Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V7: High Resolution Tunneling Spectroscopy of Dirac Fermions |
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Sponsoring Units: GIMS Chair: Eric Hudson, Massachusetts Institute of Technology Room: Ballroom C3 |
Thursday, March 24, 2011 8:00AM - 8:36AM |
V7.00001: Strain-Induced Pseudo--Magnetic Fields in Graphene: MegaGauss in Nanobubbles Invited Speaker: Recent theoretical proposals suggest that strain can be used to modify graphene electronic states through the creation of a pseudo--magnetic field. This effect is unique to graphene because of its massless Dirac fermion-like band structure and particular lattice symmetry (C3v). Scanning tunneling microscopy shows that graphene grown on a platinum (111) surface forms nanobubbles, which are highly strained due to thermal expansion mismatch between the film and the substrate. We find that scanning tunneling spectroscopy measurements of these nanobubbles exhibit Landau levels that form in the presence of strain-induced pseudo--magnetic fields greater than 300 Tesla. This demonstration of enormous pseudo--magnetic fields opens the door to both the study of charge carriers in previously inaccessible high magnetic field regimes and deliberate mechanical control over electronic structure in graphene or so-called ``strain engineering''. \\[4pt] In collaboration with S. A. Burke$^{\S ,2}$, K. L. Meaker$^{2}$, M. Panlasigui$^{2}$, A. Zettl$^{2,3}$, F. Guinea$^{4}$, A. H. Castro Neto$^{5}$ and M. F. Crommie$^{2,3}$. {\S}. Present address: Department of Physics and Astronomy and Department of Chemistry, University of British Columbia, Vancouver, BC V6T 121, Canada. 2. Department of Physics, University of California, Berkeley, CA 94720, USA. 3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. 4. Instituto de Ciencia de Materiales de Madrid (CSIC), Madrid 28049, Spain. 5. Department of Physics, Boston University, Boston, MA 02215, USA. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 9:12AM |
V7.00002: Scanning tunneling microscopy studies of topological insulators grown by molecular beam epitaxy Invited Speaker: I will summarize our recent activities of using scanning tunneling microscope (STM) to study topological insulators grown by molecular beam epitaxy (MBE). The Landau quantization in three-dimensional topological insulators was directly observed in the tunneling spectra. In particular, we discovered the zeroth Landau level, which is predicted to give rise to the half-quantized Hall effect for the topological surface states. The existence of the discrete Landau levels and the suppression of Landau levels by surface impurities strongly support the 2D nature of the topological states. In addition, we studied the quantum interference pattern formed by the topological surface states near the step edges and magnetic impurities in Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$. The decay behavior of the standing waves is in good agreement with the Dirac cone structure of the topological surface states. We show that the combination of MBE and high energy resolution scanning tunneling spectroscopy provides a powerful~way to probing the novel physics in the topological insulators. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:48AM |
V7.00003: Spatially Resolved Spectroscopy of Magnetic States in Epitaxial Graphene Invited Speaker: Graphene grown epitaxially on silicon carbide provides a potential avenue toward industrial-scale graphene electronics. A predominant aspect of the multilayer graphene produced on the carbon-terminated ($000\bar{1}$) face of SiC is the rotational stacking faults between graphene layers and their associated moire-pattern superlattice. We use scanning tunneling microscopy (STM) and spectroscopy (STS) in high magnetic fields to obtain detailed information about the massless Dirac fermions that carry charge in graphene. In agreement with prior investigations, we find that for small magnetic fields, the rotational stacking effectively decouples the electronic properties of the top graphene layer from those below. However, in maps of the wavefunction density at magnetic fields above 5 Tesla, we discover atomic-scale features that were not previously known or predicted. A phenomenological theory shows that this high-field symmetry-breaking is a consequence of small cyclotron-orbit wavefunctions, which are sensitive to the local layer stacking internal to the moire superlattice cell. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:24AM |
V7.00004: Landau-level spectroscopies of a topological insulator Invited Speaker: Topological insulators such as Bi$_2$Se$_3$ are characterized by massless Dirac surface state which would give rise to unique quantum phenomena in a magnetic field. Although it was experimentally verified by many ARPES experiments that the surface electrons are indeed massless, there has been a lack of studies exploring their quantum properties due to the inevitable contribution from the bulk electrons in a real material. Using surface-sensitive STM/STS technique, we selectively probed the surface massless electrons in Bi$_2$Se$_3$. Under magnetic field perpendicular to the cleaved surface, a series of Landau levels (LLs) has been observed in the tunneling spectra. Remarkably, there is a field-independent LL at the Dirac point, which is a hallmark of Dirac fermions. We developed a scaling analysis scheme of LLs based on the Bohr-Sommerfeld quantization condition which allowed us to determine the energy-momentum dispersion of the surface state~[1]. Width of the LL peaks in the spectra becomes smaller near the Fermi energy, which may suggest that electron-electron correlation plays a role. In addition to the narrowing of LLs, the spectra near the Fermi energy exhibit complicated fine structures, which may be responsible for the anomalous magneto-fingerprint effect~[2]. This work has been done in collaboration with K. Igarashi, M. Kawamura, H. Takagi and T. Sasagawa.\\[4pt] [1] T. Hanaguri {\it et al.}, Phys. Rev. B {\bf 82}, 081305(R) (2010).\\[0pt] [2] J. G. Checkelsky {\it et al.}, Phys. Rev. Lett. {\bf 103}, 246601 (2009). [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 11:00AM |
V7.00005: Scanning Tunneling Spectroscopy of a Gated Single- and Bilayer Graphene Devices in the Quantum Hall Regime Invited Speaker: We have performed scanning tunneling spectroscopy (STS) measurements on a gated single- and bilayer graphene devices. The combination of STM/STS capability and an electrostatic back gate enables us to investigate the interactions of Dirac particles with local impurities at the atomic scale in zero magnetic field and in the quantum Hall regime while varying the Fermi-energy with respect to a Dirac (charge neutrality) point. In an applied magnetic field, well-resolved Landau levels (LLs) following the Dirac particle scaling are observed in both single- and bilayer graphene. Additionally, in single layer graphene, spatial dispersion of LLs caused by disorder potential lead to formation of graphene quantum dots (QDs). The tunneling spectra measured as a function of gate and sample biases are governed by Coulomb blockade physics. In contrast, no QDs are seen in bilayer devices. Instead, the main feature of the spectra is an energy gap formed around the charge neutrality point. The possible origin of energy gap will be discussed in a context of broken layer symmetry caused by gate electric field and disorder potential variation. Other noticeable features in the tunneling spectra of single- and bilayer graphene such as the formation of electron- and hole-puddles and the Fermi-level pinning effect will be discussed. [Preview Abstract] |
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