H-Tphase diagrams of the double transition in thoriatedUBe13

Abstract

We present magnetic field-temperature (H-T) phase diagrams of the double transitions of superconducting U 1Ϫx Th x Be 13 with xϭ0.030 and xϭ0.022. For both samples increasing the applied magnetic field moves the two transitions to lower temperature while decreasing their separation in temperature. For the xϭ0.030 sample, the transitions remain distinct for TϾ100 mK. For xϭ0.022, however, the two transitions appear to merge near Hϭ20 kOe and Tϭ350 mK, analogous to the situation in the related heavy fermion superconductor UPt 3 .Perhaps the most compelling manifestation of the exotic nature of the superconductivity in the heavy-fermion compounds is the occurrence of not one, but two transitions in the superconducting state. In addition to the normal-statesuperconductor transition at T c , a second phase transition occurs at a lower temperature, T c2 , which maintains the distinctive features of zero-resistance and inductive shielding. The most intensively studied double transition is that of UPt 3 . With a relatively small temperature splitting ⌬Tϭ60 mK, an explanation arises naturally in terms of two superconducting states with nearly degenerate energetics but different symmetries. [1][2][3][4][5][6][7] Detailed studies of the magnetic field-temperature 8-11 (H-T) and pressure-temperature [12][13][14] (P-T) phase diagrams of UPt 3 have revealed additional phases and have provided insight into the nature of the transitions between them. Whereas the superconducting order in UPt 3 can be treated as a perturbation on the crystal symmetry, the situation in thoriated UBe 13 is not so clear. U 1Ϫx Th x Be 13 exhibits a double transition in the superconducting state for 0.018Ͻx Ͻ0.045, but with ⌬T's up to several hundred mK, a fair fraction of T c . The material remains superconducting below T c2 , with the slope of the lower critical field actually increasing. 15,16 Muon-spin-relaxation ͑SR͒ measurements have detected weak magnetic correlations below T c2 , 16 suggesting that the lower transition may correspond to the onset of some weak magnetic ordering that then coexists with the superconductivity below T c2 . Alternatively, the second transition may correspond to a change in the symmetry of the superconducting state. In one such scenario, 17 buttressed by recent explorations of both the local magnetization in a torus of U 0.97 Th 0.03 Be 13 ͑Ref. 18͒ and the general P-T phase diagram, 19 the superconducting order parameter for the state below T c2 violates time reversal invariance. It thus gives rise to weak local magnetic fields consistent with the SR result. Similarly, SR measurements on UPt 3 have seen an increase in the internal magnetic field below the lower transition. 20For the comparatively neglected U 1Ϫx Th x Be 13 system, an understanding of the nature of the double transition and an explanation of their appearance in the unusual x-T phase diagram remain incomplete. Yet, the UBe 13 -based superconductors are the only materials where the specifics of sample preparation and macroscopic sample homogeneity do ...

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