Single crystal HoBa2Cu3Ox break junctions

Physica C 153-155 (1988) 1383-1384 North-Holland, Amsterdam

SINGLE CRYSTAL HoBa2Cu30 x BREAK JUNCTIONS t

John Moreland and A. F. Clark Electromagnetic Technology Division, National Bureau of Standards,

Boulder,

Colorado 80303

M. A. Damento and K. A. Gschneidner, Jr. Ames Laboratory, Iowa 50011

Department of Materials Science and Engineering,

Iowa State University,

Ames,

Tunneling spectra of HoBa~Cu30 X single crystals using the break junction method show energy gap features. These features are variable from junction to junction possibly due to an anisotropic gap function. The I-V curves show the peculiar square law dependance of the current on voltage seen in many tunneling measurements of polycrystalline samples of 90 K superconductors. This may be an indication of an inherent "granularity" built into the superconducting matrix of a single crystal.

Several electron tunneling methods have been used to measure the energy gap of RBa~Cu30 x superconductors (R - rare earth). Summaries of some of these recent results are included in references 1 and 2. Phenomena observed in the tunneling data include a linearly increasing conductance as a function of junction bias, tunneling energy gaps with BCS temperature dependance, and complex "gap harmonics" at biases larger than the gap-sum voltage. This type of data may be caused by superconducting particles coupled by weakly-linked tunneling matrix in the vicinity of a primary tunneling contact. It may be that this junction "granularity" is actually intrinsic to single crystals of RBa~Cu30 x since tunneling measurements on single crystals and oriented thin-films have shown the phenomena mentioned above. 3,4 This paper focuses on the break junction method s and its application to single crystal samples of HoBa2Cu30 x. Break junctions in single crystals should allow for a direct measurement of gap anisotropy (A(k)) assuming that the samples can be controllably fractured along a cleavage plane. In this way electrons tunnel primarily perpendicular to the sample fracture surface along a well defined crystal direction. Similar measurements using conventional oxide barrier junctions, for example, have been used to detect small tunneling energy gap anisotropies in Pb single crystals, e HoBa2Cu30 x single crytals were grown from a Cu0 flux at the Ames Laboratory using the method developed by Damento et al. ? Four samples with approximate dimensions of 0.2 mm x 0.5 mm × 0.5 mm were mounted on one break junction substrate. The samples were mounted with their c axes perpendicular to the substrate surface so that the fracturing strain would be directed in the a-b planes. The break junction apparatus was

t Contribution of the National Bureau of Standards,

0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)

enclosed in a gold-plated copper vapor can (i atm STP helium) to insure good thermal contact between the tunnel junctions in the fractures of each of the samples and a silicon diode thermometer mounted in the vapor can. The vapor can was suspended inside of a vacuum can by a stainless tube. The temperature was controlled with a heater mounted between the vapor can and thermal contact to the surrounding liquid helium bath. The samples were broken and adjusted to form break junctions at 4 K. Figures la and ib show some of the break junction tunneling curves (I-V and the corresponding dl/dV-V) at 4 K for HoBa2CusO x single crystals. As was observed for polycrystalline YBa~Cu~O x break junctions these data show a linearly increasing background conductance with increasing junction bias (Fig. la). Some of the curves show additional gap structure on top of the sloping conductance background (Fig. ib). Figures le through If show the temperature dependance of the I-V curve of one setting in one sample. Each of the samples showed weak gap structure after extensive adjustments (resetting) of the junctions. Gap measurements as a function of temperature using mechanically adjustable junctions are usually difficult because the contact shifts during thermal cycling. The I-V trace for this one setting remained the same, however, after cycling from 4 K to i00 K and back to 4 K again. In general the conductance peak locations shift towards zero bias with increasing temperature. At temperatures above 40 K the peaks are sufficiently broadened and the spectrum noise large enough that it is difficult to determine the peak locations. Briefly comparing these results to those for polycrystalline YBaICu~O x break junctions one finds that in both cases the junction conductance

not subject to copyright.

J. Moreland et aL / Single crystal HoBa2Cu30 x break/unctions

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Fig. i. Current-voltage and the corresponding differential conductance-voltage curves for HoBa2Cu30 x single crystal break junctions. Curves a, b, and c were measured at 4 K. Curves d, e, and f were measured at 10 K, 20 K, and 40 K respectively. increases linearly with junction bias. This may be due to an intrinsic particulate tunnel junction matrix existing within a single grain or crystal as discussed by Kirtley et al. s On the other hand, gap structure in the tunneling spectra of polycrystalline YBa2CusO x samples occurred more often during the adjustment of the junctions than is the case for the junctions reported here. This may be due to the existence of a normal core in the single crystals caused by lack of complete O 2 penetration during the sample anneal. Finally, the fracture characteristics of the HoBa2CusO x single crystals were non ideal, occurring randomly along several cleavage planes. In conclusion, we hope to improve the breaking process to obtain A(k) using break junctions. Also we believe that single crystal crystals samples should be very thin (i00 ~m or less) to insure a complete 02 anneal. ACKNOWLEDGEMENTS This work was supported by the Office of Naval Research under contract No. N00014-88-F-0013 (at NBS) and by the U.S. Department of Energy, Basic Energy Sciences under contract No. W-7405-ENG-82 (at Iowa State).

REFERENCES (I) J. Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, and A. F. Clark, Advances in Cryogenic Engineering (Materials) 34, to be published. (2) K. E. Gray, M. E. Howley, and E. R. Moog, Novel Superconductivity, edited by S. A. Wolf and V. Z. Kresin (Plenum, New York, 1987) pp. 611-625. (3) J. R. Kirtley, C. C. Tsuei, S. I. Park, C. C. Chi, J. Rozen, M. W. Shafer, W. J. Gallagher, R. L. Sandstrom, T. R. Dinger, and D.A.Chance, Proceedings LTI8 (1987) 997. (4) M. Naito, D. P. E. Smith, M. D. Kirk, B. Oh, M. R. Hahn, K. Char, D. B. Mitzi, J. Z. Sun, D. J. Webb, M. R. Beasely, O. Fischer, T. H. Geballe, R. H. Hamond, A. Kapitulnik, and C. F. Quate, Phys. Rev. B 3_55 (1987) 7228. (5) J. Moreland and J. W. Ekin, J. Appl. Phys. 58 (1985) 3888. (6) B. L. Blackford and R. H. March, Phys. Rev. 186 (1969) 397. (7) M. A. Damento, K. A. Gschneidner, Jr., and R. W. McCallum, AppI. Phys. Lett. 51 (1987) 690.