Superconducting State Of Sr2RuO4 Research Articles

Unveiling unconventional magnetism at the surface of Sr2RuO4

Materials with strongly correlated electrons often exhibit interesting physical properties. An example of these materials is the layered oxide perovskite Sr2RuO4, which has been intensively investigated due to its unusual properties. Whilst the debate on the symmetry of the superconducting state in Sr2RuO4 is still ongoing, a deeper understanding of the Sr2RuO4 normal state appears crucial as this is the background in which electron pairing occurs. Here, by using low-energy muon spin spectroscopy we discover the existence of surface magnetism in Sr2RuO4 in its normal state. We detect static weak dipolar fields yet manifesting at an onset temperature higher than 50 K. We ascribe this unconventional magnetism to orbital loop currents forming at the reconstructed Sr2RuO4 surface. Our observations set a reference for the discovery of the same magnetic phase in other materials and unveil an electronic ordering mechanism that can influence electron pairing with broken time reversal symmetry.

A proposal for reconciling diverse experiments on the superconducting state in Sr2RuO4

A variety of precise experiments have been carried out to establish the character of the superconducting state in Sr2RuO4. Many of these appear to imply contradictory conclusions concerning the symmetries of this state. Here we propose that these results can be reconciled if we assume that there is a near-degeneracy between a {d}_{{x}^{2}-{y}^{2}} (B1g in group theory nomenclature) and a {g}_{xy({x}^{2}-{y}^{2})} (A2g) superconducting state. From a weak-coupling perspective, such an accidental degeneracy can occur at a point at which a balance between the on-site and nearest-neighbor repulsions triggers a d-wave to g-wave transition.

Quasiparticle interference testing the possible pairing symmetry in Sr2RuO4**Project supported by the National Natural Science Foundataion of China (Grant Nos. 11604168, 11604166, and 11604303). WSW also acknowledges the supports by K. C. Wong Magna Fund in Ningbo University.

The quasiparticle interference (QPI) patterns of the superconducting state in Sr2RuO4 are theoretically studied by taking into account the spin–orbital coupling and two different pairing modes, chiral p-wave pairing and equal d-wave pairing, in order to propose an experimental method to test them. Both of the QPI spectra for the two pairing modes have clearly peaks evolving with energy, and their locations can be determined from the tips of the constant energy contour. But the number, location, and evolution of these peaks with energy are different between the two pairing modes. The different behaviors of the QPI patterns in these two pairing modes may help to resolve whether Sr2RuO4 is a chiral p-wave or d-wave superconductor.

Evolution of the filamentary 3-Kelvin phase inPb−Ru−Sr2RuO4Josephson junctions

The evolution of the filamentary 3-Kelvin (3K) superconducting phase at the interface between Sr2RuO4 and Ru-metal inclusions is discussed for Pb-Ru-Sr2RuO4 contacts. Using the Ginzburg-Landau model, the influence of proximity-induced superconductivity in Ru on the topology of the 3K phase is analyzed. Because the s-wave order parameter in Ru favors a 3K state of trivial topology, the onset temperature of the phase with a non-trivial topology, which is compatible with the bulk phase of Sr2RuO4, is essentially reduced to the bulk transition temperature. Because the topology of the superconducting state in Sr2RuO4 is crucial for the Josephson effect through Pb-Ru-Sr2RuO4 contacts, this model qualitatively reproduces the experimental observation of the anomalous temperature dependence on the critical current.

Spin triplet superconducting phases in Sr2RuO4 in a magnetic field applied along the c-axis

We investigate the spin-triplet superconducting state of Sr2RuO4 in a magnetic field along the c-axis on the basis of the four-component Ginzburg-Landau (GL) model with a weak spin-orbit coupling. We show that the non-unitary superconducting phase is stabilized near the upper critical field owing to a coupling between the chirality and the magnetic field. The vortex lattice structures of this superconducting phase are calculated at temperatures near the superconducting transition temperature Tc. Furthermore, we show a schematic of the global phase diagram in terms of the temperature and the magnetic field.

Spin–orbit coupling and finite quasi-particle lifetime effects in normal metal/triplet superconductor junctions

The Bogoliubov–de Gennes equation and extended Blonder–Tinkham–Klapwijk model are applied to studying tunneling conductance in normal metal/triplet superconductor (N/TS) junctions. Three kinds of pairing for TS are chosen: px, py, and px+ipy wave. It is found that the tunneling conductance strongly depends on kinds of pairings, then is affected by the spin–orbit coupling (SOC) of the Rashba type and the finite quasi-particle lifetime. The obtained results may help to explore the properties of superconducting state in Sr2RuO4.

High Resolution Polar Kerr Effect Measurements ofSr2RuO4: Evidence for Broken Time-Reversal Symmetry in the Superconducting State

The polar Kerr effect in the spin-triplet superconductor Sr2RuO4 was measured with high precision using a Sagnac interferometer with a zero-area Sagnac loop. We observed nonzero Kerr rotations as big as 65 nanorad appearing below Tc in large domains. Our results imply a broken time-reversal symmetry state in the superconducting state of Sr2RuO4, similar to 3He-A.

Search for the chiral superconducting state in Sr2RuO4–Ru eutectic

Sr2RuO4 has become one of the most intensively studied transition metal oxides displaying unconventional superconductivity (Tc=1.5 K). It is known that its eutectic system Sr2RuO4–Ru shows interface superconductivity with an enhanced onset of ∼3 K. It is predicted that in the 3-K phase, a second transition to a chiral state occurs at a temperature between 1.5 and 3 K. The second transition is thought to be observable by ac-susceptibility measurements. We report the low-frequency ac-susceptibility of Sr2RuO4–Ru eutectic measured using a SQUID ac magnetometer.

Ru NMR probe of spin susceptibility in the superconducting state ofSr2RuO4

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17O Knight shift study in the superconducting state of Sr2RuO4

17O Knight shift measurements in Sr2RuO4 were performed over the wide range of magnetic field 3.2-11.4kOe parallel to the basal RuO2 planes. The spin susceptibility is totally unchanged through its Tc, evidencing that the spin-triplet superconducting state is realized in Sr2RuO4. The result indicates that the Cooper pairs consist of the parallel spin pairs | ↑↑> and | ↓↓> with their quantization axis perpendicular to the c-axis direction. The in-plane 2D nearly ferromagnetic spin fluctuations may play a role for the stabilization of this state among various representations of spin-triplet order parameter.