Emergence of Coherent Transport inUBe13: An Optical Investigation in both the Normal and Superconducting States

Abstract

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.

Electrodynamics of correlated electron materials

“…Extending previous experiments (Bommeli et al, 1997;Bonn et al, 1988a) to lower frequencies and temperatures, Holden et al (2003) observed coherent transport in UBe 13 with an abrupt decrease in scattering rate and a strong increase of the effective mass. The strongest enhancement of the effective mass is observed in heavy-fermion systems with a magnetically ordered ground state.…”

Spin gap in heavy fermion compound UBe13

“…The explanation which invokes spin gap opening at 180 K is consistent with anomalies in specific heat [43], NMR [44] and optical conductivity [14], which are discussed in terms of crystal field splitting of the f 5 3 U ion with characteristic energy ∼180 K. Below we provide additional arguments in favour of the spin gap opening.…”

“…In transverse magnetic field, if the muon precession frequency becomes much smaller than its hoprate between these sites at 180 K, the doublet would collapse into a single line. However, anomalies discovered at 180 K by several different techniques which do not involve the muon-specific heat [43], NMR [44] or optical conductivity [14]-do imply that these anomalies relate to UBe 13 system itself rather than the muon local diffusion.…”

“…Quite remarkably, the FL behaviour in UBe 13 (as in some other HF systems) is restored by application of a strong magnetic field [11]. Another remarkable feature which requires explanation is that while UBe 13 displays coherent quasiparticle behaviour at low temperature, at higher temperature it exhibits an incoherent metallic state dominated by spinflip scattering [11,14].…”

Anomalous metallic ground state inBaV13O18

“…This is nearly consistent with the possible Drude weight for the σ c (ω) spectrum at 10 K below 0.01 eV, which reaches 1200 −1 cm −1 at 0 eV. Note that in various heavy-fermion compounds, a small Drude spectrum appears below T K in the ω range far below 0.01 eV [15][16][17][18][19][20].…”