A. A. Makhnev

Metal-insulator transition in double cobaltites RBaCo2O5.5 (R = Eu, Gd): Specific features of their optical properties

The temperature dependence of the optical properties of GdBaCo2O5.5 in the region of the metal-insulator transition was studied in the spectral range λ = 4.0–0.258 μm (E = 0.3–4.8 eV) by the spectroscopic ellipsometry method. The electronic structure of the crystal at T = 300 K was calculated by the linearized muffin-tin orbital method in the local-density approximation with inclusion of on-site Coulomb interaction (LDA + U). The general features of the behavior of the optical properties of GdBaCo2O5.5 and EuBaCo2O5.5 are discussed. It is shown that the optical response on the metal side of the metal-insulator transition is determined by the redistribution of the optical-conductivity spectral weight in going from high to low energies and is significantly incoherent in character. The optical band gap width of the GdBaCo2O5.5 is experimentally determined to be Eg = 0.26 eV, which is close to a theoretical estimate. The type of interband transitions is determined from analyzing the temperature dependence of the optical functions for absorption bands.

Specific features of the behavior of optical properties upon the metal-insulator transition in EuBaCo2O5+δ

The temperature dependences of the optical properties and the electrical resistivity for EuBaCo2O5+δ single crystals are investigated. At temperatures below the metal-insulator transition (TMI = 340 K), the electrical resistivity is well approximated by the relationship ρ = ρ0exp(T/T0)1/4. The optical band gap Eg = 0.05 eV for the insulating phase is underestimated as compared to the theoretical value. The specific features in the dispersion of the optical conductivity and the real part of the complex permittivity upon the metal-insulator transition are determined. It is demonstrated that the optical response from charge carriers on the metal side of the metal-insulator transition is caused by the redistribution of the spectral weight of the optical conductivity from the high-energy range to the low-energy range and exhibits a strongly incoherent character. The revealed features are associated with the manifestation of the strongly correlated metallic state.

Evolution of the optical properties of single-crystal La1−xSrxMnO3

An ellipsometric method is used to study the dispersion of the real ɛ1(ω) and imaginary ɛ2(ω) parts of the complex dielectric permittivity of single-crystal La1−xSrxMnO3 (x=0.1, 0.2, and 0.3) for energies from 100 meV to 5 eV at room temperature. It is found that, when lanthanum is replaced by strontium, the optical spectrum changes fundamentally. A shift in the main features of the spectrum of initial LaMnO3 at 1.9 and 4.7 eV to lower energies takes place, as well as a partial redistribution of the optical-conductivity spectral weight in the band gap region E<1.7 eV. For compositions with x=0.2 and 0.3, a fine structure of the interband absorption is observed against a background of non-Drude optical conductivity at low energies.

Ellipsometric study of the optical properties of Ca1 − x La x MnO3 single crystals (x = 0–0.2) under n-type doping

The dispersion of the real, ε1(ω), and imaginary, ε2(ω), parts of the complex permittivity of Ca1 − xLaxMnO3 single crystals (x = 0, 0.05, 0.10, 0.12, 0.20) was studied at room temperature in the spectral region extending from 60 meV to 5 eV. It was found that substitution of lanthanum for calcium shifts the 3.1-eV absorption band in the optical-conductivity spectrum toward higher energies, with the spectral weight of the low-energy wing of the 2.2-eV band becoming redistributed to the band-gap region (E < 1.5 eV) of the starting CaMnO3 compound. The specific features of optical-conductivity dispersion in the mid-IR region that occur under n-type doping were established. The frequency dependence of optical conductivity was shown to differ from the Drude behavior characteristic of metals. The optical-conductivity spectra of Ca1 − xLaxMnO3 were compared with our earlier results on a series of hole-doped La1 − xSrxMnO3 single crystal.

Optical properties and electronic structure of cobalt-deficit GdBaCo1.86O5 + δ single crystals

The optical properties of cobalt-deficit GdBaCo2 − xO5 + δ single crystals with different oxygen contents have been studied using the ellipsometry method in the spectral range of 0.16–4.8 eV. The optical conductivity spectrum has been characterized by a low-energy structure with the center at ∼1.3 eV and an intense broad absorption band at 1.8–4.0 eV. It has been found that the electronic transitions in the cobalt-deficit samples are partially blocked in a narrow spectral range of 0.5–0.8 eV due to weakening of the contribution from the interstitial Co(3d)-Co(3d) transitions. The ordered GdBaCo2O5.5 crystal exhibits a strong absorption in the energy band gap (Eg < 0.26 eV) due to the structural disorder, defects, and vacancies. A significant decrease in the interband absorption intensity in the tetragonal phase of the GdBaCo1.86O5.07 compound as compared to that in the case of the orthorhombic GdBaCo1.86O5.32 phase has been explained by the existence of only the pyramidal environment of a cobalt ion.

Scattering of conduction electrons in Fe( t x , Å)/Cr(10 Å) superlattices with ultrathin iron layers

IR magnetoreflection spectra, diagonal σxx and off-diagonal σxy components of the effective optical conductivity tensor, and magnetic properties of Fe(tx, Å)/Cr(10 Å) superlattices have been studied. The abrupt decrease in the amplitude of dissipative function −ωImσxy(ω) (ω is the cyclic frequency of light wave) in the superlattices with ultrathin Fe layers (tFe = 3.2, 2.6, 2.1 Å) has been analyzed. It has been found that the magnetorefractive effect in nanostructures with ultrathin iron layers is due to scattering of conduction electrons by magnetic interfacial layers formed in the Cr matrix with complete consumption of deposited iron atoms. The parameters of the interfacial scattering of electrons in the spin-up (└) and spin-down (┌) conduction channels have been discussed.

Anisotropy of permittivity of the hexagonal multiferroic HoMnO3 in the spectral range 0.6–5.0 eV

The anisotropy of the optical properties of a single crystal of the hexagonal manganite HoMnO3 has been investigated by spectroscopic ellipsometry in the spectral range 0.6–5.0 eV. It has been demonstrated that the optical absorption edge for the polarization E ⊥ c is determined by the intense narrow transition O(2p) → Mn(3d) centered at 1.5 eV, whereas this transition for the polarization E ‖ c is strongly suppressed and shifted toward higher energies by 0.2 eV. It has been revealed that, at the temperature T = 293 K, the spectra for both polarizations E ‖ c and E ⊥ c exhibit a broad absorption band centered at ∼2.4 eV, which was earlier observed in nonlinear spectra during optical second harmonic generation.

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co3V2O8 and Ni3V2O8: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co3V2O8 and Co3V2O8 single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co3V2O8 is shifted toward low energies by 0.25–0.3 eV as compared to Co3V2O8. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.

Magnetooptical, optical, and magnetotransport properties of Co/Cu superlattices with ultrathin cobalt layers

We investigated the field dependences of the magnetization and magnetoresistance of superlattices [Co(tx, Å)/Cu(9.6 Å)]30 prepared by magnetron sputtering, differing in the thickness of cobalt layers (0.3 Å ≤ tCo ≤ 15 Å). The optical and magnetooptical properties of these objects were studied by ellipsometry in the spectral region of hω= 0.09–6.2 eV and with the help of the transverse Kerr effect (hω= 0.5–6.2 eV). In the curves of an off-diagonal component of the tensor of the optical conductivity of superlattices with tCo = 3–15 Å, a structure of oscillatory type (“loop”) was detected in the ultraviolet region, resulting from the exchange splitting of the 3d band in the energy spectrum of the face-centered cubic structure of cobalt (fcc Co). Based on magnetic experiments and measurements of the transverse Kerr effect, we found the presence of a superparamagnetic phase in Co/Cu superlattices with a thickness of the cobalt layers of 3 and 2 Å. The transition from superlattices with solid ferromagnetic layers to superparamagnetic cluster-layered nanostructures and further to the structures based on Co and Cu (tCo = 0.3–1 Å) with a Kondo-like characteristics of the electrical resistivity at low temperatures is analyzed.