A. Sacuto

Two energy scales and two distinct quasiparticle dynamics in the superconducting state of underdoped cuprates

The superconducting temperature Tc of hole-doped high-temperature superconductors has a dome-like shape as a function of hole concentration, with a maximum Tc at ‘optimal’ doping. On the underdoped side, the superconducting state is often described in terms of one energy scale, associated with the maximum of the d-wave gap (at the antinodes), which increases as the doping decreases. Here, we report electronic Raman scattering experiments that show a second energy scale in the gap function: the slope of the gap at the nodes, which decreases with decreasing doping. Our measurements also reveal two distinct quasiparticle dynamics; electronic coherence persists down to low doping levels at the nodes, whereas antinodal quasiparticles become incoherent. Using a sum-rule, we find that the low-frequency Raman response and the temperature dependence of the superfluid density, both controlled by nodal excitations, behave in a qualitatively similar manner with doping variation.

Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain.

Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property--antiferromagnetism--has not been investigated extensively, especially in thin films. Here we bring insight into the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to tune the exchange bias and giant-magnetoresistive response of spin valves.

Electric-field control of spin waves at room temperature in multiferroic BiFeO3

To face the challenges lying beyond present technologies based on complementary metal-oxide-semiconductors, new paradigms for information processing are required. Magnonics proposes to use spin waves to carry and process information, in analogy with photonics that relies on light waves, with several advantageous features such as potential operation in the terahertz range and excellent coupling to spintronics. Several magnonic analog and digital logic devices have been proposed, and some demonstrated. Just as for spintronics, a key issue for magnonics is the large power required to control/write information (conventionally achieved through magnetic fields applied by strip lines, or by spin transfer from large spin-polarized currents). Here we show that in BiFeO(3), a room-temperature magnetoelectric material, the spin-wave frequency (>600 GHz) can be tuned electrically by over 30%, in a non-volatile way and with virtually no power dissipation. Theoretical calculations indicate that this effect originates from a linear magnetoelectric effect related to spin-orbit coupling induced by the applied electric field. We argue that these properties make BiFeO(3) a promising medium for spin-wave generation, conversion and control in future magnonics architectures.

Possible observation of cycloidal electromagnons in BiFeO3.

Recently, oxide multiferroics have attracted much attention due to their large magnetoelectric effect which allows the tuning of magnetic properties with electric field and vice versa and open new venues for future spintronic applications such as multiple-state memory devices with dual magnetic and electric control. BiFeO3 (BFO) belongs to this new class of materials and shows both ferroelectric and antiferromagnetic orders at room temperature with a large electric polarization associated with a cycloidal spiral magnetic ordering. The incommensurate magnetic order induces magnon zone folding and allows investigations by optical probes of unusual spin waves which couples to optical phonons, the so called ”‘electromagnons”’. Here, we unravel for the first time the electromagnon spectra of BFO by means low energy inelastic light scattering technique. We show the existence of two species of electromagnons corresponding to spin wave excitations in and out of the cycloidal plane. The present observations present an unique opportunity to study the interplay between ferroelectric and magnetic orders.

Observation of incipient charge nematicity in Ba(Fe(1-x)Co(x))2As2.

Using electronic Raman spectroscopy, we report direct measurements of charge nematic fluctuations in the tetragonal phase of strain-free Ba(Fe(1-x)Co(x))2As2 single crystals. The strong enhancement of the Raman response at low temperatures unveils an underlying charge nematic state that extends to superconducting compositions and which has hitherto remained unnoticed. Comparison between the extracted charge nematic susceptibility and the elastic modulus allows us to disentangle the charge contribution to the nematic instability, and to show that charge nematic fluctuations are weakly coupled to the lattice.

Polar phonons and spin excitations coupling in multiferroic BiFeO 3 crystals

Raman scattering measurements on BiFeO3 single crystals show an important coupling between the magnetic order and lattice vibrations. The temperature evolution of phonons shows that the lowest energy E and A1 phonon modes are coupled to the spin order up to the Neel temperature. Furthermore, low temperature anomalies associated with the spin re-orientation are observed simultaneously in both the E phonon and the magnon. These results suggest that magnetostriction plays an important role in BiFeO3.

Raman-scattering measurements and theory of the energy-momentum spectrum for underdoped Bi2Sr2CaCuO(8+δ) superconductors: evidence of an s-wave structure for the pseudogap.

S. Sakai , S. Blanc, M. Civelli, Y. Gallais, M. Cazayous , M.-A. Méasson, J. S. Wen, Z. J. Xu, G. D. Gu, G. Sangiovanni , Y. Motome, K. Held, A. Sacuto, A. Georges , and M. Imada Centre de Physique Théorique, École Polytechnique, CNRS, 91128 Palaiseau, France, Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8656, Japan, JST-CREST, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan, Laboratoire Matériaux et Phénom̀ enes Quantiques (UMR 7162 CNRS), Université Paris Diderot-Paris 7, Bat. Condorcet, 75205 P aris Cedex 13, France, Laboratoire de Physique des Solides, Université Paris-Su d, CNRS, UMR 8502, F-91405 Orsay Cedex, France, Matter Physics and Materials Science, Brookhaven National Laboratory (BNL), Upton, NY 11973, USA, Institut für Theoretische Physik und Astrophysik, Univer sität Würzburg, Am Hubland, D-97074 Würzburg, Germany, Institute for Solid State Physics, Vienna University of Tec hnology, 1040 Vienna, Austria, Collège de France, 11 place Marcelin Berthelot, 75005 Pari s, France, DPMC, Université de Genève, 24 quai Ernest Ansermet, CH-1 211 Genève, Suisse (Dated: May 2, 2014)

Doping dependence of the lattice dynamics in Ba(Fe1-xCox)2As2 studied by Raman spectroscopy

We report Raman scattering spectra of iron-pnictide superconductor Ba(Fe

Intrinsic low temperature paramagnetism in B-DNA.

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Collapse of the normal-state pseudogap at a Lifshitz transition in the Bi(2)Sr(2)CaCu(2)O(8+δ) cuprate superconductor.

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