Superconducting and ferromagnetic phases in SrTiO3/LaAlO3 oxide interface structures: possibility of finite momentum pairing

We introduce a model to explain the observed ferromagnetism and superconductivity in LAO/STO oxide interface structures. Because of the polar catastrophe mechanism, 1/2 charge per unit cell is transferred to the interface layer. We argue that this charge localizes and orders ferromagnetically via exchange with the conduction electrons. Ordinarily, this ferromagnetism would destroy superconductivity, but, due to strong spin-orbit coupling near the interface, the magnetism and superconductivity can coexist by forming a Fulde-Ferrell-Larkin-Ovchinikov-type condensate of Cooper pairs at finite momentum, which is surprisingly robust in the presence of strong disorder.     

Transport of hydrogen species in a single crystal SrTiO3

The total conductivity and the partial hydrogen ion conductivity of a single crystal of SrTiO₃ have been investigated by transport number measurements using a hydrogen activity concentration cell/EMF method in wet atmospheres as a function of pO₂ (10^− 20–1 atm) at 1000 °C and as a function of temperature (350–1050 °C) in wet hydrogen. The single crystal exhibits a small but readily measurable proton conductivity contribution in wet oxidising atmospheres. Earlier indications of apparent – but unlikely – transport by negatively charged hydrogen ions under reducing conditions and high temperatures in polycrystalline SrTiO₃ have been reproduced for the single crystal, thus eliminating grain boundaries and porosity as a cause of such findings. The paper proposes transport of neutral hydrogen species through the oxide as an alternative cause: This results in polarization of the hydrogen activity at the interface between the electrodes and the electrolyte, in turn giving rise to an unintended gradient in oxygen activity and an EMF resulting from the relatively high oxygen ion transport number of the material.     

Lattice relaxation in oxide heterostructures: LaTiO3/SrTiO3 superlattices

Local density approximation + Hubbard U and many-body effective Hamiltonian calculations are used to determine the effects of lattice relaxation in LaTiO3/SrTiO3 superlattices. Large ferroelectric-like distortions of the TiO6 octahedra are found, which substantially affect the Ti d-electron density, bringing the calculated results into good agreement with experimental data. The relaxations also change the many-body physics, leading to a novel symmetry-breaking-induced ordering of the xy orbitals, which does not occur in bulk LaTiO3, or in the hypothetical unrelaxed structure.     

Atomic structures of reduced SrTiO3(001) surfaces

The scanning tunneling microscope has been employed to study the reduced SrTiO₃(001) surface in ultrahigh vacuum. Topographic images show a change of structural ordering for the surface annealed under three different conditions. In addition, our STM images reveal row-like structures with spacings of 12 and 20 Å. The observations are discussed in terms of the formation of intergrowths of lamellae of Sr_{n + 1} Ti_nO_{3n + 1}; a result of accommodation of oxygen and titanium deficiencies.     

Surface and defect structure of oxide nanowires on SrTiO3

Processing the SrTiO(3)(001) surface results in the self-assembly of reduced titanate nanowires whose widths are approximately 1 nm. We have imaged these nanowires and their defects at elevated temperatures by atomic resolution scanning tunneling microscopy. The nanowire structure is modeled with density functional theory, and defects observed in the center of the nanowire are determined to be Ti(4)O(3) vacancy clusters. The activation energy for Ti(4)O(3) vacancy cluster diffusion is explicitly measured as 4.98±0.17 eV with an exponential prefactor of μ=6.57×10(29) (s(-1).     

Dielectric investigations of solid solutions SrTiO3-KTaO3 and SrTiO3-KNbO3

The temperature dependences of the dielectric constant and dielectric hysteresis loops in ceramic samples of (1 ? x)SrTiO_3?x KNbO₃ and (1 ? x)SrTiO_3?x KTaO₃ (0 ≤ x ≤ 0.3) solid solutions prepared using different heat treatments have been investigated. Phase diagrams of the studied solid solutions have been constructed in the T-x coordinates. It has been shown that, after quenching of samples (spontaneous cooling at room temperature after long-term heating at the sintering temperature of the ceramic samples), the temperature of the induced phase transition increases because of the weakening of random electric fields associated with nonisovalent impurities due to their “frozen” nonequilibrium redistribution. For small concentrations x, strong dielectric relaxation is observed in the temperature range of 150–250 K. A model of relaxing centers, which is based on the local charge compensation of heterovalent impurities, has been proposed.     

Spectroscopy on perovskite-type layer structures in the submillimetre-millimetre wave range

We have studied the magnetic and dielectric properties of perovskite-type layer struc- tures in the very far-infrared, i.e. in the submillimetre range extending to millimetre waves. For these materials, this spectral range is of particular interest for two reasons: (a) Below the Néel temperature, a three-dimensional antiferromagnetic order exists in these quasi-two-dimen- sional magnetic systems and the frequency of the antiferromagnetic resonance lies within this range. (b) Perovskite-type layer structures with organic ions. e.g. (CH₃NH₃)₂ MnCl₄, undergo several structural phase transitions due to the orientation and reorientation of the organic mol- ecules. Closely above the continuous transition from the disordered to an ordered phase at 395 K, the fluctuations of the organic molecules and corresponding dielectric effects are expected in the GHz-range.We have studied the antoferromagnetic resonance of some of these materials as a function of temperature. We have used an IMPATT-diode operating at 84 GHz as the source and we varied the magnetic field of a superconducting coil in order to obtain resonance conditions.For obtaining reliable values of the static dielectric constant as a function of temperature, we have measured the shift of the ‘white light position’ in an interferometer with the sample in one arm of it.     

DFT study on electronic structures and optical absorption properties of C, S cation- doped SrTiO3

The effects of C cation and S cation doping on the electronic structures and optical properties of SrTiO₃ are investigated by density function theory (DFT) calculations. The calculated results reveal that the top of the valence band is predominately made up of the O 2p states for the pure SrTiO₃. When SrTiO₃ was doped with C cation and S cation, the top of the valence bands consists mainly of O 2p+C 2s hybrid orbitals and O 2p+S 3s hybrid orbitals, respectively. The band gap of SrTiO₃ is narrowed by the doping with C cation and S cation, especially for the C and S-codoped SrTiO₃. Moreover, the red shifts of the absorption edge are found by the calculated optical properties, which is consistent with reported experiment results. It is the explanation for their visible light respondency by the presence of C 2s and S 3s states on the upper edge of the valence band. All of these results can explain the good photocatalytic properties of C, S cation-codoped SrTiO₃ under visible light irradiation.      

Electronic processes in oxide glassy semiconductors and thin-film structures based on them

The physical nature of electronic processes in oxide glassy semiconductors (OGS) based on V₂O₅ is discussed on the basis of the theory of the small radius polaron (SRP). The most important parameters of the process of charge transfer by polarons are determined from an analysis of the temperature dependence of the static conductivity by mathematical modeling employing general theoretical expressions. A model of coupled SRP, presuming that they are localized by the Coulomb field of the charged defect centers, is proposed for describing the complex of data on the effects of a strong electric field and dielectric relaxation. On the basis of the model expressions describing the dependence of the current density and dielectric constant on the dc electric field strength and also a relation determining the dielectric relaxation time for the given model are obtained. The results of studies of the electric properties of OGS, modified by additions of a second transition-metal oxide, are discussed also.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 18–33, June, 1987.     

Second sound in SrTiO3

We study collective phonon excitations in SrTiO3 by low-frequency light scattering. We employ extended thermodynamics for phonon gas to construct a theoretical spectral function that is applicable regardless of local thermal equilibrium. Our analysis reveals the temperature dependence of tauN, the relaxation time for the momentum-conserving phonon collisions (normal processes), in SrTiO3. These results indicate that the previously reported anomalous soundlike spectrum originates from second sound, which is a wavelike propagation of heat.     

Conductivity-dependent cathodoluminescence in BaTiO3, SrTiO3 and TiO2

It is reported that similar cathodoluminescence spectra are excited by an electron beam striking BaTiO₃, SrTiO₃ and TiO₂ ceramics at room temperature. The energy location of the luminescence bands does not depend on various doping or reduction treatments. The luminescence intensity increases with the electron beam current as well as with the conduction electron density. The luminescence is interpreted as a fundamental transition of local character in the TiO₆ octahedron; the conduction electrons localized at the Ti sites in polaron states recombine with the 0–2p valence electron defects. The shape and energy location of the luminescence spectra are qualitatively in accordance with an explanation in terms of a configuration coordinate model.     

The Electronic Structure of Pristine and Doped (100) Tilt Grain Boundaries in SrTiO3

To understand the electronic properties of doped grain boundaries, we reviewed the atomic scale techniques currently available to study the electronic structure at pristine SrTiO₃ grain boundaries. The knowledge gained from the pristine boundaries is used to interpret experimental and theoretical results from a Mn doped 5 SrTiO₃ grain boundaries. Mn atoms are shown to preferentially substitute at specific Ti sites at the grain boundary core. Furthermore, the formal oxidation state of the Mn atoms at the grain boundary core was found to be reduced compared to the Mn atoms substituting for Ti in the bulk. This change of valence did not, however, significantly affect the atomic structure of the grain boundary, as determined by Z-contrast imaging and electron energy-loss spectroscopy, which revealed similar fine-structure features at both the doped and pristine grain boundary. We conclude, therefore, that composition and atomic structure have different effects on the local electronic structure and should be treated separately in any segregation and electrical conductivity models for grain boundaries.     

钙钛矿型氧离子导体KNb1-xMgxO3-δ的制备和表征

在高温高压 ( 4 0GPa ,870℃ )下合成了具有正交钙钛矿结构的KNb1 -xMgxO3-δ(x =0 0— 0 3 )系列固体电解质 ,并系统地研究了Mg掺杂对其结构相变和导电性的影响 .变温拉曼谱和DTA测量结果表明 ,随着温度的升高 ,KNb1 -xMgxO3-δ发生了结构相变 ,由铁电正交、四方相转变为顺电立方相 .由于Mg掺杂削弱了B位离子对自发极化的贡献以及A位离子与BO6 八面体间的耦合作用导致了居里温度下降 .其中KNb0 85Mg0 1 5O2 775的居里点大约下降 40℃ ,为 3 92℃ .阻抗谱测量表明 ,所有样品都具有离子导电特征 ,但晶界效应较强 ,电导主要由晶界决定 .通过掺杂 ,提高了样品的电导率 ,其中KNb0 9Mg0 1 O2 85的氧离子电导率最高 ,70 0℃时达到 1.2× 10 - 3S cm .     

Soft modes in semiconducting SrTiO3

The temperature and concentration dependence of the soft zone boundary mode in heavily reduced SrTiO₃ was studied by means of inelastic neutron scattering. It was found that the 105 K structural phase transition is suppressed completely for about 2% oxygen vacancies. The results are discussed in terms of recent models.     

钙钛矿型氧离子导体KNb1-xMgxO3-δ的制备和表征

在高温高压（4.0GPa，870℃）下合成了具有正交钙钛矿结构的KNb_1-xMg_x O_3-δ（x=0.0—0.3）系列固体电解质，并系统地研究了Mg掺杂对其结构相变和导电性的影响.变温拉曼谱和DTA测量结果表明，随着温度的升高，KNb_{1-xMgxO3-δ发生了结构相变，由铁电正交、四方相转变为顺 电立方相.由于Mg掺杂削弱了B位离子对自发极化的贡献以及A位离子与BO关键词： 钙钛矿 离子电导 1-xMgxO3-δ')" href="#">KNb1-xMgxO3-δ 高温高压 铁电相变}     

Origin of polarity in amorphous SrTiO3

Although neither SrTiO3 nor BaZrO3 has any polar crystalline polymorphs, they may form noncrystalline pyro- and piezoelectric phases [Adv. Mater. 19, 1515 (2007)10.1002/adma.200602149]. These phases and the similar phase of BaTiO3 have been called quasiamorphous. In this Letter, the structure of the quasiamorphous phase of SrTiO3 is examined by the x-ray absorption fine structure technique and found to be built of a random network of polar octahedral TiO6 local bonding units. While in crystalline SrTiO3 all TiO6 octahedra are apex sharing only, in its amorphous and quasiamorphous phases, some octahedra share edges. The polarity of the quasiamorphous phase is due to the partial alignment of the TiO6 octahedra. Such a mechanism is completely different from that of inorganic polar crystals. This mechanism should be possible in a large variety of other compounds that contain similar local bonding units.     

Electronic structures of vacancies in Co_3Sn_2S_2

Co_3Sn_2S_2 has attracted a lot of attention for its multiple novel physical properties, including topological nontrivial surface states, anomalous Hall effect, and anomalous Nernst effect. Vacancies, which play important roles in functional materials, have attracted increasing research attention. In this paper, by using density functional theory calculations, we first obtain band structures and magnetic moments of Co_3Sn_2S_2 with exchange–correlation functionals at different levels. It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co_3Sn_2S_2. We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments. The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level. For di-and tri-sulfur vacancies, the localized defect states hybridize with neighboring ones, forming bonding states as well as anti-bonding states. The Sn vacancy on the Sn-terminated surface also introduces localized bond states, which are merged with the valence bands. These results provide a reference for future experimental investigations of vacancies in Co_3Sn_2S_2.     

Normal vibrations of SrTiO3 lattice

The model of polarizable ions was used to calculate the frequencies and amplitudes of normal transverse modes of vibration (for=0) of an SrTiO₃ lattice and their temperature dependence due to the temperature dilatation of lattice. In agreement with Cochran's theory, one of the frequencies was chosen equal to zero in the assumed Curie point of 28°K. This can be done by reducing the charges of the ions by the factor 0.278. The temperature dependence of the lowest frequency is in qualitative agreement both with Cochran's assumptions and with the measurements of Cowley. A fundamental role in the character of its dependence and thus in the ferroelectric behaviour of the material is played by the long-range Coulomb forces. The series of experimental data on the values of normal frequencies, the lattice constant and elastic constants indicate a decrease in the values used for the overlap energy constants. The large reduction in the charges (necessary for frequencies aboveT _C to be real) show, however, that the model of polarizable ions is unsuitable for explaining the properties of SrTiO₃.The authors would like to thank V. Janovec C. Sc. and V. Dvoák C. Sc. for guiding the work, valuable discussions and advice, which helped to improve the work. They are also indebted to J. Fousek C. Sc. for a number of valuable remarks, and thank H. Dvoáková and E. Víchová for carefully performing some of the numerical calculations.