Nematic phase in the J1_1-J2_2 square lattice Ising model in an external field

The J$_1$-J$_2$ Ising model in the square lattice in the presence of an external field is studied by two approaches: the Cluster Variation Method (CVM) and Monte Carlo simulations. The use of the CVM in the square approximation leads to the presence of a new equilibrium phase, not previously reported for this model: an Ising-nematic phase, which shows orientational order but not positional order, between the known stripes and disordered phases. Suitable order parameters are defined and the phase diagram of the model is obtained. Monte Carlo simulations are in qualitative agreement with the CVM results, giving support to the presence of the new Ising-nematic phase. Phase diagrams in the temperature-external field plane are obtained for selected values of the parameter $\kappa=J_2/|J_1|$ which measures the relative strength of the competing interactions. From the CVM in the square approximation we obtain a line of second order transitions between the disordered and nematic phases, while the nematic-stripes phase transitions are found to be of first order. The Monte Carlo results suggest a line of second order nematic-disordered phase transitions in agreement with the CVM results. Regarding the stripes-nematic transitions, the present Monte Carlo results are not precise enough to reach definite conclusions about the nature of the transitions.Comment: 13 pages, 10 figure

A comparison of medical education in Germany and the United States: From applying to medical school to the beginnings of residency

Both Germany and the United States of America have a long tradition of science and medical excellence reaching back as far as the nineteenth century. The same tribute must be paid to the medical educational system in both countries. Despite significant initial similarities and cross-inspiration, the paths from enrolling in a medical university to graduating as a medical doctor in Germany and the US seem to have become much different. To fill a void in literature, the authors' objective therefore is to delineate both structures of medical education in an up-to-date review and examine their current differences and similarities. Recent medical publications, legal guidelines of governmental or official organizations, articles in media, as well as the authors' personal experiences are used as sources of this report.Tuition loans of over $200,000 are not uncommon for students in the US after graduating from medical schools, which are often private institutions. In Germany, however, the vast majority of medical universities are tax-funded and, for this reason, free of tuition. Significant differences and surprisingly multiple similarities exist between these two systems, despite one depending on government and the other on private organizations. Germany currently employs an integrated medical curriculum that typically begins right after high school and consists of a 2-year long pre-clinical segment teaching basic sciences and a 4-year clinical segment leading medical students to the practical aspects of medicine. On the other hand, the US education is a two-stage process. After successful completion of a Bachelor's degree in college, an American student goes through a 4-year medical program encompassing 2 years of basic science and 2 years of clinical training. In this review, we will address some of these similarities and major differences.Deutschland und die Vereinigten Staaten von Amerika haben beide eine lange Tradition der Naturwissenschaft und medizinischen Exzellenz, die bis weit in das neunzehnte Jahrhundert zurückreicht. Den gleichen Tribut muss man den medizinischen Ausbildungssystemen beider Länder zollen. Trotz zu Beginn bedeutsamer Ähnlichkeiten und gewisser Querinspiration scheinen sich die Wege von der Immatrikulation an einer medizinischen Fakultät bis zum Studienabschluss als Arzt in Deutschland und den USA getrennt zu haben. Um eine Lücke in der Fachliteratur zu schließen, ist das Ziel der Autoren, die beiden Strukturen der medizinischen Ausbildung mittels einer aktuellen Übersichtsschrift darzustellen und deren Unterschiede und Gemeinsamkeiten zu untersuchen. Die neusten medizinischen Publikationen, verbindliche Richtlinien von amtlichen oder offiziellen Organisationen, Artikel in der Presse, aber auch die persönlichen Erfahrungen der Autoren dienen als Quellen für diese Arbeit.Studienkredite von über$200.000 sind nicht selten für Studenten in den USA nach deren Abschluss an einer medizinischen Hochschule, die meist in privatem Eigentum ist. In Deutschland dagegen ist die große Mehrheit der Universitäten mit medizinischen Fakultäten in öffentlicher Hand, aus Steuern finanziert und deshalb frei von Studiengebühren. Signifikante Unterschiede doch auch überraschenderweise eine Reihe von Ähnlichkeiten existieren zwischen den Systemen der zwei Länder, obwohl eines von privaten Einrichtungen und das andere von staatlichen Hochschulen abhängig ist. Deutschland verwendet aktuell ein ganzheitliches medizinisches Curriculum, das klassischerweise direkt nach dem Abitur beginnt und aus zwei Jahren vorklinischer und vier Jahren klinischer Ausbildung besteht, wobei letzteres die Studenten an die praktischen Aspekte der Medizin heranführen soll. Auf der anderen Seite herrscht in den USA ein zweistufiger Ausbildungsprozess. Nach erfolgreichem Erreichen eines Bachelorgrads im College führt der Weg eines amerikanischen Studenten durch ein vierjähriges Medizinstudium, welches aus zwei Jahren Grundlagenlehre und zwei Jahren klinischer Ausbildung besteht. In dieser Überblicksarbeit werden wir uns mit einigen dieser Gemeinsamkeiten und Hauptunterschiede befassen

Kinetic distance and kinetic maps from molecular dynamics simulation

Characterizing macromolecular kinetics from molecular dynamics (MD) simulations requires a distance metric that can distinguish slowly-interconverting states. Here we build upon diffusion map theory and define a kinetic distance for irreducible Markov processes that quantifies how slowly molecular conformations interconvert. The kinetic distance can be computed given a model that approximates the eigenvalues and eigenvectors (reaction coordinates) of the MD Markov operator. Here we employ the time-lagged independent component analysis (TICA). The TICA components can be scaled to provide a kinetic map in which the Euclidean distance corresponds to the kinetic distance. As a result, the question of how many TICA dimensions should be kept in a dimensionality reduction approach becomes obsolete, and one parameter less needs to be specified in the kinetic model construction. We demonstrate the approach using TICA and Markov state model (MSM) analyses for illustrative models, protein conformation dynamics in bovine pancreatic trypsin inhibitor and protein-inhibitor association in trypsin and benzamidine

Sun-to-Earth Characteristics of Two Coronal Mass Ejections Interacting near 1 AU: Formation of a Complex Ejecta and Generation of a Two-Step Geomagnetic Storm

On 2012 September 30 - October 1 the Earth underwent a two-step geomagnetic storm. We examine the Sun-to-Earth characteristics of the coronal mass ejections (CMEs) responsible for the geomagnetic storm with combined heliospheric imaging and in situ observations. The first CME, which occurred on 2012 September 25, is a slow event and shows an acceleration followed by a nearly invariant speed in the whole Sun-Earth space. The second event, launched from the Sun on 2012 September 27, exhibits a quick acceleration, then a rapid deceleration and finally a nearly constant speed, a typical Sun-to-Earth propagation profile for fast CMEs \citep{liu13}. These two CMEs interacted near 1 AU as predicted by the heliospheric imaging observations and formed a complex ejecta observed at Wind, with a shock inside that enhanced the pre-existing southward magnetic field. Reconstruction of the complex ejecta with the in situ data indicates an overall left-handed flux rope-like configuration, with an embedded concave-outward shock front, a maximum magnetic field strength deviating from the flux rope axis and convex-outward field lines ahead of the shock. While the reconstruction results are consistent with the picture of CME-CME interactions, a magnetic cloud-like structure without clear signs of CME interactions \citep{lugaz14} is anticipated when the merging process is finished.Comment: 15 pages, 5 figures. Accepted for publication in ApJ Letter

Surface tension of the Widom-Rowlinson model

11 pags., 5 figs., 3 tabs.We consider the computation of the surface tension of the fluid-fluid interface for the Widom-Rowlinson [J. Chem. Phys. 52, 1670 (1970)] binary mixture from direct simulation of the inhomogeneous system. We make use of the standard mechanical route, in which the surface tension follows from the computation of the normal and tangential components of the pressure tensor of the system. In addition to the usual approach, which involves simulations of the inhomogeneous system in the canonical ensemble, we also consider the computation of the surface tension in an ensemble where the pressure perpendicular (normal) to the planar interface is kept fixed. Both approaches are seen to provide consistent values of the interfacial tension. The issue of the system-size dependence of the surface tension is addressed. In addition, simulations of the fluid-fluid coexistence properties of the mixture are performed in the semigrand canonical ensemble. Our results are compared with existing data of the Widom-Rowlinson mixture and are also examined in the light of the vapor-liquid equilibrium of the thermodynamically equivalent one-component penetrable sphere model. © 2007 American Institute of Physics.Financial support is due to the Spanish Dirección General de Investigación Project Nos. FIS2004-06627-C02-01 E.d.M. and FIS2004-02954-C03-01 N.G.A. and from Universidad de Huelva and Junta de Andalucía. Additional funding from the Dirección General de Universidades e Investigación Comunidad de Madrid, Spain under the MOSSNOHO-CM program Grant No. S0505/ESP/0299 and from the Engineering and Physical Sciences EPSRC of the UK Grant Nos. GR/N20317, GR/N03358, GR/N35991, GR/R09497, and EP/E016340, the Joint Research Equipment Initiative JREI GR/M94427, and the Royal Society Wolfson Foundation refurbishment grant is also acknowledged. Finally we are grateful to the Royal Society for the award of a International Short Visit grant which has facilitated the collaborative work

Nematic phase in the J1-J2 square-lattice Ising model in an external field

© 2015 American Physical Society. The J1-J2 Ising model in the square lattice in the presence of an external field is studied by two approaches: the cluster variation method (CVM) and Monte Carlo simulations. The use of the CVM in the square approximation leads to the presence of a new equilibrium phase, not previously reported for this model: an Ising-nematic phase, which shows orientational order but not positional order, between the known stripes and disordered phases. Suitable order parameters are defined, and the phase diagram of the model is obtained. Monte Carlo simulations are in qualitative agreement with the CVM results, giving support to the presence of the new Ising-nematic phase. Phase diagrams in the temperature-external field plane are obtained for selected values of the parameter κ=J2/|J1| which measures the relative strength of the competing interactions. From the CVM in the square approximation we obtain a line of second order transitions between the disordered and nematic phases, while the nematic-stripes phase transitions are found to be of first order. The Monte Carlo results suggest a line of second order nematic-disordered phase transitions in agreement with the CVM results. Regarding the stripes-nematic transitions, the present Monte Carlo results are not precise enough to reach definite conclusions about the nature of the transitions.Peer Reviewe

Effects of confinement on pattern formation in two dimensional systems with competing interactions

Template-assisted pattern formation in monolayers of particles with competing short-range attraction and long-range repulsion interactions (SALR) is studied by Monte Carlo simulations in a simple generic model [N. G. Almarza et al., J. Chem. Phys., 2014, 140, 164708]. We focus on densities corresponding to formation of parallel stripes of particles and on monolayers laterally confined between straight parallel walls. We analyze both the morphology of the developed structures and the thermodynamic functions for broad ranges of temperature T and the separation L between the walls. At low temperature stripes parallel to the boundaries appear, with some corrugation when the distance between the walls does not match the bulk periodicity of the striped structure. The stripes integrity, however, is rarely broken for any L. This structural order is lost at T = T(L) depending on L according to a Kelvin-like equation. Above the Kelvin temperature T(L) many topological defects such as breaking or branching of the stripes appear, but a certain anisotropy in the orientation of the stripes persists. Finally, at high temperature and away from the walls, the system behaves as an isotropic fluid of elongated clusters of various lengths and with various numbers of branches. For L optimal for the stripe pattern the heat capacity as a function of temperature takes the maximum at T = T(L).Peer Reviewe

Phase behavior of the hard-sphere Maier-Saupe fluid under spatial confinement

The Maier-Saupe hard-sphere fluid is one of the simplest models that accounts for the isotropic-nematic transition characteristic of liquid crystal phases. At low temperatures the model is known to present a gas-liquid-like transition with a large difference between the densities of the coexistence phases, whereas at higher temperature the transition becomes a weak first-order transition resembling the typical order-disorder (nematic-isotropic) phase change of liquid crystals. Spatial dimensionality directly conditions the character of the orientational phase change (i.e., the high temperature transition), that goes from a first-order transition in the purely three-dimensional case, to a Berezinskii-Kosterlitz-Thouless-like continuous transition which occurs when the three dimensional Maier-Saupe spins are constrained to lie on a plane. In the latter instance, the ordered phase is not endowed with true long-range order. In this work we investigate how the continuous transition transforms into a true first-order phase change, by analyzing the phase behavior of a system of three dimensional Maier-Saupe hard spheres confined between two parallel plates, with separations ranging from the quasi-two-dimensional regime to the bulk three-dimensional limit. Our results indicate that spatial confinement in one direction induces the change from first order to a continuous transition with a corresponding decrease of the transition temperatures. As to the gas-liquid transition, the estimated “critical” temperatures and densities also decrease as the fluid is confined, in agreement with previous results for other simple systems.Dirección General de Investigación Científica y Técnica: MAT2007-65711-C04-04 Dirección General de Universidades e Investigación de la Comunidad de Madrid under Grant No. S0505/ESP/0299 and Program MOSSNOHO-CMPeer reviewe