Similar ultrafast dynamics of several dissimilar Dirac and Weyl semimetals

Recent years have seen the rapid discovery of solids whose low-energy electrons have a massless, linear dispersion, such as Weyl, line-node, and Dirac semimetals. The remarkable optical properties predicted in these materials show their versatile potential for optoelectronic uses. However, little is known of their response in the picoseconds after absorbing a photon. Here we measure the ultrafast dynamics of four materials that share non-trivial band structure topology but that differ chemically, structurally, and in their low-energy band structures: ZrSiS, which hosts a Dirac line node and Dirac points; TaAs and NbP, which are Weyl semimetals; and Sr$_{1-y}$Mn$_{1-z}$Sb$_2$, in which Dirac fermions coexist with broken time-reversal symmetry. After photoexcitation by a short pulse, all four relax in two stages, first sub-picosecond, and then few-picosecond. Their rapid relaxation suggests that these and related materials may be suited for optical switches and fast infrared detectors. The complex change of refractive index shows that photoexcited carrier populations persist for a few picoseconds

On the possibility of magnetic Weyl fermions in non-symmorphic compound PtFeSb

Weyl fermions are expected to exhibit exotic physical properties such as the chiral anomaly, large negative magnetoresistance or Fermi arcs. Recently a new platform to realize these fermions has been introduced based on the appearance of a three-fold band crossing at high symmetry points of certain space groups. These band crossings are composed of two linearly dispersed bands that are topologically protected by a Chern number, and a at band with no topological charge. In this paper we present a new way of inducing two kinds of Weyl fermions, based on two- and three-fold band crossings, in the non-symmorphic magnetic material PtFeSb. By means of density functional theory calculations and group theory analysis we show that magnetic order can split a six-fold degeneracy enforced by non-symmoprhic symmetry to create three-fold or two-fold degenerate Weyl nodes. We also report on the synthesis of a related phase potentially containing two-fold degenerate magnetic Weyl points and extend our group theory analysis to that phase. This is the first study showing that magnetic ordering has the potential to generate new threefold degenerate Weyl nodes, advancing the understanding of magnetic interactions in topological materials.Comment: 8 pages, 5 figure

Li₀.₆[Li₀.₂Sn₀.₈S₂] – a layered lithium superionic conductor

One of the key challenges of energy research is finding solid electrolytes with high lithium conductivities comparable to those of liquid electrolytes. In this context, developing new structural families of potential Li+ ion conductors and identifying structural descriptors for fast Li+ ion conduction to occur is key to expand the scope of viable Li+ ion conductors. Here, we report that the layered material Li0.6[Li0.2Sn0.8S2] shows a Li+ ion conductivity comparable to the currently best lithium superionic conductors (LISICONs). Li0.6[Li0.2Sn0.8S2] is composed of layers comprising edge-sharing Li/SnS6 octahedra, interleaved with both tetrahedrally and octahedrally coordinated Li+ ions. Pulsed field gradient (PFG) NMR studies on powder samples show intragrain (bulk) diffusion coefficients DNMR on the order of 10−11 m2 s−1 at room temperature, which corresponds to a conductivity σNMR of 9.3 × 10−3 S cm−1 assuming the Nernst–Einstein equation, thus putting Li0.6[Li0.2Sn0.8S2] en par with the best Li solid electrolytes reported to date. This is in agreement with impedance spectroscopy on powder pellets, showing a conductivity of 1.5 × 10−2 S cm−1. Direct current galvanostatic polarization/depolarization measurements on such samples show negligible electronic contributions (less than 10−9 S cm−1) but indicate significant contact resistance (d.c. conductivity in a reversible cell is 1.2 × 10−4 S cm−1 at 298 K). Our results suggest that the partial occupation of interlayer Li+ positions in this layered material is beneficial for its transport properties, which together with tetrahedrally coordinated Li sites provides facile Li+ ion diffusion pathways in the intergallery space between the covalent Sn(Li)S2 layers. This work therefore points to a generic design principle for new layered Li+ ion conductors based on the controlled depletion of Li+ ions in the interlayer space

Atomically Sharp Internal Interface in a Chiral Weyl Semimetal Nanowire

Internal interfaces in Weyl semimetals (WSMs) are predicted to host distinct topological features that are different from the commonly studied external interfaces (crystal-to-vacuum boundaries). However, the lack of atomically sharp and crystallographically oriented internal interfaces in WSMs makes it difficult to experimentally investigate hidden topological states buried inside the material. Here, we study a unique internal interface known as merohedral twin boundary in chemically synthesized single-crystal nanowires (NWs) of CoSi, a chiral WSM of space group P213 (No. 198). High resolution scanning transmission electron microscopy reveals that this internal interface is (001) twin plane and connects two enantiomeric counterparts at an atomically sharp interface with inversion twinning. Ab-initio calculations show localized internal Fermi arcs at the (001) twin boundary that can be clearly distinguished from both external Fermi arcs and bulk states. These merohedrally twinned CoSi NWs provide an ideal material system to probe unexplored topological properties associated with internal interfaces in WSMs.Comment: 19 pages, 4 figure

Treatment of transient phenomena in analysis of slag-metal-gas reaction kinetics

Equations commonly used in describing reaction kinetics are examined and the problem of applying such equations to transient processes is discussed. Three examples of transient phenomena are examined in detail. It is shown that for carbon injection into slag, the reaction can be described by employing data for carbon oxidation in CO/CO2 by assuming reaction conditions approximately halfway between those in equilibrium with the slag and those in equilibrium with carbon. It is demonstrated that, when the time averaged interfacial area is employed, the rate of reaction between slag and iron-aluminum alloys can be described by a single first order rate equation, accommodating a 300% change in interfacial area. Creation of surface area in oxygen steelmaking is discussed and a method to determine the size distribution of droplets that are generated is proposed. It is concluded that changes in conditions during reaction complicate the analysis of kinetics. However, it should be possible to develop quantitative kinetic models to describe real processes

Anomalous Low Temperature Behavior of Superconducting Nd(1.85)Ce(0.15)CuO(4-y)

We have measured the temperature dependence of the in-plane London penetration depth lambda(T) and the maximum Josephson current Ic(T) using bicrystal grain boundary Josephson junctions of the electron-doped cuprate superconductor Nd(1.85)Ce(0.15)CuO(4-y). Both quantities reveal an anomalous temperature dependence below about 4 K. In contrast to the usual monotonous decrease (increase) of lambda(T) (Ic(T)) with decreasing temperature, lambda(T) and Ic(T) are found to increase and decrease, respectively, with decreasing temperature below 4 K resulting in a non-monotonous overall temperature dependence. This anomalous behavior was found to be absent in analogous measurements performed on Pr(1.85)Ce(0.15)CuO(4-y). From this we conclude that the anomalous behavior of Nd(1.85)Ce(0.15)CuO(4-y) is caused by the presence of the Nd3+ paramagnetic moments. Correcting the measured lambda(T) dependence of Nd(1.85)Ce(0.15)CuO(4-y) for the temperature dependent susceptibility due to the Nd moments, an exponential dependence is obtained indicating isotropic s-wave pairing. This result is fully consistent with the lambda(T) dependence measured for Pr(1.85)Ce(0.15)CuO(4-y).Comment: 4 pages including 4 figures, to appear in Phys. Rev. Let

Modal Logics of Topological Relations

Logical formalisms for reasoning about relations between spatial regions play a fundamental role in geographical information systems, spatial and constraint databases, and spatial reasoning in AI. In analogy with Halpern and Shoham's modal logic of time intervals based on the Allen relations, we introduce a family of modal logics equipped with eight modal operators that are interpreted by the Egenhofer-Franzosa (or RCC8) relations between regions in topological spaces such as the real plane. We investigate the expressive power and computational complexity of logics obtained in this way. It turns out that our modal logics have the same expressive power as the two-variable fragment of first-order logic, but are exponentially less succinct. The complexity ranges from (undecidable and) recursively enumerable to highly undecidable, where the recursively enumerable logics are obtained by considering substructures of structures induced by topological spaces. As our undecidability results also capture logics based on the real line, they improve upon undecidability results for interval temporal logics by Halpern and Shoham. We also analyze modal logics based on the five RCC5 relations, with similar results regarding the expressive power, but weaker results regarding the complexity