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When electrons in a solid are excited with light, they can alter the free energy landscape and access phases of matter that are beyond reach in thermal equilibrium. This accessibility becomes of vast importance in the presence of phase competition, when one state of matter is preferred over another by only a small energy scale that, in principle, is surmountable by light. Here, we study a layered compound, LaTe3, where a small in-plane (a-c plane) lattice anisotropy results in a unidirectional charge density wave (CDW) along the c-axis. Using ultrafast electron diffraction, we find that after photoexcitation, the CDW along the c-axis is weakened and subsequently, a different competing CDW along the a-axis emerges. The timescales characterizing the relaxation of this new CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. The new density wave represents a transient non-equilibrium phase of matter with no equilibrium counterpart, and this study thus provides a framework for unleashing similar states of matter that are "trapped" under equilibrium conditions.

Topological defects, potential information carriers, were written into and erased from a solid with femtosecond light pulses.

2The observation of chirality is ubiquitous in nature. Contrary to intuition, the population of opposite chiralities is surprisingly asymmetric at fundamental levels [1,2]. Examples range from parity violation in the subatomic weak force [1] to the homochirality in essential biomolecules [2]. The ability to achieve chiralityselective synthesis (chiral induction) is of great importance in stereochemistry, molecular biology and pharmacology [2]. In condensed matter physics, a crystalline electronic system is geometrically chiral when it lacks any mirror plane, space inversion center or roto-inversion axis [3]. Typically, the geometrical chirality is predefined by a material's chiral lattice structure, which is fixed upon the formation of the crystal. By contrast, a particularly unconventional scenario is the gyrotropic order [4][5][6][7][8], where chirality spontaneously emerges across a phase transition as the electron system breaks the relevant symmetries of an originally achiral lattice. Such a gyrotropic order, proposed as the quantum analogue of the cholesteric liquid crystals, has attracted significant interest [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. However, to date, a clear observation and manipulation of the gyrotropic order remain challenging. We report the realization of optical chiral induction and the observation of a gyrotropically ordered phase in the transition-metal dichalcogenide semimetal 1T -TiSe 2 . We show that shining mid-infrared circularly polarized light near the critical temperature leads to the preferential formation of one chiral domain. As a result, we are able to observe an out-of-plane circular photogalvanic current, whose direction depends on the optical induction. Our study provides compelling evidence for the spontaneous emergence of chirality in the correlated semimetal TiSe 2 [18]. Such chiral induction provides a new way of optical control over novel orders in quantum materials.In the presence of strong correlations, the behavior of electrons in a metal can signficantly deviate from a weakly-interacting Fermi gas, forming a wide range of complex, broken symmetry phases [19]. Recent theoretical works have highlighted their analogy with classical liquids [19], where a rich set of liquid crystalline phases that exhibit varying degrees of symmetry breaking and transport properties have been observed. A well-studied case is the nematic order [19][20][21], i.e., the spontaneous emergence of rotational anisotropy. In classical liquids, this is known as the nematic liquid crystal, whereas in quantum materials, it has recently been observed in quantum Hall systems, ruthenates, high temperature supercon-3 ductors [19], heavy-fermion superconductors [20], and correlated metallic pyrochlores [21].Another fascinating case is the gyrotropic order, i.e., the spontaneous emergence of geometrical chirality. In classical liquids, this is known as the cholesteric liquid crystal. In quantum materials, despite fundamental interest [4][5][6][7][8][9][10][11][12][13][14][15][16], a clear ...

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 photo-induced transition of a model charge-densitywave (CDW) compound, LaTe3. 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 quantitatively 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.In a second-order symmetry-breaking phase transition, the spatial extent of fluctuating regions diverges close to the critical temperature, T c . Correspondingly, the relaxation time of these fluctuations tends to infinity, a phenomenon known as critical slowing down [1,2]. The phenomenology of slowing dynamics near a critical point is much more general: it has been observed in first-order transitions [3,4], glasses [5,6], dynamical systems [7], and even microbial communities [8]. Its common occurrence makes it a robust signature of phase transitions in a vast array of complex systems [9].Close to equilibrium, critical slowing down has been well characterized in condensed matter systems. Theoretically, it is described by a dynamical critical exponent, whose value depends on the dynamic universality class [2]. Experimentally, the evidence comes from a vanishing rate of change in the order parameter close to T c , with early reports in refs. [3,10,11]. While these measurements probe the slowing dynamics in the time domain, it can be observed in the frequency domain as well. For example, inelastic neutron scattering has revealed a narrowing quasi-elastic peak along the energy axis as T c is approached, indicating a suppressed relaxation rate of critical fluctuations [12][13][14]. Moreover, if there is a collective mode associated with the phase transition, the mode softening in the vicinity of T c is also taken as a signature of critical slowing down [15].For symmetry-breaking phase transitions in a highly nonequilibrium setting, the dynamics are much less understood. Recent studies have found important features in nonequilibrium transitions, such as topological defects, which are absent in their equilibrium counterparts [16][17][18]. Despite the differences, a slowdown in dynamics is thought to carry over to systems far from equilibrium. For example, in a rapid...

Upon excitation by a laser pulse, broken-symmetry phases of a wide variety of solids demonstrate similar order parameter dynamics characterized by a dramatic slowing down of relaxation for stronger pump fluences. Motivated by this recurrent phenomenology, we develop a simple nonperturbative effective model for photoinduced dynamics of collective bosonic excitations. We find that as the system recovers after photoexcitation, it shows universal prethermalized dynamics manifesting a power-law, as opposed to exponential, relaxation, explaining the slowing down of the recovery process. For strong quenches, long-wavelength overpopulated transverse modes dominate the long-time dynamics; their distribution function exhibits universal scaling in time and space, whose universal exponents can be computed analytically. Our model offers a unifying description of order parameter fluctuations in a regime far from equilibrium, and our predictions can be tested with available time-resolved techniques.

We developed a table-top vacuum ultraviolet (VUV) laser with 113.778 nm wavelength (10.897 eV) and demonstrated its viability as a photon source for high resolution angle-resolved photoemission spectroscopy (ARPES). This sub-nanosecond pulsed VUV laser operates at a repetition rate of 10 MHz, provides a flux of 2 × 10(12) photons/s, and enables photoemission with energy and momentum resolutions better than 2 meV and 0.012 Å(-1), respectively. Space-charge induced energy shifts and spectral broadenings can be reduced below 2 meV. The setup reaches electron momenta up to 1.2 Å(-1), granting full access to the first Brillouin zone of most materials. Control over the linear polarization, repetition rate, and photon flux of the VUV source facilitates ARPES investigations of a broad range of quantum materials, bridging the application gap between contemporary low energy laser-based ARPES and synchrotron-based ARPES. We describe the principles and operational characteristics of this source and showcase its performance for rare earth metal tritellurides, high temperature cuprate superconductors, and iron-based superconductors.

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