Dynamic fluctuations and static speckle in critical X-ray scattering from SrTiO3

We report a study of critical x-ray scattering from SrTiO3 near the antiferrodistortive structural phase transition at T(C) approximately 105 K. A line shape analysis of the thermal diffuse scattering results in the most precise experimental determination to date of the critical exponent gamma. The microscopic mechanism behind the anomalous "central peak" critical scattering component is clarified here by the first-ever observation of a static coherent diffraction pattern (speckle pattern) within the anomalous critical scattering of SrTiO3. This observation allows us to directly attribute the origins of the central peak to Bragg diffraction from remnant static disorder above T(C).     

Disordered nanocrystalline superconducting PbMo6S8 with a very large upper critical field

Large increases in the upper critical field B(C2)(0) are reported in bulk superconductors that demonstrate another novel property for nanocrystalline materials. Disordered nanocrystalline PbMo6S8 superconductors were fabricated by mechanical milling and hot isostatic pressing. Conventional PbMo6S8 has B(C2)(0) approximately 50 T. The nanocrystalline materials have higher resistivity (rho(N)) and B(C2)(0) approximately 100 T. The disorder produced in these nanocrystalline materials is significantly different from that produced by doping because it increases rho(N) and, hence, B(C2)(0) without significantly reducing the electronic density of states or superconducting transition temperature (T(C)). Furthermore, the disorder reduces the electron mean-free path to approximately 1 nm which is more than an order of magnitude smaller than the grain size and necessary to achieve the unprecedented increase in B(C2)(0).     

Investigation on very large scale motions(VLSMs) and their influence in a dust storm

Based on the real-time synchronous measurements of the wind velocity,temperature,the PM10 concentration at 16 m and 47 m during a dust storm event,in which Reynolds number Re exceeds 6×106,this study reveals the existence of the very large scale motions(VLSMs) during the stable stage both in the stream velocity and the temperature field at the two heights,whose streamwise scales reach up to 10 times the thickness of the boundary layer.The streamwise velocity and the PM10 concentration display a similar frequency corresponding to the peaks of their energy spectra,which implies that the VLSMs of streamwise flow have a significant role in dust transportation.In contrast,the salient deviations of the PM10 concentration at 47 m from the Gaussian distribution are revealed,which means that 47 m is not in the dust transportation layer,but is a region where the dust transportation layer and the outer flow intersect each other.Analysis demonstrates that the energy spectra of the PM10 concentrations at 16 m and 47 m display the "-1" scaling law feature,which has the same frequency range(0.001-0.1 Hz) as that of the wind velocity.This provides a new paradigm for the existence of the self-similarity scaling region in turbulent flow.     

钛酸锶晶体的光致介电常数激增以及超顺电大极化子理论

钙钛矿类化合物钛酸锶具有明显的光致介电增长的性质,这是由于被光激发至Ti⁴⁺的3d能带上的电子具有良好的巡游特性,为了进一步揭示光致介电增长的微观机理,这里假设巡游电子同时与两类声子发生耦合作用,一方面,电子与A_1g模式的晶格呼吸子发生强相互作用,另一方面,电子还与T_1u模式的非简谐声子具有相对较弱的耦合.通过变分法计算可得,这种复杂的电声耦合作用在晶体中形成两种极化子:自陷极化子和超顺电大极化子.正是由于超顺电大极化子的形成,导致了光致介电激增的现象.     

Two-stage deep learning-based hybrid precoder design for very large scale massive MIMO systems

Wireless networking is approaching a new era, which necessitates new frequency ranges and novel strategies. With recent circuit growth, communications over the Terahertz (THz) band is proving to be a viable option because of the tremendous bandwidth and low cost. On the other hand, THz signals suffer from significant direction loss, necessitating the use of precoding. In this paper, Deep Learning (DL) based precoding techniques for upcoming 6G networks were examined, along with their complexities. Based on the signal-to-noise ratio (SNR) and spectral efficiency (SE), the proposed DL-based precoding scheme is compared to traditional model-based precoding schemes. The proposed DL-based precoding technique is ideal for 6G networks, according to simulation results. Furthermore, the proposed DL-based precoding technique has lower computational complexity, making it suitable for parallel processing and high-speed data transmission.     

Polar state in a SrTiO3-KTaO3 solid solution

Dielectric properties of ceramic samples of a 0.85SrTiO₃-0.15KTaO₃ solid solution have been studied. The temperature dependence of the dielectric permittivity exhibits maxima with a strong frequency dispersion, which is described by the Vogel-Fulcher law with a characteristic temperature T _g=26 K. These features are apparently connected with transition to a polar glasslike state. Fiz. Tverd. Tela (St. Petersburg) 39, 1838–1840 (October 1997)     

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.     

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.     

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.     

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.     

EPR spectra of a GdMnO3 thin film on a SrTiO3 substrate

Electron paramagnetic resonance (EPR) spectra of a GdMnO₃/SrTiO₃ thin film in the X band have been measured in the temperature interval from 200 to 450 K. Signals from two types of paramagnetic centers have been observed in the spectra. The first paramagnetic center is a subsystem of Gd³⁺ ions, in the EPR spectrum of which the fine structure lines are resolved below 350 K. The second paramagnetic center is a system of manganese and gadolinium ions, in the EPR spectrum of which an exchange-narrowed line is observed with the width ΔH several times less than the width ΔH of an exchange-narrowed line observed in the GdMnO₃ single crystal. Unusual magnetic properties are due to the mismatch of the lattice parameters of the GdMnO₃ thin film and the SrTiO₃ substrate.     

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.     

The possibility of a very large magnetoresistance in half-metallic oxide systems

The tunnel magnetoresistance (TMR) is analyzed for ferromagnet-insulator-ferromagnet junctions, including novel half-metallic systems with 100% spin polarization. Direct tunneling is compared with the impurity-assisted and resonant TMR. Direct tunneling in iron-group systems leads to about a 20% change in resistance, as observed experimentally. Impurity-assisted tunneling decreases the TMR to 4% with Fe-based electrodes. A resonant tunnel diode structure would give a TMR of about 8%. The model applies qualitatively to half-metallics, where the change in resistance in the absence of spin flips may be arbitrarily large and even in the case of imperfect magnetic configurations the resistance change can be several thousand percent. Examples of half-metallic ferromagnetic systems are CrO₂/TiO₂ and CrO₂/RuO₂. A discussion of their properties is presented. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 430–435 (10 March 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Adding a length scale to the polyamorphic ice debate

X-ray scattering and molecular dynamics simulations have been used to correlate the short range oxygen-oxygen structure with the intermediate range ordering (IRO) upon annealing very high density amorphous ice. While it is clear that the IRO that defines the network structure breaks down continuously to a minimum level, where there are weakened correlations extending beyond 7 Angstrom, at this point the local structure (O-O-O angles) is observed to change abruptly, allowing a continuous reemergence of a new IRO network. This is very different from a classic first order transition and helps reconcile previous data.     

Observation of a turbulence-induced large scale magnetic field

An axisymmetric magnetic field is applied to a spherical, turbulent flow of liquid sodium. An induced magnetic dipole moment is measured which cannot be generated by the interaction of the axisymmetric mean flow with the applied field, indicating the presence of a turbulent electromotive force. It is shown that the induced dipole moment should vanish for any axisymmetric laminar flow. Also observed is the production of toroidal magnetic field from applied poloidal magnetic field (the omega effect). Its potential role in the production of the induced dipole is discussed.