Low-temperature specific heat of crystalline and amorphous Eu2(MoO4)3

The specific heat C _total of crystalline and amorphous Eu₂(MoO₄)₃ is measured in the temperature interval 4.5–30 K. The amorphous state is obtained by applying pressure ∼7 GPa at room temperature. It is found that the specific heat of the crystal at T⩽7.5 K is described by a cubic function of temperature, while the specific heat of the amorphous sample has a strongly non-Debye character in the entire experimental temperature interval. The curve of C _total for amorphous europium molybdate is analyzed in a model of soft atomic potentials, and it is shown that it agrees well with universal low-temperature anomalies of the specific heat of classical glasses obtained by quenching from the liquid. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 8, 623–627 (25 October 1998)     

Giant domain walls in a ferromagnet

Giant domain walls with a width of ∼7 μm are observed on the surface of a ferromagnet — an amorphous magnetically soft alloy. A magnetooptic investigation shows that the walls have a Néel structure in the subsurface region. The subsurface structure of these walls differs substantially from that of the narrower walls previously observed in iron, Permalloy, and amorphous materials. According to the theoretical model of Scheinfein and co-workers, which relates the width of an asymmetric Bloch wall in the bulk with the width at the surface, the width of the wall in the bulk is estimated to be 3–4 μm. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 528–530 (25 October 1999)     

Irradiation of amorphous Ta<Subscript>42</Subscript>Si<Subscript>13</Subscript>N<Subscript>45</Subscript> film with a femtosecond laser pulse

Films of 260 nm thickness, with atomic composition Ta₄₂Si₁₃N₄₅, on 4″ silicon wafers, have been irradiated in air with single laser pulses of 200 femtoseconds duration and 800 nm wave length. As sputter-deposited, the films are structurally amorphous. A laterally truncated Gaussian beam with a near-uniform fluence of ∼0.6 J/cm² incident normally on such a film ablates 23 nm of the film. Cross-sectional transmission electron micrographs show that the surface of the remaining film is smooth and flat on a long-range scale, but contains densely distributed sharp nanoprotrusions that sometimes surpass the height of the original surface. Dark field micrographs of the remaining material show no nanograins. Neither does glancing angle X-ray diffraction with a beam illuminating many diffraction spots. By all evidence, the remaining film remains amorphous after the pulsed femtosecond irradiation.     

A high-intensity bremsstrahlung source for investigating radiation stability of optical samples for excimer laser windows

A bremsstrahlung X-ray source (hv ≤ 600 keV) was developed on the basis of CW electron LINAC. It provides an absorbed dose rate of ∼30 Gy/s within an area of ∼2.5 cm². The radiation stability of quartz glasses and crystals for excimer lasers windows was investigated.     

Grain boundary layers in nanocrystalline ferromagnetic zinc oxide

The complete solubility of an impurity in a polycrystal increases with decreasing grain size, because the impurity dissolves not only in the crystallite bulk but also on the grain boundaries. This effect is especially strong when the adsorption layers (or the grain boundary phases) are multilayer. For example, the Mn solubility in the nanocrystalline films (where the size of grains is ∼20 nm) is more than three times greater than that in the ZnO single crystals. The thin nanocrystalline Mn-doped ZnO films in the Mn concentration range 0.1–47 at % have been obtained from organic precursors (butanoates) by the “liquid ceramic” method. They have ferromagnetic properties, because the specific area of the grain boundaries in them is greater than the critical value [B.B. Straumal et al., Phys. Rev. B 79, 205206 (2009)]. The high-resolution electron transmission microscopy studies show that the ZnO nanocrystalline grains with the wurtzite lattice are separated by amorphous layers whose thickness increases with the Mn concentration. The morphology of these layers differs greatly from the structure of the amorphous prewetting films on the grain boundaries in the ZnO:Bi₂O₃ system.     

Study of nanostructures on a solar cell surface exposed on the <Emphasis Type="Italic">Mir</Emphasis> orbital station

It is shown that atomic-force microscopy under normal conditions makes it possible to obtain important information on the topography and features of nanostructures formed on the surface of cover glasses of solar cells exposed on the Mir orbital station for more than ten years. It is found that the nanostructures are multiscale; they are present on all vertical visualization scales from ∼1000 to ∼17 nm and on horizontal scales from ∼1000 to ∼100 nm. The nanoindentation study of mechanical properties of the exposed surface layer shows that the exposed surface at nanodepths is characterized by higher plastic deformation, but lower hardness and effective modulus, in comparison with unexposed surface.     

Investigation of longitudinal vibrations of -O-H groups in chrysotile asbestos by neutron scattering and polarized infrared spectroscopy

The specific features of the vibrational spectra of chrysotile asbestos, which is a natural mineral that represents a system of closely packed tubular fibers with an outer diameter of ∼30 nm, an inside diameter of ∼5 nm, and a length up to a centimeter and more, have been investigated using neutron scattering and polarized infrared spectroscopy. This material can serve as a natural matrix for the preparation of nanostructures by filling channels with various materials.     

Electrically switchable two-dimensional photonic crystals made of?polymer-dispersed liquid crystals based on?the?Talbot?self-imaging effect

Electrically switchable two-dimensional photonic crystals were demonstrated using polymer-dispersed liquid crystal materials based on the Talbot self-imaging effect of a single photomask. With the photomask subjected to a collimated Ar⁺ laser beam operating at 488 nm, a three-dimensional spatial light intensity pattern was created due to the Talbot self-imaging effect. The spatial light intensity pattern was then recorded inside a cell filled with the liquid crystal/prepolymer mixture to create photonic crystal structures. The surface morphology of the photonic crystals was examined by an atomic force microscopy. It showed square structures with a lattice constant of ∼0.9 μm. The diffraction and electro-optical properties were also presented. This approach shows a simple and fast fabrication.     

The magnetic phase transition in amorphous ferromagnets and in spin glasses

The magnetic phase transition in materials with exchange disorder (amorphous ferromagnets, spin glasses) is discussed. In the critical temperature range the behavior of amorphous ferromagnetic transition metal-metalloid glasses is found to be similar to the one derived for a three-dimensional homogeneous Heisenberg ferromagnet. The most prominent difference between disordered and homogeneous materials is manifested in a large temperature range of deviations from the mean field behavior beyond the critical region, as observed experimentally for the temperature dependence of the linear susceptibility of amorphous ferromagnets and of the nonlinear susceptibility of spin glasses. A molecular field theory with correlations in space and time is developed, which relates the deviations from the mean field behavior to the interplay between the temperature dependent thermal correlations in the spin system and the spatial fluctuations of the material. Application to dynamical processes (kinetic critical slowing down) is discussed.     

Elastic properties of several amorphous solids and disordered crystals below 100 K

We have measured the internal friction and speed of sound variation at temperatures between 60 mK and room temperature for amorphous CdGeAs₂, Polystyrene, and Stycast 2850FT epoxy, and the disordered crystals (ZrO₂)_0.89(CaO)_0.11 and (CaF₂)_0.74(LaF₃)_0.26. A comparison of our results with an extensive review of previously published data shows a remarkable similarity in the internal friction of disordered solids below ～5 K. The low temperature elastic behavior of these solids is adequately described by the standard tunneling model, from which one finds a nearly universal density of tunneling states for glasses. Internal friction above ～10 K for different materials, however, displays a wide range of magnitudes and temperature dependence that is far from universal. Attempts to directly link the tunneling states observed by internal friction at low temperatures to configurational states of localized oscillators existing at high temperatures must take into account this striking variation among disordered solids above 10 K.     

Segmental dynamics of poly(methyl phenyl siloxane) confined to nanoporous glasses

The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.5–20 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.     

Silicon–germanium interdiffusion and interfaces in self-assembled quantum dots

A scanning transmission electron microscope (STEM) study of silicon–germanium alloying using annular dark field (ADF) or Z-contrast imaging and electron energy loss spectroscopy (EELS) is presented. Results and techniques are discussed. Growth of 11 equivalent monolayers of germanium on silicon at 650 °C results in dome-shaped islands or quantum dots that contain up to ∼40% silicon. The interface between the as-grown island and substrate shows a highly disordered or amorphous zone ∼1.5-nm wide directly under the island. Annealing for 60 min at 650 °C gives larger pyramidal islands with diffuse crystalline interfaces and an equilibrium distribution of up to ∼70% silicon in the islands. PACS 61.16.Bg; 68.35.Dv; 68.35.Rh     

Metal-insulator transition in amorphous Si1−c Mnc obtained by ion implantation

A metal-insulator transition (MIT) induced by a change in the impurity Mn concentration in a material with topological disorder — amorphous Si_1−c Mn_c — is investigated. It is found that near the critical point the localization radius, permittivity, and conductivity vary according to a power law in accordance with the scaling theory of localization. The critical exponents are determined. It is concluded that the basic mechanisms of the MIT in disordered systems do not depend on the type of disorder and are universal. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 333–337 (25 February 1997)     

Effect of vacancies and interstitials on the dynamic properties of the La2−x SrxCuO4 superconductor

The effect of vacancies and interstitials in the CuO₂ layer on the vibrational spectrum in the La_2−x Sr_xCuO₄ system has been calculated by molecular dynamics. It is shown that the excitation probability for local ∼0.4-eV high-frequency vibrations of nonphonon origin in the vicinity of Sr impurity atoms decreases if copper vacancies are introduced at a concentration x=0.17, which corresponds to the maximum superconducting transition temperature, this decrease being still more effective (by about ten times) if interstitial atoms are present. The appearance of interstitials makes a considerable region around them (five to six nearest neighbors) quasi-amorphous. A comparison with available experimental data is made. It is concluded that the behavior of the system under irradiation is accounted for primarily by interstitials, which bring about strong perturbation in the lattice (∼1 nm) up to making it completely amorphous. Fiz. Tverd. Tela (St. Petersburg) 40, 984–988 (June 1998)     

Adiabatic frequency conversion of ultrafast pulses

A new method for efficient, broadband sum and difference frequency generation of ultrafast pulses is demonstrated. The principles of the method follow from an analogy between frequency conversion and coherent optical excitation of a two-level system. For conversion of ultrafast pulses, the concepts of adiabatic conversion are developed further in order to account for dispersion and group velocity mismatch. The scheme was implemented using aperiodically poled nonlinear crystals and a single step nonlinear mixing process, leading to conversion of near-IR (∼790 nm) ultrafast pulses into the blue (∼450 nm) and mid-IR (∼3.15 μm) spectral regions. Conversion bandwidths up to 15 THz FWHM and efficiencies up to 50% are reported.     

Nonlinear optical properties of Cu x S and CuInS2 semiconductor nanoparticles in sol-gel glass

Quartz glasses containing ultradisperse particles (with a mean radius of 5 ± 3 nm) of semiconductor compounds Cu_xS and CuInS₂ were synthesized using the sol-gel technology. The line and differential absorption spectra of such glasses were investigated. In the range 600–700 nm, the samples demonstrate absorption bands that are assigned to the lowest-lying energy transition in quantum-size particles. Excitation by laser pulses with a wavelength of 540 nm causes the effect of induced absorption in the range of 500–950 nm. The induced absorption in the samples containing Cu_xS nanoparticles undergoes relaxation which has a characteristic time of ∼500 psec, and in the samples with CuInS₂ nanoparticles it has a two-exponential form with τ₁ ≈ 30 psec and τ₂>1 nsec. Translated from Zhurnal Prikladnoi Spektroskopii, vol. 67, No. 4, pp. 507–511, July–August, 2000.     

EPR and NMR line shapes in disordered ferroelectric crystals

A theory is developed for the magnetic resonance line shape in disordered ferroelectric crystals. In a study of the random fields responsible for inhomogeneous line broadening, nonlinear and spatial correlation effects are taken into account. It is shown that homogeneous broadening depends on the temperature and on the magnitude and orientation of the external magnetic field, as well as on the nonlinearity parameters of the ferroelectric material. The resonance line shape is calculated as the envelope of homogeneously broadened Lorentzian spin packets. Analytic expressions for I ₂(ω) and I ₃(ω) are introduced to account for nonlinearity and correlation effects of second and third order, respectively. Calculations are done for centrally symmetric crystals, as well as for those without a center of symmetry. It is shown that homogeneous broadening is important near the line peak, while the wings are determined mainly by inhomogeneous broadening. Manifestations of the predicted effects in the spectrum lines of disordered ferroelectric crystals are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 340–347 (February 1998)     

Universal relationships for the phonon spectra in BCC,FCC, and HCP crystals with a short-range interatomic interaction

The frequencies of the phonon branches that correspond to the vibrations of the close-packed atomic planes in bcc, fcc, and hcp crystals with short-range interatomic interaction are shown to be described by a universal relationship, which only contains two parameters for each branch, for any polarization λ. These phonon branches correspond to the (ξ, ξ, 0) direction in bcc crystals, the (ξ, ξ, ξ) direction in fcc crystals, and the (0, 0, ξ) direction in hcp crystals. This universal relationship can only be violated by long-range interactions, namely, the interactions outside the sixth coordination shell in a bcc crystal, the fifth coordination shell in an fcc crystal, and the eleventh or tenth coordination shell in an hcp crystal. The effect of these long-range interactions for each phonon branch can be quantitatively characterized by certain parameters Δ _nλ, which are simply expressed in terms of the frequencies of three phonons of the branch. The values of these parameters are presented for all bcc, fcc, and hcp metals whose phonon spectra are measured. In most cases, the proposed relationships for the frequencies are found to be fulfilled accurate to several percent. In the cases where the Δ _nλ parameters are not small, they can give substantial information on the type and scale of long-range interaction effects in various metals.     

Structural transformations of the cumulene form of amorphous carbyne at high pressure

Structural transformations of the cumulene form of amorphous carbyne which are induced by heating at high pressure (7.7 GPa) are investigated. These can be described by the sequence amorphous phase — crystal — amorphous phase — disordered graphite. Raman scattering shows that predominately the chain structure of carbyne remains at the first three stages. It was found that the intermediate crystalline phase is an unknown modification of carbon whose structure is identified as cubic (a=3.145 Å). A mechanism of structural transformations in carbyne that involves the formation of new covalent bonds between chains is discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 237–242 (25 August 1997)     

Dielectric dispersion of ultrathin polymer Langmuir-Blodgett films

Dielectric dispersion is studied in films made of vinylidene fluoride/trifluoroethylene copolymer [P(VDF/TrFE)], which are obtained by the Langmuir-Blodgett technique in the temperature range from −90 to +132°C. The thickness of the films studied is ∼7 nm. A glass transition is observed in the temperature range from −40 to −50°C; this indicates the presence of an amorphous phase in the ultrathin Langmuir-Blodgett films.