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MIT Open Access Articles

Mostrando recursos 41 - 60 de 15,148

41. Quantum phase transitions in semilocal quantum liquids - Iqbal, Nabil; Liu, Hong; Mezei, Márk
We consider several types of quantum critical phenomena from finite-density gauge-gravity duality which to different degrees lie outside the Landau-Ginsburg-Wilson paradigm. These include: (i) a “bifurcating” critical point, for which the order parameter remains gapped at the critical point, and thus is not driven by soft order parameter fluctuations. Rather it appears to be driven by “confinement” which arises when two fixed points annihilate and lose conformality. On the condensed side, there is an infinite tower of condensed states and the nonlinear response of the tower exhibits an infinite spiral structure; (ii) a “hybridized” critical point which can be described by a...

42. Magnetic proximity effect and interlayer exchange coupling of ferromagnetic/topological insulator/ferromagnetic trilayer - Li, Mingda; Cui, Wenping; Yu, Jin; Dai, Zuyang; Wang, Zhe; Katmis, Ferhat; Guo, Wanlin; Moodera, Jagadeesh
The magnetic proximity effect between the topological insulator (TI) and ferromagnetic insulator (FMI) is considered to have great potential in spintronics. However, a complete determination of interfacial magnetic structure has been highly challenging. We theoretically investigate the interlayer exchange coupling of two FMIs separated by a TI thin film, and show that the particular electronic states of the TI contributing to the proximity effect can be directly identified through the coupling behavior between two FMIs, together with a tunability of the coupling constant. Such an FMI/TI/FMI structure not only serves as a platform to clarify the magnetic structure of the...

43. Pivotal estimation via square-root Lasso in nonparametric regression - Belloni, Alexandre; Wang, Lie; Chernozhukov, Victor V.
We propose a self-tuning √Lasso method that simultaneously resolves three important practical problems in high-dimensional regression analysis, namely it handles the unknown scale, heteroscedasticity and (drastic) non-Gaussianity of the noise. In addition, our analysis allows for badly behaved designs, for example, perfectly collinear regressors, and generates sharp bounds even in extreme cases, such as the infinite variance case and the noiseless case, in contrast to Lasso. We establish various nonasymptotic bounds for√Lasso including prediction norm rate and sparsity. Our analysis is based on new impact factors that are tailored for bounding prediction norm. In order to cover heteroscedastic non-Gaussian noise,...

44. How the Upper Bound Conjecture Was Proved - Stanley, Richard P.
We give a short history of how the Upper Bound Conjecture for Spheres was proved.

45. The Lecture Hall Parallelepiped - Liu, Fu; Stanley, Richard P.
The s-lecture hall polytopes P [subscript s] are a class of integer polytopes defined by Savage and Schuster which are closely related to the lecture hall partitions of Eriksson and Bousquet-Mélou. We define a half-open parallelopiped Par [subscript s] associated with P [subscript s] and give a simple description of its integer points. We use this description to recover earlier results of Savage et al. on the δ-vector (or h*-vector) and to obtain the connections to s-ascents and s-descents, as well as some generalizations of these results.

46. Separation Probabilities for Products of Permutations - BERNARDI, OLIVIER; DU, ROSENA R. X.; MORALES, ALEJANDRO H.; Stanley, Richard P.; Bernardi, Olivier; Morales, Alejandro H.
We study the mixing properties of permutations obtained as a product of two uniformly random permutations of fixed cycle types. For instance, we give an exact formula for the probability that elements 1,2,. . .,k are in distinct cycles of the random permutation of {1,2,. . .,n} obtained as a product of two uniformly random n-cycles.

47. The Quantum Reverse Shannon Theorem and Resource Tradeoffs for Simulating Quantum Channels - Bennett, Charles H.; Devetak, Igor; Harrow, Aram W.; Shor, Peter W.; Winter, Andreas
Dual to the usual noisy channel coding problem, where a noisy (classical or quantum) channel is used to simulate a noiseless one, reverse Shannon theorems concern the use of noiseless channels to simulate noisy ones, and more generally the use of one noisy channel to simulate another. For channels of nonzero capacity, this simulation is always possible, but for it to be efficient, auxiliary resources of the proper kind and amount are generally required. In the classical case, shared randomness between sender and receiver is a sufficient auxiliary resource, regardless of the nature of the source, but in the quantum...

48. On fields of rationality for automorphic representations - Shin, Sug Woo; Templier, Nicolas
This paper proves two results on the field of rationality Q(π) for an automorphic representation π, which is the subfield of C fixed under the subgroup of Aut(C) stabilizing the isomorphism class of the finite part of π. For general linear groups and classical groups, our first main result is the finiteness of the set of discrete automorphic representations π such that π is unramified away from a fixed finite set of places, π[subscript ∞] has a fixed infinitesimal character, and [Q(π):Q] is bounded. The second main result is that for classical groups, [Q(π):Q] grows to infinity in a family...

49. Renormalization of Critical Gaussian Multiplicative Chaos and KPZ Relation - Duplantier, Bertrand; Rhodes, Rémi; Vargas, Vincent; Sheffield, Scott Roger
Gaussian Multiplicative Chaos is a way to produce a measure on R[superscript d] (or subdomain of R[superscript d]) of the form e[superscript γX(x)]dx, where X is a log-correlated Gaussian field and γ∈[ 0, √2d) is a fixed constant. A renormalization procedure is needed to make this precise, since X oscillates between −∞ and ∞ and is not a function in the usual sense. This procedure yields the zero measure when γ=√2d. Two methods have been proposed to produce a non-trivial measure when γ=√2d. The first involves taking a derivative at γ=√2d (and was studied in an earlier paper by the current...

50. THE MORPHOLOGY AND DYNAMICS OF JET-DRIVEN SUPERNOVA REMNANTS: THE CASE OF W49B - González-Casanova, Diego F.; De Colle, Fabio; Ramirez-Ruiz, Enrico; Lopez, Laura A.
The circumstellar medium (CSM) of a massive star is modified by its winds before a supernova (SN) explosion occurs, and thus the evolution of the resulting supernova remnant (SNR) is influenced by both the geometry of the explosion as well as the complex structure of the CSM. Motivated by recent work suggesting the SNR W49B was a jet-driven SN expanding in a complex CSM, we explore how the dynamics and the metal distributions in a jet-driven explosion are modified by the interaction with the surrounding environment. In particular, we perform hydrodynamical calculations to study the dynamics and explosive nucleosynthesis of...

51. RELATIVE ASTROMETRY OF COMPACT FLARING STRUCTURES IN Sgr A* WITH POLARIMETRIC VERY LONG BASELINE INTERFEROMETRY - Johnson, Michael D.; Fish, Vincent L.; Broderick, Avery E.; Wardle, John F. C.; Marrone, Daniel P.; Doeleman, Sheperd Samuel
We demonstrate that polarimetric interferometry can be used to extract precise spatial information about compact polarized flares of Sgr A*. We show that, for a faint dynamical component, a single interferometric baseline suffices to determine both its polarization and projected displacement from the quiescent intensity centroid. A second baseline enables two-dimensional reconstruction of the displacement, and additional baselines can self-calibrate using the flare, enhancing synthesis imaging of the quiescent emission. We apply this technique to simulated 1.3 mm wavelength observations of a "hot spot" embedded in a radiatively inefficient accretion disk around Sgr A*. Our results indicate that, even with...

52. IMAGING AN EVENT HORIZON: MITIGATION OF SCATTERING TOWARD SAGITTARIUS A* - Fish, Vincent L.; Johnson, Michael D.; Lu, Ru-Sen; Doeleman, Sheperd Samuel; Bouman, Katherine L.; Zoran, Daniel; Freeman, William T.; Psaltis, Dimitrios; Narayan, Ramesh; Pankratius, Victor; Broderick, Avery E.; Gwinn, Carl R.; Vertatschitsch, Laura E.
The image of the emission surrounding the black hole in the center of the Milky Way is predicted to exhibit the imprint of general relativistic (GR) effects, including the existence of a shadow feature and a photon ring of diameter ~50 μas. Structure on these scales can be resolved by millimeter-wavelength very long baseline interferometry. However, strong-field GR features of interest will be blurred at λ ≥ 1.3 mm due to scattering by interstellar electrons. The scattering properties are well understood over most of the relevant range of baseline lengths, suggesting that the scattering may be (mostly) invertible. We simulate...

53. THE STATE OF THE WARM AND COLD GAS IN THE EXTREME STARBURST AT THE CORE OF THE PHOENIX GALAXY CLUSTER (SPT-CLJ2344-4243) - McDonald, Michael A.; Swinbank, Mark; Edge, Alastair C.; Wilner, David J.; Veilleux, Sylvain; Benson, Bradford A.; Hogan, Michael T.; Marrone, Daniel P.; McNamara, Brian R.; Wei, Lisa H.; Bayliss, Matthew B.; Bautz, Marshall W.
We present new optical integral field spectroscopy (Gemini South) and submillimeter spectroscopy (Submillimeter Array) of the central galaxy in the Phoenix cluster (SPT-CLJ2344-4243). This cluster was previously reported to have a massive starburst (~800 M ☉ yr–1) in the central, brightest cluster galaxy, most likely fueled by the rapidly cooling intracluster medium. These new data reveal a complex emission-line nebula, extending for >30 kpc from the central galaxy, detected at [O II]λλ3726, 3729, [O III]λλ4959, 5007, Hβ, Hγ, Hδ, [Ne III]λ3869, and He II λ4686. The total Hα luminosity, assuming Hα/Hβ = 2.85, is L [subscript Hα] = 7.6 ±...

54. Oncogene-triggered suppression of DNA repair leads to DNA instability in cancer - Yaglom, Julia A.; McFarland, Christopher; Mirny, Leonid A.; Sherman, Michael Y.
DNA instability is an important contributor to cancer development. Previously, defects in the chromosome segregation and excessive DNA double strand breaks due to the replication or oxidative stresses were implicated in DNA instability in cancer. Here, we demonstrate that DNA instability can directly result from the oncogene-induced senescence signaling. Expression of the activated form of Her2 oncogene, NeuT, in immortalized breast epithelial cells led to downregulation of the major DNA repair factor histone H2AX and a number of other components of the HR and NHEJ double strand DNA breaks repair pathways. H2AX expression was regulated at the transcriptional level via...

55. Power law Polya’s urn and fractional Brownian motion - Hammond, Alan; Sheffield, Scott Roger
We introduce a natural family of random walks S[subscript n] on Z that scale to fractional Brownian motion. The increments X[subscript n] := S[subscript n] − S[subscript n]−1 ∈ {±1} have the property that given {X[subscript k] : k < n}, the conditional law of X[subscript n] is that of X[subscript n−k[subscript n]] , where k[subscript n] is sampled independently from a fixed law μ on the positive integers. When μ has a roughly power law decay (precisely, when μ lies in the domain of attraction of an α-stable subordinator, for 0 < α < 1/2) the walks scale to...

56. Role of spectral non-idealities in the design of solar thermophotovoltaics - Lenert, Andrej; Nam, Youngsuk; Wang, Evelyn N.; Bierman, David Matthew
To bridge the gap between theoretically predicted and experimentally demonstrated efficiencies of solar thermophotovoltaics (STPVs), we consider the impact of spectral non-idealities on the efficiency and the optimal design of STPVs over a range of PV bandgaps (0.45-0.80 eV) and optical concentrations (1-3,000x). On the emitter side, we show that suppressing or recycling sub-bandgap radiation is critical. On the absorber side, the relative importance of high solar absorptance versus low thermal emittance depends on the energy balance. Both results are well-described using dimensionless parameters weighting the relative power density above and below the cutoff wavelength. This framework can be used...

57. HEAT TRANSFER FLUIDS - Lenert, Andrej; Nam, Youngsuk; Wang, Evelyn N.
The choice of heat transfer fluids has significant effects on the performance, cost, and reliability of solar thermal systems. In this chapter, we evaluate existing heat transfer fluids such as oils and molten salts based on a new figure of merit capturing the combined effects of thermal storage capacity, convective heat transfer characteristics, and hydraulic performance of the fluids. Thermal stability, freezing point, and safety issues are also discussed. Through a comparative analysis, we examine alternative options for solar thermal heat transfer fluids including water−steam mixtures (direct steam), ionic liquids/melts, and suspensions of nanoparticles (nanofluids), focusing on the benefits and...

58. A nanophotonic solar thermophotovoltaic device - Lenert, Andrej; Nam, Youngsuk; Chan, Walker R.; Wang, Evelyn N.; Bierman, David Matthew; Soljacic, Marin; Celanovic, Ivan
The most common approaches to generating power from sunlight are either photovoltaic, in which sunlight directly excites electron–hole pairs in a semiconductor, or solar–thermal, in which sunlight drives a mechanical heat engine. Photovoltaic power generation is intermittent and typically only exploits a portion of the solar spectrum efficiently, whereas the intrinsic irreversibilities of small heat engines make the solar–thermal approach best suited for utility-scale power plants. There is, therefore, an increasing need for hybrid technologies for solar power generation. By converting sunlight into thermal emission tuned to energies directly above the photovoltaic bandgap using a hot absorber–emitter, solar thermophotovoltaics promise...

59. Origin of Metastable Knots in Single Flexible Chains - Dai, Liang; Doyle, Patrick S.; Renner, Christopher Benjamin
Recent theoretical progress has explained the physics of knotting of semiflexible polymers, yet knotting of flexible polymers is relatively unexplored. We herein develop a new theory for the size distribution of knots on a flexible polymer and the existence of metastable knots. We show the free energy of a flexible molecule in a tube can be mapped to quantitatively reproduce the free energy distribution of a knot on a flexible chain. The size distribution of knots on flexible chains is expected to be universal and might be observed at a macroscopic scale, such as a string of hard balls.

60. Field-Theory Representation of Gauge-Gravity Symmetry-Protected Topological Invariants, Group Cohomology, and Beyond - Gu, Zheng-Cheng; Wen, Xiao-Gang; Wang, Juven
The challenge of identifying symmetry-protected topological states (SPTs) is due to their lack of symmetry-breaking order parameters and intrinsic topological orders. For this reason, it is impossible to formulate SPTs under Ginzburg-Landau theory or probe SPTs via fractionalized bulk excitations and topology-dependent ground state degeneracy. However, the partition functions from path integrals with various symmetry twists are universal SPT invariants, fully characterizing SPTs. In this work, we use gauge fields to represent those symmetry twists in closed spacetimes of any dimensionality and arbitrary topology. This allows us to express the SPT invariants in terms of continuum field theory. We show...

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