16,351 research outputs found
Robust half-metallic antiferromagnets LaVOsO and LaMoO ( = Ca, Sr, Ba; = Re, Tc) from first-principles calculations
We have theoretically designed three families of the half-metallic (HM)
antiferromagnets (AFM), namely, LaVOsO, LaMoTcO and
LaMoReO ( = Ca, Sr, Ba), based on a systematic {\it ab initio} study
of the ordered double perovskites LaO with the possible and
pairs from all the 3, 4 and 5 transtion metal elements being
considered. Electronic structure calculations based on first-principles
density-functional theory with generalized gradient approximation (GGA) for
more than sixty double perovskites LaCaO have been performed using the
all-electron full-potential linearized augmented-plane-wave method. The found
HM-AFM state in these materials survives the full {\it ab initio} lattice
constant and atomic position optimizations which were carried out using
frozen-core full potential projector augmented wave method. It is found that
the HM-AFM properties predicted previously in some of the double perovskites
would disappear after the full structural optimizations. The AFM is attributed
to both the superexchange mechanism and the generalized double exchange
mechanism via the () - O (2) - () coupling
and the latter is also believed to be the origin of the HM. Finally, in our
search for the HM-AFMs, we find LaCrTcO and LaCrReO to be AFM
insulators of an unconventional type in the sense that the two
antiferromagnetic coupled ions consist of two different elements and that the
two spin-resolved densities of states are no longer the same.Comment: To appear in Phys. Rev.
An effective spin-orbital Hamiltonian for the double perovskite SrFeW O: Derivation of the phase diagram
We formulate a superexchange theory of insulating double-perovskite compounds
such as SrFeWO. An effective spin-orbital Hamiltonian is derived in the
strong coupling limit of Hubbard model for d-electrons on Fe and W ions. The
relevant degrees of freedom are the spins S=2 and the three-fold orbital
degeneracy of Fe-ions. W-sites are integrated out by means of a
fourth-order perturbative expansion. The magnetically and orbitally ordered
ground states of the effective Hamiltonia n are discussed as a function of the
model parameters. We show that for realistic values of such parameters the
ground state is antiferromagnetic, as experimentally observed. The order found
is of type-II, consisting of \{111\} ferromagnetic planes stac ked
antiferromagnetically. The orbital order energy scale found is one order of
magnitude less than the spi n one.Comment: 12 pages, 4 figure
Fermionic representation for the ferromagnetic Kondo lattice model -- diagrammatic study of spin-charge coupling effects on magnon excitations
A purely fermionic representation is introduced for the ferromagnetic Kondo
lattice model which allows conventional diagrammatic tools to be employed to
study correlation effects. Quantum 1/S corrections to magnon excitations are
investigated using a systematic inverse-degeneracy expansion scheme which
incorporates correlation effects in the form of self-energy and vertex
corrections, while explicitly preserving the continuous spin-rotation symmetry.
Magnon self-energy is studied in the full range of interaction strength, and
shown to result in strong magnon damping and anomalous softening for zone
boundary modes, which accounts for several zone-boundary anomalies observed in
recent spin-wave measurements of ferromagnetic manganites.Comment: 16 pages, 9 figure
Induced antiferromagnetism and large magnetoresistances in RuSr2(Nd,Y,Ce)2Cu2O10-d ruthenocuprates
RuSr2(Nd,Y,Ce)2Cu2O10-d ruthenocuprates have been studied by neutron
diffraction, magnetotransport and magnetisation measurements and the electronic
phase diagram is reported. Separate Ru and Cu spin ordering transitions are
observed, with spontaneous Cu antiferromagnetic order for low hole doping
levels p, and a distinct, induced-antiferromagnetic Cu spin phase in the 0.02 <
p < 0.06 pseudogap region. This ordering gives rise to large negative
magnetoresistances which vary systematically with p in the
RuSr2Nd1.8-xY0.2CexCu2O10-d series. A collapse of the magnetoresistance (MR)
and magnetisation in the pre-superconducting region may signify the onset of
superconducting fluctuations.Comment: 22 pages, 11 figure
Effects of external radiation on biased Aharonov-Bohm rings
We consider the currents flowing in a solid-state interferometer under the
effect of both an Aharonov-Bohm phase and a bias potential. Expressions are
obtained for these currents, allowing for electronic or electron-boson
interactions, which may take place solely on a quantum dot placed on one of the
interferometer arms. The boson system can be out of equilibrium. The results
are used to obtain the transport current through the interferometer, and the
current circulating around it under the effect of the Aharonov-Bohm flux. The
modifications of both currents, brought about by coupling the quantum dot to an
incoherent sonic or electromagnetic source, are then analyzed. By choosing the
appropriate range of the boson source intensity and its frequency, the
magnitude of the interference-related terms of both currents can be controlled.Comment: 18 pages, one figur
Superconductivity in an organic insulator at very high magnetic fields
We investigate by electrical transport the field-induced superconducting
state (FISC) in the organic conductor -(BETS)FeCl. Below 4 K,
antiferromagnetic-insulator, metallic, and eventually superconducting (FISC)
ground states are observed with increasing in-plane magnetic field. The FISC
state survives between 18 and 41 T, and can be interpreted in terms of the
Jaccarino-Peter effect, where the external magnetic field {\em compensates} the
exchange field of aligned Fe ions. We further argue that the Fe
moments are essential to stabilize the resulting singlet, two-dimensional
superconducting stateComment: 9 pages 3 figure
Monitoramento automático da necessidade hÃdrica das culturas do feijoeiro e milho por meio do balanço de energia/razão de Bowen.
O trabalho consistiu no monitoramento automático da necessidade hÃdrica das culturas do feijoeiro e milho por meio do balanço de energia (BE), associado à razão de Bowen (RB), para estabelecer critérios adequados de decisão do momento de aplicação de água de irrigação. Em vários dias e horários, os resultados indicaram que o fluxo de calor latente (Le) das culturas, ultrapassaram o valor do saldo radiação (Rn) medido, devido a ocorrência de efeito de energia de advecção, para ventos acima de 1,5 m.s-1. Os valores baixos de razão de Bowen (β) ocorreram para as condições sem restrição hÃdrica das culturas e do solo, em que uma maior parte de Rn foi dissipada em forma de Le. Os valores elevados de β foram verificados com o decorrer do tempo, após a aplicação de água de irrigação ou chuva, em ocasiões que se induziu o estresse hÃdrico nas culturas, com uma maior parte de Rn sendo usada no aquecimento do ar e dissipada em forma de calor sensÃvel (H). A evapotranspiração das culturas (ETc) determinada pela equação de Penman-Monteith (PM) correlacionou bem com a ETc obtida pelas estações de RB, no entanto, subestimando os valores em magnitude, principalmente, na parte da tarde, no inÃcio e no meio dos ciclos fenológicos das culturas do feijoeiro e milho.Edição especial
Anisotropic Behavior of Knight Shift in Superconducting State of Na_xCoO_2yH_2O
The Co Knight shift was measured in an aligned powder sample of
Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The
Knight-shift components parallel (K_c) and perpendicular to the c-axis (along
the ab plane K_{ab}) were measured in both the normal and superconducting (SC)
states. The temperature dependences of K_{ab} and K_c are scaled with the bulk
susceptibility, which shows that the microscopic susceptibility deduced from
the Knight shift is related to Co-3d spins. In the SC state, the Knight shift
shows an anisotropic temperature dependence: K_{ab} decreases below 5 K,
whereas K_c does not decrease within experimental accuracy. This result raises
the possibility that spin-triplet superconductivity with the spin component of
the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.Comment: 5 pages, 5 figures, to be published in Journal of Physical Society of
Japan vol. 75, No.
Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C in the normal and superconducting state
Point-contact (PC) spectroscopy measurements on antiferromagnetic (AF)
(T_N=5.2K) HoNi2B2C single crystals in the normal and two different
superconducting (SC) states (T_c=8.5K and T_c^*=5.6K<T_c, with 2\Delta/kT_c^*=3.9. The strong coupling
Eliashberg analysis of the low-temperature SC phase with T_c^*=5.6K =T_N,
coexisting with the commensurate AF structure, suggests a sizable value of the
EPI constant \lambda_s=0.93. We also provide strong support for the recently
proposed by us ''Fermi surface (FS) separation'' scenario for the coexistence
of magnetism and superconductivity in magnetic borocarbides, namely, that the
superconductivity in the commensurate AF phase survives at a special (nearly
isotropic) FS sheet without an admixture of Ho 5d states. Above T_c^* the SC
features in the PC characteristics are strongly suppressed pointing to a
specific weakened SC state between T_c* and T_c.Comment: 11 pages, 8 figs, to be published in PRB, Vol.75, Iss.2
Interpolation in non-positively curved K\"ahler manifolds
We extend to any simply connected K\"ahler manifold with non-positive
sectional curvature some conditions for interpolation in and in
the unit disk given by Berndtsson, Ortega-Cerd\`a and Seip. The main tool is a
comparison theorem for the Hessian in K\"ahler geometry due to Greene, Wu and
Siu, Yau.Comment: 9 pages, Late
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