Atomic-scale coexistence of short-range magnetic order and superconductivity in Fe1+y_{1+y}Se0.1_{0.1}Te0.9_{0.9}

The ground state of the parent compounds of many high temperature superconductors is an antiferromagnetically (AFM) ordered phase, where superconductivity emerges when the AFM phase transition is suppressed by doping or application of pressure. This behaviour implies a close relation between the two orders. Understanding the interplay between them promises a better understanding of how the superconducting condensate forms from the AFM ordered background. Here we explore this relation in real space at the atomic scale using low temperature spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy. We investigate the transition from antiferromagnetically ordered $\mathrm{Fe}_{1+y}\mathrm{Te}$ via the spin glass phase in $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.1}\mathrm{Te}_{0.9}$ to superconducting $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.15}\mathrm{Te}_{0.85}$. In $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.1}\mathrm{Te}_{0.9}$ we observe an atomic-scale coexistence of superconductivity and short-ranged bicollinear antiferromagnetic order.Comment: 7 pages, 6 figure

Macroscopic studies of short-pulse high-order harmonic generation using the time-dependent Schrödinger equation

We consider high harmonic generation by ultrashort (27–108 fs) laser pulses and calculate the macroscopic response of a collection of atoms to such a short pulse. We show how the harmonic spectrum after propagation through the medium is significantly different from the single-atom spectrum. We use single-atom data calculated by integration of the time-dependent Schrödinger equation and propose a method, based on an adiabatic approximation, to extract the data necessary to perform a propagation calculation. © 1998 The American Physical Society

Three-dimensional Maxwell-Bloch calculation of the temporal profile of a seeded soft x-ray laser pulse

International audienceWe present three-dimensional modeling of amplification of a high-order harmonic seed by a soft x-ray laser plasma. The time-dependent evolution of the x-ray signal is determined from a fully dynamic Maxwell-Bloch calculation. At high seed intensities , a simplified one-dimensional calculation leads to strong Rabi-like temporal oscillations of the output signal. However, such oscillations have not been observed experimentally. Our three-dimensional calculations demonstrate that this is due to spatial non-uniformities in the plasma gain that cause the Rabi oscillations to dampen dramatically. Large amplitude Rabi-like oscillations are expected to appear only in long and uniform plasma. Such targets require optimized guiding techniques

High-order harmonic generation with a strong laser field and an attosecond-pulse train: the Dirac Delta comb and monochromatic limits

In recent publications, it has been shown that high-order harmonic generation can be manipulated by employing a time-delayed attosecond pulse train superposed to a strong, near-infrared laser field. It is an open question, however, which is the most adequate way to approximate the attosecond pulse train in a semi-analytic framework. Employing the Strong-Field Approximation and saddle-point methods, we make a detailed assessment of the spectra obtained by modeling the attosecond pulse train by either a monochromatic wave or a Dirac-Delta comb. These are the two extreme limits of a real train, which is composed by a finite set of harmonics. Specifically, in the monochromatic limit, we find the downhill and uphill sets of orbits reported in the literature, and analyze their influence on the high-harmonic spectra. We show that, in principle, the downhill trajectories lead to stronger harmonics, and pronounced enhancements in the low-plateau region. These features are analyzed in terms of quantum interference effects between pairs of quantum orbits, and compared to those obtained in the Dirac-Delta limit.Comment: 10 pages, 7 figures (eps files). To appear in Laser Physic

Determination of the cation site distribution of the spinel in multiferroic CoFe2O4 / BaTiO3 layers by X-ray photoelectron spectroscopy

International audienceThe properties of CoFe2O4/BaTiO3 artificial multiferroic multilayers strongly depend on the crystalline structure, the stoichiometry and the cation distribution between octahedral (Oh) and tetrahedral (Td) sites (inversion factor). In the present study, we have investigated epitaxial CoFe2O4 layers grown on BaTiO3, with different Co/Fe ratios. We determined the cation distribution in our samples by X-ray magnetic circular dichroism (XMCD), a well accepted method to do so, and by X-ray photoelectron spectroscopy (XPS), using a fitting method based on physical considerations. We observed that our XPS approach converged on results consistent with XMCD measurements made on the same samples. Thus, within a careful decomposition based on individual chemical environments it is shown that XPS is fully able to determine the actual inversion factor

Attosecond control of electrons emitted from a nanoscale metal tip

Attosecond science is based on steering of electrons with the electric field of well-controlled femtosecond laser pulses. It has led to, for example, the generation of XUV light pulses with a duration in the sub-100-attosecond regime, to the measurement of intra-molecular dynamics by diffraction of an electron taken from the molecule under scrutiny, and to novel ultrafast electron holography. All these effects have been observed with atoms or molecules in the gas phase. Although predicted to occur, a strong light-phase sensitivity of electrons liberated by few-cycle laser pulses from solids has hitherto been elusive. Here we show a carrier-envelope (C-E) phase-dependent current modulation of up to 100% recorded in spectra of electrons laser-emitted from a nanometric tungsten tip. Controlled by the C-E phase, electrons originate from either one or two sub-500as long instances within the 6-fs laser pulse, leading to the presence or absence of spectral interference. We also show that coherent elastic re-scattering of liberated electrons takes place at the metal surface. Due to field enhancement at the tip, a simple laser oscillator suffices to reach the required peak electric field strengths, allowing attosecond science experiments to be performed at the 100-Megahertz repetition rate level and rendering complex amplified laser systems dispensable. Practically, this work represents a simple, exquisitely sensitive C-E phase sensor device, which can be shrunk in volume down to ~ 1cm3. The results indicate that the above-mentioned novel attosecond science techniques developed with and for atoms and molecules can also be employed with solids. In particular, we foresee sub-femtosecond (sub-) nanometre probing of (collective) electron dynamics, such as plasmon polaritons, in solid-state systems ranging in size from mesoscopic solids via clusters to single protruding atoms.Comment: Final manuscript version submitted to Natur

Solution of the two impurity, two channel Kondo Model

We solve the two-impurity two-channel Kondo model using a combination of conformal invariance and bosonisation techniques. The odd-even symmetric case is analysed in detail. The RKKY interaction turns out to be exactly marginal, resulting in a line of non-Fermi liquid fixed points. Explicit formulae are given for the critical exponents and for the finite-size spectrum, which depend continuously on a single parameter. The marginal line spans a range of values of the RKKY coupling $I$ which goes from the infinitely strong ferromagnetic point $I=-\infty$ (associated with a 4-channel spin-1 Kondo model) to a finite antiferromagnetic critical value $I_c>0$ beyond which a Fermi liquid is recovered. We also find that, when the odd-even symmetry is broken, the marginal line is unstable for ferromagnetic $I$, while for antiferromagnetic $I$ it extends into a manifold of fixed points.Comment: 9 pages, preprint LPTENS 94/1

Nucleation of a sodium droplet on C60

We investigate theoretically the progressive coating of C60 by several sodium atoms. Density functional calculations using a nonlocal functional are performed for NaC60 and Na2C60 in various configurations. These data are used to construct an empirical atomistic model in order to treat larger sizes in a statistical and dynamical context. Fluctuating charges are incorporated to account for charge transfer between sodium and carbon atoms. By performing systematic global optimization in the size range 1<=n<=30, we find that Na_nC60 is homogeneously coated at small sizes, and that a growing droplet is formed above n=>8. The separate effects of single ionization and thermalization are also considered, as well as the changes due to a strong external electric field. The present results are discussed in the light of various experimental data.Comment: 17 pages, 10 figure

Broadband, Polarization-Sensitive Photodetector Based on Optically-Thick Films of Macroscopically Long, Dense, and Aligned Carbon Nanotubes

Increasing performance demands on photodetectors and solar cells require the development of entirely new materials and technological approaches.Wereport on the fabrication and optoelectronic characterization of a photodetector based on optically-thick films of dense, aligned, and macroscopically long single-wall carbon nanotubes. The photodetector exhibits broadband response from the visible to the mid-infrared under global illumination, with a response time less than 32 ms. Scanning photocurrent microscopy indicates that the signal originates at the contact edges, with an amplitude and width that can be tailored by choosing different contact metals. A theoretical model demonstrates the photothermoelectric origin of the photoresponse due to gradients in the nanotube Seebeck coefficient near the contacts. The experimental and theoretical results open a new path for the realization of optoelectronic devices based on three-dimensionally organized nanotubes