arXiv subjects

Miroslaw Brewczyk

Publications and source records attributed to Miroslaw Brewczyk.

16 recordsLinked to original sources

arXiv2017

Unified way for computing dynamics of Bose-Einstein condensates and degenerate Fermi gases

In this work we present a very simple and efficient numerical scheme which can be applied to study the dynamics of bosonic systems like, for instance, spinor Bose-Einstein condensates with nonlocal interactions but equally well works for Fermi gases. The method we use is a modification of well known Split Operator Method (SOM). We carefully examine this algorithm in the case of $F=1$ spinor Bose-Einstein condensate without and with dipolar interactions and for strongly interacting two-component Fermi gas. Our extension of the SOM method has many advantages: it is fast, stable, and keeps constant all the physical constraints (constants of motion) at high level.

cs.CE↗

arXiv2016

Competition between Bose Einstein Condensation and spin dynamics

We study the impact of spin-exchange collisions on the dynamics of Bose-Einstein condensation, by rapidly cooling a chromium multi-component Bose gas. Despite relatively strong spin-dependent interactions, the critical temperature for Bose-Einstein condensation is reached before the spin-degrees of freedom fully thermalize. The increase in density due to Bose-Einstein condensation then triggers spin dynamics, hampering the formation of condensates in spin excited states. Small metastable spinor condensates are nevertheless produced, and manifest strong spin fluctuations.

cond-mat.quant-gas↗

arXiv2015

Density fluctuations in a quasi-one-dimensional Bose gas as observed in free expansion

We study, within a framework of the classical fields approximation, the density correlations of a weakly interacting expanding Bose gas for the whole range of temperatures across the Bose-Einstein condensation threshold. We focus on elongated quasi-one-dimensional systems where there is a huge discrepancy between the existing theory and experimental results (A. Perrin et al., Nature Phys. 8, 195 (2012)). We find that the density correlation function is not reduced for temperatures below the critical one as it is predicted for the ideal gas or for a weakly interacting system within the Bogoliubov approximation. This behavior of the density correlations agrees with the above mentioned experiment with the elongated system. Although the system was much larger then studied here we believe that the behavior of the density correlation function found there is quite generic. Our theoretical studies indicate also large density fluctuations in the trap in the quasicondensate regime where only phase fluctuations were expected. We argue that the enhanced density fluctuations can originate in the presence of interactions in the system, or more precisely in the existence of spontaneous dark solitons in the elongated gas at thermal equilibrium.

cond-mat.quant-gas↗

arXiv2011

Tunable dipolar resonances and Einstein-de Haas effect in a Rb-87 atoms condensate

We study a spinor condensate of Rb-87 atoms in F = 1 hyperfine state confined in an optical dipole trap. Putting initially all atoms in mF = 1 component we observe a significant transfer of atoms to other, initially empty Zeeman states exclusively due to dipolar forces. Because of conservation of a total angular momentum the atoms going to other Zeeman components acquire an orbital angular momentum and circulate around the center of the trap. This is a realization of Einstein-de Haas effect in a system of cold gases. We show that the transfer of atoms via dipolar interaction is possible only when the energies of the initial and the final sates are equal. This condition can be fulfilled utilizing a resonant external magnetic field, which tunes energies of involved states via the linear Zeeman effect. We found that there are many final states of different spatial density which can be tuned selectively to the initial state. We show a simple model explaining high selectivity and controllability of weak dipolar interactions in the condensate of Rb-87 atoms.

cond-mat.quant-gas↗

arXiv2010

How to observe dipolar effects in spinor Bose-Einstein condensates

We study a spinor condensate of alkali atoms in F = 1 hyperfine state under the presence of an oscillating magnetic field. We find resonances which, due to the dipolar interactions, magnify the transfer of atoms from mF = 1 to mF = 0 Zeeman sublevel. These resonances occur at magnetic fields of the order of milligaus and are broad enough to enable observation of the famous Einstein-de Haas effect, which is solely a dipolar effect, in systems of cold alkali gases.

cond-mat.quant-gas↗

arXiv2010

Exact dynamics and decoherence of two cold bosons in a 1D harmonic trap

We study dynamics of two interacting ultra cold Bose atoms in a harmonic oscillator potential in one spatial dimension. Making use of the exact solution of the eigenvalue problem of a particle in the delta-like potential we study time evolution of initially separable state of two particles. The corresponding time dependent single particle density matrix is obtained and diagonalized and single particle orbitals are found. This allows to study decoherence as well as creation of entanglement during the dynamics. The evolution of the orbital corresponding to the largest eigenvalue is then compared to the evolution according to the Gross-Pitaevskii equation. We show that if initially the center of mass and relative degrees of freedom are entangled then the Gross-Pitaevskii equation fails to reproduce the exact dynamics and entanglement is produced dynamically. We stress that predictions of our study can be verified experimentally in an optical lattice in the low-tunneling limit.

quant-ph↗

arXiv2009

Constructing classical field for a Bose-Einstein condensate in arbitrary trapping potential; quadrupole oscillations at nonzero temperatures

We optimize the classical field approximation of the version described in J. Phys. B 40, R1 (2007) for the oscillations of a Bose gas trapped in a harmonic potential at nonzero temperatures, as experimentally investigated by Jin et al. [Phys. Rev. Lett. 78, 764 (1997)]. Similarly to experiment, the system response to external perturbations strongly depends on the initial temperature and on the symmetry of perturbation. While for lower temperatures the thermal cloud follows the condensed part, for higher temperatures the thermal atoms oscillate rather with their natural frequency, whereas the condensate exhibits a frequency shift toward the thermal cloud frequency (m=0 mode), or in the opposite direction (m=2 mode). In the latter case, for temperatures approaching critical, we find that the condensate begins to oscillate with the frequency of the thermal atoms, as in the m=0 mode. A broad range of frequencies of the perturbing potential is considered.

cond-mat.quant-gas↗

arXiv2008

Decay of multiply charged vortices at nonzero temperatures

We study the instability of multiply charged vortices in the presence of thermal atoms and find various scenarios of splitting of such vortices. The onset of the decay of a vortex is always preceded by the increase of a number of thermal (uncondensed) atoms in the system and manifests itself by the sudden rise of the amplitude of the oscillations of the quadrupole moment. Our calculations show that the decay time gets shorter when the multiplicity of a vortex becomes higher.

cond-mat.other↗

arXiv2007

Coherence properties of spinor condensates at finite temperatures

We consider a spinor condensate of 87Rb atoms in its F=1 hyperfine state at finite temperatures. Putting initially all atoms in m_F=0 component we find that the system evolves into the state of thermal equilibrium. This state is approached in a step-like process and when established it manifests itself in distinguishable ways. The atoms in states m_F=+1 and m_F=-1 start to rotate in opposite directions breaking the chiral symmetry and showing highly regular spin textures. Also the coherence properties of the system changes dramatically. Depending on the strength of spin-changing collisions the system first enters the stage where the m_F=+1 and m_F=-1 spinor condensate components periodically loose and recover their mutual coherence whereas their thermal counterparts get completely dephased. For stronger spin changing collisions the system enters the regime where also the strong coherence between other components is built up.

cond-mat.other↗

arXiv2006

Bright solitons in Bose-Fermi mixtures

We consider the formation of bright solitons in a mixture of Bose and Fermi degenerate gases confined in a three-dimensional elongated harmonic trap. The Bose and Fermi atoms are assumed to effectively attract each other whereas bosonic atoms repel each other. Strong enough attraction between bosonic and fermionic components can change the character of the interaction within the bosonic cloud from repulsive to attractive making thus possible the generation of bright solitons in the mixture. On the other hand, such structures might be in danger due to the collapse phenomenon existing in attractive gases. We show, however, that under some conditions (defined by the strength of the Bose-Fermi components attraction) the structures which neither spread nor collapse can be generated. For elongated enough traps the formation of solitons is possible even at the ``natural'' value of the mutual Bose-Fermi ($^{87}$Rb -$^{40}$K in our case) scattering length.

cond-mat.other↗

arXiv2005

Dynamics of quasisolitons in degenerate fermionic gases

We investigate the dynamics of the system of multiple bright and dark quasisolitons generated in a one-component ultracold Fermi gas via the phase imprinting technique in terms of atomic orbitals approach. In particular, we analyze the collision between two bright quasisolitons and find that quasisolitons are subject to the superposition principle.

cond-mat.other↗

arXiv2004

Classical fields approximation for cold weakly interacting bosons without free parameters

Classical fields approximation to cold weakly interacting bosons allows for a unified treatment of condensed and uncondensed parts of the system. Until now, however, the quantitative predictions were limited by a dependence of the results on a grid chosen for numerical implementation of the method. In this paper we propose replacing this unphysical ambiguity by an additional postulate: the temperature of the gas at thermal equilibrium should be the same as that of an ideal Bose gas with the same fraction of condensed atoms. As it turns-out, with this additional assumption, nearly all atoms are within the classical fields, thus the method applies to the whole system.

cond-mat.other↗

arXiv2004

Ground state of two-component degenerate fermionic gases

We analyze the ground state of the two--component gas of trapped ultracold fermionic atoms. We neglect the forces between atoms in the same hyperfine state (the same component). For the case when the forces between distinguishable atoms (i.e., atoms in different hyperfine states) are repulsive (positive mutual scattering length), we find the existence of critical interaction strength above which one atomic fraction expels the other from the center of the trap. When atoms from different components attract each other (negative mutual scattering length) the ground state of the system dramatically changes its structure for strong enough attraction -- the Cooper pairs built of atoms in different hyperfine states appear.

cond-mat.soft↗

arXiv2003

Temperature dependent Bogoliubov approximation in the classical fields approach to weakly interacting Bose gas

A classical fields approximation to the finite temperature microcanonical thermodynamics of weakly interacting Bose gas is applied to the idealized case of atoms confined in a box with periodic boundary conditions. We analyze in some detail the microcanonical temperature in the model. We also analyze the spectral properties of classical amplitudes of the plane waves -- the eigenmodes of the time averaged one--particle density matrix. Looking at the zero momentum component -- the order parameter of the condensate, we obtain the nonperturbative results for the chemical potential. Analogous analysis of the other modes yields nonperturbative temperature dependent Bogoliubov frequencies and their damping rates. Damping rates are linear functions of momenta in the phonon range and show more complex behavior for the particle sector. Where available, we make comparison with the analytic estimates of these quantities.

cond-mat↗

arXiv2002

Dynamics of optically generated vortices in a one-component ultracold fermionic gases

We show that the phase imprinting method is capable of generating vortices in a one-component gas of neutral fermionic atoms at zero and finite temperatures. We find qualitative differences in dynamics of vortices in comparision with the case of Bose-Einstein condensate. The results of the imprinting strongly depend on the geometry of the trap, e.g., in asymmetric traps no single vortex state exists. The observations could be considered as a signature of Cooper-pair based superfluidity in a Fermi gas.

cond-mat↗

arXiv1996

Multielectron dissociative ionization of molecules by intense laser radiation

We solve the hydrodynamic-ballistic equations of motion for a one-dimensional time-dependent Thomas-Fermi model of Cl_2 exposed to an intense subpicosecond laser field, and observe simultaneous multielectron ionization and molecular dissociation. The fragment kinetic energy defect with respect to the simple Coulomb explosion picture is found to originate in ejected-electron screening of the escaping fragments; its magnitude agrees with that observed in recent experiments.

physics.atom-ph↗