We report the influence on the superconducting critical temperature Tc in doped SrTiO3 of the substitution of the natural 16O atoms by the heavier isotope 18O. We observe that for a wide range of doping this substitution causes a strong (~50%) enhancement of Tc. Also the magnetic critical field Hc2 is increased by a factor ~2. Such a strong impact on Tc and Hc2, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO3.

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Structural phase transitions described by Mexican hat potentials should in principle exhibit aspects of Higgs and Goldstone physics. Here, we investigate the relationship between the phonons that soften at such structural phase transitions and the Higgs-and Goldstone-boson analogs associated with the crystallographic Mexican hat potential. We show that, with the exception of systems containing only one atom type, the usual Higgs and Goldstone modes are represented by a combination of several phonon modes, with the lowestenergy phonons of the relevant symmetry having substantial contribution. Taking the hexagonal manganites as a model system, we identify these modes using Landau theory, and predict the temperature dependence of their frequencies using parameters obtained from density functional theory. Separately, we calculate the additional temperature dependence of all phonon mode frequencies arising from thermal expansion within the quasiharmonic approximation. We predict that Higgs-mode softening will dominate the low-frequency vibrational spectrum of InMnO 3 between zero Kelvin and room temperature, whereas the behavior of ErMnO 3 will be dominated by lattice expansion effects. We present temperature-dependent Raman scattering data that support our predictions, in particular confirming the existence of the Higgs mode in InMnO 3 .

Using high resolution X-Ray diffraction (XRD) on high purity powders, we resolved the structure and ab symmetry of the intriguing compound Sr2VO4 from room temperature down to 20 K to an unprecedented level of accuracy. Upon cooling, this new set of data unambiguously reveals a second order phase transition lowering the symmetry from tetragonal to orthorhombic at a temperature Tc2 = 136 K. The observation of an orthorhombic distortion of the ab-plane is attributed to nematic phase formation supported by local Jahn-Teller (JT) dynamical instability. At TN = 105 K, spins order and at Tc1 = 100 K the tetragonal structure is recovered with an elongated c-axis.

Many complex electronic systems exhibit so-called pseudogaps, which are poorly-understood suppression of low-energy spectral intensity in the absence of an obvious gap-inducing symmetry. Here we investigate the superconductor Ba 1−x K x BiO 3 near optimal doping, where unconventional transport behavior and evidence of pseudogap(s) have been observed above the superconducting transition temperature T c , and near an insulating phase with longrange lattice distortions. Angle-resolved photoemission spectroscopy (ARPES) reveals a dispersive band with vanishing quasiparticle weight and "tails" of deep-energy intensity that strongly decay approaching the Fermi level. Upon cooling below a transition temperature T p > T c , which correlates with a change in the slope of the resistivity vs. temperature, a partial transfer of spectral weight near E F into the deep-binding energy tails is found to result from metal-insulator phase separation. Combined with simulations and Raman scattering, our results signal that insulating islands of ordered bipolarons precipitate out of a disordered polaronic liquid and provide evidence that this process is regulated by a crossover in the electronic mean free path.Pseudogaps represent a departure from the expectations of standard band theory and the Fermi liquid theory of electronic excitations, which together serve as a successful starting point for understanding many condensed matter systems. They could potentially originate from any ways in which the conventional theories might break down, e.g., due to disorder, fluctuations, strong interactions, and/or strong correlations. But it is also conceivable that some observed pseudogaps might be less mysterious than they first seem, in the sense that they are rooted a "hidden" order