Spectral fringes without subcycles in Schwinger pair production and Dirac materials

TL;DR

This paper reveals that spectral fringes in Schwinger pair production can occur even with smooth, single-lobe electric pulses, due to a transition in the dominant quantum turning points, and demonstrates this in both QED and Dirac materials.

Contribution

It uncovers a new mechanism for spectral fringes arising from turning-point dominance transition, applicable to both quantum electrodynamics and Dirac materials.

Findings

Gaussian pulses produce smooth spectra, while deformed pulses develop strong fringes near the Keldysh parameter of unity.

Numerical solutions agree with semiclassical analysis, confirming the turning-point transition as the cause.

The mechanism is demonstrated in a Dirac material model relevant to epitaxial graphene.

Abstract

Spectral fringes in Schwinger pair creation are usually attributed to structured driving, such as carrier oscillations, pulse trains, or multiple creation events. We show that pronounced fringes can arise even for smooth, carrier-free single-lobe electric-field pulses. Two bell-shaped profiles that are nearly indistinguishable in real time - a Gaussian pulse and a weakly deformed variant - produce qualitatively different longitudinal momentum spectra in the nonadiabatic crossover: the Gaussian spectrum remains smooth, whereas the deformed pulse develops strong fringes as the Keldysh parameter approaches unity. Exact numerical solutions in scalar and spinor QED agree with a semiclassical turning-point analysis and trace the effect to a turning-point dominance transition, where the leading saddle becomes irrelevant and subleading contributions interfere. We demonstrate the same mechanism in a solid-state Schwinger analog described by a gapped two-dimensional Dirac model relevant to epitaxial graphene on SiC, and discuss an energy-resolved pump-probe route to observing the predicted modulation.