6,264 research outputs found
Dirac Point Degenerate with Massive Bands at a Topological Quantum Critical Point
The quasi-linear bands in the topologically trivial skutterudite insulator
CoSb are studied under adiabatic, symmetry-conserving displacement of the
Sb sublattice. In this cubic, time-reversal and inversion symmetric system, a
transition from trivial insulator to topological point Fermi surface system
occurs through a critical point in which massless (Dirac) bands are {\it
degenerate} with massive bands. Spin-orbit coupling does not alter the
character of the transition. The mineral skutterudite (CoSb) is very near
the critical point in its natural state.Comment: 5 pages, 3 figure
Intelligence/Electronic Warfare (IEW) direction-finding and fix estimation analysis report. Volume 2: Trailblazer
An analysis of the direction finding (DF) and fix estimation algorithms in TRAILBLAZER is presented. The TRAILBLAZER software analyzed is old and not currently used in the field. However, the algorithms analyzed are used in other current IEW systems. The underlying algorithm assumptions (including unmodeled errors) are examined along with their appropriateness for TRAILBLAZER. Coding and documentation problems are then discussed. A detailed error budget is presented
Linear bands, zero-momentum Weyl semimetal, and topological transition in skutterudite-structure pnictides
It was reported earlier [Phys. Rev. Lett. 106, 056401 (2011)] that the
skutterudite structure compound CoSb displays a unique band structure with
a topological transition versus a symmetry-preserving sublattice (Sb)
displacement very near the structural ground state. The transition is through a
massless Dirac-Weyl semimetal, point Fermi surface phase which is unique in
that (1) it appears in a three dimensional crystal, (2) the band critical point
occurs at =0, and (3) linear bands are degenerate with conventional
(massive) bands at the critical point (before inclusion of spin-orbit
coupling). Further interest arises because the critical point separates a
conventional (trivial) phase from a topological phase. In the native cubic
structure this is a zero-gap topological semimetal; we show how spin-orbit
coupling and uniaxial strain converts the system to a topological insulator
(TI). We also analyze the origin of the linear band in this class of materials,
which is the characteristic that makes them potentially useful in
thermoelectric applications or possibly as transparent conductors. We
characterize the formal charge as Co , consistent with the gap, with
its site symmetry, and with its lack of moment. The Sb states are
characterized as (separately, ) -bonded ring states
occupied and the corresponding antibonding states empty. The remaining
(locally) orbitals form molecular orbitals with definite parity centered
on the empty site in the skutterudite structure. Eight such orbitals must
be occupied; the one giving the linear band is an odd orbital singlet
at the zone center. We observe that the provocative linearity of the band
within the gap is a consequence of the aforementioned near-degeneracy, which is
also responsible for the small band gap.Comment: 10 pages, 7 figure
Electronic and structural distortions in graphene induced by carbon vacancies and boron doping
We present an ab initio study on the structural and electronic distortions of
modified graphene by creation of vacancies, inclusion of boron atoms, and the
coexistence of both, by means of thermodynamics and band structure
calculations. In the case of coexistence of boron atoms and vacancy, the
modified graphene presents spin polarization only when B atoms locate far from
vacancy. Thus, when a boron atom fills single- and di-vacancies, it suppresses
the spin polarization of the charge density. In particular when B atoms fill a
di-vacancy a new type of rearrangement occurs, where a stable BC4 unit is
formed inducing important out of plane distortions to graphene. All these
findings suggest that new chemical modifications to graphene and new type of
vacancies can be used for interesting applications such as sensor and chemical
labeling.Comment: 22 pages, 9 figures and 3 table
Multiple Charge State Beam Acceleration at Atlas
A test of the acceleration of multiple charge-state uranium beams was
performed at the ATLAS accelerator. A 238U+26 beam was accelerated in the ATLAS
PII linac to 286 MeV (~1.2 MeV/u) and stripped in a carbon foil located 0.5 m
from the entrance of the ATLAS Booster section. A 58Ni9+ 'guide' beam from the
tandem injector was used to tune the Booster for 238U+38. All charge states
from the stripping were injected into the booster and accelerated. Up to 94% of
the beam was accelerated through the Booster linac, with losses mostly in the
lower charge states. The measured beam properties of each charge state and a
comparison to numerical simulations are reported in this paper.Comment: LINAC2000, MOD0
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