Structure and properties of the one-dimensional cobalt oxide CaCo2O4

We have studied crystal structure and transport properties of the quasi one-dimensional cobalt oxide CaCo₂O₄. The CaCo₂O₄ phase crystallizes in calcium-ferrite type structure, which consists of a corner- and edge-shared CoO₆ octahedron network including one-dimensional double chains. Large thermoelectric power (S  150 μV/K at 390 K) with metallic temperature dependence of S, moderate resistivity (ρ  2.9 × 10⁻¹ Ω cm at 390 K) with carrier localization at low temperature, and normal thermal conductivity (κ  6.3 W/Km at 390 K) were observed. The phonon mean-free path was calculated from the observed data, as a function of temperature. The long phonon mean-free path (l  24 Å at 300 K) implies that the thermal conductivity could be suppressed by impurity scattering of phonons with partial element substitution.     

Neutron polarization analysis on the multiferroic TbMn2O5

Polycrystalline TbMn₂O₅ was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at 45 K. A broad peak below 6 K in the heat capacity measurements corresponds to the crossover transition of Tb³⁺ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn₂O₅ with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by using neutron wavelength of 4.8 Å in the temperature range of 1.8–250 K. Magnetic scattering was separated from nuclear coherent and spin incoherent scattering contributions. Long-range ordered magnetic peaks were observed below 39 K which is consistent with the heat capacity results. The drastic increasing intensities below 6 K indicate the ferromagnetic transition in Tb³⁺ orderings.     

Generation of laser-polarized xenon using fiber-coupled laser-diode arrays narrowed with integrated volume holographic gratings

Volume holographic gratings (VHGs) can be exploited to narrow the spectral output of high-power laser-diode arrays (LDAs) by nearly an order of magnitude, permitting more efficient generation of laser-polarized noble gases for various applications. A 3-fold improvement in ¹²⁹Xe nuclear spin polarization, P_Xe, (compared to a conventional LDA) was achieved with the VHG-LDA’s center wavelength tuned to a wing of the Rb D₁ line. Additionally, an anomalous dependence of P_Xe on the xenon density within the OP cell is reported—including high P_Xe values (>10%) at high xenon partial pressures (1000 torr).     

Using multiple quantum coherence to increase the N resolution in a three-dimensional TROSY HNCO experiment for accurate PRE and RDC measurements

We present a new version of the 3D TROSY HNCO pulse scheme, referred to as HR-TROSY HNCO, with comparable resolution in the ¹⁵N dimension to a 2D ¹H–¹⁵N HSQC experiment. In the conventional 3D TROSY HNCO, the constant time period (1/2J_NC  32 ms) severely limits the maximum resolution in the ¹⁵N dimension. In the HR-TROSY HNCO experiment presented here, both constant time periods (32 ms each) for coherence forward and backward transfer between ¹⁵N and ¹³C′ are utilized to double the ¹⁵N evolution time. This leads to a dramatic enhancement in peak separation along the ¹⁵N dimension, making the HR-TROSY HNCO an ideal pulse scheme for accurate paramagnetic relaxation enhancement and residual dipolar coupling measurements.     

Scanning tunneling microscopy/spectroscopy on multi-layered cuprate superconductor Ba2Ca5Cu6O12 (O1−x Fx)2

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements on multi-layered cuprate superconductor Ba₂Ca₅Cu₆O₁₂ (O_1−x F_x)₂ are carried out. STM topographies show randomly distributed bright spot structures with a typical spot size of 0.8 nm. These bright spots are occupied about 28% per one unit cell of c-plane, which is comparable to the regular amount of apical oxygen of 20% obtained from element analysis. Tunneling spectra simultaneously show both the small and the large gap structures. These gap sizes at 4.9 K are about Δ 15 meV and 90 meV, respectively. The small gap structure disappears at the temperature close to T_C, while the large gap persists up to 200 K. Therefore, these features correspond to the superconducting gap and pseudogap, respectively. These facts give evidence for some ordered state with large energy scale even in the superconducting state. For the superconducting gap, the ratio of 2Δ/K_BT_C = 4.9 is obtained with T_C = 70 K, which is determined from temperature dependence of the tunneling spectra.     

Electronic stopping power of polymers for heavy ions in the ion energy domain of LSS theory

LSS based computed electronic stopping power values have been compared with the corresponding measured values in polymers for heavy ions with Z = 5–29, in the reduced ion velocity region, v_red ≤ 1. Except for limited v_red  0.6–0.85, the formulation generally shows significantly large deviations with the measured values. The ζ factor, which was approximated to be Z₁^1/6, involved in LSS theory has been suitably modified in the light of the available experimental stopping power data. The calculated stopping power values after incorporating modified ζ in LSS formula have been found to be in close agreement with measured values in various polymers in the reduced ion velocity range 0.35 ≤ v_red ≤ 1.0.     

Electron drag in intermediate magnetic fields with low electron density

Electron drag between two two-dimensional electron systems has been measured in intermediate magnetic fields (/τ<ω_ck_BT) with a relatively low electron density. We explore, in this sample, the unusual increase of drag in intermediate magnetic fields which was well characterized by a nearly temperature independent B³ dependence. The anomalous behavior of electron drag observed in higher density samples is found to persist for low sample density.     

Effect of the synthesis route on the structural properties and shape of the indium oxide (In2O3) nano-particles

Nano-crystalline indium oxide (In₂O₃) particles have been synthesized by sol–gel and hydro-thermal techniques. A simple hydro-alcoholic solution consisting indium nitrate hydrate and citric acid (in sol–gel method) and 1, 4-butandiol (in hydro-thermal method) have been utilized. The structural properties of indium oxide nano-powders annealed at 450 °C (for both methods) have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and specific surface area (SSA) analysis. Structural analysis of the samples shows cubic phase in sol–gel and cubic-hexagonal phase mixture in hydro-thermally prepared particles. The nano-particles prepared by sol–gel method have nearly spherical shape, whereas hydro-thermally-made ones display wire- and needle-like shape in addition to the spherical shape. The obtained In₂O₃ nano-particles surface areas were 23.2 and 55.3 in sol–gel and hydro-thermal methods, respectively. The optical direct band gap of In₂O₃ nano-particles were determined to be 4.32 and 4.24 eV for sol–gel and hydro-thermal methods, respectively. These values exhibit 0.5 eV blue shift from that the bulk In₂O₃ (3.75 eV), which is related to the particle size reduction and approaching the quantum confinement limit of nano-particles.     

One-dimensional excitons in inorganic–organic self-organized quantum-wire crystals [NHC(I)=NH]PbI and [CHSC(=NH)NH]PbI

Optical properties of lead-iodide-based one-dimensional perovskite-type crystals [NH₂C(I)=NH₂]₃PbI₅ and [CH₃SC(=NH₂)NH₂]₃PbI₅ have been investigated theoretically and experimentally. The electronic and excitonic structures are studied based on group theoretical consideration and first-principle band calculation. Strong one-dimensional anisotropy of the optical absorption spectra, large Stokes shifts (1.0 eV) and huge exchange energies (70 meV) indicate that the excitons in these crystals are one-dimensional Frenkel excitons.     

Dimensional evolution of silicon nanowires synthesized by Au–Si island-catalyzed chemical vapor deposition

This study explores the nucleation and morphological evolution of silicon nanowires (Si-NWs) on Si (0 0 1) and (1 1 1) substrates synthesized using nanoscale Au–Si island-catalyzed rapid thermal chemical vapor deposition. The Au–Si islands are formed by Au thin film (1.2–3.0 nm) deposition at room temperature followed by annealing at 700 °C, which are employed as a liquid-droplet catalysis during the growth of the Si-NWs. The Si-NWs are grown by exposing the substrates with Au–Si islands to a mixture of gasses SiH₄ and H₂. The growth temperatures and the pressures are 500–600 °C and 0.1–1.0 Torr, respectively. We found a critical thickness of the Au film for Si-NWs nucleation at a given growth condition. Also, we observed that the dimensional evolution of the NWs significantly depends on the growth pressure and temperature. The resulting NWs are 30–100 nm in diameter and 0.4–12.0 μm in length. For Si (0 0 1) substrates 80% of the NWs are aligned along the 1 1 1 direction which are 30° and 60° with respect to the substrate surface while for Si (1 1 1) most of the NWs are aligned vertically along the 1 1 1 direction. In particular, we observed that there appears to be two types of NWs; one with a straight and another with a tapered shape. The morphological and dimensional evolution of the Si-NWs is significantly related to atomic diffusion kinetics and energetics in the vapor–liquid–solid processes.     

Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode distributed Bragg reflector fiber laser

In this paper, a novel all-optical microwave generation technique based on a dual-wavelength single-longitudinal-mode (SLM) distributed Bragg reflector (DBR) fiber laser is proposed and demonstrated. By exploiting spatial hole burning (SHB) effect, this laser could provide stable dual-wavelength SLM operation with a wavelength separation of 0.088 nm corresponding to the microwave signal at 10.484 GHz with a 3 dB bandwidth of 28 kHz. By appropriately adjusting the pump power, dual-wavelength oscillation could be maintained at different temperatures. We have theoretically analyzed the mechanism for microwave generation of the proposed DBR laser, and the calculated microwave frequency is in good agreement with our experimental results. Furthermore, experimental observation shows both of the laser wavelengths and generated microwave signals have good stability at room temperature.     

High-performance pyroelectric single crystals for uncooled infrared detection applications

Uncooled pyroelectric infrared detectors based on ferroelectric single crystals 0.74Pb(Mg_1/3Nb_2/3)O₃–0.26PbTiO₃ (PMN–0.26PT) were fabricated. The performances of pyroelectric detectors dependence on detector fabrication temperature, absorption layer, and element thickness were compared. The room-temperature voltage responsivity (R_v) of 200 V/W and specific detectivity (D^*) of 10⁸ cm Hz^1/2/W at 12.5 Hz have been achieved. The results reveal that the better pyroelectric response can be expected by controlling temperature below 70 °C during the fabrication of the pyroelectric detectors, selecting absorption layer with high absorption coefficient, and decreasing the thickness of the elements.     

Microwave dielectric properties of (1 − x)(Mg0.95Co0.05)TiO3–x(Na0.5La0.5)TiO3 ceramic system

The microstructures and the microwave dielectric properties of the (1 − x)(Mg_0.95Co_0.05)TiO₃–x(Na_0.5La_0.5)TiO₃ ceramic system were investigated. Two-phase system was confirmed by the XRD patterns and the EDX analysis. A co-existed second phase (Mg_0.95Co_0.05)Ti₂O₅ was also detected. The microwave dielectric properties are strongly related to the density and the matrix of the specimen. A new microwave dielectric material 0.88(Mg_0.95Co_0.05)TiO₃–0.12(Na_0.5La_0.5)TiO₃, possessing an excellent combination of dielectric properties: ε_r  22.36, Q × f  110,000 GHz (at 9 GHz), τ_f  2.9 ppm/°C), is proposed as a candidate dielectric for GPS patch antennas.     

Transport properties of single crystal

Transport properties of BaNi₂P₂ single crystals prepared by high-pressure synthesis method have been investigated. The temperature dependence of the resistivity is that of a typical metal with the anisotropy ratio ρ/ρ of 6.3 and suggests that electron–phonon interaction dominates the scattering mechanism. We have also found that the Hall effect and the magnetoresistance can be explained by a two-carrier model which is consistent with a multiple-band structure with both hole and electron characters.     

Compact 120 TW Ti:sapphire laser system with a high gain final amplifier

A 120 TW/36 fs laser system based on Ti:sapphire chirped-pulse amplification (CPA) has been successfully established in our lab. The final four pass Ti:sapphire amplifier pumped by an energetic single-shot Nd:YAG—Nd:glass laser was designed and optimized. With 24 J/8 ns pump energy at 532 nm, 300 mJ/220 ps chirped pulse was amplified to 5.98 J in this amplifier, and a total saturated gain of 20 was achieved. The focused intensity of compressed beam could reach to 10²⁰ W/cm² with the M² of 2.0.     

Spectroscopic investigation of arsenate and selenate incorporation into hydroxylapatite

The mechanism(s) of arsenate and selenate incorporation into hydroxylapatite (HAP) using extended X-ray absorption fine structure (EXAFS) spectroscopy was investigated for As- and Se-doped HAP samples with concentrations between 200 and 2500 ppm. EXAFS data on As and Se K-edges have shown similar local coordination environments and are similar to that of P in HAP, suggesting the substitution of arsenate or selenate tetrahedra on the phosphate sites. EXAFS best-fitting for As-doped samples shows that the first shell is fitted with approximately 4 O atoms at 1.68 Å, showing As(V) in tetrahedral coordination, and Se K-edge EXAFS data are characterized by the backscattering contributions an oxygen shell at 1.2 Å in the Fourier transform, which can be fit with 4 O atoms at 1.65 ± 0.01 Å. This is characteristic of Se–O distances in SeO₄ tetrahedron. These findings suggest that arsenate and selenate substitute for phosphate groups with local distortions during the incorporation of these metals into the structure of HAP.     

Superconductivity as a Kosterlitz–Thouless transition in the two-dimensional Hubbard model

We investigate a superconducting Kosterlitz–Thouless transition in the two-dimensional (2D) Hubbard model using auxiliary quantum Monte Carlo method for the ground state. The pair susceptibility is computed for both the attractive and repulsive Hubbard model. The numerical results show that the s-wave pair susceptibility scales as χ  L² for the attractive case, in agreement with previous quantum Monte Carlo studies. The scaling χ  L² also holds for the d-wave pair susceptibility for the repulsive Hubbard model if we adjust the band parameter t′.     

Gas-phase vibrational spectroscopy and ab initio calculations of Rb(H2O)n and Rb(H2O)nAr cluster ions

The competition between ion–water electrostatic interactions and water–water hydrogen bonding in cluster ions depends on several factors, including charge density of the ion and temperature of the system. Infrared photodissociation spectra of Rb⁺(H₂O)_n=2–5 and Rb⁺(H₂O)_n=1–5Ar are presented here and compared to previous experiments involving potassium and cesium. The temperature, or internal energy, of hydrated rubidium cluster ions is controlled by varying the evaporative path available for cluster formation. Warmer clusters (with effective temperatures of 250–500 K) are formed by the evaporation of water, while colder clusters (40–120 K) can be formed by argon evaporation. Colder cluster ions tend to favor conformers with more hydrogen bonds compared to those cluster ions at warmer temperatures. Previous work from this laboratory has shown significant and dramatic differences between the spectra of hydrated potassium and cesium ions. With a charge density intermediate between that of K⁺ and Cs⁺, Rb⁺ plays an important role in bridging the gap in our previous studies.     

What can we learn about the spectrum of high-energy stray neutron fields from Bonner sphere measurements?

Extended-Range Bonner sphere spectrometers (ERBSSs) are well suited for measurements in high-energy stray neutron fields, where neutron energies extend from thermal to a few hundred MeV. These fields typically have a thermal peak, a fairly flat intermediate region, a medium energy peak at 1 MeV and a high-energy peak at a few 100 MeV. The data analysis is not straightforward: it requires unfolding with response functions that have a substantial amount of overlap, and the responses of the modified spheres (which provide information about the fluence at high energies) increase dramatically above 100 MeV. In this paper, I try to determine which of the main features of the spectrum are well determined by the data given optimal methods of analysis, and which features are subject to ambiguity. To do this, I analyse ERBSS measurements using two methods of analysis that are particularly useful for this purpose, Bayesian parameter estimation and maximum entropy unfolding.     

Tunneling conductance in a gapped graphene-based superconducting structure: Case of massive Dirac electrons

The tunneling conductance in a NG/SG graphene junction in which the graphene was grown on a SiC substrate is simulated. The carriers in the normal graphene (NG) and the superconducting graphene (SG) are treated as massive relativistic particles. It is assumed that the Fermi energy in the NG and SG are E_FN400 meV and E_FS400 meV+U, respectively. Here U is the electrostatic potential from the superconducting gate electrode. It is seen that the Klein tunneling disappears in the case where a gap exist in the energy spectrum. As U→∞, the zero bias normalized conductance becomes persistent at a minimal value of G/G₀1.2. The normalized conductance G/G₀ is found to depend linearly on U with constant slope of , where is the size of the gap Δ opening up in the energy spectrum of the graphene grown on the SiC substrate. It is found that G/G₀2+αU for potentials in the range −270 meV<U<0 meV and G=0 for potentials U<−270 meV. As α→∞, the conductance for eV=Δ (V is the bias voltage placed across the NG/SG junction) can be approximated by a unit step function G(eV=Δ,U)/G₀2Θ(U). This last behavior indicates that a NG/SG junction made with gapped graphene could be used as a nano switch having excellent characteristics.