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The excitonic insulator is an electronically-driven phase of matter thatemerges upon the spontaneous formation and Bose condensation of excitons.Detecting this exotic order in candidate materials is a subject of paramountimportance, as the size of the excitonic gap in the band structure establishesthe potential of this collective state for superfluid energy transport.However, the identification of this phase in real solids is hindered by thecoexistence of a structural order parameter with the same symmetry as theexcitonic order. Only a few materials are currently believed to host a dominantexcitonic phase, Ta$_2$NiSe$_5$ being the most promising. Here, we test thisscenario by using an ultrashort laser pulse to quench the broken-symmetry phaseof this transition metal chalcogenide. Tracking the dynamics of the material'selectronic and crystal structure after light excitation reveals surprisingspectroscopic fingerprints that are only compatible with a primary orderparameter of phononic nature. We rationalize our findings throughstate-of-the-art calculations, confirming that the structural order accountsfor most of the electronic gap opening. Not only do our results uncover thelong-sought mechanism driving the phase transition of Ta$_2$NiSe$_5$, but theyalso conclusively rule out any substantial excitonic character in thisinstability.

The notion of spontaneous symmetry breaking has been used to describe phasetransitions in a variety of physical systems. In crystalline solids, thebreaking of certain symmetries, such as mirror symmetry, is difficult to detectunambiguously. Using 1$T$-TaS$_2$, we demonstrate here thatrotational-anisotropy second harmonic generation (RA-SHG) is not only asensitive technique for the detection of broken mirror symmetry, but also thatit can differentiate between mirror symmetry-broken structures of oppositeplanar chirality. We also show that our analysis is applicable to a wide classof different materials with mirror symmetry-breaking transitions. Lastly, wefind evidence for bulk mirror symmetry-breaking in the incommensurate chargedensity wave phase of 1$T$-TaS$_2$. Our results pave the way for RA-SHG toprobe candidate materials where broken mirror symmetry may play a pivotal role.

We formulate a dynamical model to describe a photo-induced charge densitywave (CDW) quench transition and apply it to recent multi-probe experiments onLaTe$_3$ [A. Zong et al., Nat. Phys. 15, 27 (2019)]. Our approach relies oncoupled time-dependent Ginzburg-Landau equations tracking two order parametersthat represent the modulations of the electronic density and the ionicpositions. We aim at describing the amplitude of the order parameters under theassumption that they are homogeneous in space. This description is supplementedby a three-temperature model, which treats separately the electronictemperature, temperature of the lattice phonons with stronger couplings to theelectronic subsystem, and temperature of all other phonons. The broad scope ofavailable data for LaTe$_3$ and similar materials as well as the synergybetween different time-resolved spectroscopies allow us to extract modelparameters. The resulting calculations are in good agreement with ultra-fastelectron diffraction experiments, reproducing qualitative and quantitativefeatures of the CDW amplitude evolution during the initial few picosecondsafter photoexcitation.

When electrons in a solid are excited with light, they can alter the freeenergy landscape and access phases of matter that are beyond reach in thermalequilibrium. This accessibility becomes of vast importance in the presence ofphase competition, when one state of matter is preferred over another by only asmall energy scale that, in principle, is surmountable by light. Here, we studya layered compound, LaTe$_3$, where a small in-plane (a-c plane) latticeanisotropy results in a unidirectional charge density wave (CDW) along thec-axis. Using ultrafast electron diffraction, we find that afterphotoexcitation, the CDW along the c-axis is weakened and subsequently, adifferent competing CDW along the a-axis emerges. The timescales characterizingthe relaxation of this new CDW and the reestablishment of the original CDW arenearly identical, which points towards a strong competition between the twoorders. The new density wave represents a transient non-equilibrium phase ofmatter with no equilibrium counterpart, and this study thus provides aframework for unleashing similar states of matter that are "trapped" underequilibrium conditions.

Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing-down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photoinduced transition of a model charge-density-wave (CDW) compound LaTe_{3}. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing-down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems.

Upon excitation with an intense ultrafast laser pulse, a symmetry-brokenground state can undergo a non-equilibrium phase transition through pathwaysdissimilar from those in thermal equilibrium. Determining the mechanismunderlying these photo-induced phase transitions (PIPTs) has been along-standing issue in the study of condensed matter systems. To this end, weinvestigate the light-induced melting of a unidirectional charge density wave(CDW) material, LaTe$_3$. Using a suite of time-resolved probes, weindependently track the amplitude and phase dynamics of the CDW. We find that aquick ($\sim\,$1$\,$ps) recovery of the CDW amplitude is followed by a slowerreestablishment of phase coherence. This longer timescale is dictated by thepresence of topological defects: long-range order (LRO) is inhibited and isonly restored when the defects annihilate. Our results provide a framework forunderstanding other PIPTs by identifying the generation of defects as agoverning mechanism.

Domain walls (DWs) are singularities in an ordered medium that often host exotic phenomena such as charge ordering, insulator-metal transition, or superconductivity. The ability to locally write and erase DWs is highly desirable, as it allows one to design material functionality by patterning DWs in specific configurations. We demonstrate such capability at room temperature in a charge density wave (CDW), a macroscopic condensate of electrons and phonons, in ultrathin 1T-TaS2. A single femtosecond light pulse is shown to locally inject or remove mirror DWs in the CDW condensate, with probabilities tunable by pulse energy and temperature. Using time-resolved electron diffraction, we are able to simultaneously track anti-synchronized CDW amplitude oscillations from both the lattice and the condensate, where photoinjected DWs lead to a red-shifted frequency. Our demonstration of reversible DW manipulation may pave new ways for engineering correlated material systems with light.

Rapid developments in time-resolved optical spectroscopy have led to renewed interest in the nonequilibrium state of superconductors and other highly correlated electron materials. In these experiments, the nonequilibrium state is prepared by the absorption of short (less than 100 fs) laser pulses, typically in the near-infrared, that perturb the density and energy distribution of quasiparticles. The evolution of the nonequilibrium state is probed by time resolving the changes in the optical response functions of the medium that take place after photoexcitation. Ultimately, the goal of such experiments is to understand not only the nonequilibrium state, but to shed light on the still poorly understood equilibrium properties of these materials. We report nonequilibrium experiments that have revealed aspects of the cup rates that have been inaccessible by other techniques. Namely, the diffusion and recombination coefficients of quasiparticles have been measured in both YBa2Cu3O6.5 and Bi2Sr2CaCu2O8+x using time-resolved optical spectroscopy. Dependence of these measurements on doping, temperature and laser intensity is also obtained. To study the recombination of quasiparticles, we measure the change in reflectivity Delta R which is directly proportional to the nonequilibrium quasiparticle density created by the laser. From the intensity dependence, we estimate beta, the inelastic scattering coefficient and gamma_th thermal equilibrium quasiparticle decay rate. We also present the dependence of recombination measurements on doping in Bi2Sr2CaCu2O8+x. Going from underdoped to overdoped regime, the sign of Delta R changes from positive to negative right at the optimal doping. This is accompanied by a change in dynamics. The decay of Delta R stops being intensity dependent exactly at the optimal doping. We provide possible interpretations of these two observations. To study the propagation of quasiparticles, we interfered two laser pulses to introduce a spatially periodic density of quasiparticles. Probing the evolution of the initialdensity through space and time yielded the quasiparticle diffusion coefficient, and both inelastic and elastic scattering rates. Measured diffusion coefficient suggests that the quasiparticles induced by the laser occupy primarily states near the antinodal regions of the Brillouin zone.

We report measurements of the photoinduced change in reflectivity of an untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay rate of the transient change in reflectivity is found to decrease rapidly with decreasing temperature and, below Tc, with decreasing laser intensity. We interpret the decay as a process of thermalization of antinodal quasiparticles, with a rate determined by inelastic scattering of quasiparticle pairs.