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Register a new userCorrelation of anomalous scattering and superconductivity in Pb$_{.99}$Tl$_{.01}$Te doped with additional indium donors
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Recent evidence for a charge-Kondo effect in superconducting samples of Pb$_{1-x}$Tl$_x$Te [1] has brought renewed attention to the possibility of negative U superconductivity in this material, associated with valence fluctuations on the Tl impurity sites [2]. Here, we use indium as an electron-donor to counterdope Pb$_{.99}$Tl$_{.01}$Te and study the effect of the changing chemical potential on the Kondo-like physics and on the superconducting critical temperature, $T_c$. We find that, as the chemical potential moves away from the value where superconductivity, Kondo-like physics, and chemical potential pinning are expected, both $T_c$ and the low-temperature resistance anomaly are suppressed. This provides further evidence that both the superconductivity and the Kondo-like behavior are induced by the same source, as anticipated in the negative U model.
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Results of resistivity, Hall effect, magnetoresistance, susceptibility and heat capacity measurements are presented for single crystals of indium-doped tin telluride with compositions Sn$_{.988-x}$In$_x$Te where $0 leq x leq 8.4 %$, along with microstructural analysis based on transmission electron microscopy. For small indium concentrations, $x leq 0.9 %$ the material does not superconduct above 0.3 K, and the transport properties are consistent with simple metallic behavior. For $x geq 2.7 %$ the material exhibits anomalous low temperature scattering and for $x geq 6.1 %$ bulk superconductivity is observed with critical temperatures close to 2 K. Intermediate indium concentrations $2.7% leq x leq 3.8%$ do not exhibit bulk superconductivity above 0.7 K. Susceptibility data indicate the absence of magnetic impurities, while magnetoresistance data are inconsistent with localization effects, leading to the conclusion that indium-doped SnTe is a candidate charge Kondo system, similar to thallium-doped PbTe.
Upon doping with Tl the narrow band-gap semiconductor PbTe exhibits anomalously high temperature superconductivity despite a very low carrier density as well as signatures of the Kondo effect despite an absence of magnetic moments. These phenomena have been explained by invoking 2$e$ fluctuations of the valence of the Tl dopants but a direct measurement of the mixed-valency implied by such a mechanism has not been reported to date. In this work we present the unambiguous observation of multiple valences of Tl in Tl-doped PbTe via photo emission spectroscopy measurements. It is shown via our quantitative analysis that the suppression of the carrier density at compositions exhibiting superconductivity and Kondo-like behaviour is fully accounted for by mixed valency, thus arguing strongly against a self-compensation scenario proposed elsewhere for this material and strengthening the case for valence fluctuation models. In addition to the identification of Tl$^+$ and Tl$^{3+}$ a possible third intermediate local charge-density is tentatively suggested by full fits to the data, the implications of which are discussed in the context of the charge-Kondo effect.
We present a neutron scattering study of phonons in single crystals of (Pb$_{0.5}$Sn$_{0.5}$)$_{1-x}$In$_x$Te with $x=0$ (metallic, but nonsuperconducting) and $x=0.2$ (nonmetallic normal state, but superconducting). We map the phonon dispersions (more completely for $x=0$) and find general consistency with theoretical calculations, except for the transverse and longitudinal optical (TO and LO) modes at the Brillouin zone center. At low temperature, both modes are strongly damped but sit at a finite energy ($sim4$ meV in both samples), shifting to higher energy at room temperature. These modes are soft due to a proximate structural instability driven by the sensitivity of Pb-Te and Sn-Te $p$-orbital hybridization to off-center displacements of the metal atoms. The impact of the soft optical modes on the low-energy acoustic modes is inferred from the low thermal conductivity, especially at low temperature. Given that the strongest electron-phonon coupling is predicted for the LO mode, which should be similar for both studied compositions, it is intriguing that only the In-doped crystal is superconducting. In addition, we observe elastic diffuse (Huang) scattering that is qualitatively explained by the difference in Pb-Te and Sn-Te bond lengths within the lattice of randomly distributed Pb and Sn sites. We also confirm the presence of anomalous diffuse low-energy atomic vibrations that we speculatively attribute to local fluctuations of individual Pb atoms between off-center sites.
We report an angle-resolved photoemission study of a series of hole and electron doped iron-based superconductors, their parent compound BaFe2As2, and their cousins BaCr2As2 and BaCo2As2. We focus on the energy (E) dependent scattering rate Gamma(E) as a function of the 3d count and on the renormalization function Z(E) of the inner hole pocket, which is the hot spot in these compounds. We obtain a non-Fermi-liquid-like linear in energy scattering rate Gamma(E>> kBT), independent of the dopant concentration. The main result is that the slope beta=Gamma(E >> kBT)/E, reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit Gamma(E)/E~1. This result together with the energy dependence of the renormalization function Z(E) signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state.
Angle-dependent magnetoresistance measurements are used to determine the isotropic and anisotropic components of the transport scattering rate in overdoped Tl$_2$Ba$_2$CuO$_{6+delta}$ for a range of $T_c$ values between 15K and 35K. The size of the anisotropic scattering term is found to scale linearly with $T_c$, establishing a link between the superconducting and normal state physics. Comparison with results from angle resolved photoemission spectroscopy indicates that the transport and quasiparticle lifetimes are distinct.
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