Session S11: Fe-based Superconductivity Under Extreme Conditions

High pressure route to high-Tc superconductivity in the FeSe-based materials

High pressure is an effective and clean tuning knob to tip the balance of competing interactions in the strongly correlated electron systems. Although FeSe is nonmagnetic within the nematic state below T_s = 90 K, the application of high pressure can induce a static magnetic order and promote high-T_c superconductivity. These facts make FeSe an ideal platform to study the interplay of superconductivity with magnetism and nematicity. In addition, high-T_c superconductivity can be achieved in various heavily electron-doped (HED) FeSe-derived materials, including A_xFe_2-ySe₂, (Li,Fe)OHFeSe, and monolayer FeSe/SrTiO₃. More intriguingly, a second high-T_c superconducting (SC-II) phase has been reported in the pressurized A_xFe_2-ySe₂ and K-doped FeSe films.To shed more light on these intriguing issues, we recently performed detailed magneto-transport measurements on bulk FeSe and the HED FeSe-based materials under high pressure. For FeSe, we constructed a comprehensive T-P phase diagram featured by a dome-shaped magnetic phase interconnecting with the nematic order and the high-T_c superconductivity [1], and further demonstrated the presence of hole-like Fermi surface and enhanced spin fluctuations near the optimal T_c = 38.5 K at 6 GPa [2]. For the HED (Li_1-xFe_x)OHFeSe, we observed the emergence of SC-II phase above P_c = 5 GPa, and uncovered a sharp transition of the normal state from a Fermi liquid for P < P_c to a non Fermi liquid for P > P_c [3]. The emergence of SC-II phase seems to be a universal phenomenon for these HED FeSe materials as also demonstrated in Li_x(NH₃)_yFe₂Se₂ with the T_c^max = 55 K achieved under high pressure [4].
References:
[1] J. P. Sun, et al., Nat. Commu. 7, 12146 (2016).
[2] J. P. Sun, et al., Phys. Rev. Lett. 118, 147004 (2017).
[3] J. P. Sun, et al., ArXiv: 17007.06735.
[4] P. Shahi, et al., ArXiv: 1709.08455.   

Unusually stronger quantum fluctuation with larger spins: Novel phenomena revealed by emergent magnetism in pressurized high-temperature superconductor FeSe

A counter-intuitive enhancement of quantum fluctuation with larger spins, together with a few novel physical phenomena, is discovered in studying the recently observed emergent magnetism in high-temperature superconductor FeSe under pressure. Starting with experimental crystalline structure from our high-pressure X-ray refinement, we analyze theoretically the stability of the magnetically ordered state with a realistic spin-fermion model. We find surprisingly that in comparison with the magnetically ordered Fe-pnictides, the larger spins in FeSe suffer even stronger long-range quantum fluctuation that diminishes their ordering at ambient pressure. This "fail-to-order" state then develops into an ordered state above 1GPa due to weakened fluctuation accompanying the reduction of anion height and carrier density. We further clarify the controversial nature of magnetism and its interplay with nematicity in FeSe in the same unified picture for all Fe-based superconductors. Our study establishes a generic exceptional paradigm of stronger quantum fluctuation with larger spins that complements the standard knowledge of insulating magnetism.   

Transport properties of FeSe thin films with various degrees of strain

Iron chalcogenide superconductor, FeSe, is a suitable material to investigate the role of the nematic order for the superconductivity in iron pnictides and chalcogenides because it does not show a magnetic order, different from other iron based materials. In order to clarify the relation between the superconductivity and the nematicity in iron chalcogenides, we investigated effects of in-plane strain in FeSe films. This method is more favorable than studies on chemical pressure effects in that there is no need for substitution of impurity.
We have successfully grown FeSe thin films with systematically varied degrees of strain, from tensile to compressive strain, via pulsed laser deposition. Magneto-transport measurements of the grown films suggested that the carrier density increased as the a-axis length shrinks, consistent with an ARPES observation[1]. The superconducting transition temperature also varied depending on the strain from below 2 K up to 12 K, which is higher than that of bulk crystals, while only a slight change was observed in the nematic transition temperature. These results imply that the nematic order is not important for superconductivity in this material.
[1] G. N. Phan et al., PRB 95, 224507 (2017).   

Persistent Interfacial High-Tc Superconductivity in Multi-layer FeSe/SrTiO3 Films

The interface between FeSe and SrTiO3 has attracted considerable attention owing primarily to the large enhancement of Tc (70 K vs 8 K in bulk). Most studies to date have utilized surface-sensitive spectroscopic measurements, necessitating the use of monolayer films to allow access to the interfacial superconductivity. The effect of additional FeSe layers on top of the initial monolayer remains an important but open question for understanding the nature of the Tc enhancement. As the air sensitivity of FeSe thin films precludes traditional ex situ transport measurements without potentially detrimental capping layers, we utilize an in situ four-point electrical contact probe to measure superconductivity at the buried FeSe/STO interface for films ranging in thickness from 1-20uc. We show that the Tc of the interfacial monolayer is robust and largely unaffected by additional FeSe layers, and that the earlier reported reduction of Tc in thicker films can be explained by a simple parallel conduction channel model which reproduces the features of our observed thickness dependence. This demonstrates that the contribution of additional FeSe layers does not affect superconductivity at the buried interface, placing important constraints on the mechanism of the Tc enhancement.   

Phonon-enhanced superconductivity at the FeSe/SrTiO3 interface

Single-layer FeSe on a SrTiO₃ substrate, with its extraordinarily large T_c amongst all interfacial superconductors and iron based superconductors, is particularly interesting, but the mechanism underlying its high T_c has remained mysterious. Here we show through isotope effects that electrons in FeSe couple with the oxygen phonons in the substrate, and the superconductivity is enhanced linearly with the coupling strength atop the intrinsic superconductivity of optimally-electron-doped FeSe. Our observations solve the enigma of FeSe/SrTiO₃, and experimentally establish the critical role and unique behavior of electron-phonon forward scattering in a correlated high-T_c superconductor. The effective cooperation between interlayer electron-phonon interactions and correlations suggests a path forward in developing more high-T_c interfacial superconductors, and may shed light on understanding the high T_c of bulk high temperature superconductors with layered structures.   

Interfacial Polaron-Mediated Electron Pairing in Single Unit-Cell FeSe Grown on SrTiO3

The recent discovery of high-temperature superconductivity in a single unit-cell FeSe layer grown on SrTiO₃ (STO) is stimulating intensive research efforts on exploration of the dominant mechanism. Among the various competing scenarios, strong interaction between the longitude optical (LO) phonon mode in STO and electrons in FeSe is likely to enhance superconductivity. Earlier experimental studies have shown that the LO phonon energy is ~100 meV, while the FeSe Fermi energy is only ~60 meV. Based on these observations, we propose the interfacial electron-phonon interaction to be non-adiabatic, and the associated phonon-mediated electron pairing lies between the BCS-BEC crossover regime. To capture the essential physics in this regime, an interfacial polaron picture is developed to account for the enhanced superconductivity. Specifically, by considering both the polaron-induced attraction and Coulomb repulsion between electrons on adjacent Fe sites, we obtain an effective total attractive interaction between the electrons. We also provide a quantitative estimate of the superconductivity enhancement within the interfacial polaron picture using physically realistic parameters, and discuss these results in connection with our latest experimental observations.   

Pressure effects on the magnetic order of Ba(Fe0.946Co0.054)2As2

Cobalt substitution for iron in BaFe₂As₂ produces a superconducting ground state with indications that antiferromagnetism (AFM) and superconductivity (SC) compete for the same itinerant electrons. Partial cobalt substitution changes the relative sizes of the hole- and electron-like fermi surface pockets responsible for AFM in BaFe₂As₂ with two effects: 1) the magnitude of the ordered moment decreases with increasing cobalt concentration; and 2) beyond a critical concentration commensurate nesting is lost and a small transverse incommensurability appears which maintains long range magnetic order. Magnetic susceptibility and resistivity measurements in applied pressure show suppression of AFM and appearance of SC above 3.5 GPa in BaFe₂As₂ and, similarly, suppression of AFM and enhancement of SC below 2 GPa in Ba(Fe_1–xCo_x)₂As₂. Such measurements could not determine if the suppression of AFM via pressure is due to reduced fermi surface nesting, leaving open the question of whether the mechanisms which produce SC in BaFe₂As₂ and Ba(Fe_1–xCo_x)₂As₂ are the same.
Results of single crystal neutron diffraction measurements which were performed under applied pressure to track the suppression of AFM order and look for IC spin density wave order in Ba(Fe_0.946Co_0.054)₂As₂ will be discussed.   

Effects of strain and disorder nematicity of Ba1-xKxFe2As2 superconductor

Hole doping by substituting Ba for K in BaFe₂As₂ parent compound results in a very complex phase diagram that includes nematic, stripe, orthorhombic (C2) and tetragonal (C4) antiferromagnetic phases with substantial doping intervals where these phases coexist with superconductivity. We use controlled uniaxial strain (mechanical) and artificial disorder (induced by electron irradiation) to study the interplay of these phases in the under-doped compositions. Experimentally, anisotropic in-plane resistivity was measured in high quality single crystals of (Ba_1-xK_x)Fe₂As₂ in pristine state and after low-temperature electron irradiation. Polarized optical microscopy was used to visualize the evolution of structural domains and correlate it with transport studies.   

Possible interface induced superconductivity in un-doped CaFe2As2

The superconductivity with a T_c up to 25 K was induced through a proper annealing process in un-doped CaFe₂As₂. A series of low temperature X-ray diffraction (LTXRD) studies were carried out and we found that the emergence of superconductivity is closely tied to the mixture of the PI and PII phases of CaFe₂As₂. The experimental results indicate that the origin of superconductivity arises from the interface of the mesoscopically stacked layers of PI and PII. Using negative binomial distribution, we simulated the possible phase distribution of samples under different thermal treatments. The simulation results are consistent with the LTXRD data. These simulations were further supported by low temperature scanning transmission electron microscopy (LTSTEM) measurements. The LTXRD, LTSTEM, and simulation results we shall present strongly support the proposal of interface-enhanced superconductivity in un-doped CaFe₂As₂.   

Superconductivity in Undoped CaFe2As2 Single Crystals at Ambient Pressure

Among the many theoretical mechanisms proposed for higher T_c, the interfacial mechanism provides not only continual inspiration but also hope. For pristine CaAs₂Fe₂ (Ca122) single crystals, previous studies have shown the existence of complex phase, but no superconductivity was reported at ambient pressure. Recently, we identified two different non-superconducting phases in Ca122 through thermal treatments. Such thermal treatments offer a reproducible, controllable, and reversible environment in which many characterization techniques can be applied. We successfully tuned a mixed-phase in Ca122 through a proper annealing process and induced the superconductivity with a T_c up to 25 K at ambient pressure[1]. The transport, magnetic, and room temperature X-ray results lead us to the conjecture that the origin of superconductivity arises from the interface of the mesoscopically stacked layers of PI and PII. To further understand the underlying mechanism, systematic low-temperature X-ray, STEM, and high-pressure measurements were conducted over a series of thermal treated Ca122 single crystals[2]. This work provides the first direct evidence of interfacial induced superconductivity in undoped Ca122 single crystals.   

Pressure-temperature phase diagrams of CaK(Fe1-xNix)4As4

Recently, hedgehog spin-vortex crystal magnetism has been identified experimentally in Ni-doped iron-based superconductor CaKFe₄As₄[1]. In this work, the pressure dependence of the superconducting and magnetic transition temperatures was studied on Ni-doped CaKFe₄As₄ single crystals. In-plane resistivity was measured under hydrostatic pressure up to ~5 GPa with magnetic fields parallel to tetragonal c-axis. We will present the pressure-temperature phase diagrams of CaK(Fe_1-xNi_x)₄As₄. Evolution of pressure response with composition is explored and the interaction between superconductivity and magnetic order will be discussed.
[1] Reference W.R.Meier et al. arXiv:1706.01067   

Role of Spin Vortex Crystal Fluctuations in the Pressure-induced Half-collapse Transition in CaKFe4As4 Superconductor

Structural transitions under pressure are studied for the recently discovered Fe-based superconductor CaKFe₄As₄ using density functional theory. Under a hydrostatic pressure of 4 GPa, this material undergoes a transition to a half-collapsed phase, which is the first known example of such transition [PRB 96, 140501(R) (2017)]. A half-collapsed phase is marked by an enhancement of only half of the As-As bonds, which is sufficient to fully suppress bulk superconductivity. Good agreement between the simulated structures and the measured ones is obtained when a spin vortex configuration is imposed on the Fe sublattice. Interestingly, spin fluctuations of similar nature are confirmed by the NMR analysis [PRB 96, 104512 (2017)]. Furthermore, electron doping leads to condensation of these fluctuations and the formation of a spin vortex crystal [arXiv:1706.01067], which is unique among iron pnictides.

This work was done in collaboration with Paul Canfield, Anna Böhmer, Sergey L. Bud'ko, Yuji Furukawa, Udhara Kaluarachchi, William Meier, Peter Orth (Ames Lab), Rafael Fernandes (Univ Minnesota) and Christian Batista (Univ Tennessee).   

Response of CaK(Fe1-xNix)4As4 (x = 0 and 0.05) superconductor to non-magnetic disorder induced by 2.5 MeV electron irradiation

Temperature dependent electrical resistivity and London penetration depth were measured in single crystals of CaK(Fe_1-xNi_x)₄As₄ (x = 0 and 0.05, as determined by wavelength dispersive spectroscopy) before and after 2.5 MeV electron irradiation of different doses. We observed increase of residual resistivity and suppression of the superconducting transition T_c, while the low temperature behavior of the penetration depth remained exponential, consistent with a robust s_± pairing. Overall, the x=0 compound is quite similar to a slightly overdoped BaK_1-yK_yFe₂As₂ with y ≈ 0.5. The Ni-doped composition, x=0.05, can be mapped onto the underdoped BaK_1-yK_yFe₂As₂ with y ≈ 0.2 by using its T_c value and shows similar increase of the superconducting gap anisotropy, confirming that Ni doping moves the composition toward the electron doped side.