First-principles study of magnetic interactions and excitations in antiferromagnetic van der Waals material MPX(M=Mn, Fe, Co, Ni; X=S, Se).

Transition metal phosphorus trichalcogenides MPX(M = Mn, Fe, Co, Ni; X = S, Se), as layered van der Waals antiferromagnetic (AFM) materials, have emerged as a promising platform for exploring two-dimensional (2D) magnetism. Based on density functional theory, we present a comprehensive investigation of the electronic and magnetic properties of MPX. We calculated the spin exchange interactions as well as magnetocrystalline anisotropy energy.

Coherent ultrafast photoemission from a single quantized state of a one-dimensional emitter.

Femtosecond laser-driven photoemission source provides an unprecedented femtosecond-resolved electron probe not only for atomic-scale ultrafast characterization but also for free-electron radiation sources. However, for conventional metallic electron source, intense lasers may induce a considerable broadening of emitting energy level, which results in large energy spread (>600 milli-electron volts) and thus limits the spatiotemporal resolution of electron probe. Here, we demonstrate the coherent ultrafast photoemission from a single quantized energy level of a carbon nanotube.

Observation of robust zero-energy state and enhanced superconducting gap in a trilayer heterostructure of MnTe/BiTe/Fe(Te, Se).

The interface between magnetic material and superconductors has long been predicted to host unconventional superconductivity, such as spin-triplet pairing and topological nontrivial pairing state, particularly when spin-orbital coupling (SOC) is incorporated. To identify these unconventional pairing states, fabricating homogenous heterostructures that contain such various properties are preferred but often challenging. Here, we synthesized a trilayer-type van der Waals heterostructure of MnTe/BiTe/Fe(Te, Se), which combined s-wave superconductivity, thickness-dependent magnetism, and strong SOC.

High-Throughput Investigations of Topological and Nodal Superconductors.

The theory of symmetry indicators has enabled database searches for topological materials in normal conducting phases, which has led to several encyclopedic topological material databases. To date, such a database for topological superconductors is yet to be achieved because of the lack of information about pairing symmetries of realistic materials. In this Letter, sidestepping this issue, we tackle an alternative problem: the predictions of topological and nodal superconductivity in materials for each single-valued representation of point groups.

Designing light-element materials with large effective spin-orbit coupling.

Spin-orbit coupling (SOC), which is the core of many condensed-matter phenomena such as nontrivial band gap and magnetocrystalline anisotropy, is generally considered appreciable only in heavy elements. This is detrimental to the synthesis and application of functional materials. Therefore, amplifying the SOC effect in light elements is crucial.

Self-Assembly of Isostatic Self-Dual Colloidal Crystals.

Self-dual structures whose dual counterparts are themselves possess unique hidden symmetry, beyond the description of classical spatial symmetry groups. Here we propose a strategy based on a nematic monolayer of attractive half-cylindrical colloids to self-assemble these exotic structures. This system can be seen as a 2D system of semidisks.

Colossal Terahertz Photoresponse at Room Temperature: A Signature of Type-II Dirac Fermiology.

The discovery of Dirac semimetal has stimulated bourgeoning interests for exploring exotic quantum-transport phenomena, holding great promise for manipulating the performance of photoelectric devices that are related to nontrivial band topology. Nevertheless, it still remains elusive on both the device implementation and immediate results, with some enhanced or technically applicable electronic properties signified by the Dirac fermiology. By means of Pt doping, a type-II Dirac semimetal IrPtTe with protected crystal structure and tunable Fermi level has been achieved in this work.

Plasmonic evolution of atomically size-selected Au clusters by electron energy loss spectrum.

The plasmonic response of gold clusters with atom number () = 100-70 000 was investigated using scanning transmission electron microscopy-electron energy loss spectroscopy. For decreasing , the bulk plasmon remains unchanged above = 887 but then disappears, while the surface plasmon firstly redshifts from 2.4 to 2.

Anisotropic ultrasensitive PdTe-based phototransistor for room-temperature long-wavelength detection.

Emergent topological Dirac semimetals afford fresh pathways for optoelectronics, although device implementation has been elusive to date. Specifically, palladium ditelluride (PdTe) combines the capabilities provided by its peculiar band structure, with topologically protected electronic states, with advantages related to the occurrence of high-mobility charge carriers and ambient stability. Here, we demonstrate large photogalvanic effects with high anisotropy at terahertz frequency in PdTe-based devices.

Tuning Electrical Conductance in Bilayer MoS through Defect-Mediated Interlayer Chemical Bonding.

Interlayer interaction could substantially affect the electrical transport in transition metal dichalcogenides, serving as an effective way to control the device performance. However, it is still challenging to utilize interlayer interaction in weakly interlayer-coupled materials such as pristine MoS to realize layer-dependent tunable transport behavior. Here, we demonstrate that, by substitutional doping of vanadium atoms in the Mo sites of the MoS lattice, the vanadium-doped monolayer MoS device exhibits an ambipolar field effect characteristic, while its bilayer device demonstrates a heavy -type field effect feature, in sharp contrast to the pristine monolayer and bilayer MoS devices, both of which show similar -type electrical transport behaviors.

Experimental Observation of the Gate-Controlled Reversal of the Anomalous Hall Effect in the Intrinsic Magnetic Topological Insulator MnBiTe Device.

Magnetic topological insulator, a platform for realizing quantum anomalous Hall effect, axion state, and other novel quantum transport phenomena, has attracted a lot of interest. Recently, it is proposed that MnBiTe is an intrinsic magnetic topological insulator, which may overcome the disadvantages in the magnetic doped topological insulator, such as disorder. Here we report on the gate-reserved anomalous Hall effect (AHE) in the MnBiTe thin film.

Evidence for singular-phonon-induced nematic superconductivity in a topological superconductor candidate SrBiSe.

Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent s-wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of SrBiSe, a prime candidate for realizing topological superconductivity by doping the topological insulator BiSe, we find that there exist highly anisotropic phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the [001] direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic p-wave nematic superconducting pairing in the MBiSe (M = Cu, Sr, Nb) superconductor family.

Experimental Demonstration of Acoustic Chern Insulators.

We report the experimental realization of an acoustic Chern insulator (ACI), by using an angular-momentum-biased resonator array with the broken Lorentz reciprocity. High Q-factor resonance of the constituent rotors is leveraged to reduce the required rotation speed. ACI is a new topological acoustic system analogous to the electronic quantum Hall insulator, based on an effective magnetic field.

Ultrahigh conductivity in Weyl semimetal NbAs nanobelts.

In two-dimensional (2D) systems, high mobility is typically achieved in low-carrier-density semiconductors and semimetals. Here, we discover that the nanobelts of Weyl semimetal NbAs maintain a high mobility even in the presence of a high sheet carrier density. We develop a growth scheme to synthesize single crystalline NbAs nanobelts with tunable Fermi levels.

Topological materials discovery by large-order symmetry indicators.

Crystalline symmetries play an important role in the classification of band structures, and their richness leads to various topological crystalline phases. On the basis of our recently developed method for the efficient discovery of topological materials using symmetry indicators, we explore topological materials in five space groups ( s), which are diagnosed by large-order symmetry indicators (ℤ and ℤ) and support the coexistence of several kinds of gapless boundary states in a single compound. We predict many candidate materials; some representatives include PtGe ( ), graphite ( ), Pt ( , = Sn, Pb), AuTi ( ), and TiSn ( ).

Comprehensive search for topological materials using symmetry indicators.

Over the past decade, topological materials-in which the topology of electron bands in the bulk material leads to robust, unconventional surface states and electromagnetism-have attracted much attention. Although several theoretically proposed topological materials have been experimentally confirmed, extensive experimental exploration of topological properties, as well as applications in realistic devices, has been restricted by the lack of topological materials in which interference from trivial Fermi surface states is minimized. Here we apply our method of symmetry indicators to all suitable nonmagnetic compounds in all 230 possible space groups.

Theory for the negative longitudinal magnetoresistance in the quantum limit of Kramers Weyl semimetals.

Negative magnetoresistance is rare in non-magnetic materials. Recently, negative magnetoresistance has been observed in the quantum limit of β-AgSe, where only one band of Landau levels is occupied in a strong magnetic field parallel to the applied current. β-AgSe is a material that hosts a Kramers Weyl cone with band degeneracy near the Fermi energy.

NaIrCl: Spin-Orbital-Induced Semiconductor Showing Hydration-Dependent Structural and Magnetic Variations.

Iridium(IV) oxides have gained increased attention in recent years owing to the presence of competing spin-orbit coupling and Coulomb interactions, which facilitate the emergence of novel quantum phenomena. In contrast, the electronic structure and magnetic properties of Ir-based molecular materials remain largely unexplored. In this paper, we take a fresh look at an old but puzzling compound, NaIrCl, which can be hydrated to form two stable phases with formulas NaIrCl·2HO and NaIrCl·6HO.

Photoresponsivity of an all-semimetal heterostructure based on graphene and WTe.

Heterostructures based on two-dimensional (2D) materials have sparked wide interests in both fundamental physics and applied devices. Recently, Dirac/Weyl semimetals are emerging as capable functional materials for optoelectronic devices. However, thus far the interfacial coupling of an all-semimetal 2D heterostructure has not been investigated, and its effects on optoelectronic properties remain less well understood.

Band Structure Perfection and Superconductivity in Type-II Dirac Semimetal Ir Pt Te.

The discovery of a new type-II Dirac semimetal in Ir Pt Te with optimized band structure is described. Pt dopants protect the crystal structure holding the Dirac cones and tune the Fermi level close to the Dirac point. The type-II Dirac dispersion in Ir Pt Te is confirmed by angle-resolved photoemission spectroscopy and first-principles calculations.

Discovery of coexisting Dirac and triply degenerate magnons in a three-dimensional antiferromagnet.

Topological magnons are emergent quantum spin excitations featured by magnon bands crossing linearly at the points dubbed nodes, analogous to fermions in topological electronic systems. Experimental realisation of topological magnons in three dimensions has not been reported so far. Here, by measuring spin excitations (magnons) of a three-dimensional antiferromagnet CuTeO with inelastic neutron scattering, we provide direct spectroscopic evidence for the coexistence of symmetry-protected Dirac and triply degenerate nodes, the latter involving three-component magnons beyond the Dirac-Weyl framework.

Three-Dimensional Anisotropic Magnetoresistance in the Dirac Node-Line Material ZrSiSe.

The family of materials defined as ZrSiX (X = S, Se, Te) has been established as Dirac node-line semimetals, and subsequent study is urgent to exploit the promising applications of unusual magnetoresistance (MR) properties. Herein, we systematically investigated the anisotropic MR in the newly-discovered Dirac node-line material ZrSiSe. By applying a magnetic field of 3 T by a vector field, three-dimensional (3D) MR shows the strong anisotropy.

Rules for Phase Shifts of Quantum Oscillations in Topological Nodal-Line Semimetals.

Nodal-line semimetals are topological semimetals in which band touchings form nodal lines or rings. Around a loop that encloses a nodal line, an electron can accumulate a nontrivial π Berry phase, so the phase shift in the Shubnikov-de Haas (SdH) oscillation may give a transport signature for the nodal-line semimetals. However, different experiments have reported contradictory phase shifts, in particular, in the WHM nodal-line semimetals (W=Zr/Hf, H=Si/Ge, M=S/Se/Te).