Spin-dependent conduction in metals underlies all modern magnetic memory technologies, such as giant magnetoresistance (GMR). The charge current in ferromagnetic transition metals is carried by two non-mixing populations of sp-band Fermi-level electrons: one of majority-spin and one of minority-spin. These electrons experience spin-dependent momentum scattering with localized electrons, which originate from the spin-split d-band. The direct observation of magnetotransport under such fundamental conditions, however, requires magnetotransport measurements on the same timescale as the electron momentum scattering, which takes place in the sub-100 fs regime. Using terahertz electromagnetic probes, we directly observe the magnetotransport in a metallic system under the fundamental conditions, and determine the spin-dependent densities and momentum scattering times of conduction electrons. We show that traditional measurements significantly underestimate the spin asymmetry in electron scattering, a key parameter responsible for effects such as GMR. Furthermore, we demonstrate the possibility of magnetic modulation of terahertz waves, along with heat- and contact-free GMR readout using ultrafast terahertz signals

Ultrafast photoinduced carrier dynamics in prototypical low band gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The subpicosecond and few-picosecond decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM, a strong dependency of electron momentum scattering time on electron density was found, well parametrized by the empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.

We report the synthesis of silicon nanocrystals via a one-step route, namely, femtosecond laser ablation in 1-hexene under ambient conditions. The size of these silicon nanocrystals is 2.37 ± 0.56 nm as determined by transmission electron microscopy. Fourier transform infrared spectra and X-ray photoelectron spectra indicate that the surface of the silicon nanocrystals is passivated by organic molecules and is also partially oxidized by O(2) and H(2)O dissolved in the solution. These silicon nanocrystals emit stable and bright blue photoluminescence. We suggest that the photoluminescence originates from the radiative recombination of electron-hole pairs through the oxide-related centers on the surface of the silicon nanocrystals. The decay rate of the oxide-related surface recombination can be comparable to that of the direct band gap transition. In the excitation and emission spectra, a vibrational structure with nearly constant spacings (0.18 eV) is observed. We propose that the strong electron-phonon coupling between excitons and the longitudinal optical (LO) phonons of the Si-C vibration is responsible for this vibrational structure. The fluctuations in the peak resolution, about ±0.01 eV, are ascribed to the size distribution and presence of Si-O vibrations. These silicon nanocrystals offer stable luminescence and are synthesized through a "green" and simple route. They may find important applications in many fields, such as bioimaging and environmental science.

We report on the coherent control of terahertz (THz) spin waves in a canted antiferromagnet yttrium orthoferrite, YFeO3, associated with a quasiferromagnetic (quasi-FM) spin resonance at a frequency of 0.3 THz, using a single-incident THz pulse. The spin resonance is excited impulsively by the magnetic field component of the THz pulse. The intrinsic dielectric anisotropy of YFeO3 in the THz range allows for coherent control of both the amplitude and the phase of the excited spin wave. The coherent control is based on simultaneous generation of two interfering phase-shifted spin waves whose amplitudes and relative phase, dictated by the dielectric anisotropy of the YFeO3 crystal, can be controlled by varying the polarization of the incident THz pulse with respect to the crystal axes. The spatially anisotropic decay of the THz-excited FM spin resonance in YFeO3, leading to an increasingly linear polarization of the THz oscillation at the spin resonance frequency, suggests a key role of magnon-phonon coupling in spin-wave energy dissipation. 2013 American Physical Society. We report on the coherent control of terahertz (THz) spin waves in a canted antiferromagnet yttrium orthoferrite, YFeO 3 , associated with a quasiferromagnetic (quasi-FM) spin resonance at a frequency of 0.3 THz, using a single-incident THz pulse. The spin resonance is excited impulsively by the magnetic field component of the THz pulse. The intrinsic dielectric anisotropy of YFeO 3 in the THz range allows for coherent control of both the amplitude and the phase of the excited spin wave. The coherent control is based on simultaneous generation of two interfering phase-shifted spin waves whose amplitudes and relative phase, dictated by the dielectric anisotropy of the YFeO 3 crystal, can be controlled by varying the polarization of the incident THz pulse with respect to the crystal axes. The spatially anisotropic decay of the THz-excited FM spin resonance in YFeO 3 , leading to an increasingly linear polarization of the THz oscillation at the spin resonance frequency, suggests a key role of magnon-phonon coupling in spin-wave energy dissipation.

Size dependence of spin dynamics in colloidal CdSe quantum dots (QDs) are investigated with circularly polarized pump-probe transmission spectroscopy at room temperature. The excitation energy is tuned to resonance with the lowest exciton (1S(h)1S(e)) energy of the CdSe QDs. The exciton spin dynamics of CdSe QD with the diameter of 5.2 nm shows monoexponential decay with a typical time constant of about 1-3 ps depending on the excitation energy. For the cases of CdSe QDs with smaller size (with the diameter of 4.0 and 2.4 nm), the exciton spin relaxation shows biexponential decay, a fast component with time constant of several ps and a slow one with time constant of hundreds of ps to nanosecond time scale. The fast spin relaxation arises from the bright-dark transition, i.e., J = ±1 ↔ -/+2 transition. This process is dominated by the hole spin flips, while the electron spin conserves. The slow spin relaxation is attributed to the intralevel exciton transitions (J = ±1 ↔ -/+1 transition), which is relevant to the electron spin flip. Our results indicate that the exciton spin relaxation pathways in CdSe QD are controllable by monitoring the particle size, and polarized pump-probe spectroscopy is proved to be a sensitive method to probe the exciton transition among the fine structures.

We present the magnetic dipole transition at 0.299 THz excited by magnetic component of terahertz electromagnetic pulse in an antiferromagnetic YFeO3 crystal. The impulsive magnetic field of the terahertz pulse tilts the macroscopic magnetization, causing deviation from the equilibrium position, which is manifested by a sharp absorption at the frequency of the quasiferromagnetic mode of the crystal. The rotating coherent macroscopic magnetization radiates elliptically polarized emission at the frequency of the quasiferromagnetic resonance due to the dichroic absorption in the crystal.

Temperature-induced spin reorientation transition in NdFeO3 single crystal is studied by terahertz time-domain spectroscopy in the temperature range from 50 K to 290 K. Following the resonant excitation of quasi-antiferromagnetic (AF) mode, the nature of temperature dependence of emission from AF-mode is investigated systematically in the spin reorientation temperature interval. The emission frequency is observed at 0.485 THz for both Γ4 and Γ24 phases, and it shifts abruptly to 0.456 THz (around 110 K) corresponding to Γ2 phase. The evolution of vector G is obtained from the temperature-dependent polarization changes of the AF-mode excitation. Our results demonstrate that the polarized terahertz time-domain spectroscopy is a sensitive tool to explore the dynamical spin reorientation transition in RFeO3 crystals, and the terahertz magnetic pulse shows potential application for non-thermally manipulating ultrafast spin reorientation.

Interaction of pioglitazone (PGZ) and macrophages (Mps) in renal crystal formation remains unclear. In the present study, we investigated the possible mechanisms involved with Mps of PGZ in suppressing renal crystal formation. Crystal formation in the mouse kidney was detected using polarized light optical microscopy and Pizzolato staining. Gene expression was detected by Western blot analysis, quantitative RT-PCR, immunohistochemistry, immunofluorescence, and ELISA. Mp phenotypes were identified by flow cytometric analysis. Cell apoptosis was detected with TUNEL assay, and tubular injury was detected with periodic acid-Schiff staining. Interaction of peroxisome proliferator-activated receptor (PPAR)-γ and promoter was determined by chromatin immunoprecipitation assay. Luciferase reporter assay was performed to authenticate target genes of miRNA-23 (miR-23). Recombinant adenovirus was used to elucidate the role of miR-23 in vivo. Renal crystal formation, inflammation, tubular injury, and cell apoptosis were significantly marked in glyoxylic acid-treated groups and significantly decreased in PGZ-treated groups. PGZ significantly reduced Mp infiltration and M1 Mp polarization in the kidney. In vitro, PGZ shifted crystal-stimulated M1-predominant Mps to M2-predominant Mps, which were anti-inflammatory. PPAR-γ could directly bind to one PPAR-γ regulatory element in the promoter of pre-miR-23 to promote expression of miR-23 in Mps. We identified two downstream target genes of miR-23, interferon regulatory factor 1 and Pknox1. Moreover, miR-23 decreased crystal deposition, M1 Mp polarization, and injury in the kidney. This study has proven that PGZ decreased renal calcium oxalate crystal formation and renal inflammatory injury by suppressing M1 Mp polarization through a PPAR-γ-miR-23-interferon regulatory factor 1/Pknox1 axis. PGZ is liable to be a potential therapeutic medicine for treating urolithiasis.