23 research outputs found
Spin-orbit controlled quantum capacitance of a polar heterostructure
Oxide heterostructures with polar films display special electronic
properties, such as the electronic reconstruction at their internal interfaces
with the formation of two-dimensional metallic states. Moreover, the electrical
field from the polar layers is inversion-symmetry breaking and generates a
Rashba spin-orbit coupling (RSOC) in the interfacial electronic system. We
investigate the quantum capacitance of a heterostructure in which a sizeable
RSOC at a metallic interface is controlled by the electric field of a surface
electrode. Such a structure is, for example, given by a LaAlO_3 film on a
SrTiO_3 substrate which is gated by a top electrode. Such heterostructures can
exhibit a strong enhancement of their capacitance [Li et al., Science 332, 825
(2011)]. The capacitance is related to the electronic compressibility of the
heterostructure, but the two quantities are not equivalent. In fact, the
transfer of charge to the interface controls the relation between capacitance
and compressibility. We find that due to a strong RSOC, the quantum capacitance
can be larger than the classical geometric value. However, in contrast to the
results of recent investigations [Caprara et al., Phys. Rev. Lett. 109, 196401
(2012); Bucheli et al., Phys. Rev. B 89, 195448 (2014); Seibold et al.,
Europhys. Lett. 109, 17006 (2015)] the compressibility does not become negative
for realistic parameter values for LaAlO_3/SrTiO_3 and, therefore, we find that
no phase-separated state is induced by the strong RSOC at these interfaces
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
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
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
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
Superconductivity and local non-centrosymmetricity in crystal lattices
Symmetry of the crystal lattice can be a determining factor for the structure
of Cooper pairs in unconventional superconductors. In this study we extend the
discussion of superconductivity in non-centrosymmetric materials to the case
when inversion symmetry is missing locally, but is present on a global level.
Concretely, we investigate the staggered non-centrosymmetricity within a
regular sublattice structure, in some analogy to the discussion of
superconductivity in antiferromagnetic systems. Three crystal structures are
analyzed in detail as illustrative examples for the extended classification of
Cooper-pairing channels. One of the cases may be relevant for the class of
iron-pnictide superconductors
Pairing Theory of Striped Superconductivity
Striped high- superconductors such as
LaNdSrCuO and LaBaCuO near
show a fascinating competition between spin and charge order and
superconductivity. A theory for these systems therefore has to capture both the
spin correlations of an antiferromagnet and the pair correlations of a
superconductor. For this purpose we present here an effective Hartree-Fock
theory incorporating both electron pairing with finite center-of-mass momentum
and antiferromagnetism. We show that this theory reproduces the key
experimental features such as the formation of the antiferromagnetic stripe
patterns at 7/8 band filling or the quasi one-dimensional electronic structure
observed by photoemission spectroscopy.Comment: 4 pagesw, 4 figure
Disorder Induced Stripes in d-Wave Superconductors
Stripe phases are observed experimentally in several copper-based high-Tc
superconductors near 1/8 hole doping. However, the specific characteristics may
vary depending on the degree of dopant disorder and the presence or absence of
a low- temperature tetragonal phase. On the basis of a Hartree-Fock decoupling
scheme for the t-J model we discuss the diverse behavior of stripe phases. In
particular the effect of inhomogeneities is investigated in two distinctly
different parameter regimes which are characterized by the strength of the
interaction. We observe that small concen- trations of impurities or vortices
pin the unidirectional density waves, and dopant disorder is capable to
stabilize a stripe phase in parameter regimes where homogeneous phases are
typically favored in clean systems. The momentum-space results exhibit
universal features for all coexisting density-wave solutions, nearly unchanged
even in strongly disordered systems. These coexisting solutions feature
generically a full energy gap and a particle-hole asymmetry in the density of
states.Comment: 28 pages, 8 figure
Flux Periodicities in Loops of Nodal Superconductors
Supercurrents in superconducting flux threaded loops are expected to
oscillate with the magnetic flux with a period of hc/2e. This is indeed true
for s-wave superconductors larger than the coherence length xi_0. Here we show
that for superconductors with gap nodes, there is no such strict condition for
the supercurrent to be hc/2e rather than hc/e periodic. For nodal
superconductors, the flux induced Doppler shift of the near nodal states leads
to a flux dependent occupation probability of quasi-particles circulating
clockwise and counter clockwise around the loop, which leads to an hc/e
periodic component of the supercurrent, even at zero temperature. We analyze
this phenomenon on a cylinder in an approximative analytic approach and also
numerically within the framework of the BCS theory. Specifically for d-wave
pairing, we show that the hc/e periodic current component decreases with the
inverse radius of the loop and investigate its temperature dependence