K. Nenkov

Strain-induced insulator state and giant gauge factor of La0.7Sr0.3CoO3 films.

We report on the observation of a strain-induced insulator state in ferromagnetic La0.7Sr0.3CoO3 films. Tensile strain above 1% is found to enhance the resistivity by several orders of magnitude. Reversible strain of 0.15% applied using a piezoelectric substrate triggers huge resistance modulations, including a change by a factor of 10 in the paramagnetic regime at 300 K. However, below the ferromagnetic ordering temperature, the magnetization data indicate weak dependence on strain for the spin state of the Co ions. We interpret the changes observed in the transport properties in terms of a strain-induced splitting of the Co eg levels and reduced double exchange, combined with a percolation-type conduction in an electronic cluster state.

Influence of the substrate on growth and magnetoresistance of La0.7Ca0.3MnOz thin films deposited by magnetron sputtering

Off-axis radio frequency magnetron sputtering was employed to grow La0.7Ca0.3MnOz (LCMO) thin films onto three different types of substrates. The substrate strongly influences the structure and the colossal magnetoresistance effect of the obtained films. Single-crystalline thin films were prepared on LaAlO3 (100) substrates, showing a low value of residual resistivity and a metal–insulator transition at a temperature of up to Tpeak=290 K. The latter value of the transition temperature is one of the highest reported so far on thin films of the La–Ca–Mn–O system. Films deposited onto Y-stabilized ZrO2 substrates and onto MgO substrates are polycrystalline and less textured. These films are characterized by a large negative magnetoresistance ratio MR=[R(H)−R(0)]/R(0) measured for small values of the magnetic field H. For H=1.5 kOe, the MR was found to be approximately −30%, −20%, and −8% at temperatures of 20, 77, and 180 K, respectively. The magnetoresistance of polycrystalline LCMO films shows two contribu...

Formation and pinning properties of growth-controlled nanoscale precipitates in YBa2Cu3O7-δ/transition metal quasi-multilayers

To study the possibility of enhancing the pinning forces in YBa2Cu3O7?? films through the introduction of nanosized precipitates, quasi-multilayers of YBa2Cu3O7?? and a transition metal (TM = Ti,Zr,Hf) were deposited on single-crystal SrTiO3 substrates using pulsed laser deposition. The transition metal layer thickness was chosen to be less than one unit cell, resulting in separated nanoscale islands that form inclusions with perovskite structure BaTM O3 during film growth. These inclusions grow biaxially textured within the film. Whereas the Ti-doped films show a very strong decrease in the critical temperature Tc and, hence, a strong decrease in the critical current density Jc with increasing TM amount for all temperatures, Hf and Zr doping show an increase in Jc for the smallest amounts of doping. An irreversibility field as high as 10.3?T at 77?K was observed in the case of low Hf content.

High trapped fields in bulk MgB2 prepared by hot-pressing of ball-milled precursor powder

Bulk superconducting MgB2 samples, 20?mm in diameter, were prepared by hot-pressing of ball-milled Mg and B powders using fine-grained boron powders. High maximum trapped fields of B0?=?5.4?T were obtained at 12?K in one of the investigated trapped field magnets (height 8?mm) at the centre of the bulk surface. Investigating the temperature dependence of the trapped field for short MgB2 samples (height ?1.6?mm), trapped fields of up to B0?=?3.2?T at 15?K were achieved. These high trapped fields are related to extremely high critical current densities of up to 106?A?cm?2 at 15?K, indicating strong pinning due to nanocrystalline MgB2 grains. Expected trapped field data for long trapped field magnets prepared from the available MgB2 material are estimated.

Electronic structure and weak electron-phonon coupling in TaB 2

We present electronic structure calculations together with resistivity, susceptibility, and specific heat measurements for

Superconductivity and disorder in YxLu1−xNi2B2C

{\mathrm{TaB}}_{2}

Heisenberg-like critical properties in ferromagnetic Nd1−xPbxMnO3 single crystals

to search for the recently contradictorily reported superconductivity and to study related normal-state properties. We ascribe the absence of superconductivity down to 1.5 K for our

Pinning behavior and magnetic relaxation in melt-processed YBCO doped with Li, Ni and Pd

{\mathrm{TaB}}_{2}

The upper critical field in superconducting MgB2

samples to the generally weak electron-phonon coupling derived from comparison of the calculated and measured specific heat constants. For the

Fully dense MgB2 superconductor textured by hot deformation

{E}_{2g}