7 research outputs found
Probing the Upper Limit of Nonclassical Rotational Inertia
We study the effect of confinement on solid 4-He's nonclassical rotational
inertia (NCRI) in a torsional oscillator by constraining it to narrow annular
cells of various widths. The NCRI exhibits a broad maximum value of 20% for
annuli of approximately 100 micrometer width. Samples constrained to porous
media or to larger geometries both have smaller NCRI, mostly below about 1%. In
addition, we extend Kim and Chan's blocked annulus experiment to solid samples
with large supersolid fractions. Blocking the annulus suppresses the
nonclassical decoupling from 17.1% below the limit of our detection of 0.8%.
This result demonstrates the nonlocal nature of the supersolid phenomena. At 20
mK, NCRI depends on velocity history showing a closed hysteresis loop in
different thin annular cells.Comment: 5 pages, 4 figure
Absence of Pressure-Driven Supersolid Flow at Low Frequency
An important unresolved question in supersolid research is the degree to
which the non-classical rotational inertia (NCRI) phenomenon observed in the
torsional oscillator experiments of Kim and Chan, is evidence for a
Bose-condensed supersolid state with superfluid-like properties. In an open
annular geometry, Kim and Chan found that a fraction of the solid moment of
inertia is decoupled from the motion of the oscillator; however, when the
annulus is blocked by a partition, the decoupled supersolid fraction is locked
to the oscillator being accelerated by an AC pressure gradient generated by the
moving partition. These observations are in accord with superfluid
hydrodynamics. We apply a low frequency AC pressure gradient in order to search
for a superfluid-like response in a supersolid sample. Our results are
consistent with zero supersolid flow in response to the imposed low frequency
pressure gradient. A statistical analysis of our data sets a bound, at the 68%
confidence level, of 9.6 nm/s for the mass transport velocity
carried by a possible supersolid flow. In terms of a simple model for the
supersolid, an upper bound of 3.3 is set for the supersolid
fraction at 25 mK, at this same confidence level. These findings force the
conclusion that the NCRI observed in the torsional oscillator experiments is
not evidence for a frequency independent superfluid-like state. Supersolid
behavior is a frequency-dependent phenomenon, clearly evident in the frequency
range of the torsional oscillator experiments, but undetectably small at
frequencies approaching zero.Comment: 6 pages, 5 figure
Spin-Imbalance in a One-Dimensional Fermi Gas
Superconductivity and magnetism generally do not coexist. Changing the
relative number of up and down spin electrons disrupts the basic mechanism of
superconductivity, where atoms of opposite momentum and spin form Cooper pairs.
Nearly forty years ago Fulde and Ferrell and Larkin and Ovchinnikov proposed an
exotic pairing mechanism (FFLO) where magnetism is accommodated by formation of
pairs with finite momentum. Despite intense theoretical and experimental
efforts, however, polarized superconductivity remains largely elusive. Here we
report experimental measurements of density profiles of a two spin mixture of
ultracold 6Li atoms trapped in an array of one dimensional (1D) tubes, a system
analogous to electrons in 1D wires. At finite spin imbalance, the system phase
separates with an inverted phase profile in comparison to the three-dimensional
case. In 1D we find a partially polarized core surrounded by wings composed of
either a completely paired BCS superfluid or a fully polarized Fermi gas,
depending on the degree of polarization. Our observations are in quantitative
agreement with theoretical calculations in which the partially polarized phase
is found to be a 1D analogue of the FFLO state. This study demonstrates how
ultracold atomic gases in 1D may be used to create non-trivial new phases of
matter, and also paves the way for direct observation and further study of the
FFLO phase.Comment: 30 pages, 7 figure