771 research outputs found
Simulation of electron paramagnetic resonance spectra of spin-labeled molecules from replica-exchange molecular dynamics
On the magnetization of two-dimensional superconductors
We calculate the magnetization of a two-dimensional superconductor in a
perpendicular magnetic field near its Kosterlitz-Thouless transition and at
lower temperatures. We find that the critical behavior is more complex than
assumed in the literature and that, in particular, the critical magnetization
is {\it not} field independent as naive scaling predicts. In the low
temperature phase we find a substantial fluctuation renormalization of the
mean-field result. We compare our analysis with the data on the cuprates.Comment: 8 pages, 3 figure
Flows, Fragmentation, and Star Formation. I. Low-mass Stars in Taurus
The remarkably filamentary spatial distribution of young stars in the Taurus
molecular cloud has significant implications for understanding low-mass star
formation in relatively quiescent conditions. The large scale and regular
spacing of the filaments suggests that small-scale turbulence is of limited
importance, which could be consistent with driving on large scales by flows
which produced the cloud. The small spatial dispersion of stars from gaseous
filaments indicates that the low-mass stars are generally born with small
velocity dispersions relative to their natal gas, of order the sound speed or
less. The spatial distribution of the stars exhibits a mean separation of about
0.25 pc, comparable to the estimated Jeans length in the densest gaseous
filaments, and is consistent with roughly uniform density along the filaments.
The efficiency of star formation in filaments is much higher than elsewhere,
with an associated higher frequency of protostars and accreting T Tauri stars.
The protostellar cores generally are aligned with the filaments, suggesting
that they are produced by gravitational fragmentation, resulting in initially
quasi-prolate cores. Given the absence of massive stars which could strongly
dominate cloud dynamics, Taurus provides important tests of theories of
dispersed low-mass star formation and numerical simulations of molecular cloud
structure and evolution.Comment: 32 pages, 9 figures: to appear in Ap
Biot-Savart correlations in layered superconductors
We discuss the superconductor to normal phase transition in an
infinite-layered type-II superconductor in the limit where the Josephson
coupling between layers is negligible. We model each layer as a neutral gas of
thermally excited pancake vortices. We assume the dominant interaction between
vortices in the same and in different layers is the electromagnetic interaction
between the screening currents induced by these vortices. Our main result,
obtained by exactly solving the leading order renormalization group flow, is
that the phase transition in this model is a Kosterlitz--Thouless transition
despite being a three--dimensional system. While the transition itself is
driven by the unbinding of two-dimensional pancake vortices, an RG analysis of
the low temperature phase and a mean-field theory of the high temperature phase
reveal that both phases possess three-dimensional correlations. An experimental
consequence of this is that the jump in the measured in-plane superfluid
stiffness, which is a universal quantity in 2d Kosterlitz-Thouless theory, will
receive a small non--universal correction (of order 1% in
BiSrCaCuO). This overall picture places some claims
expressed in the literature on a more secure analytical footing and also
resolves some conflicting views.Comment: 16 pages, 2 figures; minor typos corrected, references adde
Localization protected quantum order
Closed quantum systems with quenched randomness exhibit many-body localized
regimes wherein they do not equilibrate even though prepared with macroscopic
amounts of energy above their ground states. We show that such localized
systems can order in that individual many-body eigenstates can break symmetries
or display topological order in the infinite volume limit. Indeed, isolated
localized quantum systems can order even at energy densities where the
corresponding thermally equilibrated system is disordered, i.e.: localization
protects order. In addition, localized systems can move between ordered and
disordered localized phases via non-thermodynamic transitions in the properties
of the many-body eigenstates. We give evidence that such transitions may
proceed via localized critical points. We note that localization provides
protection against decoherence that may allow experimental manipulation of
macroscopic quantum states. We also identify a `spectral transition' involving
a sharp change in the spectral statistics of the many-body Hamiltonian
Photophoresis of topical steroids in the treatment of severe forms of lichen ruber planus of oral mucosa
The objective of the study is to search for new methods of treatment for lichen ruber planus with high efficacy, few complications and side effects, as well as providing a long-term therapeutic effec
Multispin correlations and pseudo-thermalization of the transient density matrix in solid-state NMR: free induction decay and magic echo
Quantum unitary evolution typically leads to thermalization of generic
interacting many-body systems. There are very few known general methods for
reversing this process, and we focus on the magic echo, a radio-frequency pulse
sequence known to approximately "rewind" the time evolution of dipolar coupled
homonuclear spin systems in a large magnetic field. By combining analytic,
numerical, and experimental results we systematically investigate factors
leading to the degradation of magic echoes, as observed in reduced revival of
mean transverse magnetization. Going beyond the conventional analysis based on
mean magnetization we use a phase encoding technique to measure the growth of
spin correlations in the density matrix at different points in time following
magic echoes of varied durations and compare the results to those obtained
during a free induction decay (FID). While considerable differences are
documented at short times, the long-time behavior of the density matrix appears
to be remarkably universal among the types of initial states considered -
simple low order multispin correlations are observed to decay exponentially at
the same rate, seeding the onset of increasingly complex high order
correlations. This manifestly athermal process is constrained by conservation
of the second moment of the spectrum of the density matrix and proceeds
indefinitely, assuming unitary dynamics.Comment: 12 Pages, 9 figure
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