General scheme for stable single and multiatom nanomagnets according to symmetry selection rules

At low temperature, information can be stored in the orientation of the localized magnetic moment of an adatom. However, scattering of electrons and phonons with the nanomagnet leads its state to have incoherent classical dynamics and might cause fast loss of the encoded information. Recently, it has been understood that such scattering obeys certain selection rules due to the symmetries of the system. By analyzing the point-group symmetry of the surface, the time-reversal symmetry and the magnitude of the adatom effective spin, we identify which nanomagnets configurations are to be avoided and which are promising to encode a stable bit. A new tool of investigation is introduced and exploited: the quasi-spin quantum number. By means of this tool, our results are easily generalized to a broad class of bipartite cluster configurations where adatoms are coupled through Heisenberg-like interactions. Finally, to make contact with the experiments, numerical simulations have been performed to show how such stable configurations respond to typical scanning tunneling microscopy measurements.Comment: 15 pages, 7 figures. Published versio

Effect of chiral symmetry on chaotic scattering from Majorana zero modes

In many of the experimental systems that may host Majorana zero modes, a so-called chiral symmetry exists that protects overlapping zero modes from splitting up. This symmetry is operative in a superconducting nanowire that is narrower than the spin-orbit scattering length, and at the Dirac point of a superconductor/topological insulator heterostructure. Here we show that chiral symmetry strongly modifies the dynamical and spectral properties of a chaotic scatterer, even if it binds only a single zero mode. These properties are quantified by the Wigner-Smith time-delay matrix $Q=-i\hbar S^\dagger dS/dE$, the Hermitian energy derivative of the scattering matrix, related to the density of states by $\rho=(2\pi\hbar)^{-1}\,{\rm Tr}\,Q$. We compute the probability distribution of $Q$ and $\rho$, dependent on the number $\nu$ of Majorana zero modes, in the chiral ensembles of random-matrix theory. Chiral symmetry is essential for a significant $\nu$-dependence.Comment: 5 pages, 3 figures + appendix (3 pages, 1 figure

Effect of a tunnel barrier on the scattering from a Majorana bound state in an Andreev billiard

We calculate the joint distribution $P(S,Q)$ of the scattering matrix $S$ and time-delay matrix $Q=-i\hbar S^\dagger dS/dE$ of a chaotic quantum dot coupled by point contacts to metal electrodes. While $S$ and $Q$ are statistically independent for ballistic coupling, they become correlated for tunnel coupling. We relate the ensemble averages of $Q$ and $S$ and thereby obtain the average density of states at the Fermi level. We apply this to a calculation of the effect of a tunnel barrier on the Majorana resonance in a topological superconductor. We find that the presence of a Majorana bound state is hidden in the density of states and in the thermal conductance if even a single scattering channel has unit tunnel probability. The electrical conductance remains sensitive to the appearance of a Majorana bound state, and we calculate the variation of the average conductance through a topological phase transition.Comment: Contribution for the special issue of Physica E in memory of Markus B\"{u}ttiker. 13 pages, 7 figure

Time-delay matrix, midgap spectral peak, and thermopower of an Andreev billiard

We derive the statistics of the time-delay matrix (energy derivative of the scattering matrix) in an ensemble of superconducting quantum dots with chaotic scattering (Andreev billiards), coupled ballistically to $M$ conducting modes (electron-hole modes in a normal metal or Majorana edge modes in a superconductor). As a first application we calculate the density of states $\rho_0$ at the Fermi level. The ensemble average $\langle\rho_0\rangle=\delta_0^{-1}M[\max(0,M+2\alpha/\beta)]^{-1}$ deviates from the bulk value $1/\delta_0$ by an amount depending on the Altland-Zirnbauer symmetry indices $\alpha,\beta$. The divergent average for $M=1,2$ in symmetry class D ($\alpha=-1$, $\beta=1$) originates from the mid-gap spectral peak of a closed quantum dot, but now no longer depends on the presence or absence of a Majorana zero-mode. As a second application we calculate the probability distribution of the thermopower, contrasting the difference for paired and unpaired Majorana edge modes.Comment: 13 pages, 6 figure

The lithic assemblage of the US 13 at the middle paleolithic site of Oscurusciuto (Ginosa, Taranto, Southern Italy): technological studies

Master Erasmus Mundus em Quaternário e Pré-Históri

Continuities and discontinuities during the late Middle Palaeolithic at the Oscurusciuto rock shelter (southern Italy). An integrated study of lithic manufacture in the strata SU 15, SU 14, SU 13 and SU 11.

Aquesta investigació té com a objectiu interpretar les variacions en la producció Neandertal, així com la funcionalitat de les eines de pedra mitjançant l'anàlisi de conjunts lítics dins l'últim Paleolític Mitjà del sud d'Itàlia. L'abric Oscurusciuto és clau per a la comprensió del comportament dels neandertals, ja que ofereix un dipòsit llarg i fiable, d'uns 6 m de profunditat, format per diversos nivells que oscil•len entre 42,724 ± 716 cal BP i 55 ± 2 kyrs (40Ar/39Ar). L’objectiu principal d'aquesta investigació és realitzar un estudi del conjunt d’eines de lítica, present a la secció inferior de la sèrie investigada fins ara en aquest jaciment: SU 15, SU 14, SU 13 i SU 11. Volem individualitzar, des d'un punt de vista diacrònic, les continuïtats i discontinuïtats d'aquests tecno-complexos lítics. Tenim la intenció d'aconseguir aquest objectiu a través d'un estudi integrat del material lític que permetrà una descripció del comportament econòmic relacionat amb l'explotació de les fonts de matèria primera lítica, seguit d'una descripció detallada de les fases de la seqüència de reducció, així com una definició de conceptes, mètodes, dinàmiques i objectius de la talla. Aquesta anàlisi tecnològica fonamental s'aplica als quatre nivells: SU 15, SU 14, SU 13 i SU 11. Basant-nos en les característiques del material recollit, s'han dut a terme diversos estudis: d’unitats de matèria primera (RMU) i de remuntatges per SU 13 i SU 14, per tal de comprendre millor la fragmentació de la cadena operativa. A més, s'ha desenvolupat un protocol tecno-funcional sobre un grup seleccionat de peces de la SEU 13, per tal d'aprofundir la nostra comprensió sobre la potencialitat les eines. Els resultats demostren que aquestes unitats estratigràfiques mostren peculiaritats específiques en termes de sistemes de producció lítica, elements estructurals, distribució espacial i tipus d'ocupació.Esta investigación tiene como objetivo interpretar las variaciones en la producción y la funcionalidad de herramientas de piedra de los Neandertales a través del análisis de los conjuntos líticos referibles a la última parte del Paleolítico Medio del sur de Italia. Un sitio clave para este propósito es el abrigo Oscurusciuto. Este es esencial para la comprensión del comportamiento de los Neandertales porque ofrece un depósito largo y confiable, de unos 6 m de profundidad. El mismo se encuentra formado por varios niveles que oscilan entre 42,724 ± 716 cal BP y 55 ± 2 kyrs (40Ar/39Ar). El propósito explícito de esta investigación es realizar un estudio integrado de las herramientas líticas presentes en la sección inferior de la serie hasta ahora investigada del abrigo Oscurusciuto: SU 15, SU 14, SU 13 y SU 11. Se pretende individualizar, desde un punto de vista diacrónico, las continuidades y discontinuidades de estos tecnocomplejos líticos. Para lograr este propósito se realizará un estudio integrado del material lítico, que permita una descripción del comportamiento económico relacionado con la explotación de las fuentes de materias primas líticas, una descripción detallada de las fases de la secuencia de reducción, así como también una definición de conceptos, métodos, dinámicas y objetivos del debitage. Este análisis tecnológico fundamental se aplica a los cuatro niveles. Considerando las características del material recogido, se han llevado a cabo estudios adicionales, como Unidades de Materia Prima (RMU) y remontajes para SU 13 y SU 14, con el fin de comprender mejor la fragmentación de la cadena operativa. Además, se ha desarrollado un protocolo tecno-funcional sobre un grupo seleccionado de piezas de SU 13, con el fin de profundizar nuestra comprensión de la potencialidad de las herramientas. Los resultados demuestran que estas unidades estratigráficas muestran peculiaridades específicas en términos de sistemas de producción lítica, elementos estructurales, manejo espacial y tipo de ocupación.This research aims to interpret the variations in the Neanderthal production and functionality of stone tools through analyses of lithic assemblages referable to the last Middle Palaeolithic of southern Italy. A key site for this purpose is the Oscurusciuto rock shelter, which is essential to the understanding of Neanderthal behaviour, as it offers a long, reliable deposit, about 6 m in depth, made up of several levels ranging between 42,724 ± 716 cal BP and 55 ± 2 kyrs (40Ar/39Ar). The explicit purpose of this research is to perform an integrated study of the lithic manufacture present in the lower section of the series so far investigated the Oscurusciuto rock shelter: SU 15, SU 14, SU 13 and SU 11. We want to individuate, from a diachronic point of view, the continuities and discontinuities of these lithic techno-complexes. We intend to achieve this purpose through an integrated study of the lithic material, which in turn allows for a description of the economic behaviour involved in the exploitation of the sources of lithic raw material, followed by a detailed description of the phases in the reduction sequence, as well as a definition of concepts, methods, dynamics and objectives of the debitage. This fundamental technological analysis is applied to all four levels: SU 15, SU 14, SU 13, and SU 11 (48,382 items). Based on the characteristics of the collected material, further studies have been undertaken, such as Raw Material Units plus refitting studies for SU 13 and SU 14, in order to better understand the fragmentation of the operative chain. Moreover, a techno-functional protocol has been developed regarding a selected group of pieces from SU 13, in order to deepen our comprehension of productional vs. functional tools. The results demonstrate that these stratigraphic units show specific peculiarities in terms of lithic production systems, structural elements, spatial management, and type of occupation

Delayed gastric emptying and reduced postprandial small bowel water content of equicaloric whole meal bread versus rice meals in healthy subjects: novel MRI insights

BACKGROUND/OBJECTIVES: Postprandial bloating is a common symptom in patients with functional gastrointestinal (GI) diseases. Whole meal bread (WMB) often aggravates such symptoms though the mechanisms are unclear. We used magnetic resonance imaging (MRI) to monitor the intragastric fate of a WMB meal (11% bran) compared to a rice pudding (RP) meal. SUBJECTS/METHODS: 12 healthy volunteers completed this randomised crossover study. They fasted overnight and after an initial MRI scan consumed a glass of orange juice with a 2267 kJ WMB or an equicaloric RP meal. Subjects underwent serial MRI scans every 45 min up to 270 min to assess gastric volumes and small bowel water content and completed a GI symptom questionnaire. RESULTS: The MRI intragastric appearance of the two meals was markedly different. The WMB meal formed a homogeneous dark bolus with brighter liquid signal surrounding it. The RP meal separated into an upper, liquid layer and a lower particulate layer allowing more rapid emptying of the liquid compared to solid phase (sieving). The WMB meal had longer gastric half emptying times (132±8 min) compared to the RP meal (104±7 min), P<0.008. The WMB meal was associated with markedly reduced MRI-visible small bowel free mobile water content compared to the RP meal, P<0.0001. CONCLUSIONS: WMB bread forms a homogeneous bolus in the stomach which inhibits gastric sieving and hence empties slower than the equicaloric rice meal. These properties may explain why wheat causes postprandial bloating and could be exploited to design foods which prolong satiation

X-shaped and Y-shaped Andreev resonance profiles in a superconducting quantum dot

The quasi-bound states of a superconducting quantum dot that is weakly coupled to a normal metal appear as resonances in the Andreev reflection probability, measured via the differential conductance. We study the evolution of these Andreev resonances when an external parameter (such as magnetic field or gate voltage) is varied, using a random-matrix model for the $N\times N$ scattering matrix. We contrast the two ensembles with broken time-reversal symmetry, in the presence or absence of spin-rotation symmetry (class C or D). The poles of the scattering matrix in the complex plane, encoding the center and width of the resonance, are repelled from the imaginary axis in class C. In class D, in contrast, a number $\propto\sqrt{N}$ of the poles has zero real part. The corresponding Andreev resonances are pinned to the middle of the gap and produce a zero-bias conductance peak that does not split over a range of parameter values (Y-shaped profile), unlike the usual conductance peaks that merge and then immediately split (X-shaped profile).Comment: Contribution for the JETP special issue in honor of A.F. Andreev's 75th birthday. 9 pages, 8 figure

Assessment of motion of colonic contents in the human colon using MRI tagging

Background We have previously reported a non-invasive, semi-automated technique to assess motility of the wall of the ascending colon (AC) using Magnetic Resonance Imaging. This study investigated the feasibility of using a tagged MRI technique to visualise and assess the degree of flow within the human ascending colon in healthy subjects and those suffering from constipation. Methods An open-labelled study of 11 subjects with constipation and 11 subjects without bowel disorders was performed. MRI scans were acquired fasted, then 60 and 120 mins after ingestion of a 500ml macrogol preparation. The amount of free fluid in the small and large bowel was assessed using a heavily T2-weighted MRI sequence. The internal movement of the contents of the AC were visualised using a cine tagged MRI sequence and assessed by a novel analysis technique. Comparisons were made between fasting and postprandial scans within individuals, and between the constipation and control groups. Key results. Macrogol significantly increased the mobile, MR visible water content of the ascending colon at 60 mins post ingestion compared to fasted data (controls p=0.001, constipated group p=0.0039). The contents of the AC showed increased motion in healthy subjects but not in the constipated group with significant differences between groups at 60 minutes (p<0.002) and 120 minutes (p<0.003). Conclusions and inferences. This study successfully demonstrated the use of a novel MRI tagging technique to visualise and assess the motion of ascending colon contents following a 500ml macrogol challenge. Significant differences were demonstrated between healthy and constipated subjects

MRI assessment of the postprandial gastrointestinal motility and peptide response in healthy humans

Background: Feeding triggers inter-related gastrointestinal (GI) motor, peptide and appetite responses. These are rarely studied together due to methodological limitations. Recent MRI advances allow pan-intestinal, non-invasive assessment of motility in the undisturbed gut. This study aimed to develop a methodology to assess pan-intestinal motility and transit in a single session using MRI and compare imaging findings to GI peptide responses to a test meal and symptoms in a healthy volunteer cohort. Methods: Fifteen healthy volunteers (29.3±2.7years and BMI 20.1±1.2Kg/m2) underwent baseline and postprandial MRI scans, symptom questionnaires and blood sampling (for subsequent GI peptide analysis, Glucagon-like peptide-1 (GLP-1), Polypeptide YY (PYY), Cholecystokinin (CCK)) at intervals for 270min following a 400g soup meal (204kcal, Heinz, UK). Gastric volume, gall bladder volume, small bowel water content, small bowel motility and whole gut transit were measured from the MRI scans. Key Results: (mean±SEM) Small bowel motility index increased from fasting 39±3 arbitrary units (a.u.) to a maximum of 87±7a.u. immediately after feeding. PYY increased from fasting 98±10pg/ml to 149±14pg/ml at 30min and GLP-1 from fasting 15±3µg/ml to 22±4µg/ml. CCK increased from fasting 0.40±0.06pmol/ml to 0.94±0.1pmol/ml. Gastric volumes declined with a T1/2 of 46±5min and the gallbladder contracted from a fasting volume of 19±2ml to 12±2ml. Small bowel water content increased from 39±2ml to 51±2ml postprandial. Fullness VAS score increased from 9±5mm to 41±6mm at 30min postprandial. Conclusions and Inferences: The test meal challenge was effective in inducing a change in MRI motility end-points which will improve understanding of the pathophysiological postprandial GI response