2PO1-120 Non Resonant Microwave Absorption Studies of Vortex Dynamics in LSCO Single Crystals

A. Anand Kumar 1, S.V. Bhat 1, Amit Rastogi 2, Y. Kodama 2, K. Oka 2, and Y. Nishihara 2. 1 Department of Physics Indian Institute of Science Bangalore 560 012, India. 2 Electrotechnical Laboratory Tsukuba Ibaraki 305, Japan.

Presenting Author: A. Kumar

The technique of non resonant microwave absorption is well established as a useful technique to characterise layered HTSCs. We had earlier seen the effects of intrinsic Josephson coupling in Bi2212 [1] single crystals using this technique. Here we report our studies on La_2-xSr_xCuO₄ single crystals for various Sr (x = 0.08, 0.1, 0.14, 0.18, 0.2) doping concentrations for H_{dc½ ½} c and H_dc^ c . The under doped and the optimally doped single crystals show different behaviours for H_{dc½ ½} c and H_dc^ c. For H_{dc½ ½} c, characteristic temperature and field dependent features are observed for under and optimally doped single crystals. The surface resistance Rµ[{-1+(1+4f²B₂/B0₂)^1/2}/2]^1/2, where f, B and B₀ are fraction of free fluxons, the magnetic field and the fitting parameter respectively, is used to fit the observed features. Further, B₀ depends on the anisotropy parameter and fluxon viscosity. The fits give viscosity coefficients for under doped and optimally doped crystals for H_{dc½ ½} c. However, the signals for over doped single crystals are similar for H_{dc½ ½} c and H_dc^ c, indicating the reduction in anisotropy with the increase in dopant concentration.

[1] Amit Rastogi et al., Phys. Rev. B 53, 9366 (1996).

2PO1-121 Non-resonant Microwave Absorption Evidence for Intrinsic Josephson Coupling in Tl₂Ba₂CaCu₂O₈ Single Crystals.

Janhavi P. Joshi 1, A. Anand 1, S.V. Bhat 1, P. Chowdhury 2, and S.N. Bhatia 2. 1 Department of Physics, Indian Institute of Science, Bangalore 560 012, India. 2 Department of Physics, Indian Institute of Technology, Powai, Mumbai 400 076, India.

Presenting Author: J.P. Joshi

The layered structure and the short c axis coherence length x _c of the high T_c superconductors result in Josephson coupling of the superconducting bilayers across the intervening non-superconducting regions. Here we report the results of non-resonant microwave absorption (NRMA) studies on single crystals of Tl 2212 which reflect the occurrence of intrinsic Josephson coupling. We have studied the magnetic field induced microwave absorption at various temperatures from 4.2 K to T_c (~104K) using a standard CW EPR spectrometer (H_{dc½ ½} c and H_{mw½ ½} ab). We observe the appearance of characteristic features in the NRMA signals similar to the ones seen by us earlier in Bi 2212 [1]. These features arise due to the microwave current driven dissipative motion of the pancake vortices present in successive CuO₂ bilayers connected by the Josephson vortices formed in the intervening region. They start appearing a few degrees below T_c, and on cooling pass through a maximum in intensity before disappearing at a further low temperature. This behavior is attributed to the appearance, strengthening and disappearance of Josephson response consequent to the sensitive temperature dependence of the viscosity of the Josephson medium between CuO₂ superconducting sheets.

[1] Amit Rastogi et al., Phys. Rev. B 53, 9366 (1996).

2PO1-122 Effect of Zero Field Cooling (ZFC) and Field Cooling (FC) on Field Dependent Microwave Absorption in HIgh T_c Superconductors and Determination of Inter & Intra-granular Lower Critical Field

Mohammed Shahabuddin, Department of Physics, Jamia Millia Islamia University, New Delhi, 110025, India

Presenting Author: M. Shahabuddin

Microwave Absorption in High T_c Superconductors(HTSC) as a function of magnetic field in the low field region shows hysteresis. The effect of field exposure (to zero field cooled HTSC) and field cooling on the hysteresis of Field Dependent Microwave Absorption(FDMA) in YBa₂Cu₃O₇ and Tl₂Ba₂CaCu₂O₈ was studied. Hysteresis area in FDMA is very much sensitive to the strength of the exposure and cooling field. The hysteresis area in FDMA as a function of field exposure and FC clearly differentiate between the inter and intra-granular effect. The above measurement also gives the value of H_c1j and H_c1g. The value of the critical field measured by FDMA matches with the other technique i.e. magnetization measurement. The studies of the effect of field exposures in different sample gives the qualitative measurement of the pinning strength and inter-granular coupling strength in those samples. Field cooling effects on FDMA shows very interesting behavior and is different from that of the field exposure. The results were explained in terms of the critical state model of the random inter-granular Josephson junctions.

2PO1-123 Time Relaxation of Microwave Second Order Magnetization of Superconductors in the Critical State

A. Agliolo Gallitto, M. Guccione, and M. Li Vigni, INFM and Dipartimento di Scienze Fisiche e Astronomiche, University of Palermo, via Archirafi 36, I-90123, Palermo, Italy

Presenting Author: A. Agliolo Gallitto

It is well known that superconductors in a critical state developed by a static magnetic field, when submitted to an intense EM field, exhibit a magnetization vector containing Fourier components at harmonic frequencies of the driving field. In particular, it has been shown that superconductors exposed to microwave fields exhibit odd as well as even harmonic emission. We report and discuss a set of experimental results on relaxation of the microwave second order magnetization in different superconductors in the critical state.

The sample is placed in a bimodal cavity, resonating at 3 and 6 GHz, in a region in which the fundamental and second-harmonic magnetic fields are maximal and parallel to each other. The fundamental mode of the cavity is fed by a pulsed oscillator, with a pulse repetition rate of 1 pps and a pulse width of about 1 msec. The maximal peak power is about 1kW. The harmonic signals radiated by the sample are detected by a superheterodyne receiver. All the measurements have been performed with the static magnetic field parallel to the microwave fields, at T = 4.2 K.

The harmonic signals decay during the time for which the microwave pulse lasts. The decay time strongly depends on the type of superconductor.

2PO1-124 Multiterminal Measurements of Vortex Correlations in the (K,Ba)BiO₃ System

Andreas Rydh 1, Thierry Klein 2, Isabelle Joumard 2, and Östen Rapp 1. 1 Department of Solid State Physics, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. 2 Laboratoire d'Etudes des Propriétés Electroniqes des Solides, CNRS, FR-38042 Grenoble, France.

Presenting Author: A. Rydh

The Ba_1-xK_xBiO₃ (BKBO) superconductor (T_c ~ 30 K) is particularly interesting since it both shares common features with the high-T_c cuprates and displays significant differences. Thus it has the perovskite structure and is sensitive to the potassium and oxygen stoichiometry, but the unit cell is cubic (the electrical anisotropy parameter g = 1). This makes it a key system for the understanding and interpretation of vortex correlation

measurements in the layered high-T_c materials.

We have studied vortex correlations in single crystals of BKBO by means of multiterminal transport measurements in magnetic fields up to 12 T. The pseudo-flux transformer geometry was used to probe the vortex correlations in the vortex liquid as a function of temperature and field. Vortex correlation was estimated from the voltage ratio between the case with current going through current contacts placed on the same side of the crystal as the voltage contacts, and the case with current fed on the opposite side of the crystal. By choosing the magnetic field direction, this arrangement allows studies of correlations both along the vortices and between adjacent vortices.

The observed loss of vortex correlation in the vortex liquid above the onset of resistivity is discussed.

2PO1-125 The current contribution to the thermal generation of vortices in HTSC

L. Romano and C. Paracchini - INFM. Instituto Nazionale di Fisica della Materia Dipartimento di Fisica. Universita' di Parma, Parma, Italy

Presenting Author: L. Romano

Dissipation in superconductors is usually attributed to the motion of flux lines generated by an external magnetic field. At zero field the dissipation is still present, but the vortex origin is an open question.

In planar systems thermal fluctuations produce vortex pairs which dissociate at a temperature T_k (critical temperature) where a phase transition occurs. For T_k single vortices are free to move giving rise to an ohmic resistance proportional to the free vortex density. In this work the effect of an applied current on the KT phase transition are studied. The Lorentz force, reduces the logarithmic interaction favouring the couples thermal generation. A complete depairing occurs at lower temperature and T_k becomes current dependent. From the formal point of view, it consists in the renormalization of the potential containing the Lorentz term. For T_k(I) an equation for the resistivity is numerically studied whose results are compared with the experimental data.

2PO1-126 An intrinsic dissipative process in planar superconductors at zero magnetic field

C. Paracchini and L. Romano, INFM. Instituto Nazionale di Fisica della Materia Dipartimento di Fisica. Universita, di Parma. Parma. Italy

Presenting Author: C. Paracchini

The dependence of the resistance R vs. the temperature T is studied in ten samples including thin films and Josephson junction arrays of metallic and high temperature superconductors. The data are obtained by direct measurements or by re-examining published results. In all these planar systems non-ohmic dissipation is exhibited, where the R(t) dependence at constant current is delimited between a threshold T_o(I) and a current independent upper limit T_m. By plotting R vs. Q = [T - T_o(I)]/[T_m - T_o(I)], all the R(T) curves overlap on one line. The R(Q) trend is aproximated by R=R_o exp[-b(1/Q)-0.5], where R_o and b are related to the final resistance and to the efficiency of the process.

The obtained R(T,I) dependence is similar to that attributed to the depairing of thermally excited vortex-antivortex pairs in the Kosterlitz-Thouless (K-T) theory at zero or at very low currents. The validity of the previous R(T,I) trend in a current range of several orders of magnitude indicates that the same mechanism works also in presence of current, where it reduces the T_k value. This study suggests that:

a) the planar structure of the system has a prevalent role in the dissipative processes in respect to the composition and the aggregative state of the material.

b) the non-ohmic response is due to one process and the dependence of dissiptaion on the temperature and current suggests a mechanism based on the thermal origin and the current assisted depairing of vortex-antivortex pairs.

2PO1-127 Mixed-state Hall effect in Hg- and Tl-based superconducting thin films

W.N. Kang 1, Wan-Seon Kim 1, Sung-Ik Lee 1, B.W. Kang 2, J.Z. Wu 2, Q.Y. Chen 3, W.K. Chu 3, and C.W. Chu 3. 1 National Creative Research Initiative Center for Superconductivity, Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea. 2 Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA. 3 Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA.

Presenting Author: W.N. Kang

We have investigated the mixed-state Hall resistivity r _xy and the longitudinal resistivity r _xx in HgBa₂CaCu₂O₆, HgBa₂Ca₂Cu₃O₈, and Tl₂Ba₂CaCu₂O₈ thin films for various parameters, such as densities of columnar defects, number of CuO₂ planes, and anisotropy ratios, as functions of the magnetic field up to 18 T. We observe the universal scaling behavior between r _xy and r _xx in the regions of the clean and the moderately clean limit. The scaling exponent b in r _xy = Ar _xxb is 1.9 ± 0.1 in the clean limit at high fields and low temperatures whereas b is 1 ± 0.1 in the moderately clean limit at low fields and high temperatures. We also find the triple sign reversal in the HgBa₂CaCu₂O₆ films containing high-density columnar defects. These results can be interpreted within the context of a recent theory based on the midgap states in the vortex core.

2PO1-128 Resistive Lock-in Oscillations in YBa₂Cu₃O_y

Sergey N. Gordeev 1, Peter de Groot 1, A.G.M. Jansen 2, Robert Gagnon 3, and Louis Taillefer 4. 1 Physics Department University of Southampton, SO17 1BJ, United Kingdom. 2 Grenoble High Magnetic Field Laboratory, MPIF-CNRS, BP 166, 38042 Grenoble Cedex 09, France. 3 Department of Physics, McGill University, Montreal, 2HA 2T8, Canada. 4 Department of Physics, University of Toronto, M5S 1A7, Canada.

Presenting Author: S.N. Gordeev

We report on the first observations of oscillatory behavior of resistance caused by intrinsic pinning in YBa₂Cu₃O_y single crystals. The measurements were performed on an underdoped YBa₂Cu₃O_y crystal with y=6.55 (60-K phase) in geometry B//ab and J//ab (J transverse to B) at fields up to 28 T. We observed a series of peaks and dips on the R(B) dependence. The oscillations appear below the vortex melting transition. The positions of these features are independent of temperature and applied current. It was found that dips correspond to commensurate states at which the vortex lattice spacing, s, fulfils the condition s=nd, where d is the CuO plane period and n is an integer number. We were able to detect dips which correspond to n=2-5. The oscillations are periodic in (1/B)^0.5. The microscopic anisotropy of the crystal was extracted from the period of the oscillations and was equal to 15. The R(B) dependence showed clear hysteretic behavior. The resistance measured in the regime of increasing field was higher than for decreasing field. The shape of the R(B) dependence was also slightly different. The analysis of current-voltage characteristics shows dramatic difference in the vortex dynamics between commensurate and incommensurate states.

2PO1-129 Theory of Anomalous Hall Effect in the Mixed State Type-II Superconductors

Wei Yeu Chen 1, M.J. Chou 1, and H.Q. Lin 2. 1 Department of Physics, Tamkang University, Tamsui, 25137, Taiwan, R.O.C. 2 Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong.

Presenting Author: W.Y. Chen

We present a new model for the anomalous Hall effect in the mixed state type-II superconductors. In this model the eigenmodes and the many body correlations of the vortex lattice are considered. The sign change of the Hall resistivity is the result of the competition between the motion of thermally induced vacancies (or in other words effective antivortices) and the thermally activated motion of the pinned vortex lattice together with the motion of the thermally induced interstitial vortices. Within this model many essential experimental results can be explained.