3,698 research outputs found
A machine learning approach to the Berezinskii-Kosterlitz-Thouless transition in classical and quantum models
The Berezinskii-Kosterlitz-Thouless transition is a very specific phase
transition where all thermodynamic quantities are smooth. Therefore, it is
difficult to determine the critical temperature in a precise way. In this paper
we demonstrate how neural networks can be used to perform this task. In
particular, we study how the accuracy of the transition identification depends
on the way the neural networks are trained. We apply our approach to three
different systems: (i) the classical XY model, (ii) the phase-fermion model,
where classical and quantum degrees of freedom are coupled and (iii) the
quantum XY model.Comment: 11 pages, 7 figure
Particle-hole character of the Higgs and Goldstone modes in strongly-interacting lattice bosons
We study the low-energy excitations of the Bose-Hubbard model in the
strongly-interacting superfluid phase using a Gutzwiller approach and extract
the single-particle and single-hole excitation amplitudes for each mode. We
report emergent mode-dependent particle-hole symmetry on specific arc-shaped
lines in the phase diagram connecting the well-known Lorentz-invariant limits
of the Bose-Hubbard model. By tracking the in-phase particle-hole symmetric
oscillations of the order parameter, we provide an answer to the long-standing
question about the fate of the pure amplitude Higgs mode away from the
integer-density critical point. Furthermore, we point out that out-of-phase
oscillations are responsible for a full suppression of the condensate density
oscillations of the gapless Goldstone mode. Possible detection protocols are
also discussed.Comment: 6 pages, 3 figure
High Tc Superconductors -- A Variational Theory of the Superconducting State
We use a variational approach to gain insight into the strongly correlated
d-wave superconducting state of the high Tc cuprates at T=0. We show that
strong correlations lead to qualitatively different trends in pairing and phase
coherence: the pairing scale decreases monotonically with hole doping while the
SC order parameter shows a non-monotonic dome. We obtain detailed results for
the doping-dependence of a large number of experimentally observable
quantities, including the chemical potential, coherence length, momentum
distribution, nodal quasiparticle weight and dispersion, incoherent features in
photoemission spectra, optical spectral weight and superfluid density. Most of
our results are in remarkable quantitative agreement with existing data and
some of our predictions, first reported in Phys. Rev. Lett. {\bf 87}, 217002
(2001), have been recently verified.Comment: (Minor revisions, 1 figure added, version to appear in PRB) 23 RevTeX
pages, 11 eps figs, long version of cond-mat/0101121, contains detailed
comparisons with experiments, analytical insights, technical aspects of the
calculation, and comparison with slave boson MF
Estimation of Parameters of Misclassified Size Biased Borel Distribution
A misclassified size-biased Borel Distribution (MSBBD), where some of the observations corresponding to x = c + 1 are wrongly reported as x = c with probability α, is defined. Various estimation methods like the method of maximum likelihood (ML), method of moments, and the Bayes estimation for the parameters of the MSBB distribution are used. The performance of the estimators are studied using simulated bias and simulated risk. Simulation studies are carried out for different values of the parameters and sample size
Classification of Lipschitz simple function germs
It was shown by Henry and Parusiński in 2003 that the bi-Lipschitz right equivalence of function germs admits moduli. In this article, we introduce the notion of Lipschitz simple function germ and present the complete classification in the complex case. For this, we present several bi-Lipschitz invariants associated to functions germs. In particular, we prove that the lowest degree homogeneous part of a function germ is a bi-Lipschitz invariant and use this to show a weak version of the splitting lemma for bi-Lipschitz equivalence. We improve upon earlier results on bi-Lipschitz triviality of families to show that several families of germs in Arnold's list of unimodal singularities are bi-Lipschitz trivial. A surprising consequence of our result is that a function germ is Lipschitz modal if and only if it deforms to the smooth unimodal family of singularities called (Formula presented.) in Arnold's list
Estimation in Misclassified Size – Biased Log Series Distribution
A misclassified size-biased Log Series Distribution (MSBLSD) ............
Particle-Hole Symmetry and the Effect of Disorder on the Mott-Hubbard Insulator
Recent experiments have emphasized that our understanding of the interplay of
electron correlations and randomness in solids is still incomplete. We address
this important issue and demonstrate that particle-hole (ph) symmetry plays a
crucial role in determining the effects of disorder on the transport and
thermodynamic properties of the half-filled Hubbard Hamiltonian. We show that
the low-temperature conductivity decreases with increasing disorder when
ph-symmetry is preserved, and shows the opposite behavior, i.e. conductivity
increases with increasing disorder, when ph-symmetry is broken. The Mott
insulating gap is insensitive to weak disorder when there is ph-symmetry,
whereas in its absence the gap diminishes with increasing disorder.Comment: 4 pages, 4 figure
A Representation of Symmetry Generators for the Type IIB Superstring on a Plane Wave in the U(4) Formalism
We calculate the symmetry currents for the type IIB superstring on a
maximally supersymmetric plane wave background using the N=(2,2)
superconformally covariant U(4) formulation developed by Berkovits, Maldacena
and Maoz. An explicit realization of the U(4) generators together with 16
fermionic generators is obtained in terms of the N=(2,2) worldsheet fields.
Because the action is no longer quadratic, we use a light-cone version to
display the currents in terms of the covariant worldsheet variables.Comment: 9 pages, harvmac, Corrected some typographical errors, Added
reference
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Results of an aqueous source term model for a radiological risk assessment of the Drigg LLW Site, U.K.
A radionuclide source term model has been developed which simulates the biogeochemical evolution of the Drigg low level waste (LLW) disposal site. The DRINK (DRIgg Near field Kinetic) model provides data regarding radionuclide concentrations in groundwater over a period of 100,000 years, which are used as input to assessment calculations for a groundwater pathway. The DRINK model also provides input to human intrusion and gaseous assessment calculations through simulation of the solid radionuclide inventory. These calculations are being used to support the Drigg post closure safety case. The DRINK model considers the coupled interaction of the effects of fluid flow, microbiology, corrosion, chemical reaction, sorption and radioactive decay. It represents the first direct use of a mechanistic reaction-transport model in risk assessment calculations
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