The seminars are regularly held on Wednesday, 11:00-12:00 in S11-02-07 unless otherwise announced.
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|Date / Speaker||Title / Abstract|
|14 Dec 2016
Johannes Lischner (Imperial College London, UK)
|New techniques for modelling nano and energy materials |
A detailed theoretical understanding of microscopic energy conversion processes is needed to identify new materials for photovoltaic and photocatalytic applications. In the first part of my talk, I will discuss a new approach for modelling electronic excitations at solid-liquid interfaces. In this method, many-body perturbation theory within the GW/BSE approach is combined with accurate and efficient continuum theories of inhomogeneous liquids. I will present results for oxide-water interfaces. In the second part of my talk, I will discuss new methods for describing charged impurities in two-dimensional materials. Specifically, we developed a new first- principles method to evaluate the interaction of graphene with adsorbed atoms and molecules. Because of the slow decay of the screening response of two-dimensional substrates, this requires the use of supercells containing 100,000 atoms or more. For calcium atoms on doped graphene, this approach yields excellent agreement with recent scanning tunnelling spectroscopy measurements.
|30 Nov 2016
Daniel Leykam (NTU)&Yu Zhiming (SUTD)
|Linear and nonlinear Weyl point photonic lattices |
Topologically nontrivial photonic lattices can offer unique insights into topological phenomena difficult to probe in condensed matter. I will discuss light propagation in a class of staggered helical lattices that emulate the band structure of the recently-discovered "type-II" Weyl semimetals. Probing the lattice at various frequencies, we experimentally observe two signatures of the Weyl point: optical analogues of their "Fermi arc" surface modes, and conical diffraction associated with the Weyl point's conical isofrequency surface. Simulating nonlinear propagation at high probe beam powers, we predict that the Kerr effect can result in self-focusing or self-induced unidirectional edge states.
|23 Nov 2016
Igor Herbut (Simon Fraser University, Canada)
|Interactions and quantum phase transitions in Dirac and parabolic semimetals |
I will review recent progress in our understanding of strongly interacting electronic systems that have their Fermi surfaces reduced to points. The first example will involve the familiar Dirac electrons in two dimensions described by the Hubbard, or a Hubbard-like, model. Recent confluence of field-theoretic and numerical methods leads a rather detailed and quantitative picture of metal-insulator quantum phase transition in this system, at which sharp quasiparticles disappear and Mott gap and Neel order parameter simultaneously emerge at a critical interaction strength. A connection to the observed quantum Hall effect at the neutrality point in graphene under magnetic field will be noted. The second example will go beyond Dirac "relativistic" dispersion, and concern the effect of Coulomb interactions in systems such as gray tin or mercury telluride, where due to strong spin-orbit coupling and the ensuing band inversion electronic bands touch quadratically at the Fermi level. We will examine the stability of the putative non-Fermi liquid ground state proposed by Abrikosov and Beneslavskii in the 70's, and discuss a general mechanism for an instability of such scale-invariant interacting phases.
|16 Nov 2016
Paul Indelicato (Université Pierre et Marie Curie, France)
|The proton and deuteron size revisited: the puzzle gets more puzzling! |
In 2010, our collaboration published a measurement of the 2s Lamb shift in muonic hydrogen . This measurement allowed deriving a completely new value the proton charge radius. This value, while roughly 10 times more accurate than the one derived from hydrogen and deuterium spectroscopy or electron-proton elastic scattering was 4% lower, and 5 standard deviations away from the CODATA 2010 value. In 2013 we published the measurement of a second line in muonic hydrogen, allowing for the measurement of the hyperfine structure of the 2s level. This measurement allowed to derive the size of the Zemach's radius and of the magnetic moment distribution, and an improvement of the accuracy of the proton charge radius . The difference was then increased to 6.9 standard deviations. We have now measured, for the first time, 3 lines in muonic deuterium . I will report on this new measurement, show how it impacts the proton size puzzle. I will discuss possible reasons and implications for physics of the large disagreement between the different measurements of the proton and deuteron size.
|2 Nov 2016
Yuval Yaish (Technion, Israel Institute of Technology)
|Automated circuit fabrication and direct characterization of carbon nanotubes vibrations - optical imaging of CNTs |
Since their discovery carbon nanotubes (CNTs) have fascinated many researchers due to their unprecedented electrical, optical, thermal, and mechanical properties. Recently, a complete computer based on CNT circuits has been demonstrated. However, a major drawback in utilizing CNTs for practical applications is the difficulty in positioning or growing them at specific locations or in locating them on the substrate such that the circuit can be built around them. Here we present a simple, rapid, non-invasive, and scalable technique that enables optical imaging of CNT . Instead of relying on the CNT chemical properties to bind marker molecules we rely on the fact that the CNT is both a chemical and physical defect on the otherwise flat and uniform surface. Namely, it may serve as a seed for nucleation and growth of small size, optically visible, nano-crystals. As the CNT surface is not used to bind the molecules they can be removed completely after imaging, leaving the surface intact and thus the CNT electrical and mechanical properties are preserved. The successful and robust optical imaging allowed us to develop a dedicated image processing algorithm through which we are able to demonstrate a fully automated circuit design resulting in field effect transistors and inverters. Moreover, we demonstrate that this imaging method allows not only to locate CNTs but also, as in the case of suspended ones, to study their dynamic mechanical motion. The decorated tubes exhibit linear as well as non-linear Duffing type behavior, and for the first time transition from hardening to softening is observed.
|26 Oct 2016
Chen Jian-Hao (Peking University)
|Tunable negative magnetoresistance in hydrogenated graphene |
The problem of unconventional magnetism in materials without d and f electrons has attracted continuous attention. In particular, a lot of efforts have been devoted to understand the origin and effects of magnetic moments induced in graphene with structure defects such as missing carbon atoms, absorption of light atoms such as hydrogen or fluorine. We have measured the magnetoresistance (MR) of graphene at low temperature with in-situ hydrogenation in ultra-high vacuum environment. Large negative MR was found in hydrogenated graphene which could be tuned by carrier density and sample temperature. Depending on the density of adsorbed atomic hydrogen and carrier density, large linear negative MR was found which did not saturate up to 9 Tesla. Such negative MR could be the manifestation of local moments created by atomic hydrogen adsorbed on graphene.
|25 Oct 2016
Vladimir Falko (University of Manchester, UK)
|Electronic and optical properties of two-dimensional InSe |
We present the evolution of electronic band structure of InSe and other III-VI semiconductors) films, from stoichiometric mono-layer to multilayer films, and trace consequences of these changes for optical properties of these 2D materials. This study is based on the ab initio DFT and related multi-orbital tight-binding model analysis of the electronic band structure and wave functions in the two-dimensional N-layer InSe crystals. We find that the conduction band edge electron mass in few-layer InSe is quite light (comparable to Si), which suggests opportunities for high-mobility devices and the development of nanocircuits. In contrast, the valence band in mono-, bi- and trilayer InSe is flat, opening possibilities for strongly correlated hole gases in p-doped systems.
|19 Oct 2016
Somnath Bhattacharyya (University of the Witwatersrand, South Africa)
|Exotic Features in the Superconductivity-Insulator Phase Transition of Nanodiamond films |
Pristine nanocrystalline diamond films well known for hard coating applications are electrically insulating however can reach a metallic state through the incorporation of nitrogen and also a superconducting phase by boron doping at temperatures close to the liquid helium point. This superconducting phase displays anomalous features notably a peak in the temperature dependence of resistivity close to the superconducting transition point in the regime much below the critical current. Similarly magnetoresistance shows a peak in the low field regime with some hysteresis which can be understood as a reentrant superconductivity. Characteristics of the peak observed at zero bias current have been studied as a function of the orientation of sample with respect to the applied magnetic field. These features can be understood as an interplay between vortex and spin related states which has a strong resemblance to quantum dots and superconductors having ferromagnetic impurities. We are working towards utilizing the superconducting phenomena in nanodiamond films in making some novel quantum electronic and high speed devices. This project complements our previous work on nitrogen-doped nanodiamond films and related nanostructured carbon devices which showed interesting radio frequency features in the gigahertz range.
|12 Oct 2016
Alexandra Carvalho (Centre for Advanced 2D Materials, NUS)
|Phosphorene: From theory to applications |
Phosphorene, a monolayer of black phosphorus, earned its place among the family of 2D semiconductor materials when recent results unveiled its high carrier mobility combined with a sizable direct bandgap and other attractive properties of interest for electronic and optoelectronic applications. Unlike graphene, phosphorene has an anisotropic orthorhombic structure that is ductile along one of the in-plane crystal directions but stiff along the other. This results in unusual mechanical, electronic, optical and transport properties that reflect the anisotropy of the lattice. This talk will overview the electronic properties of phosphorene and highlight the recent progress made in understanding how it can be doped, how it interacts with the environment, and how one can make use of the intrinsic oxide. Further, we will consider the opportunities lying in the extended family of phosphorene-like group IV monochalcogenides. These compounds are electronic with phosphorene and have a similar structure - but reveal a plethora of additional functional properties including piezoelectricity, multistability, ferroelectricity and spin-orbit splitting.
|5 Oct 2016
Eoin O'Farrell (Centre for Advanced 2D Materials, NUS)
|Rashba interactions and magnetism in van der Waals heterostructures |
Van der Waals interfaces offer the opportunity to combine materials with distinct ground states into heterostructures, under relatively relaxed lattice matching constraints. In this talk I will discuss the use of confinement in heterostructures as a means to induce hybridization with non-van der Waals materials. First, I will discuss spin orbit coupling that can be induced in graphene by hybridization with heavy metals. Here we demonstrate the mechanical intercalation of Au on a dielectric substrate by using a heterostructure of graphene and hBN . Magnetotransport observes spin-splitting due to a Rashba interaction with approximate magnitude 25 meV. Additionally we observe giant negative magnetoresistance, up to 75%, that indicates Au ions form local magnetic moments at the interface. In the region of strongest Rashba coupling an anomalous Hall effect and kinks in the magnetoresistance suggest the proximity to a collective magnetic phase that leads to a strong enhancement of the electron g-factor. I will also discuss more recent unpublished non-local measurements, which show the anomalous Hall effect drives non-local currents in these structures. Second, I will discuss tunneling spectroscopy measurements of an antiferromagnetic Mott insulator. Surprisingly, in this system we observe Coulomb oscillations due to the charging of a quantum dot. While this quantum dot appears to be due to a defect state at the interface, we observe evidence it couples to intrinsic Coulomb interactions of the magnetic system and may thereby allow us a window through which to observe interactions.
|21 Sep 2016
Aris Alexandradinata (Yale University, USA)
|The first non-symmorphic topological insulator: prediction and discovery |
Spatial symmetries in crystals are distinguished by whether they preserve the spatial origin. I will show how this basic geometric property gives rise to a new topology in band insulators, which we propose (and subsequently discover) to lie in the large-gap insulators: KHgX (X=As,Sb,Bi). These insulators are described by generalized symmetries that combine space-time transformations with quasimomentum translations in the Brillouin torus. This provides a natural generalization of space groups, such that real and quasimomentum spaces are placed on equal footing. A broader consequence of our theory is a connection between band topology and group cohomology.
|14 Sep 2016
Yonatan Dubi (Ben-Gurion University, Israel)
|Structure-Environment interplay and the role of coherence in photosynthetic complexes|
The striking efficiency of energy transfer in natural photosynthetic systems and the evidence of long-lived quantum coherence in biological light harvesting complexes have triggered much excitement, due to the evocative possibility that these systems - essential to practically all life on earth – use quantum mechanical effects to achieve optimal functionality. A large body of theoretical work has addressed the role of local environments in determining the transport properties of excitons in photosynthetic networks and the survival of quantum coherence in a classical environment. Nonetheless, understanding the connection between quantum coherence, exciton network geometry and energy transfer efficiency remains a challenge. Here we address this connection from the perspective of heat transfer within the exciton network. Using a non-equilibrium open quantum system approach and focusing on the Fenna-Matthews-Olson complex (the "fruit-fly" of exciton transfer), we demonstrate that finite local dephasing can be beneficial to the overall power output. The mechanism for this enhancement of power output is identified as a gentle balance between quantum and classical contributions to the local heat flow, such that the total heat flow is directed along the shortest paths and dissipation is minimized. Strongly related to the spatial network structure of the exciton transfer complex, this mechanism elucidates how energy flows in photosyntetic excitonic complexes.
|Date / Speaker||Title / Abstract|
|24 Aug 2016
Eric Mazur (Harvard University)
|Less is More: Extreme Optics with Zero Refractive Index|
Nanotechnology has enabled the development of nanostructured composite materials (metamaterials) with exotic optical properties not found in nature. In the most extreme case, we can create materials which support light waves that propagate with infinite phase velocity, corresponding to a refractive index of zero. This zero index can only be achieved by simultaneously controlling the electric and magnetic resonances of the nanostructure. We present an in-plane metamaterial design consisting of silicon pillar arrays, embedded within a polymer matrix and sandwiched between gold layers. Using an integrated nano-scale prism constructed of the proposed material, we demonstrate unambiguously a refractive index of zero in the optical regime. This design serves as a novel on-chip platform to explore the exotic physics of zero-index metamaterials, with applications to super-coupling, integrated quantum optics, and phase matching.
|17 Aug 2016
Ajit Kembhavi (IU Centre for Astronomy and Astrophysics, Pune, India)
|Data Intensive Astronomy and Virtual Observatories|
Astronomy has always been a data driven science. Modern telescopes and state-of-the-art instruments provide astronomers enormous quantities of data on the Universe. It is now usual for astronomers to deal with many Terabytes of data, and soon this will increase to many Petabytes and more. All this data has to be managed and analyzed, so that interesting new discoveries can be made. The tasks involved require vast computing resources and sophisticated specialised skills, which may not be easily available. Astronomers have over the last several years developed the concept of Virtual Observatories (VO), which enable everyone, with some access to the internet, to use the vast storehouse of astronomical data. The data may be located anywhere in the world, and may need to be analysed using computer resources and programs which may be available in yet other locations. The development of the VO has led to a new paradigm for research in astronomy. I will describe in my talk some large astronomical facilities and the tremendous quantities of astronomical data that they make available, the development and nature of Virtual Observatories, and some exciting new discoveries made possible by them. I will mention the participation of India in some of the large projects, and the challenges and opportunities made available to young Indian students and faculty.
|22 Jun 2016
Giovanni Vignale (University of Missouri-Columbia)
|Spin-Orbital effects in spintronics|
Layered structures of materials with different magnetic and/or spin orbital properties are rapidly emerging as most promising candidates for spintronic applications. A slew of new effects is under study, which have no analogue in bulk materials or differ subtly from their three-dimensional counterparts. Spin polarization and spin transfer torque can be generated by passing an electric current near an interface. Novel types of anisotropic magnetoresistance (e.g., the recently observed "unidirectional spin Hall magnetore- sistance") are possible. Spin polarized currents may produce a nonlocal form of the anomalous Hall effect. In this talk I review our recent theoretical work on these effects vis-a-vis experiment. In particular, I present a new theory of the nonlocal anomalous Hall effect, which occurs in a non-magnetic heavy metal (eg. Pt) that is interfaced with a ferromagnet.
|2 June 2016
Norbert Klein (Imperial College, London)
|Microwave-to-terahertz sensors: from 2D materials towards health and security applications |
Passive microwave-to- terahertz resonators and transmission line structures offer a wide potential for contact-free material characterization and sensor applications. As an example, our semi-open dielectric loaded microwave cavities based in functional ceramics have been successfully commercialized for liquid explosive detection in passenger checkpoints, and are still in use for our research, for example as biosensor for accurate haemoglobin measurements on sub-microlitre blood samples and as method for contact-free characterization and quality assessment of large area graphene layers and FET structures. At terahertz frequencies, our photonic bandgap and spoof plasmon structures have been successfully used for nanolitre bioliquid detection, aiming towards label-free cancer cell detection within microfluidic lab-on-chip systems. Graphene has a huge potential to be used as material for THz modulators and detectors, potentially enabling low-cost THz communication and imaging systems. First results with transparent microwave modulators based on large-area self-gated transparent G-FET structures will be presented. In combination with graphene as biointerface, the microwave-to-terahertz frequency range offers challenging opportunities for sensor applications within health and security.
|18 May 2016
Mirco Milletari (Department of Physics&CA2DM, NUS)
|Spin-orbit torques and spin Hall effect – describing them via the gauge potential picture |
Spintronics, the science that aims at utilising the spin degrees of freedom in addition to the charge of electrons for low-power operation and novel device functionalities, has seen rapid developments in the past decade. In particular, the spin Hall (SH) effect, i.e. the emergence of a transverse spin current in response to an applied longitudinal electric field, has attracted much interest for the possibility of building all-electric spin manipulation devices . The efficiency of the Spin current generation is measured by the SH angle. While in semiconductors the SH angle is quite small (0.0001-0.001) , it was shown that a giant SH conductivity can be achieved in graphene decorated with small doses of resonant, spin-orbit active adatoms . It was argued that in this case, the effect is mostly due to the semiclassical skew-scattering mechanism, where electrons of different spins are scattered asymmetrically.
In this talk, I will present present a rigorous microscopic theory of the extrinsic spin Hall effect in disordered graphene based on a nonperturbative quantum diagrammatic treatment incorporating skew scattering and anomalous – impurity concentration-independent – quantum corrections on equal footing [4, 5]. Our self-consistent approach – where all topologically equivalent noncrossing diagrams are resummed – unveils that the skewness generated by spin-orbit-active impurities deeply influences the anomalous component of the SH conductivity, even in the weak scattering regime. This seemingly counterintuitive result is due to the symmetry structure induced by spin-orbit coupling, for which the commonly Gaussian white noise approximation is generally invalid. Our treatment shows that it is possible to experimentally access regions in parameter space where anomalous quantum contributions to the SH conductivity are dominant.
Finally, we assess the role of quantum interference corrections by evaluating an important subclass of crossing diagrams, considered only recently in the context of the anomalous Hall effect . We show that diagrams encoding quantum coherent skew scattering events, display a strong Fermi energy dependence, dominating the anomalous spin Hall component away from the Dirac point. Our findings open up the intriguing prospect of measuring quantum interference fingerprints in nonlocal spin signals.
|11 May 2016
Mansoor B. A. Jalil (Department of E&C Engineering, NUS)
|Spin-orbit torques and spin Hall effect – describing them via the gauge potential picture |
The concepts of gauge fields and gauge symmetry are pervasive in modern physics . Recently, the concepts of gauge potentials have been introduced to the field of spintronics as a means to describe spin transport and dynamics, especially in spin-orbit coupling systems [2,3]. In this talk, I will present the gauge field description of spin-orbit torques (SOT). At present, SOT is being heavily-researched as it can potentially lead to a highly efficient method of switching magnetic memories. The phenomenon of SOT was first predicted by us in a Rashba system in 2007, via the gauge field description [4,5]. The SOT formula was later re-derived via the equivalent Boltzmann model by Manchon in 2008 , and since then has been experimentally investigated in a variety of systems, beginning with a heavy metal interfacial system in 2010 . I will also present the gauge description of SOT in other two-dimensional systems like graphene  and topological insulators . Secondly, I would also describe the application of gauge theory to the spin Hall effect (SHE) [10,11], and introduce the associated concept of the Yang-Mill's spin motive force . Finally, the equivalence to the Kubo treatment will be discussed .
|Date / Speaker||Title / Abstract|
|20 Apr 2016
Hidekazu Kurebayashi (London Centre for Nanotechnology, UCL)
Spin-conversion effects using spin textures in momentum space|
The spin-orbit interaction has been providing richness and greatness of magnetism and spintronics. In solid states, it couples electron's momentum and spins, which make it possible to electrically excite or detect spin accumulation/currents. Looking at localized spins, it is the microscopic origin of magnetic anisotropies (together with the magnetic-dipole interaction) where the sample's real space symmetry, such as surface-induced two-fold and crystalline-induced four-fold, is reflected on the magnetic energy landscape. Along this line, we can also think of what will happen when we lower the sample symmetry to "inversion broken". In this case, an electron propagating along one direction is, on the symmetry argument, no longer required to be on the same state as ones moving to the opposite direction. The spin-orbit interaction picks up this and causes a preferential spin direction for each electronic state, as a whole, forming spin textures in momentum space. These spin textures are a fascinating playground for developing spin-conversion effects. Although the electric excitation of spin textured materials has been known as the Edelstein effect  for more than two decades, its real spintronic use, e.g. magnetisation control , has been much more recent interest. In this talk, I will summarise our recent results on spin-conversion effects using GaMnAs spin textures. I will show microscopic origins of current-induced magnetisation control by the Edelstein effects in single ferromagnetic layers , as well as by non-magnetic layers . In addition, I will touch upon charge pumping experiments by magnons .
|13 Apr 2016
Anthony K. Cheetham (University of Cambridge, UK)
Phase Transitions and Transformations in Metal-Organic Framework Materials|
Our current research on metal-organic frameworks (MOFs) focuses primarily on their physical properties, including their remarkable mechanical, optical, magnetic, ferroelectric and electronic behaviour. We have worked extensively on the amorphization of MOFs, which can be induced thermally, under pressure, or by milling . In certain cases we have been able to form glassy MOFs by quenching of the molten state . I shall discuss several cases of phase transitions that depend heavily on framework flexibility. These include the transition from a porous to a dense framework at 160K in the Zeolitic Imidazolate Framework, ZIF-4, which is accompanied by a decrease in volume of ~23% . A second example involves a reversible, pressure-induced phase transition in a dense rare-earth formate, which shows the breaking and making of bonds during a transition that is accompanied by a 10% change in volume . In addition, we shall explore chemical transformations that depend on flexibility. These include the topochemical dehalogenation of a copper trithiocyanurate framework that is accompanied by a change from an insulating crystalline phase to an amorphous semiconductor , an insulator to proton conductor transition that is driven by hydration , and an in situ study of the successive crystallization of MOFs with increasing stability . Finally, a brief summary of some of our recent work on hybrid perovskites will also be presented .
|06 Apr 2016
Justin Song (Nanyang Technological University, Singapore)
Chiral plasmons without magnetic field|
Plasmons, the collective oscillations of interacting electrons, possess emergent properties that dramatically alter the optical response properties of metals. We predict the existence of a new class of plasmons – chiral Berry plasmons (CBPs) – for a wide range of metallic systems including anomalous Hall metals and gapped Dirac materials. As we show, in these materials the interplay between Berry curvature and electron-electron interactions yields chiral plasmonic modes at zero magnetic field. The CBP modes are confined to system boundaries, even in the absence of topological edge states, with chirality manifested in split energy dispersions for oppositely directed plasmon waves. We unveil a rich CBP phenomenology and propose setups for realizing them, including in anomalous Hall metals and optically-pumped 2D Dirac materials. Realization of CBPs will offer a new paradigm for magnetic field-free, sub-wavelength optical non-reciprocity, in the mid IR-THz range, with tunable splittings as large as tens of THz, as well as sensitive all-optical diagnostics of topological bands.
|09 Mar 2016
László Forró (Laboratory of Physics of Complex Matter, EPFL, Switzerland)
CH3NH3PbI3 hybrid halide perovskite: beyond photovoltaics|
Recently, it has been shown by the Snaith  and Graetzel  groups that CH3NH3PbI3 is very promising material in photovoltaic devices reaching light conversion efficiency (η) up to 21%. A strong research activity has been focused on the chemistry of the material to establish the most important parameters which could further improve η and to collect photons from a broad energy window. The major trend in this field is in photovoltaic device engineering although the fundamental aspects of the material are not yet understood. In my lab we have devoted considerable effort to the growth of high quality single crystals at different length scales, ranging from large bulk crystals (up to 100 mm3) through nanowires [3,4] down to quantum dots of tens of nanometers of linear dimensions. The structural tunability of the material allows to study a broad range of physical phenomena including electrical and thermal transport, magnetism and optical properties which will be reported in this presentation together with some device applications . Acknowledgement: The work has been performed in collaboration with Endre Horvath, Massimo Spina, Balint Nafradi, Alla Araktcheva, Andrea Pisoni, Jacim Jacimovic and the Van der Marel group. This work was partially supported by the ERC Advanced Grant (PICOPROP#670918).
|02 Mar 2016
Edwin Barnes (Virginia Tech University, USA)
Many-body interaction effects in Dirac-Weyl semimetals|
Dirac materials have been at the forefront of condensed matter research over the past decade following breakthroughs in graphene transport experiments. The relativistic-like linear dispersion of quasiparticles near the Dirac point leads to a variety of intriguing phenomena, among which are many-body interaction effects that both resemble and strongly contrast with quantum electrodynamics. Most notably, graphene experiments have revealed a strong Dirac cone squeezing effect due to electron-electron interactions. This and related phenomena can arise not only in graphene, but also in three-dimensional Dirac-Weyl semimetals, albeit with many qualitative differences due to the renormalization of both charge and Fermi velocity in three dimensions. I will describe our efforts to develop a quantitative and predictive theory of many-body phenomena in both graphene and 3D Dirac-Weyl semimetals.
|17 Feb 2016
Rodney S. Ruoff (Ulsan Center for Multidimensional Carbon Materials, Korea)
Graphene, Diamond, Diamane|
We have focused recently on preparing large area Cu foils with (111) orientation by converting as- received polycrystalline Cu foils. The large area Cu (111) foils have been used to prepare large area Cu/Ni alloy (111) foils, and also foils involving other elements such that the foil surface lattice has a close lattice match to graphene and to hexagonal boron nitride (h-BN). (i) Our use of these foils to enable the growth of large area single crystal single-layer graphene and bilayer graphene will be presented. (ii) A brief update on work on attempting to make diamane from AB stacked bilayer (and trilayer and multilayer) graphene by either fluorination or hydrogenation will be given. (iii) We have recently been making polymers that can be converted to diamond and/or carbon rich sp3-bonded carbon materials, and this will be briefly presented. This work was supported by IBS-R019-D1.
|10 Feb 2016
Thomas G. Pedersen (Aalborg University, Denmark)
Exciton ionization in transition-metal dichalcogenides|
In photodetectors and solar cells, optically generated excitons must be ionized to separate electrons and holes. If the excitons are strongly bound, thermal ionization is inefficient. A strong electric field, however, can greatly enhance the ionization rate. We study this process theoretically for mono- and multilayer transition-metal dichalcogenides within a modified Wannier exciton model. The effects of dimensionality and screening on the exciton binding energy are discussed. In the presence of a strong electric field, the exciton energies become complex resonances. We extract the ionization (tunnelling) rate using two complementary approaches: complex scaling and hypergeometric resummation . By applying these techniques to Mo and W based compounds we compute the field-dependence of the ionization rate for both monolayer and multilayer photodetectors, thereby obtaining a fundamental limit for the photoresponse rate .
|3 Feb 2016
Lifa Zhang (Nanjing Normal University, China)
Chiral Phonons in 2D Systems|
Recently, a remarkable phenomenon of the phonon Hall effect was observed in a paramagnetic insulator, which is indeed a surprise since phonons as neutral quasiparticles cannot directly couple to magnetic field via Lorentz force. The following theoretical studies showed that through Raman spin-phonon interaction the magnetic field can have an effective force to distort phonon transport, and thus drive a circulating heat. Inspired by the phonon Hall effect, very recently we found chiral phonons in systems that break time reversal or spatial inversion symmetries. In magnetic systems, where time reversal symmetry is broken, phonons generally carry a nonzero angular momentum . At zero temperature, a phonon has a zero-point angular momentum in addition to a zero-point energy. With increasing temperature, the total phonon angular momentum diminishes and approaches zero in the classical limit. The nonzero phonon angular momentum can have a significant impact on the Einstein–de Haas effect. In non-magnetic crystals with inversion symmetry breaking, we find chiral phonons with valley contrasting circular polarization. At valley centers, there is a three-fold rotational symmetry endowing phonons with a quantized pseudo angular momentum, which includes spin and orbital parts. The chiral valley phonons are verified and the selection rules are predicted in monolayer Molybdenum disulfide. Due to valley contrasting phonon Berry curvature, a valley phonon Hall effect can also be observed.
|27 Jan 2016
Xiao (Renshaw) Wang (SMART, Singapore)
Control of Functionalities in Oxide Heterostructures and Interfaces|
With the current technology approaching its physical limit, enhanced functionality is the route for future devices. Among different material systems, the transition metal oxide constitutes a promising candidate in multifunctional context as it has properties such as ferroelectricity and magnetism, which are not exhibited by conventional semiconductors. With advances in thin-film deposition techniques, synthesis of oxide heterostructures and interfaces has reached an unprecedented level of sophistication with atomic-level control over the thickness. Coupled with the broad compatibility of the oxygen sublattices of different materials and the sensitivity of charge, spin and orbital degrees of freedom on the atomic structure of the heterostructures and interfaces, this technical control has made oxide heterostructures and interfaces a platform for exploring phenomena that are absent in the bulk constituents. Highly mobile two-dimensional electron gas and magnetic order have been shown to emerge at the interface between two insulating and non-magnetic oxides. In this talk I will provide a brief overview of our recent contribution to the progress of the emergent functionalities in oxide heterostructures. A particular emphasis is on our new discovery of a special effect relating to the magnetism of such oxide heterostructures.
|20 Jan 2016
Thomas Schmidt (University of Luxembourg)
Detecting and manipulating Majorana bound states in nanowires|
It has been shown recently that the interplay of spin-orbit coupling, magnetic fields, and the superconducting proximity effect can lead to the emergence of zero-energy Majorana bound states (MBS) at the ends of a nanowire. These states are interesting because of their non-Abelian exchange statistics and their potential usefulness for quantum computation applications. However, detecting and manipulating them remains a challenge.
In the first part of the talk, I will present multi-terminal networks hosting MBS which could be useful for their identification. In particular, I will discuss T-shaped junctions of two Majorana nanowires. When the wires are in the topologically nontrivial regime, three MBS are localized near the outer ends of the wires, while one MBS is localized near the crossing point, and when the lengths of the wires are finite adjacent MBS can overlap. A combination of current and cross-correlation measurements can then be used to reveal the predicted coupling of four MBS in a topological T junction. Interestingly, the elementary transport processes at the central lead are different compared to the outer leads, giving rise to characteristic nonlocal signatures in electronic transport .
MBS have also been proposed as building blocks for qubits on which certain operations can be performed in a topologically protected way using braiding. However, the set of these protected operations is not sufficient to realize universal quantum computing. In the second part of the talk, I will show that the electric field in a microwave cavity can induce Rabi oscillations between adjacent MBS. These oscillations can be used to implement an additional single-qubit gate. Supplemented with one braiding operation, this gate allows one to perform arbitrary single-qubit operations [2,3].
|13 Jan 2016
Yoshihito Hotta (University of Tokyo, Japan)
Tensor-network algorithm for nonequilibrium relaxation in the thermodynamic limit|
Tensor-network algorithms are generalization of DMRG and transfer-matrix methods to higher dimensions and can handle models in two and higher dimensions straightforwardly. We propose a tensor-network algorithm for discrete-time stochastic dynamics of a homogeneous system in the thermodynamic limit. We map a d-dimensional nonequilibrium Markov process to a (d + 1)-dimensional infinite tensor network by using a higher-order singular-value decomposition. As an application of the algorithm, we compute the nonequilibrium relaxation from a fully magnetized state to equilibrium of the one- and two- dimensional Ising models with periodic boundary conditions. Utilizing the translational invariance of the systems, we analyze the behavior in the thermodynamic limit directly. We estimated the dynamical critical exponent z = 2.16(5) for the two-dimensional Ising model. Our approach fits well with the framework of the nonequilibrium relaxation method. Our algorithm can compute time evolution of the magnetization of a large system precisely for a relatively short period. In the nonequilibrium relaxation method, on the other hand, one needs to simulate dynamics of a large system for a short time. The combination of the two provides a new approach to the study of critical phenomena.