Relaxation dynamics of an isolated large-spin Fermi gas far from equilibrium

A fundamental question in many-body physics is how closed quantum systems reach equilibrium. We address this question experimentally and theoretically in an ultracold large-spin Fermi gas where we find a complex interplay between internal and motional degrees of freedom. The fermions are initially prepared far from equilibrium with only a few spin states occupied. The subsequent dynamics leading to redistribution among all spin states is observed experimentally and simulated theoretically using a kinetic Boltzmann equation with full spin coherence. The latter is derived microscopically and provides good agreement with experimental data without any free parameters. We identify several collisional processes, which occur on different time scales. By varying density and magnetic field, we control the relaxation dynamics and are able to continuously tune the character of a subset of spin states from an open to a closed system.Comment: 18 pages, 9 figure

Determination of the Pressure and Energy Density of the Universe, Implying the Presence of Missing Mass

Neste estudo analisou-se os comportamentos anômalos de temperatura do ar nas regiões sul e sudeste do Brasil durante o verão 2013-2014 e as anomalias de circulação atmosférica em baixos níveis através dos dados de reanálise do MERRA. Tais anomalias tiveram como base a climatologia do período de 1979 a 2008 para dois trimestres: DJF (Dezembro de 2013, Janeiro e Fevereiro de 2014) e JFM (Janeiro, Fevereiro e Março de 2014). Verificou-se que o trimestre mais representativo para a denominação de “anomalamente quente” foi JFM. Foi observado no nível de 850 hPa através do campo de anomalias de linhas de corrente a intensificação da componente meridional de norte, que advectou massas de ar quente e úmida originadas na região Amazônica ocasionando um comportamento anômalo positivo de umidade específica neste nível e de temperatura em superfície. Além disso, observou-se uma provável maior frequência e/ou intensidade de ciclones extratropicais no sudeste do continente, contudo, os sistemas frontais associados aos ciclones não atingiram a região de estud

Quantum trajectories for time-local non-Lindblad master equations

For the efficient simulation of open quantum systems we often use quantum jump trajectories given by pure states that evolve stochastically to unravel the dynamics of the underlying master equation. In the Markovian regime, when the dynamics is described by a Lindblad master equation, this procedure is known as Monte Carlo wavefunction (MCWF) approach. However, beyond ultraweak system-bath coupling, the dynamics of the system is not described by an equation of Lindblad type, but rather by the Redfield equation, which can be brought into pseudo-Lindblad form. Here negative dissipation strengths prohibit the conventional approach. To overcome this problem, we propose a pseudo-Lindblad quantum trajectory (PLQT) unraveling. It does not require an effective extension of the state space, like other approaches, except for the addition of a single classical bit. We test the PLQT for the eternal non-Markovian master equation for a single qubit and an interacting Fermi Hubbard chain coupled to a thermal bath and discuss its computational effort compared to solving the full master equation

EXPLORING THE RELATION BETWEEN FIRM OWNERSHIP AND IT CAPABILITY

As IT spending continuously increased throughout the last years, it nowadays accounts for a significant amount of total corporate spending. Simultaneously, academic research validated the strategic importance and performance benefits derived from a superior IT capability the ability to successfully deploy organizational IT skills and resources. Furthermore, shareholders pursuing long-term oriented goals are interested in consistent investments to develop organizational capabilities, such as IT capability. Consequentially, the question arises how a firms ownership structure is related to its IT capability. This research question is addressed by analyzing secondary data on publicly listed U.S. companies of the last ten years. The results provide support for the hypothesized relation throughout the investigated period and withstand several control and robustness tests. This study contributes to the ongoing research on IT capability by showing that certain types of shareholders promote the development and deployment of IT capability which in turn influences strategic topics. Findings further confirm that decisions on IT investments characterized by long-term benefits must be backed by owners with a corresponding long-term investment horizon

Synthetic dimension-induced pseudo Jahn-Teller effect in one-dimensional confined fermions

We demonstrate the failure of the adiabatic Born-Oppenheimer approximation to describe the ground state of a quantum impurity within an ultracold Fermi gas despite substantial mass differences between the bath and impurity species. Increasing repulsion leads to the appearance of non-adiabatic couplings between the fast bath and slow impurity degrees of freedom which reduce the parity symmetry of the latter according to the pseudo Jahn-Teller effect. The presence of this mechanism is associated to a conical intersection involving the impurity position and the inverse of the interaction strength which acts as a synthetic dimension. We elucidate the presence of these effects via a detailed ground state analysis involving the comparison of ab initio fully-correlated simulations with effective models. Our study suggests ultracold atomic ensembles as potent emulators of complex molecular phenomena

IT Capability and Firm Performance: Findings from Periods of Economic Downturn

Information technology is crucial in many industries and is seen as a decisive factor of a firm’s performance and financial success. Based on the dynamic capabilities view, this paper aims at extending existing research on organizational IT capability and performance. In particular, this work examines if firms exhibiting superior IT capability outperform their competitors, especially during two phases of economic downturn: the burst of the dotcom-bubble in 2000 and the recent financial crisis in 2008. Applying secondary statistics on different performance measures and proxies of IT capability among publicly traded US companies, we found that firms characterized by superior IT capability outperformed their competitors during both crises in all but one performance indicator. This paper contributes to research by investigating two crisis periods and using up-to-date data to reconcile prior research

Preparation and properties of monolithically sintered silicon p-n junctions for thermoelectric applications

Die vorliegende Arbeit beschäftigt sich mit der Anwendung eines stromunterstützten Sinterverfahrens zur Herstellung thermoelektrischer Bauteile (Current-Activated Preasure-Assisted Densification -CAPAD-, auch Spark-Plasma-Sinterverfahren - SPS- genannt). Während bisher Stromsintern nur zur Erzeugung thermoelektrischer Materialien verwendet wird, die in weiteren Arbeitsschritten zu thermoelektrischen Elementen oder Bauteilen weiterverarbeitet werden, wird hier als besondere Idee die monolithische Integration eines p-n-Übergangs direkt während des Sintervorgangs verfolgt. Dies dient dem Ziel die schwierige elektrische Kontaktierung an der Heißseite des herkömmlichen Generators zu umgehen. Das neue Konzept ist bisher nur theoretisch diskutiertes. Die zugrunde liegende Idee ist die, dass ein Temperaturgradient entlang des p-n Übergangs verläuft, sodass einerseits eine Thermodiffusion auf der Grundlage des Seebeck-Effektes der jeweiligen Majoritätsladungsträger erfolgt, andererseits Ladungsträger im p-n Übergang thermisch generiert und getrennt werden. Die vorliegende Arbeit beschäftigt sich mit der Fragestellung inwieweit p-n Übergänge für die thermoelektrische Energieumwandlung genutzt werden können. Die Studie wird an p-n Übergängen aus versinterten nanokristallinem Silizium durchgeführt. Für die Dotierung wird Bor und Phosphor verwendet, wobei die Partikelsynthese in einem Plasmareaktor bzw. einem Heißwandreaktor stattfindet. Die unterschiedlich dotierten Pulver werden in einem Graphittiegel zu gleichen Anteilen in zwei Lagen übereinander geschichtet. Durch einen Stromsinterprozess wird die Pulverschüttung in eine feste Struktur überführt, wobei ein elektrischer Strom durch die Pulverschüttung geleitet wird. Der Prozess findet bei 1150°C statt, wobei Ströme in der Größenordnung von 0,1 bis 1kA fließen. Der Einfluss des Peltier-Effektes auf die Wärmeverteilung in der Probe beim Stromsinterprozess wird experimentell und theoretisch herausgearbeitet und analysiert. Es zeigt sich, dass aufgrund der hohen elektrischen Ströme, der Peltier-Effekt einen dominanten Einfluss auf die Temperaturverteilung in der Probe hat und sich damit auch auf die Homogenität der physikalischen Eigenschaften der Probe auswirkt. Mit Hilfe einer ortsaufgelöste Messung des Seebeck-Koeffizienten über dem p-n Übergang wird der Übergangsbereich zwischen den unterschiedlich dotierten Arealen analysiert und ein Modell der Ladungsträgerverteilung unter Berücksichtigung des Sinterprozesses erarbeitet. Die Untersuchungen kommen zu dem Schluss, dass während des Sinterprozesses benachbarte unterschiedlich dotierte Partikel miteinander verschmelzen, wobei es zu einer Kompensation von Ladungsträgern kommt. Die thermoelektrischen Eigenschaften des p-n Übergangs wird mit Hilfe einfacher Ersatzschaltbilder untersucht und mit einem eigens gebauten Messplatz überprüft. In einem beschränkten Temperaturbereich von 30 bis 90°C wird eine elektrische Spannung am kalten Ende des p-n Überganges gemessen. Der Betrag der gemessenen Spannung ist linear zur angelegten Temperaturdifferenz. Durch eine Metallisierung am warmen Ende kann die Thermospannung verbessert werden. Abschließend wird mit den gesammelten Erfahrungen ein Demonstrator gebaut und dessen Funktion nachgewiesen.The presented work deals with the application of a current assisted sintering process to prepare a thermoelectric device (Current-Activated Preasure-Assisted Densification -CAPAD-, also known as Spark Plasma Sintering -SPS). So far CAPAD has been used to produce thermoelectric materials which were processed in additional steps to thermoelectric devices. In this work, the discussed idea is based on a monolithic concept including a p-n junction which is directly processed during the sintering. The idea is motivated in order to avoid complicated electrical contacts at the hot side of a conventional thermoelectric generator. The concept has been discussed in literature theoretically only. The main idea is to use a temperature gradient along the p-n junction in order to achive thermodiffusion of the majority charge carriers on the basis of the Seebeck effect. On the other hand, thermally generated charges might be seperated inside the space charge region. In the study, nanocrystalline silicon is used to accomplish experiments. The silicon nanoparticles were sinthesized seperately in a microwave plasma reactor and a hot wall reactor using boron and phosphorous for p and n-doping. Before sintering, the different kinds of powder were stacked into a graphite crucible. To produce the p-n junction a double layer system of the p-doped and the n-doped powder is created. Using a current-activated preassure-assisted densification process the powder is transformed into a bulk material with a p-n junction. During the process the electric current flows through the powder. The process runs at a temperature of 1050°C. The electric current ranges from 0.1 kA to 1kA. The influence of the Peltier effect on the heat distribution of the sample is analyzed theoretically and experimentally. Due to the high electric current, the Peltier effect has a major impact on the temperature distribution. Therefore, the Peltier effect affects the homogeneity and the physical properties of the sample. Seebeck microscopy allows the analyzis of the transition of the differently doped regimes. A model, considering the sintering process, which describes the charge carrier distribution across the junction will be presented. The investigations conclude with the idea, that heated particles melt, intermix and solidify. Thus, new particles are formed. While the neighbouring particles are melted, dopants of the different partilces mix and compensate for each other. Therefore the charge carrier concentration of the new formed particle is the average of the former particles. The thermoelectric properties of the p-n junctions will be studied with simple equivalent circuits. They will be validated with a self-made measurement setup. In a small temperature intervall ranging from 30 to 90°C the electric voltage will be measured at the cold side of the p-n junction. The absolute value of the voltage is linear to the applied temperature difference. With a metallization, the thermovoltage can be improved. Finally, the collected information and ideas will be used for a demonstrator device in order to experimentally prove the concept

Estudo de Caso de Sistemas Meteorológicos em Altos Níveis Sobre a América do Sul: Análise Sinótica

O Vórtice ciclônico de altos níveis (VCAN) sobre o nordeste brasileiro (NeB) e a Alta da Bolívia (AB) são os típicos sistemas de pressão em altos níveis (250 hPa) da América do Sul, especialmente nas estações mais quentes. Somados a eles, também pode-se considerar o VCAN extratropical, de característica transiente e encontrado o ano todo. Neste trabalho, é apresentada uma análise sinótica, baseada na literatura especializada, de um caso observado em 01 de fevereiro de 2005 em que os três sistemas ocorreram simultaneamente. Algumas características encontradas na literatura foram observadas nos sistemas deste caso. O campo de temperatura mais associado ao sistema extratropical foi o de 500 hPa, e não o de 250 hPa. Os movimentos verticais mais relevantes foram encontrados somente no sistema extratropical: intensos movimentos ascendentes à sotavento do eixo do cavado em altos níveis. Não se encontrou um sistema em superfície especificamente associado ao VCAN do NeB, enquanto que se observou a baixa continental (ou Baixa do Chaco) associada à Alta da Bolívia e um intenso ciclone extratropical associado ao VCAN extratropical. Tal ciclone em superfície acarretou em vento sul e consequente resfriamento da camada, o que pôde ser observado pela amplitude do cavado em altos níveis, o que é raro para esta época do ano