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.

No abstract

We present a theoretical formulation of the coherent ultrafast nonlinear optical response of a strongly correlated system and discuss an example where the Coulomb correlations dominate. We separate out the correlated contributions to the third-order nonlinear polarization, and identify non-Markovian dephasing effects coming from the non-instantaneous interactions and propagation in time of the collective excitations of the many-body system. We discuss the signatures, in the time and frequency dependence of the four-wave-mixing (FWM) spectrum, of the inter-Landau level magnetoplasmon (MP) excitations of the two-dimensional electron gas (2DEG) in a perpendicular magnetic field. We predict a resonant enhancement of the lowest Landau level (LL) FWM signal, a strong non-Markovian dephasing of the next LL magnetoexciton (X), a symmetric FWM temporal profile, and strong oscillations as function of time delay, of quantum kinetic origin. We show that the correlation effects can be controlled experimentally by tuning the central frequency of the optical excitation between the two lowest LLs.

We use femtosecond optical pulses to induce, control and monitor magnetization precession in ferromagnetic Ga 0.965 Mn 0.035 As. At temperatures below ~40 K we observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ~T C /2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent on ambient temperature. We provide a physical interpretation of the observed light-induced collective Mn-spin relaxation and precession.The magnetic semiconductor GaMnAs has received considerable attention in recent years, largely because of its potential role in the development of spin-based devices 1,2 . In this itinerant ferromagnet, the collective magnetic order arises from the interaction between mobile valence band holes and localized Mn spins. Therefore, the magnetic properties are sensitive to external excitations that change the carrier density and distribution. Ultrafast pump-probe magneto-optical spectroscopy is an ideal technique for controlling and characterizing the magnetization dynamics in the magnetic materials, and has been applied to the GaMnAs system by several groups 3,4 .Although optically induced precessional motion of magnetization has been studied in 2 other magnetic systems 5 , magnetization precession in ferromagnetic GaMnAs has been observed only recently 4 and has yet to be adequately understood.In this paper, we report comprehensive temperature and photoexcitation intens ity dependent measurements of photoinduced magnetization precession in Ga 1-x Mn x As (x = 0.035) with no externally imposed magnetic field. By comparing and contrasting the temperature and intensity dependence of the precession frequency, damping, and amplitude, we identify the importance of light-induced nonlinear effects and obtain new information on the relevant physical mechanisms. Our measurements of the photoinduced magnetization show coherent oscillations, arising from the precession of collective Mn spins. Amplitude of the magnetization precession saturates above certain pump intensity is a strong indication that direction of the magnetic easy axis remains unchanged at temperatures above about half the Curie temperature (T C ). The precession is explained by invoking an ultrafast change in the orientation of the in-plane easy axis, due to an impulsive change in the magnetic anisotropy induced by the laser pulse. We also find that the Gilbert damping coefficient, which characterizes the Mn-spin relaxation, depends only weakly on the ambient temperature but changes dramatically with pump intensity. Our results suggest a general model for photoinduced precessional motion and relaxation of magnetization in the GaMnAs system under compressive strain.Time-resolved magneto-optical Kerr effect (MOKE) measurements were performed on a 300 nm thick f...

Topology-protected surface transport of ultimate thinness in three-dimensional topological insulators (TIs) is breaking new ground in quantum science and technology. Yet a challenge remains on how to disentangle and selectively control surface helical spin transport from the bulk contribution. Here we use the mid-infrared and terahertz (THz) photoexcitation of exclusive intraband transitions to enable ultrafast manipulation of surface THz conductivity in Bi2Se3. The unique, transient electronic state is characterized by frequency-dependent carrier relaxations that directly distinguish the faster surface channel than the bulk with no complication from interband excitations or need for reduced bulk doping. We determine the topological enhancement ratio between bulk and surface scattering rates, i.e., γBS/γSS ~3.80 in equilibrium. The ultra-broadband, wavelength-selective pumping may be applied to emerging topological semimetals for separation and control of the protected transport connected with the Weyl nodes from other bulk bands.

We study the effect of Coulomb correlations on the ultrafast optical dynamics of small metal particles. We demonstrate that a surface-induced dynamical screening of the electron-electron interactions leads to quasiparticle scattering with collective surface excitations. In noble-metal nanoparticles, it results in an interband resonant scattering of d-holes with surface plasmons. We show that this size-dependent many-body effect manifests itself in the differential absorption dynamics for frequencies close to the surface plasmon resonance. In particular, our self-consistent calculations reveal a strong frequency dependence of the relaxation, in agreement with recent femtosecond pump-probe experiments.Comment: 8 pages + 4 figures, final version accepted to PR

Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behaviour these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe1-xCox)2As2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.