Preparation, Crystal Structure, and Properties of the Kagome Metal ThVSn.

Kagome lattice materials are anticipated to exhibit unique properties stemming from the intricate interplay among geometry, magnetism, electronic correlation, and band topology. Here, we report a new ternary compound, ThVSn, which contains double-layer kagome networks composed of vanadium atoms. The compound crystallizes in an HfFeGe-type structure with cell parameters of = = 5.

Crystal structure, properties and pressure-induced insulator-metal transition in layered kagome chalcogenides.

Layered materials with kagome lattice have attracted a lot of attention due to the presence of nontrivial topological bands and correlated electronic states with tunability. In this work, we investigate a unique van der Waals (vdW) material system,(= K, Rb, Cs;= Ni, Pd;= S, Se), where transition metal kagome lattices, chalcogen honeycomb lattices and alkali metal triangular lattices coexist simultaneously. A notable feature of this material is that each Ni/Pd atom is positioned in the center of four chalcogen atoms, forming a local square-planar environment.

Observation of a Chern insulator in crystalline ABCA-tetralayer graphene with spin-orbit coupling.

Degeneracies in multilayer graphene, including spin, valley, and layer degrees of freedom, can be lifted by Coulomb interactions, resulting in rich broken-symmetry states. Here, we report a ferromagnetic state in charge-neutral ABCA-tetralayer graphene driven by proximity-induced spin-orbit coupling from adjacent tungsten diselenide. The ferromagnetic state is identified as a Chern insulator with a Chern number of 4; its maximum Hall resistance reaches 78% quantization at zero magnetic field and is fully quantized at either 0.

Superconductivity and Charge-Density-Wave-Like Transition in ThCuAs.

We report the synthesis, crystal structure, and physical properties of a novel ternary compound, ThCuAs. The material crystallizes in a tetragonal structure with lattice parameters = 4.0639(3) Å and = 24.

Time-reversal symmetry breaking superconductivity between twisted cuprate superconductors.

Twisted interfaces between stacked van der Waals (vdW) cuprate crystals present a platform for engineering superconducting order parameters by adjusting stacking angles. Using a cryogenic assembly technique, we construct twisted vdW Josephson junctions (JJs) at atomically sharp interfaces between BiSrCaCuO crystals, with quality approaching the limit set by intrinsic JJs. Near 45° twist angle, we observe fractional Shapiro steps and Fraunhofer patterns, consistent with the existence of two degenerate Josephson ground states related by time-reversal symmetry (TRS).

Low-energy quasi-circular electron correlations with charge order wavelength in BiSrCaCuO.

Most resonant inelastic x-ray scattering (RIXS) studies of dynamic charge order correlations in the cuprates have focused on the high-symmetry directions of the copper oxide plane. However, scattering along other in-plane directions should not be ignored as it may help understand, for example, the origin of charge order correlations or the isotropic scattering resulting in strange metal behavior. Our RIXS experiments reveal dynamic charge correlations over the scattering plane in underdoped BiSrCaCuO.

Geometrical frustration versus Kitaev interactions in BaCo(AsO).

Recently, Co-based honeycomb magnets have been proposed as promising candidate materials to host the Kitaev spin liquid (KSL) state. One of the front-runners is BaCo(AsO) (BCAO), where it was suggested that the exchange processes between Co ions via the surrounding edge-sharing oxygen octahedra could give rise to bond-dependent Kitaev interactions. In this work, we present and analyze a comprehensive inelastic neutron scattering (INS) study of BCAO with fields in the honeycomb plane.

A magnetic continuum in the cobalt-based honeycomb magnet BaCo(AsO).

Quantum spin liquids (QSLs) are topologically ordered states of matter that host fractionalized excitations. A particular route towards a QSL is via strongly bond-dependent interactions on the hexagonal lattice. A number of Ru- and Ir-based candidate Kitaev QSL materials have been pursued, but all have appreciable non-Kitaev interactions.

Little-Parks like oscillations in lightly doped cuprate superconductors.

Understanding the rich and competing electronic orders in cuprate superconductors may provide important insight into the mechanism of high-temperature superconductivity. Here, by measuring BiSrCaCuO in the extremely underdoped regime, we obtain evidence for a distinct type of ordering, which manifests itself as resistance oscillations at low magnetic fields (≤10 T) and at temperatures around the superconducting transition. By tuning the doping level p continuously, we reveal that these low-field oscillations occur only when p < 0.

Spin-Polarized Yu-Shiba-Rusinov States in an Iron-Based Superconductor.

Yu-Shiba-Rusinov (YSR) bound states appear when a magnetic atom interacts with a superconductor. Here, we report on spin-resolved spectroscopic studies of YSR states related with Fe atoms deposited on the surface of the topological superconductor FeTe_{0.55}Se_{0.

Observation of magnetic adatom-induced Majorana vortex and its hybridization with field-induced Majorana vortex in an iron-based superconductor.

Braiding Majorana zero modes is essential for fault-tolerant topological quantum computing. Iron-based superconductors with nontrivial band topology have recently emerged as a surprisingly promising platform for creating distinct Majorana zero modes in magnetic vortices in a single material and at relatively high temperatures. The magnetic field-induced Abrikosov vortex lattice makes it difficult to braid a set of Majorana zero modes or to study the coupling of a Majorana doublet due to overlapping wave functions.

A cleanroom in a glovebox.

The exploration of new materials, novel quantum phases, and devices requires ways to prepare cleaner samples with smaller feature sizes. Initially, this meant the use of a cleanroom that limits the amount and size of dust particles. However, many materials are highly sensitive to oxygen and water in the air.

Observation of a thermoelectric Hall plateau in the extreme quantum limit.

The thermoelectric Hall effect is the generation of a transverse heat current upon applying an electric field in the presence of a magnetic field. Here, we demonstrate that the thermoelectric Hall conductivity α in the three-dimensional Dirac semimetal ZrTe acquires a robust plateau in the extreme quantum limit of magnetic field. The plateau value is independent of the field strength, disorder strength, carrier concentration, or carrier sign.

KIrO and KIrO, Low-Dimensional Iridates with Infinite IrO Chains.

A previously unreported 1D iridate, KIrO, has been grown by a flux method in O-rich environment, and its crystal structure was determined via single crystal structural analysis. It exhibits straight chains of face-sharing [IrO] octahedra, which are arranged along the crystallographic axis, separated by nonmagnetic K ions. No magnetic transitions are observed during measured range, and the material is electrically insulating.

Weak-field induced nonmagnetic state in a Co-based honeycomb.

Layered honeycomb magnets are of interest as potential realizations of the Kitaev quantum spin liquid (KQSL), a quantum state with long-range spin entanglement and an exactly solvable Hamiltonian. Conventional magnetically ordered states are present for all currently known candidate materials, however, because non-Kitaev terms in the Hamiltonians obscure the Kitaev physics. Current experimental studies of the KQSL are focused on 4 or 5 transition metal-based honeycombs, in which strong spin-orbit coupling can be expected, yielding Kitaev interaction that dominates in an applied magnetic field.

Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor.

Majorana zero modes (MZMs) are spatially localized, zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold promise for topological quantum computing. Owing to the particle-antiparticle equivalence, MZMs exhibit quantized conductance at low temperature. By using variable-tunnel-coupled scanning tunneling spectroscopy, we studied tunneling conductance of vortex bound states on FeTeSe superconductors.

High-temperature superconductivity in monolayer BiSrCaCuO.

Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity.

Sign-Reversing Hall Effect in Atomically Thin High-Temperature Bi_{2.1}Sr_{1.9}CaCu_{2.0}O_{8+δ} Superconductors.

We developed novel techniques to fabricate atomically thin Bi_{2.1}Sr_{1.9}CaCu_{2.

Evidence for Helical Hinge Zero Modes in an Fe-Based Superconductor.

Combining topology and superconductivity provides a powerful tool for investigating fundamental physics as well as a route to fault-tolerant quantum computing. There is mounting evidence that the Fe-based superconductor FeTeSe (FTS) may also be topologically nontrivial. Should the superconducting order be s, then FTS could be a higher order topological superconductor with helical hinge zero modes (HHZMs).

Strong quantum fluctuations in a quantum spin liquid candidate with a Co-based triangular lattice.

Currently under active study in condensed matter physics, both theoretically and experimentally, are quantum spin liquid (QSL) states, in which no long-range magnetic ordering appears at low temperatures due to strong quantum fluctuations of the magnetic moments. The existing QSL candidates all have their intrinsic disadvantages, however, and solid evidence for quantum fluctuations is scarce. Here, we report a previously unreported compound, [Formula: see text], a geometrically frustrated system with effective spin-1/2 local moments for Co ions on an isotropic 2-dimensional (2D) triangular lattice.

A New Magnetic Topological Quantum Material Candidate by Design.

Magnetism, when combined with an unconventional electronic band structure, can give rise to forefront electronic properties such as the quantum anomalous Hall effect, axion electrodynamics, and Majorana fermions. Here we report the characterization of high-quality crystals of EuSnP, a new quantum material specifically designed to engender unconventional electronic states plus magnetism. EuSnP has a layered, BiTe-type structure.

Three-dimensional quantum Hall effect and metal-insulator transition in ZrTe.

The discovery of the quantum Hall effect (QHE) in two-dimensional electronic systems has given topology a central role in condensed matter physics. Although the possibility of generalizing the QHE to three-dimensional (3D) electronic systems was proposed decades ago, it has not been demonstrated experimentally. Here we report the experimental realization of the 3D QHE in bulk zirconium pentatelluride (ZrTe) crystals.

Magnetic properties of the BaNi(As V )O honeycomb system.

The Kitaev model for magnetic interactions on a honeycomb lattice, which underlies the exotic quantum spin liquid (QSL) state in such systems, has long been established and experimentally confirmed. Current studies on Kitaev QSLs have focused on heavy-metal-based Mott insulators where strong spin-orbit coupling (SOC) leads to bond-dependent spin anisotropy. Here we examine a 3d transition-metal-based honeycomb lattice material BaNi(As V )O, where the SOC is weak but spin exchange coupling is strongly frustrated due to the crystal field.

Directly visualizing the sign change of d-wave superconducting gap in BiSrCaCuO by phase-referenced quasiparticle interference.

The superconducting state is formed by the condensation of Cooper pairs and protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the gap function. Thus, it is pivotal to detect the gap structure for understanding the mechanism of superconductivity.