2,307 research outputs found
Atomic spin sensitive dissipation on magnetic surfaces
We identify the mechanism of energy dissipation relevant to spin-sensitive
nanomechanics including the recently introduced magnetic exchange force
microscopy, where oscillating magnetic tips approach surface atomic spins. The
tip-surface exchange couples spin and atom coordinates, leading to a
spin-phonon problem with Caldeira-Leggett type dissipation. In the overdamped
regime, that can lead to a hysteretic flip of the local spin with a large
spin-dependent dissipation, even down to the very low experimental tip
oscillation frequencies, describing recent observations for Fe tips on NiO. A
phase transition to an underdamped regime with dramatic drop of magnetic tip
dissipation should in principle be possible by tuning tip-surface distance.Comment: 4 pages, 4 figure
Theory of Quantum Annealing of an Ising Spin Glass
Probing the lowest energy configuration of a complex system by quantum
annealing was recently found to be more effective than its classical, thermal
counterpart. Comparing classical and quantum Monte Carlo annealing protocols on
the random two-dimensional Ising model we confirm the superiority of quantum
annealing relative to classical annealing. We also propose a theory of quantum
annealing, based on a cascade of Landau-Zener tunneling events. For both
classical and quantum annealing, the residual energy after annealing is
inversely proportional to a power of the logarithm of the annealing time, but
the quantum case has a larger power which makes it fasterComment: RevTex, 8 pages, 3 figure
Dissipation assisted Thouless pumping in the Rice-Mele model
We investigate the effect of dissipation from a thermal environment on
topological pumping in the periodically-driven Rice-Mele model. We report that
dissipation can improve the robustness of pumping quantisation in a regime of
finite driving frequencies. Specifically, in this regime, a low-temperature
dissipative dynamics can lead to a pumped charge that is much closer to the
Thouless quantised value, compared to a coherent evolution. We understand this
effect in the Floquet framework: dissipation increases the population of a
Floquet band which shows a topological winding, where pumping is essentially
quantised. This finding is a step towards understanding a potentially very
useful resource to exploit in experiments, where dissipation effects are
unavoidable. We consider small couplings with the environment and we use a
Bloch-Redfield quantum master equation approach for our numerics: Comparing
these results with an exact MPS numerical treatment we find that the quantum
master equation works very well also at low temperature, a quite remarkable
fact.Comment: 21 pages, 8 figure
Effective thermal dynamics following a quantum quench in a spin chain
We study the nonequilibrium dynamics of the Quantum Ising Model following an
abrupt quench of the transverse field. We focus on the on-site autocorrelation
function of the order parameter, and extract the phase coherence time
from its asymptotic behavior. We show that the initial state
determines only through an effective temperature set by its
energy and the final Hamiltonian. Moreover, we observe that the dependence of
on the effective temperature fairly agrees with that obtained
in thermal equilibrium as a function of the equilibrium temperature.Comment: 4 pages, 4 figures. Published versio
AFM Dissipation Topography of Soliton Superstructures in Adsorbed Overlayers
In the atomic force microscope, the nanoscale force topography of even
complex surface superstructures is extracted by the changing vibration
frequency of a scanning tip. An alternative dissipation topography with similar
or even better contrast has been demonstrated recently by mapping the
(x,y)-dependent tip damping but the detailed damping mechanism is still
unknown. Here we identify two different tip dissipation mechanisms: local
mechanical softness and hysteresis. Motivated by recent data, we describe both
of them in a onedimensional model of Moire' superstructures of incommensurate
overlayers. Local softness at "soliton" defects yields a dissipation contrast
that can be much larger than the corresponding density or corrugation contrast.
At realistically low vibration frequencies, however, a much stronger and more
effective dissipation is caused by the tip-induced nonlinear jumping of the
soliton, naturally developing bistability and hysteresis. Signatures of this
mechanism are proposed for experimental identification.Comment: 5 pages, 5 figures, Phys Rev B 81, 045417 (2010
Hysteresis from dynamically pinned sliding states
We report a surprising hysteretic behavior in the dynamics of a simple
one-dimensional nonlinear model inspired by the tribological problem of two
sliding surfaces with a thin solid lubricant layer in between. In particular,
we consider the frictional dynamics of a harmonic chain confined between two
rigid incommensurate substrates which slide with a fixed relative velocity.
This system was previously found, by explicit solution of the equations of
motion, to possess plateaus in parameter space exhibiting a remarkable
quantization of the chain center-of-mass velocity (dynamic pinning) solely
determined by the interface incommensurability. Starting now from this
quantized sliding state, in the underdamped regime of motion and in analogy to
what ordinarily happens for static friction, the dynamics exhibits a large
hysteresis under the action of an additional external driving force F_ext. A
critical threshold value F_c of the adiabatically applied force F_ext is
required in order to alter the robust dynamics of the plateau attractor. When
the applied force is decreased and removed, the system can jump to intermediate
sliding regimes (a sort of ``dynamic'' stick-slip motion) and eventually
returns to the quantized sliding state at a much lower value of F_ext. On the
contrary no hysteretic behavior is observed as a function of the external
driving velocity.Comment: 12 pages, 5 figures, ECOSS 200
Matrix Product State representation for Slater Determinants and Configuration Interaction States
Slater determinants are product states of filled quantum fermionic orbitals.
When they are expressed in a configuration space basis chosen a priori, their
entanglement is bound and controlled. This suggests that an exact
representation of Slater determinants as finitely-correlated states is
possible. In this paper we analyze this issue and provide an exact Matrix
Product representation for Slater determinant states. We also argue possible
meaningful extensions that embed more complex configuration interaction states
into the description.Comment: 16 pages, 4 figures. Published in IJMPB, focus issue on "Classical
vs. Quantum Correlations in Composite Systems
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