156 research outputs found
The Hubbard model with spin orbit coupling: a lattice gauge theory approach
We study the symmetry properties of the Hubbard model with spin-orbit
interactions of Rashba and Dresselhaus type. These interactions break the
rotational symmetry in spin space, so that the magnetic order cannot be
excluded by using the Bogoliubov inequality method. Nevertheless, we rigorously
show that the existence of the magnetic long-range orders may be ruled out when
the Rashba and Dresselhaus coupling constants are equal in modulus, whereas the
eta-pairing can be always ruled out, regardless of the microscopic parameters
of the model. These results are obtained by imposing locally the SU(2) gauge
symmetry on the lattice, and rewriting the spin-orbit interactions in such a
way that they are included in the path ordered of the gauge field on lattice.Comment: 7 page
Phenomenological model for magnetotransport in a multi-orbital system
By means of the Boltzmann equation, we have calculated some magnetotransport
quantities for the layered multi-orbital compound SrRuO. The Hall
coefficient, the magnetoresistance and the in-plane resistivity have been
determined taking into account the Fermi surface curvature and different time
collisions for the electrons in the bands. A consistent explanation of
the experimental results has been obtained assuming different relaxation rates
for the in-plane transport with and without an applied magnetic field,
respectively.Comment: 4 pages, 3 Figure; to appear in Phys. Rev.
Causality and no-go theorems
The aim of the paper is to investigate the role played by causality, and more specifically the no-signaling condition, in the assessment of the quantum theory. To this end, we discuss why it is important that even a non-relativistic theory such as Quantum Mechanics doesn’t imply a violation of this condition. Then, we use this argument to prove an original result stating that the destructive behaviour of the measurement process on the entanglement properties of quantum systems is a necessary and unavoidable feature of the quantum theory. Finally, we critically review the no-cloning theorem. The original formulation of the theorem states that a linear quantum cloning machine, designed in order to successfully clone states that coincide with appropriate basis vectors, fails to copy states that are a non-trivial superposition of those basis vectors; we will furthermore prove that such a linear cloning device, even with the hypothesis that it can only clone basis vectors successfully, may provide a violation of the no-signaling condition and therefore cannot exist
The measurement problem in quantum mechanics
In this paper, we discuss the importance of measurement in quantum mechanics and the so-called measurement problem. Any quantum system can be described as a linear combination of eigenstates of an operator representing a physical quantity; this means that the system can be in a superposition of states that corresponds to different eigenvalues, i.e., different physical outcomes, each one incompatible with the others. The measurement process converts a state of superposition (not macroscopically defined) in a well-defined state. We show that, if we describe the measurement by the standard laws of quantum mechanics, the system would preserve its state of superposition even on a macroscopic scale. Since this is not the case, we assume that a measurement does not obey to standard quantum mechanics, but to a new set of laws that form a “quantum measurement theory”
Control of magnetism in singlet-triplet superconducting heterostructures
We analyze the magnetization at the interface between singlet and triplet
superconductors and show that its direction and dependence on the phase
difference across the junction are strongly tied to the structure of the
triplet order parameter as well as to the pairing interactions. We consider
equal spin helical, opposite spin chiral, and mixed symmetry pairing on the
triplet side and show that the magnetization vanishes at only in the
first case, follows approximately a behavior for the second, and
shows higher harmonics for the last configuration. We trace the origin of the
magnetization to the magnetic structure of the Andreev bound states near the
interface, and provide a symmetry-based explanation of the results. Our
findings can be used to control the magnetization in superconducting
heterostructures and to test symmetries of spin-triplet superconductors.Comment: 5 pages, 3 figure
Spin-sensitive Long-ranged Proximity Effect for Triplet Superconductors
The discovery of noncentrosymmetric superconductors, such as CePtSi, and
chiral superconductors, such as SrRuO, calls for experimental methods
to identify the presence of spin-triplet pairing. We here demonstrate a method
which accomplishes this in an appealingly simple manner: a spin-sensitive
proximity effect in a ferromagnettriplet superconductor bilayer. It is
shown how the orientation of the field can be used to unambiguously distinguish
between different spin-triplet states. Moreover, the proximity effect becomes
long-ranged in spite of the presence of an exchange field and even without any
magnetic inhomogeneities, in contrast to conventional SF junctions. Our
results can be verified by STM-spectroscopy and could be useful as a tool to
characterize the pairing state in unconventional superconducting materials.Comment: 4 pages, 3 figure
Proximity effect between an unconventional superconductor and a ferromagnet with spin bandwidth asymmetry
We study the proximity effect within a junction made of an unconventional superconductor (US) and a ferromagnet (F) in the clean limit with high barrier transparency. Superconductivity in the US side is described by an extended Hubbard model with intersite attractive interaction, while metallic ferromagnetism in the F side is assumed to be originated by a relative change in the bandwidths of electrons with opposite spin. The effect of this mass-split mechanism is analyzed in conjunction with the usual Stoner-like one, where one band is rigidly shifted with respect to the other, due to the presence of a constant exchange field. Starting from the numerical solution of the Bogoliubov-de Gennes equations, we show that the two above mentioned mechanisms for ferromagnetism lead to different features as concerns the formation at the interface of dominant and subdominant superconducting components, as well as their propagation in the ferromagnetic side. This considerably affects the opening of gaplike structures in the local density of states for majority and minority spin electrons, leading to distinct effects as one moves toward the half-metallic regime, where the density of the minority carriers becomes vanishing
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