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Wang Group
Fakultät Physik
Student sits at workstation and looks at laptop. © Aliona Kardash​/​TU Dortmund
Publication in Nano Letters

Transient symmetry breaking in a terahertz-driven superconductor

Transient symmetry breaking in a superconductor by THz light © Siyu Duan​/​TU Dortmund
In the most recently published article in Nano Letters, Dr. Siyu Duan – Walter Benjamin Fellow funded by the DFG – and her collaborators present experimental signatures of transient symmetry breaking in a dirty superconductor NbN under a drive of a picosecond terahertz pulse. In particular, Dr. Duan observed transient even-order nonlinear responses, which are forbidden in a system with inversion symmetry such as in NbN, thereby evidencing inversion-symmetry breaking on an ultrafast time scale shorter than a picosecond.

To the publication

Symmetry provides a fundamental framework for classifying states of matter and describing phase transitions, while nonlinear dynamical responses are highly sensitive to changes in symmetry. Nonlinear optical responses in superconductors provide important information complementary to linear-response probes, with even-order nonlinear responses being particularly sensitive to inversion-symmetry breaking. While transient inversion-symmetry breaking has been demonstrated in clean-limit superconductors, whether such symmetry-breaking behavior can occur on an ultrafast timescale in dirty-limit superconductors has remained an open question. Dirty-limit superconductors are characterized by strong impurity scattering, a substantially reduced mean free path, suppressed superfluid density, and diffusive quasiparticle dynamics, which tend to suppress symmetry-breaking nonlinear optical responses.

In order to detect a possible but small characteristic nonlinear response for symmetry breaking, Dr. Duan et al. designed and fabricated split-ring micro-resonators and embedded NbN thin film between a pair of the resonators. Such a structure enhances the detection sensitivity of the transient response in NbN, especially around the resonant frequency of the microstructures, i.e., at 0.5 THz. By setting up a time-resolved terahertz pump-terahertz probe spectroscopy, the authors observed that the pump field-induced changes of the probe field exhibit clear oscillations as a function of the pump-probe delay, with Fourier spectra showing peaks at the fundamental, second, and third harmonics of the pump-pulse frequency f. The observed 1f and 3f signals correspond to even-order nonlinear responses that are not allowed by inversion symmetry at thermal equilibrium, providing evidence for transient inversion-symmetry breaking. With entering the superconducting state, these nonlinear responses are significantly enhanced, i.e., by nearly two orders of magnitude, highlighting the occurrence of transient symmetry breaking in the superconducting state.

Our results demonstrate that transient symmetry breaking can be induced by a picosecond supercurrent even in a dirty-limit superconductor with strong scattering. The cavity-engineered superconductor microstructure provides an all-optical, contact-free means of probing transient symmetry-breaking superconducting states. These findings enrich the possibilities of engineering superconductor-based symmetry-breaking metamaterials for ultrafast applications.