Tailoring the nature and strength of electron-phonon interactions in the SrTiO3(001) 2D electron liquid

Unearthing the emerging properties at buried oxide heterointerfaces: the γ-Al2O3/SrTiO3 heterostructure

The symmetry breaking that is formed when oxide layers are combined epitaxially to form heterostructures has led to the emergence of new functionalities beyond those observed in the individual parent materials. SrTiO3-based heterostructures have played a central role in expanding the range of functional properties arising at the heterointerface and elucidating their mechanistic origin. The heterostructure formed by the epitaxial combination of spinel γ-Al2O3 and perovskite SrTiO3 constitutes a striking example with features distinct from perovskite/perovskite counterparts such as the archetypical LaAlO3/SrTiO3 heterostructure. Here, non-isomorphic epitaxial growth of γ-Al2O3 on SrTiO3 can be achieved even at room temperature with the epitaxial union of the two distinct crystal structures resulting in modification of the functional properties by the broken cationic symmetry. The heterostructure features oxygen vacancy-mediated conductivity with dynamically adjustable electron mobilities as high as 140 000 cm2 V-1 s-1 at 2 K, strain-tunable magnetism and an unsaturated linear magnetoresistance exceeding 80 000% at 15 T and 2 K. Here, we review the structural, electronic and magnetic characteristics of the γ-Al2O3/SrTiO3 heterostructure with a particular emphasis on elucidating the underlying mechanistic origins of the various properties. We further show that γ-Al2O3/SrTiO3 may break new grounds for tuning the electronic and magnetic properties through dynamic defect engineering and polarity modifications, and also for band engineering, symmetry breaking and silicon integration.

Engineering ultra-strong electron-phonon coupling and nonclassical electron transport in crystalline gold with nanoscale interfaces

Electrical resistivity in good metals, particularly noble metals such as gold (Au), silver (Ag), or copper, increases linearly with temperature (T) for T > ΘD, where ΘD is the Debye temperature. This is because the coupling (λ) between the electrons and the lattice vibrations, or phonons, in these metals is weak, with λ ~ 0.1-0.2. In this work, we outline a nanostructuring strategy of crystalline Au where this concept of metallic transport breaks down. We show that by embedding a distributed network of ultra-small Ag nanoparticles (AgNPs) of radius ~ 1-2 nm inside a crystalline Au shell, the electron-phonon interaction can be enhanced, with an effective λ as high as  ≈ 20. With increasing AgNP density, the electrical resistivity deviates from T-linearity and approaches a saturation to the Mott-Ioffe-Regel scale ρMIR ~ ha/e2 for both disorder (T → 0) and phonon (T ≫ ΘD)-dependent components of resistivity (here, a = 0.3 nm, is the lattice constant of Au).

Strong electron-phonon coupling in magic-angle twisted bilayer graphene

The unusual properties of superconductivity in magic-angle twisted bilayer graphene (MATBG) have sparked considerable research interest1-13. However, despite the dedication of intensive experimental efforts and the proposal of several possible pairing mechanisms14-24, the origin of its superconductivity remains elusive. Here, by utilizing angle-resolved photoemission spectroscopy with micrometre spatial resolution, we reveal flat-band replicas in superconducting MATBG, where MATBG is unaligned with its hexagonal boron nitride substrate11. These replicas show uniform energy spacing, approximately 150 ± 15 meV apart, indicative of strong electron-boson coupling. Strikingly, these replicas are absent in non-superconducting twisted bilayer graphene (TBG) systems, either when MATBG is aligned to hexagonal boron nitride or when TBG deviates from the magic angle. Calculations suggest that the formation of these flat-band replicas in superconducting MATBG are attributed to the strong coupling between flat-band electrons and an optical phonon mode at the graphene K point, facilitated by intervalley scattering. These findings, although they do not necessarily put electron-phonon coupling as the main driving force for the superconductivity in MATBG, unravel the electronic structure inherent in superconducting MATBG, thereby providing crucial information for understanding the unusual electronic landscape from which its superconductivity is derived.

Identification of large polarons and exciton polarons in rutile and anatase polymorphs of titanium dioxide

Titanium dioxide (TiO2) is a wide-gap semiconductor with numerous applications in photocatalysis, photovoltaics, and neuromorphic computing. The unique functional properties of this material critically depend on its ability to transport charge in the form of polarons, namely narrow electron wavepackets accompanied by local distortions of the crystal lattice. It is currently well established that the most important polymorphs of TiO2, the rutile and anatase phases, harbor small electron polarons and small hole polarons, respectively. However, whether additional polaronic species exist in TiO2, and under which conditions, remain open questions. Here, we provide definitive answers to these questions by exploring the rich landscape of polaron quasiparticles in TiO2 via recently developed ab initio techniques. In addition to the already known small polarons, we identify three species, namely a large hole polaron in rutile, a large quasi-two-dimensional electron polaron in anatase, and a large exciton polaron in anatase. These findings complete the puzzle on the polaron physics of TiO2 and pave the way for systematically probing and manipulating polarons in a broad class of complex oxides and quantum materials.

Orientation-dependent electronic structure in interfacial superconductors LaAlO3/KTaO3

Emergent superconductivity at the LaAlO3/KTaO3 interfaces exhibits a mysterious dependence on the KTaO3 crystallographic orientations. Here by soft X-ray angle-resolved photoemission spectroscopy, we directly resolve the electronic structure of the LaAlO3/KTaO3 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.

Unidirectional Charge Orders Induced by Oxygen Vacancies on SrTiO3(001)

The discovery of high-mobility two-dimensional electron gas and low carrier density superconductivity in multiple SrTiO3-based heterostructures has stimulated intense interest in the surface properties of SrTiO3. The recent discovery of high-Tc superconductivity in the monolayer FeSe/SrTiO3 led to the upsurge and underscored the atomic precision probe of the surface structure. By performing atomically resolved cryogenic scanning tunneling microscopy/spectroscopy characterization on dual-TiO2-δ-terminated SrTiO3(001) surfaces with (√13 × √13), c(4 × 2), mixed (2 × 1), and (2 × 2) reconstructions, we disclosed universally broken rotational symmetry and contrasting bias- and temperature-dependent electronic states for apical and equatorial oxygen sites. With the sequentially evolved surface reconstructions and simultaneously increasing equatorial oxygen vacancies, the surface anisotropy reduces and the work function lowers. Intriguingly, unidirectional stripe orders appear on the c(4 × 2) surface, whereas local (4 × 4) order emerges and eventually forms long-range unidirectional c(4 × 4) charge order on the (2 × 2) surface. This work reveals robust unidirectional charge orders induced by oxygen vacancies due to strong and delicate electronic-lattice interaction under broken rotational symmetry, providing insights into understanding the complex behaviors in perovskite oxide-based heterostructures.

Observation of interlayer plasmon polaron in graphene/WS2 heterostructures

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed", which leads to the formation of polaronic quasiparticles. The exploration of polaronic effects on low-energy excitations is in its infancy in two-dimensional materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of single-layer WS2. By using micro-focused angle-resolved photoemission spectroscopy during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the single-layer WS2 conduction band minimum. Our results are explained by an effective many-body model in terms of a coupling between single-layer WS2 conduction electrons and an interlayer plasmon mode. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides.

Non-KAM classical chaos topology for electrons in superlattice minibands determines the inter-well quantum transition rates

We investigate the quantum-classical correspondence for a particle tunnelling through a periodic superlattice structure with an applied bias voltage and an additional tilted harmonic oscillator potential. We show that the quantum mechanical tunnelling rate between neighbouring quantum wells of the superlattice is determined by the topology of the phase trajectories of the analogous classical system. This result also enables us to estimate, with high accuracy, the tunnelling rate between two spatially displaced simple harmonic oscillator states using a classical model, and thus gain new insight into this generic quantum phenomenon. This finding opens new directions for exploring and understanding the quantum-classical correspondence principle and quantum jumps between displaced harmonic oscillators, which are important in many branches of natural science.

Giant acceleration of polaron transport by ultrafast laser-induced coherent phonons

Polaron formation is ubiquitous in polarized materials, but severely hampers carrier transport for which effective controlling methods are urgently needed. Here, we show that laser-controlled coherent phonon excitation enables orders of magnitude enhancement of carrier mobility via accelerating polaron transport in a prototypical material, lithium peroxide (Li2O2). The selective excitation of specific phonon modes, whose vibrational pattern directly overlap with the polaronic lattice deformation, can remarkably reduce the energy barrier for polaron hopping. The strong nonadiabatic couplings between the electronic and ionic subsystem play a key role in triggering the migration of polaron, via promoting phonon-phonon scattering in q space within sub-picoseconds. These results extend our understanding of polaron transport dynamics to the nonequilibrium regime and allow for optoelectronic devices with ultrahigh on-off ratio and ultrafast responsibility, competitive with those of state-of-the-art devices fabricated based on free electron transport.

Electronic Structure of Few-Layer Black Phosphorus from μ-ARPES

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based microfocus angle resolved photoemission (μ-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters, which provide an accurate description of the electronic structure for all thicknesses.

Nonlinear optical phonon spectroscopy revealing polaronic signatures of the LaAlO3/SrTiO3 interface

We report the direct observation of lattice phonons confined at LaAlO₃/SrTiO₃ (LAO/STO) interfaces and STO surfaces using the sum-frequency phonon spectroscopy. This interface-specific nonlinear optical technique unveiled phonon modes localized within a few monolayers at the interface, with inherent sensitivity to the coupling between lattice and charge degrees of freedom. Spectral evolution across the insulator-to-metal transition at LAO/STO interface revealed an electronic reconstruction at the subcritical LAO thickness, as well as strong polaronic signatures upon formation of the two-dimensional electron gas. We further discovered a characteristic lattice mode from interfacial oxygen vacancies, enabling us to probe such important structural defects in situ. Our study provides a unique perspective on many-body interactions at the correlated oxide interfaces.

Giant Enhancement of Electron-Phonon Coupling in Dimensionality-Controlled SrRuO3 Heterostructures

Electrons in crystals interact closely with quantized lattice degree of freedom, determining fundamental electrodynamic behaviors and versatile correlated functionalities. However, the strength of the electron-phonon interaction is so far determined as an intrinsic value of a given material, restricting the development of potential electronic and phononic applications employing the tunable coupling strength. Here, it is demonstrated that the electron-phonon coupling in SrRuO₃ can be largely controlled by multiple intuitive tuning knobs available in synthetic crystals. The coupling strength of quasi-2D SrRuO₃ is enhanced by ≈300-fold compared with that of bulk SrRuO₃ . This enormous enhancement is attributed to the non-local nature of the electron-phonon coupling within the well-defined synthetic atomic network, which becomes dominant in the limit of the 2D electronic state. These results provide valuable opportunities for engineering the electron-phonon coupling, leading to a deeper understanding of the strongly coupled charge and lattice dynamics in quantum materials.

Revealing the Polaron State at the MoS2/TiO2 Interface

Interfacial polarons determine the distribution of free charges at the interface and thus play important roles in manipulating the physicochemical properties of hybridized polaronic materials. In this work, we investigated the electronic structures at the atomically flat interface of the single-layer MoS₂ (SL-MoS₂) on the rutile TiO₂ surface using high-resolution angle-resolved photoemission spectroscopy. Our experiments directly visualized both the valence band maximum and the conduction band minimum (CBM) of SL-MoS₂ at the K point, which clearly defines a direct bandgap of ∼2.0 eV. Detailed analyses corroborated by density functional theory calculations demonstrated that the CBM of MoS₂ is formed by the trapped electrons at the MoS₂/TiO₂ interface that couple with the longitudinal optical phonons in the TiO₂ substrate through an interfacial Fröhlich polaron state. Such an interfacial coupling effect may register a new route for tuning the free charges in the hybridized systems of two-dimensional materials and functional metal oxides.

Perspectives on oxide heterostructures - the curious case of γ-Al2O3/SrTiO3

The heterostructure formed by depositing nanoscale thin films of spinel γ-Al₂O₃ on perovskite SrTiO₃ exhibits a range of exciting properties including room temperature epitaxial growth, high electron mobility, strain-tunable magnetic order, and a symmetry-related reordering of the conduction bands. In comparison to the benchmark LaAlO₃/SrTiO₃ heterostructure, the γ-Al₂O₃/SrTiO₃ heterostructure has been more sparsely investigated, which leaves plenty of room for scientific and technological discoveries. In this perspective article, I describe the key findings of the γ-Al₂O₃/SrTiO₃ heterostructure and propose five directions for future research: (1) an exploration of novel phenomena emerging when relaxing the conventional epitaxial constraint of matching crystal structures across the interface, (2) a dynamic switching of a strong polarization through nanoscale electromigration of aluminum vacancies, (3) autonomous and forced enhancement of the electron mobility via oxygen vacancy diffusion, (4) writing and erasing of magnetic and conducting nanolines using ferroelastic domain walls, and (5) a multiferroic state formed by combining ferroelectricity, ferromagnetism, and ferroelasticity. The proposed research directions may shed light on both fundamental aspects of the heterostructure and pave the way for applications within green energy devices and nanoelectronics.

Interfacial engineering of halide perovskites and two-dimensional materials

Recently, halide perovskites (HPs) and layered two-dimensional (2D) materials have received significant attention from industry and academia alike. HPs are emerging materials that have exciting photoelectric properties, such as a high absorption coefficient, rapid carrier mobility and high photoluminescence quantum yields, making them excellent candidates for various optoelectronic applications. 2D materials possess confined carrier mobility in 2D planes and are widely employed in nanostructures to achieve interfacial modification. HP/2D material interfaces could potentially reveal unprecedented interfacial properties, including light absorbance with desired spectral overlap, tunable carrier dynamics and modified stability, which may lead to several practical applications. In this review, we attempt to provide a comprehensive perspective on the development of interfacial engineering of HP/2D material interfaces. Specifically, we highlight the recent progress in HP/2D material interfaces considering their architectures, electronic energetics tuning and interfacial properties, discuss the potential applications of these interfaces and analyze the challenges and future research directions of interfacial engineering of HP/2D material interfaces. This review links the fields of HPs and 2D materials through interfacial engineering to provide insights into future innovations and their great potential applications in optoelectronic devices.

Quantum electron liquid and its possible phase transition

Purely quantum electron systems exhibit intriguing correlated electronic phases by virtue of quantum fluctuations in addition to electron-electron interactions. To realize such quantum electron systems, a key ingredient is dense electrons decoupled from other degrees of freedom. Here, we report the discovery of a pure quantum electron liquid that spreads up to ~3 Å in a vacuum on the surface of an electride crystal. Its extremely high electron density and weak hybridization with buried atomic orbitals show the quantum and pure nature of the electrons, which exhibit a polarized liquid phase, as demonstrated by our spin-dependent measurement. Furthermore, upon enhancing the electron correlation strength, the dynamics of the quantum electrons change to that of a non-Fermi liquid along with an anomalous band deformation, suggestive of a transition to a hexatic liquid crystal phase. Our findings develop the frontier of quantum electron systems and serve as a platform for exploring correlated electronic phases in a pure fashion.

Going beyond the Cumulant Approximation: Power Series Correction to the Single-Particle Green's Function in the Holstein System

In the context of a single-electron two orbital Holstein system coupled to dispersionless bosons, we develop a general method to correct the single-particle Green's function using a power series correction (PSC) scheme. We outline the derivations of various flavors of cumulant approximation through the PSC scheme explaining the assumptions and approximations behind them. Finally, we compare the PSC spectral function with cumulant and exact diagonalized spectral functions and elucidate three regimes of this problem-two where the cumulant explains and one where the cumulant fails. We find that the exact and the PSC spectral functions match within spectral broadening across all three regimes.

Photoinduced Small Hole Polarons Formation and Recombination in All-Inorganic Perovskite from Quantum Dynamics Simulation

We conducted ab initio molecular dynamics (AIMD) and nonadiabatic MD to simulate polaron formation and recombination in all-inorganic Cs₃Bi₂Br₉ perovskite. The meticulously designed AIMD simulations show that two types of small hole polaron, including localized and semidelocalized small hole polaron on either an intralayer or an interlayer Br dimer, are adiabatically formed within 1.71 ps. The localized small hole polaron reduces nonadiabatic coupling and decoherence time and, thus, delays charge recombination to 213 ns. In contrast, the dominant semidelocalized polaron increases nonadiabatic coupling by enhancing electron-hole overlap and restores the energy gap and decoherence time to the pristine system, accelerating recombination to 4.7 ns compared to a 10 ns charge carrier lifetime in the pristine system. All the obtained time scales agree well with experiments. The study offers a fundamental understanding of the excited-state dynamics of small hole polaron in Cs₃Bi₂Br₉ and helps to design high-performance perovskite optoelectronics and photovoltaics.

Unified Approach to Polarons and Phonon-Induced Band Structure Renormalization

Ab initio calculations of the phonon-induced band structure renormalization are currently based on the perturbative Allen-Heine theory and its many-body generalizations. These approaches are unsuitable to describe materials where electrons form localized polarons. Here, we develop a self-consistent, many-body Green's function theory of band structure renormalization that incorporates localization and self-trapping. We show that the present approach reduces to the Allen-Heine theory in the weak-coupling limit, and to total energy calculations of self-trapped polarons in the strong-coupling limit. To demonstrate this methodology, we reproduce the path-integral results of Feynman and diagrammatic Monte Carlo calculations for the Fröhlich model at all couplings, and we calculate the zero point renormalization of the band gap of an ionic insulator including polaronic effects.

Phonon signatures for polaron formation in an anharmonic semiconductor

Mechanistic studies on lead halide perovskites (LHPs) in recent years have suggested charge carrier screening as partially responsible for long carrier diffusion lengths and lifetimes that are key to superior optoelectronic properties. These findings have led to the ferroelectric large polaron proposal, which attributes efficient charge carrier screening to the extended ordering of dipoles from symmetry-breaking unit cells that undergo local structural distortion and break inversion symmetry. It remains an open question whether this proposal applies in general to semiconductors with LHP-like anharmonic and dynamically disordered phonons. Here, we study electron-phonon coupling in Bi₂O₂Se, a semiconductor which bears resemblance to LHPs in ionic bonding, spin-orbit coupling, band transport with long carrier diffusion lengths and lifetimes, and phonon disorder as revealed by temperature-dependent Raman spectroscopy. Using coherent phonon spectroscopy, we show the strong coupling of an anharmonic phonon mode at 1.50 THz to photo-excited charge carriers, while the Raman excitation of this mode is symmetry-forbidden in the ground-state. Density functional theory calculations show that this mode, originating from the A_1g phonon of out-of-plane Bi/Se motion, gains oscillator strength from symmetry-lowering in polaron formation. Specifically, lattice distortion upon ultrafast charge localization results in extended ordering of symmetry-breaking unit cells and a planar polaron wavefunction, namely a two-dimensional polaron in a three-dimensional lattice. This study provides experimental and theoretical insights into charge interaction with anharmonic phonons in Bi₂O₂Se and suggests ferroelectric polaron formation may be a general principle for efficient charge carrier screening and for defect-tolerant semiconductors.

Slow Magnetic Relaxation of Dy Adatoms with In-Plane Magnetic Anisotropy on a Two-Dimensional Electron Gas

We report on the magnetic properties of Dy atoms adsorbed on the (001) surface of SrTiO3. X-ray magnetic circular dichroism reveals slow relaxation of the Dy magnetization on a time scale of about 800 s at 2.5 K, unusually associated with an easy-plane magnetic anisotropy. We attribute these properties to Dy atoms occupying hollow adsorption sites on the TiO2-terminated surface. Conversely, Ho atoms adsorbed on the same surface show paramagnetic behavior down to 2.5 K. With the help of atomic multiplet simulations and first-principles calculations, we establish that Dy populates also the top-O and bridge sites on the coexisting SrO-terminated surface. A simple magnetization relaxation model predicts these two sites to have an even longer magnetization lifetime than the hollow site. Moreover, the adsorption of Dy on the insulating SrTiO3 crystal leads, regardless of the surface termination, to the formation of a spin-polarized two-dimensional electron gas of Ti 3dxy character, together with an antiferromagnetic Dy-Ti coupling. Our findings support the feasibility of tuning the magnetic properties of the rare-earth atoms by acting on the substrate electronic gas with electric fields.

Is There a Polaron Signature in Angle-Resolved Photoemission of CsPbBr_{3}?

The formation of large polarons has been proposed as reason for the high defect tolerance, low mobility, low charge carrier trapping, and low nonradiative recombination rates of lead halide perovskites. Recently, direct evidence for large-polaron formation has been reported from a 50% effective mass enhancement in angle-resolved photoemission of CsPbBr_{3} over theory for the orthorhombic structure. We present in-depth band dispersion measurements of CsPbBr_{3} and GW calculations, which lead to similar effective masses at the valence band maximum of 0.203±0.016 m_{0} in experiment and 0.226 m_{0} in orthorhombic theory. We argue that the effective mass can be explained solely on the basis of electron-electron correlation and large-polaron formation cannot be concluded from photoemission data.

Multipole polaron in the devil's staircase of CeSb

Rare-earth intermetallic compounds exhibit rich phenomena induced by the interplay between localized f orbitals and conduction electrons. However, since the energy scale of the crystal-electric-field splitting is only a few millielectronvolts, the nature of the mobile electrons accompanied by collective crystal-electric-field excitations has not been unveiled. Here, we examine the low-energy electronic structures of CeSb through the anomalous magnetostructural transitions below the Néel temperature, ~17 K, termed the 'devil's staircase', using laser angle-resolved photoemission, Raman and neutron scattering spectroscopies. We report another type of electron-boson coupling between mobile electrons and quadrupole crystal-electric-field excitations of the 4f orbitals, which renormalizes the Sb 5p band prominently, yielding a kink at a very low energy (~7 meV). This coupling strength is strong and exhibits anomalous step-like enhancement during the devil's staircase transition, unveiling a new type of quasiparticle, named the 'multipole polaron', comprising a mobile electron dressed with a cloud of the quadrupole crystal-electric-field polarization.

High-order replica bands in monolayer FeSe/SrTiO3 revealed by polarization-dependent photoemission spectroscopy

The mechanism of the enhanced superconductivity in monolayer FeSe/SrTiO₃ has been enthusiastically studied and debated over the past decade. One specific observation has been taken to be of central importance: the replica bands in the photoemission spectrum. Although suggestive of electron-phonon interaction in the material, the essence of these spectroscopic features remains highly controversial. In this work, we conduct angle-resolved photoemission spectroscopy measurements on monolayer FeSe/SrTiO₃ using linearly polarized photons. This configuration enables unambiguous characterization of the valence electronic structure with a suppression of the spectral background. We consistently observe high-order replica bands derived from various Fe 3d bands, similar to those observed on bare SrTiO₃. The intensity of the replica bands is unexpectedly high and different between d_xy and d_yz bands. Our results provide new insights on the electronic structure of this high-temperature superconductor and the physical origin of the photoemission replica bands.

The origin of the photoemission replica bands in monolayer FeSe/SrTiO₃ remains controversial. Here, the authors perform angle-resolved photoemission spectroscopy with polarized photon on FeSe/SrTiO₃ and observe high-order replica bands with high intensity from various Fe 3d bands, suggesting a mixed mechanism.

Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO_{3}

The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO_{3}, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO_{3} substrate might contribute to the enhanced superconducting pairing temperature. Alternatively, it has also been recently proposed that such replica bands might instead originate from extrinsic final state losses associated with the photoemission process. Here, we perform a quantitative examination of replica bands in monolayer FeSe/SrTiO_{3}, where we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects. A detailed analysis of the energy splittings and relative peak intensities between the higher-order replicas, as well as other self-energy effects, allows us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of λ=0.19±0.02, providing valuable insights into the enhancement of superconductivity in monolayer FeSe/SrTiO_{3}. The methodology employed here can also serve as a new and general approach for making more rigorous and quantitative comparisons to theoretical calculations of electron-phonon interactions and coupling constants.

Stabilization of Sr3Al2O6 Growth Templates for Ex Situ Synthesis of Freestanding Crystalline Oxide Membranes

A new synthetic approach has recently been developed for the fabrication of freestanding crystalline perovskite oxide nanomembranes, which involves the epitaxial growth of a water-soluble sacrificial layer. By utilizing an ultrathin capping layer of SrTiO₃, here we show that this sacrificial layer, as grown by pulsed laser deposition, can be stabilized in air and therefore be used as transferrable templates for ex situ epitaxial growth using other techniques. We find that the stability of these templates depends on the thickness of the capping layer. On these templates, freestanding superconducting SrTiO₃ membranes were synthesized ex situ using molecular beam epitaxy, enabled by the lower growth temperature which preserves the sacrificial layer. This study paves the way for the synthesis of an expanded selection of freestanding oxide membranes and heterostructures with a wide variety of ex situ growth techniques.

Recent Advances in 2D Superconductors

The emergence of superconductivity in 2D materials has attracted much attention and there has been rapid development in recent years because of their fruitful physical properties, such as high transition temperature (T_c ), continuous phase transition, and enhanced parallel critical magnetic field (B_c ). Tremendous efforts have been devoted to exploring different physical parameters to figure out the mechanisms behind the unexpected superconductivity phenomena, including adjusting the thickness of samples, fabricating various heterostructures, tuning the carrier density by electric field and chemical doping, and so on. Here, different types of 2D superconductivity with their unique characteristics are introduced, including the conventional Bardeen-Cooper-Schrieffer superconductivity in ultrathin films, high-T_c superconductivity in Fe-based and Cu-based 2D superconductors, unconventional superconductivity in newly discovered twist-angle bilayer graphene, superconductivity with enhanced B_c , and topological superconductivity. A perspective toward this field is then proposed based on academic knowledge from the recently reported literature. The aim is to provide researchers with a clear and comprehensive understanding about the newly developed 2D superconductivity and promote the development of this field much further.

Band-Order Anomaly at the γ-Al2O3/SrTiO3 Interface Drives the Electron-Mobility Boost

The rich functionalities of transition-metal oxides and their interfaces bear an enormous technological potential. Its realization in practical devices requires, however, a significant improvement of yet relatively low electron mobility in oxide materials. Recently, a mobility boost of about 2 orders of magnitude has been demonstrated at the spinel-perovskite γ-Al₂O₃/SrTiO₃ interface compared to the paradigm perovskite-perovskite LaAlO₃/SrTiO₃ interface. We explore the fundamental physics behind this phenomenon from direct measurements of the momentum-resolved electronic structure of this interface using resonant soft-X-ray angle-resolved photoemission. We find an anomaly in orbital ordering of the mobile electrons in γ-Al₂O₃/SrTiO₃ which depopulates electron states in the top SrTiO₃ layer. This rearrangement of the mobile electron system pushes the electron density away from the interface, which reduces its overlap with the interfacial defects and weakens the electron-phonon interaction, both effects contributing to the mobility boost. A crystal-field analysis shows that the band order alters owing to the symmetry breaking between the spinel γ-Al₂O₃ and perovskite SrTiO₃. Band-order engineering, exploiting the fundamental symmetry properties, emerges as another route to boost the performance of oxide devices.

Electron-plasmon interaction induced plasmonic-polaron band replication in epitaxial perovskite SrIrO3 films

Electron-boson interaction is fundamental to a thorough understanding of various exotic properties emerging in many-body physics. In photoemission spectroscopy, photoelectron emission due to photon absorption would trigger diverse collective excitations in solids, including the emergence of phonons, magnons, electron-hole pairs, and plasmons, which naturally provides a reliable pathway to study electron-boson couplings. While fingerprints of electron-phonon/-magnon interactions in this state-of-the-art technique have been well investigated, much less is known about electron-plasmon coupling, and direct observation of the band renormalization solely due to electron-plasmon interactions is extremely challenging. Here by utilizing integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy, we discover the long sought-after pure electron-plasmon coupling-induced low-lying plasmonic-polaron replica bands in epitaxial semimetallic SrIrO₃ films, in which the characteristic low carrier concentration and narrow bandwidth combine to provide a unique platform where the electron-plasmon interaction can be investigated kinematically in photoemission spectroscopy. This finding enriches the forms of electron band normalization on collective modes in solids and demonstrates that, to obtain a complete understanding of the quasiparticle dynamics in 5d electron systems, the electron-plasmon interaction should be considered on equal footing with the acknowledged electron-electron interaction and spin-orbit coupling.

Metal-Insulator Transition and Emergent Gapped Phase in the Surface-Doped 2D Semiconductor 2H-MoTe_{2}

Artificially created two-dimensional (2D) interfaces or structures are ideal for seeking exotic phase transitions due to their highly tunable carrier density and interfacially enhanced many-body interactions. Here, we report the discovery of a metal-insulator transition (MIT) and an emergent gapped phase in the metal-semiconductor interface that is created in 2H-MoTe_{2} via alkali-metal deposition. Using angle-resolved photoemission spectroscopy, we found that the electron-phonon coupling is strong at the interface as characterized by a clear observation of replica shake-off bands. Such strong electron-phonon coupling interplays with disorder scattering, leading to an Anderson localization of polarons which could explain the MIT. The domelike emergent gapped phase could then be attributed to a polaron extended state or phonon-mediated superconductivity. Our results demonstrate the capability of alkali-metal deposition as an effective method to enhance the many-body interactions in 2D semiconductors. The surface-doped 2H-MoTe_{2} is a promising candidate for realizing polaronic insulator and high-T_{c} superconductivity.

Angle, Spin, and Depth Resolved Photoelectron Spectroscopy on Quantum Materials

The role of X-ray based electron spectroscopies in determining chemical, electronic, and magnetic properties of solids has been well-known for several decades. A powerful approach is angle-resolved photoelectron spectroscopy, whereby the kinetic energy and angle of photoelectrons emitted from a sample surface are measured. This provides a direct measurement of the electronic band structure of crystalline solids. Moreover, it yields powerful insights into the electronic interactions at play within a material and into the control of spin, charge, and orbital degrees of freedom, central pillars of future solid state science. With strong recent focus on research of lower-dimensional materials and modified electronic behavior at surfaces and interfaces, angle-resolved photoelectron spectroscopy has become a core technique in the study of quantum materials. In this review, we provide an introduction to the technique. Through examples from several topical materials systems, including topological insulators, transition metal dichalcogenides, and transition metal oxides, we highlight the types of information which can be obtained. We show how the combination of angle, spin, time, and depth-resolved experiments are able to reveal "hidden" spectral features, connected to semiconducting, metallic and magnetic properties of solids, as well as underlining the importance of dimensional effects in quantum materials.

Growth, electronic structure and superconductivity of ultrathin epitaxial CoSi2films

We report growth, electronic structure and superconductivity of ultrathin epitaxial CoSi₂films on Si (111). At low coverages, preferred islands with 2, 5 and 6 monolayers height develop, which agrees well with the surface energy calculation. We observe clear quantum well states as a result of electronic confinement and their dispersion agrees well with density functional theory calculations, indicating weak correlation effect despite strong contributions from Co 3delectrons.Ex situtransport measurements show that superconductivity persists down to at least 10 monolayers, with reducedT_cbut largely enhanced upper critical field. Our study opens up the opportunity to study the interplay between quantum confinement, interfacial symmetry breaking and superconductivity in an epitaxial silicide film, which is technologically relevant in microelectronics.

Formation of Plasmonic Polarons in Highly Electron-Doped Anatase TiO2

The existence of various quasiparticles of polarons because of electron-boson couplings plays important roles in determining electron transport in titanium dioxide (TiO₂), which affects a wealth of physical properties from catalysis to interfacial superconductivity. In addition to the well-defined Fröhlich polarons whose electrons are dressed by the phonon clouds, it has been theoretically predicted that electrons can also couple to their own plasmonic oscillations, namely, the plasmonic polarons. Here we experimentally demonstrate the formation of plasmonic polarons in highly doped anatase TiO₂ using angle-resolved photoemission spectroscopy. Our results show that the energy separation of plasmon-loss satellites follows a dependence on √n, where n is the electron density, manifesting the characteristic of plasmonic polarons. The spectral functions enable to quantitatively evaluate the strengths of electron-plasmon and electron-phonon couplings, respectively, providing an effective approach for characterizing the interplays among different bosonic modes in the complicate many-body interactions.

Interfacial Polarons in van der Waals Heterojunction of Monolayer SnSe2 on SrTiO3 (001)

Interfacial polarons have been demonstrated to play important roles in heterostructures containing polar substrates. However, most of polarons found so far are diffusive large polarons; the discovery and investigation of small polarons at interfaces are scarce. Herein, we report the emergence of interfacial polarons in monolayer SnSe₂ epitaxially grown on Nb-doped SrTiO₃ (STO) surface using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). ARPES spectra taken on this heterointerface reveal a nearly flat in-gap band correlated with a significant charge modulation in real space as observed with STM. An interfacial polaronic model is proposed to ascribe this in-gap band to the formation of self-trapped small polarons induced by charge accumulation and electron-phonon coupling at the van der Waals interface of SnSe₂ and STO. Such a mechanism to form interfacial polaron is expected to generally exist in similar van der Waals heterojunctions consisting of layered 2D materials and polar substrates.

Strong Electron-Phonon Interaction in 2D Vertical Homovalent III-V Singularities

Highly polar materials are usually preferred over weakly polar ones to study strong electron-phonon interactions and its fascinating properties. Here, we report on the achievement of simultaneous confinement of charge carriers and phonons at the vicinity of a 2D vertical homovalent singularity (antiphase boundary, APB) in an (In,Ga)P/SiGe/Si sample. The impact of the electron-phonon interaction on the photoluminescence processes is then clarified by combining transmission electron microscopy, X-ray diffraction, ab initio calculations, Raman spectroscopy, and photoluminescence experiments. 2D localization and layer group symmetry properties of homovalent electronic states and phonons are studied by first-principles methods, leading to the prediction of a type-II band alignment between the APB and the surrounding semiconductor matrix. A Huang-Rhys factor of 8 is finally experimentally determined for the APB emission line, underlining that a large and unusually strong electron-phonon coupling can be achieved by 2D vertical quantum confinement in an undoped III-V semiconductor. This work extends the concept of an electron-phonon interaction to 2D vertically buried III-V homovalent nano-objects and therefore provides different approaches for material designs, vertical carrier transport, heterostructure design on silicon, and device applications with weakly polar semiconductors.

Electrostatically Enhanced Electron-Phonon Interaction in Monolayer 2H-MoSe2 Grown by Molecular Beam Epitaxy

The enhancement of electron-phonon interaction provides a reasonable explanation for gate-tunable phonon properties in some semiconductors where multiple inequivalent valleys are simultaneously occupied upon charge doping, especially in few-layer transition metal dichalcogenides (TMDs). In this work, we report var der Waals epitaxy of 2H-MoSe₂ by molecular beam epitaxy (MBE) and gate-tunable phonon properties in monolayer and bilayer MoSe₂. In monolayer MoSe₂, we find that out-of-plane phonon mode A_1g exhibits a strong softening and shifting toward lower wavenumbers at a high electron doping level, while in-plane phonon mode E_2g¹ remains unchanged. The softening and shifting of the out-of-plane phonon mode could be attributed to the increase of electron-phonon interaction and the simultaneous occupation of electrons in multiple inequivalent valleys. In bilayer MoSe₂, no corresponding changes of phonon modes are detected at the same doping level, which could originate from the occupation of electrons only in single valleys upon high electron doping. This study demonstrates electrostatically enhanced electron-phonon interaction in monolayer MoSe₂ and clarifies the relevance between occupation of multiple valleys and phonon properties by comparing Raman spectra of monolayer and bilayer MoSe₂ at different doping levels.

Direct Measurement of Electron-Phonon Coupling with Time-Resolved ARPES

Time- and angular- resolved photoelectron spectroscopy is a powerful technique to measure electron dynamics in solids. Recent advances in this technique have facilitated band and energy resolved observations of the effect that excited phonons, have on the electronic structure. Here, we show with the help of ab initio simulations that the Fourier analysis of the time-resolved measurements of solids with excited phonon modes enables the determination of the band- and mode-resolved electron-phonon coupling directly from the experimental data without any additional input from theory. Such an observation is not restricted to regions of strong electron-phonon coupling and does not require strongly excited or hot phonons, but can be employed to monitor the dynamical renormalization of phonons in driven phases of matter.

Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3

Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI₃, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm⁻¹. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by ~50% and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI₃. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors.

Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon (e-ph) coupling. Here, the authors show the presence of strong e-ph coupling in bilayer CrI₃ and observe a Raman feature with periodic broad modes up to the 8th order, attributed to the polaronic character of excitons.

Large Polarons as Key Quasiparticles in SrTiO_{3} and SrTiO_{3}-Based Heterostructures

Despite its simple structure and low degree of electronic correlation, SrTiO_{3} (STO) features collective phenomena linked to charge transport and, ultimately, superconductivity, that are not yet fully explained. Thus, a better insight into the nature of the quasiparticles shaping the electronic and conduction properties of STO is needed. We studied the low-energy excitations of bulk STO and of the LaAlO_{3}/SrTiO_{3} two-dimensional electron gas (2DEG) by Ti L_{3} edge resonant inelastic x-ray scattering. In all samples, we find the hallmark of polarons in the form of intense dd+phonon excitations, and a decrease of the LO3-mode electron-phonon coupling when going from insulating to highly conducting STO single crystals and heterostructures. Both results are attributed to the dynamic screening of the large polaron self-induced polarization, showing that the low-temperature physics of STO and STO-based 2DEGs is dominated by large polaron quasiparticles.

Ultrafast dynamics of hot carriers in a quasi-two-dimensional electron gas on InSe

Two-dimensional electron gases (2DEGs) are at the base of current nanoelectronics because of their exceptional mobilities. Often the accumulation layer forms at polar interfaces with longitudinal optical (LO) modes. In most cases, the many-body screening of the quasi-2DEGs dramatically reduces the Fröhlich scattering strength. Despite the effectiveness of such a process, it has been recurrently proposed that a remote coupling with LO phonons persists even at high carrier concentration. We address this issue by perturbing electrons in an accumulation layer via an ultrafast laser pulse and monitoring their relaxation via time- and momentum-resolved spectroscopy. The cooling rate of excited carriers is monitored at doping level spanning from the semiconducting to the metallic limit. We observe that screening of LO phonons is not as efficient as it would be in a strictly 2D system. The large discrepancy is due to the remote coupling of confined states with the bulk. Our data indicate that the effect of such a remote coupling can be mimicked by a 3D Fröhlich interaction with Thomas-Fermi screening. These conclusions are very general and should apply to field effect transistors (FET) with high-κ dielectric gates, van der Waals heterostructures, and metallic interfaces between insulating oxides.

Superconductivity enhancement in FeSe/SrTiO3: a review from the perspective of electron-phonon coupling

Single-layer FeSe films grown on SrTiO3, with the highest superconducting transition temperature (TC) among all the iron-based superconductors, serves as an ideal platform for studying the microscopic mechanisms of high-TCsuperconductivity. The significant role of interfacial coupling has been widely recognized, while the precise nature of theTCenhancement remains open. In this review, we focus on the investigations of the interfacial coupling in FeSe/SrTiO3from the perspective of electron-phonon coupling (EPC). The main content will include an overview of the experimental measurements associated with different theoretical models and arguments about the EPC. Especially, besides the discussions of EPC based on the measurements of electronic states, we will emphasize the analyses based on phonon measurements. A uniform picture about the nature of the EPC and its relation to theTCenhancement in FeSe/SrTiO3has still not achieved, which should be the key for further studies aiming to the in-depth understanding of high-TCsuperconductivity and the discovery of new superconductors with even enhancedTC.

Evidence of Large Polarons in Photoemission Band Mapping of the Perovskite Semiconductor CsPbBr_{3}

Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr_{3} by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of 0.26±0.02 m_{e}, 50% heavier than the bare mass m_{0}=0.17 m_{e} predicted by density functional theory. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fröhlich coupling parameter α=1.81. A good agreement with our experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.

Controlling a Van Hove singularity and Fermi surface topology at a complex oxide heterostructure interface

The emergence of saddle-point Van Hove singularities (VHSs) in the density of states, accompanied by a change in Fermi surface topology, Lifshitz transition, constitutes an ideal ground for the emergence of different electronic phenomena, such as superconductivity, pseudo-gap, magnetism, and density waves. However, in most materials the Fermi level, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${E}_{{\rm{F}}}$$\end{document}EF, is too far from the VHS where the change of electronic topology takes place, making it difficult to reach with standard chemical doping or gating techniques. Here, we demonstrate that this scenario can be realized at the interface between a Mott insulator and a band insulator as a result of quantum confinement and correlation enhancement, and easily tuned by fine control of layer thickness and orbital occupancy. These results provide a tunable pathway for Fermi surface topology and VHS engineering of electronic phases.

A singularity in a material’s density of states at the Fermi energy can drive the formation of unconventional electronic phases. Here the authors show a Van Hove singularity is tunable across the Fermi energy in an oxide heterostructure, leading to enhanced electronic correlations.

In-gap state generated by La-on-Sr substitutional defects within the bulk of SrTiO3

Local distortion in the conduction pathway has a significant influence on the conducting properties of oxides. The electronic states induced in the band gap of SrTiO3 by La doping were investigated using photoemission spectroscopy (PES) and soft X-ray emission spectroscopy (SXES); moreover, the local distortion in the conduction pathway was examined using extended X-ray absorption fine structure (EXAFS). An itinerant state and a localized state were observed as a metallic state and an in-gap state, respectively, in the PES spectra and as inelastic peaks in the SXES spectra. This implied that the itinerant state and the in-gap state coexisted within the bulk. From EXAFS results, it was observed that La doped into SrTiO3 substituted Sr and locally distorted the conduction pathway. The results showed that some electrons doped by La-on-Sr substitution are trapped/localized by the local distortion in the conduction pathway, whereas the remaining doped electrons itinerate in the pristine conduction pathway with no distortion.

Polarons from First Principles, without Supercells

We develop a formalism and a computational method to study polarons in insulators and semiconductors from first principles. Unlike in standard calculations requiring large supercells, we solve a secular equation involving phonons and electron-phonon matrix elements from density-functional perturbation theory, in a spirit similar to the Bethe-Salpeter equation for excitons. We show that our approach describes seamlessly large and small polarons, and we illustrate its capability by calculating wave functions, formation energies, and spectral decomposition of polarons in LiF and Li_{2}O_{2}.

Enhanced Superconducting State in FeSe/SrTiO_{3} by a Dynamic Interfacial Polaron Mechanism

The observation of substantially enhanced superconductivity of single-layer FeSe films on SrTiO_{3} has stimulated intensive research interest. At present, conclusive experimental data on the corresponding electron-boson interaction is still missing. Here we use inelastic electron scattering spectroscopy and angle resolved photoemission spectroscopy to show that the electrons in these systems are dressed by the strongly polarized lattice distortions of the SrTiO_{3}, and the indispensable nonadiabatic nature of such a coupling leads to the formation of dynamic interfacial polarons. Furthermore, the collective motion of the polarons results in a polaronic plasmon mode, which is unambiguously correlated with the surface phonons of SrTiO_{3} in the presence of the FeSe films. A microscopic model is developed showing that the interfacial polaron-polaron interaction leads to the superconductivity enhancement.

In-Gap States and Band-Like Transport in Memristive Devices

Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface.

Itinerant ferromagnetism of the Pd-terminated polar surface of PdCoO2

The ability to modulate the collective properties of correlated electron systems at their interfaces and surfaces underpins the burgeoning field of "designer" quantum materials. Here, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability to itinerant ferromagnetism at the Pd-terminated surface of the nonmagnetic delafossite oxide metal PdCoO₂ Combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we show how this leads to a rich multiband surface electronic structure. We find similar surface state dispersions in PdCrO₂, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.

Decoupling Carrier Concentration and Electron-Phonon Coupling in Oxide Heterostructures Observed with Resonant Inelastic X-Ray Scattering

We report the observation of multiple phonon satellite features in ultrathin superlattices of the form nSrIrO_{3}/mSrTiO_{3} using resonant inelastic x-ray scattering (RIXS). As the values of n and m vary, the energy loss spectra show a systematic evolution in the relative intensity of the phonon satellites. Using a closed-form solution for the RIXS cross section, we extract the variation in the electron-phonon coupling strength as a function of n and m. Combined with the negligible carrier doping into the SrTiO_{3} layers, these results indicate that the tuning of the electron-phonon coupling can be effectively decoupled from doping. This work both showcases a feasible method to extract the electron-phonon coupling in superlattices and unveils a potential route for tuning this coupling, which is often associated with superconductivity in SrTiO_{3}-based systems.

Orbital Ordering of the Mobile and Localized Electrons at Oxygen-Deficient LaAlO3/SrTiO3 Interfaces

Interfacing different transition-metal oxides opens a route to functionalizing their rich interplay of electron, spin, orbital, and lattice degrees of freedom for electronic and spintronic devices. Electronic and magnetic properties of SrTiO₃-based interfaces hosting a mobile two-dimensional electron system (2DES) are strongly influenced by oxygen vacancies, which form an electronic dichotomy, where strongly correlated localized electrons in the in-gap states (IGSs) coexist with noncorrelated delocalized 2DES. Here, we use resonant soft-X-ray photoelectron spectroscopy to prove the e_g character of the IGSs, as opposed to the t_2g character of the 2DES in the paradigmatic LaAlO₃/SrTiO₃ interface. We furthermore separate the d _xy and d _xz/d _xz orbital contributions based on deeper consideration of the resonant photoexcitation process in terms of orbital and momentum selectivity. Supported by a self-consistent combination of density functional theory and dynamical mean field theory calculations, this experiment identifies local orbital reconstruction that goes beyond the conventional e_g- vs-t_2g band ordering. A hallmark of oxygen-deficient LaAlO₃/SrTiO₃ is a significant hybridization of the e_g and t_2g orbitals. Our findings provide routes for tuning the electronic and magnetic properties of oxide interfaces through "defect engineering" with oxygen vacancies.