Xu Zhu-An

Crystal chemistry and structural design of iron-based superconductors

The second class of high-temperature superconductors (HTSCs), iron-based pnictides and chalcogenides, necessarily contain Fe2X2 (“X refers to a pnictogen or a chalcogen element) layers, just like the first class of HTSCs which possess the essential CuO2 sheets. So far, dozens of iron-based HTSCs, classified into nine groups, have been discovered. In this article, the crystal-chemistry aspects of the known iron-based superconductors are reviewed and summarized by employing “hard and soft acids and bases (HSAB) concept. Based on these understandings, we propose an alternative route to exploring new iron-based superconductors via rational structural design.

Relationship between Superconductivity and Antiferromagnetism in LaFe(As1−xPx)O Revealed by 31P-NMR

We performed 31P-NMR measurements on LaFe(As1−xPx)O to investigate the relationship between antiferromagnetism and superconductivity. The antiferromagnetic (AFM) ordering temperature TN and the moment µord are continuously suppressed with increasing P content x and disappear at x = 0.3 where bulk superconductivity appears. At this superconducting x = 0.3, quantum critical AFM fluctuations are observed, indicative of the intimate relationship between superconductivity and low-energy AFM fluctuations associated with the quantum-critical point in LaFe(As1−xPx)O. The relationship is similar to those observed in other isovalent-substitution systems, e.g., BaFe2(As1−xPx)2 and SrFe2(As1−xPx)2, with the “122” structure. Moreover, the AFM order reappears with further P substitution (x > 0.4). The variation of the ground state with respect to the P substitution is considered to be linked to the change in the band character of Fe-3d orbitals around the Fermi level.

Magnetic Properties of Orthorhombic Perovskite Ho1−xLaxMnO3

Single phase orthorhombic perovskite Ho1?xLaxMnO3 (x = 0.1, 0.15) compounds are successfully synthesized by using a sol-gel method. We find that the orthorhombic perovskite structure of HoMoO3 compound can be stabilized by partial substitution of smaller Ho ion by larger La ion. The magnetic properties of orthorhombic perovskite Ho1?xLaxMnO3 are investigated for the first time. For the x = 0.15 sample, a ferromagnetic-like transition is found around T = 130 K, which should correspond to partial FM ordering of Mn3+ magnetic moments. The partial substitution of Ho by La causes a switch of magnetic state of Mn3+ moments from AFM order to FM-like order. Our study indicates that La doping has significant influence on the magnetic structures of Mn ions.

Effect of Te doping on superconductivity and charge-density wave in dichalcogenides 2H-NbSe2-xTex (x = 0, 0.1, 0.2)

Single crystals of Te-doped dichalcogenides 2H-NbSe2-xTex (x = 0,0.10,0.20) were grown by vapour transport method. The effect of Te doping on the superconducting and charge-density wave (CDW) transitions has been investigated. The sharp decrease of residual resistance ratio, RRR = R(300K)/R(8K), with increasing Te content was observed, indicating that the disorder in the conducting plane is induced by Te doping. Meanwhile the superconducting transition temperature, Tc, decreases monotonically with Te content. However, the CDW transition temperature, TCDW, shown by a small jump in the temperature dependence of the resistivity near 30 K, increases slightly. The results show that the suppression of superconductivity might be caused by the enhancement of CDW ordering. The disorder has little influence on the CDW ordering.

Upper Critical Fields and Anisotropy of BaFe1.9 Ni0.1As2 Single Crystals

Temperature dependence of the upper critical magnetic field (Hc2) near Tc of 20 K in a BaFe1.9Ni0.1As2 single crystal is determined via magneto-resistance measurements, for the out-plane (H⊥ab) and in-plane (H || ab) directions in magnetic fields of up to 8 T. The upper critical fields at zero temperature estimated by the Werthamer-Helfand-Hohenberg (WHH) formula are μ0H||c2(0) = 137 T and μ0H⊥c2(0) = 51 T, both exceeding the weak-coupling Pauli paramagnetic limit (μ0Hp = 1.84Tc). However, the WHH formula could overestimate the μ0H||c2(0) value. The anisotropy of upper critical fields is around 3 in the temperature range close to Tc. The result is very similar to the Co-doped 122 superconductor BaFe2–xCoxAs2, indicating that electron-doped 122 superconductors exhibit similar superconducting properties.

Second-Order Phase Transition in the Two-Dimensional Classical Lattice Coulomb Gas of Half-Integer Charges

The second-order phase transition in the two-dimensional (2D) classical Coulomb gas of half-integer charges on a square lattice is investigated by using Monte Carlo simulations. Based on the finite-size scaling analysis, we estimate the second-order phase transition temperature Tc and the static critical exponents β and ν with a new numerical analysis method. More precise critical temperature Tc = 0.1311(2) and critical exponents β/ν = 0.1152(12) and ν = 0.857(15) are obtained. The estimated value of ν indicates that the charge lattice melting transition is different from the pure 2D Ising transition.

Research on anisotropy for single crystal GdBa2Cu3O7−y

Abstract In this paper, a series results in single crystal GdBa2Cu3O7−y, which include anisotropic behavior of resistivity in normal state and superconducting transition temperature, have been reported. The authors find that both ϱ‖ (‖ ab plane) and ϱ⊥ (⊥ab plane) are fitted very well to the Anderson-Zou relationship: ϱ = A T + BT . The superconductivity in c axis is weaker than that in ab plane: Tce⊥ is 2K lower than Tce‖. When the applied magnetic field H is parallel to the ab plane the signal of the A.C. magnetic shielding is 82% of that when H is perpendicular to the ab plane.

High Tc superconductivity in single crystal of BiCaSrCuO

Abstract In this letter, we report that superconducting Bi-Ca-Sr-Cu-O single crystals with the typical size 2×2×1 mm3 were grown with the flux methed successfully. The single crystal structure is proved by Laue photograph. Weissenberg photograph shows that it belongs to orthorhombic system with cell constants: a=5.42 A , b=27.18 A , c=30.67 A . The superconducting transition temperatures are Tc(mid)=83 K, Tce=79 K. A.C. magnetic susceptibility measurement shows the crystal has strong diamagnetic. The relation between superconductivity and the structure is discussed.

Interplay of superconductivity and d?f correlation in CeFeAs1?xPxO1?yFy

The recent discovery of high-temperature superconductivity in iron-based pnictides (chalcogenides) not only triggers tremendous enthusiasm in searching for new superconducting materials, but also opens a new avenue to the study of the Kondo physics. CeFeAsO is a parent compound of the 1111-type iron-based superconductors. It shows 3d-antiferromagnetic (AFM) ordering below ~ 139 K and 4f-AFM ordering below ~ 4 K. On the other hand, the phosphide CeFePO is a ferromagnetically correlated heavy-fermion (HF) metal with Kondo scale TK ~ 10 K. These properties set up a new platform for research of the interplay among magnetism, Kondo effect, and superconductivity (SC). In this review, we present the recent progress in the study of chemical pressure effect in CeFeAsO1−yFy (y = 0 and 0.05). This P/As-doping in CeFeAsO serves as an effective controlling parameter which leads to two magnetic critical points, xc1 0.4 and xc2 0.92, associated with suppression of 3d and 4f magnetism, respectively. We also observe a turning point of AFM-FM ordering of Ce3+ moment at xc3 0.37. The SC is absent in the phase diagram, which is attributed to the destruction to Cooper pair by Ce-FM fluctuations in the vicinity of xc1. We continue to investigate CeFeAs1−xPxO0.95F0.05. With the separation of xc1 and xc3, this chemical pressure results in a broad SC region 0≤ x ≤ 0.53, while the original HF behavior is driven away by 5% F− doping. Different roles of P and F dopings are addressed, and the interplay between SC and Ce-4f magnetism is also discussed.

Dynamic Approach to the Ground State of a Vortex System with Random Pinning

We study the ground state of a two-dimensional vortex system with random pinnings by cooling the vortex system to zero temperature. The peak of structure factor S(k) of vortices increases with vortex density B but decreases with pin density B, indicating that the vortex lattice becomes better and better ordered with increasing B. When B>B, the triangular vortex lattice is observed away from the pins. Repulsive vortex interactions destroy the Mott insulator phase which is predicted to occur at B = B. The crossover line between the strong Bose glass and the weak Bose glass is nearly independent of B for small B.