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.

We present a new Kondo-lattice system, YbNi 4 P 2 , which is a clean heavy-fermion metal with a severely reduced ferromagnetic ordering temperature at T C = 0.17 K, evidenced by distinct anomalies in susceptibility, specific-heat, and resistivity measurements. The ferromagnetic nature of the transition, with only a small ordered moment of ∼ 0.05 µ B , is established by a diverging susceptibility at T C with huge absolute values in the ferromagnetically ordered state, severely reduced by small magnetic fields. Furthermore, YbNi 4 P 2 is a stoichiometric system with a quasi-one-dimensional crystal and electronic structure and strong correlation effects which dominate the low temperature properties. This is reflected by a stronger-thanlogarithmically diverging Sommerfeld coefficient and a linear-in-T resistivity above T C which cannot be explained by any current theoretical predictions. These exciting characteristics are unique among all correlated electron systems and makes this an interesting material for further in-depth investigations. arXiv:1108.4274v1 [cond-mat.str-el]

One-sentence summary: We demonstrate that heavy-electron superconductivity develops in YbRh 2 Si 2 due to the weakening of its antiferromagnetism by the ordering of nuclear spins, providing evidence that quantum criticality is a robust mechanism for unconventional superconductivity.We report magnetic and calorimetric measurements down to T = 1 mK on the canonical heavy-electron metal YbRh 2 Si 2 . The data reveal the development of nuclear antiferromagnetic order slightly above 2 mK. The latter weakens the primary electronic antiferromagnetism, thereby paving the way for heavy-electron superconductivity below T c = 2 mK. Our results demonstrate that superconductivity driven by quantum criticality is a general phenomenon.Unconventional (i.e., non-phonon mediated) superconductivity, which has been attracting much interest since the early 1980s, is often observed at the border of antiferromagnetic (AF) order [1].As exemplified by heavy-electron (or heavy-fermion) metals, the suppression of the AF order opens up a wide parameter regime where the physics is controlled by an underlying quantum 1 arXiv:1707.03006v1 [cond-mat.str-el] 10 Jul 2017 critical point (QCP) [2,3]. A central question, then, concerns the interplay between quantum criticality and unconventional superconductivity in strongly correlated electron systems such as heavy-electron metals. In many of the latter superconductivity turns out to develop near such a QCP [2][3][4]. However, the absence of superconductivity in the prototypical quantum critical material YbRh 2 Si 2 (Ref. [5]) has raised the question as to whether the presence of an AF QCP necessarily gives rise to the occurrence of superconductivity. Because YbRh 2 Si 2 exists in the form of high-quality single crystals, it is meaningful to address this issue at very low temperatures without seriously encountering the limitations posed by disorder. We have therefore used this heavy-electron compound to carry out the first study on quantum critical metals at ultra-low temperatures.YbRh 2 Si 2 exhibits AF order below a Néel temperature T AF = 70 mK. A small magnetic field of B = 60 mT, when applied within the basal plane of the tetragonal structure, continuously suppresses the magnetic order and induces a QCP, presumably of unconventional nature [6,7].Electrical resistivity measurements down to 10 mK have failed to show any indications for superconductivity [5]. Recognizing that a critical field of 60 mT is unlikely to sustain even heavyelectron superconductivity with a T c of less than 10 mK, a different means of suppressing the antiferromagnetism is needed to eventually reveal any potential superconductivity at its border.We take advantage of the early recognition that hyperfine coupling to nuclear spins can considerably influence the electronic spin properties near a quantum phase transition [8]. Furthermore, measurements on PrCu 2 and related compounds have demonstrated a large coupling between the electronic and nuclear spins in rare-earth-based intermetallics at temperatures as high as 50 mK...

We present a design for a piezoelectric-driven uniaxial stress cell suitable for use at ambient and cryogenic temperatures, and that incorporates both a displacement and a force sensor. The cell has a diameter of 46 mm and a height of 13 mm. It can apply a zero-load displacement of up to ∼45 µm, and a zero-displacement force of up to ∼245 N. With combined knowledge of the displacement and force applied to the sample, it can quickly be determined whether the sample and its mounts remain within their elastic limits. In tests on the oxide metal Sr 2 RuO 4 , we found that at room temperature serious plastic deformation of the sample onset at a uniaxial stress of ∼0.2 GPa, while at 5 K the sample deformation remained elastic up to almost 2 GPa. This result highlights the usefulness of in situ tuning, in which the force can be applied after cooling samples to cryogenic temperatures.

YbRh2Si2 is a prototypical system for studying unconventional antiferromagnetic quantum criticality. However, ferromagnetic correlations are present which can be enhanced via isoelectronic cobalt substitution for rhodium in Yb(Rh(1-x)Co(x))2Si2. So far, the magnetic order with increasing x was believed to remain antiferromagnetic. Here, we present the discovery of ferromagnetism for x = 0.27 below T(C) = 1.30  K in single crystalline samples. Unexpectedly, ordering occurs along the c axis, the hard crystalline electric field direction, where the g factor is an order of magnitude smaller than in the basal plane. Although the spontaneous magnetization is only 0.1 μB/Yb it corresponds to the full expected saturation moment along c taking into account partial Kondo screening.

Among the frustrated magnetic materials, spin-ice stands out as a particularly interesting system. Residual entropy, freezing and glassiness, Kasteleyn transitions and fractionalization of excitations in three dimensions all stem from a simple classical Hamiltonian. But is the usual spin-ice Hamiltonian a correct description of the experimental systems? Here we address this issue by measuring magnetic susceptibility in the two most studied spin-ice compounds, Dy2Ti2O7 and Ho2Ti2O7, using a vector magnet. Using these results, and guided by a theoretical analysis of possible distortions to the pyrochlore lattice, we construct an effective Hamiltonian and explore it using Monte Carlo simulations. We show how this Hamiltonian reproduces the experimental results, including the formation of a phase of intermediate polarization, and gives important information about the possible ground state of real spin-ice systems. Our work suggests an unusual situation in which distortions might contribute to the preservation rather than relief of the effects of frustration.