transport through the single molecule magnet Mn12 Herre van der Zant H.B. Heersche, Z. de Groot (Delft) C. Romeike, M. Wegewijs (RWTH Aachen) D. Barreca, E. Tondello (Padova) L. Zobbi, A. Cornia (Modena) Source Gate Drain Molecular Electronics and Devices group interplay between molecular spin and electron transport (molecular spintronics)
transport through single molecules LUMO HOMO
molecular transistors: quantum dots in the weak coupling limit Kondo in V 2 complex: Liang et al., Nature 417, 725 (22) Co-ion:. Park et al., Nature 417, 722 (22) N N+1 several oxidation states of OPV-5 Kubatkin et al., Nature 425, 698 (23) electron-phonon coupling inc 6 H. Park et al., Nature 47 (2) 57
quantum dots: Coulomb blockade and discrete energy spectrum small box occupied by electrons (holes) box (island) has discrete energy spectrum island weakly coupled to leads: Coulomb blockade level broadening (Γ) << charging energy (E c ) single molecule self-assembled QD semiconducting QD nanotube 1 nm 1 nm 1 nm 1µm
Coulomb blockade in quantum dots SOURCE e DRAIN ISLAND GATE V sd V g I V V G C G QD R L,C L R R,C R dot coupled via tunnel barriers to source and drain reservoirs and capacitively to gate electrode µ(n +1) +E c µ (N µ ) ( N) µ S µ µd Sµ(N-1) : level spacing E c : charging energy γ: level broadening V g
stability diagram and Coulomb diamonds linear regime µ(n +1) (N µ ) ( N) µ (N ) Γ L Γ R µ S µ µ S µ S µd µ ( N) µ D Sµ(N-1) +E c two knobs : V SD and V gate state contributes to transport if µ s > µ(n) > µ d V g V g di/dv sd V sd E add N-1 N N+1 V g Coulomb diamonds with no current
level spectroscopy Non-linear regime µ(n +1) µ e (N ) E mµs S µ(n) DE m(n) mµ D µs m (N) mµ D D two knobs : V SD and V gate state contributes to transport if µ s > µ(n) > µ d V sd di/dv sd E E add N-1 N N+1 excited states result in extra lines in stability diagram (red) excited states can be electronic, vibrational, or spin related V g
single-wall nanotubes different regimes as a function of gate voltage strong coupling (R~R Q ), large γ blurred CB diamonds Fabry-Perot, Kondo physics 4-fold filling weak coupling (R>>R Q ), small γ sharp CB diamonds spectroscopy bias voltage Kondo peak singlet-triplet Kondo co-tunnel line excited state gate voltage
experimental approach sample preparation at room temperature: nanogap fabrication with electromigration of gold wires with self-assembled monolayers ( trapping a falling molecule ) measurement system: low-temperature He-4 probe down to 1.5 K with home-made low noise electronics (Desert Cryogenics Probe-Station for screening; near future dilution fridge) measurements gate sweeps and identify interesting devices stability diagram: current vs. bias and gate voltage (di/dv numerically) lock-in measurement (di/dv directly) temperature- and magnetic field dependent measurements currents typically in the pa range; electron resides on the molecule for 1-1 ns
gap fabrication: electromigration on top of an aluminium gate Al 2 O 3 Au Source Gate Drain SiO 2 V V g 1 µm
visualization of electromigration of gold wires (TEM) Au pattern Si 3 N 4 Si substrate V b R V H.B. Heersche et al., to be published
electromigration: : control of gap resistance I (ma) 3. 2.5 2. 1.5 1..5 R S = 3 Ω I (ma) 5 4 3 2 1 R S = 5 Ω...2.4.6.8 1. Vb (V) breaks all the way: gap > 1nm..1.2.3.4.5.6 Vb (V) breaks half way: few-atom contact V R S V R (Ohm) 1.E+12 1.E+1 1.E+8 1.E+6 1.E+4 1.E+2 1.E+ 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 21 22 23 24 Number
electromigration: : reproducibly single-atom contacts G (2e^2/h) 12 11 1 9 8 7 6 5 4 3 2 1 5 6 7 8 9 1 Vb (mv) Au Al 2 O 3 1 nm
sample preparation: contacting molecules with thiol end groups chip placed in solution self-assembled monolayer forms electromigration creates gap molecule trapped in gap
When do we have a molecule between the contacts? addition energy > 1 mev limited number of charge states additional molecular fingerprints: - vibrational modes - spin excitations 1 8 6 Diff: \\172.16.1.248\edgar\opv5\25_August5\7_mega.dat 12 11 1 4 9 2 8 in practice: only a few percent of the samples show molecular behavior and of those quite a few show unstable, switchy behavior V b (mv) -2-4 -6 7 6 5 4 3-8 2-1 -2.5-2 -1.5-1 -.5.5 1 1.5 2 2.5 V g (V)
excitations in OPV-5: all lines can be mapped onto Raman spectrum N-1 N N+1 17 mev 82 mev 66 mev 55 mev 42.5 mev 96 mev 81 mev 66 mev 56 mev 44 mev second derivative numerical
excitations due to vibrational modes (OPV-5) Raman Spectra cm-1 mev 1 Diff: \\172.16.1.248\edgar\opv5\25_August5\17_megac.dat 8 6 3 2.8 85 123.8 192.8 1.6 15.5 24.1 V b (mv) 4 2-2 -4 35±1 mev 25± 1meV 3.5±.5 mev 25±1meV 35±1meV 35±2 mev 25±2 mev 12±.5 mev 7.5±.5meV 12±.5meV 25±2meV 35± 2meV 56±2 mev 47±2 mev 2.6 2.4 2.2 2 1.8 216.2 266 323.2 375.4 48.6 459.4 548.4 583.3 634.9 27. 33.3 4.4 46.9 51.1 57.4 68.6 72.9 79.4 1 1.6 Diff: \\172.16.1.248\edgar\opv5\25_August5\17_megac.dat -6-8 V b (mv) 8 6 4 2 1.4 1.2 3 2.8 2.6 2.4 2.2 2-1 -2.5-2 -1.5-1 -.5 V g (V) -2-4 -6-8 1.8 1.6 1.4 1.2 1-1 -2.5-2 -1.5-1 -.5
molecular junction with OPV-3: Kondo V bias (mv) NDR Spin blockade? B = V gate (V) B = 7T V gate (V)
inelastic electron scattering probing vibrational modes in OPV-3 electron spectroscopy on a three-terminal device with a single OPV-3 molecule Raman data OPV-3 molecules in solution intensity 14 mev 12 1 wave number cm -1 15 mev
the single molecule magnet Mn-12 H = D 1 2 4 4 ( S + ) 2 N, SSz B4;N,S S+ B. Barbara et al., Journ. Magn. Magn. Mat., 2, 167(1999) Mn12: high spin (S=1) + high D- anisotropy energy barrier ~ 6K (5.6 mev) hysteretic behavior quantum tunneling of magnetization when m states are aligned (Friedman et al., 1996)
Mn-12 on a gold surface: ligands with thiols STM image Mn12- derivatives that bind to gold surface have been synthesized by Cornia et al. ligands: HO 2 (CH 2 ) 15 SAc PhSAc after cleaning the gold wires, we put samples in a.1mm solution for ~1h A. Cornia et al., Angew. Chem. Int. Ed., 42, 1645 (23)
Coulomb blockade in Mn-12 junctions H.B. Heersche et al. cond-mat/51732 Mn-12PhSAc Mn-12 with two different ligands fingerprints (not observed in any of the other more than 1 devices without or with other molecules) : excitation at 14 mev (unknown origin) low energy excitations (<1 mev ) with strong negative differential conductance V bias (mv) 2 1-1 -2-1.5-1. -.5 V gate Mn-12C15SAc
complete current suppression (CCS) and negative differential resistance (NDR) T = 3 K 2 1 V b (mv) V b (mv) -2.4 V g (V) -1-1 -.5.5 V g (V) I (pa) -5 not easily explained within usual CB theory and spin blockade! -1-1 1 V b (mv)
different sample showing NDR (3 mev) ) and excitations at the same energies 2 2 V b (mv) I (pa) - -2-2 T = 1.5 K 1 V g (V) -15 15 V b (mv)
transport: charge (±1) and spin (±1/2) change transport: charge (±1) and spin (±1/2) change 1 9 1.5 9.5 8.5 I II III IV N+1,D N+1,1.5 N,D N,1 2 1 2 1, 1 1 2 1 2 1 1 2 1 2 1 1 9.5,,, ) ( ) ( 1 (9.5) ± = = ± + + + + + + + Σ = N z N z S m N N z N N z N N z N m N m N N N N m S m S D m D m D D N z N z z z z C µ µ µ µ differences in chemical potentials matter transition rates are not the same and are determined by Clebsch-Gordon coefficients 2 z S D N, S = H Christian Romeike, Maarten R. Wegewijs RWTH Aachen
model includes different spin manifolds two spin excitations (manifolds) per charge state energy scales N= are known energy scales N=±1 not known S=9 1D=56 K S=1 S=9 4.5 K (DFT) 4±2 K (EPR-exp) N= S=1 Christian Romeike, Maarten R. Wegewijs,RWTH Aachen Park and Pederson, PRB 7 (24) 54414 K. Petukhov et al. PRB 7 (24) 54426
calculations: new kind of spin blockade 1 V b (mv) -1 n-1 n -1 1 V g (V) V b (mv) n n+1-1 -1 1 V g (V) New type of spin blockade: S z, m z selection rules instead of S selection rules only transition rates depend on Clebsch-Gordon coefficients quantum tunneling of magnetization (QTM) enhances or destroys NDC
non-degenerate spin multiplets sequential tunneling populates a blocking state that can only be depopulated slowly by a violation of spin-selection rules induced by QTM M = -8 M = -9 M = -9 S=9 S 2 n, D n, S=1 n M = -1 M=1 Cation (N = n-1) (1, 8½) (1, 9½) (, 7½) (, 8½) (, 9½) Neutral (N = n) (, 8) (, 9) (, 1)
energy scales 1 V b (mv) CSS: lifted at the energy scale of the magnetic anisotropy barrier (about 5 mev) -1-1 -.5.5 V g (V) NDR: few mev depends on choice of the anisotropy barrier and the distance between the two manifolds (not a unique combination but the MAB need to be different for the charge states involved)
conclusions First transport through a single Mn-12 molecule shows a new kind of spin blockade due to the presence of non-degenerate spin states. It would be exciting to do more systematic studies at lower temperatures in a magnetic field to probe QTM (orientation of molecule with respect to field however not known). Molecular spintronics: start working in parallel on other more simple- molecular magnetic systems. Source Gate Drain molecular quantum dots: transition rates are dependent on the wave functions (vibrational modes, spin states, different charge states) Γ L, R ΓL,R Ψafter Ψbefore 2
magnetic field B=7T V b (mv) -2-4 -6-8 1 2 Field (T) diamond edge is suppressed current increases for neg. bias; decreases for pos. bias magnetic field sweep : magnetic rearrangements?
different molecules show distinct different features V bias (mv) V_b (mv) 2 1-1 -2 -.5..5 V_g (V) V gate (V)
spin blockade Type I: Negative Differential Resistance (NDR) Type II: complete current suppression around zero excited states with maximum spin S=n/2 are involved ground states with spin differing by more than ½ (e.g. S= S =3/2) n+1, S+1/2 n+1, S n, S n, S n-1 S+1/2 n-1 S+1/2 verified in semi-conducting quantum dots (1995)
Mn-12: Coulomb-blockade blockade physics with excitations (spin?, vibrations?) 2 V_b (mv) 1-1 Mn(III), S=2 Mn(IV), S=3/2 single-molecule magnet Mn-12-2 2 I (pa) 1 -.5..5 V_g (V) -1-2 -1 1 2 V b (mv) Delft unpublished results
Coulomb blockade in Mn-12 junctions V_b (mv) 2 1-1 Manganese-12 molecule with PhSAc ligands, T=3K -2 -.5..5 V_g (V) B=7T