Nanoscale Explorations of the Insulator‐Metal Transition in VO2 Jenny Hoffman Contributors: Yi Yin Changhyun Ko XiangFeng Wang Martin Zech Shriram Ramanathan Gang Wu Tess Williams Harvard School of Engineering Xianhui Chen Jeehoon Kim & Applied Sciences USTC Alex Frenzel Harvard Physics Thanks to: Correlated Electron Materials • High-Tc superconductors: ρ = 0 up to T = 135 Kelvin • Colossal magnetoresistance: B-field changes electrical conductivity by 1000x • Multi-ferroics: magneto-electric-elastic coupling • Heavy fermion materials: m* = 1000 me • Graphene: m* = 0 • TlililtdiitilTopological insulators: dissipationless sp in; fractional charges; magnetic monopoles Science 309, 391 (2005) Jannik Meyer Hoffman Lab JARA, RWTH Aachen Zahid Hasan Physics World Metals: electronic Bloch states are homogeneous, no spatial variation Correlated electron materials: weak screening, sp atial inhomog eneit y Æ We need local probes to study these materials. Hoffman Lab Local Probes Scanning Tunneling Ultra‐high vacuum Microscope Force Microscope STM coming on line soon! • spin‐polarized tunneling • atom‐moving cuprate high‐Tc 122 iron topological metal‐insulator 1111 iron superconductor pnictide insulator transition pnictide Hoffman Lab Local Probes Scanning Tunneling Ultra‐high vacuum Microscope Force Microscope STM Themes: • needld loca l s tditudies to un ders tan dblkd bulk proper ties o f corre ltdlated e lec tron ma ter ilials • active manipulation as well as passive imaging • surface probes can be used to investigate bulk properties • for each material studied: • physics: how can we understand it? • technology: how can we apply it? Outline Strong electron correlations Æ Mott transition MttSMott Syst ems Vanadium dioxide (VO2) •history • new physics • new applications Superconducting vortices • scanning tunneling microscopy • magnetic force microscopy Outline Strong electron correlations Æ Mott transition MttSMott Syst ems Vanadium dioxide (VO2) •history • new physics • new applications Superconducting vortices • scanning tunneling microscopy • magnetic force microscopy Band Theory E Single atom: Crystal: Add momentum information: E 3s k 2p Brillouin zone edge: highest unique wavevector k 2s (larger k’s aren’t sampled by the atoms so they are aliased back to smaller k’s) λmin kmax 1s Band Theory: Metal E Single atom: Crystal: Add momentum information: E 3s k 2p Brillouin zone edge: highest unique wavevector k 2s (larger k’s aren’t sampled by the atoms so they are aliased back to smaller k’s) λmin kmax 1s Band Theory: Insulator E Single atom: Crystal: Add momentum information: E 3s k 2p Brillouin zone edge: highest unique wavevector k 2s (larger k’s aren’t sampled by the atoms so they are aliased back to smaller k’s) λmin kmax 1s Band Theory: Metal E Single atom: Crystal: Add momentum information: E 3s k 2p Brillouin zone edge: highest unique wavevector k 2s (larger k’s aren’t sampled by the atoms so they are aliased back to smaller k’s) λmin kmax 1s Band Insulator: Peierls Transition E Molecule: Crystal: Add momentum information: E 3s k 2p Brillouin zone edge: highest unique wavevector k 2s (larger k’s aren’t sampled by the atoms so they are aliased back to smaller k’s) new λmin doubles kmax halves 1s Band Theory: Metal E Single atom: Crystal: Add momentum information: E 3s k 2p 2s 1s Add e‐e Correlations E Single atom: Crystal: Add momentum information: E 3s k 2p 2s U e e e e e e e e 1s Add e‐e Correlations E Single atom: Crystal: Add momentum information: E 3s k 2p 2s U e e e e e e e e 1s Add e‐e Correlations E Single atom: Crystal: Add momentum information: E 3s k 2p 2s U e e e e e e e e 1s Mott Insulator E Single atom: Crystal: Add momentum information: E 3s k 2p 2s U e e e e e e e e 1s Mott Transition localized delocalized further delocalized Outline Strong electron correlations Æ Mott transition MttSMott Syst ems Vanadium dioxide (VO2) •history • new physics • new applications Superconducting vortices • scanning tunneling microscopy • magnetic force microscopy Mott Insulator: NiO N. F. Mott, Proceedings of the Physical Society of London A62, 416 (1949) Mott Insulator: Hydrogen best data metal? to date Mott transition? insulator Deemyad & Silvera, PRL 100, 155701 (2008) Mott Transition: Cold Atoms V0 = potential depth of lattice tighter lattice, increasing V0 sharper interference loss of coherence = Mott transition Greiner, Nature 415, 39 (2002) Mott Insulator: Undoped Cuprate max Bi2Sr2CaCu2O8+d (Tc ~ 93 K) BiO Each Cu is singly occupied Æ half-filled band, should be metal? SrO Strong Coulomb repulsion CuO2 Æ Mott insulator Ca CuO2 SrO BiO BiO SrO CuO2 Ca CuO2 SrO BiO Mott Transition: Dope the Cuprate max Bi2Sr2CaCu2O8+d (Tc ~ 93 K) BiO SrO CuO2 Ca CuO2 SrO BiO BiO SrO CuO2 Ca CuO2 SrO BiO Mott Transition: Dope the Cuprate max Bi2Sr2CaCu2O8+d (Tc ~ 93 K) BiO SrO CuO2 Ca CuO2 SrO BiO BiO SrO CuO2 Ca CuO2 SrO BiO Cuprate Phase Diagram s 3-dim cuprate phase diagram “strange metal” ) Boyer, Nat Phys Nat Phys (2007) Boyer, 1.2 density of state mm energy (V)(meV) c Ω 0.8 0.4 sistivity (m sistivity ee 0 r 0 200 400 600 800 T (K) T antiferromagnetic states insulator ff density o energy (meV) d-wave superconductor B doped Mott insulator x (doping) becomes metal Cuprate Phase Diagram 3-dim cuprate phase diagram “strange metal” “Here be dragons” T antiferromagnetic insulator d-wave superconductor B doped Mott insulator x (doping) becomes metal Cuprate Phase Diagram 3-dim cuprate phase diagram “strange metal” “Here be dragons” T antiferromagnetic insulator d-wave superconductor B doped Mott insulator x (doping) Interest in High‐Tc Cuprates funding agency frustration sets in? (price of helium skyrockets) J Supercond Nov Magn 21, 113 (2008) Outline Strong electron correlations Æ Mott transition MttSMott Syst ems Vanadium dioxide (VO2) •history • new physics • new applications Superconducting vortices • scanning tunneling microscopy • magnetic force microscopy Outline Strong electron correlations Æ Mott transition MttSMott Syst ems Vanadium dioxide (VO2) •history • new physics • new applications Superconducting vortices • scanning tunneling microscopy • magnetic force microscopy 1959: VO2 Metal‐Insulator Transition T (Kelvin) 500 250 167 125 100 83 Morin, PRL 3, 34 (1959) VO2 Metal‐Insulator Transition T (K) Tetragonal Metal 105 Heating V4+ Cooling 104 ) ΩΩ 3 R ( 10 340 K Monoclinic Insulator 102 300 320 340 360 380 T (Kelvin) Also transitions as a function of • stress (~38 kbar) • opti cal ex ci tati on • applied voltage or current (~107 V/m ??) Æ up to 5 orders of magnitude change in conductivity VO2 Metal‐Insulator Transition: Physics T (K) Tetragonal Metal 3dπ (V-O-V) EF 3d|| (V-V) 340 K Band insulator? E Monoclinic Insulator ? k 3dπ (V-O-V) • Goodenough, Phys. Rev. 117, 1442 (1960) • Wentzcovitch, PRL 72, 3389 (1994) ? E 07eV0.7 eV F Mott insulator? 3d|| (V-V) U e e e e • Zylbersztejn & Mott, PRB 11, 4383 (1975) Goodenough, J.Solid State Chem 3, 490 (1971) Outline Strong electron correlations Æ Mott transition Mott Systems Vanadium dioxide (VO2) •history • new ppyhysics • new applications Superconducting vortices • scanning tunneling microscopy • magnetic force microscopy VO2 Recent Revival Mott vs. Peierls (electronic vs. structural)? (Mott is more interesting) • m* increases, approaching metal Æ insulator transition Qazilbash, Science 318, 1750 (2007) • Δt = 80 fs, limit ed b y exactl y ½ per io d o f s truc tura l p honon Cavalleri, PRB 70, 161102 (2004) Large Gap at Low T 4000 cm-1 ~ 0.5 eV Redistribution persists up to 6 e V ~ 7 0,000 K Gap = 0.6 eV ~ 7,000 Kelvin Qazilbash, PRB 77, 115121 (2008) Phillips, RMP (2010); arXiv:1001.5270 m* Enhancement Approaching Insulator less more conducting conducting Qazilbash, Science 318, 1750 (2007) Insulator ÆMetal Transition Limited by Phonon response time >> excitation time response time ~ excitation time excitation response ½ phonon period Æ importance of band nature of transition Cavalleri, PRB 70, 161102 (2004) VO2 Recent Revival Mott vs. Peierls (electronic vs. structural)? (Mott is more interesting) • m* increases, approaching metal Æ insulator transition Qazilbash, Science 318, 1750 (2007) • Δt = 80 fs, limit ed b y exactl y ½ per io d o f s truc tura l p honon Cavalleri, PRB 70, 161102 (2004) Can Mott exist w/o Peierls? (Mott would be even faster , wouldn’ t crack crystal) • 46 years (1959 – 2005): answer is NO • Raman spectroscopy Æ yes? H.T. Kim, APL 86, 242101 (2005) • Local IR microscopy (electrical) & x-ray diffraction (structural) Æ yes? Qazilbash, unpublished APS March talk (2010) Decoupling of Structural & Electronic Transitions ? features of high T features of low T can vary R rutile structure ext monoclinic structure compliance to vary I compliance current Cr/Au metallic state transition occurs at I < 3 mA H-T Kim, APL 86, 242101 (2005) Also: Qazilbash et al infer decouping from nanoscale IR and XRD (but not simultaneous expts)… APS March Meeting (2010) VO2 Recent Revival Mott vs. Peierls (electronic vs. structural)? (Mott is more interesting) • m* increases, approaching metal Æ insulator transition Qazilbash, Science 318, 1750 (2007) • Δt = 80 fs, limit ed b y exactl y ½ per io d o f s truc tura l p honon Cavalleri, PRB 70, 161102 (2004) Can Mott exist w/o Peierls? (Mott would be even faster , wouldn’ t crack crystal) • 46 years (1959 – 2005): answer is NO • Raman spectroscopy Æ yes? H.T. Kim, APL 86, 242101 (2005) • Local IR microscopy (electrical) & x-ray diffraction (structural) Æ yes? Qazilbash, unpublished APS March talk (2010) Can it be entirely E-field driven or need heat too ? (would like zero quiescent current) • 3-terminal (field-effect) devices show transition (but questions of breakdown voltage…) Stefanovich, JPCM 12, 8837 (2000) Qazilbash, APL 92, 241906 (2008) • thermal modeling says Joule heating is not enough to drive transition Gopalakrishnan, J .