arXiv subjects

Veronique Brouet

Publications and source records attributed to Veronique Brouet.

9 recordsLinked to original sources

arXiv2023

Electronic structure evolution of the magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ with hole and electron doping

Co$_3$Sn$_2$S$_2$ has been established as a prototype of magnetic Weyl semimetal, exhibiting a ''giant'' anomalous Hall effect in its ferromagnetic phase. An attractive feature of this material is that Weyl points lie close to Fermi level, so that one can expect a high reactivity of the topological properties to hole or electron doping. We present here a direct observation with Angle Resolved Photoemission Spectroscopy of the evolution of the electronic structure under different types of substitutions : In for Sn (hole doping outside the kagome Co plane), Fe for Co (hole doping inside the kagome Co plane) and Ni for Co (electron doping inside the kagome Co plane). We observe clear shifts of selected bands, which are due both to doping and to the reduction of the magnetic splitting by doping. We discriminate between the two by studying the temperature evolution from ferromagnetic to paramagnetic state. We discuss these shifts with the help of DFT calculations using the Virtual Crystal Approximation. We find that these calculations reproduce rather well the evolution with In, but largely fail to capture the effect of Fe and Ni, where local behavior at the impurity site plays an important role.

cond-mat.str-el↗

arXiv2023

Impurity-induced phonon thermal Hall effect in the antiferromagnetic phase of Sr2IrO4

A thermal Hall effect is observed in an increasing number of insulators often attributed to phonons, but the underlying mechanism is in most cases unknown. A coupling of phonons to spins has been invoked and scattering of phonons by impurities or defects has been proposed, but there is no systematic evidence to support either scenario. Here we present a study on the effect of adding Rh impurities to the antiferromagnetic insulator Sr2IrO4, substituting for the spin-carrying Ir atoms. We find that adding small concentrations of Rh impurities increases the thermal Hall conductivity dramatically, but adding enough Rh to suppress the magnetic order eventually decreases the thermal Hall conductivity until it nearly vanishes. We conclude that the thermal Hall effect in this material is caused by the scattering of phonons by impurities embedded within a magnetic environment.

cond-mat.str-el↗

arXiv2023

Evolution of the spectral lineshape at the magnetic transition in Sr2IrO4 and Sr3Ir2O7

Sr2IrO4 and Sr3Ir2O7 form two families of spin-orbit Mott insulators with quite different charge gaps and an antiferromagnetic (AF) ground state. This offers a unique opportunity to study the impact of long-range magnetic order in Mott insulators. It appears to play a different role in the two families, as there is almost no change of the resistivity at the magnetic transition TN in Sr2IrO4 and a large one in Sr3Ir2O7. We use angle-resolved photoemission to study the evolution of the spectral lineshape through the magnetic transition. We use Ru and La substitutions to tune TN and discriminate changes due to temperature from those due to magnetic order. We evidence a shift and a transfer of spectral weight in the gap at TN in Sr3Ir2O7, which is absent in Sr2IrO4. We assign this behavior to a significantly larger coherent contribution to the spectral lineshape in Sr3Ir2O7, which evolves strongly at TN. On the contrary, the Sr2IrO4 lineshape is dominated by the incoherent part, which is insensitive to TN. We compare these findings to theoretical expections of the Slater vs Mott antiferromagnetism within Dynamical Mean Field Theory.

cond-mat.str-el↗

arXiv2016

Influence of the surface symmetry breaking on the magnetism, collapsing and three dimensional dispersion of Co pnictides ACo2As2 (A=Ba, Sr, Ca)

We use angle-resolved photoemission to study the three dimensional (3D) electronic structure of Co pnictides ACo2As2 with A=Ba, Sr, Ca or a mixture of Sr and Ca. These compounds are isostructural to Fe based superconductors, but have one more electron in the Co $3d$ orbitals. Going from Ba to Ca, they become more and more 3D, eventually forming a 'collapsed' tetragonal phase, where the distance between CoAs layers is markedly reduced. The observed periodicity of the 3D electronic structure matches in each case that expected from the distance between the planes in the bulk. However, the electronic structure is better fitted by a calculation corresponding to a slab with 2 CoAs layers than to the bulk structure. We attribute this to subtle modifications of the 2D electronic structure induced by the truncation of the 3D dispersion at the surface in the ARPES measurement. We further study how this affects the electronic properties. We show that, despite this distortion, the electronic structure of CaCo2As2 is essentially that expected for a collapsed phase. Electronic correlations produce a renormalization of the electronic structure by a factor 1.4, which is not affected by the transition to the collapsed state. On the other hand, a small shift of the Fermi level reduces the density of states in the eg bands and suppresses the magnetic transition expected in CaCo2As2. Our study evidences that observing the bulk periodicity is not sufficient to ensure bulk sensitivity. It further gives direct information on the role of 3D interactions, mostly governed by Co-As hybridization, among eg and t2g orbitals. It is also useful to better understand the electronic structure of Fe superconductors and the range of validity of ARPES measurements.

cond-mat.str-el↗

arXiv2016

Orbital-dependent Fermi Surface shrinking as a fingerprint of nematicity in FeSe

The large anisotropy in the electronic properties across a structural transition in several correlated systems has been identified as the key manifestation of electronic nematic order, breaking rotational symmetry. In this context, FeSe is attracting tremendous interest, since electronic nematicity develops over a wide range of temperatures, allowing accurate experimental investigation. Here we combine angle-resolved photoemission spectroscopy and theoretical calculations based on a realistic multi-orbital model to unveil the microscopic mechanism responsible for the evolution of the electronic structure of FeSe across the nematic transition. We show that the self-energy corrections due to the exchange of spin fluctuations between hole and electron pockets are responsible for an orbital-dependent shrinking of the Fermi Surface that affects mainly the $xz/yz$ parts of the Fermi surface. This result is consistent with our experimental observation of the Fermi Surface in the high-temperature tetragonal phase, that includes the $xy$ electron sheet that was not clearly resolved before. In the low-temperature nematic phase, we experimentally confirm the appearance of a large ($\sim$ 50meV) $xz/yz$ splittings. It can be well reproduced in our model by assuming a moderate splitting between spin fluctuations along the $x$ and $y$ crystallographic directions. Our mechanism shows how the full entanglement between orbital and spin degrees of freedom can make a spin-driven nematic transition equivalent to an effective orbital order.

cond-mat.str-el↗

arXiv2016

ARPES view of orbitally resolved quasiparticle lifetimes in iron pnictides

We study with ARPES the renormalization and quasiparticle lifetimes of the $d_{xy}$ and $d_{xz}$/$d_{yz}$ orbitals in two iron pnictides, LiFeAs and Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ (Co8). We find that both quantities depend on orbital character rather than on the position on the Fermi Surface (for example hole or electron pocket). In LiFeAs, the renormalizations are larger for $d_{xy}$, while they are similar on both types of orbitals in Co8. The most salient feature, which proved robust against all the ARPES caveats we could think of, is that the lifetimes for $d_{xy}$ exhibit a markedly different behavior than those for $d_{xz}$/$d_{yz}$. They have smaller values near $E_F$ and exhibit larger $ω$ and temperature dependences. While the behavior of $d_{xy}$ is compatible with a Fermi liquid description, it is not the case for $d_{xz}$/$d_{yz}$. This situation should have important consequences for the physics of iron pnictides, which have not been considered up to now. More generally, it raises interesting questions on how a Fermi liquid regime can be established in a multiband system with small effective bandwidths.

cond-mat.str-el↗

arXiv2012

Nanoscale textured superconductivity in Ru-substituted BaFe2As2 : a challenge to a universal phase diagram for the pnictides

75As NMR experiments were performed in Ba(Fe1-xRux)2As2 for x=0 to 80%. Magnetic fractions and NMR lineshapes demonstrate that Ru substitution destroys the antiferromagnetic (AF) order inhomogeneously with a magnetic moment distributed from 0.9 to 0 uB. Superconductivity emerges at intermediate Ru doping and coexists with AF order only in the regions where moments are smaller than ~0.3uB, resulting in an original nanoscale texture. This situation contrasts with that of Co substitution, challenging the apparent universality of the phase diagram in Fe-based superconductors.

cond-mat.str-el↗

arXiv2006

Influence of local fullerene orientation on the electronic properties of A3C60 compounds

We have investigated sodium containing fullerene superconductors Na2AC60, A = Cs, Rb, and K, by Na-23 nuclear magnetic resonance (NMR) spectroscopy at 7.5 T in the temperature range of 10 to 400 K. Despite the structural differences from the Rb3C60 class of fullerene superconductors, in these compounds the NMR line of the tetrahedrally coordinated alkali nuclei also splits into two lines (T and T') at low temperature. In Na2CsC60 the splitting occurs at 170 K; in the quenched cubic phase of Na2RbC60 and Na2KC60 we observe split lines at 80 K. Detailed investigations of the spectrum, spin-spin and spin-lattice relaxation as well as spin-echo double resonance (SEDOR) in Na2CsC60 we show that these two different tetrahedral sites are mixed on a microscopic scale. The T and T' sites differ in the orientation of first-neighbor C60 molecules. We present evidence that the orientations of neighboring molecules are uncorrelated. Thermally activated molecular reorientations cause an exchange between the T and T' sites and motional narrowing at high temperature. We infer the same activation energy, 3300 K, in the temperature range 125 to 300 K. The spin lattice relaxation rate is the same for T and T' down to 125 K but different below. Both the spin-lattice relaxation rate and Knight shift are strongly temperature dependent in the whole range investigated. We interpret this temperature variation by the effect of phonon excitations involving the rigid librational motion of the C60 molecules. By extending the understanding of the structure and molecular dynamics of C60 superconductors, these results may help in clarifying the effects of the structure on the superconducting properties.

cond-mat.supr-con↗

arXiv2000

Alkali specific effects in superconducting fullerides: the observation of a high temperature insulating phase in Na_2CsC_60

Electron Spin Resonance and optical reflectivity measurements demonstrate a metal-insulator transition in Na_2CsC_60 as the system passes from the low temperature simple cubic to the high temperature {\it fcc} structure above 300 K. The non-conducting electronic state is especially unexpected in view of the metallic character of other, apparently isostructural fullerides, like K_3C_60. The occurence of this phase in Na_2CsC_60 suggests that alkali specific effects can not be neglected in the description of the electronic properties of alkali doped fullerides. We discuss the origin of the insulating state and the relevance of our results for the anomaly observed in the magnitude of the superconducting transition temperature of Na_2AC_60 fullerides.

cond-mat.str-el↗