arXiv subjects

G. Jakob

Publications and source records attributed to G. Jakob.

43 records · Page 3Linked to original sources

arXiv1999

Hall effect of epitaxial double-perovskite Sr_2FeMoO_6 thin films

We prepared high epitaxial thin films of the compound Sr_2FeMoO_6 with narrow rocking curves by pulsed laser deposition. The diagonal and nondiagonal elements of the resistivity tensor were investigated at temperatures from 4 K up to room temperature in magnetic fields up to 8 T. An electronlike ordinary Hall effect and a holelike anomalous Hall contribution are observed. Both coefficients have reversed sign compared to the colossal magnetoresistive manganites. We found at 300 K an ordinary Hall coefficent of -1.87x10^{-10} m^3/As, corresponding to a nominal charge carrier density of four electrons per formula unit. At low temperature only a small negative magnetoresistance is observed which vanishes at higher temperatures. The temperature coefficient of the resistivity is negative over the whole temperature range. A Kondo like behavior is observed below 30 K while above 100 K variable range hopping like transport occurs.

cond-mat.mtrl-sci↗

arXiv1999

Universal relationship between the penetration depth and the normal-state conductivity in YBaCuO

The absolute values of the conductivity in the normal state sigma_n and of the low temperature penetration depths lambda(0) were measured for a number of different samples of the YBaCuO family. We found a striking correlation between sigma_n and 1/lambda^2, regardless of doping, oxygen reduction or defects, thus providing a simple method to predict the superconducting penetration depth and to have an estimate of the sample quality by measuring the normal-state conductivity.

cond-mat.supr-con↗

arXiv1999

Charge carrier density collapse in La_0.67Ca_0.33MnO_3 and La_0.67Sr_0.33MnO_3 epitaxial thin films

We measured the temperature dependence of the linear high field Hall resistivity of La_0.67Ca_0.33MnO_3 (T_C=232K) and La_0.67Sr_0.33MnO_3 (T_C=345K) thin films in the temperature range from 4K up to 360K in magnetic fields up to 20T. At low temperatures we find a charge carrier density of 1.3 and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In this temperature range electron-magnon scattering contributes to the longitudinal resistivity. At the ferromagnetic transition temperature T_C a dramatic drop in the number of current carriers $n$ down to 0.6 holes per unit cell, accompanied by an increase in unit cell volume, is observed. Corrections of the Hall data due to a non saturated magnetic state will lead a more pronounced charge carrier density collapse.

cond-mat.mtrl-sci↗

arXiv1999

Transport and magnetic properties of La_(1-x)Ca_xMnO_3-films (0.1<x<0.9)

By laser ablation we prepared thin films of the colossal magnetoresistive compound La_(1-x)Ca_xMnO_3 with doping levels 0.1<x<0.9 on MgO substrates. X-ray diffraction revealed epitaxial growth and a systematic decrease of the lattice constants with doping. The variation of the transport and magnetic properties in this doping series was investigated by SQUID magnetization and electrical transport measurements. For the nonmetallic samples resistances up to 10^13 Ohm have been measured with an electrometer setup. While the transport data indicate polaronic transport for the metallic samples above the Curie temperature the low doped ferromagnetic insulating samples show a variable range hopping like transport at low temperature.

cond-mat.str-el↗

arXiv1999

Frequency dependent relaxation rate in the superconducting YBa2Cu3O{6+delta}

The submillimeter-wave 3 cm{-1} < nu < 35 cm{-1} complex conductivity of the reduced YBa2Cu3O{6+delta} film (Tc=56.5 K) was investigated for temperatures 4 K < T < 300 K and compared to the properties of the same film in the optimally doped state. The frequency dependence of the effective quasiparticle scattering rate 1/tau*(nu) was extracted from the spectra. 1/tau*(nu) is shown to be frequency independent at low frequencies and high temperatures. A gradual change to 1/tau*~nu^{1.5} law is observed as temperature decreases. In order to explain the observed temperature dependence of the low frequency spectral weight above Tc, the quasiparticle effective mass is supposed to be temperature dependent for T>Tc.

cond-mat.supr-con↗

arXiv1999

Current dependence of grain boundary magnetoresistance in La_0.67Ca_0.33MnO_3 films

We prepared epitaxial ferromagnetic manganite films on bicrystal substrates by pulsed laser ablation. Their low- and high-field magnetoresistance (MR) was measured as a function of magnetic field, temperature and current. At low temperatures hysteretic changes in resistivity up to 70% due to switching of magnetic domains at the coercitive field are observed. The strongly non-ohmic behavior of the current-voltage leads to a complete suppression of the MR effect at high bias currents with the identical current dependence at low and high magnetic fields. We discuss the data in view of tunneling and mesoscale magnetic transport models and propose an explicit dependence of the spin polarization on the applied current in the grain boundary region.

cond-mat.str-el↗

arXiv1998

Crossovers Between Elastic, Plastic and Quantum Creep in Two Dimensional YBa_2Cu_3O_7/PrBa_2Cu_3O_7 Superlattices

Two-dimensional(2D) vortex dynamics was studied in an YBa_2Cu_3O_7/ PrBa_2Cu_3O_7 superlattice by measuring the I-V characteristics. In the high current limit, 2D collective creep was observed with an activation energy characterized by U(j)=a j^u. A dislocation mediated vortex melting happened when the temperature increased. In the low current limit, the exponential growth of energy barrier for elastic motion was prohibited by the plastic deformation of vortices. A plateau in the resistive transition was observed, which was attributed to possible quantum tunneling of vortices. Our results suggest that a 2D vortex glass can not exist at any temperature, including T=0 K.

cond-mat.supr-con↗