Harry Alles - Academia.edu (original) (raw)

Papers by Harry Alles

IOP Conference Series: Materials Science and Engineering, 2013

Sensors and Actuators B: Chemical, 2014

Recent discovery of a nonclassical rotational inertia in solid ^4He below about 0.2 K by Kim and ... more Recent discovery of a nonclassical rotational inertia in solid ^4He below about 0.2 K by Kim and Chan has initiated an intensive study on the properties of solid ^4He. As Kim and Chan have interpreted their observation as the evidence of supersolid behavior, we have decided to measure very accurately the melting curve of ^4He because, as the slope of the melting curve is proportional to the difference in the entropy of the liquid and solid phases, there should be an anomaly at the possible supersolid transition. We have measured the melting curve of ^4He with the accuracy of about 0.5 microbar with ^4He crystals which had various concentration of defects. All our samples showed only the expected T^4 dependence due to phonons without any sign of the supersolid transition in the temperature range of 80400 mK. Below 80 mK we observed a small deviation from T^4 dependence which, however, cannot be attributed to the supersolid transition because our recent measurements with the cell cont...

Jetp Lett Engl Tr, Mar 28, 2007

We have measured the melting pressure and pressure in the liquid at constant density of ultra-pur... more We have measured the melting pressure and pressure in the liquid at constant density of ultra-pure 4^44He (0.3 ppb of 3^33He impurities) with the accuracy of about 0.5 mu\mumubar in the temperature range from 10 to 320 mK. Our measurements show that the anomaly on the melting curve below 80 mK which we have recently observed is entirely due to an anomaly in the elastic modulus of Be-Cu from which our pressure gauge is made of. We thus conclude that the melting pressure of 4^44He follows the T4T^4T4 law due to phonons in the whole temperature range from 10 to 320 mK without any sign of a supersolid transition.

Physica B-condensed Matter, 2000

Torsional oscillator and NMR measurements on superfluid 3He in aerogel glass of 98% porosity are ... more Torsional oscillator and NMR measurements on superfluid 3He in aerogel glass of 98% porosity are compared with predictions of the homogeneous scattering model in the Ginzburg–Landau limit for the B-phase; ρs is less than predicted but semi-quantitative agreement between experiment and theory is obtained for the NMR data.

Reviews of Modern Physics, 2005

Physical Review Letters, 1994

Journal of Low Temperature Physics, 1995

ABSTRACT We have developed optical, interferometric methods for investigations of interfaces at u... more ABSTRACT We have developed optical, interferometric methods for investigations of interfaces at ultra low temperatures. In our scheme conventional optical windows are avoided: laser illumination (He-Ne) is guided into the cryostat via a single-mode optical fiber and images are taken using a CCD sensor mounted inside the 4-K vacuum can. A real-time video camera has been successfully used in investigations of superfluid3He down to 0.6 mK whereas a slow-scan camera has been employed for optimal contrast in low-intensity imaging of liquid/solid interfaces (reflection coefficient 10–6). The investigated topics include (1) superfluid3He surface in rotation and during rapid deceleration, (2) hydrodynamics of thin superfluid3He layers, (3) superfluid/solid interface in4He, and (4) wetting of superfluid4He by normal3He in phase separated mixtures. A vertical resolution of 10 nm and even below has been achieved in these studies.

Journal of Low Temperature Physics, 2000

Czechoslovak Journal of Physics, 1996

We have investigated the vortex core transition in3He-B by measuring the associated changes in mu... more We have investigated the vortex core transition in3He-B by measuring the associated changes in mutual fricition dissipation within the superfluid. If rotation is continuously stopped and restarted while cooling or warming then the transition occurs at a clearly defined temperature, but temperature sweeps during continuous rotation show substantial supercooling and superheating. Moreover, the high temperature vortex shows a continuum of metastable states when supercooled to a constant, arbitrary low temperature, the mutual friction dissipaton depending on the temperature at which rotation was started. Our current interpretation is that the high temperature vortex state is a temperature-dependent mixture of two vortex types.

Physical Review Letters

According to Landau, at T=0 the equilibrium crystal surface consists of an infinite number of fac... more According to Landau, at T=0 the equilibrium crystal surface consists of an infinite number of facets lying in all directions with rational Miller indices-the so-called devil's staircase phenomenon. We have discovered 11 new types of facets on the surface of 4He crystals, in addition to the three observed before. Some of the new facets are of very high order, lying at angles as small as 4 degrees to the basal c facet, thus forming the predicted devil's staircase. The estimated step energies depend rather weakly on interplanar distance which we explain by the strong anisotropy of the steps.

Physica B: Condensed Matter, 1992

... Rev. Lett. 45 (1980) 1952. [10] BS Shivaram, MW Meisel, BK Sarma, DB Mast, WP Halperin and JB... more ... Rev. Lett. 45 (1980) 1952. [10] BS Shivaram, MW Meisel, BK Sarma, DB Mast, WP Halperin and JB Ketterson, Phys. Rev. Lett. 49 (1982) 1646. [11] BS Shivaram, MW Meisel, BK Sarma, WP Hal-perin and JB Ketterson, J. Low Temp. Phys. 63 (1986) 57. ...

Carbon, 2015

ABSTRACT See up to Aug. 18th: http://authors.elsevier.com/a/1RHZ11zUA1UN6

Applied Physics Letters, 2014

ABSTRACT Photo-induced changes in the electrical conductivity and the sensitivity to oxygen gas o... more ABSTRACT Photo-induced changes in the electrical conductivity and the sensitivity to oxygen gas of graphene sheets grown by chemical vapor deposition and transferred onto Al2O3 and SiO2 thin film substrates were studied at ambient conditions. The pristine graphene sensors were initially completely insensitive to oxygen gas at room temperature but showed significant (up to 100%) response when illuminated with weak ultraviolet (300 nm or 365 nm) light. Oxygen response was governed by Langmuir law and its activation was insensitive to humidity. The mechanism of sensitization is analyzed together with other photo-induced effects-negative persistent photo-conduction and photo-induced hysteresis of field effect transistor characteristics. While the reduction of conductivity in air is persistent effect, the oxygen sensitization and enlargement of hysteresis take place only under the direct influence of light. It is concluded that the charge traps with differently adsorbed oxygen and water are involved in these phenomena. (C) 2014 AIP Publishing LLC.

Proceedings of the National Academy of Sciences of the United States of America, Jan 19, 2002

It has been predicted by Landau that, ideally at low temperatures, crystals should show many diff... more It has been predicted by Landau that, ideally at low temperatures, crystals should show many different types of facets, i.e., flat smooth faces on their surface, but this so-called "devil's staircase" phenomenon has been difficult to observe experimentally. In this paper we describe our recent experiments, in which altogether 11 different types of facets have been identified on growing (3)He crystals at the temperature of 0.55 mK by using a unique low-temperature Fabry-Pérot interferometer. Previously only 3 types of facets had been seen in this system. We have also measured the growth velocities of different facets, and our interpretation of the obtained results yields the conclusion that (3)He has much stronger coupling of the liquid-solid interface to the crystal lattice than has been expected. After an introduction we present a short theoretical background about the equilibrium crystal shape and the roughening transitions, which is followed by the description of ou...