Transport signature of the magnetic Berezinskii-Kosterlitz-Thouless transition

Abstract

Motivated by recent experimental progress in 2D magnetism, we theoretically study spin transport in 2D easy-plane magnets at finite temperatures across the Berezinskii-Kosterlitz-Thouless (BKT) phase transition, by developing a duality mapping to the 2+1D electromagnetism with the full account of spin’s finite lifetime. In particular, we find that the non-conservation of spin gives rise to a distinct signature across the BKT transition, with the spin current decaying with distance power-law (exponentially) below (above) the transition; this is detectable in the proposed experiment with NiPS_33 and CrCl_33.

Superfluid Spin Transistor

“…Berezinskii-Kosterlitz-Thouless (BKT) transition has been theoretically predicted in 2D XY model [4; 5], but substantial experimental progress has been achieved so far in nonmagentic systems such as thin films of superfluids/superconductors [6][7][8][9][10] and superconducting arrays [11; 12]. Designing unambiguous experiments in magnetic systems is of high importance and spintronic methods seem to be suitable for identifying signatures of BKT transition [13][14][15]. Recent experiments on magnetic van der Waals (vdW) materials such as NiPS 3 , CrBr 3 , and CrCl 3 further motivate theoretical research of magnetic BKT transition [16][17][18].…”

“…Above BKT transition, the behavior is determined by exponentially increasing density of unbound topological defects. Recent theoretical works have addressed behavior of spin and charge currents in the vicinity of magnetic BKT transition [13][14][15]. The behavior of spin current is of particular interest due to apparent analogy to superfluid transport described by the U (1) phase gradient [13].…”

“…Recent theoretical works have addressed behavior of spin and charge currents in the vicinity of magnetic BKT transition [13][14][15]. The behavior of spin current is of particular interest due to apparent analogy to superfluid transport described by the U (1) phase gradient [13]. Spin superfluid transport has been proposed in collinear [21][22][23][24][25][26][27][28] and noncollinear [29; 30] magnets and realized in recent experiments [31; 32].…”

“…Transport signatures can be used for identification of BKT transition, e.g., as has been established in literature on superconducting systems. While applying these ideas to magnetic insulating systems one needs to be aware of differences such as the nonconservation of spin currents [13], the coupling to phonons, and the presence of merons rather than vortices as topological defects. This warrants both identification of new physics and development of new methods for studying relevant magnetic systems.…”

“…In this work, we propose to use transport signatures of the Hall response of topological defects, such as merons and antimerons, as a hallmark of BKT transition. Starting from the Landau-Lifshitz-Gilbert type phenomenology, we derive the constitutive phenomenological relations [13] describing the vorticity and spin currents in insulating magnetic systems in the presence of Gilbert damping and spin nonconservation. We confirm these relations and a possibility of long range spin superfluidity below BKT transition by employing a combination of Monte Carlo and spin dynamics simulations.…”