53 research outputs found
Quantum phase transition in capacitively coupled double quantum dots
We investigate two equivalent, capacitively coupled semiconducting quantum
dots, each coupled to its own lead, in a regime where there are two electrons
on the double dot. With increasing interdot coupling a rich range of behavior
is uncovered: first a crossover from spin- to charge-Kondo physics, via an
intermediate SU(4) state with entangled spin and charge degrees of freedom;
followed by a quantum phase transition of Kosterlitz-Thouless type to a
non-Fermi liquid `charge-ordered' phase with finite residual entropy and
anomalous transport properties. Physical arguments and numerical
renormalization group methods are employed to obtain a detailed understanding
of the problem.Comment: 4 pages, 3 figure
Two-channel Kondo physics in tunnel-coupled double quantum dots
We investigate theoretically the possibility of observing two-channel Kondo
(2CK) physics in tunnel-coupled double quantum dots (TCDQDs), at both zero and
finite magnetic fields; taking the two-impurity Anderson model (2AIM) as the
basic TCDQD model, together with effective low-energy models arising from it by
Schrieffer-Wolff transformations to second and third order in the tunnel
couplings. The models are studied primarily using Wilson's numerical
renormalization group. At zero-field our basic conclusion is that while 2CK
physics arises in principle provided the system is sufficiently
strongly-correlated, the temperature window over which it could be observed is
much lower than is experimentally feasible. This finding disagrees with recent
work on the problem, and we explain why. At finite field, we show that the
quantum phase transition known to arise at zero-field in the two-impurity Kondo
model (2IKM), with an essentially 2CK quantum critical point, persists at
finite fields. This raises the prospect of access to 2CK physics by tuning a
magnetic field, although preliminary investigation suggests this to be even
less feasible than at zero field.Comment: 10 pages, 8 figures. Version as published in PR
Magnetic field effects in few-level quantum dots: theory, and application to experiment
We examine several effects of an applied magnetic field on Anderson-type
models for both single- and two-level quantum dots, and make direct comparison
between numerical renormalization group (NRG) calculations and recent
conductance measurements. On the theoretical side the focus is on
magnetization, single-particle dynamics and zero-bias conductance, with
emphasis on the universality arising in strongly correlated regimes; including
a method to obtain the scaling behavior of field-induced Kondo resonance shifts
over a very wide field range. NRG is also used to interpret recent experiments
on spin-1/2 and spin-1 quantum dots in a magnetic field, which we argue do not
wholly probe universal regimes of behavior; and the calculations are shown to
yield good qualitative agreement with essentially all features seen in
experiment. The results capture in particular the observed field-dependence of
the Kondo conductance peak in a spin-1/2 dot, with quantitative deviations from
experiment occurring at fields in excess of 5 T, indicating the eventual
inadequacy of using the equilibrium single-particle spectrum to calculate the
conductance at finite bias.Comment: 15 pages, 12 figures. Version as published in PR
A local moment approach to the degenerate Anderson impurity model
The local moment approach is extended to the orbitally-degenerate [SU(2N)]
Anderson impurity model (AIM). Single-particle dynamics are obtained over the
full range of energy scales, focussing here on particle-hole symmetry in the
strongly correlated regime where the onsite Coulomb interaction leads to
many-body Kondo physics with entangled spin and orbital degrees of freedom. The
approach captures many-body broadening of the Hubbard satellites, recovers the
correct exponential vanishing of the Kondo scale for all N, and its universal
scaling spectra are found to be in very good agreement with numerical
renormalization group (NRG) results. In particular the high-frequency
logarithmic decays of the scaling spectra, obtained here in closed form for
arbitrary N, coincide essentially perfectly with available numerics from the
NRG. A particular case of an anisotropic Coulomb interaction, in which the
model represents a system of N `capacitively-coupled' SU(2) AIMs, is also
discussed. Here the model is generally characterised by two low-energy scales,
the crossover between which is seen directly in its dynamics.Comment: 23 pages, 7 figure
Correlated electron physics in multilevel quantum dots: phase transitions, transport, and experiment
We study correlated two-level quantum dots, coupled in effective 1-channel
fashion to metallic leads; with electron interactions including on-level and
inter-level Coulomb repulsions, as well as the inter-orbital Hund's rule
exchange favoring the spin-1 state in the relevant sector of the free dot. For
arbitrary dot occupancy, the underlying phases, quantum phase transitions
(QPTs), thermodynamics, single-particle dynamics and electronic transport
properties are considered; and direct comparison is made to conductance
experiments on lateral quantum dots. Two distinct phases arise generically, one
characterised by a normal Fermi liquid fixed point (FP), the other by an
underscreened (USC) spin-1 FP. Associated QPTs, which occur in general in a
mixed valent regime of non-integral dot charge, are found to consist of
continuous lines of Kosterlitz-Thouless transitions, separated by first order
level-crossing transitions at high symmetry points. A `Friedel-Luttinger sum
rule' is derived and, together with a deduced generalization of Luttinger's
theorem to the USC phase (a singular Fermi liquid), is used to obtain a general
result for the T=0 zero-bias conductance, expressed solely in terms of the dot
occupancy and applicable to both phases. Relatedly, dynamical signatures of the
QPT show two broad classes of behavior, corresponding to the collapse of either
a Kondo resonance, or antiresonance, as the transition is approached from the
Fermi liquid phase; the latter behavior being apparent in experimental
differential conductance maps. The problem is studied using the numerical
renormalization group method, combined with analytical arguments.Comment: 22 pages, 18 figures, submitted for publicatio
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
Interplay between Kondo physics and spin-orbit coupling in carbon nanotube quantum dots
We investigate the influence of spin-orbit coupling on the Kondo effects in
carbon nanotube quantum dots, using the numerical renormalization group
technique. A sufficiently large spin-orbit coupling is shown to destroy the
SU(4) Kondo effects at zero magnetic field, leaving only two SU(2) Kondo
effects in the one- and three-electron Coulomb blockade valleys. On applying a
finite magnetic field, two additional, spin-orbit induced SU(2) Kondo effects
arise in the three- and two-electron valleys. Using physically realistic model
parameters, we calculate the differential conductance over a range of gate
voltages, temperatures and fields. The results agree well with measurements
from two different experimental devices in the literature, and explain a number
of observations that are not described within the standard framework of the
SU(4) Anderson impurity model.Comment: 15 pages, 11 figure
Conductance fingerprint of Majorana fermions in the topological Kondo effect
We consider an interacting nanowire/superconductor heterostructure attached
to metallic leads. The device is described by an unusual low-energy model
involving spin-1 conduction electrons coupled to a nonlocal spin-1/2 Kondo
impurity built from Majorana fermions. The topological origin of the resulting
Kondo effect is manifest in distinctive non-Fermi-liquid (NFL) behavior, and
the existence of Majorana fermions in the device is demonstrated unambiguously
by distinctive conductance lineshapes. We study the physics of the model in
detail, using the numerical renormalization group, perturbative scaling and
abelian bosonization. In particular, we calculate the full scaling curves for
the differential conductance in AC and DC fields, onto which experimental data
should collapse. Scattering t-matrices and thermodynamic quantities are also
calculated, recovering asymptotes from conformal field theory. We show that the
NFL physics is robust to asymmetric Majorana-lead couplings, and here we
uncover a duality between strong and weak coupling. The NFL behavior is
understood physically in terms of competing Kondo effects. The resulting
frustration is relieved by inter-Majorana coupling which generates a second
crossover to a regular Fermi liquid.Comment: 17 pages, 8 figure
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