Thermal Conductivity of the Iron-Based Superconductor FeSe: Nodeless Gap with a Strong Two-Band Character

Bourgeois-Hope, Patrick; Chi, Shun; Bonn, D. A.; Liang, Ruixing; Hardy, W. N.; Wolf, Th.; Meingast, C.; Doiron-Leyraud, N.; Taillefer, Louis

doi:10.1103/physrevlett.117.097003

Cited by 59 publications

(88 citation statements)

References 31 publications

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“…However, in FeSe thick films, a V-shaped spectrum has been observed suggesting the d-wave gap structure [14]. Thermal conductivity measurement has been carried out which suggests a nodeless gap but with a very small gap minimum [15]. In this paper, we report low temperature specific heat on the FeSe single crystal.…”

Section: Subject Areas: Condensed Matter Physics Superconductivitymentioning

confidence: 67%

“…The STS measurements [12,14] suggest that there might be nodes in this system, while the thermal conductivity measurement [15] and the specific heat measurement [20] support a nodeless gap. In order to obtain more information about the gap structure of FeSe, we use BCS formula to fit the electronic specific heat in supercon- ducting state, the formula based on BCS theory is shown below,…”

Section: B Measurement On Specific Heat and Fitting With Different Gmentioning

confidence: 85%

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Highly anisotropic superconducting gaps and possible evidence of antiferromagnetic order in FeSe single crystals

et al. 2017

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Specific heat has been measured in FeSe single crystals down to 0.414 K under magnetic fields up to 16 T. A sharp specific heat anomaly at about 8.2 K is observed and is related to the superconducting transition. Another jump of specific heat is observed at about 1.08 K which may either reflect an antiferromagnetic transition of the system or a superconducting transition arising from Al impurity. We would argue that this anomaly in low temperature region may be the long sought antiferromagnetic transition in FeSe. Global fitting in wide temperature region shows that the models with a single contribution with isotropic s-wave, anisotropic s-wave, and d-wave gap all do not work well, nor the two isotropic s-wave gaps. We then fit the data by a model with two components in which one has the gap function of ∆0(1 + αcos2θ). To have a good global fitting and the entropy conservation for the low temperature transition, we reach a conclusion that the gap minimum should be smaller than 0.15 meV (α = 0.9 to 1), indicating that the superconducting gap(s) are highly anisotropic. Our results are very consistent with the gap structure derived recently from the scanning tunneling spectroscopy measurements and yield specific heat contributions of about 32% weight from the hole pocket and 68% from the electron pockets. Subject Areas: Condensed Matter Physics, SuperconductivityThe iron-selenium (FeSe) is one of the iron based superconductors with the simplest structure [1]. The later effort in enhancing the superconducting transition temperature from about 8.5 K to 37 K by pressure was encouraging [2]. However, the interest has been revived recently by exploring the phase diagram under pressure [3][4][5]. In the normal state under ambient pressure, there is a structural transition from tetragonal to orthorhombic at about 90 K [6]. This transition has been proved to be accompanied by the formation of the nematic electronic state [7]. In contrast, however, there is no evidence of antiferromagnetic long range order found in the system although the normal state is dominated by very strong spin fluctuations [8]. This may help to resolve the disputes about the origin of the nematicity [9]. Under pressure, together with the enhancement of superconducting transition temperature, an antiferromagnetic (AF) order appears [3][4][5]. However, it remains unknown how the AF order extends to the superconducting state in the low pressure region. It might be possible that this AF order is hidden under the superconducting dome and most measurements have been undertaken above the possible AF transition temperature. Angle resolved photoemission (ARPES) [10,11] and scanning tunneling spectroscopy (STS) [12] have revealed the existence of both electron and hole pockets with very small area, showing the approximate semi-metal behavior. The closeness of the band edge to the Fermi energy, or the comparable scales of the Fermi energy E F and the superconducting gap ∆, suggests a possible BCS-BEC crossover [13]. The pairing order parameter has been detected...

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Section: Subject Areas: Condensed Matter Physics Superconductivitymentioning

confidence: 67%

Section: B Measurement On Specific Heat and Fitting With Different Gmentioning

confidence: 85%

Highly anisotropic superconducting gaps and possible evidence of antiferromagnetic order in FeSe single crystals

et al. 2017

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“…Recently, a monolayer of FeSe grown on SrTiO 3 has even shown a sign of T c over 100 K [5,6]. FeSe manifests some intriguing properties, including a nematic state without long-range magnetic order [7], crossover from Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein-condensation (BEC) [8], and a Diraccone-like state [9][10][11], which are crucial to understanding high-T c superconductivity.Efforts have been made to elucidate FeSe's gap structure, and the presence of nodes [8,12] or deep minima [13][14][15][16] have been proposed. Even if the multi-gap structure is established [17,18], gap nodes or minima must still be located in the corresponding bands.…”

mentioning

confidence: 99%

“…Efforts have been made to elucidate FeSe's gap structure, and the presence of nodes [8,12] or deep minima [13][14][15][16] have been proposed. Even if the multi-gap structure is established [17,18], gap nodes or minima must still be located in the corresponding bands.…”

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confidence: 99%

Gap structure of FeSe determined by angle-resolved specific heat measurements in applied rotating magnetic field

et al. 2017

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Quasiparticle excitations in FeSe were studied by means of specific heat (C) measurements on a high-quality single crystal under rotating magnetic fields. The field dependence of C shows threestage behavior with different slopes, indicating the existence of three gaps (∆1, ∆2, and ∆3). In the low-temperature and low-field region, the azimuthal-angle (φ) dependence of C shows a fourfold symmetric oscillation with sign change. On the other hand, the polar-angle (θ) dependence manifests as an anisotropy-inverted two-fold symmetry with unusual shoulder behavior. Combining the angle-resolved results and the theoretical calculation, the smaller gap ∆1 is proved to have two vertical-line nodes or gap minima along the kz direction, and is determined to reside on the electron-type ε band. ∆2 is found to be related to the electron-type δ band, and is isotropic in the ab-plane but largely anisotropic out of the plane. ∆3 residing on the hole-type α band shows a small out-of-plane anisotropy with a strong Pauli-paramagnetic effect.Superconducting (SC) gap structures are intimately related to the pairing mechanism, which is pivotal for high-temperature superconductors. This issue is crucial for FeSe because of the unexpectedly high superconducting temperature, T c , in this system. Although the initial T c is below 10 K [1], it can be easily enhanced to 37 K under pressure, [2,3] and to over 40 K by intercalating spacer layers [4]. Recently, a monolayer of FeSe grown on SrTiO 3 has even shown a sign of T c over 100 K [5,6]. FeSe manifests some intriguing properties, including a nematic state without long-range magnetic order [7], crossover from Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein-condensation (BEC) [8], and a Diraccone-like state [9][10][11], which are crucial to understanding high-T c superconductivity.Efforts have been made to elucidate FeSe's gap structure, and the presence of nodes [8,12] or deep minima [13][14][15][16] have been proposed. Even if the multi-gap structure is established [17,18], gap nodes or minima must still be located in the corresponding bands. Unfortunately, there have been few reports on this issue except for the recent Bogoliubov quasiparticle interference (BQPI) experiments, which found gap minima in both the α and ε bands [19]. However, there is still no bulk evidence of the locations of nodes or gap minima, and also no information about the gap from the δ band. More importantly, details of gap structure, including the three-dimensional (3D) locations of gap nodes or minima, remain unexplored. To solve these issues, a bulk technique capable of probing quasiparticle (QP) excitations with 3D angular resolution is needed. Field-angle-resolved specific heat (ARSH) measurement is an ideal tool for probing the density of states (DOS) of QPs without interference from surface effects; it is angle resolved because the lowlying QP excitations near the gap nodes (minima) are field-orientation dependent [20]. ARSH measurements have been well applied to investigating the locations and types of nod...

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“…A nodal gap was reported based on the observation of the V-shaped STM spectrum, a nearly linearly temperature dependent penetration depth at low temperatures, and a large residual thermal conductivity [3]. However, nodeless gap structure is also claimed by the lowtemperature specific heat [11,12], lower critical field, and thermal conductivity measurements reported by other groups [13,14]. Such controversy may come from the difference in sample quality.…”

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confidence: 99%

Effects of heavy-ion irradiation on FeSe

et al. 2017

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We report the effects of heavy-ion irradiation on FeSe single crystals by irradiating Uranium up to a dose equivalent matching field of B φ = 16 T. Almost continuous columnar defects along the c-axis with a diameter ∼10 nm are confirmed by high-resolution transmission electron microscopy. Tc is found to be suppressed by introducing columnar defects at a rate of dTc/dB φ ∼ -0.29KT −1 , which is much larger than those observed in iron pnictides. This unexpected large suppression of Tc in FeSe is discussed in relation to the large diameter of the columnar defects as well as its unique band structure with a remarkably small Fermi energy. The critical current density is first dramatically enhanced with irradiation reaching a value over ∼2×10 5 A/cm 2 (∼5 times larger than that of the pristine sample) at 2 K (self-field) with B φ = 2 T, then gradually suppressed with increasing B φ . The δl-pinning associated with charge-carrier mean free path fluctuations, and the δTc-pinning associated with spatial fluctuations of the transition temperature are found to coexist in the pristine FeSe, while the irradiation increases the contribution from δl-pinning, and makes it dominant over B φ = 4 T.

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Thermal Conductivity of the Iron-Based Superconductor FeSe: Nodeless Gap with a Strong Two-Band Character

Cited by 59 publications

References 31 publications

Highly anisotropic superconducting gaps and possible evidence of antiferromagnetic order in FeSe single crystals

Highly anisotropic superconducting gaps and possible evidence of antiferromagnetic order in FeSe single crystals

Gap structure of FeSe determined by angle-resolved specific heat measurements in applied rotating magnetic field

Effects of heavy-ion irradiation on FeSe

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