Tuning Moleculemediated Spin Coupling in Bottomup Fabricated VanadiumTCNE
Tuning Molecule-mediated Spin Coupling in Bottom-up Fabricated Vanadium-TCNE Nanostructures Daniel Wegner Institute of Physics and Center for Nanotechnology (Ce. NTech) University of Münster Germany
Why Bottom-Up ? Single molecule Magnet: Mn 12 -acetate (drosophila of SMMs) − + Challenges • Low temperatures • Leads • Molecule orientation • Deposition Controlled model systems Bottom-up STM 2
Our Goal • Bottom-up design by STM manipulation • Spin-coupling via molecules 3 d molecule 3 d 3
Flexibility Engineer spin structures (1 D, 2 D) triangular spin lattice Tune spin coupling spin chain spin ladder Kagome spin lattice square spin lattice 4
Outline Motivation: molecular spin networks V + TCNE – promising candidates Building Vx(TCNE)y molecules Magnetic properties of Vx(TCNE)y Outlook 5
Vanadium + TCNE Bulk Vx[TCNE]y • Room-temperature magnet (TC ≈ 400 K) molecule = ? 3 d = V magnetization tetracyanoethylene 300 K magnetic field Manriquez et al. , Science (1991) • Crystal structure unclear magnetic coupling unclear! 6
Sample Preparation Molecules via leak valve at 300 K UHV V deposition at low-temperature leak valve TCNE gas Ag(100) V e-beam evaporator crystalline TCNE in-situ LT-STM (T = 7 K) 7
Outline Motivation: molecular spin networks V + TCNE– promising candidates Building Vx(TCNE)y molecules Magnetic properties of Vx(TCNE)y Outlook 8
Building Vx(TCNE)y Complexes topo Wegner et al. , Nano Lett. 8, 131 (2008) d. I/d. V -0. 6 V LUMO Synthesis of V-TCNE Wegner et al. , PRL 103, 087205 (2009) Reaction of V-TCNE with V trans-V 2 TCNE V V-TCNE V V V-TCNE 20 Å -0. 3 Å 0Å 0. 5 Å TCNE d 2 I/d. V 2 TCNE -80 V 2 TCNE -40 0 40 80 V-TCNE sample bias (m. V) 1. 0 Å 1. 5 Å 9
Vx(TCNE)y Structure Analysis Model “rules” • TCNE on bridge site • V on hollow site • V-N bond 1. 8 -2. 4 Å V-V distances @27° > @11° 11. 9 Å > 10. 4 Å long V 2 TCNE -0. 3 Å 0Å 0. 5 Å short V 2 TCNE 1. 0 Å 1. 5 Å Wegner et al. , PRL 103, 087205 (2009) 10
Outline Motivation: molecular spin networks V + TCNE– promising candidates Building Vx(TCNE)y molecules Magnetic properties of Vx(TCNE)y Outlook 11
STS Probes the Spin V-TCNE V(TCNE)2 5Å 5Å @V DFT Kondo resonance 0 0. 5 1 -1 -1 conduction -0. 5 0 0 electrons 0. 5 1 1 E−E sample bias (V) spin screen impurity F (e. V) d. I/d. V -40 EF Minority sample bias (V) -80 U d. I/d. V V-LDOS d. I/d. V -0. 5 metal Majority -0. 2 V state @TCNE -1 V atom 0 40 80 sample bias (m. V) two local probes of V-spin -80 -40 0 40 80 sample bias (m. V) Wegner et al. , PRL 103, 087205 (2009) 12
Spin in V 2 TCNE long V 2 TCNE short V 2 TCNE Long V 2 TCNE • -0. 2 V state • Kondo resonance 5Å 5Å Short V 2 TCNE d. I/d. V @V -0. 5 0 0. 5 sample bias (V) d. I/d. V -1 1 -1 • -0. 2 V state • STS virtually identical -0. 5 -50 0 50 sample bias (m. V) 0 0. 5 sample bias (V) ! d. I/d. V @TCNE -50 0 1 • No Kondo resonance! Magnetic coupling! 50 sample bias (m. V) Wegner et al. , PRL 103, 087205 (2009) 13
Magnetic Coupling and Kondo Effect Two-impurity Kondo problem Jayaprakash et al. , PRL (1981) Single spin impurity Texp Two spin impurities weakly coupled weak coupling long V 2 TCNE Two spin impurities strongly coupled strong FM coupling → TKFM « TK weak strong AFM coupling → TK → no strong coupling short V 2 TCNE 14
FM vs. AFM Coupling short V 2 TCNE Symmetry arguments • Even number of electrons on TCNE (TCNE 0, TCNE 2−) V-spins couple AFM 5Å • Odd number of electrons on TCNE (TCNE−) V-spins couple FM Determine the charge state 15
DFT vs. Experiment V 2 TCNE • (V 2 TCNE)− DOS fits best short d. I/d. V • ferromagnetic coupling V 2 TCNE exp. long -1 Wegner et al. , PRL 103, 087205 (2009) -0. 5 0 0. 5 sample bias (V) 1
Double Check: V-TCNE • (V-TCNE)− DOS fits best d. I/d. V • isolated molecule: TCNE− V-TCNE exp. @V @TCNE -1 Wegner et al. , PRL 103, 087205 (2009) -0. 5 0 0. 5 sample bias (V) 1
Outline Motivation: molecular spin networks V + TCNE– promising candidates Building Vx(TCNE)y molecules Magnetic properties of Vx(TCNE)y Outlook 18
Outlook Weaken substrate coupling • reduce screening TCNE V TCNE Insulating monolayer Substrate • increase spin coupling Build larger structures • various symmetries, geometries • engineered spin structures 19
Summary Create V+TCNE complexes: - atomic-scale precision - strong chemical bond STS: two spin probes short long. V V 22 TCNE d. I/d. V @TCNE V majority d-state @V -1 -0. 5 0 0. 5 sample bias (V) 1 V 2 TCNE molecules: Kondo resonance Increase orbital overlap Ferromagnetic coupling -50 0 50 sample bias (m. V) Wegner et al. , PRL 103, 087205 (2009) 20
Acknowledgements Michael F. Crommie Ryan Yamachika Xiaowei Zhang (UC Berkeley Physics) Jeffrey R. Long Bart M. Bartlett (UC Berkeley Chemistry) Mark Pederson Tunna Baruah (NRL Washington DC / UT El Paso) Alexander von Humboldt Foundation National Science Foundation Department of Energy 21
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