Quantum Criticality and Scaling of the Magnetic Response in CeCu6−x Au x

The heavy-fermion system CeCu_6?x Au _x which exhibits antiferromagnetic order for Au concentrations x>0.1, can be easily tuned to a quantum phase transition, either by chemical composition, i.e., Au concentration, or by an external magnetic field. We present extensive inelastic neutron scattering measurements in order to study the momentum and energy dependence of the critical spin fluctuations for concentration- and magnetic field-tuned CeCu_6?x Au _x . The quasi-2D fluctuations, observed for x=0.1 at the magnetic instability, are already present in pure CeCu₆ and persist to x=0.2, i.e., well into the magnetically ordered regime. This is a strong hint that disorder does not play a major role in this system. The magnetic response for quantum critical CeCu_6?x Au _x exhibits critical slowing down and obeys scaling relations that depend on the tuning parameter. While the E/T scaling of the dynamic susceptibility in concentration-tuned CeCu_5.9Au_0.1 is an indication for local quantum criticality, the E/T ^3/2 scaling in field-tuned CeCu_5.8Au_0.2 points to a conventional spin-density-wave instability.     

Magnetic, optical, and kinetic properties of Hg1 − x − y Mn x Dy y Te crystals

We have studied the magnetic, optical, and kinetic properties of Hg_{1 ? x ? y} Mn _x Dy _y Te crystals. The behavior of their magnetic susceptibility can be accounted for by the presence of clusters of various sizes. The Hg_{1 ? x ? y} Mn _x Dy _y Te crystals are shown to be n-type. Absorption data are used to determine the optical band gap of the crystals.     

Photoinduced electric properties of Pb1−xSnxTe single crystals

It was established that the simultaneous action of laser IR radiation (ω < E_g, where E_g, is the band gap) and an external field (density of current through the crystal, j≲10²A/cm²) results in an accelerated change in electrical properties of Pb_1−xSn_xTe single crystals, which is attributed to an accelerated diffusion in the external electric field of metallic ions from associates of metal-enriched defects and dopants, arising during the crystal growth.     

Edge absorption and light propagation in single crystals of Zn1−x Cd x As2 solid solutions

The transmittance of Zn_1?x Cd _x As₂ anisotropic single crystals in their transparency region is found to depend on the incident light polarization, photon energy, and sample thickness. This effect is shown to be associated with light scattering in Zn_1?x Cd _x As₂, which can be understood in terms of the crystal structure of the monoclinic crystals. The fundamental absorption edge in Zn_1?x Cd _x As₂ is dominated by an indirect forbidden transition for the E ‖ c polarization and by a direct forbidden transition for E ⊥ c.     

Growth, characterization and transport properties of Pb x Zn1 − x S mixed crystals

The polycrystalline Pb _x Zn_1???x S semiconductor powder with (0 ≤ x ≤ 0·5) has been prepared by controlled co-precipitation method from an alkaline medium using thiourea as a sulphide ion source. Pellets are made with these powders applying 10 ton/sq cm pressure and sintered at 800 °C for 2 h in nitrogen atmosphere. X-ray studies of these samples have indicated that the compounds are polycrystalline in nature with mixed hexagonal and cubic structure of ZnS and cubic structure of PbS. Lattice parameters (a and c) of all the compounds are determined from the X-ray data and are found to decrease non-linearly with increase in Pb concentration (x). It is also observed that the grain size of the crystallites increases in samples with x = 0–0·5. Scanning electron micrographs have shown that both cubic and hexagonal crystallites are present in the mixed crystals. The electrical conductivity in Pb _x Zn_1???x S is found to decrease with increase in composition (x = 0–0·5), whereas it increases at all temperatures in all samples. Mobility of charge carrier concentration is found to increase with increasing temperature. The increase in carrier mobility in Pb _x Zn_1???x S samples may be due to reduced grain boundary potential. In Pb _x Zn_1???x S samples with x = 0–0·3, sum of the activation energy due to charge carriers and grain boundary potential is equal to the activation energy due to conductivity.     

Thermodynamic study of the magnetic ordering of Fe3−x Mn x Si alloys

The compounds Fe_3–x Mn _x Si exhibit a complex magnetic behavior for 0.6x1.75 involving ferromagnetic ordering followed by a reordering at lower temperature. Measurements of the thermal expansion and the specific heat C _p show only a weak anomaly at the Curie temperature, T _C, whereas a large, relatively sharp peak occurs at the magnetic reordering temperature, T _R. Thermal expansion measurements for the Fe_2.2Mn_0.8Si compound made about T _R are indicative of long-range ordering that can be suppressed in an applied field of 0.4T. Magnetization measurements show that low fields, 1–2 mT, strongly influence the bulk magnetization. Irreversible magnetization behavior, similar to that in spin-glass systems, is displayed.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

Effect of correlated and point-like disorder on transport properties of YBa2Cu3O7−δ single crystals

Pinning by point-like defects and twin boundaries in single-crystalline YBa₂Cu₃O_7– has been investigated by measuring resistance and current-voltage characteristics in fields up to 5 T. A number of untwinned and twinned samples with different densities of point-like defects were studied. The experimental results are compared within the framework of vortex-entanglement.     

Magnetic properties of (Dy1−xGdx)3Ga5O12 garnet single crystals

Magnetic properties of Dy_1–x Gd _x )₃Ga₅O₁₂ (DGGG) garnet single crystals were calculated using the Weiss molecular field theory and also measured using the vibrating sample magnetometer in the temperature range 4.2–40 K in the effective magnetic field from 0–7 T. The magnetic properties of DGGG single crystals are distributed between those of Dy₃Ga₅O₁₂ (DGG) and Gd₃Ga₅O₁₂ (GGG) single crystals, but are considerably closer to those of DGG. Based on the magnetic properties, the magnetic entropy change, S_M, was evaluated in the temperature range below 15 K. DGGG single crystals are a prospective material for magnetic refrigeration below 15 K.     

Thermal conductivity of tellurium-enriched Pb1 − x Mn x Te single crystals

We have studied the effect of excess tellurium atoms (up to 0.5 at %) on the thermal conductivity of single-crystal Pb_{1 ? x} Mn _x Te (x = 0.04) samples annealed at 570K for 120 h. We have evaluated the lattice and electron thermal conductivities of the samples, their additional thermal resistance due to structural defects, and the coefficient in the expression for the effective phonon scattering cross section. We assume that some of the excess Te atoms fill lead vacancies, thereby reducing the effective phonon scattering cross section.     

Macroscopic magnetic properties in TlBa2Ca3Cu4O−δ−δ−δ single crystals

The four-CuO₂-layer TlBa2Ca₃Cu₄O_11– compound has a high superconducting transition temperature of 128 K and strong interlayer coupling. We have performed magnetic experiments on single crystals of TlBa₂Ca₃Cu₄O_11–, which include hysteresis loop, irreversibility line and time relaxation measurements. From these measurements, we observed the large critical current density, high irreversibility line and strongly temperature dependent flux creep rate in the TlBa₂Ca₃Cu₄O_11-gd single crystals.     

Structural and optical properties of novel optoelectronic Tl1−xIn1−xSixSe2 single crystals

Principal optical properties of Tl_1?xIn_1?xSi_xSe₂ solid state crystalline alloys were studied. The influence of the x on the principal optical and structural features was explored. Temperature features of the band energy gap are studied. The possible explanation of the observed behavior is given within a framework of the Urbach rule approach. The role of intrinsic defective sub-system and anharmonic electron–phonon interaction is discussed. A possible application of the titled materials for the infrared optoelectronic is discussed. The replacement of In atoms by Sn is evaluated. Additionally studies of influence of x on the optoelectronic features is done. The formation of the positive charged ions (donors) and negatively charged ions is explored within the introduced intrinsic defect models.     

Magnetic and structural properties of nanophase AgxFe1 − x solid solution particles

In this study, silver and iron elements are evaporated simultaneously to form nanocrystalline solid solution particles and then quench to liquid nitrogen temperature. The average composition of the nanocrystalline Ag-Fe system analyzed by scanning electron microscopy with energy-dispersive spectroscopy is close to the gross composition of the raw materials. X-ray diffraction patterns indicate only Ag peaks in those of the nanocrystalline Ag-Fe solid solutions. Transmission electron microscopy images of the Ag-Fe system indicates a mean particle of about 10 nm for these nanocrystalline solid solutions. The magnetic properties of Ag-Fe systems depend on the mean particle's sizes and the concentration of solid solutions. The temperature dependence of the magnetic properties is analyzed and related to the microstructural changes induced by the thermal treatments. Nanocrystalline Ag-Fe system with a Curie temperature of Fe occurs at about 580 °C. The magnetization and remanence of as prepared or after heat-treatment nanocrystalline Ag-Fe solid solutions increase with increasing atomic percentage of iron.     

Incoherent transport and pseudo-gap in HoBa2Cu3O7−δ single crystals with different oxygen content

In the present experimental study the temperature dependence of the resistivity, along the c-axis is investigated in HoBa₂Cu₃O_7?δ single crystals, with different oxygen contents. It is determined that, the boundaries of the twin domains are effective centers for the scattering of normal carriers. With the reduction of the oxygen content, there is an increase of the processes of the carrier localization. This in turn accompanies the transition from a pseudo-gap regime to the variable-range-hopping regime. For HoBa₂Cu₃O_7?δ (in distinction to the YBa₂Cu₃O_7?δ, compounds) the temperature dependence for the thermally activated hopping conductivity of the perpendicular resistivity is well described, by the universal “law of ½”.     

Dielectric and optical properties of TlGa1 − x Er x S2 (x = 0, 0.001, 0.005, 0.01) single crystals

TlGa_{1 ? x} Er _x S₂ (x = 0, 0.001, 0.005, 0.01) solid solutions, based on the layered compound TlGaS₂, have been prepared by direct elemental synthesis. The effect of Er concentration on the dielectric and optical properties of the TlGa_{1 ? x} Er _x S₂ solid solutions has been studied. The results demonstrate that increasing the Er content of the TlGa_{1 ? x} Er _x S₂ solid solutions decreases the real part of their complex dielectric permittivity and increases their dielectric loss tangent. The conductivity (σ) of the TlGa_{1 ? x} Er _x S₂ solid solutions in the frequency range f = 1 to 35 MHz exhibits σ ～ f ^0.8 behavior, indicative of hopping charge transport through their band gap. We have evaluated the key parameters of this charge transport mechanism. We have studied temperature-dependent optical properties of the TlGa_{1 ? x} Er _x S₂ solid solutions. At temperatures in the range T = 77–200 K, the TlGa_0.999Er_0.001S₂ solid solution has an absorption band near its fundamental absorption edge, which is due to transitions to a direct exciton state.     

Thermodynamic properties of Ca1 − x Er x F2 + x and Ca1 − x Yb x F2 + x heterovalent solid solutions

The heat capacity of single crystals of the Ca_{1 ? x} Er _x F_{2 + x} (x = 0.05, 0.10) and Ca_0.95Yb_0.05F_2.05 fluorite solid solutions was determined by adiabatic calorimetry in the temperature range 55–300 K. The results were used to obtain temperature dependences of the Debye characteristic temperature, entropy, and enthalpy for the solid solutions.     

Magnetic properties of (Fe1−x M x )7Se8

The temperature dependence of the magnetization of the quenched and slowly cooled samples of Fe₇Se₈ and (Fe_1–x M _x )₇Se₈ samples with M = cobalt and nickel and x=0.02, 0.05 and 0.08 are given. All the thermomagnetic curves obtained belong to the Weiss ferrimagnetic type. Discontinuities indicating a magnetic transformation to antiferromagnetic order were obtained for some samples. The magnetic moment at 0 and 78 K (M ₀ and M₇₈) dependence on nickel and cobalt concentrations are given. The temperature dependence of the reciprocal susceptibility in the paramagnetic range was studied, and the asymptotic Curie points are given. The values of the effective magnetic moment, _eff, and the number of unpaired electrons were calculated. The thermal variation of the electrical conductivity of the host material, Fe₇Se₈, is given.     

Composition and predominant electron scattering mechanisms in Hg1 − x − y Cd x Eu y Se crystals

The surface of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals has been examined by electron microscopy using backscattered and secondary electron imaging, and the composition of the crystals has been determined. Using transport and optical measurements, we have identified the predominant electron scattering mechanisms in the Hg_{1 ? x ? y} Cd _x Eu _y Se crystals.     

Magnetization curves of (La1−x Sr x )2CuO4−δ single crystals as functions ofx and δ

The second peak effect in magnetization curves for overdoped (La_1–xSr_x)CuO_4– single crystals was examined by means of systematic variations of Sr content x and oxygen deficiency . Oxygen defect concentration was found to sensitively affect the critical temperature T_c and the macroscopic pinning force F_p, resulting in significant changes in magnetization hysteresis loops. Observations of dependence of M and F_p at the same reduced temperature T/T_c yielded an insight into the role of oxygen defects: increasing results in increasing the pinning center density N. The second peak field B_2pk seemed to be determined by the development of a percolating network of magnetically reversible regions from the observation that the temperature dependence of B_2pk showed similar behavior to the irreversibility field B_irr.     

Room-temperature multiferroic properties and magnetoelectric coupling in Bi4−x Sm x Ti3−x Co x O12−δ ceramics

We present the structural, dielectric, ferroelectric, magnetic and magnetoelectric studies of lead free; single phase Bi_4?x Sm _x Ti_3?x Co _x O_12?δ (0 ≤ x ≤ 0.07) ceramics, synthesized using a standard solid-state reaction technique. Raman spectroscopy analysis reveals the relaxation of distortion in TiO₆ octahedron. Field emission scanning electron microscopy confirmed the growth of plate-like grains. It is observed that with the substitution of Sm³⁺ and Co³⁺ ions the dielectric constant, loss tangent and ferroelectric transition temperature decreases. Electrical dc resistivity, remnant polarization and magnetization increases with increasing Sm³⁺ and Co³⁺ contents. The magnetoelectric coupling co-efficient, α = 0.65 mV cm^?1 Oe^?1 is realized for Bi_4?x Sm _x Ti_3?x Co _x O_12?δ (x = 0.07) ceramic sample. Our results clearly demonstrate the lead free, multiferroic nature of Sm/Co-substituted Bi₄Ti₃O₁₂, which may find useful application in designing cost-effective electromagnetic devices.     

Magnetic, Electrical and Thermodynamic Properties of UCuT x Al11−x Alloys Where T = Mn, Fe and x=4 and 5

The structure, magnetic, electrical, and thermodynamic properties of UCuT _x Al_11?x alloys, where T = Mn or Fe and x=4 or 5 are presented. The behavior of the Fe alloys is ferromagnetic-like with the Curie points amounting to 180 and 230 K, and the saturation magnetic moments under magnetic field of 5 T equal to 4.75 and 6.02 μ_B/f.u., respectively, whereas under a magnetic field of about 34 T the magnetic moments amount to 6.9 and 9.0 μ_B/f.u. for the alloys with x=4 and 5, respectively. The Curie points are reflected in the temperature dependence of the specific heat in which the anomalies are found at 180–200 and 230 K for alloys with x=4 and 5, respectively, however, it shows no reflection in the temperature dependence of the electrical resistivity. The field dependence of the magnetization at T=1.9 K for both compounds exhibits considerable hysteresis. There is a pronounced difference between ZFC and FC magnetization in its temperature dependence below the Curie point for materials with x=4 and 5. The Mn alloys exhibit ferrimagnetic-like character for which, supposedly, the interplay of the uranium and manganize sublattices is responsible. Magnetic transitions are determined at T _N =300 (x=4) and 380 K (x=5). However, those anomalies do not find confirmation in the temperature dependence of the specific heat and the electrical resistivity. Magnetic moments determined at T=1.9 K and in a magnetic field of 5 T are very low and in both cases amount to about 0.35 μ_B/f.u. and these values are slightly higher in a magnetic field of 34 T reaching a value of about 1.5 μ_B/f.u. Also for the Mn alloys the clear difference between ZFC and FC magnetization in its temperature dependence below the Curie point is observed.