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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Inducing Ferrimagnetic Exchange in 1D-FeSe 2 Chains Using Heteroleptic Amine Complexes: [Fe(en)(tren)][FeSe 2 ] 2

[Fe(en)(tren)][FeSe 2 ] 2 (en = ethylenediamine, C 2 H 8 N 2 , tren = tris(2-aminoethyl)amine, C 6 H 18 N 4 ) has been synthesized by a mixed-ligand solvothermal method. Its crystal structure contains heteroleptic [Fe(en)(tren)] 2+ complexes with distorted octahedral coordination, incorporated between 1D-FeSe 2 chains composed of edge–sharing FeSe 4 tetrahedra. The twisted octahedral coordination environment of the Fe–amine complex leads to partial dimerization of Fe–Fe distances in the FeSe 2 chains so that the FeSe 4 polyhedra deviate strongly from the regular tetrahedral geometry. 57 Fe Mössbauer spectroscopy reveals oxidation states of +3 for the Fe chain atoms and +2 for the Fe complex atoms. The close proximity of Fe atoms in the chains promotes ferromagnetic nearest neighbor interactions, as indicated by a positive Weiss constant, θ = +53.8(6) K, derived from the Curie–Weiss fitting. Magnetometry and heat capacity reveal two consecutive magnetic transitions below 10 K. DFT calculations suggest that the ordering observed at 4 K is due to antiferromagnetic intrachain interactions in the 1D-FeSe 2 chains. The combination of two different ligands creates an asymmetric coordination environment that induces changes in the structure of the Fe–Se fragments. Finally, this synthetic strategy opens new ways to explore the effects of ligand field strength on the structure of both Fe-amine complexes and surrounding Fe–Se chains.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Shot noise and differential conductance as signatures of putative topological superconductivity in FeSe 0.45 ⁢Te 0.55

We present a theory for the differential shot noise, dS/dV, as measured via shot-noise scanning tunneling spectroscopy, and the differential conductance, dI/dV, for tunneling into Majorana zero modes (MZMs) in the putative topological superconductor FeSe 0.45 Te 0.55 . We demonstrate that for tunneling into chiral Majorana edge modes near domain walls, as well as MZMs localized in vortex cores and at the end of defect lines, dS/dV vanishes whenever dI/dV reaches a quantized value proportional to the quantum of conductance. Furthermore, these results are independent of the particular orbital tunneling path, thus establishing a vanishing dS/dV concomitant with a quantized dI/dV, as universal signatures for Majorana modes in two-dimensional topological superconductors, irrespective of the material’s specific complex electronic band structure.

Iron-based superconductors↗
DOE OSTI2021

Electronic Origin of T c in Bulk and Monolayer FeSe

FeSe is classed as a Hund’s metal, with a multiplicity of d bands near the Fermi level. Correlations in Hund’s metals mostly originate from the exchange parameter J, which can drive a strong orbital selectivity in the correlations. The Fe-chalcogens are the most strongly correlated of the Fe-based superconductors, with d xy the most correlated orbital. Yet little is understood whether and how such correlations directly affect the superconducting instability in Hund’s systems. By applying a recently developed ab initio theory, we show explicitly the connections between correlations in d xy and the superconducting critical temperature T c . Starting from the ab initio results as a reference, we consider various kinds of excursions in parameter space around the reference to determine what controls T c . We show small excursions in J can cause colossal changes in T c . Additionally we consider changes in hopping by varying the Fe-Se bond length in bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO 3 substrate (M-FeSe/STO). The twin conditions of proximity of the dxy state to the Fermi energy, and the strength of J emerge as the primary criteria for incoherent spectral response and enhanced single- and two-particle scattering that in turn controls T c . Using c-RPA, we show further that FeSe in monolayer form (M-FeSe) provides a natural mechanism to enhance J. We explain why M-FeSe/STO has a high T c , whereas M-FeSe in isolation should not. Our study opens a paradigm for a unified understanding what controls Tc in bulk, layers, and interfaces of Hund’s metals by hole pocket and electron screening cloud engineering.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Interrelationships between nematicity, antiferromagnetic spin fluctuations, and superconductivity: Role of hotspots in FeSe 1 - x S x revealed by high pressure Se 77 NMR study

The sulfur-substituted FeSe, FeSe 1-x S x , is one of the unique systems that provides an independent tunability of nematicity, antiferromagnetism, and superconductivity under pressure (p). Recently, Rana et al. [K. Rana et al., Phys. Rev. B 101, 180503(R) (2020)] reported, from 77 Se nuclear magnetic resonance (NMR) measurements on FeSe 0.91 S 0.09 under pressure, that there exists a clear role of nematicity in the relationship between antiferromagnetic (AFM) spin fluctuations and the superconducting transition temperature (T c ), where the AFM spin fluctuations are more effective in enhancing T c in the absence of nematicity than with nematicity. Here, motivated by the work, we carried out 77 Se NMR measurements on FeSe 1-x S x with x=0.15 and 0.29 under pressure up to 2.10 GPa to investigate the relationship in a wide range of x in the FeSe 1-x S x system. Based on the new results together with the previously reported data for x=0 [P. Wiecki et al., Phys. Rev. B 96, 180502(R) (2017)] and 0.09 [K. Rana et al. Phys. Rev. B 101, 180503(R) (2020)], we established a p–x–temperature (T) phase diagram exhibiting the evolution of AFM spin fluctuations. From the systematic analysis of the NMR data, we found that the superconducting (SC) state in a nematic state arises from a non-Fermi-liquid state with strong stripe-type AFM spin fluctuations, while the SC state without nematicity comes from a Fermi-liquid state with mild stripe-type AFM spin fluctuations. Furthermore, we show that the previously reported impact of nematicity on the relationship between AFM fluctuations and superconductivity holds throughout the wide range of x from x=0 to 0.29 in FeSe 1-x S x under pressure. We discuss the origin of the role of nematicity in terms of the different numbers of hotspots on Fermi surfaces with and without nematicity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

On the Remarkable Superconductivity of FeSe and Its Close Cousins

Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Canted antiferromagnetism and spin reorientation in corner-shared single chain quasi-one-dimensional Ba 2 ⁢FeSe 3

Here, we report the canted antiferromagnetic (AFM) structure together with a spin reorientation in a single chain quasi-one-dimensional (Q-1D) iron chalcogenide Ba 2⁢ FeSe 3 . Ba 2 ⁢FeSe 3 crystallizes in Pnma (No. 62) orthorhombic structure with linear single iron chains consisting of corner-shared distorted FeSe 4 tetrahedra along the 𝑏 axis. Ba 2 ⁢FeSe 3 is a narrow-gap semiconductor and orders AFM below 60 K. Modeling of neutron powder diffraction data reveals a canted AFM ground state of magnetic space group 𝑃⁢𝑎⁢21/𝑐 (BNS No. 14.80) with commensurate propagation vector 𝐤 =(0, $\frac{1}{2}$, 0), where the Fe ion spins are AFM aligned with up-down-up-down (↑−↓−↑−↓) sequence along the Q-1D chain direction of the 𝑏 axis. In the magnetically ordered state, the canting of magnetic moments reorients from the 𝑎⁢𝑐 plane to the 𝑎⁢𝑏 plane below 30 K, with a 10° tilting angle toward the 𝑎 axis, and the magnetic moment does not induce a net moment in either orientation. The density functional theory results indicate that an ↑−↓−↑−↓ AFM state is stabilized along the chain direction. In this work, we elucidate the unique canted AFM of the iron chalcogenide and pave the way for searching exotic physics in Q-1D Ba 2⁢ FeSe 3 .

Gao, Fei [Univ. of Texas at Dallas, Richardson, TX↗
DOE OSTI2025

Role of Nematic Fluctuations on Superconductivity in FeSe 0.47 ⁢Te 0.53 Revealed by Nuclear Magnetic Resonance under Pressure

The relationship between antiferromagnetic (AFM) spin fluctuations (SF), nematic fluctuations, and superconductivity (SC) has been central to understanding the pairing mechanism in iron-based superconductors (IBSCs). Iron chalcogenides, which hold the simplest crystal structure in IBSCs, provide a good platform to investigate the relationship. Here, we report 77 Se and 125 Te nuclear magnetic resonance studies of FeSe 0.47⁢ Te 0.53 , which is located close to a nematic quantum critical point (QCP), under pressures up to 1.35 GPa. Further, both the superconducting critical temperature and AFMSF were found to be enhanced under pressure, which suggests a correlation between SC and AFMSF in FeSe 0.47 ⁢Te 0.53 . However, the contribution of AFMSF to SC in FeSe 0.47 ⁢Te 0.53 was found to be much less compared to that in FeSe 1−𝑥 ⁢S 𝑥 , suggesting that nematic fluctuations play a dominant role in the SC in FeSe 1−𝑥⁢ Te 𝑥 around the nematic QCP.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Impact of nematicity on the relationship between antiferromagnetic fluctuations and superconductivity in FeSe 0.91 S 0.09 under pressure

The sulfur-substituted FeSe system, FeSe 1 - x S x , provides a versatile platform for studying the relationship among nematicity, antiferromagnetism, and superconductivity. Herein, by nuclear magnetic resonance (NMR) and resistivity measurements up to 4.73 GPa on FeSe 0.91 S 0.09 , we established the pressure- ( p -) temperature ( T ) phase diagram in which the nematic state is suppressed with pressure showing a nematic quantum phase transition (QPT) around p = 0.5 GPa , two superconductivity (SC) regions separated by the QPT appear, and antiferromagnetic (AFM) phase emerges above ~ 3.3 GPa . From the NMR results up to 2.1 GPa, AFM fluctuations are revealed to be characterized by the stripe-type wave vector which remains the same for the two SC regions. Furthermore, the electronic state is discovered to change in character from non-Fermi liquid to Fermi liquid around the nematic QPT and persists up to ~ 2.1 GPa . In addition, although the AFM fluctuations correlate with T c in both SC states, demonstrating the importance of the AFM fluctuations for the appearance of SC in the system, we found that, when nematic order is absent, T c is strongly correlated with the AFM fluctuations whereas T c weakly depends on the AFM fluctuations when nematic order is present. Our findings on FeSe 0.91 S 0.09 were shown to be applied to the whole FeSe 1 - x S x system and offer an insight into the relationship between AFM fluctuations and SC in Fe-based superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Capping layer influence and isotropic in-plane upper critical field of the superconductivity at the FeSe / SrTiO 3 interface

Understanding the superconductivity at the interface of FeSe/SrTiO 3 is a problem of great contemporary interest due to the significant increase in critical temperature (T c ) compared to that of bulk FeSe, as well as the possibility of an unconventional pairing mechanism and topological superconductivity. We report a study of the influence of a capping layer on superconductivity in thin films of FeSe grown on SrTiO 3 using molecular beam epitaxy. We used in vacuo four-probe electrical resistance measurements and ex situ magnetotransport measurements to examine the effect of three capping layers that provide distinct charge transfer into FeSe: insulating FeTe, nonmetallic Te, and metallic Zr. Our results show that FeTe provides an optimal cap that barely influences the inherent T c found in pristine FeSe/SrTiO 3 , while the transfer of holes from a nonmetallic Te cap completely suppresses superconductivity and leads to insulating behavior. Finally, we used ex situ magnetoresistance measurements in FeTe capped FeSe films to extract the angular dependence of the in-plane upper critical magnetic field. Our observations reveal an almost isotropic in-plane upper critical field, providing insight into the symmetry and pairing mechanism of high-temperature superconductivity in FeSe.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Unraveling Enhanced Superconductivity in Single-Layer FeSe through Substrate Surface Terminations

Single-layer FeSe on SrTiO 3 (001) substrates shows a superconducting transition temperature much higher than that of bulk FeSe, which has been attributed to factors such as electron doping, interfacial electron–phonon coupling, and electron correlations. To pinpoint the primary driver, we grew single-layer FeSe films on SrTiO 3 (001) substrates with coexisting TiO 2 and SrO surface terminations. Scanning tunneling spectroscopy revealed a larger superconducting gap (17.0 meV) on the TiO 2 -termination than on the SrO-termination (10.5 meV). Tunneling spectroscopy also showed a larger work function on the SrO surface, resulting in reduced charge transfer to FeSe, as confirmed by angle-resolved photoemission spectroscopy. Scanning transmission electron microscopy further revealed distinctive interfacial atomic-scale structures, with the Se–Fe–Se tetrahedral angle changing from 109.5° on the SrO-termination to 104.9° on the TiO 2 -termination. Compared to dynamical mean field theory calculations, our results indicate that enhanced superconductivity in single-layer FeSe/TiO 2 arises from optimal electron correlations, in addition to sufficient charge transfer from the substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Frustrated magnetic interactions in FeSe

The structurally simplest high-temperature superconductor FeSe exhibits an intriguing superconducting nematic paramagnetic phase with unusual spin excitation spectra that are different from typical spin waves; thus, determining its effective magnetic exchange interactions is challenging. In this work, we report neutron scattering measurements of spin fluctuations of FeSe in the tetragonal paramagnetic phase. We show that the equal-time magnetic structure factor, S(Q), can be effectively modeled using the self-consistent Gaussian approximation calculation with highly frustrated nearest-neighbor (J 1 ) and next-nearest-neighbor (J 2 ) exchange couplings, and very weak further neighbor exchange interaction. Our results elucidate the frustrated magnetism in FeSe, which provides a natural explanation for the highly tunable superconductivity and nematicity in FeSe and related materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Vertex dominated superconductivity in intercalated FeSe

Bulk FeSe becomes superconducting below 9 K, but the critical temperature (T c ) is enhanced almost universally by a factor of ~4–5 when it is intercalated with alkali elements. How intercalation modifies the structure is known from in-situ X-ray and neutron scattering techniques, but why T c changes so dramatically is not known. Here we show that there is one-to-one correspondence between the enhancement in magnetic instabilities at certain q vectors and superconducting pairing vertex, even while the nuclear spin relaxation rate 1/(T 1 T) may not reflect this enhancement. Intercalation modifies electronic screening both in the plane and also between layers. We disentangle quantitatively how superconducting pairing vertex gains from each such changes in electronic screening. Intercalated FeSe provides an archetypal example of superconductivity where information derived from the single-particle electronic structure appears to be insufficient to account for the origins of superconductivity, even when they are computed including correlation effects. We show that the five-fold enhancement in T c on intercalation is not sensitive to the exact position of the d xy at Γ point, as long as it stays close to E F . Finally, we show that intercalation also significantly softens the collective charge excitations, suggesting the electron-phonon interaction could play some role in intercalated FeSe.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Role of Nematicity in Controlling Spin Fluctuations and Superconducting Tc in Bulk FeSe

FeSe undergoes a transition from a tetragonal to a slightly orthorhombic phase at 90 K and becomes a superconductor below 8 K. The orthorhombic phase is sometimes called a nematic phase because quantum oscillation, neutron, and other measurements detect a significant asymmetry in x and y. How nematicity affects superconductivity has recently become a matter of intense speculation. Here, we employ an advanced ab initio Green's function description of superconductivity and show that bulk tetragonal FeSe would, in principle, superconduct with almost the same critical temperature Tc as the nematic phase. The mechanism driving the observed nematicity is not yet understood. Since the present theory underestimates it, we simulate the full nematic asymmetry by artificially enhancing the orthorhombic distortion. For benchmarking, we compare theoretical spin susceptibilities against experimentally observed data over all energies and relevant momenta. When the orthorhombic distortion is adjusted to correlate with observed nematicity in spin susceptibility, the enhanced nematicity causes spectral weight redistribution in the Fe-3dxz and Fe-dyz orbitals, but it leads to at most a 10-15% increment in Tc. This is because the dxy orbital always remains the most strongly correlated and provides most of the source of the superconducting glue. Nematicity suppresses the density of states at the Fermi level; nevertheless, Tc increases, in contradiction to both BCS theory and the theory of Bose-Einstein condensation. We show how the increase is connected to the structure of the particle-particle vertex. Our results suggest that while nematicity may be an intrinsic property of bulk FeSe, it is not the primary force driving the superconducting pairing.

bulk FeSe↗
DOE OSTI2021

Evolution of spin excitations from bulk to monolayer FeSe

Abstract In ultrathin films of FeSe grown on SrTiO 3 (FeSe/STO), the superconducting transition temperature T c is increased by almost an order of magnitude, raising questions on the pairing mechanism. As in other superconductors, antiferromagnetic spin fluctuations have been proposed to mediate SC making it essential to study the evolution of the spin dynamics of FeSe from the bulk to the ultrathin limit. Here, we investigate the spin excitations in bulk and monolayer FeSe/STO using resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to its bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems revealing a remarkable reconfiguration of spin excitations in FeSe/STO, essential to understand the role of spin fluctuations in the pairing mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Spin correlations in the nematic quantum disordered state of FeSe

The quantum-disordered state in FeSe, intertwined with superconductivity and nematicity, has been a research focus in iron-based superconductors. However, the intrinsic spin excitations across the entire Brillouin zone in detwinned FeSe, crucial for understanding its magnetism and superconductivity, have remained unresolved. Using inelastic neutron scattering, we reveal that stripe spin excitations (Q = (1, 0)/(0, 1)) exhibit the C 2 symmetry, while Néel spin excitations (Q = (1, 1)) retain C 4 symmetry within the nematic state. Temperature-dependent differences between Q = (1, 0) and (0, 1) spin excitations above the structural transition unambiguously reveals the nematic quantum disordered state. Comparison with NaFeAs suggests the Néel excitations originate from enhanced 3d xy orbital correlations. Modeling the stripe dispersions using a J 1 -K-J 2 Heisenberg Hamiltonian, we establish a spin-interaction phase diagram, positioning FeSe near a crossover regime between the antiferroquadrupolar, Néel, and stripe orders. Our results provide key insights into the microscopic spin interactions and their role in the intertwined orders in iron-based superconductors.

Electronic properties and materials↗
DOE OSTI2020

Comparison of GW band structure to semiempirical approach for an FeSe monolayer

We present the G 0 W 0 band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn-Sham density functional theory (DFT) with the Perdew, Burke, and Ernzerhof functional. We find the GW correction increases the bandwidth of the states forming the M pocket near the Fermi energy, while leaving the Γ pocket roughly unchanged. We then compare the G 0 W 0 quasiparticle band energies with the band structure from a simple empirical + A approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory. We show that this empirical correction succeeds in approximating the GW nonlocal and dynamical self-energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).

36 MATERIALS SCIENCE↗
DOE OSTI2022

Relationship Between Nematicity, Antiferromagnetic Fluctuations, and Superconductivity in FeSe 1-x S x Revealed by NMR

The S-substituted FeSe, FeSe 1-x S x , under pressure (p), provides a versatile platform for studying the relationship among nematicity, antiferromagnetism, and superconductivity. Here we present a short review of the recent experimental evidence showing that nematicity has a remarkable impact on the relationship between antiferromagnetic fluctuations and superconductivity. This has been revealed by several 77 Se nuclear magnetic resonance studies that have tracked the variability of antiferromagnetic fluctuations and superconducting transition temperature (T c ) as a function of x and p. T c is roughly proportional to antiferromagnetic fluctuations in the presence or absence of nematic order suggesting the importance of antiferromagnetic fluctuations in the Cooper pairing mechanism in FeSe 1-x S x . However, the antiferromagnetic fluctuations are more effective in enhancing superconductivity in the absence of nematicity as compared to when it is present. These experimental observations give renewed insights into the interrelationships between nematicity, magnetism, and superconductivity in Fe-based superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Pressure-induced high-temperature superconductivity retained without pressure in FeSe single crystals

To raise the superconducting-transition temperature (T c ) has been the driving force for the long-sustained effort in superconductivity research. Recent progress in hydrides with T c s up to 287 K under pressure of 267 GPa has heralded a new era of room temperature superconductivity (RTS) with immense technological promise. Indeed, RTS will lift the temperature barrier for the ubiquitous application of superconductivity. Unfortunately, formidable pressure is required to attain such high T c s. The most effective relief to this impasse is to remove the pressure needed while retaining the pressure-induced T c without pressure. Here, we show such a possibility in the pure and doped high-temperature superconductor (HTS) FeSe by retaining, at ambient pressure via pressure quenching (PQ), its T c up to 37 K (quadrupling that of a pristine FeSe at ambient) and other pressure-induced phases. We have also observed that some phases remain stable without pressure at up to 300 K and for at least 7 d. The observations are in qualitative agreement with our ab initio simulations using the solid-state nudged elastic band (SSNEB) method. We strongly believe that the PQ technique developed here can be adapted to the RTS hydrides and other materials of value with minimal effort.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗