20 research outputs found
Strong Coupling Correction in Superfluid He in Aerogel
Effects of impurity scatterings on the strong coupling (SC) contribution,
stabilizing the ABM (axial) pairing state, to the quartic term of the
Ginzburg-Landau (GL) free energy of superfluid He are theoretically studied
to examine recent observations suggestive of an anomalously small SC effect in
superfluid He in aerogels. To study the SC corrections, two approaches are
used. One is based on a perturbation in the short-range repulsive interaction,
and the other is a phenomenological approach used previously for the bulk
liquid by Sauls and Serene [Phys.Rev.B 24, 183 (1981)]. It is found that the
impurity scattering favors the BW pairing state and shrinks the region of the
ABM pairing state in the T-P phase diagram. In the phenomenological approach,
the resulting shrinkage of the ABM region is especially substantial and, if
assuming an anisotropy over a large scale in aerogel, leads to justifying the
phase diagrams determined experimentally.Comment: 19 pages, 9 figures, Accepted for publication in Phys. Rev.
Vortex core transitions in superfluid 3He in globally anisotropic aerogels
Core structures of a single vortex in A-like and B-like phases of superfluid
3He in uniaxially compressed and stretched aerogels are studied by numerically
solving Ginzburg-Landau equations derived microscopically. It is found that,
although any uniaxial deformation leads to a wider A-like phase with the axial
pairing in the pressure-temperature phase diagram, the vortex core states in
the two phases in aerogel depend highly on the type of deformation. In a
compressed aerogel, the first-order vortex core transition (VCT) previously
seen in the bulk B phase appears at any pressure in the B-like phase while no
strange vortex core is expected in the corresponding A-like phase. By contrast,
in a stretched aerogel, the VCT in the B-like phase is lost while another VCT
is expected to occur between a nonunitary core and a polar one in the A-like
phase. Experimental search for these results is hoped to understand correlation
between superfluid 3He and aerogel structure.Comment: 7 pages, 6 figures Text was changed. Resubmitted versio
Strong Coupling Corrections to the Ginzburg-Landau Theory of Superfluid ^{3}He
In the Ginzburg-Landau theory of superfluid He, the free energy is
expressed as an expansion of invariants of a complex order parameter. Strong
coupling effects, which increase with increasing pressure, are embodied in the
set of coefficients of these order parameter invariants\cite{Leg75,Thu87}.
Experiments can be used to determine four independent combinations of the
coefficients of the five fourth order invariants. This leaves the
phenomenological description of the thermodynamics near incomplete.
Theoretical understanding of these coefficients is also quite limited. We
analyze our measurements of the magnetic susceptibility and the NMR frequency
shift in the -phase which refine the four experimental inputs to the
phenomenological theory. We propose a model based on existing experiments,
combined with calculations by Sauls and Serene\cite{Sau81} of the pressure
dependence of these coefficients, in order to determine all five fourth order
terms. This model leads us to a better understanding of the thermodynamics of
superfluid He in its various states. We discuss the surface tension of
bulk superfluid He and predictions for novel states of the superfluid
such as those that are stabilized by elastic scattering of quasiparticles from
a highly porous silica aerogel.Comment: 9 pages, 7 figures, 2 table
Superfluid He in globally isotropic random media
Recent theoretical and experimental studies of superfluid He in aerogels
with a global anisotropy, e.g., due to an external stress, have definitely
shown that the A-like phase with an equal spin pairing (ESP) in such aerogel
samples is in the ABM (or, axial) pairing state. In this paper, the A-like
phase of superfluid He in globally {\it isotropic} aerogel is studied in
details by assuming a weakly disordered system in which singular topological
defects are absent. Through calculation of the free energy, a disordered ABM
state is found to be the best candidate of the pairing state of the globally
isotropic A-like phase. Further, it is found through a one-loop renormalization
group calculation that the coreless continuous vortices (or, vortex-skyrmions)
are irrelevant to the long-distance behavior of the disorder-induced textures,
and that the superfluidity is maintained in spite of lack of the conventional
off-diagonal long range order. Therefore, the globally isotropic A-like phase
at weak disorder is, like in the case with a global stretched anisotropy, a
superfluid glass with the ABM pairing.Comment: Revised version accepted for publication in Phys.Rev.
Superfluid nanomechanical resonator for quantum nanofluidics
We have developed a nanomechanical resonator, for which the motional degree
of freedom is a superfluid 4He oscillating flow confined to precisely defined
nanofluidic channels. It is composed of an in-cavity capacitor measuring the
dielectric constant, which is coupled to a superfluid Helmholtz resonance
within nanoscale channels, and it enables sensitive detection of nanofluidic
quantum flow. We present a model to interpret the dynamics of our superfluid
nanomechanical resonator, and we show how it can be used for probing confined
geometry effects on thermodynamic functions. We report isobaric measurements of
the superfluid fraction in liquid 4He at various pressures, and the onset of
quantum turbulence in restricted geometry.Comment: 10 pages, 11 figure