Influence of the degenerate d level and of the Jahn-Teller effect on the manganite electronic structure calculated in the tight-binding approximation

E(k) dispersion curves for the charge carriers in the LaMnO₃-like perovskites were calculated for the basic types of canted antiferromagnetic ordering of the Mn sublattice in the framework of the tight-binding approximation. The E(k) spectrum of the antiferromagnetic structures was calculated for the first time taking into account the degeneracy of the Mn e _g level and the Jahn-Teller distortion of the cubic perovskite structure. This calculation involved diagonalization of the 8×8 Hamiltonian matrix. Analytical expressions for the E(k) function at separate points and symmetry lines of the Brillouin zone were derived. The calculations showed that the properties of the La_1?x Ca_xMnO₃ system do not have electron-hole symmetry.     

Anomaly in the lattice partition function

The lattice partition function Z(T)=σ_(Si) exp(−H/k_BT), where H, k_B and T are the Hamiltonian of the system, the Boltzmann constant and the absolute temperature, respectively, leads to a vanishing spontaneous magnetisation for all temperatures, independently of the lattice considered. This feature is related to the symmetry breaking in these systems. In comparing this relation to the well-known partition function Z(T)=σ_n exp(−E_n/k_BT) where E_n is energy, we observe an incompatibility which could be the reason that this partition function leads to a vanishing spontaneous magnetisation.     

Galvanomagnetic effects in graphite—II: The influence of trigonal warping of the constant energy surfaces on σxx(Bz)

The dispersion relationship for the electrons near the region of band overlap in graphite corresponds to the case where the constant energy surfaces are strongly warped in the k_x?k_y plane with three-fold symmetry (trigonal warping). The effects of this warping on the galvanomagnetic tensor component σ_xx(B_z) are examined. In the present calculation the trigonal warping is treated using a simplified mathematical model. Implications with respect to the calculation of the density of free carriers in graphite are discussed.     

The influence of microscopic parameters on the ionic conductivity of SrBi2(Nb1−xVx)2O9−δ(0≤x≤0.3) ceramics

Layered SrBi₂(Nb_1−xV_x)₂O_9−δ (SBVN) ceramics with x lying in the range 0-0.3 (30 mol%) were fabricated by the conventional sintering technique. The microstructural studies confirmed the truncating effect of V₂O₅ on the abnormal platy growth of SBN grains. The electrical conductivity studies were centred in the 573-823 K as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D₀) and the mean free path (a) were calculated by using Rice and Roth formalism. The conductivity parameters such as ion-hopping rate (ω_p) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The aforementioned microscopic parameters were found to be V₂O₅ content dependent on SrBi₂(Nb_1−xV_x)₂O_9−δ ceramics.     

Quantization and meaning of observables linear in momentum

A detailed study is made of the observablesξ ^j(x^k)p_i+f(x^k) linear in momentum on a Riemannian manifold: their quantization and (through quantum unitary transformations) physical meaning are discussed using geometrical methods.     

Low-frequency behavior of optical spatial dispersion effects

A theoretical explanation is given for the frequency independence of the nonreciprocal birefringence of light, which was recently observed in the semiconductors Cd_1?x Mn_xTe, Zn_1?x Mn_xTe, and GaAs in the frequency range below the frequency corresponding to the interband absorption edge. It is shown that the symmetry of the effect becomes higher at such frequencies if the light-induced excitation energy ?ω_n(k) only slightly depends on the photon momentum k. In this case, the nonreciprocal birefringence is completely determined by the second-rank magnetoelectric tensor. It is shown that the nonreciprocal birefringence of light can be observed in magnetic media with a tensor order parameter.     

The influence of Zn ions substitution on the transport properties of Mg-ferrite

The thermoelectric power and electrical conductivity measurements of Zn-substituted Mg-ferrites having the general formula Mg_1−xZn_xFe₂O₄ (where x=0, 0.2, 0.4 and 0.6) were carried out from room temperature to 773 K. The Seebeck coefficient is positive for all the compositions showing that these ferrites behave as p-type semiconductors and the majority charge carriers are holes. The temperature variation of the Seebeck coefficient is also discussed. The Fermi energy (E_F); the density of charge carriers (n) and the carriers mobility (μ) were determined for the studied system. The variation of log σ with reciprocal of temperature shows a discontinuity at Curie temperature. The DC electrical conductivity increases with increasing temperature ensuring the semiconducting nature of the samples. The Curie temperature determined from DC electrical conductivity was found in satisfactory agreement with that determined from initial magnetic permeability measurements. This transition temperature is found to decrease with increasing Zn concentration. The activation energy in the paramagnetic region is found to be lower than that in ferrimagnetic region. The variation of room temperature conductivity with composition indicates that conductivity increases with increasing Zn content. The dependence of the electrical conductivity of Mg-Zn ferrite on Zn content is explained on the basis of the cation distribution.     

Giant optical anisotropy in R-plane GaN/AlGaN quantum wells caused by valence band mixing effect

This study investigates the optical anisotropy spectrum in the R-plane (i.e., the -oriented layer plane) of GaN/Al_0.2Ga_0.8N quantum wells of different widths. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in -oriented GaN/Al_0.2Ga_0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x^′-polarization to y^′-polarization as the well width increases.     

Asymptotic correlation functions and FFLO signature for the one-dimensional attractive spin-1/2 Fermi gas

We investigate the long distance asymptotics of various correlation functions for the one-dimensional spin-1/2 Fermi gas with attractive interactions using the dressed charge formalism. In the spin polarized phase, these correlation functions exhibit spatial oscillations with a power-law decay whereby their critical exponents are found through conformal field theory. We show that spatial oscillations of the leading terms in the pair correlation function and the spin correlation function solely depend on ΔkF and 2ΔkF, respectively. Here ΔkF=π(n_↑−n_↓) denotes the mismatch between the Fermi surfaces of spin-up and spin-down fermions. Such spatial modulations are characteristics of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Our key observation is that backscattering among the Fermi points of bound pairs and unpaired fermions results in a one-dimensional analog of the FFLO state and displays a microscopic origin of the FFLO nature. Furthermore, we show that the pair correlation function in momentum space has a peak at the point of mismatch between both Fermi surfaces k=ΔkF, which has recently been observed in numerous numerical studies.     

Experimental verification of the commutation relation for Pauli spin operators using single-photon quantum interference

We report experimental verification of the commutation relation for Pauli spin operators using the single-photon polarization state. The experimental quantum operation corresponding to the commutator, [σz,σx]=kσy, showed process fidelity of 0.94 compared to the ideal σy operation and |k| is determined to be 2.12±0.18.     

Electronic structures and magnetocrystalline anisotropy energies of ordered Co1-xNix alloys: a first principles study

The electronic structures and magnetocrystalline anisotropy(MA) of ordered hexagonal close-packed(hcp) Co1-xNix alloys are studied using the full-potential linear-augmented-plane-wave(FLAPW) method with generalized gradient approximation(GGA).Great changes of magnetocrystalline anisotropy energy(MAE) are gained with different Ni compositions.Also,in-plane magnetocrystalline anisotropy is obtained for Co 15 Ni in which the Snoek’s limit is exceeded.It is found that the changes of the symmetry of the crystal field on Ni induce small variations in band structures around the Fermi level under different compositions,which plays an important role in modulating the magnetization direction,where the hybridization between Co-3d and Ni-3d orbits is of special importance in deciding the magnetocrystalline anisotropy of itinerant states.The rigid-band model is inapplicable to explain the evolution of magnetocrystalline anisotropy energy with Ni composition,and it is also inadequate to predict the magnetocrystalline anisotropy energy through the anisotropy of the orbital magnetic moment.     

Landau quantization of a two-dimensional electron with the nonparabolic dispersion law, pseudospin components and chirality terms

The Landau quantization of a two-dimensional electron in a perpendicular magnetic field on the basis of a Hamiltonian with two pseudospin components is considered. The diagonal elements of the Hamiltonian have non-parabolic but circular symmetric dispersion laws, whereas the off diagonal elements contain the chirality terms of different degrees. The solution of the matrix form Schrödinger equation was found following the method proposed by Rashba in his theory of spin-orbit coupling, taking into account different degrees of chirality and deviations on the parabolic dispersion law. The Landau quantization Hamiltonians were obtained by substituting the canonical momentum operators by the kinetic momentum operators. Two concrete examples were discussed. One of them concerns the Mexican hatlike dispersion law in the biased bilayer graphene with second order chirality, when the Landau quantization levels except two are characterized by two quantum numbers (n−2) and n for n≥2, corresponding to different pseudospin projections. They differ by 2 as the degree of chirality is. There are two energy levels E^±(n−2,n) with the same numbers (n−2) and n. The lower energy levels E⁻(n−2,n) have a linear decreasing behavior with dependence on the magnetic field strength H with different slopes and minima for different values of n≥2. At the intersection point H_th, two energy levels E⁻(1,3) and E⁻(0,2) have the same energy forming two degenerate LLLs. Touching the minima at different values of H, the energy branches gradually transform in the increasing quadratic dependences proportional to (2n+1)²H². The similar results were obtained in the case of cosine-type dispersion law in the frame of one-band model.     

Dispersion relation of charge gap excitations in quasi-1D Mott insulators studied by resonant X-ray scattering

Momentum dependence of charge excitations across the effective Mott gap in several quasi-low-dimensional model cuprates with different effective dimensionalities is studied using high resolution inelastic X-ray scattering by working near Copper k-edge resonance which allows us to extract the dispersion relations of the particle-hole pair excitations at the gap edge. Besides electron-electron correlation, momentum dependence of the gap-excitations is found to be strongly dependent on the effective dimensionality (or topology) of the 3dx²−y² network.     

Optical properties of GaNAs/GaAs triple quantum well structures

Optical properties of the GaNAs/GaAs triple quantum well structures were characterized by using photoreflectance and photoluminescence spectroscopy at different temperatures. The excitonic interband transitions of the triple quantum well systems were observed in the spectral range above hν=E_g(GaN_xAs_1−x). A matrix transfer algorithm was used to match the GaN_xAs_1−x/GaAs boundary conditions and calculate the triple quantum well subband energies numerically for theoretical comparison. The internal electric field in the system was extracted from Franz-Keldysh oscillations in the photoreflectance spectra. The influences of the annealing treatment on the transition energy and the internal electric field are also analyzed.     

Exact solutions for a diffusion equation with a nonlinear external force

This work is devoted to investigate the solutions of the one-dimensional diffusion equation by taking the nonlinear external force F(x,t;ρ)=−k(t)x+K/x+κx|x|^α−1η[ρ(x,t)] into account. Our investigation is first performed by considering the case α=0 and η=1, which results in a Burgers like equation with a spatial and time dependent external force. After, we consider the case α≠0 and η=α+1 and show that the solution found may be expressed in terms of the q-exponential functions present in the Tsallis formalism. In addition, we also discuss the stationary solution for α and η arbitraries.     

A magnetic, magnetostrictive and Mössbauer study of Tb0.3Dy0.7−xPrx(Fe0.9Al0.1)1.95 alloys

The effect of Pr substitution for Dy on the magnetization, magnetostriction, anisotropy and spin reorientation of a series of Tb_0.3Dy_0.7−xPr_x(Fe_0.9Al_0.1)_1.95 alloys (x=0, 0.1, 0.20, 0.25, 0.30, 0.35) at room temperature has been investigated. It was found that the magnetization and magnetostriction of the homogenized Tb_0.3Dy_0.7−xPr_x(Fe_0.9Al_0.1)_1.95 alloys decreases drastically with increasing x and the magnetostrictive effect disappears for x>0.2, but the spontaneous magnetostriction λ₁₁₁ increases approximately linearly with increasing x. Moreover, the magnetostriction exhibits slightly bigger value at x=0.1 than the free alloys and is saturated more easily with the magnetic field H, showing that a small amount of Pr substitution is beneficial to a decrease in the magnetocrystalline anisotropy. The analysis of the Mössbauer spectra indicated that the easy magnetization direction in the {1 1 0} plane deviates slightly from the main axis of symmetry with Pr concentration x, namely spin reorientation. Comparing with the Al substitution, the effect of Pr substitution for Dy on the spin reorientation is smaller.     

Finite-energy sum rule as a test of the pomeranchuk theorem

A finite-energy sum rule for π-p scattering lengths, which holds even in the case of Δσ(∞) = = σ_π⁻_p(∞) - σ_π⁺_p(∞) ≠ 0, is used to estimate the magnitude of Δσ(∞). A criterion is given of testing the presence of the Pomeranchuk-theorem-violating term.     

Structural and electrical study of Ce-substituted bismuth vanadate

Samples of Ce^IV-substituted bismuth vanadate, formulated as Bi₄Ce_xV_2−xO_{11−(x/2)−δ}; 0≤x≤0.30, were synthesized by solid-state reactions. The phase structure and electrical conductivity were investigated using X-ray powder diffraction, FT-IR, differential thermal analysis and AC impedance spectroscopy. For a low composition range, two phase transitions, α↔β and β↔γ, were exhibited in which the system mimics in most events the parent compound. Impedance analysis evidenced no relationship between the blocking effect of charge carriers and structural changes at ambient temperatures. However, the temperature dependence of conductivity was correlated with the stability region of various phases within the system.     

A variable temperature EPR study of the manganites (La1/3Sm2/3)2/3SrxBa0.33−xMnO3 (x=0.0, 0.1, 0.2, 0.33): Small polaron hopping conductivity and Griffiths phase

Four manganite samples of the series, (La_1/3Sm_2/3)_2/3Sr_xBa_0.33−xMnO₃, with x=0.0, 0.1, 0.2 and 0.33, were investigated by X-band (∼9.5 GHz) electron paramagnetic resonance (EPR) in the temperature range 4-300 K. The temperature dependences of EPR lines and linewidths of the samples with x=0.0, 0.1 and 0.2, containing Ba²⁺ ions, exhibit similar behavior, all characterized by the transition temperatures (T_C) to ferromagnetic states in the 110-150 K range. However, the sample with x=0.33 (containing no Ba²⁺ ions) is characterized by a much higher T_C=205 K. This is due to significant structural changes effected by the substitution of Ba²⁺ ions by Sr²⁺ ions. There is an evidence of exchange narrowing of EPR lines near T_min, where the linewidth exhibits the minimum. Further, a correlation between the temperature dependence of the EPR linewidth and conductivity is observed in all samples, ascribed to the influence of small-polaron hopping conductivity in the paramagnetic state. The peak-to-peak EPR linewidth was fitted to ΔB_pp(T)=ΔB_pp,min+A/Texp(−E_a/k_BT), with E_a=0.09 eV for x=0.0, 0.1 and 0.2 and E_a=0.25 eV for x=0.33. From the published resistivity data, fitted here to σ(T)∝1/T exp(−E_σ/k_BT), the value of E_σ, the activation energy, was found to be E_σ=0.18 eV for samples with x=0.0, 0.1 and 0.2 and E_σ=0.25 eV for the sample with x=0.33. The differences in the values of E_a and E_σ in the samples with x= 0.0, 0.1and 0.2 and x=0.33 has been ascribed to the differences in the flip-flop and spin-hopping rates. The presence of Griffiths phase for the samples with x=0.1 and 0.2 is indicated; it is characterized by coexistence of ferromagnetic nanostructures (ferrons) and paramagnetic phase, attributed to electronic phase separation.