Session P11: Correlations and Superconductivity in Fe chalcogenides II

BCS-BEC crossover physics in FeSe bulk superconductor

The physics of the crossover between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits gives a unified framework of quantum bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems, but is extremely difficult to be realized for electrons in solids. Through the superfluid response, transport, thermoelectric response [1], and quantum oscillations [2], we demonstrate that the Fermi energy of the bulk superconductor FeSe is extremely small, with the ratio of the gap to Fermi energy is of the order of unity, which qualifies FeSe to be deep inside the BCS-BEC crossover regime. Thus FeSe appears to be a key material to solve the longstanding issue in the crossover physics; the presence of preformed Cooper pairs giving rise to a pseudogap above the superconducting transition temperature $T_c$. We report experimental signatures of preformed Cooper pairing well above $T_c=8.5$ K in clean single crystals of FeSe. Our torque magnetometry reveals distinct diamagnetic signal below $T^*\sim 20$ K indicating that the superconducting fluctuations above the transition temperature are strongly enhanced from the standard Gaussian theory. The transport and thermoelectric coefficients also exhibit distinct anomalies at $\sim T^*$, signaling a possible pseudogap formation. The multiband nature with the electron-hole compensation in FeSe may highlight a fundamentally new aspect of the BCS-BEC crossover physics. [1] S. Kasahara {\it et al.}, PNAS {\bf 111}, 16309 (2014). [2] T. Terashima {\it et al.}, Phys. Rev. B {\bf 90}, 144517 (2014); M. D. Watson {\it et al.}, Phys. Rev. Lett. {\bf 115}, 027006 (2015).   

Enhanced superconductivity in heavily electron doped surface layer of FeSe bulk crystal

The recording setting superconducting transition temperature of as high as 100 K discovered in 1 monolayer FeSe grown on SrTiO3 immediately brought attention to the mechanism for the dramatically enhanced Tc from its original value of 7 K. At present, the two most popular views for the enhanced Tc are interfacial effect and excess electron with enhanced correlation strength. The issue is controversial and there are evidences supporting each view. Here, we report the observation of 20 K superconductivity in the electron doped surface layer mimics all the key spectroscopic aspects of the electronic structure of 1ML FeSe on STO but with a smaller superconducting gap opening of 4 meV. Our results clearly show that excess electron doping with enhanced correlation strength alone cannot induce the maximum Tc, which strongly suggests a need for additional interfacial effect.   

Low temperature scanning tunneling microscopy and spectroscopy investigation of FeSe$_{1-\delta}$ and FeSe$_{1-x}$S$_{x}$ single crystals

Due to its relatively simple crystallographic structure, investigations into FeSe$_{1-\delta}$ have held the promise to provide an avenue towards a better understanding of the mechanism of superconductivity in the iron-pnictides/chalcogenides and the relationship between nematicity and superconducting state. Here, we present low-temperature scanning tunneling microscopy and spectroscopy investigations of high purity FeSe$_{1-\delta}$ and sulfur substituted FeSe$_{1-x}$S$_x$ single crystals. Vortex core anisotropy and vortex matter in these systems will be discussed.   

Thermal conductivity of the iron-based superconductor FeSe : Nodeless gap with strong two-band character

The thermal conductivity $\kappa$ of the iron-based superconductor FeSe was measured at temperatures down to 50~mK in magnetic fields up to 17~T. In zero magnetic field, the residual linear term in the $T=0$ limit, $\kappa_0/T$, is vanishingly small. Application of a magnetic field $H$ causes no increase in $\kappa_0/T$ initially. Those two facts show that there are no zero-energy quasiparticles that carry heat and therefore no nodes in the superconducting gap of FeSe. The full field dependence of $\kappa_0/T$ has the classic shape of a two-band superconductor, such as MgB$_2$. It rises initially with a characteristic field $H^{\star} \simeq H_{\rm c2} / 25$, and then more slowly up to $H_{\rm c2} = 14$~T. We interpret this in terms of a small gap $\Delta_{\rm A} \simeq \Delta_0 / 5$ on some part of the Fermi surface, with a large gap $\Delta_{\rm B} = \Delta_0$ in the region that controls $H_{\rm c2}$.   

Pressure-dependent upper critical field of FeSe superconductor

In FeSe, the superconducting transition temperature $T_c $ ($\approx 9\,$K at ambient pressure) has a complicated pressure dependence with a local maximum near $p_1\approx$\,0.8\,GPa and a local minimum at $p_2\approx$\,1.2GPa. In this work, we study the upper critical field, $H_{c2,c}(T)$, of FeSe using c-axis resistivity measurements under hydrostatic pressure up to 1.56\,GPa with the magnetic field H$\parallel$c. Application of both current and magnetic field along the same axis reduces the flux flow motion and give sharper transition in applied fields. We observe a non-monotonic evolution of the slope of $H_{c2,c}(T)$$\mid_{T_c}$ with pressure, with changes around $p_1$ and $p_2$. We employ two-band orbital $H_{c2,c}$ calculation to show that the data can be explained using the Fermi velocities extracted from the recent quantum oscillations study\,$[1]$ over the whole pressure range. \linebreak $[1]$ Terashima {\it et al.} arXiv:1510.01840v1 [cond-mat.supr-con] (2015)   

Microwave Conductivity Spectroscopy for Fe(Se,Te) Thin Films

Iron chalcogenide superconductors Fe(Se,Te) have very small $\epsilon_F$ and are considered to be in the BCS-BEC crossover regime[1]. Since Ginzburg number, $G_i =(k_BT_c/\epsilon_F)^4$, which is the relative temperature width of the superconducting fluctuation region, is large for materials in the BCS-BEC crossover regime, large superconducting fluctuations are expected in Fe(Se,Te). In order to investigate superconducting fluctuations in these materials we have performed microwave conductivity spectroscopy on Fe(Se,Te) thin films. Superfluid density of an Fe(Se,Te) film with $T_c ^{zero}$=17 K[2] took finite values above 25 K. This temperature is much higher than $T_c$ estimated by the dc measurement, suggesting strong superconducting fluctuations in Fe(Se,Te). A dynamic scaling analysis of complex fluctuation conductivity revealed that the superconducting fluctuations of Fe(Se,Te) exhibit a 2-dimensional behavior, while BKT transition was not observed. We will also report on the thickness dependence and the Te content dependence of the superconducting fluctuation. [1] Y. Lubashevsky $et\ al$., Nat. Phys. \textbf{8} (2012) 309. [2] Y. Imai $et\ al$., PNAS \textbf{112} (2015) 1937.   

Superconductivity and spin excitations in orbitally ordered FeSe

We provide a band-structure with low-energy properties consistent with recent photoemission and quantum oscillations measurements on the Fe-based superconductor FeSe[1], including a mean-field like orbital ordering in the $d_{xz}/d_{yz}$ channel, and show that this model also accounts for the temperature dependence of the measured Knight shift and the spin-relaxation rate[2]. An RPA calculation of the dynamical spin susceptibility yields spin excitations which are peaked at wave vector $(\pi,0)$ in the 1-Fe Brillouin zone, with a broad maximum at energies of order a few meV. Furthermore, the superconducting gap structure obtained from spin fluctuation theory exhibits nodes on the electron pockets, consistent with the 'V'-shaped density of states measured by tunneling spectroscopy on this material. The redistribution of spectral weight in the superconducting state creates a $(\pi,0)$ "neutron resonance" as seen in recent experiments[3]. Comparing to various experimental results, we give predictions for further studies. [1] S. Mukherjee, et al., PRL 115, 026402 (2015); A. Kreisel, et al., arXiv:1506.03593 [2] S.-H. Baek, et al., Nat. Mater. 14, 210 (2015); A.E. B\"ohmer, et al., PRL 114, 027001 (2015) [3] M.C. Rahn, et al., PRB 91, 180501 (2015); Q. Wang, et al., arXiv:1502.07544   

Intercallation of Li$_{1-x}$Fe$_{x}$O$_{2}$ in the superconducting FeSe

The intercallation of LiFeO${_2}$ in the tetragonal lattice of the 8 K superconductor Fe$_{1-y}$Se leads to a great enhancement of the superconducting transition temperature, T$_{C}$ $\sim$ 43 K, and to an antiferromagnetic transition at 8.5 K. While the LiFeO${_2}$ layer acts as a charge reservoir, its Fe$^{3+}$ ion (3d$^{5}$) is magnetic creating a magnetic buffer layer. Most recently, we developed a new synthesis method to control the Fe concentration in the intercallating layer as well as the filling ratio of the Li$_{1-x}$Fe$_{x}$O$_{2}$ : FeSe layers. Neutron scattering measurements were carried out on powder samples of (Li$_{1-x}$Fe$_{x}$O$_{2}$)${_y}$FeSe. With the intercallation, no crystal structural transition from the P4/nmm symmetry occurs but the c-axis lattice constant expands substantially, evidence of the intercallation. At the same time, the tetrahedral FeSe layers remain intact with no compression or expansion and free of vacancies. Moreover, the intercallation along the c-axis although not uniform leads to a reduction in T${_C}$ when the ratio of Li${_1-x}$Fe${_x}$O$_{2}$ : FeSe layers is about 1 to 3. Our results also indicate that the amount of Fe in the Li${_1-x}$Fe${_x}$O$_{2}$ layer has a direct correlation to the transition temperature as well.   

The Dual Role of Fe Dopants in Enhancing Stability and Charge Transfer in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe Superconductors

The recently discovered (Li$_{0.8}$Fe$_{0.2}$)OHFeSe superconductor provides a new platform for exploiting the microscopic mechanisms of high-$T_c$ superconductivity in FeSe-derived systems. Using density functional theory calculations, we first show that substitution of Li by Fe not only significantly strengthens the attraction between the (Li$_{0.8}$Fe$_{0.2}$)OH spacing layers and the FeSe superconducting layers along the \emph{c} axis, but also minimizes the lattice mismatch between the two in the \emph{ab} plane, both favorable for stabilizing the overall structure. Next we explore the electron injection into FeSe from the spacing layers, and unambiguously identify the Fe$_{0.2}$ components to be the origin of the dramatically enhanced interlayer charge transfer. We further reveal that the system strongly favors collinear antiferromagnetic ordering in the FeSe layers, but the spacing layers can be either antiferromagnetic or ferromagnetic depending on the Fe$_{0.2}$ spatial distribution. Based on these insights, we predict (Li$_{0.8}$Co$_{0.2}$)OHFeSe to be structurally stable with even larger electron injection and potentially higher $T_c$.   

Superconductivity in binary FeS single crystals

FeS is the third recently discovered member of the superconducting binary iron-chalcogenide series that includes the well-known FeSe and FeSe$_{\mathrm{1-x}}$Te$_{\mathrm{x}}$ members. Grown via hydrothermal techniques, single crystals of FeS have been characterized using transport, thermodynamic and magnetic techniques. We will review experimental results and compare with the unconventional superconducting properties of the selenide and telluride counterparts.   

High-pressure NMR Study of Magnetism and Superconductivity on FeSe Single Crystals

Bulk FeSe has a structure transition at $T\sim$ 91 K and a superconducting transition at $T\sim$ 9.3 K, but no magnetic ordering at the ambient pressure. With increasing pressure, the structure transition is suppressed, whereas a magnetic ordering emerges. This is in contrast to most iron arsenides, where the structure transition is usually accompanied by a stripe magnetic ordering. Here we report our high-pressure NMR study on high-quality FeSe single crystals. The spin fluctuations and the magnetic ordering observed by our measurements give fresh information for understanding the interplay among the structure transition, the superconductivity and the magnetism in bulk FeSe materials.   

Cs vacancy ordering and properties of phase separated Cs$_{x}$Fe$_{2-y}$Se$_{2}$

Iron-based selenides are among the most complex and least understood superconductors. At high temperature, a \textsc{\char13}122\textsc{\char13}-type structure with random iron vacancies undergoes a complex iron vacancy ordering scheme below $\sim$500K causing the material to phase separate into A$_{2}$Fe$_{4}$Se$_{5}$, known as the 245 phase, and a minority A-site deficient and fully iron stoichiometric A$_{x}$Fe$_{2}$Se$_{2}$ phase (122). At slightly lower temperatures, the material undergoes another transition with the Fe spins of the main \textsc{\char13}245\textsc{\char13} phase ordering into an exotic checkerboard-type magnetic structure with a large magnetic moment. The minority 122 phase is reported to either remain nonmagnetic or to become magnetic below $\sim$200K. At temperatures below $\sim$30K, the magnetic material becomes superconducting and the two states appear to coexist. I will present and discuss our recent synthesis and characterization of high quality Cs$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals and bulk samples with various Tc\textsc{\char13}s that form a relatively large superconducting dome. I will discuss our findings of a previously unseen three dimensional cesium vacancy ordering in the low temperature 122 phase in addition to hosting superconductivity.   

Effects of inversion symmetry breaking in monolayer FeSe

In this talk, we will discuss the role of broken inversion symmetry due to a substrate on the electronic and magnetic correlated states of monolayer iron selenide (FeSe). We use the group theoretic method of invariants with first principles density functional theory (DFT) calculations to investigate differences between bulk and single layer superconducting and magnetic orders. We show that a spin vortex crystal phase is stabilized, and that two-gap superconductivity is stabilized (though the gaps may be of similar magnitude).