arXiv subjects

C. A. R. Sa de Melo

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

At least 19 recordsLinked to original sources

arXiv2025

Duality breaking, mobility edges, and the connection between topological Aubry-André and quantum Hall insulators in atomic wires with fermions

It is well known that the Aubry-Andr{é} model lacks mobility edges due to its energy-independent self-duality but may exhibit edge states. When duality is broken, we show that mobility regions arise and non-trivial topological phases emerge. By varying the degree of duality breaking, we identify mobility regions and establish a connection between Aubry-Andr{é} atomic wires with fermions and quantum Hall systems for a family of Hamiltonians that depends on the relative phase of laser fields, viewed as a synthetic dimension. Depending on the filling factor and the degree of duality breaking, we find three classes of non-trivial phases: conventional topological insulator, conventional topological Aubry-Andr{é} insulator, and unconventional (hybrid) topological Aubry-Andr{é} insulator. Finally, we discuss appropriate Chern numbers that illustrate the classification of topological phases of localized fermions in atomic wires.

cond-mat.mes-hall↗

arXiv2022

Tighter upper bounds on the critical temperature of two-dimensional superconductors and superfluids: Approaching the supremum

We discuss standard and tighter upper bounds on the critical temperature $T_c$ of two-dimensional superconductors and superfluids versus particle density n or filling factor $ν$, under the assumption that the transition from the normal to the superconducting (superfluid) phase is governed by the Berezinskii-Kosterlitz-Thouless (BKT) mechanism of vortex-antivortex binding and a direct relation between the superfluid density tensor and $T_c$ exists. The standard critical temperature upper bound $T_c^{\rm up1}$ is obtained from the Glover-Ferrell-Tinkham sum rule for the optical conductivity, which constrains the superfluid density tensor components. However, we show that $T_c^{\rm up1}$ is only useful in the limit of low particle/carrier density, where it may be close to the critical temperature supremum $T_c^{\rm sup}$ . For intermediate and high particle/carrier densities, $T_c^{\rm up1}$ is far beyond $T_c^{\rm sup}$ for any given interaction strength. We demonstrate that it is imperative to consider at least the full effects of phase fluctuations of the order parameter for superconductivity (superfluidity) to establish tighter bounds over a wide range of densities. Using the renormalization group, we obtain the critical temperature supremum for phase fluctuations $T_c$ and show that it is a much tighter upper bound to $T_c^{\rm sup}$ than $T_c^{\rm up1}$ for all particle/carrier densities. We conclude by indicating that if the $T_c$ is exceeded in experiments involving single band systems, then a non-BKT mechanism must be invoked.

cond-mat.supr-con↗

arXiv2021

Density induced BCS-Bose evolution in gated two-dimensional superconductors: The Berezinskii-Kosterlitz-Thouless transition as a function of carrier density

We discuss the evolution from BCS to Bose superconductivity versus carrier density in gated two-dimensional s-wave superconductors. We show that the transition from the normal to the superconducting state is controlled by the Berezinskii-Kosterlitz-Thouless vortex-antivortex pairing mechanism, and that the evolution from high carrier density (BCS pairing) to low carrier density (Bose pairing) is just a crossover in s-wave systems. We compare our results to recent experiments on the superconductor LixZrNCl, a lithium intercalated layered nitride, and obtain very good quantitative agreement at low and intermediate densities.

cond-mat.supr-con↗

arXiv2018

Chern number spectrum of ultra-cold fermions in optical lattices tuned independently via artificial magnetic, Zeeman and spin-orbit fields

We discuss the Chern number spectrum of ultra-cold fermions in square optical lattices as a function of artificial magnetic, Zeeman and spin-orbit fields that can be tuned independently. We show the existence of topological quantum phase transitions induced by Zeeman and spin-orbit fields, where the total number and chirality of edge states change for fixed magnetic flux ratio, thus leading to topological-insulator phases which are different from those found at zero Zeeman and spin-orbit fields. We construct phase diagrams of chemical potential versus Zeeman field or spin-orbit coupling and characterize all insulating phases by their topological invariants. Lastly, we obtain a staircase structure in the filling factor versus chemical potential for various Zeeman and spin-orbit fields, showing the existence of incompressible states at rational filling factors derived from a generalized Diophantine equation.

cond-mat.quant-gas↗

arXiv2017

Superfluid transition temperature of spin-orbit and Rabi coupled fermions with tunable interactions

We obtain the superfluid transition temperature of equal Rashba-Dresselhaus spin-orbit and Rabi coupled Fermi superfluids, from the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) regimes in three dimensions. Spin-orbit coupling enhances the critical temperature in the BEC limit, and can convert a first order phase transition in the presence of Rabi coupling into second order, as a function of the Rabi coupling for fixed interactions. We derive the Ginzburg-Landau equation to sixth power in the superfluid order parameter to describe both first and second order transitions as a function of spin-orbit and Rabi couplings.

cond-mat.quant-gas↗

arXiv2016

Quantum phases of interacting three-component fermions under the influence of spin-orbit coupling and Zeeman fields

We describe the quantum phases of interacting three component fermions in the presence of spin-orbit coupling, as well as linear and quadratic Zeeman fields. We classify the emerging superfluid phases in terms of the loci of zeros of their quasi-particle excitation spectrum in momentum space, and we identify several Lifshitz-type topological transitions. In the particular case of vanishing quadratic Zeeman field, a quintuple point exists where four gapless superfluid phases with surface and line nodes converge into a fully gapped superfluid phase. Lastly, we also show that the simultaneous presence of spin-orbit and Zeeman fields transforms a momentum-independent scalar order parameter into an explicitly momentum-dependent tensor in the generalized helicity basis.

cond-mat.quant-gas↗

arXiv2016

Creating spin-one fermions in the presence of artificial spin-orbit fields: Emergent spinor physics and spectroscopic properties

We propose the creation and investigation of a system of spin-one fermions in the presence of artificial spin-orbit coupling, via the interaction of three hyperfine states of fermionic atoms to Raman laser fields. We explore the emergence of spinor physics in the Hamiltonian described by the interaction between light and atoms, and analyze spectroscopic properties such as dispersion relation, Fermi surfaces, spectral functions, spin-dependent momentum distributions and density of states. Connections to spin-one bosons and SU(3) systems is made, as well relations to the Lifshitz transition and Pomeranchuk instability are presented.

cond-mat.quant-gas↗

arXiv2013

Formation of Feshbach molecules in the presence of artificial spin-orbit coupling and Zeeman fields

We derive general conditions for the emergence of singlet Feshbach molecules in the presence of artificial Zeeman fields for arbritary mixtures of Rashba and Dresselhaus spin-orbit orbit coupling in two or three dimensions. We focus on the formation of two-particle bound states resulting from interactions between ultra-cold spin-1/2 fermions, under the assumption that interactions are short-ranged and occur only in the s-wave channel. In this case, we calculate explicitly binding energies of Feshbach molecules and analyze their dependence on spin-orbit couplings, Zeeman fields, interactions and center of mass momentum, paying particular attention to the experimentally relevant case of spin-orbit couplings with equal Rashba and Dresselhaus (ERD) amplitudes.

cond-mat.quant-gas↗

arXiv2012

Antiferromagnetic Spatial Ordering in a Quenched One-dimensional Spinor Gas

We have experimentally observed the emergence of spontaneous antiferromagnetic spatial order in a sodium spinor Bose-Einstein condensate that was quenched through a magnetic phase transition. For negative values of the quadratic Zeeman shift, a gas initially prepared in the F = 1, mF = 0 state collapsed into a dynamically evolving superposition of all 3 spin projections, mF = 0, +/-1. The quench gave rise to rich, nonequilibrium behavior where both nematic and magnetic spin waves were generated. We characterized the spatiotemporal evolution through two particle correlations between atoms in each pair of spin states. These revealed dramatic differences between the dynamics of the spin correlations and those of the spin populations.

cond-mat.quant-gas↗

arXiv2010

Ultra-cold fermions in real or fictitious magnetic fields: The BCS-BEC evolution and the type-I--type-II transition

We study ultra-cold neutral fermion superfluids in the presence of fictitious magnetic fields, as well as charged fermion superfluids in the presence of real magnetic fields. Charged fermion superfluids undergo a phase transition from type-I to type-II superfluidity, where the magnetic properties of the superfluid change from being a perfect diamagnet without vortices to a partial diamagnet with the emergence of the Abrikosov vortex lattice. The transition from type-I to type-II superfluidity is tunned by changing the scattering parameter (interaction) for fixed density. We also find that neutral fermion superfluids such as $^6$Li and $^{40}$K are extreme type-II superfluids, and that they are more robust to the penetration of a fictitious magnetic field in the BCS-BEC crossover region near unitarity, where the critical fictitious magnetic field reaches a maximum as a function of the scattering parameter (interaction).

cond-mat.quant-gas↗

arXiv2008

Exploring the order parameter symmetry of p-wave Fermi condensates

We discuss the time-of-flight expansion of dilute p-wave Fermi condensates on the BEC side of Feshbach resonances, as a way to extract information about the order parameter symmetry for superfluidity. We show that the cloud profile is in general sensitive to the interaction strength between fermions, the magnitude and direction of external magnetic fields, and to the angular momentum projection of the order parameter. In particular, due to the anisotropic nature of p-wave interactions we show that the time-of-flight expansion of a p-wave superfluid is anisotropic even if the superfluid is confined to a completely isotropic trap, unlike the case of Bose or s-wave Fermi condensates, which under the same circumstances expand isotropically. Furthermore, we demonstrate that expanding p-wave superfluids released from axially symmetric traps experience anisotropy inversions, where the aspect ratio between the axial and radial directions change during expansion, while the axial symmetry can also be lost reflecting the spatial anisotropy of the underlying interaction.

cond-mat.str-el↗

arXiv2008

Matter-wave interference in s-wave and p-wave Fermi condensates

We discuss the time evolution and matter-wave interference of Fermi condensates on the BEC side of Feshbach resonances for s and p-wave superfluids, upon release from harmonic traps. In swave systems, where the order parameter is a complex scalar, we find that the interference patterns depend on the relative phase of the order parameters of the condensates. In p-wave systems involving the mixture of two-hyperfine states, we show that the interference pattern exhibits a polarization effect depending on the relative orientation of the two vector order parameters. Lastly, we also point out that p-wave Fermi condensates exhibit an anisotropic expansion, reflecting the spatial anisotropy of the underlying interaction between fermions and the orbital nature of the vector order parameter. Potential applications of our results include systems of ultra-cold atoms that exhibit p-wave Feshbach resonances such as 6Li or 40K.

cond-mat.str-el↗

arXiv2007

Quantum phases of Fermi-Fermi mixtures in optical lattices

The ground state phase diagram of Fermi-Fermi mixtures in optical lattices is analyzed as a function of interaction strength, population imbalance, filling fraction and tunneling parameters. It is shown that population imbalanced Fermi-Fermi mixtures reduce to strongly interacting Bose-Fermi mixtures in the molecular limit, in sharp contrast to homogeneous or harmonically trapped systems where the resulting Bose-Fermi mixture is weakly interacting. Furthermore, insulating phases are found in optical lattices of Fermi-Fermi mixtures in addition to the standard phase-separated or coexisting superfluid/excess fermion phases found in homogeneous systems. The insulating states can be a molecular Bose-Mott insulator (BMI), a Fermi-Pauli insulator (FPI), a phase-separated BMI/FPI mixture or a Bose-Fermi checkerboard (BFC).

cond-mat.supr-con↗

arXiv2007

Superfluid and insulating phases of fermion mixtures in optical lattices

The ground state phase diagram of fermion mixtures in optical lattices is analyzed as a function of interaction strength, fermion filling factor and tunneling parameters. In addition to standard superfluid, phase-separated or coexisting superfluid/excess-fermion phases found in homogeneous or harmonically trapped systems, fermions in optical lattices have several insulating phases, including a molecular Bose-Mott insulator (BMI), a Fermi-Pauli (band) insulator (FPI), a phase-separated BMI/FPI mixture or a Bose-Fermi checkerboard (BFC). The molecular BMI phase is the fermion mixture counterpart of the atomic BMI found in atomic Bose systems, the BFC or BMI/FPI phases exist in Bose-Fermi mixtures, and lastly the FPI phase is particular to the Fermi nature of the constituent atoms of the mixture.

cond-mat.supr-con↗

arXiv2007

Superfluid and Mott Insulating shells of bosons in harmonically confined optical lattices

Weakly interacting atomic or molecular bosons in quantum degenerate regime and trapped in harmonically confined optical lattices, exhibit a wedding cake structure consisting of insulating (Mott) shells. It is shown that superfluid regions emerge between Mott shells as a result of fluctuations due to finite hopping. It is found that the order parameter equation in the superfluid regions is not of the Gross-Pitaeviskii type except near the insulator to superfluid boundaries. The excitation spectra in the Mott and superfluid regions are obtained, and it is shown that the superfluid shells posses low energy sound modes with spatially dependent sound velocity described by a local index of refraction directly related to the local superfluid density. Lastly, the Berezinskii-Kosterlitz-Thouless transition and vortex-antivortex pairs are discussed in thin (wide) superfluid shells (rings) limited by three (two) dimensional Mott regions.

cond-mat.other↗

arXiv2007

Fermi-Fermi Mixtures in the Strong Attraction Limit

The phase diagrams of low density Fermi-Fermi mixtures with equal or unequal masses and equal or unequal populations are described at zero and finite temperatures in the strong attraction limit. In this limit, the Fermi-Fermi mixture can be described by a weakly interacting Bose-Fermi mixture, where the bosons correspond to Feshbach molecules and the fermions correspond to excess atoms. First, we discuss the three and four fermion scattering processes, and use the exact boson-fermion and boson-boson scattering lengths to generate the phase diagrams in terms of the underlying fermion-fermion scattering length. In three dimensions, in addition to the normal and uniform superfluid phases, we find two stable non-uniform states corresponding to (1) phase separation between pure unpaired (excess) and pure paired fermions (molecular bosons); and (2) phase separation between pure excess fermions and a mixture of excess fermions and molecular bosons. Lastly, we also discuss the effects of the trapping potential in the density profiles of condensed and non-condensed molecular bosons, and excess fermions at zero and finite temperatures, and discuss possible implications of our findings to experiments involving mixtures of ultracold fermions.

cond-mat.supr-con↗

arXiv2007

Mixtures of ultracold fermions with unequal masses

We analyze the phase diagram of superfluidity for two-species fermion mixtures from the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) limit as a function of scattering parameter, population imbalance and mass anisotropy. We identify regions corresponding to normal, or uniform/non-uniform superfluid phases, and discuss topological quantum phase transitions in the BCS, unitarity and BEC limits. We derive the Ginzburg-Landau equation near the critical temperature, and show that it describes a dilute mixture of paired and unpaired fermions in the BEC limit. We also obtain the zero temperature low frequency and long wavelength collective excitation spectrum, and recover the Bogoliubov relation for weakly interacting dilute bosons in the BEC limit. Lastly, we discuss the effects of harmonic traps and the resulting density profiles in the BEC regime.

cond-mat.supr-con↗

arXiv2007

Vortex Quasi-Crystals

There seems to be a one to one correspondence between the phases of atomic and molecular matter (AMOM) and vortex matter (VM) in superconductors. Crystals, liquids and glasses have been experimentally observed in both AMOM and VM. However, quasi-crystals also exist in AMOM, thus a new phase of vortex matter is proposed here: the vortex quasi-crystal. It is argued that vortex quasi-crystals are stabilized due to boundary and surface energy effects for samples of special shapes and sizes, and that a phase transition between a vortex crystal and a vortex quasi-crystal occurs as a function of magnetic field and temperature as the sample size is reduced.

cond-mat.supr-con↗