School of Physics Seminar - Professor Hua Chen

Significant developments in spintronics over the past decades have established that electron spin can be transported and manipulated by electrical means. The mechanisms of these processes are often captured qualitatively by treating spin as a conserved quantity. However, in materials that exhibit strong spintronics phenomena, such an approximation is frequently questionable. This dilemma leads us to pose the following question: Can conduction electrons carry physical observables other than spin, even if these observables are not strictly conserved, and give rise to new measurable phenomena? In this talk, I will introduce the language and tools for defining and calculating spin magnetic multipole moments of Bloch electrons in a gauge-invariant manner [1]. These magnetic multipole moments are inherently local in both real and momentum spaces, achieved through a wave-packet approach. One can then introduce a semiclassical Boltzmann theory to describe the transport of these multipole moments. Using monolayer phosphorene as a prototypical example, I will elucidate some physical consequences of spin magnetic multipole moments generated by electric fields and currents, in particular current-induced spin accumulation with staggered signs at corners of a square sample. Additionally, I will discuss how the spin Hall effect can be understood as an electrical response of spin magnetic quadrupole moment or spin density polarization [2]. In the end I will briefly mention our ongoing work on the modern theory of equilibrium higher-order spin magnetic multipole moments [3].

[1] M. Tahir and HC, Phys. Rev. Lett. 131, 106701 (2023)
[2] HC, Q. Niu, and A. H. MacDonald, arXiv:1803.01294
[3] HC, G.-Y. Guo, D. Xiao, in preparation.