# Scientific Highlights 2018

### 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.

### Re(1−x)Mox as an ideal test case of time-reversal symmetry breaking in unconventional superconductors

Non-centrosymmetric superconductors (NCSCs) are promising candidates in the search for unconventional and topological superconductivity. The α-Mn-type rhenium-based alloys represent excellent examples of NCSCs, where spontaneous magneticfields, peculiar to time-reversal symmetry (TRS) breaking, have been shown to develop in the superconducting phase. By converse, TRS is preserved in many other isostructural NCSCs, thus leaving the key question about its origin fully open. Here, we consider ...

### Strain engineering of the charge and spin-orbital interactions in Sr2IrO4

Understanding the relationship between entangled degrees of freedom (DOF) is a central problem in correlated materials and the possibility to influence their balance is promising toward realizing novel functionalities. In Sr_{2}IrO_{4}, the interaction between spin–orbit coupling and electron correlations induces an exotic ground state with magnetotransport properties promising for antiferromagnetic spintronics applications.

### Electron–phonon-driven three-dimensional metallicity in an insulating cuprate

Elucidating the role of different degrees of freedom in a phase transition is crucial in the comprehension of complex materi- als. A phase transformation that attracts significant interest is the insulator-to-metal transition of Mott insulators, in which the electrons are thought to play the dominant role. Here, we use ultrafast laser spectroscopy and theoretical calculations ....

### Magnetism and anomalous transport in the Weyl semimetal PrAlGe: possible route to axial gauge fields

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.