Normal-superfluid Transition Research Articles

Momentum Distribution of Liquid $$^{4}$$ 4 He Across the Normal–Superfluid Phase Transition

We have carried out a study of the momentum distribution and of the spectrum of elementary excitations of liquid $^4$He across the normal-superfluid transition temperature, using the path integral Monte Carlo method. Our results for the momentum distribution in the superfluid regime show that a kink is present in the range of momenta corresponding to the roton excitation. This effect disappears when crossing the transition temperature to the normal fluid, in a behavior currently unexplained by theory.

Dynamic structure factor of liquidHe4across the normal-superfluid transition

We have carried out a microscopic study of the dynamic structure factor of liquid $^4$He across the normal-superfluid transition temperature using the path integral Monte Carlo method. The ill-posed problem of the inverse Laplace transform, from the imaginary-time intermediate scattering function to the dynamic response, is tackled by stochastic optimization. Our results show a quasi-particle peak and a small and broad multiphonon contribution. In spite of the lack of strength in the collective peaks, we clearly identify the rapid dropping of the roton peak amplitude when crossing the transition temperature $T_\lambda$. Other properties such as the static structure factor, static response, and one-phonon contribution to the response are also calculated at different temperatures. The changes of the phonon-roton spectrum with the temperature are also studied. An overall agreement with available experimental data is achieved.

Depression of the normal-superfluid transition temperature in gated bilayer graphene

It is shown that the normal-superfluid transition in bilayer graphene predicted to occur at a high temperature is strongly affected not only by the dielectric constants of the substrate, interlayer, and gate insulators but also by the proximity of ideal metal gates. Even assuming optimistically a completely unscreened interlayer Coulomb interaction—thus bypassing the controversial problems regarding the proper way to screen the interlayer Coulomb interactions—it is shown that employing a gate-insulator thickness smaller than about 2-to-5 nm of equivalent SiO2-thickness pushes the transition temperature significantly below 300 K to the 1 K–1 mK range, depending on the dielectric constant of the gate insulator and on the dielectric mismatch of the insulators employed. These results imply that thicker and low-dielectric-constant gate insulators should be employed to observe the phase transition, but exploiting the superfluid state of gated graphene-bilayers in room-temperature device applications may be challenging.

A Ginzburg-Landau model in superfluidity

A dynamical model for the normal-superfluid transition in 4He, accounting for variations of temperature and pressure, is proposed. The phenomenon is analyzed in the framework of Ginzburg-Landau theory characterizing the phase of the material with an order parameter related to the concentration of the superfluid particles. The constitutive choices allow one to prove the existence of a critical velocity above which superfluid properties disappear and to recover the usual phase diagram of liquid helium. The thermodynamic restrictions in the constitutive relations which ensure the Clausius-Duhem inequality have been pointed out.

Superfluid-superfluid phase transitions in a two-component Bose-Einstein condensate.

Depending on the Hamiltonian parameters, two-component bosons in an optical lattice can form at least three different superfluid phases in which both components participate in the superflow: a (strongly interacting) mixture of two miscible superfluids (2SF), a paired superfluid (PSF) vacuum, and (at a commensurate total filling factor) the super-counter-fluid (SCF) state. We study the universal properties of the 2SF-PSF and 2SF-SCF quantum phase transitions and show that (i) they can be mapped onto each other and (ii) their universality class is identical to the (d+1)-dimensional normal-superfluid transition in a single-component liquid. The finite-temperature 2SF-PSF(SCF) transitions and the topological properties of 2SF-PSF(SCF) interfaces are also discussed.

Equations for the order parameter and effective magnetic field for nonunitary phases of superfluid Fermi liquids with spin-triplet p-wave pairing

On the basis of generalized Fermi-liquid approach we have derived the systems of coupled equations for the components of the order parameter and effective magnetic field for nonunitary phases of neutral paramagnetic superfluid Fermi liquids with spin-triplet pairing in strong static uniform magnetic field at arbitrary temperatures from the interval 0⩽ T⩽ T c ( T c is the normal–superfluid transition temperature). These equations are presented here for dense superfluid neutron matter (existing inside fluid core of neutron stars) with triplet p-wave pairing of 3 He –A 2 type and using the effective Skyrme interaction between neutrons.

3He in 99.5% Porous Aerogel at the Normal-Superfluid Transition

We have used a torsional oscillator to measure the superfluid density and dissipation near the superfluid transition of 3 He in aerogel of 99.5% porosity. We used a new cell (constructed at Penn State) for which the aerogel was grown in the pores of a ∼100 μm silver sinter. The cell was tested with 4 He and showed no signs of the second-sound resonances that have interfered with previous torsional oscillator measurements. The measurements with 3 He, presented here, were taken at pressures of 1.34 and 4.13 bars. We observed values of ρ s /ρ in the T→ 0 limit of 0.05 and 0.14 respectively. Our measurements show an increase in the dissipation on warming through T c . This series of measurements is ongoing and temperature sweeps at various pressures are planned.

On Superfluidity and Phase Transition in Atomic Nucleus

Assuming the existence of an energy gap and the validity of the shell model at low excitations, residual interactions of nucleons situated near the Fermi energy surface are calculated. From these interactions, the nuclear thermodynamic potential is calculated by variational techniques. This potential is used to determine the superfluid properties and normal-superfluid transition temperature of the nucleus.