Electron transport through Shiba states induced by magnetic adsorbates on a superconductor

Transcription

1 Electron transport through Shiba states induced by magnetic adsorbates on a superconductor Michael Ruby, Nino Hatter, Benjamin Heinrich Falko Pientka, Yang Peng, Felix von Oppen, Nacho Pascual, Katharina Franke Freie Universität Berlin

2 Classical atomic magnetism on a superconductor magnetic moment on superconductor Shiba states Quasi-particle DoS quantum phase transition at sufficiently large exchange coupling E B E B Transport processes through Shiba states? electron-like hole-like E F Energy

3 Magnetic interaction with electrons and Cooper pairs exchange interaction with Cooper pairs Kondo screening Which is the many-body ground state? Free spin vs. Kondo screened

4 Magnetism of single molecules on surfaces anisotropy split spin eigenstates M S > ZFS Role of magnetocrystalline anisotropy on Shiba states? Zitko, et al, Phys. Rev. B (2011)

5 Outline Transport processes through Shiba states Anisotropy splitting of Shiba states

6 Superconducting Pb Pb: superconductor with TT c = 7.2 K energy gap: Δ 1.35 mev Sample

7 Spectroscopy of SC-vacuum-SC junctions superconducting tip: sharp tip DoS probes surface DoS 4 /ee Sample energy resolution: beyond Fermi-Dirac broadening di/dv spectra: convoluted tip-sample DoS

8 Transport through Shiba states Single particle current: tunneling into Shiba state changes occupancy relaxation necessary II ss tt 2 Γ 1,2 Sample

9 Transport through Shiba states Single particle current: tunneling into Shiba state changes occupancy relaxation necessary II ss tt 2 Γ 1,2 Several-particle current: resonant Andreev reflections NO relaxation necessary Sample higher tunnel coupling necessary II aa tt 4

10 Impurity induced bound states Shiba states of Mn atoms on Nb(111) (T=4 K) Shiba states of Mn atoms on Pb(111) (T=0.3 K) Yazdani et al, Science 275, 1767 (1997) Ji et al, PRL 100, (2008) Shiba states interpreted as single electron tunneling Shiba height reflects density of states

11 Mn atoms on Pb(111) B C S Shiba thermal resonances Shiba B C S 2Δ 2Δ multiple Shiba resonances asymmetric peak heights

12 Mn atoms on Pb(111) B C S Shiba thermal resonances Shiba B C S 2Δ 2Δ Sample BCS peaks multiple Shiba resonances thermally excited Shiba states

13 Shiba states at different junction conductances symmetry of Shiba intensity varies with junction conductance intensity cannot be interpreted as density of states

14 Shiba states at different junction conductances sublinear increase of Shiba state conductance at high tunnel rates inversion of Shiba intensity Sample

15 Shiba states at different junction conductances sublinear increase of Shiba state conductance at high tunnel rates inversion of Shiba intensity Sample

16 Shiba states at different junction conductances 4.5 K sublinear increase of Shiba state conductance at high tunnel rates inversion of Shiba intensity higher temperatures: cross over at larger tunnel rates thermally activated relaxation Lifetimes: Sample 0.2 ns at 1.2 K 6 ps at 4.8 K

17 Outline Transport processes through Shiba states Anisotropy splitting of Shiba states

18 MnPc/Pb(111) MnPc: spin S=3/2 Organic ligand determines the magnetic moment the magnetocrystalline anisotropy the interaction with the surface 3.0nm

19 Splitting of Shiba states triplets of Shiba states

20 Shiba states on MnPc 3.0nm 3.0nm different adsorption sites lead to different alignment of Shiba states Shiba states shift as triplet through gap different regimes: I: high intensity Shiba state at neg. E III: high intensity Shiba state at pos. E

21 Kondo screening vs. Shiba bound states 1 free spin -1 Kondo screened Quantum phase transition Franke, Schulze, Pascual, Science, 332, 940 (2011) T. Matsuura, Prog. Theor. Phys. 57, 1823 (1977)

22 Kondo effect d(x²-y²) d(z²) d(xz) d(yz) d(xy) Spin-state: 3/2 Liao et al., Inorg. Chem. 44, 1941 (2005)

23 Orbital hybridization with substrate d(x²-y²) d(z²) d z2 -orbital: Strong hybridization with substrate Shiba state d(xz) d(yz) d xz, d yz -orbitals: singlet with organic ligands screened d(xy) d xy -orbital: weak interaction no Shiba state D. Jacob, et al., Phys. Rev. B 88, (2013)

24 Magnetocrystalline anisotropy in MnPc/Pb(111): S=1 gas phase: S=3/2 reduction to S=1 on surface anisotropy? anisotropic environment: D lifts degeneracy, E mixes states: D = 0 & E = 0 D > 0 & E = 0 D > 0 & E > 0 -E- ZFS

25 Excitation of Shiba states Excited state anisotropy split excited state e - Ground state

26 Excitation of Shiba states Excited state e - Ground state anisotropy split ground state

27 Peak intensities: signature of quantum phase transition equal intensity equal excitation probability different transition probabilities due to thermal occupation?

28 Peak intensities: signature of quantum phase transition different transition probabilities due to thermal occupation!

29 Conclusions Two transport mechanisms through Shiba states: Single electron tunneling Resonant Andreev reflections Thermal relaxation from Shiba states M. Ruby, F. Pientka, Y. Peng, F. von Oppen, B. W. Heinrich, K. J. Franke, arxiv , Phys. Rev. Lett, in press Magnetic interaction with superconductor: Shiba states and Kondo resonance adsorption site determines exchange coupling to surface splitting of Shiba states due to anisotropy thermal occupation of anisotropy split Shiba states

30 THANKS! Michael Ruby Benjamin Heinrich Falko Pientka Yang Peng Felix von Oppen Nino Hatter Benjamin Heinrich Michael Ruby Nacho Pascual