Quasiparticle Dynamics in FeSe Superconductors Studied by Femtosecond Spectroscopy

The ultrafast quasiparticle dynamics in FeSe single crystals were measured by using dual-color transient reflectivity measurements (ΔR/R) from 4.4 to 290 K. In general, the typical ΔR/R of FeSe includes two significant components. One is the relaxation of photoinduced quasiparticles, which has been used to estimate the electron–phonon coupling strength (λ=0.16). The other is the oscillation component due to the acoustic phonon. Moreover, the acoustic phonon’s energy estimated from the period of oscillation in ΔR/R markedly shrinks around 90 K, which is the so-called phonon softening.     

Anisotropic Photoexcited Carrier Dynamics in (100)-, (001)-, and (110)-Oriented YBCO Films by Polarized Ultrafast Optical Spectroscopy

We report, the time-resolved ultrafast optical spectroscopy results obtained from a series of well characterized (001)-, (100)- and (110)-oriented YBa₂Cu₃O_7– (YBCO) superconducting films prepared by pulsed laser deposition. The well-controlled film orientation has allowed us to probe the relaxation dynamics along various crystal orientations using respectively polarized pump and probe laser beams. The significant anisotropies in both the magnitude of the characteristic relaxation time and the temperature dependence of the photo-induced transient reflectance change indicate the nature of relaxation channel might be intrinsically different along different orientations. The implications of the results on the fundamental characteristics concerning superconducting gaps are discussed.     

Ultrafast Dynamics of BiFeO3 Thin Films Studied by Dual-Color Femtosecond Spectroscopy

The ultrafast dynamics in (100)-oriented BiFeO₃ thin films were studied from 20 K to 300 K by using the dual-color femtosecond pump-probe spectroscopy, which were performed by the 400 nm (3.1 eV) pump pulses above the energy gap of 2.67 eV and the 800 nm (1.55 eV) probe pulses below the energy gap of 2.67 eV. From the temperature-dependent transient reflectivity changes (??R/R), the anomalous changes were clearly observed below 200 K caused by magnons. Thus, the femtosecond pump-probe spectroscopy could unambiguously reveal the magnetization dynamics in BiFeO₃ with strong magnetoelectric coupling.     

Stacking-Order-Driven Optical Properties and Carrier Dynamics in ReS2

Two distinct stacking orders in ReS₂ are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm⁻¹ for AA stacking, and 20 cm⁻¹ for AB stacking, making a simple tool for determining the stacking orders in ReS₂. Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump–probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. The findings underscore the stacking-order driven optical properties and carrier dynamics of ReS₂, mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.     

Frequency-Dependent Elongational Viscosity by Nonequilibrium Molecular Dynamics

The elongational viscosity of a liquid describes the response of the liquid to simultaneous stretching and compression in various directions, subject to the restriction that the trace of the rate of the strain tensor is zero (or the density is constant). Despite the growing popularity and usefulness of nonequilibrium molecular dynamics methods in studies of the shear viscosity of simple and complex fluids, the elongational viscosity remains a relatively neglected property in computer simulation studies. This stems from some significant technical difficulties that arise when standard methods such as the constant strain rate SLLOD algorithm are applied to elongational flow. For example, if planar elongational flow with a constant elongation rate is applied in a computer simulation with periodic boundary conditions, the box size in the contracting direction quickly becomes smaller than twice the range of the potential, violating the minimum image convention. The time at which this occurs may be less than the time required for the system to reach a steady state, making it impossible to compute the steady-state elongational viscosity. This difficulty can be avoided by applying an oscillating elongational strain rate to the liquid, and computing frequency dependent elements of the stress tensor, which can then be extrapolated to zero frequency to evaluate the steady-state elongational viscosity. We have used this method to compute the elongational viscosity of a simple atomic liquid and discuss its possible application to a model polymeric liquid.     

Moisture Distribution in Partially Saturated Concrete Studied by Impedance Spectroscopy

The moisture content and its spatial distribution has a great influence on the durability properties of concrete structures. Several non-destructive techniques have been used for the determination of the total water content, but moisture distribution is difficult to determine. In this paper impedance spectroscopy is used to study the water distribution in concrete samples with controlled and homogeneously distributed moisture contents. The technique is suitable for the determination of water distribution inside the sample, using the appropriate equivalent circuits. It is shown that using the selected drying procedures there is no change in the solid phase of the samples, although the technique can only be used for the qualitative study of variations in the solid phase when samples are too thick. The results of this work show that for a wide range of concrete percentages of saturation, from full to 18 % saturation, practically all the pores keep at least a thin layer of electrolyte covering their walls, since the capacitance measurement results are practically independent of the saturation degree.     

Thermal Conductivity in Zeolites Studied by Non-equilibrium Molecular Dynamics Simulations

The thermal conductivity of zeolites is an important material property. For example, this is the case for catalysis, where chemical reactions release heat either inside zeolites or at zeolite surfaces. At zeolite surfaces, heat is released during the adsorption of guest molecules. Unfortunately, it can be difficult to determine the thermal conductivity of zeolites from experiments or from equilibrium molecular dynamics simulations. Non-equilibrium molecular dynamics (NEMD) simulation is an interesting approach to determine thermal conductivities. Inducing a thermal gradient by moving kinetic energy between different parts of the simulation box, and then studying the resulting thermal gradient, will lead to direct access to the thermal conductivity of the zeolite. In this work, we have used NEMD simulations to determine the thermal conductivity of several pure silica zeolites. The zeolites are modeled using the Demontis force field, making it possible to screen many zeolite frameworks, and study finite-size effects. In addition, we have studied the influence of adsorbed guest molecules on the thermal conductivity. The thermal conductivity of zeolites is usually in order of 0.6  $\mathrm{W}\cdot \mathrm{m}^{-1}\cdot \mathrm{K}^{-1}$ W · m ? 1 · K ? 1 to almost 4  $\mathrm{W}\cdot \mathrm{m}^{-1}\cdot \mathrm{K}^{-1}$ W · m ? 1 · K ? 1 , with large differences between different crystallographic directions. We find that the loading of guest molecules adsorbed inside the zeolite has a minor influence on the thermal conductivity, and that in general the thermal conductivity increases with increasing framework density of the zeolite.     

Optically Polarized 129Xe in NMR Spectroscopy*

¹²⁹ Xe has found widespread applications in NMR spectroscopy, but its use is often restricted by its low sensitivity and long relaxation times. Optical pumping of rubidium followed by spin exchange with xenon provides a well understood technique to enhance the ¹²⁹ XePolarization by several orders of magnitude. The exploitation of this highly spin-polarized xenon as a surface probe in NMR studies of various meterials is discussed. Applications of this technique make use not only of ¹²⁹Xe NMR detection, but also of the-high spin polarization as a magnetization reservoir for selective cross-polarization experiments to surface nuclei. A diverse variety of materials has been studied, ranging from mesoscopic structures such as semiconductor nanocrystals and porous silicon to high surface area polymers.     

Vortex Dynamics in Bi2Sr2CaCu2O8 Single Crystal with Low Density Columnar Defects Studied by Magnetic Force Microscope

We have studied vortex dynamics in Bi₂Sr₂CaCu₂O₈ single crystal with low density columnar defects by using a magnetic force microscope. Single crystal Bi₂Sr₂CaCu₂O₈ sample was irradiated by 1.3 GeV uranium ion to form artificial pinning centers along the crystalline c-axis. The irradiation dose corresponded to a matching field of 20 gauss. The radius of an individual vortex is approximately 140 nm, which is close to the penetration depth of this material. Magnetic force microscope (MFM) images show that intrinsic crystalline defects such as stacking fault dislocations are very effective pinning centers for vortices in addition to the pinning centers due to ion bombardment. By counting the number of vortex, we found that the flux trapped at each pinning center is a single flux quantum. At higher magnetic field, the vortex structure showed an Abrikosov lattice disturbed only by immobile vortices located at pinning centers. When increasing or decreasing the external magnetic field, the spatial distribution of vortices showed a Bean model like behavior.     

Anisotropy of Iodine Intercalated Bi2212 Single Crystals by Magnetic Torque

The magnetic torque of iodine intercalated Bi₂Sr₂CaCu₂O₈₊ (I-Bi2212) single crystals has been measured under magnetic fields up to 8 kOe in the temperatures from 4.2 to 100 K. The anisotropy parameter for the 1-Bi2212 determined by London model is smaller than that for the pure Bi₂Sr₂CaCu₂O₈₊ (Bi2212). A sharp peak C1 due to the intrinsic pinning and an extra peak C2 due to the dimensional crossover of the pancake vortex are observed in the magnetic field direction dependence of the torque. The reduced temperature at which the peak C1 appears with decreasing temperature is T/T _c=0.82, which is lower than T/T _c=0.97 for the pure Bi2212. The dimensional crossover field for the I-Bi2212 obtained from the peak C2 is larger than that for the pure Bi2212. According to these results, it is concluded that the iodine intercalation causes the enhancement of interlayer coupling.     

Carrier Relaxation and Multiplication in Bi Doped Graphene

By introducing different contents of Bi adatoms to the surface of monolayer graphene, the carrier concentration and their dynamics have been effectively modulated as probed directly by the time- and angle-resolved photoemission spectroscopy technique. The Bi adatoms are found to assist acoustic phonon scattering events mediated by supercollisions as the disorder effectively relaxes the momentum conservation constraint. A reduced carrier multiplication has been observed, which is related to the shrinking Fermi sea for scattering, as confirmed by time-dependent density functional theory simulation. This work gives insight into hot carrier dynamics in graphene, which is crucial for promoting the application of photoelectric devices.     

Spin Switching Mechanism Stimulated by Linearly Polarized Femtosecond Optical Pump

Journal of Superconductivity and Novel Magnetism - We propose the mechanism for the optical magnetization reversal of metallic ferrimagnets near the magnetic compensation temperature in a zero...     

Lanthanide Substitution Effects in Electron-Doped High-T c Superconductors Studied by Angle-Resolved Photoemission Spectroscopy

We have performed an angle-resolved photoemission study of the electron-doped high-T _c superconductors (HTSCs) Sm_1.85Ce_0.15CuO₄ (SCCO) and Eu_1.85Ce_0.15CuO₄ (ECCO). Around the nodal point , we observed an energy gap at the Fermi level (E _F) for both samples, resulting from the strong effects of antiferromagnetism. The magnitude of this gap in ECCO is larger than that in SCCO, implying that the effects of antiferromagnetism in ECCO are stronger than that in SCCO. Also, we quantitatively confirmed the effects of antiferromagnetism by the band dispersions to tight-binding calculation.     

Unifying Measurements in Quantitative Element Determination by Optical Spectroscopy

Unifying measurements is considered in determining trace levels of elements. Optical atomic-spectral methods are examined for use in unified apparatus together with methods for unifying that measurement area.     

Nanobubble Dynamics in Aqueous Surfactant Solutions Studied by Liquid-Phase Transmission Electron Microscopy

Nanobubbles have attracted considerable attention in various industrial applications due to their excep-tionally long lifetime and their potential as carriers a...     

Tuning Pb Content in High-T c Bi2Sr2CaCu2O y Superconductors Studied By X-ray Absorption Near-Edge Structure Spectroscopy

The variation of hole concentration within the CuO₂ planes via tuning Pb content in (Bi_2–x Pb _x )Sr₂CaCu₂O _y (0.2x0.6) has been investigated by X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation. The profile of XANES spectra indicated that Pb can be incorporated into Bi-2212 phase under nitrogen atmosphere around x=0.6. However, after annealing the nitrogen-sintered samples in oxygen, Pb solubility is found to be decreased only up to x=0.2 in (Bi_2–x Pb _x )Sr₂CaCu₂O _y , showing that compounds with high Pb content are more stable under oxygen-free atmosphere.     

Femtosecond Pump-Probe Polarization Dependent Investigation of Relaxation Dynamics in YBa2Cu4O8

Femtosecond pump probe experiments are reported on quasiparticle relaxation and recombination in YBa₂Cu₄O₈ as a function of temperature and polarization. We compare our results with the data obtained on YBa₂Cu₃O_7?δ and show that similar two-component relaxation is present in both cases. A strong polarization anisotropy of the picosecond response is observed below T _c and interpreted with the aid of a simple model which considers the anisotropy of the probe transition matrix elements.     

Inside Front Cover: Mobility and Dynamics of Charge Carriers in Rubrene Single Crystals Studied by Flash‐Photolysis Microwave Conductivity and Optical Spectroscopy (Adv. Mater. 5/2008)

Electrodeless photoconductivity measurements of a rubrene single crystal are performed by microwave conductivity and transient optical spectroscopy, demonstrating anisotropy and ambipolarity. From these comprehensive studies, Akinori Saeki, Seiichi Tagawa, and co‐workers discuss on p. 920 the optoelectronic properties of rubrene single crystal in terms of charge recombination of holes and electrons, exciton–exciton annihilation, quantum efficiency of charge carrier generation, triplet contribution, and the extinction coefficients of ionic species accessed by pulse radiolysis.