Search

It is well known that the critical temperature of multigap superconducting three-dimensional (3D) heterostructures at atomic limit (HAL) made of a superlattice of atomic layers with an electron spectrum made of several quantum subbands can be amplified by a shape resonance driven by the contact exchange interaction between different gaps. The ${T}_{C}$ amplification is achieved tuning the Fermi level near the singular nodal point at a Lifshitz transition for opening a neck. Recently high interest has been addressed to the breaking of inversion symmetry which leads to a linear-in-momentum spin-orbit induced spin splitting, universally referred to as Rashba spin-orbit coupling (RSOC) also in 3D layered metals. However the physics of multigap superconductivity near unconventional Lifshitz transitions in 3D HAL with RSOC, being in a non-BCS regime, is not known. The key result of this work getting the superconducting gaps by Bogoliubov theory and the 3D electron wave functions by solution of the Dirac equation is the feasibility of tuning multigap superconductivity by suitably matching the spin-orbit length with the 3D superlattice period. It is found that the presence of the RSOC amplifies both the $k$ dependent anisotropic gap function and the critical temperature when the Fermi energy is tuned near the circular nodal line. Our results suggest a method to effectively vary the effect of RSOC on macroscopic superconductor condensates via the tuning of the superlattice modulation parameter in a way potentially relevant for spintronics functionalities in several existing experimental platforms and tunable materials needed for quantum devices for quantum computing.

The oxygen isotope ($^{16}\mathrm{O}$/$^{18}\mathrm{O}$) effect (OIE) on the pseudogap (charge-stripe ordering) temperature ${T}^{*}$ is investigated for the cuprate superconductor ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ as a function of doping $x$ by means of x-ray absorption near edge structure studies. A strong $x$ dependent and sign reversed OIE on ${T}^{*}$ is observed. The OIE exponent ${\ensuremath{\alpha}}_{{T}^{*}}$ systematically decreases from ${\ensuremath{\alpha}}_{{T}^{*}}=\ensuremath{-}0.6(1.3)$ for $x=0.15$ to ${\ensuremath{\alpha}}_{{T}^{*}}=\ensuremath{-}4.4(1.1)$ for $x=0.06$, corresponding to increasing ${T}^{*}$ and decreasing superconducting transition temperature ${T}_{c}$. Both ${T}^{*}(^{16}\mathrm{O})$ and ${T}^{*}(^{18}\mathrm{O})$ exhibit a linear doping dependence with different slopes and critical end points [where ${T}^{*}(^{16}\mathrm{O})$ and ${T}^{*}(^{18}\mathrm{O})$ fall to zero] at ${x}_{c}(^{16}\mathrm{O})=0.201(4)$ and ${x}_{c}(^{18}\mathrm{O})=0.182(3)$, indicating a large positive OIE of ${x}_{c}$ with an exponent of ${\ensuremath{\alpha}}_{{x}_{c}}=0.84(22)$. The remarkably large and strongly doping dependent OIE on ${T}^{*}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.

We have used scanning micro x-ray diffraction to characterize different phases in superconducting KxFe2-ySe2 as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (T-c similar to 32 K), phase separation temperature (T-ps similar to 520 K), and iron-vacancy order temperature (T-vo similar to 580 K). In addition to the iron-vacancy ordered tetragonalmagnetic phase and orthorhombicmetallic minority filamentary phase, we have found clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below similar to 300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides clear evidence of the formation of protected metallic percolative paths in themajority texturewith largemagnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the T-ps and T-c is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in KxFe2-ySe2.

We have performed extended x-ray-absorption fine-structure (EXAFS) measurements to study the local atomic structure of the ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ (Hg-1201) system across the superconducting transition temperature ${(T}_{c}).$ The Debye-Waller factors of atomic pairs obtained by Cu K-edge EXAFS are compared with diffraction data to calculate atomic correlations. The atomic correlations for Cu-O bonds show anomalous temperature dependences across the ${T}_{c},$ highlighting the important role of electron-lattice interactions to derive the superconducting characteristics of the system.

Local structure of ${\mathrm{CuO}}_{2}$ plane of the superconducting ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi2212) crystal at the optimum doping has been investigated by in-plane polarized Cu K-edge x-ray-absorption spectroscopy. We have determined the quadrupole character of the $1\stackrel{\ensuremath{\rightarrow}}{s}{3d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ transition in the x-ray-absorption near-edge spectra by rotation of the in-plane polarization geometry by 45\ifmmode^\circ\else\textdegree\fi{} from the Cu-O-Cu direction. The nonzero intensity of this transition in the polarization vector parallel to the Cu-O-Cu direction indicates local structural distortion of the ${\mathrm{CuO}}_{4}$ square plane. The statistical distribution of the instantaneous Cu-O bonds is determined by polarized extended x-ray-absorption fine structure and discussed.

We have identified a set of electronic states with a one-dimensional dispersion in the $\ensuremath{\Gamma}=(0,0)$ to ${M}_{1}=(\ensuremath{\pi},0)$ direction crossing the Fermi surface at one point ${k}_{F}=0.2\ifmmode\pm\else\textpm\fi{}0.03\ensuremath{\pi}$ by angle-resolved photoemission. The one-dimensional character of these electronic states is demonstrated by missing dispersion along the orthogonal direction, observed in energy-distribution curves, and by $k$-space mapping of the Fermi surface, using angle scanning photoemission. The observed electronic states beyond the well-known large Fermi surface provoke the models based on charge and spin ordering in the ${\mathrm{CuO}}_{2}$ plane of high-${T}_{c}$ superconductors.

Temperature-dependent polarized Cu K-edge x-ray absorption has been used to investigate local structural distortions in the ${\mathrm{CuO}}_{2}$ plane of the ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ system. The Cu-O pair distribution shows the presence of a minority phase containing ${\mathrm{CuO}}_{6}$ octahedra characterized by a shorter Cu-O(apical) bond (\ensuremath{\Delta}R\ensuremath{\sim}-0.1 \AA{}) and two longer Cu-O(planar) bonds (\ensuremath{\Delta}R\ensuremath{\sim}+0.08 \AA{}) and a tilting angle of 16\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}. The temperature-dependent distortions show a maximum around T (\ensuremath{\sim}1.${6\mathrm{T}}_{\mathrm{c}}$) and a minimum at ${\mathrm{T}}_{\mathrm{c}}$ (\ensuremath{\sim}35 K). The data show the coexistence of two types of doped charges in different stripes in the superconducting phase.

The temperature-dependent amplitude of the Cu-O(planar) bond modulation in the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4.1}$ superconducting crystal has been measured by in-plane-polarized Cu K-edge extended x-ray-absorption fine structure. Minority domains characterized by a long Cu-O(planar) bond length are found at T150 K. The separation between the two Cu-O(planar) distances provides a measure of the amplitude of the ${\mathrm{CuO}}_{2}$ lattice modulation assigned to a charge-density wave that coexists with the superconducting phase, providing evidence for a broken symmetry.

The local structure and electronic properties of Rb${}_{1\ensuremath{-}x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ are investigated by means of site selective polarized x-ray absorption spectroscopy at the iron and selenium $K$ edges as a function of pressure. A combination of dispersive geometry and a nanodiamond anvil pressure cell has permitted us to reveal a steplike decrease in the Fe-Se bond distance at $p\ensuremath{\simeq}11$ GPa and, at the same pressure, the local atomic structure of Rb${}_{1\ensuremath{-}x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ becomes more ordered. Thus, at this pressure the phase separation might be suppressed, which has a large influence on the electronic structure, as evidenced by changes in the Fe $K$-edge prepeak. The present results provide compelling evidence that the reemerging superconductivity in ${A}_{1\ensuremath{-}x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ is closely related to the transition to a locally more ordered state, which has a different electronic structure.

Novel imaging methods show that the mobile dopants in optimum doped La${}_{2}$CuO${}_{4+y}$ (LCO) get self-organized, instead of randomly distributed, to form an inhomogeneous network of nanoscale metallic puddles with ordered oxygen interstitials interspersed with oxygen-depleted regions. These puddles are expected to be metallic, being far from half filling because of high dopant density, and to sustain superconductivity having a size in the range 5--20 nm. However, the electronic structure of these heavily doped metallic puddles is not known. In fact the rigid-band model fails because of ordering of dopants and supercell calculations are required to obtain the Fermi surface reconstruction. We have performed advanced band calculations for a large supercell La${}_{16}$Cu${}_{8}$O${}_{32+N}$ where $N=1$ or 2 oxygen interstitials form rows in the spacer La${}_{16}$O${}_{16+N}$ layers intercalated between the CuO${}_{2}$ layers as determined by scanning nano x-ray diffraction. The additional occupied states made by interstitial oxygen orbitals sit well below the Fermi level (${E}_{F}$) and lead to hole doping as expected. The unexpected results show that in the heavily doped puddles the altered Cu(3$d$)-O(2$p$) band hybridization at ${E}_{F}$ induces a multiband electronic structure with the formation of multiple Fermi surface spots: (a) Small gaps appear in the folded Fermi surface, (b) three minibands cross ${E}_{F}$ with reduced Fermi energies of 60, 150, and 240 meV, respectively, (c) the density of states and band mass at ${E}_{F}$ show substantial increases, and (d) spin-polarized calculations show a moderate increase of antiferromagnetic spin fluctuations. All calculated features are favorable to enhance superconductivity; however, the comparison with experimental methods probing the average electronic structure of cuprates will require the description of the electronics of a network of multigap superconducting puddles.

Oxygen chain fragments are known to appear at the insulator to superconductor transition (SIT) in YBa2Cu3O6+y. However the self organization and the size distribution of oxygen chain fragments is not known. Here we contribute to fill this gap, using scanning micro X ray diffraction which is a novel imaging method based on advances in focusing synchrotron radiation beam. This novel approach allows us to probe both real-space and k-space of a high-quality YBa2Cu3O6.33 single crystals with Tc=7K. We report compelling evidence for nanoscale striped puddles, with Ortho-II structure, made of chain fragments in the basal Cu(1) plane with local oxygen concentration 0.5. The size of the Ortho-II puddles spans a range between 2 and 9 nanometers. The real space imaging of Ortho-II puddles granular network shows that superconductivity, at low hole-doping regime, occurs in a network of nanoscale oxygen ordered patches, interspersed with oxygen depleted regions. The manipulation by thermal treatments of the striped Ortho II puddles has been investigated focusing on the spontaneous symmetry breaking near the order to disorder phase transition at 350 K.

The modulated structure of the ${\mathrm{CuO}}_{2}$ plane in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{y}}$ superconductor has been solved by Cu K-edge anomalous x-ray diffraction. The displacement of Cu ions along the c axis shows the largest amplitude and anharmonic character. The results show flat stripes of ${\mathrm{CuO}}_{2}$ lattice of width L=15.5 \AA{} with long Cu-Bi distances that are separated by stripes of bent ${\mathrm{CuO}}_{2}$ plane and shorter Cu-Bi distances. In each ${\mathrm{CuO}}_{2}$ bilayer the flat stripes in a first layer are close to the bent stripes in the second layer, suppressing the electronic coupling between the two adjacent planes. \textcopyright{} 1996 The American Physical Society.

Here we report the interpretation of all spectral features of Ce ${\mathit{L}}_{3}$ x-ray-absorption near-edge structure (XANES) of ${\mathrm{CeO}}_{2}$ over a 40 eV range. The local partial unoccupied density of states in the initial state and in the fully relaxed final state have been calculated by the full multiple-scattering approach. The wave function of the excited electron in the final states of the Ce ${\mathit{L}}_{3}$-edge XANES spectrum is found to be determined by the multiple-scattering processes in a large size cluster formed by at least 45 atoms. We predict, in good agreement with the experimental data, the crystal-field splitting of 5d states in the final state ${\mathrm{\ensuremath{\Delta}}}_{\mathit{f}}$=4.0\ifmmode\pm\else\textpm\fi{}0.2 eV and its variation from the initial to the final state. The many-body final states for the 2p\ensuremath{\rightarrow}5,\ensuremath{\varepsilon}d transition, arising from the configuration interaction due to mixing of Ce 4f and O 2p valence orbitals, have been calculated taking into account the crystal-field splitting of the 5,\ensuremath{\varepsilon}d states and they give a full explanation for the low-energy shoulder on the Ce ${\mathit{L}}_{3}$ x-ray-absorption spectrum white line that was the object of a long-standing discussion. The origin of the pre-edge peak at about 10 eV below the white line maximum is explained as due to transitions at the bottom of the conduction band.

Using scanning micro-x-ray diffraction we report a mixed real and reciprocal space visualization of the spatial heterogeneity of the lattice incommensurate supermodulation in a single crystal of Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+y}$ with ${T}_{c}$ = 84 K. The mapping shows an amplitude distribution of the supermodulation with large lattice fluctuations at a microscale with \ensuremath{\sim}50% amplitude variation. The angular distribution of the supermodulation amplitude in the a-$b$ plane shows a lattice chiral symmetry, forming a left-handed oriented striped pattern. The spatial correlation of the supermodulation is well described by a compressed exponential with an exponent of 1.5 \ifmmode\pm\else\textpm\fi{} 0.3 and a correlation length of \ensuremath{\sim}50 \ensuremath{\mu}m, showing intrinsic lattice disorder in high-temperature superconductors.

Advanced synchrotron radiation focusing down to a size of 300 nm has been used to visualize nanoscale phase separation in the K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ superconducting system using scanning nanofocus single-crystal x-ray diffraction. The results show an intrinsic phase separation in K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ single crystals at $T$ 520 K, revealing the coexistence of (i) a magnetic phase characterized by an expanded lattice with superstructures due to Fe vacancy ordering and (ii) a nonmagnetic phase with an in-plane compressed lattice. The spatial distribution of the two phases at 300 K shows a frustrated or arrested nature of the phase separation. The space-resolved imaging of the phase separation permitted us to provide direct evidence of nanophase domains smaller than 300 nm and different micrometer-sized regions with percolating magnetic or nonmagnetic domains forming a multiscale complex network of the two phases.

The local and partial density of states of the conduction band of solid neon in a large energy interval has been extracted from the analysis of the K-edge x-ray-absorption near-edge structure (XANES) spectrum by using a full multiple-scattering method. The absorption cross section for one-electron transitions from the Ne 1s level to the conduction-band final states in the solid Ne is predicted by first principles. The absorption cross section is described as due to an atomic factor and to a structure factor determined by the local and partial density of states for a large cluster formed by about 80 atoms. The experimental spectrum is obtained by describing the final state in the fully relaxed potential in the presence of the core hole. The exciton peak at threshold is shown to be due to the lowest unoccupied valence state pushed down in the bound-state region by the core hole. In the 100-eV XANES energy range the single-scattering approximation fails to predict the XANES of solid Ne indicating that the full multiple-scattering approach is required. The present analysis allows us to identify strong two-electron excitations 1${\mathit{s}}^{2}$2${\mathit{s}}^{2}$2${\mathit{p}}^{6}$\ensuremath{\rightarrow}1${\mathit{s}}^{1}$2${\mathit{s}}^{2}$2${\mathit{p}}^{5}$${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{i}}$${\mathit{p}}^{2}$ in the energy range of 30 eV above the K edge, involving monopole 2p\ensuremath{\rightarrow}\ensuremath{\varepsilon}p transitions to the lowest valence states at energy \ensuremath{\varepsilon}. The intensity of the two-electron excitations is found to be so strong that it should be considered in standard extended x-ray-absorption fine-structure--XANES data analysis for local structure investigation.

Resonances in the superconducting properties, in a regime of crossover from BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band superconductor where the chemical potential is tuned near the band edge of the second mini-band generated by quantum confinement effects. The shape resonances at T=0 in the superconducting gaps (belonging to the class of Feshbach-like resonances) is manifested by interference effects in the superconducting gap at the first large Fermi surface when the chemical potential is in the proximity of the band edge of the second mini-band. The case of a superlattice of quantum wells is considered and the amplification of the superperconducting gaps at the 3D-2D Fermi surface topological transition is clearly shown. The results are found to be in good agreement with available experimental data on a superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers., 13 pages, 9 images

The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO (La-1111) and SmFeAsO (Sm-1111) single crystals measured by high-resolution x-ray diffraction is found to be sharp while the $R\text{FeAsO}$ $(R=\text{La},\text{ }\text{Nd},\text{ }\text{Pr},\text{ }\text{Sm})$ polycrystalline samples show a broad continuous SPT. Comparing the polycrystalline and the single-crystal 1111 samples, the critical exponents of the SPT are found to be the same while the correlation length critical exponents are found to be very different. These results imply that the lattice fluctuations in 1111 systems change in samples with different surface to volume ratio that is assigned to the relieve of the temperature-dependent superlattice misfit strain between active iron layers and the spacer layers in 1111 systems. This phenomenon that is missing in the $A{\text{Fe}}_{2}{\text{As}}_{2}$ $(A=\text{Ca},\text{ }\text{Sr},\text{ }\text{Ba})$ ``122'' systems, with the same electronic structure but different for the thickness and the elastic constant of the spacer layers, is related with the different maximum superconducting transition temperature in the 1111 (55 K) versus 122 (35 K) systems and implies the surface reconstruction in 1111 single crystals.

The high-resolution Ca ${\mathit{L}}_{2,3}$ x-ray-absorption near-edge-structure (XANES) spectrum of a ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ single crystal has been measured by use of a magnetic-projection x-ray microscope probing a surface area of 200\ifmmode\times\else\texttimes\fi{}200 \ensuremath{\mu}${\mathrm{m}}^{2}$. The Ca ${\mathit{L}}_{2,3}$ XANES spectrum is analyzed by performing a multiple-scattering XANES calculation in real space and comparing the results with the spectrum of ${\mathrm{CaF}}_{2}$. Good agreement between the calculated and experimental crystal-field splitting ${\mathrm{\ensuremath{\Delta}}}_{\mathit{f}}$ of the Ca 3d final states is found and the splitting is shown to be smaller by 0.5 eV than in the initial state. The Ca 3d partial density of states is found to be close to the Fermi level in the initial state. The Ca-O(in plane) distance is shown to be a critical parameter associated with the shift of the Ca 3d states relative to the Fermi level; in particular, we have studied the effect of the out-of-plane dimpling mode of the in-plane oxygen atoms O(in plane) that will move the Ca 3d states on or off the Fermi level. This mode can therefore play a role in modulating the charge transfer between the two ${\mathrm{CuO}}_{2}$ planes separated by the Ca ions.

The polarized E\ensuremath{\perp}c oxygen K-edge x-ray-absorption near-edge structure (XANES) spectrum of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi 2:2:1:2) has been calculated by a multiple-scattering approach and compared with experimental data. The symmetry of the conduction bands over a range of 15 eV above the Fermi level has been studied by a combined analysis of five different polarized x-ray-absorption spectra at the following edges: O K edge, Cu K edge, and Cu ${\mathit{L}}_{3}$ edge. Information on the partial density of states of the conduction bands, ${\mathit{scrD}}_{\mathit{X}(\mathit{l},}$${\mathit{m}}_{\mathit{l}}$), projected on the atomic site X (Cu and O) with selected orbital angular momentum l, and its projection ${\mathit{m}}_{\mathit{l}}$ along the c axis (${\mathit{scrD}}_{\mathrm{Cu}(2,0)}$, ${\mathit{scrD}}_{\mathrm{C}\mathrm{u}(2,\ifmmode\pm\else\textpm\fi{}1)}$, and ${\mathit{scrD}}_{\mathrm{C}\mathrm{u}(2,\ifmmode\pm\else\textpm\fi{}2)}$ for the Cu site; ${\mathit{scrD}}_{\mathrm{O}(1,0)}$ and ${\mathit{scrD}}_{\mathrm{O}(1,\ifmmode\pm\else\textpm\fi{}1)}$ for the oxygen site) have been obtained. The symmetry of the itinerant states induced by doping at the Fermi level is deduced to be a mixture of 3d holes with ${\mathit{m}}_{\mathit{l}}$=\ifmmode\pm\else\textpm\fi{}2 and ${\mathit{m}}_{\mathit{l}}$=0 orbital angular momenta and holes on planar oxygen in the molecular-orbital combination of local ${\mathit{b}}_{1}$ and ${\mathit{a}}_{1}$ symmetry [the ligand holes L(${\mathit{b}}_{1}$) and L(${\mathit{a}}_{1}$)].

The linearly polarized Cu {ital L}{sub 3}-edge x-ray-absorption near-edge structure (XANES) of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} has been measured and the spectra are interpreted by the full multiple-scattering approach in real space. The polarized spectra over a range of 20 eV can be predicted in terms of the one-electron dipole ({Delta}{ital l}=+1) transition Cu 2{ital p}{r arrow}{epsilon}{ital d}, probing the unoccupied {ital d}-like ({ital l}=2) density of states projected on the Cu site with orbital angular momentum {ital m}{sub {ital l}}=0, 1 in the {bold E}( ){ital z} spectra, and the {ital m}{sub {ital l}}=2, 1, and 0 in the {bold E}{perpendicular}{ital c} spectra. The oscillator strength for the dipole allowed transitions ({Delta}{ital l}={minus}1) Cu 2{ital p}{r arrow}{var epsilon}{ital s} is shown to be a factor of 100 weaker than the 2{ital p}{r arrow}3{ital d} transitions. The Coulomb interaction in the final state between the Cu 2{ital p} core hole and the excited Cu 3{ital d} electron is found to be 5.5 eV forming a bound state below the continuum threshold, the well-known Cu {ital L}{sub 3} white line. On the contrary, the core hole induces a nearly rigid redshift about 1 eV of the high-energy conduction bands.

The linearly polarized Cu K-edge x-ray-absorption near-edge structure (XANES) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi 2:2:1:2) has been measured, and the spectra are interpreted by the multiple-scattering approach in real space. The polarized spectra can be predicted in terms of a one-electron dipole (\ensuremath{\Delta}l=+1) transition Cu 1s\ensuremath{\rightarrow}\ensuremath{\epsilon}p, probing the unoccupied p-like (l=1) density of states projected on the Cu site with orbital angular momentum ${\mathit{m}}_{\mathit{l}}$=0 in the E\ensuremath{\parallel}z spectra, and the ${\mathit{m}}_{\mathit{l}}$=1 in the E\ensuremath{\perp}c spectra. Therefore we show that the electronic structure of the high-energy conduction bands, beyond the Cu 3d band, are well described in terms of the one-electron approximation. Final-state effects induced by the core hole have been studied and calculated. A satellite 7 eV above the main K-XANES peak in both polarizations is shown to be a multielectron shake-up excitation, and it is enhanced going into the insulating phase. Structural information on the copper site in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ and in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{YCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ has been extracted from XANES spectra, providing evidence for the decrease of the Cu-O distance in the metallic phase.

Multielectron excitations involving metal deep 1s and shallow 3p core levels have been identified in the x-ray-absorption spectra in the range 50--60 eV above the K edge of transition-metal ions V, Cr, and Mn with tetrahedral coordination geometry. The many-body transitions have been found after subtraction of the dominant one-electron transitions to continuum states giving the x-ray-absorption near-edge structure. The presence of many-body excitations involving valence electrons has been found at \ensuremath{\sim}7 eV above threshold. The energy positions are in good agreement with calculated excitation energies for the many-body configurations, where the two excited electrons are in the unoccupied molecular ${\mathit{t}}_{2}^{\mathrm{*}}$ orbital. Experimental comparison of the intensities of corresponding many-body satellites between the Cr 2p x-ray photoelectron spectrum and Cr K-edge absorption spectrum is reported for ${\mathrm{K}}_{2}$${\mathrm{CrO}}_{4}$.

Here we report the synthesis and characterization of ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Sc}}_{x}{\mathrm{B}}_{2}\phantom{\rule{0.3em}{0ex}}\phantom{\rule{0.3em}{0ex}}(0.12lxl0.27)$ system, with critical temperature in the range of $30g{T}_{c}g6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We find that the $\mathrm{Sc}$ doping moves the chemical potential through the 2-D/3-D electronic topological transition (ETT) in the $\ensuremath{\sigma}$ band where the ``shape resonance'' of interband pairing occurs. In the 3-D regime beyond the ETT we observe a hardening of the ${\mathrm{E}}_{2g}$ Raman mode with a significant line-width narrowing due to the suppression of the Kohn anomaly over the range $0lql2{k}_{F}$.

We have studied local structure of the Laves phase Ce{sub 1-x}La{sub x}Ru{sub 2} superconductor by Ru K-edge extended x-ray absorption fine-structure measurements focusing on the small La concentration regime where the transition temperature T{sub c} passes through a local maximum. We find that correlated Debye-Waller factor of the Ru-Ru bonds follows T{sub c} with the varying La concentration in the system. Although, this remarkable T{sub c} correlation on the local atomic structure suggests important role of the electron-lattice interactions, the band-structure effects seem more likely the reason to drive the anomalous superconducting behavior and the T{sub c} maximum in this 4f system.

We investigate the application of atomistic models of self-interstitial defects to the simulation of Rutherford backscattering-channeling (RBS-C) spectra in ion irradiated Si. By the comparison of simulated and experimental measurements, we verify the ability of different models, either of elementary interstitials or of small clusters, to reproduce experimental spectra measured under different alignment conditions in Si lightly damaged by ${\mathrm{Si}}^{+}$ ion implantation. A model system for RBS-C simulation is built by inserting a distribution of defects in a supercell with size of $\ensuremath{\sim}{10}^{6}$ atoms. The system is then structurally relaxed by the application of the classical environment-dependent interatomic potential (EDIP). After adjusting the defect distribution in order to fit the $〈100〉$ RBS-C spectrum, simulations are performed under the other alignment conditions investigated. The scattering factors of defects are then extracted from both experimental and simulated RBS-C spectra and compared. It is shown that the anisotropy of experimental damage is not compatible with a significant presence of random (incoherent) disorder, but can be reproduced by some of the defect models under consideration: the split-$〈110〉$ interstitial, the diinterstitial formed by the addition of an interstitial to the split-$〈110〉$ interstitial, and two different configurations of the four-interstitial aggregate; one formed by two close diinterstitials and the other by the aggregation of four split-$〈100〉$ interstitials. Due to the different $〈100〉$ scattering factors of the four defect configurations which are found to reproduce experimental spectra, there is an inherent uncertainty of a factor of 2 in the estimate of the amount of interstitials by $〈100〉$ RBS-C analysis. The agreement between simulations and experiments is remarkable, considering that the method makes use of physical, although empirical, models of defects, where the only adjustable parameter is the absolute concentration of interstitials.

Temperature dependent local structure of ${\mathrm{Nb}}_{3}\mathrm{Ge}$ superconductor has been studied by Ge K-edge extended x-ray-absorption fine structure measurements. Correlated Debye-Waller factor of the Ge-Nb pairs shows a clear change across the superconducting transition temperature ${T}_{c}.$ This change appears to be similar to the one observed for the $\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}\ensuremath{-}{T}_{c}$ cuprates, indicating a common cause of pairing in the short coherence length superconductors. The results suggest an intimate relation between the local atomic displacements and the short coherence superconductivity, provoking models based on local electron-lattice interaction for the $\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}\ensuremath{-}{T}_{c}$ superconductivity.

The model of two ~s and p! channel superconductivity known to be necessary to explain the superconductivity in MgB2 has been applied to the Al12xMgxB2 diborides by tuning x from MgB2 to AlMgB4 . The interband coupling parameter ~probing the strength of the interchannel pairing due to quantum interference effects! and the two gaps in the s and p channel have been calculated as a function of x by using experimental data for the two phonon mode energies which are most relevant for the pairing interaction. It is found that the two gaps reverse in leading order around x50.6 and that in comparison with the dominant intra-s-band single channel pairing an increase in Tc of 1.5 occurs for MgB2 while in AlMgB4 the dominant p band single channel pairing leads to enhancement of Tc of 100. DOI: 10.1103/PhysRevB.67.132509

By using a single-coil technique, we study the low-temperature dependence of the in-plane magnetic penetration depth λ a b in several Bi 2 Sr 2 CaCu 2 O 8 + δ (BSCCO) single crystals prepared with various doping levels δ ranging from the under- to the overdoped regime. Four samples exhibit a linear dependence of thermally activated low-energy excitations Δλ a b ≡ λ a b (T) - λ a b (0) ∼ T. The thermal activation rate ∂λ a b /∂T is minimum, 8 A/K, at optimum doping, in good quantitative agreement with the d-wave model of superconducting order parameter. The much larger rates,? 40 A/K, observed in all under- and overdoped samples indicate that, in these samples, another type of low-energy excitations is relevant. These large rates are quantitatively consistent with a model of thermal phase fluctuations suitable for granular superconductors with short coherence length like cuprates. One underdoped sample exhibits a quadratic dependence suggesting an incipient crossover from thermal to quantum fluctuations.

We report the results of the simulation of ion-channeling spectra in a disordered silicon crystal, where lattice relaxation in the neighborhood of point defects, calculated by the application of empirical potentials, is taken into account. We show that, in general, the backscattering yield increases when the perfectly symmetrical configurations of point defects in the unperturbed lattice are allowed to relax. The yield enhancement depends on the potential used, the point defect type, and the beam-lattice alignment condition. A quantitative correlation between the microscopic disorder and the macroscopic yield measured by ion channeling, has been determined under the condition of a low concentration of weakly interacting point defects. The practical consequences of introducing relaxation in the interpretation of Rutherford backscattering-channeling spectra are pointed out and discussed. One important result is that if relaxation effects are neglected (as in damage models used so far), the amount of defects extracted from channeling analysis may be appreciably overestimated. The method developed here has been applied to the study of the damage distribution in the near surface of a Si sample implanted with high energy ions. In spite of the simplified description of damage in terms of point defects, our preliminary results show that taking into account lattice relaxation in ion-channeling simulation allows simultaneous fitting of backscattering spectra collected along different axial alignment conditions. The same result cannot be achieved using the standard description based on unrelaxed defects.

La 8 - x Sr x Cu 8 O 2 0 is a nonsuperconducting cuprate which exhibits a doubling of the elementary cell along the c axis. Its optical conductivity has been first measured here, down to 20 K, in two single crystals with x =1.56 and x = 2.24, Along c, bands are observed in both samples which correspond to strongly bound charges and confirm that the cell doubling is due to charge ordering. In the ab plane, in addition to the Drude term one observes an infrared peak at ∼0.1 eV and a midinfrared hand at 0.7 eV. The 0.1-eV peak is found at higher frequencies below 200 K, in correspondence with an anomalous increase in the dc resistivity and consistently with its assignment to localized charges. These results point out similarities and differences with respect to the optical properties of superconducting cuprates.

Raman and infrared absorption spectra of ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2}$ have been collected for $0l~xl~0.5$ in the frequency range of the phonon spectrum. The data show a remarkable dependence on the Al content. Raman spectra have been carefully analized and the x dependence of the peak frequency, the width and the intensity of the phonon modes have been obtained. The results allow us to distinguish between two regimes in the low and high Al content regions. In particular the onset of the high Al content phase evidenced in previous structural studies is marked by new spectral components at high frequencies. Finally a connection between the whole of our results and the suppression of the superconducting phase at high Al content is established.

We have used polarized Cu K-edge extended x-ray absorption fine structure to investigate the local lattice displacements in the charge-ordered ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_{4}$ system. The temperature-dependent distance broadening, given by the Debye-Waller factors of the Cu-O bonds measuring local lattice fluctuations, shows a clear steplike increase below the charge stripe order temperature. The results construct a direct evidence for the local lattice fluctuations as a response function of the charge stripe ordering in the cuprate superconductors.