Quasiparticle Excitations in the Superconducting State of FeSe Probed by Thermal Hall Conductivity in the Vicinity of the BCS–BEC Crossover

Watashige, Tatsuya; Arsenijević, Stevan; Yamashita, Takuya; Terazawa, Daiki; Onishi, Toshitake; Opherden, L.; Kasahara, S.; Tokiwa, Yutaka; Shibauchi, T.; Löhneysen, H. v.; Wosnitza, J.; Matsuda, Yuji

doi:10.7566/jpsj.86.014707

Cited by 31 publications

(38 citation statements)

References 54 publications

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“…3(a) are also illustrated as a contour plot in the phase diagram. The H-induced unconventional SC B phase reported by Kasahara 11) and Watashige 21) is also shown in the same figure. It seems that H anom is linked to the A − B phase transition line in the SC state.…”

Section: )supporting

confidence: 72%

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by ⁷⁷Se-NMR

et al. 2018

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We conducted77 Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature (T1T ) −1 increases below the structural transition temperature Ts but starts to be suppressed below T * , well above the superconducting transition temperature Tc(H), resulting in a broad maximum of (T1T ) −1 at Tp(H). This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because T * and Tp(H) decrease in the same manner as Tc(H) with increasing H, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above Tc(H).The discovery of superconductivity in iron pnictide 1)provided new research systems in which the unconventional superconductivity realized in strongly correlated electron compounds can be studied. Among the Fe-based superconductors (FeSCs), FeSe, which exhibits superconductivity at T c ∼ 9 K, has the simplest crystalline structure, which is called the 11 structure,2) but it exhibits several properties unlike those of other FeSCs. A tetragonal-to-orthorhombic structural transition occurs at T s ∼ 90 K without long-range antiferromagnetic (AFM) ordering down to T c .3) This is in stark contrast with the other FeSCs, such as the 122 and 1111 systems, where static AFM ordering was observed at or slightly below T s . 4,5) We reported that the 77 Se nuclear magnetic resonance (NMR) spectrum splits below T s and that the nuclear spin-lattice relaxation rate divided by T , (T 1 T ) −1 , of 77 Se is enhanced below T s . 6) These results suggest that orbital ordering induces the electronic nematic state below T s and also triggers the development of AFM fluctuations with stripe correlations breaking the C 4 symmetry in FeSe.6, 7) This scenario is consistent with the results of angle-resolved photoemission spectroscopy, 8,9) but it seems to be inconsistent with the AFM-fluctuation-driven nematic scenario.10) In any case, FeSe is an ideal system for studying the origin of the electronic nematic state in FeSCs.It was recently suggested that the superconductivity in FeSe may be in the Bardeen-Cooper-Schrieffer (BCS)-Bose-Einstein condensate (BEC) crossover regime, as an extremely small Fermi energy comparable to the superconducting (SC) condensation energy was revealed by several measurements.11, 12) In a schematic phase diagram of the attractive Hubbard model, the interaction between two fermions (e.g., electrons in solids) is in the weak-coupling regime in the BCS state and in the strongcoupling regime in the BEC state. The characteristic pa-

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Section: )supporting

confidence: 72%

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by ⁷⁷Se-NMR

et al. 2018

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“…Typically, a BCS superconductor has many thousands of pairs overlapping within V coh , whereas a BEC superconductor has less than one pair in V coh and there is no interaction between different pairs. With a number of 25 pairs in V coh , sample 4 is similar to FeSe (with 31 pairs [54]), which is considered to reside within the BCS-BEC crossover regime [55][56][57]. These findings therefore suggest that this new breed of semi-metals is a good playground for the observation of possible exotic superconductivity on the BCS-BEC boundary.…”

Section: Bcs-bec Crossover Regimementioning

confidence: 81%

Two- and Three-Dimensional Superconducting Phases in the Weyl Semimetal TaP at Ambient Pressure

et al. 2020

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The motivation to search for signatures of superconductivity in Weyl semi-metals and other topological phases lies in their potential for hosting exotic phenomena such as nonzero-momentum pairing or the Majorana fermion, a viable candidate for the ultimate realization of a scalable quantum computer. Until now, however, all known reports of superconductivity in type-I Weyl semi-metals have arisen through surface contact with a sharp tip, focused ion-beam surface treatment or the application of high pressures. Here, we demonstrate the observation of superconductivity in single crystals, even an as-grown crystal, of the Weyl semi-metal tantalum phosphide (TaP), at ambient pressure. A superconducting transition temperature, T c , varying between 1.7 and 5.3 K, is observed in different samples, both as-grown and microscopic samples processed with focused ion beam (FIB) etching. Our data show that the superconductivity present in the as-grown crystal is inhomogeneous yet three-dimensional. For samples fabricated with FIB, we observe, in addition to the three-dimensional superconductivity, a second superconducting phase that resides on the sample surface. Through measurements of the characteristic fields as a function of temperature and angle, we are able to confirm the dimensionality of the two distinct superconducting phases.

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“…The iron based superconductors provided new opportunities to this route. For instance, in FeSe, the low E F results in /E F = 0.3 for the hole pocket (FeSe-h) and 0.83 -1 for the electron pocket (FeSe-e) [17,18], closer to the unitary point than cuprates [ Fig. 2 (a)].…”

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confidence: 99%

Superconductivity of the FeSe/SrTiO₃ Interface in View of BCS–BEC Crossover^*

Zhang

Miao

Guan

et al. 2019

Chinese Phys. Lett.

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In paired Fermi systems, strong many-body effects exhibit in the crossover regime between the Bardeen-Cooper-Schrieffer (BCS) and the Bose-Einstein condensation (BEC) limits. The concept of the BCS-BEC crossover, which is studied intensively in the research field of cold atoms, has been extended to condensed matters. Here, by analyzing the typical superconductors within the BCS-BEC phase diagram, we find that FeSe-based superconductors are prone to shift their positions in the BCS-BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconducting gap sizes are comparable. Especially at the interface of a single-layer FeSe on SrTiO 3 substrate, the superconductivity is relocated closer to the crossover unitary than other doped FeSe-based materials, indicating that the pairing interaction is effectively modulated. We further show that hole-doping can drive the interfacial system into the phase with possible prepaired electrons, demonstrating its flexible tunability within the BCS-BEC crossover regime.Based on the assumption of Fermi systems with attractive interaction, Bardeen-Cooper-Schrieffer (BCS) theory [1,2] has been well applied not only to electronic systems, but also applicable to other Fermi systems, such as the superfluid state of paired Fermionic atoms [3]. Accordingly the BCS picture can be linked to the concept of Bose-Einstein condensation (BEC), as demonstrated in experiments by directly tuning the attractive interaction or scattering length of ultra-cold atoms [4][5][6][7]. In this context, the

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Quasiparticle Excitations in the Superconducting State of FeSe Probed by Thermal Hall Conductivity in the Vicinity of the BCS–BEC Crossover

Cited by 31 publications

References 54 publications

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by ⁷⁷Se-NMR

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by ⁷⁷Se-NMR

Two- and Three-Dimensional Superconducting Phases in the Weyl Semimetal TaP at Ambient Pressure

Superconductivity of the FeSe/SrTiO₃ Interface in View of BCS–BEC Crossover^*

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Quasiparticle Excitations in the Superconducting State of FeSe Probed by Thermal Hall Conductivity in the Vicinity of the BCS–BEC Crossover

Cited by 31 publications

References 54 publications

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by 77Se-NMR

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by 77Se-NMR

Two- and Three-Dimensional Superconducting Phases in the Weyl Semimetal TaP at Ambient Pressure

Superconductivity of the FeSe/SrTiO3 Interface in View of BCS–BEC Crossover*

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Product

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Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by ⁷⁷Se-NMR

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by ⁷⁷Se-NMR

Superconductivity of the FeSe/SrTiO₃ Interface in View of BCS–BEC Crossover^*