Onset of the Meissner effect at 65 K in Fe Se thin film grown on Nb-doped Sr TiO_3 substrate

We report the Meissner effect studies on an Fe Se thin film grown on Nb-doped Sr Ti O3 substrate by molecular beam epitaxy. Two-coil mutual inductance measurement clearly demonstrates the onset of diamagnetic screening at 65 K, which is consistent with the gap opening temperature determined by previous angle-resolved photoemission spectroscopy results. The applied magnetic field causes a broadening of the superconducting transition near the onset temperature, which is the typical behavior for quasi-two-dimensional superconductors. Our results provide direct evidence that Fe Se thin film grown on Nb-doped Sr Ti O3 substrate has an onset TC* 65 K,which is the highest among all iron-based superconductors discovered so far.

Observations of abundant structural and electronic phases in potassium-doped single-layer p-quaterphenyl film

Scanning tunneling microscopy/spectroscopy is applied herein to study the pristine and potassium(K)-doped single-layer pquaterphenyl(P4P) films grown on the Au(111) substrate at the molecular level. Abundant complex structural and electronic phases are induced by various K doping. The Fermi-level pinning effect is observed at a low doping level. On the contrary,K3P4P exhibits intriguing versatile phases and properties because charge carriers are effectively doped in. For example, two kinds of molecular vibration modes with energies below 100 meV are observed, indicating a possible strong electron-phonon coupling. The splitting of the lowest unoccupied molecular orbital state in K3P4P illustrates an electronic correlation effect, and its strength varies for four different K3P4P phases with different structures. In addition, the appearance of a Kondo resonance on the molecular vacancy/impurity implies a local molecular magnetic moment. Our results demonstrate that the complex electronic properties of an alkali metal-doped P4P/Au film stem from the existence of many competing interactions, such as electronelectron correlations and electron-vibration coupling, which can be effectively tuned via variable carrier doping and molecular structure. Our work also opens new routes toward engineering novel molecular devices and creating new electronic phases in strongly correlated molecular materials.

Two-impurity Kondo effect in potassium-doped single-layer p-sexiphenyl films

Herein, we show that a self-assembled phase of potassium(K)-doped single-layer para-sexiphenyl(PSP) film on a gold substrate is an excellent platform for studying the two-impurity Kondo model. On K-doped PSP molecules well separated from others, we observe a Kondo resonance peak close to EFwith a Kondo temperature of 30 K. The Kondo resonance peak splits when another K-doped PSP molecule is present in the vicinity, and the splitting gradually increases with the decrease in intermolecular distance without signs of phase transition. Our data demonstrate how a Kondo singlet state gradually evolves into an antiferromagnetic singlet state due to the competition between Kondo screening and antiferromagnetic Ruderman-Kittel-Kasuya-Yosida coupling,as described in the two-impurity Kondo model. Intriguingly, the antiferromagnetic singlet is quickly destroyed on increasing temperature and transforms back to a Kondo singlet below the Kondo temperature. Our data provide a comprehensive picture and quantitative constraints on related theories and calculations of the two-impurity Kondo model.

Nodeless superconducting gaps in Ca10(Pt(4-δ)As8)((Fe(1-x)Ptx)2As2)5 probed by quasiparticle heat transport

The in-plane thermal conductivity of the iron-based superconductor Ca10(Pt_(4-δ)As_8)((Fe_(1-x)Pt_x)_2As_2)_5 single crystal("10-4-8", T c= 22 K) was measured down to 80 m K. In a zero field, the residual linear term κ_0/T is negligible, suggesting the nodeless superconducting gaps in this multiband compound. In the magnetic fields, κ_0/T increases rapidly, which mimics the multiband superconductor Nb Se_2 and Lu Ni_2B_2 C with highly anisotropic gap. Such a field dependence of κ_0/T is an evidence for the multiple superconducting gaps with quite different magnitudes or highly anisotropic gap. Compared with the London penetration depth results of the Ca10(Pt_(4-δ)As_8)((Fe_(1-x)Pt_x)_2As_2)_5("10-3-8") compound, the 10-4-8 and 10-3-8 compounds may have a similar superconducting gap structure.

Carrier balance and linear magnetoresistance in type-II Weyl semimetal WTe2

Unsaturated magnetoresistance （MR） has been reported in type-II Weyl semimetal WTe2, manifested as a perfect compensation of opposite carriers. We report linear MR （LMR） in WTe2 crystals, the onset of which was identified by constructing the MR mobility spectra for weak fields. The LMR further increased and became dominant for fields stronger than 20 T, while the parabolic MR gradually decayed. The LMR was also observed in high-pressure conditions.