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ABSTRACT: The microwave surface impedance (Zs = Rs+ iXs) of in situ MgB2 thin films was measured as a function of temperature and parallel dc magnetic field at several frequencies between 5.7 GHz and 18.5 GHz using a dielectric resonator technique. The results are consistent with the expectations for a classical type-II superconductor and, consequently, quite different from those of the high-Tc cuprates. The films cooled in zero-field revealed a clear indication of the lower critical field, Bc1. At higher fields (up to 2 T at 3 K) both Rs and Xs showed a linear increase with field in good agreement with the vortex state model introduced for type-II superconductors by Coffey, Clem and Brandt. The curves measured in sweeping fields (e.g. -2 T < B < 2 T at 3 K) revealed a narrow hysteresis loop in the range |B| < (3 ÷ 4) Bc1and were perfectly symmetrical with respect to the ordinate axis.
Journal of Physics Conference Series 03/2008; 97(1):012060.
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ABSTRACT: The microwave surface impedance (Zs=Rs+iXs) of in situ MgB2 thin films was measured as a function of temperature and parallel dc magnetic field at several frequencies between 5.7 and 18.5 GHz using a dielectric resonator technique. The results are consistent with the expectations for a classical type-II superconductor and, consequently, quite different from those of the high-Tc cuprates. The films cooled in zero field revealed a clear indication of the lower critical field Bc1, with a small hysteresis around B⩽Bc1. In higher fields (B>Bc1), the losses followed the Coffey-Clem and Brandt model, including the frequency dependences, whereas high-Tc Y-Ba-Cu-O films did not show a reasonable agreement with this model. Both the relatively high values of ΔXs∕ΔRs ratio and their frequency dependence indicate a weak effect of flux creep on the measured microwave loss in MgB2 films. The temperature dependence of ΔXs∕ΔRs ratio can be described by a microscopic pinning model for BCS superconductors.
Phys. Rev. B. 06/2007; 75(21).
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ABSTRACT: Evidence of elastic tunneling has been observed in Y Ba <sub>2</sub> Cu <sub>3</sub> O <sub>7-δ</sub>( S )/ La <sub>1/3</sub> Ca <sub>2/3</sub> Mn O <sub>3</sub>( AF )/ La <sub>2/3</sub> Ca <sub>1/3</sub> Mn O <sub>3</sub>( F ) junctions. S/AF/F trilayers with thicknesses of ∼100 , ∼7 , and ∼80 nm , respectively, were grown on Sr Ti O <sub>3</sub> (001) substrates. Electrical measurements of the junctions at low temperatures under zero magnetic field display an asymmetry of the I-V and G(V)≡dI/dV characteristics: for positive voltages, we observed a sharp peak at ∼20 meV at 15 K ; for negative voltages we find at this temperature a peak around -15 meV in agreement with the value expected for Y Ba <sub>2</sub> Cu <sub>3</sub> O <sub>7-δ</sub> . The low values of the conductance G for voltages above the energy gap are discussed.
Journal of Applied Physics 06/2005; · 2.17 Impact Factor
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ABSTRACT: We have studied the exchange bias (EB) interaction in ferromagnetic La2/3Ca1/3MnO3/antiferromagnetic La1/3Ca2/3MnO3 superlattices fabricated on single crystal (001)-(LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates using pulsed laser deposition. The as-grown multilayers were characterized with respect to their magnetic and structural properties by means of dc superconducting quantum interference device magnetometry and x-ray diffraction (XRD) measurements, respectively. The field cooling loop is shifted towards negative fields compared to the zero-field cooling measurement, indicating an EB effect. The XRD profile with clearly resolved superlattice Bragg peaks and multiple satellite peaks around (00l) Bragg reflections of the LSAT substrate gives evidence of a well defined superlattice structure.
Journal of Applied Physics 05/2005; 97(10):10K116-10K116-3. · 2.17 Impact Factor
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ABSTRACT: We fabricated ∼200 nm thin (001)-oriented films of the ferromagnetic metallic perovskite La2/3Ca1/3MnO3 on single crystal (001)-SrTiO3 substrates by dc-sputtering at high oxygen pressure. The samples feature a Curie temperature TC∼260 K and a magnetic moment μ(T→0 K)∼3 μB per Mn atom. The magnetization loops are nearly square-shaped with a coercive field Hc(5 K)=0.03 T that decreases linearly in temperature down to TC. At low temperature, the magnetization shows a decrease ΔM∼T2 and the resistivity an increase Δρ∼T2 as is expected for itinerant electron ferromagnets where single particle excitations dominate. As a further indication of the high quality of the samples, the resistivity without external magnetic field peaks right at TC with a maximum value ρ(H=0,T=TC) of only ∼2 mΩ cm. The magnetoresistance ratio Δρ/ρ0=[ρ(H,T)−ρ(H=0,T)]/ρ(H=0,T) also reaches its maximum value at TC, with Δρ/ρ0(H=6 T,T=TC)∼−50%. The complete absence of this colossal magnetoresistance effect in the low-temperature limit is a further indication of the high sample quality which may be due to a high degree of oxygenation as a result of our preparation condictions. The relation ρ(H,T)=ρmexp[−M(H,T)/M0] observed below TC as well as the temperature characteristic of the resistivity above TC can both be explained by a simple magnetic polaron hopping tunneling model where the tunneling barrier height depends in a simple way on the relative orientation of the local magnetization at both ends of the hopping path. However, here it must be considered that the polarons completely change their character at TC from large and delocalized to small and highly localized entities. © 2004 American Institute of Physics.
Journal of Applied Physics 05/2004; 95(11):6239-6244. · 2.17 Impact Factor
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ABSTRACT: Current transport through ultrathin SrTiO <sub>3</sub> (STO) barriers has been studied systematically with respect to its dependence on barrier thickness d=2–30 nm , temperature T, and voltage V in state-of-the-art planar YBa <sub>2</sub> Cu <sub>3</sub> O <sub>7-x</sub>/ SrTiO <sub>3</sub>/ Au (YBCO/STO/Au) heterojunctions with c-axis oriented YBCO layer. We identified different transport regimes: Elastic tunneling was observed for samples with a nominal barrier thickness of 2 nm, which represents our experimental minimum for obtaining insulating transport characteristics. Already for slightly thicker STO barriers, resonant tunneling and hopping via a small number of localized states begins to dominate the transport behavior. For d≫20 nm , a crossover to variable range hopping behavior is observed in the high-bias voltage regime as well as in the high temperature regime. A localization length of ∼0.46 nm indicating the spread of the localized states can be derived from these experiments. This value is close to the STO lattice constant and corresponds to a high density of localized states of n<sub>L</sub>∼6×10<sup>19</sup>( eV )<sup>-1</sup> cm <sup>-3</sup>. In a free electron tunneling model, this corresponds to an average tunnel barrier height of ∼0.4 eV. © 2003 American Institute of Physics.
Journal of Applied Physics 12/2003; · 2.17 Impact Factor
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ABSTRACT: Current injection into superconducting microbridges has been studied for state-of-the-art YBa2Cu3O7−x/SrTiO3/Au (YBCO/STO/Au) planar heterostructures based on c-axis oriented YBCO layers. By injecting a gate current Ig into the YBCO channel of these three-terminal devices the critical current Ic could be reduced up to a gain factor G≡−dIc/dIg∼5 at 77 K. G is found to be roughly proportional to the superconducting quality of the samples as expressed by the critical current density Jc of the YBCO channel. At least for our high-quality samples, this Ic suppression is shown to be mainly due to the perturbation of the electronic superconductive equilibrium state by quasiparticles associated with the injection of the gate current. However, this basic mechanism for the current-injection-based transistor principle was in our experiments always accompanied by substantial parasitic effects, such as current summation and Joule heating due to electric power dissipation in the ultrathin STO barrier. This reduces the efficiency of the transistor mechanism and thus the hopes for THz operation of such devices. © 2003 American Institute of Physics.
Journal of Applied Physics 11/2003; 94(10):6667-6672. · 2.17 Impact Factor
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ABSTRACT: High-quality epitaxial thin films of the ferromagnetic metallic oxide SrRuO3 (SRO) were fabricated by dc-sputtering at high oxygen pressure and their structural and magnetoelectrical properties were carefully studied. The films featured a Curie temperature TC ∼ 160 K and a magnetic moment of ∼0.7 μB per Ru ion. The temperature dependent magnetization could be well described by the scaling relation M(T) ∝ (TC − T)β with a critical exponent β = 0.53 over the entire ferromagnetic temperature range. A negative magnetoresistance, MR, on the order of a few percent was found up to room temperature. MR showed a maximum of ∼4% right at TC where a kink structure of the resistivity, ρ, at zero field was flattened out on magnetic field application. This ρ contribution could be related to scattering due to orientational disorder of the Ru magnetic moments which become aligned by an external magnetic field. In addition, an equally strong MR effect, related to localization phenomena, could be observed at lower temperature. Particularly, the second MR peak at ∼35 K might be related to a Fermi-liquid to non-Fermi-liquid crossover. A scaling behavior dρ/dT ∝ |T − TC|α was observed only above TC. Here, values for the exponent α ≈ −0.4 and α ≈ −1.4 were obtained in zero field and in a field of 9 T, respectively. The commonly observed ρ minimum, appearing at low temperatures (∼3 K in the present case), is correlated with the structural disorder of the SRO films and is believed to have its origin in quantum corrections to the conductivity (QCC).Graphical abstract
Solid State Sciences. 11(3):700-705.
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ABSTRACT: High-quality, c-axis oriented YBa2Cu3O7 − x/SrTiO3/Au (YBCO/STO/Au) planar structures were fabricated in situ by direct current/radiofrequency inverted-cylinder magnetron sputtering on (001) STO oriented substrates. The sandwich-type structures were patterned to transistor dimensions by standard ultraviolet-photolithography and Ar etching. The current transport mechanism in the very thin STO barriers (2–30 nm) was examined by measuring the tunneling G as function of temperature (T), and bias voltage (V). It was found that resonant tunneling and hopping via a small number of localized states (LS) are responsible for electronic conduction in the insulating material. Elastic tunneling was observed for the case of a nominal 2 nm thick STO-barrier with an energy gap Δ ≈ 20 meV in the (001) direction of YBCO. On the other hand, inelastic hopping transport via n-LS dominated for STO barrier thickness d > 2 nm. G of the lowest-order hopping channel (hopping via two LS) exhibits the characteristic T and V dependences: G2hop(T) ∝ T4/3, G2hop(V) ∝ V4/3, respectively. Increasing the thickness of the STO barriers, hopping channels of higher order contribute more and more to the current transport as proven by measuring the T and V dependences. A crossover to variable range hopping behavior has been observed for junctions with thicker barriers (d ≥ 20 nm) in the high-V or high-T regime. By fitting the experimental data to theoretical models, physical parameters of the LS could be determined. For instance, the value of the localization length or radius of the localized state was determined to be ~ 4.6 × 10− 8 cm which corresponds to the lattice constant of the STO unit cell. A value of ~ 6 × 1019 (eV)− 1 cm− 3 was calculated for the density of LS and the average barrier height was estimated as ~ 0.4 eV.
Thin Solid Films 517(6):1908-1916. · 1.89 Impact Factor
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F. Perez,
K. Gross,
E. Baca,
W. Saldarriaga,
P. Prieto,
M.E. Gomez,
O. Morán, R. Hott,
K. Grube,
D. Fuchs,
R. Schneider
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ABSTRACT: Perovskite ferromagnetic La2/3Ca1/3MnO3 films with thickness between 22 and 35 unit cells were deposited on single-crystal SrTiO3 substrates with crystal plane orientation (0 0 1) and covered with ∼10 unit cells of superconducting YBa2Cu3O7−δ in order to study their microstructure and physical properties. X-ray diffraction and atomic force microscopy studies showed no dramatic change of the structural and morphological quality of the superconducting top layers with increasing of the thickness of the ferromagnetic layer. Electrical transport and magnetic measurements revealed a strong dependence of both the superconducting transition temperature and the Curie temperature for the onset of ferromagnetic ordering on the thickness of the ferromagnetic layer. Intrinsic phenomena like proximity effect or extrinsic factors like interface strain, interdiffusion of cations between the constituents layers or spin injection from the magnetic into the superconducting electrode are discussed as possible sources of the depression of the critical temperature in these bilayers.
Physica C: Superconductivity.
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ABSTRACT: We prepared in-situ Au contacts on high-quality epitaxial YBa2Cu3O7 (YBCO) films. Very high specific contact resistivity values up to ∼10−2 Ω cm2 at 4.2 K were obtained on 12×5 μm2 contact areas. This resistivity value decreased by two orders of magnitude as the temperature was raised to room temperature. In the temperature range T<200 K, the contacts showed non-ohmic behavior suggesting the presence of a well-defined insulating native Y–Ba–Cu–O barrier between the two electrodes. The electrical transport in this barrier layer was analyzed in the limit of high temperatures and high voltages to follow Mott's variable-range hopping conduction mechanism with physically reasonable parameters describing the localized states in the barrier. The high-resistivity contacts were tested successfully in quasiparticles injection experiments where the critical current Ic of the YBCO microbridge could be strongly suppressed on injection of an additional current through the contact into the superconducting channel.
Journal of Physics and Chemistry of Solids.
