Kondo effect in quantum dots coupled to ferromagnetic leads with noncollinear magnetizations

Non-equilibrium Green's function technique has been used to calculate spin-dependent electronic transport through a quantum dot in the Kondo regime. The dot is described by the Anderson Hamiltonian and is coupled either symmetrically or asymmetrically to ferromagnetic leads, whose magnetic moments are noncollinear. It is shown that the splitting of the zero bias Kondo anomaly in differential conductance decreases monotonically with increasing angle between magnetizations, and for antiparallel configuration it vanishes in the symmetrical case while remains finite in the asymmetrical one.Comment: 4 pages, 3 figure

Spin-polarized transport through a single-level quantum dot in the Kondo regime

Nonequilibrium electronic transport through a quantum dot coupled to ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium Green function technique. The system is described by the Anderson model with arbitrary correlation parameter $U$. Exchange interaction between the dot and ferromagnetic electrodes is taken into account {\it via} an effective molecular field. The following situations are analyzed numerically: (i) the dot is symmetrically coupled to two ferromagnetic leads, (ii) one of the two ferromagnetic leads is half-metallic with almost total spin polarization of electron states at the Fermi level, and (iii) one of the two electrodes is nonmagnetic whereas the other one is ferromagnetic. Generally, the Kondo peak in the density of states (DOS) becomes spin-split when the total exchange field acting on the dot is nonzero. The spin-splitting of the Kondo peak in DOS leads to splitting and suppression of the corresponding zero bias anomaly in the differential conductance.Comment: 9 pages, 7 figure

Narodowa Strategia Bezpieczeństwa Stanów Zjednoczonych 2006. Neokonserwatywna wizja świata prezydenta George'a Busha

"16 marca 2006 r. Biały Dom wydał nową, zrewidowaną wersję Narodowej Strategii Bezpieczeństwa Stanów Zjednoczonych z 2002 r. Poprzedni tekst, zredagowany pod dużym wpływem takich osobistości jak podsekretarze obrony Paul Wolfowitz i Douglas Feith czy przewodniczący Rady Polityki Obronnej Richard Perle, stanowił apogeum osiągnięć ruchu neokonserwatywnego. Zdaniem Toda Lindberga, członka Instytutu Hoovera i redaktora periodyku „Policy Review”, została tam sformułowana nowa doktryna strategiczna o historycznym znaczeniu, która będzie określać politykę bezpieczeństwa Stanów Zjednoczonych przez najbliższe 50 lat. Cóż więc się stało, iż po zaledwie 3,5 roku administracja Busha wydaje nowy dokument, w którym rewiduje swoje poprzednie założenia? Czy bolesne doświadczenia z inwazji na Irak, kiedy to Waszyngton po raz pierwszy zastosował prawo do jednostronnego uderzenia wyprzedzającego, kluczowe założenie wojowniczej strategii z 2002 r., spowodowały zmianę poglądów prezydenta, odrzucenie neokonserwatywnej koncepcji polityki zagranicznej i przyjęcie bardziej realistycznego stanowiska? Nic podobnego. Narodowa Strategia Bezpieczeństwa z 2006 r., podobnie jak wersja z 2002 r., jednoznacznie oparta jest na neokonserwatywnej wizji świata, podzielanej przez George’a Busha, a ewentualne różnice dotyczą przede wszystkim rozłożenia akcentów."(...

Electronic Structure of Y3Al5O12:V Single Crystals, Comparison with Sintered Ceramics

Y3Al5O12 (YAG) single crystals doped with vanadium ions (V3+) were obtained by the Czochralski method. The X-ray photoelectron spectra of YAG:V annealed in reducing atmospheres: H2, vacuum and H2 + vacuum are presented and compared with the spectra of the YAG ceramics. The X-ray photoelectron spectra showed that the vanadium dopant concentration in YAG:V crystals is lower than a nominal one. For the \as grown" YAG:2.8at.%V crystal vanadium exists in the mixed valence state. The increase in lattice parameters for the samples annealed in hydrogen was found

Cotunneling through a quantum dot coupled to ferromagnetic leads with noncollinear magnetizations

Spin-dependent electronic transport through a quantum dot has been analyzed theoretically in the cotunneling regime by means of the second-order perturbation theory. The system is described by the impurity Anderson Hamiltonian with arbitrary Coulomb correlation parameter $U$. It is assumed that the dot level is intrinsically spin-split due to an effective molecular field exerted by a magnetic substrate. The dot is coupled to two ferromagnetic leads whose magnetic moments are noncollinear. The angular dependence of electric current, tunnel magnetoresistance, and differential conductance are presented and discussed. The evolution of a cotunneling gap with the angle between magnetic moments and with the splitting of the dot level is also demonstrated.Comment: accepted for publication in Eur. Phys. J.

33000 Photons per MeV from Mixed (Lu0.75Y0.25)3Al5O12:Pr Scintillator Crystals

(LuxY1-x)3Al5O12:Pr (x = 0.25, 0.50, 0.75) crystals have been grown by the Czochralski method and their scintillation properties have been examined. Compared to the well-respected LuAG:Pr scintillator, which has so extensively been studied in the recent years, the new mixed LuYAG:Pr crystals display markedly higher light yields, regardless of the value of x. In particular, (Lu0.75Y0.25)3Al5O12:0.2%Pr characterized by a yield of 33000 ph/MeV, an energy resolution of 4.4% (at 662 keV), and a density of 6.2 g/cm3, seems to be an ideal candidate to supercede Lu3Al5O12:0.2%Pr (19000 ph/MeV, 4.6%, 6.7 g/cm3) in various applications. The observed enhancement of light output following the partial substitution of lutetium by yttrium is most probably related to some specific differences in distributions of shallow traps in particular materials

Transient heat generation in a quantum dot under a step-like pulse bias

We study the transient heat generation in a quantum dot system driven by a step-like or a square-shaped pulse bias. We find that a periodically oscillating heat generation arises after adding the sudden bias. One particularly surprising result is that there exists a heat absorption from the zero-temperature phonon subsystem. Thus the phonon population in non-equilibrium can be less than that of the equilibrium electron-phonon system. In addition, we also ascertain the optimal conditions for the operation of a quantum dot with the minimum heat generation.Comment: 6 pages, 4 figure

Spin-dependent thermoelectric transport through double quantum dots

We study thermoelectric transport through double quantum dots system with spin-dependent interdot coupling and ferromagnetic electrodes by means of the non-equilibrium Green function in the linear response regime. It is found that the thermoelectric coefficients are strongly dependent on the splitting of interdot coupling, the relative magnetic configurations and the spin polarization of leads. In particular, the thermoelectric efficiency can achieve considerable value in parallel configuration when the effective interdot coupling and tunnel coupling between QDs and the leads for spin-down electrons are small. Moreover, the thermoelectric efficiency increases with the intradot Coulomb interactions increasing and can reach very high value at an appropriate temperature. In the presence of the magnetic field, the spin accumulation in leads strongly suppresses the thermoelectric efficiency and a pure spin thermopower can be obtained.Comment: 5 figure

A deeper insight into (Lu,Y)AG : Pr scintillator crystals

Interior of Czochralski-grown (Lu,Y)AG:Pr crystals has been examined by means of several techniques, such as X-Ray Photoelectron Spectroscopy, X-Ray Diffraction, Time-of-Flight Secondary Ion Mass Spectrometry, and magnetic susceptibility measurements. Additionally, their luminescence has been monitored at various combinations of a double-beam (X-ray/IR) excitation