An unusual angle-dependent topological phase transition and butterfly magnetoresistance were found in the Dirac semimetal ZrSiS.

By establishing magnetic order in a square lattice compound, we introduce the first magnetic “new fermion.”

Non-symmorphic materials have recently been predicted to exhibit many different exotic features in their electronic structures. These originate from forced band degeneracies caused by the nonsymmorphic symmetry, which not only creates the possibility to realize Dirac semimetals, but also recently resulted in the prediction of novel quasiparticles beyond the usual Dirac, Weyl or Majorana fermions, which can only exist in the solid state. Experimental realization of non-symmorphic materials that have the Fermi level located at the degenerate point is difficult, however, due to the requirement of an odd band filling. In order to investigate the effect of forced band degeneracies on the transport behavior, a material that has such a degeneracy at or close to the Fermi level is desired. Here, we show with angular resolved photoemission experiments supported by density functional calculations, that ZrSiTe hosts several fourfold degenerate Dirac crossings at the X point, resulting from non-symmorphic symmetry. These crossings form a Dirac line node along XR, which is located almost directly at the Fermi level and shows almost no dispersion in energy. ZrSiTe is thus the first real material that allows for transport measurements investigating Dirac fermions that originate from non-symmorphic symmetry. OPEN ACCESS RECEIVED

Spin 1 2 honeycomb materials have gained substantial interest due to their exotic magnetism and possible application in quantum computing. However, in all current materials out-of-plane interactions are interfering with the in-plane order, hence a true 2D magnetic honeycomb system is still of demand. Here, we report the exfoliation of the magnetic semiconductor α-RuCl 3 into the first halide monolayers and the magnetic characterization of the spin 1 2 honeycomb arrangement of turbostratically stacked RuCl 3 monolayers. The exfoliation is based on a reductive lithiation/hydration approach, which gives rise to a loss of cooperative magnetism due to the disruption of the spin 1 2 state by electron injection into the layers. After an oxidative treatment, cooperative magnetism similar to the bulk is restored. The oxidized pellets of restacked single layers feature a magnetic transition at T N = 7 K in the in-plane direction, while the magnetic properties in the out-of-plane direction vastly differ from bulk α-RuCl 3 . The macro- Binary halide nanosheets have been predicted based on chemical intuition 3,4 or ab initio calculations. 7 Yet, no single layer halides have been synthesized so far, even though this class of compounds features an array of interesting electrical and magnetic properties.The magnetic semiconductor α-RuCl 3 is one such example. While it was investigated in the past as a host for intercalants 8,9 and as a lithium ion conductor, 10 current research focuses on its magnetic properties. Due to its layered honeycomb structure of spin 1 2 Ru 3+ centers in combination with spin orbit coupling (SOC), it is one of the few known materials featuring a zigzag antiferromagnetic (AF) ground state below a temperature of T N1 = 8 K. [11][12][13] In the zigzag order, the magnetic moments form ferromagnetic (FM) zigzag chains, whose magnetization direction is opposed to the neighboring chains within the plane. Additionally, there is a further magnetic phase transition observed at T N2 = 14 K. The origin of this transition is currently still under debate. This type of ordering was first observed in Na 2 IrO 3 14-16 and explained by the Kitaev-Heisenberg model, 17,18 which describes that a frustrated spin 1 2 honeycomb arrangement could lead to a variety of interesting spin structures. Based on the competition among the exchange interactions up to the third neighbor, the system could possibly be pushed into a quantum spin liquid regime by the manipulation of the competing interactions, thereby opening up applications in quantum computing. 17,19 Yet, the Na + ions in the interlayer space of Na 2 IrO 3 lead to disadvantageous interactions between the iridate layers, which interfere with theoretical predictions of a honeycomb arrange-2 ment of spin 1 2 magnetic arrays. 20 Eliminating the interlayer interaction could provide a route to manipulate the spin structure of real materials featuring a spin 1 2 honeycomb arrangement. In RuCl 3 , where no charged ions are in between the honeycomb layers, the interlayer in...

Many topological semimetals are known to exhibit exceptional electronic properties, which are the fundamental basis for design of novel devices and further applications. Materials containing the structural motif of a square net are known to frequently be topological semimetals. In this work, the synthesis and structural characterization of the square‐net‐based magnetic topological semimetal candidates GdSbxTe2−x−δ (0≤x≤1, δ indicating the vacancy level) are reported. The structural evolution of the series with Sb substitution is studied, finding a transition between a simple tetragonal square‐net structure to complex superstructure formations due to the presence of charge density waves. The structural modulations coincide with a significant modification of the magnetic order. This work thus establishes GdSbxTe2−x−δ as a platform to study the interplay between crystal symmetry, band filling, charge density wave, and magnetism in a topological semimetal candidate.

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