2015 Annual Meeting Poster Abstracts

1. Okechukwu  Abana
Undergrad, Howard University
 

2. Quinton  Barclift
Undergrad, Prince George’s Community College
The Optical Properties of Arrays of Thermoelectric Junctions
Quinton Barclift, Tito Huber
 
We developed a thermoelectric device composed of bismuth nanowire array to
investigate the response to light. First we started with a sample of bismuth nanowire
array (Bi NWA) that is atop of bulk bismuth. Next by using a Cressington 308 Sputter
System we deposit indium tin oxide (ITO) which is transparent onto the bismuth
nanowire array to act as an electrode. Copper wires were connected to the top and
bottom of the detector using silver epoxy. The detector was assembled into a metal
box designed to test for photoresponse. Using a lamp, light chopper, lock-in amplifier, filters, and pre-amp we setup the detector to capture light and record data.

3. 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.

4. Carina  Belvin
Undergrad, Wellesley College
Optically Detected Magnetic Resonance Measurements of Nitrogen-Vacancy Centers
Carina Belvin, Robbie Berg
 
Nitrogen-vacancy (NV) centers in diamond are promising candidates for qubits. The electronic ground state of an NV center is a spin triplet and has the spin sublevels ms = 0 and ms = ±1, which can function as the two states of the qubit. By applying microwave radiation at the frequency of the transition between these two spin states (2.87 GHz), we can alter the spin state of the NV center, and then read out the spin state optically. At Wellesley College, we have constructed a setup for performing optically detected magnetic resonance (ODMR) measurements on ensembles of NV centers in diamond nanocrystals.

5. Joshua  Bridger
Faculty, Dover Sherborn Public Schools
Diamond Explorations: Nitrogen Vacancy Magnetometry, Thermal Conductivity & Refractive Index
 
High School physics lab curriculum seldom introduces students to material science or contemporary research. In an effort to provide such enrichment for Advanced Placement Physics students, three exploratory investigations were developed. Two are explorations of physical properties of diamonds: index of refraction and thermal conductivity. One is an exploration of Nitrogen Vacancy (NV) centers in diamonds and the construction and testing of a diamond magnetometer using Electron Spin Resonance (ESR). The three-lab sequence encourages original experimental design and exposes students to contemporary research practices, materials & equipment.

6. 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.

7. Nixia  Chen
Undergrad,  Wellesley College
The Diamond Age of Quantum Materials: Why a Small Heat Capacity is a Good Thing
Nixia Chen, Robbie Berg
 
The Nitrogen-vacancy center (NV center) is a common defect in diamonds. It consists of a nitrogen atom that replaces a carbon atom and a lattice vacancy. Studies have found that the high lattice frequency of diamonds not only contributes to the long-lived coherent state of the spin, but also accounts for the anomalously low heat capacity of diamonds at room temperature.
Our research this summer and this semester involves measuring the heat capacity of diamonds from room temperature (300 K) to moderately low temperatures (~ 100 K). To achieve this range of temperatures, we used a cryogenic system that uses a flow of liquid nitrogen to cool the sample. We are going to use the electron beam evaporation at Harvard CNS to deposit a thin gold film on the diamond plate that has an appropriate amount of electrical resistance. The heater wire connected to the film carries a known current for a short period of time and delivers the heat to the diamond plate. The thermocouple wire enables us to measure how the temperature of the sample changes in response to the heat pulse. We also designed and crafted a diamond-mounting scheme that comprises of a stainless steel diamond plate holder, four stainless steel posts, and a copper board connected to the cold finger. This design provides mechanical support for the diamonds, as well as enough thermal resistan! ce to guarantee that the time constant of cooling down the sample is in the right order of magnitude.
So far, we have successfully mounted the diamond plates, delivered the heat to the sample. We are still in the process of carrying out the experiment under low temperatures and depositing the gold film. The data we collect will help us better understand the unconventional behavior of diamonds at room temperature and below and also their characteristics as a quantum material. In addition, the research will improve the current project design for physics 310 and set up a model for making the research on quantum materials accessible to undergraduates.

8. Michelle  Cole
Undergrad, Howard University
 
9. Sekayi  Curtis
Undergrad, Howard University
Conversion of Silicon Carbide to Diamond
Sekayi Curtis, James Griffin, Crawford Taylor
 
There is a constant demand for progress in the nano-science community. Every day, scientists are investigating new alternatives to older technologies around the world. A valuable material within the nano-science community is diamond due to its high hardness, good electric and thermal conductivity and ability to function as both an insulator and a conductor. Recognizing this value, it is easy to identify the demand of such a material and overcome the limitations in present diamond technology. This is why my research project this summer was to explore a new method in nano-diamond growth in hopes of producing larger and more cost effective diamond substrates.

10. 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.

11. 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.

12. Eric  Edwards
Undergrad, Prince George’s Community College
 

13. 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.

14. 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.
 

15. Amelework  Habtemichael
Undergrad, Gallaudet
 

16. Adrianne  Hargrove
Undergrad, Howard University
Etching Nano-pillars in Polycrystalline Diamond
 
Diamondhasamazingpropertiesthatrangefromgreatheatconductivitytotransparency.When nitrogen vacancy centers(NV-centers) are strategically incorporated in the crystal lattice structure, the possible applications increase.The surface of the diamond acts as a mirror(called the refractive index) when trying to retrieve photons from the NV-centers. The goal of this research was to etch vertical pillars on the surface of polycrystalline diamond to reduce the diamond’s refractive index.
 

17. 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


18. 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.


19. Jazzmen  Johnson
Undergrad, Howard University
 

20. Fikriye Idil (K)  Kaya
Undergrad, Mt Holyoke College
Raman Scattering in Polyacetylene
Fikriye Idil Kaya, Eric J Heller
 
The polyacetylene molecule was first introduced as a potential organic conducting polymer and it then became a model for soliton behavior. We consider an infinitely long polyacetylene polymer (C2H2)n as our system and explore its spectral features and Raman spectrum (RS). We are interested in extending the traditional Born Oppenheimer (BO) approach to calculating Raman cross-sections, through the KHD formula, to periodic systems, in particular to explain the novel characteristics of polyacetylene’s RS such as the evolution of its few peaks under increasing incident laser frequency. The KHD approach requires us to use the BO approximation in which we fix the position of the nuclei and explore the time-dependent coordinates of electrons separately from the nuclei. In order to find the transition matrix elements necessary for KHD, we employ Density Functional Theory (DFT) as well as a tight-binding model with distance dependent hopping parameters. This approach lets us! develop approximate Hamiltonians to model our system in a vastly simpler fashion by reducing dimensionality and simplifying particle interactions (e.g.; to “nearest neighbor” only).

21. Rabeb  Layouni
Undergrad, Mt. Holyoke College
STM Tip Fabrication for Scanning Capacitance Microscopy
Rabeb Layouni, Robert M. Westervelt
 
2-D materials have increasingly gained importance among the scientific community as well as in industry due to their potential applications in a multitude of fields. The exclusive properties they exhibit, that their three dimensional form does not, have made them of great interest to scientists and engineers and hence a lot of efforts and resources have been devoted to understanding these properties. Among the 2-D materials currently studied, we have a particular interest in graphene. This newly discovered material, can carry a thousand times more electricity than copper and has a mobility two hundred and fifty times more than that of silicon [1]. We aim to use a scanning capacitance microscopy technique to image edge states in graphene, electronic states in quantum dots and a wide range of other electronic properties [4]. A tremendously important factor in the scanning process is the STM tip quality. Tip sharpness determines the reliability and resolution of the images,! whereas its strength determines the repeatability of measurements [3],[2]. In this paper, we introduce a simple and reliable STM tip fabrication method based on a two-step electrochemical etching process. This method allows us to produce tungsten STM tips with controllable size and shape. We were also able to test the tips by attempting to measure capacitance with a tungsten metallic plate using a simple capacitance measurement circuit. The primary measurements are only qualitative but were physically relevant, however further work needs to be done to obtain more precise data.
References:
[1] A. H. Castro Neto,F. Guinea,N. M. R. Peres,K. S. Novoselov and A. K. Geim. (2009). The electronic properties of graphene. Reviews of Modern Physics, 81, 109.
[2] GHODRAT TAHMASEBIPOUR,VAHID AHMADI,AMIR ABDULLAH,YOUSEF HOJJAT. (2009). International journal of nanoscience. [FABRICATION OF STM TUNGSTEN NANOTIP BY ELECTROCHEMICAL ETCHING METHOD] 8, 305.
[3] Gobind Basnet. (2013). Fabrication of tungsten tips suitable for scanning probe microscopy by electrochemical etching methods. University of Arkansas).
[4] S Bhandari, R. M. W. (2014). Low-temperature scanning capacitance probe for imaging electron motion. 27th International Conference on Low Temperature Physics.

22. 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.

23. Linh Nguyen Phoung  Mai
Undergrad, Bunker Hill Community College
Rethinking The Community College Engineering Classroom Experience
Linh Mai, Rafael Cabanas, Michelle Cole
 
Redesigning the engineering curriculum of Bunker Hill Community College is the main goal of this project. More exciting and engaging courses are built to shift the focus from the instructor to the students. This will increase student retention as well as reduce the drop-out percentage in the STEM fields. By applying methods of interactive teaching and active learning, students will be able to acknowledge the importance of learning technical fundamentals, and the relevance of engineering to their lives through real-life applications, and hands-on experiences and projects. As a result, students will feel more interested in their chosen STEM professions, and have sufficient exposure to essential skills that are needed to transfer to a four-year college.

24. 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.

25. Brandt  Marcuaeux
Undergrad, Gallaudet
Exfoliation of Bismuth Telluride (Bi2Te3)
Brandt Marceaux, Paul Sabila
 
Bismuth Telluride (Bi2Te3) is a good candidate for thermoelectric applications at room temperature as it has low thermal conductivity and high electrical conductivity.1 Bi2Te3 nanomaterials have higher thermoelectricity figure of merit (ZT). In this poster, we report on the progress we have made towards chemical exfoliation of Bi2Te3 layers using n-butyl lithium (n-BuLi). The exfoliated material was deposited on silicon wafers and then analyzed using Scanning Electron Microscope (SEM), Optical Microscope, Energy Dispersive X-ray Spectroscopy (EDS), and Raman microscope.

26. 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.

27. Christopher  Mbochwa
Undergrad, Gallaudet
 

28. Tabia  Muhammad
Undergrad, Howard University
Effective Graphene Dopants
Tabia A. Muhammad, Charles Hosten
 
Graphene has zero band gap, making it a perfect semiconductor. Chemical doping is performed on graphene to induce a band gap and change its electrical properties. Chemically doped graphene has promising applications in biosensors, fuel cells and metal batteries. In this project the ability of Acridine Orange to dope graphene was probed. Raman spectroscopy was the technique utilized to monitor doping of the graphene sample.

29. Quan  Nguyen
Bright, Ordered, and Compact PbS Nanocrystal Films
 
Semiconductor nanocrystals are an interesting group of materials because their band gap, which dictates what wavelength of light they can absorb and emit, is highly dependent on their size. By synthesizing nanocrystals (2 – 8 nm diameter) of a material such as lead sulfide (PbS), we can tune its absorption / emission properties to be in the range of 800 – 2000 nm. This makes PbS nanocrystals an attractive material for solar cells, thermoelectrics, and light emitting diodes (LEDs). Furthermore, the nanocrystals are synthesized at low temperatures and, as a colloid, have the benefit of being solution-processable. However, the long hydrocarbon ligands which cover the surfaces of the nanocrystals, imparting their colloidal stability, often hinder the performance of nanocrystal devices once the nanocrystals have been deposited as a film. In this work, we strive to reduce the length of the surface ligands on our nanocrystals while maintaining all the beneficial properties ! of the nanocrystals such as solution processability and bright emission. By carefully treating the nanocrystals with acids of varying molecular lengths, we have developed a method to replace the 18-carbon long ligands that typically bind to the nanocrystal surface with as short as 6-carbon long ligands as well as any in-between length of ligand. In total we are now able to tune how closely nanocrystals pack in thin films; from 3.5 nm separation using our longest ligand to only 1.5 nm using our shortest ligand. Critically, we find that in all cases the properties such as absorption and emission peak, colloidal stability, and purity have remained unaffected by the ligand change. The next phase of this project will be to evaluate the efficiency of transport in these films as a function of the ligand length to determine which ligands are best for certain device applications.

30. 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).

31. 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.

32. 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.
 
33. Breyonna Pinkney
Undergrad, Howard University
Exploration of Boron doping in Diamond for Superconductivity
Breyonna Pinkney, Delroy Green, 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.
 

34. 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.

35. Khosro  Shirvani
Postdoc, Howard University
 

36. 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.

37. 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.

38. Jordan  Stroman
Graduate Student, Howard University
 
 
 39. Karine  Thate
Program Manager, Stragtegic Projects, Museum of Science, Boston
How Can Science Museums Tackle Quantum Materials?
...and what are they anyway?
Karine Thate, Carol Lynn Alpert
 
Atomic-layered materials, topological insulators, diamond NV centers, oh my!
This stuff goes beyond the nanoscale… and deep into new atomic-scale realms. Partnering with the NSF Center for Integrated Quantum Materials (CIQM), the Museum of Science team is figuring out ways to broadly share the enthusiasm today’s researchers have for these newly-discovered materials, their startling quantum behaviors, and their possibly revolutionary applications.


40. Ping-chun  Tsai       
Ab Initio Study of Sodium Intercalation into Disordered Carbon
Ping-chun Tsai, Yet-Ming Chiang
 
Graphite, a predominantly chosen anode material for commercial lithium ion batteries (LIBs), has been reported to have negligible intercalation capacity as an anode for sodium ion batteries (NIBs). Disordered carbon exhibits high Na intercalation capacity and emerges as a leading candidate for NIB applications. However, the mechanism of Na+ ion insertion into disordered carbon is still controversial. Here, we propose an ab initio model for disordered carbon and investigate the intercalation mechanism of Na into the layered domains. Our ab initio calculations reveal that a larger interlayer distance and the presence of defects can effectively overcome the van der Waals interaction between graphene sheets and help Na intercalation to form NaC8. The calculation results clarify the mechanism of the Na intercalation and account for the presence of sloping and flat regions of charge–discharge curves in disordered carbon reported in numerous experiments. This reveals new pros! pects for helping Na intercalation into graphite.
 

41. Bekuechukwa  Uzondu
Undergrad, Howard University
HFCVD Growth of Graphene on Silicon Carbide
Bekuechukwu Uzondu, Tina Brower-Thomas
 
Graphene is a 2D crystalline structure of sp2 carbon. Graphene exhibits fascinating electrical and mechanical properties, but we are interested in studying graphene's electrical and optical  properties, and we require large-area defect  free graphene on an insulating substrate. Currently methods for graphene growth include various chemical vapor deposition (CVD)growth methods on catalytic surfaces such as copper and require tedious transfer processes for getting it on the required surface. Graphene has also been grown on silicon carbide by sublimation a technique that requires chamber temperatures up to 1500°c which is a difficult task. There are a few efforts in literatureto grow graphene on non catalytic surfaces suchas silicon dioxide/silicon(SiO2/Si) by using a method called hot filament chemical vapor deposition (HFCVD).So the purpose of my project is to use the HFCVD reactor to determine the optimal growthconditions of a monolayer of graphene. This process involves exposing silicon carbide (SiC) substrates to high temperatures in order to sublimate the silicon and leave only carbon. For my project we used 6H semi-insulating silicon carbide which is one of the various allotropes of SiC . So because it is semi-insulating the graphene can be grown directly on top of the substrate

42. 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.

43. 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.

44. 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.

45. 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.


46. 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.


47. 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
 

48. Henny  Zandbergen
Faculty, Kavli Centre of Nanoscience Delft
In-situ, the new trend in electron microscopy
 
The performance of materials like high-strength steels, catalysts, nanomaterials is ruled by processes that take place at the nanoscale during manufacturing and use. The possibility of following these processes in a TEM in real time and under realistic environmental conditions has the potential of revealing many fundamental aspects of these processes, as well as speed up materials innovations.
One challenge for this in-situ TEM approach is to monitor materials changes during exposure to a gas of atmospheric pressure and higher. For this purpose, systems with differential pumping near the specimen area are not possible, since the quantity of gas in the electron beam path would be so large that only blurred images are obtained. In Delft we focussed on the development of nanoreactors using Micro-ElectroMechanical System (MEMS) devices. In these systems the gas is enclosed by ~ 20 nm thick electron transparent windows only 5 – 30 microns apart. In addition a heating system is present that was designed for minimal specimen drift, enabling lattice imaging at pressures up to 5 bar in the temperature range of 300 – 1000 K.
Another challenge is to perform in-situ TEM in combination with electrical measurements. For instance, current passage through thin nanowire can lead to atoms displacements at high current densities, a phenomena called electromigration. Examples will be given of the electromigration process in Pt nanobridges and InAs nanowires was investigated by in situ transmission electron microscopy (TEM), using a FEI Titan microscope operating at 300 kV. For some experiments in electrical measurement was combined with heating for which a micro-electro-mechanical-system (MEMS) based heater was used and a special electrical sample holder, built in-house, allows to visualise changes in a specimen under both dynamic conditions, i.e. heating and current passage.


49. 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.


50. 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.