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In the underdoped copper-oxides, high-temperature superconductivity condenses from a nonconventional metallic "pseudogap" phase that exhibits a variety of non-Fermi liquid properties. Recently, it has become clear that a charge density wave (CDW) phase exists within the pseudogap regime. This CDW coexists and competes with superconductivity (SC) below the transition temperature Tc, suggesting that these two orders are intimately related. Here we show that the condensation of the superfluid from this unconventional precursor is reflected in deviations from the predictions of BSC theory regarding the recombination rate of quasiparticles. We report a detailed investigation of the quasiparticle (QP) recombination lifetime, τqp, as a function of temperature and magnetic field in underdoped HgBa2CuO(4+δ) (Hg-1201) and YBa2Cu3O(6+x) (YBCO) single crystals by ultrafast time-resolved reflectivity. We find that τqp(T) exhibits a local maximum in a small temperature window near Tc that is prominent in underdoped samples with coexisting charge order and vanishes with application of a small magnetic field. We explain this unusual, non-BCS behavior by positing that Tc marks a transition from phase-fluctuating SC/CDW composite order above to a SC/CDW condensate below. Our results suggest that the superfluid in underdoped cuprates is a condensate of coherently-mixed particle-particle and particle-hole pairs.

We present far infrared reflectance measurements of UBe13 and URu2Si2 down to a temperature of 2 K. At low temperatures the optical conductivity of both materials may be described in terms of free charge carriers with frequency and temperature dependent scattering rates and mass enhancements. The 'coherent regime' occurs at too low a temperature in UBe13 to be observed in our measurements, but it can be observed in URu2Si2. In addition to the behaviour of the charge carriers URu2Si2 exhibits two strong phonon lines at 109 cm-1 and 378 cm-1 and below the magnetic transition temperature there is an additional absorbtion feature near 50 cm-1. © 1988.

We report on the optical properties of the heavy fermion superconductor UBe13 from 0.050 to 2eV. The reflectance shows sharp structure at low frequencies superimposed on a smooth decrease down to 50% at 1.6eV. The ThBe13 compound is shown for comparison as a non-heavy fermion system. Kramers-Kronig analysis of reflectance measurements yields sharp interband structure in UBe13 in the 0.1 eV region. In the far IR, above 100 K, the material shows normal Drude behavior: the optical conductivity agrees well with the d.c. conductivity. © 1987.