SciPost Phys. Core 7, 002 (2024) ·
published 22 January 2024

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We demonstrate that in Weyl semimetals, the momentumspace helical spin texture can couple to the chirality of the Weyl node to generate a frequencyindependent optical spin injection. This frequencyindependence is rooted in the topology of the Weyl node. Since the helicity and the chirality are always locked for Weyl nodes, the injected spin from a pair of Weyl nodes always add up, implying no symmetry requirements for Weyl semimetals. Finally, we show that such frequencyindependent spin injection is robust against multiband corrections and latticeregularization effect and capable of realizing alloptical magnetization switching in the THz regime.
Weicheng Ye, Meng Guo, YinChen He, Chong Wang, Liujun Zou
SciPost Phys. 13, 066 (2022) ·
published 26 September 2022

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LiebSchultzMattis (LSM) theorems provide powerful constraints on the emergibility problem, i.e. whether a quantum phase or phase transition can emerge in a manybody system. We derive the topological partition functions that characterize the LSM constraints in spin systems with $G_s\times G_{int}$ symmetry, where $G_s$ is an arbitrary space group in one or two spatial dimensions, and $G_{int}$ is any internal symmetry whose projective representations are classified by $\mathbb{Z}_2^k$ with $k$ an integer. We then apply these results to study the emergibility of a class of exotic quantum critical states, including the wellknown deconfined quantum critical point (DQCP), $U(1)$ Dirac spin liquid (DSL), and the recently proposed nonLagrangian Stiefel liquid. These states can emerge as a consequence of the competition between a magnetic state and a nonmagnetic state. We identify all possible realizations of these states on systems with $SO(3)\times \mathbb{Z}_2^T$ internal symmetry and either $p6m$ or $p4m$ lattice symmetry. Many interesting examples are discovered, including a DQCP adjacent to a ferromagnet, stable DSLs on square and honeycomb lattices, and a class of quantum critical spinquadrupolar liquids of which the most relevant spinful fluctuations carry spin$2$. In particular, there is a realization of spinquadrupolar DSL that is beyond the usual parton construction. We further use our formalism to analyze the stability of these states under symmetrybreaking perturbations, such as spinorbit coupling. As a concrete example, we find that a DSL can be stable in a recently proposed candidate material, NaYbO$_2$.
SciPost Phys. 12, 196 (2022) ·
published 14 June 2022

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We study the edge physics of the deconfined quantum phase transition (DQCP) between a spontaneous quantum spin Hall (QSH) insulator and a spinsinglet superconductor (SC). Although the bulk of this transition is in the same universality class as the paradigmatic deconfined Neel to valencebondsolid transition, the boundary physics has a richer structure due to proximity to a quantum spin Hall state. We use the parton trick to write down an effective field theory for the QSHSC transition in the presence of a boundary. We calculate various edge properties in an $N\to\infty$ limit. We show that the boundary Luttinger liquid in the QSH state survives at the phase transition, but only as "fractional" degrees of freedom that carry charge but not spin. The physical fermion remains gapless on the edge at the critical point, with a universal jump in the fermion scaling dimension as the system approaches the transition from the QSH side. The critical point could be viewed as a gapless analogue of the quantum spin Hall state but with the full $SU(2)$ spin rotation symmetry, which cannot be realized if the bulk is gapped.