Vortex States of a Superconducting Film from a Magnetic Dot Array

Using Ginzburg-Landau theory, we find novel configurations of vortices in superconducting thin films subject to the magnetic field of a magnetic dot array, with dipole moments oriented perpendicular to the film. Sufficiently strong magnets cause the formation of vortex-antivortex pairs. In most cases, the vortices are confined to dot regions, while the antivortices can form a rich variety of lattice states. We propose an experiment in which the perpendicular component of the dot dipole moments can be tuned using an in-plane magnetic field. We show that in such an experiment the vortex-antivortex pair density shows broad plateaus as a function of the dipole strength. Many of the plateaus correspond to vortex configurations which break dot lattice symmetries. In some of these states, the vortex cores are strongly distorted. Possible experimental consequences are mentioned.Comment: 4 pages, 4 figure

Skyrme and Wigner crystals in graphene

At low-energy, the band structure of graphene can be approximated by two degenerate valleys $(K,K^{\prime})$ about which the electronic spectra of the valence and conduction bands have linear dispersion relations. An electronic state in this band spectrum is a linear superposition of states from the $A$ and $B$ sublattices of the honeycomb lattice of graphene. In a quantizing magnetic field, the band spectrum is split into Landau levels with level N=0 having zero weight on the $B(A)$ sublattice for the $$ valley. Treating the valley index as a pseudospin and assuming the real spins to be fully polarized, we compute the energy of Wigner and Skyrme crystals in the Hartree-Fock approximation. We show that Skyrme crystals have lower energy than Wigner crystals \textit{i.e.} crystals with no pseudospin texture in some range of filling factor $\nu$ around integer fillings. The collective mode spectrum of the valley-skyrmion crystal has three linearly-dispersing Goldstone modes in addition to the usual phonon mode while a Wigner crystal has only one extra Goldstone mode with a quadratic dispersion. We comment on how these modes should be affected by disorder and how, in principle, a microwave absorption experiment could distinguish between Wigner and Skyrme crystals.Comment: 14 pages with 11 figure

Evaluation of MOSFETs for Terahertz Detector Arrays

The terahertz (THz) region of the electromagnetic spectrum is one of the last remaining regions that has yet to be fully characterized. THz imaging is one of the foremost drivers of this technology gap and has the potential to push development in the near term to a similar capability level as infrared (IR). Properties of THz radiation are introduced, along with promising current applications. Interest in array based imaging of THz radiation (T-Rays) has gained traction lately, specifically using a CMOS process due to its ease of manufacturability and the use of MOSFETs as a detection mechanism. The theory outlined explains that incident terahertz radiation on to the gate channel region of a properly configured MOSFET can be related to plasmonic response waves, which change the electron density and potential across the channel producing a photoinduced response. This work utilizes a test chip fabricated to investigate these effects. The 0.35 um silicon CMOS MOSFETs tested contain varying structures, providing a range of detectors to analyze. Included are individual test MOSFETs for which various operating parameters and modes are studied and results presented. The focus on single transistor-antenna testing provides a path for discovering the most efficient combination for coupling 0.2 THz band energy. Specifically introduced, is a novel source region extension which is proven to improve MOSFET response. Sensitivity analysis and responsivity are described, in parallel with theoretical expectations of the plasmonic response in room temperature conditions. A maximum responsivity of 40 kV/W and corresponding NEP of 10 pW/Hz^(-1/2) (±10% uncertainty) is demonstrated

Excitations from Filled Landau Levels in Graphene

We consider graphene in a strong perpendicular magnetic field at zero temperature with an integral number of filled Landau levels and study the dispersion of single particle-hole excitations. We first analyze the two-body problem of a single Dirac electron and hole in a magnetic field interacting via Coulomb forces. We then turn to the many-body problem, where particle-hole symmetry and the existence of two valleys lead to a number of effects peculiar to graphene. We find that the coupling together of a large number of low-lying excitations leads to strong many-body corrections, which could be observed in inelastic light scattering or optical absorption. We also discuss in detail how the appearance of different branches in the exciton dispersion is sensitive to the number of filled spin and valley sublevels.Comment: 15 pages, 19 figure

Collective Excitations of a Two-Component Bose Condensate at Finite Temperature

We compare the collective modes for Bose-condensed systems with two degenerate components with and without spontaneous intercomponent coherence at finite temperature using the time-dependent Hartree-Fock approximation. We show that the interaction between the condensate and non-condensate in these two cases results in qualitatively different collective excitation spectra. We show that at zero temperature the single-particle excitations of the incoherent Bose condensate can be probed by intercomponent excitations.Comment: 4 pages, 3 figures, Submitted to PR

Orbital maneuvering engine feed system coupled stability investigation

A digital computer model used to analyze and predict engine feed system coupled instabilities over a frequency range of 10 to 1000 Hz was developed and verified. The analytical approach to modeling the feed system hydrodynamics, combustion dynamics, chamber dynamics, and overall engineering model structure is described and the governing equations in each of the technical areas are presented. This is followed by a description of the generalized computer model, including formulation of the discrete subprograms and their integration into an overall engineering model structure. The operation and capabilities of the engineering model were verified by comparing the model's theoretical predictions with experimental data from an OMS-type engine with a known feed system/engine chugging history

Signature of Quantum Hall Effect Skyrmions in Tunneling: A Theoretical Study

We present a theoretical study of the $I-V$ tunneling characteristic between two parallel two-dimensional electron gases in a perpendicular magnetic field when both are near filling factor $\nu=1$. Finite-size calculations of the single-layer spectral functions in the spherical geometry and analytical expressions for the disk geometry in the thermodynamic limit show that the current in the presence of skyrmions reflects in a direct way their underlying structure. It is also shown that fingerprints of the electron-electron interaction pseudopotentials are present in such a current.Comment: 4 pages, 1 figur