1,917 research outputs found
Reducing Memory Cost of Exact Diagonalization using Singular Value Decomposition
We present a modified Lanczos algorithm to diagonalize lattice Hamiltonians
with dramatically reduced memory requirements, {\em without restricting to
variational ansatzes}. The lattice of size is partitioned into two
subclusters. At each iteration the Lanczos vector is projected into two sets of
smaller subcluster vectors using singular value decomposition.
For low entanglement entropy , (satisfied by short range Hamiltonians),
the truncation error is expected to vanish as . Convergence is tested for the Heisenberg model on Kagom\'e
clusters of 24, 30 and 36 sites, with no lattice symmetries exploited, using
less than 15GB of dynamical memory. Generalization of the Lanczos-SVD algorithm
to multiple partitioning is discussed, and comparisons to other techniques are
given.Comment: 7 pages, 8 figure
Even-odd correlations in capacitance fluctuations of quantum dots
We investigate effects of short range interactions on the addition spectra of
quantum dots using a disordered Hubbard model. A correlation function \cS(q) is
defined on the inverse compressibility versus filling data, and computed
numerically for small lattices. Two regimes of interaction strength are
identified: the even/odd fluctuations regime typical of Fermi liquid ground
states, and a regime of structureless \cS(q) at strong interactions. We
propose to understand the latter regime in terms of magnetically correlated
localized spins.Comment: 3 pages, Revtex, Without figure
Atomic Quantum Simulation of Dynamical Gauge Fields coupled to Fermionic Matter: From String Breaking to Evolution after a Quench
Using a Fermi-Bose mixture of ultra-cold atoms in an optical lattice, we
construct a quantum simulator for a U(1) gauge theory coupled to fermionic
matter. The construction is based on quantum links which realize continuous
gauge symmetry with discrete quantum variables. At low energies, quantum link
models with staggered fermions emerge from a Hubbard-type model which can be
quantum simulated. This allows us to investigate string breaking as well as the
real-time evolution after a quench in gauge theories, which are inaccessible to
classical simulation methods.Comment: 14 pages, 5 figures. Main text plus one general supplementary
material and one basic introduction to the topic. Published versio
QCD as a Quantum Link Model
QCD is constructed as a lattice gauge theory in which the elements of the
link matrices are represented by non-commuting operators acting in a Hilbert
space. The resulting quantum link model for QCD is formulated with a fifth
Euclidean dimension, whose extent resembles the inverse gauge coupling of the
resulting four-dimensional theory after dimensional reduction. The inclusion of
quarks is natural in Shamir's variant of Kaplan's fermion method, which does
not require fine-tuning to approach the chiral limit. A rishon representation
in terms of fermionic constituents of the gluons is derived and the quantum
link Hamiltonian for QCD with a U(N) gauge symmetry is expressed in terms of
glueball, meson and constituent quark operators. The new formulation of QCD is
promising both from an analytic and from a computational point of view.Comment: 27 pages, including three figures. ordinary LaTeX; Submitted to Nucl.
Phys.
Elucidating the structural composition of a Fe-N-C catalyst by nuclear and electron resonance techniques
FeâNâC catalysts are very promising materials for fuel cells and metalâair batteries. This work gives fundamental insights into the structural composition of an FeâNâC catalyst and highlights the importance of an inâdepth characterization. By nuclearâ and electronâresonance techniques, we are able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of αâiron and, surprisingly, iron oxide. Our work makes it questionable to what extent FeN4 sites can be present in FeâNâC catalysts prepared by pyrolysis at 900â°C and above. The simulation of the iron partial density of phonon states enables the identification of three FeN4 species in our catalyst, one of them comprising a sixfold coordination with endâon bonded oxygen as one of the axial ligands
Quantum simulation of spin ordering with nuclear spins in a solid state lattice
An experiment demonstrating the quantum simulation of a spin-lattice
Hamiltonian is proposed. Dipolar interactions between nuclear spins in a solid
state lattice can be modulated by rapid radio-frequency pulses. In this way,
the effective Hamiltonian of the system can be brought to the form of an
antiferromagnetic Heisenberg model with long range interactions. Using a
semiconducting material with strong optical properties such as InP, cooling of
nuclear spins could be achieved by means of optical pumping. An additional
cooling stage is provided by adiabatic demagnetization in the rotating frame
(ADRF) down to a nuclear spin temperature at which we expect a phase transition
from a paramagnetic to antiferromagnetic phase. This phase transition could be
observed by probing the magnetic susceptibility of the spin-lattice. Our
calculations suggest that employing current optical pumping technology,
observation of this phase transition is within experimental reach.Comment: 11 pages, 3 figues; Published versio
Statics and dynamics of weakly coupled antiferromagnetic spin-1/2 ladders in a magnetic field
We investigate weakly coupled spin-1/2 ladders in a magnetic field. The work
is motivated by recent experiments on the compound (C5H12N)2CuBr4 (BPCB). We
use a combination of numerical and analytical methods, in particular the
density matrix renormalization group (DMRG) technique, to explore the phase
diagram and the excitation spectra of such a system. We give detailed results
on the temperature dependence of the magnetization and the specific heat, and
the magnetic field dependence of the nuclear magnetic resonance (NMR)
relaxation rate of single ladders. For coupled ladders, treating the weak
interladder coupling within a mean-field or quantum Monte Carlo approach, we
compute the transition temperature of triplet condensation and its
corresponding antiferromagnetic order parameter. Existing experimental
measurements are discussed and compared to our theoretical results. Furthermore
we compute, using time dependent DMRG, the dynamical correlations of a single
spin ladder. Our results allow to directly describe the inelastic neutron
scattering cross section up to high energies. We focus on the evolution of the
spectra with the magnetic field and compare their behavior for different
couplings. The characteristic features of the spectra are interpreted using
different analytical approaches such as the mapping onto a spin chain, a
Luttinger liquid (LL) or onto a t-J model. For values of parameters for which
such measurements exist, we compare our results to inelastic neutron scattering
experiments on the compound BPCB and find excellent agreement. We make
additional predictions for the high energy part of the spectrum that are
potentially testable in future experiments.Comment: 35 pages, 26 figure
Trapdoor Memory-Hard Functions
Memory-hard functions (MHF) are functions whose evaluation provably requires a lot of memory. While MHFs are an unkeyed primitive, it is natural to consider the notion of trapdoor MHFs (TMHFs). A TMHF is like an MHF, but when sampling the public parameters one also samples a trapdoor which allows evaluating the function much cheaper.
Biryukov and Perrin (Asiacrypt\u2717) were the first to consider TMHFs and put forth a candidate TMHF construction called Diodon that is based on the Scrypt MHF (Percival, BSDCan\u2709). To allow for a trapdoor, Scrypt\u27s initial hash chain is replaced by a sequence of squares in a group of unknown order where the order of the group is the trapdoor. For a length sequence of squares and a group of order , Diodon\u27s cumulative memory complexity (CMC) is without the trapdoor and with knowledge of it.
While Scrypt is proven to be optimally memory-hard in the random oracle model (Alwen et al., Eurocrypt\u2717), Diodon\u27s memory-hardness has not been proven so far. In this work, we fill this gap by rigorously analyzing a specific instantiation of Diodon. We show that its CMC is lower bounded by which almost matches the upper bound. Our proof is based Alwen et al.\u27s lower bound on Scrypt\u27s CMC but requires non-trivial modifications due to the algebraic structure of Diodon. Most importantly, our analysis involves a more elaborate compression argument and a solvability criterion for certain systems of Diophantine equations
Quantum and classical thermal correlations in the XY spin-1/2 chain
We investigate pairwise quantum correlation as measured by the quantum
discord as well as its classical counterpart in the thermodynamic limit of
anisotropic XY spin-1/2 chains in a transverse magnetic field for both zero and
finite temperatures. Analytical expressions for both classical and quantum
correlations are obtained for spin pairs at any distance. In the case of zero
temperature, it is shown that the quantum discord for spin pairs farther than
second-neighbors is able to characterize a quantum phase transition, even
though pairwise entanglement is absent for such distances. For finite
temperatures, we show that quantum correlations can be increased with
temperature in the presence of a magnetic field. Moreover, in the XX limit, the
thermal quantum discord is found to be dominant over classical correlation
while the opposite scenario takes place for the transverse field Ising model
limit
Hydrophilic intraocular lens opacification after posterior lamellar keratoplasty - a material analysis with special reference to optical quality assessment
Background: Laboratory analysis and optical quality assessment of explanted hydrophilic intraocular lenses (IOLs) with clinically significant opacification after posterior lamellar keratoplasty (DMEK and DSAEK). Methods: Thirteen opacified IOLs after posterior lamellar keratoplasty, 8 after descemet stripping automated endothelial keratoplasty (DSAEK), 3 after descemet membrane endothelial keratoplasty (DMEK) and 2 after both DSAEK and DMEK were analysed in our laboratory. Analyses included optical bench assessment for optical quality, light microscopy, scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS). Results: In all IOLs the opacification was caused by a thin layer of calciumphosphate that had accumulated underneath the anterior optical surface of the IOLs in the area spared by the pupil/anterior capsulorhexis. The calcifications lead to a significant deterioration of the modulation transfer function across all spatial frequencies of the affected IOLs. Conclusions: The instillation of exogenous material such as air or gas into the anterior chamber increases the risk for opacification of hydrophilic IOLs irrespective of the manufacturer or the exact composition of the hydrophilic lens material. It is recommended to avoid the use of hydrophilic acrylic IOLs in patients with endothelial dystrophy that will likely require procedures involving the intracameral instillation of air or gas, such as DMEK or DS(A)EK
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