A quantum spin Hall (QSH) insulator is a novel twodimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin-orbit coupling 1,2 . By investigating the electronic structure of epitaxially grown monolayer 1T'-WTe 2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulk bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe 2 as a new class of QSH insulator with large bandgap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).A two-dimensional (2D) topological insulator (TI), or a quantum spin Hall insulator, is characterized by an insulating bulk and a conductive helical edge state, in which carriers with different spins counter-propagate to realize a geometry-independent edge conductance 2e 2 /h (refs 1,2). The only scattering channel for such helical edge current is back scattering, which is prohibited by time reversal symmetry, making QSH insulators a promising material candidate for spintronic and other applications.The prediction of the QSH effect in HgTe quantum wells sparked intense research efforts to realize the QSH state [3][4][5][6][7][8][9][10][11] . So far only a handful of QSH systems have been fabricated, mostly limited to quantum well structures of three-dimensional (3D) semiconductors such as HgTe/CdTe (ref.3) and InAs/GaSb (ref. 6). Edge conduction consistent with a QSH state has been observed 3,6,12 . However, the behaviour under a magnetic field, where time reversal symmetry is broken, cannot be explained within our current understanding of the QSH effect 13,14 . There have been continued efforts to predict and investigate other material systems to further advance the understanding of this novel quantum phenomenon 5,[7][8][9]15 . So far, it has been difficult to make a robust 2D material with a QSH state, a platform needed for widespread study and application. The small bandgaps exhibited by many candidate systems, as well as their vulnerability to strain, chemical adsorption, and element substitution, make them impractical for advanced spectroscopic studies or applications. For example, a QSH insulator candidate stanene, a monolayer analogue of graphene for tin, grown on Bi 2 Se 3 becomes topologically trivial due to the modification of its band structure by the underlying substrate 11,16 . Free-standing Bi film with 2D bonding on a cleaved surface has shown edge conduction 9 , but its topological nature is still debated 17 . It takes 3D out-of-plane bonding with the substrate and large stra...

The search for oxide materials with physical properties similar to the cuprate high Tc superconductors, but based on alternative transition metals such as nickel, has grown and evolved over time [1][2][3][4][5][6][7][8][9][10]. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) [11,12] further strengthens these efforts. With a crystal structure similar to the infinite-layer cupratestransition metal oxide layers separated by a rare-earth spacer layerformal valence counting suggests that these materials have monovalent Ni 1+ cations with the same 3d electron count as Cu 2+ in the cuprates. Here, we use x-ray spectroscopy in concert with density functional theory to show that the electronic structure of RNiO2 (R = La, Nd), while similar to the cuprates, includes significant distinctions. Unlike cuprates with insulating spacer layers between the CuO2 planes, the rare-earth spacer layer in the infinite-layer nickelate supports a weaklyinteracting three-dimensional 5d metallic state. This three-dimensional metallic state hybridizes with a quasi-two-dimensional, strongly correlated state with 3dx 2 -y 2 symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare earth intermetallics [13-15], well-known for heavy Fermion behavior, where the NiO2 correlated layers play an analogous role to the 4f states in rare-earth heavy Fermion compounds. This unique Kondo-or Anderson-lattice-like "oxide-intermetallic" replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.While the mechanism of superconductivity in the cuprates remains a subject of intense research, early on it was suggested that the conditions required for realizing high Tc superconductivity are rooted in the physics of a two-dimensional electron system subject to strong local repulsion [16,17]. This describes the Mott (charge-transfer) insulators in the stoichiometric parent compounds, characterized by spin ½ Heisenberg antiferromagnetism, from which superconductivity emerges upon doping. A long-standing question regards whether these "cuprate-Mott" conditions can be realized in other oxides; and extensive efforts to synthesize and engineer nickel oxides (nickelates) have promised such a realization [1-10]. Infinite-layer NdNiO2 became the first such nickelate superconductor following the recent discovery of superconductivity in Srdoped samples [11]. The undoped parent compound, produced by removing the apical oxygen atoms from the perovskite nickelate NdNiO3 using a metal hydride-based soft chemistry reduction process [10,[18][19][20], appears to be a close sibling of the cuprates-it is isostructural to the infinitelayer cuprates with monovalent Ni 1+ cations and possesses the same 3d 9 electron count as that of Cu 2+ cations in undoped cuprates. Yet, as we will reveal, the electronic structure of the undoped RNiO2 (R = La and Nd) remains distinct from the Mott, or charge-transfer, compounds of undoped cuprates, and even...

The solid electrolyte interphase (SEI) is a passivation layer naturally formed on battery electrodes. It protects electrodes and electrolytes from degradation and dictates charging time capabilities and lifetime. Despite its importance, it remains a poorly understood battery component. This study provides novel insights into the formation, morphology, and composition of the SEI on Si anodes through a multi-modal approach. The findings show a layered SEI and the ion and electron conductivities, as well as their relation to performance, are discussed.

How coherent quasiparticles emerge by doping quantum antiferromagnets is a key question in correlated electron systems, whose resolution is needed to elucidate the phase diagram of copper oxides. Recent resonant inelastic X-ray scattering (RIXS) experiments in hole-doped cuprates have purported to measure high-energy collective spin excitations that persist well into the overdoped regime and bear a striking resemblance to those found in the parent compound, challenging the perception that spin excitations should weaken with doping and have a diminishing effect on superconductivity. Here we show that RIXS at the Cu L 3 -edge indeed provides access to the spin dynamical structure factor once one considers the full influence of light polarization. Further we demonstrate that high-energy spin excitations do not correlate with the doping dependence of T c , while low-energy excitations depend sensitively on doping and show ferromagnetic correlations. This suggests that high-energy spin excitations are marginal to pairing in cuprate superconductors.

The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a "coherent lock-in" measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.

Despite significant progress in resonant inelastic x-ray scattering (RIXS) experiments on cuprates at the Cu L-edge, a theoretical understanding of the cross section remains incomplete in terms of elementary excitations and the connection to both charge and spin structure factors. Here, we use state-of-the-art, unbiased numerical calculations to study the low-energy excitations probed by RIXS in the Hubbard model, relevant to the cuprates. The results highlight the importance of scattering geometry, in particular, both the incident and scattered x-ray photon polarization, and they demonstrate that on a qualitative level the RIXS spectral shape in the cross-polarized channel approximates that of the spin dynamical structure factor. However, in the parallel-polarized channel, the complexity of the RIXS process beyond a simple two-particle response complicates the analysis and demonstrates that approximations and expansions that attempt to relate RIXS to less complex correlation functions cannot reproduce the full diversity of RIXS spectral features.

Recent debates on the oxygen redox behaviors in battery electrodes have triggered a pressing demand for the reliable detection and understanding of non-divalent oxygen states beyond conventional absorption spectroscopy. Here, enabled by high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) coupled with first-principles calculations, we report distinct mRIXS features of the oxygen states in Li2O, Li2CO3, and especially, Li2O2, which are successfully reproduced and interpreted theoretically. mRIXS signals are dominated by valence-band decays in Li2O and Li2CO3. However, the oxidized oxygen in Li2O2 leads to partially unoccupied O-2p states that yield a specific intra-band excitonic feature in mRIXS. Such a feature displays a specific emission energy in mRIXS, which disentangles the oxidized oxygen states from the dominating transition-metal/oxygen hybridization features in absorption spectroscopy, thus providing critical hints for both detecting and understanding the oxygen redox reactions in transition-metal oxide based battery materials.

Functional CsPbI3 perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite δ phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI3 perovskite structure to room temperature by subjecting the δ phase to pressures of 0.1 – 0.6 GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic γ-CsPbI3. Once formed, γ-CsPbI3 could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI6]4− octahedra which in turn tune the energy difference between δ- and γ-CsPbI3, leading to the preservation of γ-CsPbI3. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality.