Temperature dependence of the photoluminescence spectra in AgGaS2

The Silver Gallium Sulfide (AgGaS₂) ternary compound is a wide bandgap semiconductor (about 2.7 eV) whose photoluminescence properties are characterized by excitons and donor-acceptor pairs recombinations. We have performed photoluminescence (PL) measurement exciting with the third harmonic (3.5 eV) of a Nd:YAG laser from room temperature down to 10 K at different excitation power. In this work we report the dependence of the ‘green band’ on the excitation power at various temperatures.     

Transverse energy gap in 2H-NbSe2

The transverse energy gap in 2H-NbSe₂ has been measured using a Nb flat “point” contact tunneling technique. Its value of 1.15±0.07 milivolt yields a value of 3.78 for the ratio 2Δ(0)/kT_c.     

Anomalous temperature dependence of the paramagnetic resonance line in FeF3 · 3H2O

Anomalous line-broadening in the paramagnetic resonance of FeF₃ · 3H₂O, which has a linear chain arrangement, has been observed. The line width was found to decrease monotonically with increasing temperature in the range from 100–300°K.     

Magneto-optical studies of excitons in AgGaS2

The reflection, absorption and luminescence spectra of the lowest excitons in AgGaS₂ are measured in magnetic fields. The results can be explained by the quasicubic model. The exchange splittings of the Γ₄, Γ₅ and Γ₃ exciton state are evaluated. The Zeeman splittings of the Γ₅ and Γ₅-Γ₃ states yield g-values of + 3.8 and 3.7, respectively. They are determined mainly by the contributions of the hole and conduction electron spins.     

Anomalous temperature dependence of the band gap in AgGaSe2 and AgInSe2

The lowest band gaps of AgGaSe₂ and AgInSe₂ single crystals in the temperature range from 90 to 300 K were determined from photoconductivity measurements. Below (above ≈ 120 K in AgInSe₂ and ≈ 125 K in AgGaSe₂ the temperature coefficient of the band gap is +5 × 10⁻⁴ eV/K (−1.5 × 10⁻⁴ eV/K) and +1.1 × 10⁻⁴ eV/K (−4.28 × 10⁻⁴ eV/K), respectively. The positive value is explained with the lattice dilation effect being the dominant mechanism for the band gap variation at the temperatures less than ≈ 120–125 K.     

Anomalous temperature dependence of magnetization in polycrystalline Fe65Ni35 alloy

Magnetization at 0.3 and 140 Hz (0–10 Oe) and magnetic relaxation measurements were carried out in detail in the temperature range between 4.2 and 300 K for a polycrystalline Fe₆₅Ni₃₅ alloy. The typical temperature T_g and the magnetic field H_g which correspond to the anomalous temperature in χ-T curves and inflection field in σ-H curves, respectively, are summarized and a H-T diagram is obtained. A strong magnetic relaxation is observed along the H_g-T line. The temperature dependence of H_g is discussed by the thermal activation of 180° domain wall which is pinned strongly by the antiferromagnetic-like clusters below T_g. It is find that H _g is a linear function of T .     

Anomalous temperature dependence of magnetization in single-crystal Fe65Ni35 alloy

Magnetization (0–10 Oe) and magnetic relaxation measurements were carried out in the temperature range between 4.2 and 300 K for three picture-frame samples of Fe₆₅Ni₃₅ alloy whose edges were parallel to 100, 110 and 111, respectively. The typical temperature T_g and the magnetic field H_g which correspond to the anomalous temperature in the χ-T curve and inflection field in the σ-H curve, respectively are summarized and H-T_g and H_g-T diagrams are obtained. A strong magnetic relaxation is observed along the H_g-T line. The dependence of H_g on the crystallographic direction and on the temperature are discussed by the thermal activation process of the 180° domain wall which is pinned strongly by the antiferromagnetic clusters below T_g. The anomaly of magnetization of Fe₆₅Ni₃₅ alloy can be interpreted by the macroscopic picture of the coexistence of ferromagnetic and antiferromagnetic-like regions which may be caused by a statistical fluctuation of alloy composition.     

Edge emission in AgGaS2 single crystals

This paper reports on a study of the luminescence of single crystals of silver thiogallate (AgGaS₂). Cathodoluminescence measurements were made between liquid helium temperature and 250°K at two voltages, 25 kV and 10 kV and for 6 and ⊥ polarizations at the c axis. The cathodo-excitation technique used revealed the self-absorption phenomena, which are considerable near the bandgap.It is shown that the excitonic emission observed at liquid helium temperature remains up to 150°K and at higher temperatures there is band to band transition. In addition, a free to bound type of luminescence is indicated. This transition would appear to involve a level at 75 meV of the transport band.     

ESR and x-ray analysis of the ternary semiconductors CuAlS2, CuInS2 and AgGaS2

The electron spin resonance (ESR) of Fe³⁺-impurity ions in CuAlS₂, CuInS₂ and AgGaS₂ has been analysed. In addition, the crystallographic parameters a, c and x_f were determined by X-ray diffraction techniques.     

Absolute sense of piezoelectric coefficients in the chalcopyrites ZnGeP2 and AgGaS2

The absolute orientation of single crystals of ZnGeP₂ and AgGaS₂ on which the relative sense of the d₁₄ and d₃₆ piezoelectric coefficients had been measured, was determined by the X-ray absorption edge method. Both coefficients were thereby found to be positive, corresponding in the point charge model to positive charge associated with the metal and negative charge with the nonmetal atoms, in contrast with the negative d_33′ reported in GaP.     

Anomalous temperature dependence of phonon modes in Bi2Ti4O11 and modified Bi2Ti4O11

We have observed the anomalous temperature dependence of the phonon modes in the Raman spectra of Bi₂Ti₄O₁₁ and the modified Bi₂Ti₄O₁₁ with the dopant of PbO. The square of the lowest mode frequency in both substances decreases linearly with increasing temperature, while the damping constant increases reaching a peak value at the transition temperature. The mode frequency, however, does not tend to zero but stays finite and the damping constant decreases above the transition temperature. The mode is Raman-active both above and below the transition temperature.     

Donor-acceptor pairs and excitons recombinations in AgGaS2

Silver thiogallate (AgGaS₂) is a ternary semiconductor which crystallizes in the chalcopyrite structure. Silver thiogallate has been widely used in different applications for its interesting physical properties: wide transparency range (from 0.5 to 12 μm), high non-linear optical coefficient combined with good mechanical properties.The direct band gap in this compound is of about 2.7 eV and emissions due to free and bound excitons had been observed. Photoluminescence spectrum is also characterized by a wide emission band centred at 496 nm (2.50 eV) due to donor-acceptor pairs recombination (DAP).We performed photoluminescence (PL) measurements exciting with the third harmonic (3.5 eV) of a Nd:YAG laser from room temperature down to 10 K at different excitation power.In this work, we report the dependence of the photoluminescence features of AgGaS₂ on the excitation power at various temperatures: ionization energy of defects are estimated on the basis DAP theoretical model and of thermal quenching of the photoluminescence; evidences of non-radiative processes competitive to DAP is also presented.     

Energy levels from lattice defects in AgGaS2

The luminescence of single crystals of AgGaS₂ from shallow centers is studied. As-grown crystals and crystals annealed in S, Ga, or in vacuum, were used. We show that the S vacancy causes a donor level at 50 meV (probably due to a charge state of V_S) and that the cation vacancy (perhaps Ag) introduces an acceptor level at 110 meV. Two other levels are found: a donor level at 30 meV and an acceptor level at 200 meV. Their origin is unknown.     

A study of energy band gap versus temperature for Cu2ZnSnS4 thin films

The temperature dependent band gap energy of Cu₂ZnSnS₄ thin film was studied in the temperature range of 77-410 K. Various relevant parameters, which explain the temperature variation of the fundamental band gap, have been calculated using empirical and semi-empirical models. Amongst the models evaluated, the Varshni and Pässler models show the best agreement with experimental data in the middle temperature range. However, the Bose-Einstein model fits reasonably well over the entire temperature range evaluated. The calculated fitting parameters are in good agreement with the estimated value of the Debye temperature calculated using the Madelung-Einstein approximation and the Hailing method.     

A donor-acceptor pair broad band emission in AgGaS2

We interpret in this paper a broad band emission of silver thiogallate (AgGaS₂) peaked at 1.642 eV with a width of 395 meV. We show that the recombination kinetics are governed by donor-acceptor processes, with the band shape dominated by vibronic interaction. A theoretical calculation of the time-resolved spectra built with the parameters of donor-acceptor transitions gives good agreement with the experimental spectra.     

Anneal temperature dependence of Si/SiO2 superlattices photoluminescence

Photoluminescence spectroscopy, Fourier transform infrared spectroscopy, X-ray reflectometry and high resolution electron microscopy have been used to interpret the photoluminescence properties of annealed (3/19 nm) Si/SiO₂ multilayers grown by reactive magnetron sputtering. The multilayers show an emission in the visible and near-infrared range after heat treatment from 900°C which tends to decrease from 1200°C. Three different origins for the photoluminescence activity have been found. An anneal temperature of 1200°C is necessary to optimise the silicon crystallisation within the silicon sublayers.     

Anomalous field dependence of the inverse magnetocaloric effect in Ce(Fe0.93Ru0.07)2

A summary of detailed isothermal magnetization along with the ac susceptibility measurements on Ce(Fe_0.93Ru_0.07)₂ is presented. Near the first-order transition, i.e. near 110 K, Ce(Fe_0.93Ru_0.07)₂ displays a large positive magnetic entropy change, ΔSm, of 7.8 J/kg K in fields of only 2 T, i.e. it displays a strong inverse magnetocaloric (MCE) effect, as expected, on entering an antiferromagnetic (AF) state. However, the variation of the magnitude and width of this entropy change with field are anomalous when compared with model predictions, the former increasing with applied field below 2 T, while the latter exceeds 60 K in a field of 8 T.     

Non-linear temperature dependence of the low temperature specific heat of vitreous B2O3

The specific heat of virteous B₂O₃ has been measured between 50 mK and 1 K. The excess to the calculated acoustic term is found to vary as T^1.45 below 0.7K as opposed to the linear law generally observed. This may be attributed to the particular structure of this glass.     

Fundamental absorption of AgGaS2 single crystals and thin polycrystalline films

AgGaS₂ single crystals show an absorption edge at 300 K which follows Urbach's rule for both directions of polarized light, parallel and vertical to the optical c-axis. The UV absorption curve of a polycrystalline thin film exhibited several maxima at photon energies which are in good agreement with previous results on optical reflectance and electroreflectance measurements.     

Anomalous resistivity of iodine-free 2H-NbSe2

Resistance ratios of crystals of 2H-NbSe₂ grown with and without iodine were measured over the temperature range 6K–40K. Iodine-free crystals had higher ratios than any NbSe₂ previously reported and exhibited an abrupt anomaly at the onset of the charge density wave phase. All crystals displayed a cubic temperature dependence of their resistivities below 18K.