E(transverse optical)-mode properties in the LiTaO3: Nd crystal at room temperature

E(TO)-mode properties in LiTaO₃:Nd crystal were examined by analyzing the Raman spectra measured.E(TO) modes appear in the transverseA ₁ spectrum. Their intensities obviously increase in theE andE+A ₁ mixed-symmetry spectra but decrease in theE spectrum which shows new vibrational modes. In particular, in the transverse-E spectrum of y(xz) geometry, the properties ofE(TO) modes are similar to those of pure LiTaO₃ of the same geometry, whereas in the transverse-E spectrum in x(yz) geometry these modes are turned intoA ₁ (TO) modes. We attribute these properties to both the surface strain produced by mechanical polishing of the sample and the microstructural change of the LiTaO₃ crystal resulting from Nd doping.     

Y1Ba2Cu3O7-δ thin films from KrF laser ablation with substrate heating and in situ patterning by CO2 laser radiation

Y₁Ba₂Cu₃O_7– thin films were deposited by KrF laser ablation while replacing conventional contact heating by cw CO₂ laser irradiation of the substrate front surface. The HTSC films obtained on (100)ZrO₂ showed T _c(R=0)=90 K, T(90–10%)=0.5 K, j _c=2.5 × 10⁶ A/cm², a sharp transition in the ac susceptibility X(T), and pure c-axis orientation. Micrographs of thin films (< 0.5 m) showed a smooth morphology while thick films (>1 m) contained many crystallites sticking in the bulk material. Furthermore, in situ patterning was achieved during deposition by local laser heating of a selected substrate surface area. The resulting planar films contained amorphous, semiconducting parts only 1 mm or less apart from crystalline material showing the above HTSC quality.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Energy levels of vanadium ions in CdTe

In CdTe doped with vanadium the photoluminescence due to the ³ T ₂(F) ³ A ₂(F) transition of V³⁺(d ²) is detected. Its decay time is determined as (630±20) s, a result comparable to the analogous emissions in various host lattices. Further emissions around 5000 cm^–1 and 9000 cm^–1 are caused by charge-transfer transitions or bound-exciton decay. Excitation and sensitization spectra yield information on the positions of the energy levels within the gap, which are discussed using two different models. At T=4.2 K, the distance of the V²⁺/V³⁺ donor level is 7300 cm^–1 and 5700 cm^–1 referred to the valence and the conduction band edges, respectively. The absence of V²⁺(d ³) centres is tentatively ascribed to the existence of deeply bound excitons.     

Micro-Raman spectroscopy in polycrystalline CuInSe2 formation

The crystalline formation of CuInSe₂ thin films has been investigated using micro-Raman spectroscopy and AES composition analysis. It is confirmed that the Raman peaks are stongly dependent on the surface morphology and the Cu:In:Se ratio. In the films annealed at 315°C, crystalline grains larger than 2 m show Raman peaks at 174 cm^–1 and 258 cm^–1. The In content is very low and the Cu:Se ratio is about 1:1 in these grains. The low In concentration is thought to be due to the formation of In₂O₃ on the surface. On the other hand, random structures of 1–2 m grains found in films annealed at temperatures below 305°C show peaks at 174 cm^–1 and 186 cm^–1 instead of 258 cm^–1 and have a Cu:In:Se ratio of 1:1:3–4. Thus the 186 cm^–1 peak is thought to be related to a Cu, In-deficient phase when compared to stoichiometric CuInSe₂. The optimum annealing condition was found by analyzing the Raman spectra and composition of different crystalline CuInSe₂ grains. Films annealed under this condition exhibited a clear Raman peak at 174 cm^–1 and consisted of clusters of crystals less than 1 m in size.     

Positron annihilation study of spin-crossover in [Fe x Zn1−x (ptz)6](BF4)2 single crystals

Positron lifetime measurements were carried out on [Fe _x Zn_1–x (ptz)₆](BF₄)₂ single crystals in the temperature range of 72–300 K. Complementary Doppler broadening spectra were also recorded at room temperature. The probability of the formation of o-Ps (lifetime 500–1400 ps) was between 10–30%. Both the spin density and the lattice parameters of the sample affected the lifetime of o-Ps through the ortho-para conversion and the pick-off annihilation, respectively. The positron annihilation technique proved to be a sensitive detector of the spin-crossover⁵T₂ ¹A₁ and of the concomitant phase transformation.     

Synthesis and optical properties of single crystalline GaN nanorods with a rectangular cross-section

GaN nanorods were synthesized from the reaction of a Ga/Ga₂O₃ mixture with NH₃ on Si substrates by chemical vapor deposition. The synthesized products were characterized by scanning and transmission electron microscopy, X-ray diffraction, photoluminescence and Raman spectroscopy. The nanorods are highly single crystalline and possess uniform smooth surfaces. PL revealed only a strong emission at 3.268 eV, ascribed to free exciton (FX) transitions, at room temperature; while the well-known yellow luminescence band centered at 2.2-2.3 eV was not detected. Four first-order phonon modes, corresponding to the A₁(TO), E₁(TO), E₂(high), and A₁(LO) at ∼531, 554, 564, and 721 cm⁻¹, respectively, were observed by Raman backscattering. The red-shift of the FX emission peak and the down-shifts of the Raman modes by a few wave numbers are attributed to the presence of tensile strain inside GaN nanorods.     

Sensitivity of approximate formulas for determining optical constants of thin films

The sensitivity of approximate formulas for determining the optical constants of thin films using measurement of reflectancesR and transmittancesT at normal incidence have been investigated theoretically. The ranges of refractive indexn, absorption indexk,2nk (=₂) andn ²–k ²(=₁) within relative errors of 5%, 10%, and 20% may be obtained. Selected signs of (₁)⁺ or (₁)^– have been determined. Validity of the condition n₀ A=n _s A has been also evaluated (A=1–R –T andA=1–R–T).     

Infrared reflectivity of BaMnF4

The polarized reflectivity of BaMnF₄ single crystals was measured in the 20–600 cm^–1 range for temperatures from 10 to 300 K. The results were evaluated using Kramers-Kronig analysis to obtain the dielectric function. The number of active modes in the high temperature polar phase (T>T _i 250K) is in agreement with group-theoretical expectation. In the incommensurate phaseT<T _i the number of active modes increases more than two times. All the results are in reasonable agreement with previous Raman and far infrared data.     

Superconductivity induced phonon anomalies in the Raman spectra of Zn and Ni doped YBa2Cu3O7

Here we present Raman spectra of YBa₂(Cu_1–x Zn _x )₃O₇ and YBa₂(Cu_1–x Ni _x )₃O₇ as a function of temperature and Zn or Ni content. The temperature dependence of two modes at 340 and 440 cm^–1 is analyzed. Similarly to the infrared measurements it is found that Zn substantially suppresses the superconductivity induced phonon softening whereas, Ni does not affect much that effect. Moreover, the superconductivity induced phonon stiffening of the 440 cm^–1 mode completely disappeared with the Zn doping. We find this behaviour might support the model where Zn acts effectively as a magnetic pair breaker.     

Structural phase transition and internal fields in rubidium thiocyanate probed through vibrational spectroscopy

A comprehensive vibrational study of Rubidium thiocyanate (RbNCS), in the 298–448°K range reveals that the orthorhombic to tetragonal transition atT _c440°K is of order-disorder type exhibiting purely second order characteristics. A consistent order-parameter exponent 0.45±0.04 has been obtained from both Raman and IR measurements. In the Raman spectra, the appearance of sidebands in both the C–N&C–S stretching modes and their temperature dependence indicate the existence and a gradual diminution of internal electric field asT approachesT _c. These results have been discussed in the context of an anharmonic oscillator model. An activation energy (U) for the reorientational motion of NCS^– ions,U0.29±0.02 eV, has been calculated from large changes in the bandwidth (_C-N) nearT _c.     

The temperatures of protons andπ − mesons in central nucleus-nucleus interactions at a momentum of 4.5 GeV/c per incident nucleon

An estimate of the temperature of protons and ^– mesons in central He–Li, He–C, C–C, C–Ne, C–Cu, C–Pb, O–Pb, Mg–Mg interactions is presented. The results indicate an increase of the proton temperature with increasing mass numbers of projectile and target nuclei (A _p ,A _T ) fromT _p =(118±3) MeV for He–Li toT _p =(141±2) MeV for C–Pb. The temperature of ^– mesons does not depend onA _P ,A _T andT 95 MeV. A satisfactory fit for ^– mesons in C–Cu, C–Pb, O–Pb, Mg–Mg collisions can be achieved by using a form involving two temperatures,T ₁ andT ₂. The relative yield of the high temperature component (T ₂) is 24% for C–Cu, C–Pb, and Mg–Mg interactions. The observed results forT _P in C–Ne, C–Cu and C–Pb collisions are consistent with the prediction of the thermodynamic hagedorn model.     

Origin of the Raman mode at 379 cm−1 observed in ZnO thin films grown on sapphire

In this work, we present a detailed Raman scattering study to clarify the origin of the mode at 379 cm⁻¹ which is observed in Raman spectra of the ZnO films grown on c‐sapphire substrates and generally attributed to the A₁‐transverse optical (A₁‐TO) mode of ZnO. The studied ZnO films were deposited by metal‐organic chemical vapor deposition on c‐sapphire and (0001) ZnO substrates. In the z(−,−)z̄ backscattering configuration, the A₁‐TO mode is forbidden, while the 379 cm⁻¹ peak is still observed in the as‐deposited film grown on sapphire substrate. However, this mode is not observed in Raman spectra of the as deposited film grown on ZnO substrate. We suggest that the peak at 379 cm⁻¹ is the E_1g mode of the sapphire substrate which is allowed in z(−,−)z̄ backscattering configuration. The effects of annealing, the substrate and the collection cross‐section on Raman active modes were analyzed. Copyright © 2011 John Wiley & Sons, Ltd.     

Crystallization temperatures of thin Co−Zr amorphous alloy films from electron diffraction

The transformation of amorphous thin alloy films of Co–Zr to the crystalline state was observed with an electron microscope in the diffraction mode. This investigation elucidates the crystallization temperatures,T _x , of thin films as opposed to theT _x presented for thicker samples; usually melt-quenched in the thickness range of 20–50 m. TheT _x of the thin films are compared also with theT _x for sputtered alloys of Co–Zr at 5 m.     

Photoelectric properties of Pb1−x−y Sn x Ge y Te:In epitaxial films

Pb_1–x–y Sn _x Ge _y Te:In epitaxial films are examined in a wide temperature interval and at various background fluxes. These films have high sensitivity to infrared radiation in the spectral range <20m. The lifetime depends exponentially on temperature and varies from several seconds at T=10 K to 10^–2 s at T=20 K. The two-electron model of Jahn-Teller centers is proposed to explain the results. Multielement photoresistors based on these films are fabricated and D*=1.7×10¹³ cm Hz^1/2 W^–1 at T=25 K is achieved. Noise of the photoresistors is independent of background flux when it varies from 10¹² cm^–2 s^–1 to 10¹⁸ cm^–2 s^–1. As compared with Si:Ga and Ge:Hg photoresistors, the responsitivity is several orders larger at the operating temperature 25–30 K.     

Preparation of high J c YBa2Cu3O7−δ superconducting thin films by ion beam sputtering deposition

Preparation of high T _c and high J _c YBa₂Cu₃O_7– superconducting thin films by ion beam sputtering deposition is reported. The main factors affecting the composition of the films and the orientation of the crystal grains have been examined. Experimental results show that the Y, Ba and Cu composition of as-deposited films can be conveniently and accurately adjusted by a combined sputtering target which consists of a large sintered target of YBa₂Cu₃O_7– and a small one that is Ba and Cu rich (YBa_2.5Cu_3.3O_x). Fabrication conditions of highly oriented superconducting thin films are described. YBa₂Cu₃O_7– superconducting films with zero resistance at 88–90.5K and critical current density J _c (at 77K) of 1.5×10⁵ A/cm² are obtained.     

Variation of electronic properties and evidence for a reversible defect induced metal to semiconductor transition in PbMo6S8 observed by ion irradiation and subsequent annealing of thin films

The influence of lattice disorder varied by low temperature irradiation with 20 MeV³²S-ions and subsequent isochronal thermal annealing on Hall-effectR _H (T), resistivityp(T) and superconducting critical temperatureT _c of thin films of the Chevrel-phase PbMo₆S₈ is reported. It is found that the well known, unusual sensitivity ofT _c is correlated with drastic changes of normal state transport properties e.g.R _H (T) andp(T). In the low fluence regime (6·10¹³ cm^–2,T _c 2 K) annealing leads to a monotonous restoration of the initial properties with the main recovery occurring at temperatures as low as 500 K. Contrary to this, annealing of highly disordered samples (10¹⁵ cm^–2) creates semiconductor-like conduction behavior. This manifests itself by a strong increase of the electrical resistivity with decreasing temperature which becomes more pronounced at higher annealing temperaturesT _A . AfterT _A =800 K the resistivityp(15K) is enhanced by more than a factor of 1000 withp(15K)/p(280 K)=210 compared to as irradiated. Further annealing at 900 K and 1000K results in the reappearance of metallicp(T)-behavior and superconductivity (T _c >10K). The observed effects can be understood by systematic changes of the electronic density of states consistent with an earlier proposed defect model.     

Raman polarization studies of highly oriented organic thin films

In this work we report on the capability of polarized Raman spectroscopy to investigate the structure of thin organic films. Diindenoperylene (DIP) thin films on (1 × 1)‐rutile(110) were prepared via organic molecular beam deposition (OMBD). Raman spectra of DIP thin films showed several strong Raman modes in the wavenumber region from 1200 to 1650 cm⁻¹. The A_g mode at 1284 cm⁻¹ shows two contributions, thereby indicating the coexistence of at least two DIP film structures. Polarized Raman spectroscopy was applied to characterize the molecular orientation and the dominance of the σ‐configuration (i.e. upright standing DIP molecules) was found. Copyright © 2009 John Wiley & Sons, Ltd.     

Optic-mode coupling in ferroelectric gadolinium molybdate

Separate measurements of the A₁(TO) and A₁(LO) Raman spectra of ferroelectric gadolinium molybdate at 80°K and above have elucidated the origin of the anomalous temperature dependence of the two lowest frequency lines in the A₁(TO) spectrum. The observed behavior is postulated to be the result of coupling among modes at 44.5, 51.5, and 83 cm^?1 (at 80°K). The 44.5 and 83 cm^?1 modes become the degenerate, soft zone-boundary modes of the paraelectric phase while the 51.5 cm^?1 mode changes to B₂ symmetry. The two lowest frequency lines are the same as those observed previously in i.r. absorption.     

Influence of sputter-etching of substrate on the microstructural and optical properties of ZnO films deposited by RF magnetron sputtering

ZnO films were prepared using radio frequency magnetron sputtering on Si(1 1 1) substrates that were sputter-etched for different times ranging from 10 to 30 min. As the sputter-etching time of the substrate increases, both the size of ZnO grains and the root-mean-square (RMS) roughness decrease while the thickness of the ZnO films shows no obvious change. Meanwhile, the crystallinity and c-axis orientation are improved by increasing the sputter-etching time of the substrate. The major peaks at 99 and 438 cm⁻¹ are observed in Raman spectra of all prepared films and are identified as E₂(low) and E₂(high) modes, respectively. The Raman peak at 583 cm⁻¹ appears only in the films whose substrates were sputter-etched for 20 min and is assigned to E₁(LO) mode. Typical ZnO infrared vibration peak located at 410 cm⁻¹ is found in all FTIR spectra and is attributed to E₁(TO) phonon mode. The shoulder at about 382 cm⁻¹ appearing in the films whose substrates were sputter-etched for shorter time (10-20 min) originates from A₁(TO) phonon mode. The results of photoluminescence (PL) spectra reveal that the optical band gap (Eg) of the ZnO films increases from 3.10 eV to 3.23 eV with the increase of the sputter-etching time of the substrate.     

Raman Scattering on the Effective Soft Mode for Lithium Niobate Crystals

The evolution of Raman spectra in a wide range of temperatures that includes the ferroelectric transition point in lithium niobate single crystals is studied for polarization geometry X(ZZ)Y. In this geometry, the soft mode responsible for the phase transition distinguished by 1A₁(TO)-type symmetry should appear in the spectra. Experimental studies show that the 1A₁(TO) mode interacts resonantly with nonfundamental modes in the low-frequency region of the spectrum. Near the ferroelectric phase transition point, an isofrequency opalescence effect is observed that consists of an abrupt increase in Raman signal intensity at fixed frequencies near the excitation line.