312 research outputs found
Detection of Quantum Phase Transitions with a Lee-Yang Formalism and Many-Body Algorithms
No full textPhase transitions in many-body quantum systems arise from the collective behaviour of many degrees of freedom. For many interacting quantum many-body systems, it remains challenging to determine their phase diagram due to the exponential growth in the Hilbert space size and the difficulty of existing numerical techniques to tackle generic interacting quantum many-body systems.
In this thesis, we combine state-of-the-art numerical techniques with an approach to symmetrybreaking phase transitions inspired by the works of Lee and Yang on the zeros of partition functions in the complex plane. Specifically, we study the behaviour and distribution of zeros of the momentgenerating function of the relevant order parameters to locate the phase transitions. We refer to these zeros as Lee-Yang zeros. Our approach involves using tensor networks and neural quantum states as variational states to describe the ground states or finite temperature density matrices of the systems studied. While tensor networks allow for a direct evaluation of the moment-generating function, and therefore a direct determination of the position of these Lee-Yang zeros, this is not possible for neural quantum states. Therefore, we also present a method that uses high cumulants of the order parameter combined with knowledge about the symmetries of the Lee-Yang zeros to estimate their locations. By extrapolating the distance from the origin to these zeros to the thermodynamic limit, the presence of a phase transition can be determined. Using this Lee-Yang formalism, we map out the phase diagram of the transverse field Ising model and a fermionic chain, thereby showing its practical applicability to specific quantum many-body models.
Also using neural quantum states and tensor networks, we determine the phase diagram of a tetramerized antiferromagnetic spin-1/2 J₁-J₂ Heisenberg model on the square lattice. Without relying on our Lee-Yang formalism, we are able to trace out the phase diagram using conventional means, such as studying the susceptibility, spin structure factor and the many-body gap. This model, which has been recently realized in experiment, exhibits an intriguing competition between conventional magnetically ordered phases and a higher-order symmetry protected topological phase. By mapping out its phase diagram, we contribute to guiding experiments towards the parameter regimes of interest
A Cretaceous carbonate delta drift in the Montagna della Maiella, Italy
No full textThe Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is compared to newly discovered contourite drifts in the Maldives. Like the drift deposits in the Maldives, the Orfento Formation fills a channel and builds a Miocene delta-shaped and mounded sedimentary body in the basin that is similar in size to the approximately 350 km 2 large coarse-grained bioclastic Miocene delta drifts in the Maldives. The composition of the bioclastic wedge of the Orfento Formation is also exclusively bioclastic debris sourced from the shallow-water areas and reworked clasts of the Orfento Formation itself. In the near mud-free succession, age-diagnostic fossils are sparse. The depositional textures vary from wackestone to float-rudstone and breccia/conglomerates, but rocks with grainstone and rudstone textures are the most common facies. In the channel, lensoid convex-upward breccias, cross-cutting channelized beds and thick grainstone lobes with abundant scours indicate alternating erosion and deposition from a high-energy current. In the basin, the mounded sedimentary body contains lobes with a divergent progradational geometry. The lobes are built by decametre thick composite megabeds consisting of sigmoidal clinoforms that typically have a channelized topset, a grainy foreset and a fine-grained bottomset with abundant irregular angular clasts. Up to 30 m thick channels filled with intraformational breccias and coarse grainstones pinch out downslope between the megabeds. In the distal portion of the wedge, stacked grainstone beds with foresets and reworked intraclasts document continuous sediment reworking and migration. The bioclastic wedge of the Orfento Formation has been variously interpreted as a succession of sea-level controlled slope deposits, a shoaling shoreface complex, or a carbonate tidal delta. Current-controlled delta drifts in the Maldives, however, offer a new interpretation because of their similarity in architecture and composition. These similarities include: (i) a feeder channel opening into the basin; (ii) an excavation moat at the exit of the channel; (iii) an overall mounded geometry with an apex that is in shallower water depth than the source channel; (iv) progradation of stacked lobes; (v) channels that pinch out in a basinward direction; and (vi) smaller channelized intervals that are arranged in a radial pattern. As a result, the Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is here interpreted as a carbonate delta drift
Identification of a specific one amino acid change in recombinant human transglutaminase 2 that regulates its activity and calcium sensitivity
No full textTG2 (transglutaminase 2) is a calcium-dependent protein crosslinking enzyme which is involved in a variety of cellular processes. The threshold level of calcium needed for endogenous and recombinant TG2 activity has been controversial, the former being more sensitive to calcium than the latter. In the present study we address this question by identifying a single amino acid change from conserved valine to glycine at position 224 in recombinant TG2 compared with the endogenous sequence present in the available genomic databases. Substituting a valine residue for Gly224 in the recombinant TG2 increased its calcium-binding affinity and transamidation activity 10-fold and isopeptidase activity severalfold, explaining the inactivity of widely used recombinant TG2 at physiological calcium concentrations. ITC (isothermal titration calorimetry) measurements showed 7-fold higher calcium-binding affinities for TG2 valine residues which could be activated inside cells. The two forms had comparable substrate- and GTP-binding affinities and also bound fibronectin similarly, but coeliac antibodies had a higher affinity for TG2 valine residues. Structural analysis indicated a higher stability for TG2 valine residues and a decrease in flexibility of the calciumbinding loop resulting in improved metal-binding affinity. The results of the present study suggest that Val224 increases TG2 activity by modulating its calcium-binding affinity enabling transamidation reactions inside cells. © 2013 Biochemical Society
Aldosterone diagnosis in hypertension: comparative evaluation of radioimmunoassays for urinary aldosterone and 18-OH- corticosterone.
No full textThe pathogenesis of pseudohyperaldosteronism from carbenoxolone
No full textCarbenoxolone is a derivative of glycyrrhetinic acid used for the treatment of peptic ulcer and gastritis, with salt and water retention a very common side-effect. To investigate this drug-induced pseudohyperaldosteronism we have studied 6 male volunteers before, during and after treatment with carbenoxolone for 7 days. Serum, urinary and sweat electrolytes values were consistent with a mineralocorticoid-like effect of drug administration. PRA was suppressed, and plasma cortisol and aldosterone progressively decreased over treatment. We have also determined by radioreceptor assay the plasma levels of factors which bind to mineralocorticoid receptors in rat kidney cytosol. The levels of these factors were decreased significantly at day 3 of treatment, suggesting a local renal effect of carbenoxolone to amplify endogenous steroid action. At day 7 the radioreceptor assay values were still decreased but significantly higher than at day 3, suggesting in addition a direct mineralocorticoid effect of the drug. We conclude that the drug is initially effective by amplifying the effect of endogenous steroids, and then when the plasma concentrations of the drug or its metabolites reach a higher plasma concentration, there may also be in addition a direct mineralocorticoid-like effect
Montagna della Maiella: The sedimentary and sequential evolution of a Bahamian-type carbonate platform of the South-Tethyan continental margin.
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Diffúziós vizsgálatok hengeres nanoszerkezetekben
No full textAlumínium-oxid és cink-oxid hengeres nanoszálakon végzett diffúziós vizsgálatok pásztázó elektronmikroszkóp, pásztázó transzmissziós elektronmikroszkóp és röntgen diffraktométer segítségével.AnyagtudományMSc/M
Lithium treatment reduces the renal kallikrein excretion rate
No full textLithium treatment reduces the renal kallikrein excretion rate. Lithium salts are widely used agents for the prophylactic treatment of affective disorders. Lithium salts may be associated with distal nephron dysfunction. Kallikrein is a protease which is generated by the distal nephron. We used an amidolytic assay of chromatographically purified enzyme to determine the urinary excretion rate of active kallikrein in relation to lithium treatment. All plasma lithium concentrations were within the therapeutic range (0.4 to 0.9 mmol/liter). In 15 patients the urinary excretion rate of active kallikrein was 267.4 65.6 mU/24 hrs before lithium treatment, and fell to 117.8 39.6 mU/24 hrs (P < 0.05) on day 14 of lithium treatment. This reduction was associated with a decrease of immunoreactive kallikrein in the same urines by 66%. In another 15 patients who had undergone lithium therapy for an average period of 5.6 years, the urinary excretion rate of active kallikrein was 86.1 14.5 mU/24 hrs, while 21 age-matched healthy controls had an excretion rate of 364.1 58.4 mU/24 hrs (P < 0.05). Measurements of immunoreactive kallikrein in the same urine samples demonstrated a reduction of kallikrein after long-term lithium treatment by 78%. These observations could not be attributed to changes in creatinine clearance, renal sodium or potassium excretion rates or plasma concentrations of aldosterone and vasopressin. Addition of lithium to the urine in vitro had no demonstrable effect on kallikrein measurement by amidolytic assay. We conclude that lithium in therapeutic plasma concentrations may directly suppress the secretion of kallikrein by renal connecting tubule cells
Montagna della Maiella: The sedimentary and sequential evolution of a Bahamian-type carbonate platform of the South-Tethyan continental margin
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