# Floquet dynamical quantum phase transitions

## TL;DR

This paper introduces Floquet dynamical quantum phase transitions, a new type of DQPTs occurring within each period of a periodically driven system, linked to Floquet topological phases, and demonstrates their experimental realization.

## Contribution

It discovers Floquet DQPTs as intrinsic features of periodically driven systems, expanding the understanding of non-equilibrium quantum phase transitions beyond quenched scenarios.

## Key findings

- Floquet DQPTs occur within each period of continuous driving.
- Existence of Floquet DQPTs is linked to nontrivial Floquet topological phases.
- Experimental demonstration with a single spin in diamond.

## Abstract

Dynamical quantum phase transitions (DQPTs) are manifested by time-domain nonanalytic behaviors of many-body systems.Introducing a quench is so far understood as a typical scenario to induce DQPTs.In this work, we discover a novel type of DQPTs, termed "Floquet DQPTs", as intrinsic features of systems with periodic time modulation.Floquet DQPTs occur within each period of continuous driving, without the need for any quenches.In particular, in a harmonically driven spin chain model, we find analytically the existence of Floquet DQPTs in and only in a parameter regime hosting a certain nontrivial Floquet topological phase. The Floquet DQPTs are further characterized by a dynamical topological invariant defined as the winding number of the Pancharatnam geometric phase versus quasimomentum.These findings are experimentally demonstrated with a single spin in diamond.This work thus opens a door for future studies of DQPTs in connection with topological matter.