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Evidence for Charge Kondo Effect in Superconducting Tl-Doped

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Evidence for Charge Kondo Effect in Superconducting Tl-Doped SLAC-PUB-13857 Evidence for charge Kondo effect in superconducting Tl-doped PbTe Y. Matsushita,1 H. Bluhm,2 T. H. Geballe,3 and I. R. Fisher3 1Department of Materials Science and Engineering and Geballe Laboratory for Advanced Materials 2Department of Physics and Geballe Laboratory for Advanced Materials 3Department of Applied Physics and Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305-4045 (Dated: October 27, 2009) We report results of low-temperature thermodynamic and transport measurements of Pb1−xTlxTe single crystals for Tl concentrations up to the solubility limit of approximately x = 1.5%. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature TK ∼ 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high Tc values. PACS numbers: 74.70.Dd, 72.15.Qm The Kondo effect arises from the interaction of con- In this letter we describe measurements of PbTe doped duction electrons with degenerate degrees of freedom in with small amounts of Tl. The material exhibits a a material and is usually associated with dilute magnetic Kondo-like upturn in the resistivity in the absence of impurities in a nonmagnetic host. In such cases, the two magnetic impurities and superconducts at low temper- degenerate states correspond to the impurity spins ori- atures. We make the case that this behavior originates ented up or down. Second order scattering processes in- from a charge Kondo effect involving degenerate valence volving virtual intermediate states lead to the well-known states of the Tl impurities. This is the first evidence for logarithmic increase in resistivity at low temperatures, such an effect and is especially significant in substantiat- which saturates in the unitary scattering limit below a ing claims that the superconducting pairing mechanism characteristic Kondo temperature [1]. However, other in Tl-doped PbTe derives principally from negative-U ef- systems comprising two degenerate degrees of freedom fects. can also lead to Kondo-like phenomena [2]. In partic- PbTe is a small gap semiconductor. It has a rocksalt ular, a “charge Kondo effect,” corresponding to dilute structure and has been treated with reasonable success impurities with two degenerate charge states, has been − using ionic models (i.e., Pb2+Te2 ) [12]. The material proposed in the negative-U Anderson model [3], though can be doped to degeneracy by either vacancies or third- to date there has not been an experimental realization of element dopants, with typical carrier concentrations in such an effect. Significantly, the quantum valence fluc- − the range of 1018 − 1020 cm 3 [13, 14, 15]. In compari- tuations implicit in such a model, which involve pairs son with similar semiconducting materials such as SnTe, of electrons that tunnel on and off impurity sites, also GeTe, and InTe, it was previously anticipated that doped provide an electronic pairing mechanism for supercon- PbTe would only superconduct below approximately 0.01 ductivity [4, 5, 6, 7]. K, if at all [16]. This has been found to be the case for all dopants except thallium, for which superconductivity Thallium is one of several elements that is known to was observed with critical temperatures up to 1.5 K [17], skip valences, such that only Tl1+ and Tl3+ are observed two orders of magnitude higher than anticipated given in ionic compounds, corresponding to electron configu- the modest carrier concentrations. Given the anoma- rations 6s2 and 6s0 respectively. Compounds that one lously high T values for Tl-doped PbTe, there has been would otherwise expect to contain divalent Tl are found c considerable discussion as to the role of the Tl impu- to disproportionate. For example, TlBr is more specifi- 2 rities in this material, whether they act as negative-U cally TlITlIIIBr , and TlS is likewise TlITlIIIS [8]. This 4 2 centers, and specifically whether such impurities can en- effect is driven by the stability of a filled shell in con- hance an incipient tendency towards superconductivity junction with the polarizability of the material. In this [4, 5, 6, 18, 19]. case, Tl2+ can be characterized by a negative effective U, where Un = (En+1 − En) − (En − En−1) < 0 and Single crystals of Pb1−xTlxTe were grown by an un- n labels the valence state [9, 10, 11]. For this reason, seeded physical vapor transport method as described valence-skipping elements provide experimental access to elsewhere [20]. The thallium content was measured by negative-U behavior and are therefore suitable candidate Electron Microprobe Analysis (EMPA). Resistivity mea- impurities for realizing a charge Kondo effect in a bulk surements were made at 77 Hz and with current densities material. in the range of 25 mA/cm2 (corresponding to a current of Work supported in part by US Department of Energy contract DE-AC02-76SF00515. 2 (a) x = 1.3% 6 665 1.0 1.5 1.0 x = 1.1% cm) 4 cm) 0.5 ( x = 0.8% cm) 664 m in (m 0.5 2 o - 0 x = 0.4% (m 0.0 0 1.0 x = 0.3% 0.0 0.5 1.0 1.5 0.0 663 x (at.%) 0.0 0.5 1.0 1.5 T (K) (K) c T (b) 511 x = 0.8% 0.5 single crystals polycrystalline [21] thin films [22] 510 0.0 0.0 0.5 1.0 1.5 cm) x (at.%) ( (c) x = 0.4% FIG. 1: Variation of Tc with Tl content x for Pb1−xTlxTe. 0.0 Line shows linear fit. Inset shows representative resistivity -0.1 cm) data for single crystals. 302 ( 0 -0.2 - 2 -0.3 0 5 10 15 10 µA for low-temperature measurements) to 1 A/cm at 2 2 T (K ) higher temperatures. To check for heating effects, resis- 301 (d) tivity data were taken for different current densities and x = 0.3% for warming and cooling cycles for each sample. Several 0.0 samples were measured for each Tl concentration. 209 cm) Tc values estimated from the midpoints of supercon- -0.2 ( 0 ducting transitions in the resistivity are shown as a func- - tion of Tl concentration in Fig. 1 and agree with pub- -0.4 0 5 10 15 2 2 lished data for polycrystalline samples [21] and for thin T (K ) 208 films [22] for high Tl concentrations. By measuring Tc 0.1 1 10 for the lowest Tl concentrations, we find that there is a T (K) critical concentration of ∼ 0.3% below which the material does not superconduct above 20 mK. The inset to Fig. 1 FIG. 2: Low temperature resistivity of Pb1−xTlxTe for x = shows the sharp resistive transitions for representative 0.3, 0.4, 0.8, and 1.3%. Inset to panel (a) shows residual − samples. resistivity ρ0 (left axis, solid symbols) and ρ0 ρmin (right axis, open symbols) as a function of x. Insets to (c) and (d) The temperature dependence of the normal-state resis- show T 2 behavior at low temperatures for the smallest Tl tivity of the Tl-doped crystals is shown on an expanded concentrations. scale in Fig. 2 for temperatures below 10 K. The resis- tivity shows a distinct upturn for temperatures below approximately 9 K, following a form characteristic of the low approximately 4 K. In the following paragraphs we Kondo effect. Similar results were observed previously argue that this behavior originates from a charge Kondo by Andronik and co-workers for temperatures above 4.2 effect associated with degenerate valence states of the Tl K for a smaller subset of two Tl concentrations [23]. A impurities. First we demonstrate that the behavior is not crude estimate for the magnitude of this effect, neglect- due to localization or electron-electron effects. ing any additional temperature dependence below 10 K, Thallium impurities cause a rapid increase in residual can be made from the quantity ρ0 − ρmin, where ρ0 is the resistivity of PbTe, characterized by approximately 0.8 residual resistivity measured at our lowest temperatures mΩ cm per at.% Tl, as shown in the inset to Fig. 2(a). and ρmin is the value of the resistivity at the resistance Taking x = 0.4% as representative, and assuming that minimum. As shown in the inset to Fig. 2(a), this quan- the Tl impurities do not substantially alter the band tity scales approximately linearly with the Tl concentra- structure of PbTe [14, 15], the measured hole concentra- tion x. Insets to Fig. 2(c) and 2(d) show the resistivity tion of 7×1019 cm−3 implies that there are holes in both as a function of T 2 for the two lowest Tl contents, x = the light and heavy bands located at the L and Σ points 0.3 and 0.4%, for which Tc is less than 0.3 K. The re- in the Brillouin zone, and that the Fermi level lies ap- sistivity clearly follows a T 2 temperature dependence, as proximately 180 meV below the top of the valence band. expected for Kondo-like behavior, for temperatures be- This allows an estimate of the Fermi velocity, which has 3 5 temperature. The susceptibility is diamagnetic for all 252 (0 T) Tl concentrations due to the small density of states and (5 T) - 4.8 cm 4 becomes less diamagnetic with increasing hole concen- 251 cm) tration. The weak temperature dependence arises from ( 3 a temperature dependence of both the band gap and ef- fective mass of PbTe [24].
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