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

Mountain maple and balsam fir early response to partial and clear-cut harvesting under aspen stands of northern Quebec

This study is a component of the Sylviculture et am

Characteristics of Stipends and Their Value-Adding Potential in Design-Build US Highway Construction

A best practice tool to enhance design-build best-value procurements is stipends, yet there is very little literature dedicated to this topic. Cited benefits of stipends include incentivizing the level of effort put forth by proposers in preparing their technical proposal, encouraging the number of proposers and thereby increasing competition, and mitigating risk. This paper presents cross-validated findings from the literature, agency policies, a survey of 53 US design-build projects, six agency representative interviews, and 13 design-build industry professional interviews. This paper investigates four aspects of stipends: (1) stipend value and calculation processes; (2) impact on a contractor's decision to propose; (3) impact of stipend amount on an offeror's proposal development, and (4) stipends' ability to aid agencies in achieving best value for highway construction projects. Stipends were found to be a necessary process to achieve best value because they increase competition and often can increase the quality of a proposal based on the stipend amount. Stipends typically cover one-third to one-half of a contractor's proposal costs. Agencies should use stipends when proposal costs are expected to be high and should estimate the stipend amount on a project-by-project basis. Properly valued stipends demonstrate that an agency is serious about going forward with the project, and understands both the work required and the design-build process. Finally, stipends were found to promote a fair procurement process, building trust with contractors leading to a more collaborative and innovative project execution.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]