We demonstrate memcapacitor structures utilizing a quasi-two-dimensional electron gas, formed at the crystalline LaAlO3/SrTiO3 heterointerface, as electrodes and SiO2/SrTiO3 as dielectric layer. The observed memcapacitance originates from charge localization in a lateral floating gate, while an applied gate voltage induces a threshold voltage shift of approximately 1 V and enables reversible tuning of the zero-bias capacitance from similar to 147 to similar to 386 pF. Furthermore, preprogrammed or erased gate voltages enable controllable shifts of the capacitance hysteresis window toward positive or negative bias, leading to an enlarged capacitance gap of 243 pF compared to the initial value of 100 pF at zero bias. A theoretical model incorporating charge fluctuations within the oxide layer qualitatively reproduces the experimentally observed capacitance hysteresis and its frequency dependence, which is retained up to approximately 1 kHz. The demonstrated low voltage operation ( +/- 1 V), combined with gate tunability of oxide interface-based memcapacitors with focus on device physics bridges the path for power-efficient next generation computing architectures.

RuO_{2} has emerged as a prototypical candidate for altermagnetism. In the face of daunting evidence for magnetic order in the bulk, the focus naturally shifted to surfaces and ultrathin films, where Coulomb interactions are dimensionally quenched and electron correlations strongly enhanced. Here, we examine atomically ordered, ultrathin RuO_{2}(110) grown on Ru(0001) using a combination of scanning tunneling microscopy (STM), density functional theory, and density matrix renormalization group methods. We observe a nonmagnetic charge order that is imprinted by the incommensurate moiré stacking with the substrate and enhanced by the electronic Fermi-surface scattering within the flat-band surface state. We further identify a nonmagnetic, metastable c(2×2) surface reconstruction that arises from surface phonon softening and can be toggled reversibly via STM tip manipulation. Spin-polarized STM measurements, however, reveal no evidence of magnetic order on the RuO_{2}(110) surface. Our findings of a nonmagnetic charge modulation position ultrathin RuO_{2}(110) as an intriguing platform for exploring moiré-assisted electronic orders.

Magnetic materials with strong spin-orbit coupling (SOC) are essential for the advancement of spin-orbitronic devices, as they enable efficient spin-charge conversion, complex magnetic structures, spin-valley physics, topological phases and other exotic phenomena. 5d transition-metal oxides such as SrIrO3 feature large SOC, but usually show paramagnetic behavior due to broad bands and a low density of states at the Fermi level, accompanied by a relatively low Coulomb repulsion. Here, we unveil ferromagnetism in 5d SrIrO3 thin films grown on SrTiO3 (111). Through substrate-induced structural engineering, a zigzag stacking of three-unit-cell thick layers along the [111] direction is achieved, stabilizing a ferromagnetic state at the interfaces. Magnetotransport measurements reveal an anomalous Hall effect below similar to 30 K and hysteresis in the Hall conductivity below 7 K, indicating ferromagnetic ordering. X-ray magnetic circular dichroism further supports these results. Theoretical analysis suggests that the structural engineering of the IrO6 octahedral network enhances the density of states at the Fermi level and thus stabilizes Stoner ferromagnetism. This work highlights the potential of structurally engineered 5d oxides for spin-orbitronic devices, where efficient control of SOC-induced magnetic phases by electric currents can lead to lower energy consumption and improved performance in next-generation device technologies.

Graphene exhibits extraordinarily high carrier mobility, making it a promising platform for next-generation electronics. Scalable growth on SiC, however, suffers from limited dielectric screening at the graphene-substrate interface, degrading electronic performance. In this work, we systematically enhance dielectric screening by intercalating a bilayer of indium at the graphene-SiC interface. Using graphene's plasmaronic signature observed in angle-resolved photoemission spectroscopy as a proxy for interaction strength, we quantitatively demonstrate strong dielectric screening arising from the interplay of both indium layers. Layer-resolved density functional theory shows that the first indium layer acts as a buffer that absorbs substrate interactions, enabling the second layer to form a nearly free-electron system that efficiently screens the graphene layer above. Experiments with only a single intercalated indium layer reveal reduced screening, confirming the essential role of the second layer. Our results establish 2ML indium intercalation as a powerful route for engineering dielectric environments in graphene.

Two-dimensional quantum spin Hall insulators based on atomic monolayers offer a promising route toward dissipationless electronics, yet their practical use is often limited by environmental instability. Encapsulating the system with a graphene capping layer has been shown to be a reliable method to prevent oxidation and degradation. However, the confirmation of a successful encapsulation still relies on ultra-high vacuum techniques, that considerably slow the process. Here, we present an ex situ, rapid, nondestructive and spatially resolved Raman characterization of graphene-capped bismuthene, a honeycomb monolayer of Bi on SiC. A pronounced Raman scattering peak at around 122 cm-1 is identified as the E2g phonon of bismuthene, via a comparison with density functional perturbation theory calculations. We use excitation-energy and polarization-dependent Raman measurements to enable an unambiguous assignment of the spectral features. Tuning the excitation energy close to the excitonic transition in pristine bismuthene, we observe a strong enhancement of the Raman response and the emergence of additional scattering peaks. In this regime, higher-order phonon features, as well as interfacial modes between bismuthene and the SiC substrate, become visible, suggesting the involvement of resonant scattering processes. Our results establish Raman micro-spectroscopy as a versatile tool for probing graphene-protected quantum materials, providing access to lattice dynamics and interlayer coupling.

Aside from recent advances in artificial intelligence (AI) models, specialized AI hardware is crucial for addressing large volumes of unstructured and dynamic data. Conventional complementary metal-oxide-semiconductor (CMOS)-based AI hardware faces several critical challenges including scaling limitations, the separation of computation and memory units, and overall system energy efficiency. While emerging materials have been proposed to overcome these limitations, issues such as scalability, reproducibility, and compatibility remain critical obstacles. Here, we demonstrate polymorphic electronic devices with programmable transistor, memristor, and memcapacitor functionalities by manipulating the quasi-two-dimensional electron gas in LaAlO3/SrTiO3 heterostructures using lateral gates. A circuit utilizing transistor and memcapacitor functionalities exhibits digit recognition, enabling implementation in physical reservoir computing. An integrated circuit incorporating transistor and memristor functionalities performs logic operations with in-situ output storage and supports advanced reconfigurable synaptic logic operations for multi-input decision-making tasks such as patient monitoring. Our findings pave the way for oxide-based monolithic integrated circuits in a scalable, silicon-compatible, energy-efficient single platform for polymorphic and neuromorphic computing.

The quantum spin Hall insulator bismuthene, a two-third monolayer of bismuth on SiC(0001), is distinguished by helical metallic edge states that are protected by a groundbreaking 800 meV topological gap, making it ideal for room temperature applications. This massive gap inversion arises from a unique synergy between flat honeycomb structure, strong spin orbit coupling, and an orbital filtering effect that is mediated by the substrate. However, the rapid oxidation of bismuthene in air has severely hindered the development of applications, so far confining experiments to ultra-high vacuum conditions. Here, we successfully overcome this barrier, intercalating bismuthene between SiC and a protective sheet of graphene. As we demonstrate through scanning tunneling microscopy and photoemission spectroscopy, graphene intercalation preserves the structural and topological integrity of bismuthene, while effectively shielding it from oxidation in air. We identify hydrogen as the critical component that was missing in previous bismuth intercalation attempts. Our findings facilitate ex-situ experiments and pave the way for the development of bismuthene based devices, signaling a significant step forward in the development of next-generation technologies.

Spin-momentum-locked edge states of quantum spin Hall insulators provide a compelling platform for spintronic applications, owing to their intrinsic protection against backscattering from non-magnetic disorder. This protection emerges from time-reversal symmetry, which pairs Kramers partners of helical edge modes with opposite spin and momentum, thereby strictly forbidding elastic single-particle backscattering within the pair. Yet, contrary to the idealized notion of linear edge bands, the non-monotonic dispersions of realistic materials can host multiple Kramers pairs, reintroducing backscattering channels between them without violating time-reversal symmetry. Here, we investigate inter-Kramers pair backscattering in the non-linear edge bands of the quantum spin Hall insulator indenene, highlighting a critical aspect of edge state stability. Using quasiparticle interference in scanning tunneling spectroscopy - a direct probe of backscattering - we observe intra-band coupling between different Kramers pairs, while energy regions with only a single Kramers pair remain strictly protected. Supported by theoretical analysis, our findings provide an unprecedented experimental demonstration of edge state backscattering fully consistent with their underlying topological protection. This insight has profound implications for numerous quantum spin Hall insulator candidates, emphasizing that the mere presence of gap-traversing edge modes does not inherently guarantee their protection against backscattering.

Current memcapacitor implementations typically demand complex fabrication processes or depend on organic materials exhibiting poor environmental stability and reproducibility. Here, we demonstrate memcapacitor structures utilizing a quasi 2-dimensional electron gas, formed at the crystalline LaAlO3/SrTiO3 heterointerface, as electrodes and SiO2/SrTiO3 as dielectric layer. The observed memcapacitance originates from the charge localization in a lateral floating gate, while an applied gate voltage enables reversible tuning of the device capacitance. Furthermore, preprogrammed or erased gate biases enable controllable shifts of the capacitance hysteresis window toward positive or negative bias, leading to an enlarged capacitance gap at zero bias. A memcapacitor model developed for this system reproduces the main features of the experimental capacitance hysteresis, capturing the effects of charge fluctuations and dielectric frequency modulation within the oxide layer. The demonstrated low-voltage operation and gate tunability of oxide interface-based memcapacitors highlight their potential for power-efficient, capacitor-based neuromorphic and synaptic electronic architectures.

The three-dimensional recording scheme of time-of-flight momentum microscopes (ToF-MMs) is advantageous for fast mapping of the photoelectron distribution in (E,k) parameter space over the entire Brillouin zone. However, the 2 ns pulse period of most synchrotrons is too short for pure ToF photoelectron spectroscopy. The use of a hemispherical analyzer (HSA) as a pre-filter allows ToF-MM at such high pulse rates. The first HSA & ToF hybrid MM is operated at the soft X-ray branch of beamline I09 at the Diamond Light Source, UK. The photon energy ranges from 105 eV to 2 keV, with circular polarization available for hν ≥ 145 eV. The HSA reduces the transmitted energy band to typically 0.5 eV, which is then further analyzed by ToF recording. In initial experiments, the overall efficiency gain when switching from the standard 2D (kx,ky) mode to the 3D (kx,ky,Ekin) hybrid mode was about 24. This value is determined by the number of resolved kinetic energies (here 12) and the transmission gain of the electron optics due to the high pass energy of the HSA in hybrid mode (Epass up to 500 eV). The transmission gain depends on the size of the photon footprint on the sample. Under k-imaging conditions, the energy and momentum resolution are 10.2 meV (FWHM) (4.2 meV with 200 μm slits and Epass = 8 eV) and 0.010 Å-1. The energy filtered X-PEEM mode showed a spatial resolution of 250 nm. As examples, we show 2D band mapping of bilayer graphene, 3D mapping of the Fermi surface of Cu, circular dichroic ARPES for intercalated indenene layers, and the sp valence band of Au. Full-field photoelectron diffraction patterns of Ge show rich structure in k-field diameters of up to 6 Å-1.

Using x-ray photoelectron diffraction (XPD) and angle-resolved photoemission spectroscopy, we study photoemission intensity changes related to changes in the geometric and electronic structure in the kagome metal CsV_3Sb_5 upon transition to an unconventional charge density wave (CDW) state. The XPD patterns reveal the presence of a chiral atomic structure in the CDW phase. Furthermore, using circularly polarized x-rays, we have found a pronounced non-trivial circular dichroism in the angular distribution of the valence band photoemission in the CDW phase, indicating a chirality of the electronic structure. This observation is consistent with the proposed orbital loop current order. In view of a negligible spontaneous Kerr signal in recent magneto-optical studies, the results suggest an antiferromagnetic coupling of the orbital magnetic moments along the c-axis. While the inherent structural chirality may also induce circular dichroism, the observed asymmetry values seem to be too large in the case of the weak structural distortions caused by the CDW.

The quantum spin Hall insulator bismuthene, a two-third monolayer of bismuth on SiC(0001), is distinguished by helical metallic edge states that are protected by a groundbreaking 800 meV topological gap, making it ideal for room temperature applications. This massive gap inversion arises from a unique synergy between flat honeycomb structure, strong spin orbit coupling, and an orbital filtering effect that is mediated by the substrate. However, the rapid oxidation of bismuthene in air has severely hindered the development of applications, so far confining experiments to ultra-high vacuum conditions. Intercalating bismuthene between SiC and a protective sheet of graphene, this barrier is successfully overcome. As demonstrated by scanning tunneling microscopy and photoemission spectroscopy, graphene intercalation preserves the structural and topological integrity of bismuthene, while effectively shielding it from oxidation in air. Hereby, hydrogen is identified as the critical process gas that was missing in previous bismuth intercalation attempts. These findings facilitate ex-situ experiments and pave the way for the development of bismuthene based devices, signaling a significant step forward in the development of next-generation technologies.

RuO_2 has emerged as a prototypical candidate for altermagnetism. In the face of daunting evidence for magnetic order in the bulk, the focus naturally shifted to surfaces and ultrathin films, where Coulomb interactions are dimensionally quenched and electron correlations strongly enhanced. Here, we examine atomically ordered, ultrathin RuO_2(110) grown on Ru(0001) using a combination of scanning tunneling microscopy (STM), density functional theory, and density matrix renormalization group methods. We observe a nonmagnetic charge order that is imprinted by the incommensurate moiré stacking with the substrate and enhanced by the electronic Fermi surface scattering within the flat-band surface state. We further identify a nonmagnetic, metastable c(2 × 2) surface reconstruction that arises from surface phonon softening and can be toggled reversibly via STM tip manipulation. Spin-polarized STM measurements, however, reveal no evidence of magnetic order on the RuO_2(110) surface. Our findings of a nonmagnetic charge-modulation position ultrathin RuO_2(110) as an intriguing platform for exploring moiré-assisted electronic orders.

Atomic monolayers on semiconductor surfaces represent an emerging class of functional quantum materials in the two-dimensional limit — ranging from superconductors and Mott insulators to ferroelectrics and quantum spin Hall insulators. Indenene, a triangular monolayer of indium with a gap of ~ 120 meV is a quantum spin Hall insulator whose micron-scale epitaxial growth on SiC(0001) makes it technologically relevant. However, its suitability for room-temperature spintronics is challenged by the instability of its topological character in air. It is imperative to develop a strategy to protect the topological nature of indenene during ex situ processing and device fabrication. Here we show that intercalation of indenene into epitaxial graphene provides effective protection from the oxidising environment, while preserving an intact topological character. Our approach opens a rich realm of ex situ experimental opportunities, priming monolayer quantum spin Hall insulators for realistic device fabrication and access to topologically protected edge channels.

Epitaxial thin films of SrTiO3(100) doped with 6% and 12% Ni are studied with resonant angle-resolved photoelectron spectroscopy at the Ti and Ni L2,3-edges. We find that the Ni doping shifts the valence band of n-doped pristine SrTiO3 toward the Fermi level (in the direction of p-doping) and reducing the bandgap. In the Ti t2g-derived mobile electron system (MES), the Ni doping depopulates the out-of-plane dxz/yz-derived bands, transforming the MES to two-dimensional and progressively reduces the electron density embedded in the in-plane dxy-derived bands as reflected in their Fermi momentum. Furthermore, the Ti and Ni L2,3-edge resonant photoemission is used to identify the Ni 3d impurity state in the vicinity of the valence-band maximum and decipher the full spectrum of the in-gap states originating from the Ni atoms, Ti atoms, and from their hybridized orbitals. Our experimental information about the dependence of the valence bands, MES, and in-gap states in Ni-doped SrTiO3 may help the development of this material toward its device applications associated with the reduced optical bandgap.

Intercalating two-dimensional quantum materials beneath a sheet of graphene provides effective environmental protection and facilitates ex situ device fabrication. However, developing a functional device requires rapid, large-scale screening methods to evaluate the quality of the intercalant, which to date can be monitored only by slow, ultra-high vacuum-based surface science techniques. In this study, we utilize ex situ Raman micro-spectroscopy to optically and nondestructively identify the quantum spin Hall insulator indenene, a monolayer of indium sandwiched between a SiC(0001) substrate and a single sheet of graphene. Color modulation combined with indenene's distinctive low-frequency Raman fingerprint enables rapid assessment of its homogeneity and crystalline quality. Density functional perturbation theory indicates that this Raman signature originates mainly from indenene's shear and breathing modes, while additional higher order modes are tentatively attributed to defect-assisted and two-phonon Raman processes.

The demonstration of a topological band inversion constitutes the most elementary proof of a quantum spin Hall insulator (QSHI). On a fundamental level, such an inverted band gap is intrinsically related to the bulk Berry curvature, a gauge-invariant fingerprint of the wave function's quantum geometric properties in Hilbert space. Intimately tied to orbital angular momentum (OAM), the Berry curvature can be, in principle, extracted from circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES), were it not for interfering final state photoelectron emission channels that obscure the initial state OAM signature. Here, we outline a full-experimental strategy to avoid such interference artifacts and isolate the clean OAM from the CD-ARPES response. Bench-marking this strategy for the recently discovered atomic monolayer system indenene, we demonstrate its distinct QSHI character and establish CD-ARPES as a scalable bulk probe to experimentally classify the topology of two-dimensional quantum materials with time reversal symmetry.

2D topological insulators promise novel approaches towards electronic, spintronic, and quantum device applications. This is owing to unique features of their electronic band structure, in which bulk-boundary correspondences enforces the existence of 1D spin-momentum locked metallic edge states-both helical and chiral-surrounding an electrically insulating bulk. Forty years since the first discoveries of topological phases in condensed matter, the abstract concept of band topology has sprung into realization with several materials now available in which sizable bulk energy gaps-up to a few hundred meV-promise to enable topology for applications even at room-temperature. Further, the possibility of combining 2D TIs in heterostructures with functional materials such as multiferroics, ferromagnets, and superconductors, vastly extends the range of applicability beyond their intrinsic properties. While 2D TIs remain a unique testbed for questions of fundamental condensed matter physics, proposals seek to control the topologically protected bulk or boundary states electrically, or even induce topological phase transitions to engender switching functionality. Induction of superconducting pairing in 2D TIs strives to realize non-Abelian quasiparticles, promising avenues towards fault-tolerant topological quantum computing. This roadmap aims to present a status update of the field, reviewing recent advances and remaining challenges in theoretical understanding, materials synthesis, physical characterization and, ultimately, device perspectives.

We present a systematic growth study of epitaxial RuO2(110) and IrO2(110) on TiO2(110) substrates by pulsed laser deposition. We describe the main challenges encountered in the growth process, such as a deteriorating material flux due to laser-induced target metallization or the delicate balance of under- vs over-oxidation of the “stubborn” Ru and Ir metals. We identify growth temperatures and oxygen partial pressures of 700 K, 1 × 10−3 mbar for RuO2 and 770 K, 5 × 10−4 mbar for IrO2 to optimally balance between metal oxidation and particle mobility during nucleation. In contrast to IrO2, RuO2 exhibits layer-by-layer growth up to 5 unit cells if grown at high deposition rates. At low deposition rates, the large lattice mismatch between film and substrate fosters initial 3D island growth and cluster formation. In analogy to reports for RuO2 based on physical vapor deposition [He et al., J. Phys. Chem. C 119, 2692 (2015)], we find these islands to eventually merge and grow to continue in a step flow mode, resulting in highly crystalline, flat, stoichiometric films of RuO2(110) (up to 30 nm thickness) and IrO2(110) (up to 13 nm thickness) with well-defined line defects.

Wires having a width of one or two atoms are the smallest possible physical objects that may exhibit one-dimensional properties. In order to be experimentally accessible at finite temperatures, such wires must stabilized by interactions in two and even three dimensions. These interactions modify and partly destroy their one-dimensional properties, but introduce new phenomena of coupling and correlation that entangle both charge and spin. We explore this fascinating field by first giving an overview of the present status of theoretical knowledge on 1D physics, including coupling between chains and to the substrate, before we set out for experimental results on ordered arrays of atomic wires on both flat and vicinal Si(111) surfaces comprising Si(111)-In, Si(hhk)-Au, Si(557)-Pb, Si(557)-Ag, Ge(001)-Au and rare earth silicide wires. While for these systems structural, spectroscopic and (magneto-)conductive properties are in the focus, including temperature- and concentration-induced phase transitions, explicit dynamics on the femto- and picosecond time scales were explored for the modified Peierls transition in indium chains on Si(111). All these systems are characterized by strong correlations, including spin, that are extended over whole terraces and partly beyond, so that small geometric changes lead to large modifications of their electronic properties. Thus this coupling in one (1D), two (2D) (and even three) dimensions results in a wealth of phase transitions and transient quasi-1D conductance. As extremes, modified quasi-1D properties survive, as in the Si(111)-In system, whereas strong Fermi nesting results in entanglement of spin and charge between terraces for Si(557)-Pb, so that spin orbit density waves across the steps are formed.