Coulomb blockade and single electron tunnelling

Transcription

1 Coulomb blockade and single electron tunnelling Andrea Donarini Institute of theoretical physics, University of Regensburg

2 Three terminal device Source System Drain Gate Variation of the electrostatic energy Typical time scale

3 Single electron detection How small and how cold should the system be such that a single electron tunneling has a measurable effect? Thermal fluctuations Quantum uncertainty

4 First device (1987) Aluminum- Aluminum oxide, Au-Cr gate electrode Fulton and Dolan PRL, (1987)

5 STM set-up (1992) Schönenberger, van Houten and Donkersloot, EPL 20, 249 (1992)

6 Single electron transistor in 2DEG structures 2 dimensional electron gas (2DEG) Source Gate Gate Drain Depleting top metal gates Staring, PhD thesis (1992)

7 Coulomb oscillations Kouwenhoven et al., Z. Phys. B Condensed Matter 85, 367 (1991)

8 Coulomb staircase Kouwenhoven et al., Z. Phys. B Condensed Matter 85, 367 (1991)

9 Stability diagram H. van der Zant, unpublished

10 Coulomb blockade Source System Drain Gate

11 Single electron tunnelling Source System Drain Gate

12 Discrete vs. Continuum Averin & Likharev (1986)

13 Master equation The only relevant quantity is the probability that the system is occupied with N electrons. The system relaxes to local thermal equilibrium after each tunneling event lead-level occupation system-level occupation energy conservation Let us assume Orthodox theory: Averin & Likharev

14 Orthodox theory Particle number controlled by the gate Orthodox theory: Averin & Likharev

15 Orthodox theory Conductance peak => Coulomb oscillations Orthodox theory: Averin & Likharev

16 Orthodox theory Source N -> N+1 Resonance condition Drain N -> N-1 Resonance condition Coulomb diamonds Orthodox theory: Averin & Likharev

17 Discrete vs. Continuum Beenakker (1991)

18 Generalized Master Equation We start with the Liouville equation: We define the reduced density matrix which is block-diagonal in particle number spin energy We keep the coherences between orbitally degenerate states. The Generalized Master Equation is the equation of motion for : Coherent dynamics Effective internal dynamics Tunnelling dynamics

19 Grancanonical energy Tunnelling Liouvillean E Projector on the subspace of N particles and energy E. N - 1 N N + 1 Particle Number

20 Single particle rate matrix Effective Hamiltonian Current operator

21 Many-body rate matrix The current is proportional to the transition rate between many-body states where For uncorrelated and non-degenerate systems the many-body rate reduces to

22 Negative differential conductance (NDC) with symmetric contacts

23 Single electron transistor V b Source System Drain Small System size + weak System-Lead Tunnelling coupling Strong e-e interaction on the System Single electron control Gate V b V g V g μ N+1 μ S μ N μ N-1 μ D G N-1 N N+1 V g

24 Single electron transistor V b Source System Drain Small System size + weak System-Lead Tunnelling coupling Strong e-e interaction on the System Single electron control Gate V b V g V g μ N+1 μ S μ N μ N-1 μ D I di dv b V b

25 Negative Differential Conductance I μ S μ N+1 μ N μ D di dv b μ N-1 NDC V b Negative differential conductance (NDC) is usually associated with a strong asymmetry in the coupling to the leads

26 Grand Canonical Energy The free energy formulation F = H μ 0 N GC energy Source transition Drain transition V b N-1 N N+1 V g N-1 N N+1 Particle Number AD, G Begemann, Milena Grifoni PRB, 82, (2010)

27 Grand Canonical Energy The free energy formulation F = H μ 0 N GC energy Source transition Drain transition V b N-1 N N+1 V g N-1 N N+1 Particle Number AD, G Begemann, Milena Grifoni PRB, 82, (2010)

28 GC Energy NDC with symmetric set-up V b Γ 01 B Γ 00 A N N+1 N N+1 V g The dynamics is evaluated with the master equation: in the stationary limit P N,0 P N+1,0 P N+1,1 Current A B 1/2 1/2 0 1/3 1/3 1/3 Γ 00 /2 1/3(Γ 00 + Γ 01 ) NDC if I B < I A Γ 01 < Γ 00 /2

29 GC Energy Role of the degeneracies V b x 2 Γ 01 x 2 B Γ 00 A N N+1 N N+1 V g The dynamics is evaluated with the master equation: in the stationary limit P N,0 P N+1,0 P N+1,1 Current A B 1/3 1/3 0 1/5 1/5 1/5 2Γ 00 /3 2/5(Γ 00 + Γ 01 ) NDC if I B < I A Γ 01 < 2Γ 00 /3

30 GC Energy Role of the degeneracies V b x m Γ 01 x m B n x Γ 00 A N N+1 N N+1 V g The dynamics is evaluated with the master equation: in the stationary limit P N,0 P N+1,0 P N+1,1 Current A B 1 n+m 1 n+2m 1 n+m 1 n+2m nm 0 Γ n+m 00 1 nm (Γ 00 + Γ 01 ) n+2m n+2m NDC if I B < I A m Γ 01 < Γ n+m 00

31 GC Energy An example: a suspended CNT 2 x 1 x 1 x ћω ћω x 8 x 4 x 4 4N 4N+1 The transition rates are proportional to product of Frack-Condon coefficients ij n ( n) FC ( i, j; ) A. Yar, AD, S. Koller, and M. Grifoni, arxiv: (2011)

32 GC Energy An example: a suspended CNT 2 x 1 x 1 x x 8 x 4 x 4 4N 4N+1 λ= 0.68 λ= 0.83 λ= 1.18 A. Yar, AD, S. Koller, and M. Grifoni, arxiv: (2011)

33 Interference Single Electron Transistors

34 Para vs. Meta Para Meta G. Begemann, D. Darau, AD, M. Grifoni, Phys. Rev. B 77, (R) (2008)

35 Interacting isolated benzene The Pariser-Parr-Pople Hamiltonian for isolated benzene reads: The size of the Fock space for the many-body system 4 6 = 4096 since for each site there are 4 possibilities: Within this Fock space we diagonalize exactly the Hamiltonian.

36 Symmetry of the ground states Rotation phase factors Under rotation of an angle No phase acquired S S

37 Generalized Master Equation

38 The effective Hamiltonian The effective Hamiltonian is expressed in terms of angular momentum operators and renormalization frequencies: In particular in the Hilbert space of the 7 particle ground states Bias and gate dependent

39 Para vs. Meta Para Meta G. Begemann, D. Darau, AD, M. Grifoni, Phys. Rev. B 77, (R) (2008)

40 Conductance suppression Para Meta equal suppressed A: non- degenerate B: non- degenerate Equal A: non- degenerate E: degenerate Suppressed

41 Destructive interference Neglecting for simplicity the dynamics introduced Interference by the effective Hamiltonian, the analytical formula for the conductance reads: factor interference In particular for the transition 6-7 in the meta configuration: = + 2

42 Negative Differential Conductance The 7 particle ground state has spin and orbital degeneracies; Physical basis: the basis that diagonalizes the stationary density matrix; The physical basis depends on the bias: in whatever reference basis, coherences are essential for a correct description of the system; The visualization tool: position resolved transition probability to the physical basis: 6g 7g 2 7g 2

43 Current [eγ] Interference blockade Geometry I-V for transition 6-7 Energetics Blocking state 7 6 Non-blocking state Coulomb blockade Interference blockade current onset 7 Bias Voltage [ b /e] 6 The blocking state is an eigenstate of the effective Hamiltonian blockade

44 Normal vs. ferromagnetic leads Polarization = or Polarization =

45 Selective Interference Blocking AD, G. Begemann, and M. Grifoni Nano Lett. 9, 2897 (2009)

46 Level renormalization in presence of polarized leads We obtain a difference in the renormalization frequencies for the 2 spin directions linear in the polarization of the leads: The splitting of the level renormalization depends crucially on the Coulomb interaction on the molecule and vanishes in absence of exchange.

47 Summary Pedagogical introduction to the physics of the sequential tunnelling regime in transport through micro- and nano-junctions. Basic theory of Coulomb blockade and single electron tunneling in two flavours Metallic island Quandum dot Master equation Generalized Master equation Application of the GME to different types of quantum dot molecules

48 Acknowledgements Georg Begemann Milena Grifoni Dana Darau A. Yar S. Kolmeder B. Siegert S.Pfaller