181 research outputs found
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.
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
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
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
Pairing effects in the normal phase of a two-dimensional Fermi gas
In a recent experiment [M. Feld et al., Nature 480, 75 (2011); B. Froehlich
et al., Phys. Rev. Lett. 109,130403 (2012)], a pairing gap was detected in a
two-dimensional (2D) Fermi gas with attractive interaction at temperatures
where superfluidity does not occur. The question remains open as to whether
this gap is a pseudogap phenomenon or is due to a molecular state. In this
paper, by using a t-matrix approach, we reproduce quite well the experimental
data for a 2D Fermi gas, and set the boundary between the pseudogap and
molecular regimes. We also show that pseudogap phenomena occurring in 2D and 3D
can be related through a variable spanning the BCS-BEC crossover in a universal
way.Comment: 10 pages, 9 figures; final versio
Temperature and coupling dependence of the universal contact intensity for an ultracold Fermi gas
Physical properties of an ultracold Fermi gas in the temperature-coupling
phase diagram can be characterized by the contact intensity C, which enters the
pair-correlation function at short distances and describes how the two-body
problem merges into its surrounding. We show that the local order established
by pairing fluctuations about the critical temperature Tc of the superfluid
transition considerably enhances the contact C in a temperature range where
pseudogap phenomena are maximal. Our ab initio results for C in a trap compare
well with recently available experimental data over a wide coupling range. An
analysis is also provided for the effects of trap averaging on C.Comment: 5 pages, 5 figure
Competition between final-state and pairing-gap effects in the radio-frequency spectra of ultracold Fermi atoms
The radio-frequency spectra of ultracold Fermi atoms are calculated by
including final-state interactions affecting the excited level of the
transition, and compared with the experimental data. A competition is revealed
between pairing-gap effects which tend to push the oscillator strength toward
high frequencies away from threshold, and final-state effects which tend
instead to pull the oscillator strength toward threshold. As a result of this
competition, the position of the peak of the spectra cannot be simply related
to the value of the pairing gap, whose extraction thus requires support from
theoretical calculations.Comment: 4 pages, 3 figures, final version published in Phys. Rev. Let
Online tutoring system for programming courses to improve exam pass rate
University students enrolled in the first year of the Computer Science degree may have problems approaching programming, negatively affecting their study during the course. Tutoring programming projects are very important in helping students with difficulty in learning by providing the right approach to study, improving their knowledge and skills in computing. The aim of this work is to realize a new Java Programming tutoring online course that allows students to have an effective online tool to achieve the learning goals of the course and this will enhance the programming exam pass rate. The course we have designed consists of tools to help students with video tutorials, self- assessment quizzes, code evaluations and exercises to solve using an online Java editor. Because the Moodle platform lacks tools to check the quality of the code syntax, a new software was created. It performs a syntax analysis of the Java code and, as a tutor, automatically provides feedbacks and tips to the students to improve the quality. For each online tool the immediate feedback technique is used to amplify students' engagement. A Clustering Machine Learning technique is performed to identify different students' behaviors. A correlation between them and the final performance showed the most influential features of the completed activities. Quantitative analysis highlighted the effectiveness of the tutoring system and the online course designed in this work to enhance the final exam pass rate. At the end, students filled a questionnaire to report their perception and satisfaction about the course
Dispersions, weights, and widths of the single-particle spectral function in the normal phase of a Fermi gas
The dispersions, weights, and widths of the peaks of the single-particle
spectral function in the presence of pair correlations, for a Fermi gas with
either attractive or repulsive short-range inter-particle interaction, are
determined in the normal phase over a wide range of wave vectors, with a
twofold purpose. The first one is to determine how these dispersions identify
both an energy scale known as the pseudo-gap near the Fermi wave vector, as
well as an additional energy scale related to the contact C at large wave
vectors. The second one is to differentiate the behaviors of the repulsive gas
from the attractive one in terms of crossing versus avoided crossing of the
dispersions near the Fermi wave vector. An analogy will also be drawn between
the occurrence of the pseudo-gap physics in a Fermi gas subject to pair
fluctuations and the persistence of local spin waves in the normal phase of
magnetic materials.Comment: 18 pages, 21 figure
Evolution of the Normal State of a Strongly Interacting Fermi Gas from a Pseudogap Phase to a Molecular Bose Gas
Wave-vector resolved radio frequency (rf) spectroscopy data for an ultracold
trapped Fermi gas are reported for several couplings at Tc, and extensively
analyzed in terms of a pairing-fluctuation theory. We map the evolution of a
strongly interacting Fermi gas from the pseudogap phase into a fully gapped
molecular Bose gas as a function of the interaction strength, which is marked
by a rapid disappearance of a remnant Fermi surface in the single-particle
dispersion. We also show that our theory of a pseudogap phase is consistent
with a recent experimental observation as well as with Quantum Monte Carlo data
of thermodynamic quantities of a unitary Fermi gas above Tc.Comment: 9 pages, 9 figures. Substantially revised version (to appear in Phys.
Rev. Lett.
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