Maxwell Dworkin G115, 33 Oxford Street, Cambridge, MA 02138
|
|
Poster Abstracts
1. Sagar Bhandari
Harvard University Imaging Electrons in 2D Materials Sagar Bhandari, Gil-Ho Lee, Ke Wang, Kenji Watanabe, Takashi Taniguchi, Philip Kim, Robert M. Westervelt 2D materials such as graphene, transition metal dichalcogenides (TMDCs) and topological insulators are excellent candidates for new electronics and photonics based on quantum mechanics. To understand their physics, it is critical to image how electrons move through a 2D structure. We present images of electron flow through graphene by a cooled Scanning Gate Microscope (SGM), including cyclotron orbits of electrons, collimation of electron flow, Andreev reflection from a superconducting contact and the formation of quantum dots in MoS2. We image electron flow by deflecting electron trajectories with a capacitively coupled SGM tip. An image of flow is obtained by displaying the change in conduction as the tip is raster scanned across the sample. A quantum dot can be detected by displaying the conductance as the tip is raster scanned above, creating a bullseye pattern of Coulomb-blockade conductance peaks. We used this technique to image quantum dots in an ultrathin MoS2 device. These results help us understand the device physics in 2D materials to develop quantum devices for electronics and photonics applications. 2. Dmitri Efetov
ICFO High-speed Bolometry Based on Johnson Noise D. K. Efetov, R.-J. Shiue, Y. Gao, B. Skinner, E. Walsh, H. Choi, J. Zheng, C. Tan, G. Grosso, C. Peng, J. Hone, K. C. Fong, D. Englund Since the invention of the bolometer, its main design principles relied on efficient light absorption into a low-heat-capacity material and its exceptional thermal isolation from the environment. While the reduced thermal coupling to its surroundings allows for an enhanced thermal response, it in turn strongly reduces the thermal time constant and dramatically lowers the detector's bandwidth. With its unique combination of a record small electronic heat capacity and a weak electron-phonon coupling, graphene has emerged as an extreme bolometric medium that allows for both, high sensitivity and high bandwidths. Here, we introduce a hot-electron bolometer based on a novel Johnson noise readout of the electron gas in graphene, which is critically coupled to incident radiation through a photonic nanocavity. This proof-of-concept operates in the telecom spectrum, achieves an enhanced bolometric response at charge neutrality with a noise equivalent power NEP 5pW/ Sqrt(Hz), a thermal relaxation time of {\tau} 34ps, an improved light absorption by a factor ~3, and an operation temperature up to T=300K. 3. Shiang Fang
Harvard University Electronic Structure Modeling for 2D Layers Shiang Fang, Stephen Carr, Yuan Cao, Valla Fatemi, Pablo Jarillo-Herrero, Efthimios Kaxiras With the development of the experimental techniques to synthesize and characterize the two-dimensional van der Waals layered materials, more layer types are discovered and studied which include examples of graphene, hexagonal boron-nitride and various transition metal dichalcogenides crystals. These layers host a variety of physical properties such as charge density waves, superconductivity, magnetism, topological phases, and more. The layer geometry allows various heterostructures to be fabricated and tuning of the band structure properties. These would have implications on fundamental physics research and potential device applications. However, it also poses great challenges to the theoretical simulations when considering the modeling of various layer types, the strain effects within the layers, the stacking order between layers and the rotation angle in between. These could have dramatic effects on the electronic structure under the right conditions. For example, when the twisted bilayer graphene are rotated at the “magic angle” about one degree, the flat bands emerge from the hybridization of Dirac electrons and give rise to the superconducting states and correlated Mott phases. I will discuss the numerical multi-scale approach to the simulation of the van der Waals layers and their heterostructures. The efficient tight-binding Hamiltonians are constructed based on the Wannier transformations of the density functional theory calculations, which also enable us to derive the interlayer coupling terms relevant to the simulations of the twisted layers stacks with the given layer types. 4. Mary Keenan
Harvard University Imaging Electron Flow in Graphene Point Contacts Mary Keenan, Sagar Bhandari, Robert M. Westervelt Cooled scanning gate microscopy is used to image the flow of electrons through graphene point contacts. Point contacts in 2-dimensional materials are known to be useful to understanding mesoscopic physics, and generally exhibit conductance quantization of e^2/h. However, observation of conductance quantization in graphene often proves challenging. Using a charged scanning probe tip to manipulate the motion of electrons in graphene, we will map conductance as a function of back gate voltage in order to better understand the causes behind this challenge. 5. Cyprian Lewandowski
MIT Photoexcitation Cascade and Carrier Multiplication Cyprian Lewandowski, Leonid Levitov In Dirac materials linear band dispersion blocks momentum-conserving interband transitions, creating a bottleneck for electron-hole pair production and carrier multiplication in the photoexcitation cascade. We show that the decays are unblocked and the bottleneck is relieved by subtle many-body effects involving multiple off-shell e-h pairs. The decays result from a collective behavior due to simultaneous emission of many soft pairs. We discuss characteristic signatures of the off-shell pathways, in particular the sharp angular distribution of secondary carriers, resembling relativistic jets in high-energy physics. The jets can be directly probed using solid-state equivalent of particle detectors. Collinear scattering enhances carrier multiplication, allowing for emission of as many as ~10 secondary carriers per single absorbed photon. 6. Ziwei Qiu
Harvard University Towards Molecular Qubit Control via NV Centers Ziwei Qiu, Alexei Bylinskii, Lei Sun, Michael Graham, Chung Yu, Javier Sanchez-Yamagishi, Bo Dwyer, Elana Urbach, Danna Freedman, Mikhail Lukin and Hongkun Park While magnetic resonance is an established tool for applications ranging from molecular structure determination to quantum computing, it typically requires large ensembles of molecules to detect the weak magnetic signals. In order to push magnetic resonance spectroscopy and control to the single-molecule limit, we magnetically couple molecular spins, such as free electron spins on transition metal complexes or photoexcited triplet states, to the electron spin of an individual nitrogen-vacancy (NV) defect in diamond, which can be optically initialized and read out. The electron spin on the molecule in turn acts as a reporter of the nuclear spin positions on the complex or on a target of interest attached to it, thus producing a magnetic resonance image (MRI) of the single molecule. In addition, the molecule complex can be coherently controlled via the NV center to act as a qubit, which can be assembled into desired quantum computing/simulation architectures via chemical linking. 7. Kyle Serniak
Yale University Hot Non-equilibrium Quasiparticles in Transmon Qub K. Serniak, M. Hays, G. de Lange, S. Diamond, S. Shankar, L. D. Burkhart, L. Frunzio, M. Houzet, M. H. Devoret Non-equilibrium quasiparticle excitations degrade the performance of a variety of superconducting circuits. Understanding the energy distribution of these quasiparticles will yield insight into their generation mechanisms, the limitations they impose on superconducting devices, and how to efficiently mitigate quasiparticle-induced qubit decoherence. To probe this energy distribution, we systematically correlate qubit relaxation and excitation with charge-parity switches in an offset-charge-sensitive transmon qubit, and find that quasiparticle-induced excitation events are the dominant mechanism behind the residual excited-state population in our samples. By itself, the observed quasiparticle distribution would limit T1 to ~200 microseconds, which indicates that quasiparticle loss in our devices is on equal footing with all other loss mechanisms. Furthermore, the measured rate of quasiparticle-induced excitation events is greater than that of relaxation events, which signifies that the quasiparticles are more energetic than would be predicted from a thermal distribution describing their apparent density. |
