Exchange-biased topological transverse thermoelectric effects in a Kagome ferrimagnet.

Kagome metal TbMn6Sn6 was recently discovered to be a ferrimagnetic topological Dirac material by scanning tunneling microscopy/spectroscopy measurements. Here, we report the observation of large anomalous Nernst effect and anomalous thermal Hall effect in this compound. The anomalous transverse transport is consistent with the Berry curvature contribution from the massive Dirac gaps in the 3D momentum space as demonstrated by our first-principles calculations. Furthermore, the transverse thermoelectric transport exhibits asymmetry with respect to the applied magnetic field, i.e., an exchange-bias behavior. Together, these features place TbMn6Sn6 as a promising system for the outstanding thermoelectric performance based on anomalous Nernst effect.

Anomalous Thermal Hall Effect in an Insulating van der Waals Magnet.

Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile playground for the discovery and exploration of physical phenomena and new physics. In this Letter, we report the observation of an anomalous thermal Hall effect (THE) with κ_{xy}∼1×10^{-2} W K^{-1} m^{-1} in an insulating van der Waals ferromagnet VI_{3}. The thermal Hall signal persists in the absence of an external magnetic field and flips sign upon the switching of the magnetization. In combination with theoretical calculations, we show that VI_{3} exhibits a dual nature of the THE, i.e., dominated by topological magnons hosted by the ferromagnetic honeycomb lattice at higher temperatures and by phonons induced by the magnon-phonon coupling at lower temperatures. Our results not only position VI_{3} as the first ferromagnetic system to investigate both anomalous magnon and phonon THEs, but also render it as a potential platform for spintronics-magnonics applications.

Anisotropic Transport and Quantum Oscillations in the Quasi-One-Dimensional TaNiTe5: Evidence for the Nontrivial Band Topology.

The past decade has witnessed the burgeoning discovery of a variety of topological states of matter with distinct nontrivial band topologies. Thus far, most materials that have been studied possess two-dimensional or three-dimensional electronic structures, with only a few exceptions that host quasi-one-dimensional (quasi-1D) topological electronic properties. Here we present clear-cut evidence for Dirac Fermions in the quasi-1D telluride TaNiTe5. We show that its transport behaviors are highly anisotropic, and we observe nontrivial Berry phases via quantum oscillation measurements. The nontrivial band topology is further corroborated by first-principles calculations. Our results may help to guide the future quest for topological states in this new family of quasi-1D ternary chalcogenides.

Strong lattice correlation of non-equilibrium quasiparticles in a pseudospin-1/2 Mott insulator Sr2IrO4.

In correlated oxides the coupling of quasiparticles to other degrees of freedom such as spin and lattice plays critical roles in the emergence of symmetry-breaking quantum ordered states such as high temperature superconductivity. We report a strong lattice coupling of photon-induced quasiparticles in spin-orbital coupling Mott insulator Sr2IrO4 probed via optical excitation. Combining time-resolved x-ray diffraction and optical spectroscopy techniques, we reconstruct a spatiotemporal map of the diffusion of these quasiparticles. Due to the unique electronic configuration of the quasiparticles, the strong lattice correlation is unexpected but extends the similarity between Sr2IrO4 and cuprates to a new dimension of electron-phonon coupling which persists under highly non-equilibrium conditions.

Interfacial magnetism in complex oxide heterostructures probed by neutrons and x-rays.

Magnetic complex-oxide heterostructures are of keen interest because a wealth of phenomena at the interface of dissimilar materials can give rise to fundamentally new physics and potentially valuable functionalities. Altered magnetization, novel magnetic coupling and emergent interfacial magnetism at the epitaxial layered-oxide interfaces are under intensive investigation, which shapes our understanding on how to utilize those materials, particularly for spintronics. Neutron and x-ray based techniques have played a decisive role in characterizing interfacial magnetic structures and clarifying the underlying physics in this rapidly developing field. Here we review some recent experimental results, with an emphasis on those studied via polarized neutron reflectometery and polarized x-ray absorption spectroscopy. We conclude with some perspectives.

Ignoring your neighbors: moment correlations dominated by indirect or distant interactions in an ordered nanomagnet array.

We have studied the moment correlations within triangular lattice arrays of single-domain coaligned nanoscale ferromagnetic islands. Independent variation of lattice spacing along and perpendicular to the island axis tunes the magnetostatic interactions between islands through a broad range of relative strengths. For certain lattice parameters, the sign of the correlations between near-neighbor island moments is opposite to that favored by the pairwise interaction. This finding, supported by analysis of the total correlation in terms of direct and convoluted indirect contributions across multiple pairwise interactions, indicates that indirect interactions and/or those mediated by further neighbors can be tuned to be dominant, with implications for the wide range of systems composed of interacting nanomagnets.

The Zintl ion [As7]2-: an example of an electron-deficient As(x) radical anion.

[K(2,2,2-crypt)](2)[As(7)]·THF, 1 (2,2,2-crypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is the first well characterized seven-atom radical anion of group 15. UV-Vis spectroscopy confirms the presence and electronic structure of [As(7)](2-). Cyclic voltammetry in DMF solution shows the As(7)(3-)/As(7)(2-) redox couple as a one-electron reversible process. Theoretical investigations explore the bonding and properties of compound 1.

Effective temperature in an interacting vertex system: theory and experiment on artificial spin ice.

Frustrated arrays of interacting single-domain nanomagnets provide important model systems for statistical mechanics, as they map closely onto well-studied vertex models and are amenable to direct imaging and custom engineering. Although these systems are manifestly athermal, we demonstrate that an effective temperature, controlled by an external magnetic drive, describes their microstates and therefore their full statistical properties.

A strong ferroelectric ferromagnet created by means of spin-lattice coupling.

Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials that could give rise to new technologies in which the low power and high speed of field-effect electronics are combined with the permanence and routability of voltage-controlled ferromagnetism. Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena are insignificant in comparison with those of useful ferroelectrics or ferromagnets: their spontaneous polarizations or magnetizations are smaller by a factor of 1,000 or more. The same holds for magnetic- or electric-field-induced multiferroics. Owing to the weak properties of single-phase multiferroics, composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components are the closest to application today. Recently, however, a new route to ferroelectric ferromagnets was proposed by which magnetically ordered insulators that are neither ferroelectric nor ferromagnetic are transformed into ferroelectric ferromagnets using a single control parameter, strain. The system targeted, EuTiO(3), was predicted to exhibit strong ferromagnetism (spontaneous magnetization, approximately 7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, approximately 10 microC cm(-2)) simultaneously under large biaxial compressive strain. These values are orders of magnitude higher than those of any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression we turned to tensile strain. Here we show both experimentally and theoretically the emergence of a multiferroic state under biaxial tension with the unexpected benefit that even lower strains are required, thereby allowing thicker high-quality crystalline films. This realization of a strong ferromagnetic ferroelectric points the way to high-temperature manifestations of this spin-lattice coupling mechanism. Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition for creating multiferroics.

Chemical synthesis of air-stable manganese nanoparticles.

Elemental manganese has a complex crystal structure and unusual magnetic properties, making it an intriguing target for exploration in nanocrystalline form. However, because of its oxophilicity and the difficulty in reducing soluble metal salts to elemental Mn using the most common solution-phase reducing agents, it has been challenging to synthesize and stabilize elemental Mn nanoparticles using solution chemistry methods. Here we report the chemical synthesis of alpha-Mn nanoparticles using n-butyllithium as a reducing agent. The nanoparticles have been characterized by powder XRD, TEM, electron diffraction, infrared spectroscopy (DRIFT), XPS, and SQUID magnetometry. An amorphous manganese oxide layer bound by oleate ligands helps to render the nanoparticles air-stable. The oxide-coated alpha-Mn nanoparticles are paramagnetic.

Room-temperature chemical synthesis of shape-controlled indium nanoparticles.

Shape-controlled metal nanoparticles are of interest because of their wide range of properties that are useful for applications that include optics, electronics, magnetism, and catalysis. Indium metal is an attractive target for nanoparticle synthesis because it is superconducting, plasmonically active, and is a component in low-melting solders and solid-state lubricants. Indium nanoparticles are typically synthesized using harsh physical or chemical techniques, and rigorous shape control is difficult. Here we present a simple and robust kinetically controlled process for synthesizing shape-controlled indium nanoparticles. By controlling the rate of dropwise addition of a solution of NaBH4 in tetraethylene glycol to an alcoholic solution of InCl3 and poly(vinyl pyrrolidone), indium nanoparticles are formed with shapes that include high aspect ratio nanowires and uniform octahedra and truncated octahedra. The zero-dimensional indium nanoparticles exhibit an SPR band centered around 400 nm, and all morphologies are superconducting (Tc = 3.4 K) with higher critical fields than bulk indium.