1. Iron-based superconductor
Pressure tuned superconductivity and normal state behavior in Ba(Fe0.943Co0.057)2As2
A quantum critical point (QCP) is the point of transitionbetween two stable phases at absolute zero temperature driven by quantumfluctuations. A QCP can be achieved by tuning a magnetic order to a zerotemperature transition by applying pressure, magnetic field or chemical doping.In iron-based superconductors, it is a matter of debate in iron-basedsuperconductors whether a QCP is hidden inside the superconducting dome. Controversialresults are obtained, for example, a sharp peak of the zero-temperaturepenetration depth at optimal composition is observed in BaFe2(As1-xPx)2,suggesting the existence of a QCP at x = 0.3. While neutron scatteringmeasurements on Ba(Fe1-xNix)2As2observed a first-order-like antiferromagnetism to superconductivity transition,suggests the absence of a quantum critical point.
In this work, we probe the phase diagram of Ba(Fe1-xCox)2As2 closeto the antiferromagnetic boundary through measurements of resistivity andmagnetization by tuning the applied pressure in a sample with x = 0.057.Wefound that the resistivity shows a linear temperature dependence around acritical pressure of 3.5 GPa where Tc is maximum. In addition, theresidual resistivity and the resistivity at Tc all change around thesame critical pressure. Furthermore, we detected signs of an accompanied changein the superconducting volume. These results are most likely due to a possiblepressure tuned QCP hidden inside the superconducting dome of Ba(Fe1-xCox)2As2.In addition, we also performed I-V measurements on Ba(Fe1-xCox)2As2with various Co doping. Considerably enhanced flux-flow resistivity ρffwas detected for x = 0.06, perhaps due to enhancement of spin fluctuations nearQCP.
FigureTemperature-pressure (T-P) phase diagram of Ba(Fe1-xCox)2As2
Phys. Rev. B 97, 144515 (2018)
Optical study ofDirac fermions in antiferromagnetic compound CaFeAsF
The parentantiferromagnetic phase of the iron-based superconductors has a metalliccharacter and possesses nontrivial topological properties with Dirac fermionsnear to the Fermi energy. The observation of Dirac fermion state with protectedDirac cones in the parent compound of iron-based superconductor provide us anew kind of topological material, after cuprates, graphene, topologicalinsulators and Weyl semimetals.
CaFeAsF is anew member of 1111 family of iron-based superconductor, but with oxygen free.The optical conductivity probes the bulk band structure and carrierelectrodynamics over a broad energy range and provides rich information on theintraband and interband transitions of possible Dirac cones. Recently theavailability of sizable single crystals allows for an optical study of theintrinsic properties of this material.
In the SDWstate of CaFeAsF, we have observed a singular absorption feature and tworegions of quasilinear conductivity that have been interpreted in terms of theresponse of Dirac fermions. Note that such features disappear rapidly as afunction of electron and hole doping in the BaFe2As2system. In addition, we found in CaFeAsF that the infrared-active Fe-As phononmode shows signatures of a strong coupling with the Dirac fermions that appearin the SDW state. Very recently, the existence of Dirac fermions in CaFeAsF hasbeen confirmed by quantum oscillation measurements, Phys. Rev. X 8, 011014(2018).
FigureOptical conductivity of CaFeAsF after subtraction of the Drude and phonon termsin the antiferromagnetic state.
Phys. Rev. B 97, 195110(2018)
Pressure-inducedsuperconductivity in parent CaFeAsF single crystals
CaFeAsF parent compoundexhibits pressure-induced superconductivity and the maximumTc is higher than that inLaFeAsO, making it a promising candidate as a parent compound for high Tc superconductor. However,up to now, the phase diagram of this material is based on data of polycrystalsamples and controversial results are obtained. Here, for the first time, weperformed high pressure resistivity measurements on single crystal sample ofCaFeAsF parent compound up to about 50 GPa and we determined the change ofstructural and magnetic phase transition temperature and the superconductingtemperature with pressure. In strongly contrast to 122 family of iron-basedsuperconductor, the SC of CaFeAsF is quite robust under pressure and shows nodome-shaped behavior, which is coincident with reported monoclinic phase. Thesefindings provide valuable insights to understand the interplay between thestructure and the appearance of SC in iron-based superconductors.
Figure The pressure-temperature phase diagram ofCaFeAsF.
Phys. Rev. B 97, 174505(2018)
2. Heavy fermionsuperconductor
Universal linear-temperature resistivity: possible quantum diffusiontransport in strongly correlated superconductors
What underlies the linear resistivity in stronglycorrelated superconductors is a well known, intriguing and long-standingquestion in condensed matter physics. Based on the pressure tuned resistivitydata of heavy fermion superconductor CeCoIn5, and combined with datareported on all kinds of superconductors available in the literature, we founda universal relation
, which bridges the slope of the linear-T-dependent resistivity (dρ/dT) to the London penetration depth λLat zero temperature. In this work, we investigate nearly 70 differentsuperconductors, including cuprates, pnictides, heavy fermiion superconductors,and conventional metal superconductors as well. The combined data span nearlysix orders of magnitude. This scaling relation holds for cuprate, pnictide andheavy fermion superconductors as well, regardless of the significantdifferences in the strength of electronic correlations, transport directions,and doping levels.
Our analysis suggest the ubiquitous presence of the quantum diffusion in superconductors, whichnot only causes the scaling behavior of linear-temperature resistivity observedin this paper but also is responsible for those well-known scaling relations ofsuperconductors, like Uemura’s law and Homes’ law. The experimental finding ofquantum diffusionin superconductors indicates a fundamental inadequacy of previous understanding of superconductivity.
Figure Log-log plot of 
vs.
for various strongly correlated superconductors.
Sci. Rep. 7, 9469 (2017)