Solid-State Gravitational Redshift: Transport Signatures of Massless Dirac Fermions in Tilted Dirac Cone Heterostructures

Abstract

We investigate the propagation of electron waves in a two-dimensional tilted Dirac cone heterostructure where tilt depends on the coordinate $z$ along the junction. The resulting Dirac equation in an emergent curved spacetime for the spinor $\psi(z)$ can be efficiently solved using 4th-order Runge-Kutta numerical method by a transformation to a "suitable" spinor $\varphi$ where the resulting Dirac cone looks locally upright. The spatial texture of the tilt induces oscillatory behaviors in key physical quantities such as the norm $|\varphi(z)|^2$ of the wave function, polar and azimuthal angles $\Theta(z)$ and $\Phi(z)$ of the pseudospin, and the integrated transmission $\tau$ where oscillation wave-lengths get shorter (longer) in stronger (weaker) tilt regions. Such an oscillatory behavior reminiscent of gravitational red-shift is an indicator of an underlying spacetime metric that can be probed in tunneling experiments. We derive analytical approximations for the position-dependent wave numbers $\Delta k_z(z)$ that explain the red-shift patterns and corroborate them with numerical simulations. For a tilt "bump" spread over length scale $\ell$, upon increasing $\ell$, the amplitude of red-shifted oscillations reduces whereas the number of peaks increases. The scale invariance of Dirac equation allows to probe these aspects of $\ell$-dependence by a voltage sweep in transmission experiments. Smooth variations of the tilt reduce impedance mismatch of the electron waves, thereby giving rise to very high transmission rate. This concept can be used in combination with a sigmoid-shaped tilt texture for red-shift or blue-shift engineering of the transmitted waves, depending on whether the sigmoid is downswing or upswing.