Session E10: Fe-based Superconductors -- Bulk FeSe

Laser ARPES on Orbital Origin of Extremely Anisotropic Superconducting Gap in FeSe Superconductor

We have carried out high resolution laser-based angle-resolved photoemission (ARPES) measurements on bulk FeSe superconductor (Tc=8~9K). We will report direct observation of highly anisotropic Fermi surface and extremely anisotropic superconducting gap in the nematic state of the FeSe superconductor. We find that the low-energy excitations of the entire hole pocket at the Brillouin zone center are dominated by the single dxz orbital. The superconducting gap exhibits an anticorrelation relation with the dxz spectral weight near the Fermi level. These observations provide new insights in understanding the orbital origin of the extremely anisotropic superconducting gap in the FeSe superconductor and the relation between nematicity and superconductivity in the iron-based superconductors.   

STM studies of Iron-based Superconductor FeSe0.5Te0.5

Recent studies on the iron-based superconductor, FeSe_0.5Te_0.5 have suggested that it is in a topologically non-trivial phase and harbors topological surface states. When these states are gapped by proximity effect, Majorana bound states are expected in the vortex cores, which have been confirmed by STM. In this work we present STM studies of both the vortex core states as well as boundary states in FeSe_0.5Te_0.5. Our data provide supporting evidence for the existence of Majorana modes in this system.   

Itinerant approach to magnetic neutron scattering of FeSe: effect of orbital selectivity

Recent STM experiments and theoretical considerations have highlighted the role of interaction-driven orbital selectivity in FeSe, and its role in generating the extremely anisotropic superconducting gap structure in this material.
We study the magnetic excitation spectrum resulting from the coherent quasiparticles within the same renormalized random phase approximation approach used to explain the STM experiments, and show that it agrees well with the low-energy momentum and energy dependent response measured by inelastic neutron scattering experiments. We find a correlation-induced suppression of (π,π) scattering due to a small quasiparticle weight of states of d_xy character [1]. We compare predictions for twinned and untwinned crystals, and predict in particular a strongly (π,0)-dominated susceptibility at low energies in untwinned systems. These results will be discussed in comparison to recent inelastic neutron scattering on detwinned FeSe.
The same description of the coherent quasiparticles is used to make further predictions of physical observables in FeSe such as the magnetic penetration depth which has recently been examined in detail by µSR experiments [2].
[1] A. Kreisel, Brian M. Andersen, P. J. Hirschfeld, arXiv:1807.09482
[2] P. Biswas, et al., arXiv:1810.06269   

Pressure-induced tetragonal magnetic order in FeSe

We present a microscopic study of magnetism and lattice distortion on FeSe single crystals by high-energy x-ray diffraction and time-domain Mössbauer spectroscopy over wide temperature and pressure ranges. The magneto-structural ground state can be tuned by pressure from a paramagnetic state with an orthorhombic lattice distortion through a strongly coupled magnetically ordered and orthorhombic state to a magnetically ordered state without an orthorhombic lattice distortion. Close to the reported maximum of the superconducting critical temperature, the orthorhombic distortion suddenly disappears and FeSe remains tetragonal down to the lowest temperature measured. Analysis of the structural and magnetic order parameters suggests an independent origin of the structural and magnetic ordering phenomena, and their cooperative coupling leads to the similarity with the canonical phase diagram of iron pnictides.   

Relationship between Superconductivity and Antiferromagnetic Fluctuations in FeSe0.9S0.1 under Pressure Studied by 77Se NMR

Magnetic fluctuations and/or nematic fluctuations are considered to be candidates for the pairing mechanism in unconventional superconductors such as iron pnictide superconductors. Quite recently nuclear magnetic resonance (NMR) measurements revealed strong correlation between superconductivity and antiferromagnetic (AFM) fluctuations despite being near a nematic quantum critical point in FeSe_1−xS_x [1]. To understand the quantitative relationship between superconductivity and magnetic fluctuations, we carried out ⁷⁷Se NMR on the optimally doped FeSe_0.9S_0.1 under pressure. In this presentation, we will report the NMR results, especially temperature dependence of nuclear spin lattice relaxation rate under different pressures. Upon analyzing the new together with the previous data [1,2] we will discuss the relationship between AFM fluctuations and superconductivity in the system.

[1] P. Wiecki, et al., Phys. Rev. B 98, 020507(R) (2018).
[2] P. Wiecki, et al., Phys. Rev. B 96, 180502(R) (2017).   

Evidence for time-reversal symmetry breaking in the superconducting state of FeSe

The iron-based superconductor FeSe has attracted special attention because it uniquely has a pure nematic phase without a magnetic ordering. It is considered as a key material for investigating the influence of nematicity on superconductivity. The superconducting state inside the nematic phase also has unique properties, and it has been recently proposed that the superconducting order parameter breaks the time-reversal symmetry near the nematic twin boundaries. The lifting of superconducting gap nodes due to twin boundaries has been observed in scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy, which is consistent with the induced imaginary component. However, these measurements of the gap structure provide only indirect evidence for time-reversal symmetry breaking (TRSB), and thus the observation of spontaneous internal magnetic field generated by TRSB is indispensable. Here we report on the zero-field muon spin rotation (μSR) measurement, which is one of the most sensitive magnetic probes, in high-quality single crystals of FeSe. We find that the relaxation rate starts to grow just below T_c (=9 K). This indicates that weak but finite internal magnetic field is induced in the superconducting state, providing strong evidence for TRSB state in FeSe.   

Superconductivity and magnetism of S-doped FeSe with a high Tc (≈25-30K) studied via 77Se-NMR measurements under pressure

12%-S doped FeSe system has a high T_c of 25-30 K class under a pressure of 3.0 GPa. We have succeeded in investigating its microscopic properties for the first time via ⁷⁷Se-NMR measurements under pressure. We measured the relaxation time (T₁), the Knight shift, and the AC susceptibilty under pressures up to 3.0 GPa. Unexpectedly, the antiferromagnetic (AFM) fluctuation measured from 1/T₁T at the optimal pressure was strongly suppressed compared to the AFM fluctuation at ambient pressure, despite that the optimal pressure is close to the phase boundary of the AFM phase induced at a high pressure region. In addition, we revealed that the SC phase at an applied field of 6 T exhibited a remarkable double-dome structure in the pressure-temperature phase diagram, unlike the SC phase at zaro field. These phenomena are explained by a pressure-induced Lifshitz transition, a topological changes in Fermi surfaces under pressure. The strong AFM fluctuation at ambient pressure is attributable to a hidden AFM quantum critical point.   

Scaling of the superconducting gap with orbital character in FeSe

It is often hypothesized that superconductivity in the iron-based superconductors is mediated by a spin-fluctuation pairing mechanism, however direct evidence of this pairing is challenging to obtain. Here we present a high-resolution angle-resolved photoemission spectroscopy (ARPES) study on the three-dimensional superconducting gap of FeSe. We observe a direct scaling between the d_yz orbital weight at the Fermi level and the size of the superconducting gap at both the hole and electron pockets. Suggesting that superconducting pairing is mediated by strong, local Coulomb interactions.

We then present an ARPES-derived tight binding model which quantitatively takes into account all the features present in the nematic phase of FeSe; including spin-orbital coupling, nematic ordering, and most crucially the incoherence of an electron pocket. Using this model, we show that the calculated momentum dependence of the superconducting gap, assuming spin-fluctuation mediated superconductivity, directly reproduces the experimental results.

These findings provide both experimental and theoretical support for spin-fluctuation mediated superconductivity in FeSe.

L.C. Rhodes et. al. (2018) arXiv:1804.01436   

Ultra-long quasiparticle relaxation times in the superconducting state of FeSe

We report on measurements of the in-plane microwave conductivity of FeSe in the superconducting state. Our measurements were performed from 0.5 to 20 GHz and from 1.2 to 10 K by means of bolometric broadband microwave spectroscopy. In this frequency range, we are sensitive to the charge dynamics of thermally excited quasiparticles. We observe a dramatic narrowing in the conductivity spectra below Τ_c, corresponding to a rapid suppression of quasiparticle scattering. Using a phenomenological model, we extract a temperature-dependent quasiparticle relaxation rate Γ from the conductivity, which can be understood as the sum of two contributions. The contribution to Γ from inelastic scattering has an exponential temperature dependence. This implies that the excitation spectrum responsible for the inelastic scattering of quasiparticles is electronic in origin and gapped below Τ_c. The contribution to Γ from elastic scattering is consistent with Born-limit scattering from a dilute concentration of impurities. At 1.2 K, the quasiparticle relaxation rate reaches a value corresponding to a mean free path greater than 100 μm.


  

Anisotropic Vortex Pinning in Single Crystal FeSe

Iron-based superconductors are known for their complex interplay between magnetic, nematic, and superconducting order. FeSe is an iron-based superconductor with a structural transition at 90 K without the appearance of magnetic ordering. The lack of magnetic order allows us to probe the superconducting transition of an iron-based superconductor from a purely nematic phase. We study single crystal FeSe using a scanning SQUID microscope that has both a field coil to apply a local magnetic field and a pickup loop to measure the vortex response. We model the effect of the SQUID geometry on vortex motion in an anisotropic pinning potential and find that SQUID orientation has a significant effect. Our initial findings suggest that that the pinning potential anisotropy aligns with twin domain boundaries produced by the structural transition. We also check for correlation between spatial variation in diamagnetic response and in anisotropy. These studies will help elucidate whether the relationship between nematic and superconducting order in these materials is competing or enhancing.   

Quantum vortex core and missing pseudogap in the multi-band BCS-BEC-crossover superconductor FeSe

The iron-based superconductor FeSe is considered to be in the BCS-BEC crossover regime where the superconducting gap is comparable to the Fermi energy. We performed spectroscopic-imaging scanning tunneling microscopy to search for spectroscopic signatures of the BCS-BEC crossover. The large superconducting-gap to Fermi-energy ratio may yield well separated discrete bound states in the vortex core, as well as a pseudogap above the superconducting transition temperature due to preformed Cooper pairs. We found a signature of the discrete vortex bound states but could not detect the pseudogap. We argue that these conflicting observations can be resolved if multi-band nature is taken into account.   

Superconductivity fluctuation in FeSe investigated by magnetic torque measurement using optical interference

We investigated the superconducting fluctuation (SCF) in FeSe by a magnetic torque measurement. This material has a large ratio of the superconducting gap to Fermi Energy, and is proposed to be located in BCS-BEC crossover regime [1]. In our method, the absolute value of the cantilever displacement can be measured with high resolution by detecting the interference intensity of the Fabry-Perot type cavity formed between the cantilever and the optical fiber. We observed SCF signal originating from the surviving vortex liquid state, whose onset temperature increases up to above 1.5 Tc in high magnetic field. Its contribution to the magnetic torque is about 0.5 % of that of the total sample magnetization at 10 K and 10 T. These features are totally different, both qualitatively and quantitatively, from those of the signal observed in the previous torque magnetometry study using a piezoresistive cantilever[1], while similar to what reported in cuprate superconductors [2]. Therefore, we conclude that SCF in FeSe is not anomalous and it can be understood in the same framework as other superconductors using Ginzburg number.
[1]S. Kasahara et al., Nature Comm. 7, 12843 (2016).
[2]L. Li et al., Phys. Rev. B 81, 054510 (2010).