Exposing a Hidden State: Shining intense laser light on a material can temporarily alter its properties. The effect usually subsides after a few picoseconds, unless the system is trapped in a metastable state, in which case the transient period may last as long as microseconds. Stojchevska et al. (p. 177 ) observed that, following exposure to a 35-femtosecond laser pulse, the layered dichalcogenide 1T -TaS ₂ entered a stable “hidden” state not present in the equilibrium phase diagram and stayed there indefinitely. The switch to the hidden state could be reversed by heat or a train of laser pulses. Because the switch alters the sample's conducting properties, the phenomenon might also lead to practical applications.

In the last two decades non-equilibrium spectroscopies have evolved from avant-garde studies to crucial tools for expanding our understanding of the physics of strongly correlated materials. The possibility of obtaining simultaneously spectroscopic and temporal information has led to insights that are complementary to (and in several cases beyond) those attainable by studying the matter at equilibrium. From this perspective, multiple phase transitions and new orders arising from competing interactions are benchmark examples where the interplay among electrons, lattice and spin dynamics can be disentangled because of the different timescales that characterize the recovery of the initial ground state. For example, the nature of the broken-symmetry phases and of the bosonic excitations that mediate the electronic interactions, eventually leading to superconductivity or other exotic states, can be revealed by observing the sub-picosecond dynamics of impulsively excited states. Furthermore, recent experimental and theoretical developments have made it possible to monitor the time-evolution of both the single-particle and collective excitations under extreme conditions, such as those arising from strong and selective photo-stimulation. These developments are opening the way for new, non-equilibrium phenomena that can eventually be induced and manipulated by short laser pulses. Here, we review the most recent achievements in the experimental and theoretical studies of the non-equilibrium electronic, optical, structural and magnetic properties of correlated materials. The focus will be mainly on the prototypical case of correlated oxides that exhibit unconventional superconductivity or other exotic phases. The discussion will also extend to other topical systems, such as iron-based and organic superconductors, and charge-transfer insulators. With this review, the dramatically growing demand for novel experimental tools and theoretical methods, models and concepts, will clearly emerge. In particular, the necessity of extending the actual experimental capabilities and the numerical and analytic tools to microscopically treat the non-equilibrium phenomena beyond the simple phenomenological approaches represents one of the most challenging new frontiers in physics.

We report on the synthesis, structure, and self-assembly of single-wall subnanometer-diameter molybdenum disulfide tubes. The nanotubes are up to hundreds of micrometers long and display diverse self-assembly properties on different length scales, ranging from twisted bundles to regularly shaped “furry” forms. The bundles, which contain interstitial iodine, can be readily disassembled into individual molybdenum disulfide nanotubes. The synthesis was performed using a novel type of catalyzed transport reaction including C ₆₀ as a growth promoter.

Photoexcited quasiparticle relaxation dynamics are investigated in a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductor as a function of doping δ and temperature $T$ using ultrafast time-resolved optical spectroscopy. A model calculation is presented that describes the temperature dependence of the photoinduced quasiparticle population $n_{\mathrm{pe}},$ photoinduced transmission $\Delta \mathcal{T}/\mathcal{T},$ and relaxation time τ for three different superconducting gaps: (i) a temperature-dependent collective gap such that $\mathit{\Delta }\mathrm{}\left(T\right)\mathrm{}\to 0$ as $\overrightarrow{T}{T}_c,$ (ii) a temperature-independent gap, which might arise for the case of a superconductor with preformed pairs, and (iii) an anisotropic (e.g., $d$-wave) gap with nodes. Comparison of the theory with data of photoinduced transmission $|\Delta \mathcal{T}/\mathcal{T}|,$ reflection $|\Delta \mathcal{R}/\mathcal{R}|,$ and quasiparticle recombination time τ in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ over a very wide range of doping $(0.1<\delta <0.48)$ is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping $(\delta <0.1)$ and a temperature-independent isotropic gap in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}(0.15<\delta <0.48).\mathrm{}$ A pure $d$-wave gap was found to be inconsistent with the data.

The study of the temporal evolution of systems undergoing symmetry breaking phase transitions—whether it is in condensed-matter physics, cosmology or finance^1,2,3,4,5,6—is difficult because they are hard to repeat, or they occur very rapidly. Here we report a high-time-resolution study of the evolution of both bosonic and fermionic excitations through an electronic charge-ordering symmetry breaking phase transition. Periodically quenching our system with femtosecond optical pulses, we subsequently detect hitherto-unrecorded coherent aperiodic undulations of the order parameter, critical slowing down of the collective mode and evolution of the particle–hole gap as the system evolves through the transition. Modelling on the basis of Ginzburg–Landau theory is used to reproduce the observations without free parameters. Of particular interest is the observation of spectrotemporal distortions arising from spontaneous annihilation of topological defects, analogous to those discussed by the Kibble–Zurek cosmological model^2,3.

The gap structure in ${\mathrm{Y}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals is investigated by real-time measurements of the quasiparticle (QP) relaxation dynamics with femtosecond optical spectroscopy. We find evidence for the coexistence of two distinct gaps. One is a $T$-independent pseudogap ${\Delta }_p$ and the other is a $T$-dependent gap ${\Delta }_c\left(T\right)$ with a BCS-like $T$ dependence. From QP relaxation time measurements above $T_c$ we ascertain that fluctuations associated with the latter are consistent with time-dependent Ginzburg-Landau theory and are distinct from the pseudogap whose presence is apparent well above $T_c$ for all $x$.