Application of pseudopotential theory to magnetic semiconductor heterostructures

The results of empirical pseudopotential calculations for the semiconductor compound Cd_1 − xMn_xTe are presented. The effective electron and hole masses obtained from the pseudopotential calculations are then employed in an envelope function approximation, using two different effective mass Hamiltonians to evaluate the transition energies of the excitonic ground state in CdTe– Cd_1 − xMn_xTe quantum wells of variable width. It is shown that in non-magnetic systems it is not possible to utilize exciton energies alone to either distinguish between different model Hamiltonians or to quantify the interface roughness. However, it is shown that the latter can be quantified in magnetic systems via the resulting Zeeman effect.     

II–VI and II1−x Mn x VI semiconductor nanocrystals formed by the pressure cycle method

II–VI and II_1?x Mn _x VI nanocrystals were prepared by the pressure cycle method using the Paris–Edinburgh cell. The recovered samples are nanocrystals in the cubic phase zinc-blend (ZB) structure and were characterized using transmission electron microscopy, electron diffraction, X-ray diffraction and Raman scattering. Transmission electron micrographs show that these nanocrystals are nearly spherical with diameters ranging from 20 to 50 nm depending on the sample under investigation. The Raman scattering measurements confirm the existence of II–VI nanocrystals in the cubic phase (ZB). The magnetic properties of Cd_0.5Mn_0.5Te nanoparticles were found to vary with the particle size and were different from those observed for the Cd_0.5Mn_0.5Te bulk initial samples. The χ vs. T data show temperature hysteresis due to spin-glass form, which occurs at T _g=21 K, for both the bulk as well as for the recovered nanoparticle samples. The zero-field cooled and field-cooled χ vs. T curves for the nanoparticles showed a monotonous increase below T _g. Below ～21 K, the M vs. B curve for the recovered nanoparticle samples exhibited magnetic hysteresis, and this is attributed to a weak ferromagnetic contribution. This contribution is also observed in the χ(T) curves and is due to a large surface/volume ratio of the nanoparticles, which enhances the magnetic interaction.     

Excitonic magnetic polaron dynamics of MBE grown CdTe/Cd1 − xMnxTe,Cd1 − xMnxTe/Cd1 − vMgyTe single quantum wells in magneti fields

Excitonic lifetimes in Cd_1 − xMn_Ue2Te, Cd_1 − xMg_xTe epilayers and CdTe/Cd_1 − xMn_xTe, Cd_1 − xMn_xTe/Cd_1 − vMg_yTe single quantum wells with different well widths and Mn, Mg compositions are investigated. The excitonic lifetimes are found to reduce drastically by applying external magnetic fields to samples with giant Zeeman splittings. The observed phenomenon is interpreted in terms of the PL decay time contribution from the long-life dark excitons which can convert to excitons for recombinations by a spin-flip process. We attribute the lifetime reduction to the depletion of dark excitons due to their crossing over the exciton energies for dipole allowed transitions in magnetic fields.     

Effects of Ni and Co doping on the physical properties of tetragonal FeSe0.5Te0.5 superconductor

Isoelectronic Tellurium (Te) substitution for Selenium (Se) in the tetragonal phase of FeSe (β-FeSe) increases the superconducting transition temperature (T_c) by applying a negative pressure on the lattice. However, the normal state resistivity increases and shows semi-metallic behavior for samples with higher Te concentration. With increasing Te concentration, the T_c increases and reaches a maximum for FeSe_0.5Te_0.5 and then decreases with further increase of Te. We have investigated the effect of Cobalt (Co) and Nickel (Ni) doping in FeSe_0.5Te_0.5 in the nominal composition range Fe_1?xTM_xSe_0.5Te_0.5 (TM = Co (x = 0.05, 0.1, 0.15, 0.2) and Ni (x = 0.05, 0.1)). Both Co and Ni doping suppress T_c and drives the system to metal–insulator transition. The in-plane (‘a’) and out-of-plane (‘c’) lattice constants decrease with increasing dopant concentration.     

Pressure-induced magnetic transition in MnRhAs

The magnetic properties of a Fe₂P-type intermetallic compound MnRhAs have been investigated under high pressure up to 8.0 GPa by AC susceptibility measurement. Initially, both the antiferromagnetic (AF(I)) to the canted state magnetic transition temperature T_t and the canted state to another antiferromagnetic one (AF(II)) transition temperature T_C increase with compression. At 4.0 GPa, however, T_t decreases abruptly, while the increasing rate of T_C becomes larger above this pressure. A pressure-induced magnetic phase transition was seen at around this pressure when T_t and T_C are plotted in the pressure–temperature phase diagram. The transition from the antiferromagnetic to the ferromagnetic state observed below 160 K with increasing pressure is not frequently observed.     

Synthesis and characterization of spinel type high-power cathode materials Li MxMn2−x O4 (M=Ni,Co, Cr)

The transition metal-doped spinel cathode materials, LiM_0.5Mn_1.5O₄ (M=Ni. Co, Cr) were prepared by solid-state reaction. The structure and morphology of the samples were investigated by X-ray diffraction, Rietveld refinement and scanning electron microscopy (SEM). The diffraction peaks of all the samples corresponded to a single phase of cubic spinel structure with a space group Fd3m. Field-emission SEM shows octahedron like shapes and the primary particles size was between 500 nm and 2 μm. Oxidation states of Ni, Co and Cr were found to be 2+, 2+ and 3+ as revealed by X-ray photoelectron spectroscopy. During discharging, LiNi_0.5Mn_1.5O₄ and LiCo_0.5Mn_1.5O₄ sample shows more than 130 mAh/g between 3.5 and 5.2 V at a current density of 0.65 mA/cm² and well developed plateau around 5 V, respectively.     

Up-conversion luminescence and optical temperature-sensing properties of Er3+-doped perovskite Na0.5Bi0.5TiO3 nanocrystals

In this work we demonstrate the preparation of Er³⁺ doped perovskite ferroelectric Na_0.5Bi_0.5TiO₃ nanocrystals and their application in temperature sensing. The samples were synthesized via a facile hydrothermal method. Upconversion emission at 528 nm and 547 nm from two thermodynamically coupled excited states of Er³⁺ were recorded in the temperature from 80 K to 480 K under the excitation of a 980 nm diode laser. The emission intensity ratio (I₅₂₈/I₅₄₇) as a function of the temperature was investigated. A sensitivity of 0.0053 K⁻¹ is observed at 400 K, suggesting they are promising candidate for nanothermometers.     

Pressure-induced coordination crossover in magnetite; the breakdown of the Verwey–Mott localization hypothesis

Temperature-dependent ⁵⁷Fe Mössbauer spectroscopy to 40 GPa shows that Fe₃O₄ magnetite undergoes a coordination crossover (CC), whereby charge density is shifted from octahedral to tetrahedral sites and the spinel structure thus changes from inverse to normal with increasing pressure and decreasing temperature. A precursor to the CC is a d-charge decoupling within the octahedral sites at the inverse-spinel phase. The CC transition takes place almost exactly at the Verwey transition temperature (T_V=122 K) at ambient pressure. While T_V decreases with pressure the CC-transition temperature increases with pressure, reaching 300 K at 10 GPa. The d electron localization mechanism proposed by Verwey and later by Mott for T<T_V is shown to be unrelated to the actual mechanism of the metal–insulator transition attributed to the Verwey transition. It is proposed that a first-order phase transition taking place at ∼T_V at ambient pressure opens a small gap within the oxygen p-band, resulting in the observed insulating state at T>T_V.     

AlH3 between 65 and 110 GPa: Implications of electronic band and phonon structures

A first-principles density-functional-theory method has been used to reinvestigate the mechanical and dynamical stability of the metallic phase of AlH₃ between 65 and 110 GPa. The electronic properties and phonon dynamics as a function of pressure are also explored. We find electron–phonon superconductivity in the cubic Pm-3n structure with critical temperature T_c = 37 K at 70 GPa which decreases rapidly with the increase of pressure. Further unlike a previously calculated T_c-value of 24 K at 110 GPa, we do not find any superconductivity of significance at this pressure which is consistent with experimental observation.     

Structural,magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure

At ambient pressure CaFe₂As₂ has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T ～ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ～0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ～1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe₂As₂ transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.     

Behavioral response of pyrite structured Co0.2Fe0.8S2 nano-wires under high-pressure up to 8 GPa – Mössbauer spectroscopic and electrical resistivity studies

Pyrite-structured Co_0.2Fe_0.8S₂ nano wires with aspect ratio 45:1, synthesized using solution colloid method were studied under high pressure up to 8 GPa using ⁵⁷Fe Mössbauer spectroscopy (using diamond anvil cell) and electrical resistivity (using tungsten carbide cell) techniques. Room temperature S K-edge XANES studies at INFN-LNF synchrotron beam line signified the changes in the electronic structure owing to Co substitution. Magnetic measurements at 5 K demonstrated disordered ferromagnetic behavior similar to Griffith phase. The value of isomer shift identified Fe in divalent, low spin state corresponding to pyrite structure. Higher value of quadrupole splitting observed at ambient condition was due to large lattice strain and electric field gradient generated by large surface to volume ratio of the nano size of the system. With applied pressure, the value followed the expected trend of increase up to 4.3 GPa, then to decrease till 6.4 GPa. Such change in the trend suggested a phase transition. On decompression to ambient pressure, the system seemed to retain high pressure phase and nano structure. The pressure coefficient of electrical resistivity varying from −0.0454 to −0.166 Ω-cm/GPa across the transition pressure of ~4.5 GPa was sluggish suggesting second order phase transition. The pressure-dependent variations by Mössbauer parameters and electrical resistivity showed identical result. This is the first report of pressure effect on nano sized Co_0.2Fe_0.8S₂. Effect of particle size on transition pressure could not be evaluated due to lack of available reports on bulk system.     

Intrinsic pinning properties of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe_0.5Te_0.5, which is a superconductor with a critical temperature T_c of approximately 14 K, were studied through the analysis of magnetization curves obtained using an extended critical state model. For the magnetization measurements carried out with a superconducting quantum interference device (SQUID), external magnetic fields were applied parallel and perpendicular to the c-axis of the sample. The critical current density J_c under the perpendicular magnetic field of 1 T was estimated using the Kimishima model to be equal to approximately 1.6 × 10⁴, 8.8 × 10³, 4.1 × 10³, and 1.5 × 10³ A/cm² at 5, 7, 9, and 11 K, respectively. Furthermore, the temperature dependence of J_c was fitted to the exponential law of J_c(0) × exp(?αT/T_c) up to 9 K and the power law of J_c(0) × (1 ? T/T_c)ⁿ near T_c.     

Neutron studies of the iron-based family of high TC magnetic superconductors

We review neutron scattering investigations of the crystal structures, magnetic structures, and spin dynamics of the iron-based RFe(As, P)(O, F) (R = La, Ce, Pr, Nd), (Ba,Sr,Ca)Fe₂As₂, and Fe_1+x(Te–Se) systems. On cooling from room temperature all the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic/monoclinic structural transition occurs, below which the systems become antiferromagnets. For the first two classes of materials the magnetic structure within the a–b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than one Bohr magneton. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting and a very energetic spin wave bandwidth ～0.2 eV. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with superconducting transition temperatures up to ≈55 K. Magnetic correlations are observed in the superconducting regime, with a magnetic resonance that follows the superconducting order parameter just like the cuprates. The rare earth moments order antiferromagnetically at low T like ‘conventional’ magnetic superconductors, while the Ce crystal field linewidths are affected when superconductivity sets in. The application of pressure in CaFe₂As₂ transforms the system from a magnetically ordered orthorhombic material to a ‘collapsed’ non-magnetic tetragonal system. Tetragonal Fe_1+xTe transforms to a low T monoclinic structure at small x that changes to orthorhombic at larger x, which is accompanied by a crossover from commensurate to incommensurate magnetic order. Se doping suppresses the magnetic order, while incommensurate magnetic correlations are observed in the superconducting regime.     

Pressure-induced phase transition in wurtzite ZnS: An ab initio constant pressure study

We study the pressure-induced phase transition of wurtzite ZnS using a constant pressure ab initio technique. A first-order phase transition into a rocksalt state at 30–35 GPa is observed in the constant pressure simulation. We also investigate the stability of wurtzite (WZ) and zinc-blende (ZB) phases from energy–volume calculations and Gibbs free energies at zero temperature and find that both structures show nearly similar equations of state and transform into a rocksalt structure around 14 GPa, in agreement with experiments. Additionally, we examine the influence of pressure on the electronic structure of the wurtzite and zinc-blende ZnS crystals and find that their band gap energies exhibit similar tendency and increase with increasing pressure. The calculated pressure coefficients and deformation potential are found to be comparable with experiments.     

Theoretical investigation of the electronic structure and structural phase stability of CeGa2 under pressure

Recently, Chandra Shekar et al. (Phys. Stat. Sol. B 241(2004)2893), studied the structural stability of CeGa₂ under high pressure up to ∼32 GPa and reported a structural transition from hexagonal AlB₂-type to omega trigonal-type starting at ∼16 GPa with a volume collapse of ∼6%. The high-pressure omega triginal phase is found to coexist with the parent phase up to 32 GPa. In this paper, we report the results of our band structure calculations on this system as a function of reduced volume by the tight-binding linear muffin–tin orbital (TB-LMTO) method, in order to look into this structural transition and to understand it in terms of changes in its electronic structure. Our calculations indicate a structural transition at ∼30.6 GPa with a volume collapse of 3.5%, in good agreement with the experimental results. The possible mechanism of the phase transition may be due to f→d electron transfer under pressure. The theoretically calculated ground-state properties, namely the lattice parameters and the bulk modulus are also in good agreement with the experimental values.     

Electrochemical and magnetic properties of Li4 / 3Mn2 / 3O2–LiCoO2 solid solution

The compounds Li_(4−x)/3Mn_2(1−x)/3Co_xO₂ (0 < x < 0.5) were prepared by the sol–gel technique. X-ray diffraction patterns of these compounds were identified as α-NaFeO₂ type layered structure, though some super-structure lines, related to the ordered array of Li and transition metal ions in the transition metal layer, were observed. The magnetic susceptibility exhibited an antiferromagnetic transition around 40 K for x < 0.2, however the specimens with x > 0.3 had no magnetic transition. The magnetic percolation may explain these magnetic variations. The electrochemical performances were evaluated for the compound of x = 0.5, and it was seen that the electrochemical properties were sensitive to the potential window. Additionally, it was also found that the discharge capacity strongly depended on the preparation temperature; it took a maximum value at the preparation temperature of 900 °C. The discharge capacity is sensitive not only to the cation mixing degree but also to the particle size.     

Fabrication of Gd0.5Sr0.5CoO3 film for SOFC cathode by pulsed laser deposition

Gd_0.5Sr_0.5CoO₃ (GSCO) film has been fabricated by pulsed laser deposition (PLD) to be used as the cathode of the solid oxide fuel cell (SOFC). The GSCO thin film obtained has a columnar crystalline structure so that it will have a high permeation property. The PLD technique has been found suitable for growing a film of complex composition because of its good control of stoichiometry and thus for fabricating a GSCO film used as the cathode of the SOFC. The GSCO film has been studied for porosity electrical conductivity and power density. The GSCO film grown at a substrate temperature of 1100 K and oxygen gas pressure of 100 Pa has high electrical conductivity which is 820 S cm^− 1 at 973 K with post annealing at a rather low temperature (1000 K). This value is higher than that of the GSCO film prepared by RF-sputtering with post annealing at a higher temperature (1273 K).     

Ferromagnetism in Zn1−xCrxTe (x = 0.05, 0.15) films grown on GaAs(1 0 0) substrate

Zn_1−xCr_xTe (x = 0.05, 0.15) films were grown on GaAs(1 0 0) substrate by thermal evaporation method. X-ray diffraction analysis showed the presence of ZnCrTe phase without any secondary phase. The surface was analyzed by high resolution magnetic force microscope and profile measurements showed orientation of magnetic domains in the range of 0.5–2 nm with increase of Cr content. Magnetic moment–magnetic field measurements showed a characteristic hysteresis loop even at room temperature. The Curie temperature was estimated to be greater than 300 K. From the electron spin resonance spectra, the valence state of Cr in ZnTe was found to be +2 with d² electronic configuration. Hall effect study was done at room temperature and the result showed the presence of p-type charge carriers and hole concentration was found to increase from 5.95 × 10¹² to 6.7 × 10¹² m⁻³ when Cr content increases. We deduce the origin of ferromagnetic behavior based on the observed experimental results.     

Low-temperature magnetic ordering in the perovskites Pr1−xAxCoO3 (A=Ca,Sr)

The magnetic and electrical properties of polycrystalline Pr_1?xA_xCoO₃ cobaltites with A=Ca, Sr and 0≤x≤0.5 were studied in the temperature range 4 K≤T≤1000 K and field up to 7 T. The X-ray analyses show the presence of only one phase having monoclinic or orthorhombic symmetry. The magnetic measurements indicate that the Ca-doped samples have at low temperatures, similar properties to the frustrated magnetic materials. PrCoO₃ is a paramagnetic insulator in the range from 4 to 1000 K. The Sr-doped cobaltites exhibit two phase transitions: a paramagnetic–ferromagnetic (or magnetic phase separated state) phase transition at about 240 K and a second one at about 100 K. The magnetic measurements suggest the presence of magnetic clusters and a change in the nature of magnetic coupling between Co ions at low temperatures. A semiconducting type behavior and high negative magnetoresistance was found for the Ca-doped samples, while the Sr-doped ones were metallic and with negligible magnetoresistance. The results are analyzed in the frame of a phase separation scenario in the presence of the spin-state transitions of Co ions.