167 research outputs found
Dissipation signatures of the normal and superfluid phases in torsion pendulum experiments with 3He in aerogel
We present data for energy dissipation factor (Q^{-1}) over a broad
temperature range at various pressures of a torsion pendulum setup used to
study 3He confined in a 98% open silica aerogel. Values for Q^{-1} above T_c
are temperature independent and have a weak pressure dependence. Below T_c, a
deliberate axial compression of the aerogel by 10% widens the range of
metastability for a superfluid Equal Spin Pairing (ESP) state; we observe this
ESP phase on cooling and the B phase on warming over an extended temperature
region. While the dissipation for the B phase tends to zero as T goes to 0,
Q^{-1} exhibits a peak value greater than that at T_c at intermediate
temperatures. Values for Q^{-1} in the ESP phase are consistently higher than
in the B phase and are proportional to \rho_s/\rho until the ESP to B phase
transition is attained. We apply a viscoelastic collision-drag model, which
couples the motion of the helium and the aerogel through a frictional
relaxation time \tau_f. Our dissipation data is not sensitive to the damping
due to the presumed small but non-zero value of \tau_f. The result is that an
additional mechanism to dissipate energy not captured in the collision-drag
model and related to the emergence of the superfluid order must exist. The
extra dissipation below T_c is possibly associated with mutual friction between
the superfluid phases and the clamped normal fluid. The pressure dependence of
the measured dissipation in both superfluid phases is likely related to the
pressure dependence of the gap structure of the "dirty" superfluid. The large
dissipation in the ESP state is consistent with the phase being the A or the
Polar with the order parameter nodes oriented in the plane of the cell and
perpendicular to the aerogel anisotropy axis.Comment: 12 pages, 7 figure
Magneto-Acoustic Spectroscopy in Superfluid 3He-B
We have used the recently discovered acoustic Faraday effect in superfluid
3He to perform high resolution spectroscopy of an excited state of the
superfluid condensate. With acoustic cavity interferometry we measure the
rotation of the plane of polarization of a transverse sound wave propagating in
the direction of magnetic field from which we determine the Zeeman energy of
the excited state. We interpret the Lande g-factor, combined with the
zero-field energies of the state, using the theory of Sauls and Serene to
calculate the strength of f -wave interactions in 3He.Comment: 4 pages, 5 figures, submitted to PRL, Aug 30th, 200
Direct measurement of the 14N(p,g)15O S-factor
We have measured the 14N(p,g)15O excitation function for energies in the
range E_p = 155--524 keV. Fits of these data using R-matrix theory yield a
value for the S-factor at zero energy of 1.64(17) keV b, which is significantly
smaller than the result of a previous direct measurement. The corresponding
reduction in the stellar reaction rate for 14N(p,g)15O has a number of
interesting consequences, including an impact on estimates for the age of the
Galaxy derived from globular clusters.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
New Chiral Phases of Superfluid 3He Stabilized by Anisotropic Silica Aerogel
A rich variety of Fermi systems condense by forming bound pairs, including
high temperature [1] and heavy fermion [2] superconductors, Sr2RuO4 [3], cold
atomic gases [4], and superfluid 3He [5]. Some of these form exotic quantum
states having non-zero orbital angular momentum. We have discovered, in the
case of 3He, that anisotropic disorder, engineered from highly porous silica
aerogel, stabilizes a chiral superfluid state that otherwise would not exist.
Additionally, we find that the chiral axis of this state can be uniquely
oriented with the application of a magnetic field perpendicular to the aerogel
anisotropy axis. At suffciently low temperature we observe a sharp transition
from a uniformly oriented chiral state to a disordered structure consistent
with locally ordered domains, contrary to expectations for a superfluid glass
phase [6].Comment: 6 pages, 4 figure, and Supplementary Informatio
Superfluid Phase Stability of He in Axially Anisotropic Aerogel
Measurements of superfluid He in 98% aerogel demonstrate the existence of
a metastable \emph{A}-like phase and a stable \emph{B}-like phase. It has been
suggested that the relative stability of these two phases is controlled by
anisotropic quasiparticle scattering in the aerogel. Anisotropic scattering
produced by axial compression of the aerogel has been predicted to stabilize
the axial state of superfluid He. To explore this possiblity, we used
transverse acoustic impedance to map out the phase diagram of superfluid He
in a % porous silica aerogel subjected to 17% axial compression. We
have previously shown that axial anisotropy in aerogel leads to optical
birefringence and that optical cross-polarization studies can be used to
characterize such anisotropy. Consequently, we have performed optical
cross-polarization experiments to verify the presence and uniformity of the
axial anisotropy in our aerogel sample. We find that uniform axial anisotropy
introduced by 17% compression does not stabilize the \emph{A}-like phase. We
also find an increase in the supercooling of the \emph{A}-like phase at lower
pressure, indicating a modification to \emph{B}-like phase nucleation in
\emph{globally} anisotropic aerogels.Comment: 4 pages, 4 figures, submitted to LT25 (25th International Conference
on Low Temperature Physics
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
Mass coupling and ^3$He in a torsion pendulum
We present results of the and period shift, , for He
confined in a 98% nominal open aerogel on a torsion pendulum. The aerogel is
compressed uniaxially by 10% along a direction aligned to the torsion pendulum
axis and was grown within a 400 m tall pancake (after compression) similar
to an Andronikashvili geometry. The result is a high pendulum able to
resolve and mass coupling of the impurity-limited He over the
whole temperature range. After measuring the empty cell background, we filled
the cell above the critical point and observe a temperature dependent period
shift, , between 100 mK and 3 mK that is 2.9 of the period shift
(after filling) at 100 mK. The due to the He decreases by an order
of magnitude between 100 mK and 3 mK at a pressure of bar. We
compare the observable quantities to the corresponding calculated and
period shift for bulk He.Comment: 8 pages, 3 figure
Phase diagram of superfluid 3He in "nematically ordered" aerogel
Results of experiments with liquid 3He immersed in a new type of aerogel are
described. This aerogel consists of Al2O3 strands which are nearly parallel to
each other, so we call it as a "nematically ordered" aerogel. At all used
pressures a superfluid transition was observed and a superfluid phase diagram
was measured. Possible structures of the observed superfluid phases are
discussed.Comment: 6 pages, 8 figures. Submitted to Pis'ma v ZhETF (JETP Letters
Modification of the 3He Phase Diagram by Anisotropic Disorder
Motivated by the recent prediction that uniaxially compressed aerogel can
stabilize the anisotropic A phase over the isotropic B phase, we measure the
pressure dependent superfluid fraction of 3He entrained in 10% axially
compressed, 98% porous aerogel. We observe that a broad region of the
temperature-pressure phase diagram is occupied by the metastable A phase. The
reappearance of the A phase on warming from the B phase, before superfluidity
is extinguished at Tc, is in contrast to its absence in uncompressed aerogel.
The phase diagram is modified from that of pure 3He, with the disappearance of
the polycritical point (PCP) and the appearance of a region of A phase
extending below the PCP of bulk 3He, even in zero applied magnetic field. The
expected alignment of the A phase texture by compression is not observed.Comment: 4 pages, 4 figures. Submitted for review to PR
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