335 research outputs found
Direct solution of multiple excitations in a matrix product state with block Lanczos
Matrix product state methods are known to be efficient for computing ground
states of local, gapped Hamiltonians, particularly in one dimension. We
introduce the multi-targeted density matrix renormalization group method that
acts on a bundled matrix product state, holding many excitations. The use of a
block or banded Lanczos algorithm allows for the simultaneous, variational
optimization of the bundle of excitations. The method is demonstrated on a
Heisenberg model and other cases of interest. A large of number of excitations
can be obtained at a small bond dimension with highly reliable local
observables throughout the chain.Comment: 18 page
Characteristics of oxygen isotope substitutions in the quasiparticle spectrum of BiSrCaCuO
There is an ongoing debate about the nature of the bosonic excitations
responsible for the quasiparticle self energy in high temperature
superconductors -- are they phonons or spin fluctuations? We present a careful
analysis of the bosonic excitations as revealed by the `kink' feature at 70 meV
in angle resolved photoemission data using Eliashberg theory for a d-wave
superconductor. Starting from the assumption that nodal quasiparticles are not
coupled to the magnetic resonance, the sharp structure at meV
can be assigned to phonons. We find that not only can we account for the shifts
of the kink energy seen on oxygen isotope substitution but also get a
quantitative estimate of the fraction of the area under the electron-boson
spectral density which is due to phonons. We conclude that for optimally doped
BiSrCaCuO phonons contribute % and
non-phononic excitations %.Comment: 6 pages, 3 figure
Semiclassical description of spin ladders
The Heisenberg spin ladder is studied in the semiclassical limit, via a
mapping to the nonlinear model. Different treatments are needed if the
inter-chain coupling is small, intermediate or large. For intermediate
coupling a single nonlinear model is used for the ladder. Its predicts
a spin gap for all nonzero values of if the sum of the spins
of the two chains is an integer, and no gap otherwise. For small , a better
treatment proceeds by coupling two nonlinear sigma models, one for each chain.
For integer , the saddle-point approximation predicts a sharp drop
in the gap as increases from zero. A Monte-Carlo simulation of a spin 1
ladder is presented which supports the analytical results.Comment: 8 pages, RevTeX 3.0, 4 PostScript figure
Enhanced Two-Channel Kondo Physics in a Quantum Box Device
We propose a design for a one-dimensional quantum box device where the charge
fluctuations are described by an anisotropic two-channel Kondo model. The
device consists of a quantum box in the Coulomb blockade regime, weakly coupled
to a quantum wire by a single-mode point contact. The electron correlations in
the wire produce strong back scattering at the contact, significantly
increasing the Kondo temperature as compared to the case of non-interacting
electrons. By employing boundary conformal field theory techniques we show that
the differential capacitance of the box exhibits manifest two-channel Kondo
scaling with temperature and gate voltage, uncontaminated by the
one-dimensional electron correlations. We discuss the prospect to
experimentally access the Kondo regime with this type of device.Comment: EPL style, 5 pages, 1 figure, final published versio
Magnetism and d-wave superconductivity on the half-filled square lattice with frustration
The role of frustration and interaction strength on the half-filled Hubbard
model is studied on the square lattice with nearest and next-nearest neighbour
hoppings t and t' using the Variational Cluster Approximation (VCA). At
half-filling, we find two phases with long-range antiferromagnetic (AF) order:
the usual Neel phase, stable at small frustration t'/t, and the so-called
collinear (or super-antiferromagnet) phase with ordering wave-vector
or , stable for large frustration. These are separated by a phase with
no detectable long-range magnetic order. We also find the d-wave
superconducting (SC) phase (), which is favoured by frustration if
it is not too large. Intriguingly, there is a broad region of coexistence where
both AF and SC order parameters have non-zero values. In addition, the physics
of the metal-insulator transition in the normal state is analyzed. The results
obtained with the help of the VCA method are compared with the large-U
expansion of the Hubbard model and known results for the frustrated J1-J2
Heisenberg model. These results are relevant for pressure studies of undoped
parents of the high-temperature superconductors: we predict that an insulator
to d-wave SC transition may appear under pressure.Comment: 12 pages, 10 figure
Impact of water saturation on seismoelectric transfer functions: a laboratory study of coseismic phenomenon
Seismic waves propagating in a porous medium, under favourable conditions, generate measurable electromagnetic fields due to electrokinetic effects. It has been proposed, following experimental and numerical studies, that these so-called ‘seismoelectromagnetic' couplings depend on pore fluid properties. The theoretical frame describing these phenomena are based on the original Biot's theory, assuming that pores are fluid-filled. We study here the impact of a partially saturated medium on amplitudes of those seismoelectric couplings by comparing experimental data to an effective fluid model. We have built a 1-m-length-scale experiment designed for imbibition and drainage of an homogeneous silica sand; the experimental set-up includes a seismic source, accelerometers, electric dipoles and capacitance probes in order to monitor seismic and seismoelectric fields during water saturation. Apparent velocities and frequency spectra (in the kiloHertz range) are derived from seismic and electrical measurements during experiments in varying saturation conditions. Amplitudes of seismic and seismoelectric waves and their ratios (i.e. transfer functions) are discussed using a spectral analysis performed by continuous wavelet transform. The experiments reveal that amplitude ratios of seismic to coseismic electric signals remain rather constant as a function of the water saturation in the Sw=[0.2-0.9] range, consistently with theoretically predicted transfer function
First order Mott transition at zero temperature in two dimensions: Variational plaquette study
The nature of the metal-insulator Mott transition at zero temperature has
been discussed for a number of years. Whether it occurs through a quantum
critical point or through a first order transition is expected to profoundly
influence the nature of the finite temperature phase diagram. In this paper, we
study the zero temperature Mott transition in the two-dimensional Hubbard model
on the square lattice with the variational cluster approximation. This takes
into account the influence of antiferromagnetic short-range correlations. By
contrast to single-site dynamical mean-field theory, the transition turns out
to be first order even at zero temperature.Comment: 6 pages, 5 figures, version 2 with additional results for 8 bath
site
Nonlinear sigma model of a spin ladder containing a static single hole
In this letter we extend the nonlinear sigma model describing pure spin
ladders with an arbitrary number of legs to the case of ladders containing a
single static hole. A simple immediate application of this approach to
classical ladders is worked out.Comment: 17 pages, 2 figure
The staggered vertex model and its applications
New solvable vertex models can be easily obtained by staggering the spectral
parameter in already known ones. This simple construction reveals some
surprises: for appropriate values of the staggering, highly non-trivial
continuum limits can be obtained. The simplest case of staggering with period
two (the case) for the six-vertex model was shown to be related, in one
regime of the spectral parameter, to the critical antiferromagnetic Potts model
on the square lattice, and has a non-compact continuum limit. Here, we study
the other regime: in the very anisotropic limit, it can be viewed as a zig-zag
spin chain with spin anisotropy, or as an anyonic chain with a generic
(non-integer) number of species. From the Bethe-Ansatz solution, we obtain the
central charge , the conformal spectrum, and the continuum partition
function, corresponding to one free boson and two Majorana fermions. Finally,
we obtain a massive integrable deformation of the model on the lattice.
Interestingly, its scattering theory is a massive version of the one for the
flow between minimal models. The corresponding field theory is argued to be a
complex version of the Toda theory.Comment: 38 pages, 14 figures, 3 appendice
Two-Dimensional Quantum Spin Systems with Ladder and Plaquette Structure
We investigate low-energy properties of two-dimensional quantum spin systems
with the ladder and plaquette structures, which are described by a generalized
antiferromagnetic Heisenberg model with both of the bond and spin alternations.
By exploiting a non-linear model technique and a modified spin wave
approach, we evaluate the spin gap and the spontaneous magnetization to discuss
the quantum phase transition between the ordered and disordered states. We
argue how the spin-gapped phase is driven to the antiferromagnetic phase in the
phase diagram.Comment: 8 pages (9 figures), accepted by JPS
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