Łukasz Zawitkowski, Mariusz Gajda, and Kazimierz Rzewski
We use the classical fields approximation to study a translational flow of the condensate with respect to the thermal cloud in a weakly interacting Bose gas confined in a three-dimensional box. We study both subcritical and supercritical relative velocity cases and analyze in detail a state of stationary flow which is reached in the dynamics. This state corresponds to the thermal equilibrium, which is characterized by the relative velocity of the condensate and the thermal cloud. We observe two processes—re-thermalization and drag, both of which lead to a reduction of a relative velocity of the superflow. Yet only the drag process, which is observed above the critical velocity vcr , results in transferring a Bose-Einstein condensate to a slower moving mode. In this case the relative velocity of the flow suddenly drops to a value close to zero. Finally, we report the critical velocity to be for our parameters vcr=0.21c for the initial condition and vcreq=0.12c for re-thermalized superflow ( c being the Landau speed of sound), which is strikingly lower than Landau critical velocity, yet consistent with experiments.
Phys. Rev. A vol. 74, 043601 - 1-7 (2006) [1/3]
Dec 8, 2005
Dec 1, 2005
Dynamics of quasisolitons in degenerate fermionic gases
Emilia Witkowska and Mirosław Brewczyk
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.
Phys. Rev. A 72, 023606 (2005)
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.
Phys. Rev. A 72, 023606 (2005)
Nov 18, 2005
Superluminal propagation of solitary kinklike waves in amplifying media
Maciej Janowicz and Jan Mostowski
It is shown that solitary-wave, kinklike structures can propagate superluminally in two- and four-level amplifying media with strongly damped oscillations of coherences. This is done by solving analytically the Maxwell-Bloch equations in the kinetic limit. It is also shown that the true wave fronts—unlike the pseudo wave fronts of the kinks—must propagate with velocity c, so that no violation of special relativity is possible. The conditions of experimental verification are discussed.
Phys. Rev. E vol. 73, 046613 - 1-9 (2006) [2/2]
It is shown that solitary-wave, kinklike structures can propagate superluminally in two- and four-level amplifying media with strongly damped oscillations of coherences. This is done by solving analytically the Maxwell-Bloch equations in the kinetic limit. It is also shown that the true wave fronts—unlike the pseudo wave fronts of the kinks—must propagate with velocity c, so that no violation of special relativity is possible. The conditions of experimental verification are discussed.
Phys. Rev. E vol. 73, 046613 - 1-9 (2006) [2/2]
Nov 2, 2005
Beams of electromagnetic radiation carrying angular momentum: The Riemann–Silberstein vector and the classical–quantum correspondence
Iwo Bialynicki-Birula, and Zofia Bialynicka-Birula
All beams of electromagnetic radiation are made of photons. Therefore, it is important to find a precise relationship between the classical properties of the beam and the quantum characteristics of the photons that make a particular beam. It is shown that this relationship is best expressed in terms of the Riemann–Silberstein vector – a complex combination of the electric and magnetic field vectors – that plays the role of the photon wave function. The Whittaker representation of this vector in terms of a single complex function satisfying the wave equation greatly simplifies the analysis. Bessel beams, exact Laguerre–Gauss beams, and other related beams of electromagnetic radiation can be described in a unified fashion. The appropriate photon quantum numbers for these beams are identified. Special emphasis is put on the angular momentum of a single photon and its connection with the angular momentum of the beam.
Opt. Commun. vol. 264, 342-351 (2006) [1/2]
All beams of electromagnetic radiation are made of photons. Therefore, it is important to find a precise relationship between the classical properties of the beam and the quantum characteristics of the photons that make a particular beam. It is shown that this relationship is best expressed in terms of the Riemann–Silberstein vector – a complex combination of the electric and magnetic field vectors – that plays the role of the photon wave function. The Whittaker representation of this vector in terms of a single complex function satisfying the wave equation greatly simplifies the analysis. Bessel beams, exact Laguerre–Gauss beams, and other related beams of electromagnetic radiation can be described in a unified fashion. The appropriate photon quantum numbers for these beams are identified. Special emphasis is put on the angular momentum of a single photon and its connection with the angular momentum of the beam.
Opt. Commun. vol. 264, 342-351 (2006) [1/2]
Oct 24, 2005
Kinetic lattice-gas model approach to collective diffusion in an ordered adsorbate in two dimensions.
Magdalena A. Zauska-Kotur and Zbigniew W. Gortel
A recently developed approach to microscopic kinetics of an interacting lattice gas is applied to derive an algebraic expression for the coverage dependence of the collective diffusion coefficient in a two-dimensional (2D) adsorbate populating a square lattice of adsorption sites with strong adatom-adatom repulsive nearest-neighbor interactions. Results are valid below the critical temperature for coverages at which the adsorbate is structurally ordered. Interactions between nonactivated particles as well as those between the activated one and its nonactivated neighbors are accounted for. The starting point is Markovian master equations for the kinetics of microscopic states of the system, controlled by jumps of adatoms between adsorption sites. The diffusion coefficient is extracted in the long wavelength and the thermodynamic limits from the diffusive eigenvalue of a microscopic rate matrix associated with the equations. The eigenvalue is evaluated using an Anzatz for the left and the right eigenvectors of the matrix with the adsorbate ordering inscribed into their structure. The results are validated by Monte Carlo simulations of the diffusion process.
Phys. Rev. B 72, 235425 (2005)
A recently developed approach to microscopic kinetics of an interacting lattice gas is applied to derive an algebraic expression for the coverage dependence of the collective diffusion coefficient in a two-dimensional (2D) adsorbate populating a square lattice of adsorption sites with strong adatom-adatom repulsive nearest-neighbor interactions. Results are valid below the critical temperature for coverages at which the adsorbate is structurally ordered. Interactions between nonactivated particles as well as those between the activated one and its nonactivated neighbors are accounted for. The starting point is Markovian master equations for the kinetics of microscopic states of the system, controlled by jumps of adatoms between adsorption sites. The diffusion coefficient is extracted in the long wavelength and the thermodynamic limits from the diffusive eigenvalue of a microscopic rate matrix associated with the equations. The eigenvalue is evaluated using an Anzatz for the left and the right eigenvectors of the matrix with the adsorbate ordering inscribed into their structure. The results are validated by Monte Carlo simulations of the diffusion process.
Phys. Rev. B 72, 235425 (2005)
Oct 4, 2005
Collective diffusion in a interacting one-dimensional lattice gas: Arbitrary interactions, activation energy, and nonequilibrium diffusion
Łukasz Badowski and Magdalena A. Załuska-Kotur
Collective diffusion is investigated within the kinetic lattice gas model for a system of interacting particles in one dimension. Analytic relations between the collective diffusion coefficient, diffusion activation energy, the attempt frequency pre-exponential factor, vs the particle density for both attractive and repulsive particle-particle interactions of an arbitrary strength are derived using the recently proposed [Phys. Rev. B 70, 125431 (2004)] variational method. The analytic results agree with results of Monte Carlo simulations within a broad range of temperatures. At low coverages for strongly repulsive interactions the activation energy is roughly equal to its value for the noninteracting system but around =0.5 it decreases rapidly by more than strictly accounted for by adparticle-adparticle interactions. Only at significantly higher coverages it increases reaching the expected limiting value. Peaks in the coverage dependence of the effective attempt frequency (for both the repulsive and the attractive interactions) are interpreted to reflect peaks in the total number of microscopic configurations accessible to the system at a given coverage and temperature. It is argued that the method used in this work allows for making meaningful estimates of the diffusion coefficient for systems far from thermal equilibrium.
Phys. Rev. B 72, 245413 (2005)
Collective diffusion is investigated within the kinetic lattice gas model for a system of interacting particles in one dimension. Analytic relations between the collective diffusion coefficient, diffusion activation energy, the attempt frequency pre-exponential factor, vs the particle density for both attractive and repulsive particle-particle interactions of an arbitrary strength are derived using the recently proposed [Phys. Rev. B 70, 125431 (2004)] variational method. The analytic results agree with results of Monte Carlo simulations within a broad range of temperatures. At low coverages for strongly repulsive interactions the activation energy is roughly equal to its value for the noninteracting system but around =0.5 it decreases rapidly by more than strictly accounted for by adparticle-adparticle interactions. Only at significantly higher coverages it increases reaching the expected limiting value. Peaks in the coverage dependence of the effective attempt frequency (for both the repulsive and the attractive interactions) are interpreted to reflect peaks in the total number of microscopic configurations accessible to the system at a given coverage and temperature. It is argued that the method used in this work allows for making meaningful estimates of the diffusion coefficient for systems far from thermal equilibrium.
Phys. Rev. B 72, 245413 (2005)
Aug 16, 2005
From Randomness to Periodicity --- the Effect of Polarization in the Minority Game Strategy Space
M. Zaluska-Kotur
Properties and behavior of modified minority game are analyzed. It appears that results of the game depend strongly on the way how we draw strategies for players. The probability that given strategy will be chosen is determined by the polarization parameter P. This parameter differs between strategies that go along or against the existing market trend. Strong polarization of the space leads to the periodic dynamics and, finally, for negative P values to the domination of the single strategy in the system. When P is changed the variability of the process decreases, showing kind of the phase transition region. The dependence of the variability on the polarization parameter can be understood on the basis of the crowd--anticrowd theory.
Acta Phys B Vol. 36, No. 8, (2005)
Properties and behavior of modified minority game are analyzed. It appears that results of the game depend strongly on the way how we draw strategies for players. The probability that given strategy will be chosen is determined by the polarization parameter P. This parameter differs between strategies that go along or against the existing market trend. Strong polarization of the space leads to the periodic dynamics and, finally, for negative P values to the domination of the single strategy in the system. When P is changed the variability of the process decreases, showing kind of the phase transition region. The dependence of the variability on the polarization parameter can be understood on the basis of the crowd--anticrowd theory.
Acta Phys B Vol. 36, No. 8, (2005)
Aug 5, 2005
Emission of Thermally Activated Electrons from Rare Gas Clusters Irradiated with Intense VUV Light Pulses from a Free Electron Laser
Laarmann T, Rusek M, Wabnitz H, Schulz J, de Castro AR, Gürtler P, Laasch W, Möller T
The ionization dynamics of Ar and Xe clusters irradiated with intense vacuum ultraviolet light from a free-electron laser is investigated using photoelectron spectroscopy. Clusters comprising between 70 and 900 atoms were irradiated with femtosecond pulses at 95 nm wavelength (~13 eV photon energy) and a peak intensity of ~4×1012 W/cm2. A broad thermal distribution of emitted electrons from clusters with a maximum kinetic energy up to 30–40 eV is observed. The observation of relatively low-energy photoelectrons is in good agreement with calculations using a time-dependent Thomas-Fermi model and gives experimental evidence of an outer ionization process of the clusters, due to delayed thermoelectronic emission.
Phys Rev Lett. 2005 Aug 5;95(6):063402. Epub 2005 Aug 5
The ionization dynamics of Ar and Xe clusters irradiated with intense vacuum ultraviolet light from a free-electron laser is investigated using photoelectron spectroscopy. Clusters comprising between 70 and 900 atoms were irradiated with femtosecond pulses at 95 nm wavelength (~13 eV photon energy) and a peak intensity of ~4×1012 W/cm2. A broad thermal distribution of emitted electrons from clusters with a maximum kinetic energy up to 30–40 eV is observed. The observation of relatively low-energy photoelectrons is in good agreement with calculations using a time-dependent Thomas-Fermi model and gives experimental evidence of an outer ionization process of the clusters, due to delayed thermoelectronic emission.
Phys Rev Lett. 2005 Aug 5;95(6):063402. Epub 2005 Aug 5
Jul 11, 2005
Phase fluctuations of a Bose-Einstein condensate in low-dimensional geometry
Demascoth Kadio, Mariusz Gajda and Kazimierz Rzążewski
We investigate the phase fluctuations of a Bose-Einstein condensate in one- (1D) and three-dimensional (3D) elongated harmonic traps with the help of the numerical simulation using classical field approximation. By calculating the eigenvalues of the one-dimensional single-particle density matrix we show that the phase fluctuations of a Bose-Einstein Condensate in a 1D system are due to the thermal low-energy excitations. We find that the phase fluctuations depend on the temperature as has been predicted [Phys. Rev. Lett. 85, 3745 (2000), Phys. Rev. Lett. 87, 050404 (2001)]. Finally we show that the phase coherence length of a condensate in a harmonic trap depends on the aspect ratio of anisotropy (geometry) of the trap. We determine the border between the 3D and quasi-one-dimensional systems by calculating the phase coherence length of the condensate.
Phys. Rev. A 72, 013607 (2005)
We investigate the phase fluctuations of a Bose-Einstein condensate in one- (1D) and three-dimensional (3D) elongated harmonic traps with the help of the numerical simulation using classical field approximation. By calculating the eigenvalues of the one-dimensional single-particle density matrix we show that the phase fluctuations of a Bose-Einstein Condensate in a 1D system are due to the thermal low-energy excitations. We find that the phase fluctuations depend on the temperature as has been predicted [Phys. Rev. Lett. 85, 3745 (2000), Phys. Rev. Lett. 87, 050404 (2001)]. Finally we show that the phase coherence length of a condensate in a harmonic trap depends on the aspect ratio of anisotropy (geometry) of the trap. We determine the border between the 3D and quasi-one-dimensional systems by calculating the phase coherence length of the condensate.
Phys. Rev. A 72, 013607 (2005)
Criterion for Bose-Einstein condesation in a harmonic trap in the case with attractive interactions
Mariusz Gajda
Using a model many-body wave function I analyze the standard criterion for Bose-Einstein condensation and its relation to coherence properties of the system. I pay special attention to an attractive condensate under such a condition that a characteristic length scale of the spatial extension of its center of mass differs significantly from length scales of relative coordinates. I show that although no interference fringes are produced in the two-slit Young interference experiment performed on this system, fringes of a high visibility can be observed in a conditional simultaneous detection of two particles.
Phys. Rev. A vol. 73, 023603 - 1-5 (2006) [1/1]
Using a model many-body wave function I analyze the standard criterion for Bose-Einstein condensation and its relation to coherence properties of the system. I pay special attention to an attractive condensate under such a condition that a characteristic length scale of the spatial extension of its center of mass differs significantly from length scales of relative coordinates. I show that although no interference fringes are produced in the two-slit Young interference experiment performed on this system, fringes of a high visibility can be observed in a conditional simultaneous detection of two particles.
Phys. Rev. A vol. 73, 023603 - 1-5 (2006) [1/1]
Jun 13, 2005
On the stability of Bose-Fermi mixtures
T Karpiuk, M Brewczyk, M Gajda and K Rzążewski
We consider the stability of a mixture of degenerate Bose and Fermi gases. Even though the bosons effectively repel each other, the mixture can still collapse provided the Bose and Fermi gases attract each other strongly enough. For a given number of atoms and the strengths of the interactions between them, we find the geometry of a maximally compact trap that supports the stable mixture. We compare a simple analytical estimation for the critical axial frequency of the trap with results based on the numerical solution of hydrodynamic equations for the Bose–Fermi mixture.
J. Phys. B: At. Mol. Opt. Phys. 38 L215-L221
We consider the stability of a mixture of degenerate Bose and Fermi gases. Even though the bosons effectively repel each other, the mixture can still collapse provided the Bose and Fermi gases attract each other strongly enough. For a given number of atoms and the strengths of the interactions between them, we find the geometry of a maximally compact trap that supports the stable mixture. We compare a simple analytical estimation for the critical axial frequency of the trap with results based on the numerical solution of hydrodynamic equations for the Bose–Fermi mixture.
J. Phys. B: At. Mol. Opt. Phys. 38 L215-L221
May 24, 2005
Semi-classical approaches to the ion-atom scattering
Maciej Janowicz, Katarzyna Słabkowska, Piotr Matuszak and Marek Polasik
Three semi-classical approaches to the simulation of ion–atom scattering: the Kirschbaum–Wilets, the Cohen energy-bounded, and the wave-packet – are used to calculate the ionization and charge-transfer cross sections for the scattering of fully stripped sulfur ions on atomic hydrogen. Their comparison in the high-energy regime is provided. Elements of microscopic analysis of Kirschbaum–Wilets trajectories are given.
Nuclear Instruments and Methods in Physics Research Section B Vol 235, 1-4, (2005)
Three semi-classical approaches to the simulation of ion–atom scattering: the Kirschbaum–Wilets, the Cohen energy-bounded, and the wave-packet – are used to calculate the ionization and charge-transfer cross sections for the scattering of fully stripped sulfur ions on atomic hydrogen. Their comparison in the high-energy regime is provided. Elements of microscopic analysis of Kirschbaum–Wilets trajectories are given.
Nuclear Instruments and Methods in Physics Research Section B Vol 235, 1-4, (2005)
Apr 14, 2005
Different mechanisms of cluster explosion within an unified time-dependent Thomas-Fermi approach: optical and short-wavelength regimes compared
Marian Rusek and Arkadiusz Orlowski
The dynamics of small (55 atoms) argon clusters ionized by an intense femtosecond laser pulse is studied using a time-dependent Thomas-Fermi model. The resulting Bloch-like hydrodynamic equations are solved numerically using the smooth particle hydrodynamics method without the necessity of grid simulations. As follows from recent experiments, absorption of radiation and subsequent ionization of clusters observed in the short-wavelength laser frequency regime (98 nm) differs considerably from that in the optical spectral range (800 nm). Our theoretical approach provides a unified framework for treating these very different frequency regimes and allows for a deeper understanding of the underlying cluster explosion mechanisms. The results of our analysis following from extensive numerical simulations presented in this paper are compared both with experimental findings and with predictions of other theoretical models.
Phys. Rev. A 71, 043202 (2005)
The dynamics of small (55 atoms) argon clusters ionized by an intense femtosecond laser pulse is studied using a time-dependent Thomas-Fermi model. The resulting Bloch-like hydrodynamic equations are solved numerically using the smooth particle hydrodynamics method without the necessity of grid simulations. As follows from recent experiments, absorption of radiation and subsequent ionization of clusters observed in the short-wavelength laser frequency regime (98 nm) differs considerably from that in the optical spectral range (800 nm). Our theoretical approach provides a unified framework for treating these very different frequency regimes and allows for a deeper understanding of the underlying cluster explosion mechanisms. The results of our analysis following from extensive numerical simulations presented in this paper are compared both with experimental findings and with predictions of other theoretical models.
Phys. Rev. A 71, 043202 (2005)
Feb 16, 2005
Trojan States of Electrons Guided by Bessel Beams
Iwo Bialynicki-Birula, Zofia Bialynicka-Birula and Bartosz Chmura
Previous work [I. Bialynicki-Birula, Phys. Rev. Lett. {\bf 93}, 20402 (2004)] is extended to cover more realistic examples of electromagnetic waves, viz. the Bessel beams. It is shown that electrons may be guided by a Bessel beam with nonvanishing orbital angular momentum. The mechanism for trapping the electrons near the electromagnetic vortex line of such a wave field is the same as for the Trojan states of Rydberg electrons produced by a circularly polarized wave. The main difference is that in the present case the transverse motion of electrons in a beam is confined under the action of the electromagnetic wave alone, no additional attraction center is required. We also discuss briefly the motion of electrons in Neumann and Hankel beams.
Physics/0502025 (February 2005)
Previous work [I. Bialynicki-Birula, Phys. Rev. Lett. {\bf 93}, 20402 (2004)] is extended to cover more realistic examples of electromagnetic waves, viz. the Bessel beams. It is shown that electrons may be guided by a Bessel beam with nonvanishing orbital angular momentum. The mechanism for trapping the electrons near the electromagnetic vortex line of such a wave field is the same as for the Trojan states of Rydberg electrons produced by a circularly polarized wave. The main difference is that in the present case the transverse motion of electrons in a beam is confined under the action of the electromagnetic wave alone, no additional attraction center is required. We also discuss briefly the motion of electrons in Neumann and Hankel beams.
Physics/0502025 (February 2005)
Free Electrons Generation in the Interaction of Intense Laser Pulses with Atomic Clusters
M. Rusek and A. Orłowski
Interaction of argon clusters with intense laser pulses is studied theoretically. Free electrons energy distribution is studied. Differences between infrared and vacuum ultraviolet frequency regimes are pointed out. Clear physical interpretation of the obtained results is given.
ACTA PHYS. POLONICA A Vol. 107 (2005)
Interaction of argon clusters with intense laser pulses is studied theoretically. Free electrons energy distribution is studied. Differences between infrared and vacuum ultraviolet frequency regimes are pointed out. Clear physical interpretation of the obtained results is given.
ACTA PHYS. POLONICA A Vol. 107 (2005)
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