Optically Induced Symmetry Breaking Due to Nonequilibrium Steady State Formation in Charge Density Wave Material 1T-TiSe.

The strongly correlated charge density wave (CDW) phase of 1T-TiSe is of interest to verify the claims of a chiral order parameter. Characterization of the symmetries of 1T-TiSe is critical to understand the origin of its intriguing properties. Here we use very low-power, continuous wave laser excitation to probe the symmetries of 1T-TiSe by using the circular photogalvanic effect.

Gate and Temperature Driven Phase Transitions in Few-Layer MoTe.

MoTe has a stable hexagonal semiconducting phase (2H) as well as two semimetallic phases with monoclinic (1T') and orthorhombic (T) structures. A structural change can thus be accompanied by a significant change in electronic transport properties. The two semimetallic phases are connected by a temperature driven transition and could exhibit topological properties.

Fermi surface tomography.

Fermi surfaces are essential for predicting, characterizing and controlling the properties of crystalline metals and semiconductors. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta (k) from energy- and angular distribution of photoelectrons dislodged by monochromatic light. Existing apparatus is able to determine a number of k -vectors simultaneously, but direct high-resolution 3D Fermi surface mapping remains problematic.

Exchange coupling-mediated broken symmetries in TaNiSe revealed from quadrupolar circular photogalvanic effect.

In low-electron density materials, interactions can lead to highly correlated quantum states of matter. TaNiSe, an excitonic insulator (EI) candidate, exists in a novel broken-symmetry phase below 327 K, characterized by robust exchange interaction and electron-lattice coupling. We study this phase of TaNiSe using the quadrupole circular photogalvanic effect (QCPGE).

Imaging the coherent propagation of collective modes in the excitonic insulator TaNiSe at room temperature.

Excitonic insulators host a condensate of electron-hole pairs at equilibrium, giving rise to collective many-body effects. Although several materials have emerged as excitonic insulator candidates, evidence of long-range coherence is lacking and the origin of the ordered phase in these systems remains controversial. Here, using ultrafast pump-probe microscopy, we investigate the possible excitonic insulator TaNiSe Below 328 K, we observe the anomalous micrometer-scale propagation of coherent modes at velocities of ~10 m/s, which we attribute to the hybridization between phonon modes and the phase mode of the condensate.