5,833 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
High-J v=0 SiS Maser Emission in IRC+10216: A New Case of Infrared Overlaps
We report on the first detection of maser emission in the J=11-10, J=14-13
and J=15-14 transitions of the v=0 vibrational state of SiS toward the C-rich
star IRC+10216. These masers seem to be produced in the very inhomogeneous
region between the star and the inner dust formation zone, placed at 5-7 R*,
with expansion velocities below 10 km/s. We interpret the pumping mechanism as
due to overlaps between v=1-0 ro-vibrational lines of SiS and mid-IR lines of
C2H2, HCN and their 13C isotopologues. The large number of overlaps found
suggests the existence of strong masers for high-J v=0 and v=1 SiS transitions,
located in the submillimeter range. In addition, it could be possible to find
several rotational lines of the SiS isotopologues displaying maser emission.Comment: 4 pages, 1 figure, published in the ApJ Letter
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
Homopolar bond formation in ZnVO close to a metal-insulator transition
Electronic structure calculations for spinel vanadate ZnVO show that
partial electronic delocalization in this system leads to structural
instabilities. These are a consequence of the proximity to the
itinerant-electron boundary, not being related to orbital ordering. We discuss
how this mechanism naturally couples charge and lattice degrees of freedom in
magnetic insulators close to such a crossover. For the case of ZnVO,
this leads to the formation of V-V dimers along the [011] and [101] directions
that readily accounts for the intriguing magnetic structure of ZnVO.Comment: 5 pages, 3 figures, 1 tabl
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