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Register a new userLifting of xz/yz orbital degeneracy at the structural transition in detwinned FeSe
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We study superconducting FeSe (Tc = 9 K) exhibiting the tetragonal-orthorhombic structural transition (Ts = 90 K) without any antiferromagnetic ordering, by utilizing angle-resolved photoemission spectroscopy. In the detwinned orthorhombic state, the energy position of the dyz orbital band at the Brillouin zone corner is 50 meV higher than that of dxz, indicating the orbital order similar to NaFeAs and BaFe2As2 families. Evidence of orbital order also appears in the hole bands at the Brillouin zone center. Precisely measured temperature dependence using strain-free samples shows that the onset of the orbital ordering (To) occurs very close to Ts, thus suggesting that the electronic nematicity above Ts is considerably weaker in FeSe compared to BaFe2As2 family.
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The origin of the electronic nematicity in FeSe, which occurs below a tetragonal-to-orthorhombic structural transition temperature $T_s$ ~ 90 K, well above the superconducting transition temperature $T_c = 9$ K, is one of the most important unresolved puzzles in the study of iron-based superconductors. In both spin- and orbital-nematic models, the intrinsic magnetic excitations at $mathbf{Q}_1=(1, 0)$ and $mathbf{Q}_2=(0, 1)$ of twin-free FeSe are expected to behave differently below $T_s$. Although anisotropic spin fluctuations below 10 meV between $mathbf{Q}_1$ and $mathbf{Q}_2$ have been unambiguously observed by inelastic neutron scattering around $T_c (<<T_s)$, it remains unclear whether such an anisotropy also persists at higher energies and associates with the nematic transition $T_s$. Here we use resonant inelastic x-ray scattering (RIXS) to probe the high-energy magnetic excitations of uniaxial-strain detwinned FeSe. A prominent anisotropy between the magnetic excitations along the $H$ and $K$ directions is found to persist to $sim200$ meV, which is even more pronounced than the anisotropy of spin waves in BaFe$_2$As$_2$. This anisotropy decreases gradually with increasing temperature and finally vanishes at a temperature around the nematic transition temperature $T_s$. Our results reveal an unprecedented strong spin-excitation anisotropy with a large energy scale well above the $d_{xz}/d_{yz}$ orbital splitting, suggesting that the nematic phase transition is primarily spin-driven. Moreover, the measured high-energy spin excitations are dispersive and underdamped, which can be understood from a local-moment perspective. Our findings provide the much-needed understanding of the mechanism for the nematicity of FeSe and point to a unified description of the correlation physics across seemingly distinct classes of Fe-based superconductors.
We report high resolution ARPES measurements of detwinned FeSe single crystals. The application of a mechanical strain is used to promote the volume fraction of one of the orthorhombic domains in the sample, which we estimate to be 80$%$ detwinned. While the full structure of the electron pockets consisting of two crossed ellipses may be observed in the tetragonal phase at temperatures above 90~K, we find that remarkably, only one peanut-shaped electron pocket oriented along the longer $a$ axis contributes to the ARPES measurement at low temperatures in the nematic phase, with the expected pocket along $b$ being not observed. Thus the low temperature Fermi surface of FeSe as experimentally determined by ARPES consists of one elliptical hole pocket and one orthogonally-oriented peanut-shaped electron pocket. Our measurements clarify the long-standing controversies over the interpretation of ARPES measurements of FeSe.
We have investigated the impact of Ru substitution on the multi-band electronic structure of FeSe$_{1-x}$Te$_x$ by means of angle-resolved photoemission spectroscopy (ARPES). The ARPES results exhibit suppression of the $xy$ Fermi surface and the spectral broadening near the zone boundaries, which can be associated with the lattice disorder introduced by the Ru substitution. The degeneracy of the Fe 3$d$ $yz$/$zx$ bands at the zone center, which is broken in FeSe$_{1-x}$Te$_x$, is partly recovered with the Ru substitution, indicating coexistence of nematic and non-nematic electronic states.
We report detailed study of angular-dependent magnetoresistance (AMR) with tilting angel $theta$ from $c$-axis ranging from 0$^circ$ to 360$^circ$ on a high-quality FeSe single crystal. A pronounced AMR with twofold symmetry is observed, which is caused by the quasi two-dimensional (2D) Fermi surface. The pronounced AMR is observed only in the orthorhombic phase, indicating that the quasi-2D Fermi surface is induced by the structural transition. Details about the influence of the multiband effect to the AMR are also discussed. Besides, the angular response of a possible Dirac-cone-like band structure is investigated by analyzing the detailed magnetoresistance at different $theta$. The obtained characteristic field ($B^*$) can be also roughly scaled in the 2D approximation, which indicates that the Dirac-cone-like state is also 2D in nature.
We report a structural transition found in Ca10(Ir4As8)(Fe2-xIrxAs2)5, which exhibits superconductivity at 16 K, with a layer of divalent iridium coordinated by arsenic in between Fe2As2 layers. The c-axis parameter is doubled below a structural transition temperature of approximately 100 K, while the tetragonal symmetry with space group P4/n (No.85) is unchanged at all temperatures measured. Our synchrotron x-ray diffraction study clearly shows displacements along the z-direction occur in half of the iridium sites, resulting in a complex orbital ordering pattern. Combining our theoretical calculation of the 5d orbital energies with structural data, we propose the iridium orbital crossover transition between the dxy and dz2 orbitals.
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