The direct Cu NQR Study of the Stripe Phase in the Lanthanum Cuprates

Using Cu NQR in Eu-doped La_(2-x)Sr_xCuO_4 we find the evidence of the pinned stripe phase at 1.3K for 0.08<x<0.18. The pinned fraction increases by one order of magnitude near hole doping x=1/8. The NQR lineshape reveals three inequivalent Cu positions. A dramatic change of the NQR signal for x > 0.18 correlating with the onset of bulk superconductivity corresponds to the depinning of the stripe phase.Comment: 4 pages, 3 figures, to appear in Physica C, Proceedings of the 6th International Conference on Materials and Mechanisms of Superconductivity, Houston, February 200

Thermal conductivity of doped La2CuO4\rm\bf La_2CuO_4 as an example for heat transport by optical phonons in complex materials

We investigate the phonon thermal conductivity $\kappa_{\mathrm{ph}}$ of doped $\rm La_2CuO_4$ based on out-of-plane thermal conductivity measurements. When room temperature is approached the temperature dependence of $\kappa_{\mathrm{ph}}$ strongly deviates from the $T^{-1}$-decrease which is usually expected for heat transport by acoustic phonons. Instead, $\kappa_{\mathrm{ph}}$ decreases much weaker or even increases with rising temperature. Simple arguments suggest that such unusual temperature dependencies of $\kappa_{\mathrm{ph}}$ are caused by heat transport via dispersive optical phonons

Effect of guide field on three dimensional electron shear flow instabilities in collisionless magnetic reconnection

We examine the effect of an external guide field and current sheet thickness on the growth rates and nature of three dimensional unstable modes of an electron current sheet driven by electron shear flow. The growth rate of the fastest growing mode drops rapidly with current sheet thickness but increases slowly with the strength of the guide field. The fastest growing mode is tearing type only for thin current sheets (half thickness $\approx d_e$, where $d_e=c/\omega_{pe}$ is electron inertial length) and zero guide field. For finite guide field or thicker current sheets, fastest growing mode is non-tearing type. However growth rates of the fastest 2-D tearing mode and 3-D non-tearing mode are comparable for thin current sheets ($d_e <$half thickness $< 2\,d_e$) and small guide field (of the order of the asymptotic value of the component of magnetic field supporting electron current sheet). It is shown that the general mode resonance conditions for electron-magnetohydrodynamic (EMHD) and magnetohydrodynamic (MHD) tearing modes depend on the effective dissipation mechanism (electron inertia and resistivity in cases of EMHD and MHD, respectively). The usual tearing mode resonance condition ($\mathbf{k}.\mathbf{B}_0=0$, $\mathbf{k}$ is the wave vector and $\mathbf{B}_0$ is equilibrium magnetic field) can be recovered from the general resonance conditions in the limit of weak dissipation. Necessary conditions (relating current sheet thickness, strength of the guide field and wave numbers) for the existence of tearing mode are obtained from the general mode resonance conditions.Comment: The following article has been submitted to Physics of Plasmas. After it is published, it will be found at http://scitation.aip.org/content/aip/journal/pop. Authors gratefully acknowledges the support of the German Science Foundation CRC 96

Non-Maxwellian electron distribution functions due to self-generated turbulence in collisionless guide-field reconnection

Non-Maxwellian electron velocity space distribution functions (EVDF) are useful signatures of plasma conditions and non-local consequences of collisionless magnetic reconnection. In the past, EVDFs were obtained mainly for antiparallel reconnection and under the influence of weak guide-fields in the direction perpendicular to the reconnection plane. EVDFs are, however, not well known, yet, for oblique (or component-) reconnection in dependence on stronger guide-magnetic fields and for the exhaust (outflow) region of reconnection away from the diffusion region. In view of the multi-spacecraft Magnetospheric Multiscale Mission (MMS), we derived the non-Maxwellian EVDFs of collisionless magnetic reconnection in dependence on the guide-field strength $b_g$ from small ($b_g\approx0$) to very strong ($b_g=8$) guide-fields, taking into account the feedback of the self-generated turbulence. For this sake, we carried out 2.5D fully-kinetic Particle-in-Cell simulations using the ACRONYM code. We obtained anisotropic EVDFs and electron beams propagating along the separatrices as well as in the exhaust region of reconnection. The beams are anisotropic with a higher temperature in the direction perpendicular rather than parallel to the local magnetic field. The beams propagate in the direction opposite to the background electrons and cause instabilities. We also obtained the guide-field dependence of the relative electron-beam drift speed, threshold and properties of the resulting streaming instabilities including the strongly non-linear saturation of the self-generated plasma turbulence. This turbulence and its non-linear feedback cause non-adiabatic parallel electron acceleration and EVDFs well beyond the limits of the quasi-linear approximation, producing phase space holes and an isotropizing pitch-angle scattering.Comment: 21 pages, 8 figures. Revised to match with the version published in Physics of Plasmas. An abridged version of the abstract is shown her

Analysis of fast turbulent reconnection with self-consistent determination of turbulence timescale

We present results of Reynolds-averaged turbulence model simulation on the problem of magnetic reconnection. In the model, in addition to the mean density, momentum, magnetic field, and energy equations, the evolution equations of the turbulent cross-helicity $W$, turbulent energy $K$ and its dissipation rate $\varepsilon$ are simultaneously solved to calculate the rate of magnetic reconnection for a Harris-type current sheet. In contrast to previous works based on algebraic modeling, the turbulence timescale is self-determined by the nonlinear evolutions of $K$ and $\varepsilon$, their ratio being a timescale. We compare the reconnection rate produced by our mean-field model to the resistive non-turbulent MHD rate. To test whether different regimes of reconnection are produced, we vary the initial strength of turbulent energy and study the effect on the amount of magnetic flux reconnected in time.Comment: 10 pages, 7 figure