We report the discovery of topological magnetism in the candidate magnetic Weyl semimetal CeAlGe. Using neutron scattering we find this system to host several incommensurate, squarecoordinated multi-k magnetic phases below TN. The topological properties of a phase stable at intermediate magnetic fields parallel to the c-axis are suggested by observation of a topological Hall effect. Our findings highlight CeAlGe as an exceptional system for exploiting the interplay between the nontrivial topologies of the magnetization in real space and Weyl nodes in momentum space.

A strongly frustrated ordered state can be induced in Y3Cu9(OH)19Cl8 by slightly modifying the perfect kagome lattice YCu3(OH)6Cl3.

We explore two methods for single-crystal growth of the theoretically proposed magnetic Weyl semimetals RAlGe (R = Pr, Ce), which prove that a floating-zone technique, being both crucible-and flux-free, is crucial to obtain perfectly stoichiometric RAlGe crystals. In contrast, the crystals grown by a flux-growth technique tend to be Al-rich. We further present both structural and elemental analyses, along with bulk magnetization and electrical resistivity data on the crystals prepared by the floating-zone technique. Both systems with the intended 1:1:1 stoichiometry crystallize in the anticipated polar I4 1 md (No. 109) space group, although neither displays the theoretically expected ferromagnetic ground state. Instead PrAlGe displays a spin-glass-like transition below 16 K with an easy c axis and CeAlGe has an easy-ab-plane antiferromagnetic order below 5 K. The grown crystals provide an ideal platform for microscopic studies of the magnetic field-tunable correlation physics involving magnetism and topological Weyl nodes.

In magnetic Weyl semimetals, where magnetism breaks time-reversal symmetry, large magnetically sensitive anomalous transport responses are anticipated that could be useful for topological spintronics. The identification of new magnetic Weyl semimetals is therefore in high demand, particularly since in these systems Weyl node configurations may be easily modified using magnetic fields. Here we explore experimentally the magnetic semimetal PrAlGe, and unveil a direct correspondence between easy-axis Pr ferromagnetism and anomalous Hall and Nernst effects. With sizes of both the anomalous Hall conductivity and Nernst effect in good quantitative agreement with first principles calculations, we identify PrAlGe as a system where magnetic fields can connect directly to Weyl nodes via the Pr magnetisation. Furthermore, we find the predominantly easy-axis ferromagnetic ground state coexists with a low density of nanoscale textured magnetic domain walls. We describe how such nanoscale magnetic textures could serve as a local platform for tunable axial gauge fields of Weyl fermions.

The quantum kagome antiferromagnets YCu3(OH)6OxCl3−x (x = 0, 1/3) are produced using a unified solid state synthesis route for polycrystalline samples. From structural refinements based on neutron diffraction data, we clarify the structure of the Y3Cu9(OH)18OCl8 (x = 1/3) compound and provide a revised chemical formula. We use muon spin relaxation, as a local probe of magnetism, to investigate the exotic low temperature properties in the two compounds. In agreement with the low temperature neutron diffraction data, we find no evidence for long range ordering in both materials but they exhibit distinct ground states: while disordered static magnetism develops in the x = 0 compound, we conclude on the stabilization of a quantum spin liquid in the x = 1/3 one, since the local fields remain fully dynamical. Our findings are in contrast to previous reports based on thermodynamical measurements only. We then discuss their origin on the basis of structural details and specific heat measurements. In particular, the x = 1/3 compound appears to realize an original spatially anisotropic kagome model. arXiv:1904.04125v1 [cond-mat.str-el]

We present the hydrothermal synthesis, as well as structural and chemical analysis of single crystals on the compounds EuCu3(OH)6Cl3, ZnxCu4−x(OH)6(NO3)2 and haydeeite, MgCu3(OH)6Cl2 all arising from the atacamite family. Magnetic and specific-heat measurements down to 1.8 K are carried out for these systems. EuCu3(OH)6Cl3 has a frustrated antiferromagnetic Cu 2+ ground state with order at 15 K, a strong anisotropy and increased magnetization from Van Vleck paramagnetic Eu 3+ contributions. ZnCu3(OH)6(NO3)2 reveals antiferromagnetic order at 9 K and measurements on haydeeite single crystals confirm the ferromagnetic order at 4.2 K with the easy axis within the kagome plane. These results prove that the atacamite family presents a broad class of materials with interesting magnetic ground states.

Optical conductivity measurements are combined with density functional theory calculations in order to understand the electrodynamic response of the frustrated Mott insulators Herbertsmithite ZnCu3(OH)6Cl2 and the closely-related kagome-lattice compound Y3Cu9(OH)19Cl8. We identify these materials as charge-transfer rather than Mott-Hubbard insulators, similar to the high-Tc cuprate parent compounds. The band edge is at 3.3 and 3.6 eV, respectively, establishing the insulating nature of these compounds. Inside the gap, we observe dipole-forbidden local electronic transitions between the Cu 3d orbitals in the range 1-2 eV. With the help of ab initio calculations we demonstrate that the electrodynamic response in these systems is directly related to the role of on-site Coulomb repulsion: while charge-transfer processes have their origin on transitions between the ligand band and the Cu 3d upper Hubbard band, local d-d excitations remain rather unaffected by correlations.Since the discovery of high-T c cuprates, the physics of strongly correlated materials has been at the forefront of research in condensed matter physics. The relationship between correlations, unconventional superconductivity, quantum-spin-liquid behavior and other exotic states of matter has been intensely debated. While at the level of model theories one can introduce criteria to quantify the degree of correlation, such as the ratio U/W , where U is the on-site Coulomb repulsion and W is the singleparticle bandwidth, the quantification in many materials is not that straightforward. The effect of correlations usually shows up as mass renormalization, band narrowing, or as opening or enhancement of the band gap [1, 2]. Cu+2 ions are, arguably, the most strongly correlated among d transition metals [3][4][5][6]. They are key ingredients of high-T c cuprates and have been widely investigated in the last decades. A recent revival of interest in Cu-based materials was triggered by the discovery of geometrically frustrated cuprates that seem to exhibit spin-liquid properties [7][8][9][10][11][12][13][14][15][16] and may even harbor unconventional superconductivity with higher angular momenta than existing superconductors [17] or further topological phases [18] although synthesis seems to be difficult [19].In this work we investigate the origin of the optical excitations in the spin-liquid candidate Herbertsmithite and concentrate on the following conceptual issue: which measurable properties in correlated systems are strongly affected by correlation effects and which are not? We will show, experimentally and theoretically, that in a single experimental probe, namely optical conductivity, one can simultaneously observe properties dramatically influenced by Coulomb (Mott-Hubbard) correlation effects, and those that are hardly affected at all.One can distinguish two types of optical absorption processes, depicted in Fig. 1. On the one hand, we have the "charge-transfer process", that in its simple form can be described as creating a hole and an electron res...

Topotactic transformations between related crystal structures are a powerful emerging route for the synthesis of novel quantum materials. Whereas most such "soft chemistry" experiments have been carried out on polycrystalline powders or thin films, the topotactic modification of single crystals, the gold standard for physical property measurements on quantum materials, has been studied only sparsely. Here, we report the topotactic reduction of La 1−x Ca x NiO 3 single crystals to La 1−x Ca x NiO 2+ using CaH 2 as the reducing agent. The transformation from the three-dimensional perovskite to the quasi-two-dimensional infinite-layer phase was thoroughly characterized by x-ray diffraction, electron microscopy, Raman spectroscopy, magnetometry, and electrical transport measurements. Our work demonstrates that the infinite-layer structure can be realized as a bulk phase in crystals with micrometer-sized single domains. The electronic properties of these specimens resemble those of epitaxial thin films rather than powders with similar compositions.