arXiv subjects

P. Richard

Publications and source records attributed to P. Richard.

At least 73 records · Page 4Linked to original sources

arXiv2014

Raman scattering investigation of superconducting Ba$_{2}$Ti$_{2}$Fe$_{2}$As$_{4}$O

We have performed polarized Raman scattering measurements on the newly discovered superconductor Ba$_{2}$Ti$_{2}$Fe$_{2}$As$_{4}$O$_{2}$ ($T_c = 21$ K). We observe seven out of eight Raman active modes, with frequencies in good accordance with first-principle calculations. The phonon spectra suggest neither strong electron-phonon nor spin-phonon coupling and vary only slightly with temperature, except for one E$_g$ mode associated with large displacements of As atoms near the Ti$_2$O planes. We also identify a small anomaly around 125 K in the linewidth of a A$_{1g}$ mode involving the same As atoms. Our results suggest that the transition at 125 K is most likely driven by electronic interactions taking place in the Ti$_2$O planes.

cond-mat.supr-con↗

arXiv2014

Slow relaxation and aging phenomena at nano-scale in granular materials

Complex systems having metastable elements often demonstrate nearly log-time relaxations and a kind of aging: repeated stimuli weaken the system's relaxational response. Granular matter is known to exhibit a wealth of such behaviors, for which the role of thermal fluctuations is usually ignored. However, we demonstrate that the latter can pronouncedly affect contacting mesoscopic-scale asperities and be macroscopically observed via appropriate acoustic effects. We also propose a mechanism comprising slow relaxations and aging as intrinsic properties of a wide class of systems with metastable states.

cond-mat.soft↗

arXiv2014

Observation of strong-coupling pairing with weakened Fermi-surface nesting at optimal hole doping in Ca$_{0.33}$Na$_{0.67}$Fe$_2$As$_2$

We report an angle-resolved photoemission investigation of optimally-doped Ca$_{0.33}$Na$_{0.67}$Fe$_2$As$_2$. The Fermi surface topology of this compound is similar to that of the well-studied Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ material, except for larger hole pockets resulting from a higher hole concentration per Fe atoms. We find that the quasi-nesting conditions are weakened in this compound as compared to Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. As with Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ though, we observe nearly isotropic superconducting gaps with Fermi surface-dependent magnitudes. A small variation in the gap size along the momentum direction perpendicular to the surface is found for one of the Fermi surfaces. Our superconducting gap results on all Fermi surface sheets fit simultaneously very well to a global gap function derived from a strong coupling approach, which contains only 2 global parameters.

cond-mat.supr-con↗

arXiv2014

Observation of anomalous temperature dependence of spectrum on small Fermi surfaces in a BiS2-based superconductor

We performed an angle-resolved photoemission spectroscopy study of the BiS2-based superconductor Nd(O,F)BiS2. Two small electron-like Fermi surfaces around X (pi, 0) are observed, which enclose 2.4% and 1.1% of the Brillouin zone area, respectively, corresponding to an electron doping of 7% per Bi site. The low-energy spectrum consists of a weakly-dispersing broad hump and a dispersive branch, which follows well the calculated band dispersion. This hump is drastically suppressed with increasing temperature, while the dispersive branch is essentially unaffected. The anomalous thermal effect indicates a highly interacting electronic state, in which the superconducting pairing develops.

cond-mat.supr-con↗

arXiv2013

Observation of well-defined quasi-particle over a wide energy range in quasi-2D system

We report the observation by angle-resolved photoemission spectroscopy measurements of a highly anisotropic Dirac-cone structure in high quality SrMnBi2 crystals. We reveal a well-defined sharp quasi-particle, linearly dispersive with $v_{F_1}/v_{F_2}\sim$ 5--6, forming a hole-like anisotropic Dirac-cone. The density of states for the cone remains linear up to as high as ~650 meV of binding energy. The scattering rate of the quasi-particle (QP) increases linearly as function of binding energy, indicating a non-Fermi-Liquid behavior. Our results suggest the existence of a dilute two-dimensional electron gas system in this three-dimensional material.

cond-mat.str-el↗

arXiv2013

Structural phase transition induced by van Hove singularity in 5d transition metal compound IrTe2

Comprehensive studies of the electronic states of Ir 5d and Te 5p have been performed to elucidate the origin of the structural phase transition in IrTe2 by combining angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering. While no considerable changes are observed in the configuration of the Ir 5d electronic states across the transition, indicating that the Ir 5d orbitals are not involved in the transition, we reveal a van Hove singularity at the Fermi level (EF) related to the Te px+py orbitals, which is removed from EF at low temperatures. The wavevector connecting the adjacent saddle points is consistent with the in-plane projection of the superstructure modulation wavevector. These results can be qualitatively understood with the Rice-Scott "saddle-point" mechanism, while effects of the lattice distortions need to be additionally involved.

cond-mat.supr-con↗

arXiv2013

Coexistence of orbital degeneracy lifting and superconductivity in iron-based superconductors

In contrast to conventional superconducting (SC) materials, superconductivity in high-temperature superconductors (HTCs) usually emerges in the presence of other fluctuating orders with similar or higher energy scales, thus instigating debates over their relevance for the SC pairing mechanism. In iron-based superconductors (IBSCs), local orbital fluctuations have been proposed to be directly responsible for the structural phase transition and closely related to the observed giant magnetic anisotropy and electronic nematicity. However, whether superconductivity can emerge from, or even coexist with orbital fluctuations, remains unclear. Here we report the angle-resolved photoemission spectroscopy (ARPES) observation of the lifting of symmetry-protected band degeneracy, and consequently the breakdown of local tetragonal symmetry in the SC state of Li(Fe1-xCox)As. Supported by theoretical simulations, we analyse the doping and temperature dependences of this band-splitting and demonstrate an intimate connection between ferro-orbital correlations and superconductivity.

cond-mat.supr-con↗

arXiv2013

Angle-resolved photoemission spectroscopy observation of anomalous electronic states in EuFe$_2$As$_{2-x}$P$_x$

We used angle-resolved photoemission spectroscopy to investigate the electronic structure of EuFe$_2$As$_2$, EuFe$_2$As$_{1.4}$P$_{0.6}$ and EuFe$_2$P$_2$. We observed doubled core level peaks associated to the pnictide atoms, which are related to a surface state. Nevertheless, strong electronic dispersion along the $c$ axis, especially pronounced in EuFe$_2$P$_2$, is observed for at less one band, thus indicated that the Fe states, albeit probably affected at the surface, do not form pure two-dimensional surface states. However, this latter material shows reduced spectral weight near the Fermi level as compared to EuFe$_2$As$_2$ and EuFe$_2$As$_{1.4}$P$_{0.6}$. An anomalous jump is also found in the electronic states associated with the Eu$^{2+}$ $f$ states in EuFe$_2$P$_2$.

cond-mat.supr-con↗

arXiv2013

Possible nodal superconducting gap emerging at the Lifshitz transition in heavily hole-doped Ba0.1K0.9Fe2As2

We performed a high energy resolution ARPES investigation of over-doped Ba0.1K0.9Fe2As2 with T_c= 9 K. The Fermi surface topology of this material is similar to that of KFe2As2 and differs from that of slightly less doped Ba0.3K0.7Fe2As2, implying that a Lifshitz transition occurred between x=0.7 and x=0.9. Albeit for a vertical node found at the tip of the emerging off-M-centered Fermi surface pocket lobes, the superconducting gap structure is similar to that of Ba0.3K0.7Fe2As2, suggesting that the paring interaction is not driven by the Fermi surface topology.

cond-mat.supr-con↗

arXiv2013

Experimental investigation of the electronic structure of Ca$_{0.83}$La$_{0.17}$Fe$_2$As$_2$

We performed a combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy study of the electronic structure of electron-doped Ca$_{0.83}$La$_{0.17}$Fe$_2$As$_2$. A surface reconstruction associated with the dimerization of As atoms is observed directly in the real space, as well as the consequent band folding in the momentum space. Besides this band folding effect, the Fermi surface topology of this material is similar to that reported previously for BaFe$_{1.85}$Co$_{0.15}$As$_2$, with $Γ$-centred hole pockets quasi-nested to M-centred electron pockets by the antiferromagnetic wave vector. Although no superconducting gap is observed by ARPES possibly due to low superconducting volume fraction, a gap-like density of states depression of $7.7\pm 2.9$ meV is determined by scanning tunneling microscopy.

cond-mat.supr-con↗

arXiv2012

Observation of momentum space semi-localization in Si-doped $β$-Ga$_2$O$_3$

We performed an angle-resolved photoemission spectroscopy study of Si-doped $β$-Ga$_2$O$_3$. We observed very small photoemission intensity near the Fermi level corresponding to non-dispersive states assigned to Si impurities. We show evidence for a quantization of these states that is accompanied by a confinement in the momentum space consistent with a real-space finite confinement observed in a previous scanning tunneling microscopy study. Our results suggest that this semi-localization in the conjugate spaces plays a crucial role in the electronic conduction of this material.

cond-mat.mtrl-sci↗

arXiv2012

ARPES observation of isotropic superconducting gaps in isovalent Ru-substituted Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$

We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ ($T_c = 15$ K). Despite a strong out-of-plane warping of the Fermi surface, the magnitude of the superconducting gap observed experimentally is nearly isotropic and independent of the out-of-plane momentum. More precisely, we respectively observed 5.7 meV and 4.5 meV superconducting gaps on the inner and outer $Γ$-centered hole Fermi surface pockets, whereas a 4.8 meV gap is recorded on the M-centered electron Fermi surface pockets. Our results are consistent with the $J_1-J_2$ model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors.

cond-mat.supr-con↗

arXiv2012

Electronic band structure of BaCo$_{2}$As$_2$: a fully-doped ferropnictide with reduced electronic correlations

We report an angle-resolved photoemission spectroscopy investigation of the Fermi surface and electronic band structure of BaCo$_{2}$As$_2$. Although its quasi-nesting-free Fermi surface differs drastically from that of its Fe-pnictide cousins, we show that the BaCo$_{2}$As$_2$ system can be used as an approximation to the bare unoccupied band structure of the related BaFe$_{2-x}$Co$_x$As$_2$ and Ba$_{1-x}$K$_x$Fe$_2$As$_2$ compounds. However, our experimental results, in agreement with dynamical mean field theory calculations, indicate that electronic correlations are much less important in BaCo$_{2}$As$_2$ than in the ferropnictides. Our findings suggest that this effect is due to the increased filling of the electronic 3$d$ shell in the presence of significant Hund's exchange coupling.

cond-mat.str-el↗

arXiv2012

Observation of an isotropic superconducting gap at the Brillouin zone center of Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$

We performed a high-resolution angle-resolved photoemission spectroscopy study on superconducting (SC) Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$ ($T_c=29$ K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic $\sim$ 8.2 meV 2-dimensional (2D) SC gap ($2Δ/k_BT_c\sim7$) on quasi-2D electron Fermi surfaces (FSs) located around M$(π,0,0)$-A$(π,0,π)$, we observe a $\sim 6.2$ meV isotropic SC gap ($2Δ/k_BT_c\sim5$) on the Z-centered electron FS that rules out any d-wave pairing symmetry and rather favors an s-wave symmetry. All isotropic SC gap amplitudes can be fit by a single gap function derived from a local strong coupling approach suggesting an enhancement of the next-next neighbor exchange interaction in the ferrochalcogenide superconductors.

cond-mat.supr-con↗

arXiv2012

Angle-resolved photoemission studies of the superconducting gap symmetry in Fe-based superconductors

The superconducting gap is the fundamental parameter that characterizes the superconducting state, and its symmetry is a direct consequence of the mechanism responsible for Cooper pairing. Here we discuss about angle-resolved photoemission spectroscopy measurements of the superconducting gap in the Fe-based high-temperature superconductors. We show that the superconducting gap is Fermi surface dependent and nodeless with small anisotropy, or more precisely, a function of momentum. We show that while this observation is inconsistent with weak coupling approaches for superconductivity in these materials, it is well supported by strong coupling models and global superconducting gaps. We also suggest that the strong anisotropies measured by other probes sensitive to the residual density of states are not related to the pairing interaction itself, but rather emerge naturally from the smaller lifetime of the superconducting Cooper pairs that is a direct consequence of the momentum dependent interband scattering inherent to these materials.

cond-mat.supr-con↗

arXiv2012

Three-dimensionality and orbital characters of Fermi surface in (Tl,Rb)$_y$Fe$_{2-x}$Se$_2$

We report a comprehensive angle-resolved photoemission spectroscopy study of the tridimensional electronic bands in the recently discovered Fe selenide superconductor (Tl,Rb)$_y$Fe$_{2-x}$Se$_2$ ($T_c=32$ K). We determined the orbital characters and the $k_z$ dependence of the low energy electronic structure by tuning the polarization and the energy of the incident photons. We observed a small 3D electron Fermi surface pocket near the Brillouin zone center and a 2D like electron Fermi surface pocket near the zone boundary. The photon energy dependence, the polarization analysis and the local-density approximation calculations suggest a significant contribution from the Se 4$p_z$ and Fe 3$d_{xy}$ orbitals to the small electron pocket. We argue that the emergence of Se 4$p_z$ states might be the cause of the different magnetic properties between Fe chalcogenides and Fe pnictides.

cond-mat.supr-con↗

arXiv2012

Orbital Characters Determined from Fermi Surface Intensity Patterns using Angle-Resolved Photoemission Spectroscopy

In order to determine the orbital characters on the various Fermi surface pockets of the Fe-based superconductors Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ and FeSe$_{0.45}$Te$_{0.55}$, we introduce a method to calculate photoemission matrix elements. We compare our simulations to experimental data obtained with various experimental configurations of beam orientation and light polarization. We show that the photoemission intensity patterns revealed from angle-resolved photoemission spectroscopy measurements of Fermi surface mappings and energy-momentum plots along high-symmetry lines exhibit asymmetries carrying precious information on the nature of the states probed, information that is destroyed after the data symmetrization process often performed in the analysis of angle-resolved photoemission spectroscopy data. Our simulations are consistent with Fermi surfaces originating mainly from the $d_{xy}$, $d_{xz}$ and $d_{yz}$ orbitals in these materials.

cond-mat.supr-con↗

arXiv2012

Effects of Ru Substitution on Dimensionality and Electron Correlations in Ba(Fe_{1-x}Ru_x)_2As_2

We report a systematic angle-resolved photoemission spectroscopy study on Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ for a wide range of Ru concentrations (0.15 $\leq$ \emph{x} $\leq$ 0.74). We observed a crossover from two-dimension to three-dimension for some of the hole-like Fermi surfaces with Ru substitution and a large reduction in the mass renormalization close to optimal doping. These results suggest that isovalent Ru substitution has remarkable effects on the low-energy electron excitations, which are important for the evolution of superconductivity and antiferromagnetism in this system.

cond-mat.supr-con↗