A new ternary compound NaCu6Se4 was synthesized from the reaction of Cu in a molten sodium polyselenide flux. The compound crystallizes in trigonal space group R3̅m with a = 4.0465(3) Å and c = 41.493(5) Å. The crystal structure contains flat two-dimensional slabs of (1)/∞[Cu6Se4] with a unique structural arrangement, separated by Na cations. The compound contains mixed valency and has a high conductivity of ∼3 × 10(3) S cm(-1) at room temperature, and exhibits increasing conductivity with decreasing temperature, indicating metallic behavior. A small positive thermopower (4-11 μV K(-1) from 300 to 500 K) and Hall effect measurements indicate p-type transport with a carrier concentration of ∼2.8(3) × 10(21) cm(-3) and a hole mobility of ∼8.75 cm(2) V(-1) s(-1) at 300 K. NaCu6Se4 exhibits temperature-independent Pauli paramagnetism.

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Recently, the iridate double perovskite Sr 2 YIrO 6 has attracted considerable attention due to the report of unexpected magnetism in this Ir 5+ (5d 4 ) material, in which according to the J eff model, a nonmagnetic ground state is expected. However, in recent works on polycrystalline samples of the series Ba 2−x Sr x YIrO 6 no indication of magnetic transitions have been found. We present a structural, magnetic, and thermodynamic characterization of Sr 2 YIrO 6 single crystals, with emphasis on the temperature and magnetic field dependence of the specific heat. As determined by x-ray diffraction, the Sr 2 YIrO 6 single crystals have a cubic structure, with space group F m3m. In agreement with the expected nonmagnetic ground state of Ir 5+ (5d 4 ) in Sr 2 YIrO 6 , no magnetic transition is observed down to 430 mK. Moreover, our results suggest that the low-temperature anomaly observed in the specific heat is not related to the onset of long-range magnetic order. Instead, it is identified as a Schottky anomaly caused by paramagnetic impurities present in the sample, of the order of n ∼ 0.5(2)%. These impurities lead to non-negligible spin correlations, which nonetheless, are not associated with long-range magnetic ordering.

The layered ternary compound TaIrTe4 has been predicted to be a type-II Weyl semimetal with only four Weyl points just above the Fermi energy. Performing magnetotransport measurements on this material we find that the resistivity does not saturate for fields up to 70 T and follows a ρ ∼ B 1.5 dependence. Angular-dependent de Haas-van Alphen (dHvA) measurements reveal four distinct frequencies. Analyzing these magnetic quantum oscillations by use of density functional theory (DFT) calculations we establish that in TaIrTe4 the Weyl points are located merely ∼ 40-50 meV above the chemical potential, suggesting that the chemical potential can be tuned into the four Weyl nodes by moderate chemistry or external pressure, maximizing their chiral effects on electronic and magnetotransport properties.A recent conceptual breakthrough in the theory and classification of metals is the discovery of Weyl semimetals [1][2][3]. These semimetals have a topologically nontrivial electronic structure with fermionic Weyl quasiparticles -massless chiral fermions that play as well a fundamental role in quantum field theory and high-energy physics [4]. A consequence is that in Weyl semimetals topologically protected surface states appear in the form of Fermi lines that connect Weyl points (WPs) of opposite chirality, commonly referred to as Fermi arcs.Last year it was discovered that actually two types of Weyl fermions may exist in solids [5]. Weyl semimetals of type-I have a point-like Fermi surface and consequently zero density of states at the energy of WPs [6][7][8][9][10][11][12][13][14][15][16][17][18]. This is very different from Weyl semimetals of type-II [5,19], which have thermodynamic density of states at the energy of Weyl nodes and acquire exotic Fermi surfaces: in type-II systems Weyl nodes appear at touching points between electron and hole pockets. The presence of these very peculiar states is predicted to strongly affect magnetotransport properties of a Weyl semimetal and causes the conduction of electric current only in certain directions in presence of a magnetic field [5,20,21]. In spite of the considerable progress made by theory, only a handful of type-II Weyl semimetals have been identified on the basis of electronic band-structure calculations: WTe 2 , MoTe 2 , Ta 3 S 2 , YbMnBi 2 and, very recently, TaIrTe 4 [5,16,[22][23][24][25].Of interest is in particular the orthorhombic ternary compound TaIrTe 4 as it combines structural simplicity with topological WPs: TaIrTe 4 is a structurally layered material which hosts just four type-II WPs, the minimal number of WPs a system with time-reversal invariance can host [22]. Moreover, the WPs are well separated from each other in momentum space. Such a large momentum-space separation promises a strong impact of the Weyl fermions on the transport properties. Indeed, we present in this Letter magnetotransport and magnetic quantum oscillations studies of TaIrTe 4 that evidence a non-saturating magnetoresistance signaling the presence of Weyl nodes. Analyzing de Haas-van Alp...

The new ternary copper selenide NaCu4Se3 crystallizes in the RbCd4As3 structure type with the trigonal space group R3̅m and lattice constants a = 4.0316(4) Å and c = 31.438(8) Å. Its structure is built from two-dimensional slabs of (2)/∞[Cu4Se3] separated by Na(+) cations. The compound is formally mixed-valent with Se(2-)/Se(-) atoms and exhibits metallic properties. It is a hole conductor with an electrical conductivity of ∼300 S cm(-1) at room temperature and a thermopower of ∼10 μV K(-1). Hall effect measurements indicate holes as the dominant carrier with a concentration of ∼6.12(1) × 10(21) cm(-3) at 300 K. Density functional theory electronic structure calculations indicate p-type metallic behavior for the (2)/∞[Cu4Se3] framework, which is in a good agreement with the experimental metallic conductivity and Pauli paramagnetism.