120 research outputs found
Comment on "Quantitative Condition is Necessary in Guaranteeing the Validity of the Adiabatic Approximation" [arXiv:1004.3100]
Recently, the authors of Ref.1[arXiv:1004.3100] claimed that they have proven
the traditional adiabatic condition is a necessary condition. Here, it is
claimed that there are some mistakes and an artificial over-strong constraint
in [1], making its result inconvincible.Comment: 1 pag
Nonadiabatic Nonlinear Optics and Quantum Geometry -- Application to the Twisted Schwinger Effect
We study the tunneling mechanism of nonlinear optical processes in solids
induced by strong coherent laser fields. The theory is based on an extension of
the Landau-Zener model with nonadiabatic geometric effects. In addition to the
rectification effect known previously, we find two effects, namely perfect
tunneling and counterdiabaticity at fast sweep speed. We apply this theory to
the twisted Schwinger effect, i.e., nonadiabatic pair production of particles
by rotating electric fields, and find a nonperturbative generation mechanism of
the opto-valley polarization and photo-current in Dirac and Weyl fermions.Comment: 24 pages, Accepted by SciPos
Quantum critical dynamics for a prototype class of insulating antiferromagnets
Quantum criticality is a fundamental organizing principle for studying
strongly correlated systems. Nevertheless, understanding quantum critical
dynamics at nonzero temperatures is a major challenge of condensed matter
physics due to the intricate interplay between quantum and thermal
fluctuations. The recent experiments in the quantum spin dimer material
TlCuCl provide an unprecedented opportunity to test the theories of quantum
criticality. We investigate the nonzero temperature quantum critical spin
dynamics by employing an effective field theory. The on-shell mass and
the damping rate of quantum critical spin excitations as functions of
temperature are calculated based on the renormalized coupling strength, which
are in excellent agreements with experiment observations. Their
dependence is predicted to be dominant at very low temperatures, which is to be
tested in future experiments. Our work provides confidence that quantum
criticality as a theoretical framework, being considered in so many different
contexts of condensed matter physics and beyond, is indeed grounded in
materials and experiments accurately. It is also expected to motivate further
experimental investigations on the applicability of the field theory to related
quantum critical systems.Comment: 9 pages, 7 figure
Crossovers and critical scaling in the one-dimensional transverse-field Ising model
We consider the scaling behavior of thermodynamic quantities in the
one-dimensional transverse-field Ising model near its quantum critical point
(QCP). Our study has been motivated by the question about the thermodynamical
signatures of this paradigmatic quantum critical system and, more generally, by
the issue of how quantum criticality accumulates entropy. We find that the
crossovers in the phase diagram of temperature and (the non-thermal control
parameter) transverse field obey a general scaling ansatz, and so does the
critical scaling behavior of the specific heat and magnetic expansion
coefficient. Furthermore, the Gr\"{u}neisen ratio diverges in a power-law way
when the QCP is accessed as a function of the transverse field at zero
temperature, which follows the prediction of quantum critical scaling. However,
at the critical field, upon decreasing the temperature, the Gr\"uneisen ratio
approaches a constant instead of showing the expected divergence. We are able
to understand this unusual result in terms of a peculiar form of the quantum
critical scaling function for the free energy; the contribution to the
Gr\"uneisen ratio vanishes at the linear order in a suitable Taylor expansion
of the scaling function. In spite of this special form of the scaling function,
we show that the entropy is still maximized near the QCP, as expected from the
general scaling argument. Our results establish the telltale thermodynamic
signature of a transverse-field Ising chain, and will thus facilitate the
experimental identification of this model quantum-critical system in real
materials.Comment: 7 pages, 5 figure
Finite temperature spin dynamics in a perturbed quantum critical Ising chain with an symmetry
A spectrum exhibiting symmetry is expected to arise when a small
longitudinal field is introduced in the transverse-field Ising chain at its
quantum critical point. Evidence for this spectrum has recently come from
neutron scattering measurements in cobalt niobate, a quasi one-dimensional
Ising ferromagnet. Unlike its zero-temperature counterpart, the
finite-temperature dynamics of the model has not yet been determined. We study
the dynamical spin structure factor of the model at low frequencies and nonzero
temperatures, using the form factor method. Its frequency dependence is
singular, but differs from the diffusion form. The temperature dependence of
the nuclear magnetic resonance (NMR) relaxation rate has an activated form,
whose prefactor we also determine. We propose NMR experiments as a means to
further test the applicability of the description for CoNbO.Comment: 5 pages 2 figures - Supplementary Material 11 page
- …