Electronic structure and magnetic properties of the compound CeCuIn

Magnetization, magnetic susceptibility, electrical resistivity, thermoelectric power and X-ray photoemission measurements were performed on a polycrystalline sample of CeCuIn. This compound crystallizes in a hexagonal structure of the ZrNiAl type. The magnetic data indicate that CeCuIn remains paramagnetic down to 1.9 K with a paramagnetic Curie temperature of −13 K and an effective magnetic moment equal to 2.5 μ_B. The electrical resistivity has metallic character, yet in the entire temperature range studied here, it is a strongly nonlinear function of temperature. The temperature dependence of the thermoelectric power is dominated by a small positive maximum near 76 K and a deep negative minimum at about 16 K. Above 150 K the thermopower exhibits a Mott's type behavior. The positive sign of the Seebeck coefficient in this temperature region indicates that the holes are dominant charge and heat carriers. The structure of Ce 3d_5/2 and Ce 3d_3/2 XPS spectra has been interpreted in terms of the Gunnarsson-Schönhammer theory. Three final-state contributions f⁰, f¹ and f² are clearly observed, which exhibit a spin-orbit splitting Δ_SO≈18.7 eV. The appearance of the 3d⁹f⁰ component is a clear evidence of the intermediate valence behavior of Ce. From the intensity ratio I(f⁰)/[I(f⁰)+I(f¹)+I(f²)] the 4f-occupation number is estimated to be 0.95. In turn, the ratio I(f²)/[I(f¹)+I(f²)]=0.08 yields a measure of the hybridization energy that is equal to 45 meV.     

Optical characterization of fourfold (Td)- and sixfold (Oh)-transition-metal species in MgAl2O4:Co by time-resolved spectroscopy

This work investigates the origin of novel visible photoluminescence (PL) bands observed in the spinel MgAl₂O₄:Co²⁺. Besides the well-known fourfold-coordinated Co²⁺(T_d) PL at 670 nm [N.V. Kuleshov, V.P. Mikhailov, V.G. Scherbitsky, P.V. Prokoshin and K.V. Yumashev, J. Lumin. 55 (1993) 265.], a rich structured PL band at 686 nm was also observed that we associate with uncontrolled impurities of sixfold coordinated Cr³⁺(O_h) by time-resolved spectroscopy and lifetime measurements and their variation with temperature. We also show that the lifetime of the Co²⁺(T_d) emission at 670 nm varies from τ=6.7 μs to 780 ns on passing from T=10 to 290 K. This unexpected behaviour for T_d systems is related to the excited-state crossover (⁴T₁↔²E), making the emission band to transform from a narrow-like emission from ²E at low temperature to a broad structureless band from ⁴T₁ at room temperature.     

Studies of the phosphorescence of polycrystalline hafnia

Persistent phosphorescence induced by ultraviolet light in polycrystalline HfO₂ and enhancement of the phosphorescence by sintering are investigated. The phosphorescence afterglow emission is in the 1.8-3.2 eV spectral range, with a peak at 2.53 eV. The afterglow intensity is significantly increased by sintering in either inert atmosphere or air. The afterglow light sum measured at room temperature for samples sintered at 1500 °C is more than an order of magnitude higher than that before sintering. In the temperature range −50 to 200 °C, three thermoluminescence (TL) peaks are observed near −10, 30, and 100 °C. The relative contribution of the low-temperature TL peak to the total TL intensity decreases after sintering, and this effect is more pronounced upon sintering in inert atmosphere. Conversely, the contribution of the TL peak near 100 °C increases after sintering. The enhancement of the afterglow by sintering is associated with the observed increase in the intensity of TL peaks at and above room temperature and attributed to an increase in the number of deep charge traps. The room-temperature afterglow time decay has a form consistent with the second-order mechanism, ∝(t₀+t)⁻ⁿ, and the best-fit values of both fitting parameters t₀ and n tend to increase with the sintering temperature.     

Photoluminescence spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532-nm laser radiation and gamma-rays

We have investigated temporal behavior of the photoluminescence (PL) spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532 nm laser radiation and gamma-rays. Under ∼100 W/cm² laser radiation, the PL intensity (I_PL) increases with irradiation time upto about 500 s and thereafter declines linearly. The wavelength of the PL emission (λ_peak) exhibits a blue-shift with exposure time. Upon simultaneous irradiation by 100 W/cm² 532-nm laser, as well as 0.57 and 1.06 MeV gamma-rays, the temporal behaviors of both I_PL and λ_peak are significantly different; I_PL increases to a saturation level, and the magnitude of the blue-shift in λ_peak is reduced. We discuss possible mechanisms underlying these results.     

Synthesis,magnetic and electrical transport properties of magnetoresistance material Sr2FeMoO6 by microwave sintering

Magnetoresistance material Sr₂FeMoO₆ with double perovskite structure was synthesized by microwave sintering method using SrCO₃, Fe₂O₃ and MoO₃ as raw materials, with MnO₂ for microwave absorber. The phase structure, magnetic and electrical transport properties were investigated by X-ray powder diffraction (XRD) and vibrating-sample magnetometer. XRD analysis shows that the as-synthesized sample is Sr₂FeMoO₆ with tetragonal crystal structure and I4/mmm space group. The unit cell parameters are a=0.5587 nm, c=0.7894 nm, volume=0.2464 nm³. The calculated grain size of the sample is 31.62 nm, which is obtained by the Scherrer formula using the diffraction data. Magnetism testing results show that the sample Sr₂FeMoO₆ is ferromagnetic with the magnetic transition temperature of about 380 K. Under 1.0 T magnetic field, the saturation and spontaneous magnetization of Sr₂FeMoO₆ is 1.25 μ_B/f.u. and 1.00 μ_B/f.u. at room temperature. The magnetoresistance ratio of the sample is 28%. Electrical transport properties testing results indicate that the sample exhibits typical semiconductor behavior. The conductive mechanism of Sr₂FeMoO₆ is highly dependent on temperature: within the temperature range of 100–300 K, the mechanism is attributed to the small polaron variable-range hopping model; while it is ascribed to the adiabatic small polaron model within the temperature range of 80–100 K.     

Epitaxial growth and optical characterization of compound semiconductor β-FeSi2 prepared by pulsed laser deposition

β-FeSi₂ thin films were prepared on FZ n-Si (1 1 1) substrates by pulsed laser deposition (PLD). The structural properties and crystallographic orientation of the films were investigated by X-ray diffraction (XRD) analysis. This indicates that β-FeSi₂/Si (2 0 2/2 2 0) and the single-crystalline β-FeSi₂ can be prepared using PLD. In photoluminescence (PL) measurements at 8 K detected by Ge detector, the PL spectra of the samples annealed at 900 °C for 1, 5, 8 and 20 h showed that the PL intensity of the A-band peak increased depending on annealing time in comparison with those of as-deposited samples. The intrinsic PL intensity of the A-band peak at 0.808 eV of the β-FeSi₂ from the 20-h-annealed sample was investigated for the first time by the PLD method detected by an InGaAs detector. This result has been confirmed by temperature dependence and excitation power density of the 20-h-annealed sample with the comparison of other defect-related band peaks of the sample. Cross-sectional scanning electron microscopy (SEM) observation was also performed and the thickness of the thin films was found to be at 75 nm for 20-h-annealed. The thermal diffusion for the epitaxial growth of β−FeSi₂/Si was observed when the compositional ratio of Fe to Si was around Fe:Si=1:2 for 20-h-annealed carried out by energy dispersive X-ray spectroscopy (EDX). We discussed high crystal quality of the epitaxial growth and optical characterization of β-FeSi₂ achieved after annealing at 900 °C for 20 h.     

Verwey transition in spin polarization of field-emitted electrons from 〈1 1 0〉-oriented single crystal magnetite whisker

We measured temperature dependence of a spin polarization of field-emitted electrons from a single-crystalline magnetite (Fe₃O₄) whisker with 〈1 1 0〉 orientation. The spin polarization of emitted electrons began to increase above 130 K corresponding to the temperature of Verwey point (T_v). The increase is considered as reflection of the change of the spin state near the Fermi level due to the Verwey transition. Our experimental results support a localization of t_2g orbital electrons below the Verwey point and a model of charge ordering for magnetite.     

Generation of optically active states in a-SiNx by thermal treatment

Amorphous silicon nitride (a-SiN_x) films were deposited using plasma-enhanced chemical-vapor deposition (PECVD) and subsequently, thermal annealing processes were performed at 700-1000 °C in the ultra-high vacuum (UHV) condition. A strong photoluminescence (PL) peak induced by luminescent defect centers was observed at 710 nm for the as-deposited sample. When the sample was annealed at 700-1000 °C, the PL peak intensity became about 3-12 times stronger with no shift of the PL peak. To investigate the origin of the change in PL peak intensity after the thermal annealing, Si 2p and N 1s core-level spectra were systematically analyzed by high-resolution photoemission spectroscopy (HRPES) using synchrotron radiation. In particular, N 1s spectra were decomposed with three characteristic nitrogen-bonding states. It is revealed that the nitrogen bonding state with N-Si and NSi₂ configurations (denoted as N3) contributes mainly to the change in PL peak intensity. We note that luminescent nitrogen related defect centers such as N₄⁺ and N₂° are localized in the state N3. Detailed analysis of the experimental results shows that the state N3 is located in the interface bounded by the region of the nano-sized stoichiometric silicon nitride Si₃N₄ (denoted as N1) and is considerably influenced by the thermal annealing, which is an appropriate process to cause strong photoluminescence of the related samples as mentioned above.     

XPS, electric and photoluminescence-based analysis of the GaAs (1 0 0) nitridation

In this paper, nitridation process of GaAs (1 0 0) substrates was studied in-situ using X-ray photoelectron spectroscopy (XPS) and ex-situ by means of electrical method I-V and photoluminescence surface state spectroscopy (PLS³) in order to determine chemical, electrical and electronic properties of the elaborated GaN/GaAs interfaces.The elaborated structures were characterised by I-V analysis. The saturation current I_S, the ideality factor n, the barrier height Φ_Bn and the serial resistance R_S are determined.The elaborated GaN/GaAs structures are also exhibited a high PL intensity at room temperature. From the computer-aided analysis of the power-dependent PL efficiency measurements (PLS³ technique), the value of the interface state density N_SS(E) close to the mid-gap was estimated to be in the range of 2-4 × 10¹¹ eV⁻¹ cm⁻², indicating a good electronic quality of the obtained interfaces.Correlation among chemical, electronic and electrical properties of the GaN/GaAs interface was discussed.     

Combined STM, LEED and DFT study of Ag(1 0 0) exposed to oxygen near atmospheric pressures

We have investigated the interaction of molecular oxygen with the Ag(1 0 0) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O₂, the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O₂ at room temperature, small, p(2 × 2) reconstructed patches are formed on the surface. After oxidation at ≈470 K and 10 mbar O₂ pressure the surface undergoes a c(4 × 6) reconstruction coexisting with a (6 × 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 × 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential.