New experimental tools can provide unexpected insights that can resolve `blind men and the elephant’ type issues that we often encounter in the study of complex materials. In this work we use a newly developed instrument that combines laser light with a scanning tunnelling microscope, to obtain crucial insights into the intriguing charge order observed in the Kagome compound RbV3Sb5.
The family of Kagome compounds AV3Sb5 (where A=K, Rb, Cs) has generated huge excitement due to the possible observation of a charge density wave (CDW) state with broken time-reversal symmetry. Since these materials are non-magnetic, broken time reversal symmetry suggests the presence of an exotic loop current phase which has been proposed for many decades in the context of quantum anomalous Hall states and correlated electron systems. However, the initial results of broken time reversal symmetry are now being intensely debated due to conflicting experimental data. In this talk I will describe our attempts to pin down the symmetry of the charge order by measuring the response to both linearly polarized pulsed laser light and magnetic fields. I will first show how we were able to use light to manipulate the relative strength of the CDW in a reversible fashion. Interestingly, the laser manipulation of the CDW is accompanied by a lattice distortion (strain) that can also be controlled with linearly polarized light. Armed with the laser induced manipulation of the CDW intensities and corresponding lattice distortions, we show that a similar response occurs with perpendicular magnetic fields. The simplest CDW that satisfies the constraints imposed by the light and magnetic field data is an out-of-phase combination of bond charge order and loop currents that we dub congruent CDW flux phase. Our work thus reveals that the CDW order parameter Kagome AV3Sb5 compounds is the long sought after flux phase that breaks time-reversal symmetry. Equally importantly we have discovered that under certain circumstances, laser light can be used to manipulate the spatial symmetry of a condensed matter system.
