Thermodynamic features in the H-T plane of superconducting UBe13

Tracking aluminium impurities in single crystals of the heavy-fermion superconductor UBe13

The influence of Al incorporation on the heavy fermion superconductor UBe₁₃ was investigated to explain the sample dependence of physical properties. Clear evidence for incorporated Al in flux-grown UBe₁₃ single crystals is presented by results from X-ray diffraction, nuclear magnetic resonance and X-ray spectroscopy. The increase of the lattice parameter and the concomitant change of the superconducting properties are caused by substitution of Be in the compound by 1–2 at.% Al. The minute amounts of Al in the structure were located by atomic resolution transmission electron microscopy. Specific heat measurements reveal the strong influence of incorporated Al on the physical properties of UBe₁₃. Upon long-term annealing, Al incorporated in single crystals can leave the structure, restoring properties of Al-free polycrystalline UBe₁₃.

Field-orientation dependence of low-energy quasiparticle excitations in the heavy-electron superconductor UBe(13)

Low-energy quasiparticle excitations in the superconducting (SC) state of UBe_{13} were studied by means of specific-heat (C) measurements in a rotating field. Quite unexpectedly, the magnetic-field dependence of C(H) is linear in H with no angular dependence at low fields in the SC state, implying that the gap is fully open over the Fermi surfaces, in stark contrast to previous expectations. In addition, a characteristic cubic anisotropy of C(H) was observed above 2 T with a maximum (minimum) for H∥[001] ([111]) within the (11[over ¯]0) plane, in the normal as well as in the SC states. This oscillation possibly originates from the anisotropic response of the heavy quasiparticle bands, and might be a key to understand the unusual properties of UBe_{13}.

Observation of an unusual magnetic anomaly in the superconducting mixed state of heavy-fermion compound UBe13 by precise dc magnetization measurements

We have performed precise dc magnetization measurements for a single crystal of UBe(13) down to 0.14 K, up to 80 kOe. We observed a magnetic anomaly in the superconducting (SC) mixed state at a field, named H(Mag)(*) (~ 26 kOe, at 0.14 K), implying that UBe(13) has a magnetically unusual SC state. We studied the magnetization curves of UBe(13), assuming that the H(Mag)(*) anomaly originates from (1) and unusual SC diamagnetic response, or (2) a peculiarity of the normal-state magnetization due to vortices in the SC mixed state. The origin of the H(Mag)(*) anomaly is discussed.