V. D. Okunev

Amorphous state and pulsed laser deposition of YBa2Cu3O7−δ thin films

Pulsed laser deposition of high Tc compounds onto unheated substrates, resulting in amorphous thin films, preserves to a great extent the composition of matter ejected from the target. This composition is of primary interest, both for understanding the dynamics of laser–target interaction and for practical (optimization) reasons. We have investigated the structure of amorphous and crystalline YBaCuO films obtained both in on-axis and off-axis deposition geometries, and correlated the results with optical and transport properties of these films. X-ray scattering reveals in amorphous films the existence of: (i) amorphous continuum of spatially disordered atoms, (ii) small (10–40 A) amorphous clusters which can be considered as mesoscopic order fluctuations in the amorphous continuum, and (iii) slightly larger (50–250 A) crystalline clusters exhibiting quasi-two dimensional (00l) or (11l) long range order. Crystalline films are predominantly (00l) oriented. Optical spectra of both crystalline and amorphous f...

Dynamic Phase Transition in Amorphous YBaCuO Films under Ar Laser Irradiation

The effect of laser irradiation (Ar laser, λ = 488 nm) on the atomic order and electronic structure of amorphous YBaCuO films is studied. It is shown that the laser irradiation of amorphous YBaCuO films leads to a sequence of processes having an oscillating character: energy accumulation (Iincoh → max), accompanied by stronger atomic disordering, and relaxation (Jcoh → max) due to atomic ordering and a change in the predominant cluster configuration, (11l) – (10l), at incident powers above 2 W. This corresponds to a local topological ordering of the solid solution with the participation of copper–oxygen chains, accompanied by an increase in conductivity by two orders of magnitude.

Atomic order, electron structure, and critical parameters of epitaxial YBaCuO films

X-ray structural and optical methods were applied to make a detailed investigation of atomic and electronic structure of superconducting YBa2Cu3O7−δ films obtained by pulsed laser deposition. Optical transmission spectrum was found to depend not only on intraband (ℏω 1.95 eV) transitions, but also on contributions from charge transfer transitions O 2p⇒Cu 3d in the dielectric phase due to local order in a system of crystallographic (11l) planes. Superconducting properties of the films are determined by orientational structural transition (00l)+(11l)⇒(00l) and by Anderson type transition from a charge transfer O 2p⇒Cu 3d insulator to a strongly degenerate semiconductor with metallic conductivity. We show the effects of ordering on transition temperature Tc and demonstrate that at the concentration of (00l) clusters below the percolation threshold (20%) the behavior of Tc has catastrophic character.

Effect of a cluster structure on the interaction of electron and magnetic subsystems in LaCa(Sr)MnO epilayers

The structure, electrical, and magnetic properties of epitaxial LaCa(Sr)MnO single crystal films with a clustered structure have been studied. In films with a “metallic” phase content Cm0≤0.15, the electric conductivity is determined by the spin-dependent tunneling of charge carriers between “ metallic” clusters, and the magnetoresistance is maximum at T = 4.2 K. The correlated motion of carriers over the system of tunneling-linked clusters leads to the formation of a window in the Coulomb blockade. The interactions between atomic, magnetic, and electron subsystems increase in the vicinity of the dielectric-metal percolation transition (T = 200–210 K), where the metal phase content Cm in the samples with Cm0≥0.2 reaches a maximum (Cmcrit = 0.5) due to an increase in the cluster size upon cooling. In this case, the magnetoresistance exhibits a maximum at T = 260 K, on the dielectric side of the percolation transition. Due to the presence of space charge regions at the periphery of the clusters, the content of a ferromagnetic phase is 1.5–2 times that of the “metallic” phase. For this reason, the calculations are performed using a model combining the tunneling conductivity mechanism with the percolation approximation for the description of magnetization. Allowance for the Coulomb interaction between charge carriers and clusters improves the agreement of theory and experiment.

Changes in the electronic, optical, and magnetic properties of LaSrMnO films upon transition from the rhombohedral phase to the orthorhombic phase

The properties of LaSrMnO films are investigated in the temperature range of the transition from the rhombohedral phase to the orthorhombic phase (600–650° C). It is shown that, with a variation in the growth temperature Ts, the change in the film properties is governed by the interaction of Mn-O metallic (ferromagnetic) clusters in the dielectric (antiferromagnetic) matrix. At Ts≤600°C, the low density of eg states and the dielectric gap (Eg=0.3–0.5 eV) are responsible for the following features: (i) the optical transparency in the range ℏω=0.5–2 eV, (ii) the difference between the FC and ZFC magnetizations M(T), (iii) the high resistance, and (iv) the appearance of the portions R(T) ≈ const in the dependence R(T) due to the transformation of clusters into a system of tunnel-coupled quantum dots. At Ts≥650°C, the local increase in the atomic and electronic densities leads to a decrease in the optical transmission and the resistance by three to nine orders of magnitude, the appearance of a maximum and a minimum in the dependences R(T) of the LaSrMnO films, and an increase in the magnetization M(10 K) by one order of magnitude. The inference is drawn that magnetic ordering of the system of tunnel-coupled clusters encourages an increase in the cluster size and in the content of the metallic (ferromagnetic) phase.

Spherical cluster ensembles with fractal structure in LaSrMnO: New form of self-organization in solids

The growth of La0.7Sr0.3MnO3 films in magnetron plasma, in special conditions, leads to the appearance of ensembles of micron-sized spherical crystalline clusters with fractal structure, which we consider to be a new form of self-organization in solids. Each ensemble contains 105–106 elementary clusters, 100–250 A in diameter. Interaction of the clusters in the ensemble is realized through the interatomic chemical bonds, intrinsic to the manganites. Integration of peripheral areas of interacting clusters results in the formation of common intercluster medium in the ensemble. We argue that the ensembles with fractal structure built into paramagnetic disordered matrix have ferromagnetic properties. Absence of sharp borders between elementary clusters and the presence of common intercluster medium inside each ensemble permits to rearrange magnetic order and to change the volume of the ferromagnetic phase, providing automatically a high sensitivity of the material to the external field.

Disordering of the electronic structure of YBaCuO amorphous films upon incorporation of crystalline clusters formed in laser-induced plasma into their composition

The effect of crystalline clusters formed in a laser-induced plasma on the optical properties of YBa2Cu3O6 + x amorphous films prepared by pulsed laser deposition has been investigated. It has been demonstrated that an increase in the number of clusters leads to a gradual disappearance of interference fringes inherent in optically homogeneous media. Simultaneously, the incorporation of metallic and insulating clusters into the amorphous medium results in a decrease in the optical band gap E0 of the YBaCuO amorphous matrix from 1.28 to 1.06 eV and a considerable decrease in the probability of interband optical transitions with charge transfer O 2p → Cu 3d due to the loosening of the structure and generation of local stresses. It has been revealed that there is an additional band gap E1, which decreases from 0.25–0.30 eV to zero values with a decrease in the optical band gap E0. The additional gap has been interpreted as an energy gap between localized states that belong to the valence and conduction bands. A decrease in the density of electronic states in the narrow 3d band leads to the overlap of tails of the density of states, so that the band gap E1 becomes negative.

Quantum dots in YBa2Cu3O6+x films with a tetragonal structure

Experimental evidence is presented for the existence of clusters possessing metallic conductivity in epitaxial YBa2Cu3O6+x (x<0.4) films with a tetragonal structure. Despite the overall dielectric state, the optical transmission spectra reveal the regions of absorption due to free charge carriers. The presence of metallic clusters is also confirmed by the X-ray diffraction data. At low temperatures, these metallic clusters are capable of converting into quantum dots responsible for the θ(T) = const regions in the temperature dependence of the electric resistance. The process of cluster formation can be enhanced by high-power laser (KrF) radiation.

Phase transitions in the atomic, electronic, and magnetic subsystems of LaSrMnO films versus the synthesis temperature

The evolution of the cluster structure in amorphous LaSrMnO films as synthesis temperature Ts increases from 20 to 300°C is considered. Two order-disorder phase transitions with different scale parameters are observed. One of them, the aggregation of disordered atoms into small (∼20 Å) amorphous clusters at Ts = 100°C, shows up as a sharp increase in the intensity of diffuse X-ray scattering (diffuse halo 1) with a simultaneous suppression of incoherent (background) scattering. At Ts > 150°C, disordering dominates (Iincoh = Imax) until the next stage of ordering sets in at Ts = 250−300°C. At this stage, the crystalline phase forms from large (>100 Å) crystalline clusters. This amorphous-crystalline phase transition is characterized by the appearance of Debye lines and a reduction of the halo intensity. The structural phase transition to long-range order is accompanied by a decrease in the LaSrMnO resistivity from 1010 to 10 Ω cm and a change from the tunneling mechanism of conductivity involving metallic clusters (which is typical of granulated systems) to the hopping mechanism with a hop variable length following the Mott law ρ ∼ exp(T−1/4). In the magnetic subsystem, the paramagnetic-ferromagnetic phase transition occurs.

Formation of YBa2Cu3O6 + x clusters in laser-generated plasma

We have studied the influence of YBa2Cu3O6 + x clusters formed in the plasma generated by laser ablation of a YBa2Cu3O7 − δ target on the optical transmission spectra of amorphous YBaCuO films deposited on glass substrates arranged along the direction of predominant plasma expansion in the laser plume. It is established that intense cluster formation begins in the region of rapid decrease in the film thickness, where the temperature of plasma decreases to a level at which stable atomic complexes characteristic of the target composition can form (under the experimental conditions studied, this was observed at as distance of L > 6 cm from the target). As the amount of clusters in the deposit increases, the magnitude of the interference fringes, which are characteristic of optically homogeneous media, gradually decreases and eventually almost vanishes. At the same time, features typical of the electron structure of YBa2Cu3O7 − δ appear and grow in the optical transmission spectra of the YBaCuO films, including the absorption due to free charge carriers at ℏω < 1.2 eV (characteristic of “metallic” clusters) and the minima at ℏω = 1.4 and 1.75 eV (characteristic of a dielectric state).