly been carried out on crystal spectrometers at
high-flux reactors, which usually employ focusing Bragg optics to
concentrate the neutron beam on a typically small sample. Construction
of the high-luminosity spallation neutron source at Oak Ridge, which
will provide intense pulsed neutron beams with time-averaged fluxes
equal to those at medium-flux reactors, opens entirely new opportunities
for single crystal neutron spectroscopy. Drawing upon experience
acquired during decades of studies with both crystal and time-of-flight
(TOF) spectrometers, the IDT has developed a conceptual design for a
focused-beam, hybrid time-of-flight instrument with crystal
monochromator for the SNS called HYSPEC (an acronym for
[hy]{.underline}brid [spec]{.underline}trometer). The proposed
instrument has a potential to collect data more than an order of
magnitude faster than existing steady-source spectrometers over a wide
range of energy transfer (*ħω*) and momentum transfer (***Q***) space,
and will transform the way that data in the inelastic single crystal
spectroscopy is collected. HYSPEC is optimized to provide the highest
neutron flux on sample at a good-to-moderate energy resolution in
thermal neutron energy range. By providing flux on sample several times
higher than other inelastic instruments currently previewed for the SNS,
the proposed instrument will indeed allow unique ground-breaking
measurements, and will ultimately make polarized beam studies at a
pulsed spallation source a realistic possibility.

Even though the polarized beam option was not considered at the time, a
spectrometer with performance characteristics similar to those of HYSPEC
was identified as one of the potential "day-one" inelastic instruments
for the SNS at the Inelastic Neutron Scattering Workshop[^1] organized
by the SNS at Oak Ridge in 1999. It was included as such in the Workshop
recommendations, along with six other instruments, four of which are
currently approved, and three have already been funded and are being
constructed. This proposal is a request by the IDT for SNS approval of
HYSPEC as one of the instruments for construction, and for the
assignment of a beamline 15 served by a coupled, 20 K supercritical
hydrogen moderator to the proposed spectrometer.

# SCIENTIFIC OBJECTIVES

Much of our current understanding of atomic-scale structure and the
dynamical properties of solids and liquids was gained by virtue of
neutron scattering studies. Inelastic neutron spectroscopy provided
physicists with an unprecedented, detailed access to phonon dispersions,
magnetic excitation spectra, soft-modes and critical dynamics at phase
transitions, unrivaled by other experimental techniques. Because the
neutron only interacts very weakly with matter, it is practically a
non-perturbing probe of the matter's inner structure and dynamics, not
sensitive to charges and surface layers. Therefore, unlike techniques
where the photon electric field, or charged particles (eg electrons,
muons), which significantly modify local electronic environment, are
used, neutron spectroscopy allows determination of the intrinsic,
un-perturbed physical properties of materials. By the same virtue, the
neutron is a highly penetrating and non-destructive probe, allowing the
investigation of microscopic properties of bulk materials (and not just
their surface layers), studies of samples embedded in a complex sample
environment, such as cryostats, magnets, pressure cells, etc. In fact,
ability to accept a variety of devices creating extreme sample
environment was one of the key factors that determined tremendous
success of the modern reactor-based neutron spectrometers. Finally,
proper manipulation with neutron spin quantum number using various
beam-polarizing devices gave an unrivaled opportunity to separate the
structural and magnetic phenomena on the atomic scale. Incorporating and
optimizing these unique features of the neutron scattering technique in
design of a neutron spectrometer is extremely important.

The discovery of new materials and novel unexpected phenomena along with
advent of new experimental techniques invariably lead to major advances
in condensed matter science. One example is the discovery of strongly
correlated electron systems, from heavy-fermions to high-T~c~
superconductors to manganites exhibiting giant magneto-resistance
effects, which has added a new class of materials to those with which we
are familiar. They presented new types of macroscopic behavior with a
strong potential for future technological applications, which require
detailed understanding on the microscopic level. The neutron scattering
studies which are indispensable for such understanding, represent a
significant experimental challenge. First of all, new materials are
usually only available in small quantities. Secondly, the scattering
intensities associated with the interesting features in the electronic
structure are often intrinsically small. Finally, because several
contributions to the microscopic electronic Hamiltonian in these systems
are of equal importance for determining their macroscopic bulk
properties, direct discrimination between the magnetic scattering by
electronic spin and structural/vibrational scattering by the means of
polarization analysis is vital.

The goal of the present IDT was to design an instrument which, using the
unique, high-flux, pulsed neutron beam at the SNS, would meet the
challenge posed for neutron inelastic spectroscopy by the new scientific
discoveries and the breakthroughs in the synthesis of the new materials.
There are an extremely large number of problems that are currently at
the forefront of condensed matter physics and demonstrate the need for
an instrument with the unique capabilities of HYSPEC. Because of space
limitations we describe some of these in Appendix. They are intended to
be illustrative; there could be many more!

# THE HYSPEC SPECTROMETER

![](media/image2.png){width="5.874409448818898in"
height="3.458267716535433in"}Our primary objective in formulating the
design of the HYSPEC spectrometer was that it delivers the highest
possible monochromatic flux to few-cm-sized samples over a broad range
of thermal and sub-thermal neutron energies \[5,90\] meV.

Additionally, we wanted to develop a design that would (i) minimize
beam-related background such as that arising from sample environments,
(ii) provide reasonably good (and easily adjustable) energy and
scattering vector resolution (δE/E \~ 0.02 - 0.15; δQ/Q \~ 0.005 - 0.1),
(iii) be readily adapted to polarization analysis, (iv) permit rapid
alignment of samples and easy installation of specialized sample
environments and (v) allow for straightforward, direct, on-line
monitoring and analysis of incoming data. Ultimately these criteria led
us to the direct geometry, hybrid concept shown schematically in Fig. 1
and in a 3-D view on the cover. The instrument main design parameters
are summarized in Table I.

**Table I.** Instrument Parameters

  ------------------------------ ----------------------------------------
  Moderator                      Coupled, 20K, supercritical H~2~

  Incident energy                5 - 90 meV

  Energy resolution              0.02 \<∆E/E \<0.15 (for elastic
                                 scattering) depending on neutron energy
                                 and rotation rates of choppers

  Q resolution                   ∆Q/Q \~ 0.005 - 0.1

  Primary flight path            Guide with expander and compressor
                                 sections 15-25 m long

  Secondary flight path          4.5 m

  Energy defining choppers       Counter-rotating disk and Fermi
                                 choppers. Maximum rotation rate 300Hz

  Frame-overlap/order suppressor Disk choppers. Maximum rotation rate 60
  choppers                       Hz

  Flux focusing crystal          Segmeted, vertically curved PG and
                                 fluorinated mica. Heusler for polarized
                                 beam

  Sample position                1.8 m from crystal

  Beam size at sample (optimally 4 (w) x 2 (h) cm^2^
  focused)                       

  Detectors                      188 2.5cm diameter position sensitive
                                 ^3^He tubes. Horizontal pixel resolution
                                 20 min. Vertical pixel resolution 20
                                 min. Horizontal array acceptance: 60°.
                                 Vertical array acceptance: ±7.5°

  Sample environment             Will accept all standard sample
                                 environment equipment
  ------------------------------ ----------------------------------------

# Primary spectrometer (Monochromator)

Incident neutron energy will be defined by the time of flight in the
primary spectrometer. Reflection from the monochromator crystal in most
cases will only serve to focus the beam on sample. In addition, the
monochromator will serve in place of the pulse-shaping chopper, cutting
the unwanted high-energy tail of the spectral distribution of the
incident neutrons, as shown in the Figure 2. Primary spectrometer is
envisioned as consisting of a 21.8 meter long guide with a center
section composed of 40 mm wide by 150 mm high, supermirror-coated,
modules and with 40 mm wide expander sections at each end, Figure 1.
Whether the guide will be straight or slightly curved is still an open
question that will ultimately be resolved by detailed shielding studies.
Placed at intervals along the guide would be three disk choppers; the
T~0~ and frame-overlap choppers - rotating at either the source
frequency or a sub-multiple, and a counter-rotating disk chopper pair,
rotating at integral multiples of the source frequency. At the guide's
downstream end we plan to place a rotating drum shield containing a
vertical-focusing crystal, a beam stop (or, if a straight guide is used,
a "get lost" pipe), a vertically tapered guide, a Fermi chopper and a
Soller collimator. Attached to the shield - and moving with it -- would
be a conventional sample rotation stage mounted on air pads, with a
2-axes goniometer capable of supporting large, off-center loads. In such
an arrangement, neutron energies are determined both by the beam exit
angle (as defined by the crystal and the in-shield collimator), and by
the phase of rotation of the counter-rotating chopper pair and/or the
Fermi chopper relative to the source. Apart from its energy defining
function, the crystal - together with the section of vertically tapered
supermirror guide in the drum shield -- also has the important function
of efficiently focusing the beam at the sample position.

![](media/image3.wmf){width="4.624409448818898in"
height="3.947244094488189in"}![](media/image4.wmf){width="4.7492125984251965in"
height="3.822047244094488in"}

Each chopper has a unique role in defining the monochromatic incident
beam. The T~0~ chopper blocks the gamma rays and high energy neutrons
emitted in the earliest part of the source pulse. The frame overlap
chopper (T~3~) assures that unwanted, higher order (shorter wavelength)
neutrons are removed. Also, its rotation rate can be reduced when
necessary to block alternate source pulses in cases where the scattered
energy spectrum is so broad that spectral overlap becomes a problem. In
the main, medium-resolution, high-data-collection-rate mode of
operation, the counter-rotating disk choppers T~2~ (together with the
crystal) will define both the energy of the beam and the spectral and
time width of the pulse at the sample position, the latter being the
main factor in determining the energy resolution of the time-of-flight
analyzing section. In those cases where higher energy resolution is
needed, it is envisioned that the time width of the sample pulse would
be further reduced by the addition of a Fermi chopper (T~4~). This
chopper would have short, straight slots and a vertical axis of rotation
and be located in the shield immediately upstream of the collimator.
Like the counter-rotating disk choppers, it would rotate at integral
multiples of the source frequency.

For the focusing crystal, we propose using (for non-polarized
applications involving incident neutron energies of 5 meV and higher)
individual pyrolytic graphite (PG) plates attached to a segmented
GMI-type holder. This arrangement will provide optimal vertical focusing
over the entire 5-90 meV neutron energy range. Because a relatively
broad horizontal mosaic (probably 1.0 ^o^) will be needed for optimum
performance, we plan to use sets of three ZYA grade PG plates stacked
with spacers to slightly offset the angles between them. This so-called
"fanned arrangement" has been demonstrated to expand the horizontal
mosaic but leaves the vertical mosaic (and thus the vertical focusing
properties of the crystal) unchanged. At incident neutron energies below
5 meV, where high monochromator scattering angles hamper measurements
because of the excessively tight resolution introduced by the PG crystal
in a near backscattering geometry and space restrictions, we envision
substituting an equivalent crystal composed of fanned plates of
fluorinated synthetic mica.

### Sample Stage

As emphasized in the Scientific Objectives Section, it is important for
many of the areas of research for which HYSPEC is intended that
specialized sample environments can be easily and quickly installed.
Because the collimation upstream and downstream of the sample will
restrict the horizontal field of view of the detectors to the sample
area alone, the HYSPEC sample axis will not have to be part of the
instrument vacuum system as in other planned inelastic instruments. Thus
we expect to be able to employ conventional cryostats, magnets,
furnaces, pressure cells, etc. without creating any significant, sample-
environment-related background problems. This is another unique feature
of HYSPEC.

1.  **Secondary Spectrometer (Analyzer/Detector)**

Scattered neutron energy analysis would be done by time-of-flight alone.
We propose using a 4.5 meter radius array made up of 188
one-dimensional, position-sensitive, tube-type ^3^He detectors 2.5 cm in
diameter and 1.28 meters long, centered at the sample position. A set of
radial collimators (with horizontal angular acceptances of 20, 40, and
60 minutes) interposed between the sample and detectors would restrict
their horizontal field of view to the sample area alone. Individual
horizontal and vertical pixel resolutions are envisioned as being
respectively 20 and 20 minutes of arc. Additionally, the array would
span a 60 degree arc horizontally and have a vertical acceptance of ±
7.5 degrees with respect to the scattering plane. It would be mounted on
air pads on a "tanzboden" so that it could both move with the drum
shield and be rotated about the sample axis to permit accurate
positioning with respect to the monochromatic beam incident on the
sample. The detector area would most likely be gas filled and located in
a fixed, well shielded housing.

1.  **POLARIZATION ANALYSIS**

One of the particularly attractive features of the HYSPEC concept is
that it can quickly and easily be adapted to polarization analysis. All
that is required is that the focusing crystal used for non-polarized
studies be replaced by a crystal that both focuses and polarizes the
monochromatic beam incident on the sample and that polarization
analyzers be installed between the sample and detectors. In the
immediately following paragraphs we briefly describe the crystal we
propose using for polarized applications. The selection of a
polarization analyzer will be addressed in paragraphs that follow.

1.  **The Polarizing Crystal**

Polarizing crystals are crystals of ferromagnetic materials with low
index Bragg peaks in which the nuclear and magnetic contributions to the
coherent scattering add for one of the two neutron spin states and
subtract and cancel for the other when the atomic magnetic moments are
fully aligned by an external magnetic field. Of those investigated to
date, the consensus view is that the Heusler alloy crystal Cu~2~MnAl is
the best choice in terms of both reflectivity and polarizing efficiency.
Detailed studies - such as those made, for example, by A. Freund et al
\[Physica [120B]{.underline}, 86 (1983)\] - have shown (i) that well
annealed Cu~2~MnAl crystals have (111) Bragg reflecting efficiencies
that can approach those expected for ideally imperfect crystals, (ii)
that the polarization of the reflected neutrons is in excess of 95% in
crystals in which the Mn moments are fully aligned in an external field
(in this case produced by permanent magnets) and (iii) that there is no
significant loss of polarization when individual crystal plates are
mounted so as to form a cylindrically-curved, vertical focusing array.

We are aware that Cu~2~MnAl crystals, like all polarizing crystals, are
not without problems. One is that the procurement of crystals of good
quality has been difficult in the past. We have been advised, however,
that they can now be obtained from the ILL. A second is that the nuclear
and magnetic contributions to the (222) Bragg reflection are not
well-matched and second order contamination of the reflected beam can
significantly reduce the polarizing efficiency. Fortunately this is
ruled out as a potential problem for HYSPEC because the upstream
choppers will deliver a higher-order-free, monochromatic beam to the
crystal. And third, there is the issue of parasitic Bragg reflections
which are known to have a non-negligible impact on both the polarizing
efficiency and reflectivity and are likely to impair performance at
certain neutron energies. The time-honored way to side-step this
difficulty is simply to chose a crystal orientation that gives optimal
performance at the energy at which the spectrometer will most often be
used, in this case 15 meV, and incorporate messages into the operating
software that warn experimenters about operating at those incident
neutron energies where the polarizing efficiency and/or reflectivity is
significantly affected by parasitic reflections. Since this will
somewhat limit experimental flexibility, it is our intention to explore
the feasibility of fabricating the individual plates in the vertical
focusing array from stacks of 0.5 mm thick wafers cut from Cu~2~MnAl
single crystals. There are two potential advantages of this so-called
"composite crystal" approach. One is that composite wafer stacks are not
as spatially coherent as monolithic crystals; this reduces parasitic
reflections. The other is that introducing mosaic into the wafers by
single-axis deformation before they are bonded together creates a highly
anisotropic mosaic that improves the reflectivity without impacting the
vertical focusing.

2.  **The Polarization Analyzers**

Signal and background are typically of comparable intensity in
polarization analysis measurements. Translated into practical terms,
this means that definitive determination of the signal polarization will
only be possible if both the polarizer and polarization analyzer have
polarizing efficiencies in the 0.80--0.95 range. Heusler alloy polarizer
crystals, which produce polarizations on the order of 0.90--0.95, easily
meet this standard over the neutron energy range in which HYSPEC will
operate. Because they are of comparable efficiency and are both
well-tested and maintenance-free, our choice for analysis of the
polarization of the scattered neutron beam is the supermirror-bender
polarization analyzer. Reduced to essentials, this type of analyzer is
nothing more than a short, curved multi-channel guide with
magnetically-aligned, polarization-selective Fe-Si supermirror films on
the channel walls. Because the angles of total reflection of the + and
-- spin states differ by more than a factor of three in such films,
neutrons of one spin state tend to follow the curved channels while
those of the other continue in their original direction. The incident
beam is thus divided into divergent beams of opposite polarization. When
the beam is well-collimated and the bender optimally curved and tilted,
polarization analyzing efficiencies in the 0.80-0.95 range are easily
achieved. Moreover, at sufficient analyzer-to-detector distances the two
beams become spatially separated and both polarizations can be observed
at the same time \[S.H. Lee and C.F. Majkrzak; J. Neutron Res.
[7]{.underline}, 131 (1999)\].

To make the most effective use of the extended angular acceptance of the
HYSPEC detector array, an equivalent array of bender analyzers would
need to be installed, each directly downstream of a 20 minute angular
acceptance collimator. Like all such analyzers presently in use, we
envision that the bender channels would be formed by single crystal Si
wafers with surface coatings of Fe-Si supermirror film. For HYSPEC the
wafers would be 0.25 mm thick, 5 cm long and 14 cm high. To perform
optimally at 15 meV, they would be horizontally bent to a radius of
about 5 meters. Packed in groups of 80 into 3 cm exterior width
thin-walled aluminum alloy containers, the wafer packs would form 2 cm
wide, 80 channel benders. Room temperature single crystal Si, it should
be noted, is sufficiently transparent to neutrons in the energy range of
interest that scattering and absorption losses would be of the order of
10 percent. Permanent magnets would be used to align the Fe moments in
the Fe-Si films. Assuming the closest possible packing of the containers
along an arc 55 cm from the sample axis, 19 such bender analyzers could
be positioned within the (60 degree horizontal and 15 degree vertical)
solid angle subtended by the detector array.

![](media/image5.wmf){width="5.374409448818898in"
height="4.761811023622047in"}

From the Monte Carlo (MC) simulation shown in Figure 3, it is evident
that the spatial profiles of the two oppositely polarized beams would be
completely separated at the detector bank 4.5 meters from the sample
axis. It is also evident in the figure that the undeflected beam will
spread over two (2.5 cm diameter) detectors and the oppositely polarized
(somewhat wider) beam following the curved channels will fall on the
adjacent three detectors. Considering that each bender analyzer unit
would be centered on a group of 10 detectors, 50 percent of the detector
array would be actively collecting data in this arrangement (which is
very nearly optimum from the standpoint of maintaining adequate spatial
separation of the beams from adjacent analyzers). Note that although
each of the 19 analyzers is limited to a specific scattering angle,
rotation of the detector bank around the sample axis will make it
possible to cover - sequentially - the full scattered neutron angular
range. No more than ten rotational steps of the detector would be needed
to monitor both the flipped and unflipped spin intensities over the full
60 degree angular range covered by the detector array. It is also clear
from the Figure 3 that the flipping ratio measured in some detectors is
higher than that in the others (in which the peak of the "wrong"
polarization occurs). Therefore, if the signal is sufficiently strong,
it is possible to enhance the polarization sensitivity by restricting
the counting to the detectors with the highest flipping ratios. This is
equivalent to tightening the beam collimation after the
bender-polarizer, but could be done by the experiment analysis software
without redoing the actual measurement. Such flexibility is another
attractive feature of the supermirror-bender polarization analysis
scheme proposed for HYSPEC.

![](media/image6.wmf){width="6.0in" height="4.8in"}

Figure 4 shows a MC simulation of bender analyzer performance over a
range of energies on either side of 15 meV. From this it is evident that
benders designed for optimum polarizing efficiency at 15 meV will
operate with little loss of efficiency over an energy range extending
from roughly 8 to 18 meV. A second set of essentially identical bender
analyzers configured for optimum performance at 5 meV would cover the
energy range from about 3 to 8 meV with about the same efficiency,
making it possible to span the entire scattered neutron range from 3 to
18 meV.

Probably the most attractive feature of using bender analyzers in a
time-of-flight energy analysing system like HYSPEC's is the capability
of monitoring *both* scattered neutron spin states simultaneously over a
relatively broad scattered neutron energy range. But there are other
attractions as well: bender analyzers are extremely stable and, as noted
above, once built require little or no attention and are completely
maintenance free.

### PERFORMANCE

Of the many special features of the instrument, the most important from
the viewpoint of efficient use of source neutrons is the utilization of
the superior focusing properties of curved crystals to concentrate the
monochromatic flux on sample.

To quantify the advantage of the HYSPEC concept in this respect, a
number of MCSTAS, Monte-Carlo-based, flux-on-sample simulations were
made to compare HYSPEC's flux-on-sample with that of other proposed SNS
inelastic instruments. As is evident in Figure 4, despite the finite
reflectivity of the PG crystal, its superior focusing properties
concentrate the monochromatic flux more effectively than converging
guides over the greatest part of the energy range in which HYSPEC will
operate. For the moderate resolution, single crystal sample studies for
which it is planned, the HYSPEC approach is markedly superior.

![](media/image7.wmf){width="5.9992125984251965in"
height="5.022047244094488in"}

3.  **Moderator Choice**

Monte Carlo simulations have also shown that to perform optimally over
the incident neutron energy range of interest (5-90 meV), a spectrometer
of this type needs to be located on a beamline served by a coupled, 20
K, supercritical H~2~ moderator. Our first choice would be Beamline 15.
Because optimum performance requires that the monochromator and analyzer
energy resolutions be reasonably well matched, the length of the
monochromating section should not be much greater than 20 meters. It is
also importantant that it ends at a place on the SNS experimental floor
where there is sufficient space to accommodate both the rotating drum
shield and the relatively extended time-of-flight analyzer and its
associated shielding. Extending the instrument to an outside building
would lead to a detrimental mismatch between the monochromating and
analyzing resolutions and an unacceptable reduction of monochromatic
flux at the sample position.

2.  **ADVANTAGES OF THE HYSPEC CONCEPT**

Apart from the special features described above, HYSPEC has many other
attractive properties:

• Incident neutron energy selection would be primarily by time-of-flight
as would

analysis of scattered neutron energies. Thus full advantage is taken of
the pulsed

nature of the beam.

• Only two components - the counter-rotating chopper pair and the Fermi
chopper --

would operate at high rotation speeds. Both would be of conventional
design and

would operate well within the limits imposed by the tensile strengths of
the available

materials.

• Moving the sample out of the direct beam and placing shielding between
it and the detectors is expected to reduce beam-related background to a
minimum.

• Near-forward sample scattering will be accessible to investigation.

• Depending on the type of measurement, the spectrometer operation could
be optimized for either a wide-angular-acceptance or Q-selective mode.

• Apart from its focusing properties, the crystal also has the important
effect of substantially reducing the (source produced) spectral
asymmetry of the beam incident on the sample: the energy resolution
function will be more symmetrical than on other instruments on coupled
moderators.

• A relatively wide primary beam (4 cm) will be available thus making it
possible to fully illuminate large as well as small samples.

• A broad and continuous range of incident neutron energies (5≤E~i~≤90
meV) will be accessible without order contamination.

• Energy resolution can be incrementally varied simply by changing the
rotation rate of the counter-rotating and /or Fermi choppers.

• Changing the collimation of the incident monochromatic beam or the
angular acceptance at the analyzer (or both) is all that is needed to
vary the longitudinal Q-resolution.

• Various kinds of monochromating crystals with a variety of focussing
options could be employed such as, for example, two-dimensional
focussing, asymmetrically-cut, perfect crystal focussing, etc.

3.  **FUTURE CONSIDERATIONS**

Yet to be addressed in detail are two design issues. The first and most
important relates to keeping the background to a minimum: a choice will
have to be made between a curved guide with a beam stop inside the
monochromator drum shield and a straight guide with a "get-lost" pipe
with the beam stop a considerable distance from the data collecting
area. The second issue is purely mechanical: the large (and heavy)
analyzer of the spectrometer will have to be rigidly coupled to the
large (and heavy) monochromator drum shield and move with it when the
incident energy is changed. Additionally, the detector array will have
to rotate in a precise and reproducible fashion about the sample axis.
This, however, does not seem to pose any conceptual difficulty, as
similar task is successfully accomplished in the design of several
existing modern neutron spectrometers, such as FOCUS at Swiss Spallation
Neutron Source SINQ, or a Spin-Echo machine recently commissioned at
NIST Center for Neutron Research.

There is also a location constraint that the Committee will need to take
into consideration: because the proposed secondary flight-path is
sufficiently long (4.5 m), and in addition the sample axis is offset
from the primary beam (1.8 m), the combined drum shield, analyzer and
associated analyzer shielding will require an ample amount of space on
the experimental floor and limit the choice of beam lines.

4.  **INSTRUMENT DEVELOPMENT TEAM**

The members of the IDT are listed below and come from a variety of
national and international universities and laboratories. All have,
however, a common interest in investigation of low energy, atomic-scale
dynamical properties of crystalline solids. Many are relatively senior
members of the condensed matter community with well-established
scientific credentials such as Tranquada, Lander, Shirane, Rhyne, etc.
who expect to use HYSPEC to continue their research programs. The IDT
also includes younger faculty members such as Lee, Greven, Kiryukin,
Hirota who are still in the process of building their scientific
programs and will use HYSPEC as one of their important research tools.
Other members of the IDT have particular areas of expertise, which are
vital to the project. For example, Majkrzak and Regnault are leading
experts in polarized neutron beams, the Ames group (Stassis) has
extensive experience in building large mechanical structures, and Daemen
is a leader in Monte Carlo simulations and has extensive experience in
software development. It also goes without saying that there will have
to be close interaction with the SNS Instrument team and Abernathy is
currently has that connection.

1.  **Members of the IDT and their Institutional Affiliations**

[I. Zaliznyak]{.underline}, co-PI BNL

[S. M. Shapiro]{.underline}, co-PI BNL

G. Shirane BNL

J. Tranquada BNL

L. Passell BNL

D. Abernathy SNS

L. Daemon Los Alamos

M. Greven Stanford

B. Gaulin McMaster

K. Hirota ISSP

V. Kiryukhin Rutgers

G. Lander EITU

Y. Lee MIT

C.Majkrzak NIST

S. Nagler ORNL

R. Osborn ANL

L. P. Regnault CEN-Grenoble

J. Rhyne U. Missouri

C. Stassis Ames/Iowa St.

A. Zheludev ORNL

# Project Current Organization Chart

5.  **ESTIMATED BUDGET**

The HYSPEC estimated budget is presented in Table II below. It is
necessarily tentative and was arrived at after consultations with the
SNS Instrument team following a visit to Argonne in November 2001. Some
items which would be common for other instruments such as shielding,
detectors, choppers, etc were estimated from the HRCS Instrument Summary
for IDT prepared by G. Ganroth (July 8, 2002). These numbers include
overhead and contingency. We estimate that within a realistic funding
profile the instrument could be built in 5 years from onset of funding.
This is consistent with most instruments already funded, such as the
CNCS.

**Table II. HYSPEC Estimated Budget**

  --------------------------- ----------------------- -------- ----------
  Primary Flight Path                                           

                              Choppers                         1400

                              Incident beam shielding          2300

                              Supermirror guides               500

                              Beam monitors                    100

                                                      SUB      4300

  Monochromator                                                 

                              Shielding                        1000

                              Crystals and holder              400

                              Collimators                      50

                                                      SUB      1450

  Sample stage                                                  

                              Goniometer, sample               160
                              table                            

                              Ancillary equipment              250

                                                      SUB      410

  Analyzer, detector                                            

                              Collimator sets                  200

                              Polarization benders             1000

                              PSD, electonics mounts           900

                              Beam Controls-DAQ                140

                              Detector vessel                  1500

                              Beam stop/get lost pipe          200

                                                      SUB      3740

                              CAPITAL TOTAL                    9900

                              LABOR (1/3 Capital)              3300

                              TOTAL                            13200
  --------------------------- ----------------------- -------- ----------

**Appendix A. SCIENCE EXAMPLES**

# A.1 Complex Alloys and Nanomolecules

The proposed HYSPEC spectrometer is a world-class instrument with
polarization analysis capabilities, a feature that makes it a unique
instrument among those to be installed at SNS. Actually, the capability
of using polarized neutrons is essential in practically any detailed
study of the magnetic properties of condensed matter systems. Such
studies extend from the determination of the magnetic form factor to a
detailed analysis of magneto-vibrational scattering studies, such
studies provide invaluable information about the static magnetic
properties and, most importantly, the interactions between magnetic and
other excitations in condensed matter systems. Currently though, only
for some of the reflectometers previewed for the SNS a simple polarized
neutron capability is envisaged. Polarization analysis capability of the
instrument is of particular importance for the study of complex phases.
These phases exhibit complex arrangements of various interacting degrees
of freedom (such as spin, orbital moment, charge, etc.), with
fascinating physical properties, some of which may be exploited in
emerging technologies ranging from high-T~c~ superconductivity and spin
valves to photonic switches and quantum computing. The well-known
examples of such systems, presently under intense experimental and
theoretical scrutiny, are the various phases of superconducting cuprates
and the magnetoresistive manganites.

Among the complex systems whose physical properties can be exploited in
emerging technologies and which therefore have recently attracted
considerable attention are the magnetic molecules and the intermetallic
compounds of R~5~(Si~x~Ge~1-x~)~4~ family.

# A.1.1 (Si~x~Ge~1-x~)~4~ Intermetallic Compounds

This family was discovered over thirty years ago[^2], and has recently
opened up an unprecedented opportunity to solve the century-old problem
of the intrinsic relationships between the chemical composition, atomic
structure and properties of metallic materials. These compounds exist in
nearly every R-Si-Ge system, and the distribution and room temperature
crystallography of the R~5~(Si~x~Ge~1‑x~)~4~ phases is illustrated in
Figure 2.1.

The crystal structures of the R~5~(Si~x~Ge~1‑x~)~4~ phases consist of 36
atoms per unit cell distributed among 6 to 9 independent
crystallographic sites.[^3] These alloys exhibit a number of diverse and
unique properties[^4] associated with both their naturally layered
crystal structures and the combined magnetic-crystallographic
transformations [at low temperatures]{.underline}, driven by a
reversible breaking and reforming of specific covalent Si(Ge)-Si(Ge)
bonds (Fig. 2.2, left). The crystal structures change via a
martensitic-like collective shear movement of sub-nanometer thick slabs
by as much as 1.1 Å and are easily affected by magnetic field,
temperature, and pressure. The transitions are accompanied by a giant
magnetocaloric effect, a colossal magnetostriction, and a giant
magnetoresistance, suggesting many possible technological applications
in sensors and energy-transforming devices through intelligent
manipulation of the phase transformation by composition, applied fields,
temperature, and pressure.

For the most part, identical crystallographic transition also occurs [at
high temperature]{.underline} in the paramagnetic state (Fig. 2.2,
right) when the alloy stoichiometries are near critical (i.e. those
where compositional variations induce a crystallographic phase change).
X-ray powder diffraction measurements indicate that both the
low-temperature magnetically ordered and the high-temperature
magnetically disordered orthorhombic Gd~5~Si~4~-type phases are the
same. Preliminary first principles calculations[^5] indicate, on the
other hand, that the high temperature phase transition may be an
order-disorder transformation, during which Si and Ge atoms are
redistributed among their respective lattice sites. This
crystallographic phase change has a far-reaching effect on the magnetic
properties of the material, as illustrated in Fig. 2.3.

![](media/image11.jpeg){width="5.333333333333333in" height="2.06875in"}

# A.1.2 Nanosize Magnetic Molecules

The study of systems consisting of magnetic molecules is presently a
rapidly developing field within nanomagnetism. These are crystalline
materials are composed of identical molecules which can be manipulated
to obtain a large variety of spin designs, and can be used as prototypes
for the study of fundamental problems of magnetism at the nanoscale
level (such as quantum tunneling and quantum coherence). An
understanding of these phenomena on a fundamental level may help the
development of important technologies from spin valves to quantum
computing.

The more recently synthesized systems consist of magnetic molecules
containing a number of 3d or 4f magnetic ions connected by organic or
inorganic linking atoms. The advantage of these systems over
conventional nanosized magnetic particles is that they can be
synthesized as crystals, and this allows a systematic study of their
properties by neutron scattering techniques. Since the size as well as
well as the spin of the magnetic ions can be controlled to some extent
these systems are ideal to study the transition between atomic and
nanoscale magnetic phenomena as well as the range from S = 1/2 quantum
spins (e.g., Cu and V ions) to essentially classical spins (e.g. Fe^3+^
(s=5/2), and Tb^3+^ (J=6) ions). Systems with magnetic molecules
containing as few as two magnetic ions are presently available. To date
systems with magnetic molecules containing as many as 30 magnetic ions
have been synthesized. An example of such a system (Mo~72~Fe~30~) is
illustrated in Fig. 2.4.

To date, only a few neutron scattering experiments have been performed
on these systems, mainly because of inadequate energy resolutions over
different energy regions. For complete understanding of the dynamical
properties of these systems, it is necessary to perform elastic and
inelastic neutron scattering experiments as a function of both
temperature (down to approximately 10 mK) and magnetic field (up to
approximately 10 Tesla). The observed magnetic energy levels can be
compared directly with the results of theoretical calculations and can
be used for the determination of the parameters used in the model
Hamiltonian. Of particular interest here is the determination of the
anisotropy parameter, which is presumably due to the spin-orbit
interaction and which may be estimated theoretically. Particularly
challenging is the determination of the (small) zero-field energy
splittings of the ground state at low temperatures. Neutron scattering
studies in a magnetic field will also allow us to observe the evolution
of the ground state of the system as a function of the magnetic field.
This will provide important information about the onset of magnetic
level crossings. Again, the polarization analysis capability of the
HYSPEC spectrometer will be an invaluable in the study of these
interesting systems.

In summary, the detailed study of complex systems, which are being
synthesized so that they exhibit physical properties of potential use in
emerging technologies, is a challenging problem for which the use of the
polarization analysis capabilities of the HYSPEC spectrometer will be
invaluable. Still, for a detailed study of problems of this complexity,
data obtained in the HYSPEC spectrometer must be combined with the
information obtained in complementary instruments at SNS, such as the
proposed High Energy Resolution Chopper spectrometer and the
backscattering instrument.[^6]

# A.2 Functional Materials

Fundamental understanding of the microscopic processes governing
physical phenomena in the functional materials is key to the
technological progress. HYSPEC is an indispensable experimental tool
needed to obtain such understanding.

# A.2.1 Nanoscale Features of Functional Materials

The drastically enhanced electronic and magnetic responses occurring in
complex functional materials as a result of inhomogeneity on a
microscopic length scale have recently attracted very significant
attention. Both fundamental properties, and enhanced materials
characteristics potentially useful for industrial applications are
currently being actively investigated. During the last 5 years, it
became clear, for example, that nanoscale\
inhomogeneities are intrinsic to a number of important oxide systems,
such as superconducting cuprates[^7], magnetoresistive manganites[^8],
and lead-based relaxor ferroelectrics.[^9] Also, understanding and
controlling the giant structural, optical, and magnetic responses of
inhomogeneous materials holds great potential for the creation of
technologically advanced consumer products.

Some of the most important systems discussed above exhibit nanoscale
magnetic inhomogeneities. In magnetoresistive manganites, for example,
nanoscale magnetic inhomogeneities play the crucial role in the
so-called Colossal Magnetoresistance (CMR) effect.[^10] While it is
known that nanoscale magnetic regions exist in these materials, the
structural and magnetic properties of these regions remain largely
uncharacterized.Error: Reference source not found^,^[^11] To understand
the physics of the CMR effect, and to guide future work for the
synthesis of materials with enhanced magnetoresistive properties, it is
crucial to determine the magnetic and structural properties of the
inhomogeneous states realized in these materials.

Another class of experimental systems that has recently attracted
significant attention are doped magnetic semiconductors, such as
Mn-doped GaAs and InAs, and Co-doped ZnO and TiO~2~.[^12] ^,^[^13] The
recent claimsError: Reference source not found of synthesis of
room-temperature magnetic semiconductors have generated a significant
controversy. It is becoming clear now, that in many cases the prepared
systems are actually very fine mixtures of non-magnetic semiconductors
(e.g., GaAs or InAs) and ferromagnetic metals (e.g., MnAs or MnSb).
Physical properties drastically evolve when the mixed systems vary from
complete solid solution to macroscopically phase-separated materials.
[^14] Some of these systems exhibit very large values of
magnetoresistance.[^15] A number of these materials can now be
synthesised in the bulk form. In order to understand this intriguing
behavior, it is essential to characterize the microscopic properties of
these materials, with special attention paid to the nanoscale magnetism.
In this case, as well as in the CMR manganites discussed above,
characterization of both static and dynamic magnetic properties will be
of great importance. In particular, dynamic magnetic properties of the
nanoscale regions are expected to be significantly different from the
corresponding bulk systems. In fact, drastic changes in the lattice
dynamics properties have recently been observed in relaxor
ferroelectrics,Error: Reference source not found which can be considered
a non-magnetic analogue to the systems\
described here.

Polarized neutrons will, undoubtedly, be one of the most effective tools
to study the nanoscale magnetism in these, and related, systems. A very
important factor underlying the usefulness of this probe is that
magnetic signals from systems inhomogeneous on nanoscale are spread both
in the reciprocal space, and in energy, and are, therefore, weak. In
addition, nonmagnetic background is typically strongly enhanced in such
systems. With the proposed spectrometer (HYSPEC) it would be possible to
successfully separate diffuse magnetic signals from the background, such
as multiple Bragg scattering, phonons, and incoherent scattering due to
isotopic disorder, and the polarization analysis will be of crucial
importance.

# A.2.2 Anomalous Phonon Behavior in Functional Materials

It has long been known that structural phase transformations occurring
in materials such as ferroelectrics are driven by a lattice vibrational
mode whose frequency tends to zero at the transformation temperature.
This so-called 'soft' mode usually occurs at a well defined wavevector
in a particular optic or acoustic phonon branch and is frequently
accompanied by a 'central' peak, i.e. divergent elastic scattering. The
anomalies associated with the soft mode result in very anisotropic
dispersion curves. Recent work on ferroelectics like PZT and PMNError:
Reference source not found and on shape memory alloys such as Ni-Al,
Ti-Pd, and Ni~2~MnGa have demonstrated the need for good wave-vector
resolution and for minimization of the extraneous elastic scattering
from the sample environment. It also makes clear that single crystals
are necessary. Most often these crystals are grown explicitly for
neutron scattering experiments and tend to be small. Focussing of the
incident beam is therefore vital to assure the maximum number of
neutrons impinging on the sample; a requirement met by the proposed
HYSPEC instrument.

# A.3 Strongly Correlated Electron Systems

Strongly correlated electron systems, from heavy-Fermions to high-T~c~
to materials exhibiting giant magneto-resistance effects, has added a
new class of materials to those with which we are familiar. Of crucial
significance in these materials are the role of electron correlation
energies and the interaction of the lattice with the spin degrees of
freedom. Unlike the BCS superconductors in which vibrational degrees of
freedom (the phonons) play a key role in mediating superconductivity,
the new phenomena are classified as having a range of interactions, with
the dominant one often difficult to identify.

Neutron inelastic scattering is the single most important probe for
unraveling the energetics and spatial dependencies of these
interactions. At the same time, we have to be able to separate
interactions that are electronic from those that are vibrational. This
requires polarized neutrons, and very often polarization analysis. We
may demonstrate the argument by invoking the pioneering work by Axe et
al.[^16] on the vibrational spectra of Nb~3~Sn above and below T~c~.
These experiments, performed in the 1970s, used unpolarized neutrons.
Because superconductivity in BCS materials are dominated by vibrational
energies there has never been any doubt that the measurements were of
phonons. However, if we think today of the superconductivity of
UPd~2~Al~3~, of high-T~c~ materials, that question is much harder to
answer. In both UPd~2~Al~3~ [^17] and high-T~c~ materials[^18] full
polarization analysis has proved of enormous value in separating the
electronic and lattice effects.

We need to continue to develop the technology of polarized beams and
polarization analysis (PA), so that these experiments can extend over a
wider range of energy than presently available and not be so demanding
in terms of intensity. Very large crystals are presently required to map
out excitations in full PA ,[^19] which means that the technique cannot
be generally used. An instrument at the SNS will therefore open a
completely new field of endeavor. Not just to be able to perform PA on
selected excitations, but, more importantly, to construct S(***Q**,ω*)
maps of spin-flip and non-spin-flip intensity.

Furthermore, efforts in three-dimensional polarization analysis, now
done almost exclusively with the spherical neutron polarimeter at the
ILL, can often make unambiguous identification of magnetic
configurations[^20] and needs to be implemented on a wider scale. Its
extension into inelastic scattering is just beginning.[^21] Many
surprises in our understanding of magnetism, and in defining precise
interactions, will emerge with the use of polarized neutrons.

# A.4 Quantum Critical Points

A quantum critical point is a phase transition in which the temperature
at which long-range order is established is suppressed to zero by
quantum disorder. This may be induced either externally, e.g. with the
application of pressure or magnetic field, or internally through either
compositional variations or by increasing disorder. Close to a quantum
critical point, the energy scale of critical fluctuations is determined
by the temperature which leads to highly unusual scaling of the critical
dynamics. Universal scaling in ∆/T has been observed by inelastic
neutron scattering in at least two systems believed to be close to
quantum critical points, UCu~5-x~Pd~x~ and CeCu~6-x~Au~x~. Such
experiments are challenging because they require measurements over a
wide range of energies. Thus far, the most complete data have been
obtained on time-of-flight spectrometers at both reactor and pulsed
neutron sources. However, the need for large samples has necessitated
the use of systems in which quantum criticality is induced by sample
non-stoichiometry rather than by an extreme sample environment.
Theoretically, it is important to reproduce this scaling behavior in
high-quality single crystals that are stoichiometric so that atomic
disorder can be discounted as a contributory factor. For instance, the
most reliable examples of quantum phase transitions are in systems such
as CePd~2~Si~2~ and ZrZn~2~, occuring at pressures of 28 kbar and 21
kbar respectively. There is also evidence that metamagnetic transitions
in some heavy fermion compounds - commonly occurring in fields of
greater than 10 T -may be associated with quantum phase transitions. The
ability to utilize extreme sample environments on HYSPEC will be
especially important in investigating quantum phase transitions. Also,
the focusing properties of HYSPEC will be vitally important because of
limited sample sizes and volumes.

# A.5 Correlated Phases in Many-Electron Systems

The ground states of many-electron systems with spin, orbital and/or
charge/valence degeneracy often exhibit non-trivial arrangements of
these degrees of freedom which lead to many fascinating physical
properties. Recently much attention has been paid to exploring the
charge and/or orbitally ordered phases in transition metal oxides such
as the superconducting cuprates and magnetoresistive manganites.[^22]
Understanding the nature and origin of these phases is of both
fundamental importance and technological interest. Neutron scattering is
the most informative and least perturbing experimental way of providing
information on the correlated structural distortions and/or magnetic
correlations in the charge/orbital/spin-ordered phases. Among the unique
features inherent to the neutron scattering approach is that it provides
both good energy and wavevector resolution, which permits accurate
discrimination between the static and dynamic correlations and allows
separate characterization of the corresponding correlation ranges.
Examples where this is of crucial importance are the recent observations
of striped liquid phases in the doped layered nickelates, incommensurate
dynamical correlation in the cuprate superconductors and
characterization of dynamical slow-down in the course of the spin-glass
freezing transition in the half-doped cobaltate
La~1.5~Sr~0.5~CoO~4~.[^23]

A spectrometer, optimized for studies of the short- and long-range
static and dynamic correlations in single crystals should be designed to
have both a small elastic and negligible inelastic background, a
symmetric and well-reproduced resolution function (permitting a
straightforward and reliable (de)convolution procedure) and have a
polarization analysis capability to distinguish between structural and
magnetic scattering. Equally important, it should provide complete and
continuous coverage of a significant volume in the reciprocal space of
the crystal with both energy and wavevector resolution easily adaptable
to the type of the problem studied. Finally, it is essential that such
an instrument be easily equipped with a variety of sample environments
since definitive insight into the nature of many-electron correlated
phases is often obtained by studying how they are affected by changes in
external magnetic field, temperature, pressure, etc. As will be evident,
the proposed HYSPEC spectrometer is designed to both provide superior
performance and to address these concerns.

# A.6 Quantum Magnetism

Amazing progress has been made in the field of quantum low-dimensional
magnetism in the past twenty years, almost entirely driven by inelastic
neutron scattering experiments. The simplest fundamental model systems
are by now very well understood. Today the challenge is to learn about
more complex phenomena such as the effects of disorder and impurity
substitution, spin-lattice interactions and one- and two-dimensional to
three-dimensional crossover effects.

One example of a new research direction in quantum magnetism is the
problem of *random-exchange* spin chains. Consider the case of a *S=1/2*
quantum spin chain with nearest-neighbor Heisenberg exchange
interactions of random magnitude. In the simplest model, a given
percentage x of the bonds are of magnitude J~1~, and the remaining (1-x)
bonds correspond to a coupling strength J~2~, the strong and weak bonds
being distributed randomly. For a fixed x the limiting cases are easy to
understand. For J~1~\<\<J~2~ the chain breaks up into an ensemble of
finite-sized clusters: dimers, trimers, tetramers, etc. Each cluster has
a *discrete* excitation spectrum, that in a neutron experiment appears
as a series of sharp excitations at well-defined energies. For J~1~=J~2~
the situation is entirely different. In a *uniform* spin chain such as
this, the spectrum is an exotic continuum of states with no sharp
excitations and a strong wave vector dependence. The question is: how
does the spectrum look in the intermediate case of J~1~\<\~J~2~?

To date, the answer is not known, even theoretically. The main reason is
that an experimental realization of a model suitable for inelastic
neutron experiments has not been available until the recent preparation
of the new model compound BaCu~2~(Si~x~Ge~1-x~)~2~O~7~. Solid solutions
with 0\<x\<1 are characterized by susceptibility curves that can be
fitted to a Bonner-Fisher law with an intermediate value of J and are
expected to be realizations of the S=1/2 random-exchange model.
Inelastic neutron scattering experiments on BaCu~2~(Si~x~Ge~1-x~)~2~O~7~
single crystal samples are expected to provide unique insight into the
role of disorder in quantum magnets.

Previous work on the x=1 system was carried out on thermal and cold
triple-axis instruments at reactor sources as well as on the MARI time
of flight instrument at ISIS. It is clear from these measurements that
none of these instruments will be fully adequate for the study of the
solid solution BaCu~2~(Si~x~Ge~1-x~)~2~O~7~ . First of all, the
excitation bandwidth for x close to unity is expected to be of the order
of 150 meV and using a spallation source instrument becomes mandatory if
one wants to cover the relevant energy range. At the same time, the
small sample size (the largest existing crystals of BaCu~2~SiGeO~7~ are
about 4x4x10mm^3^) will require the ability to focus the incident beam
to a small spot size; a major feature of HYSPEC. Moreover, the weak and
diffuse nature of continuum scattering requires a very low and
well-controlled instrument background. Finally, to gain a detailed
understanding of the spin dynamics, polarized neutron experiments will
be needed. All of these needs will be satisfied by the proposed HYSPEC
instrument.

# A.7 Advanced Polarization Analysis using TAS and TOF spectrometers

Much of the present condensed matter research is devoted to
understanding the physics of \"strongly correlated electronic systems\".
This class of systems encompasses a large variety of physical
situations: un-conventional \"d-wave\" superconductivity in high-T~c~
superconductors (LSCO, YBCO\...) or spin-ladder cuprates
(Sr~14~Cu~42~O~41~...), un-conventional \"p-wave\" superconductivity
(Sr~2~RuO~4~, UGe~2~, CePd~2~Al~2~\...), charge-ordered materials
(NaV~2~O~5~, Sr~14~Cu~24~O~41~, LaNiO~4+y~\...), spin-Peierls materials
(CuGeO~3~, FeOCl, Pb\[Cu(SO~4~)(OH)~2~\]\...), exotic spin dynamics in
quantum-chain systems (J1-J2 systems, alternating chains, Haldane-gap
chains, dimer systems, magnetization-plateau systems\...), highly
frustrated systems (kagome, J~1~-J~2~-J~3~ honeycomb, pyrochlore\...),
etc.

The physics which is involved in all these systems may be understood
from the competition of different degrees of freedom: spin, orbit,
charge and lattice distortion. The understanding of mechanisms at the
origin of exotic properties is only possible if one can determine
accurately and unequivocally the wave vector and energy dependencies not
only of the \"standard\" correlation functions (i.e. those coupling
identical degrees of freedom : \<spin-spin\>, \<orbit-orbit\>,
\<displacement-displacement\>\...), but also the \"hybrid\" correlations
functions coupling different degrees of freedom (\<spin-orbit\>,
\<spin-displacement\>, \<orbit-displacement\>\...). To our knowledge,
the only way to solve this problem is to measure the magnetic-magnetic
(MM), nuclear-nuclear (NN) and magnetic-nuclear (interference) terms
using the inelastic polarized neutron scattering in conjunction with a
powerful polarization analysis technique.

The best known polarization analysis technique is the so-called
\"longitudinal polarization analysis\" (LPA) method invented long time
ago by Shull, Moon, Riste and Kohler.[^24] The LPA is invaluable to
obtain separately the magnetic and structural contributions, or the
various magnetic components at a given scattering vector. We have
recently used the LPA on IN22 at ILL to determine directly the magnetic
excitation spectrum in the high-Tc compound YBCO~6.85~ (Fig.1)[^25] or
Sr~14~Cu~24~O~41~ (unpublished results). The advantage of using a
polarized beam versus an unpolarized one is clearly demonstrated in Fig.
2.5. The magnetic response is superposed to a series of more or less
well-controled structural features (phonons, braggons, spurions\...)
which render the determination highly uncertain. The same scan performed
with a polarized beam allows a very precise determination of the
magnetic contributions (in particular the resonant peak at 41 meV and
the IC dynamic correlations existing between 25 meV and 41 meV). The
\"normal\" method would have consisted in performing constant-energy
scans at different energies to determine both the signal and the
background.

Some years ago (in the early 80\'s), the LPA was successfully used on
IN12 to demonstrate the relevance of soliton-type non-linear excitations
in the quasi-1D antiferromagnet TMMC.[^26] In particular, the use of
polarization analysis was fundamental to determine univocally and
separately the contribution S^^(***Q**,ω*), directly related to the
solitons, and S^⊥^(***Q**,ω*), related to the inter-soliton spins, Fig.
2.6. With un-polarized neutrons, the method to separate both
contributions would have consisted in measuring the Q-dependence of the
response, which is problematic if one contribution is much smaller than
the other. A recent example is measurement of the SF and NSF
contributions in CuGeO~3~, a prototype spin-Peierls system, which
allowed a spectacular determination of all the magnetic (bound states,
continuum...) and phonon modes, and the double-gap structure of the
magnetic excitation spectrum.

The LPA method can be transposed to TOF machine. At least there is no
conceptual interdiction, although the practical realization may not be
simple. As important it could appear, the LPA method is still a
relatively \"old\" and \"rustic\" method. Indeed, LPA only recovers one
part of the information. Because the incident polarization direction is
selected by applying a magnetic field on the sample, only a single
projection of the polarization vector after scattering can be
determined. In the last decade an important effort has been made at ILL
to develop a new polarization analysis technique, offering the
possibility to measure all three components of the final neutron
polarization (CRYOPAD concept).[^27] The \"spherical neutron
polarimetry\" (SNP) is a new alternative to LPA. It has demonstrated
strong capabilities in diffraction, in particular to resolve non-trivial
magnetic structures.[^28] By accessing the \"off-diagonal\" components
of the polarization tensor (i.e. P~xy~,P~xz~,P~yz~\...), the method
allows in pinciple to determine the so-called \"inelastic
nuclear-magnetic interference\" (INMI) terms.[^29] SNP is a very elegant
but difficult method to determine very-small magnetic or structural
**inelastic** contributions through their interferences with strong
ones. Although the method has not yet been demonstrated for INS, the SNP
could bring new information for, e.g., the relevance of orbital
correlations, \"hidden-order\", \"orbital-currents\" in High-T~c~
cuprates or in heavy-fermion materials (URu~2~Si~2~, etc). This field of
investigation is new for INS and almost all remains to be done.

HYSPEC offers a unique opportunity to develop new polarization analysis
techniques at a pulsed spallation source. Of course, a
broad-band/broad-angle CRYOPAD adapted to TOF spectrometry has not yet
been built (although there is a project and some ideas at ILL). On the
other hand, the flexibility of HYSPEC which owes to it \"hybrid\"
nature, will probably make the \"mono-detector/narrow beam\" SNP method
easily adaptable.

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    Document IS-1.1.8.2-8004-MM-A-00 (2000).

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[^3]: V. K. Pecharsky, K.A. Gschneidner, Jr., J. Alloys Compd. **260**,
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[^4]: V.K. Pecharsky and K.A. Gschneidner, Jr., Adv. Mater. **13**, 683
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[^5]: V.K. Pecharsky, G. Samolyuk, V.P. Antropov, A.O. Pecharsky, and
    K.A. Gschneidner, Jr., J. Solid State. Chem. (Submitted; Presented
    at the 23^rd^ Rare Earth Research Conference, UC Davis, CA, July
    13-18, 2002).

[^6]: The author is grateful to Professors Gschneidner, Jr., Pecharsky,
    and Luban, and Dr. Kögerler of the Ames Laboratory for providing
    information for this brief summary.

[^7]: Lang K.M., Madhavan V., Hoffman J.E., Hudson E.W., Eisaki H.,
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[^8]: E. Dagotto, T. Hotta, and A. Moreo, Phys. Rep. 344, 1 (2001).

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[^11]: V. Kiryukhin, T. Y. Koo, A. Borissov, Y. J. Kim, C. S. Nelson, J.
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[^14]: Y. Shon, Young Hae Kwon, Deuk Young Kim, Xiangjun Fan, Fu D, Tae
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[^21]: See article by Caciuffo et al. in ILL Annual report 2001

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