News & Events

Speaker: Brian LeRoy
Affiliation: University of Arizona, US
Abstract Details: Scanning probe microscopy is a powerful tool to probe low-dimensional systems. The local information provided by scanning probe microscopy is invaluable for studying effects such as interactions and scattering. Using this approach, we have probed the local electronic properties of graphene. We have studied the effect of charged impurities and the underlying substrate on the local density of states. We find that long‑range scattering from charged impurities locally shifts the charge neutrality point leading to electron and hole doped regions. By using boron nitride as a substrate, we observe an improvement in the electronic properties of the graphene as well as a moire pattern due to the misalignment of the graphene and boron nitride lattices. We find that the periodic potential due to the boron nitride substrate creates a set of 6 new superlattice Dirac points in graphene. More complicated graphene heterostructures can be created by adding additional layers of graphene or other two-dimensional materials. I will discuss our initial results in this direction.
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Speaker: C. Qiu, C. Cong, and P. Hajiyev
Affiliation: Yu Ting\'s group, NTU
Abstract Details: Using our custom-designed measuring setup, we performed magneto-Raman spectroscopy and imaging on high quality graphene decoupled from graphite, and on ABA-/ABC- stacked trilayer graphene (TLG) samples. For decoupled graphene, a high resolution magneto phonon resonance (MPR) Raman data are obtained, which are proven by the new observation of triplet splitting of the G mode and the appearance of eight traces showing the crossing of the G mode with the anti-crossing MPR behaviour. For trilayer graphene samples, it is found that the Raman features of the E2g phonon oscillate with the magnetic fields in both supported and suspended ABA-stacked TLG, which indicates the existence of MPR and its weak dependence on the substrate. In contrast, the E2g phonon of ABC-stacked TLG are inert to the magnetic field up to 9 T. All of these magnetic field-dependent spectral features are explained by the proposed theoretical models. We also conducted Raman studies on 2D Tantalum Dislenide (TaSe2) samples, which has a unique charge density wave (CDW) phase at 123 K and 90 K followed by possible superconductivity at even lower temperatures. Single- and few-layers of 2H-TaSe2 were successfully exfoliated and room temperature Raman measurements demonstrate MoS2-like spectral features, which are reliable for thickness determination. Furthermore, in-situ low temperature Raman study has been conducted and the CDW phase related properties of single- and bi-layered TaSe2 samples have been observed for the first time.
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Speaker: Lu Bo
Affiliation: School of Physics, Peking University, China
Abstract Details: Voltage-biased solid-state nanopores are well established in their ability to detect and characterize single polymers, such as DNA, in electrolytes. The addition of a pressure gradient across the nanopore yields a second, independent molecular driving force that provides new freedom for studying molecules in nanopores. We show that opposing pressure- and voltage-derived forces allow us to detect and resolve very short DNA molecules, as well as to detect near-neutral polymer strands. Moreover, by simply balancing pressure- and voltage-derived forces in the nanopore, we were able to detect single biomolecule charge density. The measurement of the charge density of ds-DNA from pH 4 to 10 shows that the charge density of dsDNA drops significantly below pH 6, while the effective hydrodynamic radius of the DNA decreases due to the shedding of the immobilized Stern-like layer around the DNA molecule. The technique presented here is more informative and convenient for surface charge detection than the current commercial systems based on isoelectric focusing (IEF).
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Speaker: Sergey Kravchencko
Affiliation: Northeastern University, USA
Abstract Details: The spin susceptibility of strongly correlated electrons in a low-disorder 2D electron system exhibits a sharp increase tending to a divergence at a finite electron density. Surprisingly, this behavior is due to the divergence of the effective mass rather than that of the g-factor. Our results provide clear evidence for an interaction-induced transition to a new phase that may be a precursor phase or a direct transition to the long sought-after Wigner solid.
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Speaker: Pablo Ordejón
Affiliation: ICN2, Spain
Abstract Details: Graphene is a new material which holds the promise of revolutionizing technology areas such as electronics, due to its amazing properties. A large amount of activity is now being devoted to determining the effect of chemical functionalization of graphene on its electronic transport properties. The use of first-principles (ab-initio) methods is not straightforward to tackle these problems, since the relevant sizes involved in transport in graphene devices are much larger than those that ab-initio methods can reach. In this talk, I will describe how we are dealing with this problem, by using Density Functional Theory (DFT) calculations to build accurate effective Tight Binding models. These TB models are then used to compute the transport properties in mesoscopic samples by means of an efficient Kubo formulation based on wave packet evolution. In this talk, I will present the results of applying such an approach to graphene functionalized with oxygen and hydrogen. In the first case, we find that a metal-insulator transition can be driven as a function of the concentration of oxygen impurities, explaining recent experimental data. For the case of hydrogen, we have found that the electronic transport is closely linked with the magnetic ordering induced by the presence of hydrogen. In particular, it should possible to obtain measurable magnetoresistance signals by applying a sufficiently large magnetic field, which would pinpoint the presence of localized spins in H-functionalized graphene. Such effects are due to electron correlations in graphene, which translate into magnetic order and distinct transport properties of the different magnetic phases. I will also address the issue of transport in extremely disordered, amorphized graphene, which I will show to be an Anderson insulator with localization lengths of the order of only a few nanometers.
About the Speaker: Prof. Pablo Ordejón is the Director of the Catalan Institute of Nanotechnology (ICN) (Barcelona, Spain), as well as the leader of the Simulation and Theory Group at that institute.  His research has focused on the development of efficient methods for electronic structure calculations in large and complex systems, contributing to the development of techniques for large scale atomistic  simulations based on first principles methods like SIESTA. He has also been involved in the study of the fundamental properties of materials at the atomistic level. His current interests include, among many others, electronic transport in nanoscale devices and electronic processes at surfaces. He maintains frequent collaborations with industrial laboratories on the simulation of materials processes at the atomic level.
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Speaker: Jens Martin
Affiliation: Jens Martin
Abstract Details: Fictitious magnetic fields are created due to non-homogeneous strain-patterns in the graphene membrane. I will give a brief introduction to  the theoretical background, review scanning tunneling experiments on graphene-nanobubbles, discuss the limitations of those experiments, and finally present some preliminary results on creating strained graphene membranes for transport and scanning SET experiments.
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Speaker: Jak Chakhalian
Affiliation: University of Arkansas, USA
Abstract Details: Complex oxides are a class of materials characterized by a variety of competing interactions that create a subtle balance to define the lowest energy state and lead to a wide diversity of intriguing properties ranging from high Tc superconductivity to exotic magnetism and orbital phenomena. By utilizing the bulk properties of these materials as a starting point, interfaces between different classes of complex oxides offer a unique opportunity to break the fundamental symmetries present in the bulk and alter the local environment. Utilizing our recent advances in oxide growth, we can now combine materials with distinct or even antagonistic order parameters to create new materials in the form of heterostructures with atomic layer precision. The broken lattice symmetry, strain, and altered chemical and electronic environments at the interfaces then provide a unique laboratory to manipulate this subtle balance and enable novel quantum states not attainable in bulk. Understanding of these phases however requires detailed microscopic studies of the heterostructure properties. In this talk I will summarize our recent work on unit-cell thin nickelate heterostructures to illustrate recently uncovered principles of rational materials design and control of interactions by the interface. Selected references for the talk: J. Chakhalian et al, Science, v. 314, 1114, (2007). Jian Liu et al, Phys. Rev. B 83, 161102(R) (2011). J. Freeland et al, Europhys. Letters, 96 (2011) 57004. J. Chakhalian et al, Phys. Rev. Letters, 107 116805 (2011). J. Chakhalian et al, Nature Materials 11, 92–94 (2012).
About the Speaker: 2002 – Ph.D. in condensed matter physics, The University of British Columbia, Canada. 2003 – Postdoctoral Researcher at TRIUMF Canada’s National Research Facility. 2003-2006 – Fellow at Max Planck Institute for Solid State Research, Stuttgart, Germany 2006 – Assistant Professor, University of Arkansas, Fayetteville, USA 2007 – Presidential Young Investigator Career Award, National Science Foundation. 2010 – Associate Professor, University of Arkansas, Fayetteville, USA 2010 – John Imhoff Outstanding Research Award, Sigma Xi Society. 2011 – French Government Award for synchrotron research. 2012 – Director of Laboratory for Artificial Quantum Materials. 2012 – Charles and Claudine Scharlau Professor of Physics, University of Arkansas, Fayetteville, USA.
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Speaker: Thomas C. Lang
Affiliation: Boston University, USA
Abstract Details: I will give an update on the status of the potential spin liquid phase in the Hubbard model on the honeycomb lattice and will report results from mainly quantum Monte Carlo simulations of graphene nanoribbons and the instabilities of the Bernal-stacked bilayer system within the Hubbard-model description.
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Speaker: Laurens W. Molenkamp
Affiliation: University of Würzburg, Germany
Abstract Details: HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is a semimetal, lowering the crystalline symmetry opens up a gap, turning the compound into a topological insulator. The most straightforward way to do so is by growing a quantum well with (Hg,Cd)Te barriers. Such structures exhibit the quantum spin Hall effect, where a pair of spin polarized helical edge channels develops when the bulk of the material is insulating. Our transport data provide very direct evidence for the existence of this third quantum Hall effect, which now is seen as the prime manifestation of a 2- dimensional topological insulator. To turn the material into a 3-dimensional topological insulator, we utilize growth induced strain in relatively thick (ca. 100 nm) HgTe epitaxial layers. The high electronic quality of such layers allows a direct observation of the quantum Hall effect of the 2-dimensional topological surface states. Moreover, on contacting these structures with Nb electrodes, a supercurrent is induced in the surface states.
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Speaker: Philip Kim
Affiliation: Columbia University, USA
Abstract Details: The recent advent of atomically thin 2-dimensional materials such as graphene, hexa boronitride, layered transition metal chalcogenide and many strongly correlated materials, has provide a new opportunity of studying novel quantum phenomena in low dimensional systems and their heterostructures utilizing them for novel electronic devices. In particular, graphene has been provided us opportunities to explore exotic transport effect in low-energy condensed matter systems and the potential of carbon based novel device applications. In this presentation I will first discuss the quantum carrier collimation across the single and bilayer graphene lateral heterojunctions. Then, I will discuss the new type of material classes based on 2-dimensional van der Waal materials and their heterostructures extending the graphene based research into quasi 3-dimensional systems.
About the Speaker: http://pico.phys.columbia.edu
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