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Optical properties of insulating BaSnO3 (BSO) and conducting Ba0.97La0.03SnO3 (BLSO) single crystals were studied at room temperature in a wide spectral range between 0.01 and 5.9 eV by means of spectroscopic ellipsometry. The far-infrared spectra of the optical phonons in BSO and BLSO were complemented by polarized Raman scattering measurements in BSO. The electronic band structure and the optical response (dielectric function) were calculated using density functional theory, which allowed for the interpretation of the main spectroscopic features such as optical phonons and electronic interband transitions. To reconcile the observed experimental spectra with the theory, a departure from the ideal perovskite structure on the local scale was proposed for BSO.

The polymorphic intermetallic compound Zn_5Sb_4In_(2−δ) (δ = 0.15(3)) shows promising thermoelectric properties at low temperatures, approaching a figure of merit ZT of 0.3 at 300 K. However, thermopower and electrical resistivity changes discontinuously at around 220 K. Measurement of the specific heat locates the previously unknown temperature of the order-disorder phase transition at around 180 K. Investigation of the charge carrier concentration and mobility by Hall measurements and infrared reflection spectroscopy indicate a mixed conduction behavior and the activation of charge carriers at temperatures above 220 K. Zn_5Sb_4In_(2−δ) has a low thermal stability, and at temperatures above 470 K samples decompose into a mixture of Zn, InSb, and Zn_4Sb_3.

The modes of B1u -symmetry observed in the infrared spectra of YBa2Cu3O7−δ and YBa2Cu4O8 at v ≈ 277 cm−1 and 500 cm−1, respectively, are assigned to vibrations of the chain-oxygens. This assignment is reached by analysis of the corresponding eigenvectors, influence of Au, Pr, 18O doping on these frequencies and comparison of infrared spectra of both materials. Anomalous softenings of oxygen-related phonon frequencies with decreasing temperature are found starting above T c , the most pronounced ones occuring for YBa2Cu4O8.

Comparative study of temperature dependences of the Raman spectra of rhombohedral antiferromagnetic LaMnO 3 (T N ≈ 150 K) and ferromagnetic magnetoresistive La 0.98 Mn 0.96 O 3 (T c ≈ 200 K) confirms the assignment of the two broad high frequency Raman bands to dynamical Jahn-Teller lattice distortions and allows to monitor the variations of distortion amplitudes and polaronic lifetime through the insulator-metal transition in La 0.98 Mn 0.96 O 3 .

Optical properties of La 0.7 Pb 0.3 MnO 3 single crystals, which exhibit paramagnetic-to-ferromagnetic transition at T c = 332 K, are studied by optical reflection spectroscopy over broad frequency and temperature range. It is shown that this transition is accompanied by an abrupt increase of the free charge carrier density due to delocalization. Experimental results are in good agreement with theoretical predictions of Millis et al. for electron-phonon coupling strength λ ≈ 1.

By analysis of the temperature evolution of Raman spectra, we connect an observed supression in low-energy electronic density of states to an anomalous sharpening of the Ba-phonon mode in superconducting YBa2Cu4O8 (Y-124) at around T=150 K. We link the supression of electronic excitations to an opening of a pseudogap. Similar effects observed for the Ba-phonon mode at roughly the same temperature in non-superconducting PrBa2Cu4O8 (Pr-124) suggests that a pseduogap opens up around 150 K also in this material.

Optical and electronic properties of metal (Pb, Bi, Sn, and In) doped Zn4Sb3 are reported in the temperature range 80–300 K, which covers the β, α, and α′ structural phases of this thermoelectric material. Metal doping alters the subtle balance between Zn disorder and Zn deficiency present in β-Zn4Sb3 and changes its low temperature structural behavior. Analysis of infrared reflection data shows that the formation of ordered α′-Zn4Sb3 is accompanied by a substantial increase in the free charge-carrier concentration. In contrast, for samples where doping suppresses the occurrence of the low temperature α′-phase, the free charge-carrier concentration is only weakly temperature dependent. Different degrees of structural disorder in doped β-Zn4Sb3 and the ordering processes at low temperatures leading to α- and α′-Zn4Sb3 are also recognized in the charge-carrier dynamics.

Photoluminescence and absorption spectroscopy experiments are implemented on as-grown and thermally annealed GaAs1−xNx epilayers grown on GaAs(001) having a nitrogen content in the range of 0.4%–7.1%. At low temperature, photoluminescence spectra exhibit two sets of features: (i) a relatively broad peak at low energy in the vicinity of the band gap predicted by the band anticrossing model (BAC) and (ii) sharp excitonic features at higher energy (over 100meV above the band gap for x>4%). An enhancement of the photoluminescence response of excitonic emissions and a notable intensity reduction of the deeper luminescence were systematically observed for samples subjected to high-temperature postgrowth annealing treatments. For pseudomorphically strained low nitrogen-containing epilayers (x

Optical and electronic properties of thermoelectric Zn4Sb3 across the low-temperature phase transitions

A new viologen gold iodide, (C12H14N2)Au2I4, has been prepared and characterized by X-ray crystallography and vibrational spectroscopy. The crystal structure features linear equidistant gold chains with Au−Au distances of 3.3767(3) A. The interactions within the anionic “crossed-sticks” [AuI2-]∞ chains are dominated by closed-shell “aurophilic” interactions between Au(I) atoms. The low-frequency Raman and IR spectra of the compound are reported and interpreted on the basis of the group theoretical analysis of the long-wavelength optical phonons.

The B 1g -symmetry Raman spectra of lightly Co-substituted YBa 2 (Cu 3−x Co x )O 6.95 (x=0−0.05, T c =91.4K−86K) thin films have been investigated. We find that the low temperature peak in the B 1g continuum, at −500 cm −1 , shifts to higher frequencies with increasing Co-substitution, corresponding to a reduction of T c in the slightly underdoped region. Comparing these findings with published results on slightly overdoped YBa 2 Cu 3 O 7 we conclude that the B 1g Raman continuum peak frequency scales inversely with hole doping within the T c ∼90K doping plateau of the Y123 system. The lack of a correlation between T c and the frequency of the B 1g continuum peak suggests that this peak is not due to scattering across a superconducting gap.

Superconductivity induced renormalization of Raman- and infrared-active optical phonons is studied for undoped and T c -reduced Pr- and Zn-substituted double-chain YBa 2 Cu 4 O 8 materials. The magnitude of the phonon frequency and the linewidth variation upon cooling below T c is found to be dependent on the mode symmetry; odd- end even-parity phonons respond qualitatively different to the reduction of T c . Assuming phonon coupling to the excitations participating in the formation of the superconducting gap, these experimental facts provide evidence for the gap anisotropy of YBa 2 Cu 4 O 8 .

The temperature dependence of the B 1u (z) infrared phonon frequencies, linewidths and oscillator strengths is analyzed for RBa 2 Cu 3 O 7−δ ceramics (R = Er, Tm, Dy, Pr). The values of an electron-phonon coupling constants for the odd infrared-active vibrations were estimated by comparison of the experimental data with strong-coupling theory calculations. Those values (0.01−0.02) are in good agreement withthose reported for the fully symmetric Raman-active phonons as well as with ab initio calculations.

We report on a Raman scattering study of ( Y x Pr 1 − x ) 2 Ba 4 Cu 6 + n O 14 + n − δ ( n = 1, 2) superconductors with 0 ≤ x ≤ 1 in the temperature range 10–300 K. The observed changes of the phonon frequencies, including the double chain modes, with Pr-content x in general follow from the variations of the bond distances. The apex-oxygen mode is, however, found to exhibit stronger variation with x in single ( n = 0) then in double-chain ( n = 1, 2) materials, which may indicate charge redistribution effects between chains and planes.

The results of a study of the thermodynamic, electrical and X-ray scattering properties of Cd1-xPbxF2 crystals are presented. The parameters of the transition to the superionic state have been determined. A correlation has been found between ionic conductivity and the superionic-state transition temperature in Cd1-xPbxF2 crystals.

RESONANCE OF ONE-PHONON AND TWO-PHONON STATES IN MIXED SEMICONDUCTORS V. V. Artamonov, M. Ya. Valakh, A. P. Litvinehuk, V. I. Sidorenko, and A. M. Yaremko UDC 535.343.2 Studies in recent years have revealed multifaceted resonance phenomena in the interaction of exeitons, biexcitons, plasmons, electronic excitations, and phonons in solid materials [1-5]. Interesting anomalies in the resonant anharmonic interactions have been observed in a study of polar[ton Raman scattering in crystals which lack a center of inversion symmetry [6] and in "soft" phonon modes in ferroelectric crystals [7]. In the case of polariton resonance the energy mismatch of the resonating excitations may be varied by changing the scattering angle, which in turn changes the polariton frequency. The resonance conditions in ferroelee- trics are varied by changing the crystal temperature. In this work we present the results of experimental and theoretical studies of the anharmonic resonance of phonon excitations in mixed II-VI semiconductors. In semiconducting crystals the phonon branches typically exhibit significant dispersion and, consequently, there is a broad band with a complex frequency distribution function. Therefore the spectral manifestation of the resonance of discrete and continuous excitations in such materials is rather complex and in many cases may exhibit a Fano contour [8 ] with its characteristically asymmetric bandshapes and a sharp valley in the intensity between them (the so-called "Fano antiresonance"). Since the resonant interaction is determined to a significant degree by the energy mismatch and by the relationship of the intensities of the resonating states [ 1, 8, 9], mixed sere[conducting crystals are conve- nient systems for study. In crystals with a one-mode character the rearrangement of the phonon spectrum as the composition is changed includes a significant change in the frequency of the long wavelength optic phonons. Therefore, it is most likely that in some interval of composition the one-phonon state will cross a band of two- phonon excitations of the appropriate symmetry. In the case of crystals with a two-mode character, the rear- rangement of the spectrum consists mainly of a redistribution of the intensities of the phonon lines due to scattering by the individual components, which also may influence the resonance phenomena. EXPERIMENTAL RESULTS Experimental studies of Raman scattering were carried out in a right angle geometry on an apparatus based on a DFS-24 spectrometer. The light source utilized was a He-Ne laser (~ = 632.8 nm). An FEU-79 photomultiplier was used as a detector, with a photon-counting system. In Fig. 1 we show the tlaman spectra of crystals of Zn1_xCdxSe for various compositions. In the compo- sition range 0 _< x _< 0.4 they are characterized by the structure of sphalerite. ]Preliminary experimental data on the resonance phenomena in this system have been reported previously [10]. In Zn0.89Cd0.11Se lines are ob- served at ~ = 140, 186, 204, and 250 cm -1 (curve 1, Fig. la). The latter two are due to scattering by the long lines (symmetry A1) are due to scattering processes involving simultaneous excitation of two TA phonons at the special points X and W in the Brillouin zone [11]. In the polarization studied (yy), corresponding to the fully symmetric component A1, the two-phonon peak at ~ = 140 cm -1 has a significant intensity. The appear- ance of the bands of F2 symmetry in the spectra is due to incomplete polarization of the exciting and scatter- ing light and also to some lifting of the selection rules caused by the disorder of the mixed crystals. As the CD content of the mixed crystals is increased the frequency of the long wavelength LO phonons decreases monotonically to ~ = 232 cm -t for x = 0.60 (curves 1-3, Fig. la). No significant change in the in- tensiW, halfwidth, or shape of this band is observed as this takes place. At the same time the changes in the vicinity of the transverse optical vibrations are significant. The TO band, which approaches the sharp two- phonon peak at 186 cm -~, causes a significant increase in the intensity of the latter (curve 2, Fig. la). In other words, there is a transfer of intens[ W between the one-phonon and two-phonon peaks. Such a process is known Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 38, No. 2, pp. 272-279, February, 1983. Original article submitted January 4, 1982. 0021-9037/83/3802-0223507.50 9 1983 Plenum Publishing Corporation 223

Resonant Raman scattering in bulk Zn1–x CdxSe crystals obtained from the vapour phase by the “free-growing” method is studied experimentally. The existence of seventh-order LO-phonon lines under condition of excitation above the band gap and fourth-order LO-phonon lines under condition of excitation below the band gap is observed for mixed semiconductor crystals for the first time. The obtained ratios of intensities of different lines in multiple-phonon spectra are compared with the results of the theory involving the continuous exciton spectrum as an intermediate state. [Russian Text Ignored]

Temperature studies of lattice dynamics and ionic conductivity, within the temperature range of 100 to 700 K, are performed for PbF2 and PbF2:6% LiF superionic crystals. The doped crystals exhibit a decreased activation energy of the conductivity. A decrease, by about 70 degrees, in the temperature at which the ionic conductivity changes and Raman spectra anomalies occur is observed. It is found that at temperatures significantly lower than that of the superionic state transition, the Raman line form is determined by processes of phonon scattering at thermally excited defects. Fur PbF2- und PbF2:6% LiF-Superionenkristalle werden Temperaturuntersuchungen der Gitterdynamik und Ionenleitfahigkeit im Temperaturbereich von 100 bis 700 K durchgefuhrt. Die dotierten Kristalle zeigen eine verringerte Aktivierungsenergie der Leitfahigkeit. Es wird ein Abfall von etwa 70 Grad fur die Temperatur bei der die Ionenleitfahigkeit sich andert und Anomalien im Ramanspektrum auftreten, beobachtet. Es wird gefunden, das bei Temperaturen, die betrachtlich niedriger sind als die des Superionenzustandsubergangs, die Raman-Linienform durch Prozesse der Phononenstreuung an thermisch angeregten Defekten bestimmt wird.

The results of Raman scattering studies of polariton and plasmon-polariton (plasmariton) excitations in 6H-SiC are presented. Polariton anisotropy and resonant coupling with one- and two-phonon host lattice vibrations are discussed. The effect of free charge carriers upon the dispersion of polaritons in nitrogen-doped 6H-SiC crystals is studied. Good agreement of theoretical calculations with the experimental results is obtained. [Russian Text Ignored].

The results of the determination of temperature dependence of Raman scattering and lattice parameters in Cd1−xPbxF2 crystals are presented. Anomalies of temperature dependence of the lattice parameter and Raman line width for investigated crystals have been observed.

The optical properties of mixed crystals CdTe1−x−ySexSy and ZnTe1−x−;ySexSy are investigated by methods of far infrared spectroscopy. Different types of transformation of the vibrational spectra of these solid solutions with composition are found. The longitudinal and transverse optical vibration frequencies are calculated theoretically by means of a unit cell version of the random element isodisplacement model. The calculations are in satisfactory agreement with the experimental results. The reflection spectra peculiarities with mixed crystal disordering are discussed. [Russian text ignored].

Earth-abundant and low-cost semiconductors, such as zinc phosphide (Zn3P2), are promising candidates for the next generation photovoltaic applications. However, synthesis on commercially available substrates, which favors the formation of defects, and controllable doping are challenging drawbacks that restrain device performance. Better assessment of relevant properties such as structure, crystal quality and defects will allow faster advancement of Zn3P2, and in this sense, Raman spectroscopy can play an invaluable role. In order to provide a complete Raman spectrum reference of Zn3P2, this work presents a comprehensive analysis of vibrational properties of tetragonally-structured Zn3P2 (space group P42/nmc) nanowires, from both experimental and theoretical perspectives. Low-temperature, high-resolution Raman polarization measurements have been performed on single-crystalline nanowires. Different polarization configurations have allowed selective enhancement of A1g, B1g and Eg Raman modes, while B2g modes were identified from complementary unpolarized Raman measurements. Simultaneous deconvolution of all Raman spectra with Lorentzian curves has allowed identification of 33 peaks which have been assigned to 34 (8A1g + 9B1g + 3B2g + 14 Eg) out of the 39 theoretically predicted eigenmodes. The experimental results are in good agreement with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Three separate regions were observed in the phonon dispersion diagram: (i) low-frequency region (225 cm−1) which is attributed to primarily P-related vibrations. The analysis of vibrational patterns has shown that non-degenerate modes involve mostly atomic motion along the long crystal axis (c-axis), while degenerate modes correspond primarily to in-plane vibrations, perpendicular to the long c-axis. These results provide a detailed reference for identification of the tetragonal Zn3P2 phase and can be used for building Raman based methodologies for effective defect screening of bulk materials and films, which might contain structural inhomogeneities.

Photoluminescence and absorption spectroscopy experiments were performed on as grown and thermally annealed GaAs1-xNx with nitrogen content in the range of 0.75–7.1%. At low temperature, the photoluminescence spectra exhibits two set of features: (i) a relatively broad peak at low energy and near to the vicinity of the predicted band gaps and (ii) a sharp excitonic feature at higher energy (about 100 meV for x>4%). Post growth thermal annealing processes systematically favor stronger excitonic emissions, and a notable intensity reduction of the deeper (defect related) luminescence. The low temperature binding energy of the higher energy excitonic peak is found to be consistent with the increase of the electronic effective masses. A careful examination of the data obtained in this work suggests that for higher nitrogen content (x>4%), the fundamental band gap of GaAsN is located at significantly higher energies than those commonly accepted for these alloys.

The variation of the multiphonon LO-lines polarization degree and the sublinear dependence of their intensity on the pumping level have been established. The results explained in view of both the transverse and the longitudinal relaxation of the real intermediate states.