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 ...
