arXiv subjects

V. Brouet

Publications and source records attributed to V. Brouet.

At least 37 records · Page 2Linked to original sources

arXiv2010

Significant reduction of electronic correlations upon isovalent Ru substitution of BaFe2As2

We present a detailed investigation of Ba(Fe0.65Ru0.35)2As2 by transport measurements and Angle Resolved photoemission spectroscopy. We observe that Fe and Ru orbitals hybridize to form a coherent electronic structure and that Ru does not induce doping. The number of holes and electrons, deduced from the area of the Fermi Surface pockets, are both about twice larger than in BaFe2As2. The contribution of both carriers to the transport is evidenced by a change of sign of the Hall coefficient with decreasing temperature. Fermi velocities increase significantly with respect to BaFe2As2, suggesting a significant reduction of correlation effects. This may be a key to understand the appearance of superconductivity at the expense of magnetism in undoped iron pnictides.

cond-mat.supr-con↗

arXiv2010

Experimental study of the incoherent spectral weight in the photoemission spectra of the misfit cobaltate [Bi2Ba2O4][CoO2]2

Previous ARPES experiments in NaxCoO2 reported both a strongly renormalized bandwidth near the Fermi level and moderately renormalized Fermi velocities, leaving it unclear whether the correlations are weak or strong and how they could be quantified. We explain why this situation occurs and solve the problem by extracting clearly the coherent and incoherent parts of the band crossing the Fermi level. We show that one can use their relative weight to estimate self-consistently the quasiparticle weight Z, which turns out to be very small Z=0.15 +/- 0.05. We suggest this method could be a reliable way to study the evolution of correlations in cobaltates and for comparison with other strongly correlated systems.

cond-mat.str-el↗

arXiv2010

New electronic orderings observed in cobaltates under the influence of misfit periodicities

We study with ARPES the electronic structure of CoO2 slabs, stacked with rock-salt (RS) layers exhibiting a different (misfit) periodicity. Fermi Surfaces (FS) in phases with different doping and/or periodicities reveal the influence of the RS potential on the electronic structure. We show that these RS potentials are well ordered, even in incommensurate phases, where STM images reveal broad stripes with width as large as 80Å. The anomalous evolution of the FS area at low dopings is consistent with the localization of a fraction of the electrons. We propose that this is a new form of electronic ordering, induced by the potential of the stacked layers (RS or Na in NaxCoO2) when the FS becomes smaller than the Brillouin Zone of the stacked structure.

cond-mat.str-el↗

arXiv2009

Nesting between hole and electron pockets in Ba(Fe1-xCox)2As2 (x=0-0.3) observed with angle-resolved photoemission

We present a comprehensive angle-resolved photoemission study of the three-dimensional electronic structure of Ba(Fe1-xCox)2As2. The wide range of dopings covered by this study, x=0 to x=0.3, allows to extract systematic features of the electronic structure. We show that there are three different hole pockets around the G point, the two inner ones being nearly degenerate and rather two dimensional, the outer one presenting a strong three dimensional character. The structure of the electron pockets is clarified by studying high doping contents, where they are enlarged. They are found to be essentially circular and two dimensional. From the size of the pockets, we deduce the number of holes and electrons present at the various dopings. We find that the net number of carriers is in good agreement with the bulk stoichiometry, but that the number of each species (holes and electrons) is smaller than predicted by theory. Finally, we discuss the quality of nesting in the different regions of the phase diagram. The presence of the third hole pocket significantly weakens the nesting at x=0, so that it may not be a crucial ingredient in the formation of the Spin Density Wave. On the other hand, superconductivity seems to be favored by the coexistence of two-dimensional hole and electron pockets of similar sizes.

cond-mat.str-el↗

arXiv2009

Valence band electronic structure of V2O3: identification of V and O bands

We present a comprehensive study of the photon energy dependence of the valence band photoemission yield in the prototype Mott-Hubbard oxide V2O3. The analysis of our experimental results, covering an extended photon energy range (20-6000 eV) and combined with GW calculations, allow us to identify the nature of the orbitals contributing to the total spectral weight at different binding energies, and in particular to locate the V 4s at about 8 eV binding energy. From this comparative analysis we can conclude that the intensity of the quasiparticle photoemission peak, observed close to the Fermi level in the paramagnetic metallic phase upon increasing photon energy, does not have a significant correlation with the intensity variation of the O 2p and V 3d yield, thus confirming that bulk sensitivity is an essential requirement for the detection of this coherent low energy excitation.

cond-mat.str-el↗

arXiv2008

Fermi Surface Evolution Across Multiple Charge Density Wave Transitions in ErTe3

The Fermi surface (FS) of ErTe3 is investigated using angle-resolved photoemission spectroscopy (ARPES). Low temperature measurements reveal two incommensurate charge density wave (CDW) gaps created by perpendicular FS nesting vectors. A large Delta_1 = 175 meV gap arising from a CDW with c* - q_CDW1 ~ 0.70(0) c* is in good agreement with the expected value. A second, smaller Delta_2 = 50 meV gap is due to a second CDW with a* - q_CDW2 ~ 0.68(5) a*. The temperature dependence of the FS, the two gaps and possible interaction between the CDWs are examined.

cond-mat.str-el↗

arXiv2008

ARPES Study of the Evolution of Band Structure and Charge Density Wave Properties in RTe3 for R = Y, La, Ce, Sm, Gd, Tb and Dy

We present a detailed ARPES investigation of the RTe3 family, which sets this system as an ideal "textbook" example for the formation of a nesting driven Charge Density Wave (CDW). This family indeed exhibits the full range of phenomena that can be associated to CDW instabilities, from the opening of large gaps on the best nested parts of Fermi Surface (FS) (up to 0.4eV), to the existence of residual metallic pockets. ARPES is the best suited technique to characterize these features, thanks to its unique ability to resolve the electronic structure in k-space. An additional advantage of RTe3 is that the band structure can be very accurately described by a simple 2D tight-binding (TB) model, which allows one to understand and easily reproduce many characteristics of the CDW. In this paper, we first establish the main features of the electronic structure, by comparing our ARPES measurements with Linear Muffin-Tin Orbital band calculations. We use this to define the validity and limits of the TB model. We then present a complete description of the CDW properties and, for the first time, of their strong evolution as a function of R. Using simple models, we are able to reproduce perfectly the evolution of gaps in k-space, the evolution of the CDW wave vector with R and the shape of the residual metallic pockets. Finally, we give an estimation of the CDW interaction parameters and find that the change in the electronic density of states n(Ef), due to lattice expansion when different R ions are inserted, has the correct order of magnitude to explain the evolution of the CDW properties.

cond-mat.str-el↗

arXiv2007

Direct observation of strong correlations near the band insulator regime of Bi misfit cobaltates

We present the first angle-resolved photoemission (ARPES) measurement of Fermi Surface in the "misfit" cobaltate [Bi2Ba2O4].[CoO2]~2. This compound contains the same triangular Co planes as Na cobaltates, but in a different 3D environment. Our data establish the similarity of their electronic structure. We propose that the peculiar lineshape of all cobaltates is of the "peak-dip-hump" type, due to strong many-body effects. We detect a progressive transfer of spectral weight from the quasiparticle feature near Ef to a broad hump in misfit phases where Ba is replaced by Sr or Ca. This indicates stronger many-body interactions in proximity of the band insulator regime, which we attribute to the presence of unusual magnetic excitations.

cond-mat.str-el↗

arXiv2005

Multiple Bosonic Mode Coupling in Electron Self-Energy of (La_2-xSr_x)CuO_4

High resolution angle-resolved photoemission spectroscopy data along the (0,0)-($π$,$π$) nodal direction with significantly improved statistics reveal fine structure in the electron self-energy of the underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ samples in the normal state. Fine structure at energies of (40$\sim$46) meV and (58$\sim$63)meV, and possible fine structure at energies of (23$\sim$29)meV and (75$\sim$85)meV, have been identified. These observations indicate that, in LSCO, more than one bosonic modes are involved in the coupling with electrons.

cond-mat.str-el↗

arXiv2004

Orientation-dependent C60 electronic structures revealed by photoemission

We observe, with angle-resolved photoemission, a dramatic change in the electronic structure of two C60 monolayers, deposited respectively on Ag (111) and (100) substrates, and similarly doped with potassium to half-filling of the C60 lowest unoccupied molecular orbital. The Fermi surface symmetry, the bandwidth, and the curvature of the dispersion at Gamma point are different. Orientations of the C60 molecules on the two substrates are known to be the main structural difference between the two monolayers, and we present new band-structure calculations for some of these orientations. We conclude that orientations play a key role in the electronic structure of fullerides.

cond-mat.str-el↗

arXiv2004

Space Charge Effect and Mirror Charge Effect in Photoemission Spectroscopy

We report the observation and systematic investigation of the space charge effect and mirror charge effect in photoemission spectroscopy. When pulsed light is incident on a sample, the photoemitted electrons experience energy redistribution after escaping from the surface because of the Coulomb interaction between them (space charge effect) and between photoemitted electrons and the distribution of mirror charges in the sample (mirror charge effect). These combined Coulomb interaction effects give rise to an energy shift and a broadening which can be on the order of 10 meV for a typical third-generation synchrotron light source. This value is comparable to many fundamental physical parameters actively studied by photoemission spectroscopy and should be taken seriously in interpreting photoemission data and in designing next generation experiments.

cond-mat.str-el↗

arXiv2004

Fermi Surface reconstruction in the CDW state of CeTe3 observed by photoemission

CeTe3 is a layered compound where an incommensurate Charge Density Wave (CDW) opens a large gap (400 meV) in optimally nested regions of the Fermi Surface (FS), whereas other sections with poorer nesting remain ungapped. Through Angle-Resolved Photoemission, we identify bands backfolded according to the CDW periodicity. They define FS pockets formed by the intersection of the original FS and its CDW replica. Such pockets illustrate very directly the role of nesting in the CDW formation but they could not be detected so far in a CDW system. We address the reasons for the weak intensity of the folded bands, by comparing different foldings coexisting in CeTe3.

cond-mat.str-el↗

arXiv2004

The Dichotomy between Nodal and Antinodal Quasiparticles in Underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ Superconductors

High resolution angle-resolved photoemission measurements on underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ system show that, at energies below 70 meV, the quasiparticle peak is well defined around the ($π$/2,$π$/2) nodal region and disappears rather abruptly when the momentum is changed from the nodal point to the ($π$,0) antinodal point along the underlying ``Fermi surface''. It indicates that there is an extra low energy scattering mechanism acting upon the antinodal quasiparticles. We propose that this mechanism is the scattering of quasiparticles across the nearly parallel segments of the Fermi surface near the antinodes.

cond-mat.str-el↗

arXiv2004

Direct extraction of the Eliashberg function for electron-phonon coupling: A case study of Be(1010)

We propose a systematic procedure to directly extract the Eliashberg function for electron-phonon coupling from high-resolution angle-resolved photoemission data. The procedure is successfully applied to the Be(1010) surface, providing new insights to electron-phonon coupling at this surface. The method is shown to be robust against imperfections in experimental data and suitable for wider applications.

cond-mat.str-el↗

arXiv2002

Persistence of molecular excitations in metallic fullerides and their role in a possible metal to insulator transition at high temperatures

We present 13C NMR spin-lattice relaxation measurements (1/T1) in Na2CsC60 and Rb3C60 from 10 to 700K. The large temperature range of this measurement allow to define unambiguously an increase of 1/T1T with increasing temperature, which is anomalous in a simple metallic picture, where the Korringa law predicts 1/T1T = cst. We attribute this increase to the existence of an additional relaxation channel related to singlet-triplet (ST) excitations of Jahn-Teller distorted C60^{2-} and C60^{4-}. These units are formed within the metal on very short time scales (10^{-14} sec) that do not imply static charge segregation. We show that the amplitude of the ST component depends directly on the density of states, which indicates an interplay between metallic and molecular excitations. Such an interaction is also revealed by the high temperature behavior of Na2CsC60 and CsC60, that we then discuss. A divergence between the behaviors of 1/T1, the NMR shift and the ESR susceptibility is interpreted as the result of a rapid increase of the lifetime of the charge carriers, signaling a tendency to charge localization. In our analysis, the particular stability of C60^{2n-} is then a common feature of all known metallic fullerides and allow to reconcile apparently contradicting properties of these systems.

cond-mat.str-el↗

arXiv2002

Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60

We present an NMR study of Na2C60 and K4C60, two compounds that are related by electron-hole symmetry in the C60 triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or C60^{4-}. However, the extended temperature range of the measurements presented here (10 K to 700 K) allows to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, 1/T1 deviates from the activated law that one would expect from the presence of the gap and saturates. In the same temperature range, a lowering of symmetry is detected in Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60, modifications of the 13C spectra lineshapes also indicate a structural modification. We discuss this high temperature deviation in terms of a coupling between JTD and local symmetry. At low temperatures, 1/T$_1$T tends to a constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.

cond-mat.str-el↗

arXiv2002

Coexistence of spin-singlets and metallic behavior in simple cubic CsC60

We present a detailed NMR study of simple cubic CsC60, the only metallic cubic fullerides known so far besides A3C60. 133Cs NMR signals the presence of about 12 to 15% of ``anomalous'' C60 balls, characterized by a 15 meV gap. We present different experimental observations supporting the idea that a spin-singlet (i.e. a C60^{2-}) is localized on these latter balls and stabilized by a JTD. A splitting of the 133Cs spectrum into three different lines, with distinct electronic environments, is observed which emerges naturally from such a situation. Quadrupole effects on 133Cs confirm an inhomogeneous distribution of the charge between C60 molecules. Analysis of the relative intensities of the 133Cs lines show that the spin-singlets do not form clusters at the local scale, but are diluted within the lattice. This is probably to avoid the occurrence of neighboring C60^{2-} and minimize electrostatic repulsion between these balls. Through spin-lattice relaxation measurements, we detect chemical exchange between the Cs sites above 100 K. From this, we deduce that the lifetime of a spin-singlet on a given ball decreases exponentially with increasing temperature (from 15 sec at 100 K to 3 ms at 130 K). The implications of the presence of such spin-singlets for the nature of the metallic state is discussed, in relation with a decrease of 1/T1T for 13C at low temperatures, which would be anomalous for a simple metal.

cond-mat.str-el↗

arXiv2000

Role of dynamic Jahn-Teller distortions in Na2C60 and Na2CsC60 studied by NMR

Through 13C NMR spin lattice relaxation (T1) measurements in cubic Na2C60, we detect a gap in its electronic excitations, similar to that observed in tetragonal A4C60. This establishes that Jahn-Teller distortions (JTD) and strong electronic correlations must be considered to understand the behaviour of even electron systems, regardless of the structure. Furthermore, in metallic Na2CsC60, a similar contribution to T1 is also detected for 13C and 133Cs NMR, implying the occurence of excitations typical of JT distorted C60^{2-} (or equivalently C60^{4-}). This supports the idea that dynamic JTD can induce attractive electronic interactions in odd electron systems.

cond-mat.str-el↗