1,384 research outputs found
Flux-Periodicity Crossover from hc/e in Normal Metallic to hc/2e in Superconducting Loops
The periodic response of a metallic or a superconducting ring to an external
magnetic flux is one of the most evident manifestations of quantum mechanics.
It is generally understood that the oscillation period hc/2e in the
superconducting state is half the period hc/e in the metallic state, because
the supercurrent is carried by Cooper pairs with a charge 2e. On the basis of
the Bardeen-Cooper-Schrieffer theory we discuss, in which cases this simple
interpretation is valid and when a more careful analysis is needed. In fact,
the knowledge of the oscillation period of the current in the ring provides
information on the electron interactions. In particular, we analyze the
crossover from the hc/e periodic normal current to the hc/2e periodic
supercurrent upon turning on a pairing interaction in a metal ring. Further, we
elaborate on the periodicity crossover when cooling a metallic loop through the
superconducting transition temperature Tc.Comment: To be bublished in "Superconductors", InTech (Rijeka), 2012 (ISBN
979-953-307-798-6
Superconductivity with Finite-Momentum Pairing in Zero Magnetic Field
In the BCS theory of superconductivity, one assumes that all Cooper pairs
have the same center of mass momentum. This is indeed enforced by self
consistency, if the pairing interaction is momentum independent. Here, we show
that for an attractive nearest neighbor interaction, this is different. In this
case, stable solutions with pairs with momenta q and -q coexist and, for a
sufficiently strong interaction, one of these states becomes the groundstate of
the superconductor. This finite-momentum pairing state is accompanied by a
charge order with wave vector 2q. For a weak pairing interaction, the
groundstate is a d-wave superconductor
Fractional Flux Quantization in Loops of Unconventional Superconductors
The magnetic flux threading a conventional superconducting ring is typically
quantized in units of . The factor 2 in the denominator of
originates from the existence of two different types of pairing states
with minima of the free energy at even and odd multiples of . Here we
show that spatially modulated pairing states exist with energy minima at
fractional flux values, in particular at multiples of . In such
states condensates with different center-of-mass momenta of the Cooper pairs
coexist. The proposed mechanism for fractional flux quantization is discussed
in the context of cuprate superconductors, where flux periodicities as
well as uniaxially modulated superconducting states were observed.Comment: 5 pages, 3 figure
Supercurrent as a Probe for Topological Superconductivity in Magnetic Adatom Chains
A magnetic adatom chain, proximity coupled to a conventional superconductor
with spin-orbit coupling, exhibits locally an odd-parity, spin-triplet pairing
amplitude. We show that the singlet-triplet junction, thus formed, leads to a
net spin accumulation in the near vicinity of the chain. The accumulated spins
are polarized along the direction of the local -vector for triplet
pairing and generate an enhanced persistent current flowing around the chain.
The spin polarization and the "supercurrent" reverse their directions beyond a
critical exchange coupling strength at which the singlet superconducting order
changes its sign on the chain. The current is strongly enhanced in the
topological superconducting regime where Majorana bound states appear at the
chain ends. The current and the spin profile offer alternative routes to
characterize the topological superconducting state in adatom chains and
islands.Comment: 5 pages, 3 figures, 5 pages of supplemental material
Momentum-Space Spin Texture in a Topological Superconductor
A conventional superconductor with spin-orbit coupling turns into a
topological superconductor beyond a critical strength of the Zeeman energy. The
spin-expectation values in momentum space trace this
transition via a characteristic change in the topological character of the spin
texture within the Brillouin zone. At the transition the skyrmion counting
number switches from 0 to 1/2 identifying the topological superconductor via
its meron-like spin texture. The change in the skyrmion counting number is
crucially controlled by singular points of the map
from the Brillouin zone, i.e.
a torus, to the unit sphere. The complexity of this spin-map is discussed at
zero temperature as well as for the extension to finite temperatures.Comment: 16 pages, 9 figure
All scale-free networks are sparse
We study the realizability of scale free-networks with a given degree
sequence, showing that the fraction of realizable sequences undergoes two
first-order transitions at the values 0 and 2 of the power-law exponent. We
substantiate this finding by analytical reasoning and by a numerical method,
proposed here, based on extreme value arguments, which can be applied to any
given degree distribution. Our results reveal a fundamental reason why large
scale-free networks without constraints on minimum and maximum degree must be
sparse.Comment: 4 pages, 2 figure
Route to Topological Superconductivity via Magnetic Field Rotation
The verification of topological superconductivity has become a major
experimental challenge. Apart from the very few spin-triplet superconductors
with p-wave pairing symmetry, another candidate system is a conventional,
two-dimensional (2D) s-wave superconductor in a magnetic field with a
sufficiently strong Rashba spin-orbit coupling. Typically, the required
magnetic field to convert the superconductor into a topologically non-trivial
state is however by far larger than the upper critical field H_c2, which
excludes its realization. In this article, we argue that this problem can be
overcome by rotating the magnetic field into the superconducting plane. We
explore the character of the superconducting state upon changing the strength
and the orientation of the magnetic field and show that a topological state,
established for a sufficiently strong out-of-plane magnetic field, indeed
extends to an in-plane field orientation. We present a three-band model
applicable to the superconducting interface between LaAlO_3 and SrTiO_3, which
should fulfil the necessary conditions to realize a topological superconductor
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