Sign Change ofc-Axis Magnetoconductivity in YBa2C

Abstract

The out-of-plane magnetoconductivity Ds c (BkIkcrystal c axis) of two YBa 2 Cu 3 O 72d single crystals has been measured in magnetic fields up to 12 T at temperatures ranging from T c 1 3 K to T c 1 40 K. A change of sign, from negative Ds c near T c to positive at higher temperatures, is observed in a metallic sample ͑dr c ͞dT . 0͒. Recent fluctuation theories can describe the main features of these observations with parameters that are in good agreement with other studies. These results suggest that fluctuations in the density of states have been observed in the magnetoconductivity. [S0031-9007(96)00941-6] PACS numbers: 73.50.Jt, 74.40.+k, 74.72.Bk It has recently become increasingly clear that the c-axis properties of anisotropic high-temperature superconductors are essential for understanding the anomalous superconducting as well as normal-state properties. A problematic observation, for instance, is a positive dr͞dT for the ab-plane resistivity combined with a strongly negative c-axis dr͞dT in underdoped YBa 2 Cu 3 O 72d and in Bi-based materials. Attempted descriptions include various tunneling models, fluctuations, interplanar disorder, and hopping due to resonant tunneling [1][2][3][4][5].Studies of superconducting fluctuations in the magnetoconductivity, Ds͑B, T͒ 1͞r͑B, T ͒ 2 1͞r͑0, T ͒, represent a powerful experimental technique to address related problems. The considerable effects of the shortlived electron pairs over a wide range of temperatures above T c , together with the possibility to use strong magnetic fields, provide for stringent comparisons with theories. In addition to reliable estimates of the coherence lengths and the phase-breaking time [6], measurements of Ds may allow for conclusions about the pairing state [7,8], or clarify the nature of the impurity state in certain doped alloy systems [9,10]. There is also a rapid development of the fluctuation theories, with continued discussions about what terms and what limits of existing theories to use, and with an increasing number of suggested additions and new theories [11 -13].In the present study we have measured the c-axis magnetoconductivity Ds c ͑BkIkc͒ of two YBa 2 Cu 3 O 72d single crystals in the temperature range T c 1 3 K to T c 1 40 K. A change of sign was observed, from negative Ds c near T c to positive above T c 1 10 K. In the theory by Dorin and co-workers [13], such a sign change could occur as a result of fluctuations in the normal density of states. We have adapted this theory for comparisons with experiments. Excellent descriptions of the observations with reasonable parameter values were obtained in the region where Ds c # 0, including the sign change at approximately the correct temperature. In the region where Ds c . 0, the uncertainties are somewhat larger.Two single crystals of YBa 2 Cu 3 O 72d were grown by the self-flux method [14]. Sample A was oxygenated at 460, 400, 350, and finally 300 ± C, for a few days at each temperature. Sample B was oxygenated at 400 ± C. Sample A was heavily twinned, whereas sample B ...

The role of density of states fluctuations in the normal state properties of highTcsuperconductors

“…An analysis within both the dirty and clean-limit phenomenological models revealed several inconsistencies regarding the resulting value for ¿ u [117,154,162,163]. This additional magnetoconductivity is suppressed in Zn-doped YBCO [6,117], partially Pr-substituted YBCO [179] and in the more impure compounds 2223-BSCCO [147,158] and oxygen-depleted YBCO [174]. The results obtained by various authors from the analysis of the in-plane magnetoconductivity of YBCO vary for x ab(0) from 11 to 18 A, for x c(0) from 1 to 4.6 A and, in those cases, where the anomalous MT contribution was detected, for ¿ u from 2.…”

“…In contrast to the ab-plane magnetoresistance which is positive at all temperatures, along the c-axis the magnetoresistance has been found to have a negative sign not too close to Tc0 in many HTS compoundsÐ i.e. BSSCO [5,37,148,149], LSCO [150], YBCO [6] and TBCCO [185]Ð and turn to positive values at lower temperatures. We will show how these behaviours ® nd their explanation within thē uctuation theory in the magnetic-® eld induced suppression of the AL contribution along the c-axis.…”

“…and [6], the ® t of experimental data with the theory based on thē uctuation renormalization of the one-electron density of states [29,34] is excellent for weak magnetic ® elds but meets noticeable di culties in the region of strong ® elds. This is due to the formal divergence of the DOS contribution to conductivity and to the dependence of the cut-o parameter on the magnetic ® eld itself [34].…”

“…Moreover, its competition with the positive Aslamazov± Larkin magnetoresitance gave good grounds for the prediction of a sign change in the magnetoresistance as temperature decreases towards Tc [39]. The latter e ect was very recently con® rmed experimentally on YBCO [6] and TBCCO [185] samples.…”

Positive in-plane and negative out-of-plane magnetoresistance in the overdoped high-temperature superconductorBi2Sr2Heine1, Lang2, Wang3 et al. 1999Phys. Rev. B2710140View full textAdd to dashboardCite

“…Their relative weakness is caused by the high anisotropy of BSCCO, in contrast to YBCO, where the competition between ALO and DOSO effects produces a sign change of ⌬ c at about T c ϩ10 K ( Ϸ0.1). 21 Our results can be compared to normal-state transport quantities using the simple Drude model. Unfortunately, few data of overdoped 2212-BSCCO is available and we had to use results from optimally doped BSCCO.…”