Surface polaritons in KTaO3 and SrTiO3

The upper two surface polariton dispersion branches of KTaO₃ and SrTiO₃ were measured by attenuated total reflection and compared to calculated curves based on data for the dielectric function which were derived from a Kramers-Kronig analysis of reflectivity spectra. In KTaO₃ we observe a splitting of the highest branch caused by a strong combination band. The nature of that band is discussed qualitatively.     

The wavelength-modulated spectrum of SrTiO3 and KTaO3

It is shown experimentally that the temperature variation of the band gap of SrTiO₃ is determined by the phonon population rather than by the structural phase transition. Despite the great similarity between SrTiO₃ and KTaO₃ no similar temperature variation of the band gap is found in the latter material.     

Electronic band structure of KTaO3

Calculations are presented for the electronic band structure of the perovskite-type crystal KTaO₃. The results are obtained using the LCAO method in a modified form. On comparison with experimental data good agreement was found after a reassignment of one transition.     

Superconductivity in epitaxially grown LaVO3/KTaO3(111) heterostructures

Complex oxide heterointerfaces can host a rich of emergent phenomena, and epitaxial growth is usually at the heart of forming these interfaces. Recently, a strong crystalline-orientation-dependent two-dimensional superconductivity was discovered at interfaces between KTaO₃single-crystal substrates and films of other oxides. Unexpectedly, rare of these oxide films was epitaxially grown. Here, we report the existence of superconductivity in epitaxially grown LaVO₃/KTaO_3<...     

Defect driven ferroelectricity and magnetism in nanocrystalline KTaO3

Nominally pure nanocrystalline KTaO₃ was thoroughly investigated by micro-Raman and magnetic resonance spectroscopic techniques. In all samples the defect driven ferroelectricity and magnetism are registered. Both ordering states are suggested to appear due to the iron atoms and oxygen vacancies. The concentration of defects was estimated to be 0.04 and 0.06-0.1 mol%. Note that undoped single crystals of KTaO₃ are nonmagnetic and have never exhibited ferromagnetic properties. The results enable us to refer a nanosized KTaO₃ to the class of multiferroics and assume that it could perform the magnetoelectric effect at T<29 K. It was also established that the critical concentration of impurity defects necessary to provoke the appearance of the new phase states in the material strongly correlates with the size of the particle; as the size of the particle decreases, the critical concentration decreases as well.     

Pressure dependence of the impurity soft modes in Li and Na : KTaO3

Low energy Raman scattering measurements on samples of K_0.76Na_0.24TaO₃, K_0.9Na_0.1TaO₃ and K_0.75Li_0.25TaO₃ under hydrostatic pressures of up to 60 kbar supply confirming evidence to the hypothesis that the Na⁺ ions in these crystals are displaced from the centers of the sites they occupy. The pressure Raman experiments also reveal another second order Raman line interpreted as Raman scattering by two TA phonons at M. A line induced by the substitution of Na for K is found to have an energy about equal to the energy of this new second order Raman line. It is shown that if this is a resonant mode it must be an even mode.     

An alternative interpretation of the optical spectra for Fe-doped KTaO3 crystals

In this paper, the excitation spectrum and luminescence at 14 569, 17 225, 18 829 and 14 659 cm^-1 for Fe³⁺ ion at the K⁺ site of KTaO₃ crystals are assigned, respectively, to the ⁶A₁(S)→⁴T₁(G), ⁴T₂(G), ⁴E₁(G)[⁴A₁(G)] and ⁴T₁(G)→⁶A₁(S) transitions rather than to the ⁶A₁(S)→⁴T₁(G), ²T₂(I), ⁴T₂(G) and ⁴T₁(G)→⁶A₁(S) transitions given in a previous paper [Bryknar et al., Radiat. Eff. Def. Solids 149(1999)51]. On the basis of this assignment, the reasonable optical spectrum parameters (in particular, the cubic field parameter Dq≈−640 cm⁻¹) are obtained. The validity of this assignment is discussed.     

Structural,electronic and optical properties of cubic SrTiO3 and KTaO3: Ab initio and GW calculations

We present first-principles VASP calculations of the structural, electronic, vibrational, and optical properties of paraelectric SrTiO₃ and KTaO₃. The ab initio calculations are performed in the framework of density functional theory with different exchange-correlation potentials. Our calculated lattice parameters, elastic constants, and vibrational frequencies are found to be in good agreement with the available experimental values. Then, the bandstructures are calculated with the GW approximation, and the corresponding band gap is used to obtain the optical properties of SrTiO₃ and KTaO₃.     

Anomalous Metallic State Driven by Magnetic Field at the LaAlO3/KTaO3(111) Interface

The origin of the quantum superconductor to metal transition at zero temperature in two-dimensional superconductors is still an open problem, which has caused intensely discussion. Here, we report the observation of a quantum superconductor-to-metal transition in La Al O₃/KTa O₃(111) interface, driven by magnetic field. When a small magnetic field perpendicular to the film plane is applied, the residual saturated resistance is observed,indicating the emergence of an anomalo...     

Investigations of zero-field splitting and defect structure for the tetragonal FeK-OI center in KTaO3 crystal

By using the defect structure data (characterized by the coordinates of impurity center) obtained from the shell model and the density functional theory in the generalized gradient approximation (GGA) corresponding to two supercell sizes, the zero-field splitting D of the tetragonal -O_I center in KTaO₃ crystal is calculated from the high-order perturbation formula based on the dominant spin-orbit coupling mechanism. The calculated results suggest that the sign of zero-field splitting D is negative and the defect structure data obtained from GGA method are more reasonable. Compared with that corresponding to the smaller supercell size, the calculated D value based on the GGA coordinates corresponding to the larger supercell size is closer to the observed value, suggesting that the GGA coordinates obtained from the larger supercell size are more accurate.     

Studies on the resistance switching properties of the La0.5Ca0.5MnO3/Nb : SrTiO3 heterojunction

In this study, we have investigated the resistance switching behavior of the La_0.5Ca_0.5MnO₃/Nb:SrTiO₃ heterojunction. The junction shows a negative resistance switching ratio (ER) below 140 K. When , the ER goes from negative to positive with increasing bias voltage. When T>220 K, the junction shows a positive ER. This variation from a negative to a positive value indicates that the ER is determined primarily by two phenomena: (a) the negative ER value can be attributed to a disruption of the charge-ordered insulating domains in La_0.5Ca_0.5MnO₃ under large electric fields, and (b) the positive ER value at high temperatures is due to the modulation of the interface barrier width driven by the electrochemical migration of oxygen vacancies.     

Influence of the transparency of tunnel barriers in Nb/Al2O3/Al/Al2O3/Nb junctions on transport properties

The transparency of the tunnel barriers in double-barrier junctions influences the critical current density and the form of the current–voltage characteristics (IVC). Moreover, the barrier asymmetry is an important parameter, which has to be controlled in the technological process. We have performed a systematic study of the influence of the barrier transparency on critical current, I_C, and normal resistance, R_N, by preparing SIS and SINIS junctions under identical technological conditions and comparing their transport properties. We have fabricated Nb/Al₂O₃/Nb and Nb/Al₂O₃/Al/Al₂O₃/Nb devices with different current densities using a conventional fabrication process, varying pressure and oxidation time. The thickness of the Al middle electrode in all Nb/Al₂O₃/Al/Al₂O₃/Nb junctions was 6 nm. Patterning of the multilayers was done using conventional photolithography and the selective niobium etching process. The current density of SIS junctions was changed in the range from 0.5 to 10 kA/cm². At the same conditions the current density of SINIS devices revealed 1–100 A/cm² with non-hysteretic IVC and characteristic voltages, I_CR_N, of up to 200 μV. By comparing the experimental and theoretical temperature dependence of the I_CR_N product we estimated the barrier transparency and its asymmetry. The comparison shows a good agreement of experimental data with the theoretical model of tunneling through double-barrier structures in the dirty limit and provides the effective barrier transparency parameter γ_eff≈300. A theoretical framework is developed to study the influence of the barrier asymmetry on the current–phase relationship and it is proposed to determine the asymmetry parameter by measuring the critical current suppression as function of applied microwave power. The theoretical approach to determine the non-stationary properties of double-barrier junctions in the adiabatic regime is formulated and the results of calculations of the I–V characteristics are given in relevant limits. The existence and the magnitude of a current deficit are predicted as function of the barrier asymmetry.     

Magnetization studies of TbFeO3

The spontaneous magnetization and principal magnetic susceptibilities of TbFeO₃ were measured from 4.2 to 300 K. The weak ferromagnetic moment is along the c crystallographic axis in the entire temperature range. The field dependence of the magnetization at 4.2 K was also studied. The magnetic behavior is interpreted in terms of an interaction between the ordered Fe³⁺ spin system and the electrons occupying the lowest lying “accidental” doublet of the Tb³⁺ ions. The FeTb interaction and the Tb³⁺ Van Vl eck susceptibility along the c axis play significant roles in determining the magnetic configuration of the Fe³⁺ spin system. No indication was found that the TbTb interaction plays a significant role in the magnetic behavior of TbFeO₃ at temperature above 4.2 K.     

Photocarrier injection and the I-V characteristics of La0.8Sr0.2MnO3/SrTiO3:Nb heterojunctions

Oxide heterojunctions made of p-type La_0.8Sr_0.2MnO₃ (LSMO) and niobium-doped n-type SrTiO₃ (STO:Nb) have been fabricated by the pulsed laser deposition (PLD) technique and characterized under UV light irradiation by measuring the current-voltage, photovoltaic properties and the junction capacitance. It is shown that the heterojunctions work as an efficient UV photodiode, in which photogenerated holes in the STO:Nb substrate are injected to the LSMO film. The maximum surface hole density Q/e and external quantum efficiency γ are estimated to be 8.3×10¹² cm⁻² and 11% at room temperature, respectively. They are improved significantly in a p-i-n junction of LSMO/STO/STO:Nb, where Q/e and γ are 3.0×10¹³ cm⁻² and 27%, respectively.     

Time-resolved site-selection spectroscopy studies of NdAl3(BO3)4 crystals

A pulsed, tunable dye laser was used to selectively excite Nd³⁺ ions in nonequivalent crystal field sites in NdAl₃(BO₃)₄ crystals and energy transfer between ions in different types of sites was studied by monitoring the time evolution of the fluorescence spectrum. The results show that the energy transfer rate varies as t^{$\text{-1}\text{2}$} and increases with temperature. The predictions of various models of phonon-assisted energy transfer are compared to the results.     

掺铌SrTiO_3中的逆自旋霍尔效应

采用磁控溅射法在未掺杂和掺杂的SrTiO_3基片上沉积了NiFe薄膜,通过翻转测试法分离出掺杂样品中的自旋整流电压和逆自旋霍尔电压.研究结果表明:在未掺杂的SrTiO_3基片中,翻转前后测试的电压曲线基本一致,为NiFe薄膜自旋整流效应产生的电压.对于掺Nb浓度x为0.028, 0.05, 0.1, 0.15, 0.2的SrTiO_3基片,分离出的逆自旋霍尔电压随掺杂浓度增加而减小,在掺杂浓度为0.15和0.2的样品中没有探测到明显的逆自旋霍尔电压.本文的结果表明,在SrTiO_3中掺入强自旋轨道耦合的杂质,通过掺杂浓度可以实现对SrTiO_3中逆自旋霍尔效应的调控,这类可调控的自旋相关研究为自旋电子器件的研究和开发提供了更多的可能性,具有很大的潜在应用价值.     

Structure and dielectric studies of nano-composite Fe2O3: BaTiO3 prepared by sol-gel method

Nano-structure pure barium titanate (BaTiO₃) and that was doped with iron oxide (Fe₂O₃), have been prepared by sol-gel method, using barium acetate (Ba(Ac)₂) and titanium butoxide (Ti(C₄H₉O)₄), as precursors. The as-grown prepared samples by sol-gel technique were found to be amorphous, which crystallized to the tetragonal phase after synthesized at 750 °C in air for 1 h as detected from the XRD patterns. The XRD data were confirmed by transmission electron microscope (TEM). The dielectric properties namely; dielectric constant (ε′) and loss tangent (tan δ) in the frequency range between 42 Hz and 1 MHz, at range of temperature 25-250 °C were investigated. The temperature dependence of ε′ and tan δ for the undoped and doped materials, at 1 kHz, was also investigated. As a result, tan δ increased rapidly with decreasing temperature below 125 °C (Curie temperature) while above this temperature, tan δ shows temperature independent. As a result, below and above Curie temperature, ferroelectric phase and paraelectric phase of BaTiO₃ can be obtained, respectively.     

High-pressure Raman spectroscopic studies on orthophosphates Ba3(PO4)2 and Sr3(PO4)2

By using diamond anvil cell (DAC), high-pressure Raman spectroscopic studies of orthophosphates Ba₃(PO₄)₂ and Sr₃(PO₄)₂ were carried out up to 30.7 and 30.1 GPa, respectively. No pressure-induced phase transition was found in the studies. A methanol:ethanol:water (16:3:1) mixture was used as pressure medium in DAC, which is expected to exhibit nearly hydrostatic behavior up to about 14.4 GPa at room temperature. The behaviors of the phosphate modes in Ba₃(PO₄)₂ and Sr₃(PO₄)₂ below 14.4 GPa were quantitatively analyzed. The Raman shift of all modes increased linearly and continuously with pressure in Ba₃(PO₄)₂ and Sr₃(PO₄)₂. The pressure coefficients of the phosphate modes in Ba₃(PO₄)₂ range from 2.8179 to 3.4186 cm⁻¹ GPa⁻¹ for ν₃, 2.9609 cm⁻¹ GPa⁻¹ for ν₁, from 0.9855 to 1.8085 cm⁻¹ GPa⁻¹ for ν₄, and 1.4330 cm⁻¹ GPa⁻¹ for ν₂, and the pressure coefficients of the phosphate modes in Sr₃(PO₄)₂ range from 3.4247 to 4.3765 cm⁻¹ GPa⁻¹ for ν₃, 3.7808 cm⁻¹ GPa⁻¹ for ν₁, from 1.1005 to 1.9244 cm⁻¹ GPa⁻¹ for ν₄, and 1.5647 cm⁻¹ GPa⁻¹ for ν₂.     

Luminescence studies on ZnO-P2O5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Zinc phosphate glasses doped with Gd₂O₃:Eu nanoparticles and Eu₂O₃ were prepared by conventional melt-quench method and characterized for their luminescence properties. Binary ZnO-P₂O₅ glass is characterized by an intrinsic defect centre emission around 324 nm. Strong energy transfer from these defect centres to Eu³⁺ ions has been observed when Eu₂O₃ is incorporated in ZnO-P₂O₅ glasses. Lack of energy transfer from these defect centres to Eu³⁺ in Gd₂O₃:Eu nanoparticles doped ZnO-P₂O₅ glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between the luminescent centre and Eu³⁺ ions. Both doped and undoped glasses have the same glass transition temperature, suggesting that the phosphate network is not significantly affected by the Gd₂O₃:Eu nanoparticles or Eu₂O₃ incorporation.     

Surface and transport studies on La0.7Ba0.3MnO3:SnO2 bilayer

We report on study of morphology, optical contrast and transport characteristics of La_0.7Ba_0.3MnO₃ (LBMO) manganite thin films bilayered with SnO₂ on Si (0 0 1) substrate, synthesized using pulsed laser deposition system. X-ray diffraction study reveals that both LBMO and SnO₂ show polycrystalline growth over the substrate. Atomic force microscopy shows interesting pyramidal structures of LBMO of size ∼2 μm × 1 μm × 0.1 μm. On the other hand, SnO₂ grows in the form of close packed cylindrical clusters of ∼200 nm radius. Near-field optical microscopy (NSOM) study using 532 nm laser reveal that optical NSOM output intensity in LBMO is four times less than SnO₂ signal. Transport characterizations show that this bilayer configuration exhibit non-linear current-voltage characteristics from 300 upto 50 K. The nature becomes linear below this temperature. The results project the system as a promising candidate in non-conventional device category in the area of spintronics.