Unlike the parent phases of the iron-arsenide high Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc~9K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to ~37K. We investigate the electronic properties of FeSe using 77Se NMR to search for the key to the superconducting mechanism. We demonstrate that the electronic properties of FeSe are very similar to those of electron-doped FeAs superconductors, and that antiferromagnetic spin fluctuations are strongly enhanced near Tc. Furthermore, applied pressure enhances spin fluctuations. Our findings suggest a link between spin fluctuations and the superconducting mechanism in FeSe.
We investigated chemical pressure effects in electron-doped (e-doped) FeSe by fabricating an electric-double-layer structure with single crystalline FeSe films on LaAlO$_3$ with Se substituted by isovalent Te and S. Our method enables transport measurements of e-doped FeSe. Electron doping by applying gate voltage of 5 V increases $T_{\mathrm c}$ of the FeSe$_{1-x}$Te$_x$ and FeSe$_{1-y}$S$_y$ films with $0 \leq x \leq 0.4$ and $0 \leq y \leq 0.25$, while the e-doped $x=0.5$ film showed lower $T_{\mathrm c}$ than that of the undoped one. Both positive and chemical pressure suppress $T_{\mathrm…
Diamond anvil cell using boron-doped metallic diamond electrodes covered with undoped diamond insulating layer have been developed for electrical transport measurements under high pressure. These designed diamonds were grown on a bottom diamond anvil via a nanofabrication process combining microwave plasma-assisted chemical vapor deposition and electron beam lithography. The resistance measurements of high quality FeSe superconducting single crystal under high pressure were successfully demonstrated by just putting the sample and gasket on the bottom diamond anvil directly. The superconducting…
This note reviews some of the recent progresses in the study of high temperature superconductivity in the interface between a single unit cell FeSe and SrTiO$_3$. It offers the author's personal view of why $T_c$ is high and how to further increase it.
The origin of the electronic nematicity and its remarkable material-dependence are famous longstanding unsolved issues in Fe-based superconductors. To attack these issues, we focus on the in-plane anisotropy of the resistivity: In the nematic state in FeSe, the relation $ρ_x>ρ_y$ holds, where $ρ_{x(y)}$ is the resistivity along the longer (shorter) Fe-Fe axis. In contrast, the opposite anisotropy $ρ_x<ρ_y$ is realized in other undoped Fe-based superconductors. Such nontrivial material dependence is naturally explained in terms of the strongly orbital-dependent inelastic quasiparticle scatterin…
The origin of the 90 K nematic transition in the chalcogenide FeSe, which displays no magnetic order down to T=0, remains a major puzzle for a unifying theory for the iron-based superconductors. We analyze this problem in light of recent experimental data which reveal very small Fermi pockets in this material. We show that the smallness of the Fermi energy leads to a near-degeneracy between magnetic fluctuations and fluctuations in the charge-current density-wave channel. While the two fluctuation modes cooperate to promote the same preemptive Ising-nematic order, they compete for primary orde…
A review of high-pressure studies on Fe-pnictide superconductors is given. The pressure effects on the magnetic and superconducting transitions are discussed for different classes of doped and undoped FeAs-compounds, ROFeAs (R = rare earth), AeFe2As2 (Ae = Ca, Sr, Ba), and AFeAs (A = Li, Na). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting Tc increases with pressure for underdoped FeAs-pnictides, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. The undoped LaOFeA…
Single unit cell films of iron selenide (1UC FeSe) grown on SrTiO3 (STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 K), a record for iron-based superconductors. Towards understanding why Cooper pairs form at such high temperatures, a primary question to address is the role, if any, of the STO substrate. Here, we report high resolution angle resolved photoemission spectroscopy (ARPES) results which reveal an unexpected and unique characteristic of the 1UC FeSe/STO system: shake-off bands suggesting the pre…
We demonstrate that in iron-based superconductors, the extended S_{+-} SC state coexists with the d_{x^2-y^2} state under generic conditions. The mixed S_{+-} + d_{x^2-y^2} SC is a natural nematic state in which the tetragonal symmetry C_4 is broken to C_2 explaining puzzling findings of nematic SC in FeSe films [Science 332, 1410 (2011)]. Moreover, we report the possibility of a first order transition at low-T from the nematic S_{+-} + d_{x^2-y^2} state to the pure d_{x^2-y^2} state induced by the Zeeman magnetic field proposing an original experimental strategy for identifying our mixed nema…
Bulk FeSe becomes superconducting below 9\,K, but the critical temperature (T$_{c}$) is enhanced almost universally by a factor of $\sim$4-5 when it is intercalated with alkali elements. How intercalation modifies the structure is known from in-situ X-ray and neutron scattering techniques, but why T$_{c}$ changes so dramatically is not known. Here we show that there is one-to-one correspondence between the enhancement in magnetic instabilities at certain $\bf q$ vectors and superconducting pairing vertex, even while the nuclear spin relaxation rate ${1}/{(T_{1}T)}$ may not reflect this enhance…
A rich variety of nematic/smectic orders in Fe-based superconductors is an important unsolved problem in strongly correlated electron systems. A unified understanding of these orders has been investigated for the last decade. In this article, we explain the $B_{1g}$ symmetry nematic transition in FeSe$_{1-x}$Te$_x$, the $B_{2g}$ symmetry nematicity in AFe$_2$As$_2$ (A=Cs, Rb), and the smectic state in BaFe$_2$As$_2$ based on the same framework. We investigate the quantum interference mechanism between spin fluctuations by developing the density wave equation. The observed rich variety of nemat…
Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like ($α$ and $β$) and two electron-like ($η$ and $δ$) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along $k_z$. However, a third hole-like band ($γ$) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be…
A simple two-band model is used to describe the magnitude and temperature dependence of the magnetic susceptibility, Hall coefficient and Seebeck data from undoped and Co doped BaFe2As2. Overlapping, rigid parabolic electron and hole bands are considered as a model of the electronic structure of the FeAs-based semimetals. The model has only three parameters: the electron and hole effective masses and the position of the valence band maximum with respect to the conduction band minimum. The model is able to reproduce in a semiquantitative fashion the magnitude and temperature dependence of many …
Magnetism and nematic order are the two non-superconducting orders observed in iron-based superconductors. To elucidate the interplay between them and ultimately unveil the pairing mechanism, several models have been investigated. In models with quenched orbital degrees of freedom, magnetic fluctuations promote stripe magnetism which induces orbital order. In models with quenched spin degrees of freedom, charge fluctuations promote spontaneous orbital order which induces stripe magnetism. Here we develop an unbiased approach, in which we treat magnetic and orbital fluctuations on equal footing…
We address two important issues that arise in recent studies of iron-based superconductivity. (1) Why are the T_c of A_xFe_{2-y}Se_2 and the single unit cell FeSe on SrTiO$_3$ so high despite both only have electron pockets? (2) What (if any) are the effects of orbital order and orbital fluctuation on the Cooper pairing. Our conclusions are summarized in the third paragraph of the paper.
Starting from a spin-fermion model for the cuprate superconductors, we obtain an effective interaction for the charge carriers by integrating out the spin degrees of freedom. Our model predicts a quantum critical point for the superconducting interaction coupling, which sets up a threshold for the onset of superconductivity in the system. We show that the physical value of this coupling is below this threshold, thus explaining why there is no superconducting phase for the undoped system. Then, by including doping, we find a dome-shaped dependence of the critical temperature as charge carriers …
Both FeSe and cuprate superconductors are quasi 2D materials with high transition temperatures and local fermion pairs. Motivated by such systems, we investigate real space pairing of fermions in an anisotropic lattice model with intersite attraction, V, and strong local Coulomb repulsion, U, leading to a determination of the optimal conditions for superconductivity from Bose-Einstein condensation. Our aim is to gain insight as to why high temperature superconductors tend to be quasi 2D. We make both analytically and numerically exact solutions for two body local pairing applicable to intermed…
We briefly review the recently constructed two orbital microscopic model for iron-based superconductors based on $S_4$ symmetry (PRX 2 021009(2012)). With this faithful representation of the kinematics of the tri-layer FeAs or FeSe structure, the model provides answers and physical pictures to fundamental questions related to the robustness of superconductivity and pairing symmetry, unifies different families of iron-based superconductors, casts new insight into the connections to the other high $T_c$ superconductors, cuprates, and reveals why an s-wave pairing can be stabilized by repulsive i…
Iron-Based Superconductors: A Decade of Materials, Magnetism, and Mechanisms
Beijing Normal University（北京师范大学）；Renmin University of China（中国人民大学）；Fudan University（复旦大学）；Institute of Science Tokyo（东京科学大学）；Korea University（高丽大学）；Rice University（莱斯大学）
