802 research outputs found
Tunneling spectroscopy for probing orbital anisotropy in iron pnictides
Using realistic multi-orbital tight-binding Hamiltonians and the T-matrix
formalism, we explore the effects of a non-magnetic impurity on the local
density of states in Fe-based compounds. We show that scanning tunneling
spectroscopy (STS) has very specific anisotropic signatures that track the
evolution of orbital splitting (OS) and antiferromagnetic gaps. Both
anisotropies exhibit two patterns that split in energy with decreasing
temperature, but for OS these two patterns map onto each other under 90 degree
rotation. STS experiments that observe these signatures should expose the
underlying magnetic and orbital order as a function of temperature across
various phase transitions.Comment: 12 pages, 9 figures, replacement with minor changes suggested by
referee
Origin of Gap Anisotropy in Spin Fluctuation Models of the Fe-pnictides
We discuss the large gap anisotropy found for the A1g (s-wave) state in RPA
spin-fluctuation and functional renormalization group calculations and show how
the simple arguments leading to isotropic sign-switched s-wave states in these
systems need to be supplemented by a consideration of pair scattering within
Fermi surface sheets and between the individual electron sheets as well. In
addition, accounting for the orbital makeup of the states on the Fermi surface
is found to be crucial.Comment: 6 pages, 7 figure
A CRITICAL VIEW OF DIVERSION PROGRAMMES IN CONTEXT OF RESTORATIVE JUSTICE
This article describes the background and purpose of life-skills diversion programmes for young offender and then critically explores to what extent they currently endorse theprinciples of restorative justice to which the new South African child justice system aspires. Since the advent of the life-skills programmes for youths at risk, the child justice system has evolved into a comprehensive piece of legislation called the Child Justice Bill (Bill 49[2002]). Central to this Bill is the promotion of re toralive justice, which seeks to repair damage caused by crime by returning criminal cases to the main players: the victim, the offender and the community. Through a process of negotiation, these players agree on appropriate solutions which include (]) restitution, (2) reconciliation following an acknowledgement of the circumstances around the offence and its impact, and (3) an acceptance of responsibility by the offender leading to reintegration into the community. Diversion options from criminal justice procedures are the key to installing restorative justice for crimes committed by young offenders. To date the most popular form of diversion has been life-skills programmes for youths at risk, but it is not evident to what extent this meets the criteria inherent in the philosophy of restorative justice
Tc suppression and resistivity in cuprates with out of plane defects
Recent experiments introducing controlled disorder into optimally doped
cuprate superconductors by both electron irradiation and chemical substitution
have found unusual behavior in the rate of suppression of the critical
temperature Tc vs. increase in residual resistivity. We show here that the
unexpected discovery that the rate of Tc suppression vs. resistivity is
stronger for out-of-plane than for in-plane impurities may be explained by
consistent calculation of both Tc and resistivity if the potential scattering
is assumed to be nearly forward in nature. For realistic models of impurity
potentials, we further show that significant deviations from the universal
Abrikosov-Gor'kov Tc suppression behavior may be expected for out of plane
impurities.Comment: 6 pages, 5 figure
Local modulations of the spin-fluctuation mediated pairing interaction by impurities in d-wave superconductors
We present a self-consistent real space formulation of spin-fluctuation
mediated d-wave pairing. By calculating all relevant inhomogeneous spin and
charge susceptibilities in real space within the random phase approximation
(RPA), we obtain the effective pairing interaction and study its spatial
dependence near both local potential and hopping impurities. A remarkably large
enhancement of the pairing interaction may be obtained near the impurity site.
We discuss the relevance of our result to inhomogeneities observed by scanning
tunneling spectroscopy on the surface of cuprate superconductors.Comment: 8 pages, 7 figure
Supercurrent through grain boundaries in the presence of strong correlations
Strong correlations are known to severely reduce the mobility of charge
carriers near half-filling and thus have an important influence on the current
carrying properties of grain boundaries in the high- cuprates. In this
work we present an extension of the Gutzwiller projection approach to treat
electronic correlations below as well as above half-filling consistently. We
apply this method to investigate the critical current through grain boundaries
with a wide range of misalignment angles for electron- and hole-doped systems.
For the latter excellent agreement with experimental data is found. We further
provide a detailed comparison to an analogous weak-coupling evaluation.Comment: 4 pages, 3 figure
Spin fluctuations and superconductivity in a 3D tight-binding model for BaFe2As2
Despite the wealth of experimental data on the Fe-pnictide compounds of the
KFe2As2-type, K = Ba, Ca, or Sr, the main theoretical work based on
multiorbital tight-binding models has been restricted so far to the study of
the related 1111 compounds. This can be ascribed to the more three dimensional
electronic structure found by ab initio calculations for the 122 materials,
making this system less amenable to model development. In addition, the more
complicated Brillouin zone (BZ) of the body-centered tetragonal symmetry does
not allow a straightforward unfolding of the electronic band structure into an
effective 1Fe/unit cell BZ. Here we present an effective 5-orbital
tight-binding fit of the full DFT band structure for BaFeAs including the kz
dispersions. We compare the 5-orbital spin fluctuation model to one previously
studied for LaOFeAs and calculate the RPA enhanced susceptibility. Using the
fluctuation exchange approximation to determine the leading pairing
instability, we then examine the differences between a strictly two dimensional
model calculation over a single kz cut of the BZ and a completely three
dimensional approach. We find pairing states quite similar to the 1111
materials, with generic quasi-isotropic pairing on the hole sheets and nodal
states on the electron sheets at kz = 0 which however are gapped as the system
is hole doped. On the other hand, a substantial kz dependence of the order
parameter remains, with most of the pairing strength deriving from processes
near kz = pi. These states exhibit a tendency for an enhanced anisotropy on the
hole sheets and a reduced anisotropy on the electron sheets near the top of the
BZ.Comment: 12 pages, 15 figure
Vortex core shrinkage in a two gap superconductor: application to MgB2
As a model for the vortex core in MgB2 we study a two band model with a clean
sigma band and a dirty pi band. We present calculations of the vortex core size
in both bands as a function of temperature and show that there exists a
Kramer-Pesch effect in both bands even though only one of the bands is in the
clean limit. We present calculations for different pi band diffusivities and
coherence lengths.Comment: Submitted to M2S-HTSC-VIII conference proceeding
Local density of states at polygonal boundaries of d-wave superconductors
Besides the well-known existence of Andreev bound states, the zero-energy
local density of states at the boundary of a d-wave superconductor strongly
depends on the boundary geometry itself. In this work, we examine the influence
of both a simple wedge-shaped boundary geometry and a more complicated
polygonal or faceted boundary structure on the local density of states. For a
wedge-shaped boundary geometry, we find oscillations of the zero-energy density
of states in the corner of the wedge, depending on the opening angle of the
wedge. Furthermore, we study the influence of a single Abrikosov vortex
situated near a boundary, which is of either macroscopic or microscopic
roughness.Comment: 10 pages, 11 figures; submitted to Phys. Rev.
Analogue mouse pointer control via an online steady state visual evoked potential (SSVEP) brain-computer interface
The steady state visual evoked protocol has recently become a popular paradigm in brain–computer interface (BCI) applications. Typically (regardless of function) these applications offer the user a binary selection of targets that perform correspondingly discrete actions. Such discrete control systems are appropriate for applications that are inherently isolated in nature, such as selecting numbers from a keypad to be dialled or letters from an alphabet to be spelled. However motivation exists for users to employ proportional control methods in intrinsically analogue tasks such as the movement of a mouse pointer. This paper introduces an online BCI in which control of a mouse pointer is directly proportional to a user's intent. Performance is measured over a series of pointer movement tasks and compared to the traditional discrete output approach. Analogue control allowed subjects to move the pointer faster to the cued target location compared to discrete output but suffers more undesired movements overall. Best performance is achieved when combining the threshold to movement of traditional discrete techniques with the range of movement offered by proportional control
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