Bi2Sr2CaCu2O8+d Bicrystal c-Axis Twist Josephson Junctions: A New Phase-Sensitive Test of Order Parameter Symmetry
Richard A. Klemma, Kurt Scharnbergb, Qiang Lic
aMax-Planck-Institut für Physik komplexer Systeme, Nöthnitzer Straße 38, D-01187 Dresden, Germany
bI. Institut für Theoretische Physik, Universität Hamburg, Jungiusstraße 9, D-20355 Hamburg, Germany
cDepartment of Applied Science, Brookhaven National Laboratory, Upton, New York, 11973 USA
We prepared atomically clean Bi2Sr2CaCu2O8+d Josephson junctions between identical single crystal cleaves stacked and twisted an angle f0 about the c axis. For each bicrystal, the ratio JcJ/JcS of the c axis twist junction critical current density to that across either single crystal part is unity, independent of f0 and the ratio AJ/AS of junctions areas. From extensive theoretical studies involving a variety of tunneling and order parameter forms, we conclude that the results provide strong evidence for incoherent c-axis tunneling and that the dominant superconducting order parameter is s-wave for T £ Tc.
Shapiro step effect in the vortex state of Bi2Sr2CaCu2O8+d
M.B. Gaifullina, Yu.I. Latyshevb, T. Yamashitac, Yuji Matsudaa
aInstitute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan
bInstitute of Radio-Engineering and Electronics, Mokhovaya 11, Moscow 103907, Russia
cResearch Institute of Electrical Communications, Tohoku University, Sendai 980-8577, Japan
We report the first observation of Shapiro step effect in the vortex state of Bi2Sr2CaCu2O8+d in strong magnetic field (H ³ 1 T). We observed phase locking between external microwave radiation and Josephson oscillation in whole stack (N » 67) of intrinsic Josephson junctions at fields tilted with respect to the c-axis. Steps were observed in flux-flow and excess current region of the I-V characteristic at voltage Vst=pNhn/2e, with p integer, n = 45-142 GHz. The height of Shapiro steps and Josephson critical current increase sharply with tilting angle and then decrease. We attribute this anomaly to the interaction between Josephson and pancake vortices.
Josephson Vortex Flow in Bi2Sr2CaCu2O8+y
Kazuto Hirata, Shuuichi Ooi, El Hadi Sadki, Takashi Mochiku
Nanomaterials Laboratory, NIMS, 1-2-1 Sengen, Tsukuba, 305-0047, Japan
We have studied the flow resistance of Josephson vortices(JV's) in Bi2Sr2CaCu2O8+y intrinsic Josephson junctions, fabricated with focused ion beam etching. In the flow resistance, we have found a coherent motion of JV's, which appears in periodical oscillations of the resistance as a function of magnetic field. This phenomenon is related to the matching between JV lattice and the size of the sample. From the results, it is shown that JV's take a triangular lattice as the ground state and magnetic phase diagram of JV's has been established. Furthermore, the size effect causes a new vortex phase in the JV system.
STM Studies of Individual Ti Impurity Atoms in Sr2RuO4
B. I. Barkera, S. K. Duttaa, C. Lupiena, P. L. McEuenb, N. Kikugawac, Y. Maenoc, J.C. Davisa
aDepartment of Physics, University of California, Berkeley, Berkeley, CA 94720-7300 USA
bDepartment of Physics, Cornell University, Ithaca, NY 14853-2501 USA
cDepartment of Physics, Kyoto University, Kyoto 606-8502, Japan
We report atomic resolution scanning tunneling microscopy of the unconventional superconductor, Sr2RuO4, at temperatures between ~ 20 mK and 20 K. Spectroscopic measurements show a complex density of states with multiple gap-like structures. Upon substitutional doping with Ti atoms, we find a local impurity state at -1.5 meV, outside the superconducting gap. We will discuss possible connections between this local state and results of neutron scattering experiments.
Critical state models for a granular ferromagnetic superconductor
Ludwig Bauernfeind, Thomas P. Papageorgiou, Hans F. Braun
Physikalisches Institut, Universität Bayreuth, D-95440 Bayreuth, Germany
Measurements of the magnetic moment M as a function of the magnetic field B can be used to extract information about the penetration behavior of the magnetic field in a superconducting sample. In granular superconductors the response to an applied a.c. field is governed by both intra- and intergranular shielding currents. We have employed low field a.c. susceptibility measurements to investigate the magnetic properties of a polycrystalline RuSr2GdCu2O8 weak ferromagnet in its superconducting state. For fields below ~ 10 G the results are consistent with the Bean critical state model. For B > 10 G, the Kim model is more suitable for the description of the sample's properties.