Many-Body Models for Chirality-Induced Spin Selectivity in Electron Transfer.

We present the first microscopic model for the chirality-induced spin selectivity effect in electron-transfer, in which the internal degrees of freedom of the chiral bridge are explicitly included. By exactly solving this model on short chiral chains we demonstrate that a sizable spin polarization on the acceptor arises from the interplay of coherent and incoherent dynamics, with strong electron-electron correlations yielding many-body states on the bridge as crucial ingredients. Moreover, we include the coherent and incoherent dynamics induced by interactions with vibrational modes and show that they can play an important role in determining the long-time polarized state probed in experiments.

Insulator-transition-induced Degradation of Pyrochlore Ruthenates in Electrocatalytic Oxygen Evolution and Stabilization through Doping.

Ru-based pyrochlores (e.g., Y2Ru2O7-d) are promised to replace IrO2 in polymer electrolyte membrane (PEM) electrolyzers. It is significant to reveal the cliff attenuation on the oxygen evolution reaction (OER) performance of these pyrochlores. In this work, we monitor the structure changes and electrochemical behavior of Y2Ru2O7-d over the OER process, and it is found that the reason of decisive OER inactivation is derived from an insulator transition occurred within Y2Ru2O7-d due to its inner ²perfecting² lattice induced by continuous atom rearrangement. Therefore, a stabilization strategy of the Ir-substituted Y2Ru2O7-d is proposed to alleviate this undesirable behavior. The double-exchange interaction between Ru and Ir in [RuO6] and [IrO6] octahedra leads the charge redistribution with simultaneous spin configuration adjustment. The electronic state in newly formed octahedrons centered with Ru 4d3 (with the state of eg'2--a1g-1 eg0) and Ir 5d6 (eg'4a1g-2 eg0) relieves the uneven electron distributions in [RuO6] orbital. The attenuated Jahn-Teller effect alleviates atom rearrangement, represented as the mitigation of insulator transition, surface reconstruction, and metal dissolution. As results, the Ir-substituted Y2Ru2O7-d presents the greatly improved OER stability and PEM durability. This study unveils the OER degradation mechanism and stabilization strategy for material design of Ru-based OER catalysts for electrochemical applications.

Twisted graphene superlattices: resonating valence bond states and magnetic properties.

Resonating valence bond (RVB) states are fundamental for understanding quantum spin liquids in two-dimensional (2D) systems. The RVB state is a collective phenomenon in which spins are uncoupled. 2D lattices such as triangular, honeycomb, and dice lattices were investigated using the Hubbard model and exact diagonalization method. We analyzed the total spin, spin-spin correlation functions, local magnetic moments, and spin and charge gaps as a function of on-site Coulomb repulsion, electron concentration, and electronic hopping parameters. Phase diagrams showed that RVB states can live in half-filled and hole-doped anisotropic triangular lattices. We found two types of RVB states: one in the honeycomb sublattice and the other in the centered hexagons in the triangular lattices. Owing to the novel discovery of exotic magnetic ordering in triangular moiré patterns in twisted bilayer graphene and transition metal dichalcogenide systems, our results provide physical insights into the onset of magnetism and possible spin liquid states in these layered materials.

Confinement-Induced Isosymmetric Metal-Insulator Transition in Ultrathin Epitaxial V2O3 Films.

Dimensional confinement has shown to be an effective strategy to tune competing degrees of freedom in complex oxides. Here, we achieved atomic layered growth of trigonal vanadium sesquioxide (V2O3) by means of oxygen-assisted molecular beam epitaxy. This led to a series of high-quality epitaxial ultrathin V2O3 films down to unit cell thickness, enabling the study of the intrinsic electron correlations upon confinement. By electrical and optical measurements, we demonstrate a dimensional confinement-induced metal-insulator transition in these ultrathin films. We shed light on the Mott-Hubbard nature of this transition, revealing a vanishing quasiparticle weight as demonstrated by photoemission spectroscopy. Furthermore, we prove that dimensional confinement acts as an effective out-of-plane stress. This highlights the structural component of correlated oxides in a confined architecture, while opening an avenue to control both in-plane and out-of-plane lattice components by epitaxial strain and confinement, respectively.

Mott Transition in the Hubbard Model on Anisotropic Honeycomb Lattice with Implications for Strained Graphene: Gutzwiller Variational Study.

The modification of interatomic distances due to high pressure leads to exotic phenomena, including metallicity, superconductivity and magnetism, observed in materials not showing such properties in normal conditions. In two-dimensional crystals, such as graphene, atomic bond lengths can be modified by more than 10 percent by applying in-plane strain, i.e., without generating high pressure in the bulk. In this work, we study the strain-induced Mott transition on a honeycomb lattice by using computationally inexpensive techniques, including the Gutzwiller Wave Function (GWF) and different variants of Gutzwiller Approximation (GA), obtaining the lower and upper bounds for the critical Hubbard repulsion (U) of electrons. For uniaxial strain in the armchair direction, the band gap is absent, and electron correlations play a dominant role. A significant reduction in the critical Hubbard U is predicted. Model considerations are mapped onto the tight-binding Hamiltonian for monolayer graphene by the auxiliary Su-Schrieffer-Heeger model for acoustic phonons, assuming zero stress in the direction perpendicular to the strain applied. Our results suggest that graphene, although staying in the semimetallic phase even for extremely high uniaxial strains, may show measurable signatures of electron correlations, such as the band narrowing and the reduction in double occupancies.

Half-topological state in magnetic topological insulators.

We predict a novel topological state,half-topological state, in magnetic topological insulators. The topological state is characterized by different topologies of electrons with different spin orientations, i.e., electrons with one spin orientation occupy a nontrivial topological insulating state, while electrons with opposite orientation occupy another insulating state with trivial topology. We demonstrate the occurrence of the half-topological state in magnetic topological insulators by employing a minimal model. The minimal model is a combination of the spinful Haldane and the double-exchange models. The double-exchange processes maintain a spontaneous magnetic ordering, while the next-nearest-neighbor hopping in the Haldane model gives rise to a nontrivial topological insulator. The minimal model is studied by applying the dynamical mean field theory. It is found that the long-range antiferromagnetic ordering drives the system from either topological or topologically trivial antiferromagnetic insulator to the half-topological state, and finally to topologically trivial antiferromagnetic insulator. The equations for the topological phase transitions are also explicitly derived.

Electron Dynamics and Correlations During High-Order Harmonic Generation in Be.

We investigate theoretically the high-order harmonic generation in beryllium atom irradiated by a short 1850 nm linearly polarized laser pulse in the intermediate strong-field ionization regime with the Keldysh parameter of 0.85. To this end, the respective time-dependent Schrödinger equation is solved by the time-dependent restricted-active-space configuration-interaction (TD-RASCI) method. By systematically increasing the active space of included configurations, we demonstrate an individual effect of different physical processes evoked by the pulse, which, all together, significantly enrich and extend the computed high-order harmonic generation spectrum.

Itinerant magnetism of chromium under pressure: a DFT+DMFT study.

We consider electronic and magnetic properties of chromium, a well-known itinerant antiferromagnet, by a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We find that electronic correlation effects in chromium, in contrast to its neighbors in the periodic table, are weak, leading to the quasiparticle mass enhancement factorm*/m≈ 1.2. Our results for local spin-spin correlation functions and distribution of weights of atomic configurations indicate that the local magnetic moments are not formed. Similarly to previous results of DFT at ambient pressure, the non-uniform magnetic susceptibility as a function of momentum possesses close to the wave vectorQH= (0, 0, 2π/a) (ais the lattice constant) sharp maxima, corresponding to Kohn anomalies. We find that these maxima are preserved by the interaction and are not destroyed by pressure. Our calculations qualitatively capture a decrease of the Néel temperature with pressure and a breakdown of itinerant antiferromagnetism at pressure of ∼9 GPa in agreement with experimental data, although the Néel temperature is significantly overestimated because of the mean-field nature of DMFT.

Metal Substitution Steering Electron Correlations in Pyrochlore Ruthenates for Efficient Acidic Water Oxidation.

Exploring the advanced oxygen evolution reaction (OER) electrocatalysts is highly desirable toward sustainable energy conversion and storage, yet improved efficiency in acidic media is largely hindered by its sluggish reaction kinetics. Herein, we rationally manipulate the electronic states of the strongly electron correlated pyrochlore ruthenate Y2Ru2O7 alternative through partial A-site substitution of Sr2+ for Y3+, efficiently improving its intrinsic OER activity. The optimized Y1.7Sr0.3Ru2O7 candidate observes a highly intrinsic mass activity of 1018 A gRu-1 at an overpotential of 300 mV with excellent durability in 0.5 M H2SO4 electrolyte. Combining synchrotron-radiation X-ray spectroscopic investigations with theoretical simulations, we reveal that the electron correlations in the Ru 4d band are weakened through coordinatively geometric regulation and charge redistribution by the exotic Sr2+ cation, enabling the delocalization of Ru 4d electrons via an insulator-to-metal transition. The induced Ru-O covalency promotion and band alignment rearrangement decreases the charge transfer energy to accelerate interfacial charge transfer kinetics. Meanwhile, the chemical affinity of oxygen intermediates is also rationalized to weaken the metal-oxygen binding strength, thus lowering the energy barrier of the overall reaction. This work offers fresh insights into designing advanced solid-state electrocatalysts and underlines the versatility of electronic structure manipulation in tuning catalytic activity.

Moiréless correlations in ABCA graphene.

Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene--a simple material that also exhibits a flat electronic band edge but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3-5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small-angle twisted double-bilayer graphene is an ideal programmable topological quantum material.

Large magnetoresistance in the iron-free pnictide superconductor LaRu2P2.

The magnetoresistance (MR) of iron pnictide superconductors is often dominated by electron-electron correlations and deviates from theH2or saturating behaviors expected for uncorrelated metals. Contrary to similar Fe-based pnictide systems, the superconductor LaRu2P2(Tc= 4 K) shows no enhancement of electron-electron correlations. Here we report a non-saturating MR deviating from theH2or saturating behaviors in LaRu2P2. We present results in single crystals of LaRu2P2, where we observe a MR followingH1.3up to 22 T. We discuss our result by comparing the bandstructure of LaRu2P2with that of Fe based pnictide superconductors. The different orbital structures of Fe and Ru leads to a 3D Fermi surface with negligible bandwidth renormalization in LaRu2P2, that contains a large open sheet over the whole Brillouin zone. We show that the large MR in LaRu2P2is unrelated to the one obtained in materials with strong electron-electron correlations and that it is compatible instead with conduction due to open orbits on the rather complex Fermi surface structure of LaRu2P2.

Interacting chiral electrons at the 2D Dirac points: a review.

The pseudo-relativistic chiral electrons in 2D graphene and 3D topological semimetals, known as the massless Dirac or Weyl fermions, constitute various intriguing issues in modern condensed-matter physics. In particular, the issues linked to the Coulomb interaction between the chiral electrons attract great attentions due to their unusual features, namely, the interaction is not screened and has a long-ranged property near the charge-neutrality point, in clear contrast to its screened and short-ranged properties in the conventional correlated materials. In graphene, this long-range interaction induces an anomalous logarithmic renormalization of the Fermi velocity, which causes a nonlinear reshaping of its Dirac cone. In addition, for strong interactions, it even leads to the predictions of an excitonic condensation with a spontaneous mass generation. The interaction, however, would seem to be not that large in graphene, so that the latter phenomenon appears to have not yet been observed. Contrastingly, the interaction is probably large in the pressurized organic materialα-(BEDT-TTF)2I3, where a 2D massless-Dirac-fermion phase emerges next to a correlated insulating phase. Therefore, an excellent testing ground would appear in this material for the studies of both the velocity renormalization and the mass generation, as well as for those of the short-range electronic correlations. In this review, we give an overview of the recent progress on the understanding of such interacting chiral electrons in 2D, by placing particular emphasis on the studies in graphene andα-(BEDT-TTF)2I3. In the first half, we briefly summarize our current experimental and theoretical knowledge about the interaction effects in graphene, then turn attentions to the understanding inα-(BEDT-TTF)2I3, and highlight its relevance to and difference from graphene. The second half of this review focusses on the studies linked to the nuclear magnetic resonance experiments and the associated model calculations inα-(BEDT-TTF)2I3. These studies allow us to discuss the anisotropic reshaping of a tilted Dirac cone together with various electronic correlations, and the precursor excitonic dynamics growing prior to a condensation. We see these provide unique opportunities to resolve the momentum dependence of the spin excitations and fluctuations that are strongly influenced by the long-range interaction near the Dirac points.

Spectroscopic Evidence for a Spin- and Valley-Polarized Metallic State in a Nonmagic-Angle Twisted Bilayer Graphene.

In the magic-angle twisted bilayer graphene (MA-TBG), strong electron-electron (e-e) correlations caused by the band-flattening lead to many exotic quantum phases such as superconductivity, correlated insulator, ferromagnetism, and quantum anomalous Hall effects, when its low-energy van Hove singularities (VHSs) are partially filled. Here our high-resolution scanning tunneling microscope and spectroscopy measurements demonstrate that the e-e correlation in a nonmagic-angle TBG with a twist angle θ = 1.49° still plays an important role in determining its electronic properties. Our most interesting observation on that sample is when one of its VHSs is partially filled, the one associated peak in the spectrum splits into four peaks. Simultaneously, the spatial symmetry of electronic states around the split VHSs is broken by the e-e correlation. Our analysis based on the continuum model suggests that such a one-to-four split of the VHS originates from the formation of an interaction-driven spin-valley-polarized metallic state near the VHS, which is a symmetry-breaking phase that has not been identified in the MA-TBG or in other systems.

Superconductivity in chromium nitrides Pr3Cr10-xN11 with strong electron correlations.

Exploration of superconductivity in Cr-based compounds has attracted considerable interest because only a few Cr-based superconductors (CrAs, A2Cr3As3 and ACr3As3 (A = K, Rb, Cs, Na)) have been discovered so far and they show an unconventional pairing mechanism. We report the discovery of bulk superconductivity at 5.25 K in chromium nitride in Pr3Cr10-xN11 with a cubic lattice structure. A relatively large upper critical field of H c2(0) ∼ 12.6 T is determined, which is larger than the estimated Pauli-paramagnetic pair-breaking magnetic field. The material has a large electronic specific-heat coefficient of 170 mJ K-2 mol-1-about 10 times larger than that estimated by the electronic structure calculation, which suggests that correlations between 3d electrons are very strong in Pr3Cr10-xN11, and thus quantum fluctuations might be involved. Electronic structure calculations show that the density of states at the Fermi energy are contributed predominantly by Cr 3d electrons, implying that the superconductivity results mainly from the condensation of Cr 3d electrons. Pr3Cr10-xN11 represents a rare example of possible unconventional superconductivity emerging in a 3D system with strong electron correlations. Nevertheless, clarification of the specific pairing symmetry needs more investigation.

Electron Correlations Engineer Catalytic Activity of Pyrochlore Iridates for Acidic Water Oxidation.

The development of highly efficient oxygen-evolving catalysts compatible with powerful proton-exchange-membrane-based electrolyzers in acid environments is of prime importance for sustainable hydrogen production. In this field, understanding the role of electronic structure of catalysts on catalytic activity is essential but still lacking. Herein, a family of pyrochlore oxides R2 Ir2 O7 (R = rare earth ions) is reported as acidic oxygen-evolving catalysts with superior-specific activities. More importantly, it is found that the intrinsic activity of this material significantly increases with the R ionic radius. Electronic structure studies reveal that the increased R ionic radius weakens electron correlations in these iridate oxides. This weakening induces an insulator-metal transition and an enhancement of IrO bond covalency, both of which promote oxygen evolution kinetics. This work demonstrates the importance of engineering the electron correlations to rationalize the catalytic activity toward water oxidation in strongly correlated transition-metal oxides.