585 research outputs found
Entanglement production in the dynamical Casimir effect at parametric resonance
The particles produced from the vacuum in the dynamical Casimir effect are
highly entangled. In order to quantify the correlations generated by the
process of vacuum decay induced by moving mirrors, we study the entanglement
evolution in the dynamical Casimir effect by computing the time-dependent
R\'enyi and von Neumann entanglement entropy analytically in arbitrary
dimensions. We consider the system at parametric resonance, where the effect is
enhanced. We find that, in (1+1) dimensions, the entropies grow logarithmically
for large times, , while in higher
dimensions (n+1) the growth is linear, where
can be identified with the Lyapunov exponent of a classical
instability in the system. In dimensions, strong interactions among
field modes prevent the parametric resonance from manifesting as a Lyapunov
instability, leading to a sublinear entropy growth associated with a constant
rate of particle production in the resonant mode. Interestingly, the
logarithmic growth comes with a pre-factor with which cannot occur in
time-periodic systems with finitely many degrees of freedom and is thus a
special property of bosonic field theories.Comment: 17 pages, 5 figure
Nernst effect anisotropy as a sensitive probe of Fermi surface distortions from electron-nematic order
We analyze the thermoelectric response in layered metals with spontaneously
broken rotation symmetry. We identify the anisotropy of the quasiparticle
Nernst signal as an extremely sensitive probe of Fermi surface distortions
characteristic of the ordered state. This is due to a subtle interplay of
different transport anisotropies which become additionally enhanced near
van-Hove singularities. Applied to recent experiments, our results reinforce
the proposal that the underdoped cuprate superconductor YBCO displays such
``electron-nematic'' order in the pseudogap regime.Comment: 4 pages, 3 figs; (v2) slight changes and clarification
Electron interactions and charge ordering in LaSrCuO
We present results of inelastic light scattering experiments on
single-crystalline LaSrCuO in the doping range and TlBaCuO at and . The main
emphasis is placed on the response of electronic excitations in the
antiferromagnetic phase, in the pseudogap range, in the superconducting state,
and in the essentially normal metallic state at , where no
superconductivity could be observed. In most of the cases we compare B
and B spectra which project out electronic properties close to
and , respectively. In the channel of electron-hole excitations
we find universal behavior in B symmetry as long as the material
exhibits superconductivity at low temperature. In contrast, there is a strong
doping dependence in B symmetry: (i) In the doping range we observe rapid changes of shape and temperature dependence of the
spectra. (ii) In LaSrCuO new structures appear for
which are superposed on the electron-hole continuum. The temperature dependence
as well as model calculations support an interpretation in terms of
charge-ordering fluctuations. For the response from fluctuations
disappears at B and appears at B symmetry in full agreement with
the orientation change of stripes found by neutron scattering. While, with a
grain of salt, the particle-hole continuum is universal for all cuprates the
response from fluctuating charge order in the range is so
far found only in LaSrCuO. We conclude that
LaSrCuO is close to static charge order and, for this reason,
may have a suppressed .Comment: 17 pages, 15 figure
Frustrated spin order and stripe fluctuations in FeSe
The charge and spin dynamics of the structurally simplest iron-based
superconductor, FeSe, may hold the key to understanding the physics of high
temperature superconductors in general. Unlike the iron pnictides, FeSe lacks
long range magnetic order in spite of a similar structural transition around
90\,K. Here, we report results of Raman scattering experiments as a function of
temperature and polarization and simulations based on exact diagonalization of
a frustrated spin model. Both experiment and theory find a persistent low
energy peak close to 500cm in symmetry, which softens slightly
around 100\,K, that we assign to spin excitations. By comparing with results
from neutron scattering, this study provides evidence for nearly frustrated
stripe order in FeSe.Comment: 12 pages, 12 figure
Nernst effect of iron pnictide and cuprate superconductors: signatures of spin density wave and stripe order
The Nernst effect has recently proven a sensitive probe for detecting unusual
normal state properties of unconventional superconductors. In particular, it
may sensitively detect Fermi surface reconstructions which are connected to a
charge or spin density wave (SDW) ordered state, and even fluctuating forms of
such a state. Here we summarize recent results for the Nernst effect of the
iron pnictide superconductor , whose ground state evolves
upon doping from an itinerant SDW to a superconducting state, and the cuprate
superconductor which exhibits static stripe
order as a ground state competing with the superconductivity. In , the SDW order leads to a huge Nernst response, which allows
to detect even fluctuating SDW precursors at superconducting doping levels
where long range SDW order is suppressed. This is in contrast to the impact of
stripe order on the normal state Nernst effect in . Here, though signatures of the stripe order are
detectable in the temperature dependence of the Nernst coefficient, its overall
temperature dependence is very similar to that of ,
where stripe order is absent. The anomalies which are induced by the stripe
order are very subtle and the enhancement of the Nernst response due to static
stripe order in as compared to that of the
pseudogap phase in , if any, is very small.Comment: To appear in: 'Properties and applications of thermoelectric
materials - II', V. Zlatic and A. Hewson, editors, Proceedings of NATO
Advanced Research Workshop, Hvar, Croatia, September 19 -25, 2011, NATO
Science for Peace and Security Series B: Physics and Biophysics, (Springer
Science+Business Media B.V. 2012
Wilson chains are not thermal reservoirs
Wilson chains, based on a logarithmic discretization of a continuous
spectrum, are widely used to model an electronic (or bosonic) bath for Kondo
spins and other quantum impurities within the numerical renormalization group
method and other numerical approaches. In this short note we point out that
Wilson chains can not serve as thermal reservoirs as their temperature changes
by a number of order Delta E when a finite amount of energy Delta E is added.
This proves that for a large class of non-equilibrium problems they cannot be
used to predict the long-time behavior.Comment: 2 page
Nernst Effect of stripe ordering LaEuSrCuO
We investigate the transport properties of
LaEuSrCuO (, 0.08, 0.125, 0.15, 0.2) with a
special focus on the Nernst effect in the normal state. Various anomalous
features are present in the data. For and 0.15 a kink-like anomaly is
present in the vicinity of the onset of charge stripe order in the LTT phase,
suggestive of enhanced positive quasiparticle Nernst response in the stripe
ordered phase. At higher temperature, all doping levels except exhibit
a further kink anomaly in the LTO phase which cannot unambiguously be related
to stripe order. Moreover, a direct comparison between the Nernst coefficients
of stripe ordering LaEuSrCuO and superconducting
LaSrCuO at the doping levels and reveals
only weak differences. Our findings make high demands on any scenario
interpreting the Nernst response in hole-doped cuprates
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