979 research outputs found
Subband Engineering Even-Denominator Quantum Hall States
Proposed even-denominator fractional quantum Hall effect (FQHE) states
suggest the possibility of excitations with non-Abelian braid statistics.
Recent experiments on wide square quantum wells observe even-denominator FQHE
even under electrostatic tilt. We theoretically analyze these structures and
develop a procedure to accurately test proposed quantum Hall wavefunctions. We
find that tilted wells favor partial subband polarization to yield Abelian
even-denominator states. Our results show that tilting quantum wells
effectively engineers different interaction potentials allowing exploration of
a wide variety of even-denominator states
Ramping fermions in optical lattices across a Feshbach resonance
We study the properties of ultracold Fermi gases in a three-dimensional
optical lattice when crossing a Feshbach resonance. By using a zero-temperature
formalism, we show that three-body processes are enhanced in a lattice system
in comparison to the continuum case. This poses one possible explanation for
the short molecule lifetimes found when decreasing the magnetic field across a
Feshbach resonance. Effects of finite temperatures on the molecule formation
rates are also discussed by computing the fraction of double-occupied sites.
Our results show that current experiments are performed at temperatures
considerably higher than expected: lower temperatures are required for
fermionic systems to be used to simulate quantum Hamiltonians. In addition, by
relating the double occupancy of the lattice to the temperature, we provide a
means for thermometry in fermionic lattice systems, previously not accessible
experimentally. The effects of ramping a filled lowest band across a Feshbach
resonance when increasing the magnetic field are also discussed: fermions are
lifted into higher bands due to entanglement of Bloch states, in good agreement
with recent experiments.Comment: 9 pages, 7 figure
Interacting classical dimers on the square lattice
We study a model of close-packed dimers on the square lattice with a nearest
neighbor interaction between parallel dimers. This model corresponds to the
classical limit of quantum dimer models [D.S. Rokhsar and S.A. Kivelson, Phys.
Rev. Lett.{\bf 61}, 2376 (1988)]. By means of Monte Carlo and Transfer Matrix
calculations, we show that this system undergoes a Kosterlitz-Thouless
transition separating a low temperature ordered phase where dimers are aligned
in columns from a high temperature critical phase with continuously varying
exponents. This is understood by constructing the corresponding Coulomb gas,
whose coupling constant is computed numerically. We also discuss doped models
and implications on the finite-temperature phase diagram of quantum dimer
models.Comment: 4 pages, 4 figures; v2 : Added results on doped models; published
versio
Mott Domains of Bosons Confined on Optical Lattices
In the absence of a confining potential, the boson Hubbard model in its
ground state is known to exhibit a superfluid to Mott insulator quantum phase
transition at commensurate fillings and strong on-site repulsion. In this
paper, we use quantum Monte Carlo simulations to study the ground state of the
one dimensional bosonic Hubbard model in a trap. We show that some, but not
all, aspects of the Mott insulating phase persist when a confining potential is
present. The Mott behavior is present for a continuous range of incommensurate
fillings, a very different situation from the unconfined case. Furthermore the
establishment of the Mott phase does not proceed via a quantum phase transition
in the traditional sense. These observations have important implications for
the interpretation of experimental results for atoms trapped on optical
lattices. Initial results show that, qualitatively, the same results persist in
higher dimensions.Comment: Revtex file, five figures, include
Quantum Monte Carlo Loop Algorithm for the t-J Model
We propose a generalization of the Quantum Monte Carlo loop algorithm to the
t-J model by a mapping to three coupled six-vertex models. The autocorrelation
times are reduced by orders of magnitude compared to the conventional local
algorithms. The method is completely ergodic and can be formulated directly in
continuous time. We introduce improved estimators for simulations with a local
sign problem. Some first results of finite temperature simulations are
presented for a t-J chain, a frustrated Heisenberg chain, and t-J ladder
models.Comment: 22 pages, including 12 figures. RevTex v3.0, uses psf.te
Effects of Nonmagnetic Impurity Doping on Spin Ladder System
Effects of nonmagnetic impurity doping on an AF spin-1/2 Heisenberg ladder
system are studied by the QMC method. A single nonmagnetic impurity induces a
localized spin-1/2 moment accompanied by "static" and enhanced AF correlations
around it. Small and finite concentration of impurities induces a remarkable
change of magnetic and thermodynamic properties with gapless excitations. It
also shows rather sharp but continuous crossover around the concentration of
about 4%. Above the crossover concentration, all the spins are strongly coupled
participating in the enhanced and rather uniform power-law decay of the
antiferromagnetic correlation. Below the crossover, each impurity forms an
antiferromagnetic cluster only weakly coupled each other. For random
distribution of impurities, large Curie-like susceptibility accompanied with
small residual entropy is obtained at low temperatures in agreement with recent
experimental observation in Zn-doped . Temperature dependence of
AF susceptibility shows power-law-like but weaker divergence than the single
chain AFH in the temperature range studied.Comment: 4 pages, LaTeX+epsf.sty, submitted to J.Phys.Soc.Jpn. New results of
AF susceptibility are adde
Two-dimensional epitaxial superconductor-semiconductor heterostructures: A platform for topological superconducting networks
Progress in the emergent field of topological superconductivity relies on
synthesis of new material combinations, combining superconductivity, low
density, and spin-orbit coupling (SOC). For example, theory [1-4] indicates
that the interface between a one-dimensional (1D) semiconductor (Sm) with
strong SOC and a superconductor (S) hosts Majorana modes with nontrivial
topological properties [5-8]. Recently, epitaxial growth of Al on InAs
nanowires was shown to yield a high quality S-Sm system with uniformly
transparent interfaces [9] and a hard induced gap, indicted by strongly
suppressed sub gap tunneling conductance [10]. Here we report the realization
of a two-dimensional (2D) InAs/InGaAs heterostructure with epitaxial Al,
yielding a planar S-Sm system with structural and transport characteristics as
good as the epitaxial wires. The realization of 2D epitaxial S-Sm systems
represent a significant advance over wires, allowing extended networks via
top-down processing. Among numerous potential applications, this new material
system can serve as a platform for complex networks of topological
superconductors with gate-controlled Majorana zero modes [1-4]. We demonstrate
gateable Josephson junctions and a highly transparent 2D S-Sm interface based
on the product of excess current and normal state resistance
On the nature of the transition from the spontaneously dimerized to the Neel phase in the two-dimensional J1-J2 model
We analyze the spectrum of the 2D S=1/2 frustrated Heisenberg model near the
transition from the spontaneously dimerized spin-liquid phase into the Neel
ordered phase. Two excitation branches: the triplet magnon, and the collective
singlet mode, both become gapless at the transition point. However we find that
the length scales associated with these modes are well separated at the quantum
transition. While in the quantum disordered phase the singlet excitation has
finite spectral weight and reflects the existence of spontaneous dimer order,
near the transition point the size of the singlet bound state grows
exponentially with the correlation length, and hence the quasiparticle residue
is exponentially small. Therefore the critical dynamics remains in the O(3)
universality class in spite of the four gapless modes.Comment: 5 pages, 3 figure
Interacting anyons in topological quantum liquids: The golden chain
We discuss generalizations of quantum spin Hamiltonians using anyonic degrees
of freedom. The simplest model for interacting anyons energetically favors
neighboring anyons to fuse into the trivial (`identity') channel, similar to
the quantum Heisenberg model favoring neighboring spins to form spin singlets.
Numerical simulations of a chain of Fibonacci anyons show that the model is
critical with a dynamical critical exponent z=1, and described by a
two-dimensional conformal field theory with central charge c=7/10. An exact
mapping of the anyonic chain onto the two-dimensional tricritical Ising model
is given using the restricted-solid-on-solid (RSOS) representation of the
Temperley-Lieb algebra. The gaplessness of the chain is shown to have
topological origin.Comment: 5 pages, 4 figure
Finite-temperature effects on the superfluid Bose-Einstein condensation of confined ultracold atoms in three-dimensional optical lattices
We discuss the finite-temperature phase diagram in the three-dimensional
Bose-Hubbard (BH) model in the strong correlation regime, relevant for
Bose-Einstein condensates in optical lattices, by employing a quantum rotor
approach. In systems with strong on site repulsive interactions, the rotor U(1)
phase variable dual to the local boson density emerges as an important
collective field. After establishing the connection between the rotor
construction and the the on--site interaction in the BH model the robust
effective action formalism is developed which allows us to study the superfluid
phase transition in various temperature--interaction regimes
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