5,522 research outputs found
Theory of the Magnetic Moment in Iron Pnictides
We show that the combined effects of spin-orbit, monoclinic distortion, and
p-d hybridization in tetrahedrally coordinated Fe in LaOFeAs invalidates the
naive Hund's rule filling of the Fe d-levels. The two highest occupied levels
have one electron each but as a result of the p-d hybridization have very
different on-site repulsions. As a result, electrons in the upper level are
more itinerant while those in the lower level are more localized. It is the
xy-projection of the spin in the lower level that orders antiferromagnetically
as the z-components of the spins in the two levels is shown to be vanishingly
small in the ground state. The resulting magnetic moment is highly anisotropic
with an in-plane value of per Fe and a z-projection of
, both of which are in agreement with experiment. As a consequence,
we arrive the minimal model that describes the electronic properties of
LaOFeAs.Comment: Published Versio
Strain-induced gauge and Rashba fields in ferroelectric Rashba lead chalcogenide PbX monolayers (X = S, Se, Te)
One of the exciting features of two-dimensional (2D) materials is their
electronic and optical tunability through strain engineering. Previously, we
found a class of 2D ferroelectric Rashba semiconductors PbX (X = S, Se, Te)
with tunable spin-orbital properties. In this work, based on our previous
tight-binding (TB) results, we derive an effective low-energy Hamiltonian
around the symmetry points that captures the effects of strain on the
electronic properties of PbX. We find that strains induce gauge fields which
shift the Rashba point and modify the Rashba parameter. This effect is
equivalent to the application of in-plane magnetic fields. The out-of-plane
strain, which is proportional to the electric polarization, is also shown to
modify the Rashba parameter. Overall, our theory connects strain and spin
splitting in ferroelectric Rashba materials, which will be important to
understand the strain-induced variations in local Rashba parameters that will
occur in practical applications
Spin-Orbit Dirac Fermions in 2D Systems
We propose a novel model for including spin-orbit interactions in buckled two
dimensional systems. Our results show that in such systems, intrinsic
spin-orbit coupling leads to a formation of Dirac cones, similar to Rashba
model. We explore the microscopic origins of this behaviour and confirm our
results using DFT calculations
Two-dimensional square buckled Rashba lead chalcogenides
We propose the lead sulphide (PbS) monolayer as a two-dimensional semiconductor with a large Rashba-like spin-orbit effect controlled by the out-of-plane buckling. The buckled PbS conduction band is found to possess Rashba-like dispersion and spin texture at the M and Γ points, with large effective Rashba parameters of λ∼5 eV Å and λ∼1 eV Å, respectively. Using a tight-binding formalism, we show that the Rashba effect originates from the very large spin-orbit interaction and the hopping term that mixes the in-plane and out-of-plane p orbitals of Pb and S atoms. The latter, which depends on the buckling angle, can be controlled by applying strain to vary the spin texture as well as the Rashba parameter at Γ and M. Our density functional theory results together with tight-binding formalism provide a unifying framework for designing Rashba monolayers and for manipulating their spin properties.P.Z.H., H.S.P., and D.K.C. acknowledge the support of the Physics and Mechanical Engineering Department at Boston University. P.Z.H. is grateful for the hospitality of the NUS Centre for Advanced 2D Materials and Graphene Research Centre where this work was initiated. D.K.C. acknowledges the hospitality of the Aspen Center for Physics, which is supported by the US National Science Foundation Grant No. PHY-1607611. A.S.R., A.C.,and A.H.C.N. acknowledge support by the National Research Foundation, Prime Minister Office, Singapore, under its Medium Sized Centre Programme and CRP award "Novel 2D materials with tailored properties: Beyond graphene" (Grant No. R-144-000295-281). (Physics and Mechanical Engineering Department at Boston University; PHY-1607611 - US National Science Foundation; R-144-000295-281 - National Research Foundation, Prime Minister Office, Singapore, under its Medium Sized Centre Programme and CRP award "Novel 2D materials with tailored properties: Beyond graphene")Published versio
Plasmonic Hot Spots in Triangular Tapered Graphene Microcrystals
Recently, plasmons in graphene have been observed experimentally using
scattering scanning near-field optical microscopy. In this paper, we develop a
simplified analytical approach to describe the behavior in triangular samples.
Replacing Coulomb interaction by a short-range one reduces the problem to a
Helmholtz equation, amenable to analytical treatment. We demonstrate that even
with our simplifications, the system still exhibits the key features seen in
the experiment.Comment: 4 pages, 3 figure
Algebraic computation of some intersection D-modules
Let be a complex analytic manifold, a locally
quasi-homogeneous free divisor, an integrable logarithmic connection with
respect to and the local system of the horizontal sections of on
. In this paper we give an algebraic description in terms of of the
regular holonomic D-module whose de Rham complex is the intersection complex
associated with . As an application, we perform some effective computations
in the case of quasi-homogeneous plane curves.Comment: 18 page
Quantum radiation in a plane cavity with moving mirrors
We consider the electromagnetic vacuum field inside a perfect plane cavity
with moving mirrors, in the nonrelativistic approximation. We show that low
frequency photons are generated in pairs that satisfy simple properties
associated to the plane geometry. We calculate the photon generation rates for
each polarization as functions of the mechanical frequency by two independent
methods: on one hand from the analysis of the boundary conditions for moving
mirrors and with the aid of Green functions; and on the other hand by an
effective Hamiltonian approach. The angular and frequency spectra are discrete,
and emission rates for each allowed angular direction are obtained. We discuss
the dependence of the generation rates on the cavity length and show that the
effect is enhanced for short cavity lengths. We also compute the dissipative
force on the moving mirrors and show that it is related to the total radiated
energy as predicted by energy conservation.Comment: 17 pages, 1 figure, published in Physical Review
Ferromagnetism without flat bands in thin armchair nanoribbons
Describing by a Hubbard type of model a thin armchair graphene ribbon in the
armchair hexagon chain limit, one shows in exact terms, that even if the system
does not have flat bands at all, at low concentration a mesoscopic sample can
have ferromagnetic ground state, being metallic in the same time. The mechanism
is connected to a common effect of correlations and confinement.Comment: 37 pages, 12 figures, in press at Eur. Phys. Jour.
Incommensurate Spin Ordering and Fluctuations in underdoped La_{2-x}Ba_{x}CuO_{4}
Using neutron scattering techniques, we have studied incommensurate spin
ordering as well as low energy spin dynamics in single crystal underdoped \LBCO
with x0.095 and 0.08; high temperature superconductors with T 27
K and 29 K respectively. Static two dimensional incommensurate magnetic order
appears below T=39.5 0.3 K in \LBCO (x=0.095) and a similar
temperature for x=0.08 within the low temperature tetragonal phase. The spin
order is unaffected by either the onset of superconductivity or the application
of magnetic fields of up to 7 Tesla applied along the c-axis in the x=0.095
sample. Such magnetic field {\it independent} behaviour is in marked contrast
with the field induced enhancement of the staggered magnetisation observed in
the related \LSCO system, indicating this phenomenon is not a universal
property of cuprate superconductors. Surprisingly, we find that
incommensurability is only weakly dependent on doping relative to
\LSCO. Dispersive excitations in \LBCO (x=0.095) at the same incommensurate
wavevector persist up to at least 60 K. The dynamical spin susceptibility of
the low energy spin excitations saturates below \tc, in a similar manner to
that seen in the superconducting state of LaCuO.Comment: 9 pages, 7 figures, submitted to PRB, figures update
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