442 research outputs found
Spin-driven Phonon Splitting in Bond-frustrated ZnCr2S4
Utilizing magnetic susceptibility, specific heat, thermal expansion and IR
spectroscopy we provide experimental evidence that the two subsequent
antiferromagnetic transitions in ZnCr_2S_4 at T_N1 = 15 K and T_N2= 8 K are
accompanied by significant thermal and phonon anomalies. The anomaly at T_N2
reveals a strong temperature hysteresis typical for a first-order
transformation. Due to strong spin-phonon coupling both magnetic phase
transitions induce a splitting of phonon modes, where at T_N1 the
high-frequency and at T_N2 the low-frequency modes split. The anomalies and
phonon splitting observed at T_N2 are strongly suppressed by magnetic field.
Regarding the small positive Curie-Weiss temperature Theta= 8 K, we argue that
this scenario of two different magnetic phases with concomitant different
magneto-elastic couplings results from the strong competition of ferromagnetic
and antiferromagnetic exchange of equal strength.Comment: 4 pages, 4 figure
Poly-MTO, {(CH_3)_{0.92} Re O_3}_\infty, a Conducting Two-Dimensional Organometallic Oxide
Polymeric methyltrioxorhenium, {(CH_{3})_{0.92}ReO_{3}}_{\infty} (poly-MTO),
is the first member of a new class of organometallic hybrids which adopts the
structural pattern and physical properties of classical perovskites in two
dimensions (2D). We demonstrate how the electronic structure of poly-MTO can be
tailored by intercalation of organic donor molecules, such as
tetrathiafulvalene (TTF) or bis-(ethylendithio)-tetrathiafulvalene (BEDT-TTF),
and by the inorganic acceptor SbF. Integration of donor molecules leads to
a more insulating behavior of poly-MTO, whereas SbF insertion does not
cause any significant change in the resistivity. The resistivity data of pure
poly-MTO is remarkably well described by a two-dimensional electron system.
Below 38 K an unusual resistivity behavior, similar to that found in doped
cuprates, is observed: The resistivity initially increases approximately as
ln) before it changes into a dependence below 2 K.
As an explanation we suggest a crossover from purely two-dimensional
charge-carrier diffusion within the \{ReO\} planes at high
temperatures to three-dimensional diffusion at low temperatures in a
disorder-enhanced electron-electron interaction scenario (Altshuler-Aronov
correction). Furthermore, a linear positive magnetoresistance was found in the
insulating regime, which is caused by spatial localization of itinerant
electrons at some of the Re atoms, which formally adopt a electronic
configuration. X-ray diffraction, IR- and ESR-studies, temperature dependent
magnetization and specific heat measurements in various magnetic fields suggest
that the electronic structure of poly-MTO can safely be approximated by a
purely 2D conductor.Comment: 15 pages, 16 figures, 2 table
Regularisation Techniques for the Radiative Corrections of Wilson lines and Kaluza-Klein states
Within an effective field theory framework we compute the most general
structure of the one-loop corrections to the 4D gauge couplings in one- and
two-dimensional orbifold compactifications with non-vanishing constant gauge
background (Wilson lines). Although such models are non-renormalisable, we keep
the analysis general by considering the one-loop corrections in three
regularisation schemes: dimensional regularisation (DR), Zeta-function
regularisation (ZR) and proper-time cut-off regularisation (PT). The relations
among the results obtained in these schemes are carefully addressed. With
minimal re-definitions of the parameters involved, the results obtained for the
radiative corrections can be applied to most orbifold compactifications with
one or two compact dimensions. The link with string theory is discussed. We
mention a possible implication for the gauge couplings unification in such
models.Comment: 37 pages, 1 Figure, LaTeX; minor correction
Hypercoagulability progresses to hypocoagulability during evolution of acetaminophen-induced acute liver injury in pigs
Increases in prothrombin time (PT) and international normalised ratio (INR) characterise acute liver injury (ALI) and failure (ALF), yet a wide heterogeneity in clotting abnormalities exists. This study defines evolution of coagulopathy in 10 pigs with acetaminophen (APAP)-induced ALI compared to 3 Controls. APAP administration began at 0 h and continued to ‘ALF’, defined as INR >3. In APAP pigs, INR was 1.05 ± 0.02 at 0 h, 2.15 ± 0.43 at 16 h and > 3 at 18 ± 1 h. At 12 h thromboelastography (TEG) demonstrated increased clot formation rate, associated with portal vein platelet aggregates and reductions in protein C, protein S, antithrombin and A Disintegrin and Metalloprotease with Thrombospondin type 1 repeats–13 (ADAMTS-13) to 60%, 24%, 47% and 32% normal respectively. At 18 ± 1 h, INR > 3 was associated with: hypocoagulable TEG profile with heparin-like effect; falls in thrombin generation, Factor V and Factor VIII to 52%, 19% and 17% normal respectively; further decline in anticoagulants; thrombocytopenia; neutrophilia and endotoxemia. Multivariate analysis, found that ADAMTS-13 was an independent predictor of a hypercoagulable TEG profile and platelet count, endotoxin, Protein C and fibrinogen were independent predictors of a hypocoagulable TEG profile. INR remained normal in Controls. Dynamic changes in coagulation occur with progression of ALI: a pro-thrombotic state progresses to hypocoagulability
On elliptic solutions of the quintic complex one-dimensional Ginzburg-Landau equation
The Conte-Musette method has been modified for the search of only elliptic
solutions to systems of differential equations. A key idea of this a priory
restriction is to simplify calculations by means of the use of a few Laurent
series solutions instead of one and the use of the residue theorem. The
application of our approach to the quintic complex one-dimensional
Ginzburg-Landau equation (CGLE5) allows to find elliptic solutions in the wave
form. We also find restrictions on coefficients, which are necessary conditions
for the existence of elliptic solutions for the CGLE5. Using the investigation
of the CGLE5 as an example, we demonstrate that to find elliptic solutions the
analysis of a system of differential equations is more preferable than the
analysis of the equivalent single differential equation.Comment: LaTeX, 21 page
Between seas and continents: aspects of the scientific career of Hermann Von Ihering, 1850-1930
This paper covers some periods in Hermann von Ihering’s scientific trajectory: his training in zoology in Germany and Naples, his international activities based in Brazil, and his return to Germany. It deals with aspects of the formulation of his theories on land bridges. It focuses on the network of contacts he maintained with German émigrés like himself, and primarily with Florentino Ameghino, which allowed him to interact in international scientific circles. It mentions excerpts of his letters and his publications in the periods when he began corresponding with Ameghino (1890), when he travelled to Europe in search of support for his theories (1907), and when he published his book on the history of the Atlantic Ocean (1927).Facultad de Ciencias Naturales y Muse
Duality and Spontaneously Broken Supergravity in Flat Backgrounds
It is shown that the super Higgs mechanism that occurs in a wide class of
models with vanishing cosmological constant (at the classical level) is
obtained by the gauging of a flat group which must be an electric subgroup of
the duality group. If the residual massive gravitinos which occur in the
partial supersymmetry breaking are BPS saturated, then the flat group is non
abelian. This is so for all the models obtained by a Scherk-Schwarz
supersymmetry breaking mechanism. If gravitinos occur in long multiplets, then
the flat groups may be abelian. This is the case of supersymmetry breaking by
string compactifications on an orientifold T^6/Z_2 with non trivial brane
fluxes.Comment: Version to appear on Nuclear Physics
Explicit de Sitter Flux Vacua for Global String Models with Chiral Matter
We address the open question of performing an explicit stabilisation of all
closed string moduli (including dilaton, complex structure and Kaehler moduli)
in fluxed type IIB Calabi-Yau compactifications with chiral matter. Using toric
geometry we construct Calabi-Yau manifolds with del Pezzo singularities.
D-branes located at such singularities can support the Standard Model gauge
group and matter content. In order to control complex structure moduli
stabilisation we consider Calabi-Yau manifolds which exhibit a discrete
symmetry that reduces the effective number of complex structure moduli. We
calculate the corresponding periods in the symplectic basis of invariant
three-cycles and find explicit flux vacua for concrete examples. We compute the
values of the flux superpotential and the string coupling at these vacua.
Starting from these explicit complex structure solutions, we obtain AdS and dS
minima where the Kaehler moduli are stabilised by a mixture of D-terms,
non-perturbative and perturbative alpha'-corrections as in the LARGE Volume
Scenario. In the considered example the visible sector lives at a dP_6
singularity which can be higgsed to the phenomenologically interesting class of
models at the dP_3 singularity.Comment: 49 pages, 5 figures; v2: references adde
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