Comments on "Vortex Glass and Lattice Melting Transitions in a YNi_2B_2C Single Crystal"

Recently, Mun et.al. (Phys. Rev. Lett., 76, 2790 (1996)) have published their results on single crystal YNi_2B_2C, claiming that their experimental observations can be explained in terms of formation of Vortex Glass and Lattice melting. Our experiments, carried out on samples obtained from the SAME source, reveal a much richer phase diagram and span wider regions of experimental parameter space than Mun et. al. that encompasses most of their observations. We speculate that this material has anomalous intrinsic properties and the results cannot be explained by simple models about the flux lattice.Comment: 1 page, LaTeX type, 1 PostScript figure, Uses PRABIB.STY file, 600 dpi PS file available at http://sagar.physics.neu.edu/preprints.html To appear in Physical Review Letter

Critical State Flux Penetration and Linear Microwave Vortex Response in YBa_2Cu_3O_{7-x} Films

The vortex contribution to the dc field (H) dependent microwave surface impedance Z_s = R_s+iX_s of YBa_2Cu_3O_{7-x} thin films was measured using suspended patterned resonators. Z_s(H) is shown to be a direct measure of the flux density B(H) enabling a very precise test of models of flux penetration. Three regimes of field-dependent behavior were observed: (1) Initial flux penetration occurs on very low field scales H_i(4.2K) 100Oe, (2) At moderate fields the flux penetration into the virgin state is in excellent agreement with calculations based upon the field-induced Bean critical state for thin film geometry, parametrized by a field scale H_s(4.2K) J_c*d 0.5T, (3) for very high fields H >>H_s, the flux density is uniform and the measurements enable direct determination of vortex parameters such as pinning force constants \alpha_p and vortex viscosity \eta. However hysteresis loops are in disagreement with the thin film Bean model, and instead are governed by the low field scale H_i, rather than by H_s. Geometric barriers are insufficient to account for the observed results.Comment: 20 pages, LaTeX type, Uses REVTeX style files, Submitted to Physical Review B, 600 dpi PostScript file with high resolution figures available at http://sagar.physics.neu.edu/preprints.htm

Magneto-electrodynamics at high frequencies in the antiferromagnetic and superconducting states of DyNi_2B_2C

We report the observation of novel behaviour in the radio frequency (rf) and microwave response of DyNi_2B_2C over a wide range of temperature (T) and magnetic field (H) in the antiferromagnetic (AFM) and superconducting (SC) states. At microwave frequencies of 10 GHz, the T dependence of the surface impedance Z_s=R_s+iX_s was measured which yields the T dependence of the complex conductivity \sigma_1-i\sigma_2 in the SC and AFM states. At radio frequencies (4 MHz), the H and T dependence of the penetration depth \lambda(T,H) were measured. The establishment of antiferromagnetic order at T_N=10.3 K results in a marked decrease in the scattering of charge carriers, leading to sharp decreases in R_s and X_s. However, R_s and X_s differ from each other in the AFM state. We show that the results are consistent with conductivity relaxation whence the scattering rate becomes comparable to the microwave frequency. The rf measurements yield a rich dependence of the scattering on the magnetic field near and below T_N. Anomalous decrease of scattering at moderate applied fields is observed at temperatures near and above T_N, and arises due to a crossover from a negative magnetoresistance state, possibly associated with a loss of spin disorder scattering at low fields, to a positive magnetoresistance state associated with the metallic nature. The normal state magnetoresistance is positive at all temperatures for \mu_0H>2T and at all fields for T>15K. Several characteristic field and temperature scales associated with metamagnetic transitions (H_M1(T), H_M2(T)) and onset of spin disorder H_D(T), in addition to T_c, T_N and H_c2(T) are observed in the rf measurements.Comment: 9 pages, Latex, Uses REVTeX, This and related publications also available at http://sagar.physics.neu.edu/ Submitted to Phys. Rev.

Vortex response and critical fields observed via RF penetration depth measurements on the superconductor YNi2B2C

Measurements of the RF penetration depth λ(T, H, θ) are used to study the superconducting order parameter, vortex dynamics in the mixed state and delineate critical fields in the borocarbide superconductor YNi2B2C. The lower critical field has an anomalous T dependence, Hc1(T) = 1.12[1 − (T/Tc)]kOe, which is, however, consistent with independent superfluid density measurements at microwave frequencies. The vortex response is dominated by viscous flux flow, indicative of extremely weak pinning, and is parameterized by a field scale Hc2, eff. The angular dependence of the vortex contribution λ(θ) is in good agreement with the Coffey-Clem model. Structure is seen in the depairing transition in the vicinity of the upper critical field, with the existence of well-defined critical fields Hc2α, Hc2βand Hc2γ, with the vortex field scale Hc2, eff closest to Hc2β. Overall the measurements indicate that YNi2B2C has a rich and unusual field dependence of its transport parameters.This is a manuscript of an article published as Oxx, S., D. P. Choudhury, Balam A. Willemsen, H. Srikanth, S. Sridhar, B. K. Cho, and P. C. Canfield. "Vortex response and critical fields observed via RF penetration depth measurements on the superconductor YNi2B2C." Physica C: Superconductivity 264, no. 1-2 (1996): 103-108. DOI: 10.1016/0921-4534(96)00230-4 Copyright 1996 Elsevier. Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Posted with permission

Observation of a field-driven structural phase transition in the flux line lattice in ErNi2B2C

Small-angle neutron scattering and magnetic decoration both demonstrate a topological transition in the flux line lattice (FLL) in ErNi2B2C. The high-field square lattice slowly transforms into a hexagonal lattice via an area preserving [100] rhombohedral distortion below roughly 500 Oe. The square FLL is aligned with the [110] direction of the tetragonal crystal, while the two domains of the hexagonal FLL are aligned with [100] and [010]. The differences in pinning for the two FLL topologies are reflected in the rf kinetic inductance.This article is published as Eskildsen, M. R., P. L. Gammel, B. P. Barber, U. Yaron, A. P. Ramirez, D. A. Huse, D. J. Bishop et al. "Observation of a field-driven structural phase transition in the flux line lattice in ErNi 2 B 2 C." Physical Review Letters 78, no. 10 (1997): 1968. DOI: 10.1103/PhysRevLett.78.1968. Copyright 1997 American Physical Society. Posted with permission

Observation of a field-driven structural phase transition in the flux line lattice in ErNi2B2C

Small-angle neutron scattering and magnetic decoration both demonstrate a topological transition in the flux line lattice (FLL) in ErNi2B2C. The high-field square lattice slowly transforms into a hexagonal lattice via an area preserving [100] rhombohedral distortion below roughly 500 Oe. The square FLL is aligned with the [110] direction of the tetragonal crystal, while the two domains of the hexagonal FLL are aligned with [100] and [010]. The differences in pinning for the two FLL topologies are reflected in the rf kinetic inductance.This article is published as Eskildsen, M. R., P. L. Gammel, B. P. Barber, U. Yaron, A. P. Ramirez, D. A. Huse, D. J. Bishop et al. "Observation of a field-driven structural phase transition in the flux line lattice in ErNi 2 B 2 C." Physical Review Letters 78, no. 10 (1997): 1968. DOI: 10.1103/PhysRevLett.78.1968. Copyright 1997 American Physical Society. Posted with permission

Effects of superconducting gap anisotropy on the flux flow resistivity in Y(Ni1-xPtx)2B2C

The microwave complex surface impedance Z_s of Y(Ni_{1-x}Pt_x)_2B_2C was measured at 0.5 K under magnetic fields H up to 7T. In nominally pure YNi_2B_2C, which is a strongly anisotropic s-wave superconductor, the flux flow resistivity \rho_f calculated from Z_s was twice as large as that expected from the conventional normal-state vortex core model. In Pt-doped samples where the gap anisotropy is smeared out, the enhancement of \rho_f is reduced and \rho_f approaches to the conventional behavior. These results indicate that energy dissipation in the vortex core is strongly affected by the anisotropy of the superconducting gap.Comment: 5 pages, 3 figure