Session L54: Fe-Based Superconductors: Iron Chalcogenides

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Simultaneously active charge, spin, and orbital degrees of freedom induce complex ordered states. Predicting these states is a challenge unless in ladders and chains. Fortunately, many real materials have this geometry. Here, I will review results using density matrix renormalization group (DMRG) applied to multiorbital Hubbard models. I focus on two aspects: (1) First, the “orbital selective Mott phase” (OSMP). This state occurs when orbitals with different crystal fields and hoppings are affected by the Hubbard and Hund couplings. Some orbitals become gapped and half-filled, while others do not, creating a localized and metallic mixture. Within OSMP, spin ``block’’ states were found, with FM islands, AFM coupled [1]. Powder neutron scattering for BaFe₂Se₃ confirmed their existence [2]. Dynamical DMRG predicts a combination of spin acoustic and optical modes for single crystals [3]. Even exotic “block spirals” were discussed [4]. (2) Second, in multiorbital ladders and chains, pairing was observed [5] as in BaFe₂X₃ (X=S,Se) at high pressure [6]. Singlet pairing was also found in degenerate two-orbital Hubbard models [7]. RVB states explain this pairing, with the orbital acting as an effective leg ladder index. In summary, computational tools applied to multiorbital interacting models in low dimensions have unveiled a remarkably rich landscape of exotic states.

[1] J. Rincon et al., PRL 112, 106405 (2014); J. Herbrych et al., PRL 123, 027203 (2019); P. Bradraj et al., PRB 102, 035149 (2020).
[2] M. Mourigal et al., PRL 115, 047401 (2015).
[3] J. Herbrych et al., Nat. Comm. 9, 3736 (2018); PRB 102, 115134 (2020).
[4] J. Herbrych et al., PNAS 117, 16226 (2020).
[5] N. Patel et al., PRB 94, 075119 (2016); PRB 96, 024520 (2017).
[6] H. Takahashi et al., Nat. Mater. 14, 1008 (2015); J. Ying et al., PRB 95, 241109(R) (2017).
[7] N. Patel et al., npj QM 5, 27 (2020).   

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The electrostatic doping of electrons into FeSe with the electric-double layer transistor (EDLT) structure enhances T_c of FeSe to 40 K. FeSe films with EDLT structure are also electrochemically etched by applying a gate voltage (V_g) at high temperature (>240 K). We previously suggested the existence of an electrochemical reaction layer at the surface of etched FeSe films which is responsible for the high T_c of 40 K [1]. If it is so, how T_c depends on V_g, might be complicated when compared with simple electrostatic doping.
We investigated the gate voltage dependence of T_c in electrochemically etched FeSe films. The T_c^zero value of the etched FeSe films with low gate voltages (V_g = 2.5 and 3.3 V) reaches 46 K, which is the highest value among almost all reported values from the resistivity measurements except for the data by Ge et al (Nat. Mat. 14, 285 (2015)). Our results suggest that the origin of the increase in T_c is not electrostatic doping but rather the electrochemical reaction at the surface of the etched films [2].
[1] S. Kouno et al., Sci. Rep. 8 14731 (2018).
[2] N. Shikama et al., Appl. Phys. Express 13, 083006 (2020).   

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Monolayer FeSe on a SrTiO₃ (STO) substrate is a high-temperature superconductor with reported T_c as high as 100 K, but the mechanism for such enhanced T_c remains poorly understood. Here we characterize the atomic structural and chemical composition of the FeSe/STO interface using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Our measurements reveal the presence of selenium in the top layers of STO, located on interstitial sites and in the TiO₂ layers. We support our measurement with density functional theory (DFT) calculations. We discuss implications of our findings on substrate-induced electron doping in the FeSe/STO heterostructure.   

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We measured the complex conductivity, σ, of the FeSe_1-xTe_x films below T_c which show a drastic increase of the superconducting transition temperature, T_c, when the nematic order disappears. Since the magnetic penetration depth, λ(> 400 nm) of Fe(Se,Te) is longer than the typical thickness of the film (~100 nm), we combined the coplanar-waveguide-resonator- and cavity-perturbation techniques to evaluate both the real and imaginary part of σ. The film with the nematic order showed a qualitatively different behavior in the quasiparticle scattering time compared with those without the nematic order, suggesting that the nematic order strongly influences the superconducting gap structure. On the other hand, T_c was proportional to n_s/m* (∝λ^-2) irrespective with the presence or the absence of the nematic order. This result indicates that the amount of the superfluid has a stronger impact on T_c of Fe(Se,Te) than the nematic order itself.   

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The iron-based superconductor FeSe exhibits electronic nematicity that coexists with superconductivity characterized by an anisotropic superconducting gap. Isovalent substitutions of S or Te for Se suppress the nematicity, offering opportunities to investigate the relationship between nematicity and superconductivity. It is also known that large amount of Te substitution (~ 50%) makes the band structure topologically nontrivial and creates a topological superconducting surface state, where the superconducting gap is isotropic. To investigate the roles of individual Te dopants on such electronic properties, we performed spectroscopic-imaging STM on lightly Te doped FeSe. We found that the isolated Te dopant hardly affects the anisotropic superconducting gap, whereas it alters the asymmetry in the density of states between filled and empty states. This may suggest that the change in the superconducting gap from anisotropic to isotropic upon Te substitution is related to the nematic and/or topological transitions.   

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Pairing in unconventional superconductors is one of the most important open questions in physics and materials science. Although superconductivity occurs along with magnetism in all systems having high transition temperatures (T_c) there exist various relevant proposals other than spin physics. Thus, an Fe-based material without the nesting paradigm of a central hole pocket and electron pockets encircling the X (Y) points is of crucial importance for clarifying the pairing mechanism. Here, we studied the electronic Raman spectra of (Li_1-xFe_x)OHFeSe as a function of light polarization and temperature. In the B_1g spectra two well resolved peaks appear below T_c, which originate from collective excitations. Given the experimental band structure of (Li_1-xFe_x)OHFeSe, our results favor pairing between the hybridized electron bands. Our work demonstrates the proximity of pairing states and the importance of band structure effects in the Fe-based compounds.   

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FeTe_1-xSe_x is a highly tunable high-T_c superconductor. It undergoes both a topological transition and an electronic nematic phase transition, around the same critical composition x_c where the superconducting transition temperature T_c peaks. This regime enables exploring the impact of electronic nematicity and symmetry-breaking strain on different electronic phases. Here, we use scanning tunneling microscopy and spectroscopy to visualize the electronic nematic transition in FeTe_1-xSe_x across x_c. At x_c, we discover the emergence of nanoscale regions hosting electronic nematicity with suppressed superconductivity, embedded within the non-nematic superconducting matrix. We conclude by discussing the role of anisotropic strain on the emergence of nematic regions, which could impact exotic topological phenomena reported in this system.   

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Remarkable enhancement of the superconducting transition temperature (T_c) has been observed for monolayer (ML) FeSe films grown on SrTiO₃ substrates, triggering a surge of research excitement. The mechanism for the Tc enhancement has been the central question, as it may present a new strategy for searching for higher T_c materials. In this talk, I will first show the combined studies of interfacial engineering, ARPES studies and synchrotron-based surface X-ray diffraction studies on FeSe/ SrTiO₃ interfaces, through which we reveal the electronic structure ingredients and the atomic-scale structure of ML FeSe/ SrTiO₃. Our results point to the fascinating prospect that cooperation between different Cooper pairing channels may be a general framework to understand and design high-temperature superconductors. Next, I will show the superconductivity and electronic structure studies on more interfaces consisting of FeSe and transition metal oxides without Ti-O bonds. These results not only enrich the family of superconducting interfaces, but also provides a testbed for understanding the key ingredients of interfacial high-temperature superconductivity.   

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The potassium dosing is a powerful tool to induce electron doping of surfaces and bulk electronic states that can be directly probed using angle-resolved photoemission spectroscopy. Here, we explore the effect of surface potassium doping of a tetragonal bulk FeSe_1-xS_x superconductor. We find that the system is efficiently doped with electrons and we can follow independently the changes in the band structure and electronic correlations of the electronic states that can be directly probed using angle-resolved photoemission spectroscopy. We identified the opening of the superconducting gap with a high critical temperature and enhanced electronic correlations despite the absence of nematic electronic phases.   

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FeSe has attracted much attention because it has a nonmagnetic nematic order. When selenium is substituted by the isovalent element, sulfur, a nematic quantum critical point without magnetism appears. In this respect, FeSe-based materials have been a unique platform for studying the relationship between the nematic phase and the superconductivity. However, in FeSe_1-xS_x, it has been reported that the superconducting properties abruptly change at the nematic end point (x = 0.17) and therfore understanding the role of the nematic fluctuations in superconductivity is complicated.
In this study, we focus on FeSe_1-xTe_x in which selenium is substituted by tellurium. We have synthesized bulk single crystals of FeSe_1-xTe_x (0 < x < 0.55) by using the chemical vapor transport method. The temperature dependence of the electrical resistivity was measured on the FeSe_1-xTe_x crystals under pressure up to 8 GPa. As a result, we have established the three-dimensional electronic phase diagram; temperature against pressure and Te-substitution. In the obtained phase diagram, the superconducting dome emerges close to the nematic phase and far from the magnetic phase, implying that the nematic fuctuations can enhance superconducting transition temperature in this system.   

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Uniaxial strain has emerged as a prominent tuning tool for accessing previously unexplored regions of phase space in quantum materials. In particular, in iron-based superconductors the susceptibility of the electronic structure to applied uniaxial strain has opened up new avenues to create and control varied quantum phases.
In our research programme, we have explored the magnetic and superconducting phase diagram under uniaxial strain along [110] crystallographic direction of FeTe. FeTe is an ‘11’ non-superconducting parent compound with double stripe AFM order ground state having (π,0) propagation vector contrary to the more typical (π, π) order. We combined low-temperature magnetoresistivity measurements in a force cell and direct spectroscopic imaging with STM, which revealed (π, π) ordering under uniaxial strain. In light of our results on this and other compounds we discuss the possibility of stabilising new order both on the surface and in bulk under experimentally accessible strains in iron-based superconductor.