52 research outputs found
Dynamical correlations in the spin-half two-channel Kondo model
Dynamical correlations of various local operators are studied in the
spin-half two-channel Kondo (2CK) model in the presence of channel anisotropy
or external magnetic field. A conformal field theory-based scaling approach is
used to predict the analytic properties of various spectral functions in the
vicinity of the two-channel Kondo fixed point. These analytical results compare
well with highly accurate density matrix numerical renormalization group
results. The universal cross-over functions interpolating between
channel-anisotropy or magnetic field-induced Fermi liquid regimes and the
two-channel Kondo, non-Fermi liquid regimes are determined numerically. The
boundaries of the real 2CK scaling regime are found to be rather restricted,
and to depend both on the type of the perturbation and on the specific operator
whose correlation function is studied. In a small magnetic field, a universal
resonance is observed in the local fermion's spectral function. The dominant
superconducting instability appears in the composite superconducting channel.Comment: 20 pages, 24 figures, PRB forma
Bloch oscillations and the lack of the decay of the false vacuum in a one-dimensional quantum spin chain
We consider the decay of the false vacuum, realised within a quantum quench
into an anti-confining regime of the Ising spin chain with a magnetic field
opposite to the initial magnetisation. Although the effective linear potential
between the domain walls is repulsive, the time evolution of correlations still
shows a suppression of the light cone and a reduction of vacuum decay. The
suppressed decay is a lattice effect, and can be assigned to emergent Bloch
oscillations.Comment: 12 pages, 6 figures, pdflatex file. v2: 14 pages, new material and
references added, improved discussion, main results and conclusions unchange
Theory of anisotropic Rashba splitting of surface states
We investigate the surface Rashba effect for a surface of reduced in-plane
symmetry. Formulating a k.p perturbation theory, we show that the Rashba
splitting is anisotropic, in agreement with symmetry-based considerations. We
show that the anisotropic Rashba splitting is due to the admixture of bulk
states of different symmetry to the surface state, and it cannot be explained
within the standard theoretical picture supposing just a normal-to-surface
variation of the crystal potential. Performing relativistic ab initio
calculations we find a remarkably large Rashba anisotropy for an
unreconstructed Au(110) surface that is in the experimentally accessible range.Comment: 4 pages, 5 figure
Schwinger Boson approach to the fully screened Kondo model
We apply the Schwinger boson scheme to the fully screened Kondo model and
generalize the method to include antiferromagnetic interactions between ions.
Our approach captures the Kondo crossover from local moment behavior to a Fermi
liquid with a non-trivial Wilson ratio. When applied to the two impurity model,
the mean-field theory describes the "Varma Jones" quantum phase transition
between a valence bond state and a heavy Fermi liquid.Comment: 4 pages, 4 figures. Changes to references and text in v
Is demagnetization an efficient optimization method?
Demagnetization, commonly employed to study ferromagnets, has been proposed
as the basis for an optimization tool, a method to find the ground state of a
disordered system. Here we present a detailed comparison between the ground
state and the demagnetized state in the random field Ising model, combing exact
results in and numerical solutions in . We show that there are
important differences between the two states that persist in the thermodynamic
limit and thus conclude that AC demagnetization is not an efficient
optimization method.Comment: 2 pages, 1 figur
A note on cluster methods for strongly correlated electron systems
We develop, clarify and test various aspects of cluster methods dynamical
mean field methods using a soluble toy model as a benchmark. We find that the
Cellular Dynamical Mean Field Theory (C-DMFT) converges very rapidly and
compare its convergence properties with those of the Dynamical Cluster
Approximation (DCA). We propose and test improved estimators for the lattice
self energy within C-DMFT.Comment: 5 pages, 3 figures; major change
Two-stage Kondo effect in side-coupled quantum dots: Renormalized perturbative scaling theory and Numerical Renormalization Group analysis
We study numerically and analytically the dynamical (AC) conductance through
a two-dot system, where only one of the dots is coupled to the leads but it is
also side-coupled to the other dot through an antiferromagnetic exchange (RKKY)
interaction. In this case the RKKY interaction gives rise to a ``two-stage
Kondo effect'' where the two spins are screened by two consecutive Kondo
effects. We formulate a renormalized scaling theory that captures remarkably
well the cross-over from the strongly conductive correlated regime to the low
temperature low conductance state. Our analytical formulas agree well with our
numerical renormalization group results. The frequency dependent current noise
spectrum is also discussed.Comment: 6 pages, 7 figure
- …