We report a comprehensive experimental investigation on the magnetic anisotropy in bulk single crystals of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$, a quasi-two-dimensional ferromagnet belonging to the family of magnetic layered transition metal trichalcogenides that have recently attracted a great deal of interest with regard to the fundamental and applied aspects of two-dimensional magnetism. For this purpose electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements have been carried out over a wide frequency and temperature range. A gradual change in the angular dependence of the ESR linewidth at temperatures above the ferromagnetic transition temperature $T_{\mathrm{c}}$ reveals the development of two-dimensional spin correlations in the vicinity of $T_{\mathrm{c}}$ thereby proving the intrinsically low-dimensional character of spin dynamics in ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. Angular and frequency dependent measurements in the ferromagnetic phase clearly show an easy-axis-type anisotropy of this compound. Furthermore, these experiments are compared with simulations based on a phenomenological approach, which takes into account results of static magnetization measurements as well as high temperature $g$ factors obtained from ESR spectroscopy in the paramagnetic phase. As a result the determined magnetocrystalline anisotropy energy density (MAE) $K_U$ is $(0.48\pm 0.02)\times {10}^6$ erg/${\mathrm{cm}}^3$. This analysis is complemented by density functional calculations which yield the experimental MAE value for a particular value of the electronic correlation strength $U$. The analysis of the electronic structure reveals that the low-lying conduction band carries almost completely spin-polarized, quasihomogeneous, two-dimensional states.

We have studied the magnetic ordering in ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ with $0.25\le x\le 0.4$ by unpolarized and polarized neutron diffraction using single crystals. Unlike most FeAs-based compounds that magnetically order, Na-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ exhibits two successive magnetic transitions: For $x=0.35$, upon cooling, magnetic order occurs at $\sim 70$ K with in-plane magnetic moments being arranged as in pure or Ni-, Co-, or K-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ samples. At a temperature of $\sim 46$ K a second phase transition occurs, which the single-crystal neutron-diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in ${\mathrm{Ba}}_{0.65}{\mathrm{Na}}_{0.35}{\mathrm{Fe}}_2{\mathrm{As}}_2$ point along the $c$ direction. The two nearly degenerate magnetic states document orbital degeneracy to persist in the superconducting phase.

We report low-temperature specific heat measurements for high-quality single crystalline KFe${}_2$As${}_2$ ($T_c\approx 3.5$ K). The investigated zero-field specific heat data yields an unusually large nominal Sommerfeld coefficient, ${\gamma }_n=94\left(3\right)$ mJ/mol K${}^2$, which is, however, affected by extrinsic contributions as evidenced by a sizable residual linear specific heat and various theoretical considerations, including an analysis of Kadowaki-Woods relations. These results indicate that KFe${}_2$As${}_2$ should be classified as a weakly or intermediately coupled superconductor with a total electron-boson coupling constant ${\lambda }_{\mathrm{tot}}\sim 1$ (including a calculated weak electron-phonon coupling ${\lambda }_{\mathrm{ph}}=0.17).$ From specific heat and ac susceptibility studies in external magnetic fields the magnetic phase diagram is also constructed. We confirm the high anisotropy of the upper critical fields ${\mu }_0{H}_{\mathrm{c}2}\left(T\right)$, ranging from a factor of 5 near $T_c$ to a slightly reduced value around 4.5 approaching $T=0$ for fields $B\parallel ab$ and $\parallel c$ and show that their ratio slightly exceeds the mean mass anisotropy of 4.4 derived from our full-relativistic local-density-approximation band structure calculations, also in accord with recent preliminary penetration depth data by Eskildsen et al. [Eskildsen, Forgan, and Kawano-Furukawa, Rep. Prog. Phys. 74, 124504 (2011)] near 4. Its slight reduction when approaching $T=0$ is not a consequence of Pauli limiting as in less perfect samples but points likely to a multiband effect. We also report irreversibility field data obtained from ac susceptibility measurements. The double-maximum in the $T$ dependence of its imaginary part for fields $B\parallel c$ indicates a peak effect in the $T$ dependence of critical currents.

${\mathrm{PtBi}}_2$ is a polymorphic system with interesting electronic properties. Here we report optimized crystal growth and structural characterization of pyrite-type and trigonal modification of ${\mathrm{PtBi}}_2$. Selected area electron diffraction, x-ray powder diffraction, and further Rietveld refinement confirms that trigonal ${\mathrm{PtBi}}_2$ crystallizes in the noncentrosymmetric $P31m$ space group, and pyrite-type ${\mathrm{PtBi}}_2$ crystallizes in the $Pa\overline{3}$ space group. A series of ${\mathrm{Pt}}_{1-x}{\mathrm{Rh}}_x{\mathrm{Bi}}_2$ samples was obtained for $x=0,0.03,0.35$ in the trigonal ${\mathrm{PtBi}}_2$ structure. These ${\mathrm{Pt}}_{1-x}{\mathrm{Rh}}_x{\mathrm{Bi}}_2$ compounds become superconducting where the critical temperature increases from $T_c=600$ mK for $x=0$ up to $T_c=2.7$ K for $x=0.35$. Furthermore, we calculate the electronic band structure using the obtained structure parameters. The calculated density of states shows a minimum for the stoichiometric compound at the Fermi level. These findings warrant further research using a broader array of experimental techniques, as well as on the effect of the substitution on the nontrivial band structure.

An essential ingredient for the production of Majorana fermions for use in quantum computing is topological superconductivity^1,2. As bulk topological superconductors remain elusive, the most promising approaches exploit proximity-induced superconductivity³, making systems fragile and difficult to realize^4–7. Due to their intrinsic topology⁸, Weyl semimetals are also potential candidates^1,2, but have always been connected with bulk superconductivity, leaving the possibility of intrinsic superconductivity of their topological surface states, the Fermi arcs, practically without attention, even from the theory side. Here, by means of angle-resolved photoemission spectroscopy and ab initio calculations, we identify topological Fermi arcs on two opposing surfaces of the non-centrosymmetric Weyl material trigonal PtBi₂ (ref. ⁹). We show these states become superconducting at temperatures around 10 K. Remarkably, the corresponding coherence peaks appear as the strongest and sharpest excitations ever detected by photoemission from solids. Our findings indicate that superconductivity in PtBi₂ can occur exclusively at the surface, rendering it a possible platform to host Majorana modes in intrinsically topological superconductor–normal metal–superconductor Josephson junctions.