907 research outputs found
Adaptive Design of Excitonic Absorption in Broken-Symmetry Quantum Wells
Adaptive quantum design is used to identify broken-symmetry quantum well
potential profiles with optical response properties superior to previous ad-hoc
solutions. This technique performs an unbiased stochastic search of
configuration space. It allows us to engineer many-body excitonic wave
functions and thus provides a new methodology to efficiently develop optimized
quantum confined Stark effect device structures.Comment: 4 pages, 3 encapsulated postscript figure
Effects of Strong Correlations and Disorder in d-Wave Superconductors
We use exact diagonalization techniques to study the interplay between strong
correlations, superconductivity, and disorder in a model system. We study an
extension of the t-J model by adding an infinite-range d-wave superconductivity
inducing term and disorder. Our work shows that in the clean case the magnitude
of the order parameter is surprisingly small for low-hole filling, thus
implying that mean-field theories might be least accurate in that important
regime. We demonstrate that substantial disorder is required to destroy a
d-wave superconducting state for low-hole doping. We provide the first bias
free numerical results for the local density of states of a strongly correlated
d-wave superconducting model, relevant for STM measurements at various fillings
and disorders.Comment: 4 pages, 4 figures, as publishe
Pancreatic exocrine insufficiency after bariatric surgery
Morbid obesity is a lifelong disease, and all patients require complementary follow-up including nutritional surveillance by a multidisciplinary team after bariatric procedures. Pancreatic exocrine insufficiency (PEI) refers to an insufficient secretion of pancreatic enzymes and/or sodium bicarbonate. PEI is a known multifactorial complication after upper gastrointestinal surgery, and might constitute an important clinical problem due to the large number of bariatric surgical procedures in the world. Symptoms of PEI often overlap with sequelae of gastric bypass, making the diagnosis difficult. Steatorrhea, weight loss, maldigestion and malabsorption are pathognomonic for both clinical conditions. Altered anatomy after bypass surgery can make the diagnostic process even more difficult. Fecal elastase-1 (FE1) is a useful diagnostic test. PEI should be considered in all patients after bariatric surgery with prolonged gastrointestinal complaints that are suggestive of maldigestion and/or malabsorption. Appropriate pancreatic enzyme replacement therapy should be part of the treatment algorithm in patients with confirmed PEI or symptoms suggestive of this complication
Using the J1-J2 Quantum Spin Chain as an Adiabatic Quantum Data Bus
This paper investigates numerically a phenomenon which can be used to
transport a single q-bit down a J1-J2 Heisenberg spin chain using a quantum
adiabatic process. The motivation for investigating such processes comes from
the idea that this method of transport could potentially be used as a means of
sending data to various parts of a quantum computer made of artificial spins,
and that this method could take advantage of the easily prepared ground state
at the so called Majumdar-Ghosh point. We examine several annealing protocols
for this process and find similar result for all of them. The annealing process
works well up to a critical frustration threshold.Comment: 14 pages, 13 figures (2 added), revisions made to add citations and
additional discussion at request of referee
Aperiodic nano-photonic design
The photon scattering properties of aperiodic nano-scale dielectric
structures can be tailored to closely match a desired response by using
adaptive algorithms for device design. We show that broken symmetry of
aperiodic designs provides access to device functions not available to
conventional periodic photonic crystal structures.Comment: 23 pages, LaTex, 8 postscript figure
Disorder-enhanced phase coherence in trapped bosons on optical lattices
The consequences of disorder on interacting bosons trapped in optical
lattices are investigated by quantum Monte Carlo simulations. At small to
moderate strengths of potential disorder a unique effect is observed: if there
is a Mott plateau at the center of the trap in the clean limit, phase coherence
{\it increases} as a result of disorder. The localization effects due to
correlation and disorder compete against each other, resulting in a partial
delocalization of the particles in the Mott region, which in turn leads to
increased phase coherence. In the absence of a Mott plateau, this effect is
absent. A detailed analysis of the uniform system without a trap shows that the
disordered states participate in a Bose glass phase.Comment: 4 pages, 4 figure
Statistical properties of random matrix product states
We study the set of random matrix product states (RMPS) introduced in
arXiv:0908.3877 as a tool to explore foundational aspects of quantum
statistical mechanics. In the present work, we provide an accurate numerical
and analytical investigation of the properties of RMPS. We calculate the
average state of the ensemble in the non-homogeneous case, and numerically
check the validity of this result. We also suggest using RMPS as a tool to
approximate properties of general quantum random states. The numerical
simulations presented here support the accuracy and efficiency of this
approximation. These results suggest that any generalized canonical state can
be approximated with high probability by the reduced density matrix of a random
MPS, if the average MPS coincide with the associated microcanonical ensemble.Comment: 12 pages, 17 figures; published versio
Three-Dimensional Ordering in Weakly Coupled Antiferromagnetic Ladders and Chains
A theoretical description is presented for low-temperature magnetic-field
induced three-dimensional (3D) ordering transitions in strongly anisotropic
quantum antiferromagnets, consisting of weakly coupled antiferromagnetic
spin-1/2 chains and ladders. First, effective continuum field theories are
derived for the one-dimensional subsystems. Then the Luttinger parameters,
which determine the low-temperature susceptibilities of the chains and ladders,
are calculated from the Bethe ansatz solution for these effective models. The
3D ordering transition line is obtained using a random phase approximation for
the weak inter-chain (inter-ladder) coupling. Finally, considering a Ginzburg
criterion, the fluctuation corrections to this approach are shown to be small.
The nature of the 3D ordered phase resembles a Bose condensate of integer-spin
magnons. It is proposed that for systems with higher spin degrees of freedom,
e.g. N-leg spin-1/2 ladders, multi-component condensates can occur at high
magnetic fields.Comment: RevTex, 18 pages with 7 figure
Bose glass and Mott glass of quasiparticles in a doped quantum magnet
The low-temperature states of bosonic fluids exhibit fundamental quantum
effects at the macroscopic scale: the best-known examples are Bose-Einstein
condensation (BEC) and superfluidity, which have been tested experimentally in
a variety of different systems. When bosons are interacting, disorder can
destroy condensation leading to a so-called Bose glass. This phase has been
very elusive to experiments due to the absence of any broken symmetry and of a
finite energy gap in the spectrum. Here we report the observation of a Bose
glass of field-induced magnetic quasiparticles in a doped quantum magnet
(Br-doped dichloro-tetrakis-thiourea-Nickel, DTN). The physics of DTN in a
magnetic field is equivalent to that of a lattice gas of bosons in the
grand-canonical ensemble; Br-doping introduces disorder in the hoppings and
interaction strengths, leading to localization of the bosons into a Bose glass
down to zero field, where it acquires the nature of an incompressible Mott
glass. The transition from the Bose glass (corresponding to a gapless spin
liquid) to the BEC (corresponding to a magnetically ordered phase) is marked by
a novel, universal exponent governing the scaling on the critical temperature
with the applied field, in excellent agreement with theoretical predictions.
Our study represents the first, quantitative account of the universal features
of disordered bosons in the grand-canonical ensemble.Comment: 13+6 pages, 5+6 figures; v2: Fig. 5 update
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