Rhodium and Cobalt Germanium Crystals Unlock New Routes to Advanced Electronics

Chiral semimetals possessing multifold fermions and unusual surface states are currently a major focus of condensed matter physics. Shangjie Tian from Anhui University, alongside Xiangjiang Dong and Bowen Zhang from the Center for High-Pressure Science and Technology Advanced Research, et al., present a detailed investigation into the physical properties of high-quality RhGe and CoGe single crystals with a B20 structure. Their research is significant because it establishes a robust platform for exploring the behaviour of multifold fermions and the potential instability of helicoid-arc surface states, particularly concerning the influence of spin-orbit coupling and surface conditions within these B20 systems. Transport and magnetisation measurements demonstrate metallic behaviour with Fermi-liquid characteristics and paramagnetism, respectively, alongside evidence suggesting weak electronic correlations and low carrier concentrations consistent with a semimetallic state.

Researchers have successfully grown high-quality single crystals of RhGe and CoGe, materials belonging to the chiral topological semimetal family. These compounds, possessing a B20 crystal structure, are predicted to host exotic electronic states including multifold fermions and unique surface properties. The work details a high-pressure synthesis method for these materials.

High-pressure and high-temperature synthesis using hermetically sealed hexagonal boron nitride capsules for RhGe and CoGe single crystal growth

High-pressure and high-temperature synthesis formed the basis of single crystal growth for RhGe and CoGe compounds. High-purity elemental precursors of rhodium (99.99 %), cobalt (99.95 %) and germanium (99.99 %) were ground to stoichiometric ratios of Rh:Ge = 1:1.2 and Co:Ge = 1:1, with excess germanium acting as a solvent flux.

These mixtures were sealed within hermetically sealed capsules constructed from hexagonal boron nitride to prevent reactions between the elemental precursors and capsule materials at elevated temperatures and pressures. The sealed capsules were then positioned at the centre of a cubic anvil apparatus and subjected to a compressive force of 5 GPa.

For RhGe synthesis, the temperature was increased to 1250°C at a rate of 10°C/min, maintained at this temperature for 3 hours, slowly cooled to 800°C, and then rapidly quenched to ambient temperature before decompression. CoGe synthesis followed a similar procedure, employing a peak temperature of 1100°C for 2 hours, cooling to 700°C, and subsequent quenching.

Powder X-ray diffraction analysis was performed on crushed crystals using a Bruker D8 high-resolution diffractometer with Cu Kα radiation (λ = 1.5406 Å) to confirm phase purity. Single-crystal orientations were determined using Laue diffraction in backscattering mode with a SmartLab Rigaku system. Scanning electron microscopy, utilising a JEOL JSM-7000F field-emission instrument, and energy-dispersive X-ray spectroscopy were employed to analyse the chemical composition of the crystals.

Electrical transport measurements were conducted using a Cryomagnetics C-Mag Vari-9 system, while magnetization and heat capacity were measured with a Quantum Design MPMS3 and PPMS-14T, respectively. Rietveld refinement of the powder X-ray diffraction data yielded refinement parameters of RwR = 7.504 % for RhGe and 1.046 % for CoGe, demonstrating high reliability of the structural model. The lattice parameter for RhGe was determined to be 0.4859(2) nm and for CoGe 0.4640(1) nm, consistent with previously reported values for polycrystalline samples.

Electronic characteristics and low carrier density in RhGe and CoGe single crystals

RhGe and CoGe single crystals, both possessing a B20 structure, exhibit metallic behaviour with Fermi-liquid scaling at low temperatures. Magnetization measurements confirm paramagnetic characteristics in both compounds, establishing a baseline for further investigation of their electronic properties.

Transport measurements reveal that both materials maintain metallic conductivity throughout the measured temperature range, indicative of a non-trivial electronic structure. Detailed analysis indicates low carrier concentrations in both RhGe and CoGe, coupled with a small electronic specific heat coefficient.

This combination suggests a semimetallic feature with relatively weak electronic correlations, differentiating these materials from strongly correlated electron systems. The low carrier density is a crucial factor in understanding the topological properties arising from their chiral crystal structure.

These findings establish a foundation for exploring the behaviour of multifold fermions and helicoid-arc surface states within the B20 system. Single crystals were successfully grown using a high-pressure method at 5 GPa, employing excess germanium as a solvent flux. RhGe crystals were synthesised at 1250°C and maintained at this temperature for three hours, while CoGe crystals were grown at 1100°C for two hours, followed by controlled cooling and quenching.

Powder X-ray diffraction confirmed the crystalline structure of both materials, and Laue diffraction verified the single-crystal orientation of five samples of each compound. Energy-dispersive X-ray spectroscopy, performed alongside scanning electron microscopy, was used to verify the chemical composition of the crystals.

These high-quality single crystals provide a platform for detailed studies of spin-orbit coupling and surface effects on the topological states present in these B20 materials. Preliminary results on RhGe single crystals indicate the absence of both weak itinerant ferromagnetism, previously reported at approximately 140 K, and superconductivity at 4.3 K.

Characterisation of RhGe and CoGe single crystals reveals semimetallic behaviour and topological potential

Researchers have successfully grown high-quality single crystals of rhodium germanium (RhGe) and cobalt germanium (CoGe), both possessing a B20 crystal structure. Transport measurements confirm metallic behaviour in these materials at low temperatures, alongside paramagnetic characteristics revealed by magnetic measurements.

Neither superconductivity nor long-range ferromagnetic order was detected in either compound down to 2 Kelvin. These high-quality crystals are essential for future investigations into their topological properties, particularly using angle-resolved photoemission spectroscopy. The materials exhibit low carrier concentrations and a small electronic specific heat coefficient, indicating their semimetallic nature with weak electronic correlations.

This work also underscores the continuing difficulty in identifying superconductivity within B20-structured materials, despite ongoing research efforts. The availability of these single crystals facilitates the exploration of multifold fermions and the behaviour of helicoid-arc surface states, influenced by spin-orbit coupling and surface conditions, within this class of compounds.

The authors acknowledge a limitation in not observing superconductivity, highlighting the challenges in achieving this state within the B20 structure. Future research will likely focus on utilising angle-resolved photoemission spectroscopy to further characterise the topological properties of RhGe and CoGe, potentially revealing novel quantum phenomena and advancing understanding of these chiral semimetals.