Search Results (91 total)

High-density direct currents are used to drive flux quanta via the Lorentz force toward a highly ordered “free flux flow” (FFF) dynamic state, made possible by the weak-pinning environment of high-quality, single-crystal samples of two low-T_c superconducting compounds, V₃Si and LuNi₂B₂C. We report the effect of the magnetic field-dependent fluxon-core size on flux flow resistivity ρ_f. Much progress has been made in minimizing the technical challenges associated with the use of high currents. Attainment of a FFF phase is indicated by the saturation at highest currents of flux flow dissipation levels that are well below the normal-state resistance and have field-dependent values. The field dependence of the corresponding ρ_f is shown to be consistent with a prediction based on a model for the decrease of fluxon-core size at higher fields in weak-coupling BCS s-wave materials.

We report predictions for the energy eigenstates and inelastic neutron-scattering excitations of an isotropic Heisenberg hexamer consisting of general spin S and S^′ trimers. Specializing to spin-1/2 ions, we give analytic results for the energy excitations, magnetic susceptibility, and inelastic neutron-scattering intensities for this hexamer system. To examine this model further, we compare these calculations to the measured magnetic susceptibility of a vanadium material, which is considered to be well-defined magnetically as an isolated S=1/2 V⁴⁺ trimer model. Using our model, we determine the amount of intertrimer coupling that can be accommodated by the measured susceptibility and predict the inelastic neutron-scattering spectrum for comparison with future measurements.

We report the basal-plane critical current and superfluid density of magnetically aligned NdFeAsO_0.86F_0.14 powder. This sample has individual crystallite grains permanently oriented with their c axis along the external measuring field. Magnetic irreversibilities at high field suggest strong flux pinning of basal-plane critical currents, with monotonic field dependence and no evidence of the “fishtail” effect. The small particles provide a sensitive indicator of dc flux penetration and allow analysis of the temperature dependence of ab-plane London penetration depth λ_ab,L, which is quadratic at low T. This feature may not necessarily be due to the nodes in the gap but may be rather a sign of a strong pair breaking. A quantitative determination of the absolute magnitude of λ_ab,L is hindered by the need for accurate knowledge of the particle size distribution.

We present evidence for entangled solid vortex matter in a glassy state in a layered superconductor Bi₂Sr₂CaCu₂O_8+y containing randomly splayed linear defects. The interlayer phase coherence—probed by the Josephson plasma resonance—is enhanced at high temperatures, reflecting the recoupling of vortex liquid by the defects. At low temperatures in the vortex solid state, the interlayer coherence follows a boomerang-shaped reentrant temperature path with an unusual low-field decrease in coherence, indicative of meandering vortices. We uncover a distinct temperature scaling between in-plane and out-of-plane critical currents with opposing dependencies on field and time, consistent with the theoretically proposed “splayed-glass” state.

We use clustering, an analysis method not presently common to the physics education research community, to group and characterize student responses to written questions about two-dimensional kinematics. Previously, clustering has been used to analyze multiple-choice data; we analyze free-response data that includes both sketches of vectors and written elements. The primary goal of this paper is to describe the methodology itself; we include a brief overview of relevant results.

We report measurements of the temperature dependence of the magnetic penetration depth λ(T) in noncentrosymmetric superconductor Re₃W. We employed two experimental techniques: extraction of λ(T) from magnetic dc susceptibility, measured on a powder sample, and the rf tunnel diode resonator technique, where a bulk polycrystalline sample was used. The results of both techniques agree: the temperature dependence of the penetration depth can be well described by weak-coupling, dirty-limit, s-wave BCS theory where we obtain Δ(0)∕k_BT_C=1.76±0.05. No evidence for unconventional pairing resulting from the absence of the inversion symmetry is found.

Strong quantum size effects enable the formation of crystalline Pb films that are atomically flat on a macroscopic length scale. The superconducting properties of 5–18–monolayer-(ML) thick Pb films were investigated in a superconducting quantum interference device (SQUID) magnetometer using combined ac and dc methods. Even the thinnest films (5 ML) are extraordinarily robust type-II superconductors. Despite the extreme two-dimensional geometry, the thermodynamic parameters T_c and upper critical field H_c2 are primarily controlled by the physical boundary conditions of the film and show no evidence for disorder-driven or fluctuation-driven quenching of superconductivity. A magnetically hard critical state is established as a consequence of vortex trapping by quantum growth defects.

The field dependence of the vortex core size ξ(B) is incorporated in the London model, in order to describe reversible magnetization M(B,T) for a number of materials with large Ginzburg-Landau parameter κ. The dependence ξ(B) is directly related to deviations in M(ln B) from linear behavior prescribed by the standard London model. A simple method to extract ξ(B) from the magnetization data is proposed. For most materials examined, ξ(B) so obtained decreases with increasing field and is in qualitative agreement both with behavior extracted from μSR and small-angle neutron-scattering data and with that predicted theoretically.

Enhanced in-field critical current density J_c has been achieved in YBa₂Cu₃O_7−δ (YBCO) films grown on single crystal and biaxially textured metal substrate surfaces that were pretreated with second-phase nanoscale MgO and BaZrO₃ particles. Nanoparticles were applied using wet chemical solution-based approaches. Results obtained in this study are compared with our previous investigations on surface modified substrates with sputter processed Ir nanoparticles. Cross-sectional TEM analysis and details of the field-orientation dependence of J_c are found to be similar for both sputtered and solution processed nanoparticle species. Present results show a more uniform dependence of J_c over all orientations of magnetic field, along with improved irreversibility behavior. Irrespective of the decoration technique, scaling behavior of the normalized volume pinning force density, F_p, demonstrates the similarity of pinning mechanisms for these YBCO films deposited on artificially modified substrates.

The critical current density J_c flowing in thin YBa₂Cu₃O_7−δ (YBCO) films of various thicknesses d has been studied magnetometrically, both as a function of applied field H and temperature T, with a central objective to determine the dominant source of vortex pinning in these materials. The films, grown by a BaF₂ ex situ process and deposited on buffered rolling assisted biaxially textured substrates (“RABiTS”) substrates of Ni-5% W, have thicknesses d ranging from 28 nm to 1.5 μm. Isothermal magnetization loops M(H;T) and remanent magnetization M_rem(T) in H=0 were measured with H‖c-axis (i.e., normal to film plane). The resulting J_c(d) values (obtained from a modified critical state model) increase with thickness d, peak near d∼120 nm, and thereafter decrease as the films get thicker. For a wide range of temperatures and intermediate fields, we find J_c∝H^−α with α∼(0.56–0.69) for all materials. This feature can be attributed to pinning by large random defects, which theoretically has power-law exponent α=5∕8. Calculated values for the size and density of defects are comparable with those observed by TEM in the films. As a function of temperature, we find J_c(T,sf)∼[1−(T∕T_c)²]ⁿ with n∼1.2–1.4. This points to “δT_c pinning” (pinning that suppresses T_c locally) in these YBCO materials.

We report the variable temperature vibrational properties of single crystals of the S=1∕2 Heisenberg antiferromagnet VOHPO₄∙1/2H₂O. A pair of peaks in the far infrared spectral response may be due to magnetic excitations. We invoke a dynamic Dzyaloshinskii-Moriya mechanism to explain the activation and polarization dependence of the singlet-to-triplet gap in the far infrared, and we identify the low-energy phonons that likely facilitate this coupling. The spin-gap values are compared to those obtained via magnetic susceptibility, electron spin resonance, and neutron scattering. Vibrational mode splitting in VOHPO₄∙1/2H₂O indicates a weak local symmetry breaking near 180 K, and the low-temperature redshift of VO and HO related modes demonstrates enhanced low-temperature hydrogen bonding. The low lattice symmetry is important for the proposed magnetoelastic interactions.

We present a comprehensive study of ferromagnetism and magnetotransport in Mn-doped germanium, grown with molecular-beam epitaxy. Ferromagnetism in Mn_xGe_1−x (0<x<0.09) is characterized by two different ordering temperatures T_C and T_C^* with T_C⪡T_C^*. The onset of global ferromagnetic order at T_C coincides with the percolation threshold for (activated) charge transport. Magnetism between T_C and T_C^* originates from “clustered dopants” associated with inhomogeneities. The ferromagnetic ordering temperature within the clusters is of order T_C^* while the coupling between the clusters is mediated by thermally activated carriers moving in an impurity band. The magnetoresistance exhibits nonmonotonic temperature and magnetic field dependence; both negative and positive magnetoresistance contributions are observed. The anomalous Hall effect between T_C and T_C^* appears to be influenced heavily by the large magnetoresistance. The normal and anomalous Hall coefficients both diverge at low temperature. All these observations indicate that Mn_xGe_1−x is most adequately described within an impurity band model where the ratio J∕t of the Mn hole exchange J and hole hopping t is large.

Recent advancement in bulk metallic glasses, whose properties are usually superior to their crystalline counterparts, has stimulated great interest in fabricating bulk amorphous steels. While a great deal of effort has been devoted to this field, the fabrication of structural amorphous steels with large cross sections has remained an alchemist’s dream because of the limited glass-forming ability (GFA) of these materials. Here we report the discovery of structural amorphous steels that can be cast into glasses with large cross-section sizes using conventional drop-casting methods. These new steels showed interesting physical, magnetic, and mechanical properties, along with high thermal stability. The underlying mechanisms for the superior GFA of these materials are discussed.

We report significant alteration of the equilibrium properties of the superconductor HgBa₂Ca₂Cu₃O_x when correlated disorder in the form of randomly oriented columnar tracks is introduced via induced fission of Hg nuclei. From studies of the equilibrium magnetization M_eq and the persistent current density over a wide range of temperatures, applied magnetic fields, and track densities up to a “matching field” of 3.4 T, we observe that the addition of more columnar tracks acting as pinning centers is progressively offset by reductions in the magnitude of M_eq. Invoking anisotropy-induced “refocusing” of the random track array and incorporating vortex-defect interactions, we find that this corresponds to increase in the London penetration depth λ; this reduces the vortex line energy and consequently reduces the pinning effectiveness of the tracks.

The critical current density flowing across low angle grain boundaries in YBa₂Cu₃O_7-δ thin films has been studied magnetometrically. Films (200 nm thickness) were deposited on SrTiO₃ bicrystal substrates containing a single [001] tilt boundary, with angles of 2, 3, 5, and 7 degrees, and the films were patterned into rings. Their magnetic moments were measured in applied magnetic fields up to 30 kOe at temperatures of 5–95 K; current densities of rings with or without grain boundaries were obtained from a modified critical state model. For rings containing 5 and 7 degree boundaries, the magnetic response depends strongly on the field history, which arises in large part from self-field effects acting on the grain boundary.

We report the polarized reflectance and optical conductivity of the quasi-one-dimensional conductor Li_0.9Mo₆O₁₇ as a function of temperature. The compound displays an unusual (non-Drude-type) mobile carrier response at low-energy, with partially screened vibrational features along the highly conducting b axis. In addition, we observe Mo d→d transitions near 0.42, 0.57, and 1.3 eV, and an O p→Mo d charge-transfer band near 4 eV. Perpendicular to the b axis, Li_0.9Mo₆O₁₇ exhibits semiconducting behavior with an optical gap of 0.4 eV and electronic structure similar to that of the b axis at higher energies. The substantial temperature dependence of the vibrational modes in this direction reveals that the lattice of Li_0.9Mo₆O₁₇ is not rigid. However, no noticeable change in the lattice through the 25 K metal-insulator transition is observed. Comparing x-ray and infrared data for several model materials, we establish an upper bound on the size of any lattice distortion in Li_0.9Mo₆O₁₇. Based upon these combined results, we argue that localization effects dominate the bulk and microscopic properties of this material.

The peak effect in critical current density J_c is investigated by studying the flux dynamics in V₃Si using bulk magnetometry, small-angle neutron scattering, and transport measurements on clean single-crystal samples from the same ingot. For a field-cooled history, well-defined structure in the vortex lattice was found for fields and temperatures (H,T) below the peak-effect line H_P(T); above this line, the structure disappeared. History-dependent, metastable disorder is found only for (H,T) below H_P(T) but the vortex system is reproducibly re-ordered either by field cooling or a low-frequency, pulsed “shaking” transport current. The latter is shown to attain Bardeen-Stephen flux flow. In addition, flux flow is observed at H_P(T) at high current levels. The results support the traditional picture of H_P(T) as an order-disorder transition due to the competition between elasticity and pinning.

We report the vibrational properties of quasi-two-dimensional K₂V₃O₈ as a function of temperature. Based on the splitting of the 990-cm^-1 c-axis phonon mode into a doublet, we conclude that the 110-K transition is driven by a local distortion of the VO₅ square pyramids. At lower temperature, red shifting of two ab-plane phonon modes motivates us to identify an additional, more modest relaxation near 60 K. These observations are supported by an analysis of the electronic structure as well as other physical-property measurements including the permittivity, specific heat, and magnetization.

Soft-x-ray absorption and fluorescence measurements are reported for the K edge of B in MgB₂. The measurements confirm a high density of B p_xy(σ) states at the Fermi edge and extending to approximately 0.9 eV above the edge. A strong resonance is observed in elastic scattering through a core exciton derived from out-of-plane p_z(π^*) states. Another strong resonance, observed in both elastic and inelastic spectra, is identified as a product of surface boron oxides.

For a single crystal of YNi₂B₂C superconductor, the equilibrium magnetization M in the square basal plane has been studied experimentally as a function of temperature and magnetic field. While the magnetization M(H) deviates from conventional London predictions, a recent extension of London theory (to include effects of nonlocal electrodynamics) describes the experiments accurately. The resulting superconductive parameters are well behaved. These results are compared with corresponding findings for the case with M perpendicular to the basal plane.

We have studied the pinning force density F_p of YNi₂B₂C superconductors for various field orientations. We observe anisotropies both between the c axis and the basal plane and within the plane that cannot be explained by the usual mass anisotropy. For magnetic field H‖c, the reorientation structural transition in the vortex lattice due to nonlocality, which occurs at a field H₁∼1 kOe, manifests itself as a kink in F_p(H). When H ⊥c, F_p is much larger and has a quite different H dependence, indicating that other pinning mechanisms are present. In this case the signature of nonlocal effects is the presence of a fourfold periodicity of F_p within the basal plane.

Structural, magnetic, electrical and thermal transport, and heat-capacity measurements are reported on single crystals of Eu₈Ga₁₆Ge₃₀, Sr₈Ga₁₆Ge₃₀, and Ba₈Ga₁₆Ge₃₀. These compounds all crystallize in a cubic type-I ice clathrate structure, and are of interest as potential thermoelectric materials. Neutron-diffraction measurements were made on a single crystal of Eu₈Ga₁₆Ge₃₀ that was grown using isotopically pure Eu¹⁵³. Nuclear density maps clearly show that Eu atoms at the 6d sites (1 / 4,1 / 2,0) can move away from the cage center to one of four nearby positions. Ferromagnetism is observed in Eu₈Ga₁₆Ge₃₀ for temperatures below 32 K, with the preferred direction of the Eu spins along the (100) axis. Ferromagnetism in these heavily doped semiconductors (≈10²¹ electrons/cm³) is likely due to a Rudermann-Kittel-Kasuya-Yoshida-type interaction. A large (≈10% at 8 T) negative magnetoresistance was measured near the Curie temperature of Eu₈Ga₁₆Ge₃₀. The lattice thermal conductivities of Eu₈Ga₁₆Ge₃₀ and Sr₈Ga₁₆Ge₃₀ single crystals show all of the characteristics of a structural glass. The thermal conductivity of Ba₈Ga₁₆Ge₃₀ is low at room temperature (1.3 W/m K), but exhibits a temperature dependence characteristic of a crystal. A magnetic field has no significant effect on the thermal conductivity of any of the crystals for temperatures between 2 and 300 K. Heat-capacity measurements indicated Einstein contributions from each of the rattlers, with characteristic temperatures of 60, 53, and 30 K for Ba, Sr, and Eu atoms respectively. No superconductivity was observed in heavily doped single crystals of Ba₈Ga₁₆Ge₃₀ for temperatures above 2 K, contrary to a previous report.

Cd₂Os₂O₇ crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in Cd₂Os₂O₇ using x-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. Cd₂Os₂O₇ orders magnetically at the MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on Cd₂Os₂O₇. These calculations indicate that Cd₂Os₂O₇ is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition—unlike a Mott transition—is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below T_MIT.

Magnetization and neutron diffraction studies of the 2D S  =  1/2 antiferromagnet, K₂V₃O₈, indicate an ordered state exhibiting weak ferromagnetism and field-induced spin reorientations. Of particular interest is the behavior in a basal plane magnetic field where a unique spin reorientation is observed in which the spins rotate from the easy c axis to the basal plane while remaining normal to the applied field. The experimental observations are well described by a two spin exchange model incorporating Heisenberg and Dzyaloshinskii-Moriya interactions with an additional c-axis anisotropy.