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High-resolution tunnelling spectroscopy of a graphene quartet
Song, Young Jae,Otte, Alexander F.,Kuk, Young,Hu, Yike,Torrance, David B.,First, Phillip N.,de Heer, Walt A.,Min, Hongki,Adam, Shaffique,Stiles, Mark D.,MacDonald, Allan H.,Stroscio, Joseph A. Nature Publishing Group, a division of Macmillan P 2010 Nature Vol.467 No.7312
Electrons in a single sheet of graphene behave quite differently from those in traditional two-dimensional electron systems. Like massless relativistic particles, they have linear dispersion and chiral eigenstates. Furthermore, two sets of electrons centred at different points in reciprocal space (??valleys??) have this dispersion, giving rise to valley degeneracy. The symmetry between valleys, together with spin symmetry, leads to a fourfold quartet degeneracy of the Landau levels, observed as peaks in the density of states produced by an applied magnetic field. Recent electron transport measurements have observed the lifting of the fourfold degeneracy in very large applied magnetic fields, separating the quartet into integer and, more recently, fractional levels. The exact nature of the broken-symmetry states that form within the Landau levels and lift these degeneracies is unclear at present and is a topic of intense theoretical debate. Here we study the detailed features of the four quantum states that make up a degenerate graphene Landau level. We use high-resolution scanning tunnelling spectroscopy at temperatures as low as 10??mK in an applied magnetic field to study the top layer of multilayer epitaxial graphene. When the Fermi level lies inside the fourfold Landau manifold, significant electron correlation effects result in an enhanced valley splitting for even filling factors, and an enhanced electron spin splitting for odd filling factors. Most unexpectedly, we observe states with Landau level filling factors of 7/2, 9/2 and 11/2, suggestive of new many-body states in graphene.
Invited Review Article: A 10 mK scanning probe microscopy facility.
Song, Young Jae,Otte, Alexander F,Shvarts, Vladimir,Zhao, Zuyu,Kuk, Young,Blankenship, Steven R,Band, Alan,Hess, Frank M,Stroscio, Joseph A American Institute of Physics 2010 Review of scientific instruments Vol.81 No.12
<P>We describe the design, development and performance of a scanning probe microscopy (SPM) facility operating at a base temperature of 10 mK in magnetic fields up to 15 T. The microscope is cooled by a custom designed, fully ultra-high vacuum (UHV) compatible dilution refrigerator (DR) and is capable of in situ tip and sample exchange. Subpicometer stability at the tip-sample junction is achieved through three independent vibration isolation stages and careful design of the dilution refrigerator. The system can be connected to, or disconnected from, a network of interconnected auxiliary UHV chambers, which include growth chambers for metal and semiconductor samples, a field-ion microscope for tip characterization, and a fully independent additional quick access low temperature scanning tunneling microscope (STM) and atomic force microscope (AFM) system. To characterize the system, we present the cooling performance of the DR, vibrational, tunneling current, and tip-sample displacement noise measurements. In addition, we show the spectral resolution capabilities with tunneling spectroscopy results obtained on an epitaxial graphene sample resolving the quantum Landau levels in a magnetic field, including the sublevels corresponding to the lifting of the electron spin and valley degeneracies.</P>
Creating nanostructured superconductors on demand by local current annealing
Baek, Hongwoo,Ha, Jeonghoon,Zhang, Duming,Natarajan, Bharath,Winterstein, Jonathan P.,Sharma, Renu,Hu, Rongwei,Wang, Kefeng,Ziemak, Steven,Paglione, Johnpierre,Kuk, Young,Zhitenev, Nikolai B.,Stroscio American Physical Society 2015 Physical review. B, Condensed matter and materials Vol.92 No.9
Thermoelectric imaging of structural disorder in epitaxial graphene
Cho, Sanghee,Kang, Stephen Dongmin,Kim, Wondong,Lee, Eui-Sup,Woo, Sung-Jae,Kong, Ki-Jeong,Kim, Ilyou,Kim, Hyeong-Do,Zhang, Tong,Stroscio, Joseph A.,Kim, Yong-Hyun,Lyeo, Ho-Ki Nature Publishing Group, a division of Macmillan P 2013 Nature materials Vol.12 No.10
Heat is a familiar form of energy transported from a hot side to a colder side of an object, but not a notion associated with microscopic measurements of electronic properties. A temperature difference within a material causes charge carriers, electrons or holes to diffuse along the temperature gradient inducing a thermoelectric voltage. Here we show that local thermoelectric measurements can yield high-sensitivity imaging of structural disorder on the atomic and nanometre scales. The thermopower measurement acts to amplify the variations in the local density of states at the Fermi level, giving high differential contrast in thermoelectric signals. Using this imaging technique, we uncovered point defects in the first layer of epitaxial graphene, which generate soliton-like domain-wall line patterns separating regions of the different interlayer stacking of the second graphene layer.
Electromechanical Properties of Graphene Drumheads
Klimov, N. N.,Jung, S.,Zhu, S.,Li, T.,Wright, C. A.,Solares, S. D.,Newell, D. B.,Zhitenev, N. B.,Stroscio, J. A. American Association for the Advancement of Scienc 2012 Science Vol.336 No.6088
<P>We determined the electromechanical properties of a suspended graphene layer by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, as well as computational simulations of the graphene-membrane mechanics and morphology. A graphene membrane was continuously deformed by controlling the competing interactions with a STM probe tip and the electric field from a back-gate electrode. The probe tip-induced deformation created a localized strain field in the graphene lattice. STS measurements on the deformed suspended graphene display an electronic spectrum completely different from that of graphene supported by a substrate. The spectrum indicates the formation of a spatially confined quantum dot, in agreement with recent predictions of confinement by strain-induced pseudomagnetic fields.</P>