The Nernst-Ettingshausen effect in ferromagnetic metals

A formula is given which relates the Nernst-Ettingshausen coefficient to the Hall effect and the thermoelectric power, assuming the “side jump” mechanism for the Hall effect to be dominant. Using this formula, the experimental data can be analysed to give the variation of the Hall effect with electron energy. Results of this analysis are consistent with observed behaviour of the Hall effect on alloying.     

Nernst-Ettingshausen effect in graphene

The Nernst-Ettingshausen effect corresponds to the regime of crossed magnetic and electric fields. In the current theoretical studies of this effect in graphene, the dependence of the Landau levels on the applied electric field is neglected. This dependence takes place in the case of the nonquadratic energy spectrum of the charge carriers. In this work, oscillations of the Nernst coefficient in graphene with a zero and nonzero band gap have been studied taking into account such dependence. The effect of the Coulomb interaction on these oscillations is considered.     

The Nernst-Ettingshausen coefficient in conductors with a narrow conduction band: Analysis and application of its results to HTSC materials

A theoretical analysis is made of the Nernst-Ettingshausen coefficient Q for the case of a narrow conduction band present in the band spectrum of a material. It is shown that the presence of such a band results in a qualitative change in the mechanism responsible for the Nernst-Ettingshausen effect as compared to the classical case of a broad conduction band and that the behavior of this coefficient reveals a number of specific features that are different from the case of the classical theory of transport coefficients in semiconductors and metals. It is demonstrated that the pattern of the Q(T) relation in the case of a narrow band is drastically affected by the asymmetry of the dispersion curve, whereas the other features of the band spectrum and of the properties of the carrier system, including the character of the energy dependence of the relaxation time, are less significant and, in a first approximation, can be disregarded. The calculated Q(T) curves are in qualitative agreement with the experimental relationships obtained for doped HTSCs of the YBa₂Cu₃O_y system. The possibility of using this approach for a complex analysis of the experimental temperature dependences of the four transport coefficients in the normal phase of HTSC materials is demonstrated.     

Nernst-Ettingshausen effect in thin ferromagnetic films

A new thermomagnetic effect in a ferromagnetic film is discussed. When a temperature gradient is established along the x axis in a ferromagnetic sample, a transverse electric field arises in the same plane as the spontaneous-magnetization vector (in the case H = 0, where H is the external magnetic field). A phenomenological expression is given for the transverse electric field as a function of the square of the magnetization. Theory is given for the Nernst-Ettingshausen effect due to the spin-orbit interaction in a ferromagnetic film in the effectivemass approximation, and the Nernst-Ettingshausen constant is derived. The calculation is carried out by the density-matrix method derived by Kohn and Luttinger.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 28–32, November, 1969.     

Giant Nernst-Ettingshausen oscillations in semiclassically strong magnetic fields

We consider the Nernst-Ettingshausen (NE) effect in the presence of semiclassically strong magnetic fields for a quasi-two-dimensional system with a parabolic or linear dispersion of carriers. We show that the occurring giant oscillations of the NE coefficient are coherent with the recent experimental observation in graphene, graphite, and bismuth. In the 2D case we find the exact shape of these oscillations and show that their magnitude decreases (increases) with enhancement of the Fermi energy for Dirac fermions (normal carriers). With a crossover to the 3D spectrum the phase of the oscillations shifts, their amplitude decreases, and the peaks become asymmetric.     

Strong Coulomb effects in hole-doped Heisenberg chains

Substances such as the “telephone number compound” Sr₁₄Cu₂₄O₄₁ are intrinsically hole-doped. The involved interplay of spin and charge dynamics is a challenge for theory. In this article we propose to describe hole-doped Heisenberg spin rings by means of complete numerical diagonalization of a Heisenberg Hamiltonian that depends parametrically on hole positions and includes the screened Coulomb interaction among the holes. It is demonstrated that key observables like magnetic susceptibility, specific heat, and inelastic neutron scattering cross section depend sensitively on the dielectric constant of the screened Coulomb potential.     

Magnetorefractive effect in manganites

The magnetotrans mittance and magnetoreflectance of manganite films and one-dimensional magnetophotonic crystals containing a manganite film as a defect are calculated. The calculations demonstrate that the magnetotransmittance and magnetoreflectance of manganites with a colossal magnetoresistance can be treated as a manifestation of the magnetorefractive effect. The magnitude of the magnetorefractive effect in transmission can reach 20–40% at low temperatures. The effect in reflection amounts to 1–2% for a 300-nm-thick film and can be substantially amplified through multibeam interference in the symmetric and antisymmetric structures of magnetophotonic crystals with a built-in defect.     

Transverse dislocation analog of the Nernst-Ettingshausen effect in ionic crystals

We show that in ionic crystals under the action of a temperature gradient and an external magnetic field on the cloud of point defects surrounding a charged dislocation, a phenomenon arises which is analogous to the Nernst-Ettingshausen effect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 80–83, January, 1996.     

Electronic correlations in manganites

The influence of local electronic correlations on the properties of colossal magnetoresistive manganites is investigated. To this end, a ferromagnetic two-band Kondo lattice model is supplemented with the local Coulomb repulsion missing in this model and is analyzed within dynamical mean-field theory. Results for the spectral function, optical conductivity, and the paramagnetic-to-ferromagnetic phase transition show that electronic correlations have drastic effects and may explain some experimental observations.     

Oscillating Nernst-Ettingshausen effect in bismuth across the quantum limit

In elemental bismuth, 10(5) atoms share a single itinerant electron. Therefore, a moderate magnetic field can confine electrons to the lowest Landau level. We report on the first study of metallic thermoelectricity in this regime. The main thermoelectric response is off-diagonal with an oscillating component several times larger than the nonoscillating background. When the first Landau level attains the Fermi energy, both the Nernst and the Ettingshausen coefficients sharply peak, and the latter attains a temperature-independent maximum. These features are yet to be understood. We note a qualitative agreement with a theory invoking current-carrying edge excitations.     

Model of vortex states in hole-doped iron-pnictide superconductors

Based on a phenomenological model with competing spin-density-wave (SDW) and extended s-wave superconductivity, the vortex states in Ba(1-x)K(x)Fe2As2 are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap.     

Theory of dual fermion superconductivity in hole-doped cuprates

Since the discovery of the cuprate high-temperature superconductivity in 1986, a universal phase diagram has been constructed experimentally and numerous theoretical models have been proposed. However, there remains no consensus on the underlying physics thus far. Here, we theoretically investigate the phase diagram of hole-doped cuprates based on an itinerant-localized dual fermion model, with the charge carriers doped on the oxygen sites and localized holes on the copper d _{x2 ? y2} orbitals. We analytically demonstrate that the puzzling anomalous normal state or the strange metal could simply stem from a free Fermi gas of carriers bathing in copper antiferromagnetic spin fluctuations. The short-range high-energy spin excitations also act as the “magnetic glue” of carrier Cooper pairs and induce d-wave superconductivity from the underdoped to overdoped regime, distinctly different from the conventional low-frequency magnetic fluctuation mechanism. We further sketch out the characteristic dome-shaped critical temperature T _c versus doping level. The emergence of the pseudogap is ascribed to the localization of partial carriers coupled to the local copper moments or a crossover from the strange metal to a nodal Kondo-like insulator. Our work provides a consistent theoretical framework to understand the typical phase diagram of hole-doped cuprates and paves a distinct way to the studies of both non-Fermi liquid and unconventional superconductivity in strongly correlated systems.     

Mixed-valence manganites

Mixed-valence manganese oxides (R₁-_χA_χ)MnO₃ (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO₃, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, the role of cation disorder and the Jahn-Teller effect. Sample preparation, and the properties of related ferromagnetic oxides are also discussed.     

The Nernst-Ettingshausen, Seebeck, and Hall effects in Sb2Te3 single crystals

This paper reports on measurement of the temperature dependences of the following transport coefficients: electrical conductivity in the σ₁₁ cleavage plane, the Seebeck coefficients S ₁₁ and S ₃₃ (axis 3 is along the trigonal crystal axis), the Hall coefficients R ₁₂₃ and R ₃₂₁, and the Nernst-Ettingshausen constant Q ₁₂₃; all measurements were made on high-quality Czochralski-grown Sb₂Te₃ single crystals. The results obtained are analyzed in terms of phenomenological theory. It is shown that the main features of the experimental data, including the anisotropy of the Hall and Seebeck effects, can be explained within a two-band model with notice-ably different anisotropy of the mobilities of holes of two types in the cleavage-plane and trigonal-axis directions. Estimates are made of the band-gap width (?_g?0.3 eV), as well as of the energy gap between the main and additional valence-band extrema (Δ?_v～0.1 eV).     

Magnetocaloric effect in manganites

The magnetocaloric effect (MCE) in La_{1 ? x} Sr _x MnO₃, Sm_0.55Sr_0.45MnO₃, and PrBaMn₂O₆ compounds is studied. The maximum values of MCE (??T _max) determined by a direct method in the second and third compositions and in La_0.9Sr_0.1MnO₃ are found to be much lower than those calculated from the change of the magnetic part of entropy in the Curie temperature (T _C) and the Néel temperature (T _N) range. The negative contribution of the antiferromagnetic (AFM) part of a sample in the La_{1 ? x} Sr _x MnO₃ system at 0.1 ?? x ?? 0.3 decreases ??T _max and changes the ??T(T) curve shape, shifting its maximum 20?C40 K above T _C. Lower values of ??T _max are detected in the range T _C = 130?142 K in polycrystalline and single-crystal Sm_0.55Sr_0.45MnO₃ samples cooled in air. If such samples were cooled in an oxygen atmosphere (which restores broken Mn-O-Mn bonds and, thus, increases the volume of CE-type AFM clusters), the maximum in the temperature dependence of MCE is located at T _N (243 K) for CE-type AFM clusters. A magnetic field applied to a sample during the MCE measurements transforms these clusters into a ferromagnetic (FM) state, and both types of clusters decompose at T = T _N. The PrBaMn₂O₆ composition undergoes an AFM-FM transition at 231 K, and the temperature dependence of its MCE has a sharp minimum at T = 234 K, where MCE is negative, and a broad maximum covering T _C. The absolute values of MCE at both extrema are several times lower than those calculated from the change in the magnetic entropy. These phenomena are explained by the presence of a magnetically heterogeneous FM-AFM state in these manganites.     

Solitonic phase in manganites

Orbital order present in several transition metal compounds could give rise to topological defects. Here we argue that the topological defects in orbital ordered half doped manganites are orbital solitons that carry a charge of +/-e/2. When extra charge is added to the system an array of solitons is formed and an incommensurate solitonic phase occurs. The striking experimental asymmetry in the phase diagram as electrons or holes are added to half doped manganites is explained by the energy difference between positive and negative charged solitons. The presence of nanoscale inhomogeneities in manganites is naturally explained by the existence of solitonic phases.     

Three-dimensional MgB2-type superconductivity in hole-doped diamond

We substantiate by numerical and analytical calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB2, albeit in three dimensions. Holes at the top of the zone-centered, degenerate sigma-bonding valence-band couple strongly to the optical bond-stretching modes. The increase from two to three dimensions reduces the mode softening crucial for T(c) reaching 40 K in MgB2. Even if diamond had the same bare coupling constant as MgB2, which could be achieved with 10% doping, T(c) would be only 25 K. Superconductivity above 1 K in Si (Ge) requires hole doping beyond 5% (10%).