6 research outputs found
Tunneling currents in ferromagnetic systems with multiple broken symmetries
SHORTENED ABSTRACT: A system exhibiting multiple simultaneously broken
symmetries offers the opportunity to influence physical phenomena such as
tunneling currents by means of external control parameters. In this paper, we
consider the broken SU(2) (internal spin) symmetry of ferromagnetic systems
coexisting with \textit{i)} the broken U(1) symmetry of superconductors and
\textit{ii)} the broken spatial inversion symmetry induced by a Rashba term in
a spin-orbit coupling Hamiltonian. In order to study the effect of these broken
symmetries, we consider tunneling currents that arise in two different systems;
tunneling junctions consisting of non-unitary spin-triplet ferromagnetic
superconductors and junctions consisting of ferromagnets with spin-orbit
coupling.Comment: Accepted for publication in Phys. Rev.
Thermodynamic Properties near the onset of Loop-Current Order in high- superconducting cuprates
We have performed large-scale Monte Carlo simulations on a two-dimensional
generalized Ashkin-Teller model to calculate the thermodynamic properties in
the critical region near its transitions. The Ashkin-Teller model has a pair of
Ising spins at each site which interact with neighboring spins through
pair-wise and 4-spin interactions. The model represents the interactions
between orbital current loops in -plaquettes of high- cuprates,
which order with a staggered magnetization \Mso inside each unit-cell in the
underdoped region of the phase diagram below a temperature which
depends on doping. The pair of Ising spins per unit-cell represent the
directions of the currents in the links of the current loops. The
generalizations are the inclusion of anisotropy in the pair-wise nearest
neighbor current-current couplings consistent with the symmetries of a square
lattice and the next nearest neighbor pair-wise couplings. We use the Binder
cumulant to estimate the correlation length exponent and the order
parameter exponent . Our principal results are that in a range of
parameters, the Ashkin-Teller model as well as its generalization has an order
parameter susceptibility which diverges as and an order parameter
below . Importantly, however, there is no divergence in the specific heat.
This puts the properties of the model in accord with the experimental results
in the underdoped cuprates. We also calculate the magnitude of the "bump" in
the specific heat in the critical region to put limits on its observability.
Finally, we show that the staggered magnetization couples to the uniform
magnetization such that the latter has a weak singularity at and
also displays a wide critical region, also in accord with recent experiments.Comment: 14 pages, 19 figures, to appear in Physical Review
Origin and control of spin currents in a magnetic triplet Josephson junction
We study the appearance of a Josephson spin current in a model triplet
superconductor junction with a magnetically-active tunnelling barrier. We find
three distinct mechanisms for producing a spin current, and we provide a
detailed discussion of the symmetry properties and the physical origins of
each. By combining these three basic mechanisms, we find that it is possible to
exercise fine control over the spin currents. In particular, we show that
unlike the charge current, the spin currents on either side of the barrier need
not be identical.Comment: 5 pages, 4 figures, RevTe
Superconducting spintronics
The interaction between superconducting and spin-polarized orders has recently emerged as a major research field following a series
of fundamental breakthroughs in charge transport in superconductor-ferromagnet heterodevices which promise new device
functionality. Traditional studies which combine spintronics and superconductivity have mainly focused on the injection of
spin-polarized quasiparticles into superconducting materials. However, a complete synergy between superconducting and magnetic
orders turns out to be possible through the creation of spin-triplet Cooper pairs which are generated at carefully engineered
superconductor interfaces with ferromagnetic materials. Currently, there is intense activity focused on identifying materials
combinations which merge superconductivity and spintronics in order to enhance device functionality and performance. The results
look promising: it has been shown, for example, that superconducting order can greatly enhance central effects in spintronics such as
spin injection and magnetoresistance. Here, we review the experimental and theoretical advances in this field and provide an outlook
for upcoming challenges related to the new concept of superconducting spintronics.J.L. was supported by the Research Council of Norway, Grants No. 205591 and 216700.
J.W.A.R. was supported by the UK Royal Society and the Leverhulme Trust through an
International Network Grant (IN-2013-033).This is the accepted manuscript. The final version is available at http://www.nature.com/nphys/journal/v11/n4/full/nphys3242.html