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Interesting emergent behavior in quantum materials arises when the interaction of electrons with the lattice leads to partial localization and ordering of charge at low temperatures. The triangular lattice of some transition metal dichalcogenides additionally presents an interesting case, where spin order is frustrated, leading to an additional complex interplay of interactions involving spin, charge and lattice degrees of freedom. Here we present a study of local symmetry breaking of the lattice structure in the layered dichalcogenide material 1T-TaS$_{2}$ using x-ray pair-distribution function measurements. Remarkably, we observe symmetry-breaking polaronic distortions of the lattice structure around individual localized electrons at temperatures well above any of the known long-range ordered phases. These characteristic polaronic signatures remain on cooling through the spin and charge ordered states, eventually revealing a new transition near 50 K to a state displaying partially restored symmetry and significant inter-layer dimerization. The order parameter associated with the observed local atomic displacements and symmetry-allowed polaron spin structure in the ground state suggests that charge ordering is driven by the crystallization of polarons, rather than conventional Fermi surface nesting. Symmetry analysis shows that the distorted structure is consistent with a breakup of the QSL phase at low temperature, concurrent with the disappearance of domains in the charge order., Comment: 16 pages, 4 figures

Ultrafast dynamics across the photoinduced three-dimensional Peierls-like insulator-metal (IM) transition in CuIr$_{2}$S$_{4}$ was investigated by means of the all-optical ultrafast multi-pulse time-resolved spectroscopy. The structural coherence of the low-$T$ broken symmetry state is strongly suppressed on a sub-picosecond timescale above a threshold excitation fluence of $F_{\mathrm{c}}\approx3$ mJ/cm$^{2}$ (at 1.55-eV photon energy) resulting in a structurally inhomogeneous transient state which persists for several-tens of picoseconds before reverting to the original low-$T$ state. The electronic order shows a transient gap filling at a significantly lower fluence threshold of $\sim0.6$~mJ/cm$^{2}$. The data suggest that the photoinduced-transition structural dynamics to the high-$T$ metallic phase is governed by first-order-transition nucleation kinetics that prevents the complete structural transition into the high-$T$ phase even at excitation fluences significantly larger than $F_{\mathrm{c}}$. In contrast, the dynamically-decoupled electronic order is suppressed rather independently due to a photoinduced Mott transition., By mistake resubmitted as 2104.03698

Ultrafast transient reflectivity across the unusual three-dimensional Peierls-like insulator-metal (IM) transition in CuIr_{2}S_{4} was measured as a function of temperature. The low-temperature insulating-phase transient response is dominated by broken-symmetry-induced coherent lattice oscillations that abruptly vanish at the IM transition. The coherent mode spectra are consistent with Raman spectra reported in literature. The origin of the broken-symmetry-induced is also briefly discussed.

The femtosecond transinet optical spectroscopy is employed to study the relaxation dynamics of the equilibrium and hidden metastable charge-density-wave states in single crystals of 1$T$-TaS$_{2-x}$Se$_{x}$ as a function of the Se doping $x$. Similarly to pristine 1$T$-TaS$_{2}$, the transition to a hidden phase is observed at low temperature after a quench with a single 50~fs laser pulse, in the commensurate Mott phase up to $x=0.6$. The photo-induced hidden-phase formation is accompanied by a notable change in the coherent phonon spectra, and particularly the collective amplitude mode. While the stability of the hidden phase with increased temperatures is only slightly dependent of the Se content the hidden-phase creation-treshold fluence strongly increases with the Se content from 1 to $\sim4$ mJ/cm$^{2}$.

All optical femtosecond relaxation dynamics in a single crystal of mono-phosphate tungsten bronze (PO$_{2}$)$_{4}$(WO$_{3}$)$_{2m}$ with alternate stacking m=6 of WO$_{3}$ layers was studied through the three consequent charge density wave (CDW) transitions. Several transient coherent collective modes associated to the different CDW transitions were observed and analyzed in the framework of the time dependent Ginzburg-Landau theory. Remarkably, the interference of the modes leads to an apparent rectification effect in the transient reflectivity response. A saturation of the coherent-mode amplitudes with increasing pump fluence well below the CDWs destruction threshold fluence indicates a decoupling of the electronic and lattice parts of the order parameter under strong optical drive.

Hidden states of matter with novel and unusual properties may be created if a system out of equilibrium can be induced to follow a trajectory to a state which is inaccessible or does not even exist under normal equilibrium conditions. Here we report on the discovery of a hidden (H) topologically protected electronic state in a layered dichalcogenide 1T-TaS2 crystal reached as a result of a quench caused by a single 35 fs laser pulse. The properties of the H state are markedly different from any other state of the system: it exhibits a large drop of electrical resistance, strongly modified single particle and collective mode spectra and a marked change of optical reflectivity. Particularly important and unusual, the H state is stable for an arbitrarily long time until a laser pulse, electrical current or thermal erase procedure is applied, causing it to revert to the thermodynamic ground state. Major observed events can be reproduced by a kinetic model describing the conversion of photo excited electrons and holes into an electronically ordered crystal, thus converting a Mott insulator to a conducting H state. Its long-time stability follows from the topological protection of the number of periods in the electronic crystal., 21 pages and 5 figures, separate supplementary material

The anharmonic response of charge-density wave (CDW) order to strong laser-pulse perturbations in 1T-TaS_2 and TbTe_3 is investigated by means of a multiple-pump-pulse time-resolved femtosecond optical spectroscopy. We observe remarkable anharmonic effects hitherto undetected in the systems exhibiting collective charge ordering. The efficiency for additional excitation of the amplitude mode by a laser pulse becomes periodically modulated after the mode is strongly excited into a coherently oscillating state. A similar effect is observed also for some other phonons, where the cross-modulation at the amplitude-mode frequency indicates anharmonic interaction of those phonons with the amplitude mode. By analyzing the observed phenomena in the framework of time-dependent Ginzburg-Landau theory we attribute the effects to the anharmonicity of the mode potentials inherent to the broken symmetry state of the CDW systems.

We investigate the quasiparticle relaxation and low-energy electronic structure in undoped SrFe_2As_2 exhibiting spin-density wave (SDW) ordering using optical pump-probe femtosecond spectroscopy. A remarkable critical slowing down of the quasiparticle relaxation dynamics at the SDW transition temperature T_SDW = 200K is observed. From temperature dependence of the transient reflectivity amplitude we determine the SDW-state charge gap magnitude, 2Delta_SDW/k_BT_SDW=7.2+-1. The second moment of the Eliashberg function, lambda=110+-10meV^2, determined from the relaxation time above T_SDW, is similar to SmFeAsO and BaFe_2As_2 indicating a rather small electron phonon coupling constant unless the electron-phonon spectral function (alpha^2F(omega) is strongly enhanced in the low-energy phonon region.

The 2D lattice gas model with competing short and long range interactions is appliedused for calculation of the incoherent charge transport in the classical strongly-correlated charge segregated polaronic state. We show, by means of Monte-Carlo simulations, that at high temperature the transport is dominated by hopping of the dissociated correlated polarons, where with thetheir mobility is inversely proportional to the temperature. At the temperatures below the clustering transition temperature the bipolaron transport becomes dominant. The energy barrier for the bipolaron hopping is determined by the Coulomb effects and is found to be lower than the barrier for the single-polaron hopping. This leads to drastically different temperature dependencies of mobilities for polarons and bipolarons at low temperatures.

Temperature and fluence dependence of the transient photoinduced reflectivity and the magnetooptical Kerr angle was measured in two Pr0.6Ca0.4MnO3 thin films subject to tensile and compressive substrate-induced strain. A photoinduced transient ferromagnetic metallic (TFM) phase is found to form below ~60K and ~40K in the substrate-strained and substrate-compressed film, respectively. From the hysteresis loops a difference in the TFM cluster sizes and amount of photomodulation is observed at low temperatures and low excitation fluences in the films with different strain. Surprisingly, the characteristic timescale for the TFM phase photomodulation is virtually strain independent. At high excitation fluences, the cluster sizes and amount of photomodulation are independent on the substrate-induced strain.

Magnetic properties of high purity stoichiometric La$_2$CuO$_4$ nanoparticles are systematically investigated as a function of particle size. Ferromagnetic single-domain spin clusters are shown to spontaneously form at the surface of fine grains as well as paramagnetic defects. Hysteresis loops and thermomagnetic irreversibility are observed in a wide temperature range $5 - 350$ K with the remnant moment and coercivity gradually decreasing with increasing temperature. Possible origins of the spontaneous surface ferromagnetic clusters and the relation of our data to the appearance of unusual magnetic phenomena and phase separation of doped cuprates are discussed., 10 pages, 15 figures

Self-organization of charged particles on a 2D lattice, subject to an anisotropic Jahn-Teller-type interaction and 3D Coulomb repulsion is investigated. In the mean-field approximation without Coulomb interaction, the system displays a phase transition of first order. In the presence of the Coulomb repulsion the global phase separation becomes unfavorable and the system shows a mesoscopic phase separation, where the size of the charged regions is determined by the competition between the ordering energy and the Coulomb energy. The phase diagram of the system as a function of particle density and temperature is obtained by systematic Monte Carlo simulations. With decreasing temperature a crossover from a disordered state to a state composed from mesoscopic charged clusters is observed. In the phase separated state charged clusters with even number of particles are more stable than those with odd number of particles in a large range of particle densities. With increasing particle density at low temperatures a series of crossovers between states with different cluster sizes is observed. Above half filling in addition to the low temperature clustering another higher temperature scale, which corresponds to orbital ordering of particles, appears. We suggest that the diverse functional behaviour - including superconductivity - observed in transition metal oxides can be thought to arise from the self-organization of this type., 9 pages, 7 figures

Time-resolved optical experiments can give unique information on the characteristic length scales of dynamic charge inhomogeneity on femtosecond timescales. From data on the effective quasiparticle relaxation time r in La2-xSrxCuO4 and Nd2-xCexCuO4 we derive the temperature- and doping- dependence of the intrinsic phonon escape length le, which, under certain circumstances, can be shown to be a direct measure of charge inhomogeneity. Remarkably, a common feature of both p and n-type cuprates - which has important consequences for superconductivity - is that as T  Tc from above, the escape length approaches the zero-temperature superconducting coherence length, le  s(0). In close vicinity of Tc, le appears to follow the critical behaviour of the Ginsburg-Landau coherence length, GL(T). In the normal state le is found to be in excellent agreement with the mean free path lm obtained from the resistivity data. The data on le also agree well with the data on structural coherence lengths ls obtained from neutron scattering experiments, implying the existence of complex intrinsic textures on different length scales which may have a profound effect on the functional properties of these materials., Comment: To appear in Physical Review Letters

Femtosecond pump probe experiments are reported on quasiparticle relaxation and recombination in YBa2Cu4O8 as a function of temperature and polarization. The data show a 2-component relaxation similar to YBa2Cu3O7-\delta, one component being assocated with the superconducting transition, and the other with the pseudogap below T* The relaxation time tau_p associated with the pseudogap is found to be T-independent, while the relaxation time tau_g of the component obseved only below Tc exhibits a clear divergence near Tc. A strong polarisation anisotropy of the picosecond transient is observed below Tc which is attributed to the anisotropy of the probe transition matrix elements., Comment: 4 pages, 4 figures

Measurements of the temperature dependence of the quasiparticle (QP) dynamics in Hg1223 with femtosecond time-resolved optical spectroscopy are reported. From the temperature dependence of the amplitude of the photoinduced reflection, the existence of two gaps is deduced, one temperature dependent Dc that closes at Tc, and another temperature independent ''pseudogap'' Dp. The zero-temperature magnitudes of the two gaps are Dc/kTc = 6 +/- 0.5 and Dp/kTc = 6.4 +/- 0.5 respectively. The quasiparticle lifetime is found to exhibit a divergence as T -> Tc from below, which is attributed to the existence of a superconducting gap which closes at Tc. Above Tc the relaxation time is longer than expected for metallic relaxation, which is attributed to the presence of the ''pseudogap''. The QP relaxation time is found to increase significantly at low temperatures. This behavior is explained assuming that at low temperatures the relaxation of photoexcited quasiparticles is governed by a bi-particle recombination process., accepted for publication in Phys.Rev.B

The unusual slow carrier relaxation dynamics - observed in femtosecond pump-probe experiments on high-temperature superconductors and recently also in a charge-density-wave system - is analyzed in terms of a model for relaxation of carriers in intra-gap states. The data on YBa_2Cu_3O_7-delta near optimum doping and K_0.3$MoO_3 are found to be described very well with the model using a BCS-like gap which closes at T_c. From the analysis of the data we conclude that a significant intra-gap density of localized states exists in these materials, which can be clearly distinguished from quasiparticle states by the time-resolved optical experiments because of the different time- and temperature-dependences of the photoinduced transmission or reflection. Localized charges are suggested to be the most likely origin of the intra-gap states, while the similarity of the response in the two materials appear to exclude spin and vortex excitations., 3 postscript figures are included, will be published in Phys. Rev. B

A systamatic quantitative comparison of ''pseudogap'' values obtained from the analysis of charge and spin excitation spectroscopies in underdoped YBa2Cu3O7-x using a temperature-independent gap shows two distinct excitations, one visible in spin-flip spectroscopies like NMR and spin polarized neutron scattering, and the other in charge excitation spectoscopies like single particle tunneling and time-resolved quasiparticle relaxation. Both appear to decrease with doping x approximately as 1/x and are T-independent, existing above and below Tc. We suggest that the charge excitation can be attributed to a pair-breaking local gap, while the spin excitation can be explained by an intra-gap local triplet state., 4 pages and 3 postscript figures, to be published in Phys. Rev.B (Rapid. Comm), Figure 1 is corrected

Time-resolved optical measurements give information on the quasiparticle relaxation dynamics in YBCO, from which the evolution of the gap with doping and temperature can be systematically deduced. In this paper these optical charge-channel `pseudogap' data are compared with the `pseudogap' obtained from the NMR Knight shift Ks, spin polarized neutron scattering (SPNS) and single particle tunneling measurements. A simple energy level diagram is proposed to explain the different `gap' magnitudes observed by different spectroscopies in Y1-xCaxBa2Cu3O7-y, whereby the spin gap Delta_s in NMR and SPNS corresponds to a triplet local pair state, while Delta_p in the charge excitation spectrum corresponds to the pair dissociation energy. At optimum doping and in the overdoped state, an additional T-dependent gap becomes evident, which closes at T_c, suggesting a cross-over to a more conventional BCS-like superconductivity scenario., Comment: 9 pages, 4 figures. Presented in HTS99, Miami, January 99

Ultrafast transient reflectivity changes caused by collective and single particle excitations in the quasi one-dimensional charge-density wave (CDW) semiconductor K0.3MoO3 are investigated with optical pump-probe spectroscopy. The temperature-dependence of non-equilibrium single particle excitations across the CDW gap and their recombination dynamics are reported for the first time. In addition, amplitude mode reflectivity oscillations are observed in real time. A T-dependent overdamped response is also observed which is attributed to relaxation of the phason mode., Comment: 5 pages, 4 figures, accepted for publication in Phys.Rev.Lett

We report new time-resolved data of quasiparticle relaxation and Cooper pair recombination dynamics in YBa_2Cu_3O_{7-��}, measured as a function of temperature and doping ��using femtosecond optical spectroscopy. The data show the existence of a normal state pseudo-gap for in-plane charge excitations below T* and an unusual divergence of the quasiparticle relaxation time in the superconducting state ��_s with ��towards optimum doping. In the underdoped state, no change in the amplitude of the induced transmission (which is proportional to the DOS at E_F), or relaxation time ��_s is observed at T_c. From the sum rule, both observations signify that no gap opens at T_c in underdoped YBCO for ��>0.15. T_c in this case only signifies the onset of phase coherence. The presented data thus suggest pair formation with an associated redistribution of the DOS starting at T* and the establishment of phase coherence at T_c consistent with Bose-Einstein condensation. In the optimally doped material, ��~ 0.1, on the other hand, both a divergence in lifetime and a change of the DOS occur at T_c, signifying the opening of a gap and the occurence of pairing takes place simultaneously., 5 pages, 4 figures