Session C1: New Developments in Iron Chalcogenide Superconductors

Frustrated Magnetism and Superconductivity in the Iron Chalcogenides

While studies in the early stage on the iron-based supercondoctors (FeSCs) focused on the iron pnictides, considerable efforts in the more recent past have also been directed towards iron chalcogenides. These studies are giving us renewed hope for even higher transition temperatures in the iron-based materials. In this talk, I will discuss several theoretical issues on the microscopic physics of the iron chalcogenides that teach us much about the overall physics of the FeSCs. One is the proposal we made on the orbital selective Mott phase [1], for which considerable evidence has come from ARPES [2] and other experiments. The second issue concerns magnetism, in particular the correlation-induced magnetic frustration effect. A major puzzle arises in bulk FeSe, which shows a structural phase transition similar to that seen in the iron pnictides but, unlike the latter, does not exhibit any static antiferromagnetic order. We studied the effect of magnetic frustration associated with the bilinear-biquadratic spin-exchange interactions [3]. Based on the derived phase diagram, we proposed that the structural transition in FeSe originates from an Ising-nematic order of an antiferro-quadrupolar phase. Within this picture, we have predicted that the collective modes of this quadrupolar state show (pi,0) magnetic fluctuations, which have since been verified by inelastic neutron scattering experiments [4]. These results considerably expand on the notion [5] regarding the importance of the bad-metal behavior, and provide a substantially broadened perspective on the magnetic and nematic correlations in the FeSCs. Finally, implications of the frustrated magnetism for superconductivity [5] will also be discussed. References: [1] R. Yu and Q. Si, Phys. Rev. Lett. 110, 146402 (2013). [2] M. Yi et al., Nature Commun. 6, 7777 (2015). [3] R. Yu and Q. Si, Phys. Rev. Lett. 115, 116401 (2015). [4] M. Rahn et al., Phys. Rev. B 91, 180501(R) (2015); Q. Wang et al., arXiv:1502.07544. [5] Q. Si and E. Abrahams, Phys. Rev. Lett. 101, 076401 (2008). [6] E. Nica, R. Yu and Q. Si, arXiv:1505.04170.   

On nematicity, magnetism and superconductivity in FeSe

FeSe is unique among iron-based superconductors, notably regarding the interrelationships of structure, magnetism, and superconductivity. At ambient pressure, FeSe exhibits a tetragonal-to-orthorhombic (nematic) phase transition at $T_s=90$ K, similar to other iron-based materials, but unlike them, no long-range magnetic order. One consequence is the unique possibility to study the in-plane resistivity anisotropy, arguably the most investigated ‘nematic property’, without interfering effects from the Fermi surface reconstruction induced by antiferromagnetic order. Recent findings pose the question whether nematicity in FeSe is driven by magnetic fluctuations, as often assumed in other iron-based systems. In particular, magnetic fluctuations, which are prominent at low temperatures, are not observed above $T_s$ in FeSe by NMR [1,2], even though indicated by inelastic neutron scattering. The pressure-temperature phase diagram, recently obtained in new comprehensiveness using vapor-grown single crystals [3], shows that the structural transition is suppressed at ~2 GPa and a new, likely magnetic phase is stabilized above 0.8 GPa, where $T_c$ has a local maximum. Various theoretical scenarios have been proposed to explain this nematic transition far away from the magnetic order. Surprisingly, the degree of the orthorhombic distortion does not decrease below the superconducting transition at $T_c = 8$ K, suggesting that nematic and superconducting “channels” do not compete [4]. Our new results on the superconducting state under pressure, show a non-monotonic pressure dependence of the upper critical field, which is well explained by the Fermi surface evolution. Further, we have successfully detwinned FeSe crystals and measured the in-plane resistivity anisotropy and elastoresistivity coefficients and compared them with model calculations of inelastic scattering from spin fluctuations. [1] B\"{o}hmer et al., PRL 114, 027001 (2015) [2] Baek et al., Nat. Mat. 14, 210 (2015) [3] Terashima et al., JPSJ 84, 063701 (2015) [4] B\"{o}hmer et al., PRB 87 180505 (2013)   

What Makes the T$_{\mathrm{c}} $of FeSe/SrTiO$_{\mathrm{3}}$ so High ?

Raising the superconducting transition temperature to a point where applications are practical is one of the most important challenges in science. In the history of high \textit{Tc }superconductivity there are two landmark events: the discovery of copper-oxide superconductor in 1986, and the discovery of iron-based superconductor in 2006 For the Fe-based superconductors the record of T$_{\mathrm{c}}$ was 55$K$[1] until 2012. In the interface system composed of an one unit cell thick FeSe film grown on the TiO$_{\mathrm{2}}$ terminated (001) surface of SrTiO$_{\mathrm{3\thinspace }}$an anomalously large superconducting-like energy gap was seen by scan tunneling microscopy for [2]. Later ARPES works show the gap opening temperature can reach nearly the liquid nitrogen boiling temperature [3-7]. More recently several FeSe-related bulk and thin film high T$_{\mathrm{c}}$ systems have be discovered. This talk reviews some of the recent experimental [7] and theoretical [8] progresses in the study of the mechanism for high temperature superconductivity in this interface system. It offers the author's personal view of why T$_{\mathrm{c}} $is so high and how to further increase it [9£¬10]. References: \begin{enumerate} \item Z.A. Ren \textit{et al.}, Chin. Phys. Lett. \textbf{25}, 2215-2216 (2008). \item Q.Y. Wang \textit{et al.}, Chin. Phys. Lett. \textbf{29}, 037402 (2012). \item D.F. Liu \textit{al.}, Nature Commun. \textbf{3}, 931 (2012). \item S. He \textit{et al.}, Nature Materials \textbf{12}, 605-610 (2013). \item S. Tan \textit{et al.}, Nature Materials \textbf{12}, 634-640 (2013). \item R. Peng \textit{et al.}, Nature Commun. \textbf{5}, 5044 (2014). \item J.J. Lee \textit{et al,} Nature \textbf{515,} 245 (2014). \item Zixiang Li \textit{et al}, to be published. \item D.-H. Lee, Chin. Phys. B, 2015, 24 (\textbf{11}): 117405 \textbf{doi:} 10.1088/1674-1056/24/11/117405 \item Much of the contents of this talk are stimulated by the collaborative work with Z-X Shen and his ARPES group members and T. Deveraux and his group members. \end{enumerate}   

What makes the nematic phase of FeSe different than other iron-based superconductors?

Most iron-based superconductors display, in their normal state, a transition to a magnetic stripe state that is either accompanied or preceded by a tetragonal-to-orthorhombic transition. The proximity between these two transitions has led to the proposal that they correspond to a two-stage melting of the magnetic stripe state, resulting in a vestigial orthorhombic-paramagnetic nematic phase. Despite the success of this scenario to describe many iron-based materials, the simplest of them, FeSe, displays a high-temperature nematic transition but no long-range magnetic order. Interestingly, in its monolayer form, FeSe displays the highest $T_c$ of all iron-based materials, raising the question of whether the nematic state of its bulk form could be related to the superconducting state of its monolayer form. In this talk, we investigate theoretically the microscopic origin of the nematic phase of FeSe. By extending the standard RPA formalism, we compare the orbital-order susceptibility and the spin-driven nematic susceptibility of a generic multi-orbital Hubbard model. We find that the former cannot in general drive the nematic transition, and that high-energy magnetic fluctuations play a fundamental role in stabilizing the nematic state in the absence of long-range magnetic order. Focusing on FeSe, we identify two features that distinguish it from all other iron-based materials: a very small Fermi energy and a large degeneracy of the magnetic ground state. We show that both effects enhance the nematic transition temperature at the same time as they suppress the magnetic transition. These results may explain why, in FeSe, the onset of nematic order does not require strong magnetic fluctuations, in contrast to other iron-based materials. Finally, we discuss how the interplay between magnetic fluctuations and small Fermi energy in FeSe can lead to the emergence of different types of Pomeranchuk instabilities, and discuss their experimental manifestations.   

Tuning the electronic structure of bulk FeSe with chemical pressure using quantum oscillations and angle resolved photoemission spectroscopy (ARPES)

FeSe is a unique and intriguing superconductor which can be tuned into a high temperature superconducting state using applied pressure, chemical intercalation and surface doping. In the absence of magnetism, the structural transition in FeSe is believed to be electronically driven, with the orbital degrees of freedom playing an important part [1]. This scenario supports the stabilization of a nematic state in FeSe, which manifests as a Fermi surface deformation in the presence of strong interactions, as detected by ARPES [1]. Another manifestation of the nematicity is the enhanced nematic susceptibility determined from elastoresistance measurements under applied strain [1]. Isovalent Sulphur substitution onto the Selenium site constitutes a chemical pressure, which subtly modifies the electronic structure of FeSe, suppressing the structural transition without inducing high temperature superconductivity [3]. I will present the evolution of the electronic structure with chemical pressure in FeSe, as determined from quantum oscillations [1,2] and ARPES studies [3] and I will discuss the suppression of the nematic electronic state and the role of electronic correlations. Experiments were performed at high magnetic field facilities in Tallahassee, Nijmegen and Toulouse and Diamond Light Source, UK. This work is mainly supported by EPSRC, UK (EP/I004475/1, EP/I017836/1) and I acknowledge my collaborators from Refs. [1-3] . [1] Phys. Rev. B 91, 155106 (2015); [2] Phys. Rev. Lett. 115, 027006 (2015); [3] Phys. Rev. B 92, 121108 (2015).