96 research outputs found
Complementary colors of colorons: the elementary excitations of the SU(3) Haldane--Shastry model
We propose two possible trial wave functions for the elementary excitations
of the SU(3) Haldane--Shastry model, but then argue on very general grounds
that only one or the other can be a valid excitation. We then prove explicitly
that the trial wave function describing a coloron excitation which transforms
according to representation under SU(3) rotations if the spins of the
original model transform according to representation 3, is exact. If a basis
for the spins on the chain is spanned by the colors blue, red, and green, a
basis for the coloron excitations is hence given by the complementary colors
yellow, cyan, and magenta. We obtain the dispersion and the exclusion
statistics among polarized colorons. Furthermore, we compare our results with
the asymptotic Bethe Ansatz and discuss the generalization to SU()
Dynamical phase transitions after quenches in non-integrable models
We investigate the dynamics following sudden quenches across quantum critical
points belonging to different universality classes. Specifically, we use matrix
product state methods to study the quantum Ising chain in the presence of two
additional terms which break integrability. We find that in all models the rate
function for the return probability to the initial state becomes a non-analytic
function of time in the thermodynamic limit. This so-called `dynamical phase
transition' was first observed in a recent work by Heyl, Polkovnikov, and
Kehrein [Phys. Rev. Lett. 110, 135704 (2013)] for the exactly-solvable quantum
Ising chain, which can be mapped to free fermions. Our results for `interacting
theories' indicate that non-analytic dynamics is a generic feature of sudden
quenches across quantum critical points. We discuss potential connections to
the dynamics of the order parameter
Efficiency and power of a thermoelectric quantum dot device
We study linear response and nonequilibrium steady-state thermoelectric
transport through a single-level quantum dot tunnel coupled to two reservoirs
held at different temperatures as well as chemical potentials. A fermion
occupying the dot interacts with those in the reservoirs by a short-ranged
two-particle interaction. For parameters for which particles flow against a
bias voltage from the hot to the cold reservoir this setup acts as an
energy-conversion device with which electrical energy is gained out of waste
heat. We investigate how correlations affect its efficiency and output power.
In linear response the changes in the thermoelectric properties can be traced
back to the interaction induced renormalization of the resonance line shape. In
particular, small to intermediate repulsive interactions reduce the maximum
efficiency. In nonequilibrium the situation is more complex and we identify a
parameter regime in which for a fixed lower bound of the output power the
efficiency increases.Comment: 6 pages, 6 figure
Luttinger liquid universality in the time evolution after an interaction quench
We provide strong evidence that the relaxation dynamics of one-dimensional,
metallic Fermi systems resulting out of an abrupt amplitude change of the
two-particle interaction has aspects which are universal in the Luttinger
liquid sense: The leading long-time behavior of certain observables is
described by universal functions of the equilibrium Luttinger liquid parameter
and the renormalized velocity. We analytically derive those functions for the
Tomonaga-Luttinger model and verify our hypothesis of universality by
considering spinless lattice fermions within the framework of the density
matrix renormalization group
Spin and orbital fluctuations in non-equilibrium transport through quantum dots: A renormalisation-group analysis
We study non-equilibrium current and occupation probabilities of a
two-orbital quantum dot. The couplings to the leads are allowed to be
asymmetric and orbital dependent as it is generically the case in transport
experiments on molecules and nanowires. Starting from a two-orbital Anderson
model, we perform a generalised Schrieffer-Wolff transformation to derive an
effective Kondo model. This generates an orbital potential scattering
contribution which is of the same order as the spin exchange interaction. In a
first perturbative analysis we identify a regime of negative differential
conductance and a cascade resonance in the presence of an external magnetic
field, which both originate from the non-equilibrium occupation of the
orbitals. We then study the logarithmic enhancement of these signatures by
means of a renormalisation-group treatment. We find that the orbital potential
scattering qualitatively changes the renormalisation of the spin exchange
couplings and strongly affects the differential conductance for asymmetric
couplings.Comment: 6 pages, 4 figures, revised version as publishe
Magnetic field effects on the finite-frequency noise and ac conductance of a Kondo quantum dot out of equilibrium
We present analytic results for the finite-frequency current noise and the
nonequilibrium ac conductance for a Kondo quantum dot in presence of a magnetic
field. Using the real-time renormalization group method, we determine the line
shape close to resonances and show that while all resonances in the ac
conductance are broadened by the transverse spin relaxation rate, the noise at
finite field additionally involves the longitudinal rate as well as sharp kinks
resulting in singular derivatives. Our results provide a consistent theoretical
description of recent experimental data for the emission noise at zero magnetic
field, and we propose the extension to finite field for which we present a
detailed prediction.Comment: 21 pages, 13 figure
Spin switching via quantum dot spin valves
We develop a theory for spin transport and magnetization dynamics in a
quantum-dot spin valve, i.e., two magnetic reservoirs coupled to a quantum dot.
Our theory is able to take into account effects of strong correlations. We
demonstrate that, as a result of these strong correlations, the dot gate
voltage enables control over the current-induced torques on the magnets, and,
in particular, enables voltage-controlled magnetic switching. The electrical
resistance of the structure can be used to read out the magnetic state. Our
model may be realized by a number of experimental systems, including magnetic
scanning-tunneling microscope tips and artificial quantum dot systems
Exact results for SU(3) spin chains: trimer states, valence bond solids, and their parent Hamiltonians
We introduce several exact models for SU(3) spin chains: (1) a
translationally invariant parent Hamiltonian involving four-site interactions
for the trimer chain, with a three-fold degenerate ground state. We provide
numerical evidence that the elementary excitations of this model transform
under representation 3bar of SU(3) if the original spins of the model transform
under rep. 3. (2) a family of parent Hamiltonians for valence bond solids of
SU(3) chains with spin reps. 6, 10, and 8 on each lattice site. We argue that
of these three models, only the latter two exhibit spinon confinement and a
Haldane gap in the excitation spectrum
Influence of non-magnetic impurities on hole doped two-leg Cu-O Hubbard ladders
We study the influence of non magnetic impurities on the phase diagram of
doped two-leg Hubbard Cu-O ladders. In the absence of impurities this system
posseses d-wave superconducting states and orbital current states depending on
the doping. A single, strong, scatterer modifies its environment locally and
this effect is assessed using a renormalization group analysis. At high doping,
disorder causes intraband instabilities and at low doping it promotes interband
instabilities. In the former case, we extend the boundary conformal field
theory method, developed in the context of single chains, to handle the ladder
problem, and we find exact closed-form analytical expressions for the
correlation functions. This allows us to compute experimentally measurable
local quantities such as the nuclear magnetic resonance line broadenings and
scanning tunnelling microscope profiles. We also discuss the low doping regime
where Kondo physics is at play, making qualitative predictions about its
nature. Insight into collective effects is also given in the many weak
impurities case, based on an RG approach. In this regime, one sees the
interplay between interactions and disorder. We emphasize the influence of the
O atoms on disorder effects both for the single- and for the many-defect
situations.Comment: accepted to be published in NJP special editio
On the determinant representations of Gaudin models' scalar products and form factors
We propose alternative determinant representations of certain form factors
and scalar products of states in rational Gaudin models realized in terms of
compact spins. We use alternative pseudo-vacuums to write overlaps in terms of
partition functions with domain wall boundary conditions. Contrarily to
Slavnovs determinant formulas, this construction does not require that any of
the involved states be solutions to the Bethe equations; a fact that could
prove useful in certain non-equilibrium problems. Moreover, by using an
atypical determinant representation of the partition functions, we propose
expressions for the local spin raising and lowering operators form factors
which only depend on the eigenvalues of the conserved charges. These
eigenvalues define eigenstates via solutions of a system of quadratic equations
instead of the usual Bethe equations. Consequently, the current work allows
important simplifications to numerical procedures addressing decoherence in
Gaudin models.Comment: 15 pages, 0 figures, Published versio
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