Meni Shay

Crack-like processes governing the onset of frictional slip

We perform real-time measurements of the net contact area between two blocks of like material at the onset of frictional slip. We show that the process of interface detachment, which immediately precedes the inception of frictional sliding, is governed by three different types of detachment fronts. These crack-like detachment fronts differ by both their propagation velocities and by the amount of net contact surface reduction caused by their passage. The most rapid fronts propagate at intersonic velocities but generate a negligible reduction in contact area across the interface. Sub-Rayleigh fronts are crack-like modes which propagate at velocities up to the Rayleigh wave speed, VR, and give rise to an approximate 10% reduction in net contact area. The most efficient contact area reduction (~20%) is precipitated by the passage of ‘slow detachment fronts’. These fronts propagate at ‘anomalously’ slow velocities, which are over an order of magnitude lower than VR yet orders of magnitude higher than other characteristic velocity scales such as either slip or loading velocities. Slow fronts are generated, in conjunction with intersonic fronts, by the sudden arrest of sub-Rayleigh fronts. No overall sliding of the interface occurs until either of the slower two fronts traverses the entire interface, and motion at the leading edge of the interface is initiated. Slip at the trailing edge of the interface accompanies the motion of both the slow and sub-Rayleigh fronts. We might expect these modes to be important in both fault nucleation and earthquake dynamics.

The Fermi surface and band folding in La2−xSrxCuO4, probed by angle-resolved photoemission

A systematic angle-resolved photoemission study of the electronic structure of La2−xSrxCuO4 in a wide doping range is presented in this paper. In addition to the main energy band, we observed a weaker additional band, the (π, π) folded band, which shows unusual doping dependence. The appearance of the folded band suggests that a Fermi surface reconstruction is doping dependent and could already occur at zero magnetic field.

Observation of macroscopic structural fluctuations in bcc solid {sup 4}He

We report neutron diffraction studies of low density bcc and hcp solid {sup 4}He. In the bcc phase, we observed a continuous dynamical behavior involving macroscopic structural changes of the solid. The dynamical behavior takes place in a cell full of solid, and therefore represents a solid-solid transformation. The structural changes are consistent with a gradual rotation of macroscopic grains separated by low angle grain boundaries. We suggest that these changes are triggered by random momentary vibrations of the experimental system. An analysis of Laue diffraction patterns indicates that in some cases these structural changes, once initiated by a momentary impulse, seem to proceed at a constant rate over times approaching 1 hour. The energy associated with these macroscopic changes appears to be on the order of kT. Under similar conditions (temperature and pressure), these effects were absent in the hcp phase.

Interaction between the magnetic and superconducting order parameters in a La 1.94 Sr 0.06 CuO 4 wire studied via muon spin rotation

Meni Shay, Amit Keren, Gad Koren, Amit Kanigel, Oren Shafir, Lital Marcipar, Gerard Nieuwenhuys, Elvezio Morenzoni, Andreas Suter, Thomas Prokscha, Moshe Dubman, 3 and Daniel Podolsky Department of Physics, Technion Israel Institute of Technology, Haifa 32000, Israel Paul Scherrer Institute, CH 5232 Villigen PSI, Switzerland Institut für Physik der Kondensierten Materie,TU Braunschweig,D-38106 Braunschweig,Germany Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7 (Dated: August 6, 2009)

Parallel and perpendicular susceptibility aboveTcin La2−xSrxCuO4single crystals

We report direction-dependent susceptibility and resistivity measurements on La2-xSrxCuO4 single crystals. These crystals have rectangular needle-like shapes with the crystallographic “c” direction parallel or perpendicular to the needle axis, which, in turn, is in the applied field direction. At optimal doping we find finite diamagnetic susceptibility above Tc, namely fluctuating superconductivity (FSC), only when the field is perpendicular to the planes. In underdoped samples we find FSC in both field directions. We provide a phase diagram showing the FSC region, although it is sample dependent in the underdoped cases. The variations in the susceptibility data suggest a different origin for the FSC between underdoping (below 10%) and optimal doping. Finally, our data indicate that the spontaneous vortex diffusion constant above Tc is anomalously high.

Strong- versus weak-coupling paradigms for cuprate superconductivity

Absolute resistivity measurements as a function of temperature from optimally doped YBa_2Cu_3O_(7), La_(2-x)Sr_xCuO_4, Bi_2Sr_2CaCu_2O_(8-x), and (Ca_0.1La_0.9)(Ba_1.65La_0.35)Cu_3O_y thin films are reported. Special attention is given to the measurement geometrical factors and the resistivity slope between Tc and T^{*}. The results are compared with a strong coupling theory for the resistivity derivative near T_c, which is based on hard core bosons (HCB), and with several weak coupling theories, which are BCS based. Surprisingly, our results agree with both paradigms. The implications of these findings and the missing calculations needed to distinguish between the two paradigms are discussed.