In this review article, we give a brief overview of heavy fermions, which are prototype examples of strongly correlated electron systems. We introduce the application of physical pressure in heavy fermion systems to construct their pressure phase diagrams and to study the close relationship between superconductivity（SC） and other electronic instabilities, such as antiferromagnetism（AFM）, ferromagnetism（FM）, and valence transitions. Field-angle dependent heat capacity and point-contact spectroscopic measurements under pressure are taken as examples to illustrate their ability to investigate novel physical properties of the emergent electronic states.
Recently, new Cr-based superconductors, A2Cr3As3(A = K, Rb, Cs), have gained intense interest because of their one-dimensional crystal structures and electron correlations. Here we report the crystal structure and physical properties of two related materials ACr3As3(A = Rb, Cs) which are synthesized via a soft-chemical A+ deintercalation in A2Cr3As3. The new compounds remain one-dimensional(Cr3As3)∞ linear chains, and the interchain distance can be tuned by the incorporation of the alkali-metal cations with different sizes. The physical property measurements indicate a local-moment behavior at high temperatures, and the moments freeze into a cluster spin-glass state below 5–6 K. No superconductivity was observed in both materials. We also found that, with increasing the interchain distance, the Cr effective moments increase monotonically, accompanied with the enhancement of semi-conductivity. Our results shed light on the understanding of occurrence of superconductivity in A2Cr3As3.
We have successfully synthesized Sr 2 CuO3+δ single crystals under high pressure and high temperature for the first time. The structure analysis show that this material crystallizes into tetragonal structure isostructural La 2 CuO4 with single CuO 2 plane. The magnetic susceptibility as well as resistance measurements indicates that the bulk superconductivity with the critical transition temperature 37 K is achieved in the crystal.
Iron-chalcogenide compounds with FeSe（Te, S） layers did not attract much attention until the discovery of high-Tc superconductivity （SC） in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the crystal lattice. Another reason is that the large single crystals are easily grown for the iron-chalcogenide compounds. This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on several issues, including the crystal structure, magnetic properties, superconductivity, and phase separation. Some of them reach a consensus but some important questions still remain to be answered.
Since the discovery of high-temperature superconductivity(SC) in quasi-two-dimensional copper oxides,a few layered compounds,which bear similarities to the cuprates,have also been found to host the unconventional SC. Our recent observation of SC at 6.1 K in correlated electron material K2Cr3As3 represents an obviously different paradigm,primarily because of its quasi-one-dimensional(Q1D) nature. The new material is structurally featured by the [(Cr3As3)2-]∞ double-walled subnano-tubes composed of face-sharing Cr6/2(As6/2) octahedron linear chains,which are well separated by the columns of K+ counterions. Later,an isostructural superconducting Rb2Cr3As3 was synthesized,thus forming a new superconducting family. Here we report the third member,Cs2Cr3As3,which possesses the largest interchain distance. SC appears below 2.2 K. Similar to the former two sister compounds,Cs2Cr3As3 exhibits a non-Fermi liquid behavior with a linear temperature dependence of resistivity in the normal state,and a high upper critical field beyond the Pauli limit as well,suggesting a common unconventional SC in the Q1 D Cr-based material.
