1,645 research outputs found
Constructing Dirac linear fermions in terms of non-linear Heisenberg spinors
We show that the massive (or massless) neutrinos can be described as special
states of Heisenberg nonlinear spinors. As a by-product of this decomposition a
particularly attractive consequence appears: the possibility of relating the
existence of only three species of mass-less neutrinos to such internal
non-linear structure. At the same time it allows the possibility that neutrino
oscillation can occurs even for massless neutrinos
Current-perpendicular-to-plane giant magnetoresistance of a spin valve using Co2MnSi Heusler alloy electrodes
We report the current-perpendicular-to-plane giant magnetoresistance of a
spin valve with Co2MnSi (CMS) Heusler alloy ferromagnetic electrodes. A
multilayer stack of Cr/Ag/Cr/CMS/Cu/CMS/Fe25Co75/Ir28Mn72/Ru was deposited on a
MgO (001) single crystal substrate. The bottom CMS layer was epitaxially grown
on the Cr/Ag/Cr buffer layers and was ordered to the L21 structure after
annealing at 673 K. The upper CMS layer was found to grow epitaxially on the Cu
spacer layer despite the large lattice mismatch between Cu and CMS. The highest
MR ratios of 8.6% and 30.7% for CPP-GMR were recorded at room temperature and 6
K, respectively. The high spin polarization of the epitaxial CMS layers is the
most likely origin of the high MR ratio.Comment: 14 pages, 3 figures, presented at the 53rd Annual Conference on
Magnetism and Magnetic Materials, to be published in J. Appl. Phy
Power-dependent internal loss in Josephson bifurcation amplifiers
We have studied nonlinear superconducting resonators: lambda/2
coplanar-waveguide (CPW) resonators with Josephson junctions (JJs) placed in
the middle and lambda/4 CPW resonators terminated by JJs, which can be used for
the qubit readout as "bifurcation amplifiers." The nonlinearity of the
resonators arises from the Josephson junctions, and because of the
nonlinearity, the resonators with appropriate parameters are expected to show a
hysteretic response to the frequency sweep, or "bifurcation," when they are
driven with a sufficiently large power. We designed and fabricated resonators
whose resonant frequencies were around 10 GHz. We characterized the resonators
at low temperatures, T<0.05 K, and confirmed that they indeed exhibited
hysteresis. The sizes of the hysteresis, however, are sometimes considerably
smaller than the predictions based on the loaded quality factor in the weak
drive regime. When the discrepancy appears, it is mostly explained by taking
into account the internal loss, which often increases in our resonators with
increasing drive power in the relevant power range. As a possible origin of the
power-dependent loss, the quasiparticle channel of conductance of the JJs is
discussed.Comment: 8 pages, 9 figure
Supersymmetric quantum mechanics based on higher excited states
We generalize the formalism and the techniques of the supersymmetric (susy)
quantum mechanics to the cases where the superpotential is generated/defined by
higher excited eigenstates. The generalization is technically almost
straightforward but physically quite nontrivial since it yields an infinity of
new classes of susy-partner potentials, whose spectra are exactly identical
except for the lowest m+1 states, if the superpotential is defined in terms of
the (m+1)-st eigenfunction, with m=0 reserved for the ground state. It is shown
that in case of the infinite 1-dim potential well nothing new emerges (the
partner potential is still of P\"oschl-Teller type I, for all m), whilst in
case of the 1-dim harmonic oscillator we get a new class of infinitely many
partner potentials: for each m the partner potential is expressed as the sum of
the quadratic harmonic potential plus rational function, defined as the
derivative of the ratio of two consecutive Hermite polynomials. These partner
potentials of course have m singularities exactly at the locations of the nodes
of the generating (m+1)-st wavefunction. The susy formalism applies everywhere
between the singularities. A systematic application of the formalism to other
potentials with known spectra would yield an infinitely rich class of
"solvable" potentials, in terms of their partner potentials. If the potentials
are shape invariant they can be solved at least partially and new types of
analytically obtainable spectra are expected.
PACS numbers: 03.65.-w, 03.65.Ge, 03.65.SqComment: 15 pages LaTeX file, no figures, submitted to J. Phys. A: accepted
for publication
Theory for Photon-Assisted Macroscopic Quantum Tunneling in a Stack of Intrinsic Josephson Junctions
We propose a theory for photon-assisted macroscopic quantum tunneling (MQT)
in a stack of capacitively-coupled intrinsic Josephson junctions in which the
longitudinal Josephson plasma, i.e., longitudinal collective phase oscillation
modes, is excited. The scheme of energy-level quantization in the collective
oscillatory states is clarified in the -junction system. When the MQT occurs
from the single-plasmon states excited by microwave irradiation in the
multi-photon process to the uniform voltage state, our theory predicts that the
escape rate is proportional to . This result is consistent with the recent
observation in Bi-2212 intrinsic Josephson junctions.Comment: 5 pages, 2 figure
Effect of interfacial strain on spin injection and spin polarization of Co2CrAl/NaNbO3/Co2CrAl magnetic tunneling junction
First-principles calculations were carried out to investigate interfacial
strain effects on spin injection and spin polarization of a magnetic tunnel
junction consisting of half-metallic full-Heusler alloy Co2CrAl and
ferroelectric perovskite NaNbO3. Spin-dependent coherent tunneling was
calculated within the framework of non-equilibrium Green's function technique.
Both spin polarization and tunnel magnetoresistance (TMR) are affected by the
interfacial strain but their responses to compressive and tensile strains are
different. Spin polarization across the interface is fully preserved under a
compressive strain due to stronger coupling between interfacial atoms, whereas
a tensile strain significantly enhances interface states and lead to
substantial drops in spin polarization and TMR
Magnetism and superconductivity in TaSC (M = Fe, Co, Ni, and Cu)
Magnetic properties of TaSC ( = Fe, Co, Ni, Cu) have
been studied using SQUID DC and AC magnetic susceptibility. In these systems
magnetic ions are intercalated into van der Waals gaps between
adjacent S layers of host superconductor TaSC.
FeTaSC is a quasi 2D -like antiferromagnet on the
triangular lattice. It undergoes an antiferromagnetic phase transition at
(= 117 K). The irreversible effect of magnetization occurs below
, reflecting the frustrated nature of the system. The AF phase coexists
with two superconducting phases with the transition temperatures
K and K. CoTaSC is a quasi 2D Ising-like
antiferromagnet on the triangular lattice. The antiferromagnetic phase below
K coexists with a superconducting phase below K.
Both NiTaSC and CuTaSC are
superconductors with ( K for Ni and 6.4 K for Cu) and
(= 4.6 K common to TaSC). Very small effective magnetic
moments suggest that Ni and Cu spins are partially delocalized.Comment: 15 pages, 17 figures, and 3 table
- …