C. W. Luo

Ultrafast photoinduced mechanical strain in epitaxial BiFeO3 thin films

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76 km/s.

Magnetic ordering anisotropy in epitaxial orthorhombic multiferroic YMnO3 films

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 obs...

Eliminate coherence spike in reflection-type pump-probe measurements.

The coherence spike of femtosecond laser pulses in the reflection-type pump-probe measurements has been systematically studied in the semiconductor (100) InP. By varying the setup of the pump-probe measuring system, i.e. the polarizations of pump and probe pulses, the incident angles of pump and probe beams, and the interval of delay time between pump and probe pulses, the dramatic changes in the strength of coherence spike could be clear observed. Furthermore, the proposed methods to remove the coherence spike from the transient reflectivity curves have been demonstrated in the time-domain measurements.

Fluctuation-enhanced conductivity in YBa2Cu3Ox (6.5 ≤ x ≤ 6.95) and Tl2Ba2Ca2Cu3O10±δ superconducting thin films

Abstract The fluctuation-induced conductivity (Δ σ ( T )) near T c of a sole YBa 2 Cu 3 O x (YBCO) film with various precisely controlled oxygen contents (6.5≤ x ≤6.95) was studied and compared with those obtained from its Tl-based counterparts. For YBCO films with x >6.7, Δσ( T ) displays a distinct 3D to 2D crossover as the temperature approaches T c . On the other hand, as T → T c , Δ σ ( T ) of both Tl-based and YBCO film with x ≤6.7 exhibits a reversed 2D to 3D crossover. It is suggestive that the coupling between the CuO 2 layers may have changed significantly with reducing oxygen.

Saturation of the free carrier absorption in ZnTe crystals

This study systematically investigates the influence of free carriers on the generation of THz in ZnTe crystals, over a wide range of pumping fluences. As the pumping fluence is increased (< 6.36 mJ/cm(2)), the concentration of free carriers gradually increases and the THz output power is saturated, as clearly demonstrated by the time delay in the THz temporal waveforms, the changes in the THz spectral weight and the red-shift in the PL spectra. For high pumping fluences (> 6.36 mJ/cm(2)), spectacularly, there is a significant quadratic increase in the THz output power when the pumping fluence is increased, as well as at low pumping fluences of < 0.58 mJ/cm(2), because of the saturation of free carriers.

Anomalous magnetic ordering in b-axis-oriented orthorhombic HoMnO3 thin films

Orthorhombic HoMnO3 films with well-aligned crystallographic orientations were deposited on LaAlO3(110) single crystal substrates by using pulsed laser deposition. The nearly perfect b-axis-oriented films provide the opportunity of investigating the orientation-dependent physical property of this material. The temperature dependent magnetization evidently displays an antiferromagnetic ordering near 42K, irrespective to the direction of applied field. Furthermore, the theoretically expected lock-in transition was clearly observed at around 30K when field was applied along the c axis and was undetectable along the a and b axes. The 30K transition was suppressed to 26K when the applied field increased from 100to500Oe.

Phonon dynamics in CuxBi2Se3 (x = 0, 0.1, 0.125) and Bi2Se2 crystals studied using femtosecond spectroscopy

Phonon dynamics in Bi, Bi2Se2, and CuxBi2Se3 (x = 0, 0.1, 0.125) single crystals was investigated using femtosecond pump-probe spectroscopy. Two damped oscillations in measured transient reflectivity changes were observed, respectively, attributed to coherent optical and acoustic phonons. The frequency shift of the phonon modes in Bi-rich Bi2Se2 crystals indicates the intercalation of the additional Bi atoms into the Bi2Se3 matrix and formation of the Bi2 layer between the quintuple layers (QLs). The deformation of QLs in CuxBi2Se3 crystals was also found. These phonon dynamics results suggest that Cu atoms in CuxBi2Se3 crystals are predominantly intercalated between pair of QLs.

Spatial symmetry of the superconducting gap ofYBa2Cu3O7−δobtained from femtosecond spectroscopy

The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa 2 Cu 3 O 7 - δ thin films are reported. This bulk-sensitive spectroscopy, combined with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By an orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally.

Marked enhancement of Néel temperature in strained YMnO3 thin films probed by femtosecond spectroscopy

In this paper, the (001)-oriented hexagonal YMnO3 (h-YMO) thin films with various nominal strain states were deposited on MgO(100), MgO(111) and YSZ(111) substrates by pulsed laser deposition. The ultrafast dynamics probed by wavelength-tunable femtosecond pump-probe spectroscopy was performed to disclose the effects of the epitaxial strain on the electronic structure and associated magnetism. Analyses based on the measured transient-reflectivity-change (ΔR/R) curves revealed that while the magnitude of the on-site Mn d-d transition energy Edd(T) changes only slightly with the variation of the lattice constant ratio c/a, a marked shift in the antiferromagnetic (AFM) Neel temperature TN was observed and the direction of shift was dependent on the type of strain. The possible mechanism for the observed strain effect on the AFM ordering is discussed.

Ultrafast optical probes of polaron dynamics in La0.7Ca0.3MnO3 thin films

In this work, we use ultrafast optical pump-optical probe spectroscopy to probe the polaron dynamics in La0.7Ca0.3MnO3 (LCMO) thin films. The temporal evolution in transient reflectivity change ΔR/R exhibits two relaxing components: a fast component with a time constant of subpicosecond and a slow component with time constant ranging from tens of picoseconds to hundreds of picoseconds. The amplitude of the fast component, though has been seldom discussed before, exhibits the similar temperature dependence with that of the resistivity and the neutron scattering intensity due to nanoscale correlated polarons. The results strongly suggest that the fast photoinduced reflectivity change may have been due to the photoexcitation and trapping process of correlated Jahn–Teller polarons in the paramagnetic and ferromagnetic phases and, thus, implies the presence of electronic inhomogeneity in LCMO manganites.