D. A. Chareev

Unusual band renormalization in the simplest iron-based superconductor FeSe 1 − x

The electronic structure of the iron chalcogenide superconductor FeSe_{1-x} was investigated by high- resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of FeSe_{1-x}, DFT calculations and existing data on FeSe_{x}Te_{1-x}. The bands undergo a pronounced orbital dependent renormalization, different from what was observed for FeSe_{x}Te_{1-x} and any other pnictides.

Magnetic ground state of FeSe

Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors exhibit Néel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (Ts=90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here we report inelastic neutron-scattering experiments that reveal both stripe and Néel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the Néel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ∼60% larger than that in the iron pnictide BaFe2As2. Our results suggest that FeSe is a novel S=1 nematic quantum-disordered paramagnet interpolating between the Néel and stripe magnetic instabilities.

Temperature dependence of lower critical field Hc1(T) shows nodeless superconductivity in FeSe

We investigate the temperature dependence of the lower critical field Hc1(T) of a high-quality FeSe single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding Hc1(T) was deduced by taking into account demagnetization factors. A pronounced change in the Hc1(T) curvature is observed, which is attributed to anisotopic s-wave or multiband superconductivity. The London penetration depth λab(T) calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean s-wave superconductor. Using either a two-band model with s-wave-like gaps of magnitudes � 1 = 0.41 ± 0.1 meV and � 2 = 3.33 ± 0.25 meV or a single anisotropic s-wave order parameter, the temperature dependence of the lower critical field Hc1(T) can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless.

Coexistence of isotropic and extended s-wave order parameters in FeSe as revealed by low-temperature specific heat

The comprehensive low-temperature specific heat C(T) data identify both an isotropic s-wave and an extended s-wave order parameters coexisting in a superconducting single crystal FeSe with Tc=8.11 K. The isotropic gap {\Delta}0=1.33 meV on the hole Fermi sheets and the extended s-wave gap {\Delta}={\Delta}e(1+\alpha*cos2_theta) with {\Delta}e=1.13 meV and \alpha=0.78 on the electron Fermi sheets. The extended s-wave is rather anisotropic but the low energy quasiparticle excitations demonstrate no sign of the accidental nodes. The coefficient \gamma(H) manifesting the quasiparticle contribution to C is a non-linear function of the applied magnetic field H in the mixed state in accord with the anisotropic multi-order parameters.

Quasiparticle Dynamics and Phonon Softening in FeSe Superconductors

Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (ΔR/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ΔR/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe.

Superconducting properties of sulfur-doped iron selenide

The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of

Interplay between lattice and spin states degree of freedom in the FeSe superconductor: dynamic spin state instabilities

{\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}

Highly Anisotropic and Twofold Symmetric Superconducting Gap in Nematically OrderedFeSe0.93S0.07

(

Magnetic properties of superconducting FeSe in the normal state

x=0

Anisotropy in the upper critical field of FeSe and FeSe0.33Te0.67 single crystals

, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature