Effect of isotope substitution and doping on the Raman spectrum of galena (PbS)

The Raman spectrum of natural PbS has been shown to exhibit features related to the IR-active (Raman forbidden) LO phonons and to allowed scattering by two phonons. In order to confirm the vibrational nature of these features, and their peculiar temperature dependence, we compare here the spectra obtained for natural samples (i.e. mineral as well as synthetic with the natural isotopic abundance) with those measured for crystals prepared from highly enriched ³⁴S. The observed isotopic shifts confirm the vibrational nature of the structures mentioned above. We have also measured the spectra of synthetic samples of PbS annealed at several different pressures of sulphur. Because of their non-stoichiometric sulphur content they have carrier concentrations (either electrons or holes) in the 1×10¹⁷-3×10¹⁸ carriers/cm³ range (as determined from their IR reflection spectra). The measured spectra, as well as their temperature dependence, do not depend on the bulk carrier concentration, a fact that must result from surface pinning of the Fermi level within the fundamental energy gap.We finally discuss the apparent paradox of why the IR spectra reveal coupled LO-phonon-plasmon modes whereas the Raman spectra exhibit the bare LO-phonons.     

Defining under and over-doping as bond order phenomena in the pair plaid model

Within the rules of a pair plaid model charge-lattice lock-in patterns are identified, which are responsible for the trends to characteristic hole values for a variety of features in the doping curves [onsets, kinks or optima,] of cuprates and related materials. Examples are the prevalence of optimal hole values of 0.16, 0.22 or 0.25. Selection of one of these values depends on the degree of isolation of the plane. Periods of pair plaid represent a central aspect for an algorithm of T_c or doping curve predictions when combined with the plane isolation model. Aspects of the theoretical concept of pair plaids have been made visible in recent STM graphs indicating electronic crystal behavior for the related pseudo-gap region. Pair plaids are in competition with stripes of non-superconducting singles and can be transformed into them. This competition is further elucidated with emphasis on the regions, where both orders can coexist. This further defines over and under-doping.     

Low-energy signatures of charge and spin fluctuations in Raman and optical spectra of the cuprates

We calculate the optical and Raman response within a phenomenological model of fermion quasiparticles coupled to nearly critical collective modes. We find that, whereas critical scaling properties might be masked in optical spectra due to charge conservation, distinct critical signatures of charge and spin fluctuations can be detected in Raman spectra exploiting specific symmetry properties. We compare our results with recent experiments on the cuprates.     

New picture of high-temperature superconductivity

The case for high-temperature superconductivity originating in SrO or BaO planes, or in interstitial regions, is made, including (i) four successfully predicted superconductors; (ii) evidence that the superconductivity of the major cuprates is associated with holes in these layers; (iii) data showing that Pr on one side of a cuprate-plane kills the superconductivity, but Pr on the other side does not; and (iv) evidence that doped Sr₂YRuO₆ has an onset of superconductivity at ∼45 K despite having no cuprate-planes.     

Local structure study of T′ cuprate superconductors

Undoped T′-type cuprates are believed to be charge transfer (Mott-Hubbard) insulators. Recent observation of undoped superconducting T′-type cuprates suggests half-filling weakly correlated metallic electron states (Tsukada et al.), which is against the fundamental picture. Here we report the local structures of undoped T′-(La³⁺,Y³⁺)₂CuO₄ and doped T′-(La³⁺,Ce⁴⁺)₂CuO₄ thin-film single crystals by polarized extended X-ray absorption fine structure (EXAS). The in-plane (E//ab) polarized Cu K-EXAFS data shows a remarkable Cu-O bond distance displacement. The broadened Cu-O distribution suggests a large local lattice distortion in T′-type structures, which might influence the electronic structure.     

On the contribution of nearly critical spin and charge collective modes to the Raman spectra of high-Tc cuprates

We discuss how Raman spectra are affected by nearly critical spin and charge collective modes, which are coupled to charge carriers near a stripe quantum critical point. We show that specific fingerprints of nearly critical collective modes can indeed be observed in Raman spectra and that the selectivity of Raman spectroscopy in momentum space may also be exploited to distinguish the spin and charge contribution. We apply our results to discuss the spectra of high-T_c superconducting cuprates finding that the collective modes should have masses with substantial temperature dependence in agreement with their nearly critical character. Moreover, spin modes should be more diffusive than charge modes indicating that in stripes the charge is nearly ordered, while spin modes are strongly overdamped and fluctuate with high frequency.     

Pressure broadening measurement of submillimeter-wave lines of O3

The pressure broadening coefficients and their temperature dependences for two submillimeter-wave transitions of ozone, one being monitored with Odin and the other to be monitored with JEM/SMILES and EOS-MLS, have been determined by using a BWO based submillimeter-wave spectrometer. The measurements have also been extended to one of the symmetric isotopic species, ¹⁶O¹⁸O¹⁶O. The isotopic species is observed in natural abundance and as a consequence the temperature dependence is not determined due to weak signal intensity. The pressure broadening parameters are determined with better than 1% accuracy, while the temperature dependence exponents are obtained within 1.5-3% accuracy for the normal species transitions.     

Infrared reflection absorption study of carbon monoxide adsorption on Pd/Cu(1 1 1)

Pd-Cu bimetallic surfaces formed through a vacuum-deposition of Pd on Cu(1 1 1) have been discussed on the basis of carbon monoxide (CO) adsorption: CO is used as a surface probe and infrared reflection absorption (IRRAS) spectra are recorded for the CO-adsorbed surfaces. Low energy electron diffraction (LEED) patterns for the bimetallic surfaces reveal six-fold symmetry even after the deposition of 0.6 nm. The lattice spacings estimated by the separations of reflection high-energy electron diffraction (RHEED) streaks increase with increasing Pd thickness. Room-temperature CO exposures to the bimetallic surfaces formed by the Pd depositions less than 0.3 nm thickness generate the IRRAS bands due to the three-fold-hollow-, bridge- and linear-bonded CO to Pd atoms. In particular, on the 0.1 nm-thick Pd surface, the linear-bonded CO band becomes apparent at an earlier stage of the exposure. In contrast, the bridge-bonded CO band dominates the IRRAS spectra for CO adsorption on the 0.6 nm-thick Pd surface, at which the lattice spacing corresponds to that of Pd(1 1 1). A 90 K-CO exposure to the 0.1 nm-thick Pd surface leads to the IRRAS bands caused not only by CO-Pd but also by CO-Cu, while the Cu-related band is almost absent from the spectra for the 0.3 nm-thick Pd surface. The results clearly reveal that local atomic structures of the outermost bimetallic surface can be discussed by the IRRAS spectra for the probe molecule.     

On the H-exchange of ammonia and silica hydroxyls in the presence of Rh nanoparticles

Attenuated total reflection infrared spectroscopy (ATR-FT-IR) is employed to study the H/D-exchange of planar hydroxylated silica during ND₃ and D₂ dosing, and catalyzed by Rh nanoparticles. For ND₃ dosing, it is observed that the H/D-exchange is about 10 times more efficient in the presence of Rh nanoparticles on the hydroxylated silica than for bare hydroxylated silica. When the Rh adsorption sites are blocked by CO, the H/D-exchange is similar to the case without Rh nanoparticles. No H/D-exchange is observed for exposure to D₂ regardless of the presence of Rh nanoparticles. Hydrogen spillover, involving the decomposition of D₂ on the Rh and subsequent transfer of atomic D to the oxide support, therefore does not explain the observed effects. Alternatively, we conjecture that for ND₃, the exchange is through a mechanism in which ND₃ adsorption on the edge of the Rh particles takes place, followed by direct H/D exchange with the hydroxyls of the support. This exchange is possibly aided by the formation of ammonium complexes with the help of hydrogen from the hydroxyls.     

Controlled synthesis and phase characterization of Fe-based nanoparticles obtained by thermal decomposition

Iron oxide nanoparticles were synthesized by the thermal decomposition of Fe(acac)₃ and Fe(CO)₅. Three different homogeneous procedures were used for the controlled synthesis of Fe₃O₄, γ-Fe₂O₃ and Fe₃O₄/γ-Fe₂O₃ mixture nanocrystals. A combination of characterization techniques was used in order to distinguish these oxides. The controllable size, the narrow distribution and the rhombic self-assembly of the nanoparticles were revealed by the high-resolution transmission electron microscopy images and the X-ray powder diffraction results. For the quantitative analysis of the samples manganometry was used. Preliminary magnetic measurements indicated the size and composition dependence of saturation magnetization, a superparamagnetic behavior of the samples and some ferromagnetic features.     

Phase coexistence in annealed CaMn7O12

The structural phase transition in annealed CaMn₇O₁₂ has been investigated by using high resolution synchrotron radiation powder diffraction. There is a phase coexistence phenomenon: two different crystallographic phases coexist in the material between 410 and 458 K. The first one is trigonal and it has a charge ordering (CO) of the Mn³⁺ and Mn⁴⁺ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from 22% at 418 K up to 100% at 468 K. Both phases have domains of at least 150 nm at each temperature. The annealing of CaMn₇O₁₂ relaxed a part of the strains in the lattice, but did not influence the phase coexistence phenomenon.     

Ab initio theory of complex electronic ground state of superconductors: I. Nonadiabatic modification of the Born-Oppenheimer approximation

The ARPES of high-T_c cuprates and theoretical results of low-Fermi energy band structure fluctuation for different groups of superconductors indicate that electron coupling to pertinent phonon modes drive system from adiabatic into anti-adiabatic state (ω>E_F). At these circumstances, not only Migdal-Eliashberg approximation is not valid, but basic adiabatic Born-Oppenheimer approximation (BOA) does not hold. At these circumstances, electronic structure has to be studied as explicitly dependent on instantaneous nuclear coordinates Q as well as on instantaneous nuclear momenta P.In the present paper—part I, it has been shown that Q, P-dependent modification of the BOA for ground electronic state can be derived by sequence of canonical transformations of the basis functions. The effect of nuclear coordinates and momenta on electronic structure is presented in the form of corrections to zero-, one- and two-particle terms of clamped nuclear Hamiltonian. In the anti-adiabatic state, correction to electronic ground state energy (zero-particle term correction) is negative and system can be stabilized in the anti-adiabatic state at distorted geometry with respect to adiabatic equilibrium structure and gap in one-particle spectrum of quasi-continuum states at Fermi level can be opened. Stabilization effect is solely the consequence of nuclear dynamics (P) that is crucial in anti-adiabatic state. It has been shown that nuclear dynamics also increases electron correlation until system at nuclear motion remains in a bound state. Corresponding corrections to electronic wave function are also specified.On the other hand, when system remains at vibration motion of nuclei in adiabatic state, the influence of nuclear dynamics (P-dependence) is negligible. In this case, all basic effects are covered through nuclear coordinates (Q-dependence) within the adiabatic BOA and standard results of solid-state (or molecular) physics are recovered.     

Photoluminescence and Raman studies of Xe ion-implanted diamonds: Dependence on implantation dose

We report on photoluminescence and Raman studies of Xe ion-implanted diamond. Several natural and high-purity artificial diamonds implanted within the wide dose range of 10¹⁰-5×10¹⁴ ion/cm² were studied. The room temperature luminescence of the Xe center consists of two zero phonon lines, at 813 nm (strong) and 794 nm (weak). The dose dependences of photoluminescence and Raman spectra were studied. For doses less than 10¹³ ion/cm², the luminescence intensity grows with the implantation dose linearly. The defect-induced photoluminescence quenching was observed for doses equal or more than 10¹³ ion/cm². Possible models of the Xe center will be discussed. The nature of damages induced by ion implantation at different doses was analyzed using micro-Raman spectroscopy.     

Competing phases in the cuprates: Charge vs spin order

We have investigated magnetic and charge instabilities of the cuprates within the Gutzwiller approximation RPA (GA+RPA). Interestingly, in GA the dressed Hubbard U is not a single parameter, but has different forms in the spin and charge responses, with distinct doping dependencies. While there are a number of competing magnetic instabilities for hole-doped cuprates, we fail to find any purely electronic charge density waves. The dominant magnetic instabilities are associated with ‘double nesting’, and the phase diagrams are material dependent, with LSCO differing from other cuprates.     

Anomalous isotopic effect near the charge-ordering quantum criticality

Within the Hubbard-Holstein model, we evaluate the crossover lines marking the opening of pseudogaps in the cuprates, which, in our scenario, are ruled by the proximity to a charge-ordering quantum criticality (stripe formation). We find that their isotopic dependence, due to critical fluctuations, implies a substantial positive shift of the pseudogap-formation temperature T(*). We infer that the isotopic shift of the superconducting T(c) is nearly absent in the optimally and overdoped regimes and is negative and increasing upon underdoping. The dynamical nature of the charge-ordering transition may explain the spread of the experimental values of T(*).     

Optical properties of Tm-doped GaSe single crystals

Optical properties of Tm-doped GaSe single crystals were investigated by measurements of optical absorption and photoluminescence. The single crystals were grown by the Bridgman technique. The X-ray diffraction analysis revealed that the single crystals were in the ε-type GaSe phase. The optical absorption spectra showed a sharp absorption peak at 582 nm near the band edge, which is due to direct free exciton. The temperature dependence of the energy of the exciton absorption peak was well fitted by the Varshni relation. In the photoluminescence spectrum at 10 K, we observed a very weak emission peak at 586 nm, a relatively strong emission peak centered at 613 nm, and several sharp and narrow emission peaks in the 790-840 nm region. The two emission peaks at 586 and 613 nm were associated with intrinsic emission lines due to direct free exciton and indirect bound exciton. The emission peaks in the 790-840 nm region, which were related to extrinsic emission, were assigned as due to the ³F₄→³H₆ transition of Tm³⁺ ions with a low symmetry of D₃ in the host lattice.     

The ground state and EPR spectrum in monoclinic KY(WO4)2:Nd single crystal

The electron paramagnetic resonance (EPR) of Nd³⁺ ion in KY(WO₄)₂ single crystal was investigated at T=4.2 K using an X-band spectrometer. The observed resonance absorption represents the complex superposition of three spectra corresponding to neodymium isotopes with different nuclear momenta. The EPR spectrum is characterized by a strong g-factor anisotropy. The temperature dependences of the g-factor were caused by strong spin-orbit and orbit-lattice coupling. The resonance lines become broader as temperature increases due to the short spin-lattice relaxation time.     

VUV spectroscopic properties of Ce and Pr-doped AREP2O7-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu)

Spectroscopic properties of Ce³⁺ and Pr³⁺-doped AREP₂O₇-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu) have been investigated using VUV spectroscopy technique. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 325-450 nm. The strong host absorption band starting at around 160 nm indicates that the optical band gap of AREP₂O₇ hosts is at least 7.7 eV, and the host→Ce³⁺ energy transfer process is rather efficient. However, AREP₂O₇:Pr³⁺ samples show less efficient host→Pr³⁺ energy transfer. The direct Pr³⁺ 4f²→4f¹5d¹ excitation, which are 12160±640 cm⁻¹ higher respect to that of Ce³⁺, leads to strong 4f¹5d¹→4f² emission bands in the range of 230-325 nm but no obvious 4f²→4f² emission lines.