Iron-based High-temperature Superconductors Research Articles

Superstrength permanent magnets with iron-based superconductors by data- and researcher-driven process design

Iron-based high-temperature (high-Tc) superconductors have good potential to serve as materials in next-generation superstrength quasipermanent magnets owing to their distinctive topological and superconducting properties. However, their unconventional high-Tc superconductivity paradoxically associates with anisotropic pairing and short coherence lengths, causing challenges by inhibiting supercurrent transport at grain boundaries in polycrystalline materials. In this study, we employ machine learning to manipulate intricate polycrystalline microstructures through a process design that integrates researcher- and data-driven approaches via tailored software. Our approach results in a bulk Ba0.6K0.4Fe2As2 permanent magnet with a magnetic field that is 2.7 times stronger than that previously reported. Additionally, we demonstrate magnetic field stability exceeding 0.1 ppm/h for a practical 1.5 T permanent magnet, which is a vital aspect of medical magnetic resonance imaging. Nanostructural analysis reveals contrasting outcomes from data- and researcher-driven processes, showing that high-density defects and bipolarized grain boundary spacing distributions are primary contributors to the magnet’s exceptional strength and stability.

Zero energy bound states on nano atomic line defect in iron-based high temperature superconductors

Motivated by recent scanning tunneling microscopy experiments on Fe atomic line defect in iron-based high temperature superconductors, we explore the origin of the zero energy bound states near the endpoints of the line defect by employing the two-orbit four-band tight binding model. With increasing the strength of the Rashba spin–orbit coupling along the line defect, the zero energy resonance peaks move simultaneously forward to negative energy for s+− pairing symmetry, but split for s++ pairing symmetry. The superconducting order parameter correction due to As(Te, Se) atoms missing does not shift the zero energy resonance peaks. Such the zero energy bound states are induced by the weak magnetic order rather than the strong Rashba spin–orbit coupling on Fe atomic line defect.

Theoretical investigation of the Co-occurrence of superconductivity and antiferromagnetism in iron-based high-temperature superconductors

In this research work, the Co-occurrence of superconductivity and antiferromagnetism in Ca0.74 1 La0.26 1 Fe1−xCox As2 iron-based high-temperature superconductors have been investigated. Based on the multi-band nature of Ca0.74 1 La0.26 1 Fe1−xCox As2 iron-based superconductors, a two-band model Hamiltonian that contains intra- and inter-bands is developed. By employing the matrix form of the temperature-dependent Green’s function formalism with the two-band Hamiltonian, the mathematical expressions of superconducting transition temperature as functions of the superconducting order parameter and antiferromagnetism translational temperature as a function of the magnetic order parameter are obtained, respectively. The plotted graph of the superconducting and magnetic temperature as a function of the magnetic order parameter indicates a clear possibility of Co-occurrence of superconductivity and the antiferromagnetism order parameter in Ca0.74 1 La0.26 1 Fe1−xCox As2 iron-based superconductors in the range of magnetic order parameters between 2.3 meV and 6.07 meV, which is in good agreement with experimental observations. This research contributes to understanding the complex behavior of high-temperature superconductors and provides valuable technological applications for other fields.

Phase diagrams on composition-spread FeyTe1−xSex films

FeyTe1−xSex, an archetypical iron-based high-temperature superconductor with a simple structure but rich physical properties, has attracted lots of attention because the two end compositions, Se content x = 0 and 1, exhibit antiferromagnetism and nematicity, respectively, making it an ideal candidate for studying their interactions with superconductivity. However, what is clearly lacking to date is a complete phase diagram of FeyTe1−xSex as functions of its chemical compositions since phase separation usually occurs from x ∼ 0.6 to 0.9 in bulk crystals. Moreover, fine control of its composition is experimentally challenging because both Te and Se are volatile elements. Here we establish a complete phase diagram of FeyTe1−xSex, achieved by high-throughput film synthesis and characterization techniques. An advanced combinatorial synthesis process enables us to fabricate an epitaxial composition-spread FeyTe1−xSex film encompassing the entire Se content x from 0 to 1 on a single piece of CaF2 substrate. The micro-region composition analysis and X-ray diffraction show a successful continuous tuning of chemical compositions and lattice parameters, respectively. The micro-scale pattern technique allows the mapping of electrical transport properties as a function of relative Se content with an unprecedented resolution of 0.0074. Combining with the spin patterns in literature, we build a detailed phase diagram that can unify the electronic and magnetic properties of FeyTe1−xSex. Our composition-spread FeyTe1−xSex films, overcoming the challenges of phase separation and precise control of chemical compositions, provide an ideal platform for studying the relationship between superconductivity and magnetism.

Anomalous Contribution to the Nematic Electronic States from the Structural Transition in FeSe Revealed by Time- and Angle-Resolved Photoemission Spectroscopy.

High-resolution time- and angle-resolved photoemission measurements were made on FeSe superconductors. With ultrafast photoexcitation, two critical excitation fluences that correspond to two ultrafast electronic phase transitions were found only in the d_{yz}-orbit-derived band near the Brillouin-zone center within our time and energy resolution. Upon comparison to the detailed temperature dependent measurements, we conclude that there are two equilibrium electronic phase transitions (at approximately 90 and 120K) above the superconducting transition temperature, and an anomalous contribution on the scale of 10meV to the nematic states from the structural transition is experimentally determined. Our observations strongly suggest that the electronic phase transition at 120K must be taken into account in the energy band development of FeSe, and, furthermore, the contribution of the structural transition plays an important role in the nematic phase of iron-based high-temperature superconductors.

Strongly anisotropic antiferromagnetic coupling in EuFe2As2 revealed by stress detwinning

Of all parent compounds of iron-based high-temperature superconductors, EuFe2As2 exhibits by far the largest magnetostructural coupling due to the sizable biquadratic interaction between Eu and Fe moments. While the coupling between Eu antiferromagnetic order and Fe structural/antiferromagnetic domains enables rapid field detwinning, this prevents simple magnetometry measurements from extracting the critical fields of the Eu metamagnetic transition. Here we measure these critical fields by combining x-ray magnetic circular dichroism spectroscopy with in-situ tunable uniaxial stress and applied magnetic field. The combination of two tuning knobs allows us to separate the stress-detwinning of structural domains from the field-induced reorientation of Eu moments. Intriguingly, we find a spin-flip transition which can only result from a strongly anisotropic interaction between Eu planes. We argue that this anisotropic exchange is a consequence of the strong anisotropy in the magnetically ordered Fe layer, which presents a new form of higher-order coupling between Eu and Fe magnetism.

Discovery of mesoscopic nematicity wave in iron-based superconductors

Patterned nematics Electrons in solids can break rotational symmetry, resulting in electronic nematicity. This phenomenon has been observed in both cuprate-based and iron-based high-temperature superconductors, and its relationship to superconductivity remains a subject of debate. Shimojima et al . used linear dichroism measurements to image nematicity in two iron-based superconductors. Unexpectedly, the researchers found periodic patterns with very long wavelengths. The findings could be described with a phenomenological model assuming a train of nematic domain walls. —JS

Interaction-induced topological phase transition and Majorana edge states in low-dimensional orbital-selective Mott insulators

Topological phases of matter are among the most intriguing research directions in Condensed Matter Physics. It is known that superconductivity induced on a topological insulator’s surface can lead to exotic Majorana modes, the main ingredient of many proposed quantum computation schemes. In this context, the iron-based high critical temperature superconductors are a promising platform to host such an exotic phenomenon in real condensed-matter compounds. The Coulomb interaction is commonly believed to be vital for the magnetic and superconducting properties of these systems. This work bridges these two perspectives and shows that the Coulomb interaction can also drive a canonical superconductor with orbital degrees of freedom into the topological state. Namely, we show that above a critical value of the Hubbard interaction the system simultaneously develops spiral spin order, a highly unusual triplet amplitude in superconductivity, and, remarkably, Majorana fermions at the edges of the system.

Light quantum control of persisting Higgs modes in iron-based superconductors

The Higgs mechanism, i.e., spontaneous symmetry breaking of the quantum vacuum, is a cross-disciplinary principle, universal for understanding dark energy, antimatter and quantum materials, from superconductivity to magnetism. Unlike one-band superconductors (SCs), a conceptually distinct Higgs amplitude mode can arise in multi-band, unconventional superconductors via strong interband Coulomb interaction, but is yet to be accessed. Here we discover such hybrid Higgs mode and demonstrate its quantum control by light in iron-based high-temperature SCs. Using terahertz (THz) two-pulse coherent spectroscopy, we observe a tunable amplitude mode coherent oscillation of the complex order parameter from coupled lower and upper bands. The nonlinear dependence of the hybrid Higgs mode on the THz driving fields is distinct from any known SC results: we observe a large reversible modulation of resonance strength, yet with a persisting mode frequency. Together with quantum kinetic modeling of a hybrid Higgs mechanism, distinct from charge-density fluctuations and without invoking phonons or disorder, our result provides compelling evidence for a light-controlled coupling between the electron and hole amplitude modes assisted by strong interband quantum entanglement. Such light-control of Higgs hybridization can be extended to probe many-body entanglement and hidden symmetries in other complex systems.

Angle-resolved photoemission spectroscopy studies on the electronic structure and superconductivity mechanism for high temperature superconductors

Superconductivity represents a magic macroscopic quantum phenomenon. There have been two major categories of superconductors: the conventional superconductors represented by metals or alloys; and the unconventional superconductors represented by cuprates and iron-based high-temperature superconductors. While the superconductivity mechanism of the conventional superconductors is successfully addressed by the BCS theory of superconductivity, no consensus has been reached in understanding the high temperature superconductivity mechanism for more than 30 years, which has become one of the most prominent issues in condensed matter physics. Revealing the microscopic electronic structure of unconventional superconductors is the prerequisite and foundation in understanding their superconductivity. Angle resolved photoelectron spectroscopy (ARPES) plays an important role in the study of unconventional superconductors because it can directly measure the electronic structure of materials. In this paper, our recent progress in the ARPES study of electronic structure and superconductivity mechanism of high temperature cuprate superconductors and iron-based superconductors is reviewed. It mainly includes the electronic structure of the parent compound, the non-Fermi liquid behavior in the normal state, the band and gap structure of the superconducting state, and the many-body interactions both in the normal and superconducting states. These results will provide important information in understanding the superconductivity mechanism of Cu-based and Fe-based superconductors.

Quantitative Comparison of LDA + DMFT and ARPES Spectral Functions

The emergence of angle-resolved photoemission spectroscopy (ARPES) made it possible to observe electronic dispersion directly as a spectral function map. On the other hand, a spectral function map can be obtained theoretically, for example, in the LDA+DMFT method. The electronic band on such a map is characterized not only by its energy position at a given $k$-point, but also by its width and intensity. To illustrate a way of quantitative comparison of theoretical spectral functions and ARPES data, spectral functions obtained by the LDA+DMFT method are chosen. It is shown that the theoretical spectral functions should take into account a number of experimental features: the photoionization cross section, the experimental energy and angular resolution, as well as the effects of the photohole lifetime arising in the process of photoemission. In this article, we present a robust procedure for taking these experimental features into account by the example of iron-based high-temperature superconductors (HTSC) systems: NaFeAs and FeSe on a SrTiO$_3$ substrate.

Searching for new unconventional high temperature superconductors

Based on the common properties exhibited in both cuprates and iron-based high temperature superconductors, we have recently proposed the “gene” concept for unconventional high temperature superconductors: those d-orbitals of transition metal elements with the strongest in-plane bonding to anion p-orbitals must be isolated near Fermi energy. Here we summarized recent progress in this research direction and discussed several electronic environments that meet the “gene” condition. We also discussed the challenge and the possibility in finding new unconventional high temperature superconductors.

Atomic line defects and zero-energy end states in monolayer Fe(Te,Se) high-temperature superconductors

Majorana zero-energy bound states (ZEBSs) have been proposed to exist at the ends of one-dimensional Rashba nanowires proximity-coupled to an s-wave superconductor in an external magnetic field induced Zeeman field. Such hybrid structures have been a central platform in the search for non-Abelian Majorana zero modes (MZMs) toward fault-tolerant topological quantum computing. Here we report the discovery of ZEBSs simultaneously appearing at each end of a one-dimensional atomic line defect in monolayer iron-based high-temperature superconductor FeTe0.5Se0.5 films grown on SrTiO3(001) substrates. The spectroscopic properties of the ZEBSs, including the temperature and tunneling barrier dependences, as well as their fusion induced by coupling on line defects of different lengths are found to be robust and consistent with those of the MZMs. These observations suggest a realization of topological Shockley defects at the ends of an atomic line defect in a two-dimensional s-wave superconductor that can host a Kramers pair of MZMs protected by time-reversal symmetry along the chain. Our findings reveal an unprecedented class of topological line defect excitations in two-dimensional superconductor FeTe0.5Se0.5 monolayer films and offer an advantageous platform for generating topological zero-energy excitations at higher operating temperatures, in a single material, and under zero external magnetic field.

Persistent antiferromagnetic order in heavily overdoped Ca1−xLaxFeAs2

In the Ca1−xLaxFeAs2 (1 1 2) family of pnictide superconductors, we have investigated a highly overdoped composition (x = 0.56), prepared by a high-pressure, high-temperature synthesis. Magnetic measurements show an antiferromagnetic transition at TN = 120 K, well above the one at lower doping (0.15 < x < 0.27).Below the onset of long-range magnetic order at TN, the electrical resistivity is strongly reduced and is dominated by electron–electron interactions, as evident from its temperature dependence. The Seebeck coefficient shows a clear metallic behavior as in narrow band conductors. The temperature dependence of the Hall coefficient and the violation of Kohler’s rule agree with the multiband character of the material. No superconductivity was observed down to 1.8 K. The success of the high-pressure synthesis encourages further investigations of the so far only partially explored phase diagram in this family of Iron-based high temperature superconductors.

Momentum Dependence of the Nematic Order Parameter in Iron-Based Superconductors.

The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and BaFe_{2}As_{2} without interference from either twinning or magnetic order. We find that the nematic order parameter exhibits the same momentum dependence in both compounds with a sign change between the Brillouin center and the corner. This suggests that the same microscopic mechanism drives the nematic order in spite of the very different phase diagrams.

Possible Mott transition in layeredSr2Mn3As2O2single crystals

Single crystals of ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{2}$ have been grown for the first time, for which we show a possible layer-selective Mott insulator behavior. This compound stands out as a hybrid structure of ${\mathrm{MnO}}_{2}$ and MnAs layers, analogously to the active ${\mathrm{CuO}}_{2}$ and FeAs layers, respectively, in the cuprate and iron-based high-temperature superconductors. Electrical transport, neutron diffraction measurements, together with density functional theory calculations on ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{2}$ single crystals converge toward a picture of independent magnetic order at ${T}_{1}\ensuremath{\sim}79$ K and ${T}_{2}\ensuremath{\sim}360$ K for the two Mn sublattices, with insulating behavior at odds with the metallic behavior predicted by calculations. Furthermore, our inelastic neutron-scattering studies of spin-wave dispersions for the Mn(1) sublattice reveal an effective magnetic exchange coupling of $SJ\ensuremath{\sim}3.7$ meV. This is much smaller than those for the Mn(2) sublattice.

Symmetry of superconducting order parameter in minimal model of iron-based HTSC: variational cluster approximation

Two-orbital model describing iron-based high-temperature superconductors is studied within the limits of variational cluster approximation. It is found that in the undoped regime the stripe antiferromagnetic ordering is realized, while in doped regime antiferromagnetism and superconductivity coexist. In the superconducting state, two types of symmetry are found: extended inter-orbit s-wave and intra-orbit d-wave, in agreement with known experimental results.

Antiferromagnetic and spectral properties of two-orbital model of iron-based HTSC

Two-orbital model describing iron-based high-temperature superconductors was studied within the limits of variational cluster approximation. The electron density of states for each orbital, as well as momentum distribution, were calculated; the magnetic ordering was determined. The results obtained are in a good agreement with known experimental data.

A possible family of Ni-based high temperature superconductors

We suggest that a family of Ni-based compounds, which contain [Ni2M2O]2− (M = chalcogen) layers with an antiperovskite structure constructed by mixed-anion Ni complexes, NiM4O2, can be potential high temperature superconductors (high-Tc) upon doping or applying pressure. The layer structures have been formed in many other transitional metal compounds such as La2B2Se2O3 (B = Mn, Fe, Co). For the Ni-based compounds, we predict that the parental compounds host collinear antiferromagnetic states similar to those in iron-based high temperature superconductors. The electronic physics near Fermi energy is controlled by two egd-orbitals with completely independent in-plane kinematics. We predict that the superconductivity in this family is characterized by strong competition between extended s-wave and d-wave pairing symmetries.

Uniaxial strain control of spin-polarization in multicomponent nematic order of BaFe2As2

The iron-based high temperature superconductors exhibit a rich phase diagram reflecting a complex interplay between spin, lattice, and orbital degrees of freedom. The nematic state observed in these compounds epitomizes this complexity, by entangling a real-space anisotropy in the spin fluctuation spectrum with ferro-orbital order and an orthorhombic lattice distortion. A subtle and less-explored facet of the interplay between these degrees of freedom arises from the sizable spin-orbit coupling present in these systems, which translates anisotropies in real space into anisotropies in spin space. We present nuclear magnetic resonance studies, which reveal that the magnetic fluctuation spectrum in the paramagnetic phase of BaFe2As2 acquires an anisotropic response in spin-space upon application of a tetragonal symmetry-breaking strain field. Our results unveil an internal spin structure of the nematic order parameter, indicating that electronic nematic materials may offer a route to magneto-mechanical control.