Ronghua Liu

Bio: Ronghua Liu is an academic researcher from Nanjing University. The author has contributed to research in topics: Superconductivity & Magnetization. The author has an hindex of 32, co-authored 98 publications receiving 5291 citations. Previous affiliations of Ronghua Liu include University of Science and Technology of China & Emory University.

Superconductivity at 43 K in SmFeAsO1-xFx.

Abstract: The recently discovered layered rare-earth metal oxypnictides have reinvigorated research into high-temperature superconductivity. The first of these, found only a few months ago, had a transition temperature of 26 K. A recent paper in Nature reported an iron–arsenic-based material superconducting at 43 K with the application of pressure. Previously only copper oxides superconductors had beaten the 40 K barrier. Now Chen et al. report bulk superconductivity in the samarium–arsenide oxide SmFeAsO1−xFx with a transition temperature of 43 K without this pressure. A report on the discovery of bulk superconductivity in samarium-arsenide oxides SmFeAsO1−xFx with a transition temperature as high as 43 K. Since the discovery of high-transition-temperature (high-Tc) superconductivity in layered copper oxides, extensive effort has been devoted to exploring the origins of this phenomenon. A Tc higher than 40 K (about the theoretical maximum predicted from Bardeen–Cooper–Schrieffer theory1), however, has been obtained only in the copper oxide superconductors. The highest reported value for non-copper-oxide bulk superconductivity is Tc = 39 K in MgB2 (ref. 2). The layered rare-earth metal oxypnictides LnOFeAs (where Ln is La–Nd, Sm and Gd) are now attracting attention following the discovery of superconductivity at 26 K in the iron-based LaO1-xF x FeAs (ref. 3). Here we report the discovery of bulk superconductivity in the related compound SmFeAsO1-xF x , which has a ZrCuSiAs-type structure. Resistivity and magnetization measurements reveal a transition temperature as high as 43 K. This provides a new material base for studying the origin of high-temperature superconductivity.

Coexistence of the spin-density wave and superconductivity in Ba1−xKxFe2As2

Abstract: The relation between the spin-density wave (SDW) and superconducting order is a central topic in the current research on the FeAs-based high-TC superconductors. Conflicting results exist in the LaFeAs(O, F)-class of materials, for which whether the SDW and superconductivity are mutually exclusive or they can coexist has not been settled. Here we show that for the (Ba, K)Fe2As2 system, the SDW and superconductivity can coexist in an extended range of compositions. The availability of single crystalline samples and high value of the energy gaps would make the materials a model system to investigate the high-TC ferropnictide superconductivity.

A BCS-like gap in the superconductor SmFeAsO0.85F0.15.

Abstract: Since the discovery of superconductivity in the high-transition-temperature (high-T(c)) copper oxides two decades ago, it has been firmly established that the CuO(2) plane is essential for superconductivity and gives rise to a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO(1-x)F(x) has recently been discovered and the conspicuous lack of the CuO(2) planes raises the tantalizing question of a different pairing mechanism in these oxypnictides. The superconducting gap (its magnitude, structure, and temperature dependence) is intimately related to pairing. Here we report the observation of a single gap in the superconductor SmFeAsO(0.85)F(0.15) with T(c) = 42 K as measured by Andreev spectroscopy. The gap value of 2Delta = 13.34 +/- 0.3 meV gives 2Delta/k(B)T(c) = 3.68 (where k(B) is the Boltzmann constant), close to the Bardeen-Cooper-Schrieffer (BCS) prediction of 3.53. The gap decreases with temperature and vanishes at T(c) in a manner consistent with the BCS prediction, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, the t-J model, and the like, originally designed for the high-T(c) copper oxides.

Coexistence of static magnetism and superconductivity in SmFeAsO(1-x)F(x) as revealed by muon spin rotation.

Abstract: In non-conventional superconductors, the competition of magnetic order and superconductivity seems to be a key element for the origin of superconductivity. Investigation of the newly discovered iron-pnictides superconductors challenges this picture, showing a coexistence of superconductivity and magnetism.

Different resistivity response to spin density wave and superconductivity at 20 K in $Ca_{1-x}Na_xFe_2As_2$

Abstract: We report that intrinsic transport and magnetic properties, and their anisotropy from high quality single crystal $CaFe_2As_2$. The resistivity anisotropy ($\rho_c/\rho_{ab}$) is $\sim 50 $, and less than 150 of $BaFe_2As_2$, which arises from the strong coupling along c-axis due to an apparent contraction of about 0.13 nm compared to $BaFe_2As_2$. Temperature independent anisotropy indicates that the transport in ab plane and along c-axis direction shares the same scattering mechanism. In sharp contrast to the case of parent compounds $ROFeAs$ (R=rare earth) and $MFe_2As_2$ (M=Ba and Sr), spin-density-wave (SDW) ordering (or structural transition) leads to a steep increase of resistivity in $CaFe_2As_2$. Such different resistivity response to SDW ordering is helpful to understand the role played by SDW ordering in Fe-based high-$T_c$ superconductors. The susceptibility behavior is very similar to that observed in single crystal $BaFe_2As_2$. A linear temperature dependent susceptibility occurs above SDW transition of about 165 K. Partial substitution of Na for Ca suppresses the SWD ordering (anomaly in resistivity) and induces occurrence of superconductivity at $\sim 20$ K.

Spin Hall effects

Abstract: In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.

Magnetic order close to superconductivity in the iron-based layered LaO1-xFxFeAs systems

Abstract: Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Abstract: The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

Superconductivity in iron compounds

Abstract: Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

High-temperature superconductivity in iron-based materials

Abstract: The surprising discovery of high-temperature superconductivity in a material containing a strong magnet—iron—has led to thousands of publications. By placing all the data in context, it becomes clear what we know and where we are headed.