15,589 research outputs found
Chemically-Mediated quantum criticality in NbFe_2
Laves-phase Nb{1+c}Fe_{2-c} is a rare itinerant intermetallic compound
exhibiting magnetic quantum criticality at c_{cr}=1.5%Nb excess; its origin,
and how alloying mediates it, remains an enigma. For NbFe_2, we show that an
unconventional band critical point (uBCP) above the Fermi level E_F explains
most observations, and that chemical alloying mediates access to this uBCP by
an increase in E_F with decreasing electrons (increasing %Nb), counter to
rigid-band concepts. We calculate that E_F enters the uBCP region for c_{cr} >
1.5%Nb and by 1.74%Nb there is no Nb site-occupation preference between
symmetry-distinct Fe sites, i.e., no electron-hopping disorder, making
resistivity near constant as observed. At larger Nb (Fe) excess, the
ferromagnetic Stoner criterion is satisfied.Comment: 4 pages, 5 figure
Low-energy, planar magnetic defects in BaFe2As2: nanotwins, twins, antiphase and domain boundaries
In BaFe2As2, structural and magnetic planar defects begin to proliferate
below the structural phase transition, affecting descriptions of magnetism and
superconductivity. We study using density-functional theory the stability and
magnetic properties of competing antiphase and domain boundaries, twins and
isolated twins (twin nuclei) - spin excitations proposed and/or observed.
These nanoscale defects have very low surface energy (-~Jm),
with twins favorable to the mesoscale. Defects exhibit smaller moments confined
near their boundaries -- making a uniform-moment picture inappropriate for
long-range magnetic order in real samples. {\it{Nano}}twins explain features in
measured pair distribution functions, so should be considered when analyzing
scattering data. All these defects can be weakly mobile and/or have
fluctuations that lower assessed "ordered" moments from longer spatial and/or
time averaging, and should be considered directly.Comment: 6 pages, 6 figures, 1 tabl
SET based experiments for HTSC materials: II
The cuprates seem to exhibit statistics, dimensionality and phase transitions
in novel ways. The nature of excitations
[i.e. quasiparticle or collective], spin-charge separation, stripes [static
and dynamics], inhomogeneities, psuedogap, effect of impurity dopings [e.g. Zn,
Ni] and any other phenomenon in these materials must be consistently
understood. In this note we further discuss our original suggestion of using
Single Electron Tunneling Transistor
[SET] based experiments to understand the role of charge dynamics in these
systems. Assuming that SET operates as an efficient charge detection system we
can expect to understand the underlying physics of charge transport and charge
fluctuations in these materials for a range of doping. Experiments such as
these can be classed in a general sense as mesoscopic and nano characterization
of cuprates and related materials. In principle such experiments can show if
electron is fractionalized in cuprates as indicated by ARPES data. In contrast
to flux trapping experiments SET based experiments are more direct in providing
evidence about spin-charge separation. In addition a detailed picture of nano
charge dynamics in cuprates may be obtained.Comment: 10 pages revtex plus four figures; ICMAT 2001 Conference Symposium P:
P10-0
Anomalous random correlations of force constants on the lattice dynamical properties of disordered Au-Fe alloys
Au-Fe alloys are of immense interest due to their biocompatibility, anomalous
hall conductivity, and applications in various medical treatment. However,
irrespective of the method of preparation, they often exhibit a high-level of
disorder, with properties sensitive to the thermal or magnetic annealing
temperatures. We calculate lattice dynamical properties of AuFe
alloys using density functional theory methods, where, being a multisite
property, reliable interatomic force constant (IFC) calculations in disordered
alloys remain a challenge. We follow a two fold approach: (1) an accurate IFC
calculation in an environment with nominally zero chemical pair correlations to
mimic the homogeneously disordered alloy; and (2) a configurational averaging
for the desired phonon properties (e.g., dispersion, density of states, and
entropy). We find an anomalous change in the IFC's and phonon dispersion (split
bands) near =0.19, which is attributed to the local stiffening of the Au-Au
bonds when Au is in the vicinity of Fe. Other results based on mechanical and
thermo-physical properties reflect a similar anomaly: Phonon entropy, e.g.,
becomes negative below =0.19, suggesting a tendency for chemical unmixing,
reflecting the onset of miscibility gap in the phase diagram. Our results match
fairly well with reported data, wherever available
Dead cone due to parton virtuality
A general expression for the dead cone of gluons radiated by virtual partons
has been derived. The conventional dead cone for massive on-shell quarks and
the dead cone for the massless virtual partons have been obtained by using
different limits of the general expression. Radiative suppression due to the
virtuality of initial parton jets in Heavy-Ion Collisions (HIC) has been
discussed. It is observed that the suppression caused by the high virtuality is
overwhelmingly large as compared to that on account of conventional dead-cone
of heavy quarks. The dead cone due to virtuality may play a crucial role in
explaining the observed similar suppression patterns of light and heavy quarks
jets in heavy ion collisions at Relativistic Heavy Ion Collider (RHIC)
In re Harrods Ltd.: The Brussels Convention and the Proper Application of Forum Non Conveniens to Non-Contracting States
Although the doctrine of forum non conveniens is unknown in Continental legal systems, Community law does not prevent English courts from preserving their discretion to stay proceedings, in conflicts involving a defendant domiciliary, in favor of more appropriate courts in a non-Contracting State. Where the provisions of the Brussels Convention do not address a legal question, the answer must be sought in the objectives and scheme of the Convention. The English Court of Appeals in Harrods properly understood that Community law does not require ritualistic reliance on the Convention\u27s jurisdiction conferring provisions in cases involving a defendant domiciled in a Contracting State and the jurisdiction of a court in a non-Contracting State
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