Andreev reflection in point contacts between the heavy-fermion superconductor UPt3 and ordinary superconductors

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Physica B 218 (1996) 161 164

Andreev reflection in point contacts between the heavy-fermion superconductor UPt3 and ordinary superconductors Yu.G. Naidyuk a'b' *, H.v. L6hneysen a, G. Goll a, C. Paschke a, I.K. Yanson b, A.A. Menovsky c a Physikalisches Institut, Universitiit Karlsruhe, Engesserslr. 7, 76128 Karlsruhe, Germany b Institute for Low Temperature Physics and Engineering, 310164 Kharkov, Ukraine c Van der Waals-Zeeman Laboratorium, Universiteit van Amsterdam, 1018 XE Amsterdam, The Netherlands

Abstract The differential resistance d V/dl versus applied voltage V of UPt3-Zn point contacts at very low temperatures shows the double-minimum structure around V = 0 characteristic for Andreev reflection due to superconducting Zn. In magnetic fields above Bc of Zn the d V/dI curves exhibit only a shallow minimum at V = 0 which is attributed to superconducting UPt3. A fit of the d V/dI curves on the basis of the Blonder-Tinkham-Klapwijk theory taking into account a lifetime broadening F of the quasiparticle density of states yields for Zn an energy gap A ( T ) perfectly following the BCS theory, a barrier strength Z ~ 0.45 and F ~> ~I A0. The usual Z value contradicts the notion of extremely high reflection coefficient of electrons at the interface with heavy-fermiom compounds due to their heavy masses. All this suggest that UPt3 behaves in the contacts as normal metal, possibly with strong pair breaking taking into account the high F value. This might be the reason for the failure to observe a Josephson coupling in contacts UPt3 with Zn, Pb, and NbTi.

1. Introduction Experiments carried out in the last decade on UPt3 show that superconductivity in this heavy-fermion compound is difficult to describe in the frame of conventional BCS theory. There is strong evidence that superconducting order parameter in UPt3 is anisotropic, and furthermore might not exhibit the full point symmetry of the crystal. Moreover, phonon-mediated pairing interaction has been questioned in this case. To prove this picture with certainty new experiments are still being attempted and point contact (PC) spectroscopy is among them. In conventional superconductors (SCs), the measurements of the conductance of normal metal-superconductor (N-S) contacts yield direct information about energy gap and its anisotropy via the mechanism of Andreev reflection (AR) [1]. On the other hand the Josephson effects are sensitive to the phase of the Cooper-pair wave function. As was recently suggested by Sigrist and Rice [2] one can measure in a direct way the angular dependence of the phase of the * Corresponding author. 0921-4526/96/$15.00 @ 1996 Elsevier Science B.V. All rights rese~'ed SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 5 8 3 - 8

order parameter with a special SQUID configuration. This can provide a decisive test of symmetry of the order parameter. In this paper we present PC experiments on S-S contacts between UPt3 and classical SCs. We have used different SCs in contact with UPt3, namely weak-coupling SC Zn, strongcoupling SC Pb and type-II SC NbTi in order to search for Josephson coupling of the wave function in weak links. We have made also attempts to build a SQUID on the basis of UPt3.

2. Experimental PCs between single crystal UPt3 and Zn wire were established by means of mechanical feedthrough directly into the mixing chamber of a dilution refrigerator. Contacts with Pb counterelectrode were fixed at room temperature and mounted in the mixing chamber prior to cool down. Weak links between UPt3 and NbTi were realized also at room temperature by pressing two NbTi thin wires against UPt3 crystal for preferred current flow along a and e axes in order to establish a closed loop as for a two-contact SQUID.

Yu.G. Naidyuk et al./ Physica B 218 (1996) 161 164

162

The differential resistance d V/'dI versus applied voltage V of the PCs was measured using standard modulation method. The SQUID was driven into the resistive state by the DC current and then dV/dl was measured as a function of the magnetic field produced by small superconducting coil placed directly in the mixing chamber. The mixing chamber was magnetically shielded by three p-metal coaxial cylinders.

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Fig. 1 shows d V/dl characteristics of a typical UPt3-Zn contact established along a direction. The contacts established along c direction showed the same qualitative behavior. At temperatures well below the critical temperature T~ for both materials (Tc ~" 0.5 K of UPt3 and T~ ~_ 0.85 K of Zn) the dV/dl curves exibit the double-minimum structure around V = 0 with a width corresponding to 2A for Zn. These features evolve smoothly with increasing T and vanish at T~ of Zn. It is obvious that this structure is caused by AR at the interface with superconducting Zn, i.e. it looks as if UPt3 in this case behaves as a normal-conducting metal. In a small magnetic field comparable with B~ for Zn the pronounced AR feature abruptly disappears (Fig. 2) and the dV/dI curves show only a shallow minimum at V = 0 which remains up to 0.6 T. This field corresponds to the critical field for UPt3-Pt contacts [3]. Therefore, the shallow minimum is attributed to superconducting UPb. At T > iQ of Zn the d ~ J d I curves exhibit a remarkable parabolic dependence (Fig. 1 ), which is also typical for PCs with normalconducting UPt3 [3]. This means that both normal and superconducting properties of UPt3 are reflected in the d V/dl curves, but the intensity of the superconducting feature of UPt3 is about one order of magnitude smaller than expected in N S contacts with ordinary SC and, in particular, in comparison with the observed structure caused by Zn. Moreover, no evidence of a finite critical current at V = 0 was observed at all in the UPt3-Zn contacts. The small intensity of the AR feature of UPt3 was found previously [3]. We can describe the d V/dl curves using the onedimensional BTK model [1] for the conductivity of an N - S contact with a d-function barrier at the interface. The fit parameters in the BTK model are the value of the superconducting energy gap A and the barrier strength Z which models the probabilities for Andreev and normal reflection. In our case we extend the BTK model including a broadening F of the BCS density of states

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Fig. 2. The magnetic field dependence of the dV/dl curves versus bias V for UPt3-Zn contact with R0 0.42 ~. Curves are offset vertically for clarity; T ~_ 0.1 K. F = 0 but also the absolute magnitude of d V/dl dependence turns out correctly (Fig. 3(a)). From the fit procedure averaging over several PCs we obtain for T ~ 0.1 K, A0 - (115 ± 5 ) g e V or 2Ao/kBTc 3.3 4- 0.1 in excellent agreement with literature data for Zn. Moreover, from the temperature dependence of d V/dl (see e.g. Fig. 1 ) A(T) is inferred which perfectly follows the BCS theory (Fig. 4(a)). All this confirms the validity of the modified BTK model and attaches some physical significance to the F parameter. It is interesting to note that F is about ~Ao for low-resistance PCs ( ~ 0.5 [~) and increases roughly linearly with PC resistance up to F ~ A0 (Fig. 4(b)). The Z value for the contacts falls in the range 0.45 :t: 0.02. Fig. 3(b) presents fit curves to the shallow d&/'dl minimum due to UPt3 (see e.g. Fig. 2). The fit yields A0 (see figure caption) which is comparable with values obtained in [3] in the frame of the ordinary BTK fit. The large value of F > A0 accounts for the small absolute magnitude of the minimum.

Yu. G. Naidyuk et aL / Physica B 218 (1996) 161 164 '

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As to UPt3-Pb contacts some of the contacts exibit a similar double-minimum structure as for Zn with the width of 2A corresponding to 2Ao/kBTc ~ 4.3 in agreement with strong e l e c t r o n ~ h o n o n coupling for Pb. Similarly to Zn, we did not find any evidence of a finite critical current at V-0. Turning to UPt3-NbTi contacts in SQUID geometry we obtained I - V characteristics for some PC with a quasicritical current, i.e with near zero resistance at V - 0 . We have investigated these contacts in a small magnetic field but no SQUID oscillations as found in URuzSi2 [5] were observed.

4. Discussion The main results of our experiments are the observation of clear AR features of Zn or Pb in contacts with UPt3 and the absence of Josephson coupling for both mentioned contacts and for weak links with NbTi. Furthermore, no decrease of the resistance at V = 0 in the case of Zn and Pb was observed, which would be characteristic for S-S contacts. It has been speculated that a supercurrent between conventional and unconventional SC may be impossible due

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to the different symmetry of the superconducting order parameter. However, it is now believed that at least a reduced Josephson coupling should be possible as a consequence of the symmetry-breaking effect of the interface. In the frame of our supposition for URu2Si2 [5] we believed that absence of the Josephson effects in UPt3 is due to a normal-conducting zone in the PC. The fact that the AR feature of Zn is much stronger than for UPt3 suggests that the normal layer is located on the UPt3 side of the PC. In this case AR on the UPt3 side actually occurs somewhat within heavy-fermion compound which decreases the number of Andreev-reflected particles which are retroreflected through the contact. This might result in only a shallow minimum in d [Tdl observed in a magnetic field B > Bc of Zn. The large value of F which increases linearly with PC resistance provides evidence that the N layer breaks the quantum phase coherence of the carriers and gives rise to the shortening of the lifetime. We have measured d V//dl curves for Zn Ag point contacts and found that the F parameter in this case is four times smaller for the same PC resistance or about one order of magnitude lower for the same PC size. This gives clear evidence that the interface with UPt3 produces a much stronger pair-breaking effect. One of the reasons might be magnetic moments of the U atoms, which order antiferromagnetically, albeit with a very small

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Yu. G. Naidyuk et al. / Physica B 218 (1996) 161-164

moment, in the bulk. At the surface, because of its nonideality antiferromagnetic order is destroyed and strong pair-breaking might occur. We do not believe in the suppression of the superconductivity in PC only because of the strong disorder in the constriction caused by mechanical stress. Even vacuum tunneling experiments on the UPt3 [6] did not reveal any indication of a Josephson current. It should be pointed out that the usual value of the barrier strength Z, the same as for Z n - A g contacts, supports a theory [7] that the boundary condition at the interface with heavy-fermion compounds involves Fermi velocities without a large mass-enhancement factor. In conclusion, we have observed unambiguous AR structures of ordinary SC in contact with UPt3. With a modified BTK model we obtain parameters describing properties of the interface, namely Z and F. These data can be interpreted in terms of a normal-conducting zone at the UPt3 interface leading to a typical normal-metal reflection of electrons and obstructing the possibility of the Josephson effects.

Acknowledgements This work was supported by Alexander von Humboldt Foundation and by the Deutsche Forschungsgemeinschaft through SFB 195.

References [1] G.E. Blonder, M. Tinkham and T.M. Klapwijk, Phys. Rev. B 25 (1982) 4515. [2] M. Sigrist and T.M. Rice, J. Phys. Soc. Japan 61 (1992) 4283. [3] G. Goll, H.v. L6hneysen, I.K. Yanson and L. Taillefer, Phys. Rev. Lett. 70 (1993) 2008. [4] R.C. Dynes, V. Narayanamurti and J.P. Gamo, Phys. Rev. 50 (1978) 1509. [5] A. Nowack, Yu.G. Naidyuk, E. Ulbrich, A. Freimuth, W. Schlabitz, I.K. Yanson and A.A. Menovsky, Z. Phys. B 97 (1995) 77. [6] U. Poppe, J. Magn. Magn. Mater. 52 (1985) 157. [7] G. Deutscher and P. Nozieres, Phys. Rev. B 50 (1994) 13 557.