Yu. P. Sukhorukov

Red Shift of Absorption Edge and Nonmetal–Metal Transition in Single Crystals La1—xSrxMnO3 (x = 0.1, 0.2, 0.3)

The red shift of the fundamental absorption edge (mo.16 eV) of a La 0.9 Sr 0.1 MnO 3 single crystal occurs as the crystal is cooled into the ferromagnetic phase. This result shows that there exist spin-dependent states. The reflection and absorption spectra of La 1-x Sr x MnO 3 single crystals (x = 0.1, 0.2, 0.3) indicate that the red shift may be the reason for the appearance of free charge carriers participating in the colossal magnetoresistance phenomenon. The nonmetal-metal type transition below T C confirms this.

Giant change in infrared light transmission in La0.67Ca0.33MnO3 film near the Curie temperature

Yu. P. Sukhorukov,, E. A. Gan’shina, B. I. Belevtsev, N. N. Loshkareva, A. N. Vinogradov, K. D. D. Rathnayaka, A. Parasiris, D. G. Naugle Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, 620219 Ekaterinburg, Russia Physics Department, Moscow State University, 119899, Moscow, Russia B. Verkin Institute for Low Temperature Physics & Engineering, Ukrainian Academy of Sciences, Kharkov, 61103, Ukraine Department of Physics, Texas A&M University, College Station, TX 77843-4242, USATransport, magnetic, magneto-optical (Kerr effect) and optical (light absorption) properties have been studied in an oriented polycrystalline La0.67Ca0.33MnO3 film which shows colossal magnetoresistance. The correlations between these properties are presented. A giant change in infrared light transmission (more than a thousand-fold decrease) is observed on crossing the Curie temperature (about 270 K) from high to low temperature. Large changes in transmittance in a magnetic field were observed as well. The giant changes in transmittance and the large magnetotransmittance can be used for development of IR optoelectronic devices controlled by thermal and magnetic fields. Required material characteristics of doped manganites for these devices are discussed.

Specific features of the electronic structure and optical spectra of nanoparticles with strong electron correlations

Analysis of the experimental optical spectra of CuO nanoparticles with the electronic structure characterized by strong electron correlations has revealed the appearance of unusual states inside the band gap. The intragap states and the specific features of the electronic structure of CuO nanoparticles are discussed in the framework of the generalized tight-binding method previously developed for describing the electronic structure of superconducting cuprates.

Anomalies in the optical properties of nanocrystalline copper oxides CuO and Cu2O near the fundamental absorption edge

A 0.1–0.15-eV displacement of the fundamental absorption edge in the optical absorption spectra of nanocrystalline oxide n-CuO (relative to the position of the fundamental absorption edge in the spectra of CuO single crystals) towards lower energies (red shift) is observed against the background of strong blurring. Nanocrystalline n-Cu2O exhibits a displacement of the fundamental absorption edge towards higher energies (blue shift) by approximately 0.35 eV. The size of crystallites in n-CuO and n-Cu2O ranges from 10 to 90 nm. The blue shift of the fundamental absorption edge of n-Cu2O is typical of classical wide-gap semiconductors and can be explained by size quantization upon a change in the particle size. The anomalous red shift of the fundamental absorption edge of the strongly correlated nanocrystalline oxide n-CuO can be attributed to the highly defective structure of n-CuO, anomalies in the electronic structure of strongly correlated compounds based on 3d metals, and their tendency to electronic phase separation with the formation of metal-like inclusions.

Nanocrystalline copper oxide for selective solar energy absorbers

The high-energy edge of the transparency window in nanocrystalline copper monoxide (CuO) can be shifted to 0.6 eV (against 1.45 eV in single crystals). This shift is related to a modification of the shape of the fundamental absorption edge of CuO without any change in the refractive index. The obtained nanocrystalline CuO is recommended for use as a selective solar energy absorber.

The influence of magnetic and electronic inhomogeneities on magnetotransmission and magnetoresistance of La0.67Sr0.33MnO3 films

The optical (absorption of light and magnetotransmission of IR radiation), magnetic, and transport properties of the epitaxial La0.67Sr0.33MnO3 films of different thickness grown by laser ablation on the (100) SrTiO3 and LaAlO3 substrates were investigated. The effect of magnetotransmission reaches 6% at the temperature of 350K while magnetoresistance reaches 7.6% at 354K in a magnetic field of 8kOe. The factors, which influence the values of magnetotransmission and magnetoresistance of manganite films, are discussed.

Giant magnetorefractive effect in La0.7Ca0.3MnO3 films

Complex experimental investigations of the structural, optical, and magneto-optical properties (magnetotransmission, magnetoreflection, and transversal Kerr effect, as well as the magnetoresistance, of La0.7Ca0.3MnO3 epitaxial films indicate that magnetoreflection and magnetotransmission in manganite films can reach giant values and depend strongly on the magnetic and charge homogeneity of the films, their thickness, and spectral range under investigation. It has been shown that the optical enhancement of the magnetorefractive effect occurs in thin films as compared to manganite crystals. In the region of the minimum of the reflectance near the first phonon band, the resonance-like magnetorefractive effect has been observed, which is accompanied by change of the sign of the magnetoreflection. A model based on the theory of the magnetorefractive effect has been proposed to qualitatively explain this behavior.

Effect of isovalent doping of manganite (La1−xPrx)0.7Ca0.3MnO3 films (0≤x≤1) on their optical, magnetooptical, and transport properties near the metal-insulator transition

The optical, magnetooptical, and electric properties of epitaxial (La1−xPrx)0.7Ca0.3MnO3 films (0≤x≤1) grown by MOCVD on LaAlO3 and SrTiO3 substrates were studied. It is shown that the decrease in the average cation radius resulting from isovalent substitution of the Pr for La ions brings about a lowering of the Curie temperature, the metal-insulator transition temperature, and the temperatures of the maxima in magnetotransmission (MT) and magnetoresistance (MR). These temperatures depend only weakly on the substrate type. Substitution of La by Pr does not change the shape of the spectral response of the transverse Kerr effect. For concentrations x≤0.50, the maximum values of the Kerr effect and of the MT vary insignificantly, which should be assigned to the existence of a singly connected ferromagnetic metallic region at low temperatures. In films with x=0.75, the presence of ferromagnetic metallic drops in an antiferromagnetic insulating matrix was revealed. The totality of the experimental data obtained suggest that nanoscopic magnetic and electronic nonuniformities exist both in films with a singly connected metallic region and in an x=1 film, which is an antiferromagnetic insulator.

Optical spectroscopy of phase separation in LaxMnO3

Abstract Optical, transport and magnetic properties of polycrystalline samples La x MnO 3 (0.7⩽ x ⩽1) are studied experimentally. The optical evidence for metal–insulator transition below Curie temperature is found in all samples, including that with semiconductor-like behavior of DC resistivity in the entire temperature range. Results indicate that there are metallic droplets in insulating matrix. Optical method is suggested for determining the metal–insulator transition in lanthanum manganites.

The evolution of magneto-transport and magneto-optical properties of thin La0.8Ag0.1MnO3+δ films possessing the in-plane variant structure as a function of the film thickness

Epitaxial La 0.8 Ag 0.1 MnO 3+δ films of different thicknesses (500-1000 nm) were grown on ZrO 2 (Y 2 O 3 ) substrates. Their optical, magneto-optical and transport properties were studied in order to clarify the effect of the epitaxial variant structure and Ag ion distribution on the conductivity, magnetoresistance and infrared magnetotransmission in these films. An original method was developed for separating MR contributions related to the colossal magnetoresistance near T C and the tunnelling magnetoresistance. It was established that in the La 0.8 Ag 0.1 MnO 3+δ films spin-polarization of electrons P reached ∼0.5. The transverse Kerr effect revealed the irregular distribution of Ag ions through the film thickness. The comparison of optical and electrical data implies lower silver content near the film-substrate boundary in relation to that in the domain volume.