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Register a new userSynergy and competition between superconductivity and antiferromagnetism in FeSe under pressure
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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.
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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.
A huge enhancement of the superconducting transition temperature Tc was observed in tetragonal FeSe superconductor under high pressure. The onset temperature became as high as 27 K at 1.48 GPa and the pressure coefficient showed a huge value of 9.1 K/GPa. The upper critical field Hc2 was estimated to be ~ 72 T at 1.48 GPa. Because of the high Hc2, FeSe system may be a candidate for application as superconducting wire rods. Moreover, the investigation of superconductivity on simple structured FeSe may provide important clues to the mechanism of superconductivity in iron-based superconductors.
Magnetism induced by external pressure ($p$) was studied in a FeSe crystal sample by means of muon-spin rotation. The magnetic transition changes from second-order to first-order for pressures exceeding the critical value $p_{{rm c}}simeq2.4-2.5$ GPa. The magnetic ordering temperature ($T_{{rm N}}$) and the value of the magnetic moment per Fe site ($m_{{rm Fe}}$) increase continuously with increasing pressure, reaching $T_{{rm N}}simeq50$~K and $m_{{rm Fe}}simeq0.25$ $mu_{{rm B}}$ at $psimeq2.6$ GPa, respectively. No pronounced features at both $T_{{rm N}}(p)$ and $m_{{rm Fe}}(p)$ are detected at $psimeq p_{{rm c}}$, thus suggesting that the stripe-type magnetic order in FeSe remains unchanged above and below the critical pressure $p_{{rm c}}$. A phenomenological model for the $(p,T)$ phase diagram of FeSe reveals that these observations are consistent with a scenario where the nematic transitions of FeSe at low and high pressures are driven by different mechanisms.
The pressure dependence of the structural ($T_s$), antiferromagnetic ($T_m$), and superconducting ($T_c$) transition temperatures in FeSe is investigated on the basis of the 16-band $d$-$p$ model. At ambient pressure, a shallow hole pocket disappears due to the correlation effect, as observed in the angular-resolved photoemission spectroscopy (ARPES) and quantum oscillation (QO) experiments, resulting in the suppression of the antiferromagnetic order, in contrast to the other iron pnictides. The orbital-polarization interaction between the Fe $d$ orbital and Se $p$ orbital is found to drive the ferro-orbital order responsible for the structural transition without accompanying the antiferromagnetic order. The pressure dependence of the Fermi surfaces is derived from the first-principles calculation and is found to well account for the opposite pressure dependences of $T_s$ and $T_m$, around which the enhanced orbital and magnetic fluctuations cause the double-dome structure of the eigenvalue $lambda$ in the Eliashberg equation, as consistent with that of $T_c$ in FeSe.
We offer an explanation for the recently observed pressure-induced magnetic state in the iron-chalcogenide FeSe based on textit{ab initio} estimates for the pressure evolution of the most important Coulomb interaction parameters. We find that an increase of pressure leads to an overall decrease mostly in the nearest-neighbor Coulomb repulsion, which in turn leads to a reduction of the nematic order and the generation of magnetic stripe order. We treat the concomitant effects of band renormalization and the induced interplay of nematic and magnetic order in a self-consistent way and determine the generic topology of the temperature-pressure phase diagram, and find qualitative agreement with the experimentally determined phase diagram.
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