3Spontaneous symmetry breaking gives rise to a new quantum ground state featuring characteristic low-energy elementary excitations 3,11,14,[18][19][20][21][22] Ultrashort pulses in the terahertz (1 THz = 10 12 Hz) range have been used to trace electronic order via direct coupling to such excitations 22,23 . We demonstrate that THz pulses may simultaneously also track the crystalline order during an ultrafast phase transition.This idea is tested in a prominent reference system, 1T-TiSe 2 . Within the family of layered transition-metal dichalcogenides, this material has attracted special attention: Upon cooling below T c ≈ 200 K, it undergoes a transition into a commensurate CDW accompanied by the formation of a structural (2×2×2) superlattice 21 (Fig. 1a). In its high-temperature phase, TiSe 2 is a semimetal 20 with electron and hole pockets at the L and  points of the Brillouin zone, respectively 15,24 (Fig. 1b). The spatial reconstruction due to the CDW maps these two points on top of each other and leads to the partial opening of an electronic energy gap as well as a dramatic reduction of the density of free charge carriers 20 (Fig. 1b). Superconductivity emerges when the CDW is suppressed, e.g. by Cu intercalation 7 or pressure 25 . This discovery as well as novel chiral properties 26 have intensified the interest in the nature of the CDW in 1T-TiSe 2 . Yet, the microscopic mechanisms remain elusive 24,[27][28][29] . A first hypothesis assumes electron-phonon coupling based on a Jahn-Teller effect as the driving force 27 . A competing model suggests that the transition is purely electronically driven 24,28 . Coulomb attraction may render the system unstable against the formation of excitons between the electron-and hole-like Fermi pockets, leading to lattice deformation with the corresponding wave vector. Combinations of the two scenarios have also been proposed 29 . Time-resolved x-ray diffraction 16 and photoemission 10,15 experiments have separately tracked the dynamics of either structural or electronic orders. 4Evidence for both excitonic and phononic contributions was obtained in this way, leaving a controversial picture.Here we disentangle the two coupled components of the CDW order parameter by simultaneously tracing the ultrafast THz response of PLD-related phonons and electronic conductivity while a femtosecond pulse selectively melts the excitonic order. Our data reveal a transient phase in which the PLD persists in the absence of excitonic correlations. A quantummechanical theory 29 corroborates our conclusions.In TiSe 2 , the transition to the CDW ordered phase modifies the low-frequency optical response in three distinct ways: (i) The CDW-induced energy gap introduces a broad single- (Fig. 1d). Above T c , we observe a single TO phonon resonance at 17 meV. Below T c , back-folding of the uppermost acoustic branch from the L to the  point 21 yields an additional IR-active in-plane mode at 19 meV. The weaker peak at 22 meV likely originates from a folded optical branch at the M point 21 . 5W...

The technological demand to push the gigahertz (10(9) hertz) switching speed limit of today's magnetic memory and logic devices into the terahertz (10(12) hertz) regime underlies the entire field of spin-electronics and integrated multi-functional devices. This challenge is met by all-optical magnetic switching based on coherent spin manipulation. By analogy to femtosecond chemistry and photosynthetic dynamics--in which photoproducts of chemical and biochemical reactions can be influenced by creating suitable superpositions of molecular states--femtosecond-laser-excited coherence between electronic states can switch magnetic order by 'suddenly' breaking the delicate balance between competing phases of correlated materials: for example, manganites exhibiting colossal magneto-resistance suitable for applications. Here we show femtosecond (10(-15) seconds) photo-induced switching from antiferromagnetic to ferromagnetic ordering in Pr0.7Ca0.3MnO3, by observing the establishment (within about 120 femtoseconds) of a huge temperature-dependent magnetization with photo-excitation threshold behaviour absent in the optical reflectivity. The development of ferromagnetic correlations during the femtosecond laser pulse reveals an initial quantum coherent regime of magnetism, distinguished from the picosecond (10(-12) seconds) lattice-heating regime characterized by phase separation without threshold behaviour. Our simulations reproduce the nonlinear femtosecond spin generation and underpin fast quantum spin-flip fluctuations correlated with coherent superpositions of electronic states to initiate local ferromagnetic correlations. These results merge two fields, femtosecond magnetism in metals and band insulators, and non-equilibrium phase transitions of strongly correlated electrons, in which local interactions exceeding the kinetic energy produce a complex balance of competing orders.

We report on the kinetics of the inner ring in the exciton emission pattern. The formation time of the inner ring following the onset of the laser excitation is found to be about 30 ns. The inner ring was also found to disappear within 4 ns after the laser termination. The latter process is accompanied by a jump in the photoluminescence (PL) intensity. The spatial dependence of the PL-jump indicates that the excitons outside of the region of laser excitation, including the inner ring region, are efficiently cooled to the lattice temperature even during the laser excitation. The ring formation and disappearance are explained in terms of exciton transport and cooling.