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|
Shyue Ping Ong (University of California, San Diego)
| Deep Learning Quantum Mechanics to Discover New Materials |
Powered by methodological breakthroughs and computing advances, electronic structure methods have today become an indispensable toolkit in the materials designer’s arsenal. In this talk, I will discuss two emerging trends that holds the promise to continue to push the envelope in computational design of materials. The first trend is the development of robust software and data frameworks for the automatic generation, storage and analysis of materials data sets. The second is the advent of reliable central materials data repositories, such as the Materials Project, which provides the research community with efficient access to large quantities of property information that can be mined for trends or new materials. I will show how we have leveraged on these new tools to accelerate discovery and design in energy and structural materials as well as our efforts in contributing back to the community through further tool or data development.
Alexander Tartakovskii (University of Sheffield, UK)
|Photonics and polaritonics with van der Waals heterostructures|
Monolayer films of van der Waals crystals of transition metal dichalcogenides (TMDs) are direct band gap semiconductors exhibiting excitons with very large binding energies and small Bohr radii, leading to a high oscillator strength of the exciton optical transition. Together with graphene as transparent electrode and hexagonal boron nitride (hBN) as an insulator, TMD monolayers can be used to produce so-called van der Waals heterostructures. Here we use this approach to make electrically pumped light-emitting quantum wells (LEQWs) [1,2] and single-photon emitters . We combine this new technology with optical microcavities to demonstrate control of the emitter spectral properties and directionality, making first steps towards electrically injected TMD lasers . Furthermore, by embedding MoSe2/hBN structures in tuneable microcavities, we enter the regime of the strong light-matter interaction and observe formation of exciton-polaritons . Here we demonstrate that the magnitude of the characteristic anti-crossing between the cavity modes and the MoSe2 excitons (a Rabi splitting) can be enhanced by embedding a multiple-QW structure, containing two MoSe2 monolayers separated by an hBN barrier. We extend this work to demonstrate valley addressable polaritons in both MoSe2 and WSe2, the property inherited from valley excitons, but strongly modified through changes in exciton relaxation in the strong-coupling regime . As the next step towards strongly interacting polaritons, we explore type-II semiconducting TMD heterostructures , where we observe Moire excitons and unusual optical selection rules.
Matjaž Humar (Jožef Stefan Institute, Slovenia)
|Micro-sized lasers inside live human cells |
Microlasers completely embedded within single live cells and biological tissues have been demonstrated. This includes the first laser inside a live cell. The lasers inside cells can act as very sensitive sensors, enabling us to better understand cellular processes. Each laser within a cell emits light with a slightly different fingerprint that can be used as a barcode to tag the cell. Up to a trillion cells (1,000,000,000,000) could be uniquely tagged. We have also realized that fat cells already contain lipid droplets that can work as natural lasers. Small lasers embedded in the sample can be used for novel nonlinear microscopy, including super resolution imaging. We have also embedded lasers into skin, which may enable new diagnostic, treatment and imaging tools in medicine and biology.
|13 Sep 2017
Abdullah Rasmita (NTU) & Weili Cheah (IHPC)
|Singapore Quantum Materials Seminar @ NUS |
Microsecond dark-exciton valley polarization memory in 2D heterostructures
|13 Sep 2017
Gareth Conduit (University of Cambridge, UK)
|Materials discovery with artificial intelligence |
We have developed a computational tool that employs deep learning with neural networks to discover new materials. The tool combines databases of experimental results with Density Functional Theory calculations to get high accuracy across a broad range of compositions. This enables us to propose materials that are most likely to fulfil multivariate targets. This holistic approach to materials design has allowed us to propose four new nickel-base alloys for use in jet engines, whose properties have been experimentally verified, new Lithium-ion battery cathode materials, and titanium alloys. The neural network approach to materials modelling can also assess the integrity of materials data. We have exploited this capability to automatically validate and correct entries in a commercial metal alloy and polymer database.
|Date / Speaker||Title / Abstract|
|30 Aug 2017
Eddwi Hesky Hasdeo (IHPC, Singapore)
|Novel plasmonics in topological edge states of gapped bilayer graphene |
Gapped bilayer graphene hosts topological edge states at domain walls separating two regions with opposite sign of gap. This domain walls can be created in a number of different ways; e.g. at stacking faults (AB- and BA-) or in a split dual-gate geometry wherein perpendicular applied electric field in adjacent regions have opposite signs. Interestingly, the edge states are gapless and carry valley polarized currents protected by time reversal symmetry. Here we predict that collective modes of these domain wall edge states, domain wall plasmons (DWPs), can exist even at zero bulk density, and possess a markedly different character from that of bulk plasmons. DWPs have a linear dispersion in contrast to the √q dispersion of 2D bulk plasmons. Forward and backward propagating modes of DWPs are valley polarized inherited from the topological edge states. Strikingly, the lifetimes of DWPs is very long, overcoming the transport scattering time in the bulk by orders of magnitude. DWPs possess a rich phenomenology including a wide range of frequencies (from the terahertz to the mid-infrared), sub-wavelength electro-magnetic confinement lengths, and tunable degrees of valley polarization.
|23 Aug 2017
Maxim Trushin (CA2DM, Singapore)
| Exciton spectrum in 2D transition metal dichalcogenides |
We develop an analytically solvable model able to qualitatively explain nonhydrogenic exciton spectra observed recently in two-dimensional (2D) semiconducting transition-metal dichalcogenides. Our exciton Hamiltonian explicitly includes additional angular momentum associated with the pseudospin degree of freedom unavoidable in 2D semiconducting materials with honeycomb structure. We claim that this is the key ingredient for understanding the non-hydrogenic exciton spectra that was missing so far.
|2 Aug 2017
Tobias Vogl (Australian National University)
|Experimental quantum optics using novel materials|
Although most research on 2D materials is targeting electronic applications, recent advances have opened a new platform for single photon generation based on these novel materials. Single photons are a key resource for quantum optics and optical quantum information processing. The integration of scalable room temperature quantum emitters into photonic circuits remains to be a technical challenge.
|14 Jun 2017
Philip W. Phillips (University of Illinois at Urbana-Champaign, USA)
|Detecting Anomalous Dimensions in the Strange Metal Phase of the Cuprates|
We all learned that conserved quantities such as the current in a metal cannot acquire an anomalous dimension in any theory that respects charge conservation. A recent theory of the strange metal of the cuprates has reached the conclusion that all of the properties of this phase can be understood if the current does in fact acquire an anomalous dimension. I will show how this seemingly contradictory prediction can be understood and also show that a finger print of such an anomaly is the Aharanov-Bohm flux through a strange metal ring. In the presence of an anomalous dimension, the AB phase deviates strikingly from the standard result and offers a precise diagnostic as to what is strange about the strange metal. I will also construct a Virasoro algebra for such anomalous currents and show that they correspond to a new class of non-local yet conformal theories.
|31 May 2017
Thomas Whitcher (NUS Singapore Synchrotron Light Source & CA2DM
|X-ray Diagnostic Techniques and Advanced Materials Research at the Singapore Synchrotron Light Source|
X-ray diagnostic techniques are very valuable tools when it comes to investigating and manipulating properties of novel and advanced materials. Techniques such as Resonant Soft X-ray Scattering (RSXS), X-ray Absorption Spectroscopy (XAS), X-ray Magnetic Circular Dichromism (XMCD), Spin and Angle-Resolved Photoemission Spectroscopy (S-ARPES), Spectroscopic Ellipsometry and many others provide vast amounts of information on materials such as Perovskites, topological insulators and high-transition temperature semiconductors. At the Singapore Synchrotron Light Source (SSLS), there are two beam-lines whose primary focus is research into advanced materials. These are the Surface, Interface and Nano-structure Science (SINS) beam-line and the newly completed Soft X-ray - Ultraviolet (SUV) beam-line. In this seminar I will describe the multiple X-ray diagnostic techniques that the beam-lines will be able to employ and elaborate on how they can be used to explore the fundamental properties of a variety of advanced materials.
|3 May 2017
Lee Ching Hua (A*STAR IHPC, Singapore)
|Topological phases from circuits to black holes|
In the last decade, topology has taken a prominent role in various subdisciplines of physics. On one hand, it has initiated a race for realizing topological phases in systems from quantum wells to photonic crystals, with breakthroughs leading to potentially impactful technological applications. On the other hand, topology has also helped forge connections between abstract theoretical physics and concrete experimental phenomena. In this talk, I shall illustrate how deep physics can emerge in very simple and experimentally accessible systems through four diverse yet related examples. The first system is a mechanical driven (Floquet) Chern insulator comprising a lattice of electromagnets controlled by AC currents. This mechanical realization affords an extremely intuitive visualization of topological edge state dynamics in terms of Newton's laws, with the AC input offering unprecedented tunability for the study of non-equilibrium effects. The second system is a 2D photonic crystal with line nodes and type I and II Dirac cones protected by nonsymmorphic symmetry. Photonic crystals are particularly suitable for topological bandstructure engineering since they can be relatively easily fabricated with features possessing any desired symmetry, size and periodicity. The third system showcases that generically, electric RLC circuits can be engineered to possess peculiar "topolectrical" resonances corresponding to Fermi arcs in the circuit Laplacian. In particular, I will motivate the construction of a circuit analog of a Weyl semimetal by progressively building up upon lower-dimensional RLC circuits easily produced in undergraduate labs. The fourth and final system is a 3D holographic topological insulator living on a tree-like hyperbolic lattice also realizable with RLC elements. Its construction is inspired by the famed anti deSitter space - conformal field theory (AdS-CFT) correspondence in high energy physics, where a surprising equivalence was found between 3+1 D super Yang-Mills theory and a 4+1 D gravitational theory at the partition function level. Starting from an ordinary Chern 2D insulator, an emergent 3D Z2 topological insulator can be shown to exist in the holographic bulk. By appealing to the Ryu-Takayanagi formula, this 3D bulk is shown to possess a black hole event horizon of radius proportional to its temperature. At a deeper level, the holographic mapping not only provides another notion of bulk-boundary correspondence for topological systems, but also elucidates the connection between the chiral anomaly of the 2D boundary with the parity anomaly of the 3D bulk. All in all, these four systems provide novels ways whereby deep physics can be illustrated through experimentally realistic topological phases, in or out of equilibrium.
|Date / Speaker||Title / Abstract|
|26 Apr 2017
M. Raju & Likun Shi (NTU Physics & A*STAR IHPC)
|Singapore Quantum Materials Seminar @ NUS|
Talk 1: Topological transport and microscopy studies of magnetic skyrmions in Ir/Fe/Co/Pt multilayers
M. Raju --- Christos Panagopoulos group, Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore
Magnetic skyrmions are promising candidates for memory applications due to their small size, topological stability, and mobility. However, materials and techniques for studying tunable skyrmions are yet be realized. I will discuss the development of a novel multilayer system offering tunability of skyrmion properties like size, density and ease of nucleation. Using magnetotransport and local microscopy experiments our results demonstrate the topological Hall effect as an electrical means of studying skyrmions in tailor-made magnetic multilayers for room temperature applications.
Talk 2 Dirac semimetal Fermi arc states and its optical response Likun Shi --- Institute of High Performance Computing, A*STAR
We will present a theoretical discussion on Dirac semimetal Fermi arc states and its optical response. We show that the Fermi arc states possess strong/weak optical response when the light is polarized along/transverse the Fermi arc. The large optical weight persist as a large Drude weight of Fermi arc carriers when the system is doped.
|19 Apr 2017
Slaven Garaj (NUS)
|Nanofluidics with two-dimensional materials|
Curious behavior of water and ions in constrictions with dimensions comparable to the size of ions are of particular interest for many applications, including filtration membranes, single-biomolecule analysis, supercapacitors, etc. The nanofluidic behavior of such structures depends on their dimensionality: ranging from the edge-enhanced ionic current in 0D graphene nanopores [1,2], anomalous ionic flow in 1D nanotubes, to frictionless water transport in 2D graphene  and graphene-oxide nanochannels [4, 5].
We set to investigate ionic flow in graphene-based nanostructures, including scalable GO membranes, and model systems consisting of individual graphene channels only about 1 nm in height. By measuring mobility of a wide selection of aqueous salts ions in channels of GO membranes , we demonstrated that the dominant mechanisms for the ion rejection are (a) size exclusion due to compression of the ionic hydration shell in narrow channels; and (b) electrostatic repulsion due to the membrane surface charge.
Armed with the insight into the physical mechanism governing the ionic flow, we are able to engineer new membranes with decreased the ionic cut-off size and increased charge selectivity. At the end, I will present some new results leading to promising applications in desalination and electrodialysis.
|05 Apr 2017
Martial Ducloy (Université Paris 13 & NTU)
|Atom-Metamaterial resonant coupling: a model of a quantum hybrid system|
In recent years, investigations of the coupling between atomic systems and material nanostructures have undergone fast development, with their potential applications in nanotechnologies and quantum information. When the coupling gets resonant, it involves important modifications on the radiation of quantum emitters (emission pattern, transition probabilities, branching ratios, etc.) as well as energy level shifts. I will present theoretical and experimental studies of the resonant coupling between Cesium atoms and 2D-nanostructured metallic metamaterial («metasurface»). Via the modification of the metamaterial geometry, the frequency of the localised plasmon resonance is tuned over the Cs optical resonance at 852nm, implying both dispersive and dissipative resonant coupling. Our experimental approach is based on selective reflection of resonance light at the Cs vapour – metamaterial interface. It allows us to monitor the Casimir-Polder interactions between Cs atoms and the metamaterial, and their resonant tuning via the metamaterial geometry. Extensions of this work will be discussed.
|29 Mar 2017
Frank Watt (NUS Centre for Ion Beam Applications)
|Fast protons: from super-resolution microscopy to cancer therapy|
The Centre for Ion Beam Applications, Dept of Physics, is an interdisciplinary research centre which was set up around 15 years ago to investigate the potential of fast ions [ie million volt proton and helium ions] in a wide range of disciplines. CIBA has pioneered many unique technologies since its inception, including proton beam writing, silicon micromachining, and fast ion beam microscopy [which is being used for cell imaging at super-resolutions - resolutions better than the diffraction limit of light]. Singapore is constructing a new National Cancer Centre [NCCS] building [costing around 55 0.75 billion], situated in the Outram Park Campus [56H]. The new Centre will include a state-of-the—art proton therapy facility utilising fast protons for cancer treatment. It is well known that proton beam therapy can target tumours with greater accuracy and with less collateral damage to surrounding healthy tissue compared with traditional radiation therapy using X-rays. At the cellular level however, the localised action of fast protons inside the cell nucleus is not well understood. This talk will describe the development and underlying principles of fast ion microscopy in CIBA. the basics of proton beam therapy, and show how these two programmes can be linked via a recently funded new research programme into single cell proton radiobiology to be carried out at CIBA.
|22 Mar 2017
Sarah Demers (Yale University, USA)
|An update from CERN's Energy Frontier|
Since discovering the Higgs boson in 2012, the ATLAS and CMS Experiments have been hard at work characterizing this particle and searching for other evidence of physics beyond the Standard Model. CERN's Large Hadron Collider delivered a substantial dataset in 2016, and we anticipate even more integrated luminosity in 2017. In this talk I will describe where we stand in terms of Higgs boson measurements at ATLAS, and our strategy for searches for new physics.
|1 Mar 2017
Peter Hänggi (University of Augsburg, Germany)
|(Quantum) Thermodynamics at strong coupling and its implications for Stochastic Thermodynamics|
The case of strong system-environment coupling plays an increasingly important role when it comes to describe systems of small size which are in contact with an environment. The commonly known textbook situation refers solely to a weak coupling situation for which the equilibrium state of the system is described by a Gibbs state. This situation changes drastically, however, when strong coupling is at work; then, the interaction energy can be of the order of the (sub)-system energy of interest . Let us consider first an overall thermal equilibrium of a total setup composed of a system Hamiltonian HS, coupling Hamiltonian Hint and a bath Hamiltonian HB.
|15 Feb 2017
Loren Alegria (Princeton University, USA)
|Topological Insulator Nanostructures and Devices |
2D materials afford a natural solution to interfacial issues encountered in scaling electronic devices. Among 2D materials, the layered topological insulators have extreme spin-orbit coupling and a unique quintuple layer structure, enabling the creation of unprecedented nanostructures and spintronic devices. We present measurements of topological insulator nanostructures, focusing on high quality quantum wires, heterostructures between topological insulators and ferromagnetic insulators, and self-assembled topological insulator nanotubes.
|8 Feb 2017
Peng Weng Kung (Nanyang Technological University)
|Rapid and Label-free Magnetic Resonance based Molecular Phenotyping of Oxidative Stress of Diabetes Mellitus |
Diabetes mellitus (DM) is one of the fastest growing health burdens that is projected to affect 592 million people worldwide by 2035 . In this talk, a new paradigm of molecular phenotyping using the fundamental of spin physics for clinical diabetes care and management, will be introduced. Specifically, we aim to establish high throughput oxidative stress and metabolic phenotyping platform for DM. We have recently shown that various pathological states (e.g., oxidative stress of diabetes mellitus , malaria infection [3,4], and blood oxidation/oxygenation level [5,6]) can be rapidly captured (< 5 minutes) by mapping out the redox properties of blood (< 10 µL) using inexpensive, home-built, microscale magnetic resonance (MR) technology. A range of microscale MR technology (relaxometry, spectroscopy, imaging) developed in the pipeline will be used as the platform in translating a new class of functional biomarkers for DM medical research and clinical practices. In addition, the speaker will discuss briefly the newly developed ultrafast multidimensional relaxographic imaging  for next generation of rapid and label-free molecular phenotyping. We demonstrate its clinical utilities in sub-phenotyping the hemoglobinopathies (e.g., thalassemia), assessment of glucose toxicity in pancreatic islets and sub-stratification of endometriosis disease.
|25 Jan 2017
Siddarth Saxena (University of Cambridge, UK)
|Tuneable Spin and Charge Phenomena|
Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new emergent effects, like the breakdown of the conventional Fermi-liquid theory in metals has been focused in narrow band electronic systems. Ferroelectric crystals provide a very different type of quantum criticality that arises purely from the crystalline lattice. In many cases the ferroelectric phase can be tuned to absolute zero using hydrostatic pressure. Close to such a zero temperature phase transition, the dielectric constant and other quantities change into radically unconventional forms due to the fluctuations experienced in this region. The simplest ferroelectrics may form a text-book paradigm of quantum criticality in the solid-state where there are no complicating effects of electron damping of the quantum charge fluctuations. We present low temperature high precision data demonstrating these effects in pure single crystals of SrTiO3 and KTaO3. We outline a model for describing the physics of ferroelectrics close to quantum criticality and highlight the expected 1/T2 dependence of the dielectric constant measured over a wide temperature range at low temperatures. In the neighbourhood of the quantum critical point we report the emergence of a small frequency independent peak in the dielectric constant at approximately 2K in SrTiO3 and 3K in KTaO3. Looking to the future, we imagine that quantum paraelectric fluctuations may lead to new low temperature states and mediate novel interactions in multi-ferroic systems (e.g. EuTiO3) and ferroelectric crystals supporting itinerant electrons.
|18 Jan 2017
Won Jong Yoo (Sungkyunkwan University, Korea)
|Carrier Transport at the interface of 2-dimensional materials |
Two dimensional (2D) materials are being investigated very intensively, with some of them holding great promise as semiconducting materials for future low power nano-electronics, as they present a range of achievable bandgaps and ultra-thin body with efficient electrostatic control. These properties, combined with mechanical flexibility, enable 2D materials to be very promising candidates that can meet major requirements for electronic and photonic devices operated in emerging future mobile and IoT environment.
|11 Jan 2017
Adolfo del Campo (University of Massachusetts, USA)
|Engineering Quantum Thermal Machines |
Quantum thermodynamics has the potential to impact energy science. Yet, the identification of scenarios characterized by a quantum supremacy, unmatched by the classical counterpart, remains challenging. In this talk I shall review recent advances in the engineering and optimization of quantum thermal machines. I will show that nonadiabatic many-particle effects can give rise to quantum supremacy in finite-time thermodynamics. Further, quantum heat engines can be operated at maximum efficiency and arbitrarily high output power by making use of shortcuts to adiabaticity. A thermodynamic cost of these shortcuts will be elucidated by analyzing the full work distribution function and introducing a novel kind of work-energy uncertainty relation. I shall close by discussing the identification of scenarios with a quantum-enhanced performance in thermal machines run over many cycles.