R. A. Chakalov

Doping phase diagram of Y1-xCaxBa2(Cu1-yZny)3O7-δ from transport measurements : Tracking the pseudogap below Tc

The effects of planar hole concentration, p, on the resistivity, r(T), of sintered Y1-xCaxBa2(Cu1-yZny)3O7-d samples were investigated over a wide range of Ca, Zn, and oxygen contents. Zn was used to suppress superconductivity and this enabled us to extract the characteristic pseudogap temperature, T*(p), from r(T,p) data below Tco(p) [ = Tc (y = 0)]. We have also located the characteristic temperature, Tscf, marking the onset of significant superconducting fluctuations above Tc, from the analysis of r(T,H,p) and r(T,p) data. This enabled us to identify T*(p) near the optimum doping level where the values of T*(p) and Tscf(p) are very close and hard to distinguish. We again found that T*(p) depends only on the hole concentration p, and not on the level of disorder associated with Zn or Ca substitutions. We conclude that (i) T*(p) (and therefore, the pseudogap) persists below Tco(p) on the overdoped side and does not merge with the Tco(p) line and (ii) T*(p), and thus the pseudogap energy, extrapolates to zero at the doping p = 0.19 +/- 0.01. PACS numbers: 74.25.Dw, 74.25. 74.62.Dh, 74.72.-h Key words: Pseudoap, Superconducting fluctuations

Structure, microstructure, and magneto-optical properties of laser deposited Bi3Fe5O12/Gd3Ga5O12(111) films

Bi3Fe5O12 films were deposited onto single crystal Gd3Ga5O12(111) substrates by pulsed laser deposition. X-ray diffraction patterns and cross sectional transmission electron micrographs show epitax ...

Effect of spin-polarized injection on the mixed state of YBa2Cu3O7−δ

We report measurements of the magnetic moment of YBa2Cu3O7−δ films when subjected to injection of short pulses of spin-polarized current from the colossal magnetoresistance (CMR) manganites La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3, and unpolarized current from the nonmagnetic perovskite LaNiO3(LNO). We study relaxation from the Bean critical state, with and without current pulses. For pulses longer than 100 ms, the effect of injection is similar for all samples and is due to heating. Below 1 ms, we see a clear difference between the CMR and LNO samples and attribute this to the spin polarization of the current injected from the CMR materials.

Superconductivity in thin-film YBa2Cu3O7−δ/La0.7Ca0.3MnO3 bilayers

We report the influence of the magnetic state of thin films of La0.7Ca0.3MnO3 (LCMO) on the superconducting order parameter in LCMO/YBa2Cu3O7−δ (YBCO) thin-film bilayers. We find that the number density of Cooper pairs is enhanced at the coercive field of the LCMO layer relative to that where the magnetic moment is saturated. This effect is at most ∼10% and is only observable within a few degrees of the superconducting transition temperature. Experiments with thin SrTiO3 layers between the LCMO and YBCO layers provide strong evidence that the effect is a consequence of the spin-polarized nature of the electrons at the interface, rather than a direct magnetic effect.

Deposition of YBCO films by high temperature spray pyrolysis

Abstract The fabrication of YBCO coated conductors on flexible textured metallic substrates requires the deposition of biaxially textured buffer layers and superconducting films. In this study we have prepared YBCO thin films on single crystal SrTiO 3 substrates and cube textured Ni substrates by spray pyrolysis. The Ni substrates have been pre-buffered with CeO 2 /YSZ/CeO 2 , layers deposited by pulsed laser deposition. Spray pyrolysis of nitrate solutions has been performed directly on heated substrates at temperatures between 800 and 900 °C without need for a subsequent annealing step. YBCO films deposited on both types of substrate are biaxially textured. Full width half maximum values determined from φ -scans are 8° and 20° for films on SrTiO 3 and buffered Ni substrates respectively. A transport J c value of 1.2×10 5 A/cm 2 at 77 K and zero field has been achieved on SrTiO 3 ( T c onset =91 K, ΔT c =6 K). χ ac susceptibility measurements of films on buffered Ni substrates show T c onsets of 88 K with ΔT c =18 K.

Materials aspects of laser-ablation-deposited cuprate∕manganate bilayers for spin-polarized injection devices

We report studies of cuprate∕manganate bilayers deposited onto SrTiO3 substrates by laser ablation. A significant difference in the properties of the individual layers is observed depending on the material deposition sequence. The material which is on the bottom of the multilayer shows worse properties, e.g., a critical temperature of ∼60K for YBa2Cu3O7−δ (YBCO) covered by La2∕3Ca1∕3MnO3 (LCMO) as opposed to a value of ∼90K when YBCO is on the top. Similarly, the LCMO film has its metal-insulator transition temperature shifted below the Curie temperature when the YBCO is on top. We argue that the effect is not due to degraded crystal quality, cation substitution, or an electronic interaction, but results from a reduction in the oxygen content of the underlying layer. We find that this reduction is a feature of the complete LCMO layer and is not associated with the constituent elements. We discuss possible explanations for this effect.

Spin-polarized injection into YBaCu3O7 grain boundary junctions

YBa2Cu3O7/SrTiO3/La0.67Ca0.33MnO3 trilayer thin films grown on bicrystal substrates were patterned into a structure that allows the injection of spin-polarized current into the region of a grain boundary junction (GBJ). The transfer length LT was determined to ensure that injection was over the entire device width, in contrast to previously reported experiments. Currents up to some 100 times the junction critical current I0 and area density 108 A m−2, have been injected into the GBJ region by using an offset current method. The I0(B) characteristics of the GBJ under injection suggest that the suppression of I0 can be explained by a combination of heating and self-field effects, without any need to invoke the spin-polarized nature of the injected current. We find no evidence of direct injection into the junction.

Critical current control in a superconducting thin film with a ferromagnetic layer

We report suppression of the critical current of thin film YBa/sub 2/Cu/sub 3/O/sub 7/ layers by altering the magnetization of a ferromagnetic La/sub 0.67/Ca/sub 0.33/MnO/sub 3/ overlayer. We provide evidence that the critical current suppression is related to the spin-polarized state of the magnetic layer, rather than a direct magnetic influence. We argue that the spin diffusion length in the superconducting layer is small compared with the typical domain size in the ferromagnetic layer in its demagnetized state.

Magnetic and superconducting properties of CMR/HTS multilayers for spin injection devices

Abstract We report substantial improvement in the critical current in HTS thin film due to the influence of colossal magnetoresistance (CMR) films in YBa 2 Cu 3 O x -based multilayers for spin injection devices. The effect of the CMR strongly depends on the thickness of the HTS, CMR and the intermediate insulator layers. We argue that the improvement in critical current may be due to the self-injection of the spin polarized electrons from the CMR to HTS, opposite to that normally observed in current driven spin injection devices.

Spin injection control of critical current in Y Ba2Cu3Ox films

We report on the suppression of critical current in Y Ba2Cu3Ox thin films by the injection of spin-polarized electrons from colossal magnetoresistive (CMR) materials La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3. We compare the effect of pulse and direct current injection. The gain of such devices and their temperature dependence is reported. We give evidence of the effect of spin polarization and argue that the most favourable conditions for the spin injection could be reached in contacts of sub-micron size.