arXiv subjects

Mirosław Brewczyk

Publications and source records attributed to Mirosław Brewczyk.

At least 19 recordsLinked to original sources

arXiv2026

Searching for cosmic vortices

Our study focuses on the strong tidal disruption of a cold helium white dwarf passing a black hole. We model the white dwarf as a Bose-Fermi droplet and use quantum hydrodynamic equations to simulate the binary system's evolution. As the white dwarf passes through periastron, it loses a significant amount of mass. This mass falls onto the black hole and forms an accretion disc. Quantized vortices appear in the accretion disc, manifesting as strong electromagnetic radiation signals that exhibit characteristic flickering patterns changing on a timescale of a few seconds. Meanwhile, the white dwarf moves away from the black hole. As the white dwarf moves through space, vortices run along its surface. This elongates its geometry, causing it to rotate and emit gravitational waves.

astro-ph.HE↗

arXiv2026

Three-dimensional Bose-Fermi droplets at nonzero temperatures

Using numerical methods, we study the formation of self-bound quantum Bose-Fermi droplets at nonzero temperatures. We describe an attractive atomic Bose-Fermi mixture using quantum hydrodynamics enriched by beyond-mean-field corrections and thermal fluctuations, together with a simplified self-consistent Hartree-Fock model. With these models, we determine that low-temperature droplets with finite lifetimes can exist in free space when the attraction between bosons and fermions is sufficiently strong. Additionally, Bose-Fermi droplets at nonzero temperatures can exist in a box potential in equilibrium with bosonic and fermionic vapor. We discuss the properties of Bose-Fermi droplets at nonzero temperatures in terms of the initial condensate fraction, total atom number, and interspecies attraction strength.

cond-mat.quant-gas↗

arXiv2025

Dynamic formation of supersolid phase in a mixture of ultracold bosonic and fermionic atoms

We numerically study the dynamical properties of a mixture consisting of a dipolar condensate and a degenerate Fermi gas in a quasi-one-dimensional geometry. In particular, we focus on the system's response to a temporal variation in the interaction strength between bosons and fermions. When the interspecies attraction becomes sufficiently strong, we observe a phase transition to a supersolid state. This conclusion is supported by the emergence of an out-of-phase Goldstone mode in the excitation spectrum.

cond-mat.quant-gas↗

arXiv2025

Competitive algorithms for calculating the ground state properties of Bose-Fermi mixtures

In this work we define, analyze, and compare different numerical schemes that can be used to study the ground state properties of Bose-Fermi systems, such as mixtures of different atomic species under external forces or self-bound quantum droplets. The bosonic atoms are assumed to be condensed and are described by the generalized Gross-Pitaevskii equation. The fermionic atoms, on the other hand, are treated individually, and each atom is associated with a wave function whose evolution follows the Hartree-Fock equation. We solve such a formulated set of equations using a variety of methods, including those based on adiabatic switching of interactions and the imaginary time propagation technique combined with the Gram-Schmidt orthonormalization or the diagonalization of the Hamiltonian matrix. We show how different algorithms compete at the numerical level by studying the mixture in the range of parameters covering the formation of self-bound quantum Bose-Fermi droplets.

cs.CE↗

arXiv2025

Studying the radiation of a white dwarf star falling onto a black hole

We investigate electromagnetic and gravitational radiation generated during the process of the tidal stripping of a white dwarf star circulating a black hole. We go beyond Chandrasekhar's ideas and not only consider a white dwarf itself as a quantum object, but also describe the dynamics of a produced accretion disk in a quantum way. We model the white dwarf star as a Bose-Fermi droplet and use the quantum hydrodynamic equations to simulate the evolution of the black hole-white dwarf binary system. While going through the periastron, the white dwarf loses a small fraction of its mass. The mass falling onto a black hole is a source of powerful electromagnetic and gravitational radiation. Bursts of ultraluminous radiation are flared at each periastron passage. This resembles the recurrent flaring of X-ray sources discovered recently by Irwin et al. Gravitational energy bursts occur mainly through emission at very low frequencies. The accretion disk, formed due to stripping of the white dwarf, starts at some point to contribute continuously to radiation of both electromagnetic and gravitational types.

astro-ph.HE↗

arXiv2024

Fermionic atoms in a spin-dependent optical lattice potential: topological insulators with broken time-reversal symmetry

We propose a novel approach to study the topological properties of matter. In this approach, fermionic atoms are placed in an external magnetic field and in a two-dimensional spin-dependent optical lattice (SDOL) created by intersecting laser beams with a superposition of polarizations. To demonstrate the utility of the SDOL-based technique we compute the topological invariants (Chern numbers) for the SDOL bands as a function of an external magnetic field, and show the existence of a rich topology of the energy bands for this system which does not have parity-time-reversal symmetry. We explicitly consider $^{6}$Li $F=1/2$ atoms. Using a projection matrix method we observe topological phase transitions between an ordinary insulator, an abelian topological insulator, and a non-abelian topological insulator as the external magnetic field strength is varied. Upon introducing edges for the SDOL we find topological edge states (that are correlated with the band Chern numbers) that simultaneously exhibit non-trivial density and spin currents with both a rotational flow contribution and flow along the edge of the SDOL.

cond-mat.quant-gas↗

arXiv2024

Supersolidity of dipolar Bose-Einstein condensates induced by coupling to fermions

We study a mixture of a repulsive dipolar condensate and a degenerate Fermi gas in a quasi-one-dimensional geometry. We demonstrate that the presence of fermions, which attract bosons, drastically changes the behavior of the dipolar condensate. For strong enough boson-fermion attraction, a dipolar Bose-Fermi droplet appears in the mixture, and as the attraction becomes stronger, a roton excitation develops in the Bogoliubov spectrum, leading to the formation of a supersolid, and eventually a crystal of isolated droplets. We describe the system by coupled extended Gross-Pitaevskii (bosons) and Hartree-Fock (fermions) equations. We study the excitation spectrum of the system and identify a number of Goldstone and Higgs modes in the supersolid regime.

cond-mat.quant-gas↗

arXiv2024

Mechanism for sound dissipation in a two-dimensional degenerate Fermi gas

We numerically study the transport properties of a two-dimensional Fermi gas in a weakly and strongly interacting regimes, in the range of temperatures close to the transition to a superfluid phase. For that we excite sound waves in a fermionic mixture by using the phase imprinting technique, follow their evolution, and finally determine both their speed and attenuation. Our formalism incorporates thermal fluctuations via the ground canonical ensemble description and with the help of Metropolis algoritm. From numerical simulations we extract temperature dependence of the sound velocity and diffusivity as well as the dependence on the interaction strength. We emphasize the role of virtual vortex-antivortex pairs creation in the process of sound dissipation.

cond-mat.quant-gas↗

arXiv2022

Atoms in a spin dependent optical potential: ground state topology and magnetization

We investigate a Bose-Einstein condensate of $F= 1$ $^{87}$Rb atoms in a 2D spin-dependent optical lattice generated by intersecting laser beams with a superposition of polarizations. For $^{87}$Rb the effective interaction of an atom with the electromagnetic field contains a scalar and a vector (called as fictitious magnetic field, $B_{fic}$) potentials. The Rb atoms behave as a quantum rotor (QR) with angular momentum given by the sum of the atomic rotational motion angular momentum and the hyperfine spin. The ground state of the QR is affected upon applying an external magnetic field, $B_{ext}$, perpendicular to the plane of QR motion and a sudden change of its topology occurs as the ratio $B_{ext}/B_{fic}$ exceeds critical value. It is shown that the change of topology of the QR ground state is a result of combined action of Zeeman and Einstein-de Haas effects. The first transfers atoms to the largest hyperfine component to polarize the sample along the field as the external magnetic field is increased. The second sweeps spin to rotational angular momentum, modifying the kinetic energy of the atoms.

quant-ph↗

arXiv2022

Dynamics of large samples of repulsive Fermi gases at nonzero temperatures

We develop a model of a binary fermionic mixture, consisting of large number of atoms, applicable at nonzero temperatures, in the normal phase. We use this approach to study dynamics of degenerate Fermi systems under various perturbations. For example, we analyze spin-dipole oscillations of a two-component fermionic mixture, demonstrating that the ferromagnetic phase shows up at stronger repulsion between components while the temperature raises. We study as well the radial oscillations of weakly interacting repulsive Fermi gases. We obtain a good agreement with experimental data when available. Otherwise, we compare our results with the outcome of the Hartree-Fock orbital calculations done for the system with small number of fermions.

cond-mat.quant-gas↗

arXiv2021

Berezinskii-Kosterlitz-Thouless phase induced by dissipating quasisolitons

We theoretically study the sound propagation in a two-dimensional weakly interacting uniform Bose gas. Using the classical fields approximation we analyze in detail the properties of density waves generated both in a weak and strong perturbation regimes. While in the former case density excitations can be described in terms of hydrodynamic or collisionless sound, the strong disturbance of the system results in a qualitatively different response. We identify observed structures as quasisolitons and uncover their internal complexity for strong perturbation case. For this regime quasisolitons break into vortex pairs as time progresses, eventually reaching an equilibrium state. We find this state, characterized by only fluctuating in time averaged number of pairs of opposite charge vortices and by appearance of a quasi-long-range order, as the Berezinskii-Kosterlitz-Thouless (BKT) phase.

cond-mat.quant-gas↗

arXiv2021

Spin distillation cooling of ultracold Bose gases

We study the spin distillation of spinor gases of bosonic atoms and find two different mechanisms in ${}^{52}$Cr and $^{23}$Na atoms, both of which can cool effectively. The first mechanism involves dipolar scattering into initially unoccupied spin states and cools only above a threshold magnetic field. The second proceeds via equilibrium relaxation of the thermal cloud into empty spin states, reducing its proportion in the initial component. It cools only below a threshold magnetic field. The technique was initially demonstrated experimentally for a chromium dipolar gas [B. Naylor et al., Phys. Rev. Lett. 115, 243002 (2015)], whereas here we develop the concept further and provide an in-depth understanding of the required physics and limitations involved. Through numerical simulations, we reveal the mechanisms involved and demonstrate that the spin distillation cycle can be repeated several times, each time resulting in a significant additional reduction of the thermal atom fraction. Threshold values of magnetic field and predictions for the achievable temperature are also identified.

cond-mat.quant-gas↗

arXiv2021

Modelling quantum aspects of disruption of a white dwarf star by a black hole

We study the final stages of the evolution of a binary system consisted of a black hole and a white dwarf star. We implement the quantum hydrodynamic equations and carry out numerical simulations. As a model of a white dwarf star, we consider a zero temperature droplet of attractively interacting degenerate atomic bosons and spin-polarized atomic fermions. Such mixtures are investigated experimentally nowadays. We find that the white dwarf star is stripped off its mass while passing the periastron. Due to nonlinear effects, the accretion disk originated from the white dwarf becomes fragmented and the onset of a quantum turbulence with giant quantized vortices present in the bosonic component of the accretion disk is observed. The binary system ends its life in a spectacular way, revealing quantum features underlying the white dwarf star's structure. We find a charged mass, falling onto a black hole, could be responsible for recently discovered ultraluminous X-ray bursts. The simulations show that the final passage of a white dwarf near a black hole can cause a gamma-ray burst.

astro-ph.HE↗

arXiv2020

Breathing Mode of a Bose-Einstein Condensate Immersed in a Fermi Sea

By analyzing breathing mode of a Bose-Einstein condensate repulsively interacting with a polarized fermionic cloud, we further the understanding of a Bose-Fermi mixture recently realized by Lous et al. [\textit{Phys. Rev. Lett.} \textbf{120}, 243403]. We show that a hydrodynamic description of a domain wall between bosonic and fermionic atoms reproduces experimental data of Huang et al. [\textit{Phys. Rev. A} \textbf{99}, 041602(R)]. Two different types of interaction renormalization are explored, based on lowest order constrained variational and perturbation techniques. In order to replicate nonmonotonic behavior of the oscillation frequency observed in the experiment, temperature effects have to be included. We find that the frequency down-shift is caused by the fermion-induced compression and rethermalization of the bosonic species as the system is quenched into the strongly interacting regime.

cond-mat.quant-gas↗

arXiv2020

Bistability of Bose-Fermi mixtures

We study the properties of the Bose-Fermi mixture from the perspective of reaching a state of a self-bound quantum droplet. The variational analysis shows that the system exhibits bistability. For weak repulsion between bosons, one of the equilibrium states, smaller in size, spherically symmetric, and with negative energy, corresponds to quantum droplet, the other with always positive energy represents the elongated droplet-like state immersed in the sea of a fermionic cloud. For stronger repulsion between bosons the bifurcation is seized and only the former state is left. Now it represents an elongated object which, for strong enough boson-fermion attraction, gets negative energy. It becomes an excited Bose-Fermi droplet when the trap is released, what is demonstrated by solving the quantum hydrodynamics equations for the Bose-Fermi system. To depict our ideas we consider the $^{133}$Cs-$^6$Li mixture under ideal conditions, i.e. we assume no losses.

cond-mat.quant-gas↗

arXiv2020

Fermionic quantum carpets: From canals and ridges to solitonlike structures

We report a formation of sharp, solitonlike structures in an experimentally accessible ultracold Fermi gas, as a quantum carpet solution is analyzed in a many body system. The effect is perfectly coherent in a noninteracting gas, but in the presence of repulsive interaction in a two-component system, the structures vanish at a finite time. As they disappear, the system enters a dynamical equilibrium, in which kinetic energies of atoms tend to the same average value. The coherence is revived in a strong interaction regime, with the onset of phase separation, and with a double quantum carpet appearing.

quant-ph↗

arXiv2020

Collective oscillations of a two-component Fermi gas on the repulsive branch

We calculate frequencies of collective oscillations of two-component Fermi gas that is kept on the repulsive branch of its energy spectrum. Not only is a paramagnetic phase explored, but also a ferromagnetically separated one. Both in-, and out-of-phase perturbations are investigated, showing contributions from various gas excitations. Additionally, we compare results coming from both time-dependent Hartree-Fock and density-functional approaches.

cond-mat.quant-gas↗

arXiv2020

Nonzero temperature dynamics of a repulsive two-component Fermi gas

We study spin-dipole oscillations of a binary fermionic mixture at nonzero temperatures. We apply the atomic-orbital method combined with the Monte Carlo technique based sampling to probe finite temperatures. Our results agree quantitatively with recent experiment, G. Valtolina et al., Nat. Phys. 13, 704 (2017), showing the appearance of the ferromagnetic phase at stronger repulsion between components when the temperature is increased.

cond-mat.quant-gas↗