1,735 research outputs found
Fourier's Law in a Quantum Spin Chain and the Onset of Quantum Chaos
We study heat transport in a nonequilibrium steady state of a quantum
interacting spin chain. We provide clear numerical evidence of the validity of
Fourier law. The regime of normal conductivity is shown to set in at the
transition to quantum chaos.Comment: 4 pages, 5 figures, RevTe
-Kicked Quantum Rotors: Localization and `Critical' Statistics
The quantum dynamics of atoms subjected to pairs of closely-spaced
-kicks from optical potentials are shown to be quite different from the
well-known paradigm of quantum chaos, the singly--kicked system. We
find the unitary matrix has a new oscillating band structure corresponding to a
cellular structure of phase-space and observe a spectral signature of a
localization-delocalization transition from one cell to several. We find that
the eigenstates have localization lengths which scale with a fractional power
and obtain a regime of near-linear spectral variances
which approximate the `critical statistics' relation , where is related to the fractal
classical phase-space structure. The origin of the exponent
is analyzed.Comment: 4 pages, 3 fig
Counting the Holes
Argle claimed that holes supervene on their material hosts, and that every truth about holes boils down to a truth about perforated things. This may well be right, assuming holes are perforations. But we still need an explicit theory of holes to do justice to the ordinary way of counting holes--or so says Cargle
Chaotic quantum ratchets and filters with cold atoms in optical lattices: properties of Floquet states
Recently, cesium atoms in optical lattices subjected to cycles of
unequally-spaced pulses have been found to show interesting behavior: they
represent the first experimental demonstration of a Hamiltonian ratchet
mechanism, and they show strong variability of the Dynamical Localization
lengths as a function of initial momentum. The behavior differs qualitatively
from corresponding atomic systems pulsed with equal periods, which are a
textbook implementation of a well-studied quantum chaos paradigm, the quantum
delta-kicked particle (delta-QKP). We investigate here the properties of the
corresponding eigenstates (Floquet states) in the parameter regime of the new
experiments and compare them with those of the eigenstates of the delta-QKP at
similar kicking strengths. We show that, with the properties of the Floquet
states, we can shed light on the form of the observed ratchet current as well
as variations in the Dynamical Localization length.Comment: 9 pages, 9 figure
Quantum Poincare Recurrences for Hydrogen Atom in a Microwave Field
We study the time dependence of the ionization probability of Rydberg atoms
driven by a microwave field, both in classical and in quantum mechanics. The
quantum survival probability follows the classical one up to the Heisenberg
time and then decays algebraically as P(t) ~ 1/t. This decay law derives from
the exponentially long times required to escape from some region of the phase
space, due to tunneling and localization effects. We also provide parameter
values which should allow to observe such decay in laboratory experiments.Comment: revtex, 4 pages, 4 figure
High-Throughput Design of Refractory High-Entropy Alloys: Critical Assessment of Empirical Criteria and Proposal of Novel Guidelines for Prediction of Solid Solution Stability
Refractory high-entropy alloys (RHEAs) are a new class of metallic alloys which have been extensively studied in the past decade due to their excellent high-temperature performances. However, the design of new lightweight and ductile RHEAs is a challenging task, since an extensive exploration of the immense compositional space of multicomponent systems is practically impossible. Aiming to reduce the experimental effort, several research groups have proposed different predictive criteria to design new high-performing HEAs. Nevertheless, the criteria proposed so far are often based on a limited amount of data and, generally, do not differentiate between refractory and nonrefractory HEAs. To overcome these limitations, herein, a comprehensive database of properties of 265 RHEAs reported in the open literature from 2010 to 2022 is developed. Such a database is used to assess the validity of predictive empirical criteria and new guidelines for the prediction of solid solution stability in RHEAs are proposed
Effect of Process Parameters on Laser Powder Bed Fusion of Al-Sn Miscibility Gap Alloy
Al-Sn binary system is a miscibility gap alloy consisting of an Al-rich phase and a Sn-rich phase. This system is traditionally applied in bearings and more recently found application as form-stable phase change material (PCM) exploiting solid-liquid phase transition of Sn. A careful choice of production process is required to avoid macro-segregation of the two phases, which have different densities and melting temperatures. In the present study, the additive manufacturing process known as laser powder bed fusion (LPBF) was applied to an Al-Sn alloy with 20% volume of Sn, as a rapid solidification process. The effect of process parameters on microstructure and hardness was evaluated. Moreover, feasibility and stability with thermal cycles of a lattice structure of the same alloy were experimentally investigated. An Al-Sn lattice structure could be used as container for a lower melting organic PCM (e.g., a paraffin or a fatty acid), providing high thermal diffusivity thanks to the metallic network and a "safety system" reducing thermal diffusivity if the system temperature overcomes Sn melting temperature. Even if focused on Al-Sn to be applied in thermal management systems, the study offers a contribution in view of the optimization of manufacturing processes locally involving high solidification rates and reheat cycles in other miscibility gap alloys (e.g., Fe-Cu) with similar thermal or structural applications
High order non-unitary split-step decomposition of unitary operators
We propose a high order numerical decomposition of exponentials of hermitean
operators in terms of a product of exponentials of simple terms, following an
idea which has been pioneered by M. Suzuki, however implementing it for complex
coefficients. We outline a convenient fourth order formula which can be written
compactly for arbitrary number of noncommuting terms in the Hamiltonian and
which is superiour to the optimal formula with real coefficients, both in
complexity and accuracy. We show asymptotic stability of our method for
sufficiently small time step and demonstrate its efficiency and accuracy in
different numerical models.Comment: 10 pages, 4 figures (5 eps files) Submitted to J. of Phys. A: Math.
Ge
Dynamical Localization in Quasi-Periodic Driven Systems
We investigate how the time dependence of the Hamiltonian determines the
occurrence of Dynamical Localization (DL) in driven quantum systems with two
incommensurate frequencies. If both frequencies are associated to impulsive
terms, DL is permanently destroyed. In this case, we show that the evolution is
similar to a decoherent case. On the other hand, if both frequencies are
associated to smooth driving functions, DL persists although on a time scale
longer than in the periodic case. When the driving function consists of a
series of pulses of duration , we show that the localization time
increases as as the impulsive limit, , is
approached. In the intermediate case, in which only one of the frequencies is
associated to an impulsive term in the Hamiltonian, a transition from a
localized to a delocalized dynamics takes place at a certain critical value of
the strength parameter. We provide an estimate for this critical value, based
on analytical considerations. We show how, in all cases, the frequency spectrum
of the dynamical response can be used to understand the global features of the
motion. All results are numerically checked.Comment: 7 pages, 5 figures included. In this version is that Subsection III.B
and Appendix A on the quasiperiodic Fermi Accelerator has been replaced by a
reference to published wor
Heat conduction in one dimensional systems: Fourier law, chaos, and heat control
In this paper we give a brief review of the relation between microscopic
dynamical properties and the Fourier law of heat conduction as well as the
connection between anomalous conduction and anomalous diffusion. We then
discuss the possibility to control the heat flow.Comment: 15 pages, 11 figures. To be published in the Proceedings of the NATO
Advanced Research Workshop on Nonlinear Dynamics and Fundamental
Interactions, Tashkent, Uzbekistan, Octo. 11-16, 200
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