69 research outputs found
Nanometer Scale Mapping of the Density of States in an Inhomogeneous Superconductor
Using high speed scanning tunneling spectroscopy, we perform a full mapping
of the quasiparticle density of states (DOS) in single crystals of
BiPbSrCaCuO(2212). The measurements carried out at 5 K showed a complex spatial
pattern of important variations of the local DOS on the nanometer scale.
Superconducting areas are co-existing with regions of a smooth and larger
gap-like DOS structure. The superconducting regions are found to have a minimum
size of about 3 nm. The role of Pb-introduced substitutional disorder in the
observed spatial variations of the local DOS is discussed.Comment: 4 page Letter with 3 figures (2 color figures
Multi-study analysis of learning culture, human capital and operational performance in supply chain management: The moderating role of workforce level
Purpose â The purpose of this study is to empirically evaluate the relationship between learning culture, workforce level, human capital and operational performance in two diverse supply chain populations, aircraft maintenance and logistics readiness.
Design/methodology/approach â Drawing upon competence-based view of the firm and human capital theory, this paper analyzes data from two studies.
Findings â The results provide support for the hypothesized model. Workforce level moderates the relationship between learning culture and human capital, and human capital partially mediates the relationship between learning culture and operational performance.
Research limitations/implications â The findings have implications for behavioral supply chain management research and implications for educating and training the supply chain management workforce. While the populations represent a diverse set of logistics functions and responsibilities, the participants are all military members, which may limit generalizability.
Practical implications â This study should help leaders understand the importance of learning culture and the perceived differences in its effect on human capital based upon workforce level.
Originality/value â This research is among the first to investigate the role of workforce level and answers a multitude of calls for research into the human side of supply chain management
Similarities between the Hubbard and Periodic Anderson Models at Finite Temperatures
The single band Hubbard and the two band Periodic Anderson Hamiltonians have
traditionally been applied to rather different physical problems - the Mott
transition and itinerant magnetism, and Kondo singlet formation and scattering
off localized magnetic states, respectively. In this paper, we compare the
magnetic and charge correlations, and spectral functions, of the two systems.
We show quantitatively that they exhibit remarkably similar behavior, including
a nearly identical topology of the finite temperature phase diagrams at
half-filling. We address potential implications of this for theories of the
rare earth ``volume collapse'' transition.Comment: 4 pages (RevTeX) including 4 figures in 7 eps files; as to appear in
Phys. Rev. Let
Quantum critical point in a periodic Anderson model
We investigate the symmetric Periodic Anderson Model (PAM) on a
three-dimensional cubic lattice with nearest-neighbor hopping and hybridization
matrix elements. Using Gutzwiller's variational method and the Hubbard-III
approximation (which corresponds to the exact solution of an appropriate
Falicov-Kimball model in infinite dimensions) we demonstrate the existence of a
quantum critical point at zero temperature. Below a critical value of the
hybridization (or above a critical interaction ) the system is an {\em
insulator} in Gutzwiller's and a {\em semi-metal} in Hubbard's approach,
whereas above (below ) it behaves like a metal in both
approximations. These predictions are compared with the density of states of
the - and -bands calculated from Quantum Monte Carlo and NRG
calculations. Our conclusion is that the half-filled symmetric PAM contains a
{\em metal-semimetal transition}, not a metal-insulator transition as has been
suggested previously.Comment: ReVteX, 10 pages, 2 EPS figures. Minor corrections made in the text
and in the figure captions from the first version. More references added.
Accepted for publication in Physical Review
A Quantum Monte Carlo algorithm for non-local corrections to the Dynamical Mean-Field Approximation
We present the algorithmic details of the dynamical cluster approximation
(DCA), with a quantum Monte Carlo (QMC) method used to solve the effective
cluster problem. The DCA is a fully-causal approach which systematically
restores non-local correlations to the dynamical mean field approximation
(DMFA) while preserving the lattice symmetries. The DCA becomes exact for an
infinite cluster size, while reducing to the DMFA for a cluster size of unity.
We present a generalization of the Hirsch-Fye QMC algorithm for the solution of
the embedded cluster problem. We use the two-dimensional Hubbard model to
illustrate the performance of the DCA technique. At half-filling, we show that
the DCA drives the spurious finite-temperature antiferromagnetic transition
found in the DMFA slowly towards zero temperature as the cluster size
increases, in conformity with the Mermin-Wagner theorem. Moreover, we find that
there is a finite temperature metal to insulator transition which persists into
the weak-coupling regime. This suggests that the magnetism of the model is
Heisenberg like for all non-zero interactions. Away from half-filling, we find
that the sign problem that arises in QMC simulations is significantly less
severe in the context of DCA. Hence, we were able to obtain good statistics for
small clusters. For these clusters, the DCA results show evidence of non-Fermi
liquid behavior and superconductivity near half-filling.Comment: 25 pages, 15 figure
Depleted Kondo Lattices
We consider a two dimensional Kondo lattice model with exchange J and hopping
t in which three out of four impurity spins are removed in a regular way. At
the particle-hole symmetric point the model may be studied with auxiliary field
quantum Monte Carlo methods without sign problems. To achieve the relevant
energy scales on finite clusters, we introduce a simple method to reduce size
effects by up to an order of magnitude in temperature. In this model, a
metallic phase survives up to arbitrarily low temperatures before being
disrupted by magnetic fluctuations which open a gap in the charge sector. We
study the formation of the heavy-electron state with emphasis on a crossover
scale T* defined by the maximum in the resistivity versus temperature curve.
The behavior of thermodynamic properties such as specific heat as well as spin
and charge uniform susceptibilities are studied as the temperature varies in a
wide range across T*. Within our accuracy T* compares well to the Kondo scale
of the related single impurity problem. Finally our QMC resuls are compared
with mean-field approximations.Comment: 12 pages, 13 figures. Submitted to Phys. Rev.
Doping-dependent study of the periodic Anderson model in three dimensions
We study a simple model for -electron systems, the three-dimensional
periodic Anderson model, in which localized states hybridize with
neighboring states. The states have a strong on-site repulsion which
suppresses the double occupancy and can lead to the formation of a Mott-Hubbard
insulator. When the hybridization between the and states increases, the
effects of these strong electron correlations gradually diminish, giving rise
to interesting phenomena on the way. We use the exact quantum Monte-Carlo,
approximate diagrammatic fluctuation-exchange approximation, and mean-field
Hartree-Fock methods to calculate the local moment, entropy, antiferromagnetic
structure factor, singlet-correlator, and internal energy as a function of the
hybridization for various dopings. Finally, we discuss the relevance of
this work to the volume-collapse phenomenon experimentally observed in
f-electron systems.Comment: 12 pages, 8 figure
The Cerium volume collapse: Results from the LDA+DMFT approach
The merger of density-functional theory in the local density approximation
(LDA) and many-body dynamical mean field theory (DMFT) allows for an ab initio
calculation of Ce including the inherent 4f electronic correlations. We solve
the DMFT equations by the quantum Monte Carlo (QMC) technique and calculate the
Ce energy, spectrum, and double occupancy as a function of volume. At low
temperatures, the correlation energy exhibits an anomalous region of negative
curvature which drives the system towards a thermodynamic instability, i.e.,
the -to- volume collapse, consistent with experiment. The
connection of the energetic with the spectral evolution shows that the physical
origin of the energy anomaly and, thus, the volume collapse is the appearance
of a quasiparticle resonance in the 4f-spectrum which is accompanied by a rapid
growth in the double occupancy.Comment: 4 pages, 3 figure
Plaquette operators used in the rigorous study of ground-states of the Periodic Anderson Model in dimensions
The derivation procedure of exact ground-states for the periodic Anderson
model (PAM) in restricted regions of the parameter space and D=2 dimensions
using plaquette operators is presented in detail. Using this procedure, we are
reporting for the first time exact ground-states for PAM in 2D and finite value
of the interaction, whose presence do not require the next to nearest neighbor
extension terms in the Hamiltonian. In order to do this, a completely new type
of plaquette operator is introduced for PAM, based on which a new localized
phase is deduced whose physical properties are analyzed in detail. The obtained
results provide exact theoretical data which can be used for the understanding
of system properties leading to metal-insulator transitions, strongly debated
in recent publications in the frame of PAM. In the described case, the lost of
the localization character is connected to the break-down of the long-range
density-density correlations rather than Kondo physics.Comment: 34 pages, 5 figure
Signatures of Spin and Charge Energy Scales in the Local Moment and Specific Heat of the Two-Dimensional Hubbard Model
Local moment formation driven by the on--site repulsion is one of the
most fundamental features in the Hubbard model. At the simplest level, the
temperature dependence of the local moment is expected to have a single
structure at , reflecting the suppression of the double occupancy. In
this paper we show new low temperature Quantum Monte Carlo data which emphasize
that the local moment also has a signature at a lower energy scale which
previously had been thought to characterize only the temperatures below which
moments on {\it different} sites begin to correlate locally. We discuss
implications of these results for the structure of the specific heat, and
connections to quasiparticle resonance and pseudogap formation in the density
of states.Comment: 13 pages, 19 figure
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