32 research outputs found
Dry demagnetization cryostat for sub-millikelvin helium experiments: Refrigeration and thermometry
We demonstrate successful “dry” refrigeration of quantum fluids down to T = 0.16 mK by using copper nuclear demagnetization stage that is pre-cooled by a pulse-tube-based dilution refrigerator. This type of refrigeration delivers a flexible and simple sub-mK solution to a variety of needs including experiments with superfluid 3He. Our central design principle was to eliminate relative vibrations between the high-field magnet and the nuclear refrigeration stage, which resulted in the minimum heat leak of Q = 4.4 nW obtained in field of 35 mT. For thermometry, we employed a quartz tuning fork immersed into liquid 3He. We show that the fork oscillator can be considered as self-calibrating in superfluid 3He at the crossover point from hydrodynamic into ballistic quasiparticle regime.Peer reviewe
Melting curve of He: no sign of the supersolid transition down to 10 mK
We have measured the melting curve of He in the temperature range from 10
to 400 mK with the accuracy of about 0.5 bar. Crystals of different
quality show the expected -dependence in the range from 80 to 400 mK
without any sign of the supersolid transition, and the coefficient is in
excellent agreement with available data on the sound velocity in liquid He
and on the Debye temperature of solid He. Below 80 mK we have observed a
small deviation from -dependence which however cannot be attributed to the
supersolid transition because instead of decrease the entropy of the solid
rather remains constant, about Comment: 4 pages, 2 figures, published in Physical Review Letter
Quantum degeneracy in mesoscopic matter: Casimir effect and Bose-Einstein condensation
The ground-state phonon pressure is an analogue to the famous Casimir
pressure of vacuum produced by zero-point photons. The acoustic Casimir forces
are, however, many orders of magnitude weaker than the electromagnetic Casimir
forces, as the typical speed of sound is 100 000 times smaller than the speed
of light. Because of its weakness, zero-point acoustic Casimir pressure was
never observed, although the pressure of artificially introduced sound noise on
a narrow aperture has been reported. However, the magnitude of Casimir pressure
increases as with the decrease of the sample size , and reaches
picoNewtons in the sub-micron scales. We demonstrate and measure the acoustic
Casimir pressure induced by zero-point phonons in solid helium adsorbed on a
carbon nanotube. We have also observed Casimir-like "pushing out" thermal
phonons with the decreasing temperature or the length. We also show that all
thermodynamic quantities are size-dependent, and therefore in the mesoscopic
range quadruple points are possible on the phase
diagram where four different phases coexist. Due to the smallness of solid
helium sample, temperature of Bose-Einstein condensation (BEC) of vacancies is
relatively high, mK. This allowed us to experimentally discover the
BEC in a system of zero-point vacancies, predicted more than 50 years ago
Solid 4He and the Supersolid Phase: from Theoretical Speculation to the Discovery of a New State of Matter? A Review of the Past and Present Status of Research
The possibility of a supersolid state of matter, i.e., a crystalline solid
exhibiting superfluid properties, first appeared in theoretical studies about
forty years ago. After a long period of little interest due to the lack of
experimental evidence, it has attracted strong experimental and theoretical
attention in the last few years since Kim and Chan (Penn State, USA) reported
evidence for nonclassical rotational inertia effects, a typical signature of
superfluidity, in samples of solid 4He. Since this "first observation", other
experimental groups have observed such effects in the response to the rotation
of samples of crystalline helium, and it has become clear that the response of
the solid is extremely sensitive to growth conditions, annealing processes, and
3He impurities. A peak in the specific heat in the same range of temperatures
has been reported as well as anomalies in the elastic behaviour of solid 4He
with a strong resemblance to the phenomena revealed by torsional oscillator
experiments. Very recently, the observation of unusual mass transport in hcp
solid 4He has also been reported, suggesting superflow. From the theoretical
point of view, powerful simulation methods have been used to study solid 4He,
but the interpretation of the data is still rather difficult; dealing with the
question of supersolidity means that one has to face not only the problem of
the coexistence of quantum coherence phenomena and crystalline order, exploring
the realm of spontaneous symmetry breaking and quantum field theory, but also
the problem of the role of disorder, i.e., how defects, such as vacancies,
impurities, dislocations, and grain boundaries, participate in the phase
transition mechanism.Comment: Published on J. Phys. Soc. Jpn., Vol.77, No.11, p.11101
Defects and glassy dynamics in solid He-4: Perspectives and current status
We review the anomalous behavior of solid He-4 at low temperatures with
particular attention to the role of structural defects present in solid. The
discussion centers around the possible role of two level systems and structural
glassy components for inducing the observed anomalies. We propose that the
origin of glassy behavior is due to the dynamics of defects like dislocations
formed in He-4. Within the developed framework of glassy components in a solid,
we give a summary of the results and predictions for the effects that cover the
mechanical, thermodynamic, viscoelastic, and electro-elastic contributions of
the glassy response of solid He-4. Our proposed glass model for solid He-4 has
several implications: (1) The anomalous properties of He-4 can be accounted for
by allowing defects to freeze out at lowest temperatures. The dynamics of solid
He-4 is governed by glasslike (glassy) relaxation processes and the
distribution of relaxation times varies significantly between different
torsional oscillator, shear modulus, and dielectric function experiments. (2)
Any defect freeze-out will be accompanied by thermodynamic signatures
consistent with entropy contributions from defects. It follows that such
entropy contribution is much smaller than the required superfluid fraction, yet
it is sufficient to account for excess entropy at lowest temperatures. (3) We
predict a Cole-Cole type relation between the real and imaginary part of the
response functions for rotational and planar shear that is occurring due to the
dynamics of defects. Similar results apply for other response functions. (4)
Using the framework of glassy dynamics, we predict low-frequency yet to be
measured electro-elastic features in defect rich He-4 crystals. These
predictions allow one to directly test the ideas and very presence of glassy
contributions in He-4.Comment: 33 pages, 13 figure
Finite-size effects in thermodynamics
We have measured the equilibrium melting pressure of helium-4 as a function of the crystal size. Negative compressibility of a liquid with an inclusion of solid seed is predicted theoretically and verified experimentally with helium-4 crystal-superfluid system at 0.15 K. This two-phase system is shown to be stable if the crystal size is large enough, which is proven by the experiment. Crystal seeds that are too small spontaneously either melt completely or grow to a large enough size.Peer reviewe
Massless surface waves between two different superfluid phases of He 3
An interface between two media is a topologically stable two-dimensional object where 3D-symmetry breaks which allows for the existence of many exotic excitations. A direct way to explore surface excitations is to investigate their interaction with the surface waves, such as very well known capillary-gravity waves and crystallization waves. Helium remains liquid down to absolute zero where bulk excitations are frozen out and do not mask the interaction of the waves with the surface states. Here we show the possibility of the massless wave which can propagate along the surface between two different superfluids phases of He3. The displacement of the surface in this wave occurs due to the transition of helium atoms from one phase to another, so that there is no flow of particles as densities of phases are equal. We calculate the dispersion of the wave in which the inertia is provided by spin supercurrents, and the restoring force is magnetic field gradient. We calculate the dissipation of the wave and show the preferable conditions to observe it.Peer reviewe
Quasiparticle damping of surface waves in superfluid 3He and 4He
Oscillations on free surfaces of superfluids at the inviscid limit are damped by quasiparticle scattering. We study this effect in both superfluid 3He and superfluid 4He, deep below the respective critical temperatures. Surface oscillators offer several benefits over immersed mechanical oscillators traditionally used for similar purposes. Damping is modeled as specular scattering of ballistic quasiparticles from the moving free surface. The model is in reasonable agreement with our measurements for superfluid 4He but significant deviation is found for 3He.Peer reviewe