We use time-resolved optical reflectivity and x-ray diffraction with femtosecond resolution to study the dynamics of the structural order parameter of the charge density wave phase in TiSe2. We find that the energy density required to melt the charge density wave nonthermally is substantially lower than that required for thermal suppression and is comparable to the charge density wave condensation energy. This observation, together with the fact that the structural dynamics take place on an extremely fast time scale, supports the exciton condensation mechanism for the charge density wave in TiSe2.

We use time-resolved x-ray diffraction and magneto-optical Kerr effect to study the laser-induced antiferromagnetic to ferromagnetic phase transition in FeRh. The structural response is given by the nucleation of independent ferromagnetic domains (τ(1)~30 ps). This is significantly faster than the magnetic response (τ(2)~60 ps) given by the subsequent domain realignment. X-ray diffraction shows that the two phases coexist on short time scales and that the phase transition is limited by the speed of sound. A nucleation model describing both the structural and magnetic dynamics is presented.

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic reorientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.

We directly measure by femtosecond time-resolved x-ray diffraction the E g symmetry coherent phonon excited in bismuth by strong optical excitation. The magnitude of the E g mode observed is 0.2 pm peak-to-peak, compared against the 2.7 pm initial displacement of the fully-symmetric A 1g mode. The much smaller motion of the E g mode is a consequence of the short lifetime of the electronic states that drive the atomic motion. The experimentally measured magnitude of the E g motion allows us to rule out a previously suggested scenario for explaining the dynamics in bismuth that relies on strong coupling between these modes.

We show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in La1.885Ba0.115CuO4 (LBCO). Time-Resolved Resonant Soft X-Ray Diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz timedomain spectroscopy reveals that, for the first 2ps following photoexcitation, a new Josephson Plasma Resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of ∼0.5mJ/cm 2 . Using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order and rearrangement of the crystal structure.The interplay between superconductivity and broken electronic symmetries has emerged as a central theme in cuprate physics with increasing reports of charge order in several materials [1][2][3][4][5][6][7][8][9][10]. This body of work has lead to a growing realization that understanding competing orders may be key to developing high-T C superconductivity further. An early example of electronic order competing with superconductivity is that of charge and spin (stripe) ordering [1,11] in underdoped La 2−x Ba x CuO 4 , where holes doped into the CuO 2 planes order along domain walls, separating regions of antiphased antiferromagnetic spin ordering, below T 42K. Concomitantly, the crystal structure distorts into a low-temperature tetragonal (LTT) phase which is assumed to align the hole-rich domain walls along the Cu-O-Cu bond direction with a doping dependent stripe modulation [12][13][14]. The emergence of superconductivity from this stripe phase for 0.09 x 0.16 follows a peculiar double-dome phase boundary [15], with the superconducting transition temperature, T C , greatly suppressed for x =1/8. A first step in disentangling the hierachy of cause and effect demonstrated that stripe ordering persists in the absence of the LTT distortion [16,17]. In a similar fashion mid-IR light has also been used to induce lattice distortions in La 1.875 Ba 0.125 CuO 4 revealing that the LTT distortion and stripe ordering evolve as distinct nonequilibrium phases with different timescales [18]. More recent work, argues that pair-density waves in the CuO 2 planes completely suppress Josephson coupling between neighboring planes [19,20]. The loss of interlayer coupling is, however, predicted to be highly sensitive to topological defects resulting in a complex phase diagram with the emergence of new and novel superconducting phases with increasing defect density [21].Here, we reveal how puncturing stripe order, with charge transfer defects introduced by near-infrared optical pulses, can dynamically enhance the superconducting order at the expense of stripe order. Specific...

We investigate the structural response of charge and orbitally ordered (CO/OO) manganites to ultrafast optical excitation using optical reflectivity and x-ray diffraction as a probe. We study a La0.42Ca0.58MnO3 (LCMO) thin film and a La0.25Pr0.375Ca0.375MnO3 (LPCMO) single crystal. For both materials we observe oscillations in the optical responses that are assigned to a coherent optical phonon generated via displacive excitation. The coherent phonon disappears either when increasing the temperature above T CO/OO or when raising the excitation fluence above a certain threshold. At low excitation fluences the amplitude and lifetime of this phonon behave similarly to the order parameter of the structural phase transition.

We apply grazing-incidence femtosecond x-ray diffraction to investigate the details of the atomic motion connected with a displacively excited coherent optical phonon. We concentrate on the low frequency phonon associated with the charge and orbital order in the mixed valence manganite La0.25Pr0.375Ca0.375MnO3 for T < ∼ 210 K. We measure the response of three superlattice reflections that feature different sensitivities to the motion of the unit cell constituents. The results support the assignment to a translational mode of the Mn 4+ atoms together with the oxygen atoms connecting adjacent Mn 4+ sites.

We study the two coupled components of the laser induced phase transition in FeRh. We compare structural and magnetization dynamics measured with respectively time-resolved xray diffraction and magneto optical Kerr effect.