Pulsed laser deposition of (110) oriented semiconductive SrFeO3−x thin films

 The semiconductive perovskite-type oxide SrFeO_3-x (x<0.16) (SFO) thin films have been directly fabricated on (001)SrTiO₃ and (001)LaAlO₃ single crystal substrates by pulsed laser deposition(PLD) under high oxygen partial pressure of 100 Pa. The SFO thin films were (110) oriented. The x-ray photoelectron spectroscopy (XPS) analysis showed that the surface of SFO thin film has strong gas absorption capability. The resistance versus temperature has been measured in the temperature range from 77 K to 300 K. The SFO thin film showed typical semiconductive property. Dependence of resistance of SFO thin film on oxygen pressure was measured and result showed that the SFO thin film had better oxygen sensitive property. Received: 14 May 1996/Accepted: 15 August 1996     

Evaluation of thermal conductivity of porous silicon layers by a photoacoustic method

 The paper shows that it is possible to evaluate the thermal conductivity of porous silicon layers by a conventional photoacoustic gas-microphone technique, on the basis of a simple interpretative model for stratified samples. Samples produced by electrochemical etching of crystalline wafers of different type and thickness are considered. The frequency variation of photoacoustic signal, in amplitude and phase, is studied and interpreted by means of a four-layers (air, porous silicon, crystalline silicon, air) model. Thermal conductivity values in the range 2.5–31.2 W/m K are obtained. Received: 4 July 1996/Accepted: 13 August 1996     

High-sensitivity detection of NO2 using a 740 nm semiconductor diode laser

 An AlGaAs diode laser was used to detect NO₂ absorption lines belonging to the (0 0 0)–(2 13 1) vibrational band, within the X˜² A ₁ electronic ground state, at 739 nm. A simple absorption spectrometer based on wavelength-modulation spectroscopy with second-harmonic detection was developed. The minimum detectable pressure of pure NO₂ was 0.1 μbar with 2 m absorption path-length, corresponding to an absorbance of 10^-6. High-sensitivity detection of NO₂ was also performed in the presence of N₂ and air at different total pressures: The effects on the detection limit of our apparatus were accurately investigated. The minimum NO₂ concentration at 500 mbar of air was measured to be 2 ppm. Received: 11 June 1996 / Revised version: 11 October 1996     

Phosphorus distribution profiles in 100-silicon using a spin-on source at low temperatures

 Incorporation of phosphorus into silicon from a spin-on dopant layer (SOD) at 400 ^°C is described. Annealing experiments were carried out with SOD films deposited on (100) silicon substrates by using the spin-on technique. Conventional heating on a hotplate in normal atmosphere and a temperature range up to 400 ^°C was used to study the dopant incorporation. After removing the SOD-films one part of the silicon substrates was annealed at higher temperatures. Investigations were carried out by SIMS, SAM, XPS, HTEM, stripping Hall and Van der Pauw measurements before and after the high temperature annealing. Chemical phosphorus concentration profiles obtained from low temperature annealed samples showed diffusion depths of 60–80 nm (extrapolated to a substrate doping level of 10¹⁶ cm^-3) and surface concentrations of 10¹⁹–10²⁰ cm^-3. Electron concentration profiles exhibiting maximum values around 2⋅10¹⁹ cm^-3 could be measured on high temperature annealed samples only. Received: 28 March 1996/Accepted: 19 August 1996     

Continuous frequency up-conversion in a double-Λ scheme of Na2

 In a double-Λ level configuration of Na₂ molecules, involving rotational–vibrational levels of the X, A and B bands, continuous resonant frequency mixing ω₄=ω₁−ω₂+ω₃ is demonstrated. A DCM dye laser at 661 nm (λ₁) pumps a molecular Raman laser at 746 nm (λ₂) in a sodium heatpipe, which is used to generate the molecular vapour. In the same heatpipe, both fields are mixed with the radiation of an argon-ion laser at 514 nm (λ₃) to generate up-converted laser radiation at 473 nm (λ₄). For laser powers of 200 mW (λ₁), 700 mW (λ₂, internal power) and 140 mW (λ₃), an output power of 120 μW (λ₄) has been achieved. Dependences of the generated radiation on the pump fields (powers and detunings) and polarization features are presented; influences of coherent coupling and population transfer mechanisms are discussed. Received: 7 October 1996     

Nonlinear Hanle effect in the active discharge of an argon laser

 We present experimental evidence of the NonLinear Hanle Effect (NLHE) on the 488 nm transition in an active discharge of an argon laser. Experimental data are used to estimate the homogeneous linewidth of the upper level of the transition. The role of the NLHE in the laser power enhancement is also discussed. Received: 22 February 1996 / Revised version: 17 June 1996     

External radiative quantum efficiency of 96% from a GaAs / GaInP heterostructure

 GaAs/GaInP double heterostructures are index matched with ZnSe hemispheres to increase the coupling of photoluminescence out of the device. We measure external quantum efficiencies as large as 96% at room temperature using a bolometric calibration technique. When the carriers are optically injected near the bandgap energy, the luminescence is blueshifted by up to 1.4 kT. In this case, external efficiencies exceeding 97.5% would yield optical refrigeration in the solid state. Received: 22 July 1996/Accepted: 23 September 1996     

Vacuum ultraviolet gain measurements in optically pumped LiYF4:Nd3+

 We present measurements of the net-induced gain on the 5d–4f transition at 186 nm in LiYF₄ : Nd³⁺ optically pumped by radiation from a molecular fluorine laser. It is found that for LiYF₄ : Nd³⁺, one of a series of potential continuously tunable VUV lasers, relatively strong excited-state absorption results in net-induced loss. The prospects for VUV laser operation being realised in other rare-earth-doped fluorides is discussed. Received: 4 March 1996/Revised version: 10 July 1996     

Harmonic generation in an ionized gas medium with a 100-femtosecond, high repetition rate laser source at intermediate intensities

 We report the realization of a vacuum-ultraviolet radiation source based on high-order harmonic generation in noble-gas samples, operating at high repetition rate. In particular, we observed up to the 13th harmonic (λ=61 nm) of the fundamental frequency of a short pulse, high repetition rate titanium–sapphire laser after its interaction with a Xe gas jet. The effects of the propagation of the fundamental and harmonic beams through an ionized medium are studied by analysing the spectral profile of the 9th and 7th harmonics. Finally, we report a study of the dependence of the harmonic conversion efficiency on relative position of the focus and the gas target. Received: 29 March 1996/Revised version: 25 July 1996     

Odd-isotope enrichment studies of Gd by double resonance laser-ionization for the production of burnable nuclear reactor poison

 Because of its large neutron capture cross-section, gadolinium (Gd) is widely used as burnable poison in nuclear reactors. Only two of its naturally occurring seven isotopes are strong neutron absorbers; hence, it would be desirable to enrich Gd in those isotopes. We demonstrate experimentally that significant enrichment can be achieved by two-step laser-ionization by relatively broad-band (bandwidth 1–2 GHz) lasers and by utilizing the isotope shifts only. Our results indicate that the content of the strong absorbers can be raised from the natural 30% to nearly 70%. We also compare the performance of this partially enriched Gd to natural Gd and to pure ¹⁵⁷Gd, the most strongly absorbing isotope, as burnable poison. Received: 3 April 1996/Revised version: 14 October 1996     

Erbium incorporation in LiNbO3 by diffusion-doping

 The erbium incorporation into LiNbO₃ by diffusion doping is investigated in detail by means of Secondary Ion Mass Spectrometry, Secondary Neutral Mass Spectrometry, Rutherford Backscattering, Atomic Force Microscopy, X-ray Standing Wave technique and optical site-selective spectroscopy. The diffusion of erbium in LiNbO₃ can be described by Fick’s laws of diffusion with a concentration-independent diffusion coefficient. The diffusion constants and activation energies for Z-cut (X-cut) LiNbO₃ are 4.8×10^-5 cm²/s (12.0×10^-5 cm²/s) and 2.28 eV (2.44 eV), respectively. A limited solubility of erbium in LiNbO₃ has to be taken into account increasing exponentially with rising temperature. During the first step of diffusion an Er _x Nb _y -oxide layer is formed at the surface of the sample acting as diffusion reservoir. Erbium is incorporated into LiNbO₃ on vacant Li-sites slightly shifted from the original Li-position along the (-c)-direction. Site-selective spectroscopy found four distinguishable energetically different erbium centres at this lattice site resulting from locally different symmetries of the crystal field. Received: 21 March 1996 / Accepted: 12 August 1996     

Spectral dependence of the creation and erasure of electrical structure in ruby during nonlaser irradiation. Domain conductivity of ruby

The dependence of the rate of formation and erasure of photoinduced electrical domain structure in ruby on the spectral composition of nonlaser illumination is studied. Near-ultraviolet irradiation is found to cause rapid erasure of the domain structure. The photocurrent density and photoconductivity are determined as functions of the electric field strength of the domains during irradiation with different spectral compositions at 77 K and 300 K assuming a “capacitor” model for the domains. The dark conductivity of ruby is estimated. Fiz. Tverd. Tela (St. Petersburg) 39, 295–301 (February 1997)     

The valence state of M-ions in the chemically stabilized YSr2Cu3-xMxO7+δ (M=Mo, W and Re) superconductors

 Single phase Ba-free Sr-based YSr₂Cu _3-x M _x O_7+δ (M=Mo, W and Re) compounds have been stabilized by chemical doping. Superconductivity is observed for these phases in the range 30–45 K. X-ray diffraction studies suggest a relatively small orthorhombicity compared to Ba-analogue. X-ray photoelectron spectroscopic investigations reveal that the stabilizing cations are in the hexavalent state. The observation of the higher oxidation state of M-ions accounts for the excess oxygen content in these phases which is in accordence with the diffraction results. Received: 10 June 1996 / Accepted: 20 September 1996     

Spin-lattice relaxation and Knight shift on protons in the hydrogen-doped superconducting system H0.2La1.8Sr0.2CuO4

Measurements of the spin-lattice relaxation rate and Knight shift on protons in hydrogen-doped superconducting H_0.2La_1.8Sr_0.2CuO₄ samples are performed in the temperature range 4.2–300 K. An anomalous behavior of the spin-lattice relaxation rate is observed at low temperatures T∼20 K. A model is constructed that explains the appearance of carrier-depleted regions in the bulk of the semiconductor on the basis of the formation of a charged defect (proton). Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 533–538 (10 April 1996)     

Spin-reorientation phase transition in the basal plane in α-Fe2O3:Ga,Dy

In hematite crystals doped with gallium and dysprosium ions a spin-reorientation phase transition induced in the basal plane by the introduction of dysprosium ions was observed in the temperature range T=15–20 K. The effect is associated with the competition between the anisotropy fields arising from the iron and rare-earth subsystems. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 694–697 (10 May 1996)     

Relaxation and shift of the precession frequency of the spin of a negative muon in n-type silicon

The residual polarization of negative muons in n-type silicon with impurity density (1.6±0.2) · 10¹³ cm⁻³ is investigated as a function of temperature in the range 10–300 K. The measurements are performed in an external magnetic field of 0.08 T oriented transversely to the spin of the muons. Relaxation of the muon spin and a shift of the precession frequency are observed at temperatures below 30 K. The relaxation rate at 30 K equals 0.25±0.08 μs⁻¹. The shift of the precession frequency at 20 K equals 7 · 10⁻³. Both the relaxation rate and the shift of the precession frequency increase as the temperature decreases. At temperatures below 30 K the relaxation rate is described well by the relation Λ=bT ^−q , where q=2.8±0.2. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 539–543 (10 April 1996)     

Observation of ^1H tunneling diffusion in crystalline Si

H diffusion in crystalline Si has been measured in the temperature range of 50–220 K. The temperature dependence of the diffusion coefficient follows a power law of the type , . D(H) values range between 10– cm/s up to about 200 K, where a transition to thermally activated diffusion is indicated. The low-temperature transport mechanism is attributed to tunneling. Received: 15 October 1996 / Accepted: 11 November 1996     

The invar effect and phase transitions in Cs2ZnI4 crystals

An in situ x-ray diffraction study of Cs₂ZnI₄ crystals performed in the 4.2–300 K temperature range is reported. The lattice parameter measurements have revealed three anomalies corresponding to phase transitions. The thermal expansion coefficient along the c axis was found to vanish in the region of incommensurate and commensurate modulated phases, 120–96 K (the invar effect). A possible crystallographic model relating modulated atomic displacements to the invar effect is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 137–142 (January 1999)     

Characteristics of photoconductivity in thallium monosulfide single crystals

This work elucidates the photoconductivity (PC) of thallium monosulfide single crystals. Results are obtained in the 77–300 K temperature range, 1500–4500 V lx excitation intensity, 6–18 V applied voltage, and in the 640–1500 nm wavelength range. Both the ac-photoconductivity (ac-PC) and the spectral distribution of the photocurrent are studied in different values of light intensity, applied voltage and temperature. Dependencies of carrier lifetime on light intensity, applied voltage and temperature are also investigated as a result of the ac-PC measurements. The temperature dependence of the energy gap width was described by studying the dc-photoconductivity (dc-PC).      

The photovoltaic and photorefractive effects in KDP-type ferroelectrics

By illuminating KDP-type ferroelectrics (KH₂PO₄, K(D _x H_1−x )₂PO₄,x⋟0.7) in the region of the intrinsic optical absorption edge in the absence of an external field the steady state photocurrent and photo emf, being a few orders of magnitude higher than the energy gap, are observed. Under the same conditions the photorefractive effect (“optical damage”) is observed.