3,767 research outputs found
Domain Patterns in the Microwave-Induced Zero-Resistance State
It has been proposed that the microwave-induced ``zero-resistance''
phenomenon, observed in a GaAs two-dimensional electron system at low
temperatures in moderate magnetic fields, results from a state with multiple
domains, in which a large local electric field \bE(\br) is oriented in
different directions. We explore here the questions of what may determine the
domain arrangement in a given sample, what do the domains look like in
representative cases, and what may be the consequences of domain-wall
localization on the macroscopic dc conductance. We consider both effects of
sample boundaries and effects of disorder, in a simple model, which has a
constant Hall conductivity, and is characterized by a Lyapunov functional.Comment: 19 pages, 5 figures; submitted to a special issue of Journal of
Statistical Physics, in honor of P. C. Hohenberg and J. S. Lange
Steady States of a Microwave Irradiated Quantum Hall Gas
We consider effects of a long-wavelength disorder potential on the Zero
Conductance State (ZCS) of the microwave-irradiated 2D electron gas. Assuming a
uniform Hall conductivity, we construct a Lyapunov functional and derive
stability conditions on the domain structure of the photo-generated fields. We
solve the resulting equations for a general one-dimensional and certain
two-dimensional disorder potentials, and find non-zero conductances,
photo-voltages, and circulating dissipative currents. In contrast, weak white
noise disorder does not destroy the ZCS, but induces mesoscopic current
fluctuations.Comment: 4 pages, 2 colour figure
Preparation and detection of magnetic quantum phases in optical superlattices
We describe a novel approach to prepare, detect and characterize magnetic
quantum phases in ultra-cold spinor atoms loaded in optical superlattices. Our
technique makes use of singlet-triplet spin manipulations in an array of
isolated double well potentials in analogy to recently demonstrated quantum
control in semiconductor quantum dots. We also discuss the many-body
singlet-triplet spin dynamics arising from coherent coupling between nearest
neighbor double wells and derive an effective description for such system. We
use it to study the generation of complex magnetic states by adiabatic and
non-equilibrium dynamics.Comment: 5 pages, 2 Figures, reference adde
Continuous unitary transformations and finite-size scaling exponents in the Lipkin-Meshkov-Glick model
We analyze the finite-size scaling exponents in the Lipkin-Meshkov-Glick
model by means of the Holstein-Primakoff representation of the spin operators
and the continuous unitary transformations method. This combination allows us
to compute analytically leading corrections to the ground state energy, the
gap, the magnetization, and the two-spin correlation functions. We also present
numerical calculations for large system size which confirm the validity of this
approach. Finally, we use these results to discuss the entanglement properties
of the ground state focusing on the (rescaled) concurrence that we compute in
the thermodynamical limit.Comment: 20 pages, 9 figures, published versio
Schwinger-Boson Mean-Field Theory of Mixed-Spin Antiferromagnet
The Schwinger-boson mean-field theory is used to study the three-dimensional
antiferromagnetic ordering and excitations in compounds , a large
family of quasi-one-dimensional mixed-spin antiferromagnet. To investigate
magnetic properties of these compounds, we introduce a three-dimensional
mixed-spin antiferromagnetic Heisenberg model based on experimental results for
the crystal structure of . This model can explain the experimental
discovery of coexistence of Haldane gap and antiferromagnetic long-range order
below N\'{e}el temperature. Properties such as the low-lying excitations,
magnetizations of and rare-earth ions, N\'{e}el temperatures of different
compounds, and the behavior of Haldane gap below the N\'{e}el temperature are
investigated within this model, and the results are in good agreement with
neutron scattering experiments.Comment: 12 pages, 6 figure
Spin and orbital valence bond solids in a one-dimensional spin-orbital system: Schwinger boson mean field theory
A generalized one-dimensional spin-orbital model is
studied by Schwinger boson mean-field theory (SBMFT). We explore mainly the
dimer phases and clarify how to capture properly the low temperature properties
of such a system by SBMFT. The phase diagrams are exemplified. The three dimer
phases, orbital valence bond solid (OVB) state, spin valence bond solid (SVB)
state and spin-orbital valence bond solid (SOVB) state, are found to be favored
in respectively proper parameter regions, and they can be characterized by the
static spin and pseudospin susceptibilities calculated in SBMFT scheme. The
result reveals that the spin-orbit coupling of type serves
as both the spin-Peierls and orbital-Peierles mechanisms that responsible for
the spin-singlet and orbital-singlet formations respectively.Comment: 6 pages, 3 figure
Spin-1 chain with spin-1/2 excitations in the bulk
We present a spin-1 chain with a Hamiltonian which has three exactly solvable
ground states. Two of these are fully dimerized, analogous to the
Majumdar-Ghosh (MG) states of a spin-1/2 chain, while the third is of the
Affleck-Kennedy-Lieb-Tasaki (AKLT) type. We use variational and numerical
methods to study the low-energy excitations which interpolate between these
ground states in different ways. In particular, there is a spin-1/2 excitation
which interpolates between the MG and AKLT ground states; this is the lowest
excitation of the system and it has a surprisingly small gap. We discuss
generalizations of our model of spin fractionalization to higher spin chains
and higher dimensions.Comment: 7 pages including 4 figures; this is the published version of the
pape
Renormalization algorithm for the calculation of spectra of interacting quantum systems
We present an algorithm for the calculation of eigenstates with definite
linear momentum in quantum lattices. Our method is related to the Density
Matrix Renormalization Group, and makes use of the distribution of multipartite
entanglement to build variational wave--functions with translational symmetry.
Its virtues are shown in the study of bilinear--biquadratic S=1 chains.Comment: Corrected version. We have added an appendix with an extended
explanation of the implementation of our algorith
Exploring Contractor Renormalization: Tests on the 2-D Heisenberg Antiferromagnet and Some New Perspectives
Contractor Renormalization (CORE) is a numerical renormalization method for
Hamiltonian systems that has found applications in particle and condensed
matter physics. There have been few studies, however, on further understanding
of what exactly it does and its convergence properties. The current work has
two main objectives. First, we wish to investigate the convergence of the
cluster expansion for a two-dimensional Heisenberg Antiferromagnet(HAF). This
is important because the linked cluster expansion used to evaluate this formula
non-perturbatively is not controlled by a small parameter. Here we present a
study of three different blocking schemes which reveals some surprises and in
particular, leads us to suggest a scheme for defining successive terms in the
cluster expansion. Our second goal is to present some new perspectives on CORE
in light of recent developments to make it accessible to more researchers,
including those in Quantum Information Science. We make some comparison to
entanglement-based approaches and discuss how it may be possible to improve or
generalize the method.Comment: Completely revised version accepted by Phy Rev B; 13 pages with added
material on entropy in COR
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