We report the measurement, using angle-resolved photoemission spectroscopy, of the metallic electronic structure of the hole-doped thermoelectric oxide CuRh0.9Mg0.1O2. The material is found to have a "pudding mold" type band structure, with a nearly flat band edge located near the Fermi level, which is thought to be the origin of the thermoelectric behavior of this material. The experimental data match the density functional theory of the undoped parent compound, simply corrected by a rigid shift of the bands. Transport calculations based on the observed band structure yield a Seebeck coefficient of similar to 200 mu V/K for the undoped parent material, consistent with experimental measurements. Our results show that CuRhO2 is a textbook example of how pure band-structural effects can result in a large thermoelectric figure of merit, demonstrating that flat band edges in oxides are a realistic route for the efficient conversion of thermal energy.

We report a successful synthesis of low-dimensional In2O3 nanostructures on MgO smoke particles using an innovative fabrication method. By co-combusting indium and magnesium in an oxygen-rich atmosphere, the energy released (heat) upon oxidation of Mg provides the conditions for the simultaneous evaporation of In resulting in the self-organized growth of indium oxide particles with consistent orientation. Notably, we report the stabilization of the typically unstable trigonal In2O3 polymorph on edges and corners of MgO cubes. These experimental results align well with the energetics extracted from atomistic models, which offers valuable insights into the thermodynamic factors driving the In2O3 stabilization on such less dense MgO terminations. This work marks a significant advancement in multi-metal oxide (MMO) research and nanostructure engineering.

Gradual modification of the magnetic properties in ferromagnetic films has recently been proposed as an effective method to channel and control spin waves for the development of new functionalities in magnonic devices. Here, we investigate graded FeN films prepared by low-dose nitrogen implantation of Fe epitaxial thin films. Combining Brillouin light scattering measurements and a spin-wave theoretical approach, we show that nitrogen implantation induces a graded profile of both the in-plane and the perpendicular anisotropies along the film thickness. This graduation leads to a significant modification of the spin-wave spatial localization and generates a marked frequency asymmetry in the spin-wave dispersion. Moreover, we find that the anisotropy profile, and as a consequence the dispersion relation, can be tuned on changing the implantation dose, opening a way for the potential use of the graded Fe-N films in magnonic applications.

TaTe$_4$ is a quasi-1D tetrachalcogenide that exhibits a CDW instability caused by a periodic lattice distortion. Recently, pressure-induced superconductivity has been achieved in this compound, revealing a competition between these different ground states and making TaTe$_4$ very interesting for fundamental studies. Although TaTe$_4$ exhibits CDW ordering below 475 K, transport experiments have reported metallic behavior with a resistivity plateau at temperatures lower than 10 K. In this paper, we study the electronic structure of TaTe$_4$ using a combination of high-resolution angle-resolved photoemission spectroscopy and density functional calculations. Our results reveal the existence of the long-sought metallic states. These states exhibit mixed dimensionality, while some of them might have potential topological properties.

Second-order topological insulators are characterized by helical, non-spin-degenerate one-dimensional states running along opposite crystal hinges with no backscattering. Injecting superconducting pairs therefore entails splitting Cooper pairs into two families of helical Andreev states of opposite helicity, one at each hinge. Here we provide evidence for such separation via the measurement and analysis of the switching supercurrent statistics of a crystalline nanoring of bismuth. Using a phenomenological model of two helical Andreev hinge modes, we find that pairs relax at a rate comparable to individual quasiparticles, in contrast to the much faster pair relaxation of non-topological systems. This constitutes a unique telltale sign of the spatial separation of topological helical hinges. Second-order topological insulators feature helical one-dimensional states located at crystal hinges. Running a supercurrent through such systems is now shown to lead to long-lived excited Andreev pairs due to their separation along hinges with opposite helicity.

Two-dimensional electron gases (2DEGs) based on KTaO(3 )are emerging as a promising platform for spin-orbitronics due to their high Rashba spin-orbit coupling (SOC) and gate-voltage tunability. The recent discovery of a superconducting state in KTaO(3 )2DEGs now expands their potential towards topological superconductivity. Although the band structure of KTaO3 surfaces of various crystallographic orientations has already been mapped using angle-resolved photoemission spectroscopy (ARPES), this is not the case for superconducting KTaO3 2DEGs. Here, we reveal the electronic structure of superconducting 2DEGs based on KTaO3 (111) single crystals through ARPES measurements. We fit the data with a tight-binding model and compute the associated spin textures to bring insight into the SOC-driven physics of this fascinating system.

Two-dimensional electron gases in SrTiO3-based heterostructures provide a platform to study the real-time evolution of the macroscopic state with a variation of the carrier density, and the impact of structural properties on the emergence of the superconducting state. We have explored the isothermal evolution of the electron gas in AlOx/SrTiO3 by measuring the variation of resistance with continuous gate-voltage-controlled tuning of its carrier density. It is seen that condensation of the ordered phase leads to non-monotonic isotherms within the superconducting dome. The timescale for dynamic change following changes in gate voltage is measured across the phase transition. It is found to be tens of seconds near the onset of superconductivity, significantly larger compared to the normal state. Such a large timescale governing the kinetics of the phase transition presumably arises from the strong impact of structural defects and distortions of the substrate on the development of superconducting islands.

In solids, strong repulsion between electrons can inhibit their movement and result in a “Mott” metal-to-insulator transition (MIT), a fundamental phenomenon whose understanding has remained a challenge for over 50 years. A key issue is how the wave-like itinerant electrons change into a localized-like state due to increased interactions. However, observing the MIT in terms of the energy- and momentum-resolved electronic structure of the system, the only direct way to probe both itinerant and localized states, has been elusive. Here we show, using angle-resolved photoemission spectroscopy (ARPES), that in V2O3, the temperature-induced MIT is characterized by the progressive disappearance of its itinerant conduction band, without any change in its energy-momentum dispersion, and the simultaneous shift to larger binding energies of a quasi-localized state initially located near the Fermi level.

The realization of two-dimensional electronic gases (2DEGs) in SrTiO3-based heterostructures has led to important discoveries about superconductivity [1] in low dimensions. This system exhibits a superconducting dome [2] in the phase diagram. Some other interesting aspects are the observation of pairing interactions without superconductivity [3] and density-of-states features resembling the pseudogap in cuprates [4].

We report the observation of a nontrivial spin texture in Dirac node arcs, i.e., novel topological objects formed when Dirac cones of massless particles extend along an open one-dimensional line in momentum space. We find that such states are present in all the compounds of the tetradymite M_{2}Te_{2}X family (M=Ti, Zr, or Hf and X=P or As) regardless of the weak or strong character of the topological invariant. The Dirac node arcs in tetradymites are thus the simplest possible textbook example of a type-I Dirac system with a single spin-polarized node arc.

In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many electron wave function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic and still unsolved example is the transition of the heavy fermion compound URu2Si2 (URS) into the so-called hidden-order (HO) phase when the temperature drops below T-0 = 17.5 K. Here, we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle-resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a nonrigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO to AFM phase transition.

We report on the development and implementation of methodologies dedicated to soft X-ray imaging by coherent scattering in reflection mode. Two complementary approaches are tested, based on Fourier transform holography and on ptychography. A new method for designing holographic masks has been developed. Our results represent a feasibility test and highlight the potential and limitations of imaging in reflection mode. Reflectivity is less efficient than transmission at soft X-ray wavelengths, hampering the acquisition of good quality images. Nonetheless, it has the potential to image a wider set of samples, notably those that are not transparent to soft X-rays. Although the images obtained so far are of modest quality, these results are extremely encouraging for continuing the development of coherent soft X-ray imaging in reflection mode.

Insights into the role of interactions in determining the macroscopic state of a system can be obtained by observing its evolution with an isothermal variation of density. We explore the isothermal evolution of the electron gas in AlOx/SrTiO3 by a continuous gate-controlled tuning of its carrier density across the phase diagram exhibiting a superconducting dome. It is seen that condensation of the ordered phase leads to non-monotonic isotherms within the superconducting dome. The system undergoes dynamic change lasting tens of seconds following changes in gate voltage near the onset of the transition, revealing a strong impact of structural defects and distortions of the substrate on the superconducting state. These observations suggest that AlOx/SrTiO3 is a promising platform to study time-dependent kinetic processes at the onset of superconductivity.

VO2 is a much-discussed material for oxide electronics and neuromorphic computing applications. Here, heteroepitaxy of vanadium dioxide (VO2) was realized on top of oxide nanosheets that cover either the amorphous silicon dioxide surfaces of Si substrates or X-ray transparent silicon nitride membranes. The out-of-plane orientation of the VO2 thin films was controlled at will between (011)M1/(110)R and (-402)M1/(002)R by coating the bulk substrates with Ti0.87O2 and NbWO6 nanosheets, respectively, prior to VO2 growth. Temperature dependent X-ray diffraction and automated crystal orientation mapping in microprobe TEM mode (ACOM-TEM) characterized the high phase purity, the crystallographic and orientational properties of the VO2 films. Transport measurements and soft X-ray absorption in transmission are used to probe the VO2 metal-insulator transition, showing results of a quality equal to those from epitaxial films on bulk single-crystal substrates. Successful local manipulation of two different VO2 orientations on a single substrate is demonstrated using VO2 grown on lithographically-patterned lines of Ti0.87O2 and NbWO6 nanosheets investigated by electron backscatter diffraction. Finally, the excellent suitability of these nanosheet-templated VO2 films for advanced lensless imaging of the metal-insulator transition using coherent soft X-rays is discussed.

It was recently discovered that a conductive, metallic state is formed on the surface of some insulating oxides. First observed on SrTiO3 (001), it was then found in other compounds as diverse as anatase TiO2, KTaO3, BaTiO3, ZnO, and also on different surfaces of SrTiO3 (or other oxides) with different symmetries. The spatial extension of the wave function of this electronic state is of only a few atomic layers. Experiments indicate its existence is related to the presence of oxygen vacancies induced at or near the surface of the oxide. We present a simplified model aimed at describing the effect of its small spatial extension on measurements of its threedimensional (3D) electronic structure by angular resolved photoemission spectroscopy. For the sake of clarity, we base our discussion on a simple tight-binding scheme plus a confining potential that is assumed to be induced by the oxygen vacancies. Our model parameters are, nevertheless, obtained from density functional calculations. With this methodology, we can explain, from a very simple concept of selective interference, the "wobbling," i.e., the photoemission intensity modulation and/or apparent dispersion of the Fermi surface and spectra along the out-of-plane (k(z)) direction, and the "mixed 2D/3D" characteristics observed in some experiments. We conclude that the critical model parameters for such an effect are the relative strength of the electronic hopping of each band and the height/width aspect ratio of the surface confining potential. By considering recent photoemission measurements, in light of our findings, we can get relevant information on the electronic wave functions and the nature of the confining potential.

We report the observation of a two-dimensional electron system (2DES) at the (110) surface of the transparent bulk insulator ${\mathrm{SnO}}_{2}$ and the tunability of its carrier density by means of temperature or Eu deposition. The 2DES is insensitive to surface reconstructions and, surprisingly, it survives even after exposure to ambient conditions---an extraordinary fact recalling the well known catalytic properties ${\mathrm{SnO}}_{2}$. Our data show that surface oxygen vacancies are at the origin of such 2DES, providing key information about the long-debated origin of $n$-type conductivity in ${\mathrm{SnO}}_{2}$, at the basis of a wide range of applications. Furthermore, our study shows that the emergence of a 2DES in a given oxide depends on a delicate interplay between its crystal structure and the orbital character of its conduction band.

Recovering the three-dimensional (3D) properties of artificial or biological systems using low X-ray doses is challenging as most techniques are based on computing hundreds of two-dimensional (2D) projections. The requirement for a low X-ray dose also prevents single-shot 3D imaging using ultrafast X-ray sources. Here we show that computed stereo vision concepts can be applied to X-rays. Stereo vision is important in the field of machine vision and robotics. We reconstruct two X-ray stereo views from coherent diffraction patterns and compute a nanoscale 3D representation of the sample from disparity maps. Similarly to brain perception, computed stereo vision algorithms use constraints. We demonstrate that phase-contrast images relax the disparity constraints, allowing occulted features to be revealed. We also show that by using nanoparticles as labels we can extend the applicability of the technique to complex samples. Computed stereo X-ray imaging will find application at X-ray free-electron lasers, synchrotrons and laser-based sources, and in industrial and medical 3D diagnosis methods.