645 research outputs found
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
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 nonthermodynamic 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
On bipartite Rokhsar-Kivelson points and Cantor deconfinement
Quantum dimer models on bipartite lattices exhibit Rokhsar-Kivelson (RK)
points with exactly known critical ground states and deconfined spinons. We
examine generic, weak, perturbations around these points. In d=2+1 we find a
first order transition between a ``plaquette'' valence bond crystal and a
region with a devil's staircase of commensurate and incommensurate valence bond
crystals. In the part of the phase diagram where the staircase is incomplete,
the incommensurate states exhibit a gapless photon and deconfined spinons on a
set of finite measure, almost but not quite a deconfined phase in a compact
U(1) gauge theory in d=2+1! In d=3+1 we find a continuous transition between
the U(1) resonating valence bond (RVB) phase and a deconfined staggered valence
bond crystal. In an appendix we comment on analogous phenomena in quantum
vertex models, most notably the existence of a continuous transition on the
triangular lattice in d=2+1.Comment: 9 pages; expanded version to appear in Phys. Rev. B; presentation
improve
AstroSat-CZTI detection of variable prompt emission polarization in GRB 171010A
We present spectro-polarimetric analysis of \thisgrb\ using data from \asat,
\fermi, and \swift, to provide insights into the physical mechanisms of the
prompt radiation and the jet geometry.
Prompt emission from \thisgrb\ was very bright (fluence
~ergs~cm) and had a complex structure composed of the
superimposition of several pulses. The energy spectra deviate from the typical
Band function to show a low energy peak ~keV --- which we interpret as
a power-law with two breaks, with a synchrotron origin. Alternately, the prompt
spectra can also be interpreted as Comptonized emission, or a blackbody
combined with a Band function. Time-resolved analysis confirms the presence of
the low energy component, while the peak energy is found to be confined in the
range of 100--200~keV.
Afterglow emission detected by \fermi-LAT is typical of an external shock
model, and we constrain the initial Lorentz factor using the peak time of the
emission. \swift-XRT measurements of the afterglow show an indication for a jet
break, allowing us to constrain the jet opening angle to 6\degr.
Detection of a large number of Compton scattered events by \asat-CZTI
provides an opportunity to study hard X-ray polarization of the prompt
emission. We find that the burst has high, time-variable polarization, with the
emission {\bf have higher polarization} at energies above the peak energy.
We discuss all observations in the context of GRB models and polarization
arising due to {\bf due to physical or geometric effects:} synchrotron emission
from multiple shocks with ordered or random magnetic fields, Poynting flux
dominated jet undergoing abrupt magnetic dissipation, sub-photospheric
dissipation, a jet consisting of fragmented fireballs, and the Comptonization
model.Comment: Accepted for publication in Ap
Sound Propagation in Nematic Fermi Liquid
We study the longitudinal sound propagation in the electronic nematic Fermi
liquid where the Fermi surface is distorted due to the spontaneously broken
rotational symmetry. The behavior of the sound wave in the nematic ordered
state is dramatically different from that in the isotropic Fermi liquid. The
collective modes associated with the fluctuations of the Fermi surface
distortion in the nematic Fermi liquid leads to the strong and anisotropic
damping of the sound wave. The relevance of the nematic Fermi liquid in doped
Mott insulator is discussed.Comment: 4 pages, no figur
Current Diagnostic Possibilities for the Initial Forms of External Endometriosis
Background. Routine use of ultrasound methods does not always allow detecting endometriosis in the early stages. It is necessary to develop more accurate criteria for the diagnosis of endometriosis in its initial stages of development. Objective: to assess ultrasound and biological markers of endometriosis “small” forms of different localization in infertile women.Material and methods. Ultrasound examination of 208 infertile patients with initial stages of external endometriosis (1–15 points by revised American Fertility Society (rAFS) classificatoon) was performed (main group). The control group consisted of 195 healthy women of childbearing age. To verify the diagnosis, it is suggested to use a panel of biomarkers, including interleukins (IL) 1β and 6, CA125, HE4 as well as the ROMA (Risk of Ovarian Malignancy Algorithm) index in the dynamics of the menstrual cycle.Results. On ultrasound scans, 56 (94.9%) patients had small one- and two-sided ovarian cysts corresponding to endometriosis stage 1–2 (rAFS), and in 3 (5.4%) of them, the ultrasound result was questionable. Detectability during the initial ultrasound examination in the Douglas space was 76.9%, in the rest of cases the ultrasound picture was doubtful. Peritoneal endometriosis was detected only in 5 patients. In the remaining cases (88.9%), additional studies were necessary, including surgical (laparoscopy). The average content of CA125 in the main group on days 3–5 of the menstrual cycle was 42.6 (2.1) U/ml, on days 21–23 – 39.6(2.2) U/ml (p=0.32); in the control group, 5.1 (0.4) and 4.8 (0.7) U/ml, respectively (p = 0.71). The increase in CA125 in the main group was almost 8 times higher than in the control group, however, according to the indicators of HE4 and the ROMA index, statistically significant differences in the groups were not obtained. In the dynamics of the menstrual cycle, the content of ILs in the main group significantly decreased by days 21–23 of the cycle, while the content of CA125 and HE4 remained practically unchanged. Such sharp fluctuations of the considered biomarkers were not detected in the control group. However, even on days 21–23 of the cycle, the average content of ILs and CA125 in women of the main group remained statistically significantly higher than in the control group.Conclusion. In infertile women with dubious ultrasound criteria of external endometriosis, the use of IL1β and IL-6, which stimulate the immune inflammatory response, as well as tumor markers CA125, HE4 and ROMA index allows to differentiate the disease in the initial stages
Spontaneous breaking of four-fold rotational symmetry in two-dimensional electronic systems explained as a continuous topological transition
The Fermi liquid approach is applied to the problem of spontaneous violation
of the four-fold rotational point-group symmetry () in strongly correlated
two-dimensional electronic systems on a square lattice. The symmetry breaking
is traced to the existence of a topological phase transition. This continuous
transition is triggered when the Fermi line, driven by the quasiparticle
interactions, reaches the van Hove saddle points, where the group velocity
vanishes and the density of states becomes singular. An unconventional Fermi
liquid emerges beyond the implicated quantum critical point.Comment: 6 pages, 4 figure
Local versus Nonlocal Order Parameter Field Theories for Quantum Phase Transitions
General conditions are formulated that allow to determine which quantum phase
transitions in itinerant electron systems can be described by a local
Landau-Ginzburg-Wilson or LGW theory solely in terms of the order parameter. A
crucial question is the degree to which the order parameter fluctuations couple
to other soft modes. Three general classes of zero-wavenumber order parameters,
in the particle-hole spin-singlet and spin-triplet channels, and in the
particle-particle channel, respectively, are considered. It is shown that the
particle-hole spin-singlet class does allow for a local LGW theory, while the
other two classes do not. The implications of this result for the critical
behavior at various quantum phase transitions are discussed, as is the
connection with nonanalyticities in the wavenumber dependence of order
parameter susceptibilities in the disordered phase.Comment: 9 pp., LaTeX, no figs, final version as publishe
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