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MIT Open Access Articles

Mostrando recursos 41 - 60 de 12,940

41.
Fluctuating-surface-current formulation of radiative heat transfer: Theory and applications - Rodriguez, Alejandro W.; Johnson, Steven G.; Reid, M. T. Homer
We describe a fluctuating-surface current formulation of radiative heat transfer between bodies of arbitrary shape that exploits efficient and sophisticated techniques from the surface-integral-equation formulation of classical electromagnetic scattering. Unlike previous approaches to nonequilibrium fluctuations that involve scattering matrices—relating “incoming” and “outgoing” waves from each body—our approach is formulated in terms of “unknown” surface currents, laying at the surfaces of the bodies, that need not satisfy any wave equation. We show that our formulation can be applied as a spectral method to obtain fast-converging semianalytical formulas in high-symmetry geometries using specialized spectral bases that conform to the surfaces of the...

42.
Microscopic model for the boson integer quantum Hall effect - Regnault, N.; Todadri, Senthil
In two dimensions strongly interacting bosons in a magnetic field can form an integer quantum Hall state. This state has a bulk gap, no fractional charges or topological order in the bulk, but nevertheless quantized Hall transport and symmetry-protected edge excitations. Here we study a simple microscopic model for such a state in a system of two-component bosons in a strong orbital magnetic field. We show through exact-diagonalization calculations that the model supports the boson integer quantum Hall ground state in a range of parameters.

43.
Transport dynamics of superparamagnetic microbeads trapped by mobile magnetic domain walls - Rapoport, Elizabeth; Beach, Geoffrey Stephen
The dynamics of fluid-borne superparamagnetic bead transport by field-driven domain walls (DWs) in submicrometer ferromagnetic tracks is studied experimentally together with numerical and analytical modeling. A combination of micromagnetic modeling and numerical calculation is used to determine the strength of bead-DW interaction for a range of track geometries and bead sizes. The maximum DW velocity for continuous bead transport is predicted from these results and shown to be supported by experimental measurements. Enhancement of the maximum velocity by appropriate material selection or field application is demonstrated, and an analysis of the source of statistical variation is presented. Finally, the dynamics...

44.
Quantification of Cellular Poly(ADP-ribosyl)ation by Stable Isotope Dilution Mass Spectrometry Reveals Tissue- and Drug-Dependent Stress Response Dynamics - Martello, Rita; Mangerich, Aswin; Sass, Sabine; Dedon, Peter C.; Burkle, Alexander
Poly(ADP-ribosyl)ation is an essential post-translational modification with the biopolymer poly(ADP-ribose) (PAR). The reaction is catalyzed by poly(ADP-ribose) polymerases (PARPs) and plays key roles in cellular physiology and stress response. PARP inhibitors are currently being tested in clinical cancer treatment, in combination therapy, or as monotherapeutic agents by inducing synthetic lethality. We have developed an accurate and sensitive bioanalytical platform based on isotope dilution mass spectrometry in order to quantify steady-state and stress-induced PAR levels in cells and tissues and to characterize pharmacological properties of PARP inhibitors. In contrast to existing PAR-detection techniques, the LC–MS/MS method uses authentic isotope-labeled standards, which...

45.
In Situ Analysis of 8-Oxo-7,8-dihydro-2′-deoxyguanosine Oxidation Reveals Sequence- and Agent-Specific Damage Spectra - Lim, Kok Seong; Cui, Liang; Taghizadeh, Koli; Wishnok, John S.; Chan, Wan; DeMott, Michael S.; Babu, I. Ramesh; Dedon, Peter C.; Tannenbaum, Steven Robert
Guanine is a major target for oxidation in DNA, with 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) as a major product. 8-oxodG is itself significantly more susceptible to oxidation than guanine, with the resulting damage consisting of more than 10 different products. This complexity has hampered efforts to understand the determinants of biologically relevant DNA oxidation chemistry. To address this problem, we have developed a high mass accuracy mass spectrometric method to quantify oxidation products arising site specifically in DNA. We applied this method to quantify the role of sequence context in defining the spectrum of damage products arising from oxidation of 8-oxodG by two...

46.
Biologically relevant oxidants and terminology, classification and nomenclature of oxidatively generated damage to nucleobases and 2-deoxyribose in nucleic acids - Cadet, Jean; Loft, Steffen; Olinski, Ryszard; Evans, Mark D.; Bialkowski, Karol; Richard Wagner, J.; Dedon, Peter C.; Greenberg, Marc M.; Cooke, Marcus S.; Moller, Peter
A broad scientific community is involved in investigations aimed at delineating the mechanisms of formation and cellular processing of oxidatively generated damage to nucleic acids. Perhaps as a consequence of this breadth of research expertise, there are nomenclature problems for several of the oxidized bases including 8-oxo-7,8-dihydroguanine (8-oxoGua), a ubiquitous marker of almost every type of oxidative stress in cells. Efforts to standardize the nomenclature and abbreviations of the main DNA degradation products that arise from oxidative pathways are reported. Information is also provided on the main oxidative radicals, non-radical oxygen species, one-electron agents and enzymes involved in DNA degradation...

47.
Solvent Exposure Associated with Single Abasic Sites Alters the Base Sequence Dependence of Oxidation of Guanine in DNA in GG Sequence Contexts - Lee, Young-Ae; Liu, Zhi; Dedon, Peter C.; Geacintov, Nicholas E.; Shafirovich, Vladimir
The effect of exposure of guanine in double-stranded oligonucleotides to aqueous solvent during oxidation by one-electron oxidants was investigated by introducing single synthetic tetrahydrofuran-type abasic sites (Ab) either adjacent to or opposite tandem GG sequences. The selective oxidation of guanine was initiated by photoexcitation of the aromatic sensitizers riboflavin and a pyrene derivative, and by the relatively small negatively charged carbonate radical anion. The relative rates of oxidation of the 5′- and 3′ side G in runs of 5′⋅⋅⋅GG⋅⋅⋅ (evaluated by standard hot alkali treatment of the damaged DNA strand followed by high resolution gel electrophoresis of the cleavage fragments)...

48.
One-electron Oxidation of a Pyrenyl Photosensitizer Covalently Attached to DNA and Competition Between its Further Oxidation and DNA Hole Injection - Yun, Byeong Hwa; Dedon, Peter C.; Geacintov, Nicholas E.; Shafirovich, Vladimir
The photosensitized hole injection and guanine base damage phenomena have been investigated in the DNA sequence, 5′-d(CATG[subscript 1] [superscript Py]CG[subscript 2]TCCTAC) with a site-specifically positioned pyrene-like (Py) benzo[a]pyrene 7,8-diol 9,10-epoxide-derived N[superscript 2]-guanine adduct (G[subscript 1] [superscript Py]). Generation of the Py radical cation and subsequent hole injection into the DNA strand by a 355 nm nanosecond laser pulses (∼4 mJ cm[superscript −2]) results in the transformation of G[subscript 1] [superscript Py] to the imidazolone derivative Iz[subscript 1] [superscript Py] and a novel G[subscript 1] [superscript Py*] photoproduct that has a mass larger by 16 Da (M+16) than the mass (M)...

49.
The future is now: single-cell genomics of bacteria and archaea - Blainey, Paul C.
Interest in the expanding catalog of uncultivated microorganisms, increasing recognition of heterogeneity among seemingly similar cells, and technological advances in whole-genome amplification and single-cell manipulation are driving considerable progress in single-cell genomics. Here, the spectrum of applications for single-cell genomics, key advances in the development of the field, and emerging methodology for single-cell genome sequencing are reviewed by example with attention to the diversity of approaches and their unique characteristics. Experimental strategies transcending specific methodologies are identified and organized as a road map for future studies in single-cell genomics of environmental microorganisms. Over the next decade, increasingly powerful tools for...

50.
Anomalous supercurrent from Majorana states in topological insulator Josephson junctions - Potter, Andrew C.; Fu, Liang
We propose a Josephson junction setup based on a topological insulator (TI) thin film to detect Majorana states that exploits the unique helical and extended nature of the TI surface state. When the magnetic flux through the junction is close to an integer number of flux quanta, Majorana states, present on both surfaces of the film, give rise to a narrow peak-dip structure in the current-phase relation by hybridizing at the edge of the junction. Remarkably, the maximal Majorana-state contribution to Josephson current takes a (nearly) universal value, approximately equal to the supercurrent capacity of a single quantum channel. These...

51.
Excitonic effects on coherent phonon dynamics in single-wall carbon nanotubes - Nugraha, A. R. T.; Rosenthal, E. I.; Hasdeo, E. H.; Sanders, G. D.; Stanton, C. J.; Saito, R.; Dresselhaus, Mildred
We discuss how excitons can affect the generation of coherent radial breathing modes in the ultrafast spectroscopy of single-wall carbon nanotubes. Photoexcited excitons can be localized spatially and give rise to a spatially distributed driving force in real space which involves many phonon wave vectors of the exciton-phonon interaction. The equation of motion for the coherent phonons is modeled phenomenologically by the Klein-Gordon equation, which we solve for the oscillation amplitudes as a function of space and time. By averaging the calculated amplitudes per nanotube length, we obtain time-dependent coherent phonon amplitudes that resemble the homogeneous oscillations that are observed...

52.
Superconductivity from weak repulsion in hexagonal lattice systems - Nandkishore, Rahul; Thomale, Ronny; Chubukov, Andrey V.
We analyze the pairing instabilities for fermions on hexagonal lattices (both honeycomb and triangular ones) in a wide range of fermionic densities ranging from van Hove density at which a single large Fermi surface splits into two disconnected Fermi pockets, to a density at which disconnected pockets shrink to Fermi points (half-filling for a honeycomb lattice and full filling for a triangular lattice). We argue that for a generic doping in this range, superconductivity at weak coupling is of Kohn-Luttinger type, and, due to the presence of electronic interactions beyond on-site repulsion, is a threshold phenomenon, with superconductivity emerging only...

53.
Origins of hole traps in hydrogenated nanocrystalline and amorphous silicon revealed through machine learning - Mueller, Tim; Grossman, Jeffrey C.; Johlin, Eric Carl
Genetic programming is used to identify the structural features most strongly associated with hole traps in hydrogenated nanocrystalline silicon with very low crystalline volume fraction. The genetic programming algorithm reveals that hole traps are most strongly associated with local structures within the amorphous region in which a single hydrogen atom is bound to two silicon atoms (bridge bonds), near fivefold coordinated silicon (floating bonds), or where there is a particularly dense cluster of many silicon atoms. Based on these results, we propose a mechanism by which deep hole traps associated with bridge bonds may contribute to the Staebler-Wronski effect.

54.
Direct evidence of spin filtering across MnFe[subscript 2]O[subscript 4] tunnel barrier by Meservey-Tedrow experiment - Matzen, S.; Moussy, J.-B.; Miao, G. X.; Moodera, Jagadeesh
A spin filtering effect has been evidenced in epitaxial MnFe[subscript 2]O[subscript 4] tunnel barriers directly by Meservey-Tedrow experiments. The asymmetry of the Zeeman-split tunneling conductance curves of the superconducting Al spin analyzer in Pt(111)/MnFe[subscript 2]O[subscript 4](111)/γ-Al[subscript 2]O[subscript 3](111)/Al tunnel junctions revealed a positive spin-polarization (up to +9%), proving the potential of manganese ferrite for generation of a spin-polarized current. A negatively polarized spin filtering being expected theoretically, different mechanisms are discussed to explain both sign and amplitude of the measured spin-polarization.

55.
Approaching the restricted solid-on-solid critical points through entanglement: One model for many universalities - De Luca, A.; Franchini, Fabio
We analytically compute the Renyi entropies for the RSOS models, representing a wide class of exactly solvable models with multicritical conformal points described by unitary minimal models and Z[subscript n] parafermions. The exact expressions allow for an explicit comparison of the expansions around the critical points with the predictions coming from field theory. In this way, it is possible to point out the nature of the so-called “unusual corrections,” clarifying the link with the operator content, the role of the symmetries and the boundary conditions. By choosing different boundary conditions, we can single out the ground states as well as...

56.
Two types of surface states in topological crystalline insulators - Liu, Junwei; Duan, Wenhui; Fu, Liang
Topological crystalline insulators (TCIs) are new states of matter whose topological distinction relies on the crystal symmetry of periodic solids. The first material realization of TCIs has recently been predicted and observed in IV-VI semiconductor SnTe and related alloys. By combining k⋅p theory and band structure calculation, we present a unified approach to study topological surface states on various crystal surfaces of these TCI materials based on the electronic structure of the bulk. Depending on the surface orientation, we find two types of surface states with qualitatively different properties. In particular, the (111) surface states consist of four Dirac cones...

57.
Isotropic and energy-selective electron cloaks on graphene - Liao, Bolin; Zebarjadi, Mona; Esfarjani, Keivan; Chen, Gang
We propose and investigate a design for “electron cloaks” comprised of two electrodes, one top gate and one back gate, on either side of a graphene sheet arranged in a concentric disk configuration. Dirac electrons with specific energies can flow through these electron cloaks with negligible scattering, while electrons with different energies experience significant scattering. The scattering widths of the electron cloaks are analyzed using the partial wave formalism applied to the Dirac equation, and the contributions of the first two partial waves to the scattering widths are set to zero simultaneously via a proper combination of the potentials on...

58.
Observation of tunnel magnetoresistance in a superconducting junction with Zeeman-split energy bands - Li, Bin; Miao, Guo-Xing; Moodera, Jagadeesh
The change of the quasiparticle density of states in a superconductor due to the Zeeman splitting can lead to highly responsive spintronic devices. This is demonstrated here in the magnetotunneling studies of a supercondcutor/insulator/ferromagnet tunnel junction. A tunnel magnetoresistance (TMR) as large as 36% is obtained, resulting from the conductance variation due to spin-split quasiparticle tunneling, and only occurs in the superconducting state. Our results show that in addition to the naturally existent spin imbalance in ferromagnets (upon which conventional TMR is obtained), we can manipulate tunnel conductance by tailoring spin dependent density of states with interfacial exchange fields, and...

59.
Electron-electron interactions and plasmon dispersion in graphene - Shtyk, A.; Feigelman, M.; Levitov, Leonid
Plasmons in two-dimensional electron systems with nonparabolic bands, such as graphene, feature strong dependence on electron-electron interactions. We use a many-body approach to relate plasmon dispersion at long wavelengths to Landau Fermi-liquid interactions and quasiparticle velocity. An identical renormalization is shown to arise for the magnetoplasmon resonance. For a model with N ≫ 1 fermion species, this approach predicts a power-law dependence for plasmon frequency vs carrier concentration, valid in a wide range of doping densities, both high and low. Gate tunability of plasmons in graphene can be exploited to directly probe the effects of electron-electron interaction.

60.
Lattice thermal conductivity of Bi, Sb, and Bi-Sb alloy from first principles - Lee, Sangyeop; Esfarjani, Keivan; Chen, Gang; Mendoza, Jonathan M.; Dresselhaus, Mildred
Using first principles, we calculate the lattice thermal conductivity of Bi, Sb, and Bi-Sb alloys, which are of great importance for thermoelectric and thermomagnetic cooling applications. Our calculation reveals that the ninth-neighbor harmonic and anharmonic force constants are significant; accordingly, they largely affect the lattice thermal conductivity. Several features of the thermal transport in these materials are studied: (1) the relative contributions from phonons and electrons to the total thermal conductivity as a function of temperature are estimated by comparing the calculated lattice thermal conductivity to the measured total thermal conductivity, (2) the anisotropy of the lattice thermal conductivity is...