164 research outputs found
Formation of magnetic moments and resistance upturn at grain boundaries of two-dimensional electron systems
Electronic correlations control the normal state of bulk high-Tc cuprates.
Strong correlations also suppress the charge transport through cuprate grain
boundaries (GBs). The question then arises if these correlations can produce
magnetic states at cuprate GBs. We analyze the formation of local magnetic
moments at the GB of a correlated two-dimensional electron systems which is
represented by an inhomogeneous Hubbard model. The model Hamiltonian is
diagonalized after the implementation of a mean-field decoupling. The formation
of local magnetic moments is supported by a sufficiently strong variance in the
bond kinetic energies at the GB. Local scattering potentials can assist or
suppress the formation of a magnetic GB state, depending on the details of
their spacial distribution. Grain boundary induced stripes are formed in the
vicinity the GB and decay into the bulk. Moreover, we observe the build-up of
conducting channels which are confined by magnetic clusters. The grain boundary
resistance increases at decreasing temperatures. This low-temperature behavior
is caused by the suppression of current correlations in the state with local
magnetic GB moments. The resistance upturn at low temperatures is in
qualitative agreement with experiments.Comment: 12 pages, 12 figure
Exact results in a slave boson saddle point approach for a strongly correlated electron model
We revisit the Kotliar-Ruckenstein (KR) slave boson saddle point evaluation
for a two-site correlated electron model. As the model can be solved
analytically, it is possible to compare the KR saddle point results to the
exact many particle levels. The considered two site cluster mimics an
infinite- single-impurity Anderson model with a nearest neighbor Coulomb
interaction: one site is strongly correlated with an infinite local Coulomb
repulsion which hybridizes with the second site, on which the local Coulomb
repulsion vanishes. Making use of the flexibility of the representation we
introduce appropriate weight factors in the KR saddle point scheme. Ground
state and all excitation levels agree with the exact diagonalization results.
Thermodynamics and correlation functions may be recovered in a suitably
renormalized saddle point evaluation.Comment: 4 page
Imperfect nesting and Peierls instability for a two-dimensional tight-binding model
Based on a half-filled two-dimensional tight-binding model with
nearest-neighbour and next nearest-neighbour hopping the effect of imperfect
Fermi surface nesting on the Peierls instability is studied at zero
temperature. Two dimerization patterns corresponding to a phonon vector are considered. It is found that the Peierls instability will be
suppressed with an increase of next nearest-neighbour hopping which
characterizes the nesting deviation. First and second order transitions to a
homogeneous state are possible. The competition between the two dimerized
states is discussed.Comment: 17 pages, 10 eps figure
Capacitance and compressibility of heterostructures with strong electronic correlations
Strong electronic correlations related to a repulsive local interaction
suppress the electronic compressibility in a single-band model, and the
capacitance of a corresponding metallic film is directly related to its
electronic compressibility. Both statements may be altered significantly when
two extensions to the system are implemented which we investigate here: (i) we
introduce an attractive nearest-neighbor interaction as antagonist to the
repulsive on-site repulsion , and (ii) we consider nano-structured
multilayers (heterostructures) assembled from two-dimensional layers of these
systems. We determine the respective total compressibility and
capacitance of the heterostructures within a strong coupling evaluation,
which builds on a Kotliar-Ruckenstein slave-boson technique. Whereas the
capacitance for electronic densities close to half-filling is
suppressed---illustrated by a correlation induced dip in ---it may be
appreciably enhanced close to a van Hove singularity. Moreover, we show that
the capacitance may be a non-monotonic function of close to half-filling
for both attractive and repulsive . The compressibility can differ
from substantially, as is very sensitive to internal electrostatic
energies which in turn depend on the specific set-up of the heterostructure. In
particular, we show that a capacitor with a polar dielectric has a smaller
electronic compressibility and is more stable against phase separation than a
standard non-polar capacitor with the same capacitance
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
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
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