Taking La‐ and I‐doped PbTe as an example, the current work shows the effects of optimizing the thermoelectric figure of merit, zT, by controlling the doping level. The high doping effectiveness allows the carrier concentration to be precisely designed and prepared to control the Fermi level. In addition to the Fermi energy tuning, La‐doping modifies the conduction band, leading to an increase in the density of states effective mass that is confirmed by transport, infrared reflectance and hard X‐ray photoelectron spectroscopy measurements. Taking such a band structure modification effect into account, the electrical transport properties can then be well‐described by a self‐consistent single non‐parabolic Kane band model that yields an approximate (m*T)1.5 dependence of the optimal carrier concentration for a peak power factor in both doping cases. Such a simple temperature dependence also provides an effective approximation of carrier concentration for a peak zT and helps to explain, the effects of other strategies such as lowering the lattice thermal conductivity by nanostructuring or alloying in n‐PbTe, which demonstrates a practical guide for fully optimizing thermoelectric materials in the entire temperature range. The principles used here should be equally applicable to other thermoelectric materials.

Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of () % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure.

We have investigated the resistive switching behavior in stoichiometric HfO2 and oxygen-deficient HfO2−x thin films grown on TiN electrodes using reactive molecular beam epitaxy. Oxygen defect states were controlled by the flow of oxygen radicals during thin film growth. Hard X-ray photoelectron spectroscopy confirmed the presence of sub-stoichiometric hafnium oxide and defect states near the Fermi level. The oxygen deficient HfO2−x thin films show bipolar switching with an electroforming occurring at low voltages and low operating currents, paving the way for almost forming-free devices for low-power applications.

Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO x ). Photoemission was induced by illumination with a Hg UV-lamp (photon energy cutoffhω U V = 5.0 eV, wavelength λ U V = 250 nm) and with a Ti:Sapphire femtosecond laser (hω l = 3.1 eV, λ l = 400 nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.

From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we show that BaFe2As2 possesses essentially two-dimensional electronic states, with a strong change of orbital character of two of the Γ-centered Fermi surfaces as a function of kz. Upon Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases. 74.25.Jb, Since the discovery of high T c superconductivity in Fepnictides [1], many experiments have been carried out to reveal the physical and electronic properties of these materials [2,3,4,5]. The parent compounds of Fepnictide superconductors are antiferromagnetic (AFM) metals. Both electron and hole doping suppresses the AFM order and leads to a superconducting phase. The AFM ordering is supposed to occur by nesting of hole pockets at the center of the Brillouin zone (BZ) and electron pockets at the zone corner. Nesting may be also important for the pairing mechanism in these compounds [6] although there are alternative scenarios based on the high polarizability of the As ions [7]. The nesting scenario could explain why in the SmFeAsO-based superconductors [8], predicted to have an almost twodimensional electronic structure [9, 10], higher superconducting transition temperatures T c are observed than in BaFe 2 As 2 -based systems [2] which are predicted to have a more three-dimensional electronic structure [11]. In general, reduction of the dimensionality increases the number of states that could be considered to be well nested. Furthermore, we point out that the orbital character of the states at the Fermi level E F is very important for the nesting conditions as the interband transitions which determine the electronic susceptibility, as calculated by the Lindhard function, are (in weak coupling scenarios) by far strongest when the two Fermi surfaces have the same orbital character [12]. The admixture of threedimensionality, arising from interlayer coupling, makes the materials potentially more useful in devices and other applications. Thus the dimensionality of the electronic structure, i.e., the k z dispersion of the electronic states is of great importance for the understanding and application of these new superconductors.Although angle-resolved photoemission spectroscopy (ARPES) is an ideal tool to study the dispersion of bands parallel and perpendicular to the FeAs layers there exist only a few experimental studies of these issues [13,14,15]. In this letter, we report a systematic study of the dimensionality of the electronic structure of BaFe 2−x Co x As 2 (x= 0 to 0.4) using polarization dependent ARPES, uncovering two new factors which are of great signi cance for the nesting of the Fermi surfaces of these systems. Firstly we show that the Co d...

A new undulator beamline (P22) for hard X-ray photoelectron spectroscopy (HAXPES) was built at PETRA III (DESY, Hamburg) to meet the increasing demand for HAXPES-based techniques. It provides four special instruments for high-resolution studies of the electronic and chemical structure of functional nano-materials and catalytic interfaces, with a focus on measurements under operando and/or ambient conditions: (i) a versatile solid-state spectroscopy setup with optional wide-angle lens and in-situ electrical characterization, (ii) a HAXPEEM instrument for sub-µm spectro-microscopy applications, (iii) an ambient pressure system (> 1 bar) for operando studies of catalytic reactions and (iv) a time-of-flight spectrometer as a full-field k-microscope for measurements of the 4D spectral function ρ(E B ,k). The X-ray optics were designed to deliver high brightness photon flux within the HAXPES energy range 2.4-15 keV. An LN 2-cooled double-crystal monochromator with interchangeable pairs of Si(111) and (311) crystals is optionally combined with a double channel-cut post-monochromator to generate X-rays with variable energy bandpass adapted to the needs of the experiment. Additionally, the beam polarization can be varied using a diamond phase plate integrated into the beamline. Adaptive beam focusing is realized by Be compound refractive lenses and/or horizontally deflecting mirrors down to a spot size of ~20x17 µm 2 with a flux of up to 1.1x10 13 ph/s (for Si(111) at 6 keV).

The magnetic circular dichroism ͑MCD͒ of core-level absorption ͑x-ray absorption spectroscopy, XAS͒ spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloy Co 2 Cr 0.6 Fe 0.4 Al at the Co, Fe, and Cr L II,III edges. The comparison of XAS spectra before and after in situ cleaning of polished surfaces revealed a pronounced selective oxidation of Cr in air. For clean surfaces we observed a MCD for all three elements with Fe showing the largest moment per atom. The MCD can be explained by the density of states of the 3d unoccupied states, predicted by linear muffin-tin orbital atomic sphere approximation. For Fe and Cr the orbital angular momentum component of the magnetic moment is considerably larger than the values reported for metallic alloys, whereas for Co a value close to its bulk value is observed. This observation is discussed in comparison with band structure calculations.

We present a detailed study of the electronic structure and band alignment in LaAlO 3 /SrTiO 3 oxide heterostructures by hard x-ray photoelectron spectroscopy. Our spectroscopic measurements find no evidence for the strong potential gradient within the polar LaAlO 3 film predicted by band theory. Due to the high interface sensitivity of the method, we are further able to determine the valence band offset between the LaAlO 3 film and the SrTiO 3 substrate, which is found to be independent of the number of LAO overlayers. Finally, we discuss several explanations for the apparent absence of the built-in field in ex situ prepared LaAlO 3 /SrTiO 3 heterostructures.