4,461 research outputs found
Focusing RKKY interaction by graphene P-N junction
The carrier-mediated RKKY interaction between local spins plays an important
role for the application of magnetically doped graphene in spintronics and
quantum computation. Previous studies largely concentrate on the influence of
electronic states of uniform systems on the RKKY interaction. Here we reveal a
very different way to manipulate the RKKY interaction by showing that the
anomalous focusing - a well-known electron optics phenomenon in graphene P-N
junctions - can be utilized to refocus the massless Dirac electrons emanating
from one local spin to the other local spin. This gives rise to rich spatial
interference patterns and symmetry-protected non-oscillatory RKKY interaction
with a strongly enhanced magnitude. It may provide a new way to engineer the
long-range spin-spin interaction in graphene.Comment: 9 pages, 4 figure
Unusual negative formation enthalpies and atomic ordering in isovalent alloys of transition metal dichalcogenide monolayers
Common substitutional isovalent semiconductor alloys usually form disordered
metastable phases with positive excess formation enthalpies ({\Delta}H). In
contrast, monolayer alloys of transition metal dichalcogenides (TMDs) MX2 (M =
Mo, W; X = S, Se) always have negative {\Delta}H, suggesting atomic ordering,
which is, however, not yet experimentally observed. Using first-principles
calculations, we find that the negative {\Delta}H of cation-mixed TMD alloys
results from the charge transfer from weak Mo-X to nearest strong W-X bonds and
the negative {\Delta}H of anion-mixed TMD alloys comes from the larger energy
gain due to the charge transfer from Se to nearest S atoms than the energy cost
due to the lattice mismatch. Consequently, cation-mixed and anion-mixed alloys
should energetically prefer to have Mo-X-W and S-M-Se ordering, respectively.
The atomic ordering, however, is only locally ordered but disordered in the
long range due to the symmetry of TMD monolayers, as demonstrated by many
energetically degenerate structures for given alloy compositions. Besides, the
local ordering and disordering effects on the macroscopic properties such as
bandgaps and optical absorptions are negligible, making the experimental
observation of locally ordered TMD alloys challenging. We propose to take the
advantage of microscopic properties such as defects which strongly depend on
local atomic configurations for experiments to identify the disordering and
local ordering in TMD alloys. Finally, quaternary TMD alloys by mixing both
cations and anions are studied to have a wide range of bandgaps for
optoelectronic applications. Our work is expected to help the formation and
utilization of TMD alloys.Comment: 25 pages, 6 figure
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