2,287 research outputs found
Disorder-induced pseudodiffusive transport in graphene nanoribbons.
We study the transition from ballistic to diffusive and localized transport in graphene nanoribbons in the presence of binary disorder, which can be generated by chemical adsorbates or substitutional doping. We show that the interplay between the induced average doping (arising from the nonzero average of the disorder) and impurity scattering modifies the traditional picture of phase-coherent transport. Close to the Dirac point, intrinsic evanescent modes produced by the impurities dominate transport at short lengths giving rise to a regime analogous to pseudodiffusive transport in clean graphene, but without the requirement of heavily doped contacts. This intrinsic pseudodiffusive regime precedes the traditional ballistic, diffusive, and localized regimes. The last two regimes exhibit a strongly modified effective number of propagating modes and a mean free path which becomes anomalously large close to the Dirac point
Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors
The magnetic behavior of insulating doped diluted magnetic semiconductors
(DMS) is characterized by the interaction of large collective spins known as
bound magnetic polarons. Experimental measurements of the susceptibility of
these materials have suggested that the polaron-polaron interaction is
ferromagnetic, in contrast to the antiferromagnetic carrier-carrier
interactions that are characteristic of nonmagnetic semiconductors. To explain
this behavior, a model has been developed in which polarons interact via both
the standard direct carrier-carrier exchange interaction (due to virtual
carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due
to the interactions of polarons with magnetic ions in an interstitial region).
Using a variational procedure, the optimal values of the model parameters were
determined as a function of temperature. At temperatures of interest, the
parameters describing polaron-polaron interactions were found to be nearly
temperature-independent. For reasonable values of these constant parameters, we
find that indirect ferromagnetic interactions can dominate the direct
antiferromagnetic interactions and cause the polarons to align. This result
supports the experimental evidence for ferromagnetism in insulating doped DMS.Comment: 11 pages, 7 figure
Magnetic interactions of substitutional Mn pairs in GaAs
We employ a kinetic-exchange tight-binding model to calculate the magnetic
interaction and anisotropy energies of a pair of substitutional Mn atoms in
GaAs as a function of their separation distance and direction. We find that the
most energetically stable configuration is usually one in which the spins are
ferromagnetically aligned along the vector connecting the Mn atoms. The
ferromagnetic configuration is characterized by a splitting of the topmost
unoccupied acceptor levels, which is visible in scanning tunneling microscope
studies when the pair is close to the surface and is strongly dependent on pair
orientation. The largest acceptor splittings occur when the Mn pair is oriented
along the symmetry direction, and the smallest when they are oriented
along . We show explicitly that the acceptor splitting is not simply
related to the effective exchange interaction between the Mn local moments. The
exchange interaction constant is instead more directly related to the width of
the distribution of all impurity levels -- occupied and unoccupied. When the Mn
pair is at the (110) GaAs surface, both acceptor splitting and effective
exchange interaction are very small except for the smallest possible Mn
separation.Comment: 25 figure
Spin-related magnetoresistance of n-type ZnO:Al and Zn_{1-x}Mn_{x}O:Al thin films
Effects of spin-orbit coupling and s-d exchange interaction are probed by
magnetoresistance measurements carried out down to 50 mK on ZnO and
Zn_{1-x}Mn_{x}O with x = 3 and 7%. The films were obtained by laser ablation
and doped with Al to electron concentration ~10^{20} cm^{-3}. A quantitative
description of the data for ZnO:Al in terms of weak-localization theory makes
it possible to determine the coupling constant \lambda_{so} = (4.4 +-
0.4)*10^{-11} eVcm of the kp hamiltonian for the wurzite structure, H_{so} =
\lambda_{so}*c(s x k). A complex and large magnetoresistance of
Zn_{1-x}Mn_{x}O:Al is interpreted in terms of the influence of the s-d
spin-splitting and magnetic polaron formation on the disorder-modified
electron-electron interactions. It is suggested that the proposed model
explains the origin of magnetoresistance observed recently in many magnetic
oxide systems.Comment: 4 pages, 4 figure
Spin-dependent tunneling in modulated structures of (Ga,Mn)As
A model of coherent tunneling, which combines multi-orbital tight-binding
approximation with Landauer-B\"uttiker formalism, is developed and applied to
all-semiconductor heterostructures containing (Ga,Mn)As ferromagnetic layers. A
comparison of theoretical predictions and experimental results on
spin-dependent Zener tunneling, tunneling magnetoresistance (TMR), and
anisotropic magnetoresistance (TAMR) is presented. The dependence of spin
current on carrier density, magnetization orientation, strain, voltage bias,
and spacer thickness is examined theoretically in order to optimize device
design and performance.Comment: 9 pages, 13 figures, submitted to PR
Influence of band structure effects on domain-wall resistance in diluted ferromagnetic semiconductors
Intrinsic domain-wall resistance (DWR) in (Ga,Mn)As is studied theoretically
and compared to experimental results. The recently developed model of spin
transport in diluted ferromagnetic semiconductors [Van Dorpe et al., Phys. Rev.
B 72, 205322 (2005)] is employed. The model combines the disorder-free
Landauer-B\"uttiker formalism with the tight-binding description of the host
band structure. The obtained results show how much the spherical 4x4 kp model
[Nguyen, Shchelushkin, and Brataas, cond-mat/0601436] overestimates DWR in the
adiabatic limit, and reveal the dependence of DWR on the magnetization profile
and crystallographic orientation of the wall.Comment: 4 pages, 4 figures, submitted to Phys. Rev. B - Rapid Com
Cubic anisotropy in high homogeneity thin (Ga,Mn)As layers
Historically, comprehensive studies of dilute ferromagnetic semiconductors,
e.g., -type (Cd,Mn)Te and (Ga,Mn)As, paved the way for a quantitative
theoretical description of effects associated with spin-orbit interactions in
solids, such as crystalline magnetic anisotropy. In particular, the theory was
successful in explaining {\em uniaxial} magnetic anisotropies associated with
biaxial strain and non-random formation of magnetic dimers in epitaxial
(Ga,Mn)As layers. However, the situation appears much less settled in the case
of the {\em cubic} term: the theory predicts switchings of the easy axis
between in-plane and directions as a
function of the hole concentration, whereas only the
orientation has been found experimentally. Here, we report on the observation
of such switchings by magnetization and ferromagnetic resonance studies on a
series of high-crystalline quality (Ga,Mn)As films. We describe our findings by
the mean-field - Zener model augmented with three new ingredients. The
first one is a scattering broadening of the hole density of states, which
reduces significantly the amplitude of the alternating carrier-induced
contribution. This opens the way for the two other ingredients, namely the
so-far disregarded single-ion magnetic anisotropy and disorder-driven
non-uniformities of the carrier density, both favoring the
direction of the apparent easy axis. However, according to our results, when
the disorder gets reduced a switching to the orientation
is possible in a certain temperature and hole concentration range.Comment: 12 pages, 9 figure
Tailoring ferromagnetic chalcopyrites
If magnetic semiconductors are ever to find wide application in real
spintronic devices, their magnetic and electronic properties will require
tailoring in much the same way that band gaps are engineered in conventional
semiconductors. Unfortunately, no systematic understanding yet exists of how,
or even whether, properties such as Curie temperatures and band gaps are
related in magnetic semiconductors. Here we explore theoretically these and
other relationships within 64 members of a single materials class, the Mn-doped
II-IV-V2 chalcopyrites, three of which are already known experimentally to be
ferromagnetic semiconductors. Our first-principles results reveal a variation
of magnetic properties across different materials that cannot be explained by
either of the two dominant models of ferromagnetism in semiconductors. Based on
our results for structural, electronic, and magnetic properties, we identify a
small number of new stable chalcopyrites with excellent prospects for
ferromagnetism.Comment: 6 pages with 4 figures, plus 3 supplementary figures; to appear in
Nature Material
- …