Faculty Publications: October, 2017

Query Results from the Smithsonian/NASA Astrophysics Data System (ADS)

The following are Harvard Physics faculty members' publications, added to the ADS database last month. Please note that some publications which appeared in print last month may not be included in the database (and therefore may not appear on this list) until the following month.

Title:
Improved Point-source Detection in Crowded Fields Using Probabilistic Cataloging
Authors:
Portillo, Stephen K. N.; Lee, Benjamin C. G.; Daylan, Tansu; Finkbeiner, Douglas P.
Publication:
The Astronomical Journal, Volume 154, Issue 4, article id. 132, 24 pp. (2017). (AJ Homepage)
Publication Date:
10/2017
Origin:
IOP
Astronomy Keywords:
catalogs, globular clusters: individual: M2, methods: data analysis, methods: statistical
DOI:
10.3847/1538-3881/aa8565
Bibliographic Code:
2017AJ....154..132P

Abstract

Cataloging is challenging in crowded fields because sources are extremely covariant with their neighbors and blending makes even the number of sources ambiguous. We present the first optical probabilistic catalog, cataloging a crowded (˜0.1 sources per pixel brighter than 22nd mag in F606W) Sloan Digital Sky Survey r-band image from M2. Probabilistic cataloging returns an ensemble of catalogs inferred from the image and thus can capture source-source covariance and deblending ambiguities. By comparing to a traditional catalog of the same image and a Hubble Space Telescope catalog of the same region, we show that our catalog ensemble better recovers sources from the image. It goes more than a magnitude deeper than the traditional catalog while having a lower false-discovery rate brighter than 20th mag. We also present an algorithm for reducing this catalog ensemble to a condensed catalog that is similar to a traditional catalog, except that it explicitly marginalizes over source-source covariances and nuisance parameters. We show that this condensed catalog has a similar completeness and false-discovery rate to the catalog ensemble. Future telescopes will be more sensitive, and thus more of their images will be crowded. Probabilistic cataloging performs better than existing software in crowded fields and so should be considered when creating photometric pipelines in the Large Synoptic Survey Telescope era.

 

Title:
The Complete Light-curve Sample of Spectroscopically Confirmed Type Ia Supernovae from Pan-STARRS1 and Cosmological Constraints from The Combined Pantheon Sample
Authors:
Scolnic, D. M.; Jones, D. O.; Rest, A.; Pan, Y. C.; Chornock, R.; Foley, R. J.; Huber, M. E.; Kessler, R.; Narayan, G.; Riess, A. G.; Rodney, S.; Berger, E.; Challis, P. J.; Drout, M.; Finkbeiner, D.; Lunnan, R.; Kirshner, R. P.; Sanders, N. E.; Schlafly, E.; Smartt, S.; Stubbs, C. W.; Tonry, J.; Wood-Vasey, W. M.; Foley, M.; Hand, J.; Johnson, E.; Burgett, W. S.; Chambers, K. C.; Draper, P. W.; Hodapp, K. W.; Kaiser, N.; Kudritzki, R. P.; Magnier, E. A.; Metcalfe, N.; Bresolin, F.; Gall, E.; Kotak, R.; McCrum, M.; Smith, K. W.
Publication:
eprint arXiv:1710.00845
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Astrophysics - Cosmology and Nongalactic Astrophysics
Comment:
Submitted to ApJ. Revised has line in Table 11,13 fixed
Bibliographic Code:
2017arXiv171000845S

Abstract

We present optical light curves, redshifts, and classifications for 361 spectroscopically confirmed Type Ia supernovae (SNeIa) discovered by the Pan-STARRS1 (PS1) Medium Deep Survey. We detail improvements to the PS1 SN photometry, astrometry and calibration that reduce the systematic uncertainties in the PS1 SN Ia distances. We combine the subset of 276 PS1 SN Ia ($0.03 < z < 0.65$) with distance estimates of SN Ia from SDSS, SNLS, various low-z and HST samples to form the largest combined sample of SN Ia consisting of a total of 1049 SN Ia ranging from $0.01 < z < 2.3$, which we call the 'Pantheon Sample'. Photometric calibration uncertainties have long dominated the systematic error budget of every major analysis of cosmological parameters with SNIa. We have reduced these calibration systematics to the point where they are similar in size to the other major sources of known systematic uncertainties: the nature of the intrinsic scatter of SNIa and modeling of selection effects. When combining Planck 2015 CMB measurements with the Pantheon SN sample, we find $\Omega_m = 0.306 \pm 0.012$ and $w = -1.031 \pm 0.040$ for the wCDMmodel. When the SN and CMB constraints are combined with constraints from BAO and local H0 measurements, the analysis yields the most precise measurement of dark energy to date: $w_0 = -1.011 \pm 0.087$ and $w_a = -0.215 \pm 0.402$ for the w0waCDM model. Tension with a cosmological constant previously seen in an analysis of PS1 and low-z SNe has diminished after an increase of $2\times$ in the statistics of the PS1 sample, improved calibration and photometry, and stricter light-curve quality cuts. We find the systematic uncertainties in our measurements of dark energy are almost as large as the statistical uncertainties, primarily due to limitations of modeling the low-redshift sample. This must be addressed for future progress in using SN Ia to measure dark energy.

 

Title:
Relationships between cycles spaces, gain graphs, graph coverings, path homology, and graph curvature
Authors:
Kempton, Mark; Munch, Florentin; Yau, Shing-Tung
Publication:
eprint arXiv:1710.01264
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Mathematics - Combinatorics
Bibliographic Code:
2017arXiv171001264K

Abstract

We prove a homology vanishing theorem for graphs with positive Bakry-\'Emery curvature, analogous to a classic result of Bochner on manifolds \cite{Bochner}. Specifically, we prove that if a graph has positive curvature at every vertex, then its first homology group is trivial, where the notion of homology that we use for graphs is the path homology developed by Grigor'yan, Lin, Muranov, and Yau \cite{Grigoryan2}. The proof draws on several separate areas of graph theory. We study graph coverings, gain graphs, and cycle spaces of graphs, in addition to the Bakry-\'Emery curvature and the path homology. The main result follows as a consequence of several different relationships developed among these different areas. Specifically, we show that a graph with positive curvature can have no non-trivial infinite cover preserving 3-cycles and 4-cycles, and give a combinatorial interpretation of the first path homology in terms of the cycle space of a graph. We relate cycle spaces of graphs to gain graphs with abelian gain group, and relate these to coverings of graphs. Along the way, we prove other new facts about gain graphs, coverings, and cycles spaces that are of related interest.

 

Title:
The DECam Plane Survey: Optical photometry of two billion objects in the southern Galactic plane
Authors:
Schlafly, E. F.; Green, G. M.; Lang, D.; Daylan, T.; Finkbeiner, D. P.; Lee, A.; Meisner, A. M.; Schlegel, D.; Valdes, F.
Publication:
eprint arXiv:1710.01309
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Astrophysics - Astrophysics of Galaxies
Comment:
19 pages, 13 figures, interactive viewer with beautiful images at http://decaps.legacysurvey.org
Bibliographic Code:
2017arXiv171001309S

Abstract

The DECam Plane Survey is a five-band optical and near-infrared survey of the southern Galactic plane with the Dark Energy Camera at Cerro Tololo. The survey is designed to reach past the main-sequence turn-off at the distance of the Galactic center through a reddening E(B-V) of 1.5 mag. Typical single-exposure depths are 23.7, 22.8, 22.3, 21.9, and 21.0 mag in the grizY bands, with seeing around 1 arcsecond. The footprint covers the Galactic plane with |b| < 4 degrees, 5 degrees > l > -120 degrees. The survey pipeline simultaneously solves for the positions and fluxes of tens of thousands of sources in each image, delivering positions and fluxes of roughly two billion stars with better than 10 mmag precision. Most of these objects are highly reddened and deep in the Galactic disk, probing the structure and properties of the Milky Way and its interstellar medium. The full survey is publicly available.

 

Title:
Band Structure Engineering of 2D Materials using Patterned Dielectric Superlattices
Authors:
Forsythe, Carlos; Zhou, Xiaodong; Taniguchi, Takashi; Watanabe, Kenji; Pasupathy, Abhay; Moon, Pilkyung; Koshino, Mikito; Kim, Philip; Dean, Cory R.
Publication:
eprint arXiv:1710.01365
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
15 pages, 4 figures
Bibliographic Code:
2017arXiv171001365F

Abstract

The ability to manipulate two-dimensional (2D) electrons with external electric fields provides a route to synthetic band engineering. By imposing artificially designed and spatially periodic superlattice (SL) potentials, 2D electronic properties can be further engineered beyond the constraints of naturally occurring atomic crystals. Here we report a new approach to fabricate high mobility SL devices by integrating surface dielectric patterning with atomically thin van der Waals materials. By separating the device assembly and SL fabrication processes, we address the intractable tradeoff between device processing and mobility degradation that constrains SL engineering in conventional systems. The improved electrostatics of atomically thin materials moreover allows smaller wavelength SL patterns than previously achieved. Replica Dirac cones in ballistic graphene devices with sub 40nm wavelength SLs are demonstrated, while under large magnetic fields we report the fractal Hofstadter spectra from SLs with designed lattice symmetries vastly different from that of the host crystal. Our results establish a robust and versatile technique for band structure engineering of graphene and related van der Waals materials with dynamic tunability.

 

Title:
Charge Diffusion Variations in Pan-STARRS\,1 CCDs
Authors:
Magnier, Eugene A.; Tonry, J. L.; Finkbeiner, D.; Schlafly, E.; Burgett, W. S.; Chambers, K. C.; Flewelling, H. A.; Hodapp, K. W.; Kaiser, N.; Kudritzki, R.-P.; Metcalfe, N.; Wainscoat, R. J.; Waters, C. Z.
Publication:
eprint arXiv:1710.01819
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Astrophysics - Instrumentation and Methods for Astrophysics
Comment:
submitted to PASP
Bibliographic Code:
2017arXiv171001819M

Abstract

Thick back-illuminated deep-depletion CCDs have superior quantum efficiency over previous generations of thinned and traditional thick CCDs. As a result, they are being used for wide-field imaging cameras in several major projects. We use observations from the Pan-STARRS $3\pi$ survey to characterize the behavior of the deep-depletion devices used in the Pan-STARRS1 Gigapixel Camera. We have identified systematic spatial variations in the photometric measurements and stellar profiles which are similar in pattern to the so-called "tree rings" identified in devices used by other wide-field cameras (e.g., DECam and Hypersuprime Camera). The tree-ring features identified in these other cameras result from lateral electric fields which displace the electrons as they are transported in the silicon to the pixel location. In contrast, we show that the photometric and morphological modifications observed in the GPC1 detectors are caused by variations in the vertical charge transportation rate and resulting charge diffusion variations.

 

Title:
Differential-activity driven instabilities in biphasic active matter
Authors:
Weber, Christoph; Rycroft, Chris H.; Mahadevan, L.
Publication:
eprint arXiv:1710.03633
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Physics - Biological Physics, Condensed Matter - Soft Condensed Matter
Bibliographic Code:
2017arXiv171003633W

Abstract

Active stresses can cause instabilities in contractile gels and living tissues. Here we describe a generic hydrodynamic theory that treats these systems as a mixture of two phases of varying activity and different mechanical properties. We find that differential activity between the phases provides a mechanism causing a demixing instability. We follow the nonlinear evolution of the instability and characterize a phase diagram of the resulting patterns. Our study complements other instability mechanisms in mixtures such as differential growth, shape, motion or adhesion.

 

Title:
A Geometric View of Optimal Transportation and Generative Model
Authors:
Lei, Na; Su, Kehua; Cui, Li; Yau, Shing-Tung; Xianfeng Gu, David
Publication:
eprint arXiv:1710.05488
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Computer Science - Learning, Statistics - Machine Learning
Bibliographic Code:
2017arXiv171005488L

Abstract

In this work, we show the intrinsic relations between optimal transportation and convex geometry, especially the variational approach to solve Alexandrov problem: constructing a convex polytope with prescribed face normals and volumes. This leads to a geometric interpretation to generative models, and leads to a novel framework for generative models. By using the optimal transportation view of GAN model, we show that the discriminator computes the Kantorovich potential, the generator calculates the transportation map. For a large class of transportation costs, the Kantorovich potential can give the optimal transportation map by a close-form formula. Therefore, it is sufficient to solely optimize the discriminator. This shows the adversarial competition can be avoided, and the computational architecture can be simplified. Preliminary experimental results show the geometric method outperforms WGAN for approximating probability measures with multiple clusters in low dimensional space.

 

Title:
Search for sterile neutrinos in MINOS and MINOS+ using a two-detector fit
Authors:
Adamson, P.; Anghel, I.; Aurisano, A.;... Feldman, G. J.;... and 116 coauthors
Publication:
eprint arXiv:1710.06488
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Experiment
Comment:
6 pages, 4 figures
Bibliographic Code:
2017arXiv171006488A

Abstract

A search for mixing between active neutrinos and light sterile neutrinos has been performed by looking for muon neutrino disappearance in two detectors at baselines of 1.04 km and 735 km, using a combined MINOS and MINOS+ exposure of $16.36\times10^{20}$ protons-on-target. A simultaneous fit to the charged-current muon neutrino and neutral-current neutrino energy spectra in the two detectors yields no evidence for sterile neutrino mixing using a 3+1 model. The most stringent limit to date is set on the mixing parameter $\sin^2\theta_{24}$ for most values of the sterile neutrino mass-splitting $\Delta m^2_{41} > 10^{-4}$ eV$^2$.

 

Title:
iSIRA: Integrated Shift-Invert Residual Arnoldi Method for Graph Laplacian Matrices from Big Data
Authors:
Huang, Wei-Qiang; Lin, Wen-Wei; Horng-Shing Lu, Henry; Yau, Shing-Tung
Publication:
eprint arXiv:1710.07080
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Mathematics - Numerical Analysis, 15B99, 65F50, 65N25
Bibliographic Code:
2017arXiv171007080H

Abstract

The eigenvalue problem of a graph Laplacian matrix $L$ arising from a simple, connected and undirected graph has been given more attention due to its extensive applications, such as spectral clustering, community detection, complex network, image processing and so on. The associated graph Laplacian matrix is symmetric, positive semi-definite, and is usually large and sparse. Computing some smallest positive eigenvalues and corresponding eigenvectors is often of interest. However, the singularity of $L$ makes the classical eigensolvers inefficient since we need to factorize $L$ for the purpose of solving large and sparse linear systems exactly. The next difficulty is that it is usually time consuming or even unavailable to factorize a large and sparse matrix arising from real network problems from big data such as social media transactional databases, and sensor systems because there is in general not only local connections. In this paper, we propose an eignsolver based on the inexact residual Arnoldi method together with an implicit remedy of the singularity and an effective deflation for convergent eigenvalues. Numerical experiments reveal that the integrated eigensolver outperforms the classical Arnoldi/Lanczos method for computing some smallest positive eigeninformation provided the LU factorization is not available.

 

Title:
Controlled Electrochemical Intercalation of Graphene/h-BN van der Waals Heterostructures
Authors:
Zhao, S. Y. Frank; Elbaz, Giselle A.; Kwabena Bediako, D.; Yu, Cyndia; Efetov, Dmitri K.; Guo, Yinsheng; Ravichandran, Jayakanth; Min, Kyung-Ah; Hong, Suklyun; Taniguchi, Takashi; Watanabe, Kenji; Brus, Louis E.; Roy, Xavier; Kim, Philip
Publication:
eprint arXiv:1710.07877
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science, Physics - Applied Physics
Bibliographic Code:
2017arXiv171007877Z

Abstract

Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electro-chemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (h-BN). We demonstrate that encapsulating graphene with h-BN eliminates parasitic surface side reactions while simultaneously creating a new hetero-interface that permits intercalation between the atomically thin layers. To monitor the electrochemical process, we employ the Hall effect to precisely monitor the intercalation reaction. We also simultaneously probe the spectroscopic and electrical transport properties of the resulting intercalation compounds at different stages of intercalation. We achieve the highest carrier density $> 5 \times 10^{13} cm^{-2}$ with mobility $> 10^3 cm^2/(Vs)$ in the most heavily intercalated samples, where Shubnikov-de Haas quantum oscillations are observed at low temperatures. These results set the stage for further studies that employ intercalation in modifying properties of vdW heterostructures.

 

Title:
Coherent Bichromatic Force Deflection of Molecules
Authors:
Kozyryev, Ivan; Baum, Louis; Aldridge, Leland; Yu, Phelan; Eyler, Edward E.; Doyle, John M.
Publication:
eprint arXiv:1710.08525
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Physics - Atomic Physics
Bibliographic Code:
2017arXiv171008525K

Abstract

We demonstrate the coherent optical bichromatic force on a molecule, the polar free radical strontium monohydroxide (SrOH). A dual-frequency retro-reflected laser beam addressing the $\tilde{X}^2\Sigma^+\leftrightarrow\tilde{A}^2\Pi_{1/2}$ electronic transition coherently imparts momentum onto a cryogenic beam of SrOH. This directional photon exchange creates a bichromatic force that transversely deflects the molecules. By adjusting the relative phase between the forward and counter propagating laser beams we reverse the direction of the applied force. A momentum transfer of $70\hbar k$ is achieved with minimal loss of molecules to dark states. Modeling of the bichromatic force is performed via direct numerical solution of the time-dependent density matrix and is compared with experimental observations. Our results open the door to further coherent manipulation of molecular motion, including the efficient optical deceleration of diatomic and polyatomic molecules with complex level structures.

 

Title:
Angle-resolved photoemission spectroscopy with quantum gas microscopes
Authors:
Bohrdt, A.; Greif, D.; Demler, E.; Knap, M.; Grusdt, F.
Publication:
eprint arXiv:1710.08925
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases, Condensed Matter - Strongly Correlated Electrons
Comment:
16+7 pages, 10+2 figures
Bibliographic Code:
2017arXiv171008925B

Abstract

Quantum gas microscopes are a promising tool to study interacting quantum many-body systems and bridge the gap between theoretical models and real materials. So far they were limited to measurements of instantaneous correlation functions of the form $\langle \hat{O}(t) \rangle$, even though extensions to frequency-resolved response functions $\langle \hat{O}(t) \hat{O}(0) \rangle$ would provide important information about the elementary excitations in a many-body system. For example, single particle spectral functions, which are usually measured using photoemission experiments in electron systems, contain direct information about fractionalization and the quasiparticle excitation spectrum. Here, we propose a measurement scheme to experimentally access the momentum and energy resolved spectral function in a quantum gas microscope with currently available techniques. As an example for possible applications, we numerically calculate the spectrum of a single hole excitation in one-dimensional $t-J$ models with isotropic and anisotropic antiferromagnetic couplings. A sharp asymmetry in the distribution of spectral weight appears when a hole is created in an isotropic Heisenberg spin chain. This effect slowly vanishes for anisotropic spin interactions and disappears completely in the case of pure Ising interactions. The asymmetry strongly depends on the total magnetization of the spin chain, which can be tuned in experiments with quantum gas microscopes. An intuitive picture for the observed behavior is provided by a slave-fermion mean field theory. The key properties of the spectra are visible at currently accessible temperatures.

 

Title:
Operator spreading and the emergence of dissipation in unitary dynamics with conservation laws
Authors:
Khemani, Vedika; Vishwanath, Ashvin; Huse, D. A.
Publication:
eprint arXiv:1710.09835
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Statistical Mechanics, Condensed Matter - Strongly Correlated Electrons, High Energy Physics - Theory, Quantum Physics
Bibliographic Code:
2017arXiv171009835K

Abstract

We study the scrambling of local quantum information in chaotic many-body systems in the presence of a locally conserved quantity like charge or energy that moves diffusively. The interplay between conservation laws and scrambling sheds light on the mechanism by which unitary quantum dynamics, which is reversible, gives rise to diffusive hydrodynamics, which is a dissipative process. We obtain our results in a random quantum circuit model that is constrained to have a conservation law. We find that a generic spreading operator consists of two parts: (i) a conserved part which comprises the weight of the spreading operator on the local conserved densities, whose dynamics is described by diffusive charge spreading. This conserved part also acts as a source that steadily emits a flux of (ii) non-conserved operators. This emission leads to dissipation in the operator hydrodynamics, with the dissipative process being the conversion of operator weight from local conserved operators to nonconserved, at a rate set by the local diffusion current. The emitted nonconserved parts then spread ballistically at a butterfly speed, thus becoming highly nonlocal and hence essentially non-observable, thereby acting as the "reservoir" that facilitates the dissipation. In addition, we find that the nonconserved component develops a power law tail behind its leading ballistic front due to the slow dynamics of the conserved components. This implies that the out-of-time-order commutator (OTOC) between two initially separated operators grows sharply upon the arrival of the ballistic front but, in contrast to systems with no conservation laws, it develops a diffusive tail and approaches its asymptotic late-time value only as a power of time instead of exponentially. We also derive these results within an effective hydrodynamic description which contains multiple coupled diffusion equations.

 

Title:
Two-photon photoactivated voltage imaging in tissue with an Archaerhodopsin-derived reporter
Authors:
Chien, Miao-Ping; Brinks, Daan; Adam, Yoav; Bloxham, William; Kheifets, Simon; Cohen, Adam E.
Publication:
eprint arXiv:1710.10080
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Quantitative Biology - Quantitative Methods, Physics - Biological Physics
Bibliographic Code:
2017arXiv171010080C

Abstract

Robust voltage imaging in tissue remains a technical challenge. Existing combinations of genetically encoded voltage indicators (GEVIs) and microscopy techniques cannot simultaneously achieve sufficiently high voltage sensitivity, background rejection, and time resolution for high-resolution mapping of sub-cellular voltage dynamics in intact brain tissue. We developed a pooled high-throughput screening approach to identify Archaerhodopsin mutants with unusual photophysical properties. After screening ~105 cells, we identified a novel GEVI, NovArch, whose 1-photon near infrared fluorescence is reversibly enhanced by weak 2-photon excitation. Because the 2-photon excitation acts catalytically rather than stoichiometrically, high fluorescence signals, optical sectioning, and high time resolution are achieved simultaneously, at modest 2- photon laser power. We developed a microscopy system optimized for NovArch imaging in tissue. The combination of protein and optical engineering enhanced signal contrast sufficiently to enable optical mapping of back-propagating action potentials in dendrites in acute mouse brain slice.

 

Title:
Imaging electron flow from collimating contacts in graphene
Authors:
Bhandari, Sagar; Lee, Gil-Ho; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip; Westervelt, Robert M
Publication:
eprint arXiv:1710.10186
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
9 pages, 6 figures
Bibliographic Code:
2017arXiv171010186B

Abstract

The ballistic motion of electrons in graphene encapsulated in hexagonal boron nitride (hBN) promises exciting opportunities for electron-optics devices. A narrow electron beam is desired, with both the mean free path and coherence length exceeding the device size. One can form a collimating contact in graphene by adding zigzag contacts on either side of the electron emitter that absorb stray electrons to form a collimated electron beam [23]. Here we provide images of electron flow from a collimating contact that directly show the width and shape of the electron beam, obtained using a Scanning Gate Microscope (SGM) cooled to 4.2 K. The device is a hBN-encapsulated graphene hall bar with narrow side contacts on either side of the channel that have an electron emitter at the end and absorbing zig-zag contacts at both side. To form an image of electron flow, the SGM tip is raster scanned at a constant height above the sample surface while the transmission to a receiving contact on opposite sides of the channel is measured. By displaying the change {\Delta}T vs. tip position, an image of ballistic flow is obtained. The angular width of the electron beam leaving the collimating contact is found by applying a perpendicular magnetic field B that bends electron paths into cyclotron orbits. SGM images reveal that electron flow from a collimating contact disappears quickly at B = 0.05T while the flow from a non-collimating contact persists up to B = 0.19 T. Ray tracing simulations agree well with the experimental images over a range of B and electron density n. By fitting the half-width at half-max (HWHM) of the magnitude of electron flow in the experimental SGM images, we find a narrow half angular width {\Delta}{\theta} = 9.2{\deg} for the electron flow from the collimating contact, compared with a wide flow {\Delta}{\theta} = 54{\deg} from the non-collimating contact.

 

Title:
Tunneling Devices with Perpendicular Magnetic Anisotropy Electrodes on Atomically Thin van der Waals Heterostructures
Authors:
Idzuchi, H.; Taniguchi, T.; Watanabe, K.; Kim, P.
Publication:
eprint arXiv:1710.10743
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Bibliographic Code:
2017arXiv171010743I

Abstract

We report the fabrication of perpendicular ferromagnetic electrodes for tunneling devices consist of van der Waals heterostructure. We found MgO/Co/Pt films on Hexagonal BN shows perpendicular magnetic anisotropy (PMA) with the easy axis perpendicular to the substrate. Vacuum annealing enhances the perpendicular anisotropy. The easy axis along the perpendicular direction persists up to room temperature with the Pt layer thickness ranges from 1.5 to 5 nm. We employed the PMA electrodes to construct tunneling devices on graphene and monolayer WSe2, where spin injection characteristics and field effect transistor behavior were demonstrated without strong Schottky barrier formation.

 

Title:
Observational Signature of High Spin at the Event Horizon Telescope
Authors:
Gralla, Samuel E.; Lupsasca, Alexandru; Strominger, Andrew
Publication:
eprint arXiv:1710.11112
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Astrophysics - High Energy Astrophysical Phenomena, General Relativity and Quantum Cosmology, High Energy Physics - Theory
Comment:
34 pages, 5 figures. Related videos available at https://youtu.be/ZRfTj5JKPkA and https://youtu.be/K0jEa0Q9NJA
Bibliographic Code:
2017arXiv171011112G

Abstract

We analytically compute the observational appearance of an isotropically emitting point source orbiting near a rapidly spinning black hole. The primary image moves on a vertical line segment, in contrast to the primarily horizontal motion of the spinless case. Secondary images, also on the vertical line, display a rich caustic structure. If detected, this unique signature could serve as a "smoking gun" for a high spin black hole in nature.

 

Title:
Conformal Bootstrap At Large Charge
Authors:
Jafferis, Daniel; Mukhametzhanov, Baur; Zhiboedov, Alexander
Publication:
eprint arXiv:1710.11161
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Theory
Comment:
50 pages, 1 figure
Bibliographic Code:
2017arXiv171011161J

Abstract

We consider unitary CFTs with continuous global symmetries in $d>2$. We consider a state created by the lightest operator of large charge $Q \gg 1$ and analyze the correlator of two light charged operators in this state. We assume that the correlator admits a well-defined large $Q$ expansion and, relatedly, that the macroscopic (thermodynamic) limit of the correlator exists. We find that the crossing equations admit a consistent truncation, where only a finite number $N$ of Regge trajectories contribute to the correlator at leading nontrivial order. We classify all such truncated solutions to the crossing. For one Regge trajectory $N=1$, the solution is unique and given by the effective field theory of a Goldstone mode. For two or more Regge trajectories $N \geq 2$, the solutions are encoded in roots of a certain degree $N$ polynomial. Some of the solutions admit a simple weakly coupled EFT description, whereas others do not. In the weakly coupled case, each Regge trajectory corresponds to a field in the effective Lagrangian.

 

Title:
Enhanced Superconductivity and Suppression of Charge-density Wave Order in 2H-TaS$_2$ in the Two-dimensional Limit
Authors:
Yang, Yafang; Fang, Shiang; Fatemi, Valla; Ruhman, Jonathan; Navarro-Moratalla, Efrén; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
Publication:
eprint arXiv:1711.00079
Publication Date:
10/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Superconductivity
Bibliographic Code:
2017arXiv171100079Y

Abstract

As superconductors are thinned down to the 2D limit, their critical temperature $T_c$ typically decreases. Here we report the opposite behavior, an enhancement of $T_c$ with decreasing thickness, in the 2D crystalline superconductor 2H-TaS$_2$. Remarkably, in the monolayer limit, $T_c$ increases by over a factor of four compared to bulk crystals. Accompanying this trend in superconductivity, we observe progressive weakening and suppression of the charge-density wave (CDW) transition with decreasing thickness. To explain these trends, we perform electronic structure calculations showing that a reduction of the CDW amplitude results in a substantial increase of the density of states at the Fermi energy, which contributes to the enhancement of $T_c$. Our results establish ultra-thin 2H-TaS$_2$ as an ideal platform to study the competition between CDW order and superconductivity.

 

Title:
The Titan Haze Simulation Experiment: Latest Laboratory Results and Dedicated Plasma Chemistry Model
Authors:
Sciamma-O'Brien, Ella; Raymond, Alexander; Mazur, Eric; Salama, Farid
Publication:
American Astronomical Society, DPS meeting #49, id.304.05
Publication Date:
10/2017
Origin:
AAS
Abstract Copyright:
(c) 2017: American Astronomical Society
Bibliographic Code:
2017DPS....4930405S

Abstract

Here, we present the latest results on the gas- and solid phase analyses in the Titan Haze Simulation (THS) experiment, developed at the NASA Ames COSmIC simulation chamber. The THS is a unique experimental platform that allows us to simulate Titan’s complex atmospheric chemistry at Titan-like temperature (200 K) by cooling down N2-CH4-based mixtures in a supersonic expansion before inducing the chemistry by plasma. Because of the accelerated gas flow in the expansion, the residence time of the gas in the active plasma region is less than 3 µs. This results in a truncated chemistry that enables us to control how far in the chain of chemical reactions chemistry processes[1], by adding, in the initial gas mixture, heavier molecules that have been detected as trace elements on Titan.We discuss the results of recent Mid-infrared (MIR) spectroscopy[2] and X-ray Absorption Near Edge Structure spectroscopy studies of THS Titan tholins produced in different gas mixtures (with and without acetylene and benzene). Both studies have shown the presence of nitrogen chemistry, and differences in the level and nature of the nitrogen incorporation depending on the initial gas mixture. A comparison of THS MIR spectra to VIMS data has shown that the THS aerosols produced in simpler mixtures, i.e., that contain more nitrogen and where the N-incorporation is in isocyanide-type molecules instead of nitriles, are more representative of Titan’s aerosols.In addition, a new model has been developed to simulate the plasma chemistry in the THS. Electron impact and chemical kinetics equations for more than 120 species are followed. The calculated mass spectra[3] are in good agreement with the experimental THS mass spectra[1], confirming that the short residence time in the plasma cavity limits the growth of larger species and results in a truncated chemistry, a main feature of the THS.References:[1] Sciamma-O'Brien E. et al., Icarus, 243, 325 (2014)[2] Sciamma-O'Brien E. et al., Icarus, 289, 214 (2017)[3] Raymond, A. et al., submitted

 

Title:
Five-dimensional maximally supersymmetric Yang-Mills in supergravity backgrounds
Authors:
Córdova, Clay; Jafferis, Daniel L.
Publication:
Journal of High Energy Physics, Volume 2017, Issue 10, article id.3, 20 pp.
Publication Date:
10/2017
Origin:
SPRINGER
Keywords:
Field Theories in Higher Dimensions, Supergravity Models, Supersymmetric Gauge Theory
Abstract Copyright:
(c) 2017: The Author(s)
DOI:
10.1007/JHEP10(2017)003
Bibliographic Code:
2017JHEP...10..003C

Abstract

We determine the action for five-dimensional maximally supersymmetric Yang-Mills in off-shell supergravity backgrounds. The resulting theory contains novel five-dimensional BF type couplings as well as cubic scalar interactions which vanish in flat space.

 

Title:
Sub-cellular mRNA localization modulates the regulation of gene expression by small RNAs in bacteria
Authors:
Teimouri, Hamid; Korkmazhan, Elgin; Stavans, Joel; Levine, Erel
Publication:
Physical Biology, Volume 14, Issue 5, article id. 056001 (2017).
Publication Date:
10/2017
Origin:
IOP
DOI:
10.1088/1478-3975/aa69ac
Bibliographic Code:
2017PhBio..14e6001T

Abstract

Small non-coding RNAs can exert significant regulatory activity on gene expression in bacteria. In recent years, substantial progress has been made in understanding bacterial gene expression by sRNAs. However, recent findings that demonstrate that families of mRNAs show non-trivial sub-cellular distributions raise the question of how localization may affect the regulatory activity of sRNAs. Here we address this question within a simple mathematical model. We show that the non-uniform spatial distributions of mRNA can alter the threshold-linear response that characterizes sRNAs that act stoichiometrically, and modulate the hierarchy among targets co-regulated by the same sRNA. We also identify conditions where the sub-cellular organization of cofactors in the sRNA pathway can induce spatial heterogeneity on sRNA targets. Our results suggest that under certain conditions, interpretation and modeling of natural and synthetic gene regulatory circuits need to take into account the spatial organization of the transcripts of participating genes.

 

Title:
Measurement of the W+W- production cross section in pp collisions at a centre-of-mass energy of √{ s} = 13 TeV with the ATLAS experiment
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2850 coauthors
Publication:
Physics Letters B, Volume 773, p. 354-374.
Publication Date:
10/2017
Origin:
ELSEVIER
Abstract Copyright:
(c) 2017 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2017.08.047
Bibliographic Code:
2017PhLB..773..354A

Abstract

The production of opposite-charge W-boson pairs in proton-proton collisions at √{ s} = 13 TeV is measured using data corresponding to 3.16 fb-1 of integrated luminosity collected by the ATLAS detector at the CERN Large Hadron Collider in 2015. Candidate W-boson pairs are selected by identifying their leptonic decays into an electron, a muon and neutrinos. Events with reconstructed jets are not included in the candidate event sample. The cross-section measurement is performed in a fiducial phase space close to the experimental acceptance and is compared to theoretical predictions. Agreement is found between the measurement and the most accurate calculations available.

 

Title:
Auxiliary fermion approach to the resonant inelastic x-ray scattering response in an underdoped cuprate
Authors:
Shi, Yifei; James, Andrew J. A.; Demler, Eugene; Klich, Israel
Publication:
Physical Review B, Volume 96, Issue 15, id.155101 (PhRvB Homepage)
Publication Date:
10/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevB.96.155101
Bibliographic Code:
2017PhRvB..96o5101S

Abstract

We describe a method for calculating the resonant inelastic x-ray scattering (RIXS) response—including the dynamics of the transient core hole—of many-body systems with nontrivial gap structure encoded in their single particle Green's function. Our approach introduces auxiliary fermions in order to obtain a form amenable to the determinant method of Benjamin et al., [Phys. Rev. Lett. 112, 247002 (2014), 10.1103/PhysRevLett.112.247002], and is applicable to systems where interactions are most strongly felt through a renormalization of the single particle propagator. As a test case we consider the Yang-Rice-Zhang ansatz for cuprate phenomena in the underdoped "pseudogap" regime, which remains a popular tool for interpreting the results of experimental probes. We show that taking the core hole dynamics into account for a system described by this ansatz pushes the RIXS peaks towards higher energy transfer, improving agreement with experiments.

 

Title:
Jet energy scale measurements and their systematic uncertainties in proton-proton collisions at √{s }=13 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2845 coauthors
Publication:
Physical Review D, Volume 96, Issue 7, id.072002 (PhRvD Homepage)
Publication Date:
10/2017
Origin:
APS
Abstract Copyright:
2017: CERN
DOI:
10.1103/PhysRevD.96.072002
Bibliographic Code:
2017PhRvD..96g2002A

Abstract

Jet energy scale measurements and their systematic uncertainties are reported for jets measured with the ATLAS detector using proton-proton collision data with a center-of-mass energy of √{s }=13 TeV, corresponding to an integrated luminosity of 3.2 fb-1 collected during 2015 at the LHC. Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells, using the anti-kt algorithm with radius parameter R =0.4. Jets are calibrated with a series of simulation-based corrections and in situ techniques. In situ techniques exploit the transverse momentum balance between a jet and a reference object such as a photon, Z boson, or multijet system for jets with 20 <pT<2000 GeV and pseudorapidities of |η |<4.5 , using both data and simulation. An uncertainty in the jet energy scale of less than 1% is found in the central calorimeter region (|η |<1.2 ) for jets with 100 <pT<500 GeV . An uncertainty of about 4.5% is found for low-pT jets with pT=20 GeV in the central region, dominated by uncertainties in the corrections for multiple proton-proton interactions. The calibration of forward jets (|η |>0.8 ) is derived from dijet pT balance measurements. For jets of pT=80 GeV , the additional uncertainty for the forward jet calibration reaches its largest value of about 2% in the range |η |>3.5 and in a narrow slice of 2.2 <|η |<2.4.

 

Title:
Infrared divergences in QED revisited
Authors:
Kapec, Daniel; Perry, Malcolm; Raclariu, Ana-Maria; Strominger, Andrew
Publication:
Physical Review D, Volume 96, Issue 8, id.085002 (PhRvD Homepage)
Publication Date:
10/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevD.96.085002
Bibliographic Code:
2017PhRvD..96h5002K

Abstract

Recently, it has been shown that the vacuum state in QED is infinitely degenerate. Moreover, a transition among the degenerate vacua is induced in any nontrivial scattering process and determined from the associated soft factor. Conventional computations of scattering amplitudes in QED do not account for this vacuum degeneracy and therefore always give zero. This vanishing of all conventional QED amplitudes is usually attributed to infrared divergences. Here, we show that if these vacuum transitions are properly accounted for, the resulting amplitudes are nonzero and infrared finite. Our construction of finite amplitudes is mathematically equivalent to, and amounts to a physical reinterpretation of, the 1970 construction of Faddeev and Kulish.

 

Title:
Local Pore Size Correlations Determine Flow Distributions in Porous Media
Authors:
Alim, Karen; Parsa, Shima; Weitz, David A.; Brenner, Michael P.
Publication:
Physical Review Letters, Volume 119, Issue 14, id.144501 (PhRvL Homepage)
Publication Date:
10/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevLett.119.144501
Bibliographic Code:
2017PhRvL.119n4501A

Abstract

The relationship between the microstructure of a porous medium and the observed flow distribution is still a puzzle. We resolve it with an analytical model, where the local correlations between adjacent pores, which determine the distribution of flows propagated from one pore downstream, predict the flow distribution. Numerical simulations of a two-dimensional porous medium verify the model and clearly show the transition of flow distributions from δ -function-like via Gaussians to exponential with increasing disorder. Comparison to experimental data further verifies our numerical approach.

 

 

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