Superconductivity In FeSe Research Articles

Iron Selenide Intercalated by Uncharged Molecules: Superconductivity in FeSe(C2H7NO)0.3

AbstractThe charge‐neutral intercalated iron selenide compound FeSe(C2H7NO)0.3 is formed from FeSe(C2H8N2)0.3 through amine exchange under solvothermal conditions. Although the exchanged molecules are of similar size, the distance of the FeSe‐layers increases from 10.68 Å to 13.58 Å. The combination of X‐ray diffraction and DFT calculations suggests a chemically reasonable, albeit somewhat simplified, model for the orientation of the molecules between the FeSe‐layers, which interact via weak hydrogen bonds. The neutral guest species is unable to transfer charge to the FeSe layers, which remain electronically undoped. While the starting compound is non‐superconducting, the resulting product exhibits a transition to superconductivity at Tc=14 K. The topology of the calculated Fermi surface undergoes only minimal alteration with increased layer spacing, a change that is considerably less pronounced than that observed with electron doping. Our findings indicate that undoped FeSe layers become superconducting when the interlayer distance is sufficiently large, while the highest critical temperatures necessitate additional electron doping.

Evidence for nematic fluctuations in FeSe superconductor: a 57Fe Mössbauer spectroscopy study

There has been controversy about the driving force of the nematic order in the FeSe superconductor. Here, we present a detailed study of the57Fe Mössbauer spectra of FeSe single-crystal powders, focusing on the temperature dependences of the hyperfine parameters in the vicinity of the nematic transition temperature,Ts∼ 90 K. The nematicity-induced splitting ofdxzanddyzbands, obtained from the anomalous increase in quadrupole splitting nearTs, starts at 143 K. The temperature evolution of the lattice dynamics, deduced from the recoilless fractions and second-order Doppler shifts, is found to undergo successively two segments of phonon-softening (160 K-105 K) and phonon-hardening (105 K-90 K), related to the appearance of local orthorhombic distortions aboveTsand the establishing way of the associated nematic correlations. Analysis of the linewidths shows that spin fluctuations occur not only below 70 K but also acrossTs(105 K-70 K), accompanied by the non-Fermi liquid behavior of the electrons. The results demonstrate the strong interactions between lattice, spin, and electron degrees of freedom in the vicinity ofTsand that the lattice degrees of freedom may play an essential role in driving the nematic order for FeSe.

Picosecond Trajectory of Two-Dimensional Vortex Motion in FeSe_{0.5}Te_{0.5} Visualized by Terahertz Second Harmonic Generation.

We have investigated the vortex dynamics in a thin film of an iron-based superconductor FeSe_{0.5}Te_{0.5} by observing second-harmonic generation (SHG) in the terahertz frequency range. We visualized the picosecond trajectory of two-dimensional vortex motion in a pinning potential tilted by Meissner shielding current. The SHG perpendicular to the driving field is observed, corresponding to the nonreciprocal nonlinear Hall effect under the current-induced inversion symmetry breaking, whereas the linear Hall effect is negligible. The estimated vortex mass, as light as a bare electron, suggests that the vortex core moves independently from quasiparticles at such a high frequency and large velocity ≈300 km/s.

The origin of the large Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$T_{\\mathrm{c}}$\\end{document} variation in FeSe thin films probed by dual-beam pulsed laser deposition

FeSe is one of the most enigmatic superconductors. Among the family of iron-based compounds, it has the simplest chemical makeup and structure, and yet it displays superconducting transition temperature (Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$T_{\ ext{c}}$\\end{document}) spanning 0 to 15 K for thin films, while it is typically 8 K for single crystals. This large variation of Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$T_{\ ext{c}}$\\end{document} within one family underscores a key challenge associated with understanding superconductivity in iron chalcogenides. Here, using a dual-beam pulsed laser deposition (PLD) approach, we have fabricated a unique lattice-constant gradient thin film of FeSe which has revealed a clear relationship between the atomic structure and the superconducting transition temperature for the first time. The dual-beam PLD that generates laser fluence gradient inside the plasma plume has resulted in a continuous variation in distribution of edge dislocations within a single film, and a precise correlation between the lattice constant and Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$T_{\ ext{c}}$\\end{document} has been observed here, namely, Tc∝c−c0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$T_{\ ext{c}} \\propto \\sqrt{c- c_{0}}$\\end{document}, where c is the c-axis lattice constant (and c0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$c_{0}$\\end{document} is a constant). This explicit relation in conjunction with a theoretical investigation indicates that it is the shifting of the dxy\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$d_{\ ext{xy}}$\\end{document} orbital of Fe which plays a governing role in the interplay between nematicity and superconductivity in FeSe.

High-Temperature Anomalous Metal States in Iron-Based Interface Superconductors.

The nature of the anomalous metal state has been a major puzzle in condensed matter physics for more than three decades. Here, we report systematic investigation and modulation of the anomalous metal states in high-temperature interface superconductor FeSe films on SrTiO_{3} substrate. Remarkably, under zero magnetic field, the anomalous metal state persists up to 20K in pristine FeSe films, an exceptionally high temperature standing out from previous observations. In stark contrast, for the FeSe films with nanohole arrays, the characteristic temperature of the anomalous metal state is considerably reduced. We demonstrate that the observed anomalous metal states originate from the quantum tunneling of vortices adjusted by the Ohmic dissipation. Our work offers a perspective for understanding the origin and modulation of the anomalous metal states in two-dimensional bosonic systems.

Orbital-Selective Mott Transition Effects and Nontrivial Topology of Iron Chalcogenide.

The iron-based superconductor FeSe_{1-x}Te_{x} has recently gained significant attention as a host of two distinct physical phenomena: (i)Majorana zero modes that can serve as potential topologically protected qubits, and (ii)a realization of the orbital-selective Mott transition. In this Letter, we connect these two phenomena and provide new insights into the interplay between strong electronic correlations and nontrivial topology in FeSe_{1-x}Te_{x}. Using linearized quasiparticle self-consistent GW plus dynamical mean-field theory, we show that the topologically protected Dirac surface state has substantial Fe(d_{xy}) character. The proximity to the orbital-selective Mott transition plays a dual role: it facilitates the appearance of the topological surface state by bringing the Dirac cone close to the chemical potential but destroys the Z_{2} topological superconductivity when the system is too close to the orbital-selective Mott phase. We derive a reduced effective Hamiltonian that describes the topological band. Its parameters capture all the chemical trends found in the first principles calculation. Our findings provide a framework for further study of the interplay between strong electronic correlations and nontrivial topology in other iron-based superconductors.

Effect of interstitial iron on Jc and Hc2 anisotropy and magnetic relaxation of Fe1+yTe0.6Se0.4 single crystal

For a long time, the second peak effect in FeTe0.6Se0.4 has attracted the interest of researchers. The role of interstitial iron which is unique to FeTe0.6Se0.4 compared to FeSe superconductors, in the second peak effect remains unknown. In this study, we controlled the content of interstitial iron through flowing oxygen and prolonged vacuum annealing, and subsequently conducted investigations on the magnetic flux dynamics and superconducting properties with anisotropies. After reducing the interstitial iron content, Tc increased from 12.6 K to 13.8 K. The critical current density at 4.5 K increased from 1.2×105 A/cm2 to 1.76×105 A/cm2. At low fields (H<Honset), interstitial iron as weak pinning centers can rapidly trap magnetic flux and exhibit fast relaxation through elastic motion. At high fields, strong pinning centers lead to collective pinning of magnetic flux and demonstrate a reduction in relaxation rate through plastic motion. The presence of interstitial iron hinders the transition from elastic to plastic motion for magnetic flux. Furthermore, reducing the interstitial iron expands the field range of the second peak effect and leads to a noticeable change in the pinning behavior, with a transition from δl-type to δTc-type pinning and enhances the anisotropy of the system. Finally, our study provides a reference for understanding the origin of the second peak in FeTe0.6Se0.4 and offers guidance for the practical application of FeTe0.6Se0.4.

Investigation of pressure impact on physical characteristics of FeSe chalcogenide superconductor: Insights from first principles calculation

Understanding the high-pressure physical characteristics of iron selenium (FeSe) superconductors is crucial to comprehending their superconducting abilities. First-principles Density Functional Theory has been computed in the present calculations to analyze the pressure impact on structural, mechanical, electronic and superconducting characteristics along with the Debye and melting temperatures of tetragonal FeSe. The analysis of mechanical characteristics suggests that this compound remains mechanically stable up to a pressure of 20 GPa. The applied hydrostatic pressure is observed to have a significant impact on the elastic moduli and the anisotropic behaviour of FeSe at all pressure conditions. Similarly, the first indication of ductility is observed approximately at 8 GPa and then it increases as pressure rises. At 0 GPa, the optimized structural values are seen to line up with the available data. The variation of structural parameters and electronic factors of FeSe with applied pressure is also mentioned here. Moreover, the assessment of the compound’s electronic characteristics provides strong evidence for its metallic nature and the rise in a number of states at the Fermi energy level with applied pressure may be the cause for the increase in the superconducting critical temperature of FeSe with pressure.

Iron Vacancy Tunable Superconductor-Insulator Transition in FeSe/SrTiO_{3} Monolayer.

The Fe_{4}Se_{5} with a sqrt[5]×sqrt[5] Fe vacancy order is suggested to be a Mott insulator and the parent state of bulk FeSe superconductor. The iron vacancy ordered state has been considered as a Mott insulator and the parent compound of bulk FeSe-based superconductors. However, for the superconducting FeSe/SrTiO_{3} monolayer (FeSe/STO) with an interface-enhanced high transition temperature (T_{c}), the electronic evolution from its Fe vacancy ordered parent phase to the superconducting state, has not been explored due to the challenge to realize an Fe vacancy order in the FeSe/STO monolayer, even though important to the understanding of superconductivity mechanism. In this study, we developed a new method to generate Fe vacancies within the FeSe/STO monolayer in a tunable fashion, with the assistance of atomic hydrogen. As a consequence, an insulating sqrt[5]×sqrt[5] Fe vacancy ordered monolayer is realized as the parent state. By using scanning tunneling microscopy and scanning tunneling spectroscopy, the spectral evolution from superconductivity to insulator is fully characterized. Surprisingly, a prominent spectral weight transfer occurs, thus implying a strong electron correlation effect. Moreover, the Fe vacancy induced insulating gap exhibits no Mott gap-like features. This work provides new insights in understanding the high-T_{c} superconductivity in FeSe/STO monolayer.

Study of the Electronic Structure for Te-Doped FeSe Superconductor Prepared by Solvothermal Method

Study of the Electronic Structure for Te-Doped FeSe Superconductor Prepared by Solvothermal Method

Metal Chalcogenide-Hydroxide Hybrids as an Emerging Family of Two-Dimensional Heterolayered Materials: An Early Review.

Two-dimensional (2D) materials and phenomena attract huge attention in modern science. Herein, we introduce a family of layered materials inspired by the minerals valleriite and tochilinite, which are composed of alternating "incompatible", and often incommensurate, quasi-atomic sheets of transition metal chalcogenide (sulfides and selenides of Fe, Fe-Cu and other metals) and hydroxide of Mg, Al, Fe, Li, etc., stacked via electrostatic interaction rather than van der Waals forces. We survey the data available on the composition and structure of the layered minerals, laboratory syntheses of such materials and the effect of reaction conditions on the phase purity, morphology and composition of the products. The spectroscopic results (Mössbauer, X-ray photoelectron, X-ray absorption, Raman, UV-vis, etc.), physical (electron, magnetic, optical and some others) characteristics, a specificity of thermal behavior of the materials are discussed. The family of superconductors (FeSe)·(Li,Fe)(OH) having a similar layered structure is briefly considered too. Finally, promising research directions and applications of the valleriite-type substances as a new class of prospective multifunctional 2D materials are outlined.

Pure nematic state in the iron-based superconductor FeSe

Pure nematic state in the iron-based superconductor FeSe

Atomic-Scale Andreev Probe of Unconventional Superconductivity.

Recent emergence of low-dimensional unconventional superconductors and their exotic interface properties calls for new approaches to probe the pairing symmetry, a fundamental and frequently elusive property of the superconducting condensate. Here, we introduce the unique capability of tunneling Andreev reflection (TAR) to probe unconventional pairing symmetry, utilizing the sensitivity of this technique to specific Andreev reflections. Specifically, suppression of the lowest-order Andreev reflection due to quantum interference but emergence of the higher-order Andreev processes provides direct evidence of the sign-changing order parameter in the paradigmatic FeSe superconductor. TAR spectroscopy also reveals two superconducting gaps, points to a possibility of a nodal gap structure, and directly confirms that superconductivity is locally suppressed along the nematic twin boundary, with preferential and near-complete suppression of the larger energy gap. Our findings therefore enable new, atomic-scale insight into microscopic, inhomogeneous, and interfacial properties of emerging quantum materials.

Turbulent Structure and Characterization of ‘11’-type Iron-based Superconductors by Magneto-optical Imaging

Magneto-optical imaging is employed to investigate the vortex penetration modes in ‘11’-type iron-based superconductors (IBSs) Fe(Te0.6Se0.4) and FeSe single crystals. In Fe(Te0.6Se0.4), vortices penetrate mostly homogeneously from all edges, forming an unstable turbulent interface with pre-existing anti-vortices, where ‘Meissner hole’ emerges as those have been reported in ‘122’-type IBSs. In FeSe, however, the penetration is restricted to numerous mesoscopic stripe-like regions. Further experiments revealed an inhomogeneous vortex propagation pattern, where vortices are most likely to be channeled along the twin boundaries.

Microwave Flux-flow Hall Effect in a Multi-band Superconductor FeSe

We have measured the flux-flow Hall effect in a multi-band superconductor FeSe pure single crystal to investigate the nature of the vortex core state by means of the cross-shaped bimodal cavity technique. We found that the flux-flow Hall angle of FeSe at low temperatures is about 0.5, which is equal to or smaller than that evaluated by the effective viscous drag coefficient measurements. This feature is in contrast to the cuprate superconductors. The conductivity tensor of multi-band superconductors that are contributed from holes and electrons shows partial cancellation of the flux-flow Hall voltage by the electrons and holes, which can explain the observed feature. Therefore, our study suggests the appearance of the multi-band nature in the vortex dynamics.

The effect of long-term exposure at room temperature on the thermal conductivity of the FeSe superconductor in the normal state

The temperature dependence of the thermal conductivity of the iron-based FeSe0.94 superconductor sample has been studied in the range of 4.2–200 K after long-term exposure for 7 years. The temperature dependence of thermal conductivity is predominantly phonon. This dependence was approximated taking into account phonon scattering by the boundaries of the sample, as well as scattering by dislocations, point defects, and phonon-phonon scattering. When comparing the parameters of this approximation with similar parameters for literature data for a freshly prepared polycrystalline sample, it turned out that long-term exposure led to a significant increase in all parameters. That is, long-term exposure causes a significant increase in the number of dislocations, and point defects and intensifies phonon-phonon scattering. For both samples, we found no effect of the structural phase transition at T ≈ 75 K from the tetragonal phase into the orthorhombic one on the thermal conductivity.

Direct Observation of Quantum Anomalous Vortex in Fe(Se,Te)

Vortices are topological defects of type-II superconductors in an external magnetic field. In a similar fashion to a quantum anomalous Hall insulator, quantum anomalous vortex (QAV) spontaneously nucleates due to orbital-and-spin exchange interaction between vortex core states and magnetic impurity moment, breaking time-reversal symmetry (TRS) of the vortex without an external field. Here, we used scanning superconducting quantum interference device microscopy (sSQUID) to search for its signatures in iron-chalcogenide superconductor Fe(Se,Te). Under zero magnetic field, we found a stochastic distribution of isolated anomalous vortices and antivortices with flux quanta $\Phi_0$. By applying a small local magnetic field under the coil of the nano-SQUID device, we observed hysteretic flipping of the vortices reminiscent of the switching of ferromagnetic domains, suggesting locally broken-TRS. We further showed vectorial rotation of a flux line linking a paired vortex-antivortex with the local field. These unique properties of the anomalous vortices satisfied the defining criteria of QAV. Our observation suggests a quantum vortex phase with spontaneous broken-TRS in a high-temperature superconductor.

Disordered hyperuniform vortex matter with rhombic distortions in FeSe at low fields

In the current quest to synthesize hyperuniform materials with interesting applications, addressing the coupling of the objects composing the system to the physical properties of the host medium is crucial. With this aim we study a model system: vortices in the FeSe superconductor subject to a considerable magneto-elastic coupling with the host sample. We reveal that the low-field FeSe vortex structure is of the weakest hyperuniform type possibly due to the relevance of the anisotropic and long-ranged interaction term introduced by the magneto-elastic coupling. This work indicates that it is possible to tailor the hyperuniformity class of material systems by tuning the coupling of interacting objects with elastic properties of the host medium.

Boundary Effect and Critical Temperature of Two-Band Superconducting FeSe Films

Based on two-band Bogoliubov–de Gennes theory, we study the boundary effect of an interface between a two-gap superconductor FeSe and insulator (or vacuum). New boundary terms are introduced into two-band Ginzburg–Landau formalism, which modifies the boundary conditions for the corresponding order parameters of superconductor. The theory allows for a mean-field calculation of the critical temperature suppression with the decrease in FeSe film thickness. Our numerical results are in good agreement with the experimental data observed in this material.

Enhancement of superconductivity in multilayer FeSe film by Nb coating

Recently, the superconductivity of FeSe has been greatly enhanced in one-unit-cell films and ultrathin surface doped films grown on STO substrates, which lead to extensive research on the interfacial effect and charge carriers doping in FeSe films. Herein, by using ion beam sputtering deposition, transition metal (Nb) is successfully coated on the multilayer FeSe films and the superconductivity of FeSe is significantly enhanced compared to pristine FeSe. The superconducting transition temperature (Tc) varies with the thickness of Nb coatings and the highest Tc up to 13.0 K is obtained in the FeSe film with Nb coating about 10 nm. X-ray diffraction analysis (XRD) demonstrates Nb incorporates in FeSe lattice. In particular, the existence of Nb5+ in FeSe films is confirmed by the X-ray photoelectron spectroscopy (XPS). The salient change of Nb binding energy measured by XPS demonstrate the binding and hole-carriers doping mechanisms between Nb5+ and FeSe layers. In particular, the positive Hall coefficients demonstrate the hole carriers dominant in Nb-doped FeSe films. This work demonstrates the doping characteristics of Fe site in FeSe, providing an avenue for exploring diverse superconductivity of FeSe by transition metal doping.