Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in ${\mathrm{A}}_{x}{\mathrm{Fe}}_{2\mathrm{\text{\ensuremath{-}}}y}{\mathrm{Se}}_{2}$ ($A\mathbf{=}\mathrm{K}$, Rb) Superconductors

Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in $A_{x} {Fe}_{2 − y} {Se}_{2}$ ( $A = K$ , Rb) Superconductors

M. Yi, D. H. Lu, R. Yu, S. C. Riggs, J.-H. Chu, B. Lv, Z. K. Liu, M. Lu, Y.-T. Cui, M. Hashimoto, S.-K. Mo, Z. Hussain, C. W. Chu, I. R. Fisher, Q. Si, and Z.-X. Shen

Phys. Rev. Lett. 110, 067003 – Published 5 February 2013

Abstract

Using angle-resolved photoemission spectroscopy, we observe the low-temperature state of the $A_{x} {Fe}_{2 − y} {Se}_{2}$ ( $A = K$ , Rb) superconductors to exhibit an orbital-dependent renormalization of the bands near the Fermi level—the $d_{x y}$ bands heavily renormalized compared to the $d_{x z} / d_{y z}$ bands. Upon raising the temperature to above 150 K, the system evolves into a state in which the $d_{x y}$ bands have depleted spectral weight while the $d_{x z} / d_{y z}$ bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature-induced crossover from a metallic state at low temperatures to an orbital-selective Mott phase at high temperatures. Moreover, the fact that the superconducting state of $A_{x} {Fe}_{2 − y} {Se}_{2}$ is near the boundary of such an orbital-selective Mott phase constrains the system to have sufficiently strong on-site Coulomb interactions and Hund’s coupling, highlighting the nontrivial role of electron correlation in this family of iron-based superconductors.

Received 1 September 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.067003

Authors & Affiliations

M. Yi^1,2, D. H. Lu³, R. Yu⁴, S. C. Riggs^1,2, J.-H. Chu^1,2, B. Lv⁵, Z. K. Liu^1,2, M. Lu^1,6, Y.-T. Cui¹, M. Hashimoto³, S.-K. Mo⁷, Z. Hussain⁷, C. W. Chu⁵, I. R. Fisher^1,2, Q. Si⁴, and Z.-X. Shen^1,2

¹Stanford Institute of Materials and Energy Sciences, Stanford University, Stanford, California 94305, USA
²Departments of Physics and Applied Physics, and Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA
³Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
⁴Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA
⁵Department of Physics, Texas Center for Superconductivity, University of Houston, Houston, Texas 77204, USA
⁶National Laboratory of Solid-State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China
⁷Advanced Light Source, Lawrence Berkeley National Lab, Berkeley, California 94720, USA

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Vol. 110, Iss. 6 — 8 February 2013

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Figure 1
Electronic structure of $K_{x} {Fe}_{2 − y} {Se}_{2}$ . (a) FS mapping with the 2-Fe BZ boundary marked in green. (b),(d) Spectral images and (c), (e) second derivative in energy along the $Γ − X$ direction. (f)–(g) Similar measurements as (b)–(c) on $Ba ({Co}_{0.07} {Fe}_{0.93})_{2} {As}_{2}$ . (h) Schematic of the dominant orbital characters of the two electron pockets at $X$ , with one pocket (dotted) imploded for clarity. (i) LDA calculations [30] for KFS. (j)–(l) Spectral images across $X$ under different polarizations and cut directions. All light polarizations are marked. Dominant orbital characters indicated by color: blue ( $d_{x y}$ ), red ( $d_{x z}$ ), green ( $d_{y z}$ ). All data taken at 30 K, with 47.5 eV photons except (d), (e), and (k), which were taken with 26 eV photons. We note very little $k_{z}$ dispersion exists in the electron bands [27].Reuse & Permissions
Figure 2
Temperature dependence of electron bands at X. (a) Spectral images taken with 26 eV photons along cut1 as marked in Fig. 1h. (b) Spectral images and (c) corresponding EDCs taken with 47.5 eV photons along cut2. Last column shows the 30 K data with an artificially introduced 210 K thermal broadening, in comparison to the real 210 K data.Reuse & Permissions
Figure 3
Temperature dependence analysis. (a)–(b) 90 K spectral image of Fig. 2a, 2b. (c) EDCs at $X$ from (a) for selected temperatures, fitted to a Gaussian background (dotted line) and a Lorentzian peak (shaded region). (d) The integrated Lorentzian spectral weight is plotted against temperature. (e) Temperature dependence of the averaged intensity of the colored boxes in (a)–(b), with background (dotted box of the same energy window marked by same color) for each box subtracted. The resulting temperature-dependent curve is then normalized by the initial value. The blue (green) region has dominant spectral weight from the $d_{x y}$ ( $d_{y z}$ ) band, whereas the magenta (cyan) region is of mixed $d_{x y}$ and $d_{x z}$ ( $d_{y z}$ ) characters.Reuse & Permissions
Figure 4
Theoretical calculation based on a five-orbital Hubbard model. (a)–(b) Slave-spin mean-field phase diagrams of the five-orbital Hubbard model for KFS at $J / U = 0.15$ and two electron densities. OSMP and MI refer to orbital-selective Mott phase and Mott insulator, respectively. (c) The evolution of the orbital-resolved quasiparticle spectral weight $Z_{α}$ with temperature at $n = 6.15$ , $U = 3.75 eV$ , and $J / U = 0.15$ . (d)–(g) show the quasiparticle spectral functions along the $Γ − X$ direction at $n = 6.15$ . The color curves highlight the dominant orbital of the band, with $d_{x z} / d_{y z}$ in red and $d_{x y}$ in blue.Reuse & Permissions

Physical Review Letters

Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in AxFe2−ySe2 (A=K, Rb) Superconductors

M. Yi, D. H. Lu, R. Yu, S. C. Riggs, J.-H. Chu, B. Lv, Z. K. Liu, M. Lu, Y.-T. Cui, M. Hashimoto, S.-K. Mo, Z. Hussain, C. W. Chu, I. R. Fisher, Q. Si, and Z.-X. Shen

Phys. Rev. Lett. 110, 067003 – Published 5 February 2013

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Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in $A_{x} {Fe}_{2 − y} {Se}_{2}$ ( $A = K$ , Rb) Superconductors