On the structure of wave fields in the region of linear interaction between ordinary and extraordinary waves in two-dimensionally inhomogeneous magnetoactive plasmas

The spectra of the low-temperature photodissociation (photoionization) of Landau-Pekar polarons are calculated using the theory of quantum-coherent states and a new method of variation with respect to the parameters of phonon vacuum deformation. It is shown that the final polaron states upon photodissociation may have different numbers of phonons produced in a single dissociation event and different momenta of charge carriers. The spectrum of optical absorption related to the photodissociation of polarons exhibits a superposition of bands corresponding to various numbers of phonons formed as a result of dissociation of a single polaron. Due to a large width of the energy region corresponding to the final states of charge carriers, the halfwidth of each band is on the order of the energy of polaron coupling and is much greater than the phonon energy. For this reason, the individual phonon bands exhibit strong overlap. The very broad and, probably, structureless band formed as a result of the superposition of all these components begins at an energy equal to the sum of the polaron coupling energy (E _p) and the phonon energy. This band has a maximum at a frequency of about 5.6E _p/? and a halfwidth on the order of 5.6E _p/? at a unit effective mass (m* = m _e) of band electrons. For an effective charge carrier mass within m* = (1–3)m _e, the energy of the polaron band maximum can be estimated as 5E _p with an error of about 10%, and the halfwidth falls within 3.4E _p < ?Ω_1/2 < 5.6E _p. The multiphonon character of this band is related to a decay of the phonon condensate after the escape of charge carrier from a polaron. Such polarons are likely to be observed in the spectra of complex metal oxides, including high-temperature superconductors. Examples of such polaron bands in the reported absorption and photoconductivity spectra of nonstoichiometric cuprates, manganites, nickelates, and titanates are presented. A theory of the formation of Landau-Pekar polarons with the participation of branches of the polarization oscillations of the medium is developed. It is shown that, under certain conditions, such a multiphonon-dressed polaron can possess a coupling energy on the order of 0.2–0.3 eV, so that the maximum of the corresponding absorption band may occur at 1–1.5 eV.     

Phase separation in oxygen-doped cuprates

Experimental investigations of phase separation in high-T_c cuprates are reviewed with special emphasis on recent results obtained on doped La₂CuO_4+x single crystals. Particular attention is paid to the magnetic properties. The experiments give very strong evidence that magnetic polarons are responsible for macroscopic phase separation observed in La₂CuO_4+x for x>0.     

Non-adiabatic polaron hopping conduction in CaMn1−xCrxO3 (0?x?0.3)

DC electrical conductivity (σ_dc) of electron-doped antiferromagnetic CaMn_1−xCr_xO₃ (0?x?0.3) has been discussed elaborately in the light of polaron hopping conduction. The increase in Cr doping concentration increases the conductivity and decreases the activation energy. Non-adiabatic polaron hopping conduction is observed in all the manganites at high temperatures. The analysis of σ_dc data shows that small polarons are formed at lower concentrations (?5%) of Cr doping and undoped samples. However, large polarons are materialized at higher doping (?10%) concentrations. This is consistent with the fact that doped Cr³⁺ has larger ionic size compared to that of Mn⁴⁺. Again, strong electron-phonon (e-ph) interaction is perceived in undoped and 5% Cr-doped samples but not in manganites with larger doping concentration. This also confirms the formation of larger polarons with the increase of x. Mott's variable range hopping (VRH) model can elucidate the dc conductivity at very low temperatures. It has been detected that single phonon-assisted hopping is responsible for the dc conduction in the Cr-doped CaMnO₃ manganites.     

Influence of vacancies on the electrical properties of the ZnCr2−xNixSe4 spinels

The effect of vacancies on the electronic transport in the ZnCr_2−xNi_xSe₄ paramagnetic single crystals is considered. For this purpose, the structure refinements, the high temperature electrical conductivity and the thermoelectric power measurements as well as the calculations of the vacancy model parameters were used. The electrical measurements have been done in the temperature range from 290 to 520 K for single crystals with x=0.001, 0.05, and 0.065. The above investigations provide evidence for polaron conduction in defective spinel materials. In particular: (1) at high temperatures a linear dependence between thermopower S and the electrical resistivity (ln ρ), a characteristic of small polarons, was observed, (2) an origin of small polarons in this case could be associated with a crystalline distortion, which is characterized by larger values of the anion parameter than u=0.375, which describes an ideal spinel structure, and (3) a large defectiveness, which is identified by the large value of the vacancy parameter of about β=11.5%. These effects are explained in terms of a polaron mechanism of the electrical conductivity including structural defects.     

Optical observation of oxidation and reduction of small supported copper particles

In situ and real-time optical absorption measurements of supported copper particles (4–10 nm) at wavelengths of 300 to 800 nm are carried out under H₂, CO, and O₂ respectively as ambient gases in the temperature range of 300 to 673 K. We observe a reversible change in the optical spectra caused by oxidation of copper and reduction of copper oxide. The data strongly indicate that the oxidation of small copper particles is composed of a fast process of Cu to CuO_x (x ≈ 0.67) and a slow process of CuO_x (x≈ 0.67) to CuO.     

Thermal dissociation of intercalated titanium selenides Fe x TiSe2 (x = 0.10, 0.25, 0.50)

Using static tensimetry, the selenium pressure during dissociation of intercalated compounds Fe _x TiSe₂ has been measured in a temperature range of stability of the homogeneous material and in the region of decomposition caused by thermal expansion of the polaron band. It has been shown that covalent centers in the stability region of the homogeneous state, which are stabilized by the polaron state of conduction electrons, behave as the effective oxidants lowering the Fermi level. Broadening of the polaron band in the region of a high iron concentration leads to weakening the oxidizing influence of polarons. Iron selenides are formed during the decomposition of intercalated compounds, which leads to an essential non-quasi-binarity of the Fe-TiSe₂ system.     

Small polaron migration in LixMn2O4: From first principles calculations

Migration of small polarons in λ-MnO₂, Li_0.5Mn₂O₄ and LiMn₂O₄ is studied via first principles calculations. Migration energy barriers of single small polaron migrations in λ-MnO₂, Li_0.5Mn₂O₄ and LiMn₂O₄ are 0.22 eV, 0.45 eV and 0.35 eV, respectively. The energy level changes of Mn-3d states along the polaron migration path are analyzed in detail. Results indicate that the activation energy barrier of polaron migration is strongly associated with the energy level shift of Mn-3dz² orbital, which is dependent on the short range structural arrangement of Mn³⁺/Mn⁴⁺ in the crystal. The electrical conduction properties of Li_xMn₂O₄ at room temperature are then discussed.     

Fermi surface evolution in the ensemble of spin-polarized quasiparticles in La2 − x Sr x CuO4

For the spin-fermion model, it has been shown that the concept of spin polarons makes it possible to reproduce fine details of the Fermi surface evolution in the nodal direction of La_{2 ? x} Sr _x CuO₄ occurring with changes in the doping level x. The physics here is determined by the spin-correlated hopping of charge carriers and by the doping dependence of the inverse magnetic correlation length.     

Charge transfer near the Néel temperature in La2CuO4+x

The electrical resistance and the thermoelectric power in the ab plane of a weakly oxygen-doped La₂CuO_4+x crystal (0.001<x<0.007) and its static magnetic susceptibility were studied in the vicinity of antiferromagnetic transition. The electrical resistance and the thermoelectric power behave anomalously near the Néel temperature, indicating that the transport is strongly affected by the establishment of long-range antiferromagnetic order. Analysis of the obtained data allows the conclusion to be drawn that the doping gives rise to a conduction band as a result of the overlap between the wave functions of deep impurity states that are strongly renormalized due to the correlation and polaron effects.     

The interplay of superconductivity and localization in Nd2−xCexCuO4+δ single crystal films

The influence of nonstoichiometric disorder on the in-plane resistivity and SC-transition has been investigated for Nd_2−xCe_xCuO_4+δ single crystal films (x=0.15 and 0.18). It is shown that with increasing of δ the in-plane normal state resistivity increases (the mean free path diminishes) and SC-transition temperature decreases with essential broadening of the transition region. The observed evolution from homogeneous metallic (and superconducting) Nd_2−xCe_xCuO_4+δ system to inhomogeneous dielectric one is described as Anderson-type disorder-induced transition in a two-dimensional electron system.     

Effect of O-O transfer upon a hole in CuO2 planes of HTS

The formation of spin polarons (spin-polarized clusters) in the weakly doped antiferromagnetically ordered CuO₂ planes of high-T _c cuprates is investigated. The influence of additional holes is calculated within a self-consistent mean-field approximation, by taking O-O hybridization (characterized by the parameter t) into account. It is shown that the binding energy and the polarization of clusters strongly depend on the value and the sign of t, i.e., a positive t promotes the spin polaron formation, while a negative t suppresses it. For the common value t ～ 0.3.0.6 eV two polaron states with binding energies ～ 0.5 eV and ～ 0.3 eV are obtained.     

Optical spectra of strong-coupling polarons

The photodissociation spectra of Landau-Pekar polarons are calculated using the theory of quantum coherent states. It is shown that the number of phonons emitted in one dissociation event can differ and that their energy is equal to the doubled polaron binding energy E _p only on the average. It is established that the absorption spectrum is a superposition of bands corresponding to different numbers of phonons emitted during the dissociation of one polaron and that the half-width of each of the bands is much greater than the distance between the bands (which is equal to the phonon energy ?θ). Therefore, the absorption spectrum looks like a very wide unstructured band with the low-frequency edge lying at E _p + ?θ, a maximum at an energy of about 5E _p (for the band carrier mass equal to (1–3)m _e ), and the half-width being of the order of the energy corresponding to the maximum.     

Polaron cross-overs and <Emphasis Type="Italic">d</Emphasis>-wave superconductivity in Hubbard-Holstein model

We present a theoretical study of superconductivity of polarons in the Hubbard-Holstein model. A residual kinematic interaction proportional to the square of the polaron hopping energy between polarons and phonons provides a pairing field for the polarons. We find that superconducting instability in the d-wave channel is possible with small transition temperatures which is maximum in the large to small polaron cross-over region. An s-wave instability is found to be not possible when the effective on-site interaction between polarons is repulsive.Received: 9 July 2004, Published online: 12 October 2004PACS: 63.20.Kr Phonon-electron and phonon-phonon interactions - 74.20.Mn Polarons and bipolarons - 74.20.Fg BCS theoy and its developmentR. Ramakumar: Present address (on leave from SINP): Department of Physics and Astrophysics, Delhi University, Delhi-110017, India     

Cooper instability of nonlocal spin polarons in the CuO<Subscript>2</Subscript> plane of high-temperature superconductors

The energy structure of nonlocal spin polarons has been obtained for the real structure of the CuO₂ plane of cuprate superconductors in the ensemble of such Fermi quasiparticles. A nonlocal spin polaron is formed due to the exchange interaction of the spin of an oxygen hole with the spins of the two nearest copper ions. The scattering amplitude of nonlocal spin polarons in the cooper channel calculated using the diagrammatic technique indicates that the spin and charge degrees of freedom are strongly correlated.     

Effect of substitution of K ions on the structural and electrical properties of nanocrystalline MgAl2O4 spinel oxide

Potassium substituted nanosized magnesium aluminates having a nominal composition Mg_1−xK_xAl₂O₄ where x=0.0, 0.25, 0.5, 0.75, 1.0 have been synthesized by the chemical co-precipitation method. The samples have been characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and dc electrical resistivity measurements. The XRD results reveal that the samples are spinel single phase cubic close packed crystalline materials. The calculated crystallite size ranges between 6 and 8 nm. The behaviour of the lattice constant seems to deviate from the Vegard's law. While X-ray density clearly increases, the bulk density and consequently, the percentage porosity do not exhibit a significant change on increasing the K⁺ content. The SEM micrographs suggest homogeneous distribution of the nanocrystallites in the samples. The dc electrical resistivity exhibits a typical semiconducting behaviour. Substitution of a Mg²⁺ ion by a K⁺ ion provides an extra hole to the system, which forms small polaron. Thermally activated hopping of these small polarons is believed to be the conduction mechanism in the Mg_1−xK_xAl₂O₄. The activation energy of hopping of small polarons has been calculated and found K⁺ ions content dependent.     

Influence of Sr doping on the free carrier absorption in high-T c superconductors La2−x SrxCuO4

The reflectance spectra of polycrystalline La_2–x Sr _x CuO₄ samples were investigated in the energy range between 50 meV and 4 eV in dependence of the Sr content. The spectra are attributed to free carrier absorption of the Drude type, superimposed by optical phonon excitations below 0.1 eV and intrinsic absorption above 1 eV. From the influence of Sr doping onto the plasma energy it is deduced that La_2–x Sr _x CuO₄ is ap-type conductor with a maximum carrier concentration of 2.0×10²¹ cm^–3 forx=0.15. The results are interpreted in terms of a Hubbard model with an empty upper and ap-doped lower Hubbard band with a width of 1.9 eV.     

Light-induced charge transfer and kinetics of the NIR absorption of Nb4+ polarons in SBN crystals at low temperatures

Sr_1-xBa_xNb₂O₆ (SBN) crystals with open tungsten-bronze structure show enhanced photorefractive properties with doping of impurities such as Ce, Cr, Rh etc. Under illumination with Kr⁺ laser (647 nm) or Ar⁺ laser light (488 nm or 514 nm) or UV light at low temperature, pure and doped SBN crystals show a broad polaron absorption band around 0.7 eV (6000 cm^-1). The first step of a theoretical model involves the excitation of electrons by illumination from Cr³⁺/Ce³⁺ to higher excited states or the conduction band. The excited electrons can then be trapped by Nb⁵⁺ to form Nb⁴⁺ polarons and further on can directly tunnel through or hop over the potential barrier (with a value Δ≈0.15±0.02 eV) to recombine with Cr⁴⁺/Ce⁴⁺ ions. The experimental intensity dependence, temperature dependence, and decay process of the light-induced Nb⁴⁺ polarons can be fitted with the help of this model. Small, but systematic, differences lead to the additional assumption of different recombination rates of polarons at distinct distances from the Cr⁴⁺/Ce⁴⁺ recombination centers and therefore many parallel decay channels are active where each decay channel obeys a monoexponential decay law. A stretched exponential decay function is employed to fit in this case the decay process of the Nb⁴⁺ polarons at different temperatures and under illumination with different intensities. Due to the high dielectric constant value (ε₃₃ and ε₁₁ have values in the 10²-10³ range) at low temperature, the long range Coulomb attraction (to Ce³⁺ _Sr/Ba) or repulsion (from Cr³⁺ _Nb) of the electronic polaron is suppressed. The leading role in the attraction and the following trapping of the electronic Jahn–Teller polaron, both on Cr³⁺ _Nb and Ce³⁺ _Sr/Ba centers, is played by the indirect dipole–dipole interaction via the soft TO-mode. Received: 27 November 1998 / Revised version: 22 January 1999 / Published online: 7 April 1999