Torsion-controlled spin transport and tunable intersubband absorption in a screw-dislocated semiconductor nanowire

We develop an effective-mass theory for spin-resolved transport and intersubband optical absorption in a finite semiconductor nanowire containing a uniform screw-dislocation-induced torsion. The central transport quantity is the band bottom of each torsion-dependent one-dimensional subband: torsion lowers this band bottom while an axial magnetic field resolves the two Zeeman branches, so that a spin-selective torsion interval opens when the Fermi level lies between the two branch minima. We distinguish this band-bottom transport threshold from the fixed-momentum transverse energy that governs the vertical intersubband optical transition, and obtain a design rule for the width of the spin-selective torsion interval that is linear in the Zeeman splitting in the small-splitting limit. Finite-size effects are incorporated by treating the active region as an open scattering region of length $L$ and radius $R$, with Fabry-Perot interference, radial boundary conditions, and surface-induced linewidth broadening. The spin-resolved response remains well defined when the effective linewidth satisfies $\Gamma_{\mathrm{eff}}\ll\Delta_Z$, a criterion quantified through polarization and conductance-contrast maps. In the optical sector, finite-radius intersubband absorption gives a torsion-tunable resonance in the THz range; surface boundary conditions shift the absolute resonance frequency and modify the oscillator strength, while, for the dominant unit-branch transition, the leading fixed-momentum torsional slope $\partial\omega/\partial\tau\simeq\hbar k_F/m^*$ is preserved. The resulting framework connects geometric torsion, spin-resolved mesoscopic transport, spin thermopower, and tunable intersubband absorption within a single open-quantum-wire model.