The electronic band structure of heavily boron doped diamond was investigated by angle-resolved photoemission spectroscopy on (100)-oriented epilayers. A unique set of Luttinger parameters was deduced from a comparison of the experimental band structure of metallic diamond along the $\mathrm{\ensuremath{\Delta}}\phantom{\rule{0.28em}{0ex}}(\mathrm{\ensuremath{\Gamma}}X)$ and $\mathrm{\ensuremath{\Sigma}}\phantom{\rule{0.16em}{0ex}}(\mathrm{\ensuremath{\Gamma}}K)$ high-symmetry directions of the reciprocal space, with theoretical band structure calculations performed both within the local density approximation and by an analytical $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ approach. In this way, we were able to describe the experimental band structure over a large three-dimensional region of the reciprocal space and to estimate hole effective masses in agreement with previous theoretical and experimental papers.

The purple molybdenum bronze KMo6O17 is a quasi-two-dimensional compound which shows a Peierls transition towards a commensurate metallic charge density wave (CDW) state. High magnetic field measurements have revealed several transitions at low temperature and have provided an unusual phase diagram “temperature-magnetic field”. Angular studies of the interlayer magnetoresistance are now reported. The results suggest that the orbital coupling of the magnetic field to the CDW is the most likely mechanism for the field induced transitions. The angular dependence of the magnetoresistance is discussed on the basis of a warped quasi-cylindrical Fermi surface and provides information on the geometry of the Fermi surface in the low temperature density wave state.

We report high magnetic field magnetoresistance measurements performed in pulsed fields up to 55T on the quasi-two dimensional charge density wave conductor KMo6O17. Magnetoresistance curves show several anomalies below 28T. First order transitions to smaller gap states take place at low temperature above 30T. A phase diagram T(B) has been obtained. The angular dependence of the anomalies is reported.

The purple molybdenum bronze KMo6O17 is a quasi-two-dimensional compound which shows a Peierls transition towards a commensurate metallic CDW state. Electron spectroscopy (ARUPS), Scanning Tunnelling Microscopy (STM) and spectroscopy (STS) as well as high magnetic field studies are reported. ARUPS studies corroborate the model of the hidden nesting and provide a value of the CDW vector in good agreement with other measurements. STM studies visualize the triple-q CDW in real space. This is consistent with other measurements of the CDW vector. STS studies provide a value of several 10 meV for the average CDW gap. High magnetic field measurements performed in pulsed fields up to 55 T establish that first order transitions to smaller gap states take place at low temperature. These transitions are ascribed to Pauli type coupling. A phase diagram summarizing all observed anomalies and transitions is presented.

We have investigated Fermi surfaces of a low-dimensional Mo oxide, η-Mo4O11, using Compton scattering. The Fermi surfaces projected onto the a* plane, obtained from the two-dimensional reconstruction method, show a good agreement with those predicted by a previous band-theory study. Hidden one dimensionality, which is the most important feature in explaining the charge-density wave behavior in this material, is clearly observed in the experimentally obtained Fermi surfaces.

Magnetic torque and magnetoresistance measurements have been performed in high magnetic field on the quasi-two-dimensional charge density wave (CDW) oxide bronze KMo6O17 . Several anomalies have been found below 28T either on the torque or on the magnetoresistance data. They can be attributed predominantly to orbital effects. Magnetoresistance data obtained up to 55T show that a transition takes place above 30T. This transition may be due to the Pauli coupling. The new field-induced density wave state exhibits Shubnikov–de Haas (SdH) oscillations.

The purple potassium bronze of molybdenum is a quasi two-dimensional compound showing a Peierls transition at 120 K. This transition is driven by the properties of the conduction electrons. In order to confirm the nature of the transition, we have investigated at room temperature the electronic structure of this oxide and established its band structure in the ΓK direction. A weak conduction band is detected, well separated from the valence band by a depleted region. The valence band shows several structures attributed to oxygen-type states and to the K3p shallow core level. The structures of the conduction band reveal the presence of at least two bands crossing the Fermi level, in relatively good agreement with the calculated band structure.

We present scanning tunneling microscopy (STM) measurements of the two-dimensional eta -Mo4O11 com pound, which exhibits Peierls transitions at 30 and 109 K. Using a homemade UHV low-temperature STM, we observe at 50 K (between the two transitions) the formation of an incommensurate charge-density wave (CDW) along the b direction. STM images provide an accurate measurement of the wave vector of this CDW, which is the "nesting'' vector characterizing the transition at 109 K. The measured value is in perfect agreement with previous electron diffraction and x-ray diffuse scattering measurements. Finally, recording simultaneously STM images at positive and negative sample bias, we observe the expected pi phase shift of the CDW, due to a full spatial separation of the occupied and empty electronic states of the CDW.

The electronic structures of some molybdenum and tungsten oxides or bronzes exhibiting Peierls transitions are investigated at room temperature. The detection of a weak conduction band, well separated from a large valence band, evidences the metallic character of each oxide. The distributions of the valences of the different transition metals are analyzed by XPS. In each oxide, the presence of atleast two contributive components to the main core levels reveals a mixed valence state of the transition metal. But the proportions of the different components do not reflect the distribution of the cationic valences, as expected from the crystallographic structures. To understand this disagreement, we suggest that two alternative ways, including or rejecting a screening effect generated by the conduction electrons contribute to the photoemission processes and alter the real distribution of the cationic charges.

η-Mo4O11 presents a weak conduction band, well separated from a valence band which develops below 2eV. The Mo 3d core level spectrum clearly exhibits two contributions, which are not related to a valence distribution of hexavalent and tetravalent ions, but reflects the confinement of the conduction electrons in the layered crystallographic structure of this oxide.

The electronic structure of the Fe/BaBiO 3 interface is investigated by XPS. This interface is found to be reactive and iron is immediately oxidized at the surface of the bismuthate. A passivating buffer iron oxide is built between the metal and the bismuthate, while a oxygen-depleted zone is developed in the bismuthate

Electron tunneling studies have been performed on junctions made with the quasi bidimensional oxide eta-Mo4O11 and a lead film separated by an insulating oxide layer. The density of states in the vicinity of the Fermi level has been obtained along two directions (parallel and perpendicular to the layers) in the temperature range I K-90 K. The data from the differential conductance as a function of de bias voltage are consistent with gap openings taking place at T-p1 = 100 K and T-p2 = 30 K. Both electron instabilities are attributed to charge density waves and the anisotropy of the density of states is related to the anisotropy of the Fermi surface. For tunneling current in the plane of the layers, weak oscillations are found in the density of states at low temperatures. They are possibly due to the existence of several types of small electron and hole pockets induced on the Fermi surface by the charge density wave gap openings.

Measured on crystalline surfaces free of inter-grain contribution, the intrinsic electronic structures of superconducting K-substituted barium bismuthates show only single barium, bismuth and oxygen lines, according to the absence of disproportionnation. Potassium core level line exhibits two independent components, reflecting either the possibility of two substitutional sites or the evidence of a potassium segregation.

The evolutions of the core level lines of pure BaBiO3-delta or weakly Pb-substituted barium bismuthate are analyzed as a function of surface treatments. The air contamination and the Ar+ ion etching generate respectively one couple of high binding energy oxygen and bismuth satellites, while the barium line is weakly affected by these treatments. A well reconstructed surface and a fractured surface are characterized by a set of single Ba-, Bi- and O-lines, which indicate die absence of any measurable O- or Bi-disproportionation at the sample surface. Nevertheless, XANES spectra clearly show a valence of the bismuth higher than +3 and the presence of empty 6s Bi-states.

Changes in the electronic structures of BaPbxBi1−xO3−δ(x = 0 and 0.05) single crystals are analyzed as a function of sev surface treatments. The untreated surface exhibits a relatively weak presence of spoiling phases, which expresses a weak reactivity of these compounds. The fractured surface is characterized by a double O line, a double Bi line and a single Ba line, while the reconstructed surface exhibits essentially one set of single lines. Ar+ ion etching and surface oxygenation give rise to two additional O components and one additional Bi component. The evolution of the shape of the Bi line does not allow one to conclude about the presence of any bismuth disproportionation in BaBiO3−δ.

The electronic structure of the superconductor is analyzed as a function of the oxygen content adjusted by reducing and oxidizing anneals. The modifications of the Cu2p, O1s, Ba4d core level lines are found to be reversible and directly related to the presence of oxygen vacancies in the crystallographic structure. Two O-lines and two Ba-doublets specific to the cuprate are detected with variable intensities and extrinsic high binding energy O- and Ba-components are attributed to an oxygen adsorption at the surface of the samples. The texture of the samples varies the relative weight of the extrinsic lines and modifies the kinetic of the oxygen diffusion.

The thermal conductivity (kT) of the charge density wave (CDW) compounds gamma -Mo4O11, eta -Mo4O11 and KMo6O17 was measured in the range 20-300 K. At the CDW transition there is an anomalous decrease of kT in all compounds as T decreases. In gamma -Mo4O11 the anomaly can be ascribed to the decrease of the electronic contribution due to the decrease of the carrier concentration at the transition, the lattice remaining smooth, but in eta -Mo4O11 there is evidence for an additional anomalous increase of the lattice contribution below the CDW transition. In KMo6O17 there is a very large anomaly which is ascribed almost entirely to the lattice.

The electronic structure of the cuprate is analyzed by XPS in different states of the surface realized by reducing and oxidizing treatments. The evolution of the Cu2p, O1s and Ba4d core level lines are interpreted as directly related to the oxygen content and to the presence of the oxygen vacancies. An intrinsic oxygen line varying from 531.5 to 530.5 eV (BE) is associated to the disordering of the oxygen vacancies. The efficiency of the treatments on samples of different texture is determined by the oxygen mobility and could be limited by the amount of grain boundaries.

Infrared reflectivity of the quasi-two dimensional oxides η-Mo4O11, has been investigated on single crystals, using polarized light, in the range of 300 – 4000 cm−1, at room temperature and in the charge density wave (CDW) state. At 300 K, the anisotropy of the reflectivity emphasizes the anisotropy of the conduction electron gas, with a metallic behavior within the (100) plane and a semiconducting behavior in the [100] direction. In this direction, the IR reflectivity is fitted with the phonon oscillators. At low temperature, the decrease of the IR reflectivity is consistent with the loss of carriers, induced by the CDW gap opening.