377 research outputs found

### First-principles Theory of Nonlocal Screening in Graphene

Using the quasiparticle self-consistent GW (QSGW) and local-density (LD)
approximations, we calculate the q-dependent static dielectric function, and
derive an effective 2D dielectric function corresponding to screening of point
charges. In the q-to-0 limit, the 2D function is found to scale approximately
as the square root of the macroscopic dielectric function. Its value is ~4, a
factor approximately 1.5 larger than predictions of Dirac model. Both kinds of
dielectric functions depend strongly on q, in contrast with the Dirac model.
The QSGW approximation is shown to describe QP levels very well, with small
systematic errors analogous to bulk sp semiconductors. Local-field effects are
rather more important in graphene than in bulk semiconductors.Comment: 9 pages, 2 figure

### Optimizing Tc in the (Mn,Cr,Ga)As and (Mn,Ga)(As,P) Ternary Alloys

We explore two possible ways to enhance the critical temperature $T_c$ in the
dilute magnetic semiconductor Mn$_{0.08}$Ga$_{0.92}$As. Within the context of
the double-exchange and RKKY pictures, the ternary alloys
Mn$_{x}$Cr$_{0.08-x}$Ga$_{0.92}$As and Mn$_{0.08}$Ga$_{0.92}$As$_y$P$_{1-y}$
might be expected to have $T_c$ higher than the pseudobinary
Mn$_{0.08}$Ga$_{0.92}$As. To test whether the expectations from model pictures
are confirmed, we employ linear response theory within the local-density
approximation to search for theoretically higher critical temperatures in these
ternary alloys. Our results show that neither co-doping Mn with Cr, nor
alloying As with P improves $T_c$. Alloying with Cr is found to be deleterious
to the $T_c$. Mn$_{0.08}$Ga$_{0.92}$As$_y$P$_{1-y}$ shows almost linear
dependence of $T_c$ on $y$.Comment: 10 pages, 5 figure

### Many-body effects in iron pnictides and chalcogenides -- non-local vs dynamic origin of effective masses

We apply the quasi-particle self-consistent GW (QSGW) approximation to some
of the iron pnictide and chalcogenide superconductors. We compute Fermi
surfaces and density of states, and find excellent agreement with experiment,
substantially improving over standard band-structure methods. Analyzing the
QSGW self-energy we discuss non-local and dynamic contributions to effective
masses. We present evidence that the two contributions are mostly separable,
since the quasi-particle weight is found to be essentially independent of
momentum. The main effect of non locality is captured by the static but
non-local QSGW effective potential. Moreover, these non-local self-energy
corrections, absent in e.g. dynamical mean field theory (DMFT), can be
relatively large. We show, on the other hand, that QSGW only partially accounts
for dynamic renormalizations at low energies. These findings suggest that QSGW
combined with DMFT will capture most of the many-body physics in the iron
pnictides and chalcogenides.Comment: 4+ pages, 3 figure

### First-principles calculation of spin-orbit torque in a Co/Pt bilayer

The angular dependence of spin-orbit torque in a disordered Co/Pt bilayer is
calculated using a first-principles non-equilibrium Green's function formalism
with an explicit supercell averaging over Anderson disorder. In addition to the
usual dampinglike and fieldlike terms, the odd torque contains a sizeable
planar Hall-like term $(\mathbf{m\cdot
E})\mathbf{m}\times(\mathbf{z}\times\mathbf{m})$ whose contribution to
current-induced damping is consistent with experimental observations. The
dampinglike and planar Hall-like torquances depend weakly on disorder strength,
while the fieldlike torquance declines with increasing disorder. The torques
that contribute to damping are almost entirely due to spin-orbit coupling on
the Pt atoms, but the fieldlike torque does not require it.Comment: 11 pages, 5 figure

### Theory of spin loss at metallic interfaces

Interfacial spin-flip scattering plays an important role in magnetoelectronic
devices. Spin loss at metallic interfaces is usually quantified by matching the
magnetoresistance data for multilayers to the Valet-Fert model, while treating
each interface as a fictitious bulk layer whose thickness is $\delta$ times the
spin-diffusion length. By employing the properly generalized circuit theory and
the scattering matrix approaches, we derive the relation of the parameter
$\delta$ to the spin-flip transmission and reflection probabilities at an
individual interface. It is found that $\delta$ is proportional to the square
root of the probability of spin-flip scattering. We calculate the spin-flip
transmission probability for flat and rough Cu/Pd interfaces using the
Landauer-B\"uttiker method based on the first-principles electronic structure
and find $\delta$ in reasonable agreement with experiment.Comment: 5 pages + supplementary material, 3 figures, version accepted in
Phys. Rev. Let

### Ab initio transport calculations: from normal to superconducting current

Applying the Bogoliubov-de Gennes equations with density-functional theory,
it is possible to formulate first-principles description of current-phase
relationships in superconducting/normal (magnetic)/superconducting trilayers.
Such structures are the basis for the superconducting analog of
Magnetoresistive random access memory devices (JMRAM). In a recent paper [1] we
presented results from the first attempt to formulate such a theory, applied to
the Nb/Ni/Nb trilayers. In the present work we provide computational details,
explaining how to construct key ingredient (scattering matrices $S_N$) in a
framework of linear muffin-tin orbitals (LMTO).Comment: Proceeding for the Spintronics XVI - SPIE 2023 conferenc

### Quasiparticle Self-Consistent GW Theory

In past decades the scientific community has been looking for a reliable
first-principles method to predict the electronic structure of solids with high
accuracy. Here we present an approach which we call the quasiparticle
self-consistent GW approximation (QpscGW). It is based on a kind of
self-consistent perturbation theory, where the self-consistency is constructed
to minimize the perturbation. We apply it to selections from different classes
of materials, including alkali metals, semiconductors, wide band gap
insulators, transition metals, transition metal oxides, magnetic insulators,
and rare earth compounds. Apart some mild exceptions, the properties are very
well described, particularly in weakly correlated cases. Self-consistency
dramatically improves agreement with experiment, and is sometimes essential.
Discrepancies with experiment are systematic, and can be explained in terms of
approximations made.Comment: 12 pages, 3 figure

### Role of Disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN

We present calculations of magnetic exchange interactions and critical
temperature T_c in Mn:GaAs, Cr:GaAs and Cr:GaN. The local spin density
approximation is combined with a linear-response technique to map the magnetic
energy onto a Heisenberg hamiltonion, but no significant further approximations
are made. Special quasi-random structures in large unit cells are used to
accurately model the disorder. T_c is computed using both a spin-dynamics
approach and the cluster variation method developed for the classical
Heisenberg model.
We show the following: (i) configurational disorder results in large
dispersions in the pairwise exchange interactions; (ii) the disorder strongly
reduces T_c; (iii) clustering in the magnetic atoms, whose tendency is
predicted from total-energy considerations, further reduces T_c. Additionally
the exchange interactions J(R) are found to decay exponentially with distance
R^3 on average; and the mean-field approximation is found to be a very poor
predictor of T_c, particularly when J(R) decays rapidly. Finally the effect of
spin-orbit coupling on T_c is considered. With all these factors taken into
account, T_c is reasonably predicted by the local spin-density approximation in
MnGaAs without the need to invoke compensation by donor impurities.Comment: 10 pages, 3 figure

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