CIQM 2015 Micro-Presentation Abstracts
9:10-10:00 Micro-Presentation Group 1
1. Kwabena Bediako
Postdoc, Harvard University
Electrochemical Doping of hBN-encapsulated Few-layer Black Phosphorus
D. Kwabena Bediako, Philip Kim
Black phosphorus (bP) has emerged in recent years as a particularly exciting material for optoelectronic applications. Its unique anisotropic transport behavior also offers an interesting handle for fundamental studies of charge transport in two-dimensional layered systems. We are interested in investigating the transport behavior of bP upon electrochemical doping of few-layer phosphorene crystals. In this regard we are investigating both electrochemical double-layer doping as well as electrochemically controlled intercalation reactions. Encapsulation of few-layer bP flakes in an inert Van der Waals material like hexagonal boron nitride is an important step to avoid potentially detrimental electrochemical side reactions. In addition, the anisotropic transport and ion diffusion behavior of bP demands a knowledge of the crystallographic orientation of each flake, which we identify by angle-resolved polarized Raman spectroscopy. Here we present our early forays into the afo! rementioned studies.
2. Cui-Zu Chang
Postdoc, MIT
High-precision Realization of Robust Quantum Anomalous Hall State in a Hard Ferromagnetic Topological Insulator
Cui-Zu Chang, Jagadeesh S. Moodera
The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two dimensional electron layer under a strong magnetic field. The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon as the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism.Here we report the experimental observation of the QAH state in V-doped (Bi,Sb)2Te3 films with the zero-field longitudinal resistance down to 0.00013±0.00007h/e2 (~3.35±1.76W), Hall conductance reaching 0.9998±0.0006e2/h and the Hall angle becoming as high as 89.993±0.004º at T=25mK. Further advantage of this system comes from the fact that it is a hard ferromagnet with a large coercive field (Hc>1.0T) and a relative high Curie temperature. This realization of robust QAH state in hard FMTIs is a major step towards dissipationless electronic a! pplications without external fields.
3. Michelle Chavis
Postdoc, Howard University
Diblock Copolymers for Diamond Patterning and Applications
Michelle A. Chavis, Lorelis Gonzalez-Lopez, Crawford Taylor, James Griffin, Kimberly Jones, Gary L. Harris
Nitrogen-vacancy (NV-) centers have been recently forecasted as possible quantum bits – the fundamental unit of quantum information. Their properties include long spin lifetimes and the ability to store quantum information and transmit it in the form of light. However, interaction with their environment leads to decoherence in the presence of nearby nitrogen spins. The key to enhancing NV- centers is to control their location and reduce their interaction with the environment. Here, we present an innovative method that allows for control over the size and location of diamond nanopillars (DNPs).
4. Nathalie de Leon
Postdoc, Harvard University
Diamond nanophotonics for Solid State Quantum Optics
Nathalie de Leon, Mikhail Lukin
Large-scale quantum networks will require efficient interfaces between photons and stationary quantum bits. Nitrogen vacancy (NV) centers in diamond are a promising candidate for quantum information processing because they are optically addressable, have spin degrees of freedom with long coherence times, and as solid-state entities, can be integrated into nanophotonic devices. An enabling feature of the NV center is its zero-phonon line (ZPL), which acts as an atom-like cycling transition that can be used for coherent optical manipulation and read-out of the spin. However, the ZPL only accounts for 3-5% of the total emission, and previously demonstrated methods of producing high densities of NV centers yield unstable ZPLs.
I will present methods and technologies for gaining both spectral and spatial control over NV emission by coupling NV centers to nanophotonic devices. In particular, we have developed a method to create a high-density device layer of NVs with stable ZPLs in high purity diamond, and have devised a fabrication scheme to carve single mode waveguides out of the surface of the bulk diamond substrate. Using this technique, we are able to fabricate high quality factor, small mode volume photonic crystal cavities directly out of diamond, and deterministically position these photonic crystal cavities so that a stable NV center sits at the maximum electric field. We observe an enhancement of the spontaneous emission at the cavity resonance by a factor of up to 100. The NV emission is guided efficiently into a single optical mode, enabling integration with other photonic elements, as well as networks of cavities, each with their own optically addressable qubit. These nanophotonic eleme! nts in diamond will provide key building blocks for quantum information processing such as single photon transistors, enabling distribution of entanglement over quantum networks.
5. Ahmet Demir
Graduate Student, MIT
Capacitance Measurements on Transition Metal Dichalcogenides
Ahmet Demir, Raymond Ashoori
Transition metal dichalcogenides(TMDCs) are layered materials with tunable direct bandgap structure. This feature allows them to be open to a variety of applications and rich physics. We investigate atomically thin tungsten diselenide(WSe2) using capacitance measurements since it overcomes the high contact resistance problem to monolayer flakes. Here we present the capacitance data that shows the carrier population and depopulation as a function of gate voltage. We also observed some unpredicted capacitance peaks which needs further investigation.
6. Chen Fang
Postdoc, MIT
Nonsymmorphic Topological Crystalline Insulators in Three Dimensions
Chen Fang, Ling Lu, Marin Soljacic, Liang Fu
We study a new class of three-dimensional topological crystalline insulators protected by a single glide reflection symmetry. The Z2 classification is shown using both an analysis of the surface spectral flow as well as a 3D bulk invaraint. We propose one realization for this nontrivial phase in the photon bands of a photonic crystal.
7. Shiang Fang
Graduate Student, Harvard University
Ab-initio Tight-Binding Hamiltonian for 2D Materials
Shinag Fang, Bertrand I. Halperin, Efthimios Kaxiras
With the advances of material fabrications and measurements, 2D layered materials have been the focus of investigations, especially the interplay between topological phases, superconductivity, magnetism, charge density waves. We present accurate ab initio tight-binding hamiltonians for the 2D materials of the transition-metal dichalcogenides and graphene bilayers. We also elucidate the role played by the interlayer coupling, which is a crucial element in utilizing these layered materials. These tight-binding schemes of calculations are efficient methods of van der Walls heterostructure design.
10:20-11:10 Micro-Presentation Group 2
8. Jundong Wu
Graduate Student, Harvard University
Hydrodynamic Oscillator with 2D Electron Gas
Jundong Wu, Donhee Ham
The rapid oscillations of the electron density in conductors form plasmonic waves. A typical example of plasmonic wave would be the surface plasmons on bulk metals. While the surface plasmons usually appears in optics regime, plasmonic waves in 2D conductors can achieve a frequency that lies in the GHz regime. For electrons in such a system, the equation of motion contains a hydrodynamic term under certain DC current, which would give a reflection coefficient larger than unity on an AC open boundary. This gain mechanism provides our group an idea, that we can make use of such gain and build an amplifier, or even an oscillator in GHz regime.
9. Delroy Green
Graduate Student, Howard University
Exploration of Boron doping in Diamond for Superconductivity
Delroy Green, Breyonna Pinkney, Gary Harris
Diamond has extremely rare properties that are impressive to scientist and the public at large. Diamond is an insulator, however when doped with impurities, can exemplify properties of a semiconductor. However when diamond is very heavily doped it can behave as a superconductor at very low temperatures. In this work we investigated two different techniques for doping diamond. The first technique included pulsed laser deposition of boron onto a silicon wafer followed by nano-diamond seeding and subsequent growth. The second technique involved the insertion of boron powder around the sample holder to dope diamond during growth. Both techniques yielded highly doped p-type diamond films.
10. Jin-Yong Hong
Postdoc, MIT
Synthesis of High-quality Multi-layer h-BN and Large Single-crystal Mono-layer Graphene
Jin-Yong Hong, Jing Kong
Large-area and high-quality h-BN films were obtained by using CVD on a Fe foil with a slow cooling rate, which allows boron and nitrogen to diffuse out of the iron surface to form multi-layer h-BN. Graphene, MoS2, and WSe2 FET devices on a h-BN substrate were also fabricated, resulting in carrier mobilities of graphene, MoS2, and WSe2 to improve up to ~ 24,000, 40, and 9 cm2V-1s-1, respectively, indicating the compatibility of our h-BN substrate for 2D electronics.
In addition, uniform bi-/multi-layers-free monolayer graphene is obtained using Cu enclosures with a W foil enclosed. The bi-/multi-layers underneath the monolayer graphene are selectively removed while the monolayer remains intact. The W foil plays an important role in maintaining a low carbon
11. Dennis Huang
Graduate Student, Harvard University
Defect Engineering of Superconducting FeSe Thin Films
Dennis Huang, Efthimios Kaxiras, Jennifer E. Hoffman
FeSe possesses the simplest structure among the iron-based superconductors, consisting of triple layers (Se-Fe-Se) stacked by van der Waals forces, with no buffer layers. The ability to grow FeSe films layer-by-layer using molecular beam epitaxy (MBE) has opened the door to engineering novel structures that enhance or harness its superconducting properties. For example, by interfacing a single layer of FeSe with SrTiO$_3$(001), the superconducting transition temperature is increased from 9 K up to 110 K. Given its short coherence length, a further possibility is to utilize defects that locally weaken superconductivity to define nanostructures. In this poster, we report the growth of FeSe films under excess Se flux. Using scanning tunneling microscopy (STM), we image prevalent defects with dumbbell signatures. We employ density functional theory (DFT) to investigate candidate defect sites, their formation energies, and diffusion barriers. These results shed light on the e! pitaxial growth of 2D-layered FeSe and suggest new avenues of defect engineering in this material for superconducting applications.
12. Changmin Lee
Graduate Student, MIT
Direct Measurement of Ferromagnetism Induced
at the Interface of a Magnetic Topological Insulator
Changmin Lee, Nuh Gedik
When a topological insulator (TI) is brought in contact with a ferromagnetic insulator (FMI), both time reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-ranged exchange interactions. These magnetic topological insulators can host various exotic phenomena, such as massive Dirac fermions, the topological magnetoelectric effect, Majorana fermions, and the quantum anomalous Hall effect (QAHE). However, selectively measuring magnetism induced at the buried interface has remained a challenge. Using magnetic second harmonic generation (MSHG), we directly probe magnetism induced at the interface between the ferromagnetic insulator EuS and the three-dimensional TI Bi2Se3. We simultaneously measure both the in-plane and out-of-plane magnetizations at the interface through the nonlinear Faraday effect. Our findings not only allow us to characterize magnetism at the TI! -FMI interface, but also reveal the possible existence of quantum well states (QWS) confined within the finite size of the thin film.
13. Sidi Maiga
Graduate Student, Howard University
Simulations of Adsorption of Carbon Dioxide and Methane on Graphene Sheet
Sidi M. Maiga, Silvina Gatica
Adsorption is defined as the attachment of atoms, or molecules of a gas, liquid or dissolved solid onto a surface, creating a film or monolayer of material onto the adsorbing surface. Using the
Method of Grand Canonical Monte Carlo we computed the adsorption of carbon dioxide (CO2) and methane (CH4) on a monolayer graphene sheet, at various temperatures for each gas. For each temperature, we compute the adsorption isotherm, Energy gas-surface and Energy gas-gas. We compare the uptake pressures of CO2 and CH4.
14. Thomas Markovich
Graduate Student, Harvard University
Enabling Large-scale Simulation of Many-Body Dispersion Forces in Condensed Phase Systems
Thomas Markovich, Alan Aspur-Guzik
Dispersion interactions are ubiquitous in nature, and extremely important for explaining the structure and function of many systems, from soft matter to surfaces and solids. Due to their long-range and scaling with system size, dispersion interactions can prove particularly important in modeling nanostructured systems where reduced dimensionality creates large polarizable surfaces. Standard pairwise approximations are insufficient for such systems, and the true non-additive and many-body character of dispersion plays a crucial role. The many-body dispersion (MBD) method of Tkatchenko and co-workers [A. Tkatchenko et al., Phys. Rev. Lett. 108, 236402 (2012); A. Ambrossetti et al., J. Chem. Phys. 2014, 140, 18A508] seeks to address this behavior by computing the full many-body correlation energy for a fictutious set of coupled quantum harmonic oscillators that mimic the fluctuations of the real polarizable valence-electron density. Much of the work on MBD, to date, has foc! used on the energetics of various molecules and materials, with all necessary gradient information being obtained through numeric differentiation. We recently presented an implementation of the relevant analytic gradients with respect to nuclear displacements, which permitted fast and accurate geometry optimizations of many gas-phase systems and showed that PBE+MBD geometries matched those of highly accurate wavefunction theories at a fraction of the cost. In this talk I will describe an efficient implementation of the MBD energy and analytic gradients, which has enabled their application to larger simulations of condensed-phase systems. I will show examples of geometry and unit-cell optimizations with MBD corrections in the condensed and gas phase.
11:30 – 12:30 Micro Presentation Group 3
15. Raymond Otchere-Adjei
Graduate Student, Howard University
Doping of Graphene by Non-covalent Chemical Functionalization
Raymond Otchere-Adjei, Charles M. Hosten
Graphene is a single layer of graphite, made up of sp2 hybridized carbon atoms arranged in a honeycomb lattice. Graphene exhibits exceptional properties such as high conductivity, high carrier mobility, and optical transparency unmatched by any material. These properties along with its unique chemical structure make graphene ideal for electronic applications. The goal of our study is to produce graphene based materials with potential applications in fabrication of Nanoelectronic and Optoelectronic devices. This requires developing the capability to tune the electrical properties of Single Layer Graphene (SLG) while maintaining its unique chemical structure. Doping by non-covalent chemical functionalization has proven to be a feasible method for tuning graphene’s fermi level without perturbing the carbon lattice. Non-covalent doping of graphene with organic molecules can occur either by spontaneous charge transfer or π-π interaction of the dopant molecule with graphen! e. By applying surface transfer doping by techniques such as drop-casting, dip-coating, and spin-coating Acridine Orange hydrochloride hydrate and Sodium anthraquinone-2-sulfonate have shown significant effect as n and p type dopants of graphene respectively. The extrinsic semiconductor properties are characterized with Raman Spectroscopy, charge transfer by X-ray Photoelectron Spectroscopy (XPS), and conductivity by field effect transistor (FET).
16. Maoz Ovadia
Postdoc, Harvard University
Dual STM/SFM for Imaging of Vortex Bound States in Superconducting Graphene
Maoz Ovadia, Jennifer E. Hoffman
Low energy bound states are predicted to exist in the core of superconducting vortices. In the case of graphene coupled a superconductor, the energy spacing is expected to be large, allowing spectroscopy of these states. To this end we have built a scanning probe microscope which used a sharp tip on one prong of a quartz resonator to sense both force and tunneling current. The microscope system allows UHV sample exchange and operation at temperatures from 2K to 340K and at magnetic fields up to 5T.
17. Falko Pientka
Postdoc, Harvard University
Localization of Majorana States in Atomic Chains
Falko Pientka, Bertrand I. Halperin
A recent experiment [Nadj-Perge et al. Science 346, 602 (2014)] gives possible evidence for Majorana bound states in chains of magnetic adatoms placed on a superconductor. While many features of the observed end states are naturally interpreted in terms of Majoranas, their strong localization remained puzzling. Here, we show that Majorana states in proximity-coupled systems may be localized on scales much smaller than the coherence length of the host superconductor as a consequence of a strong velocity renormalization.
18. Hechen Ren
Graduate Student, Harvard University
Controlled Finite Momentum Pairing and Spatially Varying Order Parameter in Proximitized HgTe Quantum Wells
Hechen Ren, Amir Yacoby
Conventional s-wave superconductivity is understood to arise from singlet pairing of electrons with opposite Fermi momenta, forming Cooper pairs whose net momentum is zero. Several recent studies have focused on structures where such conventional s-wave superconductors are coupled to systems with an unusual configuration of electronic spin and momentum at the Fermi surface. Under these conditions, the nature of the paired state can be modified and the system may even undergo a topological phase transition. Here we present measurements and theoretical calculations of several HgTe quantum wells coupled to either aluminum or niobium superconductors and subject to a magnetic field in the plane of the quantum well. By studying the oscillatory response of Josephson interference to the magnitude of the in-plane magnetic field, we find that the induced pairing within the quantum well is spatially varying. Cooper pairs acquire a tunable momentum that grows with magnetic field str! ength, directly reflecting the response of the spin-dependent Fermi surfaces to the in-plane magnetic field. In addition, in the regime of high electron density, nodes in the induced superconductivity evolve with the electron density in agreement with our model based on the Hamiltonian of Bernevig, Hughes, and Zhang. This agreement allows us to quantitatively extract the ratio of the effective g-factor to the Fermi velocity. However, at low density our measurements do not agree with our model in detail. Our new understanding of the interplay between spin physics and superconductivity introduces a way to spatially engineer the order parameter, as well as a general framework within which to investigate electronic spin texture at the Fermi surface of materials.
19. Khosro Shirvani
Postdoc, Howard University
Fabrication of Bismuth Telliride (Bi2Te3) Nanowire Arrays (NWA) by High Pressure Injection (HPI)
Khosro Shirvani, Tito Huber, Henry Snyder
20. Oles Shtanko
Graduate Student, MIT
Guided Electron Waves in Graphene
Oles Shtanko, Leonid Levitov
Significant progress in quantum optics in the last decade stimulate the search for solid state analogs in order of compactification and easy scaling. One of the most promising materials is graphene which spectrum of charge carriers resemble light dispersion. Similar to photons, electrons in graphene nanostructures propagate ballistically over macroscopic distances, and this regime persist up to room temperatures. We show that one-dimensional potential in graphene engineered in the bulk either naturally arising at the edge can provide an analogue of photon fibers in optics. Recent experimental data using superconducting Josephson junctions give a strong evidence of existence of such modes at the graphene edge. Also, we will show that for gapped case guided states can exist in the gap which makes them easily detectable separately from bulk states.
21. Young-Ik Sohn
Graduate Student, Harvard University
Enhanced Strain Coupling of Nitrogen Vacancy Spins Tto NanoscaleDiamond Cantilevers
Young-Ik Sohn, Marko Loncar
Nitrogen vacancy (NV) centers coupled to diamond mechanical resonators via strain have recently gained attention as a novel system, which can enable hybrid quantum networks and quantum metrology. Access to the strong coupling regime of this system requires mechanical resonators with nanoscale dimensions in order to overcome the inherently weak strain response of the NV ground state spin. In this work, we incorporate NV centers in diamond cantilevers with sub-micron dimensions. Coupling of the NV ground state spin to the mechanical mode is detected in electron spin resonance (ESR) spectra, and its temporal dynamics are measured via spin echo. Our small mechanical mode volumes lead to a significantly improved spin-phonon coupling strength over previous NV-strain coupling demonstrations.
22. Jordan Stroman
Graduate Student, Howard University
Optically Detected Magnetic Resonance of NV Diamond Centers
Jordan Stroman, Gary Harris
2:30 – 3:20 Micro Presentation Group 4
23. Felix von Cube
Postdoc, Harvard University
Imaging Quantum Materials with Electron Microscopy
Felix von Cube, David C. Bell
We investigate quantum materials with high resolution transmission electron microscopy (HR-TEM). The microscopes allow for atomic resolution imaging, which helps to understand the astonishing mechanical and electric properties of hybrid quantum materials. Our research currently focuses on graphene-based quantum materials, as well as topological insulators and nitrogen vacancies in diamond nano-particles.
24. Amber Wingfield
Postdoc, Howard University
Nitrogen and Boron Doping of HF-CVD Diamond
Amber Wingfield, Gary Harris
Doping diamond for the production of nitrogen-vacancy (NV) centers has recently come to the forefront as a potential candidate for quantum computing. In a similar light, boron doping in diamond has become of interest due to its potential to improve electronic applications. Growth and doping of diamond can be achieved by way of employing hot filament chemical vapor deposition (HF-CVD). Thus, understanding how dopant concentration is effected during diamond growth is of interest. Therefore, the focus of our current research is to separately determine nitrogen and boron incorporation in relation to diamond growth conditions. This will allow leverage into determining the relationship between nitrogen concentration and NV-center incorporation, as well as, determining the relationship between boron incorporation and activated boron concentration. Prepared silicon and silicon carbide substrates are placed into a HF-CVD environment with conditions set for diamond production. Wit! hin this environment, the substrates are doped with nitrogen or boron and subjected to varying growth conditions, such as pressure and flow rate changes. Secondary ion mass spectroscopy (SIMS analysis) is then conducted to provide insight on how nitrogen and boron concentration changes occurred during growth.
25. Yuan Yang
Graduate Student, Harvard University
Theory of Graphene Raman Spectroscopy
Yuan Yang, Eric J. Heller, Lucas Kocia, Wei Chen, Shiang Fang, Mario Borunda, Efthimios Kaxiras
Raman spectroscopy plays a key role in studies of graphene and related carbon systems. Graphene is perhaps the most promising material of recent times for many novel applications, including electronics. The traditional and well established Kramers-Heisenberg-Dirac (KHD) Raman scattering theory (1925-1927) is extended to crystalline graphene for the first time. It demands different phonon production mechanisms and phonon energies than does the popular "double resonance" Raman scattering model. The latter has never been compared to KHD. Within KHD, phonons are produced instantly along with electrons and holes, in what we term an electron-hole-phonon triplet, which does not suffer Pauli blocking. A new mechanism for double phonon production we name "transition sliding" explains the brightness of the 2D mode and other overtones, as a result of linear (Dirac cone) electron dispersion. Direct evidence for sliding resides in hole doping experiments performed in 2011. Whole rang! es of electronic transitions are permitted and may even constructively interfere for the same laser energy and phonon q, explaining the dispersion, bandwidth, and strength of many two phonon Raman bands. Graphene's entire Raman spectrum, including dispersive and fixed bands, missing bands not forbidden by symmetries, weak bands, overtone bands, Stokes anti-Stokes anomalies, individual bandwidths, trends with doping, and D-2D band spacing anomalies emerge naturally and directly in KHD theory.
26. Linda Ye
Graduate Student, MIT
Anomalous Hall Effect in a Kagome Ferromagnet
Linda Ye, Joseph Checkelsky
Ferromagnetic Kagome lattice is know theoretically to possess topological electronic structures. We have synthesized large single crystals of a Kagome ferromagnet Fe3Sn2 which orders well above room temperature. We have observed a large anomalous Hall effect, which is an sensitive probe to the topological and geometrical structure of the electronic states. In addition, we also find signatures of unconventional ferromagnetism in the magnetic susceptibility.
27. Lili Yu
Graduate Student, MIT
CVD Mos2 Electronics: Devices to Systems
Lili Yu, Dina El-Damak, Sungjae Ha, Elaine McVay, Xi Ling, Anantha Chandrakasan,Jing Kong and Tomas Palacios
28. Xingyu (Alex) Zhang
Graduate Student, Harvard University
Nanopillar-based Optical Cavities for Nitrogen-vacancy Centers in Diamond
Xingyu (Alex) Zhang, Evelyn Hu, Shanying Cui, Tsung-li Liu, Jonathan Lee, David Bracher, Kenichi Ohno, David Awschalom
Nitrogen-vacancy (NV) centers in diamond have diverse applications in quantum information processing and field sensing. We have designed nanopillar-based photonic crystal (PhC) and hybrid plasmonic-PhC cavities to provide selective enhancement and improve collection efficiency of NV emission. The hybrid cavities were fabricated using thinned diamond membranes and characterized using confocal photoluminescence (PL) spectroscopy. The PhC structures were created using patterned growth of diamond via chemical vapor deposition on a single-crystal substrate. Such a technique minimizes damage introduced to the NV host lattice (e.g. during top-down fabrication). Preliminary results for the PhC structures show cavity modes as well as bright luminescence from NVs and silicon vacancy centers.
29. Shu Yang Frank Zhao
Graduate Student, Harvard University
Lithium Intercalation of Single-Layer Graphene / Boron Nitride Heterostructures
S.Y.F. Zhao, G.A. Elbaz, C. Yu, D.K. Bediako, Y. Guo, K. Watanabe, T. Taniguchi, L. Brus, X. Roy, & P. Kim
Few layer graphene (FLG) intercalate compounds form a new generation of graphene derivative systems where carrier densities are expected, and novel physical phenomena such as superconductivity and magnetism may emerge. Experimental realization of intercalated FLGs have been limited by harsh intercalation processes which are often incompatible with mesoscopic device fabrication techniques. We developed techniques to electrochemically intercalate FLGs in-situ in a controlled manner, minimizing sample degradation from parasitic reactions in the electrolyte by passivating sample surfaces using a combination of boron nitride (BN) and photoresist. By monitoring intercalation in real time via Hall measurements and Raman spectroscopy, we show that the interface between graphene and BN can be intercalated with lithium.
1. Kwabena Bediako
Postdoc, Harvard University
Electrochemical Doping of hBN-encapsulated Few-layer Black Phosphorus
D. Kwabena Bediako, Philip Kim
Black phosphorus (bP) has emerged in recent years as a particularly exciting material for optoelectronic applications. Its unique anisotropic transport behavior also offers an interesting handle for fundamental studies of charge transport in two-dimensional layered systems. We are interested in investigating the transport behavior of bP upon electrochemical doping of few-layer phosphorene crystals. In this regard we are investigating both electrochemical double-layer doping as well as electrochemically controlled intercalation reactions. Encapsulation of few-layer bP flakes in an inert Van der Waals material like hexagonal boron nitride is an important step to avoid potentially detrimental electrochemical side reactions. In addition, the anisotropic transport and ion diffusion behavior of bP demands a knowledge of the crystallographic orientation of each flake, which we identify by angle-resolved polarized Raman spectroscopy. Here we present our early forays into the afo! rementioned studies.
2. Cui-Zu Chang
Postdoc, MIT
High-precision Realization of Robust Quantum Anomalous Hall State in a Hard Ferromagnetic Topological Insulator
Cui-Zu Chang, Jagadeesh S. Moodera
The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two dimensional electron layer under a strong magnetic field. The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon as the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism.Here we report the experimental observation of the QAH state in V-doped (Bi,Sb)2Te3 films with the zero-field longitudinal resistance down to 0.00013±0.00007h/e2 (~3.35±1.76W), Hall conductance reaching 0.9998±0.0006e2/h and the Hall angle becoming as high as 89.993±0.004º at T=25mK. Further advantage of this system comes from the fact that it is a hard ferromagnet with a large coercive field (Hc>1.0T) and a relative high Curie temperature. This realization of robust QAH state in hard FMTIs is a major step towards dissipationless electronic a! pplications without external fields.
3. Michelle Chavis
Postdoc, Howard University
Diblock Copolymers for Diamond Patterning and Applications
Michelle A. Chavis, Lorelis Gonzalez-Lopez, Crawford Taylor, James Griffin, Kimberly Jones, Gary L. Harris
Nitrogen-vacancy (NV-) centers have been recently forecasted as possible quantum bits – the fundamental unit of quantum information. Their properties include long spin lifetimes and the ability to store quantum information and transmit it in the form of light. However, interaction with their environment leads to decoherence in the presence of nearby nitrogen spins. The key to enhancing NV- centers is to control their location and reduce their interaction with the environment. Here, we present an innovative method that allows for control over the size and location of diamond nanopillars (DNPs).
4. Nathalie de Leon
Postdoc, Harvard University
Diamond nanophotonics for Solid State Quantum Optics
Nathalie de Leon, Mikhail Lukin
Large-scale quantum networks will require efficient interfaces between photons and stationary quantum bits. Nitrogen vacancy (NV) centers in diamond are a promising candidate for quantum information processing because they are optically addressable, have spin degrees of freedom with long coherence times, and as solid-state entities, can be integrated into nanophotonic devices. An enabling feature of the NV center is its zero-phonon line (ZPL), which acts as an atom-like cycling transition that can be used for coherent optical manipulation and read-out of the spin. However, the ZPL only accounts for 3-5% of the total emission, and previously demonstrated methods of producing high densities of NV centers yield unstable ZPLs.
I will present methods and technologies for gaining both spectral and spatial control over NV emission by coupling NV centers to nanophotonic devices. In particular, we have developed a method to create a high-density device layer of NVs with stable ZPLs in high purity diamond, and have devised a fabrication scheme to carve single mode waveguides out of the surface of the bulk diamond substrate. Using this technique, we are able to fabricate high quality factor, small mode volume photonic crystal cavities directly out of diamond, and deterministically position these photonic crystal cavities so that a stable NV center sits at the maximum electric field. We observe an enhancement of the spontaneous emission at the cavity resonance by a factor of up to 100. The NV emission is guided efficiently into a single optical mode, enabling integration with other photonic elements, as well as networks of cavities, each with their own optically addressable qubit. These nanophotonic eleme! nts in diamond will provide key building blocks for quantum information processing such as single photon transistors, enabling distribution of entanglement over quantum networks.
5. Ahmet Demir
Graduate Student, MIT
Capacitance Measurements on Transition Metal Dichalcogenides
Ahmet Demir, Raymond Ashoori
Transition metal dichalcogenides(TMDCs) are layered materials with tunable direct bandgap structure. This feature allows them to be open to a variety of applications and rich physics. We investigate atomically thin tungsten diselenide(WSe2) using capacitance measurements since it overcomes the high contact resistance problem to monolayer flakes. Here we present the capacitance data that shows the carrier population and depopulation as a function of gate voltage. We also observed some unpredicted capacitance peaks which needs further investigation.
6. Chen Fang
Postdoc, MIT
Nonsymmorphic Topological Crystalline Insulators in Three Dimensions
Chen Fang, Ling Lu, Marin Soljacic, Liang Fu
We study a new class of three-dimensional topological crystalline insulators protected by a single glide reflection symmetry. The Z2 classification is shown using both an analysis of the surface spectral flow as well as a 3D bulk invaraint. We propose one realization for this nontrivial phase in the photon bands of a photonic crystal.
7. Shiang Fang
Graduate Student, Harvard University
Ab-initio Tight-Binding Hamiltonian for 2D Materials
Shinag Fang, Bertrand I. Halperin, Efthimios Kaxiras
With the advances of material fabrications and measurements, 2D layered materials have been the focus of investigations, especially the interplay between topological phases, superconductivity, magnetism, charge density waves. We present accurate ab initio tight-binding hamiltonians for the 2D materials of the transition-metal dichalcogenides and graphene bilayers. We also elucidate the role played by the interlayer coupling, which is a crucial element in utilizing these layered materials. These tight-binding schemes of calculations are efficient methods of van der Walls heterostructure design.
10:20-11:10 Micro-Presentation Group 2
8. Jundong Wu
Graduate Student, Harvard University
Hydrodynamic Oscillator with 2D Electron Gas
Jundong Wu, Donhee Ham
The rapid oscillations of the electron density in conductors form plasmonic waves. A typical example of plasmonic wave would be the surface plasmons on bulk metals. While the surface plasmons usually appears in optics regime, plasmonic waves in 2D conductors can achieve a frequency that lies in the GHz regime. For electrons in such a system, the equation of motion contains a hydrodynamic term under certain DC current, which would give a reflection coefficient larger than unity on an AC open boundary. This gain mechanism provides our group an idea, that we can make use of such gain and build an amplifier, or even an oscillator in GHz regime.
9. Delroy Green
Graduate Student, Howard University
Exploration of Boron doping in Diamond for Superconductivity
Delroy Green, Breyonna Pinkney, Gary Harris
Diamond has extremely rare properties that are impressive to scientist and the public at large. Diamond is an insulator, however when doped with impurities, can exemplify properties of a semiconductor. However when diamond is very heavily doped it can behave as a superconductor at very low temperatures. In this work we investigated two different techniques for doping diamond. The first technique included pulsed laser deposition of boron onto a silicon wafer followed by nano-diamond seeding and subsequent growth. The second technique involved the insertion of boron powder around the sample holder to dope diamond during growth. Both techniques yielded highly doped p-type diamond films.
10. Jin-Yong Hong
Postdoc, MIT
Synthesis of High-quality Multi-layer h-BN and Large Single-crystal Mono-layer Graphene
Jin-Yong Hong, Jing Kong
Large-area and high-quality h-BN films were obtained by using CVD on a Fe foil with a slow cooling rate, which allows boron and nitrogen to diffuse out of the iron surface to form multi-layer h-BN. Graphene, MoS2, and WSe2 FET devices on a h-BN substrate were also fabricated, resulting in carrier mobilities of graphene, MoS2, and WSe2 to improve up to ~ 24,000, 40, and 9 cm2V-1s-1, respectively, indicating the compatibility of our h-BN substrate for 2D electronics.
In addition, uniform bi-/multi-layers-free monolayer graphene is obtained using Cu enclosures with a W foil enclosed. The bi-/multi-layers underneath the monolayer graphene are selectively removed while the monolayer remains intact. The W foil plays an important role in maintaining a low carbon
11. Dennis Huang
Graduate Student, Harvard University
Defect Engineering of Superconducting FeSe Thin Films
Dennis Huang, Efthimios Kaxiras, Jennifer E. Hoffman
FeSe possesses the simplest structure among the iron-based superconductors, consisting of triple layers (Se-Fe-Se) stacked by van der Waals forces, with no buffer layers. The ability to grow FeSe films layer-by-layer using molecular beam epitaxy (MBE) has opened the door to engineering novel structures that enhance or harness its superconducting properties. For example, by interfacing a single layer of FeSe with SrTiO$_3$(001), the superconducting transition temperature is increased from 9 K up to 110 K. Given its short coherence length, a further possibility is to utilize defects that locally weaken superconductivity to define nanostructures. In this poster, we report the growth of FeSe films under excess Se flux. Using scanning tunneling microscopy (STM), we image prevalent defects with dumbbell signatures. We employ density functional theory (DFT) to investigate candidate defect sites, their formation energies, and diffusion barriers. These results shed light on the e! pitaxial growth of 2D-layered FeSe and suggest new avenues of defect engineering in this material for superconducting applications.
12. Changmin Lee
Graduate Student, MIT
Direct Measurement of Ferromagnetism Induced
at the Interface of a Magnetic Topological Insulator
Changmin Lee, Nuh Gedik
When a topological insulator (TI) is brought in contact with a ferromagnetic insulator (FMI), both time reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-ranged exchange interactions. These magnetic topological insulators can host various exotic phenomena, such as massive Dirac fermions, the topological magnetoelectric effect, Majorana fermions, and the quantum anomalous Hall effect (QAHE). However, selectively measuring magnetism induced at the buried interface has remained a challenge. Using magnetic second harmonic generation (MSHG), we directly probe magnetism induced at the interface between the ferromagnetic insulator EuS and the three-dimensional TI Bi2Se3. We simultaneously measure both the in-plane and out-of-plane magnetizations at the interface through the nonlinear Faraday effect. Our findings not only allow us to characterize magnetism at the TI! -FMI interface, but also reveal the possible existence of quantum well states (QWS) confined within the finite size of the thin film.
13. Sidi Maiga
Graduate Student, Howard University
Simulations of Adsorption of Carbon Dioxide and Methane on Graphene Sheet
Sidi M. Maiga, Silvina Gatica
Adsorption is defined as the attachment of atoms, or molecules of a gas, liquid or dissolved solid onto a surface, creating a film or monolayer of material onto the adsorbing surface. Using the
Method of Grand Canonical Monte Carlo we computed the adsorption of carbon dioxide (CO2) and methane (CH4) on a monolayer graphene sheet, at various temperatures for each gas. For each temperature, we compute the adsorption isotherm, Energy gas-surface and Energy gas-gas. We compare the uptake pressures of CO2 and CH4.
14. Thomas Markovich
Graduate Student, Harvard University
Enabling Large-scale Simulation of Many-Body Dispersion Forces in Condensed Phase Systems
Thomas Markovich, Alan Aspur-Guzik
Dispersion interactions are ubiquitous in nature, and extremely important for explaining the structure and function of many systems, from soft matter to surfaces and solids. Due to their long-range and scaling with system size, dispersion interactions can prove particularly important in modeling nanostructured systems where reduced dimensionality creates large polarizable surfaces. Standard pairwise approximations are insufficient for such systems, and the true non-additive and many-body character of dispersion plays a crucial role. The many-body dispersion (MBD) method of Tkatchenko and co-workers [A. Tkatchenko et al., Phys. Rev. Lett. 108, 236402 (2012); A. Ambrossetti et al., J. Chem. Phys. 2014, 140, 18A508] seeks to address this behavior by computing the full many-body correlation energy for a fictutious set of coupled quantum harmonic oscillators that mimic the fluctuations of the real polarizable valence-electron density. Much of the work on MBD, to date, has foc! used on the energetics of various molecules and materials, with all necessary gradient information being obtained through numeric differentiation. We recently presented an implementation of the relevant analytic gradients with respect to nuclear displacements, which permitted fast and accurate geometry optimizations of many gas-phase systems and showed that PBE+MBD geometries matched those of highly accurate wavefunction theories at a fraction of the cost. In this talk I will describe an efficient implementation of the MBD energy and analytic gradients, which has enabled their application to larger simulations of condensed-phase systems. I will show examples of geometry and unit-cell optimizations with MBD corrections in the condensed and gas phase.
11:30 – 12:30 Micro Presentation Group 3
15. Raymond Otchere-Adjei
Graduate Student, Howard University
Doping of Graphene by Non-covalent Chemical Functionalization
Raymond Otchere-Adjei, Charles M. Hosten
Graphene is a single layer of graphite, made up of sp2 hybridized carbon atoms arranged in a honeycomb lattice. Graphene exhibits exceptional properties such as high conductivity, high carrier mobility, and optical transparency unmatched by any material. These properties along with its unique chemical structure make graphene ideal for electronic applications. The goal of our study is to produce graphene based materials with potential applications in fabrication of Nanoelectronic and Optoelectronic devices. This requires developing the capability to tune the electrical properties of Single Layer Graphene (SLG) while maintaining its unique chemical structure. Doping by non-covalent chemical functionalization has proven to be a feasible method for tuning graphene’s fermi level without perturbing the carbon lattice. Non-covalent doping of graphene with organic molecules can occur either by spontaneous charge transfer or π-π interaction of the dopant molecule with graphen! e. By applying surface transfer doping by techniques such as drop-casting, dip-coating, and spin-coating Acridine Orange hydrochloride hydrate and Sodium anthraquinone-2-sulfonate have shown significant effect as n and p type dopants of graphene respectively. The extrinsic semiconductor properties are characterized with Raman Spectroscopy, charge transfer by X-ray Photoelectron Spectroscopy (XPS), and conductivity by field effect transistor (FET).
16. Maoz Ovadia
Postdoc, Harvard University
Dual STM/SFM for Imaging of Vortex Bound States in Superconducting Graphene
Maoz Ovadia, Jennifer E. Hoffman
Low energy bound states are predicted to exist in the core of superconducting vortices. In the case of graphene coupled a superconductor, the energy spacing is expected to be large, allowing spectroscopy of these states. To this end we have built a scanning probe microscope which used a sharp tip on one prong of a quartz resonator to sense both force and tunneling current. The microscope system allows UHV sample exchange and operation at temperatures from 2K to 340K and at magnetic fields up to 5T.
17. Falko Pientka
Postdoc, Harvard University
Localization of Majorana States in Atomic Chains
Falko Pientka, Bertrand I. Halperin
A recent experiment [Nadj-Perge et al. Science 346, 602 (2014)] gives possible evidence for Majorana bound states in chains of magnetic adatoms placed on a superconductor. While many features of the observed end states are naturally interpreted in terms of Majoranas, their strong localization remained puzzling. Here, we show that Majorana states in proximity-coupled systems may be localized on scales much smaller than the coherence length of the host superconductor as a consequence of a strong velocity renormalization.
18. Hechen Ren
Graduate Student, Harvard University
Controlled Finite Momentum Pairing and Spatially Varying Order Parameter in Proximitized HgTe Quantum Wells
Hechen Ren, Amir Yacoby
Conventional s-wave superconductivity is understood to arise from singlet pairing of electrons with opposite Fermi momenta, forming Cooper pairs whose net momentum is zero. Several recent studies have focused on structures where such conventional s-wave superconductors are coupled to systems with an unusual configuration of electronic spin and momentum at the Fermi surface. Under these conditions, the nature of the paired state can be modified and the system may even undergo a topological phase transition. Here we present measurements and theoretical calculations of several HgTe quantum wells coupled to either aluminum or niobium superconductors and subject to a magnetic field in the plane of the quantum well. By studying the oscillatory response of Josephson interference to the magnitude of the in-plane magnetic field, we find that the induced pairing within the quantum well is spatially varying. Cooper pairs acquire a tunable momentum that grows with magnetic field str! ength, directly reflecting the response of the spin-dependent Fermi surfaces to the in-plane magnetic field. In addition, in the regime of high electron density, nodes in the induced superconductivity evolve with the electron density in agreement with our model based on the Hamiltonian of Bernevig, Hughes, and Zhang. This agreement allows us to quantitatively extract the ratio of the effective g-factor to the Fermi velocity. However, at low density our measurements do not agree with our model in detail. Our new understanding of the interplay between spin physics and superconductivity introduces a way to spatially engineer the order parameter, as well as a general framework within which to investigate electronic spin texture at the Fermi surface of materials.
19. Khosro Shirvani
Postdoc, Howard University
Fabrication of Bismuth Telliride (Bi2Te3) Nanowire Arrays (NWA) by High Pressure Injection (HPI)
Khosro Shirvani, Tito Huber, Henry Snyder
20. Oles Shtanko
Graduate Student, MIT
Guided Electron Waves in Graphene
Oles Shtanko, Leonid Levitov
Significant progress in quantum optics in the last decade stimulate the search for solid state analogs in order of compactification and easy scaling. One of the most promising materials is graphene which spectrum of charge carriers resemble light dispersion. Similar to photons, electrons in graphene nanostructures propagate ballistically over macroscopic distances, and this regime persist up to room temperatures. We show that one-dimensional potential in graphene engineered in the bulk either naturally arising at the edge can provide an analogue of photon fibers in optics. Recent experimental data using superconducting Josephson junctions give a strong evidence of existence of such modes at the graphene edge. Also, we will show that for gapped case guided states can exist in the gap which makes them easily detectable separately from bulk states.
21. Young-Ik Sohn
Graduate Student, Harvard University
Enhanced Strain Coupling of Nitrogen Vacancy Spins Tto NanoscaleDiamond Cantilevers
Young-Ik Sohn, Marko Loncar
Nitrogen vacancy (NV) centers coupled to diamond mechanical resonators via strain have recently gained attention as a novel system, which can enable hybrid quantum networks and quantum metrology. Access to the strong coupling regime of this system requires mechanical resonators with nanoscale dimensions in order to overcome the inherently weak strain response of the NV ground state spin. In this work, we incorporate NV centers in diamond cantilevers with sub-micron dimensions. Coupling of the NV ground state spin to the mechanical mode is detected in electron spin resonance (ESR) spectra, and its temporal dynamics are measured via spin echo. Our small mechanical mode volumes lead to a significantly improved spin-phonon coupling strength over previous NV-strain coupling demonstrations.
22. Jordan Stroman
Graduate Student, Howard University
Optically Detected Magnetic Resonance of NV Diamond Centers
Jordan Stroman, Gary Harris
2:30 – 3:20 Micro Presentation Group 4
23. Felix von Cube
Postdoc, Harvard University
Imaging Quantum Materials with Electron Microscopy
Felix von Cube, David C. Bell
We investigate quantum materials with high resolution transmission electron microscopy (HR-TEM). The microscopes allow for atomic resolution imaging, which helps to understand the astonishing mechanical and electric properties of hybrid quantum materials. Our research currently focuses on graphene-based quantum materials, as well as topological insulators and nitrogen vacancies in diamond nano-particles.
24. Amber Wingfield
Postdoc, Howard University
Nitrogen and Boron Doping of HF-CVD Diamond
Amber Wingfield, Gary Harris
Doping diamond for the production of nitrogen-vacancy (NV) centers has recently come to the forefront as a potential candidate for quantum computing. In a similar light, boron doping in diamond has become of interest due to its potential to improve electronic applications. Growth and doping of diamond can be achieved by way of employing hot filament chemical vapor deposition (HF-CVD). Thus, understanding how dopant concentration is effected during diamond growth is of interest. Therefore, the focus of our current research is to separately determine nitrogen and boron incorporation in relation to diamond growth conditions. This will allow leverage into determining the relationship between nitrogen concentration and NV-center incorporation, as well as, determining the relationship between boron incorporation and activated boron concentration. Prepared silicon and silicon carbide substrates are placed into a HF-CVD environment with conditions set for diamond production. Wit! hin this environment, the substrates are doped with nitrogen or boron and subjected to varying growth conditions, such as pressure and flow rate changes. Secondary ion mass spectroscopy (SIMS analysis) is then conducted to provide insight on how nitrogen and boron concentration changes occurred during growth.
25. Yuan Yang
Graduate Student, Harvard University
Theory of Graphene Raman Spectroscopy
Yuan Yang, Eric J. Heller, Lucas Kocia, Wei Chen, Shiang Fang, Mario Borunda, Efthimios Kaxiras
Raman spectroscopy plays a key role in studies of graphene and related carbon systems. Graphene is perhaps the most promising material of recent times for many novel applications, including electronics. The traditional and well established Kramers-Heisenberg-Dirac (KHD) Raman scattering theory (1925-1927) is extended to crystalline graphene for the first time. It demands different phonon production mechanisms and phonon energies than does the popular "double resonance" Raman scattering model. The latter has never been compared to KHD. Within KHD, phonons are produced instantly along with electrons and holes, in what we term an electron-hole-phonon triplet, which does not suffer Pauli blocking. A new mechanism for double phonon production we name "transition sliding" explains the brightness of the 2D mode and other overtones, as a result of linear (Dirac cone) electron dispersion. Direct evidence for sliding resides in hole doping experiments performed in 2011. Whole rang! es of electronic transitions are permitted and may even constructively interfere for the same laser energy and phonon q, explaining the dispersion, bandwidth, and strength of many two phonon Raman bands. Graphene's entire Raman spectrum, including dispersive and fixed bands, missing bands not forbidden by symmetries, weak bands, overtone bands, Stokes anti-Stokes anomalies, individual bandwidths, trends with doping, and D-2D band spacing anomalies emerge naturally and directly in KHD theory.
26. Linda Ye
Graduate Student, MIT
Anomalous Hall Effect in a Kagome Ferromagnet
Linda Ye, Joseph Checkelsky
Ferromagnetic Kagome lattice is know theoretically to possess topological electronic structures. We have synthesized large single crystals of a Kagome ferromagnet Fe3Sn2 which orders well above room temperature. We have observed a large anomalous Hall effect, which is an sensitive probe to the topological and geometrical structure of the electronic states. In addition, we also find signatures of unconventional ferromagnetism in the magnetic susceptibility.
27. Lili Yu
Graduate Student, MIT
CVD Mos2 Electronics: Devices to Systems
Lili Yu, Dina El-Damak, Sungjae Ha, Elaine McVay, Xi Ling, Anantha Chandrakasan,Jing Kong and Tomas Palacios
28. Xingyu (Alex) Zhang
Graduate Student, Harvard University
Nanopillar-based Optical Cavities for Nitrogen-vacancy Centers in Diamond
Xingyu (Alex) Zhang, Evelyn Hu, Shanying Cui, Tsung-li Liu, Jonathan Lee, David Bracher, Kenichi Ohno, David Awschalom
Nitrogen-vacancy (NV) centers in diamond have diverse applications in quantum information processing and field sensing. We have designed nanopillar-based photonic crystal (PhC) and hybrid plasmonic-PhC cavities to provide selective enhancement and improve collection efficiency of NV emission. The hybrid cavities were fabricated using thinned diamond membranes and characterized using confocal photoluminescence (PL) spectroscopy. The PhC structures were created using patterned growth of diamond via chemical vapor deposition on a single-crystal substrate. Such a technique minimizes damage introduced to the NV host lattice (e.g. during top-down fabrication). Preliminary results for the PhC structures show cavity modes as well as bright luminescence from NVs and silicon vacancy centers.
29. Shu Yang Frank Zhao
Graduate Student, Harvard University
Lithium Intercalation of Single-Layer Graphene / Boron Nitride Heterostructures
S.Y.F. Zhao, G.A. Elbaz, C. Yu, D.K. Bediako, Y. Guo, K. Watanabe, T. Taniguchi, L. Brus, X. Roy, & P. Kim
Few layer graphene (FLG) intercalate compounds form a new generation of graphene derivative systems where carrier densities are expected, and novel physical phenomena such as superconductivity and magnetism may emerge. Experimental realization of intercalated FLGs have been limited by harsh intercalation processes which are often incompatible with mesoscopic device fabrication techniques. We developed techniques to electrochemically intercalate FLGs in-situ in a controlled manner, minimizing sample degradation from parasitic reactions in the electrolyte by passivating sample surfaces using a combination of boron nitride (BN) and photoresist. By monitoring intercalation in real time via Hall measurements and Raman spectroscopy, we show that the interface between graphene and BN can be intercalated with lithium.
