arXiv subjects

Kui Jin

Publications and source records attributed to Kui Jin.

58 records · Page 4Linked to original sources

arXiv2015

(Li0.84Fe0.16)OHFe0.98Se superconductor: Ion-exchange synthesis of large single crystal and highly two-dimensional electron properties

A large and high-quality single crystal (Li0.84Fe0.16)OHFe0.98Se, the optimal superconductor of newly reported (Li1-xFex)OHFe1-ySe system, has been successfully synthesized via a hydrothermal ion-exchange technique. The superconducting transition temperature (Tc) of 42 K is determined by magnetic susceptibility and electric resistivity measurements, and the zero-temperature upper critical magnetic fields are evaluated as 79 and 313 Tesla for the field along the c-axis and the ab-plane, respectively. The ratio of out-of-plane to in-plane electric resistivity,\r{ho}c/\r{ho}ab, is found to increases with decreasing temperature and to reach a high value of 2500 at 50 K, with an evident kink occurring at a characteristic temperature T*=120 K. The negative in-plane Hall coefficient indicates that electron carriers dominate in the charge transport, and the hole contribution is significantly reduced as the temperature is lowered to approach T*. From T* down to Tc, we observe the linear temperature dependences of the in-plane electric resistivity and the magnetic susceptibility for the FeSe layers. Our findings thus reveal that the normal state of (Li0.84Fe0.16)OHFe0.98Se becomes highly two-dimensional and anomalous prior to the superconducting transition, providing a new insight into the mechanism of high-Tc superconductivity.

cond-mat.supr-con↗

arXiv2014

Phase diagram of (Li1-xFex)OHFeSe: a bridge between iron selenide and arsenide superconductors

Previous experimental results have shown important differences between iron selenide and arsenide superconductors, which seem to suggest that the high temperature superconductivity in these two subgroups of iron-based family may arise from different electronic ground states. Here, we report the complete phase diagram of a newly synthesized superconducting (SC) system (Li1-xFex)OHFeSe with a similar structure to FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ~127 K. This is the first example to demonstrate such an SDW phase in FeSe-based superconductor system. Transmission electron microscopy (TEM) shows that a well-known sqrr(5) x sqrr(5) iron vacancy ordered state, resulting in an AFM order at ~ 500K in AyFe2-xSe2 (A = metal ions) superconductor system, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q=1/2(1, 1, 0), identical to that seen in SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors.

cond-mat.supr-con↗

arXiv2014

Anomalous magnetoresistance in the spinel superconductor LiTi2O4

Transition-metal oxides offer an opportunity to explore unconventional superconductors, where the superconductivity (SC) is often interrelated with novel phenomena such as spin/charge order, fluctuations, and Fermi surface instability (1-3). LiTi2O4 (LTO) is a unique compound in that it is the only known spinel oxide superconductor. In addition to electron-phonon coupling, electron-electron and spin fluctuation contributions have been suggested as playing important roles in the microscopic mechanism for its superconductivity (4-8). However, the lack of high quality single crystals has thus far prevented systematic investigation of their transport properties (9). Here, we report a careful study of transport and tunneling spectroscopy in epitaxial LTO thin films. In the superconducting state, the energy gap was found to decrease as a quadratic function of magnetic field. In the normal state, an unusual magnetoresistance (MR) was observed where it changes from anisotropic positive to isotropic negative as the temperature is increased. A constant charge carrier concentration without any abrupt change in lattice parameters as a function of temperature suggests that the isotropic MR stems from the suppression of spin scattering/fluctuations, while the anisotropic term originates from an orbital contribution. These observations point to an important role strong correlations play in this unique superconductor.

cond-mat.supr-con↗

arXiv2013

Combinatorial search of superconductivity in Fe-B composition spreads

We have fabricated Fe-B thin film composition spreads in search of possible superconducting phases following a theoretical prediction by Kolmogorov et al.^1 Co-sputtering was used to deposit spreads covering a large compositional region of the Fe-B binary phase diagram. A trace of superconducting phase was found in the nanocrystalline part of the spread, where the film undergoes a metal to insulator transition as a function of composition in a region with the average composition of FeB_2. The resistance drop occurs at 4K, and a diamagnetic signal has also been detected at the same temperature. The superconductivity is suppressible in the magnetic field up to 2 Tesla.

cond-mat.supr-con↗