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

Abstract

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...

Discovery of orbital-selective Cooper pairing in FeSe

“…A recent specific heat experiment finds excellent agreement using our reported gap structure [43]. This is important as specific heat probes the bulk properties while STM and ARPES are surface sensitive probes, and the less twodimensional nature of the iron-based superconductors compared to the cuprates necessitates more detailed comparisons of bulk and surface sensitive probes.…”

“…and as a function of applied pressure. It is believed that pure FeSe only exhibits magnetic order when pressure is applied [42], but there exists the possibility that AFM order develops at extremely low temperatures based on a specific heat experiment published two months ago [43]. In any case, this is very different from pnictide superconductors where the nematic phase and stripe order go hand in hand, and develop at very similar temperatures [20].…”

Superconductivity in monocrystalline YNiSi3 and LuNiSi3

“…Here,h is the reduced Planck constant, β = 1/k B T, ω D is the Debye frequency, α is the anisotropy parameter (α = 0 corresponds to an isotropic s-wave gap) [46], and a is a constant that depends on the coupling strength and the geometry of the gap [47,48]. Table I presents the fit parameters for all four models.…”

Anisotropic effect of a magnetic field on the neutron spin resonance in FeSe

“…In recent electric and thermal transport measurements [54], it was argued that FeSe is in a BCS-BEC cross over regime, and a large magnetic field along the c-axis might induce a new superconducting phase coexisting with magnetic order, possibly the FFLO state [54][55][56][57].…”