Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (∆R/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ∆R/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe. , tremendous experimental and theoretical effort has been devoted to exploring their characteristics. These Fe-based pnictide compounds exhibit a very interesting phase diagram, with antiferromagnetism (or spin-density wave) at low doping and superconductivity at intermediate doping [2]. The simultaneous presence of magnetism and superconductivity in the phase diagram implies that magnetism plays an important role in the superconductivity mechanism. The existence of precursor superconductivity above T c which competes with the spin-density wave order [3], and a pseudogaplike feature with onset around 200 K [4] were observed on underdoped (Ba, K)Fe 2 As 2 and nearly optimally doped SmFeAsO 0.8 F 0.2 , respectively. Additionally, a coherent lattice oscillation was also found in Co-doped BaFe 2 As 2 using time-resolved pump-probe reflectivity with 40 fs time resolution [5]. Among various FeSCs, the iron chalcogenide FeSe [6] stands out due to its structure simplicity, which consists of iron-chalcogenide layers stacking one by another with the same Fe +2 charge state as the iron pnictides. This so-called "11" system is so simple that it could be the key structure to understanding the origin of high-T c superconductivity [7]. There has been considerable concern over the interplay between electronic structure, phonons, magnetism, and superconductivity in 11-type FeSe. Therefore, further studies of their quasiparticle dynamics are indispensable to understanding the high-T c mechanism in FeSCs. Here we report the first time-resolved femtosecond spectroscopy study of FeSe single crystals to elucidate the electronic structure and the quasiparticle (QP) dynamics.In this study, FeSe single crystals were grown in evacuated quartz ampoules using a KCl/AlCl 3 flux [8]. The crystalline structure of the samples was examined by xray diffraction. The superconducting transition temperature T c of the FeSe single crystal was determined to be 8.8 K by the middle point of the resistive transition. The femtosecond spectroscopy measurement was performed using a dual-color pump-probe system (for light source, the repetition rate: 5.2 MHz, the wavelength: 800 nm, and the pulse duration: 100 fs) and an avalanche photodetector with the standard lock-in te...

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Orthorhombic YMnO3 thin films with (200), (020), and (001) orientations were, respectively, obtained by pulsed laser deposition on SrTiO3(110), LaAlO3(110), and SrTiO3(001) substrates. The results demonstrate that the strain between film and substrate can serve as an alternative in transforming the thermodynamically stable hexagonal YMnO3 into the orthorhombic phase, which previously could be obtained with high-pressure high temperature syntheses and epitaxy-stabilized thin film processes. More importantly, these films allow us, for the first time, to unambiguously disclose the intrinsic magnetic property along different crystallographic orientations. Our results show that, although the antiferromagnetic (AFM) ordering remains the same, there is an additional spin reordering transition which is very much dependent on the crystallographic orientation along which the measuring field was applied and on the in-plane crystallographic alignment of the films. Detailed analyses indicate that the origin of the observed second reordering may be due to some strain-induced canted AFM state instead of the incommensurate lock-in transitions conceived previously.

Single phase TiO 2 thin films, of either rutile or anatase structure, have been prepared on SrTiO 3 ͑STO͒͑100͒ substrates by in situ pulsed laser deposition ͑PLD͒. Thermodynamically unfavorable, for films deposited on STO͑100͒ substrate directly, pure anatase TiO 2 ͑00l͒ films were formed even when a rutile TiO 2 ͑110͒ substrate was used as a target. On the other hand, pure rutile TiO 2 ͑110͒ films were obtained by oxidizing PLD TiN films in-situ at temperatures higher than 700°C. The optimized deposition conditions for preparing TiN and TiO 2 films were reported. The crystalline structure, surface morphology, and electronic structure of these films were examined. A mechanism of the process of film formation is also proposed.