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Register a new userEvidence for Competition between Superconductivity and Kondo Effect in CeFeAsO0.7F0.3 under High Pressure
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We discover a pressure induced quantum phase transition from the superconducting state to the non-superconducting Kondo screened phase associated with a 2% volume collapse in CeFeAsO0.7F0.3 through measurements of high-pressure resistance, synchrotron x-ray diffraction, and x-ray absorption spectroscopy (XAS) in a diamond anvil cell. Our XAS data of Ce-L3 in CeFeAsO0.7F0.3 clearly show a spectral weight transfer from the main line to the satellite line after the transition, demonstrating the formation of the Kondo singlets under pressure in CeFeAsO1-xFx. Our results have revealed a physical picture of a pressure-induce competition between Kondo singlets and BCS singlets in the Ce-pnictide superconductors.
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Temperature dependence of resistivity under high pressures with magnetic fields parallel and perpendicular to the FeSe planes are measured in FeSe single crystals. It is found that the structural transition (nematic) temperature is suppressed by pressure and ends at around $P$ = 1.18 GPa. Below around 0.85 GPa, the superconducting transition shows a narrow width with no indication of antiferromagnetic order. While above this pressure, the superconducting transition temperature drops slightly forming a small dome of superconducting region with the maximum $T_c$ at around 0.825 GPa. Furthermore, just above this pressure, the superconducting transition exhibits an unusual large transition width which reaches about 6-8 K. This wide transition width is an intrinsic feature and does not change with magnetic field. In the high pressure region above 1.18 GPa, just accompanying the onset of superconducting transition, an upturn of resistivity immediately occurs, which is attributed to the formation of an antiferromagnetic order. This closely attached behavior of superconductivity and antiferromagnetic order indicates that these two orders have a synergy feature. Near the critical pressure 0.825 GPa and below, our data illustrate that an antiferromagnetic order emerges when superconductivity is suppressed. From the weak influence of magnetic field to the antiferromagnetic order, we conclude that it exists already below the small superconducting dome in the low pressure region. This shows a competing feature between superconductivity and antiferromagnetic order. Our results show duality features, namely synergy and competition between superconductivity and antiferromagnetic order under pressure in FeSe.
Electrical resistivity measurements under high pressures up to 29 GPa were performed for oxypnictide compound LaFeAsO. We found a pressure-induced superconductivity in LaFeAsO. The maximum value of Tc is 21 K at ~12 GPa. The pressure dependence of the Tc is similar to those of LaFeAsO1-xFx series reported previously.
SrxBi2Se3 is recently reported to be a superconductor derived from topological insulator Bi2Se3. It shows a maximum resistive Tc of 3.25 K at ambient pressure. We report magnetic (upto 1 GPa) and transport properties (upro 8 Gpa) under pressure for single crystalline Sr0.1Bi2Se3 superconductor. Magnetic measurements show that Tc decreases from ~2.6 K (0 GPa) to ~1.9 K (0.81 GPa). Similar behavior is observed in transport properties as well without much change in the metallic characteristics in normal state resistivity. No reentrant superconducting phase (Physical Review B 93, 144514 (2016)) is observed at high pressure. Normal state resistivity near Tc is explained by Fermi liquid model. Above 100 K, a polaronic hopping conduction mechanism with two parallel channels for current flow is indicated. Band structure calculations indicate decreasing density of states at Fermi level with pressure. In consonance with transition temperature suppression in conventional BCS low Tc superconductors, the pressure effect in SrxBi2Se3 is well accounted by pressure induced band broadening.
The long-sought goal of room-temperature superconductivity has reportedly recently been realized in a carbonaceous sulfur hydride compound under high pressure, as reported by Snider et al. [1]. The evidence presented in that paper is stronger than in other similar recent reports of high temperature superconductivity in hydrides under high pressure [2-7], and has been received with universal acclaim [8-10]. Here we point out that features of the experimental data shown in Ref. [1] indicate that the phenomenon observed in that material is not superconductivity. This observation calls into question earlier similar claims of high temperature conventional superconductivity in hydrides under high pressure based on similar or weaker evidence [2-7].
Pressure effect on superconducting properties of two YB6 samples (Tc = 5.9 and 7.5 K) were investigated by measurements of electrical resistivity, magnetic susceptibility, and X-ray diffraction in the pressure range up to 320 kbar. Magnetoresistivity measurements down to 60 mK and up to 47 kbar have shown a negative pressure effect on Tc as well as on the third critical field Hc3 with the slopes dlnTc/dp = -0.59%/kbar and dlnHc3/dp = -1.1%/kbar, respectively. The magnetic susceptibility measurements evidenced that the slope of dlnTc/dp gradually decreases with pressure reaching 3 times smaller value at 112 kbar. The lattice parameter measurements revealed the volume reduction of 14% at 320 kbar. The pressure-volume dependence is described by the Rose-Vinet equation of state. The obtained relative volume dependence dlnTc/dlnV analyzed by the McMillan formula for Tc indicates that the reduction of the superconducting transition temperature is mainly due to hardening of the Einstein-like phonon mode responsible for the superconducting coupling. This is confirmed by the analysis of the resistivity measurements in the normal state up to T = 300 K performed at pressures up to 28 kbar.
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