Probing optically silent superfluid stripes in cuprates

TL;DR

This study demonstrates that nonlinear Terahertz optics can detect hidden superfluid charge order in cuprates, revealing superfluid-like behavior in the normal state through third harmonic generation.

Contribution

It introduces a novel method using Terahertz pulses to probe optically silent superfluid charge stripes in cuprates, providing evidence of hidden superfluid order.

Findings

Giant Terahertz third harmonic signal observed above Tc and up to TCO.

Nonlinear mixing of tunneling modes drives large super-current oscillations.

Supports presence of hidden superfluid order in the normal state of cuprates.

Abstract

Unconventional superconductivity in the cuprates emerges from, or coexists with, other types of electronic order. However, these orders are sometimes invisible because of their symmetry. For example, the possible existence of superfluid charge stripes in the normal state of single layer cuprates cannot be validated with infrared optics, because interlayer tunneling fluctuations vanish on average. Similarly, it is not easy to establish if charge orders are responsible for dynamical decoupling of the superconducting layers over broad ranges of doping and temperatures. Here, we show that TeraHertz pulses can excite nonlinear tunneling currents between linearly de-coupled charge-ordered planes. A giant TeraHertz third harmonic signal is observed in La1.885Ba0.115CuO4 far above Tc=13 K and up to the charge ordering temperature TCO = 55 K. We model these results by considering large order-parameter-phase oscillations in a pair density wave condensate, and show how nonlinear mixing of optically silent tunneling modes can drive large dipole-carrying super-current oscillations. Our results provide compelling experimental support for the presence of hidden superfluid order in the normal state of cuprates. These experiments also underscore the power of nonlinear TeraHertz optics as a sensitive probe of frustrated excitations in quantum solids.