1. Coulomb Blockade in Quantum Dots • conductance through a nearly isolated system 2. Kondo Effect: the basics, with quantum dots in mind • localized, doubly degenerate level interacting w/ continuum 3. Kondo Effect in Quantum Dots – selected topics Title Physique de la Boite Quantique: Blocage de Coulomb et Effect Kondo Harold U. Baranger, Duke University
44
Embed
Physique de la Boite Quantique: Blocage de Coulomb et ... · Blocage de Coulomb et Effect Kondo Harold U. Baranger, Duke University . Outline-Summary OUTLINE Comp. Tech. The Kondo
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1. Coulomb Blockade in Quantum Dots • conductance through a nearly isolated system
2. Kondo Effect: the basics, with quantum dots in mind • localized, doubly degenerate level interacting w/ continuum
3. Kondo Effect in Quantum Dots – selected topics
Title Physique de la Boite Quantique:
Blocage de Coulomb et Effect Kondo Harold U. Baranger, Duke University
Outline-Summary OUTLINE
Comp. Tech.
The Kondo Effect in Quantum Dots
Things not covered: multi-level dots, nearly open dots, multiple dots, nonlinear regime, singlet-triplet Kondo, noise, …
Spectroscopy of a Kondo Box • can probe in much more detail the correlated Kondo state • spin and evolution of excited states from pert. theory & QMC • tune through weak-coupling to strong-coupling cross-over
Mesoscopic Kondo Problem• realization-dependent TK works as scaling parameter in high-T regime (from modified poor-man’s scaling) • low-T: non-universal! (theory?)
SU(4) Kondo in Carbon Nanotubes (G. Finkelstein group)• seen with 1, 2, and 3 electrons above a closed shell • smooth shape when dot is open
Kondo Qdot Expt 1 Kondo Effect in Conductance of Quantum Dots
Comp. Tech.
[L. Kouwenhoven group, Delft]
lowest T
highest T
Behavior in “odd” valleys contradicts Coloumb blockade! [Kouwenhoven group]
Qdot Qimp? 1 Kondo Effect in Conductance of Quantum Dots: Theory
Comp. Tech.
[Glazman and Raikh; Ng and Lee]
Ng
Nonlin Expt Nonlinear I-V in the Kondo Regime
Comp. Tech.
[Kouwenhoven group,`98]
Qdot Qimp? 2 Quantum Dots Are Quantum Impurities??
Comp. Tech.
• Can look at a single dot: real Single Impurity physics! • Vg (gate voltage) and shape are tunable • Random Confinement • Energy Scales: Δ, the mean level spacing; ETh, the Thouless energy
Quantum Dot Impurity Leads Conduction Electrons
BUT Quantum Dots are not Atoms!
Previous work concerning Δ: Thimm, Kroha, von Delft (99); Simon & Affleck (02); Cornaglia & Balseiro (03)
Spectr. of Kondo Spectroscopy of Kondo State: Large dot / small dot
(R. Kaul, G. Zaránd, D. Ullmo, S. Chandrasekharan, HUB)
Comp. Tech.
At temperatures « Δ, physical properties dominated by the (many-body) ground state and low energy spectra
Ground state spin? Finite-size spectra?
Parametric evolution of spectrum with J?Effect of parity, randomness… ?
Tool5: non-lin. Coulomb Blockade: Non-linear Transport
Apply a bias voltage between the two leads connected to the dot:
dot L lead R lead
tunneling spectroscopy of individual quantum states
Small/Large dot Small dot / Large dot System
Comp. Tech.
No Leads.Isolated R-S system.
Kondo-couplingbetween R-S
exact one-bodystates on R
electrostaticenergy on R
Comp. Tech.
Thm: g.s. spin
Theorem: Ground State Spin
Comp. Tech.
Mattis;Marshallʼs Sign Theorem
For fixed ground state is never degenerate!
Ground state spin fixed in parametric evolution:Calculate ground state spin in perturbation theory!
e.g. Auerbach’s book `94
Wilson `75
Thm: 1-body Exact Theorem: One-body basis
Comp. Tech.
Wilson `75
Thm: many-body Exact Theorem: Many-body basis
Comp. Tech.
… Marshallʼs Sign Theorem
For fixed ground state is never degenerate!
e.g. Auerbach `94
Spectr. S=1/2 (1) Finite Size Spectra: Ssmall-dot=1/2, N odd, J>0
Comp. Tech. Comp. Tech. from GS theorem, for all J>0
Construct excited state spectrum fromperturbation theory
Spectr. S=1/2 (2) Finite Size Spectra: S=1/2, N odd, J>0
weak coupling: expand in J
Δ{
Comp. Tech. ~Δ
Triplet: simply flip spin Higher states: a single-particle excitation is necessary
Spectr. S=1/2 (3) Finite Size Spectra: S=1/2, N odd, J>0
4 degenerate levels (↑, ↓, ↑, ↓) in the dot are coupled one-to-one to 4 modes (↑, ↓, ↑, ↓) in the leads
The modes are not mixed by tunneling; t amplitude does not depend on α or σ Hamiltonian has SU(4) symmetry
two leads ν = L, R
spin σ, orbital α N number of electrons
SU(4) tunneling Hamiltonian
Kondo effect in Nanotubes
G
Vgate 1e 2e 3e
SU(4) theories for 1 electron: Double dots, dots with symmetries: D. Boese et al., PRB (2002) L. Borda et al., PRL (2003) K. Le Hur and P. Simon, PRB (2003) G. Zarand et al., SSC (2003) W. Izumida et al., J.P.Soc.Jpn. (1998) A. Levy Yeyati et al., PRL (1999) Nanotubes: M.S. Choi et al., PRL (2005)
1 electron SU(4) Kondo experiment: Quantum dots: S. Sasaki et al., PRL (2004) Nanotubes: P. Jarillo-Herrero et al., Nature (2005)
Kondo effect in Nanotubes
G
Vgate 1e 2e 3e
2 electron SU(4) theory M.R. Galpin, D.E. Logan and H.R. Krishnamurthy, PRL (2005) C.A. Busser and G.B. Martins, PRB (2007)
2-e Kondo in nanotubes – triplet or SU(4) W.J. Liang, M. Bockrath and H. Park, PRL (2002) B. Babic, T. Kontos and C. Schonenberger, PRB (2004)
Growth of the signal in the valleys due to the Kondo effect
Ec, Δ ~ 100K
1 2
3 1
2 3
Spectroscopy at finite bias
E
Vgate
ΓL >> ΓR
Tunneling from the weakly coupled lead probes the density of states in the Kondo / Mixed Valence system 1e 2e 3e
Tunneling density of states
EF
2.25e2/h 0 Width of the resonance: 10 K ~ T0
T = 3.3 K
Conclusions CONCLUSIONS
Quantum dots can be used to probe Kondo-type correlations in several interesting contexts
Spectroscopy of a Kondo Box • can probe in much more detail the correlated Kondo state • spin and evolution of excited states from pert. theory & QMC • tune through weak-coupling to strong-coupling cross-over
Mesoscopic Kondo Problem• realization-dependent TK works as scaling parameter in high-T regime (from modified poor-man’s scaling) • low-T: non-universal! (theory?)
SU(4) Kondo in Carbon Nanotubes (G. Finkelstein group)• seen with 1, 2, and 3 electrons above a closed shell • smooth shape when dot is open
Credits: Ribhu Kaul, Jaebeom Yoo, G. Zaránd, D. Ullmo, S. Chandrasekharan, HUB