Effect of CDW and magnetic interactions on the electrons of the manganite systems

We address a model study which includes the co-existence of the charge density wave (CDW) and ferromagnetic interactions in order to explain the colossal magnetoresistance (CMR) in manganites. The Hamiltonian consists of the ferromagnetic Hund's rule exchange interaction between e_g and t_2g spins, Heisenberg core spin interactions and the CDW interaction present in the e_g band electrons. The core electron magnetization, induced e_g electron magnetization and the CDW gap are calculated using Zubarev's Green's function technique and determined self-consistently. The effect of core electron magnetization and the CDW interaction on the induced magnetization as well as on the occupation number in the different spin states of the e_g band electrons are investigated by varying the model parameters of the system like the CDW coupling, the exchange coupling, the Heisenberg coupling and the external field. It is observed that the induced magnetization exhibits re-entrant behaviour and exists within a narrow temperature range just below the Curie temperature. This unusual behaviour of the e_g band electrons will throw some new insights on the physical properties of the manganite systems.     

The influence of band Jahn-Teller effect and magnetic order on the magneto-resistance in manganite systems

A model calculation is presented in order to study the magneto-resistivity through the interplay between magnetic and structural transitions for the manganite systems. The model consists of an orbitally doubly degenerate conduction band and a periodic array of local moments of the t_2g electrons. The band electrons interact with the local t_2g electrons via the s-f hybridization. The phonons interact with the band electrons through static and dynamic band Jahn-Teller (J-T) interaction. The model Hamiltonian including the above terms is solved for the single particle Green's functions and the imaginary part of the self-energy gives the electron relaxation time. Thus the magneto-resistivity (MR) is calculated from the Drude formula. The MR effect is explained near the magnetic and structural transition temperatures.     

Giant magnetoresistance in Ce-doped manganite systems

The fascinating properties like giant magnetoresistance (GMR) effect, metal-insulator transition, charge ordering phenomenon etc. have made the divalent ion doped RMnO₃ (R = rare-earth elements) an attractive system for investigation. Resistivity of these compounds shows a peak near the ferromagnetic transition temperature (T _c ). The application of magnetic field inhibits the spin-disorder scattering and the resistivity decreases drastically. Keeping electrondoped superconductor Nd_2?x Ce _x CuO₄ in mind we have doped RMnO₃ (R = La, Pr, Nd) with tetravalent Ce ion. These compounds are very susceptible to the annealing treatment and belong to the orthorhombic perovskite phase. They show a very high value of resistivity at the peak and under the magnetic field the GMR effect is observed. For La_0.7Ce_0.3MnO₃ and Pr_0.7Ce_0.3MnO₃ the magnetoresistance ratio reaches about 54% and 82.5% respectively at 7.7 T. With the increase of the temperature the magnetic state changes from ferromagnetic to paramagnetic regime. This magnetic transition is not very sharp and the resistivity peak appears at a temperature higher than T _c .     

Review of the magnetocaloric effect in manganite materials

A thorough understanding of the magnetocaloric properties of existing magnetic refrigerant materials has been an important issue in magnetic refrigeration technology. This paper reviews a new class of magnetocaloric material, that is, the ferromagnetic perovskite manganites (R_1−xM_xMnO₃, where R=La, Nd, Pr and M=Ca, Sr, Ba, etc.). The nature of these materials with respect to their magnetocaloric properties has been analyzed and discussed systematically. A comparison of the magnetocaloric effect of the manganites with other materials is given. The potential manganites are nominated for a variety of large- and small-scale magnetic refrigeration applications in the temperature range of 100–375 K. It is believed that the manganite materials with the superior magnetocaloric properties in addition to cheap materials-processing cost will be the option of future magnetic refrigeration technology.     

Spin-dependent localization of electrons in low-dimensional systems

Possibilities of spatial localization of spin-polarized electrons due to spin–orbit interaction are investigated theoretically. Situations most interesting for us are the ones where electrons of one spin state (helicity) are localized while the opposite helicity relates to extended wave functions. On examples of simplest short range potentials it is shown that such spin separation is, in principle, possible. Magnetic properties of electrons bound to a shallow 2D axially symmetric well are considered. Accounting for the spin–orbit contribution results in an anomalously large effective g-factor of this system.     

Theories of electrons in one-dimensional disordered systems

A critical survey of the literature on the theory of electronic states in, and electron transmission through, models of one-dimensional disordered solids and liquids is given in the first part. Reference to work on three-dimensional systems is included, especially where exact results have been obtained. The relationship between this subject and the problem of elastic vibrations in disordered solids is pointed out. A complete exposition of the authors' work on one-dimensional conductivity is then presented. It provides a rigorous solution of the problem of average resistance, and of the variance (fluctuations) of resistance, for important classes of disorder which are carefully and precisely defined. Conclusions regarding the role of disorder with respect to the transmission properties are presented and discussed. It is also pointed out that, with appropriate modifications, the results apply generally to wave propagation in inhomogeneous media.     

Thermodynamic properties of electrons in finite systems

The aim of this work is to study quantitatively how the Coulomb interaction of the electrons leads to the canonical ensemble average instead of the grand canonical one. Within the “equal level spacing” model we are able to give explicit expressions for a finite model interaction, and it turns out that for real systems at low temperatures the canonical average is an appropriate approximation. Further, we compare our theoretical results to experimental datas on TCNQ salts, which we regard as a collection of interrupted strands.     

Review of photoinduced effect in manganite films and their heterostructures

Light–matter interaction plays an important role in the non-equilibrium physics, especially in strongly correlated electron systems with complex phases. Photoinduced effect can cause the variation in the physical properties and produce some emergent phases. As a classical archetype, manganites have received much attention due to their colossal magnetoresistance(CMR) effect and the strong interaction of charge, spin, orbital, and lattice degrees of freedom. In this paper, we give an overview of photoinduced effect in manganites and their heterostructures. In particular, some materials, including ZnO, Si,BiFeO_3(BFO), titanate-based oxides, and 0.7 Pb(Mg_(1/3) Nb_(2/3))O_3-0.3 PbTiO_3(PMN-PT) have been integrated with manganites. Heterostructures composed of these materials display some exciting and intriguing properties. We do hope that this review offers a guiding idea and more meaningful physical phenomena will be discovered in active areas of solid state physics and materials science.     

Interaction between the magnetic moments of the 3d and the 4f electrons in manganite, probed by Ga substitution

The substitution of Ga for Mn in manganite Nd_0.6Dy_0.1Sr_0.3MnO₃ with a ferromagnetic (FM) ground state has been performed to study the influence of the Mn-sublattice magnetic ordering on the magnetic rare-earth sublattice. It is found that the substitution of Mn³⁺ with Ga³⁺ ions results in a sharp decrease of T_C, reflecting the reduction of the double-exchange interactions strength J_Mn-Mn. At the same time, a depinning effect of the rare-earth magnetic moment has been observed. This behavior unambiguously proves that the exchange interaction between Mn and rare-earth ions J_Mn-R strongly influences the rare-earth magnetic ordering at temperatures below T_C and stabilizes the rare-earth magnetic ground state.     

Photoemission of electrons from many-body systems

The energy distribution of photoelectrons emitted from a solid is expressed exactly in terms of time-ordered correlation functions. The expression is valid for arbitrary electron-electron, electron-lattice and electron-surface interactions.     

Radiation of systems with relativistic electrons

Different methods of generating electromagnetic radiation in connection with the motion of electrons in external electromagnetic fields are discussed in this paper.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 5–31, February, 1980.     

Correspondence principle in the self-energy problem

It has been shown by [2b.] that in the classical limit, , there is a correspondence between the quantum and classical electron self-energies in spinor electrodynamics. In the present work this result is extended to the cases of scalar electrodynamics and scalar meson theory. The apparent violation of the correspondence principle in the self-energy problem, noted in the literature, can be ascribed to the inadequacy of the usual expansion procedure in terms of the parameters and as (here Λ is the usual cutoff parameter, r₀ is the cutoff radius).     

6s and 8d state self-energy for hydrogen-like ions and new results on the self-energy screening

We use a recently published method for the renormalization of the self-energy to calculate the self-energy of 6s and 8d levels to all orders in Zα. We demonstrate the accuracy of the method and its potential for high-n, low-Z applications. We also show that this method is perfectly suited for the evaluation of the two-electron self-energy (self-energy screening). For the first time, evaluation of the screening of the 1s electron by a second one in either the 1s,2s, 2p_1/2 or 2p_3/2 shells has been performed, for 30 ⩽ Z ⩽92. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of some co-precipitated manganite perovskite samples-doped iron

Polycrystalline La_0.70Sr_0.30Mn_1−yFe_yO₃ (0.05?y?0.07) samples are prepared by the co-precipitation method and have been studied. The substitution of Mn³⁺ by Fe³⁺ reduces the number of available hopping sites for the Mn e_g(↑) electron and suppresses the double exchange (DE), resulting in the reduction of the metal–semiconductor transition temperature (T_P) and the flux density saturation (B_s). Low-temperature resistivity (ρ) data (below T_P) well fit with the relation ρ(T)=ρ₀+ρ₂T², indicating the importance of grain/domain boundary effects and electron–electron scattering processes in the conduction of these materials. On the other hand, at high temperature (T_P<T<θ_D/2) conductivity data satisfy the variable range hopping (VRH) model. For T>θ_D/2 the small polaron hopping model is more appropriate than the VRH model.     

On the nucleon self-energy in nuclear matter

We consider the nucleon self-energy in nuclear matter in the absence of Pauli blocking. It is evaluated using the partial-wave analysis of scattering data. Our results are compared with that of a realistic calculation to estimate the effect of this blocking. It is also possible to use our results as a check on the realistic calculations.PACS: 21.30.-x Nuclear forces - 21.65. + f Nuclear matter     

Hall effect of hot electrons in n-PbTe

Measurements of the Hall-coefficient were performed on n-PbTe at 80K in different crystallographic directions as a function of electric field up to 1.1kV/cm and magnetic fields up to 0.6T. The observed behaviour gives evidence for the presence of strong equivalent intervalley transfer. As the crystallographic orientation had no significant influence on the threshold and the appearance of an instability, equivalent intervalley scattering was excluded as a possible mechanism for this instability.     

Enhanced magneto-electric effect in manganite tricolor superlattice with artificially broken symmetry

The magneto-electric effect in magnetic materials has been widely investigated, but obtaining an enhanced magnetoelectric effect is challenging. In this study, tricolor superlattices composed of manganese oxides—Pr_(0.9)Ca_(0.1)MnO_3,La_(0.9)Sr_(0.1)MnO_3, and La_(0.9)Sb_(0.1)MnO_3—on(001)-oriented Nb:SrTiO_3 substrates with broken space-inversion and timereversal symmetries are designed. Regarding the electric polarization in the hysteresis loops of the superlattices at different external magnetic fields, both coercive electric field E_c and remnant polarization intensity P_r clearly show strong magneticfield dependences. At low temperatures( 120 K), a considerable magneto-electric effect in the well-defined tricolor superlattice is observed that is absent in the single compounds. Both maxima of the magneto-electric coupling coefficients ?E_c and ?P_r appear at 30 K. The magnetic dependence of the dielectric constant further supports the magneto-electric effect. Moreover, a dependence of the magneto-electric effect on the periodicity of the superlattices with various structures is observed, which indicates the importance of interfaces. Our experimental results verify previous theoretical results regarding magneto-electric interactions, thereby paving the way for the design and development of novel magneto-electric devices based on manganite ferromagnets.