Atomic collapse in graphene

Transcription

1 Atomic collapse in graphene Andrey V. Shytov (BNL) Work done in collaboration with: L.S. Levitov MIT M.I. Katsnelson University of Nijmegen, Netherlands * Phys. Rev. Lett. 99, ; ibid. 99, (2007)

2 Outline 1. Atomic collapse and Dirac vacuum reconstruction in high-energy physics 2. Charged impurities in graphene: manifestations of «atomic collapse» a. Local DOS (STM) b. Transport (conductivity) c. Vacuum polarization 3. Conclusion

3 Electrons in graphene Two sublattices Pseudo-spin (sublattice) Slow, but «ultrarelativistic» Dirac fermions! Chirality conservation => no localization (Klein paradox)

4 Why graphene? High mobility, tunable carrier density Linear spectrum => high quantization energies, room temperature mesoscopic physics Realization of relativistic quantum physics in table-top experiments => unusual transport properties Large fine structure constant => strong field regime, inaccessible in high-energy physics

5 Charged impurities Dominant contribution to resistivity (RPA screening is not essential) Nomura, McDonald (2005)

6 Charged impurities Dominant contribution to resistivity (RPA screening is not essential) Nomura, McDonald (2005) Any interesting non-linear effects??? Yes, Dirac vacuum reconstruction

7 Stability of Atom Classical physics: unstable (energy is unbounded)

8 Stability of Atom Classical physics: unstable (energy is unbounded) QM: stable orbits, zero point motion stops the collapse

9 Stability of Atom Classical physics: unstable (energy is unbounded) QM: stable orbits, zero point motion stops the collapse Relativity: collapsing orbits v < c

10 Stability of Atom Classical physics: unstable (energy is unbounded) QM: stable orbits, zero point motion stops the collapse Relativity: collapsing orbits Relativity + QM:??? v < c?

11 Stability of Atom Classical physics: unstable (energy is unbounded) QM: stable orbits, zero point motion stops the collapse Relativity: collapsing orbits Relativity + QM: Collapse? v < c??? But: position uncertainty:

12 Dirac Kepler problem D = 3 Part I (Z < 137) Dirac (1929) What happens at Z > 137?

13 Dirac Kepler problem Part II (137 < Z < 170) Pomeranchuk and Smorodinskii (1945) Finite size of nucleus (regularization) Solution can be continued to Z > 137.

14 Atomic collapse: semiclassical picture Relativistic particle may fall to the Coulomb center

15 Atomic collapse: semiclassical picture Relativistic particle may fall to the Coulomb center

16 Dirac Kepler problem Part II (137 < Z < 170) Pomeranchuk and Smorodinskii (1945) Finite size of nucleus (regularization) 1S level merges into Dirac sea at Z = 170 Z > 170?

17 Dirac Kepler problem Part II (137 < Z < 170) Pomeranchuk and Smorodinskii (1945) Finite size of nucleus (regularization) Solution can be continued to Z > S level merges into hole continuum at Z = 170 Z > 170?

18 Dirac Kepler problem Part III (Z > Zc = 170) Gershteyn, Zeldovich (1969) Popov (1970) Resonant electron state in Dirac sea Screening by pair production? Quasilocalized state

19 Atomic collapse in graphene Can be modeled by charged impurities: No mass => no discrete states, continuous spectrum Manifestations: quasistationary states, resonances Strong effects in vacuum polarization, no cutoff at Compton wavelength

20 Charged impurity in graphene Dirac equation: Ansatz:

21 Two regimes Subcritical: Supercritical: Oscillations small r! Critical value:

22 Exact solution Subcritical case: Scale-invariant solution (depends only on kr )

23 Density of states Standing waves Bound states

24 Density of states Standing waves Bound states

25 Density of states Standing waves Bound states

26 Density of states Standing waves Localized state Standing waves Bound states

27 Quasi-bound states Type of carriers is reversed at small r Klein tunneling couples electron-like states at small r to hole-like states at large r

28 Quasi-bound states Classical motion Quantization condition: Transparency: Exact result near criticality:

29 Drude conductivity Transport Scattering phases (extracted from exact solution) Quasi-bound states show up as Fano resonances

30 Vacuum polarization General form: RPA (Mirlin et al): Thomas-Fermi (Katsnelson): RPA and TF give conflicting results, which is unusual

31 Friedel sum rule Total charge: Lin (2006): Phases extracted from exact solution:

32 RPA formula is valid. Subrcritical case Numericall results: Only deep states contribute to polarization charge (confirmed by a direct calculation) Terekhov et al: exact formula for polarization charge

33 Supercritical case (only s-channel is overcritical) Thomas-Fermi limit RG flow of the effective charge: The flow terminates at finite distance: The charge of an overcritical impurity is always screened to the critical value.

34 Open questions: Excitons? Effect of vacuum polarization on localized states? Other many-body effects? Kondo? Role of water? Capacitance? Gapped systems (bilayers, epitaxial graphene)?

35 Conclusions: Two distinct regimes for Coulomb impurities: subcritical and supercritical Local DOS: quasilocalized states, standing waves Fano-like resonances in transport cross-section, conductivity Screening cloud in supercritical regime Divalent and tri-valent impurities are needed (Ca, Yb, La, Gd can be intercalated, e.g., McChesney et al, (2007)). Alternatively, one can use the charge on STM tip. Signatures of «atomic collapse» can be observed.