Magnetoresistance oscillation study of the half-quantum vortex in doubly connected mesoscopic superconducting cylinders of Sr2RuO4

Abstract

The observation of the highly unusual half-quantum vortex (HQV) in a single crystalline superconductor excludes unequivocally the spin-singlet symmetry of the superconducting order parameter. HQVs were observed previously in mesoscopic samples of Sr2RuO4 in cantilever torque magnetometry measurements, thus providing direct evidence for spin-triplet pairing in the material. In addition, it raised important questions on HQV, including its stability and dynamics. These issues have remained largely unexplored, in particular, experimentally. We report in this paper the detection of HQVs in mesoscopic, doubly connected cylinders of single-crystalline Sr2RuO4 of a mesoscopic size and the examination of the effect of the in-plane magnetic field needed for the observation of the HQV by magnetoresistance (MR) oscillations measurements. Several distinct features found in our data, especially a dip and secondary peaks in the MR oscillations seen only in the presence of a sufficiently large in-plane magnetic field as well as a large measurement current, are linked to the formation of the HQV fluxoid state in the sample and crossing of an Abrikosov HQV through the sample. The conclusion is drawn from the analysis of our data using a model of thermally activated vortex crossing overcoming a free-energy barrier which is modulated by the applied magnetic flux enclosed in the cylinder as well as the measurement current. Evidence for the trapping of an HQV fluxoid state in the sample was also found. Our observation of the HQV in mesoscopic Sr2RuO4 provided not only additional evidence for spin-triplet superconductivity in Sr2RuO4 but also insights into the physics of HQV, including its spontaneous spin polarization, stability, and dynamics. Our study also revealed a possible effect of the measurement current on the magnitude of the spontaneous spin polarization associated with the HQV.

A microscopic Ginzburg--Landau theory and singlet ordering in Sr$_2$RuO$_4$

“…A peak in the accumulated evidence was reached when the NMR Knight shift experiment was repeated 14,15 , now finding a substantial reduction in the spin susceptibility at low temperature. This has launched a renewed focus on the compound, both experimentally [16][17][18][19][20][21] and theoretically [22][23][24][25][26][27][28][29][30][31][32] .…”

Little-Parks oscillation and ${\bf d}$-vector texture in spin-triplet superconducting rings with bias current

“…3 (b)] should be observed, being distinguished from the domain-induced unconventional LP pattern which is half-quantum shifted from the beginning [8,27]. Although such a field-induced half-quantum shift has not been reported so far even in the LP experiments conducted for the spin-triplet candidate Sr 2 RuO 4 [29][30][31][32] (its pairing symmetry is still controversial [28]), we believe that our result presented here will promote the exploration and further understanding of not only spin-triplet but also various classes of superconductors.…”