1,086 research outputs found
Singular conductance of a spin 1 quantum dot
We interpret the recent observation of a zero-bias anomaly in spin-1 quantum
dots in terms of an underscreened Kondo effect. Although a spin-1 quantum dots
are expected to undergo a two-stage quenching effect, in practice the log
normal distribution of Kondo temperatures leads to a broad temperature region
dominated by underscreened Kondo physics. General arguments, based on the
asymptotic decoupling between the partially screened moment and the leads,
predict a singular temperature and voltage dependence of the conductance
and differential conductance , resulting in and . Using a Schwinger boson approach, we show how these qualitative
expectations are borne out in a detailed many body calculation.Comment: Four pages, four figures. Paper revised with additional references
added in response to feedback from reader
Induced interactions in dilute atomic gases and liquid helium mixtures
In dilute mixtures of two atomic gases, interactions between two minority
atoms acquire a contribution due to interaction with the majority component.
Using thermodynamic arguments, we derive expressions for this induced
interaction for both fermions and bosons for arbitrary strength of the
interaction between the two components. Implications of the work for the theory
of dilute solutions of He in liquid He are discussed.Comment: 7 pages, 1 figure, NORDITA-2012-3
Spectral properties of a two-orbital Anderson impurity model across a non-Fermi liquid fixed point
We study by NRG the spectral properties of a two-orbital Anderson impurity
model in the presence of an exchange splitting which follows either regular or
inverted Hund's rules. The phase diagram contains a non-Fermi liquid fixed
point separating a screened phase, where conventional Kondo effect occurs, from
an unscreened one, where the exchange-splitting takes care of quenching the
impurity degrees of freedom. On the Kondo screened side close to this fixed
point the impurity density of states shows a narrow Kondo-peak on top of a
broader resonance. This narrow peak transforms in the unscreened phase into a
narrow pseudo-gap inside the broad resonance. Right at the fixed point only the
latter survives. The fixed point is therefore identified by a jump of the
density of states at the chemical potential. We also show that particle-hole
perturbations which simply shift the orbital energies do not wash out the fixed
point, unlike those perturbations which hybridize the two orbitals.
Consequently the density-of-state jump at the chemical potential remains finite
even away from particle-hole symmetry, and the pseudo-gap stays pinned at the
chemical potential, although it is partially filled in. We also discuss the
relevance of these results for lattice models which map onto this Anderson
impurity model in the limit of large lattice-coordination. Upon approaching the
Mott metal-insulator transition, these lattice models necessarily enter a
region with a local criticality which reflects the impurity non-Fermi liquid
fixed point. However, unlike the impurity, the lattice can get rid of the
single-impurity fixed-point instability by spontaneously developing
bulk-coherent symmetry-broken phases, which we identify for different lattice
models.Comment: 43 pages, 11 figures. Minor corrections in the Appendi
Local electromigration model for crystal surfaces
We analyze the dynamics of crystal surfaces in the presence of
electromigration. From a phase field model with a migration force which depends
on the local geometry, we derive a step model with additional contributions in
the kinetic boundary conditions. These contributions trigger various surface
instabilities, such as step meandering, bunching and pairing on vicinal
surfaces. Experiments are discussed
Adiabatic Pumping in Interacting Systems
A dc current can be pumped through an interacting system by periodically
varying two independent parameters such as magnetic field and a gate potential.
We present a formula for the adiabatic pumping current in general interacting
systems, in terms of instantaneous properties of the system, and find the
limits for its applicability. This formula generalizes the scattering approach
for noninteracting pumps. We study the pumped spin in a system that exhibits
the two-channel Kondo effect as an application of the adiabatic pumping
formula. We find that a quantized spin of is transferred between the
two channels as the temperature approaches zero, and discuss the non-Fermi
liquid features of this system at finite temperatures.Comment: 4 pages and 1 figur
Forbidden island heights in stress-driven coherent Stranski-Krastanov growth
The observed height distribution of clusters obtained in strained epitaxy has
been often interpreted in terms of electronic effects. We show that some
aspects can be explained classically by the interplay of strain and edge
energies. We find that soft materials can transform directly from monolayer
into thicker islands by two-dimensional (2D) multilayer nucleation and growth.
There is a critical thickness decreasing with the force constant. Thinner
islands are thermodynamically forbidden, due to the insufficient stress
relaxation upon clustering particularly under tensile stress. At sufficiently
large misfits the barrier for 2D multilayer nucleation is significantly smaller
than the barrier for subsequent single-layer nucleation. The effects are found
to be quantitatively reasonable and offer a plausible explanation for the
absence of thin islands and 2D growth of flattop islands usually attributed to
quantum size effects.Comment: 4 pages, 4 figures. Accepted version. Includes quantitative
estimations comparing with experiments plus minor change
Crossover in the two-impurity Kondo model induced by direct charge tunneling
Quantum critical behavior in the two-impurity Kondo model requires the
distinct separation of two scales, T_K >> T*, where T_K is the Kondo
temperature and T* is the scale at which the system renormalizes away from the
quantum critical point to a stable Fermi liquid fixed point. We provide a
derivation of T* based on the renormalization group to lowest order. This
result is confirmed by a numerical renormalization group (NRG) analysis which
supplements the analytic derivation with additional quantitative precision. The
form of the low-energy Fermi liquid fixed point is derived and subsequently
confirmed by the NRG. We discuss implications for series double quantum dot
systems.Comment: 10 pages, 6 figures; resubmitted Oct. 31, 2011 to include corrections
discovered after original submissio
One-Particle Anomalous Excitations of Gutzwiller Projected BCS Superconductors and Bogoliubov Quasi-Particle Characteristics
Low-lying one-particle anomalous excitations are studied for Gutzwiller
projected strongly correlated BCS states. It is found that the one-particle
anomalous excitations are highly coherent, and the numerically calculated
spectrum can be reproduced quantitatively by a renormalized BCS theory, thus
strongly indicating that the nature of low-lying excitations described by the
projected BCS states is essentially understood within a renormalized Bogoliubov
quasi-particle picture. This finding resembles a well known fact that a
Gutzwiller projected Fermi gas is a Fermi liquid. The present results are
consistent with numerically exact calculations of the two-dimensional t-J model
as well as recent photoemission experiments on high-T_{\rm C} cuprate
superconductors.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. B 74,
180504(R) (2006
Quantum replica approach to the under-screened Kondo model
We extend the Schwinger boson large N treatment of the underscreened Kondo
model in a way that correctly captures the finite elastic phase shift in the
singular Fermi liquid. The new feature of the approach, is the introduction of
a flavor quantum number with K possible values, associated with the Schwinger
boson representation. The large N limit is taken maintaining the ratio k=K/N
fixed. This approach differs from previous approaches, in that we do not
explicitly enforce a constraint on the spin representation of the Schwinger
bosons. Instead, the energetics of the Kondo model cause the bosonic degrees of
freedom to ``self assemble'' into a ground-state in which the spins of K bosons
and N-K conduction electrons are antisymmetrically arranged into a Kondo
singlet. With this device, the large N limit can be taken, in such a way that a
fraction K/N of the Abrikosov Suhl resonance is immersed inside the Fermi sea.
We show how this method can be used to model the full energy dependence of the
singular Abrikosov Suhl resonance in the underscreened Kondo model and the
field-dependent magnetization.Comment: Revised draft, with plots explicitly showing logarithmic scaling of
inverse coupling constant. Small corrections prior to submission to journa
Nonequilibrium dynamics in a two-channel Kondo system due to a quantum quench
Recent experiments by Potok et al. have demonstrated a remarkable tunability
between a single-channel Fermi liquid fixed point and a two-channel non-Fermi
liquid fixed point. Motivated by this we study the nonequilibrium dynamics due
to a sudden quench of the parameters of a Hamiltonian from a single-channel to
a two-channel anisotropic Kondo system. We find a distinct difference between
the long time behavior of local quantities related to the impurity spin as
compared to that of bulk quantities related to the total (conduction electrons
+ impurity) spin of the system. In particular, the local impurity spin and the
local spin susceptibility are found to equilibrate, but in a very slow
power-law fashion which is peculiar to the non-Fermi liquid properties of the
Hamiltonian. In contrast, we find a lack of equilibration in the two particle
expectation values related to the total spin of the system.Comment: 5 pages, 1 fig. Accepted in PR
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