Qbservation of Negative s-%ave Proximity Effect in Superconducting UBe13

The Josephson I, between a Ta-wire probe and an induced, surface, singlet, superconducting state in UBei3 decreases with decreasing temperature below the bulk UBei3 T„ in contrast to the increase seen in comparison Mo samples. This shows that the bulk UBei3 superconductivity suppresses the induced singlet superconductivity. Such suppression is evidence of a triplet superconducting state in UBei3. Evidence is presented for phase slip between weakly coupled singlet and triplet order parameters.

Such suppression is evidence of a triplet superconducting state in UBei3. Evidence is presented for phase slip between weakly coupled singlet and triplet order parameters.
PACS numbers: 74.50. +r, 74.70.Rv The nature of the superconducting state in the heavy-fermion metal UBet3' is a question which has generated considerable current interest and experimental activity. The resistivity and specific heat, ultrasonic attenuation, and other properties in UBet3 are anomalous and have supported suggestions that its pairing is odd-parity (OP) spin-triplet, as in superfluid 3He, or even-parity d wave. 5 Several authors have proposed experiments to detect a characteristic "negative proximity effect" between conventional and OP superconductors. Measurements of the Josephson I, 7'o and the quasiparticle tunneling spectrum" are also pertinent.
Interpretation of such experiments is complicated by inherent surface breaking of OP pairs and by the effects of spin-orbit interaction near an interface. 6 It has also been proposed that order parameters of different symmetry will weakly suppress each other by competing for phase space. s In this Letter we describe an experiment on UBet3 which gives evidence of such suppression, and we argue that this suppression indicates that the superconductivity in UBet3 is odd parity. We have previously reported'2 s-wave superconductivity induced in the surface of UBet3 above its T, by exchange of pairs from an s-wave probe and observed by the Josephson effect. A surface, singlet, order parameter 6, is thus established, extending a coherence length g into the bulk. This singlet state, whose magnitude can be monitored by the Josephson I" represents a probe of the bulk order parameter as in the proposed proximity-effect experiments.
New comparative measurements of the dc and ac Josephson effects have been carried out on ingots of UBet3 and Mo contacted by Ta wires. The samples are mounted outside a hole in the wide face of a E-band microwave guide, and the wire driven across its interior by an externally controlled screw. Four wires separately contact the ingot and the Ta wire. Contact is made at 4.2 K, with only millivolt bias. The apparatus can discriminate between a milliohm resistance and a short; the latter is observed on Mo below 0.92 K. Electropolished surfaces of UBet3 give significantly clearer Shapiro steps than were obtained previously. " Nb and Ta tips on 1-mm wire are mechanically ground and result in typical contact diameters of 1-10 iA, m.
Study of the surface region of UBet3 contacted by the probe in a scanning Auger microprobe (microscope) reveals no damage.  = dV/dl~o 2 x. The residual resistance at 2 K is 8'=I II = p/2a, which, using p(2 K) =200 p, Qwm, implies a =1 p. m. R(T) falls in a manner similar to the bulk resistivity (open circles), after recent measurements of Remenyi er al. '6 8 (T) below T, drops more sharply, but returns to a reduced slope and reaches 0. 28" at the lowest temperature, 0.5 K.
The observations on UBer3 may be understood if the bulk superconductivity in UBer3 is odd parity. ' Then one expects, even near an interface, the following: (1) the direct Josephson coupling between tantalum and the odd-parity superconductivity is negligible7 '0; (2) the odd-parity bulk superconductivity competes with the proximity-induced singlet superconductivity for electrons, 8 thus causing the magnitude of the proximity-induced singlet order parameter b, , to decrease as the temperature decreases below the T, of UBe», (3) phase slippage occurs in the region where the induced and bulk order parameters overlap, leading to a finite resistance in series with the Josephson junction even though the UBet3 is superconducting in bulk.
A complete discussion of these effects would involve formulating and solving a nonlocal, nonlinear equation. However, the essential question is the validity of point (2) above, and this may be demonstrated within the same simple model used for the T) T, data. r2 We assume that the magnitude of the inducedsinglet order parameter is fixed by balancing two energies: the free-energy cost SFr to impose s. type superconductivity on the UBer3 and another energy SF2 which represents coupling to the Ta wire. We have 5 F2 = q(0 (hT, -5,) 2, where SF2 is phenomenological; q is a measure of the transmissivity of the interface; b, T, and b, , are the order parameters at the interface in the Ta and UBer3, respectively; and go is the coherence length in UBer3.   Fig. 2, obtained from Eq.
(3) with X=2. 8, provides the observed behavior. '8 The data for UBer3 from four other contacts (not shown) can also be fitted by Eq. (3), by use of the same material parameter X, altering only the contacttransmissivity parameter q Furt. her work is in progress in improving and generalizing this treatment.
We note that were the superconductivity in UBer3 spin-singlet (e.g. , "d wave") our analysis would not apply. s Direct Josephson coupling between the Ta wire and the bulk UBer3 superconductivity would be possible. Also, within -(0 of the interface, i.e. , the region where the presence of the barrier is expected to destroy rotational invariance, the linearized gap equation would couple sand d. symmetry gap functions. '9 The suppression of the induced s-wave gap caused by the competition for electrons would therefore be much weaker. Therefore, one may obtain a suppression of I, below the UBer3 T, from a model assuming d-wave superconductivity only if the Josephson coupling to the bulk d-wave order parameter is anomalously small, or if the induced-s-wave order parameter extends a distance ( )) $0 into the bulk. We therefore believe the superconductivity in UBer3 is odd parity.
In summary, a negative proximity effect has been observed between the bulk superconductivity of UBer3 and a proximity-induced surface singlet state. This effect has been accounted for by a model of tripletsinglet phase competition in UBer3 below its T, . As we have argued, these new results support an oddparity superconducting ground state in UBer3.