Introduction to Mesoscopic Physics and Quantum Dots (15466-01 and 20400-01)
Fall semester 2008
Prof. Dr. Dominik Zumbühl,
last updated Dec 3, 2008
previous semesters courses:
Fall 2007, Winter 2006/7, Spring 2006
Organization of Lecture, HS 2008
lectures Thursday, 8:15am to 10am, weekly, Room 4.1
exercises Wednesday, 12:15pm to 1pm, weekly, Seminar Room 1.09
syllabus 2D electron gas, quantum point contacts, quantum dots, conductance fluctuations, quantum phase coherence, Coulomb blockade, Kondo effect, few electron dots, spin blockade, spin relaxation, charge sensing, single spin measurement, spin manipulation and coherence, nuclear spins, electron spin resonance, spin qubits, quantum computation. syllabus pdf
for: 6. semester bachelor and masters students in nanoscience and physics. physics III lecture is a prerequisite
credit points 2 credit points lecture and (optional) 2 credit points exercises.
Lecture is a two hours per week course, no problem sets, final presentation, pass/fail.
The exercises will consist of reading a series of scientific publications relevant to class, meeting one hour per week where papers discussed by the students in depth.
purpose of this lecture is to bring the audience up to date on current experimental research in the field of quantum transport through nanostructures, focusing mainly on quantum dots in GaAs, spin-quibts in coupled dots and quantum computation. The lecture will inevitably discuss some simple condensed matter theory but will mainly focus on experiments.
structure will be a combination of lectures on my part to intruduce the basics and subsequent presentations done by students towards the end of the semester on topics that can be chosen by students from a selection. There will be no problem sets, but moderate amounts of reading as preparation for lectures will be assigned. The class is pass/fail, and the evaluation will be based on the presentation given by each student taking the class for credit. Sit-ins/students auditing the class (not for credit) are welcome.
Lecture Schedule and Content, HS 2008
date | lecture content |
---|---|
Sep 16, Tue, 9:15am | preliminary meeting, flier pdf, syllabus pdf |
Thur, Sep 18, 8:15am | solid state physics background crystals, bands, effective mass approximation, spin-orbit coupling, GaAs conduction and valence bands hand out: Title and Introduction pdf |
Thur, Sep 25, 8:15am | solid state physics background II GaAs valence band, spin-orbit coupling, occupation of bands, density of states, chemical potential, intrinsic carrier concentration, semiconductor doping |
Thur, Oct 2, 8:15am | semiconductor surfaces and interfaces surface states, semiconductor-vacuum interface, band bending, Fermi level pinning, Schottky barrier and diode lecture notes Chapter 2 pdf 2D electron gases (GaAs) properties of 2DEGs, scattering mechanisms in 2DEGs and bulk GaAs lecture notes pdf |
Thur, Oct 9, 8:15am | 2D electron gases screening in 2D and 3D, Friedel Oscillations, ohmic contacts to 2DEGs, lateral gating and nanoscale devices quantum point contacts van Wees et al., PRL60, 848 (1988) pdf van Wees et al., PRB38, 3625 (1988) pdf |
Thur, Oct 16, 8:15am | experimental techniques sample fabrication, low temperature methods (chapter 4, Heinzel), properties of 3He and 4He and phase diagrams, cryostats, evaporative cooling,lecture slides pdf |
Thur, Oct 23, 8:15am | experimental techniques (II) 4He and 3He fridges, 3He-4He mixtures, dilution refrigeration concepts in mesoscopic physics Drude conductivity, Einstein relation, mesoscopic time and length scales, lecture notes chapter 3 pdf |
Thur, Oct 30, 8:15am | concepts in mesoscopic physics classical Hall effect, (integer) quantum Hall effect, 1D subbands, quantum point contacts, 0.7 structure lecture slides pdf Cronenwett et al., PRL88, 226805 (2002) pdf |
Thur, Nov 6, 8:00am | quantum dots I: Introduction types of quantum dots, open and closed dot regimes, Coulomb blockade and diamonds, quantum confinement energy, constant interaction model, ground state and excited state spectroscopy, sequential and cotunneling transport, Coulomb peak lineshapes lecture notes pdf articles: Kouwenhoven et al., dot review article, pp 1-28 pdf van Houten et al., CB review, NATO ASI, pp 1-15 pdf Foxman et al., PRB47, 10020 (1993) pdf Foxman et al., PRB50, 14193 (1994) pdf |
Thur, Nov 13, 8:00am | quantum dots II: Open Dots open dot regime, conductance fluctuations, weak localization, phase coherence, random matrix theory, spin-orbit coupling in semiconductors and quantum dots, weak antilocalization lecture notes pdf literature: A. Huibers, Ph. D. Thesis (1999) and S. Cronenwett, Ph. D. Thesis (2001) are available on the marcuslab[page does not exist anymore] webpage. open dot regime Huibers et al., PRL83, 5090 (1999) pdf |
Thur, Nov 20, 8:00am | quantum dots III: Kondo Effect Kondo effect (metals, quantum dots), Kondo screening cloud, zero bias peak, logarithmic temperature dependence, even-odd structure lecture notes pdf Kondo effect Goldhaber-Gordon et al., Nature 391, 156 (1998) pdf Cronenwett et al., Science 281, 540 (1998) pdf |
Wed, Nov 26, 12:00pm | presentations I 1. charge sensing, Andreas Trüsel 2. charge manipulation, Melanie Burkhardt |
Thur, Nov 27, 8:00am | quantum dots IV: Few Electron Dots lateral / vertical few electron structures, circular symmetry: shell filling, Fock-Darwin states, Singlet-Triplet states, Singlet Triplet ground state transition, lecture notes pdf Few Electron Dots Tarucha et al., PRL77, 3613 (1996) pdf Ciorga et al., PRB61, 16315 (2000) pdf Kouwenhoven et al., Science 278, 1788 (1997) pdf Kouwenhoven, Austing & Tarucha, RPP 64, 701 (2002) pdf |
Tue, Dec 2, 8:15am | double quantum dots double quantum dots, honey comb stability diagrams, finite bias transport, anticrossings lecture notes pdf |
Wed, Dec 3, 12:00pm | presentations II 3. singlet triplet states, Mathias Wipf 4. spin blockade, Lukas Greuter |
Thur, Dec 4, 8:00am | presentations III 5. spin relaxation I, Raphael Widmer 6. spin relaxation II, Susanne Baumann 7. spin filtering, Michael Sicher 8. single spin readout, Franz Knuth |
Wed, Dec 10, 12:00pm | presentations IV 9. fast spin readout, Dario Maradan 10. nuclear spins, Samuel d'Hollosy |
Thur, Dec 11, 8:00am | presentations V 11. spin manipulation, Cornelia Nef 12. electron spin resonance (ESR), Markus Langer 13. dynamic nuclear polarization, Lajko Zweifel |
Wed, Dec 17, 12:00pm | presentations VII 14. ESR with SO, Dominic Giss 15. ESR with nuclei, Andreas Wepf |
Thur, Dec 18, 8:00am | presentations VIII 17. carbon nanotubes: SO coupling, Lucas Casparis 18. carbon nanotubes: Wigner crystal, Petar Jurcevic 19. graphene dots, Florian Kehl 20. hole spins, Robert Zielke |
Exercise Schedule and Content, HS 2008
Organization of the exercises: As a preparation for the exercises, the students thoroughly read the one or two papers assigned before the exercise, see list below for a schedule of papers. The assigned papers are typically 3-4 pages long. It is expected that every student has at least two questions ready as a basis or starting point for a discussion. In class, the papers are discussed in detail. Focus is on depth, not breadth. In every exercise class, one student is randomly picked to be the discussion leader / mediator. To assist the discussion, I will prepare slides containing the graphs of the paper. The exercise is pass fail, based on individual preparedness.
date | papers |
---|---|
Wed, Sept. 24, 12:15pm Aharonov-Bohm (AB) Effect | Significance of Electromagnetic Potentials in the Quantum Theory Y. Aharonov and D. Bohm The Physical Review 115, 485 (August 1959), pdf cited 2529 times (ISI, 24.9.08) read the first three pages Observation of h/e Aharonov-Bohm Oscillations in Normal-Metal Rings R. A. Webb, S. Washburn, C. P. Umbach, and R. B. Laibowitz Physical Review Letters 54, 2696 (June 1985), pdf cited 524 times (ISI, 24.9.08) additional reading (not mandatory): Aharonov-Bohm Chapter, Delft Mesoscopic Physics Course pdf |
Wed, Oct. 1, 12:15pm Aharonov-Bohm (AB) Effect (2) | Direct Observation of Ensemble Averaging of the Aharonov-Bohm Effect in Normal-Metal Loops C. P. Umbach, C. Van Haesendonck, R. B. Laibowitz, S. Washburn, and R. A. Webb Physical Review Letters 56, 386 (January 1986), pdf cited 110 times (ISI, 24.9.08) Asymmetry in the Magnetoconductance of Metal Wires and Loops A. D. Benoit, S. Washburn, C. P. Umbach, R. B. Laibowitz, and R. A. Webb, Physical Review Letters 57, 1765 (October 1986), pdf cited 104 times (ISI, 24.9.08) additional reading (not mandatory): "Four Terminal Phase-Coherent Conductance", M. Büttiker, PRL 57, 1761 (1986). (1290 citations, ISI 30.9.08) pdf |
Wed, Oct. 8, 12:15pm Weak Localization (WL) | Scaling Theory of Localization: Absence of Quantum Diffusion in Two Dimensions E. Abrahams, P. W. Anderson, D. C. Licciardello, and T. V. Ramakrishnan Physical Review Letters 42, 673 (March 1979), pdf, gang of 4 paper 3'090 citations (ISI, 30.9.08) Nonmetallic Conduction in Thin Metal Films at Low Temperatures G. J. Dolan and D. D. Osheroff, Physical Review Letters 43, 721 (September 1979), pdf 288 citations (ISI, 30.9.08) additional reading (not mandatory): Weak-Localization Section, Delft Mesoscopic Physics Course pdf |
Wed, Oct. 15, 12:15pm Weak Localization (WL) (2) | Quantitative analysis of weak localization in thin Mg films by magnetoresistance measurements Gerd Bergmann Physical Review B 25, 2937 (February 1982), pdf, 51 citations (ISI, 30.9.08) Dephasing time and one-dimensional localization of two-dimensional electrons in GaAs/AlGaAs heterostructures K. K. Choi, D. C. Tsui, and K. Alavi Physical Review B 36, 7751 (November 1987), pdf 59 citations (ISI, 30.9.08) additional reading (not mandatory): "Weak Localization in Thin Films: a time of flight experiment with conduction electrons", Gerd Bergmann, Physics Reports 107, 1 (November 1983). (1'069 citations, ISI 30.9.08) pdf |
Wed, Oct. 22, 12:15pm Universal Conductance Fluctuations (UCF) | Magnetoresistance of small, quasi one-dimensional, normal-metal rings and lines C. P. Umbach, S. Washburn, R. B. Laibowitz, and R. A. Webb Physical Review B 30, 4048 (October 1984), pdf, 260 citations (ISI, 30.9.08) Magnetoresistance Fluctuations in Mesoscopic Wires and Rings A. Douglas Stone Physical Review Letters 54, 2692 (June 1985), pdf 294 citations (ISI, 30.9.08) additional reading (not mandatory): "Quantum Transport in Semiconductor Nanostructures", C. W. J. Beenakker and H. van Houten, published in Solid State Physics, 44, 1-228 (1991). (709 citations, ISI 30.9.08) pdf |
Wed, Oct. 29, 12:15pm | |
Wed, Nov. 5, 12:15pm Universal Conductance Fluctuations (UCF) (2) | Aperiodic Magnetoresistance Oscillations in Narrow Inversion Layers in Si J. C. Licini, D. J. Bishop, M. A. Kastner, and J. Melngailis Physical Review Letters 55, 2987 (December 1985), pdf, 105 citations (ISI, 30.9.08) Universal Conductance Fluctuations in Silicon Inversion-Layer Nanostructures W. J. Skocpol, P. M. Mankiewich, R. E. Howard, L. D. Jackel, D. M. Tennant, and A. Douglas Stone Physical Review Letters 56, 2865 (June 1986), pdf 190 citations (ISI, 30.9.08) |
Wed, Nov. 19, 12:15pm Coulomb Blockade (CB) | Observation of Single-Electron Charging Effects in Small Tunnel Junctions T. A. Foulton and G. J. Dolan Physical Review Letters 59, 109 (July 1987), pdf, 648 citations (ISI, 30.9.08) Conductance Oscillations Periodic in the Density of a One-Dimensional Electron Gas J. H. F. Scott-Thomas, Stuart B. Field, M. A. Kastner, Henry I. Smith, D. A. Antoniadis Physical Review Letters 62, 583 (January 1989), pdf 230 citations (ISI, 30.9.08) additional reading (not mandatory): Comment to Scott-Thomas et al., H. van Houten and C. W. J. Beenakker, PRL 63, 1893 (Oct 1989), pdf Authors reply, PRL63, 1894 (Oct 1989), pdf |
Projects and Presentations, HS08
Presentations should be no longer than about 20 to 25 minutes, plus 5 to 10 minutes for questions.
Goal of the presentation is to explain to your fellow classmates the topic you have chosen
as clearly and as concisely as possible while keeping a critical attitude towards the reported research.
List of available topics (below) will be modified / adapted, but will vaguely consist of the topics listed below.
Topics with more than one article:
choose one article for in-depth presentation, mention highlights of other paper(s).
topic | refs | presenter |
---|---|---|
1. charge sensing | Measurements of Coulomb Blockade with a Noninvasive Voltage Probe M. Field, C. G. Smith, M. Pepper, D. A. Ritchie, J. E. F. Frost, G. A. C. Jones, and D. G. Hasko, Phys. Rev. Letters 70, 1311 (1993) pdf Few-electron quantum dot circuit with integrated charge read out J. M. Elzerman, R. Hanson, J. S. Greidanus, L. H. Willems van Beveren, S. De Franceschi, L. M. K. Vandersypen, S. Tarucha, and L. P. Kouwenhoven Phys. Rev. B 67, 161308R (Apr 2003) pdf | Andreas Trüsel |
2. charge manipulation | Coherent Manipulation of Electronic States in a Double Quantum Dot T. Hayashi, T. Fujisawa, H. D. Cheong, Y. H. Jeong, and Y. Hirayama, Phys. Rev. Letters 91, 226804 (Nov 2003) pdf Manipulation of a Single Charge in a Double Quantum Dot J. R. Petta, A. C. Johnson, C.M. Marcus, M. P. Hanson, and A. C. Gossard Phys. Rev. Letters 93, 186802 (Oct 2004) pdf | Melanie Burkhardt |
3. singlet triplet states | Voltage-tunable singlet-triplet transition in lateral quantum dots Jordan Kyriakidis, M. Pioro-Ladriere, M. Ciorga, A. S. Sachrajda, and P. Hawrylak Phys. Rev. B 66, 35320 (Jul 2002) pdf Cotunneling Spectroscopy in Few-Electron Quantum Dots D.M. Zumbuühl C.M. Marcus, M. P. Hanson and A. C. Gossard, Phys. Rev. Letters 93, 256801 (Dec 2004) pdf | Mathias Wipf |
4. spin blockade | Current Rectification by Pauli Exclusion in a Weakly Coupled Double Quantum Dot System | Lukas Greuter |
5. spin relaxation I | Allowed and forbidden transitions in artificial hydrogen and helium atoms Toshimasa Fujisawa, David Guy Austing, Yasuhiro Tokura, Yoshiro Hirayama & Seigo Tarucha Nature 419, 278 (2002) pdf Zeeman Energy and Spin Relaxation in a One-Electron Quantum Dot R. Hanson, B.Witkamp, L.M. K. Vandersypen, L. H.Willems van Beveren, J.M. Elzerman, and L. P. Kouwenhoven Phys. Rev. Letters 91, 196802 (2003) pdf | Raphael Widmer |
6. spin relaxation II | Electrical Control of Spin Relaxation in a Quantum Dot S. Amasha, K. MacLean, Iuliana P. Radu, D. M. Zumbühl, M. A. Kastner, M. P. Hanson, and A. C. Gossard Phys. Rev. Letters 100, 046803 (Feb 2008) pdf Optically programmable electron spin memory using semiconductor quantum dots Miro Kroutvar, Yann Ducommun, Dominik Heiss, Max Bichler, Dieter Schuh, Gerhard Abstreiter & Jonathan J. Finley Nature 432, 81 (Nov 2004) pdf | Susanne Baumann |
7. spin filtering | Detecting Spin-Polarized Currents in Ballistic Nanostructures R.M. Potok, J. A. Folk, C.M. Marcus, and V. Umansky Phys. Rev. Letters 89, 266602 (Dec 2002) pdf Semiconductor few-electron quantum dot operated as a bipolar spin filter R. Hanson, L. M. K. Vandersypen, L. H. Willems van Beveren, J. M. Elzerman, I. T. Vink, and L. P. KouwenhovenHanson, Phys. Rev. B 70, 241304R (Dec 2004) pdf | Michael Sicher |
8. single spin readout | Single-shot read-out of an individual electron spin in a quantum dot J. M. Elzerman, R. Hanson, L. H. Willems van Beveren, B. Witkamp, L. M. K. Vandersypen & L. P. Kouwenhoven Nature 430, 431 (Jul 2004) pdf Single-Shot Readout of Electron Spin States in a Quantum Dot Using Spin-Dependent Tunnel Rates R. Hanson, L. H. Willems van Beveren, I. T. Vink, J. M. Elzerman, W. J. M. Naber, F.H. L. Koppens, L. P. Kouwenhoven, and L. M. K. Vandersypen, Phys. Rev. Letters 94, 196802 (May 2005) pdf | Franz Knuth |
9. fast spin readout | Cryogenic amplifier for fast real-time detection of single-electron tunneling I. T. Vink, T. Nooitgedagt, R. N. Schouten, and L. M. K. Vandersypen, W. Wegscheider App. Phys. Lett. 91, 123512 (Sep 2007) pdf Fast single-charge sensing with a rf quantum point contact D. J. Reilly, C. M. Marcusa, M. P. Hanson and A. C. Gossard, App. Phys. Lett. 91, 162101 (Oct 2007) pdf | Dario Maradan |
10. nuclear spins | Triplet–singlet spin relaxation via nuclei in a double quantum dot A. C. Johnson, J. R. Petta, J. M. Taylor, A. Yacoby, M. D. Lukin, C. M. Marcus, M. P. Hanson & A. C. Gossard, Nature 435, 925 (Jun 2005) pdf Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field F. H. L. Koppens, J. A. Folk, J. M. Elzerman, R. Hanson, L. H. Willems van Beveren, I. T. Vink, H. P. Tranitz, W. Wegscheider, L. P. Kouwenhoven, L. M. K. Vandersypen, Science 309, 134 (Aug 2005) pdf | Samuel d'Hollosy |
11. spin manipulation | Coherent Manipulation of Coupled Electron Spins in Semiconductor Quantum Dots J. R. Petta, A. C. Johnson, J. M. Taylor, E. A. Laird, A. Yacoby, M. D. Lukin, C. M. Marcus, M. P. Hanson, A. C. Gossard, Science 309, 2180 (Sep 2005) pdf | Cornelia Nef |
12. electron spin resonance (ESR) | Driven coherent oscillations of a single electron spin in a quantum dot F. H. L. Koppens, C. Buizert, K. J. Tielrooij, I. T. Vink, K. C. Nowack, T. Meunier, L. P. Kouwenhoven & L. M. K. Vandersypen, Nature 442, 766 (Aug 2006) pdf | Markus Langer |
13. dynamic nuclear polarization | Dynamic Nuclear Polarization with Single Electron Spins J. R. Petta, J. M. Taylor, A. C. Johnson, A. Yacoby, M. D. Lukin, C. M. Marcus, M. P. Hanson, and A. C. Gossard, Phys. Rev. Lett. 100, 067601 (Feb 2008) pdf | Lajko Zweifel |
14. ESR with SO | Coherent Control of a Single Electron Spin with Electric Fields K. C. Nowack, F. H. L. Koppens, Yu. V. Nazarov, and L. M. K. Vandersypen, Science 318, 1430 (Nov 2007) pdf | Dominic Giss |
15. ESR with nuclei | Hyperfine-mediated gate-driven electron spin resonance E. A. Laird, C. Barthel, E. I. Rashba, C. M. Marcus, M. P. Hanson, A. C. Gossard, Phys. Rev. Lett. 99, 246601 (Dec 2007) pdf | Andreas Wepf |
16. ESR slanting field | Electrically driven single-electron spin resonance in a slanting Zeeman field M. Pioro-Ladriere, T. Obata, Y. Tokura, Y.-S. Shin, T. Kubo, K. Yoshida, T. Taniyama, S. Tarucha, Nature Physics 4, 776 (Oct 2008) pdf | |
17. carbon nanotubes: SO coupling | Coupling of spin and orbital motion of electrons in carbon nanotubes F. Kuemmeth, S. Ilani, D. C. Ralph, P. L. McEuen, Nature 452, 448 (Mar 2008) pdf News and Views: Carbon nanotubes: Old nanotubes, new tricks, Jesper Nygaard, Nature Physics 4, 266 (Apr 2008) pdf | Lucas Casparis |
18. carbon nanotubes: Wigner crystal | The one-dimensional Wigner crystal in carbon nanotubes Vikram V. Deshpande, Marc Bockrath, Nature Physics 4, 314 (Apr 2008) pdf News and Views: Carbon nanotubes: Old nanotubes, new tricks, Jesper Nygaard, Nature Physics 4, 266 (Apr 2008) pdf | Petar Jurcevic |
19. graphene dots | Chaotic Dirac Billiard in Graphene Quantum Dots L. A. Ponomarenko, F. Schedin, M. I. Katsnelson, R. Yang, E. W. Hill, K. S. Novoselov, A. K. Geim Science 320, 356 (Apr 2008) pdf News and Views: Graphene Nanoelectronics, Robert M. Westervelt, Science 320, 324 (Apr 2008) pdf | Florian Kehl |
20. Hole Spins | Optical pumping of a single hole spin in a quantum dot Brian D. Gerardot, Daniel Brunner, Paul A. Dalgarno, Patrik Ahberg, Stefan Seidl, Martin Kroner, Khaled Karrai, Nick G. Stoltz, Pierre M. Petroff, Richard J. Warburton Nature 451, 441 (Jan 2008) pdf | Robert Zielke |
21. Coherence Control | Suppressing Spin Qubit Dephasing by Nuclear State Preparation D. J. Reilly, J. M. Taylor, J. R. Petta, C. M. Marcus, M. P. Hanson, and A. C. Gossard Science 321, 817 (Aug 2008) pdf |
Literature
books
“Mesoscopic Electronics in Solid State Nanostructures”, Thomas Heinzel, Wiley-VCH (2003)
“Electronic Transport in Mesoscopic Systems”, Supriyo Datta, Cambridge Universy Press (1995)
“The Physics of Low-Dimensional Semicond.”, John H. Davies, Cambridge University Press (1998)
review articles
“Quantum Transport in Semiconductor Nanostructures”, C. W. J. Beenakker and H. van Houten, published in Solid State Physics, 44, 1-228 (1991) (out of print, available at arXive:cond-mat/0412664) pdf
“Electron Transport in Quantum Dots”, L. P. Kouwenhoven, C. M. Marcus, P. L. McEuen, S. Tarucha, R. M. Westervelt and N. Wingreen, NATO ASI conference proceedings, edited by L. L. Sohn, L. P. Kouwenhoven and G. Schön (Kluwer, Dordrecht, 1997). pdf
“Coulomb Blockade Oscillations in Semiconductor Nanostructures”, H. van Houten, C. W. J. Beenakker and A. A. M. Staring, published in Single Charge Tunneling, edited by H. Grabert and M. H. Devoret, NATO ASI series B294 (Plenum, New York, 1992), (out of print, available at arXive:cond-mat/0508454). pdf
“Few-Electron Quantum Dots”, L. P. Kouwenhoven, D. G. Austing and S. Tarucha, Rep. Prog. Phys. 64, 701 (2001). pdf
“Electron transport through double quantum dots”, W. G. van der Wiel, S. De Franceschi, J. M. Elzerman, T. Fujisawa, S. Tarucha, and L. P. Kouwenhoven, Rev. Mod. Phys. 75, 1-22 (2003) pdf
“Recipes for Spin Based Quantum Computing”, V. Cerletti, W. A. Coish, O. Gywat and D. Loss, Nanotechnology 16, R27 (2005). pdf
“Spins in few-electron quantum dots”, R. Hanson, L. P. Kouwenhoven, J. R. Petta, S. Tarucha, and L. M. Vandersypen, Rev. Mod. Phys. 79, 1217-1265 (2007) pdf
“Coherent manipulation of single spins in semiconductors”, R. Hanson and D. Awschalom, Nature 453, 1043 (June 2008, Insight Review) pdf