We report experimental studies of a series of BaFe$_2$S$_{3-x}$Se$_x$ ($0\leq x\leq3$) single crystals and powder specimens using x-ray diffraction, neutron diffraction, muon spin relaxation, and electrical transport measurements. A structural transformation from Cmcm (BaFe$_2$S$_3$) to Pnma (BaFe$_2$Se$_3$) was identified around $x = 0.7\sim 1$. Neutron diffraction measurements on the samples with $x$ = 0.2, 0.4, and 0.7 reveal that the N$é$el temperature of the stripe antiferromagnetic order is gradually suppressed from $\sim$120 to 85 K, while the magnitude of the ordered Fe$^{2+}$ moments shows very little variation. Similarly, the block antiferromagnetic order in BaFe$_2$Se$_3$ remains robust for $1.5\leq x\leq3$ with negligible variation in the ordered moment and a slight decrease of the N$é$el temperature from 250 K ($x=3$) to 225 K ($x=1.5$). The sample with $x=1$ near the Cmcm and Pnma border shows coexisting, two-dimensional, short-range stripe- and block-type antiferromagnetic correlations. The system remains insulating for all $x$, but the thermal activation gap shows an abrupt increase when traversing the boundary from the Cmcm stripe phase to the Pnma block phase. The results demonstrate that the crystal structure, magnetic order, and electronic properties are strongly coupled in the BaFe$_2$S$_{3-x}$Se$_x$ system.
The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the Néel temperature $T_N$. Direct exchange interactio…
The majority of the iron-based superconductors (FeSCs) exhibit a two-dimensional square lattice structure. Recent reports of pressure-induced superconductivity in the spin-ladder system, BaFe$_2$X$_3$ (X =S,Se), introduce a quasi-one-dimensional prototype and an insulating parent compound to the FeSCs. Here we report X-ray, neutron diffraction and muon spin relaxation experiments on BaFe$_2$Se$_3$ under hydrostatic pressure to investigate its magnetic and structural properties across the pressure-temperature phase diagram. A structural phase transition was identified at a pressure of 3.7(3) GP…
Quasi-one-dimensional iron chalcogenides possess various magnetic states depending on the lattice distortion, electronic correlations, and presence of defects. We present neutron diffraction and inelastic neutron scattering experiments on the spin ladder compound BaFe$_{2-δ}$S$_{1.5}$Se$_{1.5}$ with $\sim$6% iron vacancies. The data reveal that long-range magnetic order is absent, while the characteristic magnetic excitations that correspond to both the stripe- and block-type antiferromagnetic correlations are observed. First-principles calculations support the existence of both stripe and blo…
The iron-based superconductors had a significant impact on condensed matter physics. They have a common structural motif of a two-dimensional square iron lattice and exhibit fruitful physical properties as a strongly correlated electron system. During the extensive investigations, quasi-one-dimensional iron-based spin-ladder compounds attracted much attention as a platform for studying the interplay between magnetic and orbital ordering. In these compounds, BaFe2S3 and BaFe2Se3 were found to exhibit superconductivity under high pressure, having a different crystal and magnetic structure at low…
We have grown single crystals of the iron-based ladder material BaFe_2S_3, which is superconductive under high pressure, adopting different conditions. By comparing the behaviors of these samples, it is found that the in situ annealing process can affect the crystal structure and the electrical transport, enhance the antiferromagnetic transition temperature, and reduce the extrinsic ferromagnetic component of the system. An in-depth analysis indicates that the crystal quality is improved by the in situ annealing in terms of reducing both the Fe deficiency and the Fe impurity in the samples. Th…
We report a comprehensive study of the spin ladder compound BaFe$_2$S$_{2.5}$Se$_{0.5}$ using neutron diffraction, inelastic neutron scattering, high pressure synchrotron diffraction, and high pressure transport techniques. We find that BaFe$_2$S$_{2.5}$Se$_{0.5}$ possesses the same $Cmcm$ structure and stripe antiferromagnetic order as does BaFe$_2$S$_3$, but with a reduced N{é}el temperature of $T_N=98$ K compared to 120 K for the undoped system, and a slightly increased ordered moment of 1.40$μ_B$ per iron. The low-energy spin excitations in BaFe$_2$S$_{2.5}$Se$_{0.5}$ are likewise similar …
The pressure effects on the antiferromagentic orders in iron-based ladder compounds CsFe$_2$Se$_3$ and BaFe$_2$S$_3$ have been studied using neutron diffraction. With identical crystal structure and similar magnetic structures, the two compounds exhibit highly contrasting magnetic behaviors under moderate external pressures. In CsFe$_2$Se$_3$ the ladders are brought much closer to each other by pressure, but the stripe-type magnetic order shows no observable change. In contrast, the stripe order in BaFe$_2$S$_3$, undergoes a quantum phase transition where an abrupt increase of N$\acute{e}$el t…
Motivated by the recent discovery of superconductivity in the iron-based ladder compound BaFe$_2$S$_3$ under high pressure, we derive low-energy effective Hamiltonians from first principles. We show that the complex band structure around the Fermi level is represented only by the Fe 3$d_{xz}$ (mixed with 3$d_{xy}$) and 3$d_{x^2-y^2}$ orbitals. The characteristic band degeneracy allows us to construct a four-band model with the band unfolding approach. We also estimate the interaction parameters and show that the system is more correlated than the 1111 family of iron-based superconductors. Prov…
We report pressure-dependent neutron diffraction and muon spin relaxation/rotation measurements combined with first-principles calculations to investigate the structural, magnetic, and electronic properties of BaFe$_2$S$_3$ under pressure. The experimental results reveal a gradual enhancement of the stripe-type ordering temperature with increasing pressure up to 2.6 GPa and no observable change in the size of the ordered moment. The ab initio calculations suggest that the magnetism is highly sensitive to the Fe-S bond lengths and angles, clarifying discrepancies with previously published resul…
Topological properties of spin block magnetic ladders in proximity of a superconductor: application to BaFe$_{2}$S$_{3}$
Institute of Nuclear Physics, Polish Academy of Sciences（波兰科学院核物理研究所）；Université Paris-Saclay, Centre National de la Recherche Scientifique, Laboratoire de Physique des Solides（巴黎萨克雷大学，法国国家科学研究中心，固体物理实验室）
Motivated by the discovery of superconductivity in the two-leg, quasi-one dimensional ladder compound, BaFe$_2$S$_3$ we present a renormalization group study of electrons moving on a two leg, two orbital ladder, subjected to Hubbard repulsion $U$ and Hund's coupling $J$. In our calculations, we adopt tightbinding parameters obtained from ab-initio studies on this material. At incommensurate filling, the long wavelength analysis displays four phases as a function of $0 \leq J/U <1$. We show that a fully gapped superconductor is stabilized at sufficiently large Hund's coupling, the relative phas…
The iron-selenides are important because of their superconducting properties. Here, an unexpected phenomenon is predicted to occur in an iron-selenide compound with a quasi-one-dimensional ladder geometry: BaFe$_2$Se$_3$ should be a magnetic ferrielectric system, driven by its magnetic block order via exchange striction. A robust performance (high $T_{\rm C}$ and large polarization) is expected. Different from most multiferroics, BaFe$_2$Se$_3$ is ferrielectric, with a polarization that mostly cancels between ladders. However, its strong magnetostriction still produces a net polarization that …
The discovery of superconductivity in the two-leg ladder compound BaFe$_2$S$_3$ has established the 123-type iron chalcogenides as a novel and interesting subgroup of the iron-based superconductors family. However, in this 123 series, BaFe$_2$Se$_3$ is an exceptional member, with a magnetic order and crystalline structure different from all others. Recently, an exciting experiment reported the emergence of superconductivity in BaFe$_2$Se$_3$ at high pressure [J.-J. Ying, et al., Phys. Rev. B 95, 241109 (R) (2017)]. In this publication, we report a first principles study of BaFe$_2$Se$_3$. Our …
Inelastic neutron scattering measurements have been performed to investigate the spin waves of the quasi-one-dimensional antiferromagnetic ladder compound BaFe$_2$S$_3$, where a superconducting transition was observed under pressure [H. Takahashi {\it et al.}, Nat. Mater. 14, 1008-1012 (2015); T. Yamauchi {\it et al.}, Phys. Rev. Lett. 115, 246402 (2015)]. By fitting the spherically averaged experimental data collected on a powder sample to a Heisenberg Hamiltonian, we find that the one-dimensional antiferromagnetic ladder exhibits a strong nearest neighbor ferromagnetic exchange interaction (…
Iron pnictides and selenides display a variety of unusual magnetic phases originating from the interplay between electronic, orbital, and lattice degrees of freedom. Using powder inelastic neutron scattering on the two-leg ladder BaFe2Se3, we fully characterize the static and dynamic spin correlations associated with the Fe4 block state, an exotic magnetic ground state observed in this low-dimensional magnet and in Rb0.89Fe1.58Se2. All the magnetic excitations of the Fe4 block state predicted by an effective Heisenberg model with localized spins are observed below 300 meV and quantitatively re…
The recent discovery of superconductivity in BaFe$_2$S$_3$ [Takahashi {\it et al.}, Nat. Mater. {\bf 14}, 1008 (2015)] has stimulated considerable interest in 123-type iron chalcogenides. This material is the first reported iron-based two-leg ladder superconductor, as opposed to the prevailing two-dimensional layered structures of the iron superconductors family. Once the hydrostatic pressure exceeds $11$ GPa, BaFe$_2$S$_3$ changes from a semiconductor to a superconductor below $24$~K. Although previous calculations correctly explained its ground state magnetic state and electronic structure, …
Motivated by recent experimental progress on iron-based ladder compounds, we study the doped two-orbital Hubbard model for the two-leg ladder BaFe$_2$S$_3$. The model is constructed by using {\it ab initio} hopping parameters and the ground state properties are investigated using the density matrix renormalization group method. We show that the $(π,0)$ magnetic ordering at half-filling, with ferromagnetic rungs and antiferromagnetic legs, becomes incommensurate upon hole doping. Moreover, depending on the strength of the Hubbard $U$ coupling, other magnetic patterns, such as $(0,π)$, are also …
