Electronic structure and exotic exchange splitting in spin-density-wave states of BaFe$_2$As$_2$
L. X. Yang, Y. Zhang, H. W. Ou, J. F. Zhao, D. W. Shen, B. Zhou, J. Wei, F. Chen, M. Xu, C. He, Y. Chen, Z. D. Wang, X. F. Wang, T. Wu, G. Wu, X. H. Chen, M. Ar… 展开作者
L. X. Yang, Y. Zhang, H. W. Ou, J. F. Zhao, D. W. Shen, B. Zhou, J. Wei, F. Chen, M. Xu, C. He, Y. Chen, Z. D. Wang, X. F. Wang, T. Wu, G. Wu, X. H. Chen, M. Arita, K. Shimada, M. Taniguchi, Z. Y. Lu, T. Xiang, D. L. Feng
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, BaFe$_2$As$_2$, which provides a foundation for further studies. We show that the energy of the spin density wave (SDW) in BaFe$_2$As$_2$ 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.
We present a comprehensive infrared spectroscopic study of lattice dynamics in the pnictide parent compound BaFe$_2$As$_2$. In the tetragonal structural phase, we observe the two degenerate symmetry-allowed in-plane infrared active phonon modes. Following the structural transition from the tetragonal to orthorhombic phase, we observe splitting into four non-degenerate phonon modes and a significant phonon strength enhancement. These detailed data allow us to provide a physical explanation for the anomalous phonon strength enhancement as the result of anisotropic conductivity due to Hund's coup…
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 in-plane thermal conductivity $κ$ of overdoped FeAs-based superconductor BaFe$_{1.73}$Co$_{0.27}$As$_2$ ($T_c$ = 8.1 K) single crystal was measured down to 80 mK. In zero field, the residual linear term $κ_0/T$ is negligible, suggesting a nodeless superconducting gap in the $ab$-plane. In magnetic field, $κ_0/T$ increases rapidly, very different from that of conventional s-wave superconductors. This anomalous $κ_0/T(H)$ may reveal an exotic superconducting gap structure in overdoped BaFe$_{1.73}$Co$_{0.27}$As$_2$: the vanishing hole ($β$) pocket has a much larger gap than the electron ($γ$…
A rich variety of nematic/smectic orders in Fe-based superconductors is an important unsolved problem in strongly correlated electron systems. A unified understanding of these orders has been investigated for the last decade. In this article, we explain the $B_{1g}$ symmetry nematic transition in FeSe$_{1-x}$Te$_x$, the $B_{2g}$ symmetry nematicity in AFe$_2$As$_2$ (A=Cs, Rb), and the smectic state in BaFe$_2$As$_2$ based on the same framework. We investigate the quantum interference mechanism between spin fluctuations by developing the density wave equation. The observed rich variety of nemat…
The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase transition at 140 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility. In the course of this transition, the space group symmetry changes from tetragonal (I4/mmm) to orthorhombic (Fmmm). 57Fe Moessbauer spectroscopy experiments show a singl…
We report results of 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown single crystal of BaFe2As2. A first-order antiferromagnetic (AF) transition near 135 K was detected by the splitting of NMR lines, which is accompanied by simultaneous structural transition as evidenced by a sudden large change of the electric field gradient tensor at the As site. The NMR results lead almost uniquely to the stripe spin structure in the AF phase. The data of spin-lattice relaxation rate indicate development of anisotropic spin fluctuations of the stripe-type with decreasing temperature i…
Neutron diffraction experiments have been carried out on a Sn-flux grown BaFe2As2 single crystal, the parent compound of the A-122 family of FeAs-based high-Tc superconductors. A tetragonal to orthorhombic structural phase transition and a three dimensional long-range antiferromagnetic ordering of the iron moment, with a unique magnetic propagation wavevector k = (1, 0, 1), have been found to take place at ~90 K. The magnetic moments of iron are aligned along the long a axis in the low temperature orthorhombic phase (Fmmm with b<a<c). Our results thus demonstrate that the magnetic structure of…
Neutron diffraction studies of Ba(Fe[1-x]Co[x])2As2 reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, +-e, 0) from the nominal commensurate antiferromagnetic propagation vector Q[AFM] = (1, 0, 1) (in orthorhombic notation) where e = 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of m…
The BaFe2(As1-xPx)2 compounds with x = 0 (parent), x = 0.10 (under-doped), x = 0.31, 0.33, 0.53 (superconductors with Tc = 27.3 K, 27.6 K, 13.9 K, respectively) and x = 0.70, 0.77 (over-doped) have been investigated versus temperature using 57Fe Mössbauer spectroscopy. Special attention was paid to regions of the spin-density-wave (SDW) antiferromagnetic order, spin-nematic phase, and superconducting transition. The BaFe2(As0.90P0.10)2 compound exhibits a reduced amplitude of SDW as compared to the parent compound and preserved universality class of two-dimensional magnetic planes with one-dim…
The optical properties of a large, detwinned single crystal of BaFe$_2$As$_2$ have been examined over a wide frequency range above and below the structural and magnetic transition at $T_{\rm N}\simeq 138$ K. Above $T_{\rm N}$ the real part of the optical conductivity and the two infrared-active lattice modes are almost completely isotropic; the lattice modes show a weak polarization dependence just above $T_{\rm N}$. For $T<T_{\rm N}$, the optical conductivity due to the free-carrier response is anisotropic, being larger along the $a$ axis than the $b$ axis below $\simeq 30$ meV; above this en…
We use inelastic neutron scattering to study the effect of an in-plane magnetic field on the magnetic resonance in optimally doped superconductors FeSe$_{0.4}$Te$_{0.6}$ ($T_c=14$ K) and BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_c=20$ K). While the magnetic field up to 14.5 Tesla does not change the energy of the resonance, it particially suppresses $T_c$ and the corresponding superconductivity-induced intensity gain of the mode. However, we find no direct evidence for the field-induced spin-1 Zeeman splitting of the resonance. Therefore, it is still unclear if the resonance is the long-sought single…
Nematicity plays an important role in the physics of iron-based superconductors (IBS). Its microscopic origin and in particular its importance for the mechanism of high-temperature superconductivity itself are highly debated. A crucial knowledge in this regard is the degree to which the nematic order influences the electronic structure of these materials. Earlier angle-resolved photoemission spectroscopy (ARPES) studies found that the effect is dramatic in three families of IBS including 11, 111 and 122 compounds: energy splitting reaches 70 meV and Fermi surface becomes noticeably distorted. …
Recent neutron scattering experiments addressing the magnetic state of the two-leg ladder selenide compound BaFe$_2$Se$_3$ have unveiled a dominant spin arrangement involving ferromagnetically ordered 2$\times$2 iron-superblocks, that are antiferromagnetically coupled among them (the "block-AFM" state). Using the electronic five-orbital Hubbard model, first principles techniques to calculate the electronic hopping amplitudes between irons, and the real-space Hartree-Fock approximation to handle the many-body effects, here it is shown that the exotic block-AFM state is indeed stable at realisti…
3D non-abelian anyons have been theoretically proposed to exist in heterostructures composed of type II superconductors and topological insulators. We use realistic material parameters for a device derived from Bi$_2$Se$_3$ to quantitatively predict the temperature and magnetic field regimes where an experiment might detect the presence of these exotic states by means of a cooling effect. Within the appropriate parameter regime, an adiabatic increase of the magnetic field will result in a decrease of system temperature when anyons are present. If anyons are not present, the same experiment wou…
We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe1.85Co0.15As2 and BaFe1.91Ni0.09As2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the 42/m screw symmetry around the (1/2 1/2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of…
Understanding the overall features of magnetic excitation is essential for clarifying the mechanism of Cooper pair formation in iron-based superconductors. In particular, clarifying the relationship between magnetism and superconductivity is a central challenge because magnetism may play a key role in their exotic superconductivity. BaFe2As2 is one of ideal systems for such investigation because its superconductivity can be induced in several ways, allowing a comparative examination. Here we report a study on the spin fluctuations of the hole-overdoped iron-based superconductors Ba1-xKxFe2As2 …
The recently discovered layered kagome metals AV$_3$Sb$_5$(A=K, Rb, Cs) have attracted much attention because of their unique combination of superconductivity, charge density wave (CDW) order, and nontrivial band topology. The CDW order with an in-plane 2x2 reconstruction is found to exhibit exotic properties, such as time-reversal symmetry breaking and rotational symmetry breaking. However, the nature of the CDW, including its dimensionality, structural pattern, and effect on electronic structure, remains elusive despite intense research efforts. Here, using angle-resolved photoemission spect…
Although recent experiments and theories have shown a variety of exotic transport properties of non-equilibrium quasiparticles (QPs) in superconductor (SC)-based devices with either Zeeman or exchange spin-splitting, how QP interplays with magnon spin currents remains elusive. Here, using non-local magnon spin-transport devices where a singlet SC (Nb) on top of a ferrimagnetic insulator (Y3Fe5O12) serves as a magnon spin detector, we demonstrate that the conversion efficiency of magnon spin to QP charge via inverse spin-Hall effect (iSHE) in such an exchange-spin-split SC can be greatly enhanc…
