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|
|09 Dec 2015
Meera Parish (Monash University)
Pairing phenomena in quasi-2D Fermi gases|
Quasi-two-dimensional Fermi systems are of both fundamental interest and technological importance, with graphene and the high-temperature superconductors being classic examples. Recent advances in cold-atom experiments have now made it possible to investigate model quasi-2D Fermi gases in a controlled manner. In this talk, I will discuss the different pairing regimes in the attractive Fermi gas and how these can be dramatically modified by the finite transverse width of the quasi-2D system. In particular, I will show that the critical temperature for pairing and superfluidity can be enhanced by relaxing the transverse confinement and perturbing away from the 2D limit, thus raising the tantalising possibility that superfluidity is most favourable when the gas lies between 2D and 3D.
|02 Dec 2015
Aires Ferreira (York University, UK)
Overcoming Anderson localization in chiral-disordered graphene|
Graphene subjected to chiral disorder is believed to host zero energy modes resilient to localization, as dictated by the renormalization group analysis of the underlying field theory . For disorder in the BDI chiral orthogonal class – such as vacancies and bond disorder – a line of fixed points with conductivity ~e^2/h is predicted. Such an unconventional quantum transport regime is found at variance with recent numerical works, however, which report the localization of all states, including the zero energy modes . In this talk, I introduce an exact polynomial expansion of quantum-mechanical lattice response functions, whose implementation in large-memory machines allows tackling disordered systems with multi-billion (>10^9) atoms and fine meV resolutions. Its application to the honeycomb lattice with random vacancy defects reveals an unprecedentedly robust metallic state in two dimensions. The Kubo conductivity of zero energy modes is found to match graphene’s universal ballistic conductivity - 4e^2/(pi h) - within 1% accuracy, over a wide range of energy level broadenings and vacancy concentrations . These results testify to the power of the novel polynomial expansion, and shed new light on the nature of electronic transport at the Dirac point of graphene.
 P.M. Ostrovsky, I.V. Gornyi&A.D. Mirlin, PRB 74, 235443 (2006). P.M. Ostrovsky, et al., PRL 105, 266803 (2010).this two-dimensional material.  G.T. de Laissardiere&D. Mayou, PRL 111, 146601 (2013). A. Cresti, F. Ortmann, T. Louvet, D.V. Tuan&S. Roche, PRL 110, 196601 (2013). Z. Fan, A. Uppstu&A. Harju, PRB 89, 245422 (2014).this two-dimensional material.  A. Ferreira&E. Mucciolo, PRL 115, 106601 (2015).
|25 Nov 2015
R. Mahendiran (National University of Singapore)
Revival of magneto-electric coupling in non-polar and multiferroic oxides|
Since the discoveries of giant magnetoresistance in magnetic metallic multilayers and colossal magnetoresistance in Mn based oxides, understanding the interplay between spin and charge degrees of freedom of electron received a major boost under the umbrella of “Spintronics”. Many spintronic based devices, e.g., GMR sensor, MRAM, Spin Transfer Torque(STT) RAM, are already in the market or close to commercialization in next few years. In contrast to spin-charge coupling, the interaction between spin degree of freedom and electrical polarization of materials had been overlooked for a long time. In the last few years, there has been a growing interest so called magnetoelectric or multiferroic materials that show electrical control of magnetization or magnetically tunable electrical polarization. In this talk, I give an overview of the current developments in this field and highlight some of the interesting results obtained on oxide perovskites from my laboratory.
|18 Nov 2015
Timo Laehde (Institute for Advanced Simulation, Juelich, Germany)
From carbon nanotubes to graphene - A Monte Carlo approach|
I discuss how Quantum Monte Carlo methods developed for studies of the electronic structure of graphene can be applied to carbon nanotubes, and show first results for the interaction-induced energy gap. I also review previous Monte Carlo results for the critical coupling of the semimetal-insulator transition in graphene. I also discuss how our results for carbon nanotubes could be used to constrain the strength and form of the electron-electron interaction in graphene.
|11 Nov 2015
Ben Hu (Akron University, USA)
Plasma wave instabilities in non-equilibrium graphene|
In plasma physics, two-stream instabilities can occur when two sets of charge carriers drift with respect to one another. I will discuss the theory of two-stream instabilities in the non-equilibrium situation in which electrons are injected into a doped single-layer graphene sample. As with equivalent non-equilibrium parabolic band systems, we find that these graphene systems theoretically can support unstable charge-density waves whose amplitudes grow with time. However, the linear dispersion of graphene leads to qualitative differences in the region of wavevectors of the modes which are unstable. In particular, in contrast to parabolic-band systems, in graphene the modes with wavevectors that are parallel to the direction of the injected electrons are not unstable. Possibilities of experimental observation of these modes will be briefly discussed.
|04 Nov 2015
Bertrand Roehner (Univ. Paris 6, Pierre and Marie Curie, France)
How can one measure the interaction strength in systems of living organisms|
For any system, one of its most crucial characteristics is the strength of the interaction which binds together its elements. From an ideal gas where gas molecules have very rare and short-range interactions, to a family or to the brain where the neurones have permanent long-range interactions, there is a broad range of cases. A key-problem is how to measure the strength and range of the interaction because these two characteristics deeply affect the macroscopic properties of the system. So far, this problem is largely unresolved.
|28 Oct 2015
Roberto Raimondi (Roma Tre University, Italy)
Spin current swapping and spin Hall effect in a 2DEG|
In this talk I will present some of the results obtained over the last year about spin-orbit induced effects in a two-dimensional electron gas. At first I will discuss the spin current swapping effect according to which a spin current flowing in the i direction with spin polarization along the j axis is converted into a spin current flowing in the j direction with spin polarization along the i axis. I will analyze the circumstances under which to observe the effect and its connection with the spin Hall effect. As a second topic I will focus on the spin Hall effect due to the skew-scattering mechanism induced by phonon scattering. A comparison will be made with the standard skew-scattering due to impurities and the consequences for the temperature dependence of the spin Hall angle will be analyzed. Finally, I will present a model with a striped Rashba spin-orbit coupling, which could possibly be realized in LAO/STO interfaces. Such a non homogeneous spin-orbit coupling may give rise to a spin Hall effect robust with respect to impurity scattering.
|22 Oct 2015
Allan MacDonald (University of Texas at Austin, USA)
Excitons, Polaritons, and Their Condensates in 2D materials|
2D materials are interesting in part because they can be arranged at will within the three-dimensional world in which we live. I will discuss the possibility of realizing new electrical and electro-optical properties based on the properties of multi-layers formed from two-dimensional semiconductors and two-dimensional insulators. I will focus mainly on the case of spatially indirect exciton condensates, but comment briefly on coupling between two-dimensional excitons and two-dimensional photons defined by vertical cavities.
|14 Oct 2015
Boris Narozhny (Karlsruher Institut für Technologie)
Collision-dominated hydrodynamics in graphene|
We present an effective hydrodynamic theory of electronic transport in graphene in the interaction-dominated regime. We derive the emergent hydrodynamic description from the microscopic Boltzmann kinetic equation taking into account dissipation due to Coulomb interaction and find the viscosity of Dirac fermions in graphene for arbitrary densities. The viscous terms have a dramatic effect on transport coefficients in clean samples at high temperatures. Within linear response, we show that viscosity manifests itself in the nonlocal conductivity as well as dispersion of hydrodynamic plasmons. Beyond linear response, we apply the derived nonlinear hydrodynamics to the problem of hot-spot relaxation in graphene.
|30 Sep 2015
Kedar Hippalgaonkar (IMRE, A*STAR)
Thermoelectric Powerfactor and Interface Thermal Resistance in 2D MoS2|
Thermoelectric devices require large Seebeck and simultaneously large electrical conductivity, while maintaining a low thermal conductivity. Significant progress in the thermoelectric performance of materials has been made by exploring ultralow thermal conductivity at high temperature, reducing thermal conductivity by nanostructuring, resonant doping and energy-dependent scattering. For a given thermal conductivity and temperature, thermoelectric powerfactor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to high powerfactor due to the unique density of states (DOS) of confined electrons and holes. Emerging 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only because of their discretized density of states, but also due to their large effective masses and high carrier mobilities, different from gapless semi-metallic graphene. We have observed 2D crystals of MoS2 with a powerfactor as large as 8.5 mWm−1K−2 at room temperature, which is the highest among all thermoelectric materials. Moreover, measurement of thermoelectric properties of monolayer MoS2 in the metallic regime allows us to determine the confined 2D DOS near the conduction band edge for the first time, which cannot be measured by electrical conductivity alone. Further, we measure the interlayer thermal resistance in MoS2/hBN heterostructures, which can be used to tune the in-plane thermal conductivity allowing for an additional tunable knob for future thermoelectrics. The demonstrated high electronically modulated powerfactor in 2D TMDCs with tunable thermal conductivity holds promise for efficient thermoelectric energy conversion.
|16 Sep 2015
Benoit Gremaud (Center for Quantum Technologies, NUS)
The extended Bose-Hubbard model in 1D: new results for an old problem|
In a first part, I will present the phase diagram of the one-dimensional bosonic Hubbard model with contact (U) and near neighbor (V) interactions: (i) reviewing the well-known phases, such as the Mott insulating phase, the Haldane insulating phase and the charge density wave (ii) discuss new phases, in particular the supersolid (SS) phase, .i.e, depicting both a diagonal long range order and an off-diagonal (quasi-) long range order. In addition, I will show that, at fixed integer density, the system exhibits phase separation in the (U,V) plane. Our results have been obtained with the Stochastic Green Function quantum Monte Carlo algorithm as well as the density matrix renormalization group.
In a second part, I will present the excitation spectrum above the ground state in different phases, obtained with the time evolving block decimation method. At unit filling, I will discuss the properties of the structure factor across the Mott insulating phase, the Haldane insulating phase and the charge density wave phase. In particular, in the Haldane phase, I'll emphasize the competition between the single particle and two particle excitations, resulting in different values for the neutral and the charge gap. In the supersolid phase appearing by doping the charge density wave phase, I'll show that the structure factor depicts additional gapless modes at a finite momentum that depends on the density. This feature and the low energy spectrum can be explained by a mapping of the system onto the Heisenberg model for a spin 1/2 chain in a finite magnetic field.
|09 Sep 2015
Wang Jian-Sheng (Department of Physics, NUS)
Isn't there a negative absolute temperature?|
In 1956, Ramsey, based on experimental evidence of nuclear spin, developed a theory of negative temperature. The concept is challenged recently by Dunkel and Hilbert [Nature Physics 10, 67 (2014)] and others. In this talk, we review what thermodynamics is and present our support that negative temperature is a valid concept in thermodynamics and statistical mechanics.
 Swendsen and J.-S. Wang, Phys. Rev. E 92, 020103(R) (2015)
|26 Aug 2015
Justin Song (Caltech, USA)
Curveball electrons: Valley transport in gapped Dirac materials|
Charge carriers in materials are often described as quasiparticles similar to free electrons and can be characterized by effective quantities such as an effective mass. However, electrons in topological materials acquire an additional quantum mechanical property - Berry curvature - that is the key ingredient in a range of new phenomena. A striking example is carrier dynamics in gapped Dirac systems, such as graphene on hexagonal-boron-nitride (G/h-BN). I will discuss how Berry curvature gives rise to transverse valley currents even in the absence of a magnetic field in these systems. Crucially, these valley currents do not depend on the presence of edge states, and persist even in the gapped system bulk. These anomalous carrier dynamics manifest naturally in G/h-BN, displaying large non-local resistances mediated by valley currents in G/h-BN devices, yielding a new platform/scheme to access topological characteristics in layered 2D stacks of materials.
|25 Aug 2015
Siegfried Eigler (Friedrich Alexander Universität, Germany)
Controlled Oxo-functionalization of graphene|
Note: Seminar held from 14:00 to 15:00 at the Graphene Theory Commons (S16 L6)
The bulk chemistry of graphene is based on the oxidation of graphite, with a tradition of more than 150 years. It turned out that all developed oxidation methods resulted in functionalized layers of graphene with plenty of defects within the carbon framework. Thereby the procedures are accompanied by over-oxidation leading to more amorphous structures with manifold functional groups. This over-oxidation can even lead to disintegration of flakes and the generation of oxidative debrides. Thus, chemical functionalization of graphene oxide is dominated by reactivity of edge-groups of defects. Approximately one missing carbon atom was determined in graphene oxide on about 25 lattice atoms using a conventional synthetic protocol. Thus, chemistry was predominantly driven by the functionalization of defects. We optimized the oxidative procedure to avoid the formation of defects and now a residual average density of defects of about 0.05% was reached. Statistical Raman microscopy is now available to monitor the impact of reaction on the integrity of the carbon lattice. With such materials, the chemistry of graphene can be explored while defects play a minor role. We used oxo-functionalized graphene with organosulfate groups to prepare a composite that can be used as charge storing layer in floating gate memory devices operating at 3V. This goal was only reached by controlled chemistry. In another recent study we used graphite sulfate, which is a long known intercalation compound that bears a C24 subunit with one positive charge. This charge can be used for chemical functionalization and an oxo-functionalized graphene derivative with an idealized subunit of C24(OH)(H2O)2 is yielded. Chemical reduction and statistical Raman analysis reveals that the best average quality of graphene can be prepared from this novel derivative of graphene. Overcoming performance limits, as determined for graphene oxide, is in reach using controlled chemistry of oxo-functionalized graphene.
 S. Eigler, A. Hirsch, Angew. Chem. Int. Ed. 2014, 53, 7720.
|19 Aug 2015
Peter Hänggi (University of Augsburg, Germany)
Anomalous Heat Diffusion|
The field of Statistical Physics provides many different tools, which are based either on microscopic approaches or on a more phenomenological level to describe the spread of heat in classical and quantum regimes in realistic and more idealized model systems of arbitrary dimensions. Typical such powerful tools are first principles Linear Response Theory for transport coefficients, yielding the celebrated Green-Kubo formulas, the stochastic theory of Random Walks, or mesoscopic approaches such as the more practical treatments in terms of kinetic transport equations. In low dimensional systems the transport of heat in form of diffusive spread or heat flux between reservoirs of differing ambient temperatures typically may exhibit anomalous features such as the violation of the Fourier Law with length-dependent heat conductivities or the diffusive spread of heat that occurs faster than normal [1, 2].
In this talk we discuss recent results how the dynamics of energy spread occurring in one-dimensional nonlinear lattices relates to anomalous diffusion behavior and heat conductivities. Moreover we explain how the carriers of heat, typically referred to as phonons, may be given meaning in a regime with nonlinear interaction forces beyond the ballistic behavior originating from solely harmonic (linear) interaction forces. The underlying physical mechanism of scattering then renders corresponding mean free paths of such effective phonons finite .
 A.V. Zarbudaev, S. Denisov and P. Hänggi, Phys. Rev. Lett. 106: 180061(2011); ibid, Phys. Rev. Lett. 109, 069903 (2012).
|12 Aug 2015
Hilmi Volkan Demir (CA2DM&Luminous, NTU)
Nanocrystal optoelectronics: from solution-processed quantum dots to wells|
Solution-processed semiconductor nanocrystals have attracted great interest in photonics including color conversion and enrichment in quality lighting and display backlighting . In this talk, we will present architectures of colloidal nanocrystals obtained by tailoring and controlling the dimensionality, size, and composition of these nanostructures in an effort to realize high performance in light generation and lasing . These cover types of colloidal quantum dots, quantum rods and quantum wells. Based on the rational design and control of excitonic processes in these nanocrystals, we successfully demonstrated highly efficient light- emitting diodes  and lasers [4,5]. To this end, we systematically studied and showed that electronic-type tuning in colloidal quantum heterostructures allows for fine tunability . Here ultra-low threshold stimulated emission was achieved using engineered core/shell architectures enabling substantially suppressed Auger recombination, enabling the first liquid laser of nanocrystals . Also, we developed an all-colloidal solid laser using these nanocrystals as the optical gain media for the first time in a fully colloidal resonator . As an extreme case of solution-processed highly-confined quasi-2D colloids, we showed that the atomically flat heteronanoplatelets uniquely combine ultra-low threshold stimulated emission and record high optical gain coefficients and the controlled stacking of these nanoplatelets further tune their excitonic properties . The recent progress in the colloidal optoelectronics suggest that solution-processed quantum materials hold great promise to challenge epitaxial counterparts in the near future.
 H. V. Demir et al., Nano Today 6, 632 (2011); T. Erdem and H. V. Demir, Nature Photonics 5, 126 (2011).
|Date / Speaker||Title / Abstract|
|11 Aug 2015
Daniel Ucko (University of Birmingham)
|How to publish in PRL|
Physical Review Letters (PRL) is the Letters journal of the Physical Review series, and is published by the American Physical Society. It is one of the most prestigious journals in physics. As a Letters journal publishing short reports of high importance, impact, and interest across all of physics, it is a unique publication, and no other journal has PRL’s scope and coverage of physics. We enjoy a healthy amount of submissions and recently celebrated our 50th anniversary.
However, the inner workings of PRL are however sometimes a bit mysterious to the authors and referees that make the journal what it is. The role of editor is really concerned with triad of author, referee, and editor, and managing the relationships between each of these. I will be explaining what is expected of each of these, and how they can best work together. I will also be talking about the state of the journal, and show on some new developments at PRL in view of the changing publication landscape. A question and answer session will follow the main presentation.
|01 Aug 2015
Clas Persson (University of Oslo)
|Theoretical modeling of solar energy materials: Research activities at UiO Norway|
The talk will briefly present the different projects and research activities within theoretical modeling of solar energy materials at the Univesity of Oslo in Norway. One of the main research project is on Cu-based solar cell materials, like Cu(In,Ga)Se2 and Cu2ZnSnS4, and the talk will therefore discuss a little more deepdly why this type of materials has fascinated researchers for the last three decades.
 S. Siebentritt, M. Igalson, C. Persson, and S. Lany Prog. Photovoltaics Res. Appl. 18, 390 (2010).
|29 Jun 2015
D. N. Basov (University of California San Diego, USA)
|Nano-photonic phenomena in van der Waals heterostructures|
Layered van der Waals (vdW) crystals, which consist of individual atomic planes weakly coupled by vdW interaction similar to graphene monolayers in solid graphite, can harbor remarkable properties: viz. superconductivity and ferromagnetism with high transition temperatures, light emission, and topologically protected surface states. Such artificial crystals provide building blocks for stacked heterostructures where each such block delivers layer-specific attributes. In examples assembled from atomically thin layers of graphene and hexagonal boron nitride (hBN), a rich variety of optical effects arise from the confluence of unusual elementary excitations: viz. surface plasmons in graphene and hyperbolic phonon polaritons in hBN. We have launched, detected and imaged plasmonic, phonon polaritonic and hybrid plasmon-phonon polariton waves in a setting of an antenna based nano-infrared apparatus. The nano-scale exploration of polaritonic modes has offered a new perspective on fundamental physics behind electronic phenomena in graphene. For example, by interferometric infrared imaging of plasmonic standing waves we were able to quantify the electronic losses in graphene. This latter result highlights the important role of many body effects that were not anticipated theoretically.
|17 Jun 2015
Ming Yang (CA2DM&IMRE, Singapore)
|The origin of 2EDG and magnetism at LaAlO3/SrTiO3 interface|
Abstract not available
|10 Jun 2015
Hridis Pal (Georgia Tech, USA)
|Graphene with a twist|
Graphene has been at the forefront of research for almost a decade, thanks to its unusual electronic properties. It is well known that the properties of this single layer material get modified substantially with the addition of a second layer. The most commonly studied form of graphene bilayers is one where the two layers are mutually rotated by sixty degrees--the so called Bernal stacking. However, recently there has been a surge of interest in bilayer graphene structures where the layers are rotated by an arbitrary angle instead of sixty degrees. The current understanding of these systems may be summarized as follows: at large angles the layers are essentially decoupled, and the system manifests single layer properties. With decreasing angle, however, the Fermi velocity gets renormalized, and at small enough angles, bands flatten and electrons get localized. In this talk, we will show that this current understanding is incomplete: non-trivial physics such as band flattening and localization can happen at large angles as well, as long as the system is sufficiently close to some commensuration. To this end, we formulate a long-wavelength theory near commensuration valid at arbitrary angles of rotation, generalizing existing long-wavelength theories valid only at small angles. We then use our theory to show that in the stong coupling limit and at large angles, the system becomes locally gapped and mimics the properties of a Kagome-like lattice. We discuss the implications of our model.
|09 Jun 2015
David Prendergast (The Molecular Foundry, Lawrence Berkeley Natl. Lab., USA)
|Providing molecular-scale details at electrode interfaces through interpretation of X-ray spectroscopy|
The control of energy flow in devices is often dictated by materials interfaces and a detailed understanding of such interfaces under working conditions is required for their further optimization. Soft X-ray spectroscopy is an intrinsic surface sensitive probe capable of revealing molecular scale details at interfaces. Coupled with first-principles modeling of the structure and dynamics at the junction between two materials, one can begin to interpret measured spectra and to make direct connections between structure and function. Here we present details of recent studies relevant to various model electrode interfaces in the context of electrochemistry  and photoelectrochemistry , indicating the importance of first-principles theoretical simulation in the interpretation of X-ray spectroscopy and the associated insight into the processes behind interfacial energy transfer and storage.
 The structure of interfacial water on gold electrodes studied by x-ray absorption spectroscopy J.-J. Velasco-Velez, et al., Science 346, 831 (2014).
|03 Jun 2015
Marko Kralj (Institute of Physics Zagreb, Croatia)
|Engineering epitaxial graphene by adsorption, intercalation and strain|
The properties of graphene can be exploited in various applications e.g. by controlling the density of states at the Fermi energy or the strain and related pseudomagnetic fields. Besides electric field, a chemical adsorption either "on top" or "underneath" graphene, where typically charge transfer processes take place, is a suitable tool for the charge carrier and many-body interaction modifications. In epitaxial graphene systems deposition of atoms and molecules often leads to intercalation where species are pushed between graphene and its support. Besides the charge donation, the intercalation can affect the binding interaction and more subtle properties of graphene, e.g. spin-polarization. In fact, properties of many layered materials, including copper- and iron-based superconductors, dichalcogenides, topological insulators, graphite and epitaxial graphene, can be manipulated by intercalation. Another direction of graphene electronic structure tailoring is related to a precise stress control which can be realized by graphene growth on flat or specifically on stepped surfaces and we focus to such systems in order to exploit uniaxial strain engineering.
We studied the intercalation and entrapment of alkali atoms under epitaxal graphene on Ir(111) in real and reciprocal space by means of LEEM, STM, ARPES, LEED and vdW-DFT. The microscopic mechanism and dynamics of intercalation process is explained, where we find that the intercalation is adjusted by the van der Waals interaction, with the dynamics governed by defects anchored to graphene wrinkles. Graphene wrinkles, their structure, ordering and formation dynamics are characterized in great detail on relevant nano- and micrometer scales. Finally, the way to obtain uniaxially strained graphene by growing it on a stepped single crystal substrate and subsequently transferring it to a dielectric support by preserving uniaxial pattern will be presented.
|25 May 2015
Leonardo Degiorgi (ETH-Zürich, Switzerland)
|Electrodynamic response in the electronic nematic phase of BaFe2As2|
The ferropnictides harbor a structural tetragonal-to-orthorhombic transition at Ts that may either coincide or precede a transition into a long-range antiferromagnetic order at TN, usually ascribed to a spin-density-wave (SDW) state. We measure the in-plane optical reflectivity of BaFe2As2 over a broad spectral range, covering the energy interval from the far infrared (FIR) to the ultraviolet (UV), at several combinations of uniaxial pressure, used to detwin the specimen, and temperature. Our goal is to probe the anisotropic response in the real part σ1(ω) of the optical conductivity, extracted from the reflectivity data via Kramers-Kronig transformations. We thus elucidate how the anisotropic optical metallic response evolves as a function of stress, considered as an external symmetry breaking field, and across the ferro-elastic structural transition at Ts = TN = 135 K. The infrared response reveals that the dc transport anisotropy in the orthorhombic antiferromagnetic state is determined by the interplay between the Drude spectral weight and scattering rate, but that the dominant effect is clearly associated with the metallic spectral weight. In the paramagnetic tetragonal phase, though, the dc resistivity anisotropy of strained samples is almost exclusively due to stress-induced changes in the Drude weight rather than anisotropy in the scattering rate. This result definitively establishes that the primary effect driving the resistivity anisotropy in the paramagnetic orthorhombic phase (i.e., the electronic nematic state) is the anisotropy of the Fermi surface.
|13 May 2015
Jaroslav Fabian (University of Regensburg, Germany)
|Spin phenomena in two dimensional materials|
Note: Seminar starts at 11:30
Two dimensional materials, such as graphene, transition metal dichalcogenides, or black phosphorous, offer immense opportunities for electronics and spintronics . Being ultimately thin these materials could make the thinnest diodes and transistors, or the thinnest magnetic sensors and read heads. Being essentially a surface, they are also susceptible to adatoms and admolecules which can induce local magnetic moments and giant spin-orbit coupling . This is in fact a great opportunity, allowing us to decorate (functionalize) graphene and like materials with specific defects to make desired properties. I will review the essential spin physics of novel two dimensional materials, including spin-orbit coupling and magnetic moments, and discuss the ramifications of the (intended and non-intended) functionalization for spin transport experiments. Most of the results are obtained by performing first principles calculations on large atomic supercells, necessary to study the physics in the dilute defect limit. These calculations show a nice agreement with experiments regarding spin relaxation in single  and bilayer  graphene, but also make authoritative predictions for future realistic charge and spin based device---an example is given by optospintronics in graphene/TMC structures.
 W. Han, R. Kawakami, M. Gmitra, and J. Fabian, Nature Nanotechnology 9, 794 (2014).
|Date / Speaker||Title / Abstract|
|15 Apr 2015
Xu Qing-Hua (Department of Chemistry, NUS)
|Plasmon Enhanced Nonlinear Optical Properties for Biomedical Applications|
Noble metal nanoparticles, such as gold and silver, display unique properties known as localized surface Plasmon resonance, which could be utilized to enhance linear and nonlinear optical properties of nearby chromophores and metal nanoparticles themselves. Our group has done extensive work on plasmon enhanced one- and two-photon excitation fluorescence and their applications. In particular, we found an interesting phenomenon that non-fluorescence metal nanoparticles started to emit strong two-photon photoluminescence (TPPL) upon plasmon coupling in the aggregated state. We have demonstrated that this kind of plasmon coupling enhanced TPPL is a general phenomenon for Au and Ag nanoparticles of different morphologies. TPPL of these metal nanoparticles was found to be enhanced by up to hundreds of times in the colloid solution and five orders of magnitude on single particle level upon plasmon coupling. As many biologically important species can induce aggregation of metal nanoparticles, this phenomenon has been further utilized to develop various two-photon sensing and imaging applications to take their unique advantages of deep penetration into biological tissues and 3-dimensional confined excitation. We have also employed ultrafast spectroscopy techniques to understand the underlying enhancement mechanisms.
|01 Apr 2015
Thomas G. Pedersen (Aalborg University, Denmark)
|Excitonic nonlinear optical response of 2D materials|
Abstract not available
|08 Apr 2015
John Robertson (Cambridge University, UK)
|Schottky barrier heights of Transition Metal Dichalcogenides and other layered systems|
The Schottky barrier heights of the layered transition metal dichalcogenides (TMDs) have been calculated using density functional theory. The calculated Schottky Barrier heights are found to depend quite weakly on the metal work function, with a pinning factor of S~0.3. This indicates that TMD Schottky barriers follow the metal induced gap state (MIGS) model, like three-dimensional semiconductors, despite the two-dimensional bonding of TMD layers. This is because the bonding between the contact metal atoms and the TMD chalcogen atoms is covalent/ionic, and not of a van der Waals type. The metal Fermi level is pinned in the upper gap in MoS2, but pinned near midgap in most other TMDs. Thus, ambipolar contacts can be achieved by avoiding MoS2, or by using a high work function electrodes like MoO3 on MoS2. Contacts for black phosphorus are also covered.
|25 Mar 2015
Silvija Gradecak (MIT, USA)
|Nanomaterials on demand: from controlled growth to devices|
Functionality of novel nanomaterials – including two dimensional (2D) ultrathin films, one dimensional (1D) nanowires/nanotubes, and zero dimensional (0D) nanocrystals – and their impact on society will be ultimately dictated by our understanding and ability to precisely control their structural properties, size uniformity, and dopant distribution at the atomic level. Over the past several years, we have revealed several important insights into the fundamental growth mechanisms in several nanowire compound materials systems, and more recently, we have used this knowledge to control composition and morphology of nanowires in-situ during the growth (“nanowires on demand”). These findings were enabled by the development of a unique growth system, as well as by direct structure-property correlation using state of the art electron microscopy techniques, some of which we have developed. In this talk, I will also discuss development of flexible and transparent nanostructured photovoltaic devices with power conversion efficiencies exceeding 9%. This new class of solar cells takes advantage of transparent graphene electrodes, quantum dot absorbers, and nanowire transport layers.
|13 Mar 2015
|2D Materials Forum (13 March 2015)|
Note: Seminar held from 12:15 to 13:15 at the Theory Common (S16-06) Abstract not available
|11 Mar 2015
Alexander Petrović (Nanyang Technological University)
|Tunable Multifractality in Quantum Matter|
Fractals are scale-invariant spatial distributions which are ubiquitous in nature, governing the shapes of objects as diverse as snowflakes, trees and coastlines. A less well-known instance of fractal ordering occurs in strongly disordered materials, where electronic wavefunctions develop multifractal spatial distributions in the vicinity of the Anderson transition between extended and localised states. Such multifractal ordering has been predicted to enhance electron-electron interactions. In materials with instabilities to correlated electron phase formation (such as ferromagnets or superconductors), one may therefore envisage the possibility of tuning the quantum ground state via disorder.
In disordered superconductors, a large multifractal enhancement is expected in the pairing interaction (and hence the critical temperature Tc), provided that the Coulomb repulsion is weak. However, no such rise in Tc has ever been observed experimentally, due to the suppression of superconductivity by emergent granularity and a dynamically-augmented Coulomb repulsion in highly disordered materials. Using a range of experimental and numerical techniques (including electrical transport, magnetization, X-ray diffraction/scattering and density functional theory), we demonstrate that multifractal pairing enhancement does in fact occur in the quasi-one-dimensional superconductor Na2-δMo6Se6, due to the combination of random Na vacancy disorder with an intrinsically screened Coulomb repulsion. The pairing temperature Tons rises monotonically as the Anderson-Mott mobility edge is approached from the metallic side, in quantitative agreement with a multifractal enhancement model. Strikingly, Tons continues to rise in the localised phase after crossing the mobility edge, in accordance with theoretical predictions. The upper critical field Hc2 exceeds the weak-coupling Pauli limit by a factor of at least 4 in the localised regime, indicating a large increase in the superconducting gap energy.
Our results provide the first experimental perspective onto the unknown physics of correlated electron materials in the absence of Coulomb repulsion. We also show that the unique interplay between superconductivity and localisation in nanofilamentary materials renders them ideal building blocks for functional superconductors. Electron delocalisation drives an intrinsic stabilisation of phase fluctuations upon raising the temperature, magnetic field or electric current, in direct contrast to the behaviour of conventional homogeneous superconductors.
|13 Feb 2015
Minoru Osada (NIMS and Waseda University, Japan)
|Oxide Nanosheets: Old 2D Materials, New Challenges? |
Note: Seminar held from 14:00 to 15:00 at the Theory Common (S16-06)
2D nanosheets with atomic or molecular thickness have been emerging as important due to their unique properties. Inspired by the intriguing properties of graphene, many efforts have been devoted to synthesising 2D inorganic nanosheets of various materials including metal oxides, hydroxides, and transition-metal chalcogenides as well as primarily investigating their unique electronic structures and physical properties. Among various types of inorganic nanosheets, oxide nanosheets are important, fascinating research targets because of the virtually infinite varieties of layered oxide materials with interesting functional properties. We are working on the creation of new oxide nanosheets and the exploration of their novel functionalities in electronic applications [1,2].
A variety of oxide nanosheets (such as Ti1-O2, Ti1-xCoxO2, MnO2, and perovskites) were synthesized by delaminating appropriate layered precursors into their molecular single sheets via soft- chemical process. These oxide nanosheets have distinct differences and advantages compared with graphene because of their potential to be used as insulators, semiconductors, and even conductors, depending on their composition and structures. Recently, we found that titania- or perovskite-based nanosheets exhibit superior high- performance (r = 100–320) even at a few-nm thicknesses, essential for next-generation electronics. Additionally, nanosheet-based high- capacitors exceeded textbook limits, opening a route to new capacitors and energy storage devices.
Another attractive aspect is that oxide nanosheets can be organized into various nanoarchitectures by applying solution-based layer-by-layer assembly. Sophisticated functionalities or nanodevices can be designed through the selection of nanosheets and combining materials, and precise control over their arrangement at the molecular scale. We utilized oxide nanosheets as building blocks in the LEGO-like assembly, and successfully developed various functional nanodevices such as all nanosheet FETs, artificial ferroelectrics, spinelectronic devices, magneto-plasmonic materials, Li-ion batteries, etc. Our work is a proof-of-concept, showing that new functionalities and nanodevices can be made from nanosheet architectonics.
 M. Osada and T. Sasaki, J. Mater. Chem. 19, 2503 (2009) [Review].
|04 Feb 2015
Mark A. Reed (Yale University, USA)
Electron devices containing molecules as the active region have been an active area of research over the last few years. In molecular-scale devices, a long standing challenge has been to create a true three-terminal device; e.g., one that operates by modifying the internal energy structure of the molecule, analogous to conventional FETs. Here we report  the observation of such a solid- state molecular device, in which transport current is directly modulated by an external gate voltage. We have realized a molecular transistor made from the prototype molecular junction, benzene dithiol, and have used a combination of spectroscopies to determine the internal energetic structure of the molecular junction, and demonstrate coherent transport. [2,3] Resonance- enhanced coupling to the nearest molecular orbital is revealed by electron tunnelling spectroscopy, demonstrating for the first time direct molecular orbital gating in a molecular
 H. Song et al., Nature 462, 1039 (2009)
|28 Jan 2015
Damien Voiry (Materials Science and Engineering, Rutgers University)
|Chemical strategies for tuning the properties of transition metal dichalcogenides|
Among the family of 2D materials, transition metal dichalcogenides (TMDs) have demonstrated original opto-electronic properties. For example, the band structure of TMDs can be largely tuned by changing the elemental composition, the thickness or the atomic structure. Single layer TMDs exist in 2 crystalline forms depending on the coordination of the metal atoms: either a trigonal prismatic coordination (2H phase: semiconducting) or an octahedral coordination (1T phase: metallic) . In our group we have actively studied the controlled synthesis of the 1T phase using a well-established procedure based on the lithium intercalation of the 2H phase . The metallic 1T phase has been used for improving the electrocatalytic activity [3,4] or reducing the contact resistance of electronic TMD-based devices . In addition we have recently demonstrated that the 1T phase of group 6 TMDs is chemically activated and can be functionalized covalently in order to further modify the nanosheets . In this presentation, I will introduce the results of our investigations on the phase engineering of exfoliated TMDs in order to ellucidate the importance of the structure in low dimension materials.
 Chhowalla, M. et al. Nat. Chem. 5, 263–275 (2013).
|23 Jan 2015
Alexandra Carvalho (Graphene Research Center)
|Defect engineering in 2D materials - what we can learn from DFT|
Note: Seminar starts at 12:15 at the Graphene Theory Common (S1606)
Due to its predictive power, density functional theory is widely applied today in many areas of materials science. However, it is most useful when used as a complementary tool alongside experiment, as often is the case in the field of defect identification and engineering. In this talk, we will discuss how defects can be identified by comparing experimental observables with first principles calculations, and will analyse some examples of defect engineering in 2D materials. These include the chalcogen vacancy in transition metal dicalcogenides- a defect harmful for most electronic and optoelectronic applications but than can be explored in spintronics- and some of the defects behind the degradation of phosphorene in air.
|21 Jan 2015
Tzen Ong (Rutgers University, USA)
|Entangled Orbital Triplet Pairs in Iron-Based Superconductors|
Note: Seminar held at Graphene Theory Common (S1606)
A key question in high temperature iron-based superconductivity is the mechanism by which the paired electrons minimize their strong mutual Coulomb repulsion. While electronically paired superconductors generally avoid the Coulomb interaction through the formation of nodal, higher angular momentum pairs, iron based superconductors appear to form singlet s-wave (s) pairs. By taking the orbital degrees of freedom of the iron atoms into account, here we argue that the s state in these materials possesses internal d-wave structure, in which a relative d-wave (L = 2) motion of the pairs entangles with the (I = 2) internal angular momenta of the d-orbitals to form a low spin J = L + I = 0 singlet. We discuss how the recent observation of a nodal gap with octahedral structure in KFe2As2 can be understood as a high spin (J = L + I = 4) configuration of the orbital and isospin angular momenta; the observed pressure-induced phase transition into a fully gapped state can then interpreted as a high-to-low spin phase transition of the Cooper pairs.
|19 Jan 2015
Anthony Leggett (University of Illinois at Urbana-Champaign)
|Some thoughts on all-electronic superconductivity|
Abstract not available.
|16 Jan 2015
Rahul Nandkishore (Princeton University)
|Many body localization: a new frontier for quantum statistical physics|
Note: Seminar starts at 16:30 and held at the Graphene Theory Common (S16-06)
The existing theory of quantum statistical mechanics describes open systems in contact with large reservoirs. However, experimental advances in the construction and control of isolated quantum systems have highlighted the need for an analogous theory of isolated systems. It has been realized that isolated quantum systems can support behavior which has no analogue in open quantum systems. A prominent example is the phenomenon of many body localization.
Many body localization occurs in isolated quantum systems, usually with strong disorder, and is marked by absence of dissipation, absence of thermal equilibration, a strictly zero DC conductivity (even at energy densities corresponding to high temperatures), and a memory of the initial conditions that survives in local observables for arbitrarily long times. Recently, my co-workers and I have demonstrated that many body localization also opens the door to new states of matter which cannot exist in thermal equilibrium, such as topological order at finite energy density, or broken symmetry states below the equilibrium lower critical dimension. We have also uncovered a host of unexpected properties, such as a set of universal spectral features and a non-local charge response, that have striking implications for fields as diverse as quantum Hall based quantum computation and quantum control. In this talk, I review the essential features of the many body localization phenomenon, and present some of the recent progress that I have made in this field. I also discuss the implications of these results for both theory and experiment.
|14 Jan 2015
Jong-Hyun Ahn (Yonsei University, Republic of Korea)
|Nanomaterials for Flexible and Transparent Electronics: Graphene and Silicon Nanomembrane|
Note: Seminar starts at 12:45 p.m.
With the emergence of unusual format electronics such as flexible and wearable devices, an effort has been made to integrate devices with various functions in smart clothing for providing enhanced flexibility and convenience for the users. Thus, many experts believe that an important future in electronics is with systems that avoid the rigid, brittle and planar nature of existing classes of electronics, to enable new applications. However, it is very difficult to accomplish such electronics with conventional electronic materials. Graphene, the thinnest elastic material, has superb electronic properties that make it a promising host for device applications. In particular, graphene has an extremely good mechanical property, offering a great opportunity to flexible and stretchable electronics that should maintain a stable operation under a high strain. The recent advances in large-scale synthesis of graphene films by chemical vapour deposition are expected to enable various macroscopic applications such as semiconducting and transparent conducting films useful for flexible and stretchable electronics. In addition, to overcome the limitation of conventional materials, we developed the fabrication method of ultra-thin Si nanomembrane with thickness of nanometer scale from a single crystal wafer using the top town process. The resulting materials display outstanding electrical, optical and mechanical properties for high performance flexible and transparent electronics.
1. J.-H. Ahn et al., "Graphene for displays that bend", Nature Nanotechnology, 9, 737 (2014)
|14 Jan 2015
Inti Sodemann (MIT, USA)
|Non-local transport in neutral superfluids: exciton condensates and XY magnets|
Note: Seminar starts at 12:00 p.m.
In quantum Hall bilayers at total filling fraction one the electrons can develop a many-body state in which they occupy a common coherent combination of both layers. This state behaves as a superfluid for the layer density imbalance and displays several unusual transport properties including an analogue to the Josephson effect in which dissipationless tunneling between the layers can occur. In the first part of this talk, I will describe how a simple macrospin picture is able to capture the tunneling current-voltage characteristics of this state, and in particular those in which current is simultaneously injected through both rims of a Corbino annular device. In the second, I will describe an analogue of this system realized in XY magnets, and in particular how this principle can be exploited in the development of novel spintronic devices with non-local transport characteristics.
|13 Jan 2015
Keola Wierschem (National Taiwan University)
|Characterizing the Haldane Phase in Quasi-1D Systems|
Note: Seminar held at S16-06 (Graphene Theory Common)
The Haldane phase in antiferromagnetic spin-1 Heisenberg chains is an early example of a nontrivial symmetry protected topological (SPT) state. The topological nature of this phase is manifest by a hidden symmetry breaking characterized by nonlocal string order. The Haldane phase has been experimentally observed in low dimensional spin-1 quantum magnets, such as NENP and NDMAP. These systems possess weak interchain couplings that prevent any meaningful characterization by string order, so it is unclear whether or not they remain SPT states. Recently, a so-called strange correlator has been proposed to detect SPT states. Defined as a correlation function at the temporal boundary between an SPT state and a trivial product state, the strange correlator has been shown to be long-range or quasi-long-range in one or two dimensions. In this talk, I will describe how to measure the strange correlator using quantum Monte Carlo and present some results for the strange correlator in quasi-one-dimensional Haldane gap systems.