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Here we employed optical spectroscopy and ultrafast reflectivity measurements to investigate the density wave instability of trilayer nickelate La4Ni3O10 at ambient pressure. Our optical spectroscopy measurements indicate that La4Ni3O10 is metallic with a large plasma frequency at room temperature. As the temperature decreases, we observe the formation of an energy gap in reflectivity below TDW, signaling the charge/spin density wave transition. The Drude component was largely removed due to the gap opening in the Fermi surface. Our Drude-Lorentz analysis reveals that the energy gap in La4Ni3O10 is approximately 61 meV, which is three times larger than that obtained from ARPES measurements. The density wave gap feature is more prominent than that observed in bilayer nickelate La3Ni2O7, suggesting more carriers are gapped at the Fermi surface across the density wave transition. By comparing the measured plasma frequency with the first-principles calculation, we categorize La4Ni3O10 as a moderately electronic correlation material, similar to the parent compound of iron-based superconductors, however, being weaker than the bilayer nickelate La3Ni2O7. Our ultrafast pump-probe experiments also show that the relaxation time diverges near the transition temperature. By analyzing the amplitude and relaxation time with the Rothwarf-Taylor model, we estimate the energy gap to be 58 meV, which agrees with the result of optical spectroscopy. The more prominent gap feature and weaker electronic correlation might be the cause of a lower superconductivity transition temperature in La4Ni3O10 under high pressure. These findings significantly contribute to understanding the origin of density wave and superconductivity in trilayer nickelate La4Ni3O10.

Low-dimensional magnetic structures coupled with superconductors are promising platforms for realizing Majorana zero modes, which have potential applications in topological quantum computing. Here, we report a two-dimensional (2D) magnetic-superconducting heterostructure consisting of single-layer chromium diiodide (CrI2) on a niobium diselenide (NbSe2) superconductor. Single-layer CrI2 nanosheets, which hold antiferromagnetic (AFM) ground states by our first-principles calculations, were epitaxially grown on the layered NbSe2 substrate. Using scanning tunneling microscopy/spectroscopy, we observed robust in-gap states spatially located at the edge of the nanosheets and defect-induced zero-energy peaks inside the CrI2 nanosheets. Magnetic-flux vortices induced by an external field exhibit broken threefold rotational symmetry of pristine NbSe2 superconductor, implying the efficient modulation of the interfacial superconducting states by the epitaxial CrI2 layer. A phenomenological model suggests the existence of chiral edge states in a 2D AFM-superconducting hybrid system with an even Chern number, providing a qualitatively plausible understanding for our experimental observation., Comment: 21 pages, 5 figures

Recently, the discovery of superconductivity in Ruddlesden-Popper (RP) $\mathrm{La_4Ni_3O_{10}}$ under pressure has further expanded the realm of nickelate-based superconductor family. In this paper, we performed a first-principle study of $\mathrm{La_4Ni_3O_{10}}$ for both $P2_1/a$ phase at ambient pressure and $I4/mmm$ phase at high pressure, with $U$=0, 3.5\ eV. Our results confirmed the characteristic upward shift of Ni-$d_{z^2}$ bonding band under pressure. Moreover, our analysis of electronic spectrum and orbital occupancy unveil the dynamic mechanism of electronic reconstructions under pressure, embedded in a critical dual effect. Based on our results, we further proposed a trilayer two-orbital model by performing Wannier downfolding on Ni-$e_g$ orbitals. Our model reveals four Fermi surface sheets with $\alpha,\beta,\beta^\prime,\gamma$ pockets, bearing resemblance to that of bilayer $\mathrm{La_3Ni_2O_7}$. According to the model, our calculated spin susceptibility under random phase approximation shows that $d_{x^2-y^2}$ orbital is also important for the magnetic fluctuation in RP series. Finally, a high energy sixteen-orbital model with direct $dp,pp$ hoppings is proposed, which implies that $\mathrm{La_4Ni_3O_{10}}$ also lies in charge-transfer picture within Zaanen-Sawatzky-Allen scheme. Our exposition of electronic reconstructions and multi-orbital models shed light on theoretical electronic correlation study and experimental exploration of lower pressure superconductor in RP series., Comment: 11 pages, 6 figures

Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Ruddlesden-Popper nickelate La3Ni2O7, the atomic structure and electronic band structure of the trilayer nickelate La4Ni3O10 under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P21/a space group to the tetragonal I4/mmm around 12.6-13.4 GPa is identified, accompanying with a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni 3dz2 orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La4Ni3O10. The trilayer nickelate La4Ni3O10 shows some similarities with the bilayer La3Ni2O7 and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates., Comment: 19 pages, 4 figures

Transition-metal honeycomb compounds are capturing scientific attention due to their distinctive electronic configurations, underscored by the triangular-lattice spin-orbit coupling and competition between multiple interactions, paving the way for potential manifestations of phenomena such as Dirac semimetal, superconductivity, and quantum spin liquid states. These compounds can undergo discernible pressure-induced alterations in their crystallographic and electronic paradigms, as exemplified by our high-pressure (HP) synthesis and exploration of the honeycomb polymorph of ReO3 (P6322). This HP-P6322 polymorph bears a phase transition from P6322 to P63/mmc upon cooling around Tp = 250 K, as evidenced by the evolution of temperature-dependent magnetization (M-T curves), cell dimension, and conductivity initiated by an inherent bifurcation of the oxygen position in the ab plane. Insightful analysis of its band structure positions suggests this HP-P6322 polymorph being a plausible candidate for Dirac semimetal properties. This phase transition evokes anomalies in the temperature-dependent variation of paramagnetism (non-linearity) and a crossover from semiconductor to temperature-independent metal, showing a temperature independent conductivity behavior below ~200 K. Under increasing external pressure, both the Tp and resistance of this HP-polymorph is slightly magnetic-field dependent and undergo a "V"-style evolution (decreasing and then increasing) before becoming pressure independent up to 20.2 GPa. Theoretical calculations pinpoint this anionic disorder as a probable catalyst for the decrement in the conductive efficiency and muted temperature-dependent conductivity response.

The latest discovery of high temperature superconductivity near 80K in La3Ni2O7 under high pressure has attracted much attention. Many proposals are put forth to understand the origin of superconductivity.The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking. Here we report our direct measurement of the electronic structures of La3Ni2O7 by high-resolution angle resolved photoemission spectroscopy. The Fermi surface and band structures of La3Ni2O7 are observed and compared with the band structure calculations. Strong electron correlations are revealed which are orbital- and momentum dependent. A flat band is formed from the Ni-3dz2 orbitals around the zone corner which is ~50meV below the Fermi level and exhibits the strongest electron correlation. In many theoretical proposals, this band is expected to play the dominant role in generating superconductivity in La3Ni2O7. Our observations provide key experimental information to understand the electronic structure and origin of high temperature superconductivity in La3Ni2O7., Comment: 20 pages, 5 figures

Two-dimensional (2D) magnetic materials with nontrivial topological states have drawn considerable attention recently. Among them, 2D metal-organic frameworks (MOFs) are standing out due to their advantages, such as the easy synthesis in practice and less sensitivity to oxidation that are distinctly different from inorganic materials. By means of density-functional theory calculations, we systematically investigate the electronic and topological properties of a class of 2D MOFs X(C21H15N3) (X = transition metal element from 3d to 5d). Excitingly, we find that X(C21H15N3) (X = Ti, Zr, Ag, Au) are Chern insulators with sizable band gaps (~7.1 meV). By studying a four-band effective model, it is revealed that the Chern insulator phase in X(C21H15N3) (X = Ti, Zr, Ag, Au) is caused cooperatively by the band inversion of the p orbitals of the C21H15N3 molecule and the intrinsic ferromagnetism of X(C21H15N3). Additionally, Mn(C21H15N3) is a Dirac half-metal ferromagnet with a high Curie temperature up to 156 K. Our work demonstrates that 2D MOFs X(C21H15N3) are good platforms for realizing Quantum anomalous Hall effect and designing novel spintronic devices based on half-metals with high-speed and long-distance spin transport., Comment: 16 pages, 4 figures

Recently, the topological nature of an antiferromagnet YbMnBi2 has been controversial. YbMnBi2 is regarded as a candidate of Type-II Weyl semimetals with magnetic moments of Mn atoms canting about 10{\deg} in some studies but as a Dirac semimetal without canting in others. By means of systematical density functional theory calculations, we show the perfect YbMnBi2 bulk has a collinear antiferromagnetic ordering and, naturally, it is a Dirac semimetal. Considering the vital role of magnetic moment canting in generating the Type-II Weyl state, we artificially cant the magnetic moments of Mn atoms and find that YbMnBi2 enters into the Type-II Weyl state from about 2{\deg}. Inspired by this and taking into account the possible defects in experiments, we suggest that Bi vacancies in Mn-Bi-Mn bonds, which produce sizable Dzyaloshinskii-Moriya interactions and thereby cant the magnetic moments of Mn atoms, can tune the topological nature of YbMnBi2 from Dirac semimetals to Type-II Weyl semimetals. Our work unveils the possible underlying mechanism for the Type-II Weyl state in YbMnBi2, providing insights into the Weyl state in other magnetic topological materials., Comment: 13 pages, 3 figures, 1 table