We study with Angle Resolved PhotoElectron Spectroscopy (ARPES) the evolution of the electronic structure of Sr2IrO4, when holes or electrons are introduced, through Rh or La substitutions. At low dopings, the added carriers occupy the first availabl
e states, at bottom or top of the gap, revealing an anisotropic gap of 0.7eV in good agreement with STM measurements. At further doping, we observe a reduction of the gap and a transfer of spectral weight across the gap, although the quasiparticle weight remains very small. We discuss the origin of the in-gap spectral weight as a local distribution of gap values.
We investigate the quasiperiodic crystal (LaS)1.196(VS2) by angle and time resolved photoemission spectroscopy. The dispersion of electronic states is in qualitative agreement with band structure calculated for the VS2 slab without the incommensurate
distortion. Nonetheless, the spectra display a temperature dependent pseudogap instead of quasiparticles crossing. The sudden photoexcitation at 50 K induces a partial filling of the electronic pseudogap within less than 80 fs. The electronic energy flows into the lattice modes on a comparable timescale. We attribute this surprisingly short timescale to a very strong electron-phonon coupling to the incommensurate distortion. This result sheds light on the electronic localization arising in aperiodic structures and quasicrystals.
Intriguing properties of the misfit layered chalcogenide (LaS)$_{1.196}$VS$_2$ crystals were investigated by transport, optical measurements, angle-resolved photoemission (ARPES) and x-ray diffraction. Although no clear anomaly is found in transport
properties as a function of temperature, a large spectral weight transfer, up to at least 1 eV, is observed by both optical and photoemission spectroscopies. ARPES reveals that a nearly filled band with negative curvature, close enough from the Fermi level at 300K to produce metallic-like behaviour as observed in optical conductivity spectra. At low temperature, the band structure is strongly modified, yielding to an insulating state with a optical gap of 120 meV. An accurate (3+1)D analysis of x-ray diffraction data shows that, although a phase transition does not occur, structural distortions increase as temperature is decreased, and vanadium clusterization is enhanced. We found that the changes of electronic properties and structure are intimately related. This indicates that structural distorsion play a major role in the insulating nature of (LaS)$_{1.196}$VS$_2$ and that electronic correlation may not be important, contrary to previous belief. These results shed a new light on the mechanism at the origin of non-linear electric properties observed in (LaS)$_{1.196}$VS$_2$.
In all iron pnictides, the positions of the ligand alternatively above and below the Fe plane create 2 inequivalent Fe sites. This results in 10 Fe 3d bands in the electronic structure. However, they do not all have the same status for an ARPES exper
iment. There are interference effects between the 2 Fe that modulate strongly the intensity of the bands and that can even switch their parity. We give a simple description of these effects, notably showing that ARPES polarization selection rules in these systems cannot be applied by reference to a single Fe ion. We show that ARPES data for the electron pockets in Ba(Fe0.92Co0.08)2As2 are in excellent agreement with this model. We observe both the total suppression of some bands and the parity switching of some other bands. Once these effects are properly taken into account, the structure of the electron pockets, as measured by ARPES, becomes very clear and simple. By combining ARPES measurements in different experimental configurations, we clearly isolate each band forming one of the electron pockets. We identify a deep electron band along one ellipse axis with the dxy orbital and a shallow electron band along the perpendicular axis with the dxz/dyz orbitals, in good agreement with band structure calculations. We show that the electron pockets are warped as a function of kz as expected theoretically, but that they are much smaller than predicted by the calculation.
We present a detailed comparison of the electronic structure of BaFe2As2 in its paramagnetic and antiferromagnetic (AFM) phases, through angle-resolved photoemission studies. Using different experimental geometries, we resolve the full elliptic shape
of the electron pockets, including parts of dxy symmetry along its major axis that are usually missing. This allows us to define precisely how the hole and electron pockets are nested and how the different orbitals evolve at the transition. We conclude that the imperfect nesting between hole and electron pockets explains rather well the formation of gaps and residual metallic droplets in the AFM phase, provided the relative parity of the different bands is taken into account. Beyond this nesting picture, we observe shifts and splittings of numerous bands at the transition. We show that the splittings are surface sensitive and probably not a reliable signature of the magnetic order. On the other hand, the shifts indicate a significant redistribution of the orbital occupations at the transition, especially within the dxz/dyz system, which we discuss.
Despite many ARPES investigations of iron pnictides, the structure of the electron pockets is still poorly understood. By combining ARPES measurements in different experimental configurations, we clearly resolve their elliptic shape. Comparison with
band calculation identify a deep electron band with the dxy orbital and a shallow electron band along the perpendicular ellipse axis with the dxz/dyz orbitals. We find that, for both electron and hole bands, the lifetimes associated with dxy are longer than for dxz/dyz. This suggests that the two types of orbitals play different roles in the electronic properties and that their relative weight is a key parameter to determine the ground state.
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 dopi
ng. 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.
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.
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 ass
ociated 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.
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 est
ablish 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.
