The nature of superconductivity in the dilute semiconductor SrTiO has remained an open question for more than 50 y. The extremely low carrier densities ([Formula: see text]-[Formula: see text] cm) at which superconductivity occurs suggest an unconventional origin of superconductivity outside of the adiabatic limit on which the Bardeen-Cooper-Schrieffer (BCS) and Migdal-Eliashberg (ME) theories are based. We take advantage of a newly developed method for engineering band alignments at oxide interfaces and access the electronic structure of Nb-doped SrTiO, using high-resolution tunneling spectroscopy. We observe strong coupling to the highest-energy longitudinal optic (LO) phonon branch and estimate the doping evolution of the dimensionless electron-phonon interaction strength ([Formula: see text]). Upon cooling below the superconducting transition temperature ([Formula: see text]), we observe a single superconducting gap corresponding to the weak-coupling limit of BCS theory, indicating an order of magnitude smaller coupling ([Formula: see text]). These results suggest that despite the strong normal state interaction with electrons, the highest LO phonon does not provide a dominant contribution to pairing. They further demonstrate that SrTiO is an ideal system to probe superconductivity over a wide range of carrier density, adiabatic parameter, and electron-phonon coupling strength.

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Resonant inelastic x-ray scattering (RIXS) has become an important tool for studying elementary excitations in correlated materials. Here, we present a systematic theoretical investigation of the Cu L-edge RIXS spectra of undoped and doped cuprate two-leg spin-ladders in both the non-spinconserving (NSC) and spin-conserving (SC) channels. The spectra are rich and host many exotic excitations. In the NSC-channel of the undoped case, we identify one-triplon and bound triplet two-triplon excitations in the strong-rung coupling limit, as well as confined spinons in the weakrung coupling limit. In the doped case, we observe a quasiparticle excitation formed from a bound charge and spin-1 2 in the strong-rung coupling limit. In the SC-channel, we also identify several new features, including bound singlet two-triplon excitations and confined spinons in the undoped ladders in the strong-and weak-rung coupling limits, respectively. Conversely, in the doped case, the SC channel primarily probes both gapless and gapped charge excitations. Finally, we revisit the available data for the ladder compound Sr14Cu24O41 in the context of our results. arXiv:1904.00530v1 [cond-mat.str-el] 1 Apr 2019