Compressibility Of A Two-Dimensional Hole Gas In A Tilted Magnetic Field,
2011
Northeastern University
Compressibility Of A Two-Dimensional Hole Gas In A Tilted Magnetic Field, Maryam Rahimi, M. Sakr, S. Kravchenko, S. Dultz, H. Jiang
Sergey Kravchenko
We have measured compressibility of a two-dimensional hole gas in p-GaAs/AlGaAs heterostructure, grown on a (100) surface, in the presence of a tilted magnetic field. It turns out that the parallel component of magnetic field affects neither the spin splitting nor the density of states. We conclude that: (a) g-factor in the parallel magnetic field is nearly zero in this system; and (b) the level of the disorder potential is not sensitive to the parallel component of the magnetic field.
Conductivity Of A Spin-Polarized Two-Dimensional Electron Liquid In The Ballistic Regime,
2011
Institute of Solid State Physics, Russia
Conductivity Of A Spin-Polarized Two-Dimensional Electron Liquid In The Ballistic Regime, A. A. Shashkin, E. V. Deviatov, V. T. Dolgopolov, A. A. Kapustin, S. Anissimova, A. Venkatesan, S. V. Kravchenko, T. M. Klapwijk
Sergey Kravchenko
In the ballistic regime, the metallic temperature dependence of the conductivity in a twodimensional electron system in silicon is found to change non-monotonically with the degree of spin polarization. In particular, it fades away just before the onset of complete spin polarization but reappears again in the fully spin-polarized state, being, however, suppressed relative to the zero-field case. Analysis of the degree of the suppression allows one to distinguish between the screening and the interaction-based theories.
A Metal-Insulator Transition In 2d: Established Facts And Open Questions,
2011
Northeastern University
A Metal-Insulator Transition In 2d: Established Facts And Open Questions, S. V. Kravchenko, M. P. Sarachik
Sergey Kravchenko
The discovery of a metallic state and a metal-insulator transition (MIT) in two-dimensional (2D) electron systems challenges one of the most influential paradigms of modern mesoscopic physics, namely, that "there is no true metallic behavior in two dimensions". However, this conclusion was drawn for systems of noninteracting or weakly interacting carriers, while in all 2D systems exhibiting the metal-insulator transition, the interaction energy greatly exceeds all other energy scales. We review the main experimental findings and show that, although significant progress has been achieved in our understanding of the MIT in 2D, many open questions remain.
Transport In Strongly Correlated Two Dimensional Electron Fluids,
2011
Northeastern University
Transport In Strongly Correlated Two Dimensional Electron Fluids, S. Kravchenko, S. Kivelson, X. Gao, X. Gao
Sergey Kravchenko
We present an overview of the measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities, and of some of the theories that have been proposed to account for them. Many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium. Rather, they reflect new physics associated with strong correlation effects, which warrant further study.
"Forbidden" Transitions Between Quantum Hall And Insulating Phases In P-Sige Heterostructures,
2011
Northeastern University
"Forbidden" Transitions Between Quantum Hall And Insulating Phases In P-Sige Heterostructures, M. R. Sakr, Maryam Rahimi, S. V. Kravchenko, P. T. Coleridge, R. L. Williams, J. Lapointe
Sergey Kravchenko
We show that in dilute metallic p-SiGe heterostructures, magnetic field can cause multiple quantum Hall-insulator-quantum Hall transitions. The insulating states are observed between quantum Hall states with filling factors = 1 and 2 and, for the first time, between = 2 and 3 and between = 4 and 6. The latter are in contradiction with the original global phase diagram for the quantum Hall effect. We suggest that the application of a (perpendicular) magnetic field induces insulating behaviour in metallic p-SiGe heterostructures in the same way as in Si MOSFETs. This insulator is then in competition with, and interrupted by, …
Comment On "Theory Of Metal-Insulator Transitions In Gated Semiconductors",
2011
Northeastern University
Comment On "Theory Of Metal-Insulator Transitions In Gated Semiconductors", S. V. Kravchenko, M. P. Sarachik, D. Simonian
Sergey Kravchenko
No abstract provided.
Coupled Dynamics Of Voltage And Calcium In Paced Cardiac Cells,
2011
Northeastern University
Coupled Dynamics Of Voltage And Calcium In Paced Cardiac Cells, Yohannes Shiferaw, Daisuke Sato, Alain Karma
Alain Karma
We investigate numerically and analytically the coupled dynamics of transmembrane voltage and intracellular calcium cycling in paced cardiac cells using a detailed physiological model and its reduction to a three-dimensional discrete map. The results provide a theoretical framework to interpret various experimentally observed modes of instability ranging from electromechanically concordant and discordant alternans to quasiperiodic oscillations of voltage and calcium.
Amplitude Equations For Polycrystalline Materials With Interaction Between Composition And Stress,
2011
Northeastern University
Amplitude Equations For Polycrystalline Materials With Interaction Between Composition And Stress, Alain Karma
Alain Karma
We investigate the ability of frame-invariant amplitude equations [G. H. Gunaratne, Q. Ouyang, and H. Swinney, Phys. Rev. E 50, 2802 (1994)] to describe quantitatively the evolution of polycrystalline microstructures and we extend this approach to include the interaction between composition and stress. Validations for elemental materials include studies of the Asaro-Tiller-Grinfeld morphological instability of a stressed crystal surface, polycrystalline growth from the melt, grain boundary energies over a wide range of misorientation, and grain boundary motion coupled to shear deformation. Amplitude equations with accelerated strain relaxation in the solid are shown to model accurately the Asaro-Tiller-Grinfeld instability. Polycrystalline growth …
Black Hole Chromosphere At The Cern Lhc,
2011
Northeastern University
Black Hole Chromosphere At The Cern Lhc, Luis Anchordoqui, Haim Goldberg
Haim Goldberg
If the scale of quantum gravity is near a TeV, black holes will be copiously produced at the LHC. In this work we study the main properties of the light descendants of these black holes. We show that the emitted partons are closely spaced outside the horizon, and hence they do not fragment into hadrons in vacuum but more likely into a kind of quark-gluon plasma. Consequently, the thermal emission occurs far from the horizon, at a temperature characteristic of the QCD scale. We analyze the energy spectrum of the particles emerging from the "chromosphere", and find that the hard …
Constraints On Unparticle Physics From Solar And Kamland Neutrinos,
2011
University of Wisconsin - Milwaukee
Constraints On Unparticle Physics From Solar And Kamland Neutrinos, Luis Anchordoqui, Haim Goldberg
Haim Goldberg
Interest has been directed recently towards low energy implications of a non-trivial conformal sector of an effective field theory with an IR fixed point (ΛU), manifest in terms of "unparticles" with bizarre properties. We re-examine the implications of the limits on decay lifetimes of solar neutrinos for unparticle interactions. We study in detail the fundamental parameter space (ΛU,M) and derive bounds on the energy scale M characterizing the new physics. We work strictly within the framework where conformal invariance holds down to low energies. We first assume that couplings of the unparticle sector to the Higgs field are suppressed and …
Can Hidden Gauginos Form Condensates?,
2011
Northeastern University
Can Hidden Gauginos Form Condensates?, Haim Goldberg
Haim Goldberg
Supersymmetry breaking may be linked to the formation of gaugino condensates in a hidden sector. In this work, the process of formation of the condensate is examined in a cosmological context, using an effective field theory of the gaugino bilinear which provides a reasonable interpolation between the high- and low-temperature phases. The implementation of anomaly requirements generates a large potential barrier between the zero-condensate configuration and that of the true (SUSY-breaking) vacuum. As a consequence, the transition to bubbles of true vacuum may be subject to an enormous exponential suppression. This leads to the same difficulties with inhomogeneity of the …
Auger Test Of The Cen A Model Of Highest Energy Cosmic Rays,
2011
Northeastern University
Auger Test Of The Cen A Model Of Highest Energy Cosmic Rays, Luis A. Anchordoqui, Haim Goldberg, Thomas J. Weiler
Haim Goldberg
If, as recently proposed by Farrar and Piran, Cen A is the source of cosmic rays detected above the Greisen-Zatsepin-Kuz’min cutoff, neutrons are ≈ 140 more probable than protons to be observed along its line of sight. This is because the proton flux is rendered nearly isotropic by O(μG) intergalactic magnetic fields. With the anticipated aperture of the Southern Auger Observatory, one may expect on the order of 2 neutron events/year above 1020 eV in the line of sight of Cen A.
Cosmology From String Theory,
2011
University of Wisconsin - Milwaukee
Cosmology From String Theory, Luis Anchordoqui, Haim Goldberg, Satoshi Nawata, Carlos Nuñez
Haim Goldberg
We explore the cosmological content of Salam-Sezgin six dimensional supergravity, and find a solution to the field equations in qualitative agreement with observation of distant supernovae, primordial nucleosynthesis abundances, and recent measurements of the cosmic microwave background. The carrier of the acceleration in the present de Sitter epoch is a quintessence field slowly rolling down its exponential potential. Intrinsic to this model is a second modulus which is automatically stabilized and acts as a source of cold dark matter, with a mass proportional to an exponential function of the quintessence field (hence realizing VAMP models within a String context). However, …
Decay Widths Of Lowest Massive Regge Excitations Of Open Strings,
2011
University of Wisconsin - Milwaukee
Decay Widths Of Lowest Massive Regge Excitations Of Open Strings, Luis A. Anchordoqui, Haim Goldberg, Tomasz R. Taylor
Haim Goldberg
With the advent of the LHC there is widespread interest in the discovery potential for physics beyond the standard model. In TeV-scale open string theory, the new physics can be manifest in the excitation and decay of new resonant structures, corresponding to Regge recurrences of standard model particles. An essential input for the prediction of invariant mass spectra of the decay products (which could serve to identify the resonance as a string excitation) are the partial and total widths of the decay products. We present a parameter-free calculation of these widths for the first Regge recurrence of the SU(3) gluon …
Anisotropy At The End Of The Cosmic Ray Spectrum?,
2011
Northeastern University
Anisotropy At The End Of The Cosmic Ray Spectrum?, Luis A. Anchordoqui, Haim Goldberg, Diego F. Torres
Haim Goldberg
The starburst galaxies M82 and NGC253 have been proposed as the primary sources of cosmic rays with energies above 1020.3 eV. For energies ≳ 1020.3 eV the model predicts strong anisotropies. We calculate the probabilities that the latter can be due to chance occurrence. For the highest energy cosmic ray events in this energy region, we find that the observed directionality has less than 1% probability of occurring due to random fluctuations. Moreover, during the first 5 years of operation at Auger, the observation of even half the predicted anisotropy has a probability of less than 10−5 to occur by …
Black Holes From Cosmic Rays: Probes Of Extra Dimensions And New Limits On Tev-Scale Gravity,
2011
Northeastern University
Black Holes From Cosmic Rays: Probes Of Extra Dimensions And New Limits On Tev-Scale Gravity, Luis Anchordoqui, Jonathan Feng, Haim Goldberg, Alfred Shapere
Haim Goldberg
If extra spacetime dimensions and low-scale gravity exist, black holes will be produced in observable collisions of elementary particles. For the next several years, ultra-high energy cosmic rays provide the most promising window on this phenomenon. In particular, cosmic neutrinos can produce black holes deep in the Earth’s atmosphere, leading to quasi-horizontal giant air showers. We determine the sensitivity of cosmic ray detectors to black hole production and compare the results to other probes of extra dimensions. With n ≥ 4 extra dimensions, current bounds on deeply penetrating showers from AGASA already provide the most stringent bound on low-scale gravity, …
Critical Fluctuations In Spatial Complex Networks,
2011
International School for Advanced Studies, Italy
Critical Fluctuations In Spatial Complex Networks, Serena Bradde, Fabio Caccioli, Luca Dall'asta, Ginestra Bianconi
Ginestra Bianconi
An anomalous mean-field solution is known to capture the non trivial phase diagram of the Ising model in annealed complex networks. Nevertheless the critical fluctuations in random complex networks remain mean-field. Here we show that a break-down of this scenario can be obtained when complex networks are embedded in geometrical spaces. Through the analysis of the Ising model on annealed spatial networks, we reveal in particular the spectral properties of networks responsible for critical fluctuations and we generalize the Ginsburg criterion to complex topologies.
Bose-Einstein Condensation In Complex Networks,
2011
University of Notre Dame
Bose-Einstein Condensation In Complex Networks, Ginestra Bianconi, Albert-László Barabási
Ginestra Bianconi
The evolution of many complex systems, including the World Wide Web, business, and citation networks, is encoded in the dynamic web describing the interactions between the system's constituents. Despite their irreversible and nonequilibrium nature these networks follow Bose statistics and can undergo Bose-Einstein condensation. Addressing the dynamical properties of these nonequilibrium systems within the framework of equilibrium quantum gases predicts that the "first-mover-advantage," "fit-get-rich," and "winner-takes-all" phenomena observed in competitive systems are thermodynamically distinct phases of the underlying evolving networks.
Air Shower Simulation Using Geant4 And Commodity Parallel Computing,
2011
Northeastern University
Air Shower Simulation Using Geant4 And Commodity Parallel Computing, L. A. Anchordoqui, G. Cooperman, V. Grinberg, T. P. Mccauley, T. Paul, S. Reucroft, J. D. Swain, G. Alverson
George Alverson
We present an evaluation of a simulated cosmic ray shower, based on GEANT4 and TOP-C, which tracks all the particles in the shower. TOP-C (Task Oriented Parallel C) provides a framework for parallel algorithm development which makes tractable the problem of following each particle. This method is compared with a simulation program which employs the Hillas thinning algorithm.
Anomalous Scattering Rate And Microwave Absorption In Bi₂Sr₂Cacu₂O₈₊Δ And Y Ba₂Cu₃O₇₋Δ,
2011
Northeastern University
Anomalous Scattering Rate And Microwave Absorption In Bi₂Sr₂Cacu₂O₈₊Δ And Y Ba₂Cu₃O₇₋Δ, C. Kusko, Z. Zhai, R. S. Markiewicz, S. Sridhar
Robert Markiewicz
We determine the scattering rate from microwave measurements for an optimally doped Bi-2212 single crystal, using a simple two fluid model with a d-wave symmetry order parameter. In the superconducting state, the calculated scattering rate is three orders of magnitude smaller than that determined from ARPES experiments. A similar anomalously large decrease in the scattering rate is also required to explain the data within a gap-quasiparticle scenario for other HTS, such as Y Ba₂Cu₃O₇₋δ. The results suggest that the assumption of normal excitations vanishing at low T is invalid and an additional charge mode is responsible for the microwave absorption.
