Quantum valley pseudospin controlled by strain

Abstract

Valleytronics, as an alternative to traditional electronics or spintronics, is based on the encoding of quantum information in pseudospin valley quantum numbers, rather than in charge or spin states. A key ingredient is the (optical) manipulation of valley states before loss of coherence, which can be as fast as 100 femtoseconds. Previous works have shown the possibility of valley state manipulation using external fields. Here we propose uniaxial strain as a more flexible and robust scheme to manipulate the valley state through the breaking of the crystal symmetry and the concomitant lifting of the degeneracy of the 1s exciton energy. Our theory is corroborated by state-of-the-art numerical simulations in monolayer hBN and shows the possibility to control valley pseudospin at the attosecond time scale.

Figures (3)

• Figure 1: a) Scheme of the energy degeneracy lifting of $1s$ exciton states due to strain, together with the pulse designed to excite the two relevant states. b) View of the rotations of the excitonic quantum states on the pseudospin Bloch sphere induced by strain. c) Circularly polarized optical conductivity for unstrained system (black line) as well as for the system under armchair (blue line) and zigzag strain (red line), as obtained through XATU URIAALVAREZ2024109001. Inset represents the lattice deformation for each corresponding strain.
• Figure 2: Time evolution (left--to--right) of the exciton wavefunction $\rho_{cv}({\bf k})$, from $t\approx t_f$ to $t\approx t_f + \tau$, right after excitation under $\epsilon=0.1$ strain, computed via EDUS doi:10.1021/acs.jctc.2c00674. Top row: armchair ($\theta_{s}=0$) strain. Bottom row: zigzag ($\theta_{s}=\pi/2$) strain. Fifth panel in each row corresponds to $t\approx t_f + \tau$, where the $\left|\psi^{+}\right>$ state has fully transitioned into the $\left|\psi^{-}\right>$ state. Red outline denotes the first Brillouin zone, with points green, white, and blue representing the ${\bf K}/{\bf K}'$, $\mathbf{\Gamma}$, and $\mathbf{M}/\mathbf{M}^\prime$ points, respectively.
• Figure 3: a) Illustration of the relevant transitions involved in the pump-probe signal. b) Transient absorption spectrum in the energy range corresponding to transitions from core to valence states. Fast oscillations are observed with a period $T_{\tau_+}\approx0.8\,\mathrm{fs}$, consistent with the probe pulse capturing the global phase of an exciton coherent state. Data for hBN strained along the armchair direction ($\epsilon=0.1$, $\theta_s=0$). The integrated area around the peak at -4.4 eV is highlighted in green. c) Oscillations in time delay $\tau_\mathrm{d}$ for the peak around -4.4 eV for both armchair (solid lines) and zigzag (dashed lines) strain directions. d) and e) Transient absorption spectrum in hBN strained along the armchair ($\epsilon=0.1$, $\theta_s=0$) and zigzag ($\epsilon=0.1$, $\theta_s=\pi/2$) direction, respectively.