J. L. Smith

Properties of YBa2Cu3O7−δ thick films on flexible buffered metallic substrates

We report superconducting and mechanical properties of YBa2Cu3O7−δ (YBCO) thick films on Ni‐based alloys with a textured yttria‐stabilized zirconia (YSZ) buffer layer. The YBCO and YSZ layers were deposited by pulsed laser deposition and ion beam assisted deposition, respectively. It was found that the transport critical current density (Jc) correlates very well with the YBCO mosaic spread. Jc over 1×10 6 A/cm2 at 75 K and ∼1×107 A/cm2 at 4 K were obtained in the 1‐μm thick YBCO films. Zero field critical current of 120 amps at 75 K was obtained in a 2‐μm‐thick and 1‐cm‐wide YBCO film. Angular dependence measurement revealed Jc peaks for both H∥c and H∥a‐b. The peak for H∥c implies additional pinning due to defects such as small angle grain boundaries or twin boundaries. Bending tests at 75 K showed that the YBCO thick films on the metallic substrates could sustain a strain of 0.4% and over 1% for tension and compression, respectively.

Critical examination of heat capacity measurements made on a Quantum Design physical property measurement system

Abstract We examine the operation and performance of an automated heat-capacity measurement system manufactured by Quantum Design (QD). QD’s physical properties measurement system (PPMS) employs a thermal-relaxation calorimeter that operates in the temperature range of 1.8–395 K. The accuracy of the PPMS specific-heat data is determined here by comparing data measured on copper and synthetic sapphire samples with standard literature values. The system exhibits an overall accuracy of better than 1% for temperatures between 100 and 300 K, while the accuracy diminishes at lower temperatures. These data confirm that the system operates within the ±5% accuracy specified by QD. Measurements on gold samples with masses of 4.5 and 88 mg indicate that accuracy of ±3% or better can be achieved below 4 K by using samples with heat capacities that are half or greater than the calorimeter addenda heat capacity. The ability of a PPMS calorimeter to accurately measure sharp features in Cp(T) near phase transitions is determined by measuring the specific heat in the vicinity of the first-order antiferromagnetic transition in Sm2IrIn8 (T0=14 K) and the second-order hidden order (HO) transition in URu2Si2 (TN=17 K). While the PPMS measures Cp(T) near the second-order transition accurately, it is unable to do so in the vicinity of the first-order transition. We show that the specific heat near a first-order transition can be determined from the PPMS-measured decay curves by using an alternate analytical approach. This correction is required because the latent heat liberated/absorbed at the transition results in temperature–decay curves that cannot be described by a single relaxation time constant. Lastly, we test the ability of the PPMS to measure the specific heat of Mg11B2, a superconductor of current interest to many research groups, that has an unusually strong field-dependent specific heat in the mixed state. At the critical temperature the discontinuity in the specific heat is nearly 15% lower than measurements made on the same sample using a semi-adiabatic calorimeter at Lawrence Berkeley National Laboratory.

Superconductivity of rare earth-barium-copper oxides

Abstract We report the superconductivity of R-Ba-Cu-O compounds for rare earths R = Sm through Ho. The Nd and Tm compounds were not observed to be superconducting. The Tc onsets for the Eu and Gd compounds are comparable to that observed for Y-Ba-Cu-O. An approximate value for the upper critical field slope of the Gd compound is given.

Direct observation of heavy quasiparticles in UPt3 via the dHvA effect

We present the results of an investigation of the de Haas-van Alphen (dHvA) effect in the heavy fermion superconductor UPt3. Oscillations composed of up to 8 frequency components, corresponding to cyclotron orbits in a plane normal to the a-axis, have been detected in a high purity single crystal and a study of their amplitude as a function of temperature and magnetic field has been performed in the intervals 20–150 mK and 40–115 kG, respectively. From this study we obtain estimates of the cyclotron masses, found to range approximately from 25 to 90 times the bare electron mass, and of the mean free path, found to be in excess of 1000 A. The relationship between these findings and the results of conventional energy band calculations is discussed.

Superconductivity above 90 K in magnetic rare earth-barium-copper oxides

Abstract We report measurements of the superconducting and magnetic behavior of GdBa2Cu3Ox. Superconductivity occurs below Tc = 95 and 93 K, respectively, for these compounds with large Meissner effects observed. The Gd-based compound has an upper critical field slope -dH c2 d T greater than 1.2 T/K. Both compounds exhibit Curie-Weiss behavior above Tc with free-ion effective moments and small, negative temperature intercepts. In addition, the Gd-based compound orders antiferromagnetically below 2.24 K with an entropy consistent with the ordering of seven unpaired spins.

Thermally stimulated luminescence from x-irradiated porous silicon

We have measured thermally stimulated luminescence from light-emitting porous silicon that has been x irradiated at room temperature and heated to 400 °C. The glow curve exhibits peaks at 103, 155, 219, and 271 °C, with additional maxima occurring above 400 °C. Each of the peaks emits similar emission spectra characterized by a band with a maximum near 720 nm and 0.39 eV full width at half-maximum. Following x irradiation at room temperature, the sample exhibits well-known photoluminescence, but after heating to 400 °C, the loss of hydrogen renders the sample nonphotoluminescent. However, thermally stimulated luminescence can be repeatedly induced. Observation of thermally stimulated luminescence is unambiguous evidence for the existence of an insulating surface layer on porous silicon.

Combined Experimental and Theoretical Investigation of the Premartensitic Transition in Ni2MnGa

Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity, and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni2MnGa over the temperature range 100<T<250 K. All measurements detect clear signatures of the premartensitic transition (T(PM) approximately 247 K) and the martensitic transition (T(M) approximately 196 K). Temperature-dependent UPS shows a dramatic depletion of states (pseudogap) at T(PM) located 0.3 eV below the Fermi energy. First-principles electronic structure calculations show that the peak observed at 0.3 eV in the UPS spectra for T>T(PM) is due to the Ni d minority-spin electrons. Below T(M) this peak disappears, resulting in an enhanced density of states at energies around 0.8 eV. This enhancement reflects Ni d and Mn d electronic contributions to the majority-spin density of states.

Epitaxial growth of highly conductive RuO2 thin films on (100) Si

Conductive RuO2 thin films have been heteroepitaxially grown by pulsed laser deposition on Si substrates with yttria‐stabilized zirconia (YSZ) buffer layers. The RuO2 thin films deposited under optimized processing conditions are a‐axis oriented normal to the Si substrate surface with a high degree of in‐plane alignment with the major axes of the (100) Si substrate. Cross‐sectional transmission electron microscopy analysis on the RuO2/YSZ/Si multilayer shows an atomically sharp interface between the RuO2 and the YSZ. Electrical measurements show that the crystalline RuO2 thin films are metallic over a temperature range from 4.2 to 300 K and are highly conductive with a room‐temperature resistivity of 37±2 μΩ cm. The residual resistance ratio (R300 K/R4.2 K) above 5 for our RuO2 thin films is the highest ever reported for such films on Si substrates.

Superconductivity and heavy fermions

B BATLOGG, D J BISHOP, E BUCHER, B GOLDING, Jr, A P RAMIREZ AT& T Bell Laboratorles, Murray Hill, NJ 07974, USA Z FISK, J L SMITH Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos NM 87545 USA and H R OTT Laboratonum fur FestkorperphysIk, ETH-Honggerberg 8093, Zurich, Switzerland Experimental studies are discussed which shed hght on the nature of the heavy fermlon normal state, the occurrence of heavy fermion superconductivity and anomalous properties of the superconducting state l) Magnetization and mag- netoreslstance of the Kondo lattice UBel~ demonstrate the breakdown of a description m terms of a single energy scale TK Here T K becomes temperature dependent below ~ 10 K, and the canomcal relationship between R(H) and M(H) does not hold n) Superconductivity in UPta occurs at the borderline to magnetism spin-density wave-hke instabihtJes are induced by a few percent of Th or Au, whereas mass enhancement and susceptlbihty are rapidly reduced by Ir subsntution This suggests that spin fluctuations, rather than phonons, are ~mportant in medmtmg superconducting pairing of heavy fermions in UPt~ lU) The posstbillhes of noncubic strain induced by amsotropic superconductivity in UBe~ ~ is investigated by ultrasound experiments The suggestion of tetragonal strata in (U Th)Be~ is found to be not applicable, and observed frequency and amplitude dependent absorption anomalies at T,~ T, might reflect domain wall motion

SiOx luminescence from light‐emitting porous silicon: Support for the quantum confinement/luminescence center model

Measurements of hydrogen loss and luminescence as a function of annealing temperature in porous silicon suggest that luminescence is attributable to electron‐hole recombination in SiOx surface layers with an intensity that is dependent upon the surface hydrogen content. The luminescence is composed of three Gaussian bands similar to those found in amorphous SiO2. X‐ray photoelectron spectroscopy and scanning electron microscopy show porous silicon has SiOx on the surface, which is comprised of many particles of about 10 nm size. Collectively, the data strongly support the previously proposed quantum confinement/luminescence center model.