Magnetic-order-driven metal-insulator transitions in the quasi-one-dimensional spin-ladder compounds BaFe2S3 and BaFe2Se3 (original) (raw)
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Physical Review B
Because the iron-ladder compounds BaFe 2 X 3 (X = S or Se) possess highly anisotropic crystal, electronic, and magnetic structures, the order in certain degrees of freedom leads to interesting interplays among them. Here we present a systematic study of the dielectric behavior of BaFe 2 S 3 and BaFe 2 Se 3 along all crystallographic directions revealing the conduction mechanism and its relation to the underlying magnetic ordering. The temperature dependence of the DC conductivity indicates that the antiferromagnetic order is accompanied by a transition in the conduction mechanism from a simple activated behavior above T N to three-dimensional variable range hopping below T N. Although the magnetic structures that develop on BaFe 2 S 3 and BaFe 2 Se 3 are rather different-despite nearly identical crystal structures-we do not find significant difference in the conduction mechanism.
2008
The magnetic properties in the parent compounds are often intimately related to the microscopic mechanism of superconductivity. Here we report the first direct measurements on the electronic structure of a parent compound of the newly discovered iron-based superconductor, BaFe2As2, which provides a foundation for further studies. We show that the energy of the spin density wave (SDW) in BaFe2As2 is lowered through exotic exchange splitting of the band structure, rather than Fermi surface nesting of itinerant electrons. This clearly demonstrates that a metallic SDW state could be solely induced by interactions of local magnetic moments, resembling the nature of antiferromagnetic order in cuprate parent compounds.
Site specific spin dynamics in BaFe2As2: tuning the ground state by orbital differentiation
The role of orbital differentiation on the emergence of superconductivity in the Fe-based superconductors remains an open question to the scientific community. In this investigation, we employ a suitable microscopic spin probe technique, namely Electron Spin Resonance (ESR), to investigate this issue on selected chemically substituted BaFe 2 As 2 single crystals. As the spin-density wave (SDW) phase is suppressed, we observe a clear increase of the Fe 3d bands anisotropy along with their localization at the FeAs plane. Such an increase of the planar orbital content is interestingly independent of the chemical substitution responsible for suppressing the SDW phase. As a consequence, the magnetic fluctuations in combination with this particular symmetry of the Fe 3d bands are propitious ingredients for the emergence of superconductivity in this class of materials.
Orbital order and fluctuations in the two-leg ladder materials BaFe2X3 ( X=S and Se) and CsFe2Se3
Physical Review B
The electronic structure of BaFe 2 X 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 2 X 3 exhibit two components, indicating that itinerant and localized Fe 3d sites coexist. Substantial x-ray linear dichroism 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 interaction along the legs of the Fe ladder stabilizes the orbital and antiferromagnetic orders in BaFe 2 S 3 , while the ferromagnetic molecular orbitals are realized between the rungs in CsFe 2 Se 3 .
Physical Review B, 2010
We use elastic and inelastic neutron scattering to systematically investigate the evolution of the low-energy spin excitations of the iron arsenide superconductor BaFe 2−x Ni x As 2 as a function of nickel doping x. In the undoped state, BaFe 2 As 2 exhibits a tetragonal-to-orthorhombic structural phase transition and simultaneously develops a collinear antiferromagnetic ͑AF͒ order below T N = 143 K. Upon electron doping of x = 0.075 to induce bulk superconductivity with T c = 12.2 K, the AF ordering temperature reduces to T N Ϸ 58 K. We show that the appearance of bulk superconductivity in BaFe 1.925 Ni 0.075 As 2 coincides with a dispersive neutron spin resonance in the spin excitation spectra and a reduction in the static ordered moment. For optimally doped BaFe 1.9 Ni 0.1 As 2 ͑T c =20 K͒ and overdoped BaFe 1.85 Ni 0.15 As 2 ͑T c =14 K͒ superconductors, the static AF longrange order is completely suppressed and the spin excitation spectra are dominated by a resonance and spin gap at lower energies. We determine the electron-doping dependence of the neutron spin resonance and spin gap energies and demonstrate that the three-dimensional nature of the resonance survives into the overdoped regime. If spin excitations are important for superconductivity, these results would suggest that the threedimensional characters of the electronic superconducting gaps are prevalent throughout the phase diagram and may be critical for superconductivity in these materials.
Magnetic states of the two-leg-ladder alkali metal iron selenidesAFe2Se3
Physical Review B, 2013
Recent neutron scattering experiments addressing the magnetic state of the two-leg ladder selenide compound BaFe2Se3 have unveiled a dominant spin arrangement involving ferromagnetically ordered 2×2 iron-superblocks, that are antiferromagnetically coupled among them (the "block-AFM" state). Using the electronic five-orbital Hubbard model, first principles techniques to calculate the electronic hopping amplitudes between irons, and the real-space Hartree-Fock approximation to handle the many-body effects, here it is shown that the exotic block-AFM state is indeed stable at realistic electronic densities close to n ∼ 6.0. Another state (the "CX" state) with parallel spins along the rungs and antiparallel along the legs of the ladders is close in energy. This state becomes stable in other portions of the phase diagrams, such as with hole doping, as also found experimentally via neutron scattering applied to KFe2Se3. In addition, the present study unveils other competing magnetic phases that could be experimentally stabilized varying either n chemically or the electronic bandwidth by pressure. Similar results were obtained using two-orbital models, studied here via Lanczos and DMRG techniques. A comparison of the results obtained with the realistic selenides hoppings amplitudes for BaFe2Se3 against those found using the hopping amplitudes for pnictides reveals several qualitative similarities, particularly at intermediate and large Hubbard couplings.
Phonon and magnetic dimer excitations in Fe-basedS=2spin-ladder compound BaFe2Se2O
Physical Review B, 2014
Raman scattering spectra of new Fe-based S=2 spin ladder compound BaFe 2 Se 2 O are measured in a temperature range between 15 K and 623 K. All six A 1g and two B 1g Raman active modes of BaFe 2 Se 2 O, predicted by the factor-group analysis, have been experimentally observed at energies that are in a rather good agreement with the lattice dynamics calculation. The antiferromagnetic long-range spin ordering in BaFe 2 Se 2 O below T N =240 K leaves a fingerprint both in the A 1g and B 1g phonon mode linewidth and energy. In the energy range between 400 and 650 cm −1 we have observed magnetic excitation related structure in the form of magnon continuum, with the peaks corresponding to the singularities in the one dimensional density of magnon states. The onset value of magnetic continuum (2∆ S) is found at about 437 cm −1 at 15 K. The magnetic continuum disappears at about 623 K, which lead us to conclude that the short-range magnetic ordering in BaFe 2 Se 2 O exists apparently up to 2.6T N .
Nematicity, magnetic fluctuation and ferro-spin-orbital ordering in BaFe2As2 family
Journal of Alloys and Compounds, 2016
Through detailed electronic structure simulations we show that the electronic orbital ordering (between d yz and d xz bands) takes place due to local breaking of in-plane symmetry that generates two non-equivalent a, b directions in 122 family of Fe-based superconductors. Orbital ordering is strongly anisotropic and the temperature dependence of the corner zone orbital order maps to that of the orthorhombicity parameter. Orbital anisotropy results in two distinct spin density wave nesting wave vectors and causes inter-orbital charge and spin fluctuations. Temperature dependence of the orbital order is proportional to the nematic order and it sets in at a temperature where magnetic fluctuation starts building. Magnetic fluctuations in the orthorhombic phase is characterized through evolution of Stoner factor which reproduces experimental findings very accurately. Orbital ordering becomes strongly spin dependent in presence of magnetic interaction. Occupation probabilities of all the Fed -orbitals exhibit temperature dependence indicating their possible contribution in orbital fluctuation. This need to be contrasted with the usual definition of nematic order parameter (n dxz-n dyz). Relationship among orbital fluctuations, magnetic fluctuations and nematicity are established.
Pressure-induced superconductivity in BaFe 2 As 2 single crystal
EPL (Europhysics Letters), 2009
The evolution of pressure induced superconductivity in single crystal as well as polycrystalline samples of BaFe 2 As 2 has been investigated through temperature dependent electrical resistivity studies in 0-7 GPa pressure range. While the superconducting transition remains incomplete in polycrystalline sample, a clear pressure induced superconductivity with zero resistivity at the expense of magnetic transition, associated with spin density wave (SDW), is observed in the single crystal sample. The superconducting transition temperature (T C ) is seen to increase upto a moderate pressure of about ~1.5 GPa and decreases monotonically beyond this pressure. The SDW transition temperature T SDW decreases rapidly with increasing pressure and vanishes above ~1.5 GPa. PACS Number(s): 74.62.Fj, 74.70.Dd, 74.25.Fy * Corresponding author's email: mani@igcar.gov.in The recent discovery of superconductivity in the iron pnictides [1-3] has evoked immense response from scientific community which has culminated in synthesis of several new superconductors pertaining to these classes of compounds. Amazingly, in a limited span of time four homologous series of the Fe-pnictides, namely, 1111 (ROFeAs with R = rareearth; AeFFeAs with Ae = alkaline earth), 122 (AeFe 2 As 2 with Ae = Ba, Ca, Sr), 111 (AFeAs with A=Li, Na) and 011 (FeSe) have been discovered. Proper doping by electrons or holes in these parent compounds has resulted in the maximum superconducting transition temperatures (T C ) of ~ 55 K for 1111 [4], ~ 38 K for 122 [5, 6], and 12-25 K for 111 [7, 8], and ~ 9-14 K for 011 [9], respectively. One distinguishing feature of these pnictide superconductors is the existence of spin density wave (SDW) in the parent compounds, which gives way to superconductivity with electron or hole doping [4-9]. Apart from the ample future scope of enhancing the T C, as indicated in several doping studies, the occurrence of superconductivity in the presence of large concentration of the magnetic Fe in these layered compounds has provided an avenue to investigate the interplay of magnetism and superconductivity which may help in unraveling a long standing mystery of high temperature superconductivity. High pressure techniques provide valuable tools for altering and understanding of the electronic, structural and ground state properties of materials without introducing any chemical complexity. Several such studies carried out on above classes of FeAs -compounds have revealed the following general trends [10]: (1) Pressure tends to decrease the SDW transition temperature in the undoped or slightly doped compounds. (2) T C increases with
Physical Review Letters, 2013
We use polarized neutron scattering to demonstrate that in-plane spin excitations in electron doped superconducting BaFe 1:904 Ni 0:096 As 2 (T c ¼ 19:8 K) change from isotropic to anisotropic in the tetragonal phase well above the antiferromagnetic (AFM) ordering and tetragonal-to-orthorhombic lattice distortion temperatures (T N % T s ¼ 33 AE 2 K) without an uniaxial pressure. While the anisotropic spin excitations are not sensitive to the AFM order and tetragonal-to-orthorhombic lattice distortion, superconductivity induces further anisotropy for spin excitations along the [110] and ½110 directions. These results indicate that the spin excitation anisotropy is a probe of the electronic anisotropy or orbital ordering in the tetragonal phase of iron pnictides.