Large Hole Concentration Research Articles

Polaron and bipolaron defects in a charge density wave: A model for lightly dopedBaBiO3

${\mathrm{BaBiO}}_{3}$ is a prototype ``charge ordering system'' forming interpenetrating sublattices with nominal valence ${\mathrm{Bi}}^{3+}$ and ${\mathrm{Bi}}^{5+}.$ It can also be regarded as a three-dimensional version of a Peierls insulator, the insulating gap being a consequence of an ordered distortion of oxygen atoms. When holes are added to ${\mathrm{BaBiO}}_{3}$ by doping, it remains insulating until a very large hole concentration is reached, at which point it becomes superconducting. The mechanism for insulating behavior of more lightly doped samples is formation of small polarons or bipolarons. These are self-organized point defects in the Peierls order parameter, which trap carriers in bound states inside the Peierls gap. We calculate properties of the polarons and bipolarons using the Rice-Sneddon model. Bipolarons are the stable defect; the missing pair of electrons come from an empty midgap state built from the lower Peierls band. Each bipolaron distortion also pulls down six localized states below the bottom of the unoccupied upper Peierls band. The activation energy for bipolaron hopping is estimated.

Electrical transport properties and magnetoresistance ofLaMn1-xMxO3+δ substituted with diamagnetic ions(M = Li, Mg, Ti)

Substitution of aliovalent diamagnetic ions with fixed valence states (Li+, Mg2+ and Ti4+) at the Mn site in LaMnO3(LaMn1-xMxO3+δ) drastically decreases the roomtemperature resistivity (from ~368 to ~10 Ω cm). Theconcentration of Mn4+ ions increases with increase in diamagneticaddition. The presence of diamagnetic ions in LaMnO3 samples shows lowvalues of δ (0-0.045) in comparison to LaMnO3+δ(δ = 0-0.26). Therefore, the cation vacancies (La, Mn) in the La andMn sublattices will be much lower than that of LaMnO3+δ;consequently the scattering of holes at the vacancies will be reduced,resulting in lower resistivity. There is no transition to metallic state atlow temperatures even for samples with large hole concentration as in the caseof LaMn0.8Mg0.2O3+δ. However, there is atransition from the para- to ferromagnetic state. In resistivity characteristics,the high temperature region satisfies the small polaron model and theactivation energy calculated is less than 0.3 eV. Thermopower measurementsexhibit a change in behaviour at low temperatures, which is not observed inresistivity curves. Magnesium added samples(LaMn0.8Mg0.2O3+δ) show a large negative magnetoresistance (MR)coefficient (0.8) at low magnetic fields (H = 1 T) and the MR increaseswith Mg concentration (x = 0.05-0.2). Samples treated at low partialpressures of oxygen at 1050-1300 K display large variation in resistivityand MR due to the reduction in holes.

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

A comparative study of the longitudinal ρxx and transverse ρxy resistivities and magnetic susceptibility χac of La0.8Sr0.2MnO3 single crystals and ceramic samples has been conducted in a wide range of temperatures T=1.7–370 K and magnetic fields, H=0–13.6 T. It turned out that the relation ρxy∼ρxx, which is expected to hold in the case of carrier scattering by magnetic fluctuations, applies to the single crystals. In polycrystals, an additional H-dependent contribution to the resistivity tentatively attributed to plane (near grain boundaries) and bulk “defects” of the magnetic sublattice has been detected. The scattering of carriers by these defects does not make a notable contribution to the anomalous Hall effect and magnetic susceptibility χac. As a result, the curve of ρxy versus ρxx seems to be steeper than a linear dependence. Under the assumption that the materials under investigation are metals with constant carrier concentrations, the conductivity σ=1/ρxx due to the critical magnetic scattering calculated in the molecular field approximation reproduces the main features of experimental data, namely, the drop in the amplitude and shift of the resistivity peak near the Curie point with increasing magnetic field H and also a relatively slow change in the derivative dσ/dH with increasing temperature in the region T⩽TC. The large hole concentration of about two per unit cell derived from Hall measurements indicates that carriers of opposite signs can coexist in these materials.

Polaron defects in a charge density wave: a model for lightly doped BaBiO3

A model for BaBiO3 was introduced by Rice and Sneddon, which treats this material as a simple three-dimensional version of a Peierls insulator, where the insulating gap is a consequence of the ordered distortion of the oxygen atoms. Charge accumulates on half the atoms and depletes from the other half. Experimentally, when holes are added to BaBiO3 by doping, it remains insulating until a very large hole concentration is reached, at which point it becomes superconducting. In the Rice-Sneddon model, at large enough electron-phonon coupling, a mechanism for insulating behavior of doped samples is formation of small polarons or bipolarons which trap carriers in bound states in the Peierls gap. A variational calculation of the polaron binding in this model is given, and compared with “exact” numerical results on large clusters with periodic boundary conditions.

Die entscheidende Mitwirkung der Metall- und Sauerstoffionen von Nickeloxid beim Ablauf der Oxidation von o-Xylol

The Decisive Cooperation of Metal and Oxygen Ions of Nickel Oxide During the Oxidation of o-Xylene As can be concluded from the experimental results at 450°C in the reaction mixtures consisting of N2O2-o-Xylene, both nickel oxide pure and doped with Li2O or In2O3 is unsuited as catalyst for the oxidation of o-xylene to phthalic anhydride. In contrast to NiO which ionosorbs both oxygen and o-xylene, NiOLi2O, a strong ionosorbent for o-xylene, prevents the ionosorption of oxygen because of the large concentration of holes. Since gaseous oxygen does not react with ionosorbed o-xylene but a reduction of nickel oxide to metallic nickel has been observed in spite of the fact of enough oxygen in the gas phase it can be assumed that o-xylene is forced to remove oxygen ions from the NiO lattice under generation of oxygen-ion vacanxies and nickel atoms. The predominant portions of the reaction products are H2O and CO2. With undoped nickel oxide and NiOIn2O3 which were not reduced under the same experimental conditions, the reaction products had roughly the same composition. The reduction of NiOLi2O however will be prevented in a gas mixture with a high oxygen pressure which oxidizes the formed nickel atoms on the surface of NiOLi2O to nickel oxide making possible the entrance of oxygen from the gas phase and, therefore, the oxidation of o-xylene. A turbulent motion of a 2-component catalyst powder from NiO-1mole% Li2O covered with ionosorbed o-xylene and ZnO-1mole% In2O3 covered with ionosorbed oxygen in the same gas mixture resulted in the same reaction products as in the presence of sole NiOLi2O under simultaneous reduction of nickel oxide. From that we can conclude that the oxidation of o-xylene by oxygen ions of NiO occurs more easily than the reaction with ionosorbed oxygen on ZnOIn2O3 which obviously seems to be bounded too strongly. This result is also confirmed by the prevention of the oxidation of gaseous o-xylene in the presence of only ZnOIn2O3. Finally, the operation of the carrier catalyst V2O5/TiO2 which is employed for the oxidation of o-xylene to phthalic anhydride will be prevented to a large extent in simultaneous presence of nickel oxide either pure or doped with Li2O and In2O3 in roughly the same amount. This result can be mentioned as a proof for the interaction of a 2-component catalyst the mechanism of which is at present not satisfactorily understood.

Continuum excitons in semiconductors.

The notion of a continuum exciton as an autolocalized state is presented. A state of this kind arises from interactions between an electron (a hole) in the conduction band (valence band) and a ``continuum'' of holes (electrons) in the p-type (n-type) semiconductor. The ground and the first excited states of the exciton are calculated. The possibility for a continuum exciton to form a bound state with the impurity center is also analyzed. The value of the shift in energy, given by the formation of a continuum exciton, exhibits correct values for a narrowing of the band gap in semiconductors and describes correctly the dependence of this narrowing on the concentration of the impurity dopant. It is also shown that in a semiconductor with sufficiently large hole (electron) concentration, the formation of a bielectron (bihole) state is possible. The formation of such states can result in superconductivity.

Calculated acoustic plasmon spectra in SnTe: effect on anisotropy

The dispersion curves of acoustic plasmons in strongly p-type SnTe are calculated using a model dielectric tensor based on the random phase approximation with exact analytic continuation into the lower half of the complex frequency plane. The crystalline character of the system is allowed for in the effective-mass approximation. For sufficiently large hole concentration the carriers occupy ellipsoidal pockets around the Sigma as well as the L maxima. Four systems with hole concentrations from 0.25 to 0.46 nm-3 have been studied. The acoustic plasmon is predicted to exist for some but not all directions of the wavevector. The most favourable direction is (111). For wavevectors less than the cut-off, the Landau damping by the 'faster' component is calculated and an estimate of the damping by Sn-vacancy scattering is also given. For SnTe with 0.38 holes/nm3, acoustic plasmons are predicted to exist in the (111) direction up to a maximum wavevector of 180 mu m-1, when the quantum energy has become 23 meV.

Ferromagnetic phase transitions in the single-band Hubbard model

The authors investigate the zero-temperature phase diagram of the single-band Hubbard model in two and three dimensions, with an emphasis on the region where ferromagnetism occurs. To determine the equilibrium value of the magnetization m, the authors compute the expectation value of the Hamiltonian using a Gutzwiller variational wavefunction for arbitrary values of m. Analytic calculations are performed using an improved Gutzwiller-type approximation. In two dimensions the authors predict a first-order transition from a saturated ferromagnetic state to a paramagnetic state. In three dimensions they find both a first-order and a second-order transition depending on the values of the Coulomb repulsion U and the hole concentration delta . They also discuss the possibility that the system phase separates into a region with a large hole concentration and a region with few holes.

Polaronic band-width for a strongly correlated Hubbard system

The X-operator technique, which preserves the local constraint of no double occupancy, is used to study the polaronic bandwidth for a strongly correlated system with a local electron-phonon interaction. A two-phonon coherent state, with a variational parameter in order to minimize the total energy of the system, is used for the phonon subsystem. The results are compared with those that follow from the Gutzwiller approximation. The X-operator technique and the Gutzwiller approximation predict the same qualitative variation of the polaronic band-width with the hole concentration. The band-width, being very small for low hole concentration, increases rapidly within a narrow range of the hole concentration for reasonable values of the parameters. The polaronic band-width is almost constant in the intermediate hole concentration region and then decreases for a large concentration of holes. The variation of the two-phonon coherent state parameter as a function of the electron-phonon coupling strength is also studied. The obtained results for the polaronic band-width are consistent with those predicted from the normal state properties of the high- T c oxide systems.

Instability of the Nagaoka ferromagnetic state in the kinematic field approximation at large hole concentrations

Abstract Within the scope of the approximation of a kinematic field we show that the Nagaoka ferromagnetic state is unstable at large enough hole concentrations.

Instability of the Nagaoka ferromagnetic state of the U=

We identify a ``${k}_{F}$ instability'' of the Nagaoka ferromagnetic state of the U=\ensuremath{\infty} Hubbard model. We show rigorously that for a large enough hole concentration the ferromagnet possesses an instability with respect to overturning an up-spin electron at the Fermi surface and placing it at the bottom of a down-spin band made very narrow by correlation effects. We find a low-energy scale for spin waves in this strong-coupling limit, in the form of a spin-wave stiffness that is much smaller than its random-phase-approximation value.

Solid State Electrochemical Study of Oxygen Nonstoichiometry and Transport in Superconducting Ceramics

AbstractZirconia based solid oxygen ion conductors were employed to control the oxygen content and activity of YBa2Cu3O6+x at elevated temperatures at which the material is commonly prepared and the superconducting properties are adjusted. Experimental problems of the reactivity between the electrolyte and the sample and the exchange of oxygen with the surrounding gas phase due to the high oxygen equilibrium partial pressure became eliminated. The lowest oxygen concentration appears at the composition YBa2Cu3O6 which corresponds at the same time to the ideal stoichiometry. The oxygen activity increases rapidly for small oxygen excess concentrations and turns finally into a very slow increase at large stoichiometric deviations; this is related to a semiconductor‐metal transition. The maximum oxygen concentration is limited to 60% of the structurally expected amount. The Wagner factor drops from nearly 200 at ideal stoichiometry to a value close to 1 at large oxygen excess indicating the increasing role of screening effects by large hole concentrations which are generated to compensate the negative charge of the oxygen ions. Accordingly, the chemical diffusion coefficient of YBa2Cu3O6%plus;x drops rapidly with increasing deviation from the ideal stoichiometry from about to 10−9.8 to 10−12.3 cm2 at 700°C. The diffusivity is less stoichiometry dependent, but still decreases with increasing oxygen content. The partial ionic (minority) conductivity is several orders of magnitude lower than in ceramic solid electrolytes.

PAIRING OF HOLES INDUCED BY MAGNETIC INTERACTIONS

We consider a planar Hubbard model with three sites per primitive cell. This model is mapped into a magnetic Hamiltonian. The charged holes interact via the magnetic interaction. The effect of this interaction is to create a magnetic background for the holes. Defining new quasi-particles we find that the many-body interaction becomes attractive and gives rise to conventional superconductivity. At large hole concentrations the critical temperature is determined by the antiferromagnetic exchange between copper-copper. In addition, the Increase of hole concentration reduces the exchange coupling which vanishes at some critical values. At small hole concentrations the critical temperature is determined by the Bose Einstein condensation temperature.