16,014 research outputs found
Conductance of Quantum Impurity Models from Quantum Monte Carlo
The conductance of two Anderson impurity models, one with two-fold and
another with four-fold degeneracy, representing two types of quantum dots, is
calculated using a world-line quantum Monte Carlo (QMC) method. Extrapolation
of the imaginary time QMC data to zero frequency yields the linear conductance,
which is then compared to numerical renormalization group results in order to
assess its accuracy. We find that the method gives excellent results at low
temperature (T<Tk) throughout the mixed valence and Kondo regimes, but it is
unreliable for higher temperature.Comment: 5 pages, 7 figure
Quantum Phase Transition and Dynamically Enhanced Symmetry in Quadruple Quantum Dot System
We propose a system of four quantum dots designed to study the competition
between three types of interactions: Heisenberg, Kondo and Ising. We find a
rich phase diagram containing two sharp features: a quantum phase transition
(QPT) between charge-ordered and charge-liquid phases, and a dramatic resonance
in the charge liquid visible in the conductance. The QPT is of the
Kosterlitz-Thouless type with a discontinuous jump in the conductance at the
transition. We connect the resonance phenomenon with the degeneracy of three
levels in the isolated quadruple dot and argue that this leads to a Kondo-like
dynamical enhancement of symmetry from U(1) x Z_2 to U(1) x U(1).Comment: 4 pages main text + 4 pages supplementary materia
The Benefits of Stress: Resolution of the Lifshitz Singularity
Through the AdS/CFT correspondence, Lifshitz spacetimes describe field
theories with dynamical scaling (). Although curvature invariants are
small, the Lifshitz metric exhibits a null singularity in the IR with a large
tidal force that excites string oscillator modes. However, Lifshitz is not a
vacuum solution of the Einstein equations -- the metric is supported by
nontrivial matter content which must be taken into account in analyzing the
propagation of test objects. In this paper, we consider the interaction of a
string with a D0-brane density in the IR which supports a class of UV-complete
Lifshitz constructions. We show that string/D-brane scattering in the
Regge limit slows the string significantly, preventing divergent mode
production and resolving the would-be singularity in string propagation.Comment: 16 pages; v2: new references adde
Mesoscopic Anderson Box: Connecting Weak to Strong Coupling
Both the weakly coupled and strong coupling Anderson impurity problems are
characterized by a Fermi-liquid theory with weakly interacting quasiparticles.
In an Anderson box, mesoscopic fluctuations of the effective single particle
properties will be large. We study how the statistical fluctuations at low
temperature in these two problems are connected, using random matrix theory and
the slave boson mean field approximation (SBMFA). First, for a resonant level
model such as results from the SBMFA, we find the joint distribution of energy
levels with and without the resonant level present. Second, if only energy
levels within the Kondo resonance are considered, the distributions of
perturbed levels collapse to universal forms for both orthogonal and unitary
ensembles for all values of the coupling. These universal curves are described
well by a simple Wigner-surmise type toy model. Third, we study the
fluctuations of the mean field parameters in the SBMFA, finding that they are
small. Finally, the change in the intensity of an eigenfunction at an arbitrary
point is studied, such as is relevant in conductance measurements: we find that
the introduction of the strongly-coupled impurity considerably changes the wave
function but that a substantial correlation remains.Comment: 17 pages, 7 figure
The Third Law of Quantum Thermodynamics in the Presence of Anomalous Couplings
The quantum thermodynamic functions of a harmonic oscillator coupled to a
heat bath through velocity-dependent coupling are obtained analytically. It is
shown that both the free energy and the entropy decay fast with the temperature
in relation to that of the usual coupling from. This implies that the
velocity-dependent coupling helps to ensure the third law of thermodynamics.Comment: 4 pages, 3 figures, 22 conference
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