Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDirect spectroscopic evidence for mixed-valence Tl in the low carrier-density superconductor Pb$_{1-x}$Tl$_x$Te
66
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
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.
rate research
Read More
Density functional studies of 26K superconducting LaFeAs(O,F) are reported. We find a low carrier density, high density of states, $N(E_F)$ and modest phonon frequencies relative to $T_c$. The high $N(E_F)$ leads to proximity to itinerant magnetism, with competing ferromagnetic and antiferromagnetic fluctuations and the balance between these controlled by doping level. Thus LaFeAs(O,F) is in a unique class of high $T_c$ superconductors: high $N(E_F)$ ionic metals near magnetism.
We report measurements of the thermoelectric power (TEP) for a series of Pb(1-x)Tl(x)Te crystals with x = 0.0 to 1.3%. Although the TEP is very large for x = 0.0, using a single band analysis based on older work for dilute magnetic alloys we do find evidence for a Kondo contribution of 11 - 18 uV/K. This analysis suggests that Tk is ~ 50 - 70 K, a factor 10 higher than previously thought.
High-pressure synthesis techniques have allowed for the growth of Sn$_{1-x}$In$_x$Te samples beyond the ambient In-saturation limit of $x$ = 0.5 (T$_c sim$ 4.5 K). In this study, we present measurements of the temperature dependence of the London penetration depth $Deltalambda(T)$ in this superconducting doped topological insulator for $x$ = 0.7, where T$_{c,onset}approx 5$ K. The results indicate fully gapped BCS-like behavior, ruling out odd-parity $A_{2u}$ pairing; however, odd-parity $A_{1u}$ pairing is still possible. Critical field values measured below 1 K and other superconducting parameters are also presented.
We present a study of the Seebeck and Nernst coefficients of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ extended up to 28 T. The large magnitude of the Seebeck coefficient in the optimally doped sample tracks a remarkably low normalized Fermi temperature, which, like other correlated superconductors, is only one order of magnitude larger than T$_c$. We combine our data with other experimentally measured coefficients of the system to extract a set of self-consistent parameters, which identify Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ as a low-density correlated superconductor barely in the clean limit. The system is subject to strong superconducting fluctuations with a sizeable vortex Nernst signal in a wide temperature window.
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.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
