arXiv subjects

C. A. R. Sá de Melo

Publications and source records attributed to C. A. R. Sá de Melo.

At least 37 records · Page 2Linked to original sources

arXiv2011

Density fluctuations and compressibility matrix for population or mass imbalanced Fermi-Fermi mixtures

We describe the relation between the isothermal atomic compressibility and density fluctuations in mixtures of two-component fermions with population or mass imbalance. We derive a generalized version of the fluctuation-dissipation theorem which is valid for both balanced and imbalanced Fermi-Fermi mixtures. Furthermore, we show that the compressibility, its critical exponents, and phase boundaries can be extracted via an analysis of the density fluctuations as a function of population imbalance, interaction parameter or temperature. Lastly, we demonstrate that in the presence of trapping potentials, the local compressibility and local density-density correlations can be extracted via a generalized fluctuation-dissipation theorem valid within the local density approximation.

cond-mat.quant-gas↗

arXiv2011

Topological phase transitions in ultra-cold Fermi superfluids: the evolution from BCS to BEC under artificial spin-orbit fields

We discuss topological phase transitions in ultra-cold Fermi superfluids induced by interactions and artificial spin orbit fields. We construct the phase diagram for population imbalanced systems at zero and finite temperatures, and analyze spectroscopic and thermodynamic properties to characterize various phase transitions. For balanced systems, the evolution from BCS to BEC superfluids in the presence of spin-orbit effects is only a crossover as the system remains fully gapped, even though a triplet component of the order parameter emerges. However, for imbalanced populations, spin-orbit fields induce a triplet component in the order parameter that produces nodes in the quasiparticle excitation spectrum leading to bulk topological phase transitions of the Lifshitz type. Additionally a fully gapped phase exists, where a crossover from indirect to direct gap occurs, but a topological transition to a gapped phase possessing Majorana fermions edge states does not occur.

cond-mat.quant-gas↗

arXiv2011

Evolution from BCS to BEC superfluidity in the presence of spin-orbit coupling

We discuss the evolution from BCS to BEC superfluids in the presence of spin-orbit coupling, and show that this evolution is just a crossover in the balanced case. The dependence of several thermodynamic properties, such as the chemical potential, order parameter, pressure, entropy, isothermal compressibility and spin susceptibility tensor on the spin-orbit coupling and interaction parameter at low temperatures are analyzed. We studied both the case of equal Rashba and Dresselhaus (ERD) and the Rashba-only (RO) spin-orbit coupling. Comparisons between the two cases reveal several striking differences in the corresponding thermodynamic quantities. Finally we propose measuring the spin susceptibility as a means to detect the spin-orbit coupling effect.

cond-mat.quant-gas↗

arXiv2010

The evolution from BCS to BEC superfluidity in the presence of disorder

We describe the effects of disorder on the critical temperature of $s$-wave superfluids from the BCS to the BEC regime, with direct application to ultracold fermions. We use the functional integral method and the replica technique to study Gaussian correlated disorder due to impurities, and we discuss how this system can be generated experimentally. In the absence of disorder, the BCS regime is characterized by pair breaking and phase coherence temperature scales which are essentially the same allowing strong correlations between the amplitude and phase of the order parameter for superfluidity. As non-pair breaking disorder is introduced the largely overlapping Cooper pairs conspire to maintain phase coherence such that the critical temperature remains essentially unchanged, and Anderson's theorem is satisfied. However in the BEC regime the pair breaking and phase coherence temperature scales are very different such that non-pair breaking disorder can affect dramatically phase coherence, and thus the critical temperature, without the requirement of breaking tightly-bound fermion pairs simultaneously. In this case, Anderson's theorem does not apply, and the critical temperature can be more easily reduced in comparison to the BCS limit. Lastly, we find that the superfluid is more robust against disorder in the intermediate region near unitarity between the two regimes.

cond-mat.quant-gas↗

arXiv2010

Enhancing decoherence times in superconducting qubits via circuit design

We study decoherence effects in qubits coupled to environments that exhibit resonant frequencies in their spectral function. We model the coupling of the qubit to its environment via the Caldeira-Leggett formulation of quantum dissipation/coherence, and study the simplest example of decoherence effects in circuits with resonances such as a dc SQUID phase qubit in the presence of an isolation circuit, which is designed to enhance the coherence time. We emphasize that the spectral density of the environment is strongly dependent on the circuit design, and can be engineered to produce longer decoherence times. We begin with a general discussion of superconducting qubits such as the flux qubit, the Cooper pair box and the phase qubit and show that in these kinds of systems appropriate circuit design can greatly modify the spectral density of the environment and lead to enhancement of decoherence times. In the particular case of the phase qubit, for instance, we show that when the frequency of the qubit is at least two times larger than the resonance frequency of the environmental spectral density, the decoherence time of the qubit is a few orders of magnitude larger than that of the typical ohmic regime, where the frequency of the qubit is much smaller than the resonance frequency of the spectral density. In addition, we demonstrate that the environment does not only affect the decoherence time, but also the frequency of the transition itself, which is shifted from its environment-free value. Second, we show that when the qubit frequency is nearly the same as the resonant frequency of the environmental spectral density, an oscillatory non-Markovian decay emerges, as the qubit and its environment self-generate Rabi oscillations of characteristic time scales shorter than the decoherence time.

cond-mat.supr-con↗

arXiv2009

Electrostatically induced phase transitions in superconducting complex oxides

We describe quantum phase transitions in superconducting complex oxides which could be tuned by electrostatic charge transfer. Using a simple model for the superconductivity of a thin film or surface of a bulk copper oxide, we show that tuning the carrier density may allow the visitation of several superconducting phases with different pairing symmetries such as extended $s$- $(se)$, $d$- and $(se \pm id)$-wave. We construct a universal phase diagram for single-band superconductors with $se$- and d-wave components of the order parameter based on symmetry considerations alone. For a specific model with nearest neighbor attraction, we obtain the phase diagram in the interaction versus filling factor space showing the boundaries of the possible phases. Finally, we calculate the superfluid density and penetration depth as characteristic properties of each phase.

cond-mat.supr-con↗

arXiv2009

Hexatic, Wigner Crystal, and Superfluid Phases of Dipolar Bosons

The finite temperature phase diagram of two-dimensional dipolar bosons versus dipolar interaction strength is discussed. We identify the stable phases as dipolar superfluid (DSF), dipolar Wigner crystal (DWC), dipolar hexatic fluid (DHF), and dipolar normal fluid (DNF). We also show that other interesting phases like dipolar supersolid (DSS) and dipolar hexatic superfluid (DHSF) are at least metastable, and can potentially be reached by thermal quenching. In particular, for large densities or strong dipolar interactions, we find that the DWC exists at low temperatures, but melts into a DHF at higher temperatures, where translational crystaline order is destroyed but orientational order is preserved. Upon further increase in temperature the DHF phase melts into the DNF, where both orientational and translational lattice order are absent. Lastly, we discuss the static structure factor for some of the stable phases and show that they can be identified via optical Bragg scattering measurements.

cond-mat.other↗

arXiv2008

Superfluid and Fermi liquid phases of Bose-Fermi mixtures in optical lattices

We describe interacting mixtures of ultracold bosonic and fermionic atoms in harmonically confined optical lattices. For a suitable choice of parameters we study the emergence of superfluid and Fermi liquid (non-insulating) regions out of Bose-Mott and Fermi-band insulators, due to finite Boson and Fermion hopping. We obtain the shell structure for the system and show that angular momentum can be transferred to the non-insulating regions from Laguerre-Gaussian beams, which combined with Bragg spectroscopy can reveal all superfluid and Fermi liquid shells.

cond-mat.other↗

arXiv2008

Topological quantum phase transitions of ultracold fermions in optical lattices

We consider the possibility of topological quantum phase transitions of ultracold fermions in optical lattices, which can be studied as a function of interaction strength or atomic filling factor (density). The phase transitions are connected to the topology of the elementary excitation spectrum, and occur only for non-zero angular momentum pairing (p-wave, d-wave and f-wave), while they are absent for s-wave. We construct phase diagrams for the specific example of highly anisotropic optical lattices, where the proposed topological phase transitions are most pronounced. To characterize the existence of these topological transitions, we calculate several measurable quantities including momentum distribution, quasi-particle excitation spectrum, atomic compressibility, superfluid density, and sound velocities.

cond-mat.str-el↗

arXiv2008

Spectral densities of superconducting qubits with environmental resonances

In this paper we derive the environmental spectral density for flux, phase and charge qubits when each of them is coupled to an environment with a resonance. From the spectral density we obtain the characteristic spontaneous emission (relaxation) lifetimes $T_1$ for each of these qubits, and show that there is a substantial enhancement of $T_1$ beyond the resonant frequency of the environment.

cond-mat.supr-con↗

arXiv2007

Environmentally-Induced Rabi Oscillations and Decoherence in Phase Qubits

We study decoherence effects in a dc SQUID phase qubit caused by an isolation circuit with a resonant frequency. The coupling between the SQUID phase qubit and its environment is modeled via the Caldeira-Leggett formulation of quantum dissipation/coherence, where the spectral density of the environment is related to the admittance of the isolation circuit. When the frequency of the qubit is at least two times larger than the resonance frequency of the isolation circuit, we find that the decoherence time of the qubit is two orders of magnitude larger than the typical ohmic regime, where the frequency of the qubit is much smaller than the resonance frequency of the isolation circuit. Lastly, we show that when the qubit frequency is on resonance with the isolation circuit, an oscillatory non-Markovian decay emerges, as the dc SQUID phase qubit and its environment self-generate Rabi oscillations of characteristic time scales shorter than the decoherence time.

cond-mat.supr-con↗

arXiv2006

Two-band superfluidity from the BCS to the BEC limit

We analyze the evolution of two-band superfluidity from the weak coupling Bardeen-Cooper-Schrieffer (BCS) to the strong coupling Bose-Einstein condensation (BEC) limit. When the interband interaction is tuned from negative to positive values, a quantum phase transition occurs from a 0-phase to a $π$-phase state, depending on the relative phase of two order parameters. Furthermore, population imbalances between the two bands can be created by tuning the intraband or interband interactions. We also find two undamped low energy collective excitations corresponding to in-phase and out-of-phase modes. Lastly, we derive the coupled Ginzburg-Landau equations, and show that they reduce to coupled Gross-Pitaevskii equations for two types of bosons in the BEC limit.

cond-mat.supr-con↗

arXiv2006

Nonzero orbital angular momentum superfluidity in ultracold Fermi gases

We analyze the evolution of superfluidity for nonzero orbital angular momentum channels from the Bardeen-Cooper-Schrieffer (BCS) to the Bose-Einstein condensation (BEC) limit in three dimensions. First, we analyze the low energy scattering properties of finite range interactions for all possible angular momentum channels. Second, we discuss ground state ($T = 0$) superfluid properties including the order parameter, chemical potential, quasiparticle excitation spectrum, momentum distribution, atomic compressibility, ground state energy and low energy collective excitations. We show that a quantum phase transition occurs for nonzero angular momentum pairing, unlike the s-wave case where the BCS to BEC evolution is just a crossover. Third, we present a gaussian fluctuation theory near the critical temperature ($T = T_{\rm c}$), and we analyze the number of bound, scattering and unbound fermions as well as the chemical potential. Finally, we derive the time-dependent Ginzburg-Landau functional near $T_{\rm c}$, and compare the Ginzburg-Landau coherence length with the zero temperature average Cooper pair size.

cond-mat.supr-con↗

arXiv2005

Density Induced Quantum Phase Transitions in Triplet Superconductors

We consider the possibility of quantum phase transitions in the ground state of triplet superconductors where particle density is the tunning parameter. For definiteness, we focus on the case of one band quasi-one-dimensional triplet superconductors but many of our conclusions regarding the nature of the transition are quite general. Within the functional integral formulation, we calculate the electronic compressibility and superfluid density tensor as a function of the particle density for various triplet order parameter symmetries and find that these quantities are non-analytic when a critical value of the particle density is reached.

cond-mat.supr-con↗

arXiv2005

Evolution from BCS to BEC superfluidity in p-wave Fermi gases

We consider the evolution of superfluid properties of a three dimensional p-wave Fermi gas from weak (BCS) to strong (BEC) coupling as a function of scattering volume. We analyse the order parameter, quasi-particle excitation spectrum, chemical potential, average Cooper pair size and the momentum distribution in the ground state ($T = 0$). We also discuss the critical temperature $T_{\rm c}$, chemical potential and number of unbound, scattering and bound fermions in the normal state ($T = T_{\rm c}$). Lastly, we derive the time-dependent Ginzburg-Landau equation for $T \approx T_{\rm c}$ and extract the Ginzburg-Landau coherence length.

cond-mat.other↗

arXiv2005

Superfluidity of p-wave and s-wave atomic Fermi gases in optical lattices

We consider p-wave pairing of single hyperfine state and s-wave pairing of two hyperfine states ultracold atomic gases trapped in quasi-two-dimensional optical lattices. First, we analyse superfluid properties of p-wave and s-wave symmetries in the strictly weak coupling BCS regime where we discuss the order parameter, chemical potential, critical temperature, atomic compressibility and superfluid density as a function of filling factor for tetragonal and orthorhombic optical lattices. Second, we analyse superfluid properties of p-wave and s-wave superfluids in the evolution from BCS to BEC regime at low temperatures ($T \approx 0$), where we discuss the changes in the quasiparticle excitation spectrum, chemical potential, atomic compressibility, Cooper pair size and momentum distribution as a function of filling factor and interaction strength for tetragonal and orthorhombic optical lattices.

cond-mat.supr-con↗

arXiv2005

Exotic p-wave superfluidity of single hyperfine state Fermi gases in optical lattices

We consider p-wave (triplet) pairing of single hyperfine state ultracold atomic gases trapped in quasi-two-dimensional optical lattices. We find that the critical temperatures in the lattice model is considerably higher and experimentally attainable around half-filling in contrast to the predictions of continuum model for p-wave superfluids. In tetragonal lattices, we show that the atomic compressibility and spin susceptibility have a peak at low temperatures exactly at the half-filling, but this peak splits into two in the orthorhombic lattices. These peaks reflect the p-wave structure of the order parameter for superfluidity and they disappear as the critical temperature is approached from below. We also calculate the superfluid density tensor, and show that for the orthorhombic case there is no off-diagonal component, however in the tetragonal case an off-diagonal component develops, and becomes a key signature of the exotic p-wave state.

cond-mat.supr-con↗

arXiv2005

The Lifshitz Transition in d-wave Superconductors

The BCS to BEC evolution has been recently the focus of studies in superconductors and cold atomic gases. For a d-wave system, we show that a Lifshitz transition occurs at a critical particle density which separates two topologically distinct phases: a BCS-like gapless superconductor/superfluid in the higher density limit and a BEC-like fully gapped superconductor/superfluid in the lower density limit. This transition is second order according to Ehrenfest's classification, but it occurs without a change in the symmetry of the order parameter, and thus can not be classified under Landau's scheme. To illustrate the nature of the transition, we compute the compressibility and the superfluid density as functions of particle density.

cond-mat.supr-con↗