2,712 research outputs found
Retrieving information from a noisy "knowledge network"
We address the problem of retrieving information from a noisy version of the
``knowledge networks'' introduced by Maslov and Zhang. We map this problem onto
a disordered statistical mechanics model, which opens the door to many
analytical and numerical approaches. We give the replica symmetric solution,
compare with numerical simulations, and finally discuss an application to real
datas from the United States Senate.Comment: 10 pages, 4 figures. Writing of the last section improved; version
accepted in JSTA
Non-linear optical effects and third-harmonic generation in superconductors: Cooper-pairs vs Higgs mode contribution
The recent observation of a transmitted Thz pulse oscillating at three times
the frequency of the incident light paves the way to a new protocol to access
resonant excitations in a superconductor. Here we show that this non-linear
optical process is dominated by light-induced excitation of Cooper pairs, in
analogy with a standard Raman experiment. The collective amplitude (Higgs)
fluctuations of the superconducting order parameter give in general a smaller
contribution, unless one designs the experiment by combining properly the light
polarization with the lattice symmetry.Comment: Slightly revised introduction, to appear on Phys. Rev. B. as Rapid
Communicatio
Doping dependence of the vortex-core energy in bilayer films of cuprates
The energy needed to create a vortex core is the basic ingredient to address
the physics of thermal vortex fluctuations in underdoped cuprates. Here we
theoretically investigate its role on the occurrence of the
Beresinskii-Kosterlitz-Thouless transition in a bilayer film with
inhomogeneity. From the comparison with recent measurements of the penetration
depth in two-unit cell thin films of
YCaBaCuO_{7-\d} (YBCO) by Hetel et al. [Nat. Phys.
3, 700 (2007)] we can extract the value of the vortex-core energy , and
show that scales linearly with at low doping.Comment: 4pages, 3 figures. References added, final versio
Polarization dependence of the third-harmonic generation in multiband superconductors
In a superconductor the third-harmonic generation (THG) of a strong THz pulse
is enhanced below Tc by the resonant excitation of lattice-modulated charge
fluctuations (LCF), which modulate the response according to the polarization
of the field. Here we compute the THG within a multiband model for the
prototype NbN superconductor. We show that the non-resonant contribution coming
from the instantaneous electronic response and the finite width of the pulse
significantly suppress the polarization dependence of the signal, challenging
its observation in real systems.Comment: Final version, as publishe
Broadening of the Beresinkii-Kosterlitz-Thouless superconducting transition by inhomogeneity and finite-size effects
We discuss the crucial role played by finite-size effects and inhomogeneity
on the Beresinkii-Kosterlitz-Thouless (BKT) transition in two-dimensional
superconductors. In particular, we focus on the temperature dependence of the
resistivity, that is dominated by superconducting fluctuations above the BKT
transition temperature and by inhomogeneity below it. By means of a
renormalization-group approach we establish a direct correspondence between the
parameter values used to describe the BKT fluctuation regime and the distance
between and the mean-field Ginzburg-Landau transition temperature.
Below a resistive tail arises due to finite-size effect and
inhomogeneity, that reflects also on the temperature dependence of the
superfluid density. We apply our results to recent experimental data in
superconducting LaAlO/SrTiO heterostructures, and we extract several
informations on the microscopic properties of the system from our BKT fitting
parameters. Finally, we compare our approach to recent data analysis presented
in the literature, where the physical meaning of the parameter values in the
BKT formulas has been often overlooked.Comment: 11 pages, 9 figures, final versio
Optical excitation of phase modes in strongly disordered superconductors
According to the Goldstone theorem the breaking of a continuous U(1) symmetry
comes along with the existence of low-energy collective modes. In the context
of superconductivity these excitations are related to the phase of the
superconducting (SC) order parameter and for clean systems are optically
inactive. Here we show that for strongly disordered superconductors phase modes
acquire a dipole moment and appear as a subgap spectral feature in the optical
conductivity. This finding is obtained with both a gauge-invariant random-phase
approximation scheme based on a fermionic Bogoliubov-de Gennes state as well as
with a prototypical bosonic model for disordered superconductors. In the
strongly disordered regime, where the system displays an effective granularity
of the SC properties, the optically active dipoles are linked to the isolated
SC islands, offering a new perspective for realizing microwave optical devices
Comment on: Calculation of an Enhanced Symmetry Mode Induced by Higgs Oscillations in the Raman Spectrum of High-Temperature Cuprate Superconductors
In a recent manuscript Phys. Rev. Lett. 127, 197001 (2021) Puviani et al.
claim that the Higgs mode gives an enhancement of the Raman response in the
symmetric Raman channel of cuprates. Here we report a series of
technical mistakes in their derivation that completely invalidate this
conclusion. In particular, the Raman response function contains two
renormalized vertices, with a clearly overcounting of diagrams and uncontrolled
results for the Higgs-mode contribution.Comment: Slightly shorter version, accepted for publication in Physical Review
Letter
Supergravity and IOSp(3,1|4) gauge theory
A new formulation of simple D=4 supergravity in terms of the geometry of
superspace is presented. The formulation is derived from the gauge theory of
the inhomogeneous orthosymplectic group IOSp(3,1|4) on a (4,4)-dimensional base
supermanifold by imposing constraints and taking a limit. Both the constraints
and the limiting procedure have a clear {\it a priori} physical motivation,
arising from the relationship between IOSp(3,1|4) and the super Poincar\'{e}
group. The construction has similarities with the space-time formulation of
Newtonian gravity.Comment: 17 pages. Expanded version. To appear in Class. Quantum Gra
Quantum critical behavior in disordered itinerant ferromagnets: Logarithmic corrections to scaling
The quantum critical behavior of disordered itinerant ferromagnets is
determined exactly by solving a recently developed effective field theory. It
is shown that there are logarithmic corrections to a previous calculation of
the critical behavior, and that the exact critical behavior coincides with that
found earlier for a phase transition of undetermined nature in disordered
interacting electron systems. This confirms a previous suggestion that the
unspecified transition should be identified with the ferromagnetic transition.
The behavior of the conductivity, the tunneling density of states, and the
phase and quasiparticle relaxation rates across the ferromagnetic transition is
also calculated.Comment: 15pp., REVTeX, 8 eps figs, final version as publishe
Surface tension fluctuations and a new spinodal point in glass-forming liquids
The dramatic slowdown of glass-forming liquids has been variously linked to
increasing dynamic and static correlation lengths. Yet, empirical evidence is
insufficient to decide among competing theories. The random first order theory
(RFOT) links the dynamic slowdown to the growth of amorphous static order,
whose range depends on a balance between configurational entropy and surface
tension. This last quantity is expected to vanish when the temperature
surpasses a spinodal point beyond which there are no metastable states. Here we
measure for the first time the surface tension in a model glass-former, and
find that it vanishes at the energy separating minima from saddles,
demonstrating the existence of a spinodal point for amorphous metastable order.
Moreover, the fluctuations of surface tension become smaller for lower
temperatures, in quantitative agreement with recent theoretical speculation
that spatial correlations in glassy systems relax nonexponentially because of
the narrowing of the surface tension distribution.Comment: 6 pages, 5 figure
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