Emergence of coherent transport in UBe13: an optical investigation in both the normal and superconducting states

The far-infrared reflectance of heavy-fermion superconductor UBe13 has been measured from 0.5 K in the superconducting state, to 300 K. Coherent transport, characterized by an optical conductivity that exhibits a renormalized Drude peak at low frequencies, emerges between 45 and 85 K although the dc resistivity of UBe13 continues to increase with decreasing temperature down to approximately 2.5 K. In the superconducting state, the low frequency scattering rate falls abruptly, implying that the formation of the condensate alters dramatically the interaction between the heavy electrons.     

Observation of antiferromagnetic correlations in UBe13

The wavevector and energy dependence of the paramagnetic response in the normal phase of the Heavy Fermion system UBe₁₃ has been investigated between 10 K and 300 K using polarized neutrons and polarization analysis. At 10 K the response was found to be enhanced at non zero wave-vectors indicating the presence of strong antiferromagnetic correlations. The peaks in the scattering occured at positions expected for incipient type G antiferromagnetism of the simple cubic uranium sublattice. At room temperature the spatial correlations completely disappeared and the response was wave vector independent. Constant Q scans carried out at 10 K confirmed the Lorentzian dependence proposed by Goldman et al. [1].     

Heavy-ion irradiation of UBe13 superconductors

We irradiate the heavy fermion superconductors (U,Th)Be₁₃ with high-energy heavy ions. Damage from the ions affects both heat capacity and magnetization measurements, although much less dramatically than in other superconductors. From these data and from direct imaging, we conclude that the irradiation does not create the amorphous columnar defects observed in high-temperature superconductors and other materials. We also find that the damage suppresses the two superconducting transitions of U_0.97Th_0.03Be₁₃ by comparable amounts, unlike their response to other types of defects.     

The superconductive states in the dislocation structures

On the basis of an analysis of the Ginzburg-Landau equations it is shown that in the dislocation structures above the critical temperature and the critical magnetic field of homogeneous superconductor there is an unhomogeneous superconductive order parameter. Such states may be either localized for the single dislocation and the dislocations pile-up or delocalized for periodical systems. The temperature shift, the critical field and the current are investigated.     

Spectroscopic evidence for unconventional superconductivity in UBe13

We report on measurements of the differential conductivity G of UBe13-Au contacts, which reveal the existence of low-energy Andreev surface bound states. These bound states are identified via huge conductance peaks at zero bias that may form only in superconductors with nontrivial energy-gap functions. From the voltage dependence of G at T6.7, much in excess of the weak coupling BCS value of 3.5, and directly indicating strong coupling effects in superconducting UBe13.     

Structural differences between single crystal and polycrystalline UBe13

We report on observations of structural and chemical differences between samples of UBe₁₃ that were synthesised using two different methods. Unexplained discrepancies in properties between samples with differing synthesis had previously been found in this heavy fermion superconductor. A polycrystalline UBe₁₃ sample was made by arc-melting the constituents. Single crystals were grown using an aluminium flux and had a consistently slightly larger lattice parameter than the polycrystals, which merited further study. Neutron diffraction data were collected at the Lujan Center at LANSCE on the HIPPO diffractometer. Aluminium was detected by inductively coupled plasma mass spectrometry (ICP-MS) in the flux-grown single crystal (0.803 wt%), and small amounts (~0.2 wt%) of thorium were detected in the UBe₁₃ polycrystalline sample. In order to probe the implications of the presence of Al, calculations by spin-polarised DFT-GGA+U show that the incorporation of Al onto the 96i site (the lowest symmetry site in the structure) is energetically more favourable than on other sites. In general, the trends calculated by DFT for bond lengths and lattice parameter increases are consistent with bond lengths experimentally observed by neutron diffraction, but specific percentage changes with aluminium incorporation may be obscured by the unexpected thorium in the polycrystalline sample. The aggregate of our initial observations suggests that incorporation of aluminium from the flux into single crystal UBe₁₃ is significant.     

Magnetoacoustic quantum oscillations in the heavy fermion superconductor UBe13

In UBe₁₃ magnetoacoustic quantum oscillations (MAQO) are observed for the attenuation- and the sound velocity-changes in the superconducting and normal state. The observed extremal areas and the effective masses are small.