Critical Field Measurements Research Articles

Ground state of the three-band Hubbard model

The ground state of the two-dimensional three-band Hubbard model in oxide superconductors is investigated by using the variational Monte Carlo method. The Gutzwiller-projected BCS and spin density wave (SDW) functions are employed in the search for a possible ground state with respect to dependences on electron density. Antiferromagnetic correlations are considerably strong near half-filling. It is shown that the d-wave state may exist away from half-filling for both the hole and electron doping cases. The overall structure of the phase diagram obtained by our calculations qualitatively agrees with experimental indications. The superconducting condensation energy is in reasonable agreement with the experimental value obtained from specific heat and critical magnetic field measurements for optimally doped samples. The inhomogeneous SDW state is also examined near 1/8 doping. Incommensurate magnetic structures become stable due to hole doping in the underdoped region, where the transfer ${t}_{\mathrm{pp}}$ between oxygen orbitals plays an important role in determining a stable stripe structure.

Growth of c-oriented MgB2 thin films by pulsed laser deposition: structuralcharacterization and electronic anisotropy

MgB2 thin films weredeposited using pulsed laser deposition (PLD) and ex situ annealing inMg atmosphere. The films presented critical temperatures up to 36 K and turnedout to be preferentially c-oriented both on Al2O3 (r-cut) and MgO(100)substrates. Synchrotron analyses also gave some indications of in-planetexturing. The films exhibit very fine grain size (1200 Å in the basal planeand 100 Å along the c-axis) but the general resistivity behaviour and theremarkable extension of the irreversible region confirm that the grains'boundaries are not barriers for supercurrents. Upper critical fieldmeasurements with the magnetic field perpendicular and parallel with respectto the film surface evidenced a field anisotropy ratio of 1.8. The Hc2values are considerably higher with respect to the bulk ones, namely when thefield lies in the basal plane, and the field-temperature phase diagram forthe two magnetic field orientations suggest the possibility of stronglyenhancing the pinning region by means of texturing.

Critical fields and specific heat ofLuNi2B2C

We have measured the upper critical magnetic field, lower critical magnetic field, and low-temperature specific heat of ${\mathrm{LuNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$ Our upper critical-field measurements above $\ensuremath{\sim}2\mathrm{K}$ are consistent with earlier measurements by other groups. The upper critical field exhibits a finite slope as $\stackrel{\ensuremath{\rightarrow}}{T}0,$ and we discuss possible origins of this very unusual behavior. We observe a linear temperature dependence for the lower critical field of our single-crystal sample, a temperature dependence that disagrees with earlier measurements on polycrystals and suggests that the lower critical field may be anisotropic. The Sommerfeld coefficient increases with magnetic field as ${H}^{\ensuremath{\epsilon}}$ with $\ensuremath{\epsilon}=0.63\ifmmode\pm\else\textpm\fi{}0.12$ in qualitative agreement with earlier work. The temperature dependence of the zero-field electronic specific heat is exponential well below ${T}_{c}.$ We discuss our results in light of recent proposals of both anisotropic s-wave and d-wave superconductivity for ${\mathrm{LuNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$ Our data seem most consistent with strong-coupling superconductivity and a weakly anisotropic s-wave energy gap.

A study on the relation between the strain scaling law and the temperature scaling law on flux pinning in Nb 3Sn superconducting wires

The relation between the strain scaling law and the temperature scaling law in flux pinning force has been studied by measuring the temperature dependence of upper critical fields of bronze-processed Nb 3Sn monocore-wires. Measurement of upper critical fields has been carried out for specimens with and without the bronze matrix by measuring the resistive transition in the temperature range from 12 to 18 K and measuring the critical current density at 4.2 K. Removing the bronze matrix, the critical temperature and the slope of the upper critical field vs. temperature curve near the critical temperature are increased. Based on these data and the electron scattering model of grain boundary flux pinning, the cause of difference in the upper critical field dependence of the volume flux pinning force between the strain scaling law and the temperature scaling law has been discussed. Furthermore, experimental formulae to describe the volume flux pinning force have been proposed when both strain and temperature are simultaneously varied.

The effect of thermal treatment on superconductivity in HoNi 2B 2C

We present detailed studies of the dependence of the low-temperature properties of HoNi 2B 2C on the annealing temperature. We could shift the superconducting transition temperature T c reversibly and continuously by changing the thermodynamic state of a sample via thermal treatment. The magnitude of this shift is as large as about 10% of T c. The superconducting transition temperature is correlated with the occurrence and strength of re-entrant behaviour. Upper critical field measurements of two samples annealed at 800°C and 1100°C indicate that the magnetic ordering temperatures are not affected by the thermal treatment. A reversible shift of T c with annealing temperature has also been observed for YNi 2B 2C and LuNi 2B 2C with non-magnetic rare earth ions.

A Global Terrestrial Monitoring Network Integrating Tower Fluxes, Flask Sampling, Ecosystem Modeling and EOS Satellite Data

Accurate monitoring of global scale changes in the terrestrial biosphere has become acutely important as the scope of human impacts on biological systems and atmospheric chemistry grows. For example, the Kyoto Protocol of 1997 signals some of the dramatic socioeconomic and political decisions that may lie ahead concerning CO 2 emissions and global carbon cycle impacts. These decisions will rely heavily on accurate measures of global biospheric changes (Schimel, 1998; IGBP TCWG, 1998). An array of national and international programs have inaugurated global satellite observations, critical field measurements of carbon and water fluxes, and global model development for the purposes of beginning to monitor the biosphere. The detection by these programs of interannual variability of ecosystem fluxes and of longer term trends will permit early indication of fundamental biospheric changes which might otherwise go undetected until major biome conversion begins. This article describes a blueprint for more comprehensive coordination of the various flux measurement and modeling activities into a global terrestrial monitoring network that will have direct relevance to the political decision making of global change.

Critical Magnetic Field Measurements of the Misfit LayerCompound (SnS)1.17(NbS2) down to50mK

We report on magnetoresistance measurements of the misfit layercompound (SnS)1.17(NbS2) in the superconductingstate down to 50mK. Its anisotropy was found to be in the range of 10and remains constant down to the lowest temperature of measurement. Onthe other hand, the coherence length in the direction perpendicular tothe superconducting planes at zero temperature is higher than thedistance separating the centers of two successive superconductinglayers, so this compound behaves as a3D-superconductor.

Transport and superconducting properties ofRNi2B2C (R=Y,nLu) single crystals

The in-plane resistivity, in-plane absolute thermopower, and upper critical field measurements are reported for single-crystal samples of ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C and ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C superconductors. The in-plane resistivity shows metallic behavior and varies approximately linearly with temperature near room temperature (RT) but shows nearly quadratic behavior in temperature at low temperatures. The ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C and ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C single-crystal samples exhibit large transverse magnetoresistance (\ensuremath{\approx}6--8 % at 45 kOe) in the ab plane. The absolute thermopower S(T) is negative from RT to the superconducting transition temperature ${\mathrm{T}}_{\mathrm{c}}$. Its magnitude at RT is a few times of the value for a typical good metal. S(T) is approximately linear in temperature between \ensuremath{\approx}150 K and RT. Extrapolation to T=0 gives large intercepts (few \ensuremath{\mu}V/K) for both samples suggesting the presence of a much larger 'knee' than would be expected from electron-phonon interaction renormalization effects. The upper critical fields for H parallel and perpendicular to the c axis and the superconducting parameters derived from it do not show any anisotropy for the ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C single-crystal samples in agreement with magnetization and torque magnetometry measurements, but a small anisotropy is observed for the ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C single crystals. The analysis shows that these are moderately strong-coupling type-II superconductors (similar to the A-15 compounds) with a value of the electron-phonon coupling parameter \ensuremath{\lambda}(0) approximately equal to 1.2 for ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C and 1.0 for ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C, the Ginzburg-Landau coherence length \ensuremath{\xi}(0) approximately equal to 70 \AA{}, and ${\mathrm{H}}_{\mathrm{c}2}$(0)\ensuremath{\sim}60--70 kOe. The temperature dependence of the upper critical field shows a positive curvature near ${\mathrm{T}}_{\mathrm{c}}$ in disagreement with the Werthamer, Helfand, Hohenberg, and Maki (WHHM) theory but in agreement with a recent solution of the Gor'kov equation using a basis formed by Landau levels (Bahcall); however, the data show a severe disagreement between the observed low-temperature behavior of ${\mathrm{H}}_{\mathrm{c}2}$(T) and that predicted either by WHHM or Bahcall's expressions.

The specific heat and upper critical field of amorphous - electronic and vibrational excitations

The specific heat C of amorphous films was investigated between 0.3 K and 7 K in the superconducting state as well as in the normal state using a modified AC method. The analysis of the data allows one to determine the electronic density of states at the Fermi energy , the density of states of low-energy excitations and the Debye temperature . The concentration dependence of shows a pronounced minimum at x = 60 at.% in comparison to the free-electron model, in agreement with specific heat and UPS measurements and theoretical calculations published recently. The specific heat of the low-energy excitations is of the same order of magnitude as data available from the literature, and shows a linear temperature dependence. increases with copper concentration and the values agree with those published recently. In addition, the critical magnetic field of the same films was measured. The analysis of these data which should be valid for strongly disordered superconductors yields values of which are about 20% higher than those determined by specific heat measurements. Arguments are given as to why values taken from critical field measurements might be too high. Finally, the specific heat of an amorphous film was measured. was found to be 30% below the free-electron model value, in contrast to results published previously by Rieger and Baumann who found to be in accordance with the free-electron model.

Critical magnetic fields of the misfit layer compounds (LaSe)1.14 (NbSe2) m down to 50 mK

Upper critical magnetic field measurements of the misfit layer compounds (LaSe)1.14 (NbSe2)m (m=1,2) perpendicular to the layer structure have been extended down to 50 mK by means of magnetoresistance measurements. For both compoundsHc2τ(T) does not show any sign of saturation down to the lowest temperature and has an upward curvature which varies with temperature as (Tc-T)1.5.

Superconducting and structural properties of Nb/Pd(Mn) multilayers

Nb/Pd(Mn) multilayers have been fabricated by sputtering and Molecular Beam Epitaxy (MBE). The samples were characterized by X-Ray analyses which showed in the case of sputtered samples oxygen contamination at the Nb interfaces. The MBE films exhibited higher residual resistivity ratios (RRR) and superconducting critical temperatures versus the Pd thicknesses in agreement with the de Gennes-Werthamer theory. Critical magnetic field measurements were also performed on these samples. Nb/PdMn multilayers were analyzed to study the superconductivity and the magnetism interplay.

The VAMAS intercomparison on the upper critical field measurement in Nb-Ti wire

The intercomparison on the upper critical field measurement in the Nb-Ti wire (NIST-SRM 1457) has been performed through the VAMAS (Versailles Project on Advanced Materials and Standards) TWA 6 activity. The homogeneity in the H/sub c2/ of the wire is fairly good, while the coefficient of scatter in the H/sub c2/ intercomparison is about 0.60%. The scatter in high field measurement may cause that in critical current measurement at high magnetic fields. Effects of several factors on the H/sub c2/ of the Nb-Ti wire have also been studied. The coiled specimen shows slightly lower H/sub c2/ than the straight sample. The H/sub c2/ of the wire shows an appreciable anisotropy with respect to the direction of applied field. >

Review on the phase diagram and the upper critical field anisotropy of uranium-based heavy fermion superconductors

The phase diagrams and anisotropies of the upper-critical field of the uranium-based heavy-fermion superconductors UPt 3, URu 2Si 2, UBe 13 and UPd 2Al 3 are reviewed. Special attention is given to the identification of the multi-component order-parameter from critical field measurements in the case of UPt 3 and URu 2Si 2. The role of the anti-ferromagnetic order in determining the symmetry of the gap function and the H c2 anisotropy is discussed with reference to URu 2Si 2. The experimental evidence for a Fulde-Ferrel-Larkin-Ovchinikov modulated superconducting phase is also examined.

The superconducting proximity effect in Nb/Fe multilayers

We prepared Nb/Fe multilayers with varying Nb or Fe thickness by means of MBE. The x-ray diffraction patterns show good layering quality. Critical field measurements as a function of temperature and angle demonstrate that very thin Fe layers (≈ 12 Å) decouple the Nb layers completely thus leading to 2D behaviour of the multilayers. The dependence of T c as a function of Nb thickness and the critical fields are well described by the theoretical model derived for the proximity effect in superconductor/ferromagnet multilayers.

Superconducting fluctuation conductivity in granular Al-Ge films above the metal-insulator transition

Superconducting fluctuation conductivities and magnetoconductances have been measured in thin granular Al-Ge films above the superconducting transition temperature. For very metallic films located well above the metal-insulator transition, the two-dimensional Aslamazov-Larkin and Maki-Thompson theories describe the conductivity and magnetoconductance data well. The Maki-Thompson expressions involve the temperature-dependent pair-breaking parameter delta . For films located near the metal-insulator transition, we have observed a dimensional crossover from three dimensions to two dimensions to a fractal dimension with decreasing temperatures. Values for the diffusion constant, ranging from 0.4 to 1.4 cm2 s-1, were obtained from the magnetoconductance fits, and these magnitudes compared favourably with values obtained from critical-field measurements taken below Tc.

Superconductivity to 21 K in intermetallic thorium-based boride carbides

We report the observation of superconductivity in intermetallic thorium-based boride carbides. For ThM 2B 2C, superconducting T c's of 6 K (M=Ni), 6.5 K (M=Pt), and 14.5 K (M=Pd) have been measured. These compounds are isostructural with the recently reported LnNi 2B 2C superconductors. We have also observed bulk superconductivity in an as-yet-undetermined thorium-palladium-boron-carbon quaternary phase at 21.5 K. Critical-field measurements on the two palladium-based superconductors, as well as the pressure dependence of T c for the 21.5 K superconductor, are reported.

Antiferromagnetically coupled clusters and superconductors in pulsed magnetic fields

Pulsed field differential magnetization, dM/ dB, and Faraday rotation measurements are reviewed in selected dilute magnetic semiconductors which required fields above 30 T for determining the antiferromagnetic exchange interactions between isolated pairs of magnetic ions or larger clusters. In some cases, nonequilibrium effects result in a narrowing (or broadening) of the peaks in dM/ dB at the level crossings which produce the magnetization steps at high fields. Similar measurements on a molecular ferric wheel cluster of 10 Fe 3+ ions are also discussed. Critical field measurements in K 3C 60 and RbC 60 and other superconductors are briefly summarized. The pulsed user facility is outlined.

Factors affecting the upper critical field of Nb [sbnd]Ti wires

The intercomparison on the upper critical field measurement in the Nb wire(NST SRM 1457) has been performed through the VAMAS activity. The homogeneity in the Hc 2 of the wire was good enough, and the coefficient of scatter in the Hc 2 intercomparison was about 0.60 %. Effects of several factors on the HC of the Nb Ti wire are reported in the present paper. The coiled specimen shows slightly lower Hc 2 than the straight sample. The Hc 2 of the wire decreases by annealing, and recovers by wire drawing, The HC 2 of the wire shows an appreciable anisotropy with respect to the direction of applied field. The anisotropy in HC 2 is eliminated by annealing, and reintroduced by the subsequent wire drawing.

The electron mean free path and superconductivity in Cu/Pb modulated films

The electron mean free path L in the normal state and the critical temperature Tc as well as the parallel critical magnetic field Hc// and perpendicular critical magnetic field Hc perpendicular to were measured in equilayered Cu/Pb modulated films. The modulation period lambda varied from 59 to 1620 AA. Our films were fabricated by thermal evaporation. The electron mean free path in the component layers estimated following the Gurvitch model is markedly shortened as a result of electron scattering both at interfaces and at grain boundaries. The dependence of Tc on component layer thickness was compared with existing theoretical models. Our measurements of the parallel critical fields as a function of temperature revealed the crossover of superconductivity from three-dimensional to two-dimensional behaviour.

Theoretical investigation of the behavior of ferroelectric liquid crystals in a magnetic or in a high-frequency electric field.

A theoretical explanation of the phase diagram of ferroelectric liquid crystals in an external magnetic or high-frequency electric field is given. The investigation is based on a rigorous treatment of the problem within the phenomenological model, being an extension of the Pikin-Indenbom model [Ferroelectrics 20, 151 (1978)]. The phase transition from the modulated Sm-${\mathit{C}}^{\mathrm{*}}$ phase to the homogeneously ordered Sm-C phase is studied numerically. The relevant parameters of the model are chosen in the best way to fit the experimentally obtained critical-field lines on two ferroelectric liquid-crystalline compounds. As a result, the observed relationship between the critical field and the helical pitch in zero field is theoretically confirmed. The critical-field line is investigated in the vicinity of the Lifshitz point where the metastable states appear. In both compounds, the tricritical point is found to be very close to the Lifshitz point, which seems to disagree with measurements showing hysteresis at all temperatures. This contradiction is removed by showing that, in a sample of a finite length along the helical axis, a slow pitch-relaxation process exists. Consequently, during the critical-field measurements, the system is probably not in an equilibrium, which prevents the continuous phase transition from taking place.