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Understanding and controlling the electronic structure of thin layers of quantum materials is a crucial first step towards designing heterostructures where new phases and phenomena, including the metal-insulator transition (MIT), emerge. Here, we demonstrate control of the MIT via tuning electronic bandwidth and local site environment through selection of the number of atomic layers deposited. We take CaVO3, a correlated metal in its bulk form that has only a single electron in its V4+ 3d manifold, as a representative example. We find that thick films and ultrathin films (≤6 unit cells, u.c.) are metallic and insulating, respectively, while a 10 u.c. CaVO3 film exhibits a clear thermal MIT. Our combined X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) study reveals that the thickness-induced MIT is triggered by electronic bandwidth reduction and local moment formation from V3+ ions, that are both a consequence of the thickness confinement. The thermal MIT in our 10 u.c. CaVO3 film exhibits similar changes in the RIXS response to that of the thickness-induced MIT in terms of reduction of bandwidth and V 3d–O 2p hybridization. Correlated metals: A thickness-dependent metal–insulator transition A sharp thickness-dependent metal–insulator transition has been observed in CaVO3, which is a correlated metal in its bulk form, but an insulator in its ultrathin film form. Milan Radovic and Thorsten Schmitt from the Paul Scherrer Institute in Switzerland, and colleagues, used X-ray absorption spectroscopy and resonant inelastic X-ray scattering to study CaVO3 films deposited on a SrTiO3 substrate. They found that with a growing number of layers the electronic bandwidth changed continuously by up to 40%, affecting the degree of electron localization and eventually resulting in a thermal metal–insulator transition at a thickness of 10 unit cells. The ability to control this phase transition is potentially useful for applications, for example in transparent conductors and field-effect transistors, while understanding the role of reduced dimensionality is a necessary step towards the design of functional quantum materials. [ABSTRACT FROM AUTHOR]

Unconventional superconductivity arises at the border between the strong coupling regime with local magnetic moments and the weak coupling regime with itinerant electrons, and stems from the physics of criticality that dissects the two. Unveiling the nature of the quasiparticles close to quantum criticality is fundamental to understand the phase diagram of quantum materials. Here, using resonant inelastic x-ray scattering (RIXS) and Fe − K β emission spectroscopy (XES), we visualize the coexistence and evolution of local magnetic moments and collective spin excitations across the superconducting dome in isovalently-doped BaFe 2 (As 1 − x P x ) 2 (0.00 ≤ x ≤ 0.52 ). Collective magnetic excitations resolved by RIXS are gradually hardened, whereas XES reveals a strong suppression of the local magnetic moment upon doping. This relationship is captured by an intermediate coupling theory, explicitly accounting for the partially localized and itinerant nature of the electrons in Fe pnictides. Finally, our work identifies a local-itinerant spin fluctuations channel through which the local moments transfer spin excitations to the particle-hole (paramagnons) continuum across the superconducting dome. Understanding the effect of the subtle interplay between strong and weak coupling regimes on the properties of quasiparticles is key to understanding unconventional superconductivity. Here, X-ray spectroscopies reveal the partially localized and itinerant magnetic character of quasiparticles in a doped iron pnictide material. [ABSTRACT FROM AUTHOR]

Tunable electronic properties of transition metal oxides and their interfaces offer remarkable functionalities for future devices. The interest in these materials has been boosted with the discovery of a 2D electron gas (2DEG) at SrTiO3 (STO)‐based interfaces. For the majority of these systems, oxygen vacancies play a crucial role in the emergence of interface conductivity, ferromagnetism, and high electron mobility. Despite its great importance, controlling the density and spatial distribution of oxygen vacancies in a dynamic way remains extremely challenging. Here, lithography‐like writing of a metallic state at the interface between SrTiO3 and amorphous Si using X‐ray irradiation is reported. Using a combination of transport techniques and in operando photoemission spectroscopy, it is revealed in real time that the X‐ray radiation induces transfer of oxygen across the interface leading to the on‐demand formation of oxygen vacancies and a 2DEG in STO. The formed 2DEG stays stable in ambient conditions as the interface oxygen vacancies are stabilized by the capping of Si. The study provides a fundamental understanding of X‐ray‐induced redox reactions at the SrTiO3‐based interfaces and in addition shows the potential of X‐ray radiation for patterning stabile conductive pathways for future oxide‐based electronic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) systems, such as high-temperature superconductors, surface states of topological insulators, and layered materials, have been intensively studied using vacuum-ultraviolet (VUV) angle-resolved photoemission spectroscopy (ARPES). In semiconductor films (heterostructures), quantum well (QW) states arise due to electron/hole accumulations at the surface (interface). The quantized states due to quantum confinement can be observed by VUV-ARPES, while the periodic intensity modulations along the surface normal (k z) direction of these quantized states are also observable by varying incident photon energy, resembling three-dimensional (3D) band dispersion. We have conducted soft X-ray (SX) ARPES measurements on thick and ultrathin III-V semiconductor InSb(001) films to investigate the electronic dimensionality reduction in semiconductor QWs. In addition to the dissipation of the k z dispersion, the SX-ARPES observations demonstrate the changes of the symmetry and periodicity of the Brillouin zone in the ultrathin film as 2D QW compared with these of the 3D bulk one, indicating the electronic dimensionality reduction of the 3D bulk band dispersion caused by the quantum confinement. The results provide a critical diagnosis using SX-ARPES for the dimensionality reduction in semiconductor QW structures. • The electronic structures of the thick and ultrathin InSb films are observed by SX-ARPES. • The SX-ARPES observations demonstrate the changes in the symmetry and periodicity of the Brillouin zone. • SX-ARPES provides a critical diagnosis for the dimensionality reduction in semiconductor quantum well structures. [ABSTRACT FROM AUTHOR]

Two‐dimensional dopant layers (δ‐layers) in semiconductors provide the high‐mobility electron liquids (2DELs) needed for nanoscale quantum‐electronic devices. Key parameters such as carrier densities, effective masses, and confinement thicknesses for 2DELs have traditionally been extracted from quantum magnetotransport. In principle, the parameters are immediately readable from the one‐electron spectral function that can be measured by angle‐resolved photoemission spectroscopy (ARPES). Here, buried 2DEL δ‐layers in silicon are measured with soft X‐ray (SX) ARPES to obtain detailed information about their filled conduction bands and extract device‐relevant properties. This study takes advantage of the larger probing depth and photon energy range of SX‐ARPES relative to vacuum ultraviolet (VUV) ARPES to accurately measure the δ‐layer electronic confinement. The measurements are made on ambient‐exposed samples and yield extremely thin (< 1 nm) and dense (≈1014 cm−2) 2DELs. Critically, this method is used to show that δ‐layers of arsenic exhibit better electronic confinement than δ‐layers of phosphorus fabricated under identical conditions. [ABSTRACT FROM AUTHOR]

The microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn5 and other Ce-based heavy fermion materials, depends strongly on the efficiency with which f electrons are delocalized from the rare earth sites and participate in a Kondo lattice. Replacing Ce3+ (4f1, J = 5/2) with Sm3+ (4f5, J = 5/2), we show that a combination of the crystal electric field and on-site Coulomb repulsion causes SmCoIn5 to exhibit a Γ7 ground state similar to CeCoIn5 with multiple f electrons. We show that with this single-ion ground state, SmCoIn5 exhibits a temperature-induced valence crossover consistent with a Kondo scenario, leading to increased delocalization of f holes below a temperature scale set by the crystal field, Tv ≈ 60 K. Our result provides evidence that in the case of many f electrons, the crystal field remains the dominant tuning knob in controlling the efficiency of delocalization near a heavy fermion quantum critical point, and additionally clarifies that charge fluctuations play a general role in the ground state of "115" materials. In heavy fermion metals, exotic states such as superconductivity depend to a large extent on whether unpaired electrons can become delocalized from the magnetic sites. The authors demonstrate that the primary driving force behind delocalization is the local environment of the rare earth ions and the strength of the resulting charge fluctuations. [ABSTRACT FROM AUTHOR]

Studying multi-particle elementary excitations has provided unique access to understand collective many-body phenomena in correlated electronic materials, paving the way towards constructing microscopic models. In this work, we perform O K-edge resonant inelastic X-ray scattering (RIXS) on the quasi-one-dimensional cuprate Sr 14 Cu 24 O 41 with weakly-doped spin ladders. The RIXS signal is dominated by a dispersing sharp mode ~ 270 meV on top of a damped incoherent component ~ 400-500 meV. Comparing with model calculations using the perturbative continuous unitary transformations method, the two components resemble the spin-conserving ΔS = 0 two-triplon bound state and continuum excitations in the spin ladders. Such multi-spin response with long-lived ΔS = 0 excitons is central to several exotic magnetic properties featuring Majorana fermions, yet remains unexplored given the generally weak cross-section with other experimental techniques. By investigating a simple spin-ladder model system, our study provides valuable insight into low-dimensional quantum magnetism. Quasi-one-dimensional spin ladders are useful models to study collective many-body phenomena. Here, the authors investigate the excitations of weakly hole-doped cuprate ladders using oxygen K-edge resonant inelastic X-ray scattering revealing fingerprints of spin singlet multi-triplon bound state and continuum modes that are relevant to understanding various collective spin phenomena, such as Majorana fermions. [ABSTRACT FROM AUTHOR]

Weyl semimetal is a unique topological phase with topologically protected band crossings in the bulk and robust surface states called Fermi arcs. Weyl nodes always appear in pairs with opposite chiralities, and they need to have either time‐reversal or inversion symmetry broken. When the time‐reversal symmetry is broken the minimum number of Weyl points (WPs) is two. If these WPs are located at the Fermi level, they form an ideal Weyl semimetal (WSM). In this study, intrinsic ferromagnetic (FM) EuCd2As2 are grown, predicted to be an ideal WSM and studied its electronic structure by angle‐resolved photoemission spectroscopy, and scanning tunneling microscopy which agrees closely with the first principles calculations. Moreover, anomalous Hall conductivity and Nernst effect are observed, resulting from the non‐zero Berry curvature, and the topological Hall effect arising from changes in the band structure caused by spin canting produced by magnetic fields. These findings can help realize several exotic quantum phenomena in inorganic topological materials that are otherwise difficult to assess because of the presence of multiple pairs of Weyl nodes. [ABSTRACT FROM AUTHOR]

Pb(Zr,Ti)O3 (PZT) is the most common ferroelectric (FE) material widely used in solid‐state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr0.2Ti0.8)O3 using soft‐X‐ray angle‐resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out‐of‐plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin‐film PZT samples, where a thickness‐induced self‐doping results in their heavy doping. For the first time, the soft‐X‐ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE‐polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO3 and LaxSrMn1−xO3 substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in‐gap states below the Fermi energy. For the opposite polarization state (P−), the positive charges near the surface are identified as cation vacancies resulting from non‐ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements. [ABSTRACT FROM AUTHOR]

Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework based on screening of the polarization field and depolarizing charges. Additional effects still unexplored are the band dependence of the interfacial charge modulation and the associated changes of the electron-phonon interaction, coupling the charge and lattice degrees of freedom. Here, multiferroic heterostructures of the colossal-magnetoresistance manganite La1-xSrxMnO3 buried under ferroelectric BaTiO3 and PbZrxTi1-xO3 are investigated using soft-X-ray angle-resolved photoemission. The experimental band dispersions from the buried La1-xSrxMnO3 identify coexisting two-dimensional hole and three-dimensional electron charge carriers. The ferroelectric polarization modulates their charge density, affecting the coupling of the 2D holes and 3D electrons with the lattice which forms large Fröhlich polarons inherently reducing mobility of the charge carriers. Our k-resolved results on the orbital occupancy, band filling and electron-lattice interaction in multiferroic oxide heterostructures modulated by the ferroelectric polarization disclose most fundamental physics of these systems needed for further progress of beyond-CMOS ferro-functional electronics. A deeper understanding of the coupling at the interface of multiferroics heterostructures is being achieved by the use of synchrotron radiation techniques. Here, the authors use k-resolved soft X-ray photoemission spectroscopy and first principles calculations to investigate the band structure of several multiferroic heterostructures, isolating the distinct signature of the interface. [ABSTRACT FROM AUTHOR]

The article presents a study that explores the momentum-resolved electronic structure and band offsets in an epitaxial NbN/GaN superconductor/semiconductor heterojunction. It mentions that electronic structure of heterointerfaces as a pivotal factor for their device functionality. It discusses that momentum-resolved understanding of electronic properties of interfaces elucidated in work opens up new frontiers for the quantum materials where interfacial states play a defining role.

The design of epitaxial semiconductor–superconductor and semiconductor–metal quantum devices requires a detailed understanding of the interfacial electronic band structure. However, the band alignment of buried interfaces is difficult to predict theoretically and to measure experimentally. This work presents a procedure that allows to reliably determine critical parameters for engineering quantum devices; band offset, band bending profile, and number of occupied quantum well subbands of interfacial accumulation layers at semiconductor‐metal interfaces. Soft X‐ray angle‐resolved photoemission is used to directly measure the quantum well states as well as valence bands and core levels for the InAs(100)/Al interface, an important platform for Majorana‐zero‐mode based topological qubits, and demonstrate that the fabrication process strongly influences the band offset, which in turn controls the topological phase diagrams. Since the method is transferable to other narrow gap semiconductors, it can be used more generally for engineering semiconductor–metal and semiconductor–superconductor interfaces in gate‐tunable superconducting devices. [ABSTRACT FROM AUTHOR]

We show with systematic photoemission spectroscopy and scanning tunneling spectroscopy data that a spin helical surface state appears on the (110) surface of noncentrosymmetric, three-dimensional HgTe. The topological surface state in HgTe exhibits sharp, linear dispersion without κz variation, as well as clear, left-right imbalanced spin polarization and circular dichroism. Chemical gating by alkali metal deposition on the surface causes the unexpected opening and/or increase of a surface insulating gap without changing its topological property. Such an unusual behavior we uncover in three-dimensional HgTe sheds light on a convenient control of the Fermi surface and quantum transport in a topological insulator. [ABSTRACT FROM AUTHOR]

Fractionalization of an electronic quasiparticle into spin, charge, and orbital parts is a fundamental and characteristic property of interacting electrons in one dimension. However, real materials are never strictly one dimensional and the fractionalization phenomena are hard to observe. Here we studied the spin and orbital excitations of the anisotropic ladder material CaCu2O3, whose electronic structure is not one dimensional. Combining high-resolution resonant inelastic x-ray scattering experiments with theoretical model calculations, we show that (i) spin-orbital fractionalization occurs in CaCu2O3 along the leg direction x through the xz orbital channel as in a 1D system, and (ii) no fractionalization is observed for the xy orbital, which extends in both leg and rung direction, contrary to a 1D system. We conclude that the directional character of the orbital hopping can select different degrees of dimensionality. Using additional model calculations, we show that spin-orbital separation is generally far more robust than the spin-charge separation. This is not only due to the already mentioned selection realized by the orbital hopping, but also due to the fact that spinons are faster than the orbitons. [ABSTRACT FROM AUTHOR]

The structural and optical properties of (In,Mn)As obtained by molecular beam epitaxy using Mn selective doping were investigated. Despite relatively high growth temperature, the (In,Mn)As quantum dot structures have a high crystalline quality. The synthesis of multi-layered quantum dot structure, as well as p-i-n structure with embedded (In,Mn)As quantum dot layer was carried out. The results obtained can be of importance for the creation of novel light emitting devices. [ABSTRACT FROM AUTHOR]

(Ga,Mn)As is a paradigm of a diluted magnetic semiconductor which shows ferromagnetism induced by doped hole carriers. With a few controversial models emerging from numerous experimental and theoretical studies, the mechanism of the ferromagnetism in (Ga,Mn)As still remains a puzzling enigma. In this article, we use soft x-ray angle-resolved photoemission spectroscopy to positively identify the ferromagnetic Mn 3d-derived impurity band (IB) in (Ga,Mn)As. The band appears dispersionless and hybridized with the light-hole band of the host GaAs. These findings conclude the picture of the valence-band structure of (Ga,Mn)As disputed for more than a decade. The nondispersive character of the IB and its location in vicinity of the valence-band maximum indicate that the Mn 3d-derived IB is formed as a split-off Mn-impurity state predicted by the Anderson impurity model. Responsible for the ferromagnetism is predominantly the transport of hole carriers in the IB. [ABSTRACT FROM AUTHOR]

Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic electronic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them, and determine their respective time scales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are, to a very good approximation, independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence. [ABSTRACT FROM AUTHOR]

We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin- chain materials Li2CuO2 and CuGeO3. By tuning the incoming photon energy to the oxygen K edge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring CuO4 plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables us to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity. [ABSTRACT FROM AUTHOR]

The important source of interface conductivity in LaAlO3/SrTiO3 heterostructures, the Ti 3d carriers, is probed with resonant inelastic x-ray scattering at the Ti 2p3/2 edge of epitaxially grown superlattices. We reveal unambiguously the generation of both localized and delocalized Ti 3d carriers as a result of the built-up heterointerface. Furthermore, we determine that the interface Ti3+O6 octahedra are orthorhombically distorted and quantify the crystal-field splitting energies. We argue that for as-grown superlattices, both types of Ti 3d carriers originate mainly from oxygen vacancies, whereas for fully oxidized samples they result from electronic reconstruction. [ABSTRACT FROM AUTHOR]

Unconventional superconductivity arises at the border between the strong coupling regime with local magnetic moments and the weak coupling regime with itinerant electrons, and stems from the physics of criticality that dissects the two. Unveiling the nature of the quasiparticles close to quantum criticality is fundamental to understand the phase diagram of quantum materials. Here, using resonant inelastic x-ray scattering (RIXS) and Fe − K β emission spectroscopy (XES), we visualize the coexistence and evolution of local magnetic moments and collective spin excitations across the superconducting dome in isovalently-doped BaFe 2 (As 1 − x P x ) 2 (0.00 ≤ x ≤ 0.52 ). Collective magnetic excitations resolved by RIXS are gradually hardened, whereas XES reveals a strong suppression of the local magnetic moment upon doping. This relationship is captured by an intermediate coupling theory, explicitly accounting for the partially localized and itinerant nature of the electrons in Fe pnictides. Finally, our work identifies a local-itinerant spin fluctuations channel through which the local moments transfer spin excitations to the particle-hole (paramagnons) continuum across the superconducting dome. Understanding the effect of the subtle interplay between strong and weak coupling regimes on the properties of quasiparticles is key to understanding unconventional superconductivity. Here, X-ray spectroscopies reveal the partially localized and itinerant magnetic character of quasiparticles in a doped iron pnictide material. [ABSTRACT FROM AUTHOR]

Resonant inelastic x-ray scattering (RIXS) experiments performed at the oxygen K edge on the iridate perovskites Sr2 IrO4 and Sr3 Ir2 O7 reveal a sequence of well-defined dispersive modes over the energy range up to ~0.8 eV . The momentum dependence of these modes and their variation with the experimental geometry allows us to assign each of them to specific collective magnetic and/or electronic excitation processes, including single and bimagnons, and spin-orbit and electron-hole excitons. We thus demonstrate that dispersive magnetic and electronic excitations are observable at the O K edge in the presence of the strong spin-orbit coupling in the 5 d shell of iridium and strong hybridization between Ir 5 d and O 2 p orbitals, which confirm and expand theoretical expectations. More generally, our results establish the utility of O K-edge RIXS for studying the collective excitations in a range of 5 d materials that are attracting increasing attention due to their novel magnetic and electronic properties. Especially, the strong RIXS response at O K edge opens up the opportunity for investigating collective excitations in thin films and heterostructures fabricated from these materials. [ABSTRACT FROM AUTHOR]

We present an experimental study of the local and collective magnetism of EuFe2As2 that is isostructural with the high-temperature superconductor parent compound BaFe2As2. In contrast to BaFe2As2, where only Fe spins order, EuFe2As2 has an additional magnetic transition below 20 K due to the ordering of the Eu2+ spins (J=7/2, with L=0 and S=7/2) in an A-type antiferromagnetic texture (ferromagnetic layers stacked antiferromagnetically). This may potentially affect the FeAs layer and its local and correlated magnetism. FeKβ x-ray emission experiments on EuFe2As2 single crystals reveal a local magnetic moment of 1.3±0.15μB at 15 K that increases slightly to 1.45±0.15μB at 300 K. Resonant inelastic x-ray scattering experiments performed on the same crystals show dispersive broad (in energy) magnetic excitations along (0,0)→(1,0) and (0,0)→(1,1) with a bandwidth on the order of 170-180 meV. These results on local and collective magnetism are in line with other parent compounds of the AFe2As2 series (A=Ba, Ca, and Sr), especially the well-characterized BaFe2As2. Thus, our experiments lead us to the conclusion that the effect of the high magnetic moment of Eu on the magnitude of both the Fe local magnetic moment and spin excitations is small and confined to low-energy excitations. [ABSTRACT FROM AUTHOR]

Cuprate materials, such as those hosting high-temperature superconductivity, represent a famous class of materials where the correlations between the strongly entangled charges and spins produce complex phase diagrams. Several years ago, the Zhang-Rice singlet was proposed as a natural quasiparticle in hole-doped cuprates. The occurrence and binding energy of this quasiparticle, consisting of a pair of bound holes with antiparallel spins on the same CuO4 plaquette, depends on the local electronic interactions, which are fundamental quantities for understanding the physics of the cuprates. Here, we employ state-of-the-art resonant inelastic x-ray scattering (RIXS) to probe the correlated physics of the CuO4 plaquettes in the quasi-one-dimensional chain cuprate Li2CuO2. By tuning the incoming photon energy to the O K edge, we populate bound states related to the Zhang-Rice quasiparticles in the RIXS process. Both intra- and interchain Zhang-Rice singlets are observed and their occurrence is shown to depend on the nearest-neighbor spin-spin correlations, which are readily probed in this experiment. We also extract the binding energy of the Zhang-Rice singlet and identify the Zhang-Rice triplet excitation in the RIXS spectra. [ABSTRACT FROM AUTHOR]

We study the fate of the surface states of Bi2Se3 under disorder with strength larger than the bulk gap, caused by neon sputtering and nonmagnetic adsorbates. We find that neon sputtering introduces strong but dilute defects, which can be modeled by a unitary impurity distribution, whereas adsorbates, such as water vapor or carbon monoxide, are best described by Gaussian disorder. Remarkably, these two disorder types have a dramatically different effect on the surface states. Our soft x-ray angle-resolved photoemission spectroscopy (ARPES) measurements combined with numerical simulations show that unitary surface disorder pushes the Dirac state to inward quintuplet layers, burying it below an insulating surface layer. As a consequence, the surface spectral function becomes weaker but retains its quasiparticle peak. This is in contrast to Gaussian disorder, which smears out the quasiparticle peak completely. At the surface of Bi2Se3, neon sputtering adds additional unitary scatterers to the Gaussian disorder of the adsorbates. Since the introduced unitary disorder pushes the surface state to inward layers, the effects of Gaussian disorder are reduced. As a result the ARPES signal becomes sharper upon sputtering. [ABSTRACT FROM AUTHOR]

Emergent magnetic phenomena at interfaces represent a frontier in materials science, pivotal for advancing technologies in spintronics and magnetic storage. In this Letter, we utilize a suite of advanced X-ray spectroscopic and scattering techniques to investigate emergent interfacial ferromagnetism in oxide superlattices composed of antiferromagnetic CaMnO 3 and paramagnetic CaRuO 3 . Our findings demonstrate that ferromagnetism exhibits an asymmetric profile and may extend beyond the interfacial layer into multiple unit cells of CaMnO 3 . Complementary density functional calculations reveal that the interfacial ferromagnetism is driven by the double exchange mechanism, facilitated by charge transfer from Ru to Mn ions. Additionally, defect chemistry, particularly the presence of oxygen vacancies, can play a crucial role in modifying the magnetic moments at the interface, possibly leading to the observed asymmetry between the top and bottom CaMnO 3 interfacial magnetic layers. Our findings underscore the potential of manipulating interfacial ferromagnetism through point defect engineering.

Honeycomb and kagome lattices can host propagating excitations with non-trivial topology as defined by their evolution along closed paths in momentum space. Excitations on such lattices can also be momentum-independent, and the associated flat bands are of interest due to strong interactions between heavy quasiparticles. Here, we report the discovery - using circularly polarized X-rays for the unambiguous isolation of magnetic signals - of a nearly flat spin-wave band and large (compared to elemental iron) orbital moment in the metallic ferromagnet Fe 3 Sn 2 with compact AB-stacked kagome bilayers. As a function of out-of-plane momentum, the nearly flat optical mode and the global rotation symmetry-restoring acoustic mode are out of phase, consistent with a bilayer exchange coupling that is larger than the already large in-plane couplings. The defining units of this topological metal are therefore triangular lattices of octahedral iron clusters rather than weakly coupled kagome planes. The spin waves are strongly damped when compared to elemental iron, opening the topic of topological boson-fermion interactions for deeper exploration within this material platform., (© 2024. The Author(s).)

Owing to their exceptional mechanical, electronic, and phononic transport properties, compositionally complex alloys, including high-entropy alloys, represent an important class of materials. However, the interplay between chemical disorder and electronic correlations, and its influence on electronic structure-derived properties, remains largely unexplored. This is addressed for the archetypal CrMnFeCoNi alloy using resonant and valence band photoemission spectroscopy, electrical resistivity, and optical conductivity measurements, complemented by linear response calculations based on density functional theory. Utilizing dynamical mean-field theory, correlation signatures and damping in the spectra are identified, highlighting the significance of many-body effects, particularly in states distant from the Fermi edge. Electronic transport remains dominated by disorder and potentially short-range order, especially at low temperatures, while visible-spectrum optical conductivity and high-temperature transport are influenced by short quasiparticle lifetimes. These findings improve our understanding of element-specific electronic correlations in compositionally complex alloys and facilitate the development of advanced materials with tailored electronic properties., (© 2024. The Author(s).)

Emergent superconductivity at the LaAlO 3 /KTaO 3 interfaces exhibits a mysterious dependence on the KTaO 3 crystallographic orientations. Here by soft X-ray angle-resolved photoemission spectroscopy, we directly resolve the electronic structure of the LaAlO 3 /KTaO 3 interfacial superconductors and the non-superconducting counterpart. We find that the mobile electrons that contribute to the interfacial superconductivity show strong k ⊥ dispersion. Comparing the superconducting and non-superconducting interfaces, the quasi-three-dimensional electron gas with over 5.5 nm spatial distribution ubiquitously exists and shows similar orbital occupations. The signature of electron-phonon coupling is observed and intriguingly dependent on the interfacial orientations. Remarkably, the stronger electron-phonon coupling signature correlates with the higher superconducting transition temperature. Our observations help scrutinize the theories on the orientation-dependent superconductivity and offer a plausible and straightforward explanation. The interfacial orientation effect that can modify the electron-phonon coupling strength over several nanometers sheds light on the applications of oxide interfaces in general., (© 2024. The Author(s).)