7,844 research outputs found
Electronic Instabilities of the AA-Honeycomb Bilayer
We use a functional renormalization group approach to study the instabilities
due to electron-electron interactions in a bilayer honeycomb lattice model with
AA stacking, as it might be relevant for layered graphene with this structure.
Starting with a tight- binding description for the four -bands, we
integrate out the modes of the dispersion by successively lowering an infrared
cutoff and determine the leading tendencies in the effective interactions. The
antiferromagnetic spin-density wave is an expected instability for dominant
local repulsion among the electrons, but for nonlocal interaction terms also
other instabilities occur. We discuss the phase diagrams depending on the model
parameters. We compare our results to single-layer graphene and the more common
AB-stacked bilayer, both qualitatively and quantitatively.Comment: 9 pages, 3 figures, Annalen der Physik, online available (2014
Homotopy exponents for large H-spaces
We show that H-spaces with finitely generated cohomology, as an algebra or as
an algebra over the Steenrod algebra, have homotopy exponents at all primes.
This provides a positive answer to a question of Stanley.Comment: 4 page
Photonic microstructures as laser mirrors
Deeply etched 1-D third-order Bragg reflectors have been used as mirrors for broad-area semiconductor lasers operating at 975-nm wavelength. From a threshold and efficiency analysis, we determine the mirror reflectivity to be approximately 95%. The design of the GaAs-based laser structure features three InGaAs quantum wells placed close (0.5 ÎŒm) to the surface in order to reduce the required etch depth and facilitate high-quality etching. Despite the shallow design and the proximity of the guided mode to the metal contact, the threshold current density (J_(th) = 220âA/cm^2 for infinite cavity length) and internal loss (α_i = 9±1âcm^(â1)) are very low
Non-Volatile Memory Characteristics of Submicrometre Hall Structures Fabricated in Epitaxial Ferromagnetic MnAl Films on GaAs
Hall-effect structures with submicrometre linewidths (<0.3pm) have been fabricated in ferromagnetic thin films of Mn[sub 0.60]Al[sub 0.40] which are epitaxially grown on a GaAs substrate. The MnAl thin films exhibit a perpendicular remanent magnetisation and an extraordinary Hall effect with square hysteretic behaviour. The presence of two distinct stable readout states demonstrates the potential of using ultrasmall ferromagnetic volumes for electrically addressable, nonvolatile storage of digital information
On Reachable Sets of Hidden CPS Sensor Attacks
For given system dynamics, observer structure, and observer-based
fault/attack detection procedure, we provide mathematical tools -- in terms of
Linear Matrix Inequalities (LMIs) -- for computing outer ellipsoidal bounds on
the set of estimation errors that attacks can induce while maintaining the
alarm rate of the detector equal to its attack-free false alarm rate. We refer
to these sets to as hidden reachable sets. The obtained ellipsoidal bounds on
hidden reachable sets quantify the attacker's potential impact when it is
constrained to stay hidden from the detector. We provide tools for minimizing
the volume of these ellipsoidal bounds (minimizing thus the reachable sets) by
redesigning the observer gains. Simulation results are presented to illustrate
the performance of our tools
Long-distance practical quantum key distribution by entanglement swapping
We develop a model for practical, entanglement-based long-distance quantum
key distribution employing entanglement swapping as a key building block.
Relying only on existing off-the-shelf technology, we show how to optimize
resources so as to maximize secret key distribution rates. The tools comprise
lossy transmission links, such as telecom optical fibers or free space,
parametric down-conversion sources of entangled photon pairs, and threshold
detectors that are inefficient and have dark counts. Our analysis provides the
optimal trade-off between detector efficiency and dark counts, which are
usually competing, as well as the optimal source brightness that maximizes the
secret key rate for specified distances (i.e. loss) between sender and
receiver.Comment: 14 pages, 9 figures; published in Optics Expres
The extraordinary Hall effect in coherent epitaxial tau (Mn,Ni)Al thin films on GaAs
Ultrathin coherent epitaxial films of ferromagnetic tau(Mn,Ni)0.60Al0.40 have been grown by molecular beam epitaxy on GaAs substrates. X-ray scattering and cross-sectional transmission electron microscopy measurements confirm that the c axis of the tetragonal tau unit cell is aligned normal to the (001) GaAs substrate. Measurements of the extraordinary Hall effect (EHE) show that the films are perpendicularly magnetized, exhibiting EHE resistivities saturating in the range of 3.3-7.1 muOMEGA-cm at room temperature. These values of EHE resistivity correspond to signals as large as +7 and -7 mV for the two magnetic states of the film with a measurement current of 1 mA. Switching between the two magnetic states is found to occur at distinct field values that depend on the previously applied maximum field. These observations suggest that the films are magnetically uniform. As such, tau(Mn,Ni)Al films may be an excellent medium for high-density storage of binary information
Epitaxial-tau(Mn,Ni)Al/(Al,Ga)As heterostructures: Magnetic and magneto-optic properties
Ferromagnetic Perpendicularly magnetized epitaxial thin films of tau (Mn,Ni)AI have been successfully grown on AlAs/GaAs heterostructures by molecular beam epitaxy. We have investigated the polar Kerr rotation and magnetization of tau MnAl and (Mn,Ni) Al as a function of Mn and Ni concentration. The largest polar Kerr rotation and remnant magnetization were obtained for Mn0.5Al0.5 thin films with values of 0.16-degrees and 224 emu/cm3, respectively. We observed that the Kerr rotation and magnetization remained constant with Ni additions up to about 12 at. % and subsequently decreased with further Ni additions. We discuss these results and one possible method of enhancing the Kerr rotation
Suspended Nanoscale Field Emitter Devices for High-Temperature Operation
In this work, we demonstrate suspended two- and four-terminal field emission devices for high-temperature operation. The planar structures were fabricated with tungsten on a 200-nm silicon nitride membrane. The insulator in the vicinity of the terminals was removed to minimize undesirable FrenkelâPoole emission and increase the resistance of leakage current pathways. The effects of temperatures up to 450 °C on FowlerâNordheim emission characteristics and parasitic leakage resistance were studied. Turn-on voltages with magnitudes under 15 V that further decreased as a function of increasing temperature for the two-terminal device were reported. Gating at temperatures of 150 °C and 300 °C was shown for the four-terminal device, and corresponding transconductance and cutoff frequency values were computed
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