Effect of thickness on the structure, morphology and optical properties of sputter deposited Nb2O5 films

Nb₂O₅ films with the thickness (d) ranging from 55 to 2900 nm were deposited on BK-7 substrates at room temperature by a low frequency reactive magnetron sputtering system. The structure, morphology and optical properties of the films were investigated by X-ray diffraction, atomic force microscopy and spectrophotometer, respectively. The experimental results indicated that the thickness affects drastically the structure, morphology and optical properties of the film. There exists a critical thickness of the film, d_cri =2010 nm. The structure of the film remains amorphous as d < d_cri. However, it becomes crystallized as d > d_cri. The root mean square of surface roughness increases with increasing thickness as d > 1080 nm. Widths and depths of the holes on film surface increase monotonously with increasing thickness, and widths of the holes are larger than 1000 nm for the crystalline films. Refractive index increases with increasing thickness as d < d_cri, while it decreases with increasing thickness as d > d_cri. In addition, the extinction coefficient increases with increasing thickness as d > d_cri.     

Enhancement of the anomalous Hall effect and spin glass behavior in the bilayered manganite La2−2xSr1+2xMn2O7

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of 1 mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system.     

Scintillation and anomalous emission in elpasolite Cs2LiLuCl6:Ce

The luminescence and scintillation properties of Cs₂LiLuCl₆:0.5%Ce³⁺ are presented. Special attention is devoted to a 9.4 ns fast emission at 275 nm that can only be excited via the highest cubic field 5d_e state of Ce. Contrary to Cs₃LuCl₆ and Cs₂LiYCl₆, where the same type of fast emission was observed, the emission in Cs₂LiLuCl₆ is still observed at room temperature. Assuming that the 5d_e state is located inside the host conduction band (CB), we propose that the emission originates from a mixed state at or just below the bottom of the CB and ends at the 4f ground state of Ce³⁺. To proof this model we studied the thermal quenching of the anomalous luminescence and performed X-ray photoelectron spectroscopy. A model for a temperature-activated energy transfer from the anomalous state to the lowest 5d_t excited state of Ce³⁺ explains most of the results. Besides the 275 nm emission, the material shows 5d_t-4f Ce³⁺ emission at 370 and 406 nm and 2 ns fast core-valence luminescence when excited with 16-22 eV photons. The scintillation properties of Cs₂LiLuCl₆:Ce are briefly discussed.     

Beyond the pulse-area theorem: Role of the absorption and the dispersion in the propagation of weak ultrashort resonant pulses

We present here some fundamental but yet underlooked features of the propagation of weak ultrashort pulses (with Δ_dτ << 1, where Δ_d is the Doppler width and τ is the pulse duration) in resonant atomic media. We show that the pulse area behaviour and the pulse spectrum at resonance are governed by the usual optical depth (α₀L, where α₀ is the absorption coefficient at resonance and L the length of the medium), whereas the pertinent parameter that governs the severity of the dispersion effects and the distortion of the pulse is the dispersion parameter e_disp = (α₀L)Δ_dτ that we introduce. Paradoxical effect such as distortionless propagation (e.g. e_disp << 1) with vanishing pulse area (when α₀L >> 1) can then explained within this formalism.     

Anomalous behavior of optogalvanic signal in a miniature neon discharge lamp

A strong optogalvanic effect has been observed in a negative glow of a miniature neon discharge lamp using tunable pulse dye laser pumped by a copper vapor laser. A comparative study on temporal evolution of optogalvanic signal in a positive and negative dynamic resistance region of the discharge is described. Dye laser beam was tuned to various neon transitions 1s_i → 2p_j (Paschen notations) within 570-617 nm wavelength range. Anomalous behavior of optogalvanic signal was observed at 588.2 nm for (1s₅ → 2p₂) neon transition at low discharge current (<220 μA). This anomalous behavior is the attributes of damped oscillations of optogalvanic signal that correlate with negative dynamic resistance (dV/di < 0) of the discharge. Penning ionization at low discharge current and small energy mismatch is assumed to be the main cause of the negative dynamic resistance. Penning ionization process has been explained by resonantly ionizing energy transfer via collisions between neon buffer gas atoms in the lowest metastable state (1s₅) and electrode sputtered atoms in ground state using their partial energy level diagram.     

Quantum-mechanical vs. semi-classical spectral-line widths and shifts from the line core in the non-impact region for the Ar-perturbed/ K-radiator system

New quantum-mechanical (QM) and semi-classical (SC) shifts (d's) and widths (HWHM's, w's) were measured from the line core of computed full spectral-line shapes for the Ar-perturbed/K-radiator system (K/Ar). The initial state of our model was based on a 4p²P_3/2,1/2 pseudo-potential for the K/Ar system, and the final state on a zero potential. The Fourier transform of the line shape formed the basis for the computations. Excellent agreement was found between the QM and SC values of d and of w in a high-pressure (P) non-impact region, which was characterized by a √P dependence of w and a P dependence of d. These agreements were shown to be another example of a correspondence between classical (SC) quantities and QM quantities in the limit of large quantum numbers. Typically at P=1×10⁶ Torr and T=400 K, w_QM=448 cm⁻¹ and w_SC=479 cm⁻¹, where the deviation from the mean is ±3.3%. Also, d_QM=−3815 cm⁻¹ and d_SC=−3716 cm⁻¹, where the deviation from the mean is ±1.3%. A new general method was formulated which yielded a definite pressure P₀, which was defined as an upper limit to the low-pressure impact approximation and a lower limit to the non-impact region.     

Magnetic-field-driven quantum critical behavior in graphite and bismuth

We study magnetotransport properties of graphite and rhombohedral bismuth samples and found that in both materials applied magnetic field induces the metal-insulator- (MIT) and reentrant insulator-metal-type (IMT) transformations. The corresponding transition boundaries plotted on the magnetic field-temperature (B − T) plane nearly coincide for these semimetals and can be best described by power laws T ∼ (B − B_c)^κ, where B_c is a critical field at T = 0 and κ = 0.45 ± 0.05. We show that insulator-metal-insulator (I-M-I) transformations take place in the Landau level quantization regime and illustrate how the IMT in quasi-3D graphite transforms into a cascade of I-M-I transitions, related to the quantum Hall effect in quasi-2D graphite samples. We discuss the possible coupling of superconducting and excitonic correlations with the observed phenomena, as well as signatures of quantum phase transitions associated with the M-I and I-M transformations.     

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E⊥c the photoconductivity is observed in the hν < E_g and hν > E_g regions. In the case of hv < E_g, the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E||c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton-exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.     

Phase and electrical behaviour in the Bi14W1 − xLaxO24 − 3x/2 system

The compositional and thermal dependencies of phase and electrical behaviour of compositions in the system Bi₁₄W_1 − xLa_xO_{24 − 3x/2} (0.00 < x < 1.00) have been studied by X-ray powder diffraction, differential thermal analysis and a.c. impedance spectroscopy. The system exhibits polymorphism and phase separation, which shows both compositional and thermal dependence. Compositions with x = 0.25 and x = 0.50 exhibit a single phase tetragonal structure at room temperature. In contrast, the x = 0.75 composition at room temperature shows a mixture of a cubic phase and a secondary β-Bi₂O₃ related tetragonal phase. A full solid solution is observed at high temperatures, corresponding to the occurrence of a δ-Bi₂O₃ type phase. The appearance of the various phases correlates well with the observed electrical behaviour. The x = 0.75 composition exhibits exceptionally high conductivity at high temperatures (σ₈₀₀ = 1.34 S cm^− 1), but also shows significant phase separation at lower temperatures.     

Surface structure of K/Pd(1 0 0) and the effect of H coadsorption: Density functional theory calculations

We have performed density-functional theory calculations to study the atomic structure of the K/Pd(1 0 0)-p(2 × 2) and -c(2 × 2) surfaces formed at 0.25 ML and 0.5 ML, respectively. We find that K atoms prefer the hollow site with the K adsorption height 2.44 Å for p(2 × 2) and 2.50 Å c(2 × 2). The first interlayer spacing (d₁₂) of the Pd(1 0 0) substrate appears slightly contracted from the bulk value as Δd₁₂ = −0.8% and −0.3% for p(2 × 2) and c(2 × 2), respectively. The calculated contraction Δd₁₂ = −0.3% for c(2 × 2) is not in accord with the expansion Δd₁₂ = +1.3% reported by a low-energy electron diffraction (LEED) study. As the origin of this difference, a possibility of hydrogen contamination of the surface sample used in the LEED study is suggested: Our calculations show that the d₁₂ of K/Pd(1 0 0)-c(2 × 2) increases linearly with the coverage of H coadsorption, which leads to an estimation for the H coverage of the surface sample as 0.1-0.4 ML.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

The impact of annealing temperature and time on the electrical performance of Ti/Pt thin films

In this study, we focus on the influence of annealing time t_PDA (i.e. 30 min and 630 min) on the room-temperature resistivity of electron-beam-evaporated titanium/platinum thin films when exposed to thermal loads up to temperatures T_PDA of 700 °C. The titanium has a fixed thickness of 5 nm and serves as an adhesion layer. The thickness d_f,Pt of the platinum top layer is varied between 21 and 97 nm. Up to annealing temperatures of 450 °C, the film resistivity of the bi-layer system is linearly correlated with the reciprocal platinum film thickness independent of t_PDA, as expected from the size effect. At t_PDA = 30 min, the change in intrinsic film stress dominates the electrical behavior in this annealing regime, predominantly at large d_f,Pt values. Compared to t_PDA = 630 min, however, the increase in resistivity especially at low platinum film thickness is substantially larger demonstrating that titanium starts to diffuse at these long annealing times even at moderate temperatures. At T_PDA = 600 °C, the diffusion of titanium into the top layer leads to an enhanced increase in film resistivity ρ_f, especially at low platinum thicknesses and low annealing times, as the mean penetration depth of diffused titanium is under these conditions in order of d_f,Pt. Above T_PDA = 600 °C, ρ_f is slightly increased at t_PDA = 30 min. At t_PDA = 630 min, however, the film resistivity is decreased at d_f,Pt < 58 nm. This is attributed to grain growth and re-crystallization effects. Furthermore, the mean penetration depths of titanium substantially exceed d_f,Pt resulting predominantly in Ti_xO_y formation on the top film surface and hence, having low impact on ρ_f.     

Gigantic uniaxial pressure effect in single crystals of iron-based superconductors

In order to elucidate the anisotropic pressure effect on superconductivity in an iron-based superconductor, magnetization measurements have been performed in Ba(Fe_0.92Co_0.08)₂As₂ single crystals under uniaxial pressures applied along the c-axis. Gigantic T_c suppression, dT_c/dP_//c = −15 K/GPa, was observed when the anisotropic deformation with the a-expansion and c-compression was induced by the c-pressure, which should be compared with dT_c/dP < +1 K/GPa in the isotropic pressure case. This suggests that the a-axis (c-axis) compression has a positive (negative) contribution to T_c.     

Highly concentrated toluene decomposition on the dielectric barrier discharge (DBD) plasma-photocatalytic hybrid system with Mn-Ti-incorporated mesoporous silicate photocatalyst (Mn-Ti-MPS)

This study investigates the Mn-Ti-incorporated mesoporous silicate (Mn-Ti-MPS) as a photocatalyst for highly concentrated toluene removal in a plasma-photocatalytic hybrid system. Various Mn-Ti-MPS [Ti/Si molar ratio = 1/4, Mn/Ti molar ratio = 0.01/1 (1 mol%), 0.05/1 (5 mol%) and 0.1/1 (10 mol%)] photocatalysts were successfully synthesized using a common hydrothermal method without causing any structural damage. In the X-ray diffraction (XRD) pattern, the main peaks of the TiO₂ anatase structure and MnO did not show. All samples displayed hexagonal specific peaks at 2.5° (d_1 0 0 plane), 4.1° (d_1 1 0 plane) and 4.7° (d_2 0 0 plane). This indicates that the Ti ions and Mn ions were well substituted into the Si ion sites in the framework of MCM-41. Their surface areas decreased compared with that of pure MCM-41, while the hexagonal straight pore size was distributed in a range of 2.5-3.5 nm. In the Mn-Ti-MPS, much more water and toluene molecules were absorbed compared to the Ti-MPS. From the X-ray photoelectron spectroscopy (XPS) result, it was determined that the hydrophilicity of the Mn-Ti-MPS was stronger than that of the Ti-MPS. Photocatalytic decomposition for highly concentrated toluene of 1000 ppm increased in the Mn-Ti-MPS when compared with the Ti-MPS, while toluene decomposition on 5 mol% Mn-Ti-MPS was remarkably enhanced to 80% in the plasma system. The conversion to CO₂, however, did not improve in the case of the plasma-only system. Nonetheless, in the plasma-photocatalytic hybrid system, the conversion to CO₂ for 5 mol% Mn-Ti-MPS reached 43% (in an 800 ppm toluene conversion).     

Dependence of the interband transitions on the in mole fraction and the applied electric field in InxGa1−xAs/In0.52Al0.48As multiple quantum wells

Transmission electron microscopy (TEM) and photocurrent (PC) measurements were carried out to investigate the microstructural properties and excitonic transitions in In_xGa_1−xAs/In_0.52Al_0.48As multiple quantum wells (MQWs) for x = 0.54, 0.57 and 0.60. TEM images showed that high-quality 11-period In_xGa_1−xAs/In_0.52Al_0.48As MQWs had high-quality heterointerfaces. The results for the PC spectra at 300 K showed that the peaks corresponding to the excitonic transitions from the ground state electronic sub-band to the ground state heavy-hole band (E₁-HH₁) and the ground state electronic sub-band to the ground state light-hole band (E₁-LH₁) became closer to each other with decreasing In mole fraction and that E₁-HH₁ and E₁-LH₁ excitonic peaks shifted to longer wavelength with increasing applied electric field. The calculated values of the E₁-HH₁ interband transition energies were in qualitative agreement with those obtained form the PC measurements with and without applied electric field. These results can be helpful in understanding potential applications of In_xGa_1−xAs/In_yAl_1−yAs MQWs dependent on In mole fraction and applied electric field in long-wavelength optoelectronic devices.     

Hall effect in a GdBa2Cu3O7−δ/La0.75Sr0.25MnO3 perovskite bilayer

We present results on the Hall coefficient R_H in the normal state for a GdBa₂Cu₃O_7−δ/La_0.75Sr_0.25MnO₃ bilayer and a La_0.75Sr_0.25MnO₃ film grown by dc magnetron sputtering on (1 0 0) SrTiO₃. We find that the electric transport on the bilayer can be qualitatively described using a simple parallel layers model. The GdBa₂Cu₃O_7−δ layer presents a carrier density approximately equal to that reported for 7 − δ = 6.85 oxygen doping. Also we observe an unexpected presence of two Hall resistivity regimes, effects that may be associated with the internal magnetic field induced on the superconducting layer by the ferromagnetic layer.     

Experimental determination of air-broadening parameters of pure rotational transitions of HNO3: intercomparison of measurements by using different techniques

We report on the air-broadening of the 44_d, 25 ← 44_d, 26, 43_d, 25 ← 43_d, 26, 25_d, 25 ← 24_d, 24, 27_d, 25 ← 27_d, 26, 26_1, 25 ← 26_0, 26, 15_13, 3 ← 14_13, 2, and 27_d, 27 ← 26_d, 26 transitions of HNO₃ at room temperature, which are useful for measurements of nitric acid in the Earth’s atmosphere by microwave limb sounders. In fact, the lines selection was essentially made on the basis of the spectral range requirements of MASTER, a limb sounding instrument managed by ESA. It is well established that the collisional-broadening parameters have a crucial influence on reduction of remote sensing data, but it is also well known that systematic errors make the evaluation of pressure-broadening parameters accuracy difficult. To this purpose, the investigation has been carried out in two different laboratories employing different techniques: the intercomparison allowed us to check the accuracy of the retrieved parameters as well as to estimate the extent of systematic errors affecting them.     

Universal explicit expression for quantum Hall conductance at any temperatures and chemical potentials

This paper generalises the theorem already obtained [Solid State Commun. 127 (2003) 505] for the high mobility, dissipationless, integer quantum Hall systems at T=0 K to the T>0 K situations. The results obtained are again suitable at both microscopic and macroscopic scales. In comparison [Solid State Commun. 127 (2003) 505], this generalised form gives a universal explicit expression for the Hall conductance σ_xy(μ,T) between any two points selected in such a system as a function of chemical potential and temperature. Further, thermal deviation Δσ_xy(μ,T) from the exact quantised values of σ_xy(μ,T) and the minimum slopes of Hall plateaux in the T>0 cases, observed already in experiments, are also derived in theory. Similar to those in the T=0 K case [Solid State Commun. 127 (2003) 505], the overall quantum Hall behaviour of the system can again be obtained from this theory by simply selecting two points on the two Hall contacts.     

Fabrication and characterization of sputtered titanium dioxide films

TiO₂ thin films were prepared under various conditions by using a reactive RF sputtering technique. The structural, optical and electrical characteristics of the films have been investigated. All as-deposited films were amorphous. After annealing at T > 673 K, the crystallinity of the observed tetragonal anatase phase appeared improved. The optical band gap, determined by using Tauc plot, has been found to amount to 3.38 ± 0.03 and 3.21 ± 0.03 eV for the direct and indirect transition, respectively. Also the complex optical constants for the wavelength range 300-2500 nm are reported. Using the two-point probe technique, the dark resistivity has been measured as a function of the film thickness, d. The resistivity, ρ, of the samples has been found to decrease markedly with increasing thickness, but only for d < 100 nm. The behaviour of ρd versus d was found to fit properly with the Fuchs and Sondheimer relation with parameters ρ_o = 4.95 × 10⁶ Ω cm and mean free path, l = 310 ± 2 nm. The log ρ versus 1/T curves show three distinct regions with values for the activation energy of 0.03 ± 0.01, 0.17 ± 0.01 and 0.50 ± 0.02 eV, respectively.