A series of layered ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x\mathrm{FeAs}$ compounds with $x=0$ to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. $F$ doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high $T_c$ strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce $4f$ electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.

In the Kondo insulator samarium hexaboride (SmB ₆ ), strong correlation and band hybridization lead to an insulating gap and a diverging resistance at low temperature. The resistance divergence ends at about 3 kelvin, a behavior that may arise from surface conductance. We used torque magnetometry to resolve the Fermi surface topology in this material. The observed oscillation patterns reveal two Fermi surfaces on the (100) surface plane and one Fermi surface on the (101) surface plane. The measured Fermi surface cross sections scale as the inverse cosine function of the magnetic field tilt angles, which demonstrates the two-dimensional nature of the conducting electronic states of SmB ₆ .

We performed optical spectroscopy measurement on single crystals of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{SrFe}}_2{\mathrm{As}}_2$, the parent compounds of FeAs-based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the spin-density-wave state, the formation of partial energy gaps was clearly observed with the surprising presence of two different energy scales. A large part of the Drude component was removed by the gapping of Fermi surfaces. Meanwhile, the carrier scattering rate was even more dramatically reduced. We elaborate that the spin-density-wave instability is more likely to be driven by the Fermi surface nesting of itinerant electrons than a local-exchange mechanism.

The synthesis and characterization of ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$, which has a layered array of ${\mathrm{Cu}}^{2+}$ spins in a triangular lattice, are reported. The magnetic susceptibility and neutron scattering experiments of this material show no magnetic ordering down to 0.2 K with a ${\theta }_{\mathrm{CW}}=-55\mathrm{K}$. The magnetic specific heat reveals a $T$-linear dependence with a $\gamma =43.4\mathrm{mJ}{\mathrm{K}}^{-2}{\mathrm{mol}}^{-1}$ below 1.4 K. These observations suggest that ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ is a new quantum spin liquid candidate with a $S=1/2$ triangular lattice.