Current distribution inside Py/Cu lateral spin-valve device

We have investigated experimentally the non-local voltage signal (NLVS) in the lateral permalloy (Py)/Cu/Py spin valve devices with different width of Cu stripes. We found that NLVS strongly depends on the distribution of the spin-polarized current inside Cu strip in the vicinity of the Py-detector. To explain these data we have developed a diffusion model describing spatial (3D) distribution of the spin-polarized current in the device. The results of our calculations show that NLVS is decreased by factor of 10 due to spin flip-scattering occurring at Py/Cu interface. The interface resistivity on Py/Cu interface is also present, but its contribution to reduction of NLVS is minor. We also found that most of the spin-polarized current is injected within the region 30 nm from Py-injector/Cu interface. In the area at Py-detector/Cu interface, the spin-polarized current is found to flow mainly close on the injector side, with 1/e exponential decay in the magnitude within the distance 80 nm.Comment: 10 pages, 14 figure

Mechanism of Lattice-Distortion-Induced Electric-Polarization Flop in the Multiferroic Perovskite Manganites

Magnetoelectric phase diagrams of the perovskite manganites, Eu1-xYxMnO3 and Gd1-xTbxMnO3, are theoretically studied. We first construct a microscopic model, and then analyze the model using the Monte-Carlo method. We reproduce the diagrams, which contain two different multiferroic states, i.e., the ab-plane spin cycloid with electric polarization P//a and the bc-plane spin cycloid with P//c. We reveal that their competition originates from a conflict between the single-ion anisotropy and the Dzyaloshinsky-Moriya interaction, which is controlled by the second-neighbor spin exchanges enhanced by the GdFeO3-type distortion. This leads to a P flop from a to c with increasing x in agreement with the experiments.Comment: 5 pages, 5 figures. Recalculated results after correcting errors in the assignment of DM vectors. The conclusion is not affecte

Unbounded-error One-way Classical and Quantum Communication Complexity

This paper studies the gap between quantum one-way communication complexity $Q(f)$ and its classical counterpart $C(f)$, under the {\em unbounded-error} setting, i.e., it is enough that the success probability is strictly greater than 1/2. It is proved that for {\em any} (total or partial) Boolean function $f$, $Q(f)=\lceil C(f)/2 \rceil$, i.e., the former is always exactly one half as large as the latter. The result has an application to obtaining (again an exact) bound for the existence of $(m,n,p)$-QRAC which is the $n$-qubit random access coding that can recover any one of $m$ original bits with success probability $\geq p$. We can prove that $(m,n,>1/2)$-QRAC exists if and only if $m\leq 2^{2n}-1$. Previously, only the construction of QRAC using one qubit, the existence of $(O(n),n,>1/2)$-RAC, and the non-existence of $(2^{2n},n,>1/2)$-QRAC were known.Comment: 9 pages. To appear in Proc. ICALP 200

Pressure Effects in Manganites with Layered Perovskite Structure

Pressure effects on the charge and spin dynamics in the bilayer manganite compounds $La_{2-2x}Sr_{1+2x}Mn_2O_7$ are studied theoretically by taking into account the orbital degrees of freedom. The orbital degrees are active in the layered crystal structure, and applied hydrostatic pressure stabilizes the $3d_{x^2-y^2}$ orbital in comparison with $3d_{3z^2-r^2}$. The change of the orbital states weakens the interlayer charge and spin couplings, and suppresses the three dimensional ferromagnetic transition. Numerical results, based on an effective Hamiltonian which includes the energy level difference of the orbitals, show that the applied pressure controls the dimensionality of the spin and charge dynamics through changes of the orbital states.Comment: 5 pages, 2 figure

Maximum principle and mutation thresholds for four-letter sequence evolution

A four-state mutation-selection model for the evolution of populations of DNA-sequences is investigated with particular interest in the phenomenon of error thresholds. The mutation model considered is the Kimura 3ST mutation scheme, fitness functions, which determine the selection process, come from the permutation-invariant class. Error thresholds can be found for various fitness functions, the phase diagrams are more interesting than for equivalent two-state models. Results for (small) finite sequence lengths are compared with those for infinite sequence length, obtained via a maximum principle that is equivalent to the principle of minimal free energy in physics.Comment: 25 pages, 16 figure

Kraus representation in the presence of initial correlations

We examine the validity of the Kraus representation in the presence of initial correlations and show that it is assured only when a joint dynamics is locally unitary.Comment: REVTeX4, 12 page

A new application of emulsions to measure the gravitational force on antihydrogen

We propose to build and operate a detector based on the emulsion film technology for the measurement of the gravitational acceleration on antimatter, to be performed by the AEgIS experiment (AD6) at CERN. The goal of AEgIS is to test the weak equivalence principle with a precision of 1% on the gravitational acceleration g by measuring the vertical position of the anni- hilation vertex of antihydrogen atoms after their free fall in a horizontal vacuum pipe. With the emulsion technology developed at the University of Bern we propose to improve the performance of AEgIS by exploiting the superior position resolution of emulsion films over other particle de- tectors. The idea is to use a new type of emulsion films, especially developed for applications in vacuum, to yield a spatial resolution of the order of one micron in the measurement of the sag of the antihydrogen atoms in the gravitational field. This is an order of magnitude better than what was planned in the original AEgIS proposal.Comment: 17 pages, 14 figure

Phonon Bloch oscillations in acoustic-cavity structures

We describe a semiconductor multilayer structure based in acoustic phonon cavities and achievable with MBE technology, designed to display acoustic phonon Bloch oscillations. We show that forward and backscattering Raman spectra give a direct measure of the created phononic Wannier-Stark ladder. We also discuss the use of femtosecond laser impulsions for the generation and direct probe of the induced phonon Bloch oscillations. We propose a gedanken experiment based in an integrated phonon source-structure-detector device, and we present calculations of pump and probe time dependent optical reflectivity that evidence temporal beatings in agreement with the Wannier-Stark ladder energy splitting.Comment: PDF file including 4 figure

Ferromagnetism and large negative magnetoresistance in Pb doped Bi-Sr-Co-O misfit-layer compound

Ferromagnetism and accompanying large negative magnetoresistance in Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail. Recent structural analysis of (Bi,Pb)${}_2$Sr${}_{3}$Co${}_2$O${}_9$, which has been believed to be a Co analogue of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta}$, revealed that it has a more complex structure including a CoO${}_2$ hexagonal layer [T. Yamamoto {\it et al.}, Jpn. J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only introduces holes into the conducting CoO${}_2$ layers but also creates a certain amount of localized spins. Ferromagnetic transition appears at $T$ = 3.2 K with small spontaneous magnetization along the $c$ axis, and around the transition temperature large and anisotropic negative magnetoresistance was observed. This compound is the first example which shows ferromagnetic long-range order in a two-dimensional metallic hexagnonal CoO${}_2$ layer.Comment: 8 pages including eps figures. To be published in J. Phys. Soc. Jp

Anomalous Coexistence of Ferroelectric Phases (P∥aP\parallel a and P∥cP\parallel c) in Orthorhombic Eu1−y_{1-y}Yy_yMnO3_3 (y>0.5y>0.5) Crystals

We have investigated the magnetic and dielectric properties of orthorhombic Eu$_{1-y}$Y$_y$MnO$_3$ ($0\leq y\leq 0.6$) single crystals without the presence of the 4$f$ magnetic moments of the rare-earth ions. In $y\geq 0.2$, the magnetic-structure driven ferroelectricity is observed. The ferroelectric transition temperature is steeply reducing with increasing $y$. In $y\geq 0.52$, two ferroelectric phases ($P\parallel a$ and $P\parallel c$) are coexistent at low temperatures. In these phases, ferroelectricity has different origin, which is evidenced by the distinctive poling-electric-field dependence of electric polarization. Namely, the electric polarization along the c axis ($P_c$) is easily saturated by a poling electric field, therefore $P_c$ is caused by the $bc$ spiral antiferromagnetic order. On the other hand, the electric polarization along the a axis ($P_a$) is probably attributed to the collinear $E$-type antiferromagnetic order, because $P_a$ is unsaturated even in a poling field of $10^6$ V/m.Comment: 10 pages, 4figures, to be published in Journal of the Physical Society of Japa