ODMR Study of Zn1−x Mn x Se/Zn1−y Be y Se and (Cd1−x ,Mn)Te/Cd1−y Mg y Te Diluted Magnetic Semiconductor Quantum Wells

The effects of microwave pumping with a frequency of 60 GHz on the magneto-optical properties of diluted magnetic semiconductors (DMSs) are studied in (Zn,Mn)Se/(Zn,Be)Se and (Cd,Mn)Te/(Cd,Mg)Te quantum wells. Resonant heating of the Mn²⁺ ions in the electron spin resonance conditions leads to an increase in the Mn-spin temperature, which exceeds the bath temperature by up to 5.2 K, as detected by the shift of exciton emission line and decrease of its integral intensity. Nonresonant heating mediated by free carriers is also observed through variation of the polarization degree of emission. Direct measurements of spin–lattice relaxation times for both materials using time-resolved optically detected magnetic resonance (ODMR) technique have been performed. The mechanisms of ODMR in nanostructures of DMSs are discussed.     

Martensitic transformation in La1−x Yb x Ag1−y In y

The structural phase transformation of La_1–x Yb _x Ag_1–y In _y has been studied on single crystals by low temperature Laue-technique. The martensitic transformation in this pseudobinary intermetallic alloy has to be characterized as a weak orthorhtombic distortion of a single I centered unit cell (c/a1.04;a/b1.006) and a collective slipping or twinning of these cells that gives a fixed orientation between the remaining cubic room temperature structure and the martensitic phase. Above room temperature exists an order-disorder transformation from the CsCl-B2 structure to an at room temperature metastable W-A2 structure. There is no dramatic change in the physical properties of this alloy by substituting La by Yb, so we may approximate our results to LaAg_1–y In _y .This paper is dedicated to Prof. Dr. S. Methfessel on the occasion of his 60^th birthday     

Al,F-doped new perovskite lithium fast ion conductor Li3x La2/3−x□1/3−2x Ti1−y Al y O3−y F y (x=0.11)

Al, F-doped new perovskite lithium ion conductors (x=0.11) have been prepared by solid state reaction. It is found that a pure perovskite-structured phase with space group of P4mm(99) exits in the composition range of 0<y≤0.10. The sample with y=0.02 possesses the highest ionic conductivity of 1.06×10⁻³ S/cm at room temperature, and its decomposing voltage is 2.3 V. The factors affecting the conductivity of this system are discussed.     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

Elastic constants and mechanical stability of InxAl1 − xAsySb1 − y lattice-matched to different substrates

Based on a pseudopotential approach under the virtual crystal approximation, the elastic modulus of In_xAl_1???xAs_ySb_1???y quaternaries lattice-matched to InP, GaSb and InAs substrates has been investigated. Our findings show a reasonably good accord with experiment. The dependence of the elastic features of interest on the indium concentration x shows a monotonic behaviour when In_xAl_1???xAs_ySb_1???y is lattice-matched to InP substrate. In that case, the elastic constants have larger values and the material system of interest becomes less harder and its rigidity becomes weaker. The mechanical stability criteria is verified in terms of elastic constants and shows that In_xAl_1???xAs_ySb_1???y is mechanically stable for each x and substrate being considered here. The change in indium content x and the substrate is found to have no much effect on both the Poisson ratio and machinability. The present study showed that a proper choice of the indium composition x and substrate may provide more diverse opportunities as regards the elastic modulus of In_xAl_1???xAs_ySb_1???y.     

Concentration phase transitions in Li x Na1 − x Ta y Nb1 − y O3 solid solutions

The ferroelectric-antiferroelectric concentration phase transitions in solid solutions of Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃, which are promising environmentally friendly (lead-free) ferropiezoelectric materials, are studied by means of Raman spectroscopy.     

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.     

Morphology and magnetic properties of Fe x Co1−x /Co y Fe3−y O4 nanocomposites prepared by surfactants-assisted-hydrothermal process

Recently, we have demonstrated the successful synthesis of Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites with various alkaline solutions by using surfactants-assisted-hydrothermal (SAH) process. In this article, the synthesis of Fe _x Co_1−x /Co_yFe_3−y O₄ nanocomposites with their sizes varying between 20 nm and 2 μm was reported. X-ray powder diffraction (XRD) analyses showed that the surfactants, pH, precipitator, and temperature of the system play important roles in the nucleation and growth processes. The magnetic properties tested by vibrating sample magnetometer (VSM) at room temperature exhibit ferromagnetic behavior of the nanocomposites. These Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.     

High performance p−n junctions in the LPE MnxCdyHg1−x−y Te layers

The electric properties of n⁺−p Mn_xCd_yHg,_1−x−yTe photodiodes with a cut-off wavelength of λ_co ∼ 3.3–3.8 μm at 77 K have been studied. The p−n junctions were created using ion implantation of boron into the p-type LPE epitaxial layers. Zero bias resistance-area product at T = 85 K is equal to 1.7 × 10⁷ Ω ·. cm² for the best sample (λ_co = 3.8 μm). For the material under study, this surpasses the values which have been reported up until now for a given spectral region, and is comparable with those of the best Cd_xHg_1−xTe diodes with similar energy gap values. Mechanisms of current flow through the p−n junctions at 77 < T < 200 K without and under background illumination are discussed.     

Density functional calculations of Pb1−xCaxSySe1−y alloys lattice matched to different substrates

Density functional calculations are performed to study the structural and electronic properties of technologically important Pb_1?xCa_xS_ySe_1?y quaternary alloys. The calculations are based on the total-energy calculations within the full-potential augmented plane-wave (FP-LAPW) method. For exchange-correlation energy and corresponding potential, the generalized gradient approximation (GGA) by Perdew–Burke–Ernzerhof (PBE) and Engel–Vosko (EVGGA) have been used. We investigated the effect of composition on lattice constant, bulk modulus and band gap for pseudobinary as well as for quaternary alloys, which showed non-linear dependence on the composition x and y. The presented contour maps of energy band gap and lattice constants versus concentrations could be useful for designing new structures with the desired optical properties. In addition, the energy band gap and natural band offset of simple cubic Pb_1?xCa_xS_ySe_1?y quaternary alloys lattice matched to PbS and SrS substrates are investigated. The obtained results show that the quaternary alloys of interest could be appropriate materials for designing heterostructures with desired optical and interfacial properties.     

Mössbauer Study of La1−x Ca x Mn1−y 57Fe y O3 with x=0,0.25; y=0.01

Temperature dependent Mössbauer measurements are done on the samples of La_1–x Ca _x Mn_1–y ⁵⁷Fe _y O₃ with x=0 and 0.25, and y=0.01. With decreasing temperature, the specimen with x=0.25 shows a paramagnetic to ferromagnetic transition around 175 K. In the specimen x=0.0, the temperature dependence of both the center shift () and the recoilless fraction (f) can be fitted very well with the Debye theory with a _D=320±50 K. But for the specimens with x=0.25, f and show distinct deviations from the Debye behavior in the temperature range in which the resistivity shows a sharp decrease. Dips observed in both the f and around the transition temperature suggest that the Jahn–Teller distortion observed in these systems is dynamic in nature.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

Tunable growth of (GaxIn1−x)2O3 nanowires by water vapor

The tunable growth of In-doped Ga₂O₃ (Ga₂O₃:In) and Ga-doped In₂O₃ (In₂O₃:Ga) nanowires (NWs) on Au-coated Si substrates was achieved by modulating the amount of water vapor in flowing Ar at 700–750 °C via carbothermal reduction of Ga₂O₃/In₂O₃ powders with a fixed weight ratio. In Ar, only the Ga₂O₃:In NWs were grown, while in wet Ar the In₂O₃:Ga NWs were synthesized instead. The Ga concentration in In₂O₃ NWs decreased with the increment of water vapor in flowing Ar. The growth of both Ga₂O₃:In and In₂O₃:Ga NWs followed the vapor–liquid–solid process. The In and Ga doping induced a redshift and a blueshift in the optical bandgaps of Ga₂O₃ NWs and In₂O₃ NWs, respectively. The growth mechanisms and optical properties of Ga₂O₃:In and In₂O₃:Ga NWs were discussed.     

Investigating ionic mobility in solid solutions of Li x Na1 − x Ta y Nb1 − y O3 via Raman spectroscopy

The ionic mobility in Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions (SSes) is investigated by means of Raman spectroscopy. The average lifetime of an Li⁺ ion in the equilibrium position and the magnitude of the potential barrier are estimated from the temperature dependence of the line width corresponding to the vibrations of Li⁺ cations.     

Magnetic behavior of the alloys CeCuyGa4−y and Ce1−xLaxCuGa3

The results of magnetic susceptibility (x), electrical resistivity (ρ) and heat-capacity (C) measurements of the alloys CeCu_yGa_4−y (y = 0.5, 1.0, 1.25 and 1.5) and Ce_1−xLa_xCuGa₃ (x = 0.2 and 0.9), crystallizing in the BaAl₄-type tetragonal structure, are reported. The Kondo effect tends to dominate with increasing y. All the data for the y = 0.5 alloy are consistent with the onset of ferromagnetic ordering at 6 K. For y = 1.0 and 1.25, though ρ gradually drops below 6 K, the C data do not show the existence of magnetic ordering above 2 K; presumably, for y = 1.0, 1.25 and 1.5, long-range magnetic ordering sets in below 2 K. Above all, C tends to increase with decreasing temperature before the onset of long range magnetic ordering and the values of C/T per Ce mol are considerably diminished for x = 0.9. It is proposed that this C/T enhancement lies in the magnetic precursor effects and not on the heavy-fermion behaviour.     

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca_{1 ? x ? y} Y _x Gd _y F_{2 + x + y} single crystals has revealed spectra that are not typical of gadolinium-doped CaF₂ crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd³⁺ ions localized in yttrium clusters. Weak spectra of tetragonal Gd³⁺ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd³⁺ ion, have been also detected. These centers are attributed to isolated Gd³⁺ ions localized near octahedral rare-earth clusters or their associations.     

Spin polarons at high temperatures in nonstoichiometric La1 − x Mn1 − y O3 manganites

The magnetic properties of the La_{1 ? x} Mn_{1 ? y} O₃ family of nonstoichiometric lanthanum manganites are studied in the 80 K < T < 640 K temperature range. The Curie temperature varies nonmonotonically with the growth in the density of Mn⁴⁺ ions. In the existence range for the paramagnetic phase, magnetic polarons appear in both the orthorhombic and rhombohedral phases. In the range of paramagnetism, the temperature dependence of the magnetic susceptibility is nonlinear and can be fitted by the Curie law with the temperature-dependent Curie constant.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

Local atomic arrangement in mechanosynthesized Co x Fe1−x−y Ni y alloys studied by Mössbauer spectroscopy

Mechanosynthesized Co _x Fe_1?x?y Ni _y alloys were examined using X-ray diffraction (XRD) and Mössbauer spectroscopy. In order to explain the shape of hyperfine magnetic field (HMF) distributions for the alloys, a local environment model based on a multinomial distribution was proposed. The model was in agreement with the XRD data and confirmed that the studied alloys were disordered solid solutions. It was successfully applied to describe the samples with bcc and fcc crystalline lattice type within the relatively broad range of components concentration. The results showed that the change of the crystalline lattice type does not cause an abrupt change of the HMF value. Moreover, a mean number of unpaired spins for the first coordination sphere may be used as a parameter to describe the HMF value experienced by ⁵⁷Fe nucleus. Finally, a set of the most probable atomic configurations and their corresponding contributions to the HMF distribution were obtained.     

EPR study on the La2/3Ca1/3Mn1−x Cu x O y system

The interaction between Mn and Cu ions is studied by measuring the resistance and electron paramagnetic resonance (EPR) at different temperatures of the Cu-doped compound La_2/3Ca_1/3Mn_1−xCu_xO_y. A new transition inR-T curve and substantial enhancement in magnetoresistance are induced by the substitution of Mn ions by Cu ions. The EPR measurement shows that two resonance signals appear at temperature lower than the spin-ordering temperature of Mn ions. A tentative interpretation for the observed phenomena is proposed by considering the interaction between the Cu/Mn ions besides the Mn³⁺/Mn⁴⁺ ions.