Interplay between antiferrodistortive, ferroelectric and superconducting instabilities in Sr_{1-x}Ca_{x}$TiO_{3-delta}

SrTiO$_{3}$ undergoes a cubic-to-tetragonal phase transition at 105K. This antiferrodistortive transition is believed to be in competition with incipient ferroelectricity. Substituting strontium by isovalent calcium induces a ferroelectric order. Introducing mobile electrons to the system by chemical non-isovalent doping, on the other hand, leads to the emergence of a dilute metal with a superconducting ground state. The link between superconductivity and the other two instabilities is an open question, which gathers momentum in the context of the growing popularity of the paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutron-scattering and dielectric permittivity and polarization measurements on Sr$_{1-x}$Ca$_{x}$TiO$_{3}$ ($0<x<0.009$) and a low-temperature electric conductivity in Sr$_{0.9978}$Ca$_{0.0022}$TiO$_{3-delta}$. Calcium substitution was found to enhance the transition temperature for both anti-ferrodistortive and ferroelectric transitions. Moreover, we find that Sr$_{0.9978}$Ca$_{0.0022}$TiO$_{3-delta}$ has a superconducting ground state. The critical temperature in this rare case of a superconductor with a ferroelectric parent, is slightly lower than in SrTiO$_{3-delta}$ of comparable carrier concentration. A three-dimensional phase diagram for Sr$_{1-x}$Ca$_{x}$TiO$_{3-delta}$ tracking the three transition temperatures as a function of x and $delta$ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbours.

Multiple nodeless superconducting gaps in optimally-doped SrTi$_{1-x}$Nb$_{x}$O$_{3}$

We present the first study of thermal conductivity in superconducting SrTi$_{1-x}$Nb$_{x}$O$_{3}$, sufficiently doped to be near its maximum critical temperature. The bulk critical temperature, determined by the jump in specific heat, occurs at a significantly lower temperature than the resistive T$_{c}$. Thermal conductivity, dominated by the electron contribution, deviates from its normal-state magnitude at bulk T$_{c}$, following a Bardeen-Rickayzen-Tewordt (BRT) behavior, expected for thermal transport by Bogoliubov excitations. Absence of a T-linear term at very low temperatures rules out the presence of nodal quasi-particles. On the other hand, the field dependence of thermal conductivity points to the existence of at least two distinct superconducting gaps. We conclude that optimally-doped strontium titanate is a multigap nodeless superconductor.

Critical doping for the onset of a two-band superconducting ground state in SrTiO$_{3-delta}$

In doped SrTiO$_{3}$ superconductivity persists down to an exceptionally low concentration of mobile electrons. This restricts the relevant energy window and possible pairing scenarios. We present a study of quantum oscillations and superconducting transition temperature, $T_{c}$ as the carrier density is tuned from $10^{17}$ to $10^{20}$ $cm^{-3}$ and identify two critical doping levels corresponding to the filling thresholds of the upper bands. At the first critical doping, which separates the single-band and the two-band superconducting regimes in oxygen-deficient samples, the steady increase of T$_{c}$ with carrier concentration suddenly stops. Near this doping level, the energy dispersion in the lowest band displays a downward deviation from parabolic behavior. The results impose new constraints for microscopic pairing scenarios.

Thermoelectric response of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$: evidence for strong correlation and low carrier density

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.

Transport anomalies across the quantum limit in semimetallic Bi$_{0.96}$Sb$_{0.04}$

We report on a study of electronic transport in semi-metallic Bi$_{0.96}$Sb$_{0.04}$. At zero field, the system is a very dilute Fermi liquid displaying a T$^{2}$ resistivity with an enhanced prefactor. Quantum oscillations in resistivity as well as in Hall, Nernst and Seebeck responses of the system are detectable and their period quantifies the shrinking of the Fermi surface with antimony doping. For a field along the trigonal axis, the quantum limit was found to occur at a field as low as 3T. An ultraquantum anomaly at twice this field was detected in both charge transport and Nernst response. Its origin appears to lie beyond the one-particle picture and linked to unidentified many-body effects.