21 research outputs found
Evidence for even parity unconventional superconductivity in Sr2RuO4
Funding: A.C. is grateful for support from the Julian Schwinger Foundation for Physics Research. A.P. acknowledges support by the Alexander von Humboldt Foundation through the Feodor Lynen Fellowship. Work at Los Alamos was funded by Laboratory Directed Research and Development (LDRD) program, and A.P. acknowledges partial support through the LDRD. N.K. acknowledges the support by the Grants-in-Aid for Scientific Research (KAKENHI, Grant JP18K04715 and JP21H01033) from Japan Society for the Promotion of Science (JSPS). The work at Dresden was funded by the Deutsche Forschungsgemeinschaft - TRR 288 - 422213477 (projects A10 and B01). The work at University of California, Los Angeles, was supported by NSF Grants 1709304 and 2004553.Unambiguous identification of the superconducting order parameter symmetry in Sr2RuO4 has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid 3He-A was ruled out only very recently, other proposed triplet-pairing scenarios are still viable. Establishing the condensate magnetic susceptibility reveals a sharp distinction between even-parity (singlet) and odd-parity (triplet) pairing since the superconducting condensate is magnetically polarizable only in the latter case. Here field-dependent 17O Knight shift measurements, being sensitive to the spin polarization, are compared to previously reported specific heat measurements for the purpose of distinguishing the condensate contribution from that due to quasiparticles. We conclude that the shift results can be accounted for entirely by the expected field-induced quasiparticle response. An upper bound for the condensate magnetic response of < 10% of the normal state susceptibility is sufficient to exclude all purely odd-parity candidates. PostprintPeer reviewe
Strong peak in Tc of Sr2RuO4Â under uniaxial pressure
Sr2RuO4 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 gigapascals along a 〈100〉 direction (a axis) of the crystal lattice results in the transition temperature (Tc) increasing from 1.5 kelvin in the unstrained material to 3.4 kelvin at compression by ≈0.6%, and then falling steeply. Calculations give evidence that the observed maximum Tc 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, Tc = 3.4 K Sr2RuO4 has an even-parity, rather than an odd-parity, order parameter.PostprintPeer reviewe
Upper Critical Field of SrRuO under In-Plane Uniaxial Pressure
In-plane uniaxial pressure has been shown to strongly tune the
superconducting state of SrRuO by approaching a Lifshitz transition and
associated Van Hove singularity (VHS) in the density of states. At the VHS,
and the in- and out-of-plane upper critical fields are all strongly
enhanced, and the latter has changed its curvature as a function of temperature
from convex to concave. However, due to strain inhomogeneity it has not been
possible so far to determine how the upper critical fields change with strain.
Here, we show the strain dependence of both upper critical fields, which was
achieved due to an improved sample preparation. We find that the in-plane upper
critical field is mostly linear in . On the other hand, the out-of-plane
upper critical field varies with a higher power in , and peaks strongly at
the VHS. The strong increase in magnitude and the change in form of
occur very close to the Van Hove strain, and points to a
strong enhancement of both the density of states and the gap magnitude at the
Lifshitz transition
Elastocaloric determination of the phase diagram of SrRuO
One of the main developments in unconventional superconductivity in the past two decades has been the discovery that most unconventional superconductors form phase diagrams that also contain other strongly correlated states. Many systems of interest are therefore close to more than one instability, and tuning between the resultant ordered phases is the subject of intense research1. In recent years, uniaxial pressure applied using piezoelectric-based devices has been shown to be a particularly versatile new method of tuning, leading to experiments that have advanced our understanding of the fascinating unconventional superconductor SrRuO. Here we map out its phase diagram using high-precision measurements of the elastocaloric effect in what we believe to be the first such study including both the normal and the superconducting states. We observe a strong entropy quench on entering the superconducting state, in excellent agreement with a model calculation for pairing at the Van Hove point, and obtain a quantitative estimate of the entropy change associated with entry to a magnetic state that is observed in proximity to the superconductivity. The phase diagram is intriguing both for its similarity to those seen in other families of unconventional superconductors and for extra features unique, so far, to SrRuO
Uniaxial Pressure Studies of the Unconventional Superconductor Srâ‚‚RuOâ‚„
This thesis concentrates on the effect of Lifshitz transitions and associated Van Hove singularities on the superconducting state of Srâ‚‚RuOâ‚„. I will start by giving a short summary of the unconventional superconducting state of Srâ‚‚RuOâ‚„, and discuss how a Lifshitz transition can be accessed using uniaxial pressure. I will then discuss recent results of measurements under uniaxial stress, which have changed our view of the nature of the superconductivity of this material considerably