We observed bulk superconductivity in the FeTe${}_{1-x}$Se${}_x$ crystals with lower Se concentrations after annealed in air. A revised magnetism and superconductivity phase diagram is obtained via resistance and magnetic-susceptibility measurements for the FeTe${}_{1-x}$Se${}_x$ system. It is found that bulk superconductivity coexists with antiferromagnetic order in the crystals with 0.05 $\le$ x $\le$ 0.18. The phase diagram is very similar to the case of the K-doping or Co-doping BaFe${}_2$As${}_2$ as well as that of the SmFeAsO${}_{1-x}$F${}_x$ system, perhaps indicating that all the iron-based systems have a generic phase diagram, although the antiferromagnetic wave vectors in the parent compounds of both iron pnictides and FeTe${}_{1-x}$Se${}_x$ systems are remarkably different.

Extremely large magnetoresistance (XMR) was recently discovered in many nonmagnetic materials, while its underlying mechanism remains poorly understood due to the complex electronic structure of these materials. Here we report an investigation of the $\alpha$-phase ${\mathrm{WP}}_2$, a topologically trivial semimetal with monoclinic crystal structure ($C2/m$), which contrasts with the recently discovered robust type-II Weyl semimetal phase in $\beta -{\mathrm{WP}}_2$. We found that $\alpha -{\mathrm{WP}}_2$ exhibits almost all the characteristics of XMR materials: the near-quadratic field dependence of MR, a field-induced up-turn in resistivity followed by a plateau at low temperature, which can be understood by the compensation effect, and high mobility of carriers confirmed by our Hall effect measurements. It was also found that the normalized MRs under different magnetic fields have the same temperature dependence in $\alpha -{\mathrm{WP}}_2$, the Kohler scaling law can describe the MR data in a wide temperature range, and there is no obvious change in the anisotropic parameter $\gamma$ value with temperature. The resistance polar diagram has a peanut shape when the field is rotated in the $ac$ plane, which can be understood by the anisotropy of the Fermi surface. These results indicate that both field-induced-gap and temperature-induced Lifshitz transition are not the origin of up-turn in resistivity in the $\alpha -{\mathrm{WP}}_2$ semimetal. Our findings establish $\alpha -{\mathrm{WP}}_2$ as a new reference material for exploring the XMR phenomena.

The magnetic properties of ThCr${}_2$Si${}_2$-type single crystals $A$Co${}_2$Se${}_2$ ($A=\text{K}$, Rb, and Cs) were investigated by means of magnetic susceptibilities and isothermal magnetizations measurements. The ferromagnetic phase transition temperatures are $\approx$74 and 76 K for $A=\text{K}$ and Rb, respectively. A possible antiferromagnetic phase transition occurs around 80 K in susceptibility and a metamagnetismlike behavior is first discovered at a field of $\approx$3.5 T for $H\parallel ab$ plane in CsCo${}_2$Se${}_2$ below 62 K. The susceptibilities in the paramagnetic state obey the modified Curie-Weiss law quite well in all samples. Moreover, the derived effective magnetic moments of the Co atom are about 2.21, 2.04, and 2.04 ${\mu }_B$/Co for $A=\text{K}$, Rb, and Cs, respectively. The generalized Rhodes-Wohlfarth ratio indicates an itinerant magnetism in this series. Finally, we discussed the magnetic properties in this series within the frameworks of the self-consistent renormalization and Takahashi's theory of spin fluctuations.

We have made the first observation of superconductivity in ${\mathrm{TlNi}}_{2}{\mathrm{Se}}_{2}$ at ${T}_{C}=3.7\text{ }\text{ }\mathrm{K}$, and it appears to involve heavy electrons with an effective mass ${m}^{*}=(14\char21{}20){m}_{b}$, as inferred from the normal-state electronic specific heat and the upper critical field, ${H}_{C2}(T)$. We found that the zero-field electronic specific-heat data, ${C}_{\mathrm{es}}(T)$ ($0.5\text{ }\text{ }\mathrm{K}$\le${}T<3.7\text{ }\text{ }\mathrm{K}$) in the superconducting state can be fitted with a two-gap BCS model, indicating that ${\mathrm{TlNi}}_{2}{\mathrm{Se}}_{2}$ seems to be a multiband superconductor, which is consistent with the band calculation for the isostructural ${\mathrm{KNi}}_{2}{\mathrm{S}}_{2}$. It is also found that the electronic specific-heat coefficient in the mixed state ${$\gamma${}}_{N}(H)$ exhibits a ${H}^{1/2}$ behavior, which is considered as a common feature of the $d$-wave superconductors. ${\mathrm{TlNi}}_{2}{\mathrm{Se}}_{2}$, as a $d$-electron system with heavy electron superconductivity, may be a bridge between cuprate- or iron-based and conventional heavy-fermion superconductors.