446 research outputs found
Convergence acceleration and stabilization for dynamical-mean-field-theory calculations
The convergence to the self-consistency in the dynamical-mean-field-theory
(DMFT) calculations for models of correlated electron systems can be
significantly accelerated by using an appropriate mixing of hybridization
functions which are used as the input to the impurity solver. It is shown that
the techniques and the past experience with the mixing of input charge
densities in the density-functional-theory (DFT) calculations are also
effective in DMFT. As an example, the increase of the computational
requirements near the Mott metal-insulator transition in the Hubbard model due
to critical slowing down can be strongly reduced by using the modified
Broyden's method to numerically solve the non-linear self-consistency equation.
Speed-up factors as high as 3 were observed in practical calculations even for
this relatively well behaved problem. Furthermore, the convergence can be
achieved in difficult cases where simple linear mixing is either not effective
or even leads to divergence. Unstable and metastable solutions can also be
obtained. We also determine the linear response of the system with respect to
the variations of the hybridization function, which is related to the
propagation of the information between the different energy scales during the
iteration.Comment: 9 pages, 8 figure
Zero-bias conductance in carbon nanotube quantum dots
We present numerical renormalization group calculations for the zero-bias
conductance of quantum dots made from semiconducting carbon nanotubes. These
explain and reproduce the thermal evolution of the conductance for different
groups of orbitals, as the dot-lead tunnel coupling is varied and the system
evolves from correlated Kondo behavior to more weakly correlated regimes. For
integer fillings of an SU(4) model, we find universal scaling
behavior of the conductance that is distinct from the standard SU(2) universal
conductance, and concurs quantitatively with experiment. Our results also agree
qualitatively with experimental differential conductance maps.Comment: 4 pages, 5 figure
Anderson impurity in a semiconductor
We consider an Anderson impurity model in which the locally correlated
orbital is coupled to a host with a gapped density of states. Single-particle
dynamics are studied, within a perturbative framework that includes both
explicit second-order perturbation theory and self-consistent perturbation
theory to all orders in the interaction. Away from particle-hole symmetry the
system is shown to be a generalized Fermi liquid (GFL) in the sense of being
perturbatively connectable to the non-interacting limit; and the exact Friedel
sum rule for the GFL phase is obtained. We show by contrast that the
particle-hole symmetric point of the model is not perturbatively connected to
the non-interacting limit, and as such is a non-Fermi liquid for all non-zero
gaps. Our conclusions are in agreement with NRG studies of the problem.Comment: 7 pages, 4 figure
Hanbury-Brown-Twiss correlations and noise in the charge transfer statistics through a multiterminal Kondo dot
We analyze the full counting statistics of charge transfer through a quantum
dot in the Kondo regime, when coupled to an arbitrary number of terminals N.
At the unitary Kondo fixed point and for N>2 we recover distinct
anticorrelations of currents in concurring transport channels, which are
related to the fermionic
Hanbury Brown and Twiss (HBT) antibunching.
This effect weakens as one moves away from the fixed point.
Furthermore, we identify a special class of current correlations that are due
entirely to the virtual polarization of the Kondo singlet.
These can be used for extracting information on the parameters of the
underlying Fermi-liquid model.Comment: 5 page
Quantum simulation of Anderson and Kondo lattices with superconducting qubits
We introduce a mapping between a variety of superconducting circuits and a
family of Hamiltonians describing localized magnetic impurities interacting
with conduction bands. This includes the Anderson model, the single impurity
one- and two-channel Kondo problem, as well as the 1D Kondo lattice. We compare
the requirements for performing quantum simulations using the proposed circuits
to those of universal quantum computation with superconducting qubits, singling
out the specific challenges that will have to be addressed.Comment: Longer versio
Kondo Stripes in an Anderson-Heisenberg Model of Heavy Fermion Systems
We study the interplay between the spin-liquid and Kondo physics, as related
to the non-magnetic part of the phase diagram of heavy fermion materials.
Within the unrestricted mean-field treatment of the infinite- 2D
Anderson-Heisenberg model, we find that there are two topologically distinct
non-degenerate uniform heavy Fermi liquid states that may form as a consequence
of the Kondo coupling between spinons and conduction electrons. For certain
carrier concentrations the uniform Fermi liquid becomes unstable with respect
to formation of a new kind of anharmonic "Kondo stripe" state with
inhomogeneous Kondo screening strength and the charge density modulation. These
feature are experimentally measurable, and thus may help to establish the
relevance of the spin-liquid correlations to heavy fermion materials.Comment: 4+ pages, 5 figure
Salmonella Typhimurium in the Australian egg industry: multidisciplinary approach to addressing the public health challenge and future directions
In Australia, numerous egg related human Salmonella Typhimurium outbreaks have prompted significant interest amongst public health authorities and the egg industry to jointly address this human health concern. Nationwide workshops on Salmonella and eggs were conducted in Australia for egg producers and regulatory authorities. State and National regulators representing Primary Production, Communicable Disease Control, Public Health and Food Safety and Food Standards Australia and New Zealand. All attendees participated in discussions aimed at evaluating current evidence based information, issues related to quality egg production and how to ensure safe eggs in the supply chain, identifying research gaps and practical recommendations. The perceptions from egg producers and regulatory authorities from various states were recorded during the workshops. We presented the issues discussed during the workshops including Salmonella in the farm environment, Salmonella penetration across egg shell, virulence in humans, food/egg handling in the supply chain and intervention strategies. We also discussed the perceptions from egg producers and regulators. Recommendations placed emphasis on future research needs, communication between industry and regulatory authorities and education of food handlers. Communication between regulators and industry is pivotal to control egg borne S. Typhimurium outbreaks and collaborative efforts are required to design effective and appropriate control strategies.Kapil K. Chousalkar, Margaret Sexton, Andrea McWhorter, Kylie Hewson, Glen Martin, Craig Shadbolt & Paul Goldsmit
Dynamics of capacitively coupled double quantum dots
We consider a double dot system of equivalent, capacitively coupled
semiconducting quantum dots, each coupled to its own lead, in a regime where
there are two electrons on the double dot. Employing the numerical
renormalization group, we focus here on single-particle dynamics and the
zero-bias conductance, considering in particular the rich range of behaviour
arising as the interdot coupling is progressively increased through the strong
coupling (SC) phase, from the spin-Kondo regime, across the SU(4) point to the
charge-Kondo regime; and then towards and through the quantum phase transition
to a charge-ordered (CO) phase. We first consider the two-self-energy
description required to describe the broken symmetry CO phase, and implications
thereof for the non-Fermi liquid nature of this phase. Numerical results for
single-particle dynamics on all frequency scales are then considered, with
particular emphasis on universality and scaling of low-energy dynamics
throughout the SC phase. The role of symmetry breaking perturbations is also
briefly discussed.Comment: 14 pages, 6 figure
Single-particle dynamics of the Anderson model: a two-self-energy description within the numerical renormalization group approach
Single-particle dynamics of the Anderson impurity model are studied using
both the numerical renormalization group (NRG) method and the local moment
approach (LMA). It is shown that a 'two-self-energy' description of dynamics
inherent to the LMA, as well as a conventional 'single-self-energy'
description, arise within NRG; each yielding correctly the same local
single-particle spectrum. Explicit NRG results are obtained for the broken
symmetry spectral constituents arising in a two-self-energy description, and
the total spectrum. These are also compared to analytical results obtained from
the LMA as implemented in practice. Very good agreement between the two is
found, essentially on all relevant energy scales from the high-energy Hubbard
satellites to the low-energy Kondo resonance.Comment: 12 pages, 6 figure
Renormalization group study of capacitively coupled double quantum dots
The numerical renormalization group is employed to study a double quantum
(DQD) dot system consisting of two equivalent single-level dots, each coupled
to its own lead and with a mutual capacitive coupling embodied in an interdot
interaction U', in addition to the intradot Coulomb interaction U. We focus on
the regime with two electrons on the DQD, and the evolution of the system on
increasing U'/U. The spin-Kondo effect arising for U'=0 (SU(2) x SU(2)) is
found to persist robustly with increasing U'/U, before a rapid but continuous
crossover to (a) the SU(4) point U'=U where charge and spin degrees of freedom
are entangled and the Kondo scale strongly enhanced; and then (b) a
charge-Kondo state, in which a charge-pseudospin is quenched on coupling to the
leads/conduction channels. A quantum phase transition of Kosterlitz-Thouless
type then occurs from this Fermi liquid, strong coupling (SC) phase, to a
broken symmetry, non-Fermi liquid charge ordered (CO) phase at a critical U'_c.
Our emphasis in this paper is on the structure, stability and flows between the
underlying RG fixed points, on the overall phase diagram in the (U,U')-plane
and evolution of the characteristic low-energy Kondo scale inherent to the SC
phase; and on static physical properties such as spin- and
charge-susceptibilities (staggered and uniform), including universality and
scaling behaviour in the strongly correlated regime. Some exact results for
associated Wilson ratios are also obtained.Comment: 27 pages, 12 figure
- …