Nature of contact resistance in high-T c YBa2Cu3O7−δ Superconductors

The contacts between various metals and the high-T_c superconductor YBa₂Cu₃O_7– are characterized by contact resistance,R _c, andI–V measurements from 300 K to 90 K. The contacts with bulk superconductor were made by vacuum deposition. Four metals, Au, Ag, Al, and Bi were investigated. The current transport across the contact is by carrier tunneling. All contacts were ohmic as theirI–V characteristics were symmetrical with respect to current direction.R _c values range between 10^–2 to 10¹ cm² and increase linearly as the temperature is lowered. The contact resistance originates from two distinct physical processes. One is the modification of the carrier concentration at the interface by the contact metal. The second is the nature of carrier injection at the free surface of the superconductor. TheR _c values depend on the contact metal-oxygen interaction parameter signifying the need for oxygen passivation for obtaining low contact resistances.     

Tunneling and point-contact spectroscopy of high-T c superconductors M−Ba2Cu3O7 (M=Y,La, Eu)

Tunneling and point-contact measurements have been performed on different high-T _c superconductors M–Ba₂Cu₃O₇ with M=Y, La, Eu. The average energy gaps deduced from the tunneling spectra are about 2=33 meV for the Y- and La-type samples and about 2=38 meV for the Eu-type samples, which give ratios 2/k _B T _c in the range from 4 to 6. The point-contact characteristics show a distinct minimum in the differential resistance about zero bias and additional sharp spikes up to 40 mV. From these pointcontacts we can give an estimate of the critical current, yielding values in the range from 0.2 to 3 mA.     

On theg≃2 ESR resonance in YBa2Cu3O7-y high-T c superconductor

Various samples of single-phase YBa₂Cu₃O_7–y have been examined by X-band ESR in theg2 region forTT _c .The observed spectra are attributed to oxygen coordinated Cu²⁺ ions with a large delocalization of the unpaired spins. These are probably due, in part, to the inclusion of some impurity phase. However, an analysis of the intensity as a function of the temperature in different samples indicates that a possible intrinsic contribution to the ESR signal of Cu²⁺ ions from the superconducting phase is due, at most, to 1% of the total copper in the formula. This small quantity of isolated Cu²⁺ ions may be due to the presence of low symmetry Cu³⁺ ions or, equivalently, non-magnetic centers such as Cu²⁺–O^– pairs in the chains.CISM (M.P.I.) and GNSM (C.N.R.)     

Magnetic properties of superconducting EuBa2Cu3O7−x

The magnetic properties of superconducting EuBa₂Cu₃O_7–x with resistiveT _c 96.5 K are measured. The normal state magnetic susceptibility is analyzed within the framework of the Van Vleck-Frank theory, leading to the conclusion that the strong moments of the Eu³⁺ ions are uncorrelated, and do not affect the superconducting state.     

μSR magnetic response in frustrated antiferromagnets of type RMn2 (R = rare earth)

Zero longitudinal and transverse fieldSR was carried out in the antiferromagnets YMn₂, Y_0.95 Tb_0.15 Mn₂, Y_0.9Tb_0.1Mn₂, Y_0.99 Sc_0.01 Mn₂, Y_0.98Sc_0.02Mn₂ and TbMn₂. The dynamics of Mn magnetic moments above T _N is typical for an itinerant antiferromagnet. Within a certain temperature range above T _N part of the material enters a randomly ordered (spin glass like) magnetic state as an outcome of frustration. At temperatures above 150 K the muon spin relaxation rate indicates that the muon has become mobile.This work was supported by the German Federal Minister for Research and Technology under Contract Nr. 03KA2-TUM-4.     

Magnetic properties of the R1+eFe4B4 compounds (R = rare earth) from magnetization and Mössbauer measurements

The magnetic properties of rare earth-iron-boron alloys with composition R_1+eFe₄B₄ have been determined using Mössbauer and magnetization measurements. Magnetic ordering occurs at temperatures between 4.2 and 25 K for the compounds with R = Pr, Nd, Sm, Gd, Tb, Dy, Ho. The Curie temperature scales very well with the de Gennes factor for the heavy rare earth members of the series, while significant deviations are observed for the light rare earths indicating the presence of strong CEF effects. The absence of magnetic hyperfine splitting even at 4.2 K indicates that the Fe ion has a zero magnetic moment. This is confirmed by Mössbauer spectra in an applied magnetic field.     

Magnetic properties of the high Tc superconductor YBa2Cu3O7−δ

Magnetic measurements have been performed on the YBa₂Cu₆O_7−δ superconducting system in the temperature range 4.2–320 K. A hystereris loop indicating a coexistence of the superconducting and magnetic ordering below H_c1 at 4.2 K has been recorded.     

Magnetic properties of RSr2RuCu2O8+δ (R=Eu and Gd)

The non superconducting RSr₂RuCu₂O_8+δ (R=Eu and Gd) compounds are magnetically ordered below T_N=168 and 185 K, respectively. Magnetic susceptibility (ac and dc) studies indicate that the magnetic ordering is due to the Ru sublattice. The Gd sublattice in GdSr₂RuCu₂O₈ is antiferromagnetically ordered at 2.8 K. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields, originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The magnetic behavior of this system and that of the well-known itinerant ferromagnet SrRuO₃ are compared.     

Lack of delocalized Cup states at the fermi level in the high-T c superconductor YBa2Cu3O∼7 by XANES spectroscopy

The x-ray absorption spectra of the single phase high temperature superconducting YBa₂Cu₃O₇ withT _{c, onset}=97 K have been measured between 30 K and 300 K by using synchrotron radiation.The copperK-edge x-ray absorption near edge structure (XANES) spectrum gives experimental evidence of the lack of delocalized Cup states close to the Fermi level, in fact a spectral distribution of local (at the Cu site) partial (l=1) density of states as in the insulating CuO has been found.The EXAFS (extended x-ray absorption fine structure) spectrum shows evidence of the formation of layers formed by Cu ions coordinated by oxygen square planes sharing corners. The average Cu–O distance at 30 K in the layers has been found to be 1.93 i.e. 0.02 shorter than in CuO and the average distance between Cu ions at the centers of the square planes is 3.85.The present local structure suggests that superconductivity should occur in the crystalline planes formed by CuO₄ square plane units. The lack of delocalized Cu states is in agreement with the hypothesis that conductivity is due to itinerant holesL in the oxygen derived valence band.The results of this experiment have been presented at the Symposium on the Latest Developments in Superconductivity at High Temperatures of the European Physical Society Meeting held in Pisa april 8, 1987     

Above-T c anomalies of the infrared-active phonons in RBa2Cu4O8 (R=Dy,Ho) and Y2Ba4Cu7O15−δ superconductors

We report a far-infrared reflectivity study of DyBa₂Cu₄O₈, HoBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– superconductors in the temperature range 10–300 K. The assignment of thez-polarized infrared-acitve vibrations of the rare-earth-ion (R) was verified by the substitution ofR. Since the structure of the Y₂Ba₄Cu₇O_15– compound inz-direction is an ordered sequence of YBa₂Cu₃O_7– and YBa₂Cu₄O₈ blocks, the phonon spectrum of Y₂Ba₄Cu₇O_15– is a superposition of their respective phonon modes. The temperature dependence of the infrared-active phonons in RBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– was analyzed. A narrowing of the plane-oxygen phonon upon sample cooling was found to occur atT _c , as it was reported for single-chain YBa₂Cu₃O₇ materials; however a pronounced frequency softening of this mode for both RBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– was found to start well aboveT _c , suggesting the interaction of infrared-active phonons with an excitation gap similar to that observed in spin-dependent measurements.     

Crystal field effects on the magnetic and hyperfine properties of rare earth ions in RE2 (SO4)3 · 8H2O (RE = Pr,Dy and Er)

An analysis of the observed magnetic properties of Pr-sulfate, Dy-sulfate and Er-sulfate has been undertaken on the basis of the interaction of the rare earth ion with the lattice. An effective crystal field conforming to tetragonal symmetry, which is the approximate site symmetry of the RE³⁺ ion, gives an excellent simulation of the observed behavior. The g-factors and Schottky anomalies in the heat capacity are determined theoretically. Various hyperfine properties including the thermal variation of Mössbauer spectra are predicted in light of the proposed crystal field.     

Magnetic properties of Yb Mo O and Gd Mo O from rare earth Mössbauer measurements

Using ¹⁷⁰Yb and ¹⁵⁵Gd M?ssbauer measurements down to ∼ 0.03 K, we have examined the semiconducting pyrochlore Yb₂Mo₂O₇ where the Mo intra-sublattice interaction is anti-ferromagnetic and the metallic pyrochlore Gd₂Mo₂O₇ where this interaction is ferromagnetic. Additional information was obtained from susceptibility, magnetisation and ¹⁷²Yb perturbed angular correlation measurements. The microscopic measurements evidence lattice disorder which is important in Yb₂Mo₂O₇ and modest in Gd₂Mo₂O₇. Magnetic irreversibilities occur at 17 K in Yb₂Mo₂O₇ and at 75 K in Gd₂Mo₂O₇ and below these temperatures the rare earths carry magnetic moments which are induced through couplings with the Mo sublattice. In Gd₂Mo₂O₇, we observe the steady state Gd hyperfine populations at 0.027 K are out of thermal equilibrium, indicating that Gd and Mo spin fluctuations persist at very low temperatures. Frustration is thus operative in this essentially isotropic pyrochlore where the dominant Mo intra-sublattice interaction is ferromagnetic. Received 13 January 2003 Published online 4 June 2003 RID="a" ID="a"e-mail: hodges@drecam.saclay.cea.fr     

Magnetic field orientation dependence of flux pinning in (Gd,Y)Ba2Cu3O7−x coated conductor with tilted lattice and nanostructures

The dependence of the critical current density (J_c) on the orientation of an applied magnetic field was studied for a prototype (Gd,Y)Ba₂Cu₃O_7?x (GdYBCO) coated conductor fabricated by MOCVD on an IBAD-MgO template. Additional rare-earth cations (Y and Gd) and Zr were incorporated into the superconducting film to form (Y,Gd)₂O₃ and BaZrO₃ nanoparticles extended nearly parallel to the a–b planes and to the c-axis, respectively, to enhance the flux pinning. In-field measurement of J_c was carried out with electrical current flowing either along or perpendicular to the longitudinal axis of the tape, while a maximum Lorentz force configuration was always maintained. Details in the angular dependence of J_c were related to the unique structure of the film, specifically the tilt in the GdYBCO lattice and the tilts in the extended (Y,Gd)₂O₃ and BaZrO3 nanoparticles. XRD and TEM were used to study the structure of the coated conductor. The effect of the misalignment between the external field H and the internal field B on the angular dependence of J_c is discussed.     

Magnetic ordering on Yb3+ in YbBa2Cu3O7−x

The high critical temperature superconductor YbBa₂Cu₃O_7−x has been studied over the range 0.05 to 95K using ¹⁷⁰Yb Mössbauer absorption spectroscopy. Magnetic ordering occurs within the Yb³⁺ sublattice at 0.35K. The saturated magnetic moments are 1.7μ_B. At all temperatures the hyperfine parameters show the presence of distributions attributed to some disordering of the oxygen vacancies.     

Magnetostriction of REBa2Cu3O7-δ

The magnetostriction (MS) of REBa₂Cu₃O_7– (RE=Y, Nd, Eu, Gd, Ho, Er, Yb) is reported as a function of magnetic field and temperature forH<5 T and 1.5H>1 T. The MS is much larger when the rare earth carries a magnetic moment, as usual, but the volume MS observed with moment carrying REBa₂Cu₃O_7– is larger than that of nonsuperconducting intermetalics. This effect is explained by the shielding currents which are induced by the 4f magnetic moments in the superconducting CuO-lattice. The thermal expansion of GdBa₂Cu₃O_7– shows a large peak at the ordering temperature of 2.2 K in both the superconducting and the nonsuperconducting samples.     

Magnetic properties of GdBa2Cu3O7−x in the normal and superconducting states

The magnetic properties of superconducting GdBa₂Cu₃O_7–x withT _c95 K are measured. In the normal state, its susceptibility closely follows the Curie law with an effective moment of 7.77 _B .H _c1 is determined to be 650 Oe. No superconducting fluctuation can be delineated. Flux pinning indicated behavior of type II superconductors. A 25–405 diamagnetic state indicating bulk superconductivity in GdBa₂Cu₃O_7–x .     

Temperature dependence of the Cu(2) NQR line width in YBa2Cu3O7−y

Precision X-ray structural studies were carried out for LiNbO₃:Zn_x single crystals with x=0.0, 2.87, 5.20, and 7.60 at. %. It was found that the insertion of the Zn atoms into the Li position was accompanied by a decrease in the concentration of intrinsic Nb_Li defects. At x=7.6%, the Zn atoms change their locations in the lattice and partially occupy the Nb positions. This clarifies the structural nature of the “threshold” Zn concentration, which manifests itself as singularities in the concentration dependences of various optical properties. The structural origin of the threshold concentration is likely a common feature of all nonphotorefractive impurities (Mg, Zn, In, and Sc) in LiNbO₃. A change in the intrinsic defect structure of the LiNbO₃ crystals with different Zn concentrations is discussed.     

Process of magnetic field penetration into the high-temperature superconductor YBa2Cu3O7−δ

The processes of magnetic field penetration into the ceramic samples of the HTSC YB₂Cu₃O_～6.95 at T<T _c are studied by the methods of internal friction and magnetization measurements. A clearly manifested correlation is observed between the field dependences of the internal friction spectrum parameters (the logarithmic damping decrement Q ^?1 and the resonance frequency f) and the trapped magnetic flux ΔM. The magneto-mechanical approach we used reveals a significant difference in the field dependences of the densities of pinned (N _p) and free (N _f) Abrikosov vortices for H>H _c1.     

Electric field effect on superconducting YBa2Cu3O7−δ films

Experimental evidence for a significant electric field effect in thin superconducting films of YBa₂Cu₃O_7– is presented. MISFET-type structures have been developed which allow the application of electric fields larger than 4×10⁶ V/cm across insulating SrTiO₃ barriers into thin cpitaxial YBa₂Cu₃O_7– channel layers. The results demonstrate that with these structures the electrical resistivity aboveT _c (R=0) and the density of free carriers in the YBa₂Cu₃O_7– films can be modified by 1–2% with gate voltages smaller than 50 V.