Rheology of water and ammonia-water ices

Creep experiments on fine-grained water and ammonia-water ices have been performed at one atmosphere and high confining pressure in order to develop constitutive relationships necessary to model tectonic processes and interpret surface features of icy moons of the outer solar system. The present series of experiments explores the effects of temperature, strain rate, grain size, and melt fraction on creep strength. In general, creep strength decreases with increasing temperature, decreasing strain rate, and increasing melt fraction. A transition from dislocation creep to diffusion creep occurs at finer grain sizes, higher temperatures, and lower strain rates

0-pi Josephson tunnel junctions with ferromagnetic barrier

We fabricated high quality Nb/Al_2O_3/Ni_{0.6}Cu_{0.4}/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a step-like thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microns (1.3 lambda_J) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying ~6.7% of the magnetic flux quantum Phi_0. The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.Comment: submitted to PR

High quality ferromagnetic 0 and pi Josephson tunnel junctions

We fabricated high quality \Nb/\Al_2\O_3/\Ni_{0.6}\Cu_{0.4}/\Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic \Ni_{0.6}\Cu_{0.4} layer and on the ambient temperature, the junctions were in the 0 or $\pi$ ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The \Al_2\O_3 tunnel barrier allows to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter $\beta_c$ increases exponentially with decreasing temperature and reaches $\beta_c\approx700$ at $T=2.1 {\rm K}$. The critical current density in the $\pi$ state was up to $5\:\rm{A/cm^2}$ at $T=2.1 {\rm K}$, resulting in a Josephson penetration depth $\lambda_J$ as low as $160\:\rm{\mu m}$. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the \Ni_{0.6}\Cu_{0.4} layer and different transparencies of the interfaces.Comment: Changed content and Corrected typo

Experimental evidence of a {\phi} Josephson junction

We demonstrate experimentally the existence of Josephson junctions having a doubly degenerate ground state with an average Josephson phase \psi=\pm{\phi}. The value of {\phi} can be chosen by design in the interval 0<{\phi}<\pi. The junctions used in our experiments are fabricated as 0-{\pi} Josephson junctions of moderate normalized length with asymmetric 0 and {\pi} regions. We show that (a) these {\phi} Josephson junctions have two critical currents, corresponding to the escape of the phase {\psi} from -{\phi} and +{\phi} states; (b) the phase {\psi} can be set to a particular state by tuning an external magnetic field or (c) by using a proper bias current sweep sequence. The experimental observations are in agreement with previous theoretical predictions

Theoretical current-voltage characteristics of ferroelectric tunnel junctions

We present the concept of ferroelectric tunnel junctions (FTJs). These junctions consist of two metal electrodes separated by a nanometer-thick ferroelectric barrier. The current-voltage characteristics of FTJs are analyzed under the assumption that the direct electron tunneling represents the dominant conduction mechanism. First, the influence of converse piezoelectric effect inherent in ferroelectric materials on the tunnel current is described. The calculations show that the lattice strains of piezoelectric origin modify the current-voltage relationship owing to strain-induced changes of the barrier thickness, electron effective mass, and position of the conduction-band edge. Remarkably, the conductance minimum becomes shifted from zero voltage due to the piezoelectric effect, and a strain-related resistive switching takes place after the polarization reversal in a ferroelectric barrier. Second, we analyze the influence of the internal electric field arising due to imperfect screening of polarization charges by electrons in metal electrodes. It is shown that, for asymmetric FTJs, this depolarizing-field effect also leads to a considerable change of the barrier resistance after the polarization reversal. However, the symmetry of the resulting current-voltage loop is different from that characteristic of the strain-related resistive switching. The crossover from one to another type of the hysteretic curve, which accompanies the increase of FTJ asymmetry, is described taking into account both the strain and depolarizing-field effects. It is noted that asymmetric FTJs with dissimilar top and bottom electrodes are preferable for the non-volatile memory applications because of a larger resistance on/off ratio.Comment: 14 pages, 8 figure

Velocity Distributions of Granular Gases with Drag and with Long-Range Interactions

We study velocity statistics of electrostatically driven granular gases. For two different experiments: (i) non-magnetic particles in a viscous fluid and (ii) magnetic particles in air, the velocity distribution is non-Maxwellian, and its high-energy tail is exponential, P(v) ~ exp(-|v|). This behavior is consistent with kinetic theory of driven dissipative particles. For particles immersed in a fluid, viscous damping is responsible for the exponential tail, while for magnetic particles, long-range interactions cause the exponential tail. We conclude that velocity statistics of dissipative gases are sensitive to the fluid environment and to the form of the particle interaction.Comment: 4 pages, 3 figure

Magnetic interference patterns in 0-Pi SIFS Josephson junctions: effects of asymmetry between 0 and Pi regions

We present a detailed analysis of the dependence of the critical current I_c on the magnetic field B of 0, Pi, and 0-Pi superconductor-insulator-ferromagnet-superconductor Josephson junctions. I_c(B) of the 0 and Pi junction closely follows a Fraunhofer pattern, indicating a homogeneous critical current density j_c(x). The maximum of I_c(B) is slightly shifted along the field axis, pointing to a small remanent in-plane magnetization of the F-layer along the field axis. I_c(B) of the 0-Pi junction exhibits the characteristic central minimum. I_c however has a finite value here, due to an asymmetry of j_c in the 0 and Pi part. In addition, this I_c(B) exhibits asymmetric maxima and bumped minima. To explain these features in detail, flux penetration being different in the 0 part and the Pi part needs to be taken into account. We discuss this asymmetry in relation to the magnetic properties of the F-layer and the fabrication technique used to produce the 0-Pi junctions

A subgrain‐size piezometer calibrated for EBSD

We calibrate a subgrain‐size piezometer using electron backscatter diffraction (EBSD) data collected from experimentally deformed samples of olivine and quartz. Systematic analyses of angular and spatial resolution test the suitability of each dataset for inclusion in calibration of the subgrain‐size piezometer. To identify subgrain boundaries, we consider a range of critical misorientation angles and conclude that a 1° threshold provides the optimal piezometric calibration. The mean line‐intercept length, equivalent to the subgrain‐size, is found to be inversely proportional to the von Mises equivalent stress for datasets both with and without the Holyoke and Kronenberg (2010) correction. These new piezometers provide stress estimates from EBSD analyses of polymineralic rocks without the need to discriminate between relict and recrystallised grains and therefore greatly increase the range of rocks that may be used to constrain geodynamic models

A ferroelectric memristor

Memristors are continuously tunable resistors that emulate synapses. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, but the mechanism remains controversial. Purely electronic memristors have recently emerged based on well-established physical phenomena with albeit modest resistance changes. Here we demonstrate that voltage-controlled domain configurations in ferroelectric tunnel barriers yield memristive behaviour with resistance variations exceeding two orders of magnitude and a 10 ns operation speed. Using models of ferroelectric-domain nucleation and growth we explain the quasi-continuous resistance variations and derive a simple analytical expression for the memristive effect. Our results suggest new opportunities for ferroelectrics as the hardware basis of future neuromorphic computational architectures