Tian-Long Xia

Superconductivity at 44 K in K intercalated FeSe system with excess Fe

We report here that a new superconducting phase with much higher Tc has been found in K intercalated FeSe compound with excess Fe. We successfully grew crystals by precisely controlling the starting amount of Fe. Besides the superconducting (SC) transition at ~30 K, we observed a sharp drop in resistivity and a kink in susceptibility at 44 K. By combining thermodynamic measurements with electron spin resonance (ESR), we demonstrate that this is a new SC transition. Structural analysis unambiguously reveals two phases coexisting in the crystals, which are responsible respectively for the SC transitions at 30 and 44 K. The structural experiments and first-principles calculations consistently indicate that the 44 K SC phase is close to a 122 structure, but with an unexpectedly large c-axis of 18.10 Å. We further find a novel monotonic dependence of the maximum Tc on the separation of neighbouring FeSe layers.

Resistivity plateau and extremely large magnetoresistance in NbAs 2 and TaAs 2

In topological insulators (TIs), metallic surface conductance saturates the insulating bulk resistance with de- creasing temperature, resulting in resistivity plateau at low temperatures as a transport signature originating from metallic surface modes protected by time reversal symmetry (TRS). Such characteristic has been found in several materials including Bi2Te2Se, SmB6 etc. Recently, similar behavior has been observed in metallic com- pound LaSb, accompanying an extremely large magetoresistance (XMR). Shubnikov-de Hass (SdH) oscillation at low temperatures further confirms the metallic behavior of plateau region under magnetic fields. LaSb[1] has been proposed by the authors as a possible topological semimetal (TSM), while negative magnetoresistance is absent at this moment. Here, high quality single crystals of NbAs2/TaAs2 with inversion symmetry have been grown and the resistivity under magnetic field is systematically investigated. Both of them exhibit metallic behavior under zero magnetic field, and a metal-to-insulator transition occurs when a nonzero magnetic field is applied, resulting in XMR (1.0*105% for NbAs2 and 7.3*105% for TaAs2 at 2.5 K & 14 T). With tempera- ture decreased, a resistivity plateau emerges after the insulator-like regime and SdH oscillation has also been observed in NbAs2 and TaAs2.

Na-23 and As-75 NMR study of antiferromagnetism and spin fluctuations in NaFeAs single crystals

We report the Na-23 and As-75 nuclear magnetic resonance (NMR) studies on NaFeAs single crystals. The structure transition temperature T-S (55 K) and the spin density wave (SDW) transition temperature T-SDW (40.5 K) are determined by the NMRline splits. The spin-lattice relaxation rates indicate that the spin fluctuations are strongly enhanced just below T-S and drive a second order SDW transition. A fluctuating feature of the SDW ordering is also seen below the T-SDW. We further performed high-pressure NMR studies on NaFeAs, and found that the T-SDW increases by similar to 7 K and the magnetic moment increases by similar to 30% under 2.5 GPa pressure.

As-75 NMR study of single crystals of the heavily overdoped pnictide superconductors Ba1-xKxFe2As2 (x=0.7 and 1)

We performed As-75 NMR studies on two overdoped high-quality Ba1-xKxFe2As2 (x=0.7 and 1.0) single crystals. In the normal states, we found a dramatic increase in the spin-lattice relaxation (1/T-75(1)) from the x =0.7 to the x=1.0 samples. In KFe2As2, the ratio of 1/(T1TKn2)-T-75, where K-75(n) is the Knight shift, increases as temperature drops. These results indicate the existence of another type of spin fluctuations in KFe2As2 which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, the temperature scalings of 1/T-75(1) below T-c in the overdoped samples are significantly different from those in the under or optimally doped ones. A power-law scaling behavior 1/(T1T)-T-75 similar to T-0.5 is observed, which indicates universal strong low-energy excitations in the overdoped hole-type superconductors.

Raman phonons of α -FeTe and Fe 1.03 Se 0.3 Te 0.7 single crystals

The polarized Raman scattering spectra of nonsuperconducting

77Se NMR study of the pairing symmetry and the spin dynamics in K(y)Fe(2-x)Se2.

\alpha

Effects on superconductivity of transition-metal doping in FeSe0.5Te0.5

-FeTe and of the newly discovered, As-free superconductor Fe

Magnetoresistance and Shubnikov-de Haas oscillation in YSb

_{1.03}

Interplay of Dirac electrons and magnetism in CaMnBi2 and SrMnBi2

Se

Large linear magnetoresistance in a new Dirac material BaMnBi2

_{0.3}