arXiv subjects

H. Heiselberg

Publications and source records attributed to H. Heiselberg.

At least 19 recordsLinked to original sources

arXiv2012

Inhomogeneous spin diffusion in traps with cold atoms

The spin diffusion and damped oscillations are studied in the collision of two spin polarized clouds of cold atoms with resonant interactions. The strong density dependence of the diffusion coefficient leads to inhomogeneous spin diffusion that changes from central to surface spin flow as the temperature increases. The inhomogeneity and the smaller finite trap size significantly reduce the spin diffusion rate at low temperatures. The resulting spin diffusion rates, spin drag and initial damped oscillations are compatible with measurements at low to high temperatures for resonant attractive interactions but are incompatible with a metastable ferromagnetic phase.

cond-mat.quant-gas↗

arXiv2009

Induced interactions and superfluidity in optical lattices with multi-component Fermi gases

Many-body effects on superfluidity and transition temperatures are calculated for optical lattices and uniform systems with ultracold multi-component Fermi gases. The induced interactions depend sensitively on the interactions between the multi-components and their densities. The s- and d-wave pairing gaps and critical temperatures are calculated for optical lattices at low (dilute) filling as well as near half filling to leading orders in the interaction strength with and without induced interactions. They can deviate strongly from dilute two-component systems and affect the phase diagram. In two dimensional optical lattices the induced interactions are singular at half filling and strongly affect s- and d-wave superfluidity.

cond-mat.supr-con↗

arXiv2008

Hubbard model calculations of phase separation in optical lattices

Antiferromagnetic, Mott insulator, d-wave and gossamer superfluid phases are calculated for 2D square lattices from the extended Hubbard (t-J-U) model using the Gutzwiller projection method and renormalized mean field theory. Phase separation between antiferromagnetic and d-wave superfluid phases is found near half filling when the on-site repulsion exceeds $U\ga7.3t$, and coincides with a first order transition in the double occupancy. Phase separation is thus predicted for 2D optical lattices with ultracold Fermi atoms whereas it is inhibited in cuprates by Coulomb frustration which instead may lead to stripes. In a confined optical lattice the resulting density distribution is discontinuous an with extended Mott plateau which enhances the antiferromagnetic phase but suppresses the superfluid phase. Observation of Mott insulator, antiferromagnetic, stripe and superfluid phases in density and momentum distributions and correlations is discussed.

cond-mat.supr-con↗

arXiv2007

Super-shell structures and pairing in ultracold trapped Fermi gases

We calculate level densities and pairing gaps for an ultracold dilute gas of fermionic atoms in harmonic traps under the influence of mean field and anharmonic quartic trap potentials. Super-shell structures, which were found in Hartree-Fock calculations, are calculated analytically within periodic orbit theory as well as from WKB calculations. For attractive interactions, the underlying level densities are crucial for pairing and super-shell structures in gaps are predicted.

cond-mat.other↗

arXiv2006

Phases of Bosons or Fermions in confined optical lattices

Phases of Bose or Fermi atoms in optical lattices confined in harmonic traps are studied within the Thomas-Fermi approximation. Critical radii and particle number for onset of Mott insulator states are calculated and phase diagrams shown in 1D, and estimated for 2 and 3D. Methods to observe these and novel phases such as d-wave superconductivity is discussed. Specifically the collective modes are calculated.

cond-mat.supr-con↗

arXiv2005

Extended Bose-Hubbard model with incompressible states at fractional numbers

The Bose-Hubbard model is extended to include nearest and far neighbor interactions and is related to the fractional quantum Hall effect (FQHE). Both models may be studied in optical lattices with quantum gases. The ground state is calculated for the extended Bose-Hubbard model with strong repulsive interactions (weak hopping). Incompressible Mott insulator states are found at rational filling fractions compatible with the principal and secondary FQHE filling fractions of the lowest Landau levels observed experimentally. It is discussed to which extent these states at fractional filling survive or undergoes a Mott insulator transition to a superfluid as hopping terms are included.

cond-mat.mes-hall↗

arXiv2005

Sound modes at the BCS-BEC crossover

First and second sound speeds are calculated for a uniform superfluid gas of fermi atoms as a function of temperature, density and interaction strength. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature. The sound modes and their dependence on density, scattering length and temperature are calculated in the BCS, molecular BEC and unitarity limits and a smooth crossover is extrapolated. It is found that first and second sound undergo avoided crossing on the BEC side due to mixing. Consequently, they are detectable at crossover both as density and thermal waves in traps.

cond-mat.supr-con↗

arXiv2004

Second sound in Fermi gases at the BCS-BEC crossover

The thermodynamic potential is calculated for a uniform superfluid gas of fermi atoms from the mean field BCS equations including corrections from induced interactions, Hartree-Fock energies and quasiparticle selfenergies. The entropy, specific heat and sound modes are calculated as function of temperature, density and interaction strength from the BCS to the unitarity limit and around the BCS-BEC crossover. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature.

cond-mat.supr-con↗

arXiv2004

Pairing gaps in atomic gases at the BCS-BEC crossover

Strong evidence for pairing and superfluidity has recently been found in atomic Fermi gases at the BCS-BEC crossover both in collective modes and RF excitation energies. It is argued that the scale for the effective pairing gaps measured in RF experiments is set by the lowest quasiparticle in-gap excitation energies. These are calculated at the BCS-BEC crossover from semiclassical solutions to the Bogoliubov-deGennes equations. The strong damping of the radial breathing mode observed in the BCS limit occur when the lowest quasiparticle excitation energies coincide with the radial frequency, which indicates that a coupling between them take place.

cond-mat.supr-con↗

arXiv2004

Collective modes of trapped gases at the BEC-BCS crossover

The collective mode frequencies in isotropic and deformed traps are calculated for general polytropic equation of states, $P\propto n^{γ+1}$, and expressed in terms of $γ$ and the trap geometry. For molecular and standard Bose-Einstein condensates and Fermi gases near Feshbach resonances, the effective power $γ\simeq0.5-1.3$ is calculated from Jastrow type wave-function ansätze, and from the crossover model of Leggett. The resulting mode frequencies are calculated for these phases around the BCS-BEC crossover.

cond-mat.soft↗

arXiv2004

Bosons and Fermions near Feshbach resonances

Near Feshbach resonances, $n|a|^3\gg 1$, systems of Bose and Fermi particles become strongly interacting/dense. In this unitary limit both bosons and fermions have very different properties than in a dilute gas, e.g., the energy per particle approach a value $\hbar^2n^{2/3}/m$ times an universal many-body constant. Calculations based upon an approximate Jastrow wave function can quantitatively describe recent measurements of trapped Bose and Fermi atoms near Feshbach resonances. The pairing gap between attractive fermions also scales as $Δ\sim\hbar^2n^{2/3}/m$ near Feshbach resonances and is a large fraction of the Fermi energy - promising for observing BCS superfluidity in traps. Pairing undergoes several transitions depending on interaction strength and the number of particles in the trap and can also be compared to pairing in nuclei.

cond-mat.supr-con↗

arXiv2003

Pairing of fermions in atomic traps and nuclei

Pairing gaps for fermionic atoms in harmonic oscillator traps are calculated for a wide range of interaction strengths and particle number, and compared to pairing in nuclei. Especially systems, where the pairing gap exceeds the level spacing but is smaller than the shell splitting $\hbarω$, are studied which applies to most trapped Fermi atomic systems as well as to finite nuclei. When solving the gap equation for a large trap with such multi-level pairing, one finds that the matrix elements between nearby harmonic oscillator levels and the quasi-particle energies lead to a double logarithm of the gap, and a pronounced shell structure at magic numbers. It is argued that neutron and proton pairing in nuclei belongs to the class of multi-level pairing, that their shell structure follows naturally and that the gaps scale as $\sim A^{-1/3}$ - all in qualitative agreement with odd-even staggering of nuclear binding energies. Pairing in large systems are related to that in the bulk limit. For large nuclei the neutron and proton superfluid gaps approach the asymptotic value in infinite nuclear matter: $Δ\simeq 1.1$ MeV.

physics.atom-ph↗

arXiv2002

Cooper pairing and single particle properties of trapped Fermi gases

We calculate the elementary excitations and pairing of a trapped atomic Fermi gas in the superfluid phase. The level spectra and pairing gaps undergo several transitions as the strength of the interactions between and the number of atoms are varied. For weak interactions, the Cooper pairs are formed between particles residing in the same harmonic oscillator shell. In this regime, the nature of the paired state is shown to depend critically on the position of the chemical potential relative to the harmonic oscillator shells and on the size of the mean field. For stronger interactions, we find a region where pairing occur between time-reversed harmonic oscillator states in different shells also.

cond-mat↗

arXiv2002

The few-body problem for trapped bosons with large scattering length

We calculate energy levels of two and three bosons trapped in a harmonic oscillator potential with oscillator length $a_{\mathrm osc}$. The atoms are assumed to interact through a short-range potential with a scattering length $a$, and the short-distance behavior of the three-body wave function is characterized by a parameter $\theta$. For large positive $a/a_{\mathrm osc}$, the energies of states which, in the absence of the trap, correspond to three free atoms approach values independent of $a$ and $\theta$. For other states the $\theta$ dependence of the energy is strong, but the energy is independent of $a$ for $|a/a_{\mathrm osc}|\gg1$.

cond-mat↗

arXiv2001

Cold Bose gases with large scattering lengths

We calculate the energy and condensate fraction for a dense system of bosons interacting through an attractive short range interaction with positive s-wave scattering length $a$. At high densities, $n>>a^{-3}$, the energy per particle, chemical potential, and square of the sound speed are independent of the scattering length and proportional to $n^{2/3}$, as in Fermi systems.

cond-mat↗

arXiv2001

Event-by-event physics in relativistic heavy ion collisions

Motivated by forthcoming experiments at RHIC and LHC, and results from SPS, a review is given of the present state of event-by-event fluctuations in ultrarelativistic heavy-ion collisions. Fluctuations in particle multiplicities, ratios, transverse momenta, rapidity, etc. are calculated in participant nucleon as well as thermal models. The physical observables, including multiplicity, kaon to pion ratios, and transverse momenta agree well with recent NA49 data at the SPS, and indicate that such studies do not yet reveal the presence of new physics. Predictions for RHIC and LHC energies are given. The centrality dependence with and without a phase transition to a quark-gluon plasma is discussed - in particular, how the physical quantities are expected to display a qualitative different behavior in case of a phase transition, and can be signaled by anomalous fluctuations and correlations in a number of observables.

nucl-th↗

arXiv2001

Anomalous Multiplicity Fluctuations from Phase Transitions in Heavy Ion Collisions

Event-by-event fluctuations and correlations between particles produced in relativistic nuclear collisions are studied. The fluctuations in positive, negative, total and net charge are closely related through correlations. In the event of a phase transitions to a quark-gluon plasma, fluctuations in total and net charge can be enhanced and reduced respectively which, however, is very sensitive to the acceptance and centrality. If the colliding system experiences strong density fluctuations due, e.g., to droplet formation in a first-order phase transition, all fluctuations can be enhanced substantially. The importance of fluctuations and correlations is exemplified by event-by-event measurement of the multiplicities of $J/Ψ$'s and charged particles since these observables should anti-correlate in the presence of co-mover or anomalous absorption.

nucl-th↗

arXiv2000

Elliptic flow at SPS and RHIC: from kinetic transport to hydrodynamics

Anisotropic transverse flow is studied in Pb+Pb and Au+Au collisions at SPS and RHIC energies. The centrality and transverse momentum dependence at midrapidity of the elliptic flow coefficient v_2 is calculated in the hydrodynamic and low density limits. Hydrodynamics is found to agree well with the RHIC data for semicentral collisions up to transverse momenta of 1-1.5 GeV/c, but it considerably overestimates the measured elliptic flow at SPS energies. The low density limit LDL is inconsistent with the measured magnitude of v_2 at RHIC energies and with the shape of its p_t-dependence at both RHIC and SPS energies. The success of the hydrodynamic model points to very rapid thermalization in Au+Au collisions at RHIC and provides a serious challenge for kinetic approaches based on classical scattering of on-shell particles.

hep-ph↗