378 research outputs found
Ab Initio Theory of Scattering-Independent Anomalous Hall Effect
We report on first-principles calculations of the side-jump contribution to
the anomalous Hall conductivity (AHC) directly from the electronic structure of
a perfect crystal. We implemented our approach for a short-range scattering
disorder model within the density functional theory and computed the full
scattering-independent AHC in elemental bcc Fe, hcp Co, fcc Ni, and L1o FePd
and FePt alloys. The full AHC thus calculated agrees systematically with
experiment to a degree unattainable so far, correctly capturing the previously
missing elements of side-jump contributions, hence paving the way to a truly
predictive theory of the anomalous Hall effect and turning it from a
characterization tool to a probing tool of multi-band complex electronic band
structures.Comment: PRL in pres
Non-local field-like spin-orbit torques in Rashba systems: an ab-initio study of AgBi/Ag/Fe film
We investigate from first principles the field-like spin-orbit torques (SOTs)
in a AgBi-terminated Ag(111) film grown on ferromagnetic Fe(110). We find
that a large part of the SOT arises from the spin-orbit interaction (SOI) in
the AgBi layer far away from the Fe layers. These results clearly hint at
a long range spin transfer in the direction perpendicular to the film that does
not originate in the spin Hall effect. In order to bring evidence of the
non-local character of the computed SOT, we show that the torque acting on the
Fe layers can be engineered by the introduction of Bi vacancies in the
AgBi layer. Overall, we find a drastic dependence of the SOT on the
disorder type, which we explain by a complex interplay of different
contributions to the SOT in the Brillouin zone
Communicating the threat of emerging infections to the public.
Communication theory and techniques, aided by the electronic revolution, provide new opportunities and challenges for the effective transfer of laboratory, epidemiologic, surveillance, and other public health data to the public who funds them. We review the applicability of communication theory, particularly the audience-source-message-channel meta-model, to emerging infectious disease issues. Emergence of new infectious organisms, microbial resistance to therapeutic drugs, and increased emphasis on prevention have expanded the role of communication as a vital component of public health practice. In the absence of cure, as in AIDS and many other public health problems, an effectively crafted and disseminated prevention message is the key control measure. Applying communication theory to disease prevention messages can increase the effectiveness of the messages and improve public health
Anisotropy of spin relaxation and transverse transport in metals
Using first principles methods we explore the anisotropy of the spin
relaxation and transverse transport properties in bulk metals with respect to
the direction of the spin quantization axis in paramagnets or of the
spontaneous magnetization in ferromagnets. Owing to the presence of the
spin-orbit interaction the orbital and spin character of the Bloch states
depends sensitively on the orientation of the spins relative to the crystal
axes. This leads to drastic changes in quantities which rely on interband
mixing induced by the spin-orbit interaction. The anisotropy is particularly
striking for quantities which exhibit spiky and irregular distribution in the
Brillouin zone, such as the spin-mixing parameter or the Berry curvature of the
electronic states. We demonstrate this for three cases: (i) the Elliott-Yafet
spin-relaxation mechanism in paramagnets with structural inversion symmetry;
(ii) the intrinsic anomalous Hall effect in ferromagnets; and (iii) the spin
Hall effect in paramagnets. We discuss the consequences of the pronounced
anisotropic behavior displayed by these properties for spin-polarized transport
applications.Comment: Psi-k Highlight 111 (2012
Thermal Conductivity, Thermopower, and Figure of Merit of La_{1-x}Sr_xCoO_3
We present a study of the thermal conductivity k and the thermopower S of
single crystals of La_{1-x}Sr_xCoO_3 with 0<= x <= 0.3. For all Sr
concentrations La_{1-x}Sr_xCoO_3 has rather low k values, whereas S strongly
changes as a function of x. We discuss the influence of the temperature- and
the doping-induced spin-state transitions of the Co ions on both, S and k. From
S, k, and the electrical resistivity rho we derive the thermoelectric figure of
merit Z=S^2/(k*rho). For intermediate Sr concentrations we find notably large
values of Z indicating that Co-based materials could be promising candidates
for thermoelectric cooling.Comment: 7 pages, 5 figures included, submitted to Phys. Rev.
Spin-State Transition and Metal-Insulator Transition in LaEuCoO}
We present a study of the structure, the electric resistivity, the magnetic
susceptibility, and the thermal expansion of LaEuCoO. LaCoO
shows a temperature-induced spin-state transition around 100 K and a
metal-insulator transition around 500 K. Partial substitution of La by
the smaller Eu causes chemical pressure and leads to a drastic increase
of the spin gap from about 190 K in LaCoO to about 2000 K in EuCoO, so
that the spin-state transition is shifted to much higher temperatures. A
combined analysis of thermal expansion and susceptibility gives evidence that
the spin-state transition has to be attributed to a population of an
intermediate-spin state with orbital order for and without orbital
order for larger . In contrast to the spin-state transition, the
metal-insulator transition is shifted only moderately to higher temperatures
with increasing Eu content, showing that the metal-insulator transition occurs
independently from the spin-state distribution of the Co ions. Around
the metal-insulator transition the magnetic susceptibility shows a similar
increase for all and approaches a doping-independent value around 1000 K
indicating that well above the metal-insulator transition the same spin state
is approached for all .Comment: 10 pages, 6 figure
Heat transport in SrCu_2(BO_3)_2 and CuGeO_3
In the low dimensional spin systems and the
thermal conductivities along different crystal directions show pronounced
double-peak structures and strongly depend on magnetic fields. For
the experimental data can be described by a purely phononic
heat current and resonant scattering of phonons by magnetic excitations. A
similar effect seems to be important in , too but, in addition, a
magnetic contribution to the heat transport may be present.Comment: 4 pages, 2 figures; appears in the proceedings of the SCES2001
(Physica B
Separation of Quasiparticle and Phononic Heat Currents in YBCO
Measurements of the transverse (k_{xy}) and longitudinal (k_{xx}) thermal
conductivity in high magnetic fields are used to separate the quasiparticle
thermal conductivity (k_{xx}^{el}) of the CuO_2-planes from the phononic
thermal conductivity in YBa_2Cu_3O_{7-\delta}. k_{xx}^{el} is found to display
a pronounced maximum below T_c. Our data analysis reveals distinct transport
(\tau) and Hall (\tau_H) relaxation times below T_c: Whereas \tau is strongly
enhanced, \tau_H follows the same temperature dependence as above T_c
Helical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 FeCoGe films
Epitaxial films of the B20-structure alloy FeCoGe were grown by
molecular beam epitaxy on Si (111) substrates. The magnetization varied
smoothly from the bulk-like values of one Bohr magneton per Fe atom for FeGe to
zero for non-magnetic CoGe. The chiral lattice structure leads to a
Dzyaloshinskii-Moriya interaction (DMI), and the films' helical magnetic ground
state was confirmed using polarized neutron reflectometry measurements. The
pitch of the spin helix, measured by this method, varies with Co content
and diverges at . This indicates a zero-crossing of the DMI, which
we reproduced in calculations using first principle methods. We also measured
the longitudinal and Hall resistivity of our films as a function of magnetic
field, temperature, and Co content . The Hall resistivity is expected to
contain contributions from the ordinary, anomalous, and topological Hall
effects. Both the anomalous and topological Hall resistivities show peaks
around . Our first principles calculations show a peak in the
topological Hall constant at this value of , related to the strong
spin-polarisation predicted for intermediate values of . Half-metallicity is
predicted for , consistent with the experimentally observed linear
magnetoresistance at this composition. Whilst it is possible to reconcile
theory with experiment for the various Hall effects for FeGe, the large
topological Hall resistivities for are much larger then expected
when the very small emergent fields associated with the divergence in the DMI
are taken into account
Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds
We present a study of the magnetoresistance, the specific heat and the
magnetocaloric effect of equiatomic Mg intermetallics with , Eu, Gd, Yb and , Au and of GdAuIn. Depending on the
composition these compounds are paramagnetic (, Yb) or they
order either ferro- or antiferromagnetically with transition temperatures
ranging from about 13 to 81 K. All of them are metallic, but the resistivity
varies over 3 orders of magnitude. The magnetic order causes a strong decrease
of the resistivity and around the ordering temperature we find pronounced
magnetoresistance effects. The magnetic ordering also leads to well-defined
anomalies in the specific heat. An analysis of the entropy change leads to the
conclusions that generally the magnetic transition can be described by an
ordering of localized moments arising from the half-filled
shells of Eu or Gd. However, for GdAgMg we find clear evidence
for two phase transitions indicating that the magnetic ordering sets in
partially below about 125 K and is completed via an almost first-order
transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets
and rather pronounced for the ferromagnets for low magnetic fields around the
zero-field Curie temperature.Comment: 12 pages, 7 figures include
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