17,820 research outputs found
Thermodynamics of the L\'evy spin glass
We investigate the L\'evy glass, a mean-field spin glass model with power-law
distributed couplings characterized by a divergent second moment. By combining
extensively many small couplings with a spare random backbone of strong bonds
the model is intermediate between the Sherrington-Kirkpatrick and the
Viana-Bray model. A truncated version where couplings smaller than some
threshold \eps are neglected can be studied within the cavity method
developed for spin glasses on locally tree-like random graphs. By performing
the limit \eps\to 0 in a well-defined way we calculate the thermodynamic
functions within replica symmetry and determine the de Almeida-Thouless line in
the presence of an external magnetic field. Contrary to previous findings we
show that there is no replica-symmetric spin glass phase. Moreover we determine
the leading corrections to the ground-state energy within one-step replica
symmetry breaking. The effects due to the breaking of replica symmetry appear
to be small in accordance with the intuitive picture that a few strong bonds
per spin reduce the degree of frustration in the system
Probing the Melting of a Two-dimensional Quantum Wigner Crystal via its Screening Efficiency
One of the most fundamental and yet elusive collective phases of an
interacting electron system is the quantum Wigner crystal (WC), an ordered
array of electrons expected to form when the electrons' Coulomb repulsion
energy eclipses their kinetic (Fermi) energy. In low-disorder, two-dimensional
(2D) electron systems, the quantum WC is known to be favored at very low
temperatures () and small Landau level filling factors (), near the
termination of the fractional quantum Hall states. This WC phase exhibits an
insulating behavior, reflecting its pinning by the small but finite disorder
potential. An experimental determination of a vs phase diagram for
the melting of the WC, however, has proved to be challenging. Here we use
capacitance measurements to probe the 2D WC through its effective screening as
a function of and . We find that, as expected, the screening
efficiency of the pinned WC is very poor at very low and improves at higher
once the WC melts. Surprisingly, however, rather than monotonically
changing with increasing , the screening efficiency shows a well-defined
maximum at a which is close to the previously-reported melting temperature
of the WC. Our experimental results suggest a new method to map out a vs
phase diagram of the magnetic-field-induced WC precisely.Comment: The formal version is published on Phys. Rev. Lett. 122, 116601
(2019
A new result on the Klein-Gordon equation in the background of a rotating black hole
This short paper should serve as basis for further analysis of a previously
found new symmetry of the solutions of the wave equation in the gravitational
field of a Kerr black hole. Its main new result is the proof of essential
self-adjointness of the spatial part of a reduced normalized wave operator of
the Kerr metric in a weighted L^2-space. As a consequence, it leads to a purely
operator theoretic proof of the well-posedness of the initial value problem of
the reduced Klein-Gordon equation in that field in that L^2-space and in this
way generalizes a corresponding result of Kay (1985) in the case of the
Schwarzschild black hole. It is believed that the employed methods are
applicable to other separable wave equations
Spin Hall effect in a system of Dirac fermions in the honeycomb lattice with intrinsic and Rashba spin-orbit interaction
We consider spin Hall effect in a system of massless Dirac fermions in a
graphene lattice. Two types of spin-orbit interaction, pertinent to the
graphene lattice, are taken into account - the intrinsic and Rashba terms.
Assuming perfect crystal lattice, we calculate the topological contribution to
spin Hall conductivity. When both interactions are present, their interplay is
shown to lead to some peculiarities in the dependence of spin Hall conductivity
on the Fermi level.Comment: 7 pages, 5 figure
Plasmodium falciparum glutamate dehydrogenase a is dispensable and not a drug target during erythrocytic development
<p>Background: Plasmodium falciparum contains three genes encoding potential glutamate dehydrogenases. The protein encoded by gdha has previously been biochemically and structurally characterized. It was suggested that it is important for the supply of reducing equivalents during intra-erythrocytic development of Plasmodium and, therefore, a suitable drug target.</p>
<p>Methods: The gene encoding the NADP(H)-dependent GDHa has been disrupted by reverse genetics in P. falciparum and the effect on the antioxidant and metabolic capacities of the resulting mutant parasites was investigated.</p>
<p>Results: No growth defect under low and elevated oxygen tension, no up-or down-regulation of a number of antioxidant and NADP(H)-generating proteins or mRNAs and no increased levels of GSH were detected in the D10(Delta gdha) parasite lines. Further, the fate of the carbon skeleton of [(13)C] labelled glutamine was assessed by metabolomic studies, revealing no differences in the labelling of a-ketoglutarate and other TCA pathway intermediates between wild type and mutant parasites.</p>
<p>Conclusions: First, the data support the conclusion that D10(Delta gdha) parasites are not experiencing enhanced oxidative stress and that GDHa function may not be the provision of NADP(H) for reductive reactions. Second, the results imply that the cytosolic, NADP(H)-dependent GDHa protein is not involved in the oxidative deamination of glutamate but that the protein may play a role in ammonia assimilation as has been described for other NADP(H)dependent GDH from plants and fungi. The lack of an obvious phenotype in the absence of GDHa may point to a regulatory role of the protein providing glutamate (as nitrogen storage molecule) in situations where the parasites experience a limiting supply of carbon sources and, therefore, under in vitro conditions the enzyme is unlikely to be of significant importance. The data imply that the protein is not a suitable target for future drug development against intra-erythrocytic parasite development.</p>
Influence of Combined Hard and Fine Machining on the Surface Properties of Cemented Carbides
As a result of recent developments in cold forging cemented carbides are increasingly used as tool materials. Due to their high hardness only electrical discharge machining (EDM) and grinding are suitable for tool machining. The structure of tool surface has significant influence on dominating failure mechanisms wear and fatigue. For improvement of tribological conditions the surface is polished in a finale processing step. The result of hard and fine machining is a specific combination of coarse and fine structure which is determined by processing parameters. The different surface structures lead to a particular tool behavior in forming process. This paper aims to show the influence of combined hard and fine machining on the surface properties of cemented carbides
The Effect of Fatigue on Essential Element Distribution in the Rabbit Masseter
The rabbit masseter is composed of mainly fast fatiguable type IIB fibers which give a positive mATPase reaction at pH 9.3. To study the effect of fatigue on the distribution of some crucial elements, the masseter was stimulated electrically for 45 minutes. The contralateral muscle served as a control. Control and experimental tissues were rapidly excised, and frozen in isopentane cooled to -150°C in liquid nitrogen. Sections were cut in a cryostat and prepared by freeze-drying. Water content of fatigued muscle was significantly higher than controls: 79.3% versus 77.3%. The fatigued state was verified by glycogen depletion viewed in PAS-stained sections. Ice crystal artifacts were more prominent in stimulated muscle, suggesting cytoplasmic edema and/or a change in water state. Element analysis of myofibers, using an scanning electron microscope equipped with a computer assisted energy dispersive X-ray spectrometer was performed at 15 kV for 100 seconds. Counts were recorded for Na, Mg, P, S, Cl, K and Ca ( Mg and Ca counts were too low to evaluate). Counts were converted to concentrations using a gelatin element standard. The most striking finding in fatigued muscle was a decrease in K concentration (33%) and an increase in Na (100%) and Cl (75%). P and S values were slightly lower. An interpretation of these findings is made in relation to a) changes in water structure and b) conformational changes in myofibril macromolecules
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