Absence of 3a0 charge density wave order in the infinite-layer nickelate NdNiO2.

A hallmark of many unconventional superconductors is the presence of many-body interactions that give rise to broken-symmetry states intertwined with superconductivity. Recent resonant soft X-ray scattering experiments report commensurate 3a0 charge density wave order in infinite-layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both cuprates and nickelates. Here we present X-ray scattering and spectroscopy measurements on a series of NdNiO2+x samples, which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO2. The 3a0 superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with three-unit-cell periodicity. The absence of any observable charge density wave order in NdNiO2 highlights a crucial difference between the phase diagrams of cuprate and nickelate superconductors.

Observation of room-temperature polar skyrmions.

Complex topological configurations are fertile ground for exploring emergent phenomena and exotic phases in condensed-matter physics. For example, the recent discovery of polarization vortices and their associated complex-phase coexistence and response under applied electric fields in superlattices of (PbTiO3)n/(SrTiO3)n suggests the presence of a complex, multi-dimensional system capable of interesting physical responses, such as chirality, negative capacitance and large piezo-electric responses1-3. Here, by varying epitaxial constraints, we discover room-temperature polar-skyrmion bubbles in a lead titanate layer confined by strontium titanate layers, which are imaged by atomic-resolution scanning transmission electron microscopy. Phase-field modelling and second-principles calculations reveal that the polar-skyrmion bubbles have a skyrmion number of +1, and resonant soft-X-ray diffraction experiments show circular dichroism, confirming chirality. Such nanometre-scale polar-skyrmion bubbles are the electric analogues of magnetic skyrmions, and could contribute to the advancement of ferroelectrics towards functionalities incorporating emergent chirality and electrically controllable negative capacitance.

Gapped Collective Charge Excitations and Interlayer Hopping in Cuprate Superconductors.

We use resonant inelastic x-ray scattering to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr_{0.9}La_{0.1}CuO_{2}. We detect a plasmon gap of ∼120 meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in Sr_{0.9}La_{0.1}CuO_{2} are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered t-J-V model calculations. A similar analysis performed on recent resonant inelastic x-ray scattering data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping t_{z}. Our work signifies the three dimensionality of the charge dynamics in layered cuprates and provides a new method to determine t_{z}.

Single-Crystal Alkali Antimonide Photocathodes: High Efficiency in the Ultrathin Limit.

The properties of photoemission electron sources determine the ultimate performance of a wide class of electron accelerators and photon detectors. To date, all high-efficiency visible-light photocathode materials are either polycrystalline or exhibit intrinsic surface disorder, both of which limit emitted electron beam brightness. In this Letter, we demonstrate the synthesis of epitaxial thin films of Cs_{3}Sb on 3C-SiC (001) using molecular-beam epitaxy. Films as thin as 4 nm have quantum efficiencies exceeding 2% at 532 nm. We also find that epitaxial films have an order of magnitude larger quantum efficiency at 650 nm than comparable polycrystalline films on Si. Additionally, these films permit angle-resolved photoemission spectroscopy measurements of the electronic structure, which are found to be in good agreement with theory. Epitaxial films open the door to dramatic brightness enhancements via increased efficiency near threshold, reduced surface disorder, and the possibility of engineering new photoemission functionality at the level of single atomic layers.

Subterahertz Momentum Drag and Violation of Matthiessen's Rule in an Ultraclean Ferromagnetic SrRuO_{3} Metallic Thin Film.

SrRuO_{3}, a ferromagnet with an approximately 160 K Curie temperature, exhibits a T^{2}-dependent dc resistivity below ≈30 K. Nevertheless, previous optical studies in the infrared and terahertz range show non-Drude dynamics at low temperatures, which seem to contradict Fermi-liquid predictions. In this work, we measure the low-frequency THz range response of thin films with residual resistivity ratios, ρ_{300K}/ρ_{4K}≈74. At temperatures below 30 K, we find both a sharp zero frequency mode which has a width narrower than k_{B}T/ℏ as well as a broader zero frequency Lorentzian that has at least an order of magnitude larger scattering. Both features have temperature dependences consistent with a Fermi liquid with the wider feature explicitly showing a T^{2} scaling. Above 30 K, there is a crossover to a regime described by a single Drude peak that we believe arises from strong interband electron-electron scattering. Such two channel Drude transport sheds light on reports of the violation of Matthiessen's rule and extreme sensitivity to disorder in metallic ruthenates.

Deterministic switching of ferromagnetism at room temperature using an electric field.

The technological appeal of multiferroics is the ability to control magnetism with electric field. For devices to be useful, such control must be achieved at room temperature. The only single-phase multiferroic material exhibiting unambiguous magnetoelectric coupling at room temperature is BiFeO3 (refs 4 and 5). Its weak ferromagnetism arises from the canting of the antiferromagnetically aligned spins by the Dzyaloshinskii-Moriya (DM) interaction. Prior theory considered the symmetry of the thermodynamic ground state and concluded that direct 180-degree switching of the DM vector by the ferroelectric polarization was forbidden. Instead, we examined the kinetics of the switching process, something not considered previously in theoretical work. Here we show a deterministic reversal of the DM vector and canted moment using an electric field at room temperature. First-principles calculations reveal that the switching kinetics favours a two-step switching process. In each step the DM vector and polarization are coupled and 180-degree deterministic switching of magnetization hence becomes possible, in agreement with experimental observation. We exploit this switching to demonstrate energy-efficient control of a spin-valve device at room temperature. The energy per unit area required is approximately an order of magnitude less than that needed for spin-transfer torque switching. Given that the DM interaction is fundamental to single-phase multiferroics and magnetoelectrics, our results suggest ways to engineer magnetoelectric switching and tailor technologically pertinent functionality for nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.

Functional electronic inversion layers at ferroelectric domain walls.

Ferroelectric domain walls hold great promise as functional two-dimensional materials because of their unusual electronic properties. Particularly intriguing are the so-called charged walls where a polarity mismatch causes local, diverging electrostatic potentials requiring charge compensation and hence a change in the electronic structure. These walls can exhibit significantly enhanced conductivity and serve as a circuit path. The development of all-domain-wall devices, however, also requires walls with controllable output to emulate electronic nano-components such as diodes and transistors. Here we demonstrate electric-field control of the electronic transport at ferroelectric domain walls. We reversibly switch from resistive to conductive behaviour at charged walls in semiconducting ErMnO3. We relate the transition to the formation-and eventual activation-of an inversion layer that acts as the channel for the charge transport. The findings provide new insight into the domain-wall physics in ferroelectrics and foreshadow the possibility to design elementary digital devices for all-domain-wall circuitry.

Direct Observation of Electrostatically Driven Band Gap Renormalization in a Degenerate Perovskite Transparent Conducting Oxide.

We have directly measured the band gap renormalization associated with the Moss-Burstein shift in the perovskite transparent conducting oxide (TCO), La-doped BaSnO_{3}, using hard x-ray photoelectron spectroscopy. We determine that the band gap renormalization is almost entirely associated with the evolution of the conduction band. Our experimental results are supported by hybrid density functional theory supercell calculations. We determine that unlike conventional TCOs where interactions with the dopant orbitals are important, the band gap renormalization in La-BaSnO_{3} is driven purely by electrostatic interactions.

Strain Control of Fermiology and Many-Body Interactions in Two-Dimensional Ruthenates.

Here we demonstrate how the Fermi surface topology and quantum many-body interactions can be manipulated via epitaxial strain in the spin-triplet superconductor Sr_{2}RuO_{4} and its isoelectronic counterpart Ba_{2}RuO_{4} using oxide molecular beam epitaxy, in situ angle-resolved photoemission spectroscopy, and transport measurements. Near the topological transition of the γ Fermi surface sheet, we observe clear signatures of critical fluctuations, while the quasiparticle mass enhancement is found to increase rapidly and monotonically with increasing Ru-O bond distance. Our work demonstrates the possibilities for using epitaxial strain as a disorder-free means of manipulating emergent properties, many-body interactions, and potentially the superconductivity in correlated materials.

Formation and Observation of a Quasi-Two-Dimensional dxy Electron Liquid in Epitaxially Stabilized Sr(2-x)La(x)TiO4 Thin Films.

We report the formation and observation of an electron liquid in Sr(2-x)La(x)TiO4, the quasi-two-dimensional counterpart of SrTiO3, through reactive molecular-beam epitaxy and in situ angle-resolved photoemission spectroscopy. The lowest lying states are found to be comprised of Ti 3d_{xy} orbitals, analogous to the LaAlO3/SrTiO3 interface and exhibit unusually broad features characterized by quantized energy levels and a reduced Luttinger volume. Using model calculations, we explain these characteristics through an interplay of disorder and electron-phonon coupling acting cooperatively at similar energy scales, which provides a possible mechanism for explaining the low free carrier concentrations observed at various oxide heterostructures such as the LaAlO3/SrTiO3 interface.

Interplay of spin-orbit interactions, dimensionality, and octahedral rotations in semimetallic SrIrO(3).

We employ reactive molecular-beam epitaxy to synthesize the metastable perovskite SrIrO(3) and utilize in situ angle-resolved photoemission to reveal its electronic structure as an exotic narrow-band semimetal. We discover remarkably narrow bands which originate from a confluence of strong spin-orbit interactions, dimensionality, and both in- and out-of-plane IrO(6) octahedral rotations. The partial occupation of numerous bands with strongly mixed orbital characters signals the breakdown of the single-band Mott picture that characterizes its insulating two-dimensional counterpart, Sr(2)IrO(4), illustrating the power of structure-property relations for manipulating the subtle balance between spin-orbit interactions and electron-electron interactions.

Nature of the metal insulator transition in ultrathin epitaxial vanadium dioxide.

We have combined hard X-ray photoelectron spectroscopy with angular dependent O K-edge and V L-edge X-ray absorption spectroscopy to study the electronic structure of metallic and insulating end point phases in 4.1 nm thick (14 units cells along the c-axis of VO2) films on TiO2(001) substrates, each displaying an abrupt MIT centered at ~300 K with width <20 K and a resistance change of ΔR/R > 10(3). The dimensions, quality of the films, and stoichiometry were confirmed by a combination of scanning transmission electron microscopy with electron energy loss spectroscopy, X-ray spectroscopy, and resistivity measurements. The measured end point phases agree with their bulk counterparts. This clearly shows that, apart from the strain induced change in transition temperature, the underlying mechanism of the MIT for technologically relevant dimensions must be the same as the bulk for this orientation.

Magnon interactions in a moderately correlated Mott insulator.

Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO2, single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single-magnon dispersion is only described satisfactorily when including significant quantum corrections stemming from magnon-magnon interactions. Comparative results on La2CuO4 indicate that quantum fluctuations are much stronger in SrCuO2 suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations reveal that other magnetic orders may compete with the antiferromagnetic Néel order as the ground state. Our results indicate that SrCuO2-due to strong quantum fluctuations-is a unique starting point for the exploration of novel magnetic ground states.

Is Ba3In2O6a high-Tcsuperconductor?

It has been suggested that Ba3In2O6might be a high-Tcsuperconductor. Experimental investigation of the properties of Ba3In2O6was long inhibited by its instability in air. Recently epitaxial Ba3In2O6with a protective capping layer was demonstrated, which finally allows its electronic characterization. The optical bandgap of Ba3In2O6is determined to be 2.99 eV in-the (001) plane and 2.83 eV along thec-axis direction by spectroscopic ellipsometry. First-principles calculations were carried out, yielding a result in good agreement with the experimental value. Various dopants were explored to induce (super-)conductivity in this otherwise insulating material. NeitherA- norB-site doping proved successful. The underlying reason is predominately the formation of oxygen interstitials as revealed by scanning transmission electron microscopy and first-principles calculations. Additional efforts to induce superconductivity were investigated, including surface alkali doping, optical pumping, and hydrogen reduction. To probe liquid-ion gating, Ba3In2O6was successfully grown epitaxially on an epitaxial SrRuO3bottom electrode. So far none of these efforts induced superconductivity in Ba3In2O6,leaving the answer to the initial question of whether Ba3In2O6is a high-Tcsuperconductor to be 'no' thus far.

Quasiparticle mass enhancement and temperature dependence of the electronic structure of ferromagnetic SrRuO3 thin films.

We report high-resolution angle-resolved photoemission studies of epitaxial thin films of the correlated 4d transition metal oxide ferromagnet SrRuO(3). The Fermi surface in the ferromagnetic state consists of well-defined Landau quasiparticles exhibiting strong coupling to low-energy bosonic modes which contributes to the large effective masses observed by transport and thermodynamic measurements. Upon warming the material through its Curie temperature, we observe a substantial decrease in quasiparticle coherence but negligible changes in the ferromagnetic exchange splitting, suggesting that local moments play an important role in the ferromagnetism in SrRuO(3).

Temperature dependence of the electronic structure and Fermi-surface reconstruction of Eu(1-x)Gd(x)O through the ferromagnetic metal-insulator transition.

We present angle-resolved photoemission spectroscopy of Eu(1-x)Gd(x)O through the ferromagnetic metal-insulator transition. In the ferromagnetic phase, we observe Fermi surface pockets at the Brillouin zone boundary, consistent with density functional theory, which predicts a half-metal. Upon warming into the paramagnetic state, our results reveal a strong momentum-dependent evolution of the electronic structure, where the metallic states at the zone boundary are replaced by pseudogapped states at the Brillouin zone center due to the absence of magnetic long-range order of the Eu 4f moments.

Orthorhombic BiFeO3.

A new orthorhombic phase of the multiferroic BiFeO3 has been created via strain engineering by growing it on a NdScO(3)(110)(o) substrate. The tensile-strained orthorhombic BiFeO3 phase is ferroelectric and antiferromagnetic at room temperature. A combination of nonlinear optical second harmonic generation and piezoresponse force microscopy revealed that the ferroelectric polarization in the orthorhombic phase is along the in-plane {110}(pc) directions. In addition, the corresponding rotation of the antiferromagnetic axis in this new phase was observed using x-ray linear dichroism.

Infrared phonon spectroscopy of a compressively strained (001) SrTiO3 film grown on a (110) NdGaO3 substrate.

Polarized infrared reflectivity was measured between 5 and 300 K on a 17 nm thick, 1.1% compressively strained epitaxial (001) SrTiO(3) film and the orthorhombic (110) NdGaO(3) substrate upon which it was grown. A strong in-plane infrared anisotropy of the NdGaO(3) substrate was observed and polar modes with B(1u)-and a mixture of B(2u) + B(3u)-symmetry were seen. At low temperatures three new modes arose in the 90-130 cm( - 1) range, which we assigned to 4f Nd electronic transitions. The in-plane SrTiO(3) film phonons showed strong stiffening compared to the phonon frequencies of bulk unstrained SrTiO(3), particularly the soft mode, and the in-plane phonon peaks were found to split. No anomalies were detected as a function of temperature in either the infrared response or lattice parameters of the compressively strained SrTiO(3) film, providing an absence of evidence for the out-of-plane ferroelectric phase transition predicted by theory.

Strain-stabilized superconductivity.

Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.