T. Timusk

Bosons in high-temperature superconductors: an experimental survey

We review a number of experimental techniques that are beginning to reveal fine details of the bosonic spectrum α2F(Ω) that dominates the interaction between the quasiparticles in high-temperature superconductors. Angle-resolved photoemission spectroscopy (ARPES) shows kinks in electronic dispersion curves at characteristic energies that agree with similar structures in the optical conductivity and tunnelling spectra. Each technique has its advantages. ARPES is momentum resolved and offers independent measurements of the real and imaginary part of the contribution of the bosons to the self-energy of the quasiparticles. The optical conductivity can be used on a larger variety of materials and with the use of maximum entropy techniques reveals rich details of the spectra including their evolution with temperature and doping. Scanning tunnelling spectroscopy offers spatial resolution on the unit cell level. We find that together the various spectroscopies, including recent Raman results, are pointing to a unified picture of a broad spectrum of bosonic excitations at high temperatures which evolves, as the temperature is lowered, into a peak in the 30–60 meV region and a featureless high-frequency background in most of the materials studied. This behaviour is consistent with the spectrum of spin fluctuations as measured by magnetic neutron scattering. However, there is evidence for a phonon contribution to the bosonic spectrum as well.

Surface impedance studies of YBCO

This paper summarizes recent measurements of the penetration depth and surface resistance of YBCO, with particular emphasis on the asymptotic behaviour at low temperatures and its dependence on hole doping and cation substitution. A complete set of measurements of the penetration depth tensor indicates that λ(T) has a predominantly linear temperature dependence in both the a and b directions at low temperatures, but the c-axis penetration depth is much flatter, without a clear linear term. Ni and Ca substitution are found to have little effect on the linear behaviour of λ(T), but Zn impurities alter the behaviour; from T linear to T2 and then to a non-monotonic temperature dependence at a Zn concentration of 1%. The surface resistance in the ab-plane also seems to vary linearly with temperature at low temperatures, except when the sample is doped with Zn. Substitution of Zn for Cu leads to upturns in the surface resistance below 4 K that are seen at 3.8 GHz, but not at 35 GHz, suggesting impurity effects at a very low energy scale.

Far-infrared properties of ab -plane oriented YBa 2 Cu 3 O 7 − δ

Polycrystalline samples of YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ with a variety of surface treatments show differences in absolute reflectance and width of phonon lines. Samples that are not polished and are measured immediately after annealing have largely grains with the c axis normal to the surface. Such oriented samples show a gaplike depression of conductivity in the far infrared that sets in below the superconducting transition temperature but no true gap. Phonon lines at 195 cm/sup -1/ and at 155 cm/sup -1/ narrow in the superconducting state, in analogy with the effect of the electron-phonon interaction in Bardeen-Cooper-Schrieffer superconductors. In the normal state the background conductivity is Drudelike with a plasma frequency of 0.75 eV and a relaxation rate of 200 cm/sup -1/. The extrapolated far-infrared conductivity agrees with the measured dc conductivity.

Optical Spectroscopy of Superconducting Ba0.55K0.45Fe2As2: Evidence for Strong Coupling to Low-Energy Bosons

Normal state optical spectroscopy on single crystals of the new iron arsenide superconductor Ba0.55K0.45Fe2As2 shows that the infrared spectrum consists of two major components: a strong metallic Drude band and a well-separated midinfrared absorption centered at 0.7 eV. It is difficult to separate the two components unambiguously but several fits using Lorentzian peaks suggest a model with a Drude peak having a plasma frequency of 1.6 to 2.1 eV and a midinfrared peak with a plasma frequency of 2.5 eV. Detailed analysis of the frequency dependent scattering rate shows that the charge carriers interact with a broad bosonic spectrum extending beyond 100 meV with a very large coupling constant lambda=3.4 at low temperature. As the temperature increases this coupling weakens to lambda=0.78 at ambient temperature. This suggests a bosonic spectrum that is similar to what is seen in the lower Tc cuprates.

The midinfrared absorption in RTiO3 perovskites (R = La, Ce, Pr, Nd, Sm, Gd): The Hubbard gap?

Abstract We have measured the room temperature reflectance of the rare earth titanites (RTiO 3 , R=La, Ce, Pr, Nd, Sm and Gd) between 50 and 40000 cm −1 . A midinfrared absorption exists in all members of the series which systematically shifts to higher frequencies as the Ti-O-Ti bond angle decreases. The absorption may be related to the Hubbard gap. Slightly oxidized samples of LaTiO 3 exhibit a change from insulating to metallic resistivity near the Neel temperature. The reflectance of one such sample has been measured between 20 and 300 K and the Kramers-Kronig derived optical conductivity suggests that the onset of the midinfrared absorption tends toward zero frequency in the metallic phase.

Evaluation of LaSrGaO4 as a substrate for YBa2Cu3O7-δ

Abstract We present a comparison study between YBa2Cu3O7−δ thin films deposited on LaSrGaO4 substrates. The LaSrGaO4 crystals were grown, cut and polished in-house while the LaAlO3 substrates were prepared commercially. The deposited films were characterized by a variety of techniques including DC resistivity, critical current density, X-ray diffraction including rocking-curve analysis and scanning electron microscopy. Growth temperatures above 780°C on both substrates yielded highly oriented c-axis material with excellent transport properties. Because LaSrGaO4 has the advantage of being untwinned, a desirable property for microwave-device applications, we conclude that it has much promise as a potential substitute for LaAlO3.

Temperature evolution of the pseudogap state in the infrared response of underdoped La 2 − x Sr x CuO 4

The ab-plane optical spectra of two single crystals of underdoped

a -axis optical conductivity of detwinned ortho-II Y Ba 2 Cu 3 O 6.50

{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}

In situ growth of PbSrYCaCuO films by laser ablation

were investigated. The reflectivity of

Optical spectroscopy shows that the normal state of URu2Si2 is an anomalous Fermi liquid

{\mathrm{La}}_{1.87}{\mathrm{Sr}}_{0.13}{\mathrm{CuO}}_{4}