Sign control of photocurrents by spin-group-symmetry breaking in altermagnetic insulators

Abstract

Controlling physical responses through symmetry breaking is a central paradigm in quantum materials, enabling novel functionalities. Here we determine the effects of spin-group-symmetry breaking on nonlinear optical responses of collinear altermagnetic insulators. Using shear strain as an example, we show that the direction of symmetry-breaking induced components of charge and spin photocurrents are locked to the sign of the strain. In the absence of spin-orbit coupling, this effect is intuitively captured by the spin-gap asymmetry--an imbalance between spin-up and spin-down direct band gaps which couples trilinearly with the Néel order and the strain. We demonstrate this mechanism with density functional theory calculations on the recently proposed altermagnet CuWP$_2$S$_6$. Having symmetry-guided control of both charge and spin photocurrents allows, vice versa, to reveal and investigate altermagnetism in insulating materials by exploration of their optical responses.

Figures (4)

• Figure 1: (a) Schematic band structure of a two-dimensional $d$-wave altermagnetic insulator, where red (blue) denotes the spin up (down) bands. The rectified photocurrents obey corresponding symmetry constraints, which, in this example, forbid a spin current along $x$ and charge current along $y$. (b) Upon spin-group symmetry breaking induced by shear strain, the band structure is shifted and develops a finite spin-gap asymmetry $\Delta_a$. The spin and charge photocurrent components forbidden in (a) are are thereby activated.
• Figure 2: (a–b) Crystal and magnetic structure of the CuWP$_2$S$_6$ monolayer in the altermanget-antiferroelectric phase. (c) Band structure of the CuWP$_2$S$_6$ monolayer without SOC; blue and red lines denote spin-down and spin-up bands, respectively. (d) Isoenergy contours at $E= E_f-0.2$ eV for pristine and strained monolayers. (e) Band structures under strain (dashed lines) and without strain (solid lines) plotted for the boxed region in (c). (f) NLOR of CuWP$_2$S$_6$ under linearly polarized (blue) and circularly polarized (red) light; solid lines represent charge conductivities, while dashed lines represent spin conductivities. (g-h) Representative strain-induced charge and spin components in NLOR. The responses are odd upon reversal of shear strain $\epsilon_{xy}$.
• Figure 3: (a) Momentum-resolved imbalance in the spin-resolved joint density of states $\Delta \rho (\mathbf{k}, \omega)$ at fixed frequency $\hbar\omega = 1.25$ eV for pristine and strained unit cells. The unit cells under positive and negative shear strain are related by altermagnetic symmetries, which manifest in the corresponding $\Delta \rho$ patterns. (b) Momentum-integrated $\Delta \rho (\omega)$ as a function of the incident photon energy, which is odd under strain reversal over the entire frequency range.
• Figure 4: Polar angle plots of the charge (left panels) and spin (right panels) currents generated with linearly polarized light at $\hbar\omega = 1.2$ eV for: (a) $\epsilon_{xy} = 0$, (b) $\epsilon_{xy} > 0$ and (c) $\epsilon_{xy} < 0$. The color of the points indicate the value of the angle associated to the direction $\theta$ of the generated current, with respect to the its average value.