arXiv subjects

Emil Polturak

Publications and source records attributed to Emil Polturak.

15 recordsLinked to original sources

arXiv2021

Anomalously Small Excitation Gaps as a Precursor of Dislocation Core Superfluidity in Solid Helium-4

In the vicinity of the insulator-to-superfluid quantum phase transition in its core, a dislocation in a He-4 crystal supports particle-hole excitations with arbitrary small gaps. These exotic analogs of Frenkel interstitial-vacancy pairs should manifest themselves in various threshold and thermoactivation effects. In Worm Algorithm simulations, we reveal the presence of corresponding small gaps via anomalous thermoactivation behavior of particle number fluctuations, which we unambiguously associate with dislocations by "visualization" techniques. Experimentally, the related threshold and thermoactivation dependencies could be observed in the ultrasound absorption.

cond-mat.soft↗

arXiv2021

Thermal and structural properties of topological defects in solid Helium-4: Strain induced martensitic transformation from hcp to fc orthorhombic lattice

Several experimental studies have reported thermally activated behavior of the mechanical response of solid Helium-4 which does not fit into the model of thermally activated Frenkel pairs in an ideal crystal. The purpose of the present work is to investigate how structural topological defects modify the response. Using quantum Monte Carlo Worm Algorithm, we study temperature dependence of fluctuations of the total number of particles in samples of hcp solid Helium-4 containing several types of dislocations. Such fluctuations can be described by the thermal activation law with the activation energy dependent strongly on the type of the dislocation and the crystal density. The extracted values cover a range from about 1K to 20K. It is also found that annihilation of a jog-antijog pair can produce a local region of the solid characterized by the orthorhombic symmetry. This serendipitous observation suggests that hcp solid Helium-4 can undergo a displacive phase transition into the orthorhombic crystal under uniaxial stress of about 10-15%

cond-mat.other↗

arXiv2019

Is the surface fluid during surface melting, a study of the surface shear modulus of solid Gallium

Melting of 3D solids is often preceded by a melting of their surface, a distinct process which begins at a temperature lower than the melting temperature. Until now, surface melting was investigated mostly by diffraction or other non-contact techniques, revealing how the surface becomes progressively disordered with temperature. We designed a method to measure an effective shear modulus of the surface, another property which should change at melting. This property was not measured before. We applied this method to surface melting of Gallium, and found that this surface shear modulus vanishes abruptly near the onset of premelting, about 9K below melting temperature.

cond-mat.mtrl-sci↗

arXiv2016

Observation of quantum friction in solid 4He

Classical sliding friction is dominated by the slip-stick mechanism, where contacts between two bodies are alternately formed and sheared as the bodies move past each other. When the interface between two bodies is perfectly smooth, classical friction goes to zero, a state called superlubricity. In this limit, much weaker mechanisms, called quantum friction are predicted. These mechanisms are based on an exchange of elementary excitations between two bodies moving relatively to each other. For the friction to be called quantum, the excitations must arise from the quantum mechanical description of the bodies. Photons and phonons are such excitations. Friction results from an irreversible momentum transfer from these excitations to the bodies, affecting their motion. We measured the friction force between crystallites of solid 4He moving relative to each other at low temperatures. Our data are in excellent agreement with the concept of quantum phonon friction.

cond-mat.mtrl-sci↗

arXiv2015

Observation of a Dislocation Related Interfacial Friction Mechanism in Mobile Solid $^4$He

We report a study of the temperature and stress dependence of the friction associated with a relative motion of two masses of solid $^4$He in contact. The situation where "two masses" coupled only by friction exists emerges spontaneously during a disordering of a single crystal contained inside a annular sample space of torsional oscillator (TO). Under the torque applied by the oscillating walls of the the TO these "masses" move relative to each other, generating measurable dissipation at their interface. We studied this dissipation between 0.5K and 1.8K in solid samples grown from commercially pure $^4$He and from a 100 ppm $^3$He-$^4$He mixture. The data were analyzed by modelling the TO as a driven harmonic oscillator. In this model, analysis of the resonant frequency and amplitude of the TO yields the temperature dependence of the friction coefficient. By fitting the data to specific forms, we found that over our temperature range, the dominant friction mechanism associated with the interfacial motion results from climb of individual dislocations. The characteristic energy scale associated with this internal friction is between 3K and 6K, depending on the sample. The fact that a single value of this energy accounts for the data of a given sample supports the idea that the interface between the moving and static solid is well oriented. The relative motion of the solid in this case can perhaps be described as the low limit of "slip-stick" motion.

cond-mat.other↗

arXiv2015

Characterization of the Surface of Moving solid 4He

Crystal grains of solid $^4$He can move in relation to each other even when embedded inside the solid. In this work, we characterize a macroscopic motion of solid hcp $^4$He composed of such grains. Motion is induced by applying an external torque to the solid contained inside an annular channel mounted on a torsional oscillator. In order to characterize the surface of the moving solid, we developed an in-situ flow detection method using a sensitive "microphone" embedded in the wall of the channel. Motion is detected by counting the vibrations induced by rows of He atoms moving past the microphone. Such vibrations were detected only at T=0.5K, our lowest temperature. At this temperature, the measured dissipation associated with the solid He is zero within our accuracy. Our results indicate that the orientation of the surface of the moving solid is the (0001) basal plane of the hcp structure. At T=0.5 K, we found that for speeds < 7 micrometer/sec, the solid flows without detectable friction.

cond-mat.mtrl-sci↗

arXiv2012

BCC vs. HCP - The Effect of Crystal Symmetry on the High Temperature Mobility of Solid $^4$He

We report results of torsional oscillator (TO) experiments on solid $^4$He at temperatures above 1K. We have previously found that single crystals, once disordered, show some mobility (decoupled mass) even at these rather high temperatures. The decoupled mass fraction with single crystals is typically 20- 30%. In the present work we performed similar measurements on polycrystalline solid samples. The decoupled mass with polycrystals is much smaller, $\sim$ 1%, similar to what is observed by other groups. In particular, we compared the properties of samples grown with the TO's rotation axis at different orientations with respect to gravity. We found that the decoupled mass fraction of bcc samples is independent of the angle between the rotation axis and gravity. In contrast, hcp samples showed a significant difference in the fraction of decoupled mass as the angle between the rotation axis and gravity was varied between zero and 85 degrees. Dislocation dynamics in the solid offers one possible explanation of this anisotropy.

cond-mat.other↗

arXiv2011

Measurement of the vortex mass in a superconducting film

We have combined high resolution magneto-optical imaging with an ultra-fast heating/cooling technique to measure the movement of individual vortices in a superconducting film. The motion took place while the film was heated close to $T_c$, where pinning and viscous forces are relatively small. Under these conditions, vortices move due to the magnetic repulsion between them. We found that a finite vortex mass has to be included in the analysis in order to account for the experimental results. The extent of the motion is consistent with a vortex mass being 3 orders of magnitude smaller than the mass of all the electrons in the core.

cond-mat.supr-con↗

arXiv2011

On The Mobile Behavior of Solid $^4$He at High Temperatures

We report studies of solid helium contained inside a torsional oscillator, at temperatures between 1.07K and 1.87K. We grew single crystals inside the oscillator using commercially pure $^4$He and $^3$He-$^4$He mixtures containing 100 ppm $^3$He. Crystals were grown at constant temperature and pressure on the melting curve. At the end of the growth, the crystals were disordered, following which they partially decoupled from the oscillator. The fraction of the decoupled He mass was temperature and velocity dependent. Around 1K, the decoupled mass fraction for crystals grown from the mixture reached a limiting value of around 35%. In the case of crystals grown using commercially pure $^4$He at temperatures below 1.3K, this fraction was much smaller. This difference could possibly be associated with the roughening transition at the solid-liquid interface.

cond-mat.other↗

arXiv2011

Experimental determination of correlations between spontaneously formed vortices in a superconductor

We have imaged spontaneously created arrays of vortices (magnetic flux quanta), generated in a superconducting film quenched through its transition temperature at rates around $10^9 K/s$. From these images, we calculated the positional correlation functions for two vortices and for 3 vortices. We compared our results with simulations of the time dependent Ginzburg Landau equation in 2D. The results are in agreement with the Kibble-Zurek scenario of spontaneous vortex creation. In addition, the correlation functions are insensitive to the presence of a gauge field.

cond-mat.stat-mech↗

arXiv2010

Evidence for a High-Temperature Disorder-Induced Mobility in Solid $^4$He

We have carried out torsional oscillator experiments on solid $^4$He at temperatures between 1.3K and 1.9K. We discovered phenomena similar to those observed at temperatures below 0.2K, which currently are under debate regarding their interpretation in terms of supersolidity. These phenomena include a partial decoupling of the solid helium mass from the oscillator, a change of the dissipation, and a velocity dependence of the decoupled mass. These were all observed both in the bcc and hcp phases of solid $^4$He. The onset of this behavior is coincidental with the creation of crystalline disorder but does not depend strongly on the crystalline symmetry or on the temperature.

cond-mat.other↗

arXiv2009

Correlations beyond the horizon

We have imaged spontaneously created arrays of vortices (magnetic flux quanta), generated in a superconducting film quenched through its transition temperature at rates around $10^9 K/s$. Spontaneous appearance of vortices is predicted by Kibble-Zurek and by Hindmarsh-Rajantie models of phase transitions under non-equilibrium conditions. Differentiating between these models requires a measurement of the internal correlations within the emerging vortex array. In addition to short range correlations predicted by Kibble and Zurek, we found unexpected long range correlations which are not described by any of the existing models.

cond-mat.supr-con↗

arXiv2001

Influence of Point Defects on the Shear Elastic Coefficients and on the Melting Temperature of Copper

We present molecular dynamics simulations of the influence of point defects on the shear elastic coefficients of copper. We find that vacancies do not influence these coefficients at all, while the introduction of interstitials causes a large reduction in the elastic coefficients. The simulations establish a phase diagram of the melting temperature as a function of the density of interstitials. A crystal having no free surface undergoes bulk mechanical melting as a result of the vanishing of C'=(C/sub/11-C/sub/12)/2 once the specific volume reaches a critical value, equal to the experimental volume of liquid phase. This critical volume is history independent, in the sense that it can be reached either by heating the crystal or by adding defects at a constant temperature. These results generalize the Born model of melting for the case where point defects are present.

cond-mat.mtrl-sci↗

arXiv1999

Search for Spontaneous Nucleation of Magnetic Flux During Rapid Cooling of YBCO films Through Tc

We describe an experimental search for spontaneous formation of flux lines during a rapid quench of thin YBaCuO films through Tc. This effect is expected according to the Kibble-Zurek mechanism of a creation of topological defects of the order parameter during a symmetry breaking phase transition. Spontaneously formed vortices were previously observed in superfluid 3He, while a similar experiment in superfluid 4He gave negative results. Using a high Tc SQUID, we measured both the magnetic flux in the sample during a quench with a sensitivity of 20 phi-0/cm^2, and the field noise which one would expect from flux lines pinned in the film. The sensitivity was sufficient to detect spontaneous flux at a level corresponding to 10^(-3) of the prediction. Within our resolution, we saw no evidence for this effect.

cond-mat.supr-con↗

arXiv1998

Role of Interfaces in the Proximity Effect in Anisotropic Superconductors

We report measurements of the critical temperature of YBCO-Co doped YBCO Superconductor-Normal bilayer films. Depending on the morphology of the S-N interface, the coupling between S and N layers can be turned on to depress the critical temperature of S by tens of degrees, or turned down so the layers appear almost totally decoupled. This novel effect can be explained by the mechanism of quasiparticle transmission into an anisotropic superconductor.

cond-mat.supr-con↗