Spin Casimir effect in noncollinear quantum antiferromagnets: Torque equilibrium spin wave approach

Abstract

The Casimir effect is a general phenomenon in physics, which arises when the vacuum fluctuation of an arbitrary field is modified by static or slowly varying boundary. However, its spin version is rarely addressed, mainly due to the fact that a macroscopic boundary in quantum spin systems is hard to define. In this article, we explore the spin Casimir effect induced by the zero-point fluctuation of spin waves in a general non-collinear ordered quantum antiferromagnet. This spin Casimir effect results in a spin torque between local spins and further causes various singular and divergent results in the framework of spin-wave theory, which invalidate the standard 1/S expansion procedure. Based on the spin Casimir torque interpretation, we develop a spin-wave expansion approach named as torque equilibrium spin wave theory (TESWT). In this approach, the spin Casimir effect is treated in a self-consistent way, and the spin-wave expansion results are free from singularities and divergences. A detailed spin-wave analysis of the antiferromagnetic spin-1/2 Heisenberg model on a spatially anisotropic triangular lattice is undertaken within our approach. Our results indicate that the spiral order is only stable in the region 0.5 < α < 1.2, where α is the ratio of the coupling constants. In addition, the instability in the region 1.2 < α < 2 is owing to the spin Casimir effect instead of the vanishing sublattice magnetization. Furthermore, our method provides an efficient and convenient tool that can estimate the correct exchange parameters and outline the quantum phase diagrams, which can be useful for experimental fitting processes in frustrated quantum magnets.

SU(N) spin-wave theory: Application to spin-orbital Mott insulators

“…To study other spin dynamics, such as magnon decay effects 38,39 , one should goes beyond the linear order approximation. We note that some modifications of the spin-wave theory 40,41 have been developed in the SU(2) case, their generalizations to the SU(N ) case deserve further study.…”

Raman spectroscopy of bimagnon and trimagnon excitations and rotonlike points in a distorted triangular lattice antiferromagnet

“…The modified interactions result in a shift of the ordering wave vector, a behavior supported by theoretical investigation [2] and experimental observations [2,20,21]. The TLAF has been investigated for its spin order and ground states using spin wave theory [8,14,15]. It has also been well studied theoretically and experimentally for its singlemagnon excitation behavior using inelastic neutron scattering (INS) experiments [22,23].…”

“…Experimental [1][2][3][4] and theoretical [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] studies of triangular lattice antiferromagnet (TLAF) suggest the stabilization of a long-range-ordered ground state that can be noncollinear and non-coplanar. The non-collinear ordering pattern is due to competing exchange interactions which in a perfect undistorted triangular lattice geometry manifests itself as a 120 • magnetic structure [17,18].…”

“…The spectroscopic features of these multimagnon excitations have been investigated theoretically using RIXS at the K-edge [24]. Additionally, the multimagnon Raman spectra have been investigated for the model undistorted TLAF compound using the torque equilibrium spin wave theory (TESWT) [14,17,18].…”

Quantum Damping of Skyrmion Crystal Eigenmodes due to Spontaneous Quasiparticle Decay

“…However, many-body interactions can lead to spontaneous quasiparticle decay (SQD), dressing quasiparticles with intrinsic lifetimes and potentially wiping out their spectral weight. Such a quantum-mechanical many-body phenomenon is known from phonons in crystals [2,3] and liquid helium [4][5][6][7][8][9], or magnetic excitations in spin liquids [10][11][12] and quantum (anti)ferromagnets [13][14][15][16][17][18][19][20][21][22][23][24]. Magnetic systems are particularly attractive platforms for SQD, because magnetic fields serve as handles to manipulate both the magnonic dispersion and interactions [25][26][27][28][29][30][31][32][33][34][35].…”