Granularity-induced gapless superconductivity in NbN films: evidence of thermal phase fluctuations

Using a single coil mutual inductance technique, we measure the low temperature dependence of the magnetic penetration depth in superconducting NbN films prepared with similar critical temperatures around 16 K but with different microstructures. Only (100) epitaxial and weakly granular (100) textured films display the characteristic exponential dependence of conventional BCS s-wave superconductors. More granular (111) textured films exhibit a linear dependence, indicating a gapless state in spite of the s-wave gap. This result is quantitatively explained by a model of thermal phase fluctuations favored by the granular structure.Comment: 10 pages, 4 figures, to appear in Phys. Rev.

Theory for Electron-Doped Cuprate Superconductors: d-wave symmetry order parameter

Using as a model the Hubbard Hamiltonian we determine various basic properties of electron-doped cuprate superconductors like ${Nd}_{2-x}{Ce}_{x}{CuO}_{4}$ and ${Pr}_{2-x}{Ce}_{x}{CuO}_{4}$ for a spin-fluctuation-induced pairing mechanism. Most importantly we find a narrow range of superconductivity and like for hole-doped cuprates $d_{x^{2}-y^{2}}$ - symmetry for the superconducting order parameter. The superconducting transition temperatures $T_{c}(x)$ for various electron doping concentrations $x$ are calculated to be much smaller than for hole-doped cuprates due to the different Fermi surface and a flat band well below the Fermi level. Lattice disorder may sensitively distort the symmetry $d_{x^{2}-y^{2}}$ via electron-phonon interaction

Angle-resolved photoemission study of quasi-one-dimensional superconductor β\beta-Na0.33_{0.33}V2_2O5_5

We have studied the electronic structure of $\beta$-Na$_{0.33}$V$_2$O$_5$, which becomes a superconductor under pressure, by angle-resolved photoemission spectroscopy. Clear band dispersions is observed only along the chain direction, indicating the quasi-one-dimensional (1D) electronic structure. The spectra of the V 3d band are dominated by a Gaussian-like broad feature at $\sim$ 1 eV below the Fermi level ($E_F$) with negligible intensity at $E_F$, which we attribute to strong electron-phonon coupling as in other 1D polaronic metals. In the momentum space, the spectra show a maximum intensity at $k \sim \pm \pi/4b$, where $b$ is the V-V distance along the b-axis, reflecting the band filling of the chain and/or ladder.Comment: 4 pages, 4 figure

Bipolarons and superconductivity

The problem of deformation induced attraction leading to localized pairs of electrons, the so-called bipolarons, is studied in detail. The analogy between bipolarons and itinerant Cooper pairs in a superconductor is explored. It is shown that the two types of pairing result from the microscopic nature of the electron-phonon Hamiltonian and that following the coupling constant the ground-state becomes a bipolaronic insulator beyond a critical coupling strength. A phase diagramme between a metal, superconductor and bipolaronic insulator is proposed.Nous étudions de façon détaillée le problème de l'attraction électronique induite par une déformation entraînant une paire d'électrons localisés, appelée « bipolaron ». Nous explorons l'analogie entre les bipolarons et les paires de Cooper itinérantes dans un supraconducteur. Nous montrons que les deux types de paires résultent de la nature microscopique de l'interaction électron-phonon et que suivant la constante de couplage, l'état fondamental devient un isolant bipolaronique au-delà d'une certaine valeur critique. Nous proposons un diagramme de phase entre un métal, un supraconducteur et un isolant bipolaronique

IMPERFECT AMORPHOUS SOLID AND BIPOLARONIC GROUND-STATE

It is generally considered that the Polk-model of an infinite random network of perfectly bonded amorphous solid does not explain the "real" amorphous silicon which contains dangling bonds, vacancies, voids and fine scale inhomogeneities. The question naturally arises as to the nature of the predominant defect in α-Si that would explain possible Fermi-level pinning and the recombination characteristics. Presuming an isolated dangling bond (T03) to be such a defect, Adler-Elliott have postulated negative U centers on such defects to explain Fermi-level pinning and other properties. Recent experiments as well as detailed calculations do not however support the existence of negative U in α-silicon. In this communication I like to point out that basic defect "unit" is not an isolated dangling bond, but a dangling bond cluster of which the simplest unit is a vacancy center V02 that has only two near-neighbours i.e. a center with two dangling bonds that can locally deform to give bonding. This stable bonded V02 center is a bipolaron and provides all the ingredients to understand a variety of two-electron states phenomena in α-Si. The neutral centers will be deep in the gap. The two related charge states V+2 and V-2 with respectively one and three electrons will also be gap states and are responsible for light induced e.s.r. signal. The V02 centers can trap two hydrogen atoms to give (SiH)2 complexes in contrast to Si-H bonds. A doubly ionised V02 center (V++2) can trap two electrons to go into a metastable He-like configuration and can explain Staebler-Wronski effect. α-Si in general will contain both V02 centers as well as isolated dangling bonds (T03) whose mutual interaction can explain a variety of radiative and non-radiative phenomena. The neutral centers will tend to empty nearby T03 centers, will cause light-induced e.s.r. quenching of the T03 centers. They will be also responsible for Staebler-Wronski effect as well as for quenching of luminescence and photoconductivity

Possibility of insulator to superconductor phase transition

A phase diagram is postulated where the ground-state of a strongly coupled electron-phonon system is a bipolaronic insulator beyond a critical coupling strength. This opens up the possibility of phase transition from a superconductor to insulator at T = 0, as the coupling is varied.On propose qu'un système avec un fort couplage électrons-phonons est un isolant bipolaronique au-delà d'une constante de couplage critique. Ceci donne la possibilité d'une transition de phase d'un état supraconducteur à l'état isolant à T = 0 quand on varie la constante de couplage

Frustration and (111) silicon surface

Careful analysis of the currently available experimental data on (111) surface of silicon leads us to believe that a positive correlation energy and positive dispersion are responsible for a split surface band of dangling electrons. The resultant ground-state is paramagnetic and insulating but cannot order antiferromagnetically because of the pathological frustration of the triangular lattice. Instead a ground-state results of two correlated zig-zag Heisenberg chains of S = 1/2 electrons for the unconstructed (1×1) surface. Surface reconstruction lifts the degeneracy between the two chains, stabilizes the well-known (2×1) surface and gives it a pronounced one-dimensional character.Nous montrons que toutes les données expérimentales actuelles sur la surface (111) de Si sont consistantes avec une bande d'énergie des liaisons pendantes ayant une dispersion positive et un dédoublement à cause de l'énergie de corrélation. L'état isolant qui résulte est un état paramagnétique à cause de la frustration pathologique du réseau triangulaire. En l'absence d'ordre antiferromagnétique, l'état fondamental est deux chaînes « zig-zag » de Heisenberg pour la surface (1×1) non reconstruite. La reconstruction en enlevant la dégénérescence entre les deux chaines stabilise la surface (2×1), et lui donne un caractère profondément unidimensionnel

ON THE EXISTENCE OF BIPOLARONS IN Ti4O7

Nous avons établi dans des études antérieures que les phases basse température (T < 130 K) et intermédiaire (130 K < T < 150 K) de Ti4O7 contiennent des paires Ti3+-Ti3+ On peut considérer que ces paires sont des bipolarons gelés et ordonnés sur un réseau pour T < 130 K, désordonnés et mobiles comme dans un liquide de bipolarons pour 130 K < T < 150 K. Nous montrons maintenant que dans un système comme Ti4O7 avec un électron 3d par deux cations, le réseau de bipolarons correspondant à un arrangement ordonné de paires de Ti3+ et d'ions Ti4+ est stabilisé à la fois par la répulsion coulombienne intersite entre premiers voisins et par les couplages électrons-phonons. Ces deux mécanismes peuvent conduire à une interaction effective intersite site Veff négative, qui est analogue à la répulsion coulombienne intra-atomique Ueff négative, récemment proposée par P.W. Anderson. Nous suggérons aussi, que bien que le mouvement des bipolarons par l'intermédiaire d'un état activé correspondant à la moitié de leur énergie de liaison ne soit pas tsès probable, il existe un autre mécanisme de transport par l'intermédiaire d'un état excité où les bipolarons sont dissociés en deux polarons séparés. Nous discutons nos résultats de relaxation RPE de centres Ti3+ et de V4+ dans (Ti1-xVx)4O7 en fonction de ce modèle.We have previously established that Ti3+-Ti3+ pairs are formed in both the low (T < 130 K) and the intermediate (130 K < T < 150 K) temperature phases of Ti4O7. These pairs may be viewed as bipolarons frozen and ordered on a lattice below 130 K, disordered and mobile for 130 K < T < 150 K, as in a bipolaron liquid. We now show that in a system such as Ti4O7 with one 3d electron per two cations, the bipolaron lattice with an ordered arrangement of Ti3+ pairs and Ti4+ ions, is stabilized through both the nearest neighbour intersite Coulomb repulsion V and the electron-phonon coupling. These two mechanisms may lead to a negative effective intersite interaction Veff, hich has a strong analogy with the negative effective intraatomic Coulomb repulsion Ueff recently proposed by P.W. Anderson. We also suggest that, if the motion of bipolarons through an activated state corresponding to half their binding energy is not very likely, another process through an excited state where the bipolarons are dissociated into two separated single polarons should provide an easier transport. We discuss our EPR data of relaxation of Ti3+ centers and of V4+ centers in (Ti1-xVx)4O7 the scope of this model

Chemisorption of indium on (111) silicon by a mass spectrometric technique

Kinetics of chemisorption of Indium on (111) Silicon was measured, in ultra-high vacuum, by a mass-spectrometric technique, where the desorption signal was monitored. A coverage independent chemisorption energy of 57 K.Cal/mole was obtained upto 7 × 1014 atom/cm2. A localized resonating covalent bond between the metal and the substrate provides a reasonably simple explanation of the observed chemisorption energy.La cinétique de la chimisorption de l'Indium sur du Silicium (111) a été mesurée, sous ultra-vide, par une technique de spectrométrie de masse, dans laquelle le signal de désorption a été mesuré. Une énergie de chimisorption indépendante de couverture de 57 K . cal/mole a été obtenue jusqu'à 7 × 10 14 atome/cm2. Une liaison de résonance covalente et localisée entre le métal et le substrat fournit une explication simple et raisonnable de l'énergie de chimisorption observée