Faculty Publications: April, 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:
Inference of Unresolved Point Sources at High Galactic Latitudes Using Probabilistic Catalogs
Authors:
Daylan, Tansu; Portillo, Stephen K. N.; Finkbeiner, Douglas P.
Publication:
The Astrophysical Journal, Volume 839, Issue 1, article id. 4, 22 pp. (2017). (ApJ Homepage)
Publication Date:
04/2017
Origin:
IOP
Astronomy Keywords:
gamma rays: diffuse background, gamma rays: galaxies, gamma rays: general, methods: data analysis, methods: statistical
DOI:
10.3847/1538-4357/aa679e
Bibliographic Code:
2017ApJ...839....4D

Abstract

The detection of point sources in images is a fundamental operation in astrophysics, and is crucial for constraining population models of the underlying point sources or characterizing the background emission. Standard techniques fall short in the crowded-field limit, losing sensitivity to faint sources and failing to track their covariance with close neighbors. We construct a Bayesian framework to perform inference of faint or overlapping point sources. The method involves probabilistic cataloging, where samples are taken from the posterior probability distribution of catalogs consistent with an observed photon count map. In order to validate our method, we sample random catalogs of the gamma-ray sky in the direction of the North Galactic Pole (NGP) by binning the data in energy and point-spread function classes. Using three energy bins spanning 0.3-1, 1-3, and 3-10 GeV, we identify {270}-10+30 point sources inside a 40^\circ × 40^\circ region around the NGP above our point-source inclusion limit of 3× {10}-11 cm-2 s-1 sr-1 GeV-1 at the 1-3 GeV energy bin. Modeling the flux distribution as a power law, we infer the slope to be -{1.92}-0.05+0.07 and estimate the contribution of point sources to the total emission as {18}-2+2%. These uncertainties in the flux distribution are fully marginalized over the number as well as the spatial and spectral properties of the unresolved point sources. This marginalization allows a robust test of whether the apparently isotropic emission in an image is due to unresolved point sources or of truly diffuse origin.

 

Title:
Dynamics of transient microbubbles generated by fs-laser irradiation of plasmonic micropyramids
Authors:
Chen, Jun; Saklayen, Nabiha; Courvoisier, Sébastien; Shen, Zhonghua; Lu, Jian; Ni, Xiaowu; Mazur, Eric
Publication:
Applied Physics Letters, Volume 110, Issue 15, id.153102 (ApPhL Homepage)
Publication Date:
04/2017
Origin:
AIP
Abstract Copyright:
2017: Author(s)
DOI:
10.1063/1.4979886
Bibliographic Code:
2017ApPhL.110o3102C

Abstract

We investigated the dynamics of microbubbles induced by fs-laser irradiation of plasmonic micropyramids in water. We simulated the localized plasmonic enhancement on the micropyramids using a finite-difference time-domain (FDTD) technique and experimentally confirmed the enhancement by observing the laser-induced damage pattern on the substrate. Finally, we experimentally observed the generation of micrometer-sized bubbles on our fabricated structures. We find that the maximum bubble diameter and bubble lifetime depend on power, exposure time, and repetition rate of the laser. The maximum bubble diameter increases with laser exposure time until a balance is reached between the surface tension and the pressure inside and outside the bubble.

 

Title:
Three dimensional canonical singularity and five dimensional N=1 SCFT
Authors:
Xie, Dan; Yau, Shing-Tung
Publication:
eprint arXiv:1704.00799
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Theory
Comment:
35 pages, 17 figures
Bibliographic Code:
2017arXiv170400799X

Abstract

We conjecture that every three dimensional canonical singularity defines a five dimensional N=1 SCFT. Flavor symmetry can be found from singularity structure: non-abelian flavor symmetry is read from the singularity type over one dimensional singular locus. The dimension of Coulomb branch is given by the number of compact crepant divisors from a crepant resolution of singularity. The detailed structure of Coulomb branch is described as follows: a): A chamber of Coulomb branch is described by a crepant resolution, and this chamber is given by its Nef cone and the prepotential is computed from triple intersection numbers; b): Crepant resolution is not unique and different resolutions are related by flops; Nef cones from crepant resolutions form a fan which is claimed to be the full Coulomb branch.

 

Title:
Graphene and active metamaterials: theoretical methods and physical properties
Authors:
Mattheakis, Marios; Tsironis, Giorgos P.; Kaxiras, Efthimios
Publication:
eprint arXiv:1704.01912
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Physics - Optics
Comment:
This is a Chapter of the book: Nanoplasmonics - Fundamentals and Applications. ISBN 978-953-51-5231-6
Bibliographic Code:
2017arXiv170401912M

Abstract

The interaction of light with matter has triggered the interest of scientists for long time. The area of plasmonics emerges in this context through the interaction of light with valence electrons in metals. The random phase approximation in the long wavelength limit is used for analytical investigation of plasmons in three-dimensional metals, in a two-dimensional electron gas and finally in the most famous two-dimensional semi-metal, namely graphene. We show that plasmons in bulk metals as well as in a two-dimensional electron gas originate from classical laws, whereas, quantum effects appear as non-local corrections. On the other hand, graphene plasmons are purely quantum modes and, thus, they would not exist in a "classical world". Furthermore, under certain circumstances, light is able to couple with plasmons on metallic surfaces, forming a surface plasmon polariton, which is very important in nanoplasmonics due to its subwavelength nature. In addition, we outline two applications that complete our theoretical investigation. Firstly, we examine how the presence of gain (active) dielectrics affects surface plasmon polariton properties and we find that there is a gain value for which the metallic losses are completely eliminated resulting to lossless plasmon propagation. Secondly, we combine monolayers of graphene in a periodic order and construct a plasmonic metamaterial that provides tunable wave propagation properties, such as epsilon-near-zero behavior, normal and negative refraction.

 

Title:
Thermal and electrical transport in metals and superconductors across antiferromagnetic and topological quantum transitions
Authors:
Chatterjee, Shubhayu; Sachdev, Subir; Eberlein, Andreas
Publication:
eprint arXiv:1704.02329
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Superconductivity
Comment:
36 + 11 pages, 15 figures
Bibliographic Code:
2017arXiv170402329C

Abstract

We study thermal and electrical transport in metals and superconductors near a quantum phase transition where antiferromagnetic order disappears. The same theory can also be applied to quantum phase transitions involving the loss of certain classes of intrinsic topological order. For a clean superconductor, we recover and extend the well-known universal results. The heat conductivity for commensurate and incommensurate antiferromagnetism coexisting with superconductivity shows a markedly different doping dependence near the quantum critical point, thus allowing to distinguish the states. In the dirty limit, the results for the conductivities are qualitatively similar for the metal and the superconductor. In this regime, the geometric properties of the Fermi surface allow for a very good phenomenological understanding of the numerical results on the conductivities. In the simplest model, we find that the conductivities do not track the doping evolution of the Hall coefficient, in contrast to recent experimental findings. We propose a doping dependent scattering rate, possibly due to quenched short-range charge fluctuations below optimal doping, to consistently describe both the Hall data and the longitudinal conductivities.

 

Title:
Strong coupling Bose polarons in a BEC
Authors:
Grusdt, Fabian; Schmidt, Richard; Shchadilova, Yulia E.; Demler, Eugene A.
Publication:
eprint arXiv:1704.02605
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases, Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Other Condensed Matter, Quantum Physics
Comment:
17 pages, 9 figures, 9 pages appendices
Bibliographic Code:
2017arXiv170402605G

Abstract

We use a non-perturbative renormalization group approach to develop a unified picture of the Bose polaron problem, where a mobile impurity is strongly interacting with a surrounding Bose-Einstein condensate (BEC). A detailed theoretical analysis of the phase diagram is presented and the polaron-to-molecule transition is discussed. For attractive polarons we argue that a description in terms of an effective Fr\"ohlich Hamiltonian with renormalized parameters is possible. Its strong coupling regime is realized close to a Feshbach resonance, where we predict a sharp increase of the effective mass. Already for weaker interactions, before the polaron mass diverges, we predict a transition to a regime where states exist below the polaron energy and the attractive polaron is no longer the ground state. On the repulsive side of the Feshbach resonance we recover the repulsive polaron, which has a finite lifetime because it can decay into low-lying molecular states. We show for the entire range of couplings that the polaron energy has logarithmic corrections in comparison with predictions by the mean-field approach. We demonstrate that they are a consequence of the polaronic mass renormalization which is due to quantum fluctuations of correlated phonons in the polaron cloud.

 

Title:
Bose polarons in ultracold atoms in one dimension: beyond the Fr\"ohlich paradigm
Authors:
Grusdt, Fabian; Astrakharchik, Gregory E.; Demler, Eugene A.
Publication:
eprint arXiv:1704.02606
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases, Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Other Condensed Matter, Quantum Physics
Comment:
21 pages, 18 figures, 4 pages appendices
Bibliographic Code:
2017arXiv170402606G

Abstract

Mobile impurity atoms immersed in Bose-Einstein condensates provide a new platform for exploring Bose polarons. Recent experimental advances in the field of ultracold atoms make it possible to realize such systems with highly tunable microscopic parameters and to explore equilibrium and dynamical properties of polarons using a rich toolbox of atomic physics. In this paper we present a detailed theoretical analysis of Bose polarons in one dimensional systems of ultracold atoms. By combining a non-perturbative renormalization group approach with numerically exact diffusion Monte Carlo calculations we obtain not only detailed numerical results over a broad range of parameters but also qualitative understanding of different regimes of the system. We find that an accurate description of Bose polarons requires the inclusion of two-phonon scattering terms which go beyond the commonly used Fr\"ohlich model. Furthermore we show that when the Bose gas is in the strongly interacting regime, one needs to include interactions between the phonon modes. We use several theoretical approaches to calculate the polaron energy and its effective mass. The former can be measured using radio-frequency spectroscopy and the latter can be studied experimentally using impurity oscillations in a harmonic trapping potential. We compare our theoretical results for the effective mass to the experiments by Catani et al. [PRA 85, 023623 (2012)]. In the weak-to-intermediate coupling regimes we obtain excellent quantitative agreement between theory and experiment, without any free fitting parameter. We supplement our analysis by full dynamical simulations of polaron oscillations in a shallow trapping potential. We also use our renormalization group approach to analyze the full phase diagram and identify regions that support repulsive and attractive polarons, as well as multi-particle bound states.

 

Title:
Properties of In-Plane Graphene/MoS2 Heterojunctions
Authors:
Chen, Wei; Yang, Yuan; Zhang, Zhenyu; Kaxiras, Efthimios
Publication:
eprint arXiv:1704.02669
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Materials Science
Bibliographic Code:
2017arXiv170402669C

Abstract

The graphene/MoS2 heterojunction formed by joining the two components laterally in a single plane promises to exhibit a low-resistance contact according to the Schottky-Mott rule. Here we provide an atomic-scale description of the structural, electronic, and magnetic properties of this type of junction. We first identify the energetically favorable structures in which the preference of forming C-S or C-Mo bonds at the boundary depends on the chemical conditions. We find that significant charge transfer between graphene and MoS2 is localized at the boundary. We show that the abundant 1D boundary states substantially pin the Fermi level in the lateral contact between graphene and MoS2, in close analogy to the effect of 2D interfacial states in the contacts between 3D materials. Furthermore, we propose specific ways in which these effects can be exploited to achieve spin-polarized currents.

 

Title:
Dynamics of D-branes II. The standard action --- an analogue of the Polyakov action for (fundamental, stacked) D-branes
Authors:
Liu, Chien-Hao; Yau, Shing-Tung
Publication:
eprint arXiv:1704.03237
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Theory, Mathematics - Algebraic Geometry, Mathematics - Differential Geometry, Mathematics - Symplectic Geometry, 81T30, 35J20, 16S50, 14A22, 35R01
Comment:
75+2 pages, 6 figures
Bibliographic Code:
2017arXiv170403237L

Abstract

We introduce a new action $S_{standard}^{(\rho,h; \Phi,g,B,C)}$ for D-branes that is to D-branes as the Polyakov action is to fundamental strings. This `standard action' is abstractly a non-Abelian gauged sigma model --- based on maps $\varphi: (X^{\!A\!z},E;\nabla)\rightarrow Y$ from an Azumaya/matrix manifold $X^{\!A\!z}$ with a fundamental module $E$ with a connection $\nabla$ to $Y$ --- enhanced by the dilaton term, the gauge-theory term, and the Chern-Simons/Wess-Zumino term that couples $(\varphi,\nabla)$ to Ramond-Ramond field. In a special situation, this new theory merges the theory of harmonic maps and a gauge theory, with a nilpotent type fuzzy extension. With the analysis developed in D(13.1) (arXiv:1606.08529 [hep-th]) for such maps and an improved understanding of the hierarchy of various admissible conditions on the pairs $(\varphi,\nabla)$ beyond D(13.2.1) (arXiv:1611.09439 [hep-th]) and how they resolve the built-in obstruction to pull-push of covariant tensors under a map from a noncommutative manifold to a commutative manifold, we develop further in this note some covariant differential calculus needed and apply them to work out the first variation --- and hence the corresponding equations of motion for D-branes --- of the standard action and the second variation of the kinetic term for maps and the dilaton term in this action. Compared with the non-Abelian Dirac-Born-Infeld action constructed in D(13.1) along the same line, the current note brings the Nambu-Goto-string-to-Polyakov-string analogue to D-branes. The current bosonic setting is the first step toward the dynamics of fermionic D-branes (cf. D(11.2): arXiv:1412.0771 [hep-th]) and their quantization as fundamental dynamical objects, in parallel to what happened to the theory of fundamental strings during years 1976--1981.

 

Title:
Calabi-Yau Volumes and Reflexive Polytopes
Authors:
He, Yang-Hui; Seong, Rak-Kyeong; Yau, Shing-Tung
Publication:
eprint arXiv:1704.03462
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Theory, Mathematics - Algebraic Geometry
Comment:
70 pages, 22 figures, 6 tables
Bibliographic Code:
2017arXiv170403462H

Abstract

We study various geometrical quantities for Calabi-Yau varieties realized as cones over Gorenstein Fano varieties, obtained as toric varieties from reflexive polytopes in various dimensions. Focus is made on reflexive polytopes up to dimension 4 and the minimized volumes of the Sasaki-Einstein base of the corresponding Calabi-Yau cone are calculated. By doing so, we conjecture new bounds for the Sasaki-Einstein volume with respect to various topological quantities of the corresponding toric varieties. We give interpretations about these volume bounds in the context of associated field theories via the AdS/CFT correspondence.

 

Title:
Spatial gene drives and pushed genetic waves
Authors:
Tanaka, Hidenori; Stone, Howard A.; Nelson, David R.
Publication:
eprint arXiv:1704.03525
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Quantitative Biology - Populations and Evolution, Condensed Matter - Soft Condensed Matter, Physics - Biological Physics
Bibliographic Code:
2017arXiv170403525T

Abstract

Gene drives have the potential to rapidly replace a harmful wild-type allele with a gene drive allele engineered to have desired functionalities. However, an accidental or premature release of a gene drive construct to the natural environment could damage an ecosystem irreversibly. Thus, it is important to understand the spatiotemporal consequences of the super-Mendelian population genetics prior to potential applications. Here, we employ a reaction-diffusion model for sexually reproducing diploid organisms to study how a locally introduced gene drive allele spreads to replace the wild-type allele, even though it posses a selective disadvantage $s>0$. Using methods developed by N. Barton and collaborators, we show that socially responsible gene drives require $0.5<s<0.697$, a rather narrow range. In this "pushed wave" regime, the spatial spreading of gene drives will be initiated only when the initial frequency distribution is above a threshold profile called "critical propagule", which acts as a safeguard against accidental release. We also study how the spatial spread of the pushed wave can be stopped by making gene drives uniquely vulnerable ("sensitizing drive") in a way that is harmless for a wild-type allele. Finally, we show that appropriately sensitized drives in two dimensions can be stopped even by imperfect barriers perforated by a series of gaps.

 

Title:
Positive Scalar Curvature and Minimal Hypersurface Singularities
Authors:
Schoen, Richard; Yau, Shing-Tung
Publication:
eprint arXiv:1704.05490
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Mathematics - Differential Geometry
Comment:
42 pages
Bibliographic Code:
2017arXiv170405490S

Abstract

In this paper we develop methods to extend the minimal hypersurface approach to positive scalar curvature problems to all dimensions. This includes a proof of the positive mass theorem in all dimensions without a spin assumption. It also includes statements about the structure of compact manifolds of positive scalar curvature extending the work of \cite{sy1} to all dimensions. The technical work in this paper is to construct minimal slicings and associated weight functions in the presence of small singular sets and to show that the singular sets do not become too large in the lower dimensional slices. It is shown that the singular set in any slice is a closed set with Hausdorff codimension at least three. In particular for arguments which involve slicing down to dimension $1$ or $2$ the method is successful. The arguments can be viewed as an extension of the minimal hypersurface regularity theory to this setting of minimal slicings.

 

Title:
Reassessing Graphene Absorption and Emission Spectroscopy
Authors:
Yang, Yuan; Kolesov, Grigory; Kocia, Lucas; Heller, Eric J.
Publication:
eprint arXiv:1704.07500
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Materials Science
Bibliographic Code:
2017arXiv170407500Y

Abstract

We present a new paradigm for understanding optical absorption and hot electron dynamics experiments in graphene. Our analysis pivots on assigning proper importance to phonon assisted indirect processes and bleaching of direct processes. We show indirect processes figure in the excess absorption in the UV region. Experiments which were thought to indicate ultrafast relaxation of electrons and holes, reaching a thermal distribution from an extremely non-thermal one in under 5-10 fs, instead are explained by the nascent electron and hole distributions produced by indirect transitions. These need no relaxation or ad-hoc energy removal to agree with the observed emission spectra and fast pulsed absorption spectra. The fast emission following pulsed absorption is dominated by phonon assisted processes, which vastly outnumber direct ones and are always available, connecting any electron with any hole any time. Calculations are given, including explicitly calculating the magnitude of indirect processes, supporting these views.

 

Title:
Quantum Spin Lenses in Atomic Arrays
Authors:
Glaetzle, A. W.; Ender, K.; Wild, D. S.; Choi, S.; Pichler, H.; Lukin, M. D.; Zoller, P.
Publication:
eprint arXiv:1704.08837
Publication Date:
04/2017
Origin:
ARXIV
Keywords:
Quantum Physics
Comment:
13 pages, 9 figures
Bibliographic Code:
2017arXiv170408837G

Abstract

We propose and discuss `quantum spin lenses', where quantum states of delocalized spin excitations in an atomic medium are `focused' in space in a coherent quantum process down to (essentially) single atoms. These can be employed to create controlled interactions in a quantum light-matter interface, where photonic qubits stored in an atomic ensemble are mapped to a quantum register represented by single atoms. We propose Hamiltonians for quantum spin lenses as inhomogeneous spin models on lattices, which can be realized with Rydberg atoms in 1D, 2D and 3D, and with strings of trapped ions. We discuss both linear and non-linear quantum spin lenses: in a non-linear lens, repulsive spin-spin interactions lead to focusing dynamics conditional to the number of spin excitations. This allows the mapping of quantum superpositions of delocalized spin excitations to superpositions of spatial spin patterns, which can be addressed by light fields and manipulated. Finally, we propose multifocal quantum spin lenses as a way to generate and distribute entanglement between distant atoms in an atomic lattice array.

 

Title:
Measurement of jet activity produced in top-quark events with an electron, a muon and two b-tagged jets in the final state in pp 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 2841 coauthors
Publication:
The European Physical Journal C, Volume 77, Issue 4, article id. #220, 38 pp. (EPJC Homepage)
Publication Date:
04/2017
Origin:
SPRINGER
Abstract Copyright:
(c) 2017: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-017-4766-0
Bibliographic Code:
2017EPJC...77..220A

Abstract

Measurements of jet activity in top-quark pair events produced in proton-proton collisions are presented, using 3.2 fb^{-1} of pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Events are chosen by requiring an opposite-charge eμ pair and two b-tagged jets in the final state. The normalised differential cross-sections of top-quark pair production are presented as functions of additional-jet multiplicity and transverse momentum, p_{ {T}}. The fraction of signal events that do not contain additional jet activity in a given rapidity region, the gap fraction, is measured as a function of the p_{ {T}} threshold for additional jets, and is also presented for different invariant mass regions of the eμ b\bar{b} system. All measurements are corrected for detector effects and presented as particle-level distributions compared to predictions with different theoretical approaches for QCD radiation. While the kinematics of the jets from top-quark decays are described well, the generators show differing levels of agreement with the measurements of observables that depend on the production of additional jets.

 

Title:
Metallic Hydrogen
Authors:
Silvera, Isaac F.; Dias, Ranga; Noked, Ori; Salamat, Ashkan; Zaghoo, Mohamed
Publication:
Journal of Low Temperature Physics, Volume 187, Issue 1-2, pp. 4-19 (JLTP Homepage)
Publication Date:
04/2017
Origin:
SPRINGER
Abstract Copyright:
(c) 2017: Springer Science+Business Media New York
DOI:
10.1007/s10909-017-1748-4
Bibliographic Code:
2017JLTP..187....4S

Abstract

One of the great challenges in condensed matter physics has been to produce metallic hydrogen (MH) in the laboratory. There are two approaches: solid molecular hydrogen can be compressed to high density at extreme pressures of order 5-6 megabars. The transition to MH should take place at low temperatures and is expected to occur as a structural first-order phase transition with dissociation of molecules into atoms, rather than the closing of a gap. A second approach is to produce dense molecular hydrogen at pressures of order 1-2 megabars and heat the sample. With increasing temperature, it was predicted that molecular hydrogen first melts and then dissociates to atomic metallic liquid hydrogen as a first-order phase transition. We have observed this liquid-liquid phase transition to metallic hydrogen, also called the plasma phase transition. In low-temperature studies, we have pressurized HD to over 3 megabars and observed two new phases. Molecular hydrogen has been pressurized to 4.2 megabars. A new phase transition has been observed at 3.55 megabars, but it is not yet metallic.

 

Title:
The Creation of Long-Lived Multielectron Bubbles in Superfluid Helium
Authors:
Fang, Jieping; Tempere, J.; Silvera, Isaac F.
Publication:
Journal of Low Temperature Physics, Volume 187, Issue 1-2, pp. 54-61 (JLTP Homepage)
Publication Date:
04/2017
Origin:
SPRINGER
Keywords:
Multi electron bubbles, Mebs, Mebs in superfluid helium
Abstract Copyright:
(c) 2017: Springer Science+Business Media New York
DOI:
10.1007/s10909-017-1747-5
Bibliographic Code:
2017JLTP..187...54F

Abstract

Multielectron bubbles (MEBs) in liquid helium were first observed in the late 1970s, but their properties have never been explored experimentally due to their short lifetimes. MEBs in liquid helium are predicted to have dynamic instabilities for zero or positive pressures, and stability for negative pressures. We report the production of long-lived MEBs in a novel cell filled with helium at static negative pressures. MEBs were extracted from the vapor sheath of a heated filament loop embedded in the superfluid helium and were observed by high-speed photography as they rose in the helium under buoyant forces. In earlier studies we found that MEBs created in this way had large amplitude oscillations and were unstable to decay. By creating MEBs at temperatures just under the lambda point, these oscillations are rapidly damped and the MEBs relax toward a spherical shape and stability as they rise in the helium.

 

Title:
On-chip all-dielectric fabrication-tolerant zero-index metamaterials
Authors:
Kita, Shota; Li, Yang; Camayd-Muñoz, Philip; Reshef, Orad; Vulis, Daryl I.; Day, Robert W.; Mazur, Eric; Lončar, Marko
Publication:
Optics Express, vol. 25, issue 7, p. 8326
Publication Date:
04/2017
Origin:
CROSSREF
DOI:
10.1364/OE.25.008326
Bibliographic Code:
2017OExpr..25.8326K

Abstract

Zero-index metamaterials (ZIMs) offer unprecedented ways to manipulate the flow of light, and are of interest for wide range of applications including optical cloaking, super-coupling, and unconventional phase-matching properties in nonlinear optics. Impedance-matched ZIMs can be obtained through a photonic Dirac-cone (PDC) dispersion induced by an accidental degeneracy of two linear bands - typically an electric monopole mode and a transverse magnetic dipole mode - at the center of the Brillouin zone. Consequently, PDC can only be achieved for a particular combination of geometric parameters of the metamaterial, and hence is sensitive to fabrication imperfections. These fabrication imperfections may limit the usefulness in practical applications. In this work we overcome this obstacle and demonstrate robust all-dielectric (AD) ZIM that supports PDC dispersion over wide parameter space. Our structure, consisting of an array of Si pillars on silica substrate, is fabricated in silicon-oninsulator (SOI) platform and operates at telecom wavelengths.

 

Title:
Efficient quantum computation in a network with probabilistic gates and logical encoding
Authors:
Borregaard, J.; Sørensen, A. S.; Cirac, J. I.; Lukin, M. D.
Publication:
Physical Review A, Volume 95, Issue 4, id.042312 (PhRvA Homepage)
Publication Date:
04/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevA.95.042312
Bibliographic Code:
2017PhRvA..95d2312B

Abstract

An approach to efficient quantum computation with probabilistic gates is proposed and analyzed in both a local and nonlocal setting. It combines heralded gates previously studied for atom or atomlike qubits with logical encoding from linear optical quantum computation in order to perform high-fidelity quantum gates across a quantum network. The error-detecting properties of the heralded operations ensure high fidelity while the encoding makes it possible to correct for failed attempts such that deterministic and high-quality gates can be achieved. Importantly, this is robust to photon loss, which is typically the main obstacle to photonic-based quantum information processing. Overall this approach opens a path toward quantum networks with atomic nodes and photonic links.

 

Title:
Direct single-shot observation of millimeter-wave superradiance in Rydberg-Rydberg transitions
Authors:
Grimes, David D.; Coy, Stephen L.; Barnum, Timothy J.; Zhou, Yan; Yelin, Susanne F.; Field, Robert W.
Publication:
Physical Review A, Volume 95, Issue 4, id.043818 (PhRvA Homepage)
Publication Date:
04/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevA.95.043818
Bibliographic Code:
2017PhRvA..95d3818G

Abstract

We have directly detected millimeter-wave (mm-wave) free-space superradiant emission from Rydberg states (principal quantum number n ˜30 ) of barium atoms in single-shot experiments. We trigger the cooperative effects with a weak initial pulse and detect the time-dependent amplitude and phase of the emitted radiation with single-shot sensitivity and 20 ps time resolution. This allows measurement and shot-by-shot analysis of the statistical distribution of decay rates, time delays, and time-dependent frequency shifts. Cooperative line shifts and decay rates are observed that exceed the 250 kHz Doppler width by a factor of 20 and the 50 Hz spontaneous emission rate by a factor of 105.

 

Title:
Magnetic noise spectroscopy as a probe of local electronic correlations in two-dimensional systems
Authors:
Agarwal, Kartiek; Schmidt, Richard; Halperin, Bertrand; Oganesyan, Vadim; Zaránd, Gergely; Lukin, Mikhail D.; Demler, Eugene
Publication:
Physical Review B, Volume 95, Issue 15, id.155107 (PhRvB Homepage)
Publication Date:
04/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevB.95.155107
Bibliographic Code:
2017PhRvB..95o5107A

Abstract

We develop the theoretical framework for calculating magnetic noise from conducting two-dimensional (2D) materials. We describe how local measurements of this noise can directly probe the wave-vector dependent transport properties of the material over a broad range of length scales, thus providing new insight into a range of correlated phenomena in 2D electronic systems. As an example, we demonstrate how transport in the hydrodynamic regime in an electronic system exhibits a unique signature in the magnetic noise profile that distinguishes it from diffusive and ballistic transport and how it can be used to measure the viscosity of the electronic fluid. We employ a Boltzmann approach in a two-time relaxation-time approximation to compute the conductivity of graphene and quantitatively illustrate these transport regimes and the experimental feasibility of observing them. Next, we discuss signatures of isolated impurities lodged inside the conducting 2D material. The noise near an impurity is found to be suppressed compared to the background by an amount that is directly proportional to the cross-section of electrons/holes scattering off of the impurity. We use these results to outline an experimental proposal to measure the temperature dependent level shift and linewidth of the resonance associated with an Anderson impurity.

 

Title:
Measurement of the t t ¯ production cross section in the τ +jets final state in p p collisions at √{s }=8 TeV using the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... ; and 2848 coauthors
Publication:
Physical Review D, Volume 95, Issue 7, id.072003 (PhRvD Homepage)
Publication Date:
04/2017
Origin:
APS
Abstract Copyright:
2017: CERN
DOI:
10.1103/PhysRevD.95.072003
Bibliographic Code:
2017PhRvD..95g2003A

Abstract

A measurement of the inclusive p p →t t ¯+X production cross section in the τ +jets final state using only the hadronic decays of the τ lepton is presented. The measurement is performed using 20.2 fb-1 of proton-proton collision data recorded at a center-of-mass energy of √{s }=8 TeV with the ATLAS detector at the Large Hadron Collider. The cross section is measured via a counting experiment by imposing a set of selection criteria on the identification and kinematic variables of the reconstructed particles and jets, and on event kinematic variables and characteristics. The production cross section is measured to be σt t ¯=239 ±29 pb , which is in agreement with the measurements in other final states and the theoretical predictions at this center-of-mass energy.

 

Title:
Precision decay rate calculations in quantum field theory
Authors:
Andreassen, Anders; Farhi, David; Frost, William; Schwartz, Matthew D.
Publication:
Physical Review D, Volume 95, Issue 8, id.085011 (PhRvD Homepage)
Publication Date:
04/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevD.95.085011
Bibliographic Code:
2017PhRvD..95h5011A

Abstract

Tunneling in quantum field theory is worth understanding properly, not least because it controls the long-term fate of our Universe. There are, however, a number of features of tunneling rate calculations which lack a desirable transparency, such as the necessity of analytic continuation, the appropriateness of using an effective instead of classical potential, and the sensitivity to short-distance physics. This paper attempts to review in pedagogical detail the physical origin of tunneling and its connection to the path integral. Both the traditional potential-deformation method and a recent, more direct, propagator-based method are discussed. Some new insights from using approximate semiclassical solutions are presented. In addition, we explore the sensitivity of the lifetime of our Universe to short-distance physics, such as quantum gravity, emphasizing a number of important subtleties.

 

Title:
Parallelization of microfluidic flow-focusing devices
Authors:
Amstad, Esther; Chen, Xiaoming; Eggersdorfer, Max; Cohen, Noa; Kodger, Thomas E.; Ren, Carolyn L.; Weitz, David A.
Publication:
Physical Review E, Volume 95, Issue 4, id.043105 (PhRvE Homepage)
Publication Date:
04/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevE.95.043105
Bibliographic Code:
2017PhRvE..95d3105A

Abstract

Microfluidic flow-focusing devices offer excellent control over fluid flow, enabling formation of drops with a narrow size distribution. However, the throughput of microfluidic flow-focusing devices is limited and scale-up through operation of multiple drop makers in parallel often compromises the robustness of their operation. We demonstrate that parallelization is facilitated if the outer phase is injected from the direction opposite to that of the inner phase, because the fluid injection flow rate, where the drop formation transitions from the squeezing into the dripping regime, is shifted towards higher values.

 

Title:
Measurement of the Neutrino Mixing Angle θ23 in NOvA
Authors:
Adamson, P.; Aliaga, L.; Ambrose, D.;... Feldman, G. J.;... and 178coauthors
Publication:
Physical Review Letters, Volume 118, Issue 15, id.151802 (PhRvL Homepage)
Publication Date:
04/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevLett.118.151802
Bibliographic Code:
2017PhRvL.118o1802A

Abstract

This Letter reports new results on muon neutrino disappearance from NOvA, using a 14 kton detector equivalent exposure of 6.05 ×1020 protons on target from the NuMI beam at the Fermi National Accelerator Laboratory. The measurement probes the muon-tau symmetry hypothesis that requires maximal θ23 mixing (θ23=π /4 ). Assuming the normal mass hierarchy, we find Δ m322 =(2.67 ±0.11)×10-3 eV2 and sin2θ23 at the two statistically degenerate values 0.40 4-0.022+0.030 and 0.62 4-0.030+0.022, both at the 68% confidence level. Our data disfavor the maximal mixing scenario with 2.6 σ significance.

 


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