scholarly journals Photoinduced possible superconducting state with long-lived disproportionate band filling in FeSe

2019 ◽  
Vol 2 (1) ◽  
Author(s):  
Takeshi Suzuki ◽  
Takashi Someya ◽  
Takahiro Hashimoto ◽  
Shoya Michimae ◽  
Mari Watanabe ◽  
...  
Keyword(s):  
Superconducting State ◽  
Degrees Of Freedom ◽  
Extreme Ultraviolet ◽  
High Harmonic ◽  
Photoemission Spectroscopy ◽  
Iron Based Superconductors ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Dynamical Behaviors ◽  
Band Filling ◽  
Iron Based

Abstract Photoexcitation is a very powerful way to instantaneously drive a material into a novel quantum state without any fabrication, and variable ultrafast techniques have been developed to observe how electron, lattice, and spin degrees of freedom change. One of the most spectacular phenomena is photoinduced superconductivity, and it has been suggested in cuprates that the transition temperature Tc can be enhanced from the original Tc with significant lattice modulations. Here, we show a possibility for another photoinduced high-Tc superconducting state in the iron-based superconductor FeSe. The transient electronic state over the entire Brillouin zone is directly observed by time- and angle-resolved photoemission spectroscopy using extreme ultraviolet pulses obtained from high harmonic generation. Our results of dynamical behaviors from 50 fs to 800 ps consistently support the favourable superconducting state after photoexcitation well above Tc. This finding demonstrates that multiband iron-based superconductors emerge as an alternative candidate for photoinduced superconductors.

scholarly journals Bose-Einstein condensation superconductivity induced by disappearance of the nematic state

2020 ◽  
Vol 6 (45) ◽  
pp. eabb9052
Author(s):  
Takahiro Hashimoto ◽  
Yuichi Ota ◽  
Akihiro Tsuzuki ◽  
Tsubaki Nagashima ◽  
Akiko Fukushima ◽  
...  
Keyword(s):  
Ultracold Atoms ◽  
Degrees Of Freedom ◽  
Photoemission Spectroscopy ◽  
Einstein Condensation ◽  
Energy Bands ◽  
Iron Based Superconductors ◽  
Bose Einstein Condensation ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Nematic State ◽  
Bose Einstein

The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC crossover in iron-based superconductors FeSe1 − xSx from laser-excited angle-resolved photoemission spectroscopy. The system enters the BEC regime with x = 0.21, where the nematic state that breaks the orbital degeneracy is fully suppressed. The substitution dependence is opposite to the expectation for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in this system.

scholarly journals Momentum dependent $$d_{xz/yz}$$ band splitting in LaFeAsO

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
S. S. Huh ◽  
Y. S. Kim ◽  
W. S. Kyung ◽  
J. K. Jung ◽  
R. Kappenberger ◽  
...  
Keyword(s):  
Temperature Dependence ◽  
Nematic Phase ◽  
Photoemission Spectroscopy ◽  
Temperature Dependent ◽  
Band Shift ◽  
Iron Based Superconductors ◽  
Band Splitting ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Important Clue ◽  
Iron Based

Abstract The nematic phase in iron based superconductors (IBSs) has attracted attention with a notion that it may provide important clue to the superconductivity. A series of angle-resolved photoemission spectroscopy (ARPES) studies were performed to understand the origin of the nematic phase. However, there is lack of ARPES study on LaFeAsO nematic phase. Here, we report the results of ARPES studies of the nematic phase in LaFeAsO. Degeneracy breaking between the $$d_{xz}$$ d xz and $$d_{yz}$$ d yz hole bands near the $$\Gamma$$ Γ and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the $$\Gamma$$ Γ and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the $$\Gamma$$ Γ point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo device. Since a momentum dependent splitting behavior has been observed in other iron based superconductors, our observation confirms that the behavior is a universal one among iron based superconductors.

scholarly journals Revealing the intrinsic superconducting gap anisotropy in surface-neutralized BaFe2(As0.7P0.3)2

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Ziming Xin ◽  
Yudi Wang ◽  
Cong Cai ◽  
Zhengguo Wang ◽  
Lei Chen ◽  
...  
Keyword(s):  
Alkaline Earth ◽  
Theoretical Models ◽  
Photoemission Spectroscopy ◽  
Superconducting Gap ◽  
Iron Based Superconductors ◽  
Future Studies ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Gap Anisotropy ◽  
Iron Based ◽  
Potassium Deposition

AbstractAlkaline-earth iron arsenide (122) is one of the most studied families of iron-based superconductors, especially for angle-resolved photoemission spectroscopy. While extensive photoemission results have been obtained, the surface complexity of 122 caused by its charge-non-neutral surface is rarely considered. Here, we show that the surface of 122 can be neutralized by potassium deposition. In potassium-coated BaFe2(As0.7P0.3)2, the surface-induced spectral broadening is strongly suppressed, and hence the coherent spectra that reflect the intrinsic bulk electronic state recover. This enables the measuring of superconducting gap with unpreceded precision. The result shows the existence of two pairing channels. While the gap anisotropy on the outer hole/electron pockets can be well fitted using an s± gap function, the gap anisotropy on the inner hole/electron shows a clear deviation. Our results provide quantitative constraints for refining theoretical models and also demonstrate an experimental method for revealing the intrinsic electronic properties of 122 in future studies.

scholarly journals Importance of $${{d}}_{{{xy}}}$$ orbital and electron correlation in iron-based superconductors revealed by phase diagram for 1111-system

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Tsuyoshi Kawashima ◽  
Shigeki Miyasaka ◽  
Hirokazu Tsuji ◽  
Takahiro Yamamoto ◽  
Masahiro Uekubo ◽  
...  
Keyword(s):  
Electron Correlation ◽  
Weak Coupling ◽  
Structural Parameters ◽  
Structural Flexibility ◽  
Superconducting Transition ◽  
Iron Based Superconductors ◽  
Wide Range ◽  
Band Filling ◽  
Iron Based ◽  
Correlation Strength

AbstractThe structural flexibility at three substitution sites in LaFeAsO enabled investigation of the relation between superconductivity and structural parameters over a wide range of crystal compositions. Substitutions of Nd for La, Sb or P for As, and F or H for O were performed. All these substitutions modify the local structural parameters, while the F/H-substitution also changes band filling. It was found that the superconducting transition temperature $$T_{\text{c}}$$ T c is strongly affected by the pnictogen height $$h_{Pn}$$ h Pn from the Fe-plane that controls the electron correlation strength and the size of the $$d_{xy}$$ d xy hole Fermi surface (FS). With increasing $$h_{Pn}$$ h Pn , weak coupling BCS superconductivity switches to the strong coupling non-BCS one where electron correlations and the $$d_{xy}$$ d xy hole FS may be important.

Angle-Resolved Spectroscopy Study of Ni-Based Superconductor SrNi2P2

2019 ◽  
Vol 295 ◽  
pp. 99-103
Author(s):  
Y. F. Yan
Keyword(s):  
Structural Transition ◽  
Photoemission Spectroscopy ◽  
Temperature Phase ◽  
Spectroscopy Study ◽  
Iron Based Superconductors ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Different Shapes ◽  
Length Reduction ◽  
Low Temperature Phase ◽  
Collapse Phase

We performed an angle-resolved photoemission spectroscopy (ARPES) study of the Ni-based superconductors SrNi2P2. We observe both electron and hole Fermi surface pockets with different shapes and sizes which leads to very poor nesting conditions. Moreover, we observe a band structure reconstruction below the structural transition temperature (325 K), with bands shifting downwards and one extra hole-like band appearing around Г. These behaviors might be attributed to the length reduction of one third of P-P bonds between the adjacent NiP layers. The low temperature phase in SrNi2P2 can be regarded as a partially collapse phase. Our result may facilitate understanding the collapsed behavior which is important to unveil superconductivity mechanism in iron-based superconductors.

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