9 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
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
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Nuclear Magnetic Resonance Investigations of the Highly Correlated Unconventional Superconductor Strontium Ruthenate
Sr2RuO4 is the cleanest and most well-characterized example of unconventional superconductivity known to date. Early experimental reports found strong evidence for the “chiral p-wave” d = z(kx � iky) superconducting state, an electronic analog to the chiral A-phase of superfluid Helium-3. As a result, Sr2RuO4 was widely accepted as the paradigmatic example of a topological quasi-two-dimensional superconductor and this colored the analysis of experimental reports for over two decades. The NMR measurements presented in this thesis directly contradict this interpretation. A pronounced drop is observed in the 17O Knight shift, incompatible with the chiral p-wave state as well as a previous body of NMR work. The discrepancy is shown to arise from systematic heating of the sample due to the high amplitude NMR pulses. Through quantitative measurements of the residual Knight shift as a function of applied in-plane field we additionally derive an upper bound on the magnetic response of the superconducting condensate of less than 10% that of the normal state. This is sufficient to further rule out all pure p-wave order parameter candidates for Sr2RuO4 and provides strong evidence for even parity superconductivity. As such, these results represent a fundamental advancement in understanding the nature of superconductivity in this archetypal system.The normal metallic state of Sr2RuO4 is also a subject of interest due to its strong correlations as well as the proximity of the Fermi energy to a quasi-two-dimensional singularity in the density of states. Application of uniaxial stress is known to be able to tune the band structure through the singularity and is accompanied by profound changes to the physical properties, including a more than doubling of the superconducting critical temperature. We show by way of the 17O Knight shift that the Fermi liquid crossover scale in Sr2RuO4 can be driven to vanishing temperature with the application of in-plane uniaxial stress approaching the van Hove singularity. The behavior is then successfully described via the strain dependent dispersion of a non-interacting quasiparticle model. Finally, a recently reported magnetic phase appearing at applied stresses beyond the van Hove singularity is investigated with 17T1 measurements. Enhanced fluctuations are found in a small region of phase space near the purported line of transitions but do not extend to superconducting dome, making it unlikely they are important for the superconducting pairing mechanism
Evidence for even parity unconventional superconductivity in Sr<sub>2</sub>RuO<sub>4</sub>
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.
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Evidence for even parity unconventional superconductivity in Sr2RuO4
Unambiguous identification of the superconducting order parameter symmetry in [Formula: see text] 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
Tuning the Fermi Liquid Crossover in SrRuO with Uniaxial Stress
We perform nuclear magnetic resonance (NMR) measurements of the oxygen-17 Knight shifts for SrRuO, while subjected to uniaxial stress applied along [100] direction. The resulting strain is associated with a strong variation of the temperature and magnetic field dependence of the inferred magnetic response. A quasi-particle description based on density-functional theory calculations, supplemented by many-body renormalizations, is found to reproduce our experimental results, and highlights the key role of a van-Hove singularity. The Fermi liquid coherence scale is shown to be tunable by strain, and driven to low values as the associated Lifshitz transition is approached