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At least 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Raman spectroscopy of isotopically pure ({sup 12}C, {sup 13}C) and isotopically mixed ({sup 12.5}C) diamond single crystals at ultrahigh pressures

The Raman scattering by isotopically pure {sup 12}C and {sup 13}C diamond single crystals and by isotopically mixed {sup 12.5}C diamond single crystals is studied at a high accuracy. The studies are performed over a wide pressure range up to 73 GPa using helium as a hydrostatic pressure-transferring medium. It is found that the quantum effects, which determine the difference between the ratio of the Raman scattering frequencies in the {sup 12}C and {sup 13}C diamonds and the classical ratio (1.0408), increase to 30 GPa and then decrease. Thus, inversion in the sign of the quantum contribution to the physical properties of diamond during compression is detected. Our data suggest that the maximum possible difference between the bulk moduli of the {sup 12}C and {sup 13}C diamonds is 0.15%. The investigation of the isotopically mixed {sup 12.5}C diamond shows that the effective mass, which determines the Raman frequency, decreases during compression from 12.38 au at normal pressure to 12.33 au at 73 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Diffracted transition radiation of an ultra-high-energy relativistic electron beam in a thin single-crystal wafer

We consider diffracted transition radiation (DTR) emitted by high-energy relativistic electrons crossing a thin single-crystal wafer in the Laue geometry. The expression describing the DTR angular density is derived for the case where the electron path length in the target is much smaller than the X-ray wave extinction length in the crystal and the kinematic nature of this expression is demonstrated. It is shown that the DTR angular density in a thin target is proportional to the target thickness.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Electronic Raman scattering and the renormalization of the electron spectrum in LuB{sub 12}

The electronic Raman scattering in LuB{sub 12} single crystals of various isotope compositions is studied in the temperature range 10–650 K. The shape and the energy position of spectral maxima depend on the direction and magnitude of a probe wavevector, the temperature, and the excitation symmetry and remain unchanged when the isotope composition changes. Experimental spectra are compared with the spectra simulated on the basis of a calculated electronic structure. The experimental results are successfully described when the electron spectrum renormalization effects caused by electron–phonon coupling are taken into account. This confirms that the origin of the observed spectra in LuB{sub 12} is due to Raman scattering by electrons. A comparison of the calculated and experimental data makes it possible to determine the coupling constant (λ{sub ep} = 0.32) that gives the correct superconducting transition temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

NMR study of the paramagnetic state of low-dimensional magnets LiCu{sub 2}O{sub 2} and NaCu{sub 2}O{sub 2}

A comprehensive NMR study of the magnetic properties of single crystal LiCu{sub 2}O{sub 2} (LCO) and NaCu{sub 2}O{sub 2} (NCO) is carried out in the paramagnetic region of the compounds for various orientations of single crystals in an external magnetic field. The values of the electric-field gradient (EFG) tensor, as well as the dipole and transferred hyperfine magnetic fields for {sup 63,65}Cu, {sup 7}Li, and {sup 23}Na nuclei are determined. The results are compared with the data obtained in previous NMR studies of the magnetically ordered state of LCO/NCO cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Anomalous Hall effect in a diluted p-InAs〈Mn〉 magnetic semiconductor

The dependences of the electrical resistivity and the Hall coefficient of single-crystal p-InAs〈Mn〉 bulk samples with an acceptor concentration of about 10{sup 18} cm{sup –3} on uniform pressure P = 4–6 GPa at T = 300 K in the region of impurity conduction are quantitatively analyzed. The anomalous Hall effect is shown to occur in p-InAs〈Mn〉. Its contribution is negative and correlates with the deionization of acceptors and an increase in the magnetic susceptibility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Magnetic properties of Co{sub 2}{sup 2+}Co{sub 1−} {sub x}{sup 3+}Fe{sub x}{sup 3+}BO{sub 5} (x = 0.10) single crystals with a ludwigite structure

The investigation of mixed Co–Fe ludwigite single crystals shows that their magnetic properties are close to the magnetic properties of Fe{sub 3}BO{sub 5} despite the predominance of cobalt ions. The magnetic properties of Co{sub 3–} {sub x}Fe{sub x}BO{sub 5} single crystals with x = 0.10 are studied in detail. Magnetometric measurements demonstrate a strong magnetic anisotropy with easy magnetization axis b, and the orbital magnetic moment of cobalt is in a frozen state. The detected temperature dependence of the absorption of Mössbauer spectra allowed us to determine the magnetic ordering temperature, which agrees with the results of magnetization measurements (T{sub C} = 84 K).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Analysis of the exchange magnetic structure in Pb{sub 3}Mn{sub 7}O{sub 15}

The indirect-coupling model is used to analyze the exchange magnetic structure of Pb{sub 3}Mn{sub 7}O{sub 15} in the hexagonal setting. The ratios of manganese ions Mn{sup 4+}/Mn{sup 3+} in each nonequivalent position are determined. Pb{sub 3}(Mn{sub 0.95}Ge{sub 0.05}){sub 7}O{sub 15} and Pb{sub 3}(Mn{sub 0.95}Ga{sub 0.05}){sub 7}O{sub 15} single crystals are grown by the solution–melt method in order to test the validity of the proposed model. The structural and magnetic properties of the single crystals are studied. The magnetic properties of the grown single crystals are compared with those of nominally pure Pb{sub 3}Mn{sub 7}O{sub 15}.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Two-dimensional antiferromagnetic correlations in an La{sub 1.4}Sr{sub 1.6}(Mn{sub 0.9}Co{sub 0.1}){sub 2}O{sub 7} single crystal

The temperature and field dependences of the magnetization, the electrical resistivity, and the magnetostriction of bilayer lanthanum manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} single crystals and cobalt-doped La{sub 1.4}Sr{sub 1.6}(Mn{sub 0.9}Cu{sub 0.1}){sub 2}O{sub 7} are measured. The magnetostriction of the cobalt-doped compound increases as compared to the initial La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} compound, and the magnetization and the magnetoresistance of the former compound change substantially. Powder and single-crystal neutron diffraction patterns are used to detect ferromagnetic ordering in La{sub 1.4}Sr{sub 1.6}(Mn{sub 0.9}Co{sub 0.1}){sub 2}O{sub 7} at a temperature below T{sub C} ~ 45(2) K, and this ordering coexists with antiferromagnetic correlations, which develop at temperatures below T{sub C} ~ 80(5) K.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Transport characteristics of phonons and the specific heat of Y{sub 2}O{sub 3}:ZrO{sub 2} solid solution single crystals

The temperature dependences of the specific heat and transport characteristics of phonons in single crystals of yttrium-stabilized zirconium dioxide Y{sub 2}O{sub 3}:ZrO{sub 2} solid solutions have been studied. It has been shown that the temperature dependences of the specific heat at T > 5 K are almost identical at the degree of stabilization of a solid solution with an Y{sub 2}O{sub 3} content of 5–20 mol %. Differences in the temperature dependences of the specific heat of samples from different sources at T < 5 K are due to the presence of low-energy two-level systems. The features of the transport characteristics of thermal phonons at liquid helium temperatures reflect not only the presence of two-level systems but also the scattering of phonons on low-dimensional domains of another phase coherently conjugate to the main phase of the Y{sub 2}O{sub 3}:ZrO{sub 2} solid solution.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mictomagnetic State in an EuBaCo{sub 2–x}O{sub 5.5–δ} Single Crystal

The magnetic properties of an EuBaCo{sub 1.9}O{sub 5.36} single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo{sub 2}O{sub 5.5} composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below T{sub C} = 242 K. At T < 300 K, all three spin states of the Co{sup 3+} ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co{sup 3+} ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co{sup 3+} spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo{sub 1.9}O{sub 5.36} single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo{sub 1.9}O{sub 5.36} is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Specific Heat and Phonon Transport in Er-Containing Rare-Earth–Aluminum Garnets at Liquid-Helium Temperatures

The temperature dependences of the specific heat and the transport characteristics of thermal-frequency phonons in single crystals of the solid solutions of rare-earth aluminum garnets are studied at liquid-helium temperatures in the presence of Schottky-type low-energy excitations. The kinetic characteristics of phonons as functions of the solid solution composition are measured. The relation between the kinetic and thermophysical characteristics of the material in the solid solutions of rare-earth aluminum garnets is analyzed under conditions of a nonstationary process and the spatial inhomogeneity caused by the coordinate dependence of the state of low-energy excitations. The thermalization conditions in the nonequilibrium phonon–low-energy excitation system are estimated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

The Second Moments of the Line Shapes of Multiple Quantum NMR Coherences in One-Dimensional Systems

The second moments of the line shapes of the zeroth- and second-order multiple quantum NMR coherences determined by the dipole–dipole interactions of nuclear spins in crystals have been calculated. The second moments determined by the zz part of the dipole–dipole interaction have been derived both by a direct calculation and based on the exact solution (in terms of the zz model) for the decay of the multiple quantum coherence intensities on the evolution period of a multi-quantum NMR experiment. The decay of the multiple quantum NMR coherence intensities in a single crystal of calcium fluorapatite is well described by the Gaussian curves with the calculated second moments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Some Properties of Fe{sub 1 +y}Se{sub x}Te{sub 1 –x} Single Crystals in the Superconducting and Normal States

Despite a large number of publications proving that a number of different materials can be classified as topological insulators, unambiguous proofs of the existence of promising topological superconductors have not been obtained as yet. The implementation of such systems is important not only for solid state physics, but also for applications, because topological superconductors can be used in quantum computers. We have made an attempt to confirm experimentally the assumption put forth in a number of theoretical publications that FeSe{sub x}Te{sub 1 –x} with the s-wave pairing, which is the simplest iron-containing superconductors, can become, under certain conditions, a topological superconductor with the π-wave pairing, which must have a superconducting gap in the bulk and the Majorana gapless surface states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Quantum Isotope Effect in Silicon at Low Temperatures

High-precision studies of Raman scattering in isotopically pure {sup 28}Si, {sup 29}Si, {sup 30}Si single crystals have been performed in the temperature range from 8 to 300 K. It has been found that a difference of about (0.4 ± 0.1) cm{sup –1} observed at temperatures T ≤ 100 K between the normalized Raman frequencies of the {sup 28}Si and {sup 30}Si isotopes measured with an accuracy of 0.1 cm{sup –1} is certainly due to quantum effects damping with increasing temperature. The possibility of observing quantum isotope effects in germanium has been estimated using experimental data on isotope effects in diamond and silicon.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Neutron Spin Rotation at Laue Diffraction in a Weakly Deformed Transparent Crystal with No Center of Symmetry

The effect of neutron spin rotation at Laue diffraction in a weakly deformed neutron-transparent noncentrosymmetric crystal has been described theoretically and studied experimentally. The effect is due to the bending of a Kato trajectory of the neutron in the deformed crystal. At a certain type of deformation, one of two neutron waves excited at Laue diffraction, which propagate in opposite crystal fields in the crystal with no center of symmetry, leaves the crystal. As a result, the spin of the remaining neutron wave is rotated by a certain angle with respect to initial direction due to the interaction of the magnetic moment of a moving neutron with the crystalline electric field. This effect is absent in an undeformed perfect crystal, where only the depolarization of a beam occurs because both waves in opposite electric fields have the same amplitude. A method has been developed for controlled deforming the perfect single crystal by creating a temperature gradient in it. Thereby, a new possibility of measuring electric fields acting on a neutron in noncentrosymmetric crystals, as well as a method of controlling these fields in experiments on studying the fundamental properties of the neutron, has been implemented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Influence of a Strong Magnetic Field on Magnetic Order in Polycrystalline Layered Cobaltites EuBaCo{sub 2 –x}O{sub 5.5 – δ}

The magnetic properties of polycrystalline EuBaCo{sub 2 –x}O{sub 5.5 – δ} (x ≤ 0.1, δ ≤ 0.14) are studied in the temperature range 2 K < T < 300 K in magnetic fields H ≤ 90 kOe. A comparison is made of the dependencies of magnetization M on temperature T and magnetic field H for polycrystalline and single crystal EuBaCo{sub 1.90}O{sub 5.36}, in which only Cu(III) ions are present. It is shown that in the magnetically ordered temperature range of magnetic order, the samples are ferrimagnetic, in which antiferromagnetic interactions between the nearest Co{sup 3+} ions with the same spins compete with the ferromagnetic exchange. With a decrease in temperature, a strong magnetic field affects the magnetic order. At T = 2 K in fields H = 50–90 kOe, all samples exhibit the properties of mictomagnetism. There are a shift in the hysteresis loops M(H) and a large difference in the values of the magnetization of the sample cooled in a magnetic field M (FC) and without a field M (ZFC) due to unidirectional magnetic anisotropy. In this case, there is no saturation of magnetization. The field dependences of magnetization taken in the cooling and heating in unequal fields (CHUF) mode indicate a metastable magnetic state. Near T = 20–30 K, an orientational transition characteristic of micromagnets takes place and a complex ferrimagnetic structure is formed. The main reasons for the manifestation of mictomagnetism properties in layered cobaltites are the presence of magnetoactive Co{sup 3+} ions in the states with intermediate and high spins at low temperatures and structural defects that affect the exchange interaction between the nearest neighbors. Nonmagnetic Co{sup 3+} ions in the low-spin state can also act as defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Coherent X-ray Radiation Excited by a Beam of Relativistic Electrons in a Single Crystal in the Direction of Beam Axis

In the framework of the two-wave approximation of the dynamic diffraction theory, a theory of coherent X-ray radiation of a relativistic electron beam in a single crystal in the direction close to the direction of the beam axis in the Bragg scattering geometry is developed for the general case of asymmetric reflection of the electron field relative to the target surface. Expressions describing the spectral-angular characteristics of parametric X-ray radiation near the velocity direction, transition radiation, and their interference term are obtained and studied.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Shubnikov–de Haas Effect and Electrophysical Properties of the Topological Insulator Sb{sub 2 –x}Cu{sub x}Te{sub 3}

The results of the study of the influence of Cu on the Shubnikov–de Haas effect and the electrical properties of p-Sb{sub 2 –x}Cu{sub x}Te{sub 3} single crystals (0 ≤ x ≤ 0.1) are presented. In the framework of the parabolic model of the energy spectrum, the hole concentrations and the Fermi energy in Sb{sub 2 –x}Cu{sub x}Te{sub 3} are calculated based on the oscillation frequencies of the magnetoresistance. It is shown that doping by Cu has an acceptor effect and significantly increases the frequencies of Shubnikov–de Haas oscillations. A plateau is observed in the dependences of the Hall resistance on magnetic field. The dependences of the resistance on temperature obey the power law with the exponent m = 1.2, which is characteristic of scattering on phonons with the contribution of scattering on ionized impurities. Unlike other impurities, the exponent remains almost unchanged when doped by Cu to the maximum concentrations studied.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗