News & Events

Speaker: Associate Professor Tatiana G. Rappoport
Affiliation: Instituto de Fisica Universidade Federal do Rio de Janeiro Rio de Janeiro - Brasil
Abstract Details:

Quantum spin hall insulators (QSHI), have edge states that are extremely robust against disorder because the only available backscattering channel is forbidden by topology. Consequently, they are good candidates for the development of the next generation of electronic and spintronic devices.

One possible route to obtaining QSHIs with large band-gaps is the search for 2D materials where the electronic structure can be modeled by a px-py honeycomb Hamiltonian. In this case, the band structure presents a Dirac cone similar to their pz counterpart, accompanied by two extra narrow bands symmetrically located at low and high energies. When the spin-orbit coupling is taken into account, the band structure presents three topological gaps that have sizes of the order of the atomic level splitting.[1].

The fabrication of bismuthene on top of SiC paved the way for substrate engineering of room temperature quantum spin Hall insulators made of group V atoms. They reported the realization of a condensed matter analogue of px-py honeycomb systems in flat bismuthene[2].

Motivated by these results, we consider a minimal model that consists of a px-py honeycomb lattice with spin-orbit coupling. We perform a systematic analysis of the influence of the ?-bonding in the band structure, as its strength depends on both material and substrate. In addition to this, we perform quantum transport calculations in presence of Anderson disorder and vacancies and analyze the robustness of the topological gaps against disorder and Rashba spin-orbit coupling [3].

[1] C. Wu, Phys. Rev. Lett. 101, 186807 (2008)

[2] F. Reis, G. Li, L. Dudy, M. Bauernfeind,?S. Glass, W. Hanke, R. Thomale, J. Scha ?fer, and?R. Claessen, Science 357, 287 (2017)

[3] Luis M. Canonico, Tatiana G. Rappoport, R. B. Muniz, arXiv:1811.05054

About the Speaker:

Tatiana G. Rappoport received her PhD from Universidade Federal Fluminense in Brazil. After spending a few years as a visiting student and postdoc at the University of Notre Dame, working with diluted magnetic semiconductors, she joined Universidade Federal do Rio de Janeiro as a faculty in 2007. She received an L'Oreal Unesco fellowship for young women in science in 2007 and a Newton Advanced Fellowship from the Royal Society in 2015. Tatiana is a theoretical condensed matter physicist working on quantum transport and different spin-related phenomena in solid-state systems, in particular, Dirac materials. Her current research interests are the effects of disorder on quantum materials such as topological insulators, Chern insulators, and novel low-dimensional systems.

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Speaker: Prof Ji Wei
Affiliation: Department of Physics, Renmin University of China
Abstract Details:

Van der Waals forces were believed dominant for interlayer interactions in layered two-dimensional (2D) materials. We recently found an emergent type of interlayer interactions, namely covalent-like quasi-bonding (CLQB) [1,2], in various 2D materials like black phosphorus [1,2], PtS2 [3], PtSe2 [4] and few-layer Te [5]. This interlayer interaction is, most likely, directly observed in TiS2 by x-ray scattering [6,7]. Such technique also suggested charge accumulation in hydrogen bonds. We thus imaged intermolecular hydrogen bonds in real space using a qPlus non-contact Atomic Force Microscope with a CO terminated tip [8], which suggests a covalent characteristic of hydrogen bonds. However, it was argued that the contrast between molecules is a result of CO tip tilting [9], which boost extensive discussions in the field. In collaboration with our experimental and theoretical coworkers [10], we found an O atom modified Cu tip is of strong lateral stability which is an order of magnitude higher than the usual CO modified tip. It does not show the fake contract between two S atoms in the adsorbed DBTH molecule on Cu(110), but does show the N…N bond in BPPA and hydrogen bonds in PTCDA. In addition, we discussed the interlayer magnetic couplings (IMC) of CrS2 and CrI3 bilayers. It is interesting that these two materials are at two extremes of IMC. The interlayer FM coupling in CrS2 is very robust and is nearly unable to be tuned under usual external fields, but the intra-layer magnetism does be varied under layer stacking [11]. An opposite case was found in CrI3 bilayers [12] where the intra-layer FM coupling is, however, very strong but the interlayer magnetism was found governed by a subtle change of interlayer stacking; this shows a decoupled magnetic interaction between intra- and inter-layer directions.

About the Speaker:
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Speaker: Associate Professor Zhengtang Luo
Affiliation: Department of Chemical and Biological Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Abstract Details:

We have demonstrated multiple strategies to obtain grain-size, layer number and morphology controlled high quality single-crystal graphene domains on large scale. For example, we developed a backside carbon gettering (BCG) approach for CVD growth of graphene, which regulates the nucleation of graphene domains on the top side of the Cu substrate. In addition, we have improvised the configuration of Cu foil and form a bridge-shaped geometry which generates a confined space for the CVD reaction. More recently, we demonstrated a novel way of recrystallization the polycrystalline Cu substrate surface to single-crystal Cu(111) plane through melting and resolidification phase prior to growth step. Cu (111) surface promoted the epitaxial growth of graphene during CVD and highly orientated single layer as well as bilayer graphene domains were obtained over a large area. Consequently, our multiple approaches of engineering the growth substrate paved the way toward achieving high quality single crystal graphene for high performance ultrathin optoelectronic devices.

About the Speaker:

Prof. Zhengtang Tom Luo is currently an associate professor at the Hong Kong University of Science and Technology. He has obtained his bachelor degree from South China University of Technology and PhD degree (in Polymer Science) from University of Connecticut, followed by postdoctoral training (Physics) at University of Pennsylvania. He focused on nanomaterials synthesis and product development for applications in the areas of materials chemistry and nanofabrication. He is co-inventor of the two US patents. He serves as Editorial Board member for Journal of Macromolecule Science, Functional Materials Letters, Scientific reports, ACS sensors and as associate editor for AIP Advances. In 2010, he founded Graphene Frontiers LLC, a Pennsylvania-based company, which has attracted ~US $2M investments. His current research interest focuses on graphene and 2D materials.

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Speaker: Dr. Zhifeng Huang
Affiliation: Department of Physics, Hong Kong Baptist University Kowloon Tong, Kowloon, Hong Kong SAR, China
Abstract Details: Metallic helices with a characteristic helical pitch (P) in the micro- or nano-scale have been proposed for diverse chirality-related primary applications. However, limited development of nanofabrication techniques leads to P >20 nm; molecules are too small in size to effectively perceive the helical chirality, and such the dimensional mismatch will substantially prohibit the development of those applications. In this talk, I will device a method to tackle this fundamental problem. Glancing angle deposition with fast substrate rotation is performed to produce chiral nanoparticles (CNPs) that have sub-10-nm P (as small as 2 nm) and are composed of controllable plasmonic materials and helicity. CNPs are used to induce enantiospecific adsorption of molecules, mediate the enantiopreferential photocyclodimerization of 2-anthracenecarboxylic acid, and markedly enhance optical activity of chiral molecules in roughly one order of magnitude. These studies will pave the way to developing CNPs for significant chirality-related applications, such as heterogeneous asymmetric catalysis and sensitive detection of absolute configuration of enantiomers that is practically desired by the production of single-enantiomer drugs.
About the Speaker: Dr. Z. F. Huang completed his PhD from Arizona State University (US, 2007) and postdoctoral studies from University of Alberta (Canada, 2009), and then joined Department of Physics at Hong Kong Baptist University as an Assistant Professor and was promoted to Associate Professor in 2015. Dr. Huang contributed to two book chapters, and published his studies in Nat. Nanotechnol., Annu. Rev. Phys. Chem., Adv. Mater., Nano Lett., J. Am. Chem. Soc., Small, Nanoscale, and so on. Dr. Huang was presented Outstanding Research Achievement (APSMR, 2017 and 2018), and the Prof. Rudolph A. Marcus Award 2016. He is serving as an Associate Editor for Science Advances Today and Science Letters Journal (Cognizure), and funded a spin-off company to commercialize nanomaterial-based medical devices for cell replacement therapy.
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Speaker: Maciej Koperski
Affiliation: School of Physics and Astronomy, University of Manchester
Abstract Details:

The magnetism of chromium has been investigated for almost a century now, providing substantial knowledge about its electronic configuration. Extensive research has been conducted regarding the physics of valence electrons from d-shell, which is fundamentally important for understanding the mechanisms of magnetic ordering. Interestingly, chromium atom, exhibiting a stable electronic configuration exempting from Hund’s rules, has half-filled 3d shell, which leads to manifestation of robust magnetic effects in a variety of structures. Recently, attention has been refocused on chromium trihalides (CrCl3, CrBr3 and CrI3), which constitute a group of electrically insulating layered materials displaying magnetic ordering at low temperatures, as established by inspection of bulk crystals carried out few decades ago. The progress of mechanical exfoliation techniques, performed in a controlled argon atmosphere, enables now isolation of thin layers (down to monolayers) and their incorporation in van der Waals heterostructures.
Initial reports demonstrated layer-dependent ferromagnetic and anti-ferromagnetic order below Curie temperature using Kerr rotation measurements as magnetization probe. These appealing findings motivate further study to uncover the underlying microscopic mechanisms. One possible path to learn about the electronic structure and characteristics of electronic states via optical methods involves investigations of emission and absorption processes. Here, we present detailed optical studies of exfoliated films of CrBr3 and CrI3 to demonstrate that the emergent interband luminescence has molecular-like character (most likely due to formation of Frenkel-type excitons) and the details of the structure of emission resonances can be explained by Franck-Condon principle involving multiple phonon modes. The photoluminescence studies unveil unambiguous signatures of coupling between the magnetic moments of Cr3+ ions with band carriers, offering insight into fundamental properties of these novel magnetic structures and opening up new routes for potential applications of 2D systems.

About the Speaker:

Maciej Koperski was born in 1988 in S?awno (Poland), a small town 30 km away from the coast of the Baltic Sea. He is currently holding a post-doctoral position at the University of Manchester (UK) in the Condensed Matter Physics Group. After defending PhD dissertation on optical properties of transition metal dichalcogenides in High Magnetic Field Laboratory in Grenoble (France) in 2017, he shifted his research focus on explorations of novel phenomena related to magnetism in 2D, uncovering electronic properties of less understood materials (InSe) by combining optical and electrical investigations and devising novel methods of introducing light into other areas of low dimensional physics (optical detection of fluids in 2D channels).

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Speaker: Mohammad Zarenia
Affiliation: Department of Physics & Astronomy, University of Missouri, Columbia (MO), USA.
Abstract Details: Closely coupled two-dimensional electron-hole sheets are attracting great interest as they should generate novel quantum phases driven by the strong Coulomb attractions between the sheets. In the first part of my talk, I demonstrate that coupled electron-hole bilayer graphene as well as coupled few-layer graphene sheets with carrier densities in a range accessible to experiments can access the regime of strong pairing necessary for superfluidity [2]. For the coupled bilayer graphene system, we find two new inhomogeneous ground states, a one-dimensional Charge Density Wave (1D-CDW) phase, i.e. density modulations in one planar direction, and a coupled electron-hole Wigner crystal (c-WC) in association with the superfluid phase [3]. A very interesting aspect of the system is that the elementary crystal structure of bilayer graphene plays no role in generating these new quantum phases, which are completely determined by the electrons and holes simply interacting through the Coulomb interaction. To account for the strong inter-layer correlation energy accurately, I introduce a new approach which is based on a random phase approximation at high densities and an interpolation between the weakly- and strongly-interacting regimes. The approach gives excellent agreement with available Quantum Monte Carlo calculations for single layer two-dimensional-electron-gas systems [3]. Coulomb drag of carriers in one sheet by carriers moving in the other is a powerful tool to study Fermi liquid properties and identify the formation of these phases. Two independent Coulomb drag experiments on electron-hole sheets in graphene double bilayers have reported an unexplained and puzzling sign reversal of the Coulomb drag signal. In the next part of my talk, I show that this unusual effect can be explained by the multiband character of bilayer graphene and the temperature dependence of effective mass at low densities caused by the electron-electron interactions [4]. The theory produces excellent agreement with the observed structure in the Coulomb drag resistance, capturing the key features of the recent experiments over the full reported range of temperatures. References: [1] M. Zarenia, A. Perali, D. Neilson, and F.M. Peeters, Scientific Reports 4, 7319 (2014). [2] M. Zarenia, D. Neilson, and F.M. Peeters, Scientific Reports 7, 11510 (2017). [3] M. Zarenia, D. Neilson, B. Partoens, and F. M. Peeters, Phys. Rev. B 95, 115438 (2017). [4] M. Zarenia, A. R. Hamilton, F. M. Peeters, and D. Neilson, Phys. Rev. Lett. 121, 036601 (2018).
About the Speaker: Dr Mohammad Zarenia obtained his PhD in 2013 in the Condensed Matter Theory group (heading by Professor Francois Peeters) at the University of Antwerp (Belgium). Since 2008, he has been involved in the study of electronic properties of 2D materials, particularly graphene. After his PhD, he received the prestigious postdoctoral Flanders Research Foundation fellowship at the University of Antwerp. In recent years he expanded his research interests further from the single-particle physics towards the many-body aspects of 2D materials. To pursue this, he recently moved to the group of Professor Giovanni Vignale at the University of Missouri (Columbia, USA).
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Speaker: Dr Mark Edmonds
Affiliation: Monash University
Abstract Details:

Na3Bi in bulk form represents a zero-bandgap topological Dirac semimetal (TDS), but when confined to few-layers is predicted to be a quantum spin Hall insulator with a bulk bandgap of 300 meV.1 Furthermore, application of an electric field to few-layer Na3Bi has been predicted to induce a topological phase transition from conventional to topological insulator.2

I will discuss our efforts to grow epitaxial few-layer Na3Bi via molecular beam epitaxy, and probe its electronic structure and response to an electric field using scanning probe microscopy/spectroscopy and angle-resolved photoelectron spectroscopy. We are able to demonstrate that monolayer and bilayer Na3Bi are quantum spin Hall insulators with bandgaps >300 meV. Furthermore, via application of an electric field, the bandgap can be tuned to semi-metallic and then re-opened as a conventional insulator with bandgap ~100 meV.3 The demonstration of an electric field tuned topological phase transition in ultra-thin Na3Bi provides a viable platform for the creation of a topological transistor.

About the Speaker:

Dr Mark Edmonds received his Ph.D. from La Trobe University in 2014. From 2014-2016 he was a postdoc fellow at Monash University working with Prof. Michael Fuhrer. In 2016 he was awarded an Australian Research Council Discovery Early Career Research Award to realise novel electronic phases in two-dimensional materials and is now a lecturer in the Department of Physics and Astronomy at Monash University. His research focuses on the growth and characterization of novel electronic materials for the development of next-generation electronic devices, using spectroscopic tools such as scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES).

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Speaker: Dr. Arief Suriadi Budiman
Affiliation: Extreme Materials Laboratory (XML), Singapore University of Technology & Design (SUTD)
Abstract Details: Plastic deformation mechanisms in metal-metal nanolayer composites (nanolaminates) have been studied extensively during the last decade, but their fracture responses (especially in an in situ set up) have not been reported. It has been widely observed that, for the case of metal-metal nanolaminates with a semicoherent interface, such as Cu/Nb, low interface shear strength increases the interface barrier to dislocation crossing, which improves nanolaminate plasticity. In this study, we use Cu(63nm)/Nb(63nm) accumulative roll-bonded nanolaminates, which have a large anisotropy of the interface shear strength between rolling and transverse directions (RD and TD, respectively), to study the effect of interface shear strength on the plasticity mechanisms and fracture events leading to the final catastrophic failure in metal-metal nanolaminates with a semicoherent interface during in situ clamped beam bending. Further, finite element analysis is used to understand the observed behavior. The results show a substantial difference between the fracture behaviors along the RD and TD owing to differences in the interface shear strength and grain size. For the RD beams, the slip bands originate from the Nb layers at the notch/crack tip followed by crack propagation along these bands. For the TD beams, the crack propagation is inhibited by interface shear. The results reveal that low interface shear strength can be utilized to improve fracture resistance in the studied nanolaminates, though weak grain boundaries can suppress the interface shear and the associated crack resistance improvement. This knowledge can be used to further optimize the nanolaminate fabrication process and achieve good strength, ductility, and crack resistance at the same time.
About the Speaker: Arief Suriadi Budiman received his B.S. in mechanical engineering from Institute of Technology, Bandung (ITB), Indonesia, his M.EngSc in materials engineering from Monash Univ., Australia and his Ph.D. in Materials Science and Engineering from Stanford University, CA in 2008. During his doctoral candidacy at Stanford’s Department of Materials Science & Engineering under the supervision of Professor William D. Nix (MRS Von Hippel Award 2007), Dr. Budiman received several research awards (MRS Graduate Silver Award 2006, MRS Best Paper 2006) and contributed to several high-impact journal publications (Acta Materialia, Applied Physics Letters, Journal of Electronic Materials). He gave two symposium invited talks as well in the MRS spring and fall meetings in 2006. More recently Dr. Budiman has been awarded the prestigious Los Alamos National Laboratory (LANL) Director's Research Fellowship to conduct top strategic research for the energy and national security missions of the Los Alamos National Laboratory's. At the Center for Integrated Nanotechnologies (CINT) at Los Alamos, Dr. Budiman’s research program involves nanomaterials for extreme environments with potential applications in advanced energy systems including for next-generation nuclear power reactors. Currently, at Singapore University of Technology & Design (SUTD), Prof. Budiman is leading a dynamic, young group researching nanomaterials and nanomechanics and their implications for extending the extreme limits of materials as well as their applications in the next generation energy technologies (solar PV, extreme environments, energy storage, etc.). His work has also recently received the famed Berkeley Lab Scientific Highlights twice in May 2010 and June 2013 (the latter was for his novel, innovative characterization technique that enables thin silicon solar PV technology). His deep expertise in the synchrotron X-ray microdiffraction technique was also recently utilized to enable the first ever in situ measurements of mechanical stresses in the 3-D through-silicon via (TSV) Cu interconnect schemes in the world – the findings were reported in a publication in Microelectronics Reliability (2012) and now one of the most highly cited references in the field of TSV/3D Interconnect stress measurements. He has been invited to give invited lectures/seminars on 3D/TSV Interconnect in various international conferences (including IEEE IITC 2012, AVS Thin Films Users Group 2012, TMS Symposium for Emerging Interconnects and Packaging Technologies 2011 and SEMATECH Workshop on 3D Interconnect Metrology at SEMICON 2011). Dr. Budiman has authored/coauthored several high-impact journal publications (Acta Materialia, Solar Energy Materials & Solar Cells, Materials Science Engineering A), and contributed a book chapter on “Electromigration in Thin Films and Electronic Devices: Materials and Reliability,” Woodhead Publishing, Cambridge, 2011. He has also recently published a book “Probing Crystal Plasticity at the Nanoscales – Synchrotron X-ray Microdiffraction” (Springer 2015). He has two U. S. Patents and one pending.
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Speaker: Michael Mayer
Affiliation: University of Fribourg, Switzerland
Abstract Details: This talk will give a short overview of the research activities in the Biophysics Group in the Adolphe Merkle Institute of the University of Fribourg in Switzerland and then describe the use of electrolyte-filled synthetic nanopores with self-assembled lipid membrane coatings to determine, simultaneously and in real time, the shape, volume, charge, dipole moment, and rotational diffusion coefficient of individual proteins and protein complexes in solution. The talk introduces the main concepts for a quantitative understanding and analysis of modulations in ionic current that arise from rotational dynamics of single proteins as they move through the electric field inside a nanopore. The resulting multi-parametric information raises the possibility to characterize, identify, and count individual proteins and protein complexes in a mixture with implications for protein folding studies, biomarker detection, routine protein analysis, and characterization of protein amyloids that are involved in neurodegenerative diseases such as Alzheimer’s disease.
About the Speaker: Michael Mayer studied Biotechnology at the University in Braunschweig, Germany. He conducted his Ph.D. thesis in Physical Chemistry under the guidance of Prof. Horst Vogel at the Swiss Federal Institute of Technology in Lausanne (EPFL), Switzerland, followed by postdoctoral research in Biological Chemistry in the group of Prof. George M. Whitesides at Harvard University, Cambridge, USA. In 2004, he started a tenure-track faculty position in the Department of Biomedical Engineering at the University of Michigan, Ann Arbor, USA. In fall of 2015, he moved his research group to the Adolphe Merkle Institute at the University of Fribourg, Switzerland where he holds the chair of Biophysics. His research takes inspiration from nature to solve problems in biophysics ranging from understanding signaling and transport processes in biological membranes and detecting protein complexes relevant for neurodegenerative diseases to engineering biocompatible electrical power sources.
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Speaker: Dr Ahmet Avsar
Affiliation: École Polytechnique Fédérale de Lausanne (EPFL), Switzerland
Abstract Details: Van der Waals heterostructure devices, composed from 2D crystals, could enable new spintronics functionalities that are not accessible in individual crystals or any other bulk materials. In this talk, I will present our spin dependent (opto) electronic transport measurements in such hetero devices. Firstly, I will discuss our results on inducing optospintronics functionality in semi-metallic graphene by bringing it in a proximity to monolayer WSe2. Then, I will show that semiconducting black phosphorus-based van der Waals heterostructures exhibit remarkable spin transport properties after an in-situ h-BN encapsulation process. Finally, I will provide an outlook about 2D van der Waals spin devices after briefly reviewing recently discovered 2D magnets.
About the Speaker: Ahmet Avsar is an experimental condensed matter physicist specializing in the emerging fields of spintronics and two-dimensional crystals-based nanotechnology. He obtained PhD degree from Physics Department of National University of Singapore under the supervision of Prof. Barbaros Özyilmaz and he is currently a research fellow at the group of Prof. Andras Kis (EPFL). He is a recipient of 2016- EPFL Fellows fellowship award co-fund by Marie Skladowska-Curie.
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