Role of the Octahedra Rotation on the Electronic Structures of 4d Transition Metal Oxides

Transcription

1 Role of the Octahedra Rotation on the Electronic Structures of 4d Transition Metal Oxides Changyoung Kim Dept. Physics, Yonsei University B. J. Kim 1, J. Yu 1, S. J. Oh 1, H. Koh 2, I. Nagai 3, S. I. Ikeda 3, Eun Jung Ko 4, Hyung Joon Choi 4 1 School of Physics and Center for Strongly Correlated Materials Research, Seoul National University, Seoul, Korea 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 3 National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki , Japan 4 Department of Physics, Yonsei University

2 Outline Background - (Sr,Ca) 2 RuO 4 ARPES data from Sr 2 RhO 4 Missing d xy Fermi Surface Comparison with Band Calculation Implication to (Sr,Ca) 2 RuO 4 (Sr,Ba) 2 RhO 4 - Band Structure Manipulation Summary

3 Sr 2 RuO 4 : spin-triplet superconductor Layered perovskite superconductor like La 2 CuO 4 CuO 2 -plane RuO 2 -plane Y. Maeno et al., Nature 372, 532 (1994) LaO CuO 2 LaO SrO RuO 2 SrO

4 Phase diagram of Ca 2-x Sr x RuO 4 S. Nakatsuji et al, Phys. Rev. Lett. 84, 2666 (2000). Mott transition Orbital Selective Mott Transition (OSMT)?

5 Structural distortion of Ca 2-x Sr x RuO 4 Various ground states are realized by structural distortions.

6 4d transition-metal oxide Large spatial extent of 4d orbitals large bandwidth, large 10Dq. tends to be weakly-correlated. Low-spin configuration is expected. x 2 -y 2 3z 2 -r 2 Ca 2-x Sr x RuO 4 Sr 2 RhO 4 xy yz, zx

7 Rotation of Octahedra Rotation brings about: Doubling of the unit cell Decrease of M-O-M bond angle which cause: Band folding Bandwidth narrowing

8 Unit cell doubling and band folding 1D π/2a π/2a a 2a 2D π/a 0 π/a (π,π) Γ

9 Band width narrowing Octahedra rotation t Decrease of M-O-M bond angle Metal, small U/W Decrease in hopping energy t t Increase in U/t Insulator, large U/W

10 Angle-resolved photoemission spectroscopy (ARPES) hν x z ϕ ϑ From momentum/energy conservation rules: y Direct Mapping of Band (ARPES) E f Energy Final State hν Kinetic Energy BE Initial State Momentum k pe = kn 1 E hv E pe = E N 1 E N k y k x

11 ARPES data on Ca-doped SRO ARPES is a powerful tool to study the electronic structure. S.-C. Wang et al. PRL 93, (2004) Sr 2 RuO 4 Ca 1.5 Sr 0.5 RuO 4 However, the disorder effects introduced by doping have discomforting effects in ARPES: the signals are generally broad and weak.

12 Sr 2 RhO 4 Share same crystal structure with Sr 2 RuO 4. 5 electrons in 4d orbitals. Rotation angle ~ 10º. No supeconductivity. Sr 2 RhO 4 presents an opportunity to study the effect of rotation without disorder.

13 Electrical resistivity Similar to ρ(t) in Sr 2 RuO 4 Large anisotropy ρ c / ρ ab (3K) = 2400 ρ ab (µωcm ) (b) ρ c (mωcm) 30 0 Sr 2 RhO 4 T (K) Sr 2 RhO ρ ab (µωcm) ρ c T (K) T 2 (K 2 ) T 2 (K 2 ) ρ c (Ωcm ) T 2 - dependence Fitting with ρ = ρ 0 +AT 2 ρ ab (T ) ρ c (T ) ρ 0 = 8.6 µωcm A ab = µωcm/k 2 ρ 0 = 20.1 mωcm A c = mωcm/k 2 Below ~250 K, ρ c decreases with lowering temperature ρ ab T (K) 0.1 Sr 2 RhO 4 is a two-dimensional Fermi liquid. because of suppression of thermal scattering between quasiparticles and phonon? No superconducting transition was observed down to 36 mk.

14 Expected FS of Sr2RhO4 FS of Sr 2 RuO 4 By doping one electron: (rigid-band model) α Sr 2 RuO 4 Sr 2 RhO 4 β A. Damascelli et al. Phys. Rev. Lett. 85, (2000) γ Hase et al. J. of solid state chemistry 123,186 (1996) C Bergemann et al, PRL 84, 2662 (2000) We expect basically similar FS topology in Sr 2 RhO 4

15 ARPES measurements High energy ARPES ALS BL 7 Analyzer : Scienta 100 Temperature : 40K Total Energy Resolution : 40 mev Angular Resolution : 0.25 o Photon energy : 85 ev low energy ARPES Sample cleaved in situ SSRL BL Analyzer : Scienta 2002 Temperature : 20K Total Energy Resolution : 40 mev Angular Resolution : 0.25 o Photon energy : 20 ev Sample cleaved in situ

16 FS of Sr 2 RhO 4 (ALS ARPES) M X X Fermi Surface Mapping Γ M Γ M Γ X Binding Energy (ev) Missing xy-band(γ) FS in Sr 2 RhO 4! B.J. Kim et al., to be published in PRL

17 LDA calculation (I4/mmm) WITHOUT distortion (rotation of octahedra) a hole pocket formed by xz/yz orbital band. (α) an electron pocket formed by x 2 -y 2 orbital band. (δ) two electron pockets formed by xy (γ) and yz,zx band (β) Occupation : α 94.8% β 66.8% γ 72.5% δ 7.1%

18 Effects of the rotational distortion undistorted undistorted +band folding distorted exp and calc. LDA calculation shows disappearance of xy-band (γ) FS.

19 F. Baumberger et al., PRL 96, (2006)

20 Effects of the rotational distortion

21 Effects of the rotational distortion

22 FS of Sr 2 RhO 4 (ARPES) Γ Γ X X Observation of xy-band sunken under E f

23 Effects of the rotational distortion Complete filling of the xy-band transfer of electrons from the yz/zx band to xy band xy yz, zx Electron occupation α 94.8% β 66.8% γ 72.5% δ 7.1% α 96.2% β 51.5% γ 100% δ 0%

24 Effects of the rotational distortion Hybridization between the xy and x 2 -y 2 band Increase in the orbital occupation of the x 2 -y 2 state x 2 -y 2 3z 2 -r 2 Strongly hybridized xy yz, zx Destabilizes the elongation of the octahedra along c-axis

25 Structural instability As temperature is lowered: a-axis contracts by 0.35% c-axis expands by 0.08% rotation angle increases HOWEVER, Rh-O(1) contracts only by 0.2% O(2) O(1) Rh-O(2) contracts by 0.26%

26 Summary Sr 2 RhO 4 Rotation of the octahedra leads to hybridization of xy and x 2 -y 2 bands. Hybridization of xy and x 2 -y 2 bands results in: (1) transfer of electrons from yz/zx to xy band and (2) disappearance of the xy Fermi surface. e g states play vital role in determining electronic structures near E f, and therefore should be included in the theoretical models that deals with 4d TMOs.

27 Implications to CSRO system Orbital-selective Mott-transition at x=0.5? Magnetic ground state and origin of localized spin. depends critically on n xy,n yz/zx,and the crystal structure.

28 Band structure (Sr end) Same physics apply here!

29 Experimental evidences S.-C. Wang et al. PRL 93, (2004) γ Sheet changes from electron-like to hole-like

30 Orbital-selective Mott transition? Anisimov et al. Eur. Phys. J. B 25,191 (2002) Region I (0.2>x>0) Region II (0.5>x>0.2) Region III (2>x>0.5) yz, zx 2 xy 1 xy 4/3 xy yz, zx 2 3 yz, zx 8/3 Contradicts with our finding!

31 Structural phase transition (x=0.2) Flattening of octahedra at x=0.2 3z 2 -r 2 x 2 -y 2 x 2 -y 2 3z 2 -r 2 yz, zx xy xy yz, zx Unstable! Strong hybridization with e g states drives the structural phase transition and thus the Mott transition.

32 Summary Rotation of the octahedra leads to hybridization of xy and x 2 -y 2 bands also in CSRO. Hybridization of xy and x 2 -y 2 bands results in dramatic change in the Fermi surface topology. e g states play vital role in determining electronic structures near E f, and therefore should be included in the theoretical models that deals with 4d TMOs.