# Scientific Highlights 2018

### Fate of charge order in overdoped La-based cuprates

In high-temperature cuprate superconductors, stripe order refers broadly to a coupled spin and charge modulation with a commensuration of eight and four lattice units, respectively. How this stripe order evolves across optimal doping remains a controversial question. Here we present a systematic resonant inelastic x-ray scattering study of weak charge correlations in La_{2−x}Sr_{x}CuO_{4} and La_{1.8−x}Eu_{0.2}Sr_{x}CuO_{4}. Ultra high energy resolution experiments demonstrate the importance of the separation of inelastic and elastic scattering processes. Long-range temperature-dependent stripe order is only found below optimal doping. At higher doping, short-range temperature-independent correlations are present up to the highest doping measured. This transformation is distinct from and preempts the pseudogap critical doping. We argue that the doping and temperature-independent short-range correlations originate from unresolved electron–phonon coupling that broadly peaks at the stripe ordering vector. In La_{2−x}Sr_{x}CuO_{4}, long-range static stripe order vanishes around optimal doping and we discuss both quantum critical and crossover scenarios.

### Topological magnetic structures in MnGe: Neutron diffraction and symmetry analysis

From new neutron powder diffraction experiments on the chiral cubic (P213) magnet manganese germanide (MnGe), we analyze all of the possible crystal symmetry-allowed magnetic superstructures that are determined successfully from the data. The incommensurate propagation vectors k of the magnetic structure are found to be aligned with the [100] cubic axes, and correspond to a magnetic periodicity of about 30 Å at 1.8 K. Several maximal crystallographic symmetry magnetic structures are found to fit the data equally well and are presented. These include topologically nontrivial magnetic hedgehog and “skyrmion” structures in multi-k cubic or orthorhombic 3+3 and orthorhombic 3+2 dimensional magnetic superspace groups respectively, with either potentially responsible for topological Hall effect. The presence of orthorhombic distortions in the space group P212121 caused by the transition to the magnetically ordered state does not favor the cubic magnetic hedgehog structure, and leave both orthorhombic hedgehog and skyrmion models as equal candidates for the magnetic structures. We also report on a combined mechanochemical and solid-state chemical route to synthesize MnGe at ambient pressures and moderate temperatures, and compare with samples obtained by the traditional high pressure synthesis.

### Magnetic structure of R1/3Sr2/3FeO (R = Pr, Nd)

We present magnetization and neutron powder diffraction studies in the temperature range 2K to 300K for oxygen stoichiometric R_{1/3}Sr_{2/3}FeO (R = Pr and Nd). From full symmetry analysis, we proposed two magnetic models by a combined application of irreducible representations and magnetic space groups. Both models fit equally well the neutron powder diffraction data.

### Magnetic and crystal structure of the antiferromagnetic skyrmion candidate GdSb0.71Te1.22

GdSb_{0.46}Te_{1.48}, a nonsymmorphic Dirac semimetal with Dirac nodes at the Fermi level, has a rich magnetic phase diagram with one of the phases predicted to be an antiferromagnetic skyrmion state. In the current work, we investigate GdSb_{0.71}Te_{1.22} through bulk magnetization measurements, single-crystal, and powder synchrotron X-ray diffraction, as well as single-crystal hot-neutron diffraction. We resolve a weak orthorhombic distortion with respect to the tetragonal structure and charge density wave (CDW) satellites due to incommensurate modulations of the crystal structure. At 2 K the magnetic structure is modulated with two propagation vectors, *k*_{I} = (0.45 0 0.45) and *k*_{II} = (0.4 0 0), with all their arms visible. While *k*_{I} persists up to the transition to the paramagnetic state at T_{N} = 11.9 K, *k*_{II} disappears above an intermediate magnetic transition at T_{1} = 5 K. Whereas magnetic field applied along the *c*-axis has only a weak effect on the intensity of antiferromagnetic reflections, it is effective in inducing an additional ferromagnetic component on Gd atoms. We refine possible magnetic structures of GdSb_{0.71}Te_{1.22} and discuss the possibility of hosting magnetic textures with non-trivial 3D+ 2 topologies in the GdSb_{1−x}Te_{1+x} series.

### Crossover of high-energy spin fluctuations from collective triplons to localized magnetic excitations in Sr14−xCaxCu24O41 ladder

We studied the magnetic excitations in the quasi-one-dimensional (q-1D) ladder subsystem of Sr_{14−x}Ca_{x}Cu_{24}O_{41} (SCCO) using Cu L_{3}-edge resonant inelastic X-ray scattering (RIXS). By comparing momentum-resolved RIXS spectra with high (x = 12.2) and without (x = 0) Ca content, we track the evolution of the magnetic excitations from collective two-triplon (2 T) excitations (x = 0) to weakly- dispersive gapped modes at an energy of 280 meV (x = 12.2)...

### Competing Magnetic Phases in LnSbTe (Ln = Ho and Tb)

The interplay between a topological electronic structure and magnetism may result in intricate physics. In this work, we describe a case of rather peculiar coexistence or competition of several magnetic phases below seemingly single antiferromagnetic transition in *Ln*SbTe (*Ln* = Ho and Tb) topological semimetals, the magnetic members of the ZrSiS/PbFCl structure type (space group *P*4/*nmm*). Neutron diffraction experiments reveal a complex multi-step order below *T*_{N} = 3.8 K (*Ln* = Ho) and *T*_{N} = 6.4 K (*Ln* = Tb). Magnetic phases can be described using four propagation vectors *k*_{1} = (^{1}/_{2} 0 0) and *k*_{2} = (^{1}/_{2} 0 ^{1}/_{4}) at a base temperature of 1.7 K, which transform into incommensurate vectors *k*_{1}′ = (^{1}/_{2} – δ 0 0) and *k*_{3} = (^{1}/_{2} – δ 0 ^{1}/_{2}) at elevated temperatures in both compounds. Together with the refined models of magnetic structures, we present the group theoretical analysis of magnetic symmetry of the proposed solutions. These results prompt further investigations of the relation between the electronic structure of those semimetals and the determined antiferromagnetic ordering existing therein.

### Cu-doping effects on the ferromagnetic semimetal CeAuGe

We present a study of Cu-substitution effects in 4*f*-Ce intermetallic compound CeAu_{1-}* _{x}*Cu

*Ge, with potentially unusual electronic states, in the whole concentration range (*

_{x}*x*= 0.0 – 1.0). The parent CeAuGe compound, crystallizing in a non-centrosymmetric hexagonal structure, is a ferromagnetic semimetal with Curie temperature 10 K. Cu-doping on Au-site of CeAuGe, CeAu

_{1-}

*Cu*

_{x}*Ge, changes the crystal structure from the non-centrosymmetric (*

_{x}*P*6

_{3}

*mc*) to centrosymmetric (

*P*6

_{3}/

*mmc*) space group at the concentration

*x*∼ 0.5, where the

*c*-lattice constant has a maximum value. Magnetic susceptibility and electrical resistivity measurements reveal that all Cu-doped compounds undergo magnetic phase transition near 10 K, with the maximum transition temperature of 12 K for

*x*= 0.5. The neutron powder diffraction experiments show the ferromagnetic ordering of Ce

^{3+}magnetic moments with a value of about 1.2 μ

_{B}at 1.8 K, oriented perpendicular to the hexagonal

*c*-axis. By using symmetry analysis, we have found the solutions for the magnetic structure in the ferromagnetic Shubnikov space groups

*Cmc'*2

_{1}

*′*and

*P*2

_{1}

*′/m'*for

*x*< 0.5 and

*x*≥ 0.5, respectively. Electrical resistivity

*ρ*(

*T*) exhibits a metallic temperature behaviour in all compositions. The resistivity

*ρ*(

*T*) has a local minimum in the paramagnetic state due to Kondo effects at high doping

*x*= 0.8 and 1.0. At the small Cu-doping level,

*x*= 0.2, the resistivity shows a broad feature at the ferromagnetic transition temperature and an additional transition-like peculiarity at 2.5 K in the ferromagnetic state.

### RENiO3 Single Crystals (RE = Nd, Sm, Gd, Dy, Y, Ho, Er, Lu) Grown from Molten Salts under 2000 bar of Oxygen Gas Pressure

Schematic representation of the method used to grow RENiO_{3} nickelate single crystals covering the full 4f series and Y. This novel procedure, based on the use of moderate oxygen gas pressures (2000 bar), solvothermal growth in a temperature gradient, and highly reactive eutectic salt mixtures as fluxes, yields prismatic-shaped crystals with flat facets and sizes up to ∼75 μm.

### A quantum magnetic analogue to the critical point of water

At the liquid–gas phase transition in water, the density has a discontinuity at atmospheric pressure; however, the line of these first-order transitions defined by increasing the applied pressure terminates at the critical point, a concept ubiquitous in statistical thermodynamics. In correlated quantum materials, it was predicted and then confirmed experimentally that a critical point terminates the line of Mott metal–insulator transitions, which are also first-order with a discontinuous charge carrier density. In quantum spin systems, continuous quantum phase transitions have been controlled by pressure, applied magnetic field and disorder, but discontinuous quantum phase transitions have received less attention.