## Research Activity: Theoretical Condensed Matter Physics

Division: Materials Sciences and Engineering

Primary Contact(s): Dale Koelling (<Dale.Koelling@science.doe.gov> ,
301-903-2187)

Team Leader: William Oosterhuis

Division Director: Harriet Kung

**Portfolio Description:**

The Theoretical Condensed Matter Physics activity includes a broad
spectrum of theoretically research complementing all parts of the
Materials Science and Engineering Division. Research areas include
structure and properties of quantum dots and nanotubes, two dimensional
electron gas, quantum transport, tribology at the atomic level,
superconductivity, magnetism, and optics. A significant effort within
the portfolio is the development of advanced computer algorithms to
treat many-particle systems. An important facilitating component is the
Computational Materials Science Network (CMSN), which enables groups of
scientists from DOE laboratories, universities, and (to a lesser extent)
industry to address materials problems requiring larger-scale
collaboration across disciplinary and organizational boundaries.

**Unique Aspects:**

New areas of materials science are being identified and studied. New
technology is enabling a much closer examination of the existing ones.
This healthy progress dictates that new theories be developed and that
established ones be reexamined and possibly extended. A very important
contribution of the theorist is enforcing a rational, consistent
understanding of experimental observations so that we can go forward.
Often, this involves working out implications of a theory for a specific
material or situation. In materials, this can be an extremely difficult
task because of the very many atoms involved. Many conceptual tools such
as quasiparticles, entities defined to examine phenomena at different
length scales, or summary statistical approaches have been developed.
Further development of such conceptual tools continues to be a very
important aspect of this theoretical program. However, for many
phenomena now being studied, large scale computation must be utilized to
perform the complex calculations dictated by the fundamental theory or
to perform the simulations of systems with many interacting components.
The rapid advance in computational capabilities now enables research at
such a level of sophistication that computational science has become a
"third way of doing science", but at a price. The complexity of such
research very often requires larger groups of collaborating researchers
from a diversity of disciplines. One response has been formation of the
Computational Materials Science Network to assemble multi-disciplinary
groups of scientists from DOE laboratories, universities, and industry
to collaborate on computational materials science projects. At present,
CMSN consists of five sub-projects: Excited States and Response
Functions (testing the accuracy of current levels of fundamental
theory); Microstructural Effects on the Mechanics of Materials
(computational study of the fundamental basics of metallurgy);
Fundamentals of Dirty Interfaces: From Atoms to Alloy Microstructures

(the development of microstructure in heterogeneous materials); Magnetic
Materials Bridging Basic and Applied Science (an attempt to interconnect
different scales of magnetic behavior from quantum mechanical electronic
behavior all the way to continuum micro-mechanical properties);
Predictive Capability for Strongly Correlated Systems (an attempt to
advance capabilities in many body theory).

**Relationship to Others:**

This activity interacts with all the other research activities within
the Division of Materials Science and Engineering driven by mutual
interest Also, because the computational resources at the National
Energy Research Scientific Computing facility utilized by the division
are administered here, there is an enhanced awareness of opportunity.
Within DOE, frequent interaction occurs with the Mathematics,
Information and Computer Sciences division. Information on university
grants is shared with NSF, peer reviews are sometimes shared, and on
occasion there is joint funding of grants. On the international level
participation in organizing and steering committees is frequent, as are
exchanges of experts between foreign and domestic institutions.

**Significant Accomplishments:**

Consistent with the emphasis on nanoscience enabled by developments in
technology and computational techniques, notable achievements in this
area have been made within the Condensed Matter Theory activity.

Research into low dimensional materials has revealed exciting new
information and has pointed to new possibilities in creating new
tailored materials and devices. Highlights include:

1.  By judiciously attaching molecules to nanoclusters, one can guide
    them to assemble in a specified manner. (The rule book remains to be
    written but it is under construction.)

2.  Gold nanoparticles behave much more like their platinum group
    neighbors in the periodic table including exhibiting interesting
    catalytic behavior. When passivated by dodecane thiols, they can
    self-assemble into nanocrystalline superlattices with unique
    properties.

3.  \[Nanocrystalline diamond can form with a bucky-ball-like surface
    reconstruction. The carbon nanoparticles exhibit very weak quantum
    confinement unlike silicon and germanium. \[Galli\]

4.  Silicon nanotubes can be fo stabilization with a core of nickel
    atoms. Unlike carbon, silicon nanotubes are not stable without such
    help. \[Menon\]

Significant progress has also been made in other areas as illustrated by
the following examples.

Dynamic mean-field theory, which is exact for infinite dimensions, has
been successfully coupled with three dimensional band theory. The
resulting hybrid theory has been used to elucidate the spin polarization
of CrO~2~ \-\-- a famous magnetic recording material that might find new
use is spintronics. A competition between quasiparticle behavior and
local --moment behavior is found.

A way has been found to see diffraction data for molecules adsorbed on
surfaces. The conventional methods of low energy electron diffraction
will not work: the molecules produce no Bragg spots because they are
randomly distributed on the surface. However, it has been shown that the
information is present in the intensity variation of the spots
originating from the substrate. The resulting technique has successfully
revealed the geometry of small hydrocarbons on a palladium surface.

The origin of the light-induced conductivity in the transparent oxide
12CaO·7Al~2~O~3~ is traced to electrons excited off H ions present. To
accomplish this, it was first necessary to accomplish calculating the
Coulomb gap leading to hopping conductivity.

Progress has been made on the question of how to treat core-hole effects
in x-ray absorption spectra by a collaboratory research team of the
Computational Materials Science Network. The team, which focuses on
Excited States and Response Functions, brought together experts of all
applicable approaches to compare against each other and elucidate the
formal relationships between them. What resulted was an excellent
prescription for success but with identified places for improvement.
This is needed not only for fundamental understanding of the details of
x-ray absorption phenomena but technological applications \-\-- an
example of which is the measurement of the thickness of integrated
circuit interconnects.

**Mission Relevance:**

The program's ultimate purpose is to understand the properties of
existing materials and to reveal new ones that are more efficient in
producing, storing, and using energy. To this end, the programs in this
portfolio have the common goal of achieving a basic understanding of
matter at all scales ranging all the way from the atomic to the bulk.
The experimental and theoretical programs work closely together, but
there are also more independent modes of research. The theorists try to
establish a theoretical basis for experimentally observed results, which
almost always suggests further experiments, and thus leads to new
results. New experiments and experimental techniques are often
suggested. New science is also produced by simulating processes on
computers. "Computer experiments" can be performed which are difficult
or impossible perform in the laboratory. They are also much easier to
dissect and to vary the conditions in order to isolate the effective
mechanisms. For example, the behavior of the surface layers of materials
sliding on each other and a new understanding of the role of lubricants
has been obtained in this way. Other examples include investigations
into the behavior of electrons flowing in nano wires and nanotubes and
in the properties of matter at extreme conditions of temperature and
pressure.

**Scientific Challenges:**

The close relationship between the experimental and theoretical programs
dictates that many challenges are common to both. Examples are exploring
the behavior of complex systems, investigating nano-scale systems, and
understanding superconductivity. New ways of conceptually visualizing
and characterizing phenomena will broaden our horizons. Stripes
occurring in cuprate superconductors and two dimensional electron gasses
are an excellent example. Bridging length scales is a major thrust. The
tactic of dividing up the effects in materials according to the length
scale at which they occur has greatly facilitated our understanding. But
for the theorists, this creates the problem of how to pass needed
information between the different constructs used at the different
length scales. Only in that way can one calculate parameters rather than
make phenomenological fits. Such is the basis for improved understanding
and greater precision of our modeling. It is a continuing major goal on
which limited progress has been made. Bridging time scales is similarly
important but far less progress has been made. Basic theory improvements
are also needed. For example, density functional theory is our most
computationally tractable many body theory but it defines many
functionals both for the ground state or ensemble energy and separately
for the properties that must be determined. Whereas knowledge of the
exchange-correlation functional for the ground state energy is
reasonably advanced, knowledge of all other functionals is still quite
rudimentary. Other many body approaches, although far more
computationally intensive, provide important information and require
further development. Improvements are also needed in our computational
tools. Materials theory is a very heavy consumer of computer resources
even if not so visibly as other disciplines. (This is because materials
theory deals with many dissimilar problems rather than a few overarching
ones.) The materials community could very productively make use of vast
increases in computational capability. Because the phenomenal growth due
to hardware improvements is actually overshadowed by those due to clever
algorithm design, further improvements in "tool development" will
significantly impact future development of science in a qualitative way.

**Funding Summary:**

**Dollars in Thousands**

**[FY 2003]{.underline} [FY 2004]{.underline} [FY 2005]{.underline}**
**[Request]{.underline}**

16,993 17,000 17,975

> **[Performer]{.underline} [Funding Percentage]{.underline}**
>
> DOE Laboratories 75%
>
> Universities 25%

The program provides funding for 54 university grants supporting about
as many students and partially supporting about 60 faculty and senior
staff. There are approximately 70 postdocs fully or partially supported
by this CRA. This program supports research at LBNL, AMES, BNL, ANL,
LLNL, MRS, LANL, ORNL, and NREL. Programs at the laboratories are
multi-investigator efforts on problems that require extensive
participation by experimental and theoretical scientists. Many of the
research efforts at national laboratories involve interfaces with the
university and industrial communities and with user facilities.
Additionally, about \$1.4M is provided for projects of the Computational
Materials Science Network.

**Projected Evolution:**

Materials will be modeled with ever-greater sophistication and realism
and complexity. Needs and opportunities will drive the effort inexorably
in this direction. Science at the nanoscale will continue a major
example, although it is only one of many. With the Chemistry Division, a
cooperative effort will be begun with Mathematics, Information, and
Computer Sciences seeking to enhance our capabilities to model and
simulate at the nanoscale. As a way to bring together teams adequate to
address the more complex problems envisioned, the Computational
Materials Science Network will be enhanced.