Tunneling spectroscopic measurements on boride carbide superconductors

Physlca C 235-240 (1994) 1859-1860

PHYSICA

North-Holland

Tunneling Spectroscopic Measurements on Boride Carbide Superconductors T. Hasegawa a, M. Oginoa, A. Takagi a, E. Watanabe a, M. Nantoh a, H. Takagi b, S. Uchtda b, R. J. Cava c and K. Kitazawa a

aDepartment of Applied Chemistry, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan bDepartment of Applied Physics, Universtty of Tokyo, Bunkyo-ku, Tokyo 113, Japan CAT&T Bell Laboratories, Murray lhli, New Jersey 07974, U.S.A. The new boride carbide superconductor LuNi2B2C has been investigated by electron tunnehng techntque using cryogenic temperature scanning tunnehng microscopy. A gap parameter of A(4.7K)=2.1-2.5 meV (2A/kBTc=2.9-3.5) was obtained by fitting the Dynes' function to the experimental conductance curves. These values are consistent with the BCS weak coupling superconductivity.

1. INTRODUCTION Recently, superconducuwty with relatively high critical temperature (Tc) up to 23 K was found in the series of quaternary mtermetalhc compounds L n N I 2 B 2 C (Ln=Y, Ho-Lu) [1-3]. Various superconducting propemes of these bonde carbtde compounds have been extensively studted t~, elucidate the origin of superconductivtty [4]. Energy gap is an important parameter that has not been reported so far. Here, we report the tunneling spectroscopic measurements of LuNi2B2C using cryogenic temperature s c a n m n g tunnehng microscop2~ (CT-STM),

2. EXPERIMENTAL Polycrystalhne samples of single phase L u N i 2 B 2 C were prepared by arc melting and anneahng. The sampies showed very sharp resistive transition at 16.6 K w~th the narrow transmon wtdth less than 0.5 K. The CT-STM instrument employed in this experiment was laboratory-constructed. The STM unit was sustained in the dual concentric stainless steel tubes which were directly immersed into liqutd He. Specimens were cooled down to 4.2 K through He exchange gas Surface of the samples were pohshed by an A1203 sheet m mr just prior to 0921-4534/94/507 00 © 1994 - l-lsevmr Scmncc B V All rights reserved SSt 21-4534(94)01496-5

the tunnehng measurements. 3, RESULTS The observed ~pectra had rather complex background conductance dependent on temperature. In order to determine ~hc gap value quant~tatwely, the experimental conductance curves was dwlded by an appropriate background assumed at each temperature independently. Fig. ! shows the b a c k g r o u n d corrected t u n n e l i n g spectra g(V)=os(V)/oN(V) in the temperature range of 4.717.8K. The conductance curves a' lower temperatures showed overshooting peaks at V=+_3.3 inV. The gap structure dtsappeared around 15 K, which ~s close to the bulk Tc=16 K Thts gtves a clear evtdcnce for that the superconducting gap structure was observed m F~g 1. F;g. 2 :,hows an exan.aple of g(V) at 4 7 K. It is noted from F~g. 2 that the gap structure ~s ~ubslanttally smeared m comparison wtth the original BCS profile, which g~ves flat bottom rcgmn at V=0 w~th qmte low zero bins conductance g(0) of 0 15 One may presume that the Andrecv reflection occurred s~multaneously w~th the normal electron tunnehng, since ~t apparcntly enhances the conductance inside the gap edges However, lhe conductance areas above and below g=l are 1 1.02

T. Hasegawa eta/./Phystca C 235-240 (1994) 1859-1860

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This fact excludes the possibility that the Andreev reflection w.kes place in our measurements. The gap smearing may arise from the degradation of superconductivity near the surface. By fitting the Dynes' function to the background corrected conductance curves, we have deduced the energy gap A. For the data aw4.7 K, the best fit was obtamed by assuming A=2.10+0.02 meV and F=0 66+0.02 meV. Th~s yields gap rat~o 2A/kBTc of 2.93, which is shghtly smaller than the BCS weak coupling value, 3.52. As exhibited m Fig 2, however, the Dynes function assummg relatively large F described above substantially deviates from the exper mental curve outside the conductance peaks. Betl "r fit to the conductance outside the gap regton g~ves rise to lager A - 2.53

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Tip Bias voltage [mV] Fig. 2 Tunneling spectra of LuNi2B2C at 4.7 K. Solid line represents an experimental conductance curve g(V). Dotted and broken hnes denote Dynes' functions with A=2.11 meV, F=0.66 meV and A=2.52 meV, I'--0.49 meV, respectively.

meV. Therefore, we tentatively conclude that the gap is in the range of 2.10-2.53 (2A/kBTc=2.93-3.53), which is consistent with the BCS weak coupling theory. 4, C O N C L U S I O N S In summary, we report the tunneling measurement of bonde carbide superconductor, LuNi2B2C. The observed tunneling spectra showed a clear superconducting gap structure, which disappeared near bulk Tc The gap ratio estimated by the curve-fitting of Dynes' function ranged from 2.9 to 3.5. This result is consistent with the BCS weak coupling theory. REFERENCES I. R. J. Cava et al Nature 367 (1994) 146. 2. R J Cava et al Nature 367 (i994) 252. 3. T Slegnst H. W. Zandbergen, R..I. Cava, J. J. Krjcwskl and W F Peck, Jr, Nature 367 (1994) 254. 4. H. Takagt, R. J. Cava, H. E~saki, J. O. Lee, K. MlzuhashJ, B. Batlogg, S Uchlda, J. J. rxi,.tj~,.sM and F recK, Jr., preprint. rr

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