A sensitive probe of unconventional order is its response to a symmetry-breaking field. To probe the proposed p(x) ± ip(y) topological superconducting state of Sr2RuO4, we have constructed an apparatus capable of applying both compressive and tensile strains of up to 0.23%. Strains applied along ⟨100⟩ crystallographic directions yield a strong, strain-symmetric increase in the superconducting transition temperature T(c). ⟨110⟩ strains give a much weaker, mostly antisymmetric response. As well as advancing the understanding of the superconductivity of Sr2RuO4, our technique has potential applicability to a wide range of problems in solid-state physics.

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

We report the design and construction of piezoelectric-based apparatus for applying continuously tuneable compressive and tensile strains to test samples. It can be used across a wide temperature range, including cryogenic temperatures. The achievable strain is large, so far up to 0.23% at cryogenic temperatures. The apparatus is compact and compatible with a wide variety of experimental probes. In addition, we present a method for mounting high-aspect-ratio samples in order to achieve high strain homogeneity. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License

Cuprates exhibit antiferromagnetic, charge density wave (CDW), and high-temperature superconducting ground states that can be tuned by means of doping and external magnetic fields. However, disorder generated by these tuning methods complicates the interpretation of such experiments. Here, we report a high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa2Cu3O6.67under uniaxial stress, and we show that a three-dimensional long-range-ordered CDW state can be induced through pressure along theaaxis, in the absence of magnetic fields. A pronounced softening of an optical phonon mode is associated with the CDW transition. The amplitude of the CDW is suppressed below the superconducting transition temperature, indicating competition with superconductivity. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials.

or most of its history, the superconductivity of strontium ruthenate (Sr 2 RuO 4) (ref. 1) has been understood in terms of an odd-parity two-component order parameter with equal-spin pairing in the RuO 2 planes: p x ± ip y (refs. 2-5). This order parameter is chiral: the Cooper pairs have angular momentum l = ±1. The evidence for chirality comes from the zero-field muon spin relaxation (ZF-μSR) data 6 , observation of a non-zero Kerr rotation below the critical temperature T c (ref. 7) and signs in the junction experiments of domains in the superconducting state 8,9 , while evidence for equal-spin pairing came from the absence of a change in the Knight shift below T c in nuclear magnetic resonance 10 and polarized neutron scattering 11 measurements. The Knight shift is related to the spin susceptibility; in conventional opposite-spin-pairing superconductors, it is suppressed below T c. However, in new measurements, it has been found that the Knight shift is, in fact, suppressed below T c (refs. 12-14), by a magnitude that is unlikely to be reconcilable with equal-spin pairing. This revision has called into question a number of other results on Sr 2 RuO 4. It raises a particular challenge for experiments that indicate chirality, because opposite-spin pairing implies an even-parity momentum-space gap structure. If the order parameter is constrained to be even parity, chiral, and composed of components that are degenerate on the tetragonal lattice of Sr 2 RuO 4 , the only possibility is d xz ± id yz order 15. Under conventional understanding, this is a highly unlikely order parameter because it

We report the results of a combined study of the normal-state resistivity and superconducting transition temperature T_{c} of the unconventional superconductor Sr_{2}RuO_{4} under uniaxial pressure. There is strong evidence that, as well as driving T_{c} through a maximum at ∼3.5  K, compressive strains ϵ of nearly 1% along the crystallographic [100] axis drive the γ Fermi surface sheet through a van Hove singularity, changing the temperature dependence of the resistivity from T^{2} above, and below the transition region to T^{1.5} within it. This occurs in extremely pure single-crystals in which the impurity contribution to the resistivity is <100  nΩ cm, so our study also highlights the potential of uniaxial pressure as a more general probe of this class of physics in clean systems.

Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems 1,2 , and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr 2 RuO 4 for example 3-5 , leading to a pronounced peak in T c vs. strain whose origin is still under active debate 4,6,7 . Here, we develop a simple and compact method to apply large uniaxial pressures passively in restricted sample environments, and utilize this to study the evolution of the electronic structure of Sr 2 RuO 4 using angle-resolved photoemission. We directly visualize how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in T c 7 . Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr 2 RuO 4 . More generally, our novel experimental approach opens the door to future studies of straintuned phase transitions not only using photoemission, but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.The layered perovskite Sr 2 RuO 4 has been extensively studied both because of its celebrated unconventional superconductivity 5,8-11 and the accuracy with which its normal state properties can be measured 12-15 and analysed [16][17][18][19] . In spite of a quarter of a century of work, there is still no consensus on the symmetry of its superconducting order parameter, or the mechanism by which the superconductivity condenses 5 . This is a major unsolved problem because its electronic structure, which is relatively simple compared to that of many other unconventional superconductors, is now known in considerable detail and its metallic state is firmly established to be a Fermi liquid below approximately 30 K 13 . A full understanding of the Sr 2 RuO 4 problem is therefore a benchmark for the progress of the fields of strongly interacting systems and unconventional superconductivity.

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.