737 research outputs found
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
Nernst effect, quasiparticles, and d-density waves in cuprates
We examine the possibility that the large Nernst signal observed in the
pseudogap regime of hole-doped cuprates originates from quasiparticle transport
in a state with d-density wave (DDW) order, proposed by S. Chakravarty et al.
[Phys. Rev. B 63, 094503 (2001)]. We find that the Nernst coefficient can be
moderately enhanced in magnitude by DDW order, and is generally of negative
sign. Thus, the quasiparticles of the DDW state cannot account for the large
and positive Nernst signal observed in the pseudogap phase of the cuprates.
However, the general considerations outlined in this paper may be of broader
relevance, in particular to the recent measurements of Bel et al. in NbSe_2 and
CeCoIn_5 [Phys. Rev. Lett. 91, 066602 (2003); ibid. 92, 217002 (2004)].Comment: 9 pages, 3 figures; published versio
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
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
GRB 190114C: from prompt to afterglow?
GRB 190114C is the first gamma-ray burst detected at Very High Energies (VHE,
i.e. >300 GeV) by the MAGIC Cherenkov telescope. The analysis of the emission
detected by the Fermi satellite at lower energies, in the 10 keV -- 100 GeV
energy range, up to ~ 50 seconds (i.e. before the MAGIC detection) can hold
valuable information. We analyze the spectral evolution of the emission of GRB
190114C as detected by the Fermi Gamma-Ray Burst Monitor (GBM) in the 10 keV --
40 MeV energy range up to ~60 sec. The first 4 s of the burst feature a typical
prompt emission spectrum, which can be fit by a smoothly broken power-law
function with typical parameters. Starting on ~4 s post-trigger, we find an
additional nonthermal component, which can be fit by a power law. This
component rises and decays quickly. The 10 keV -- 40 MeV flux of the power-law
component peaks at ~ 6 s; it reaches a value of 1.7e-5 erg cm-2 s-1. The time
of the peak coincides with the emission peak detected by the Large Area
Telescope (LAT) on board Fermi. The power-law spectral slope that we find in
the GBM data is remarkably similar to that of the LAT spectrum, and the GBM+LAT
spectral energy distribution seems to be consistent with a single component.
This suggests that the LAT emission and the power-law component that we find in
the GBM data belong to the same emission component, which we interpret as due
to the afterglow of the burst. The onset time allows us to estimate the initial
jet bulk Lorentz factor Gamma_0 is about 500, depending on the assumed
circum-burst density.Comment: 7 pages, 2 figures, in press, accepted for publication in A&
The rise and fall of the high-energy afterglow emission of GRB 180720B
The Gamma Ray Burst (GRB) 180720B is one of the brightest events detected by
the Fermi satellite and the first GRB detected by the H.E.S.S. telescope above
100 GeV. We analyse the Fermi (GBM and LAT) and Swift (XRT and BAT) data and
describe the evolution of the burst spectral energy distribution in the 0.5 keV
- 10 GeV energy range over the first 500 seconds of emission. We reveal a
smooth transition from the prompt phase, dominated by synchrotron emission in a
moderately fast cooling regime, to the afterglow phase whose emission has been
observed from the radio to the GeV energy range. The LAT (0.1 - 100 GeV) light
curve initially rises (), peaks at 78 s, and
falls steeply () afterwards. The peak, which we
interpret as the onset of the fireball deceleration, allows us to estimate the
bulk Lorentz factor under the assumption of a
wind-like (homogeneous) circum-burst medium density. We derive a flux upper
limit in the LAT energy range at the time of H.E.S.S. detection, but this does
not allow us to unveil the nature of the high energy component observed by
H.E.S.S. We fit the prompt spectrum with a physical model of synchrotron
emission from a non-thermal population of electrons. The 0 - 35 s spectrum
after its peak (at 1 - 2 MeV) is a steep power law extending to
hundreds of MeV. We derive a steep slope of the injected electron energy
distribution . Our fit parameters point towards a
very low magnetic field ( G) in the emission region.Comment: 10 pages, 6 figures, submitted to A&
Bi-stable tunneling current through a molecular quantum dot
An exact solution is presented for tunneling through a negative-U d-fold
degenerate molecular quantum dot weakly coupled to electrical leads. The tunnel
current exhibits hysteresis if the level degeneracy of the negative-U dot is
larger than two (d>2). Switching occurs in the voltage range V1 < V < V2 as a
result of attractive electron correlations in the molecule, which open up a new
conducting channel when the voltage is above the threshold bias voltage V2.
Once this current has been established, the extra channel remains open as the
voltage is reduced down to the lower threshold voltage V1. Possible
realizations of the bi-stable molecular quantum dots are fullerenes, especially
C60, and mixed-valence compounds.Comment: 5 pages, 1 figure. (v2) Figure updated to compare the current
hysteresis for degeneracies d=4 and d>>1 of the level in the dot, minor
corrections in the text. To appear in Phys. Rev.
Quantum melting of incommensurate domain walls in two dimensions
Quantum fluctuations of periodic domain-wall arrays in two-dimensional
incommensurate states at zero temperature are investigated using the elastic
theory in the vicinity of the commensurate-incommensurate transition point.
Both stripe and honeycomb structures of domain walls with short-range
interactions are considered. It is revealed that the stripes melt and become a
stripe liquid in a large-wall-spacing (low-density) region due to dislocations
created by quantum fluctuations. This quantum melting transition is of second
order and characterized by the three-dimensional XY universality class.
Zero-point energies of the stripe and honeycomb structures are calculated. As a
consequence of these results, phase diagrams of the domain-wall solid and
liquid phases in adsorbed atoms on graphite are discussed for various
domain-wall masses. Quantum melting of stripes in the presence of long-range
interactions that fall off as power laws is also studied. These results are
applied to incommensurate domain walls in two-dimensional adsorbed atoms on
substrates and in doped antiferromagnets, e.g. cuprates and nickelates.Comment: 11 pages, 5 figure
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