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(194) 1083-1084 B Physica 194-196 North-Holland

ALLOY NEAR TIIE LOCALIZATION THRESHOLD ST'PERCONDUCTING RESPONSEIN BULK CKTSb V. F.Gantnakher, Institute V.M.Teplinskii, V.N.Zverev, and O. I.Barkalov Russia

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Physica 194-196(1994) 1083-1084 B North-Holland

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SUPERCONDUCTING RESPONSE BULK CdSb ALLOY NEAR THE LOCALIZATION THRESHOLD IN V. F.Gantmakher, V.M.Teplinskii, Institute

of Solid State Physics,

Experiments with rnetastable CdSb alloy (S) demonstrate superconducting response on the insulating side of the metal(M)-insulator(I) transition. This response dissappears with lowering the temperature. Superconductivity behaves differently in disordered 2D and 3D electron systems. According to the scaling theory disorder always [1], Ieads to localization in 2D at T=0. As a result, S-I transition appears without intermediate M-state [2]. It has been observed and investigated experimentally [3,4]. The situation in 3D is complicated by M-state coming into play. Those who make experiments near the localization threshold are always asked about the uniformity of their material. However, this may turn to be not very important when dealing with S-transition because S-interaction stimulates phase separation near the localization threshoId even in an initially uniform material [5,6]. Our experiments wene performed with Cdnfbrralloy [7,8]. Their advantage was that the whole sequence of states was obtained with one sample while its amorphization from a metastable M-phase. high-pressure Heating to room temperatures irreversibly transformed it into an amorphous I phase. Dosing the heating allows one to obtain a sequence of quasistable states. intermediate by the These states are labelled ratio logarithm of the resistance is R/R. as measured at T=6K. R. ].n tn samthe resistance of the initial ple. The state g=4 separates M- and I-states [7]. of Fig.1 displays the evolution in the S-response. lJhen g>4, i.e. the curves demonstrate the I-states, guasireentrant behavior. The magnetic field does not change the type of the curves: for q<4 minimum in R(T) does not appear (cf. [4] for 2D). The curves for the separating ,state g=4 are shown in Fig.2.

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Fig.4. Fig. 3. Magnetoresistance explanation The natural classical quasireentrof the curves R(T) with ant behavior would suppose that Sare of the material and I-regions quantum the series, connected in suggests that one deals explanation of the Sformation with partial to the whole condensate attributed sample. Some arguments in favour of the second assumption were obtained T from measuring R(H) at different 171. Usual S-response at 3K (increase of R with H) gives waY to negati(NI'IR) at T<1K. ve magnetoresistance One may conclude from the curves at Sthat temperatures intermediate response is not simply masked by Nl'lR but disappeares at low T. The competition between S-response and Nlfi is by Fig.3, RO and R,, are itlustrated resistances at the fields O and H=47 I{e summarize by the diagram in on the diagram Fig.4. The origin corresponds to the M-I transition point, the quantity f along the xaxis is defined through the correlation length { which tends to infinity from whichever side the transition is approached. NameIY, f=L/Q on the I-side of the diagram (at right) on the M-side (at left). and f=-!/{ At the M-side, was deduced from of the conductance the extrapolation U/R) to its value at T=0. The same procedure determines the state f=0.

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So the data concerning the supercontemonset and reentrant ductivity: the inside peratures, are placed from the normal established .frame the way rle Fig.3 explains state. data. the superconducting extract When we reduce the temPerature on the I-side r.re must reach a hoPPing crossover this regime. Apparently, branch lower the with coincides on the diagram. So, T T(f)= {f) reenL if the whole approach is valid and we indeed deal with a single quantum state of the material, then it follows that crossover to hopping conduthe superconducting destroys ction response. REFERENCES 1. E.Abrahams et aI., Phis.Rev.Lett. 42 (L979) 673 Phis.Rev. 2. M.P.A.Fisher et dl., Lett. 64 U.99O) SeZ 3. D.B.Haviland et al. Physica B 169

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4. A. F. Hebard and M. A. Paalinen' Phys.Rev.Lett. 65 U'990) 927 5. L.Bulaevskii et aI., Sov.Ph.-JETP 6s (1987) 380 6. A.A.Gorbatsevich et al., Sov.Ph.JETP 74 (1.992) s1l 7. V.M.TepIinskii et 81., Sov.Ph.JETP 74 (L992) 90s 8. V.F.Gantmakher et aI., JETP Lett. s6 (t992) 30e