Bose Fermi Research Articles

Optimal preparation of Bose and Fermi atomic gas mixtures of 87Rb and 40K in a crossed optical dipole trap

We report on the optimal production of the Bose and Fermi mixtures with 87Rb and 40K in a crossed optical dipole trap (ODT). We measure the atomic number and lifetime of the mixtures in combination of the spin state |F = 9/2, m F = 9/2〉 of 40K and |1, 1〉 of 87Rb in the ODT, which is larger and longer compared with the combination of the spin state |9/2, 9/2〉 of 40K and |2, 2〉 of 87Rb in the ODT. We observe the atomic numbers of 87Rb and 40K shown in each stage of the sympathetic cooling process while gradually reducing the depth of the optical trap. By optimizing the relative loading time of atomic mixtures in the MOT, we obtain the large atomic number of 40K (∼6 × 106) or the mixtures of atoms with an equal number (∼1.6 × 106) at the end of evaporative cooling in the ODT. We experimentally investigate the evaporative cooling in an enlarged volume of the ODT via adding a third laser beam to the crossed ODT and found that more atoms (8 × 106) and higher degeneracy (T/T F = 0.25) of Fermi gases are obtained. The ultracold atomic gas mixtures pave the way to explore phenomena such as few-body collisions and the Bose–Fermi Hubbard model, as well as for creating ground-state molecules of 87Rb40K.

Quench dynamics of a Tonks-Girardeau gas in one dimensional anharmonic trap

The quench dynamics of strongly interacting bosons on quartic and sextic traps are studied by exactly solving the time-dependent many-boson Schrödinger equation numerically. The dynamics are addressed by the key measures of one-body density in conjugate space and information entropy. For both cases, rich many-body dynamics are exhibited and the loss of the Bose–Fermi oscillation in the Tonks–Girardeau limit is also attributed.

Where Charged Sectors are Localizable: A Viewpoint from Covariant Cohomology

Given a Haag–Kastler net on a globally hyperbolic spacetime, one can consider a family of regions where quantum charges are supposed to be localized. Assuming that the net fulfils certain minimal properties (factoriality of the global observable algebra and relative Haag duality), we give a geometric criterion that the given family must fulfil to have a superselection structure with charges localized on its regions. Our criterion is fulfilled by all the families used in the theory of sectors (double cones, spacelike cones, diamonds, hypercones). In order to take account of eventual spacetime symmetries, our superselection structures are constructed in terms of covariant charge transporters, a novel cohomological approach generalizing that introduced by J. E. Roberts. In the case of hypercones, with the forward light cone as an ambient spacetime, we obtain a superselection structure with Bose–Fermi parastatistics and particle-antiparticle conjugation. It could constitute a candidate for a different description of the charged sectors introduced by Buchholz and Roberts for theories including massless particles.

Bose glass and Fermi glass

It is known that two-dimensional superconducting materials undergo a quantum phase transition from a localized state to superconductivity. When the disordered samples are cooled, bosons (Cooper pairs) are generated from Fermi glass and reach superconductivity through Bose glass. However, there has been no universal expression representing the transition from Fermi glass to Bose glass. Here, we discovered an experimental renormalization group flow from Fermi glass to Bose glass in terms of simple beta-function analysis. To discuss the universality of this flow, we analyzed manifestly different systems, namely a Nd-based two-dimensional layered perovskite and an ultrathin Pb film. We find that all our experimental data for Fermi glass fall beautifully into the conventional self-consistent beta-function. Surprisingly, however, flows perpendicular to the conventional beta-function are observed in the weakly localized regime of both systems, where localization becomes even weaker. Consequently, we propose a universal transition from Bose glass to Fermi glass with the new two-dimensional critical sheet resistance close to R_Box = h/e^{2}.

Transition from a polaronic condensate to a degenerate Fermi gas of heteronuclear molecules

The interplay of quantum statistics and interactions in atomic Bose–Fermi mixtures leads to a phase diagram markedly different from pure fermionic or bosonic systems. However, investigating this phase diagram remains challenging when bosons condense due to the resulting fast interspecies loss. Here we report observations consistent with a phase transition from a polaronic to a molecular phase in a density-matched degenerate Bose–Fermi mixture. The condensate fraction, representing the order parameter of the transition, is depleted by interactions, and the build-up of strong correlations results in the emergence of a molecular Fermi gas. The features of the underlying quantum phase transition represent a new phenomenon complementary to the paradigmatic Bose–Einstein condensate/Bardeen–Cooper–Schrieffer crossover observed in Fermi systems. By driving the system through the transition, we produce a sample of sodium–potassium molecules exhibiting a large molecule-frame dipole moment in the quantum-degenerate regime.

Correlation in momentum space of Tonks–Girardeau gas

In momentum space, we investigate the correlation properties of the ground state of Tonks–Girardeau gases. With Bose–Fermi mapping method, the exact ground state wavefunction in coordinate space can be obtained based on the wavefunction of spin-polarized Fermions. By Fourier transformation we obtain the ground state wavefunction in momentum space, and therefore the pair correlation and the reduced one-body density matrix (ROBDM) in momentum space, whose diagonal part is the momentum distribution. The ROBDM in momentum space is the Fourier transformation of the ROBDM in coordinate space and the pair correlation in momentum space is the Fourier transformation of the reduced two-body density matrix in coordinate space. The correlations in momentum space display larger values only in small momentum region and vanish in most other regions. The lowest natural orbital and occupation distribution in momentum space are also obtained.

Bose-Fermi mixtures in 2D solid-state superstructures.

Bose-Fermi mixtures in 2D solid-state superstructures.

Instantons in AdS$$_4$$ from (anti)membranes wrapping $$S^7$$ to Bose–Fermi duality in CFT$$_3$$’s

We present new SO(4)-invariant and non-supersymmetric instanton solutions for the conformally coupled m^2=-2 and massive m^2=+4 (pseudo)scalars arising from a consistent truncation of 11-dimensional supergravity over AdS_4 x S^7/Z_k when the internal space is a S^1 Hopf fibration on CP^3, and we consider backreaction. In fact, the bulk configurations associate with (anti)membranes wrapped around mixed internal (and external) directions, which in turn probe the Wick-rotated or skew-whiffed background, break all supersymmetries as well as parity invariance. From near the boundary behavior of the closed solution for the coupled bulk (pseudo)scalar, we get a marginal triple-trace deformation with mixed boundary condition (valid also for the bulk massless m^2=0 (pseudo)scalar, raised when considering the external space backreaction, with Dirichlet boundary condition) and as a result, the corresponding boundary effective potential is unbounded from below and causes an instability because of the Fubini-like instanton. Presenting dual effective actions, we see that the boundary solutions and counterparts realize in singlet sectors of three-dimensional U(N) and O(N) Chern--Simons-matter field theories. In particular, we use versions of massless and mass-deformed regular and critical boson and fermion models, find instantons and confirm state-operator AdS_4/CFT_3 correspondence and also Bose-Fermi duality at the level of the solutions. In addition, we discuss on relations of our setups with Vasiliev's Higher-Spin theories, deformations of the Aharony-Bergman-Jafferis-Maldacena model and other related studies.

Exciton density waves in Coulomb-coupled dual moiré lattices.

Strongly correlated bosons in a lattice are a platform that can realize rich bosonic states of matter and quantum phase transitions1. While strongly correlated bosons in a lattice have been studied in cold-atom experiments2-4, their realization in a solid-state system has remained challenging5. Here we trap interlayer excitons-bosons composed of bound electron-hole pairs, in a lattice provided by an angle-aligned WS2/bilayer WSe2/WS2 multilayer. The heterostructure supports Coulomb-coupled triangular moiré lattices of nearly identical period at the top and bottom interfaces. We observe correlated insulating states when the combined electron filling factor of the two lattices, with arbitrary partitions, equals [Formula: see text] and [Formula: see text]. These states can be interpreted as exciton density waves in a Bose-Fermi mixture of excitons and holes6,7. Because of the strong repulsive interactions between the constituents, the holes form robust generalized Wigner crystals8-11, which restrict the exciton fluid to channels that spontaneously break the translational symmetry of the lattice. Our results demonstrate that Coulomb-coupled moiré lattices are fertile ground for correlated many-boson phenomena12,13.

Generalized Bose–Fermi mapping and strong coupling ansatz wavefunction for one dimensional strongly interacting spinor quantum gases

Quantum many-body systems in one dimension (1D) exhibit some peculiar properties. In this article, we review some of our work on strongly interacting 1D spinor quantum gas. First, we discuss a generalized Bose–Fermi mapping that maps the charge degrees of freedom to a spinless Fermi gas and the spin degrees of freedom to a spin chain model. This also maps the strongly interacting system into a weakly interacting one, which is amenable for perturbative calculations. Next, based on this mapping, we construct an ansatz wavefunction for the strongly interacting system, using which many physical quantities can be conveniently calculated. We showcase the usage of this ansatz wavefunction by considering the collective excitations and quench dynamics of a harmonically trapped system.

Quantum degenerate Bose-Fermi atomic gas mixture of 23Na and 40K

We report a compact experimental setup for producing a quantum degenerate mixture of Bose 23Na and Fermi 40K gases. The atoms are collected in dual dark magneto–optical traps (MOT) with species timesharing loading to reduce the light-induced loss, and then further cooled using the gray molasses technique on the D 2 line for 23Na and D 1 line for 40K. The microwave evaporation cooling is used to cool 23Na in | F = 2,mF = 2〉 in an optically plugged magnetic trap, meanwhile, 40K in | F = 9/2,mF = 9/2〉 is sympathetically cooled. Then the mixture is loaded into a large volume optical dipole trap where 23Na atoms are immediately transferred to |1,1〉 for further effective cooling to avoid the strong three-body loss between 23Na atoms in |2,2〉 and 40K atoms in |9/2,9/2〉. At the end of the evaporation in optical trap, a degenerate Fermi gas of 40K with 1.9 × 105 atoms at T/TF = 0.5 in the |9/2,9/2〉 hyperfine state coexists with a Bose–Einstein condensate (BEC) of 23Na with 8 × 104 atoms in the |1,1〉 hyperfine state at 300 nK. We also can produce the two species mixture with the tunable population imbalance by adjusting the 23Na magneto–optical trap loading time.

Repulsive Fermi and Bose Polarons in Quantum Gases

Polaron quasiparticles are formed when a mobile impurity is coupled to the elementary excitations of a many-particle background. In the field of ultracold atoms, the study of the associated impurity problem has attracted a growing interest over the last fifteen years. Polaron quasiparticle properties are essential to our understanding of a variety of paradigmatic quantum many-body systems realized in ultracold atomic gases and in the solid state, from imbalanced Bose–Fermi and Fermi–Fermi mixtures to fermionic Hubbard models. In this topical review, we focus on the so-called repulsive polaron branch, which emerges as an excited many-body state in systems with underlying attractive interactions such as ultracold atomic mixtures, and is characterized by an effective repulsion between the impurity and the surrounding medium. We give a brief account of the current theoretical and experimental understanding of repulsive polaron properties, for impurities embedded in both fermionic and bosonic media, and we highlight open issues deserving future investigations.

Thermodynamic geometry of harmonically trapped ideal quantum gases

In this paper, we consider the thermodynamic geometry of harmonic trapped Bose and Fermi gases. We construct a thermodynamic geometry for these systems and evaluate the corresponding metric tensor and thermodynamic curvature. By observing that the thermodynamic curvature of the trapped Bose (Fermi) gas is always positive (negative), we conclude that the trapping potential does not affect the intrinsic statistical interactions. By inspecting the singular points of the thermodynamic curvature, we argue that the confinement dimension and the frequency of the trapping potential change the critical value of fugacity for the Bose gas.

Notes on a vanishing cosmological constant without Bose–Fermi cancellation

Abstract We discuss how one can systematically construct point particle theories that realize the vanishing one-loop cosmological constant without Bose–Fermi cancellation. Our construction is based on the asymmetric (or non-geometric) orbifolds of supersymmetric string vacua. Using the building blocks of their partition functions and their modular properties, we construct theories which would be naturally identified with certain point particle theories including infinite mass spectra, but not with string vacua. They are obviously non-supersymmetric due to the mismatch of the bosonic and fermionic degrees of freedom at each mass level. Nevertheless, it is found that the one-loop cosmological constant vanishes, after removing the parameter effectively playing the role of the UV cut-off. As concrete examples we demonstrate the construction of models based on toroidal asymmetric orbifolds with Lie algebra lattices (Englert–Neveu lattices) by making use of the analysis given in Satoh and Sugawara (2017)

Emergent force and work fluctuations in a bilayer superfluid Bose–Fermi mixture in mixed dimensions

We investigate a system of two-atomic species in mixed dimensions, in which one species is spread in a three-dimensional space and the other species is confined in two parallel layers. The presence of atoms in three-dimensions creates an induced potential for the ones confined in layers. Depending on the scattering (coherence) length and the layer separation, the formation of p-wave pairing within the same layer or s-wave pairing between different layers has been suggested. It was shown that these pairs cannot coexist when time-reversal symmetry is on, and there appears a transition from p-wave to s-wave as the ratio of the layer separation and the effective scattering length decreases. Here, we study the thermodynamical signatures of such transition and find that with the formation of the inter-layer pairing, an emergent force to be present at the critical point and show that it can be derived from the ground state energy. This result offers a tool for experimentally realizing such transitions, and can find notable potential in the field of quantum-thermodynamics.

Achieving ultracold Bose–Fermi mixture of 87Rb and 40K with dual dark magnetic-optical-trap

We demonstrate that dual dark magnetic-optical-traps (MOTs) have great importance in the two-species 87Rb and 40K mixture compared with dual bright MOTs. The dark MOT has a little improvement in the trapping of single-species 87Rb or 40K gases compared with bright MOT. For the case of loading two-species 87Rb and 40K simultaneously, the improvement of 40K in the dual dark MOTs is mainly from the reduction of light-assisted collision losses. The dual dark MOTs employ a pair of conical lenses to produce the hollow beam for repump laser with high efficiency. The number and density of 87Rb and 40K atoms after evaporative cooling in the hybrid magnetic trap with dark MOT loading are compared with those in bright MOT. The atoms with large number and high density make it easier to realize the quantum degenerate of Bose–Fermi mixture.

Dynamics of a trapped ion in a quantum gas: Effects of particle statistics

We study the quantum dynamics of an ion confined in a radiofrequency trap in interaction with either a Bose or spin-polarized Fermi gas. To this end, we derive quantum optical master equations in the limit of weak coupling and the Lamb-Dicke approximations. For the bosonic bath, we also include the so-called "Lamb-shift" correction to the ion trap due to the coupling to the quantum gas as well as the extended Fr\"ohlich interaction within the Bogolyubov approximation that have been not considered in previous studies. We calculate the ion kinetic energy for various atom-ion scattering lengths as well as gas temperatures by considering the intrinsic micromotion and we analyse the damping of the ion motion in the gas as a function of the gas temperature. We find that the ion's dynamics depends on the quantum statistics of the gas and that a fermionic bath enables to attain lower ionic energies.

Optically induced hybrid Bose-Fermi system in quantum wells with different charge carriers.

It is demonstrated theoretically that the circularly polarized irradiation of two-dimensional conducting systems can produce composite bosons consisting of two electrons with different effective masses (different charge carriers), which are stable due to the Fermi sea of conduction electrons. As a result, an optically induced mixture of paired electrons and normal conduction electrons (the hybrid Bose-Fermi system) appears. Elementary excitations in such a hybrid system are analyzed, and possible manifestations of the light-induced electron pairing are discussed for semiconductor quantum wells.

Erratum to: Unexpected transitional paths in the prolate to oblate shape phase transitions for Bose–Fermi systems

Erratum to: Unexpected transitional paths in the prolate to oblate shape phase transitions for Bose–Fermi systems

Magnetoplasmon resonance in two-dimensional fluctuating superconductors

We develop a theory of the magnetoplasmon resonance (MPR) in two-dimensional superconductors in the fluctuating regime, where the temperature is slightly above the critical temperature of the superconducting transition. In this regime, unpaired electrons and fluctuating Cooper pairs coexist in the system and interact with each other via long-range Coulomb forces, forming a Bose–Fermi mixture. The sample is considered to be under the influence of an external time-dependent electromagnetic field with a frequency in sub-terahertz range and a permanent magnetic field. It is shown that the MPR of the system is strongly modified in the presence of superconducting fluctuations in the vicinity of the superconducting transition. In particular, the fluctuating Cooper pairs dramatically change the broadening of the MPR, which is reflected in the optical response of the system.