6,134 research outputs found

### The interaction between a superconducting vortex and an out-of-plane magnetized ferromagnetic disk: influence of the magnet geometry

The interaction between a superconducting vortex in a type II superconducting
film (SC) and a ferromagnet (FM) with out-of-plane magnetization is
investigated theoretically within the London approximation. The dependence of
the interaction energy on the FM-vortex distance, film thickness and different
geometries of the magnetic structures: disk, annulus(ring), square and triangle
are calculated. Analytic expressions and vectorplots of the current induced in
the SC due to the presence of the FM are presented. For a FM disk with a
cavity, we show that different local minima for the vortex position are
possible, enabling the system to be suitable to act as a qubit. For FMs with
sharp edges, like e.g. for squares and triangles, the vortex prefers to enter
its equilibrium position along the corners of the magnet.Comment: Preprint, 10 pages, 10 figures, submitted to Phys. Rev.

### Field-enhanced critical parameters in magnetically nanostructured superconductors

Within the phenomenological Ginzburg-Landau theory, we demonstrate the
enhancement of superconductivity in a superconducting film, when nanostructured
by a lattice of magnetic particles. Arrays of out-of-plane magnetized dots
(MDs) extend the critical magnetic field and critical current the sample can
sustain, due to the interaction of the vortex-antivortex pairs and surrounding
supercurrents induced by the dots and the external flux lines. Depending on the
stability of the vortex-antivortex lattice, a peak in the Hext-T boundary is
found for applied integer and rational matching fields, which agrees with
recent experiments [Lange et al., Phys. Rev. Lett. 90, 197006 (2003)]. Due to
compensation of MDs- and Hext-induced currents, we predict the field-shifted
jc-Hext characteristics, as was actually realized in previous experiment but
not commented on [Morgan and Ketterson, Phys. Rev. Lett. 80, 3614 (1998)].Comment: 8 pages, 5 figures, to appear in Europhysics Letter

### Fluxonic Cellular Automata

We formulate a new concept for computing with quantum cellular automata
composed of arrays of nanostructured superconducting devices. The logic states
are defined by the position of two trapped flux quanta (vortices) in a 2x2
blind-hole-matrix etched on a mesoscopic superconducting square. Such small
computational unit-cells are well within reach of current fabrication
technology. In an array of unit-cells, the vortex configuration of one cell
influences the penetrating flux lines in the neighboring cell through the
screening currents. Alternatively, in conjoined cells, the information transfer
can be strengthened by the interactions between the supercurrents in adjacent
cells. Here we present the functioning logic gates based on this fluxonic
cellular automata (FCA), where the logic operations are verified through
theoretical simulations performed in the framework of the time-dependent
Ginzburg-Landau theory. The input signals are defined by current loops placed
on top of the two diagonal blind holes of the input cell. For given
current-polarization, external flux lines are attracted or repelled by the
loops, forming the '0' or '1' configuration. The read-out technology may be
chosen from a large variety of modern vortex imaging methods, transport and
LDOS measurements.Comment: Featured on the cover page of APL, November 2007 issu

### Evolution of multi-gap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB$_2$

Starting from first principles, we show the formation and evolution of
superconducting gaps in MgB$_2$ at its ultrathin limit. Atomically thin MgB$_2$
is distinctly different from bulk MgB$_2$ in that surface states become
comparable in electronic density to the bulk-like $\sigma$- and $\pi$-bands.
Combining the ab initio electron-phonon coupling with the anisotropic
Eliashberg equations, we show that monolayer MgB$_2$ develops three distinct
superconducting gaps, on completely separate parts of the Fermi surface due to
the emergent surface contribution. These gaps hybridize nontrivially with every
extra monolayer added to the film, owing to the opening of additional coupling
channels. Furthermore, we reveal that the three-gap superconductivity in
monolayer MgB$_2$ is robust over the entire temperature range that stretches up
to a considerably high critical temperature of 20 K. The latter can be boosted
to $>$50 K under biaxial tensile strain of $\sim$ 4\%, which is an enhancement
stronger than in any other graphene-related superconductor known to date.Comment: To appear in Phys. Re

### Irreducible Representations of Diperiodic Groups

The irreducible representations of all of the 80 diperiodic groups, being the
symmetries of the systems translationally periodical in two directions, are
calculated. To this end, each of these groups is factorized as the product of a
generalized translational group and an axial point group. The results are
presented in the form of the tables, containing the matrices of the irreducible
representations of the generators of the groups. General properties and some
physical applications (degeneracy and topology of the energy bands, selection
rules, etc.) are discussed.Comment: 30 pages, 5 figures, 28 tables, 18 refs, LaTex2.0

### Tunable magnon topology in monolayer CrI$_\mathbf{3}$ under external stimuli

Two-dimensional (2D) honeycomb ferromagnets, such as monolayer
chromium-trihalides, are predicted to behave as topological magnon insulators -
characterized by an insulating bulk and topologically protected edge states,
giving rise to a thermal magnon Hall effect. Here we report the behavior of the
topological magnons in monolayer CrI$_3$ under external stimuli, including
biaxial and uniaxial strain, electric gating, as well as in-plane and
out-of-plane magnetic field, revealing that one can thereby tailor the magnetic
states as well as the size and the topology of the magnonic bandgap. These
findings broaden the perspective of using 2D magnetic materials to design
topological magnonic devices

### Enhancing superconductivity in MXenes through hydrogenation

Two-dimensional transition metal carbides and nitrides (MXenes) are an
emerging class of atomically-thin superconductors, whose characteristics are
highly prone to tailoring by surface functionalization. Here we explore the use
of hydrogen adatoms to enhance phonon-mediated superconductivity in MXenes,
based on first-principles calculations combined with Eliashberg theory. We
first demonstrate the stability of three different structural models of
hydrogenated Mo- and W-based MXenes. Particularly high critical temperatures of
over 30 K are obtained for hydrogenated Mo$_2$N and W$_2$N. Several mechanisms
responsible for the enhanced electron-phonon coupling are uncovered, namely (i)
hydrogen-induced changes in the phonon spectrum of the host MXene, (ii)
emerging hydrogen-based phonon modes, and (iii) charge transfer from hydrogen
to the MXene layer, boosting the density of states at the Fermi level. Finally,
we demonstrate that hydrogen adatoms are moreover able to induce
superconductivity in MXenes that are not superconducting in pristine form, such
as Nb$_2$C

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