Sr & RuO ' is an unconventional superconductor that has attracted widespread study because of its high purity and the possibility that its superconducting order parameter has odd parity. We study the dependence of its superconductivity on anisotropic strain. Applying uniaxial pressures of up to ~1 GPa along a 〈100〉 direction ( -axis) of the crystal lattice results in . increasing from 1.5 K in the unstrained material to 3.4 K at compression by ≈0.6%, and then falling steeply. Calculations give evidence that the observed maximum . occurs at or near a Lifshitz transition when the Fermi level passes through a Van Hove singularity, and open the possibility that the highly strained, . =3.4 K Sr & RuO ' has an even-rather than an odd-parity order parameter.The formation of superconductivity by the condensation of electron pairs into a coherent

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation (BEC) and superfluidity, which have been tested experimentally in a variety of different systems. When bosons are interacting, disorder can destroy condensation leading to a so-called Bose glass. This phase has been very elusive to experiments due to the absence of any broken symmetry and of a finite energy gap in the spectrum.Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (Br-doped dichloro-tetrakis-thiourea-Nickel, DTN).The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand-canonical ensemble; Br-doping introduces disorder in the hoppings and interaction strengths, leading to localization of the bosons into a Bose glass down to zero field, where it acquires the nature of an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the BEC (corresponding to a magnetically ordered phase) is marked by a novel, universal exponent governing the scaling on the critical temperature with the applied field, in excellent agreement arXiv:1109.4403v2 [cond-mat.str-el] 21 Sep 2011 2 with theoretical predictions. Our study represents the first, quantitative account of the universal features of disordered bosons in the grand-canonical ensemble.PACS numbers: 03.75. Lm, 71.23.Ft, 68.65.Cd, 72.15.Rn Introduction. Disorder can have a very strong impact on quantum fluids. Due to their wave-like nature, quantum particles are subject to destructive interference when scattering against disordered potentials. This leads to their quantum localization (or Anderson localization), which prevents e.g.electrons from conducting electrical currents in strongly disordered metals [1], and non-interacting bosons from condensing into a zero-momentum state [2]. Yet interacting bosons represent a matter wave with arbitrarily strong non-linearity, whose localization properties in a random environment cannot be deduced from the standard theory of Anderson localization. For strongly interacting bosons it is known that Anderson localization manifests itself in the Bose glass: in this phase the collective modes of the system -and not the individual particles -are Anderson-localized over arbitrarily large regions, leading to a gapless energy spectrum, and a finite compressibility of the fluid [3, 4]. Moreover nonlinear bosonic matter waves can undergo a localization-delocalization quantum phase transition in any spatial dimension when the interaction strength is varied [3, 4]; the transition brings the system from a non-interacting Anderson insulator to an interacting superfluid condensate, or from a superfluid to a Bose glass. Such a transition is relevant for a large variety of physical systems, including superfluid helium in porous media [6], Cooper pairs in disordered superconductors [7], and cold atoms in random optical potenti...

Unconventional superconductivity and other previously unknown phases of matter exist in the vicinity of a quantum critical point (QCP): a continuous phase change of matter at absolute zero. Intensive theoretical and experimental investigations on itinerant systems have shown that metallic ferromagnets tend to develop via either a first-order phase transition or through the formation of intermediate superconducting or inhomogeneous magnetic phases. Here, through precision low-temperature measurements, we show that the Grüneisen ratio of the heavy fermion metallic ferromagnet YbNi(4)(P(0.92)As(0.08))(2) diverges upon cooling to T = 0, indicating a ferromagnetic QCP. Our observation that this kind of instability, which is forbidden in d-electron metals, occurs in a heavy fermion system will have a large impact on the studies of quantum critical materials.