Observation of half-height magnetization steps in Sr2RuO4

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Abstract

Spin-triplet superfluids can support exotic objects, such as half-quantum vortices characterized by the nontrivial winding of the spin structure. We present cantilever magnetometry measurements performed on mesoscopic samples of Sr2RuO4, a spin-triplet superconductor. For micron-sized annular-shaped samples, we observe transitions between integer fluxoid states, as well as a regime characterized by "half-integer transitions," i.e., steps in the magnetization with half the height of the ones we observe between integer fluxoid states. These half-height steps are consistent with the existence of half-quantum vortices in superconducting Sr2RuO4.

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Paired states of fermions in two dimensions with breaking of parity and time-reversal symmetries, and the fractional quantum Hall effect

N Read, Dmitry Green (1999)

We analyze pairing of fermions in two dimensions for fully-gapped cases with broken parity (P) and time-reversal (T), especially cases in which the gap function is an orbital angular momentum (\(l\)) eigenstate, in particular \(l=-1\) (p-wave, spinless or spin-triplet) and \(l=-2\) (d-wave, spin-singlet). For \(l\neq0\), these fall into two phases, weak and strong pairing, which may be distinguished topologically. In the cases with conserved spin, we derive explicitly the Hall conductivity for spin as the corresponding topological invariant. For the spinless p-wave case, the weak-pairing phase has a pair wavefunction that is asympototically the same as that in the Moore-Read (Pfaffian) quantum Hall state, and we argue that its other properties (edge states, quasihole and toroidal ground states) are also the same, indicating that nonabelian statistics is a {\em generic} property of such a paired phase. The strong-pairing phase is an abelian state, and the transition between the two phases involves a bulk Majorana fermion, the mass of which changes sign at the transition. For the d-wave case, we argue that the Haldane-Rezayi state is not the generic behavior of a phase but describes the asymptotics at the critical point between weak and strong pairing, and has gapless fermion excitations in the bulk. In this case the weak-pairing phase is an abelian phase which has been considered previously. In the p-wave case with an unbroken U(1) symmetry, which can be applied to the double layer quantum Hall problem, the weak-pairing phase has the properties of the 331 state, and with nonzero tunneling there is a transition to the Moore-Read phase. The effects of disorder on noninteracting quasiparticles are considered.

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Mutual friction in superfluidHe3: Effects of bound states in the vortex core

N. Kopnin, M. Salomaa (1991)

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Stability of half-quantum vorticies in p_x+ip_y superconductors

Hendrik Bluhm, Eun-Ah Kim, Suk Chung (2007)

We consider the stability conditions for half-quantum vortices in quasi two dimensional \(p_x+ip_y\) superconductor (such as \({\rm Sr_2 RuO_4}\) is believed to be) through energetics calculations. Although the predicted exotic nature of these excitations have recently attracted much attention but they have not been observed yet. We pay particular attention to the fact that an isolated half quantum vortex has a divergent energy cost in the bulk due to the associated {\it unscreened} spin current, which requires two half-quantum vortices with opposite spin winding to pair. We show that the stability of such a pair in bulk is enhanced when the ratio of spin superfluid density to superfluid density \(\rho_{{\rm sp}}/\rho_{\rm s}\) is small. As a step towards stabilizing and observing these exotic excitations, we propose using various mesoscopic geometries. We find that certain mesoscopic geometries can selectively allow only single half-quantum vortices to enter the sample for small \(\rho_{{\rm sp}}/\rho_{\rm s}\).