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The unusual observation of the Kondo effect in the two-dimensional electron gas (2DEG) of magnetically undoped AlGaN/GaN heterostructures is reported. The temperature-dependent zero-field resistivity data exhibits an upturn below 120 K, while the standard low-temperature weak localization and then weak antilocalization behaviour is revealed at T → 0. Magnetic transport investigations of the system are performed in the temperature range of 0.1–300 K and at magnetic fields up to 8 T, applied perpendicularly to the 2DEG plane. The experimental data are analyzed in terms of the multichannel Kondo model for d0 magnetic materials and weak localization theory taking into account the spin-orbit interaction.

Angle-resolved photoelectron spectroscopy (ARPES) is the main experimental tool to explore electronic structure of solids resolved in the electron momentum k . Soft-X-ray ARPES (SX-ARPES), operating in a photon energy range around 1 keV, benefits from enhanced probing depth compared to the conventional VUV-range ARPES, and elemental/chemical state specificity achieved with resonant photoemission. These advantages make SX-ARPES ideally suited for buried heterostructure and impurity systems, which are at the heart of current and future electronics. These applications are illustrated here with a few pioneering results, including buried quantum-well states in semiconductor and oxide heterostructures, their bosonic coupling critically affecting electron transport, magnetic impurities in diluted magnetic semiconductors and topological materials, etc. High photon flux and detection efficiency are crucial for pushing the SX-ARPES experiment to these most photon-hungry cases.

Using spin-resolved and angle-resolved photoemission spectroscopy and first-principles calculations, we have identified bulk band inversion and the spin-polarized surface state evolved from a weak topological insulator (TI) phase in van der Waals materials ${\mathrm{Nb}}_{3}X{\mathrm{Te}}_{6}$ $(X=\mathrm{Si}, \mathrm{Ge})$. The fingerprints of weak TIs homologically emerge with hourglass fermions as multinodal chains composed by the same pair of valence and conduction bands gapped by spin-orbit coupling. The unique topological state, with a pair of valence and conduction bands encoding both weak TI and hourglass semimetal nature, is essential and guaranteed by nonsymmorphic symmetry. It is distinct from TIs studied previously based on band inversions without symmetry protections.

Electronic structure of LaAlO3/SrTiO3 (LAO/STO) samples, grown at low oxygen pressure and post-annealed ex situ, was investigated by soft-x-ray ARPES focussing on the Fermi momentum (k F) of the mobile electron system (MES). X-ray irradiation of these samples at temperatures below 100 K creates oxygen vacancies (VOs) injecting Ti t 2g-electrons into the MES. At this temperature the oxygen out-diffusion is suppressed, and the VOs should appear mostly in the top STO layer. The x-ray generated MES demonstrates, however, a pronounced three-dimensional (3D) behavior as evidenced by variations of its experimental k F over different Brillouin zones. Identical to bare STO, this behavior indicates an unexpectedly large extension of the x-ray generated MES into the STO depth. The intrinsic MES in the standard LAO/STO samples annealed in situ, in contrast, demonstrates purely two-dimensional (2D) behaviour. The relevance of our ARPES data analysis is supported by model calculations to compare the intensity vs gradient methods of the k F determination as a function of the energy resolution ratio to the bandwidth. Based on self-interaction-corrected DFT calculations of the MES induced by VOs at the interface and in STO bulk, we discuss possible scenarios of the puzzling 3D-ity. It may involve either a dense ladder of quantum-well states formed in a long-range interfacial potential or, more likely, x-ray-induced bulk metallicity in STO accessed in the ARPES experiment through a short-range interfacial barrier. The mechanism of this metallicity may involve remnant VOs and photoconductivity-induced metallic states in the STO bulk, and even more exotic mechanisms such as x-ray induced formation of Frenkel pairs.

Nanostructures based on buried interfaces and heterostructures are at the heart of modern semiconductor electronics as well as future devices utilizing spintronics, multiferroics, topological effects and other novel operational principles. Knowledge of electronic structure of these systems resolved in electron momentum k delivers unprecedented insights into their physics. Here, we explore 2D electron gas formed in GaN/AlGaN high-electron-mobility transistor (HEMT) heterostructures with an ultrathin barrier layer, key elements in current high-frequency and high-power electronics. Its electronic structure is accessed with angle-resolved photoelectron spectroscopy whose probing depth is pushed to a few nm using soft-X-ray synchrotron radiation. The experiment yields direct k-space images of the electronic structure fundamentals of this system: the Fermi surface, band dispersions and occupancy, and the Fourier composition of wavefunctions encoded in the k-dependent photoemission intensity. We discover significant planar anisotropy of the electron Fermi surface and effective mass connected with relaxation of the interfacial atomic positions, which translates into non-linear (high-field) transport properties of the GaN/AlGaN heterostructures as an anisotropy of the saturation drift velocity of the 2D electrons., In press with Nature Comm. (2018)

We investigate the three-dimensional electronic structure of the seminal charge-density-wave (CDW) material $2H\ensuremath{-}\mathrm{NbS}{\mathrm{e}}_{2}$ by soft x-ray angle-resolved photoelectron spectroscopy and density functional theory. Our results reveal the pronounced 3D character of the electronic structure formed in the quasi-two-dimensional layered crystal structure. In particular, we find a strong dispersion along ${k}_{z}$ excluding a nesting-driven CDW formation based on experimental data. The 3D-like band structure of $2H\ensuremath{-}\mathrm{NbS}{\mathrm{e}}_{2}$ has strong implications for the intriguing phase competition of CDW order with superconductivity.

The development of novel nano-oxide spintronic devices would benefit greatly from interfacing with emergent phenomena at oxide interfaces. In this paper, we integrate highly spin-split ferromagnetic semiconductor EuO onto perovskite SrTiO3 (001). A careful deposition of Eu metal by molecular beam epitaxy results in EuO growth via oxygen out-diffusion from SrTiO3. This in turn leaves behind a highly conductive interfacial layer through generation of oxygen vacancies. Below the Curie temperature of 70 K of EuO, this spin-polarized two-dimensional t 2g electron gas at the EuO/SrTiO3 interface displays very large positive linear magnetoresistance (MR). Soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) reveals the t 2g nature of the carriers. First principles calculations strongly suggest that Zeeman splitting, caused by proximity magnetism and oxygen vacancies in SrTiO3, is responsible for the MR. This system offers an as-yet-unexplored route to pursue proximity-induced effects in the oxide two-dimensional t 2g electron gas.

Semiconductor spintronics provides a framework for hybrid devices combining logic, communication and storage, circumventing limitations of the current electronics, especially with respect to the energy efficiency. Enormous efforts have been invested worldwide into the development of spintronics based on Si, the mainstream semiconductor platform. Notwithstanding remarkable pace, Si spintronics still experiences a technological bottleneck – creation of significant spin polarization in nonmagnetic Si. An emerging approach based on direct electrical spin injection from a ferromagnetic semiconductor – EuO being the prime choice – avoids problems inherent to metallic injectors. The functionality of the EuO/Si spin contact is controlled by the interface band alignment. To be competitive with charge electronics, the EuO/Si interface should exhibit a band offset which falls within the 0.5–2 eV range. We employ a soft-X-ray ARPES technique, using synchrotron radiation with photon energies around 1 keV, to probe the electronic structure of the buried EuO/Si interface with momentum resolution and chemical specificity. The band structure reveals a conduction band offset of 1.0 eV attesting the technological potential of the EuO/Si system.

The photoelectron spectra and the electron energy loss spectra for clean surfaces of construction zirconium alloys E110 and E635, as well as the surfaces of these alloys with different levels of oxidation starting from submonolayers of oxygen, were measured. The electronic structures of the valence band, Zr4p and Sn4d states, as well as changes in the electronic structure upon on oxidation were investigated. The results are compared with the data obtained upon the oxidation of pure zirconium. The surface oxidation rates of the E110 and E635 alloys are practically the same but significantly lower than the oxidation rate of pure zirconium.

The fundamental advantage of soft-X-ray ARPES (SX-ARPES) with photon energies around 1 keV compared to the conventional vacuum-ultraviolet ARPES (VUV-ARPES) is an increase of the photoelectron escape depth. This enhances the bulk sensitivity of the ARPES experiment as well as enables access to buried systems such as interfaces, heterostructures, and impurities. This advantage combines with resolution in three-dimensional (3D) momentum k, which, on the other hand, degrades upon further increase of energy into the hard-X-ray range. The main challenge of SX-ARPES is the valence band (VB) photoemission cross-section decreasing by a few orders of magnitude compared to the VUV-ARPES energy range. This problem can only be overpowered with advanced synchrotron radiation instrumentation delivering high photon flux. In this article, after a recap of the main virtues and current challenges of SX-ARPES, we report on the recent instrumental progress at the Swiss Light Source (SLS), where a new SX-ARPES facility was in...

The photoelectron spectra and electron energy loss spectra in reflection geometry have been measured for the clean (010) surface of NaxV2O5 compounds with x = 0.23, 0.28 and 0.33. The investigations made it possible to reveal some specific features of the electronic structure of such compounds. Ionization parameters of trinitrotoluene molecules have been measured at the surface of polycrystalline Na0.33V2O5 and the (010) surface of the single crystals Na0.28V2O5 and Na0.33V2O5. The results of investigations can be successfully used in a construction of the model of electronic structure of alkaline oxide bronzes and also in the development of materials for selective surface ionization sources of organic nitro-compounds.

Unusual observation of the Kondo effect in two-dimensional electron gas (2DEG) of magnetically undoped AlGaN/GaN high-electron-mobility transistor heterostructures is reported. The temperature-dependent zero-field resistivity data exhibited an upturn below 120 K, while the standard low-temperature weak-localization behaviour was revealed at T → 0. Magnetotransport characterization was carried out in the magnetic fields up to 80 kOe, applied perpendicular to the 2DEG plane, in the temperature range 1.8 ÷ 300 K. Negative low-temperature magnetoresistance with a magnitude of order of 1 % was detected. The data set is analysed in the frame of the multichannel Kondo model for d0-magnetic materials.

The electronic structure of titanium with different oxidation levels of the surface was studied by photoelectron spectroscopy and electron energy loss spectroscopy (EELS). The spectra of the valence band and the core level of Ti3p and EEL spectra were recorded for a titanium surface with oxidation exposures from 0.12 to 900 L at room temperature. In general, the titanium oxidation process is similar to that for zirconium, which was investigated earlier; however, certain features of the electronic structure that were not observed earlier were revealed for titanium. Even at high oxidation exposures of ∼1000 L, both O2p valence band and valence band corresponding to titanium metal states were observed. During consecutive oxidation, one can see four different positions of the Ti3p peak corresponding to titanium atoms with different valences. It is probable that, at the initial oxidation stage, islands of complex composition oxide were formed on the surface and grew into the bulk during further oxidation.

Photoelectron spectra of aluminum complexes with 8-oxyquinoline (Alq3) and zinc complexes with N, N′-(o-phenylene)-bis(salicylidenediamine) (ZnSaloph) were measured on a photoelectron spectroscopy station at the Kurchatov Centre for Synchrotronic Radiation over the range of photon energies from 10 to 100 eV. Thin films of these compounds (∼20 nm thick) prepared by thermal evaporation onto glass substrates covered with transparent electroconductive layer of solid solution of tin and indium oxides (In2O3, SnO2, ITO) were used as the objects of this study. The spectra of the valence band of these compounds and Al 2p level were measured. The data of quantum chemical calculations of the electronic structure obtained by the density functional theory are in good agreement with the experimental photoelectron spectra. Analysis of performance efficiency of the device was carried out on the basis of the results of calculation. This procedure does not result in decomposition of samples in the process of investigation and may be used for studying the electronic structure of coordination compounds with organic ligands in films.

Photoelectron spectra and optical absorption spectra for clean surfaces of quasicrystalline samples of the Ti—Zr—Ni system are measured. The resonant photoelectron spectra are also measured in the range of photon energies corresponding to Ni 2p and Zr 3d absorption thresholds. The change in the intensities of the spectrum of the valence band near the Zr 3d threshold is insignificant. The emission of Auger electrons increases near the Ni 2p threshold in a resonance way. Our studies make it possible to reveal certain specific features of the electronic structure of quasicrystals, which can be used to construct models of actual electronic structures of quasi-crystalline materials.

The photoelectron spectra, photon absorption spectra, and electron energy loss spectra have been measured in reflection geometry for the clean surface of quasicrystalline samples and ω-phase samples of identical chemical composition that belong to the Al-Cu-Fe system. The investigations performed made it possible to reveal some specific features of the electronic structure of quasicrystals; these features can be successfully used in construction of the model of the actual electronic structure of quasicrystalline materials. The resonant photoelectron spectra have been measured in the range of photon energies corresponding to the Fe2p and Cu2p absorption edges. A sharp increase (by a factor of 3–4) in the spectral intensity is found in the valence band near the Fe2p edge. Near the Cu2p edge, the Auger electron emission resonantly increased, while the emission from the valence band almost did not change.

Electronic structure of the three-dimensional colossal magnetoresistive perovskite La1−xSrxMnO3 has been established using soft-x-ray angle-resolved photoemission spectroscopy with its intrinsically sharp definition of three-dimensional electron momentum. The experimental results show much weaker polaronic coupling compared to the bilayer manganites and are consistent with the theoretical band structure including the empirical Hubbard parameter U. The experimental Fermi surface unveils the canonical topology of alternating three-dimensional electron spheres and hole cubes, with their shadow contours manifesting the rhombohedral lattice distortion. This picture has been confirmed by one-step photoemission calculations including displacement of the apical oxygen atoms. The rhombohedral distortion is neutral to the Jahn-Teller effect and thus polaronic coupling, but affects the double-exchange electron hopping and thus the colossal magnetoresistance effect. DOI: 10.1103/PhysRevLett.114.237601 PACS numbers: 79.60.-i, 71.18.+y, 75.47.Gk, 75.47.Lx Hole-doped manganites with the general chemical formula ðLa; SrÞ x MnOy (LSMO) are typical transition metal oxides (TMOs) with perovskite structure, which have attracted tremendous interest due to the discovery of their colossal magnetoresistance (CMR). Coupling of charge, orbital, spin, and lattice degrees of freedoms results in a rich phase diagram of these materials, extending over antiferromagnetic, ferromagnetic (FM), and paramagnetic (PM) insulating and metallic states. The electron transport in manganites is coupled to their ferromagnetism and is generally described in the framework of the double-exchange (DE) mechanism. However, an important role in physics of these materials can be played by polaronic effects coupling of the electron and lattice

We have observed a feature at similar to2.5 eV binding energy (BE) in the valence band of La1-xSrxMnO3 single crystals not previously reported in the literature. The relative intensity of this feature decreases with increasing Sr concentration x and has a strong photon energy dependence. It also exhibits a complicated angle dependence. In this report we present a full set of angular-resolved photoemission spectra performed on (100) La1-xSrxMnO3 single crystals. The shape of the valence band has a strong dependence on emission and incident angles. In particular, at hupsilon = 130 eV the intensity of the 2.5 eV BE feature increases when the emission angle changes from 0 to 20degrees, and then sharply drops. The shape of the valence band changes with incident angle as well, and the feature is more pronounced at larger incident angles. A strong dependences on Sr doping as well as photon energy is also established. (C) 2004 Elsevier B.V. All rights reserved. (Less)

Electronic structure of the three-dimensional colossal magnetoresistive perovskite La(1-x)Sr(x)MnO3 has been established using soft-x-ray angle-resolved photoemission spectroscopy with its intrinsically sharp definition of three-dimensional electron momentum. The experimental results show much weaker polaronic coupling compared to the bilayer manganites and are consistent with the theoretical band structure including the empirical Hubbard parameter U. The experimental Fermi surface unveils the canonical topology of alternating three-dimensional electron spheres and hole cubes, with their shadow contours manifesting the rhombohedral lattice distortion. This picture has been confirmed by one-step photoemission calculations including displacement of the apical oxygen atoms. The rhombohedral distortion is neutral to the Jahn-Teller effect and thus polaronic coupling, but affects the double-exchange electron hopping and thus the colossal magnetoresistance effect.

Photoelectron spectroscopy results on single crystals of the superconductors Bi 2 Sr 2 CaCu 2 O 8 ,Bi 2 Sr 2 CuO 6 , Ba 0.6 K 0.4 BiO 3 and the semiconductor Ba 0.9 K 0.1 BiO 3 are reported for the photon energy region around the O K absorption threshold. The development of the O- KVV Auger structure has been carefully monitored as a function of photon energy. A non-monotonic behavior displaying a feature at a constant binding energy of about 14 eV was found for Bi 2 Sr 2 CaCu 2 O 8 and Bi 2 Sr 2 CuO 6 in a narrow photon energy region of 1 eV at the main edge of the O K absorption spectrum around 530 eV. The corresponding enhancement, connected with the autoionization of O 2 p states, is absent in Ba 1− x K x BiO 3 in contrast to Bi 2 Sr 2 CaCu 2 O 8 and Bi 2 Sr 2 CuO 6 . The resonant enhancement is more pronounced for Bi 2 Sr 2 CuO 6 as compared to Bi 2 Sr 2 CaCu 2 O 8 , which can be explained by a lower charge carrier concentration in the former case, leading to a more localized nature of intermediate O 2 p states. The model parameters Cu d – d and O p – p Coulomb interactions and the charge transfer energy Δ are estimated from the experiments.

Electronic structure of a molecular beam epitaxy-grown system of (In,Mn)As quantum dots (QDs) buried in GaAs is explored with soft-x-ray angle-resolved photoelectron spectroscopy (ARPES) using photon energies around 1 keV. This technique, ideally suited for buried systems, extends the momentum-resolving capabilities of conventional ARPES with enhanced probing depth as well as elemental and chemical state specificity achieved with resonant photoexcitation. The experimental results resolve the dispersive energy bands of the GaAs substrate buried in ∼2 nm below the surface, and the impurity states (ISs) derived from the substitutional Mn atoms in the (In,Mn)As QDs and oxidized Mn atoms distributed near the surface. An energy shift of the Mn ISs in the QDs compared to (In,Mn)As DMS is attributed to the band offset and proximity effect at the interface with the surrounding GaAs. The absence of any ISs in the vicinity of the VBM relates the electron transport in (In,Mn)As QDs to the prototype (In,Mn)As diluted magnetic semiconductor. The SX-ARPES results are supported by measurements of the shallow core levels under variation of probing depth through photon energy. X-ray absorption measurements identify significant diffusion of interstitial Mn atoms out of the QDs towards the surface, and the role of magnetic circular dichroism is to block the ferromagnetic response of the (In,Mn)As QDs. Possible routes are drawn to tune the growth procedure aiming at practical applications of the (In,Mn)As based systems.

Some heavy fermion materials show so-called hidden-order phases which are invisible to many characterization techniques and whose microscopic origin remained controversial for decades. Among such hidden-order compounds, CeB6 is of model character due to its simple electronic configuration and crystal structure. Apart from more conventional antiferromagnetism, it shows an elusive phase at low temperatures, which is commonly associated with multipolar order. Here we show that this phase roots in a Fermi surface instability. This conclusion is based on a full 3D tomographic sampling of the electronic structure by angle-resolved photoemission and comparison with inelastic neutron scattering data. The hidden order is mediated by itinerant electrons. Our measurements will serve as a paradigm for the investigation of hidden-order phases in f-electron systems, but also generally for situations where the itinerant electrons drive orbital or spin order., In compounds containing 4f and 5f elements, hidden-order phases exist which are undetectable by many methods, the origins of which are debated. Here, the authors use photoemission and neutron scattering methods to show how such a multipolar-ordered phase emerges due to Fermi surface instability in CeB6.

Quasicrystals (QCs) are intermetallic alloys that have excellent long-range order but lack translational symmetry in at least one dimension. The valence band electronic structure near the Fermi energy EF in such materials is of special interest since it has a direct relation to their unusual physical properties. However, the Fermi surface (FS) topology as well as the mechanism of QC structure stabilization are still under debate. Here we report the first observation of the three-dimensional FS and valence band dispersions near EF in decagonal Al70Ni20Co10 (d-AlNiCo) QCs using soft X-ray angle-resolved photoemission spectroscopy. We show that the FS, formed by dispersive Al sp-states, has a multicomponent character due to a large contribution from high-order bands. Moreover, we discover that the magnitude of the gap at the FS related to the interaction with Brillouin zone boundary (Hume–Rothery gap) critically differs for the periodic and quasiperiodic directions., Quasicrystals show long-range order but lack the symmetry of conventional crystals. Here, the authors study the bulk electronic structure of quasicrystals and their three-dimensional Fermi surface topology, providing links to the structural stability of these materials.