A type-II superconductor is in the "superclean" regime when the spacing hcuo between energy levels in the vortex core greatly exceeds the width of the levels h/r A.key signature of this regime is a fiux-fiow Hall angle 8n that approaches~m/2. We report the first observation of this trend in a high-temperature superconductor.In "60 K" YBa2Cu, 06+, crystals, itan8"i is observed to increase dramatically when the temperature decreases below -40 K, attaining the magnitude 0.85 at 15 K. By extrapolation, we estimate that QUOT increases to 2 -3 at 3 K. Below 40 K, the Hall conductivity 0' y ls negative and decreases as I/H at high fields. PACS numbers: 74.72.Bk, 74.60.6e When a vortex moves in a type-II superconductor, dissipation is produced by the scattering of quasiparticles within the vortex core [1]. The finite lifetime r of quasiparticles trapped in the core leads to a broadening of the energy levels. Usually, the spacing hcoo between the core levels is so much smaller than the widths (QfpT (( 1) that it is a good approximation to treat the core as a column of "normal" material. This picture fails in the opposite limit cdpT » 1. The discrete nature of the levels becomes important, and dissipative processes have to be reconsidered (this is called the "superclean" regime) [2 -5]. In terms of the quasiparticle's mean free path l and the Pippard coherence length g, the superclean condition is equivalent to 1/g » aF/b, (aF is the Fermi energy and 5 the gap parameter). The high-T, superconductors are promising candidates because of the large gap and small aF (5 = 20 -30 meV, aF -500 meV). Recent surface impedance measurements on YBa2Cu307 s (YBCO) indicate that, at 4.2 K, the lifetime is very long in zero field [6 -8], as well as in intense fields (r is estimated to be 2 -6 ps in the core) [9]. The parameter cour is estimated to be -14 in 90K YBCO at temperatures below 15 K, which places YBCO firmly in the superclean regime [9]. The growing evidence notwithstanding, a key signature, namely the distinctive behavior of the Hall effect, has yet to be observed. Microscopic calculations [3,4] predict that tan 8tt in a pure type-II superconductor increases linearly as~coor when &07 )& 1 (8tt is the Hall angle). The vortex velocity u changes from near alignment with J x B towards parallelism with J as pr coincreases (J is the current density and 8 the field). Thus, a Hall angle that diverges towards~m . /2 is an essential piece of evidence for the superclean regime. Up to now, however, the values of 8~observed in YBCO (mostly at temperatures T ) 70 K) have been disappointingly small (i8tt i (0. 05 rad) [10 -12]. Using high-purity "60 K" YBa2Cu 306+y single crystals, we have performed flux-flow measurements down to 11 K. We have uncovered a remarkable increase in 8~that starts near 40 K but shows no sign of saturation at 15 K. The Hall-angle measurements provide direct evidence for the approach to the superclean regime, as well as an estimate of coos-. This is the first demonstration of a diverging Hall angle in a high-T, supe...
The Josephson plasma resonance of Bi 2 Sr 2 CaCu 2 O 8ϩ␦ in parallel magnetic fields has been measured by angular alignment with ϳ0.02 degree precision to reveal the phase collective modes in the Josephson vortex lattice. When the angle between the field and the CuO 2 plane is tilted from the c axis, the resonance field scales as H sin in the range տ4°. At lower angles, the resonance field shows anomalous decreases with decreasing . A disappearance of the resonance has been unexpectedly found for angles within ϳ0.2°, which may correspond to the angle where the lock-in transition takes place. The origin of the phenomenon is discussed in terms of the Josephson plasma oscillation and Josephson vortex oscillation modes.
Fe 3 O 4 is a ferrimagnetic spinel ferrite that exhibits electric conductivity at room temperature (RT). Although the material has been predicted to be a half metal according
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