55 research outputs found
Low-energy spin excitations in optimally doped CaFeCoAsF superconductor studied with inelastic neutron scattering
There are few inelastic neutron scattering (INS) reports on the
superconducting single crystals of FeAs-1111 system, even though it was first
discovered in 2008, due to the extreme difficulty in large single crystal
growth. In this paper, we have studied the low-energy spin excitations in the
optimally electron-doped CaFeCoAsF single crystals with
= 21 K by INS. The resonance energy of the superconducting spin
resonant mode with = 12 meV amounts to 6.6
, which constitutes the largest
/ ratio among iron-based
superconductors reported to date. The large ratio implies a strong coupling
between conduction electrons and magnetic excitations in
CaFeCoAsF. The resonance possesses a magnonlike upward
dispersion along transverse direction due to the anisotropy of spin-spin
correlation length within plane in the normal-state, which points to a
spin fluctuation mediated sign-reversed wave pairing in
CaFeCoAsF
Reduced phase space of heat-carrying acoustic phonons in single-crystalline InTe
Chalcogenide semiconductors and semimetals are a fertile class of efficient thermoelectric materials, which, in most cases, exhibit very low lattice thermal conductivity κph despite lacking a complex crystal structure such as the tetragonal binary compound InTe. Our measurements of κph(T) in single-crystalline InTe along the c axis show that κph exhibits a smooth temperature dependence upon cooling to about 50 K, the temperature below which a strong rise typical for dielectric compounds is observed. Using a combination of first-principles calculations, inelastic neutron scattering (INS), and low-temperature specific heat and transport properties measurements on single-crystalline InTe, we show that the phonon spectrum exhibits well-defined acoustic modes, the energy dispersions of which are constrained to low energies due to distributions of dispersionless, optical modes, which are responsible for a broad double peak structure in the low-temperature specific heat. The latter are assigned to the dynamics of In+ cations in tunnels formed by edge-sharing (In3+Te42−)− tetrahedra chains, the atomic thermal displacement parameters of which, probed as a function of temperature by means of single-crystal x-ray diffraction, suggest the existence of a complex energy potential. Indeed, the In+-weighted optical modes are not observed by INS, which is ascribed to the anharmonic broadening of their energy profiles. While the low κph value of 1.2Wm−1K−1 at 300 K originates from the limited energy range available for acoustic phonons, we show that the underlying mechanism is specific to InTe and argue that it is likely related to the presence of local disorder induced by the In+ sit
Inelastic neutron scattering study of spin excitations in the superconducting state of high temperature superconductors
Large Anomalous Hall effect in a silicon-based magnetic semiconductor
Magnetic semiconductors are attracting high interest because of their
potential use for spintronics, a new technology which merges electronics and
manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently
emerged as the most popular materials for this new technology. While Curie
temperatures are rising towards room temperature, these materials can only be
fabricated in thin film form, are heavily defective, and are not obviously
compatible with Si. We show here that it is productive to consider transition
metal monosilicides as potential alternatives. In particular, we report the
discovery that the bulk metallic magnets derived from doping the narrow gap
insulator FeSi with Co share the very high anomalous Hall conductance of
(GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens
up a new arena for spintronics, involving a bulk material based only on
transition metals and Si, and which we have proven to display a variety of
large magnetic field effects on easily measured electrical properties.Comment: 19 pages with 5 figure
Atomic dynamics of the i-ScZnMg and its 1/1 approximant phase: experiment and simulation
International audienceQuasicrystals are long range ordered materials which lack translational invariance so that the study of their physical properties remains a challenging problem. In order to study the respective influence of the local order and of the long range order (periodic or quasiperiodic) on lattice dynamics, we have carried out inelastic x-ray and neutron scattering experiments on single grain samples of the Zn-Mg-Sc icosahedral quasicrystal and of the Zn-Sc periodic cubic 1/1 approximant. Besides the overall similarities and the existence of a pseudo gap in the transverse dispersion relation, marked differences are observed, the pseudo gap being larger and better defined in the approximant than in the quasicrystal. This can be qualitatively explained using the concept of pseudo Brillouin zone in the quasicrystal. These results are compared to simulations on atomic models and using oscillating pair potentials which have been fitted against ab-initio data. The simulated response function reproduces both the dispersion relation but also the observed intensity distribution in the measured spectra. The partial vibrational density of states, projected on the cluster shells, is computed from this model
Magnétisme de bande et Supraconductivité : l'apport de la diffusion inélastique des neutrons
La technique de diffusion des neutrons permet d'étudier les corrélations magnétiques statiques et dynamiques sur une large gamme d'énergies et de vecteurs d'onde dans l'espace des phases. Elle offre une mesure directe de la partie imaginaire de la susceptibilité magnétique généralisé qui contient toutes les informations relatives aux excitations magnétiques d'un système. Dans un métal, les propriétés magnétiques sont principalement véhiculés par les spins des électrons itinérants. Nous décrirons comment la susceptibilité magnétiques est déterminé à partir des propriétés d'un liquide d'électrons, puis comment les excitations magnétiques rétro-agissent sur les propriétés des électrons. En passant d'un état métallique à un état supraconducteur, les électrons s'apparient pour former des paires d'électrons condensés dans un état quantique macroscopique. Aux électrons, se substituent de nouvelles quasiparticules. Nous décrirons comment la susceptibilité magnétique est modifiée et pour quelle raison de nouvelles excitations collectives peuvent apparaitre. Nous insisterons sur le fait que les électrons engendrent des excitations magnétiques qu'ils peuvent ensuite absorber, émettre ou encore (théoriquement) s'échanger pour induire une interaction attractive effective conduisant à la formation de paires d'électrons supraconductrices. Une fois ces concepts présentés, nous donnerons quelques exemples de la mesure de la partie imaginaire de la susceptibilité magnétique par diffusion inélastique des neutrons dans un métal normal, puis dans des matériaux supraconducteurs. Nous conclurons par quelques remarques d'ordre général permettant au lecteur de s'ouvrir à des problématiques que nous n'aurons pas abordés
Les supraconducteurs à base de fer
Vingt-deux ans après la découverte de la supraconductivité dans des oxydes de cuivre, une
deuxième famille de matériaux supraconducteurs à haute température critique, cette fois-ci
à base de fer, a été mise au jour. Plus qu’une heureuse surprise, cette nouvelle
découverte prouve qu’il y a différentes voies pour atteindre des températures critiques
élevées.
Les applications de ces pnictures et chalcogénures de fer demeurent pour l’instant
confidentielles. Mais l’origine de leur supraconductivité, potentiellement différente de
celle des cuprates, souligne la grande fécondité de la physique des solides, et constitue
un nouveau défi de taille pour la théorie
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