IrGe4: A Predicted Weyl-Metal with a Chiral Crystal Structure.

Polycrystalline IrGe4 was synthesized by annealing elements at 800 °C for 240 h, and the composition was confirmed by energy-dispersive X-ray spectroscopy. IrGe4 adopts a chiral crystal structure (space group P3121) instead of a polar crystal structure (P31), which was corroborated by the convergent-beam electron diffraction and Rietveld refinements using synchrotron powder X-ray diffraction data. The crystal structure features layers of IrGe8 polyhedra along the b axis, and the layers are connected by edge- and corner-sharing. Each layer consists of corner-shared [Ir3Ge20] trimers, which are formed by three IrGe8 polyhedra connected by edge-sharing. Temperature-dependent resistivity indicates metallic behavior. The magnetoresistance increases with increasing applied magnetic field, and the nonsaturating magnetoresistance reaches 11.5% at 9 T and 10 K. The Hall resistivity suggests that holes are the majority carrier type, with a carrier concentration of 4.02 × 1021 cm-3 at 300 K. Electronic band structures calculated by density functional theory reveal a Weyl point with a chiral charge of +3 above the Fermi level.

CoTe2 : A Quantum Critical Dirac Metal with Strong Spin Fluctuations.

Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long-range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity. Here, by means of ab initio calculations and magnetic, thermal, and transport measurements, it is shown that the orthorhombic CoTe2 is close to ferromagnetism, which appears suppressed by spin fluctuations. Calculations and transport measurements reveal nodal Dirac lines, making it a rare combination of proximity to quantum criticality and Dirac topology.

Multifunctional Cu2TSiS4 (T = Mn and Fe): Polar Semiconducting Antiferromagnets with Nonlinear Optical Properties.

Cu2TSiS4 (T = Mn and Fe) polycrystalline and single-crystal materials were prepared with high-temperature solid-state and chemical vapor transport methods, respectively. The polar crystal structure (space group Pmn21) consists of chains of corner-sharing and distorted CuS4, Mn/FeS4, and SiS4 tetrahedra, which is confirmed by Rietveld refinement using neutron powder diffraction data, X-ray single-crystal refinement, electron diffraction, energy-dispersive X-ray spectroscopy, and second harmonic generation (SHG) techniques. Magnetic measurements indicate that both compounds order antiferromagnetically at 8 and 14 K, respectively, which is supported by the temperature-dependent (100-2 K) neutron powder diffraction data. Additional magnetic reflections observed at 2 K can be modeled by magnetic propagation vectors k = (1/2,0,1/2) and k = (1/2,1/2,1/2) for Cu2MnSiS4 and Cu2FeSiS4, respectively. The refined antiferromagnetic structure reveals that the Mn/Fe spins are canted away from the ac plane by about 14°, with the total magnetic moments of Mn and Fe being 4.1(1) and 2.9(1) µB, respectively. Both compounds exhibit an SHG response with relatively modest second-order nonlinear susceptibilities. Density functional theory calculations are used to describe the electronic band structures.

LiMo8O10: Polar Crystal Structure with Infinite Edge-Sharing Molybdenum Octahedra.

Polycrystalline LiMo8O10 was prepared in a sealed Mo crucible at 1380 °C for 48 h using the conventional high-temperature solid-state method. The polar tetragonal crystal structure (space group I41md) is confirmed based on the Rietveld refinement of powder neutron diffraction and 7Li/6Li solid-state NMR. The crystal structure features infinite chains of Mo4O5 (i.e., Mo2Mo4/2O6/2O6/3) as a repeat unit containing edge-sharing Mo6 octahedra with strong Mo-Mo metal bonding along the chain. X-ray absorption near-edge spectroscopy of the Mo-L3 edge is consistent with the formal Mo valence/configuration. Magnetic measurements reveal that LiMo8O10 is paramagnetic down to 1.8 K. Temperature-dependent resistivity [ρ(T)] measurement indicates a semiconducting behavior that can be fitted with Mott's variable range hopping conduction mechanism in the temperature range of 215 and 45 K. The ρ(T) curve exhibits an exponential increase below 5 K with a large ratio of ρ1.8/ρ300 = 435. LiMo8O10 shows a negative field-dependent magnetoresistance between 2 and 25 K. Heat capacity measurement fitted with the modified Debye model yields the Debye temperature of 365 K.

Iridate Li8IrO6: An Antiferromagnetic Insulator.

A polycrystalline iridate Li8IrO6 material was prepared via heating Li2O and IrO2 starting materials in a sealed quartz tube at 650 °C for 48 h. The structure was determined from Rietveld refinement of room-temperature powder neutron diffraction data. Li8IrO6 adopts the nonpolar space group R3̅ with Li atoms occupying the tetrahedral and octahedral sites, which is supported by the electron diffraction and solid-state 7Li NMR. This results in a crystal structure consisting of LiO4 tetrahedral layers alternating with mixed IrO6 and LiO6 octahedral layers along the crystallographic c-axis. The +4 oxidation state of Ir4+ was confirmed by near-edge X-ray absorption spectroscopy. An in situ synchrotron X-ray diffraction study of Li8IrO6 indicates that the sample is stable up to 1000 °C and exhibits no structural transitions. Magnetic measurements suggest long-range antiferromagnetic ordering with a Néel temperature (TN) of 4 K, which is corroborated by heat capacity measurements. The localized effective moment µeff (Ir) = 1.73 µB and insulating character indicate that Li8IrO6 is a correlated insulator. First-principles calculations support the nonpolar crystal structure and reveal the insulating behavior both in paramagnetic and antiferromagnetic states.

Isotropic Nature of the Metallic Kagome Ferromagnet Fe3Sn2 at High Temperatures.

Anisotropy and competing exchange interactions have emerged as two central ingredients needed for centrosymmetric materials to exhibit topological spin textures. Fe3Sn2 is thought to have these ingredients as well, as it has recently been discovered to host room temperature skyrmionic bubbles with an accompanying topological Hall effect. We present small-angle inelastic neutron scattering measurements that unambiguously show that Fe3Sn2 is an isotropic ferromagnet below TC≈660 K to at least 480 K - the lower temperature threshold of our experimental configuration. Fe3Sn2 is known to have competing magnetic exchange interactions, correlated electron behavior, weak magnetocrystalline anisotropy, and lattice anisotropy; all of these features are thought to play a role in stabilizing skyrmions in centrosymmetric systems. Our results reveal that at elevated temperatures, there is an absence of magnetocrystalline anisotropy and that the system behaves as a typical exchange ferromagnet with a spin stiffness DT=0 K=271±9 meV Å2.

Competing magnetic phases and fluctuation-driven scalar spin chirality in the kagome metal YMn6Sn6.

Identification, understanding, and manipulation of novel magnetic textures are essential for the discovery of new quantum materials for future spin-based electronic devices. In particular, materials that manifest a large response to external stimuli such as a magnetic field are subject to intense investigation. Here, we study the kagome-net magnet YMn6Sn6 by magnetometry, transport, and neutron diffraction measurements combined with first-principles calculations. We identify a number of nontrivial magnetic phases, explain their microscopic nature, and demonstrate that one of them hosts a large topological Hall effect (THE). We propose a previously unidentified fluctuation-driven mechanism, which leads to the THE at elevated temperatures. This interesting physics comes from parametrically frustrated interplanar exchange interactions that trigger strong magnetic fluctuations. Our results pave a path to chiral spin textures, promising for novel spintronics.

A New Three-Dimensional Subsulfide Ir2In8S with Dirac Semimetal Behavior.

Dirac and Weyl semimetals host exotic quasiparticles with unconventional transport properties, such as high magnetoresistance and carrier mobility. Recent years have witnessed a huge number of newly predicted topological semimetals from existing databases; however, experimental verification often lags behind such predictions. Common reasons are synthetic difficulties or the stability of predicted phases. Here, we report the synthesis of the type-II Dirac semimetal Ir2In8S, an air-stable compound with a new structure type. This material has two Dirac crossings in its electronic structure along the Γ-Z direction of the Brillouin zone. We further show that Ir2In8S has a high electron carrier mobility of ∼10 000 cm2/(V s) at 1.8 K and a large, nonsaturating transverse magnetoresistance of ∼6000% at 3.34 K in a 14 T applied field. Shubnikov de-Haas oscillations reveal several small Fermi pockets and the possibility of a nontrivial Berry phase. With its facile crystal growth, novel structure type, and striking electronic structure, Ir2In8S introduces a new material system to study topological semimetals and enable advances in the field of topological materials.

Resonant torsion magnetometry in anisotropic quantum materials.

Unusual behavior in quantum materials commonly arises from their effective low-dimensional physics, reflecting the underlying anisotropy in the spin and charge degrees of freedom. Here we introduce the magnetotropic coefficient k = ∂2F/∂θ2, the second derivative of the free energy F with respect to the magnetic field orientation θ in the crystal. We show that the magnetotropic coefficient can be quantitatively determined from a shift in the resonant frequency of a commercially available atomic force microscopy cantilever under magnetic field. This detection method enables part per 100 million sensitivity and the ability to measure magnetic anisotropy in nanogram-scale samples, as demonstrated on the Weyl semimetal NbP. Measurement of the magnetotropic coefficient in the spin-liquid candidate RuCl3 highlights its sensitivity to anisotropic phase transitions and allows a quantitative comparison to other thermodynamic coefficients via the Ehrenfest relations.

Large anomalous Hall effect in the chiral-lattice antiferromagnet CoNb3S6.

An ordinary Hall effect in a conductor arises due to the Lorentz force acting on the charge carriers. In ferromagnets, an additional contribution to the Hall effect, the anomalous Hall effect (AHE), appears proportional to the magnetization. While the AHE is not seen in a collinear antiferromagnet, with zero net magnetization, recently it has been shown that an intrinsic AHE can be non-zero in non-collinear antiferromagnets as well as in topological materials hosting Weyl nodes near the Fermi energy. Here we report a large anomalous Hall effect with Hall conductivity of 27 Ω-1 cm-1 in a chiral-lattice antiferromagnet, CoNb3S6 consisting of a small intrinsic ferromagnetic component (≈0.0013 µB per Co) along c-axis. This small moment alone cannot explain the observed size of the AHE. We attribute the AHE to either formation of a complex magnetic texture or the combined effect of the small intrinsic moment on the electronic band structure.

Inducing superconductivity in Weyl semimetal microstructures by selective ion sputtering.

By introducing a superconducting gap in Weyl or Dirac semimetals, the superconducting state inherits the nontrivial topology of their electronic structure. As a result, Weyl superconductors are expected to host exotic phenomena, such as nonzero-momentum pairing due to their chiral node structure, or zero-energy Majorana modes at the surface. These are of fundamental interest to improve our understanding of correlated topological systems, and, moreover, practical applications in phase-coherent devices and quantum applications have been proposed. Proximity-induced superconductivity promises to allow these experiments on nonsuperconducting Weyl semimetals. We show a new route to reliably fabricate superconducting microstructures from the nonsuperconducting Weyl semimetal NbAs under ion irradiation. The significant difference in the surface binding energy of Nb and As leads to a natural enrichment of Nb at the surface during ion milling, forming a superconducting surface layer (Tc ~ 3.5 K). Being formed from the target crystal itself, the ideal contact between the superconductor and the bulk may enable an effective gapping of the Weyl nodes in the bulk because of the proximity effect. Simple ion irradiation may thus serve as a powerful tool for the fabrication of topological quantum devices from monoarsenides, even on an industrial scale.

Magnetic torque anomaly in the quantum limit of Weyl semimetals.

Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we show a pronounced anomaly in the magnetic torque of the Weyl semimetal NbAs upon entering the quantum limit state in high magnetic fields. The torque changes sign in the quantum limit, signalling a reversal of the magnetic anisotropy that can be directly attributed to the topological nature of the Weyl electrons. Our results establish that anomalous quantum limit torque measurements provide a direct experimental method to identify and distinguish Weyl and Dirac systems.

Distinct Electronic Structure for the Extreme Magnetoresistance in YSb.

An extreme magnetoresistance (XMR) has recently been observed in several nonmagnetic semimetals. Increasing experimental and theoretical evidence indicates that the XMR can be driven by either topological protection or electron-hole compensation. Here, by investigating the electronic structure of a XMR material, YSb, we present spectroscopic evidence for a special case which lacks topological protection and perfect electron-hole compensation. Further investigations reveal that a cooperative action of a substantial difference between electron and hole mobility and a moderate carrier compensation might contribute to the XMR in YSb.

Observation of long-lived interlayer excitons in monolayer MoSe2-WSe2 heterostructures.

Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of ~1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers.

In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques.

Electrically tunable excitonic light-emitting diodes based on monolayer WSe2 p-n junctions.

The development of light-emitting diodes with improved efficiency, spectral properties, compactness and integrability is important for lighting, display, optical interconnect, logic and sensor applications. Monolayer transition-metal dichalcogenides have recently emerged as interesting candidates for optoelectronic applications due to their unique optical properties. Electroluminescence has already been observed from monolayer MoS2 devices. However, the electroluminescence efficiency was low and the linewidth broad due both to the poor optical quality of the MoS2 and to ineffective contacts. Here, we report electroluminescence from lateral p-n junctions in monolayer WSe2 induced electrostatically using a thin boron nitride support as a dielectric layer with multiple metal gates beneath. This structure allows effective injection of electrons and holes, and, combined with the high optical quality of WSe2, yields bright electroluminescence with 1,000 times smaller injection current and 10 times smaller linewidth than in MoS2 (refs 17,18). Furthermore, by increasing the injection bias we can tune the electroluminescence between regimes of impurity-bound, charged and neutral excitons. This system has the required ingredients for new types of optoelectronic device, such as spin- and valley-polarized light-emitting diodes, on-chip lasers and two-dimensional electro-optic modulators.

Optical generation of excitonic valley coherence in monolayer WSe2.

As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges ('valleys') to have additional spin-like quantum numbers. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not observed to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addition to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.

Electrical control of neutral and charged excitons in a monolayer semiconductor.

Monolayer group-VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts), which have thus far been challenging to generate and control in the ultimate two-dimensional limit. Utilizing high-quality monolayer molybdenum diselenide, we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X(+)), neutral (X(o)) and negatively charged (X(-)) excitons in field-effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, whereas the linewidth is narrow (5 meV) at temperatures <55 K. This is greater spectral contrast than in any known quasi-two-dimensional system. We also find the charging energies for X(+) and X(-) to be nearly identical implying the same effective mass for electrons and holes.