318 research outputs found
Curvatronics with bilayer graphene in an effective spacetime
We show that in AB stacked bilayer graphene low energy excitations around the
semimetallic points are described by massless, four dimensional Dirac fermions.
There is an effective reconstruction of the 4 dimensional spacetime, including
in particular the dimension perpendicular to the sheet, that arises dynamically
from the physical graphene sheet and the interactions experienced by the
carriers. The effective spacetime is the Eisenhart-Duval lift of the dynamics
experienced by Galilei invariant L\'evy-Leblond spin particles
near the Dirac points. We find that changing the intrinsic curvature of the
bilayer sheet induces a change in the energy level of the electronic bands,
switching from a conducting regime for negative curvature to an insulating one
when curvature is positive. In particular, curving graphene bilayers allows
opening or closing the energy gap between conduction and valence bands, a key
effect for electronic devices. Thus using curvature as a tunable parameter
opens the way for the beginning of curvatronics in bilayer graphene.Comment: 8 pages, 3 figures. Revised version with additional materia
Multi-patch model for transport properties of cuprate superconductors
A number of normal state transport properties of cuprate superconductors are
analyzed in detail using the Boltzmann equation. The momentum dependence of the
electronic structure and the strong momentum anisotropy of the electronic
scattering are included in a phenomenological way via a multi-patch model. The
Brillouin zone and the Fermi surface are divided in regions where scattering
between the electrons is strong and the Fermi velocity is low (hot patches) and
in regions where the scattering is weak and the Fermi velocity is large (cold
patches). We present several motivations for this phenomenology starting from
various microscopic approaches. A solution of the Boltzmann equation in the
case of N patches is obtained and an expression for the distribution function
away from equilibrium is given. Within this framework, and limiting our
analysis to the two patches case, the temperature dependence of resistivity,
thermoelectric power, Hall angle, magnetoresistance and thermal Hall
conductivity are studied in a systematic way analyzing the role of the patch
geometry and the temperature dependence of the scattering rates. In the case of
Bi-based cuprates, using ARPES data for the electronic structure, and assuming
an inter-patch scattering between hot and cold states with a linear temperature
dependence, a reasonable agreement with the available experiments is obtained.Comment: 18 pages, 18 figures, to be published on Eur. Phys. J.
BCS-BEC crossover at finite temperature for superfluid trapped Fermi atoms
We consider the BCS-BEC crossover for a system of trapped Fermi atoms at
finite temperature, both below and above the superfluid critical temperature,
by including fluctuations beyond mean field. We determine the superfluid
critical temperature and the pair-breaking temperature as functions of the
attractive interaction between Fermi atoms, from the weak- to the
strong-coupling limit (where bosonic molecules form as bound-fermion pairs).
Density profiles in the trap are also obtained for all temperatures and
couplings.Comment: revised version, to be published in Phys. Rev. Let
Extracting the condensate density from projection experiments with Fermi gases
A debated issue in the physics of the BCS-BEC crossover with trapped Fermi
atoms is to identify characteristic properties of the superfluid phase.
Recently, a condensate fraction was measured on the BCS side of the crossover
by sweeping the system in a fast (nonadiabatic) way from the BCS to the BEC
sides, thus ``projecting'' the initial many-body state onto a molecular
condensate. We analyze here the theoretical implications of these projection
experiments, by identifying the appropriate quantum-mechanical operator
associated with the measured quantities and relating them to the many-body
correlations occurring in the BCS-BEC crossover. Calculations are presented
over wide temperature and coupling ranges, by including pairing fluctuations on
top of mean field.Comment: 4 pages, 4 figure
Enhancement of electron-hole superfluidity in double few-layer graphene
We propose two coupled electron-hole sheets of few-layer graphene as a new
nanostructure to observe superfluidity at enhanced densities and enhanced
transition temperatures. For ABC stacked few-layer graphene we show that the
strongly correlated electron-hole pairing regime is readily accessible
experimentally using current technologies. We find for double trilayer and
quadlayer graphene sheets spatially separated by a nano-thick hexagonal
boron-nitride insulating barrier, that the transition temperature for
electron-hole superfluidity can approach temperatures of 40 K.Comment: 17 pages, 5 figure
Screening of pair fluctuations in superconductors with coupled shallow and deep bands: a route to higher temperature superconductivity
A combination of strong Cooper pairing and weak superconducting fluctuations
is crucial to achieve and stabilize high-Tc superconductivity. We demonstrate
that a coexistence of a shallow carrier band with strong pairing and a deep
band with weak pairing, together with the Josephson-like pair transfer between
the bands to couple the two condensates, realizes an optimal multicomponent
superconductivity regime: it preserves strong pairing to generate large gaps
and a very high critical temperature but screens the detrimental
superconducting fluctuations, thereby suppressing the pseudogap state.
Surprisingly, we find that the screening is very efficient even when the
inter-band coupling is very small. Thus, a multi-band superconductor with a
coherent mixture of condensates in the BCS regime (deep band) and in the
BCS-BEC crossover regime (shallow band) offers a promising route to higher
critical temperatures.Comment: 8 pages, 1 figure, including supplemental material
Boosting Monte Carlo simulations of spin glasses using autoregressive neural networks
The autoregressive neural networks are emerging as a powerful computational
tool to solve relevant problems in classical and quantum mechanics. One of
their appealing functionalities is that, after they have learned a probability
distribution from a dataset, they allow exact and efficient sampling of typical
system configurations. Here we employ a neural autoregressive distribution
estimator (NADE) to boost Markov chain Monte Carlo (MCMC) simulations of a
paradigmatic classical model of spin-glass theory, namely the two-dimensional
Edwards-Anderson Hamiltonian. We show that a NADE can be trained to accurately
mimic the Boltzmann distribution using unsupervised learning from system
configurations generated using standard MCMC algorithms. The trained NADE is
then employed as smart proposal distribution for the Metropolis-Hastings
algorithm. This allows us to perform efficient MCMC simulations, which provide
unbiased results even if the expectation value corresponding to the probability
distribution learned by the NADE is not exact. Notably, we implement a
sequential tempering procedure, whereby a NADE trained at a higher temperature
is iteratively employed as proposal distribution in a MCMC simulation run at a
slightly lower temperature. This allows one to efficiently simulate the
spin-glass model even in the low-temperature regime, avoiding the divergent
correlation times that plague MCMC simulations driven by local-update
algorithms. Furthermore, we show that the NADE-driven simulations quickly
sample ground-state configurations, paving the way to their future utilization
to tackle binary optimization problems.Comment: 13 pages, 14 figure
Entanglement between pairing and screening in the Gorkov-Melik-Barkhudarov correction to the critical temperature throughout the BCS-BEC crossover
The theoretical description of the critical temperature Tc of a Fermi
superfluid dates back to the work by Gor'kov and Melik-Barkhudarov (GMB), who
addressed it for a weakly-coupled (dilute) superfluid in the BCS
(weak-coupling) limit of the BCS-BEC crossover. The point made by GMB was that
particle-particle (pairing) excitations, which are responsible for
superfluidity to occur below Tc, and particle-hole excitations, which give rise
to screening also in a normal system, get effectively disentangled from each
other in the BCS limit, thus yielding a reduction by a factor 2.2 of the value
of Tc obtained when neglecting screening effects. Subsequent work on this
topic, aimed at extending the original GMB argument away from the BCS limit
with diagrammatic methods, has kept this disentangling between pairing and
screening throughout the BCS-BEC crossover, without realising that the
conditions for it to be valid are soon violated away from the BCS limit. Here,
we reconsider this problem from a more general perspective and argue that
pairing and screening are intrinsically entangled with each other along the
whole BCS-BEC crossover but for the BCS limit considered by GMB. We perform a
detailed numerical calculation of the GMB diagrammatic contribution extended to
the whole BCS-BEC crossover, where the full wave-vector and frequency
dependence occurring in the repeated in-medium two-particle scattering is duly
taken into account. Our numerical calculations are tested against analytic
results available in both the BCS and BEC limits, and the contribution of the
GMB diagrammatic term to the scattering length of composite bosons in the BEC
limit is highlighted. We calculate Tc throughout the BCS-BEC crossover and find
that it agrees quite well with Quantum Monte Carlo calculations and
experimental data available in the unitarity regime.Comment: 21 pages, 11 figure
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