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We determine the surface reconstruction of $\mathrm{SrTi}{\mathrm{O}}_{3}$ used to achieve superconducting FeSe films in experiments, which is different from the $1\ifmmode\times\else\texttimes\fi{}1\mathrm{Ti}{\mathrm{O}}_{2}$-terminated $\mathrm{SrTi}{\mathrm{O}}_{3}$ assumed by most previous theoretical studies. In particular, we identify the existence of a double $\mathrm{Ti}{\mathrm{O}}_{2}$ layer at the FeSe/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient $\mathrm{SrTi}{\mathrm{O}}_{3}$ surface to FeSe when the double $\mathrm{Ti}{\mathrm{O}}_{2}$ layer is present. The double layer helps to remove the hole pocket in the FeSe at the \ensuremath{\Gamma} point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this superconductor.

We find signatures of superconductivity at ${T}_{c}=12$ K in a molybdenum arsenide sample containing \ensuremath{\sim}4% of ${\mathrm{MoO}}_{2\ensuremath{-}\ensuremath{\delta}}$ impurity by weight. Below is a report on the electronic structure calculations, structures analyses, and thermodynamic and transport properties on ${\mathrm{Mo}}_{2}{\mathrm{As}}_{3}$, ${\mathrm{Mo}}_{5}{\mathrm{As}}_{4}$, and ${\mathrm{MoO}}_{2}$, in order to uncover the reasons for superconductivity for the sample that contains a mixture of these phases. The experimental and theoretical evidence suggest that superconductivity in the ${\mathrm{MoO}}_{2\ensuremath{-}\ensuremath{\delta}}$ phase is likely and may be caused by nesting-type fluctuations.

We have presented a new structural model of the hole-trapping oxygen deficient center in Ge-doped silica glass on the basis of ab initio molecular orbital calculations. This charged center comprises the hole $(\ensuremath{\equiv}{\mathrm{Ge}}^{+}$ or $\ensuremath{\equiv}{\mathrm{Si}}^{+})$ and the paramagnetic $(\ensuremath{\equiv}\mathrm{Ge}\ifmmode\bullet\else\textbullet\fi{})$ parts that are bridged by a common oxygen atom. The isotropic ${}^{73}\mathrm{Ge}$ hyperfine coupling calculated for this paramagnetic center reproduces the observed value better than that obtained for the triplet-state delocalized center proposed in a previous paper [T. Uchino et al., Phys. Rev. Lett. 84, 1475 (2000)].

In our experiments, fully oxygenated ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals were annealed in flowing oxygen at various temperatures and quenched in liquid nitrogen to become oxygen deficient. These single crystals, with transition temperatures ranging from 35 K to 82 K, were then fabricated into mesalike devices along c axis with $a\ensuremath{-}b$ plane dimensions down to 2 $\ensuremath{\mu}\mathrm{m}.$ We measured dc current-voltage $(I\ensuremath{-}V)$ curves, and observed typical intrinsic Josephson effects such as hysteresis, voltage jumps, etc., in all oxygen deficient samples. However, the oxygen contents and $a\ensuremath{-}b$ plane sizes of the samples greatly affected the appearances of the $I\ensuremath{-}V$ characteristics, supplying important information on the intrinsic Josephson effects. Microwave responses were also explored for possible high-frequency applications.

From neutron powder diffraction studies performed on the ordered oxygen deficient perovskite ${\mathrm{HoBaCo}}_{2}{\mathrm{O}}_{5},$ we have observed two structural phase transitions at ${T}_{N}\ensuremath{\sim}340$ K and ${T}_{\mathrm{CO}}\ensuremath{\sim}210$ K, corresponding to the occurrence of magnetism and the onset of charge ordering, respectively. The charge ordered phase exhibits chains of ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ along the a axis and an alternate stacking of these ions along the b and c axes. The observed magnetic moments, ${\ensuremath{\mu}}_{{\mathrm{Co}}^{3+}}=3.7{\ensuremath{\mu}}_{B}$ and ${\ensuremath{\mu}}_{{\mathrm{Co}}^{2+}}=2.7{\ensuremath{\mu}}_{B},$ confirm this charge ordering and are in good agreement with the expected values for high-spin Co ions in pyramidal environment.

Thermoelectric power (S) and lattice parameter $(a,$ b, $c)$ measurements are used to determine the change in the carrier concentration (n) in both isotope exchanged and oxygen deficient samples of ${\mathrm{La}}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}.$ It is observed upon substitution of the ${}^{18}\mathrm{O}$ isotope for ${}^{16}\mathrm{O}$ that the Curie temperature ${T}_{C}$ decreases but no significant changes in S at 290 K and a, b, or c are seen. In contrast, for samples that are oxygen deficient, significant changes in S at 290 K and a, b, and c are observed, in addition to decreases in ${T}_{C}.$ It is found that the estimated $\ensuremath{\Delta}n$ correlating to the observed $\ensuremath{\Delta}S$ cannot account for the isotope-induced change in ${T}_{C}.$ Therefore it is concluded that any oxygen deficiencies introduced by the isotope exchange are negligible and do not account for the ${T}_{C}$ change observed.

Muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ measurements of the magnetic-field distribution $n(B)$ in the vortex state of the oxygen deficient high-${T}_{c}$ superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.60}$ reveal a vortex-lattice melting transition at much lower temperature than that in the fully oxygenated material. The transition is best described by a model in which adjacent layers of ``pancake'' vortices decouple in the liquid phase. In some samples we also observe changes in $n(B)$ which are consistent with a vortex lattice which becomes disordered along the field direction above a crossover field ${B}_{\mathrm{cr}}.$ A transition of this nature has previously been reported in the highly anisotropic superconductor ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+y}.$

The oxygen deficient perovskite ${\mathrm{La}}_{8\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Cu}}_{8}{\mathrm{O}}_{20}$ is unique in that the system can be viewed as a hybrid of parallel one-dimensional chains with localized $S=1/2$ spins and surrounding three-dimensional Cu-O network with itinerant electrons. For $x\ensuremath{\lesssim}2.1,$ two successive phase transitions are observed, where complicated anomalies appear in magnetic and transport properties. We discuss the roles of these two subsystems in the complicated magnetism of the present compound.

In-plane tunneling measurements of superconductor-insulator-superconductor (SIS)-type oxygen-deficient edge junctions made of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.6}$ as the superconductor and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{2.55}{\mathrm{Fe}}_{0.45}{\mathrm{O}}_{y}$ as the barrier are reported. Two types of junctions were prepared in which transport is either along the (100) or the (110) direction. The dynamic conductance $dI/dV$ shows a gaplike structure in the (100) junctions, while a zero-bias peak is observed in the (110) junctions. At energies below $\ensuremath{\sim}2.5{\ensuremath{\Delta}}_{0}$ the results were found to be consistent with the Blonder, Tinkham, and Klapwijk model, extended by Tanaka and Kashiwaya to the case of ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductivity. At higher energies the behavior of the dynamic conductance is explained as due to a self-heating effect inside the junction.

${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}(x\ensuremath{\approx}6.6)$ thin films were deposited by pulsed-laser ablation on ${\mathrm{SrTiO}}_{3}$ substrates, that were cut with a tilt angle $\ensuremath{\alpha}=10\ifmmode^\circ\else\textdegree\fi{}$ with respect to the [001] direction, inducing a steplike growth of the ${\mathrm{CuO}}_{2}$ layers. The films appeared semiconducting along the projection of the c axis to the film surface, but metallic in the perpendicular direction. From the anisotropic resistance, the in-plane $({\ensuremath{\rho}}_{\mathrm{ab}})$ and the out-of-plane $({\ensuremath{\rho}}_{\mathrm{c}})$ resistivities were calculated. Photodoping with white light was performed at various temperatures from 70--305 K. We have found strongly temperature dependent photoinduced decreases of ${\ensuremath{\rho}}_{\mathrm{ab}}$ and ${\ensuremath{\rho}}_{\mathrm{c}}.$ At low temperatures, the relative reduction of ${\ensuremath{\rho}}_{\mathrm{c}}$ was smaller than that of ${\ensuremath{\rho}}_{\mathrm{ab}},$ whereas, near room temperature, they were nearly the same. The electrical anisotropy ${\ensuremath{\rho}}_{\mathrm{c}}/{\ensuremath{\rho}}_{\mathrm{ab}}$ increased monotonously at 70 K, but, at higher temperatures, a fast increase was followed by a long-term decay. This unusual behavior resembles that of the in-plane Hall mobility in a previous study, suggesting that both quantities are changed by the same physical process. The results are discussed within the models previously proposed for the photodoping effect in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}.$

The paper describes a study of the antiferromagnetic ordering of oxygen-deficient ${\mathrm{Nd}}_{1+y}{\mathrm{Ba}}_{2\ensuremath{-}y}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ single crystals. In pure, stoichiometric samples, $y=0,$ with different oxygen contents x in the Cu(1) plane $(0.02lxl0.2),$ the antiferromagnetic I (AFI) phase appears to be stable down to 316 mK. The magnetic ordering within the Cu(2) sublattice of the pure ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ system is therefore similar to that of the pure ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ parent compound. With increasing oxygen content, the N\'eel temperature drops significantly and the critical exponent changes from 0.26 for ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.09}$ to about 0.5 for ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.23}.$ Magnetic ordering of the ${\mathrm{Nd}}^{3+}$ moments sets in at 1.7 K with a critical wave vector ${\mathbf{q}}_{\mathrm{Nd}}=(\frac{1}{2}\frac{1}{2}\frac{1}{2}).$ Reordering to the AFII phase is observed in a crystal, which has a significant amount (4%) of ${\mathrm{Nd}}^{3+}$ ions substituted on the Ba sites. The relatively high temperature ${T}_{2}=95\mathrm{K}$ of this reordering suggests that the ${\mathrm{Nd}}^{3+}$ ions on Ba sites are very effective defects forcing the $\mathrm{AFI}\ensuremath{\leftrightarrow}\mathrm{AFII}$ reordering. The mechanism of reordering is explained in terms of the creation of ${\mathrm{Cu}}^{2+}$ moments within the Cu(1) layer due to the ${\mathrm{Nd}}^{3+}$ ions on Ba sites, which via polarization lead to an effective ferromagnetic coupling between the moments on next-nearest neighboring Cu(2) layers. The ${\mathrm{Nd}}^{3+}$ doping on the ${\mathrm{Ba}}^{2+}$ sites increases the N\'eel temperature compared to the stoichiometric compound.

We find experimentally that the irreversibility line, H{sub irr}(T) , and the reversible magnetization of twinned YBa{sub 2}Cu {sub 3}O{sub 7} crystals both show 3D XY scaling, which suggests a common origin for both quantities. Analysis of this result and of a correlation between the condensation energy and H{sub irr}(T) of oxygen deficient YBa{sub 2}Cu {sub 3}O{sub 7{minus}{delta}} for 0{lt}{delta}{lt}0.6 , implies that H{sub irr}(T) is strongly influenced by critical fluctuations. It is possible that these values of H{sub irr}(T) and the recently discovered vortex melting line, H{sub m}(T) , both correspond to an upper critical field line which has been suppressed by thermodynamic fluctuations. {copyright} {ital 1997} {ital The American Physical Society}

We study the spin fluctuations of the oxygen deficient planes of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$. The Cu-O chains that constitute these planes are described by a model that includes antiferromagnetic interactions between spins and Kondo-like scattering of oxygen holes. The spectrum of magnetic excitations shows the presence of incommensurate dynamic fluctuations along the direction of the chains. The presence of itinerant holes is responsible for the existence of important differences between the spin dynamics of this system and that of a quasi-one-dimensional localized antiferromagnet. We comment on the possibility of experimental observation of these fluctuations.

We measured the influence of magnetic centers (MC{close_quote}s) on the nuclear relaxation in 123 compounds using {sup 169}Tm NMR and {sup 63}Cu NQR in TmBa{sub 2}Cu{sub 3}O{sub 6+x} (x=0, 0.2, 0.3, 0.4, 0.5, 0.6), and {sup 89}Y NMR and {sup 63}Cu NQR in YBa{sub 2}Cu{sub 3}O{sub 6.5}. Particular attention is paid to the region 0.5{le}x{le}0.6 in order to deal with both a well-defined ortho II structure and a high enough MC concentration. The experiments reveal a two-component nuclear relaxation in the superconducting compounds at temperatures above 1 K, especially pronounced in the x=0.5 samples. The relaxation data give evidence for microphase separation due to oxygen disorder in the basal CuO{sub x} planes. In the well-annealed samples, the MC{close_quote}s predominating the relaxation have S=1/2. The concentrations of these MC{close_quote}s are of the order of 3{percent} in the disordered (nonsuperconducting) microphase and {approximately}0.3{percent} in the ordered (superconducting) one. {copyright} {ital 1997} {ital The American Physical Society}

In this work we present a study of the magnetic and transport properties of the system LaNiO3-x (0≤x≤0.5) where the x=0 member is a well-known metallic oxide. We found that the x=0.5 compound is an antiferromagnetic insulator at room temperature. The metal-insulator transition is observed for an x value around x.25. The role played by the sample inhomogeneities on the occurrence of this transition is dis-cussed. © 1996 The American Physical Society.

Based on electrical resistivity and Hall-coefficient measurements from 2 to 370 K on slightly reduced semiconductive rutile single crystals with various oxygen deficiencies ${\mathrm{O}}_{\mathit{d}}$ from 3.7\ifmmode\times\else\texttimes\fi{}${10}^{18}$ to 1.3\ifmmode\times\else\texttimes\fi{}${10}^{19}$/${\mathrm{cm}}^{3}$, electronic conduction above 4 K is discussed in terms of two conduction bands separated by 0.03--0.05 eV. In this range of ${\mathrm{O}}_{\mathit{d}}$, defects are mostly Ti interstitial ions. Ionization of Ti interstitial donors takes place above about 4 K. The ionization energy ${\mathit{E}}_{2}$ depends on the donor concentration ${\mathit{N}}_{\mathit{D}}$ and decreases with increasing ${\mathit{N}}_{\mathit{D}}$ for ${\mathrm{O}}_{\mathit{d}}$1\ifmmode\times\else\texttimes\fi{}${10}^{19}$/${\mathrm{cm}}^{3}$. The density of states effective mass ${\mathit{m}}_{\mathit{d}}$** of the lower conduction band is estimated to be (7--8)${\mathit{m}}_{\mathit{e}}$ (${\mathit{m}}_{\mathit{e}}$ is the free electron mass). The anisotropy of the effective mass is also estimated to be roughly ${\mathit{m}}_{\mathit{c}}$ **\ensuremath{\simeq}(2--4)${\mathit{m}}_{\mathit{e}}$ and ${\mathit{m}}_{\mathit{a}}$ **\ensuremath{\simeq}(10--16)${\mathit{m}}_{\mathit{e}}$ in the c and a directions, respectively. The mobility above 50 K of the lower conduction band electrons is discussed. \textcopyright{} 1996 The American Physical Society.

We examine some models of the strongly correlated \ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}Cu-O\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot} chain fragments that are the main components of the oxygen-deficient planes of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$. We use quantum Monte Carlo methods, exact numerical diagonalization as well as an analytical approach in order to study the thermodynamic properties of the chain fragments and the charge and magnetic correlations within them as functions of the concentration of carriers and of the temperature. Only the fermionic degrees of freedom of the chain fragments are considered. \textcopyright{} 1996 The American Physical Society.

We study some models of the strongly correlated \ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}Cu-O\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot} chain fragments that form on the oxygen-deficient planes of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$. We use analytical and numerical methods to investigate the stability of the chain fragments as a function of their size. We discuss also the size and x dependence of the O-O intrachain interactions that enter in the phenomenological descriptions of the ${\mathrm{CuO}}_{\mathit{x}}$ planes. Only the fermionic degrees of freedom of the chain fragments are considered. \textcopyright{} 1996 The American Physical Society.

We have measured the temperature dependence of the longitudinal resistivity of an oxygen-deficient YBa{sub 2}Cu{sub 3}O{sub 6.45} thin film in perpendicular applied magnetic fields {ital B}{lt}12 T. A crossover from a three-dimensional (3D) flux-line liquid to a quasi-2D liquid of vortices is identified which is characterized by a crossover line {ital B}{sub cr}=136.13{sup *}(1{minus}{ital T}/{ital T}{sub {ital c}})/{ital T} (T{sub c}=30.35 K, {ital B}{sub cr} in T). In both 3D and 2D regimes, the dissipations are thermally activated. Above the crossover temperatures, the vortices are in a quasi-2D liquid state and the dissipation is dominated by the motion of edge dislocation pairs or vortex cutting and the activation energy is proportional to (1{minus}{ital T}/{ital T}{sub {ital c}})ln{ital B}. Below the crossover temperatures and above the irreversibility line, the vortices are in a 3D line liquid state, and the dissipation is governed by plastic deformation of the vortices through double kink nucleations in the vortices with activation energies {ital U}{sub 0}{proportional_to}(1{minus}{ital T}/{ital T}{sub {ital c}}){ital B}{sup {minus}0.46}.

The oxygen defects induced phase transition from nonergodic to ergodic state in superconductors with intragrain granularity is considered within the superconductive glass model. The model predictions are found to be in a qualitative agreement with some experimental observations in deoxygenated high-T_c single crystals.

The effects of photoinduced hole doping, caused by persistent photoconductivity, on the normal and superconducting properties of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ (6.35\ensuremath{\le}y\ensuremath{\le}6.70) thin films are studied using a high-power-density He-Ne laser light. Substantial photoinduced enhancements of both the normal conductivity and ${\mathit{T}}_{\mathit{c}}$ are found for all the films. Hall measurements reveal that the number of photoinduced holes depends only on the photon dose and is independent of the oxygen content. The enhanced ${\mathit{T}}_{\mathit{c}}$ has a nearly quadratic correlation with the hole density similar to that observed in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ when the hole density is varied by adding oxygen to the ${\mathrm{CuO}}_{\mathit{x}}$ chain.

First-principles theory has been developed to understand the unique material state underlying the ultra stability of O-enriched (with Ti and Y solute additions) nanoclusters (NCs) observed in nanostructured Fe. We show that an essential condition for the formation of these 2--4 nm diameter NCs is the existence of their exceptionally stable interface, as manifested in a higher (i.e., more negative) O-binding energy on the interface (${E}_{S}$) than in the interior (${E}_{B}$) of the NCs. The difference between ${E}_{S}$ and ${E}_{B}$ is due to the effect of strain, originating from the solute-solute repulsion and the presence of O in the form of O-vacancy (O:V) pair. A Friedel-type oscillation pattern is found for the distance dependence of the solute-(O:V) and (O:V)-(O:V) interaction energies. The strain is also important in controlling the size of NCs, which are predicted to exist only within a narrow O concentration range.

We report optical, electrical and magnetotransport properties of oxygen deficient ${\mathrm{SrTiO}}_{3}$ (${\mathrm{SrTiO}}_{3\ensuremath{-}x}$) thin films fabricated by pulsed laser deposition technique. The oxygen vacancies (${\mathrm{O}}_{\mathrm{vac}}$) in the thin film are expected to be uniform. By comparing its electrical properties to those of bulk ${\mathrm{SrTiO}}_{3\ensuremath{-}x}$, it was found that ${\mathrm{O}}_{\mathrm{vac}}$ in bulk ${\mathrm{SrTiO}}_{3\ensuremath{-}x}$ is far from uniform over the whole material. The metal-insulator transition (MIT) observed in the ${\mathrm{SrTiO}}_{3\ensuremath{-}x}$ film was found to be induced by the carrier freeze-out effect. The low temperature frozen state can be reexcited by Joule heating, electric and intriguingly magnetic field.

The simultaneous ordering of Gd and Cu magnetic moments was investigated in oxygen-deficient ${\mathrm{GdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ (x\ensuremath{\approxeq}0.3). The internal magnetic fields measured by the muon-spin-rotation technique show strong deviations from a mere superposition of the magnetic fields produced by the two magnetic subsystems separately. Above the ordering temperature (${\mathit{T}}_{\mathit{N}}$=2.3 K) of the Gd moments an increased relaxation rate and a negative frequency shift compared to the behavior of the ordered Cu system alone are observed. These deviations indicate the persistence of long-lived Gd spin fluctuations up to a temperature of approximately 100 K. The observed negative frequency shift is attributed to an interaction of the Gd moments with the Cu(II) magnetic subsystem.

Transport properties and magnetization measurements of the Kx MoO2-δ (0≤x≤0.25) compound are reported. The compound crystallizes in the oxygen deficient MoO2 monoclinic structure with potassium atoms occupying interstitial positions. An unconventional metallic behavior with power-law temperature dependence is related to a magnetic ordering. Superconducting transition with small volume fraction is also observed near 7 K for a sample with low potassium composition. © 2010 The American Physical Society.

A wide-range doping was achieved by a high-pressure method for ${\text{TbFeAsO}}_{1\ensuremath{-}\ensuremath{\delta}}$ from ``underdoped'' to ``overdoped'' superconducting compositions throughout the optimized superconductivity (${T}_{\text{c}}$ of 44 K). ${T}_{\text{c}}$ vs $\ensuremath{\delta}$ shows a dome-shaped feature, while ${T}_{\text{c}}$ vs the lattice constant likely follows a unique empirical curve over the doping range. The relatively large amount of oxygen vacancies up to $\ensuremath{\sim}0.3$ per the formula unit was introduced possibly because of the smaller replacement Tb than the other Ln (rare-earth element) in the ${\text{LnFeAsO}}_{1\ensuremath{-}\ensuremath{\delta}}$ system.

Etude par diffraction de neutrons pour δ compris entre 0,09 et 0,74. La supraconductivite disparai←t a la transition de phase orthorhombique tetragonale pour δ=0,45. Les parametres de structure varient lentement avec δ mais presentent un changement de pente a la transition. Analyse de l'energie de repulsion des atomes d'oxygene entre les sites O(1) et O(5)

We apply a quantum-mechanics/molecular-mechanics (QM/MM) scheme to simulate the migration of charged defects in alpha-quartz. A QM/MM framework together with a self-interaction correction (SIC) scheme was used to investigate the stability of the charged dimer defect and the E'(1) center in alpha-quartz. The use of a SIC scheme makes the charged dimer center unstable in alpha-quartz, in agreement with the experimental failure of identifying this defect, while the effects of the correction of the self-interaction error on the E-1(') center are negligible. Moreover, we conjecture that by overcoming a low energy barrier, the puckering mechanism (ideally heading from the charged dimer defect to the E'(1) center in SIC-free calculations) can be reiterated allowing the drift of the positive charge localized on an overcoordinated oxygen atom. This process can be regarded as an important channel of structural reorganization of oxygen deficient silica in the presence of strong polarizing electric fields.

Physical review / B 67, 033202 (2003). doi:10.1103/PhysRevB.67.033202, Published by APS, College Park, Md.

The line shape and Knight shift of the $^{89}\mathrm{Y}$ NMR resonance was obtained for fully oxygenated (x=7), oxygen deficient (x=6.7), and nonsuperconducting oxygen deficient (x=6) fluorine-doped YBCO (${\mathrm{Yba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$${\mathrm{F}}_{\mathit{y}}$). The results show that while fluorine can increase the onset superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ in oxygen deficient material, the Knight shift is practically independent of the fluorine concentration. The consequences of this result as far as ${\mathit{T}}_{\mathit{c}}$ is concerned are discussed.

Light illumination produces major, interesting changes of the electrical properties of oxygen deficient ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films. At small oxygen deficiency, the classical insulator-metal $(I\ensuremath{-}M)$ transition at a temperature ${T}_{p}$ decreases with increasing oxygen deficiency. The low-temperature metallic behavior in darkness shows a persistent increase of the conductivity with light. This photoconductivity increases with oxygen deficiency (from a few to $60%).$ At large oxygen deficiency, the films are semiconducting in darkness, from helium to room temperature and light induces a transient photoconductivity and a low-temperature $I\ensuremath{-}M$ transition. The intriguing possibility of collective light induced magnetism is discussed.

Room-temperature persistent spectral hole burning was observed in ${\mathrm{Eu}}^{3+}\ensuremath{-}\mathrm{d}\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{d}$ ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}{\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ glasses. The sol-gel-derived ${\mathrm{Eu}}^{3+}\ensuremath{-}\mathrm{d}\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{d}$ glasses were heated in hydrogen gas and irradiated with x ray, in which the oxygen deficient centers were induced in Al-O bonds and some ${\mathrm{Eu}}^{3+}$ ions were reduced into ${\mathrm{Eu}}^{2+}$ by heating in hydrogen gas. The spectral holes were burned in the excitation spectra of the ${}^{7}{\stackrel{\ensuremath{\rightarrow}}{{F}_{0}}}^{5}{D}_{0}$ transition of ${\mathrm{Eu}}^{3+}.$ The maximum depth of the hole burned at room temperature was found to be $\ensuremath{\sim}10%$ of the total excitation intensity for both the glasses and independent of the treatment conditions of glasses. A proposed model for hole burning is the excitation of the ${\mathrm{Eu}}^{3+}$ ion and subsequent hole trapping in the oxygen-defect centers in the Al-O bonds. The burnt holes were easily erased by laser irradiation with the energy different from burning one.

Meissner- and shielding-effect measurements were made on an oxygen-deficient single crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ as a function of aging time at room temperature following a quench from 225 \ifmmode^\circ\else\textdegree\fi{}C. It is seen that the Meissner fraction systematically increases from 11.1% to 13.2% as ${\mathit{T}}_{\mathit{c}}$ changes from 11.2 to 25.0 K. All aging was done within the helium exchange gas environment of the sample chamber. The results point to oxygen vacancy ordering rather than oxygen absorption as the mechanism for both the ${\mathit{T}}_{\mathit{c}}$ and Meissner-fraction increase.

The effects of ArF excimer laser irradiation on dehydrated high-purity silica glass were investigated on both the optical-absorption bands due to oxygen-deficient centers (ODC) and the formation of ${E}^{\ensuremath{'}}$ centers. With an intense uv flux from an excimer laser, an ${E}^{\ensuremath{'}}$-center density of the order of ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ was created. The 7.6-eV absorption band remains at the original level, while the 5.0-eV absorption band having the 4.3-eV emission band decreased. Both bands were reduced by heat treatment in an ${\mathrm{O}}_{2}$ atmosphere. These results suggest that there exist two types of ODC: ODC(I), which is responsible for the 7.6-eV band, and ODC(II), for the 5.0-eV band. The concentrations of ODC(I) and ODC(II) were evaluated to be 1 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through an analysis of the gas treatment data and of the order of ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through the growth curve of the ${E}^{\ensuremath{'}}$ centers, respectively. The structural origin of ODC(I) has been attributed to the Si-Si bond, judging from the fact that the peak energy and cross section of the absorption were in close agreement with those of the ${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$ molecule. Theoretical calculations on defect energy levels by O'Relly and Robertson supported the structural model for IDC(I) and suggested an unrelaxed oxygen vacancy for ODC(II). These assignments were also consistent with the results of a quenching experiment in which the fictive temperature of samples was changed; the equilibrium between concentrations of ODC(I) and ODC(II) shifted in a reasonable manner.