Magnetoelectric interactions and phase transitions in a new class of multiferroics with improper electric polarization

Symmetry analysis of magnetoelectric interactions in rare-earth orthoferrites and orthochromites has been performed. It has been shown that symmetry allows the appearance of spontaneous electric polarization or the magnetic-field-induced polarization in the region of antiferromagnetic (centro-asymmetric) ordering of the rare-earth ions. The analysis reveals a number of pronounced anomalies in the behavior of the electric polarization at the metamagnetic and spin-reorientation transitions in DyCrO₃ and TbFeO₃. This behavior points to a strong sensitivity of the magnetoelectric properties of such magnets to the antiferromagnetic state of the rareearth subsystem and the spin orientation of the d ions.     

Electronic structure of modulation-doped heterostructures: electric field effects

Self-consistent energy levels of electrons in modulation-doped GaAs/Ga_1−xAl_xAs heterostructures are presented and their dependence on various device parameters are examined. The results of the calculation of the electric field effects on the shape of the confinement potential, the electron concentration and the shape of the wavefunction are presented.     

The structure of He and the neutron electric form factor from polarization observables

As there are no free neutron targets one has to resort to the nuclear targets deuterium or helium. In order to reduce nuclear effects blurring the information on nuclear form factors the technique of double polarization experiments had been developed. Recent experiments at MAMI measuring the electric form factor of the neutron are discussed. In addition, other single and double nucleon knockout experiments are described which support the buildup of a consistent interpretation of nuclear structure in ³He.     

Electronic structure,spin polarization and high critical fields in Chevrel compounds

Results are presented of an extensive theoretical study of the origin of high field superconductivity and/or magnetism in a number of Chevrel phase ternary compounds, MMo₆X₈ (with M=Sn, Eu, Gd and X=S and/or Se) based on self-consistent linear muffin-tin orbital (LMTO) energy band calculations using the local density approach (Hedin et al. exchange correlation) for the paramagnetic structures and local spin density formalism (Gunnarsson and Lundqvist) for the ferromagnetic structures. All electrons and all 15 atoms/cell are included with the core electrons (including the 4f's) recalculated in each iteration in a fully relativistic representation and the conduction electrons treated semirelativistically (all relativistic terms except spin-orbit). Superconductivity is found to be due to the high Mo d-band density of states (DOS) at E_F resulting from the unusual large charge transfer of Mo electrons to the chalcogen sites. There is also a large charge transfer from the metal site to the cluster (≈2 electrons in Sn and Eu) giving essentially no occupied conduction bands, for example, at the Eu site and a divalent ion isomer shift in very good agreement with the experiments of Dunlap et al. The conduction-electron DOS at the Eu site is found to be reduced by an order of magnitude from its metallic state value - in close agreement with their spin - lattice relaxation rate measurements. This low conduction-electron DOS yields very weak coupling of the 4f electrons to the conduction electrons and only a very weak Ruderman-Kittel-Kasuya-Yosida magnetic interaction showing why all the Chevrel rare-earth compounds - except Ce and Eu - are superconducting despite their having large local magnetic moments. The unusually high upper critical fields, H_c2, in these materials is found to be due to the unusully flat energy bands near F_F. The ferromagnetic (spin polarized) results for the Eu- and Gd-compounds show a net small but positive magnetic moment on the metal site and a small but negative induced spin magnetic moment on the Mo site in the Eu compound. Fermi-contact contributions to the hyperfine field are calculated and found to be in good agreement with the Eu Mössbauer results and the negative NMR Knights shift results of Fradin et al. These results demonstrate theoretically for the first time the validity of the Fischer et al. and Fradin et al. conclusion that the Jaccarino-Peter mechanism is responsible for the large increase in the H_c2 when large concentrations of Eu magnetic impurities are substituted in SnMo₆S₈. Finally, calculated Stoner factors for the paramegnetic phase and spin magnetization densities for the ferromagnetic phase are used to discuss qualitatively the origin of the different behavior observed for GdMo₆S₈ and EuMo₆S₈.     

Electronic structure and surface structure of Cu2S nanorods from polarization dependent X-ray absorption spectroscopy

Highly aligned Cu₂S nanorods have been studied by polarization dependent X-ray absorption spectroscopy. In contrast to bulk Cu₂S, strong s, p, and d hybridization is found in the nanorods. The polarization dependence shows a predominant d_z2${\text{d}}_{z^2}$ character of Cu 3d states. Ab initio multiple-scattering calculations confirm the strong hybridization, and reveal that Cu₂S nanorods are grown along the z-axis of chalcocite structure with Cu₇ and Cu₁₀ sites being the main building blocks. The hybridized absorption peak in the nanorods is shifted towards lower energies for smaller diameter of nanorods, which is attributed to surface reconstruction due to strong Cu–Cu interactions on the Cu-rich surface of the nanorods.     

Electronic structure and spin polarization at the NiMnSb/GaAs(110) interface

Within the density functional theory, ab initio calculations of the electronic structure and magnetic properties of the (110) interface between the NiMnSb alloy and GaAs in dependence on configuration of contact atoms are carried out. It is found that two out of six possible atomic configurations of the interface exhibit a high degree of spin polarization, which attains 100% for one of the interfacial structures studied here. It is shown that contacts with a high degree of spin polarization are the most stable with an adhesion energy of about 1.3 J/m².     

Solitons of electric polarization in multiferroics

Complete analysis of the properties of breathers in spiral multiferroic structures in the sine-Gordon model has been presented. The methods of the excitation and detection of the breathers in the external electric and magnetic fields have been discussed.     

Magnetic-field-induced electric polarization in multiferroic nanostructures

Magnetic-field-induced electric polarization in nanostructured multiferroic composite films was studied by using the Green's function approach. The calculations showed that large magnetic-field-induced polarization could be produced in multiferroic nanostructures due to enhanced elastic coupling interaction. Especially, the 1-3 type films with ferromagnetic nanopillars embedded in a ferroelectric matrix exhibited large magnetic-field-induced polarization responses, while the 2-2 type films with ferroelectric and ferromagnetic nanolaminates showed much weaker magnetoelectric coupling and lower magnetic induced polarization due to large in-plane constraint effect, which was in agreement with the recent observations.     

Electronic structure of GaAs/AlGaAs quantum double rings in lateral electric field

A three-dimensional model of GaAs/A1GaAs quantum double rings in the lateral static electric field is investigated theoretically.The eigenvalue problem with the effective-mass approximation is solved by means of the finite-element method.The energy levels and wave functions of quantum-confined electrons and heavy holes are obtained and show an agreement with our previous theoretical and experimental studies.It is shown in the approximation of neglecting the Coulomb attraction between the electron and heavy hole that a relatively large Stark shift of exciton emission of 4 meV is attainable with an applied electric field of 0.7 kV/cm.     

Electronic polarization in SbSI near electrodes

An experimental study has been made of the residual-polarization charge [q] at various temperatures in darkness and with illumination. Illumination of the anode has the greatest effect on the q = f(E) dependence. The potential distribution in the sample and the q = f(T) dependences reveal a high electron density near the anode, giving rise to polarization charge of up to 10^-5 C/cm², in SbSI with In-Ga electrodes. The time dependence of the discharge current reveals three slow-relaxation components, with relaxation times of 1.4 · 10², 1.1·10³, and 2.1·10³ sec.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 45–49, December, 1969.The authors thank M. S. Kosman and V. A. Izvozchikov for discussion of these results.     

High-temperature heat capacity of samarium and erbium titanates with pyrochlore structure

Titanates Sm₂Ti₂O₇ and Er₂Ti₂O₇ with pyrochlore structure have been prepared by solid-phase synthesis in air from stoichiometric Sm₂O₃ (Er₂O₃)–TiO₂ mixtures sequentially at 1673 and 1773 K. Hightemperature heat capacity of the oxide compounds has been determined by differential scanning calorimetry. Their thermodynamic properties have been calculated from experimental temperature dependence C _p = f(T).     

外加横向电场作用下石墨烯纳米带电子结构的密度泛函紧束缚计算

采用基于密度泛函理论的紧束缚方法计算研究了外加横向电场对边缘未加氢/加氢钝化的扶手椅型石墨烯纳米带的电子结构及电子布居数的影响.计算结果表明,石墨烯纳米带的能隙变化受其宽带影响.当施加沿其宽度方向的横向外加电场时,纳米带的能带结构及态密度都会产生较大的变化.对于具有半导体性的边缘未加氢纳米带,随着所施加电场强度的增加,...     

Theory of electric polarization in multiorbital Mott insulators

The interaction between the electric field E and spins in multiorbital Mott insulators is studied theoretically. We find a generic coupling mechanism, which works for all crystal lattices and which does not involve relativistic effects. It couples E to the "internal" electric field e originating from the dynamical Berry phase. We discuss several effects of this interaction: (i) an unusual electron spin resonance, (ii) the displacement of spin textures in an applied electric field, and (iii) the resonant absorption of circularly polarized light by Skyrmions, magnetic bubbles, and magnetic vortices.     

The electric polarization of rod-like,charged macromolecules

The polarizability in the longitudinal direction of rod-like, highly charged macromolecules has been evaluated. A simple model, emphasizing the discrete distribution of the fixed charges along the macromolecule, but neglecting interionic repulsion, has been used to calculate the statistical distribution of the counterions which gives rise to this polarization. For weak electric fields the expression obtained for the polarizability is analogous to that previously derived by Schwarz, although it contains less implicit information about the temperature dependence and the influence of the valency of the counterions. The possibility that saturation effects may appear with high electric fields is left open. An approximate formula for the relaxation time of the longitudinal polarization is also derived.     

Out-of-plane spin polarization from in-plane electric and magnetic fields

We show that the joint effect of spin-orbit and magnetic fields leads to a spin polarization perpendicular to the plane of a homogeneous two-dimensional electron system with Rashba spin-orbit coupling and in-plane parallel dc magnetic and electric fields, for angle-dependent impurity scattering or nonparabolic energy spectrum, while only in-plane polarization persists for simplified models. We derive Bloch equations, describing the main features of recent experiments, including the magnetic field dependence of static and dynamic responses.     

Electronic and structural transitions in NdFeO<Subscript>3</Subscript> orthoferrite under high pressures

The effect of high pressure up to 65 GPa on the crystal structure and optical absorption spectra of NdFeO₃ orthoferrite single crystals is studied in diamond anvil cells. At P～37.5 GPa, an electronic transition at which the optical absorption edge jumps from ～2.2 to ～0.75 eV is observed. The equation of state V(P) is studied on the basis of the X-ray diffraction data obtained under pressure. This study reveals a first-order structural phase transition at P～37 GPa with a jump of ～4% in the unit cell volume. It is shown that the phase transition observed in rare-earth orthoferrites at 30–40 GPa is a transition of the insulator-to-semiconductor type.