arXiv subjects

Jiangang Guo

Publications and source records attributed to Jiangang Guo.

30 records · Page 2Linked to original sources

arXiv2017

Superconductivity in alkaline earth metal-filled skutterudites BaxIr4X12 (X = As, P)

We report superconductive iridium pnictides BaxIr4X12 (X = As and P) with a filled skutterudite structure, demonstrating that Ba filling dramatically alters their electronic properties and induces a nonmetal-to-metal transition with increasing the Ba content x. The highest superconducting transition temperatures are 4.8 and 5.6 K observed for BaxIr4As12 and BaxIr4P12, respectively. The superconductivity in BaxIr4X12 can classified into the Bardeen-Cooper-Schrieffer type with intermediate coupling.

cond-mat.supr-con↗

arXiv2017

Spin excitations and the Fermi surface of superconducting FeS

High-temperature superconductivity occurs near antiferromagnetic instabilities and nematic state. Debate remains on the origin of nematic order in FeSe and its relation with superconductivity. Here, we use transport, neutron scatter- ing and Fermi surface measurements to demonstrate that hydro-thermo grown superconducting FeS, an isostructure of FeSe, is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Only stripe-type spin excitation is observed up to 100 meV. No direct coupling between spin excitation and superconductivity in FeS is found, suggesting that FeS is less correlated and the nematic order in FeSe is due to competing checkerboard and stripe spin fluctuations.

cond-mat.supr-con↗

arXiv2015

Narrow Bandgap in beta-BaZn2As2 and Its Chemical Origins

Beta-BaZn2As2 is known to be a p-type semiconductor with the layered crystal structure similar to that of LaZnAsO, leading to the expectation that beta-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of beta-BaZn2As2 (previously-reported value ~0.2 eV) is one order of magnitude smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap value of beta-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of the tem-perature dependence of electrical conductivity. The origins of this narrow bandgap are discussed based on the chemi-cal bonding nature probed by 6 keV hard X-ray photoemission spectroscopy, hybrid density functional calculations, and the ligand theory. One origin is the direct As-As hybridization between adjacent [ZnAs] layers, which leads to a secondary splitting of As 4p levels and raises the valence band maximum. The other is that the non-bonding Ba 5dx2-y2 orbitals form unexpectedly deep conduction band minimum (CBM) in beta-BaZn2As2 although the CBM of LaZnAsO is formed mainly of Zn 4s. These two origins provide a quantitative explanation for the bandgap difference between beta-BaZn2As2 and LaZnAsO.

physics.chem-ph↗

arXiv2014

Emergence of magnetism and controlling factors of superconductivity in Li/Na-ammonia co-intercalated FeSe1-zTez

The discovery of superconductivity in alkali-ammonia co-intercalated FeSe has generated intensive interest because of their highest Tc (~ 45 K) among iron-chalcogenide superconductors with bulk form. Here, we report the phase diagrams of two series of Li/Na-ammonia co-intercalated FeSe1-zTez. When superconductivity is suppressed by Te doping, the magnetic ordering states are emergent. Moreover, a novel phase Lix(NH3)yFe2-δTe2 with possible antiferromagnetism is discovered. This strongly indicates the intimate relation between superconductivity and magnetism in these materials, like in other iron-based superconductors. On the other hand, comparative structure analysis with FeSe1-zTez suggests that although there is remarkable similarity in phase diagrams for both iron-chalcogenide and iron-pnictide superconductors, the different types of anions with different charges lead to the dissimilar controlling factors in superconductivity for both classes of materials. This opens up an opportunity to tune the superconductivity in iron-chalcogenide superconductors in more ways than just one.

cond-mat.supr-con↗

arXiv2014

Superconductivity and phase instability of NH3-free Na-intercalated FeSe1-zSz

The discovery of ThCr2Si2-type AxFe2-ySe2 (A = K, Rb, Cs and Tl) with Tc ~ 30K make much progress in iron-based superconducting field, but their multiple-phase separations are disadvantageous for understanding the origin. On the other hand, for small alkali metals, studies on (Li,Na)FeCu(S,Se)2 and NaFe2-δS2 shows that these compounds possess CaAl2Si2-type structure, implying that ThCr2Si2-type structure is unstable for small alkali-metal intercalated FeSe under high-temperature. Here we report a new intercalate Na0.65(1)Fe1.93(1)Se2 with Tc ~ 37 K, synthesized by low-temperature ammonothermal method. The notable finding is that the Na0.65(1)Fe1.93(1)Se2 shows a ThCr2Si2-type structure, which is the first instance of small-sized alkali metal intercalates without NH3 co-intercalation. Besides, the NH3-poor Na0.80(4)(NH3)0.60Fe1.86(1)Se2 and NH3-rich phase with Tcs at 45 K and 42 K are identified by tuning the concentration of Na-NH3 solutions. The modulation of interlayer spacing reveals the versatile evolution of structural stability and superconductivity in these intercalates.

cond-mat.supr-con↗

arXiv2014

Layered Compounds BaM2Ge4Ch6 (M = Rh, Ir and Ch = S, Se) with Pyrite-Type Building Blocks and Ge-Ch Heteromolecule-Like Anions

Structure and chemical features of layered compounds BaM2Ge4Ch6 (M = Rh, Ir; Ch = S, Se) synthesized under high-pressure and high-temperature method are systematically studied. These compounds crystallize in an orthorhombic phase with space group Pbca (No. 61). The remarkable structural feature is to have the M-Ge-Ch pyrite-type building units, stacking with Ba-Ch layers alternatively along c axis. It is very rare and novel that pyrite-type subunits as the building blocks in the layered compounds. Theoretical calculations and experimental results indicate that there are strongly polarized covalent bonds between Ge and Ch atoms, forming heteromolecule-like anions in these compounds. Moreover, Ge atoms in this structure exhibit unusual valence state (~ +1) due to the tetrahedral coordination environment of Ge atoms along with M and Ch atoms simultaneously.

cond-mat.mtrl-sci↗

arXiv2012

Tc Maximum in Solid Solution of Pyrite IrSe2-RhSe2 Induced by Destabilization of Anion Dimers

We have established a well-defined dome shape Tc curve in Ir0.94-xRhxSe2 superconductors. The maximum Tconset of 9.6K was obtained at x=0.36, at which the Se-Se separation in the dimer anion is the longest. Simultaneously, the destabilization of Se-Se dimers accompanied by partial elec-tron-transfer from the Ir/Rh to the chalcogenide ions result-ed in the emergence of optimal Tc.

cond-mat.supr-con↗

arXiv2012

Superconductivity in defective pyrite-type iridium chalcogenides IrxCh2 (Ch = Se and Te)

We report superconductivity in defective pyrite-type iridium chalcogenides IrxCh2. Maximum values of Tc of 6.4 K for Ir0.91Se2 and 4.7 K for Ir0.93Te2 were observed. It was found that Ir0.75Ch2 is close to the boundary between metallic and insulating states and IrxCh2 systems undergo nonmetal to metal transitions as x increases. On the basis of density functional theory calculations and the observed large variation in the Ch-Ch distance with x, we suggest that Ir0.75Ch2 is the parent compound for the present superconductors.

cond-mat.supr-con↗

arXiv2012

Pressure-Driven Quantum Criticality in An Iron-Selenide Superconductor

The discovery of superconductivity of about 30 K in iron selenides with very large magnetic moments simulates the examination of completing orders. Here we report a finding of pressure- induced suppression of the superconducting transition temperature Tc and enhancement of the temperature of the resistance hump TH through charge transfer between two iron sites with different occupancies. The activation energy for the electric transport of the high-temperature resistance is observed to go to zero at a critical pressure of 8.7 GPa, at which superconductivity tends to disappear and the semiconductor-to-metal transition takes place. Beyond the critical point, the resistance exhibits a metallic behavior over the whole temperature range studied. All these features indicate the existence of quantum criticality in iron-selenide superconductors.

cond-mat.supr-con↗

arXiv2011

Superconductivity on the verge of Mott localization in ternary iron sulfide

We report the results of electrical and magnetic properties on two new compounds, K0.8Fe1.7S2 and K0.8Fe1.7SeS, both having similar structures to newly discovered superconducting K0.8Fe1.7Se2. K0.8Fe1.7S2 exhibits a semiconductor-like electrical property and undergoes an anti-ferromagnetic transition at about 260 K. Upon replacing half of S with Se, K0.8Fe1.7SSe becomes a superconductor at 25 K, implying the superconductivity evolves from a Mott AFM state in Fe-Se based superconductors.

cond-mat.supr-con↗

arXiv2011

Quenching of superconductivity by Co doping in K0.8Fe2Se2

We synthesized a series of K0.8Fe2-xCoxSe2 samples with nominal compositions 0\leq x\leq 0.035 and investigated their physical properties. The results show that the superconductivity in K0.8Fe2-xCoxSe2 is quenched down to 5 K by 0.5 at. % Co doping, the fastest quenching rate ever-reported. The role played here by Co is in contrast with the one in FeAs based superconductors where Co usually induces superconductivity from parent compounds. Such a rapid quenching favors a localized 3d model against the itinerant one for iron pnictide superconductors.

cond-mat.supr-con↗

arXiv2010

Superconductivity in the iron selenide KxFe2Se2 (0 <= x <= 1)

We report the superconductivity at above 30 K in a new FeSe-layer compound K0.8Fe2Se2 (nominal composition) achieved by metal K intercalating in between FeSe layers. It is isostructural to BaFe2As2 and possesses the highest Tc for FeSe-layer materials so far under ambient pressure. Hall effect indicates the carriers are dominated by electron in this superconductor. We confirm that the observed superconductivity at above 30 K is due to this new FeSe-based 122 phase. Our results demonstrate that FeSe-layer materials are really remarkable superconductors via structure and carrier modulation.

cond-mat.supr-con↗