Effect of orthorhombic distortion and electron correlation on the electronic structure of SrMnO3 from first principles

The total energies and electronic structures of SrMnO₃ are studied from first principles calculations within the generalized-gradient approximation (GGA) framework. The orthorhombic structure with AFM1 configuration (see Fig. 1) is found to be the ground state of SrMnO₃, consistent with the latest experimental observation. The orthorhombic distortion effect of SrMnO₃ is studied by comparing its electronic structure and that of cubic structure, while correlation effect is evaluated by comparing GGA and GGA+U calculations. In contrast to previous theoretical studies, our calculations show that both the orthorhombic distortion and the electron correlation play important roles in the electronic structure of SrMnO₃.     

The static and dynamic properties of PuCoGa5 and NpCoGa5 investigated by neutron scattering experiments

Neutron scattering results on single crystals shed light on the static and dynamic properties of the superconductor () PuCoGa₅ and its isostructural but antiferromagnetic () homologue NpCoGa₅. By polarized neutron diffraction the magnetization density in the paramagnetic state of both compounds has been inferred. The microscopic magnetization of NpCoGa₅ is consistent with the orbital and spin components of a localized Np³⁺ magnetic moment. In the case of PuCoGa₅ the microscopic magnetization is small, temperature-independent and cannot be described as a localized Pu³⁺ magnetic moment. For NpCoGa₅ a dynamic magnetic signal has been observed by three-axis spectroscopy in the antiferromagnetically ordered state. The magnetic signal is strongest at the antiferromagnetic zone center and an energy transfer of about 5 meV. Magnetic fluctuations persist at this position in the paramagnetic state. The dynamic response is similar to the dynamic response observed in other actinide intermetallic compounds. This supports the possibility that magnetic fluctuations could also be present in the paramagnetic superconductor PuCoGa₅.     

CoO under pressure from first principles

Density functional theory and the generalized gradient approximation with correction for Hubbard energy was used to study the behavior of cobaltous oxide (CoO) under pressure. CoO undergoes an insulator-metal transition which is accompanied by a magnetic collapse. The antiferromagnetic phase of CoO transforms to nonmagnetic phase with the 6-7% reduction in the fractional volume. The magnetic collapse and the energy band gap closure are driven by the lost of correlation which results from the delocalization of 3d electrons. Delocalization process is due to the band broadening with compression. The Hubbard energy influences the transitions pressure. The lower Hubbard terms result in the lower values of transition pressure. The evolution of magnetic moment, energy band gap, and the bandwidth versus increasing pressure is analyzed. The results of calculations are compared to the existing theoretical and experimental data.     

Measurement of the F5/2 and H(2)9/2 manifold lifetime in Nd:YLiF4

We present direct measurements of the lifetime of the ⁴F_5/2 and ²H(2)_9/2 manifold in Nd³⁺:YLiF₄, using a fluorescence pump-probe technique. The technique populates the ⁴F_5/2 and ²H(2)_9/2 manifold directly with a pump pulse. Via excited state absorption from this excited manifold, the ²F(2)_5/2 manifold of Nd³⁺ is populated with a delayed probe pulse. The population in the ⁴F_5/2 and ²H(2)_9/2 manifold is monitored as a function of time by observing the change in integrated UV fluorescence from the ²F(2)_5/2 manifold for each time delay between pump and probe pulses. The pump and probe beams come from the fundamental and second harmonic wavelengths of a femtosecond Ti:sapphire regenerative amplifier. The measured lifetime agrees well with the energy gap law, based on other nonradiative lifetime measurements from the literature for Nd³⁺:YLiF₄.     

Nanoscale observations of the operational failure for phase-change-type nonvolatile memory devices using Ge2Sb2Te5 chalcogenide thin films

In this study, a phase-change memory device was fabricated and the origin of device failure mode was examined using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). Ge₂Sb₂Te₅ (GST) was used as the active phase-change material in the memory device and the active pore size was designed to be 0.5 m. After the programming signals of more than 2×10⁶ cycles were repeatedly applied to the device, the high-resistance memory state (reset) could not be rewritten and the cell resistance was fixed at the low-resistance state (set). Based on TEM and EDS studies, Sb excess and Ge deficiency in the device operating region had a strong effect on device reliability, especially under endurance-demanding conditions. An abnormal segregation and oxidation of Ge also was observed in the region between the device operating and inactive peripheral regions. To guarantee an data endurability of more than 1×10¹⁰ cycles of PRAM, it is very important to develop phase-change materials with more stable compositions and to reduce the current required for programming.     

High pressure structural studies on SrRuO3

We have performed room temperature high pressure structural studies on the ferromagnetic perovskite SrRuO₃ to 34 GPa. We have also used three different pressure media (silicone fluid, 4:1 methanol:ethanol and argon) to test for possible effects of pressure media on compression data. The orthorhombic perovskite structure is stable to the highest pressure, and the data can be fit with a bulk modulus of with a pressure derivative of for all of the pressure media. We have also examined the high pressure behavior of the RuO₆ octahedra using a model that assumes the octahedra are not distorted. Various tilt angles around the ideal cubic perovskite axes are found and can be used to estimate the Ru-O-Ru bond angle that is known to be directly related to the ferromagnetic Curie temperature. For all pressure media, there appears to be a minimum in the Ru-O-Ru bond angle around 15 GPa. Implications for the observed high pressure magnetic behavior of SrRuO₃ will be discussed.     

Point-contact study of the heavy-fermion superconductor CeCoIn5

We have investigated the differential conductance spectra of the point contacts between the heavy-fermion superconductor CeCoIn₅ and Pt. Many of them show a double-maximum structure that indicates the superconducting energy gap Δ. The Δ values derived using Blonder-Tinkham-Klapwijk model, however, varies from 0.47 to 0.77 meV, and yet they are within the scatter of the reported values. The evolution of Δ below T_c is slow as compared with that of BCS gap probably reflecting the unconventional superconductivity in CeCoIn₅.     

Magnetization and characteristic fields in Nd0.8Ba0.2FeAsO0.6F0.4 with binary doping

In the present work, the iron oxypnictide superconductor of the namely Nd_0.8Ba_0.2FeAsO_0.6F_0.4 is prepared with respect to the observation of distinct magnetization characteristics arising from both electron and hole doping. A magnetothermal phase diagram is given for the present iron oxypnictide system, based on the irreversibility fields (H_irr) and the upper critical fields (H_c2) obtained from magnetotransport measurements, as well as the lower critical fields (H_c1) evaluated by magnetization loops at various temperatures. High H_c2(0) are revealed at low temperature range, which is in consistent with the observations of comparatively high critical current (J_c) and the temperature dependent peak effect in the J_c vs H curves.     

Enhanced nonlinear current-voltage behavior in Au nanoparticle dispersed CaCu3Ti4O12 composite films

An enhanced nonlinear current-voltage behavior has been observed in Au nanoparticle dispersed CaCu₃Ti₄O₁₂ composite films. The double Schottky barrier model is used to explain the enhanced nonlinearity in I-V curves. According to the energy-band model and fitting result, the nonlinearity in Au: CCTO film is mainly governed by thermionic emission in the reverse-biased Schottky barrier. This result not only supports the mechanism of double Schottky barrier in CCTO, but also indicates that the nonlinearity of current-voltage behavior could be improved in nanometal composite films, which has great significance for the resistance switching devices.     

Submillimeter measurements of N2 and air broadening of hypochlorous acid

The pressure induced broadening of a several pure rotational transitions of hypochlorous acid, HOCl, have been measured as a function of temperature. This set of rotational transitions is the dominant feature of the submillimeter spectrum in the range where several remote sensing instruments currently operate. Additional features throughout the submillimeter spectrum have been recorded at the full-resolution of the room temperature Doppler linewidth using multiplier chains in the 110- wavelengths.     

Band alignments at SrZrO3/Ge(0 0 1) interface: Thermal annealing effects

High-κ dielectrics SrZrO₃ were prepared on Ge(0 0 1) substrate using pulse laser deposition, and band alignments and thermal annealing effects were studied with high resolution X-ray photoemission spectroscopy. Valence and conduction band offsets at this interface were measured to be 3.26 eV and 1.77 eV, respectively. Interfacial Ge oxide layers were found at the interface. After annealing at 600 °C, the interfacial Ge oxide layers were eliminated, and the valence band offset increased to 3.50 eV, but the amorphous SrZrO₃ became polycrystalline in the meantime.     

Specific heat of EuIn2P2 at high magnetic fields

We have carried out specific heat measurements on EuIn₂P₂ at high magnetic fields perpendicular to the c-axis in the hexagonal crystal structure in order to understand its thermal properties. The temperature dependence of the specific heat exhibits a clear λ-type anomaly due to a magnetic transition at , indicating that the magnetic transition is of second-order. The λ-type anomaly becomes markedly broader with increasing the magnetic field. This remarkable field-dependence is consistent with the results of previous magnetization measurements which suggest that Eu²⁺ magnetic moments align ferromagnetically perpendicular to the c-axis below T_C. In addition, a hump in the specific heat is observed around 7 K, which can be ascribed to the Zeeman splitting of the Eu²⁺ multiplet by internal magnetic fields.     

Atomic structure of CaF2/MnF2-Si(1 1 1) superlattices from X-ray diffraction

X-ray reflectivity and non-specular crystal truncation rod scans have been used to determine the three-dimensional atomic structure of the buried CaF₂-Si(1 1 1) interface and ultrathin films of MnF₂ and CaF₂ within a superlattice. We show that ultrathin films of MnF₂, below a critical thickness of approximately four monolayers, are crystalline, pseudomorphic, and adopt the fluorite structure of CaF₂. High temperature deposition of the CaF₂ buffer layer produces a fully reacted, CaF₂-Si(1 1 1) type-B interface. The mature, “long” interface is shown to consist of a partially occupied layer of CaF bonded to the Si substrate, followed by a distorted CaF layer. Our atomistic, semi-kinematical scattering method extends the slab reflectivity method by providing in-plane structural information.     

Nanostructural and surface morphological evolution of chemically sprayed SnO2 thin films

Physical properties of a nanocrystalline thin film is greatly influenced by its morphological and structural evolution. We try to understand the transition of SnO₂ thin films from amorphous to nanocrystalline structure with XRD, IR, SEM, AFM and surface profiler studies. A 2D layer like structure resulting from quantum confinement is found for the films prepared at 400 °C. We observed a new IR band at 530 cm⁻¹ that was theoretically predicted and report it for the first time. A correlation of population of defects in SnO₂ films with change in lattice parameters and FWHM of IR bands are reported. The electric and optical properties of the films have been discussed.     

Growth, coalescence, and electrical resistivity of thin Pt films grown by dc magnetron sputtering on SiO2

Ultra thin platinum films were grown by dc magnetron sputtering on thermally oxidized Si (1 0 0) substrates. The electrical resistance of the films was monitored in situ during growth. The coalescence thickness was determined for various growth temperatures and found to increase from 1.1 nm for films grown at room temperature to 3.3 nm for films grown at 400 °C. A continuous film was formed at a thickness of 2.9 nm at room temperature and 7.5 nm at 400 °C. The room temperature electrical resistivity decreases with increased growth temperature, while the in-plain grain size and the surface roughness, measured with a scanning tunneling microscope (STM), increase. Furthermore, the temperature dependence of the film electrical resistance was explored at various stages during growth.     

Anomalies of the phonon properties in multiferroic BiFeO3 thin films near the magnetic phase transition temperature

Size, substrate, doping and magnetic field effects on the phonon properties in multiferroic BiFeO₃ thin films are studied based on a microscopic model. We obtain an anomaly near the magnetic phase transition temperature TN which can be attributed to the magnetoelectric nature of BiFeO₃ and strong anharmonic spin-phonon interaction. It is shown that due to crystal lattice distortion for dopants with ionic radius smaller than that of the host ions the phonon energy decreases (for example Tb or Ti), whereas for the opposite case (larger radius of the doping ions, for example Co or Ni) it increases. The phonon damping is always enhanced compared to the undoped thin film.     

The first measured Mn I Stark widths

The shapes of the astrophysically interesting neutral manganese (Mn I) resonance spectral lines (403.075, 403.306, 403.448, 279.481, 279.826 and 280.108 nm) have been observed together with six other prominent Mn I lines in the laboratory helium plasma at a 47 000 K electron temperature and electron density. With these plasma parameters the Stark broadening has been found to be an important mechanism in the Mn I line shape formation. Our measured Mn I Stark widths (W) are the first data in the literature. Stark widths are compared with line hyperfine structure splittings (Δ_hfs). At above mentioned helium plasma conditions the line broadening due to hyperfine structure splitting of the lines is less than that of the Stark and Doppler broadening for the case of the Mn I lines under investigation. We estimate that at electron densities below and electron temperatures below 4000 K the components in the hyperfine structure play an important role in the mentioned Mn I line shape formation.     

Nonlinear statistical coupling

By considering a nonlinear combination of the probabilities of a system, a physical interpretation of Tsallis statistics as representing the nonlinear coupling or decoupling of statistical states is proposed. The escort probability is interpreted as the coupled probability, with Q=1−q defined as the degree of nonlinear coupling between the statistical states. Positive values of Q have coupled statistical states, a larger entropy metric, and a maximum coupled-entropy distribution of compact-support coupled-Gaussians. Negative values of Q have decoupled statistical states and for −2<Q<0 a maximum coupled-entropy distribution of heavy-tail coupled-Gaussians. The conjugate transformation between the heavy-tail and compact-support domains is shown to be for coupled-Gaussian distributions. This conjugate relationship has been used to extend the generalized Fourier transform to the compact-support domain and to define a scale-invariant correlation structure with heavy-tail limit distribution. In the present paper, we show that the conjugate is a mapping between the source of nonlinearity in non-stationary stochastic processes and the nonlinear coupling which defines the coupled-Gaussian limit distribution. The effects of additive and multiplicative noise are shown to be separable into the coupled-variance and the coupling parameter Q, providing further evidence of the importance of the generalized moments.     

Molecular beam epitaxy of superconducting Pr2CuO4 films

Recent reports on superconductivity in parent compounds [RE = Pr, Nd, Sm, Eu, Gd] prepared by metal-organic decomposition (MOD) shed new light on the electronic and magnetic phase diagram of electron-doped cuprates. A thorough reduction process is the key to inducing superconductivity in square-planar coordinated cuprates. In this work, Pr₂CuO₄ films were grown epitaxially by molecular beam epitaxy (MBE) on (001)SrTiO₃, (110)GdScO₃ and (110)DyScO₃ substrates. A modified two-step reduction process is required in order to obtain single-phase superconducting samples. The optimized superconducting properties are , while the room-temperature resistivity is about . Hence, we show that even amorphous Pr₂CuO_y films deposited on (110)DyScO₃ become superconducting after the application of the two-step annealing process. These results indicate that superconductivity is induced to Pr₂CuO₄ by systematic optimization of the reduction conditions, irrespective of the synthesis route.