Effect of charge carriers on the magnetic properties of the CdCr<Subscript>2</Subscript>Se<Subscript>4</Subscript> ferromagnetic semiconductor

The results of an investigation into how the composition of annealing mixtures influences the Curie temperature of single crystals of the CdCr₂Se₄ ferromagnetic semiconductor are reported. The mechanism of doping of single crystals is analyzed, and the role played by each of the components of the annealing batch is established. It is concluded that the indirect exchange involving n-type carriers near impurity gallium ions leads to a sharp increase in the phase transition temperature (from 130 to 172 K) of the samples. The dependences of the magnetization of samples with different phase transition temperatures on the temperature and magnetic field are compared.     

Electrochemical investigation of lanthanide octa-tert-butylsubstituted diphthalocyanine complexes in solutions. Approximation of their redox transitions by semiempirical calculations for yttrium diphthalocyanine

The electrochemical potentials of seven redox transitions for green forms and eight redox transitions for blue forms of neutral octa-tert-butylsubstituted diphthalocyanine cornplexes of lanthanides Pc^t ₂Ln (Ln=Pr, Sm, Dy, and Lu) in solutions were measured by cyclic voltammetry and rotating disc electrode techniques. The spectroelectrochemical investigation of the products of two-electron oxidation and reduction of the green form of Pc^t ₂Lu was performed. The frontier molecular orbitals, total charge densities, total spin densities, electrostatic potentials, and heats of ion formation for (Pc₂Y) ^m+,n− (m=0, 1, 2 and 3;n=1, 2, 3 and 4), which can model the products of the redox transitions of the diphthalocyanines under study, were calculated using the semiempirical ZINDO/1 method. The calculations for (Pc₂Y) ^m+,n− and absorption spectra show that the electron changes in all redox transitions of the green forms of Pc^t ₂Ln are mainly localized on the ligands. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya No. 12, pp. 2149–2156, December, 1997.     

Oscillatory melting temperature of the vortex smectic phase in layered superconductors

We report on transport measurements of YBa 2Cu 3O (7-delta) single crystals with different oxygen contents in the geometry B, J ||ab (J perpendicularB). Our data show that the vortices become confined between the Cu-O planes below a well-defined temperature at which the effective size 2xi of the vortex core is approximately equal to the period of the Cu-O layers. This confinement strongly increases the vortex liquid freezing temperature. A new melting line is found separating a vortex liquid and a smectic phase, which shows an oscillatory field dependence reflecting differences between commensurate and incommensurate smectic states.     

Magnetocaloric effect and multifunctional properties of Ni–Mn-based Heusler alloys

The studies of magnetocaloric properties, phase transitions, and phenomena related to magnetic heterogeneity in the vicinity of the martensitic transition (MT) in Ni–Mn–In and Ni–Mn–Ga off-stoichiometric Heusler alloys are summarized. The crystal structure, magnetocaloric effect (MCE), and magnetotransport properties were studied for the following alloys: Ni₅₀Mn_50−xIn_x, Ni_50−xCo_xMn₃₅In₁₅, Ni₅₀Mn_35−xCo_xIn₁₅, Ni₅₀Mn₃₅In₁₄Z (Z=Al, Ge), Ni₅₀Mn₃₅In_15−xSi_x, Ni_50−xCo_xMn_25+yGa_25−y, and Ni_50–xCo_xMn_32−yFeyGa₁₈. It was found that the magnetic entropy change, ΔS, associated with the inverse MCE in the vicinity of the temperature of the magneto-structural transition, TM, persists in a range of (125-5) J/(kg K) for a magnetic field change ΔH=5 T. The corresponding temperature varies with composition from 143 to 400 K. The MT in Ni₅₀Mn_50−xIn_x (x=13.5) results in a transition between two paramagnetic states. Associated with the paramagnetic austenite-paramagnetic martensite transition ΔS=24 J/(kg K) was detected for ΔH=5 T at T=350 K. The variation in composition of Ni₂MnGa can drastically change the magnetic state of the martensitic phase below and in the vicinity of TM. The presence of the martensitic phase with magnetic moment much smaller than that in the austenitic phase above TM leads to the large inverse MCE in the Ni₄₂Co₈Mn_32−yFeyGa₁₈ system. The adiabatic change of temperature (ΔT_ad) in the vicinity of TC and TM of Ni₅₀Mn₃₅In₁₅ and Ni₅₀Mn₃₅In₁₄Z (Z=Al, Ge) was found to be ΔT_ad=−2 K and 2 K for ΔH=1.8 T, respectively. It was observed that |ΔT_ad|≈1 K for ΔH=1 T for both types of transitions. The results on resistivity, magnetoresistance, Hall resistivity in some In-based alloys are discussed.     

Luminescence Excitation Spectroscopy of InAs/InGaAs/GaAs Quantum-Dot Arrays in the Temperature Range between 20 and 300 K

Optics and Spectroscopy - Evolution of photoluminescence (PL) and PL excitation spectra in structures containing InAs/InGaAs/GaAs quantum-dot (QD) arrays is investigated in detail as a function of...     

Long-term stability of long-wavelength (>1.25 µm) quantum-dot lasers fabricated on GaAs substrates

Accelerated degradation testing of long-wavelength (>1.25 µm) quantum-dot lasers made on GaAs substrates is carried out at a fixed current of 1.7 A, initial optical output of about 0.3 W, and a heat sink temperature of 60°C. No signs of degradation are revealed after testing for 450 h. The test bed is not sealed, inert gas purging is not performed, and the laser faces are not passivated.     

Magnetism in disordered magnetic Fe82−xNixCr18

The zero field μSR-method has been used to study the magnetism in the disordered magnetic alloy Fe_82−xNi_xCr₁₈ near the three-critical, pointx=25. The dynamic and static local field distributions are analyzed. The difference between spin-glass states obtained either from the paramagnetic or after the double transition is discussed.     

Ramification in Elementary Abelian Extensions

Let D be a quadrangle on a surface S of negative Gaussian curvature K. The boundary of D is formed by arcs of asymptotic lines and D is regarded as a surface with border ?D cut out of S. One obtains the conditions of fixing a part of ?D that ensure the rigidity of D. In addition, a version of the generalized solution of the equation of infinitesimal bending for surfaces of negative curvature is suggested.