pinecone/core-2020-05-10-deduplication huggingface dataset:
The limits of the total crystal-field splittings

The crystal-fields causing $|J>$ electron states splittings of the same
second moment $\sigma^{2}$ can produce different total splittings $\Delta E$
magnitudes. Based on the numerical data on crystal-field splittings for the
representative sets of crystal-field Hamiltonians ${\cal H}_{\rm
CF}=\sum_{k}\sum_{q}B_{kq}C_{q}^{(k)}$ with fixed indexes either $k$ or $q$,
the potentials leading to the extreme $\Delta E$ have been identified. For all
crystal-fields the admissible ranges $(\Delta E_{min},\Delta E_{max})$ have
been found numerically for $1\leq J\leq 8$. The extreme splittings are reached
in the crystal-fields for which ${\cal H}_{\rm CF}s$ are the definite
superpositions of the $C_{q}^{(k)}$ components with different rank $k=2,4$ and
6 and the same index $q$. Apart from few exceptions, the lower limits $\Delta
E_{min}$ occur in the axial fields of ${\cal H}_{\rm
CF}(q=0)=B_{20}C_{0}^{(2)}+B_{40}C_{0}^{(4)}+B_{60}C_{0}^{(6)}$, whereas the
upper limits $\Delta E_{max}$ in the low symmetry fields of ${\cal H}_{\rm
CF}(q=1)=B_{21}C_{1}^{(2)}+B_{41}C_{1}^{(4)}+B_{61}C_{1}^{(6)}$. Mixing the
${\cal H}_{\rm CF}$ components with different $q$ yields a secondary effect and
does not determine the extreme splittings. The admissible $\Delta E_{min}$
changes with $J$ from $2.00\sigma$ to $2.40\sigma$, whereas the $\Delta
E_{max}$ from $2.00\sigma$ to $4.10\sigma$. The maximal gap $\Delta
E_{max}-\Delta E_{min}=2.00\sigma$ has been found for the states $|J=4>$. Not
all the nominally allowed total splittings, preserving $\sigma^{2}=const$
condition, are physically available, and in consequence not all virtual
splittings diagrams can be observed in real crystal-fields.Comment: 30 pages, Appendix (8 pages) and 11 tables; submitted to pss(b
Quantum modeling of semiconductor gain materials and vertical-external-cavity surface-emitting laser systems

This article gives an,overview of the microscopic theory,theory used to quantitatively model a wide range of semiconductor laser gain materials. As a snapshot of the current state of research, applications to a variety of actual quantum-well systems are presented. Detailed theory experiment comparisons are shown and it is analyze how the theory can be used to extract poorly known material parameters. The intrinsic laser loss processes due to radiative and nonradiative Auger recombination are evaluated microscopically. The results are used for realistic simulations of vertical-external-cavity surface-emitting laser systems. To account for nonequilibrium effects, a simplified model is presented using pre-computed microscopic scattering and dephasing rates. Prominent deviations from quasi-equilibrium carrier distributions are obtained under strong in-well pumping conditions
Finite Element Simulation of the Optical Modes of Semiconductor Lasers

In the present article we investigate optical near fields in semiconductor
lasers. We perform finite element simulations for two different laser types,
namely a super large optical waveguide (SLOW) laser, which is an edge emitter,
and a vertical cavity surface emitting laser (VCSEL). We give the mathematical
formulation of the different eigenvalue problems that arise for our examples
and explain their numerical solution with the finite element method. Thereby,
we also comment on the usage of transparent boundary conditions, which have to
be applied to respect the exterior environment, e.g., the very large substrate
and surrounding air.
  For the SLOW laser we compare the computed near fields to experimental data
for different design parameters of the device. For the VCSEL example a
comparison to simplified 1D mode calculations is carried out
Exchange coupling in transition-metal nano-clusters on Cu(001) and
  Cu(111) surfaces

We present results of density-functional calculations on the magnetic
properties of Cr, Mn, Fe and Co nano-clusters (1 to 9 atoms large) supported on
Cu(001) and Cu(111). The inter-atomic exchange coupling is found to depend on
competing mechanisms, namely ferromagnetic double exchange and
antiferromagnetic kinetic exchange. Hybridization-induced broadening of the
resonances is shown to be important for the coupling strength. The cluster
shape is found to weaken the coupling via a mechanism that comprises the
different orientation of the atomic d-orbitals and the strength of
nearest-neighbour hopping. Especially in Fe clusters, a correlation of binding
energy and exchange coupling is also revealed
Sorting and transmission electron microscopy analysis of single or double wall carbon nanotubes

On the basis of the recent progress on the sorting of carbon\ud
nanotubes’ structure with respect to their diameter or number of walls, we investigate by transmission electron microscopy the sorting efficiency, with a comparison with optical absorption spectroscopy measurements. We study density gradient ultracentrifugation sorted single walled or double walled carbon nanotubes, showing obviously the ability to separate carbon nanotubes of different diameters or/and number of walls. This microscopic approach affords accurate information about the sorted samples such as the real mean diameter, the relative concentration of double walled carbon nanotubes over single walled carbon nanotubes, standard deviation, and the real diameter distribution of carbon nanotubes, even beyond any possible accurate analysis from optical absorption spectroscopy. Therefore, we demonstrate that the diameter analysis of the sorted samples by TEM can indeed afford some information about the relevant optical properties of carbon nanotubes
Development of the critical exponent at the antiferromagnetic phase
  transition of YbRh2Si2 under chemical pressure

We present specific-heat measurements in the vicinity of the
antiferromagnetic phase transition on single crystals of the alloy
Yb(Rh_{1-x}Co_x)2Si2 for x<= 0.38. This study was motivated by the violation of
critical universality in the undoped YbRh2Si2 (Krellner et al., Phys. Rev.
Lett. 102, 196402) where we have found a large critical exponent a=0.38. For
Co-doped samples we observe a drastic change of the critical fluctuations
resulting in a negative a, explainable within the universality classes of phase
transitions. The development of a under chemical pressure gives strong
indication that the violation of critical universality in YbRh2Si2 is due to
the nearby quantum critical point.Comment: Accepted for the QCNP proceedings 200
Gd3+ rattling triggered by a "weak" M-I transition at 140-160 K in the
  Ce1-xGdxFe4$P12 x ~ 0.001 skutterudite compounds: an ESR study

In this work we report electron spin resonance (ESR) measurements in the
semiconducting Ce1-xGdxFe4P12 (x ~ 0.001) filled skutterudite compounds.
Investigation of the temperature (T) dependence of the ESR spectra and
relaxation process suggests, that in the T-interval of 140-160 K, the onset of
a "weak" metal-insulator (M-I) transition takes place due to the increasing
density of thermally activated carriers across the semiconducting gap of ~ 1500
K. In addition, the observed low-T fine and hyperfine structures start to
collapse at ~ 140 K and is completely absent for > 160 K. We claim that the
increasing carrier density is able to trigger the rattling of the Gd3+ ions
which in turn is responsible, via a motional narrowing mechanism, for the
collapse of the ESR spectra.Comment: 3 pages, 3 figures, presented at QCNP2009, to appear in pss
Magnetic impurity transition in a 'd+s'-wave superconductor

We consider the superconducting state of $d+s$ symmetry with finite
concentration of Anderson impurities in the limit $\Delta_s/\Delta_d \ll 1$.
The model consists of a BCS-like term in the Hamiltonian and the Anderson
impurity treated in the self-consistent large-$N$ mean field approximation.
  Increasing impurity concentration or lowering the ratio $\Delta_s/\Delta_d$
drives the system through a transition from a state with two sharp peaks at low
energies and exponentially small density of states at the Fermi level to one
with $N(0) \simeq (\Delta_s/\Delta_d)^2$. This transition is discontinuous if
the energy of the impurity resonance is the smallest energy scale in the
problem.Comment: 3 pages, 2 figures, presented at the International Conference on
  Quantum Criticality and Novel Phases (QCNP09, Dresden
Coupled Spin-Phonon Excitations in Helical Multiferroics

Both the Dzyaloshiskii-Moriya interaction and the exchange-striction are
shown to affect dynamically the magnetoelectric excitations in the perovskite
multiferroic RMnO3. The exchange-striction results in a biquadratic interaction
between the spins and the transverse phonons, giving rise to quantum
fluctuations of the ferroelectric polarization P. This leads to low-lying
phonon modes that are perpendicular to P and to the helical spins at small wave
vector but are parallel to P at a wave vector close to the magnetic modulation
vector. For spin-1/2 helimagnet, the local polarization can be completely
reversed by the spin fluctuation, and so does the direction of the on-site spin
chirality, which allows for a finite differential scattering intensity of
polarized neutrons from a cycloidal magnet.Comment: 7 page
Scanning Tunneling Microscopy studies on CeCoIn$_5$ and CeIrIn$_5$

High--quality single crystals of the heavy fermion superconductors CeCoIn$_5$
and CeIrIn$_5$ have been studied by means of low--temperature Scanning
Tunneling Microscopy. Methods were established to facilitate \textit{in-situ}
sample cleaving. Spectroscopy in CeCoIn$_5$ reveals a gap which persists to
above $T_c$, possibly evidencing a precursor state to SC. Atomically resolved
topographs show a rearrangement of the atoms at the crystal surface. This
modification at the surface might influence the surface properties as detected
by tunneling spectroscopy
Charge Density Wave in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$

Recent experimental results [A. Rusydi et al., Phys. Rev. Lett. 97, 016403
(2006)] revealed the stability of a peculiar charge density wave (CDW) with odd
period (three or five) in the coupled Cu$_2$O$_5$ spin ladders in
Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$, while the CDW with even period is unstable.
Here we resolve this contradiction by solving the effective t-J model for the
two coupled chains supplemented by the intersite interaction term V. Crucial to
this finding is the novel interaction term V (responsible for the interladder
repulsion between nearest neighbor holes), which is derived from the on-site
Hubbard-like repulsion between electrons on the same oxygen sites but within
two distinct orbitals which belong to two neighboring Cu$_2$O$_5$ ladders in
the charge transfer model.Comment: 4 pages, 3 figure
Low temperature magnetization of the quantum critical heavy fermion
  superconductor $\beta$-YbAlB$_4$

$\beta$-YbAlB$_4$ is the first Yb-based heavy fermion superconductor with
$T_{\rm c} = 80$ mK. We measured low temperature magnetization of high-purity
single crystals down to $T=$ 25 mK. The measurements have revealed a
considerable amount of volume fractions of the superconductivity and the upper
critical field $B_{c2}$ curve under field along the c axis, consistent with the
previous results. In the normal state, the previously observed divergent
behavior in the temperature dependence of the magnetization has been confirmed
using higher quality samples and under a low field of 22 mT. In addition, the
measurements have revealed a power law behavior, namely, $dM/dT\propto
T^{3/2}$, which has a slightly higher exponent than the previous results.Comment: 3 pages, 4 figures, a proceedings paper of QCNP 200
Effect of chemical substitution and pressure on YbRh2Si2

We carried out electrical resistivity experiments on (Yb,La)Rh2Si2 and on
Yb(Rh,Ir)2Si2 under pressure and in magnetic fields. YbRh2Si2 exhibits a weak
antiferromagnetic transition at atmospheric pressure with a N\'eel temperature
of only T_N = 70 mK. By applying a small magnetic field T_N can be continuously
suppressed to T=0 at B_c = 60 mT (B_|_c) driving the system to a quantum
critical point (QCP). On applying external pressure the magnetic phase is
stabilized and T_N(p) is increasing as usually observed in Yb-based
heavy-fermion metals. Substituting Yb by La or Rh by Ir allows to create a
negative chemical pressure, La (Ir) being smaller than Yb (Rh), and eventually
to drive YbRh2Si2 to a pressure controlled QCP. In this paper we compare the
effect of external hydrostatic pressure and chemical substitution on the
ground-state properties of YbRh2Si2.Comment: 4 pages, 5 figures, proceedings paper of the QCNP0
Quantum phase transition between antiferromagnetic and charge order in
  the Hubbard-Holstein model

We explore the quantum phase transitions between two ordered states in the
infinite dimensional Hubbard-Holstein model at half filling. Our study is based
on the dynamical mean field theory (DMFT) combined with the numerical
renormalization group (NRG), which allows us to handle both strong
electron-electron and strong electron-phonon interactions. The transition line
is characterized by an effective electron-electron interaction. Depending on
this effective interaction and the phonon frequency $\omega_0$ one finds either
a continuous transition or discontinuous transition. Here, the analysis focuses
on the behavior of the system when the electron-electron repulsion $U$ and the
phonon-mediated attraction $\lambda$ are equal. We first discuss the adiabatic
and antiadiabatic limiting cases. For finite $\omega_0$ we study the
differences between the antiferromagnetic (AFM) and charge order, and find that
when present the AFM state has a lower energy on the line
Low frequency spin dynamics in the quantum magnet copper pyrazine
  dinitrate

The S=1/2 antiferromagnetic Heisenberg chain exhibits a magnetic field driven
quantum critical point. We study the low frequency spin dynamics in copper
pyrazine dinitrate (CuPzN), a realization of this model system of quantum
magnetism, by means of $^{13}$C-NMR spectroscopy. Measurements of the nuclear
spin-lattice relaxation rate $T_1^{-1}$ in the vicinity of the saturation field
are compared with quantum Monte Carlo calculations of the dynamic structure
factor. Both show a strong divergence of low energy excitations at temperatures
in the quantum regime. The analysis of the anisotropic $T_1^{-1}$-rates and
frequency shifts allows one to disentangle the contributions from transverse
and longitudinal spin fluctuations for a selective study and to determine the
transfer of delocalized spin moments from copper to the neighboring nitrogen
atoms.Comment: 5 pages, 6 figures, published in Physica Status Solidi
Observation of the J-sheet of the Fermi surface of YbRh2Si2

We report a previously unobserved quantum oscillation frequency in YbRh2Si2.
This is the first quantum oscillation that can definitely be assigned to the
larger of the two major sheets of the Fermi surface predicted by band structure
calculations. Previously observed frequencies were interpreted in terms of a
large Fermi surface, which includes the Yb 4f-hole in the Fermi volume, that
has been so strongly spin-split that it closely resembles the small Fermi
surface, which does not include the Yb 4f -hole in the Fermi volume. The new
frequency can also be incorporated into this picture, however there are some
indications that the a need for more advanced energy band calculations.Comment: 5 pages, 3 figures, submitted to Physica Status Solidi on 24
  September 2009, fixed lingering issues with figure
Quantum Criticality and Global Phase Diagram of Magnetic Heavy Fermions

Quantum criticality describes the collective fluctuations of matter
undergoing a second-order phase transition at zero temperature. It is being
discussed in a number of strongly correlated electron systems. A prototype case
occurs in the heavy fermion metals, in which antiferromagnetic quantum critical
points have been explicitly observed. Here, I address two types of
antiferromagnetic quantum critical points. In addition to the standard
description based on the fluctuations of the antiferromagnetic order, a local
quantum critical point is also considered. It contains inherently quantum modes
that are associated with a critical breakdown of the Kondo effect. Across such
a quantum critical point, there is a sudden collapse of a large Fermi surface
to a small one. I also consider the proximate antiferromagnetic and
paramagnetic phases, and these considerations lead to a global phase diagram.
Finally, I discuss the pertinent experiments on the antiferromagnetic heavy
fermions, briefly address the case of ferromagnetic heavy fermions, and outline
some directions for future studies.Comment: (v2) reference added, and typos corrected; (v1) 10 pages, 2 figures,
  based on a plenary talk given at the International Conference on Quantum
  Criticality and Novel Phases (QCNP09, Dresden
Quantum Criticality and Novel Phases: Summary and Outlook

This conference summary and outlook provides a personal overview of the
topics and themes of the August 2009 Dresden meeting on quantum criticality and
novel phases. The dichotomy between the local moment and the itinerant views of
magnetism is revisited and refreshed in new materials, new probes and new
theoretical ideas. New universality and apparent zero temperature phases of
matter move us beyond the old ideas of quantum criticality. This is accompanied
by alternative pairing interactions and as yet unidentified phases developing
in the vicinity of quantum critical points. In discussing novel order, the
magnetic analogues of superconductivity are considered as candidate states for
the hidden order that sometimes develops in the vicinity of quantum critical
points in metallic systems. These analogues can be thought of as "pairing" in
the particle-hole channel and are tabulated. This analogy is used to outline a
framework to study the relation between ferromagnetic fluctuations and the
propensity of a metal to nematic type phases which at weak coupling correspond
to Pomeranchuk instabilities. This question can be related to the fundamental
relations of Fermi liquid theory.Comment: Conference summary for the 2009 Dresden Meeting on Quantum
  Criticality and Novel Phases. 7 pages and 4 figures. The associated
  presentation may be found at
  http://www.theory.bham.ac.uk/staff/schofield/talks/Dresden
Fast construction of the Kohn--Sham response function for molecules

The use of the LCAO (Linear Combination of Atomic Orbitals) method for
excited states involves products of orbitals that are known to be linearly
dependent. We identify a basis in the space of orbital products that is local
for orbitals of finite support and with a residual error that vanishes
exponentially with its dimension. As an application of our previously reported
technique we compute the Kohn--Sham density response function $\chi_{0}$ for a
molecule consisting of $N$ atoms in $N^{2}N_{\omega}$ operations, with
$N_{\omega}$ the number of frequency points. We test our construction of
$\chi_{0}$ by computing molecular spectra directly from the equations of
Petersilka--Gossmann--Gross in $N^{2}N_{\omega}$ operations rather than from
Casida's equations which takes $N^{3}$ operations. We consider the good
agreement with previously calculated molecular spectra as a validation of our
construction of $\chi_{0}$. Ongoing work indicates that our method is well
suited for the computation of the GW self-energy $\Sigma=\mathrm{i}GW$ and we
expect it to be useful in the analysis of exitonic effects in molecules
Local-fields and disorder effects in free-standing and embedded Si
  nanocrystallites

The case study of a 32-atoms Si nanocrystallite (NC) embedded in a SiO2
matrix, both crystalline and amorphous, or free-standing with different
conditions of passivation and strain is analyzed through ab-initio approaches.
The Si32/SiO2 heterojunction shows a type I band offset highlighting a
separation between the NC plus the interface and the matrix around. The
consequence of this separation is the possibility to correctly reproduce the
low energy electronic and optical properties of the composed system simply
studying the suspended NC plus interface oxygens with the appropriate strain.
Moreover, through the definition of an optical absorption threshold we found
that, beside the quantum confinement trend, the amorphization introduces an
additional redshift that increases with increasing NC size: i.e. the gap tends
faster to the bulk limit. Finally, the important changes in the calculated
DFT-RPA optical spectra upon inclusion of local fields point towards the need
of a proper treatment of the optical response of the interface region
Catastrophe optics of caustics in single and bilayer graphene: fine
  structure of caustics

We theoretically study the scattering of a plane wave of a ballistic electron
on a circular n-p junction in single and bilayer graphene. We compare the exact
wave function inside the junction to that obtained from a semiclassical formula
developed in catastrophe optics. In the semiclassical picture short-wavelength
electrons are treated as rays of particles that can get reflected and refracted
at the n-p junction according to Snell's law with negative refraction index. We
show that for short wavelength and close to caustics this semiclassical
approximation gives good agreement with the exact results in the case of
single-layer graphene. We also verify the universal scaling laws that govern
the shrinking rate and intensity divergence of caustics in the semiclassical
limit. It is straightforward to generalize our semiclassical method to more
complex geometries, offering a way to efficiently design and model
graphene-based electron-optical systems.Comment: 4 pages, 5 figures This manuscript should be published in the
  proceedings volume of the conference IWEPNM 2010, in the Physica Status
  Solidi
Single electron tunneling through high-Q single-wall carbon nanotube
  NEMS resonators

By first lithographically fabricating contact electrodes and then as last
step growing carbon nanotubes with chemical vapour deposition across the
ready-made chip, many potential contamination mechanisms for nanotube devices
can be avoided. Combining this with pre-defined trenches on the chip, such that
the nanotubes are freely suspended above the substrate, enables the formation
of highly regular electronic systems. We show that, in addition, such suspended
ultra-clean nanotubes provide excellent high-frequency and low-dissipation
mechanical resonators. The motion detection mechanism of our experiment is
discussed, and we measure the effect of Coulomb blockade and the back-action of
single electron tunneling on the mechanical motion. In addition data on the
mechanical higher modes is presented.Comment: 7 pages, 6 figure
On the interpretation of wave function overlaps in quantum dots

The spontaneous emission rate of excitons strongly confined in quantum dots
is proportional to the overlap integral of electron and hole envelope wave
functions. A common and intuitive interpretation of this result is that the
spontaneous emission rate is proportional to the probability that the electron
and the hole are located at the same point or region in space, i.e. they must
coincide spatially to recombine. Here we show that this interpretation is not
correct even loosely speaking. By general mathematical considerations we
compare the envelope wave function overlap, the exchange overlap integral, and
the probability of electrons and holes coinciding and find that the frequency
dependence of the envelope wave function overlap integral is very different
from that expected from the common interpretation. We show that these
theoretical considerations lead to predictions for measurements. We compare our
qualitative predictions with recent measurements of the wave function overlap
and find good agreement.Comment: 4 pages, 3 figure