Pairing gap and in-gap excitations in trapped fermionic superfluids

We consider trapped atomic Fermi gases with Feshbach-resonance enhanced interactions in pseudogap and superfluid temperatures. We calculate the spectrum of RF(or laser)-excitations for transitions that transfer atoms out of the superfluid state. The spectrum displays the pairing gap and also the contribution of unpaired atoms, i.e. in-gap excitations. The results support the conclusion that a superfluid, where pairing is a many-body effect, was observed in recent experiments on RF spectroscopy of the pairing gap.Comment: Journal versio

A Study of Cluster Impacts and the Future of the Shipping Industry in Ã…land Islands

Ã…land, a small autonomous island province of Finland with 28,000 inhabitants, is base for commercial shipping employing - in and outside Ã…land - about 6 100 people, onboard and ashore, with an annual gross salary volume of 243 million euros and capital income of 275 million euros (2009). In the same year, gross sales of the shipping sector amounted to more than 870 million euros. These are impressive figures in relation to the total labour market and economy of Ã…land. Ã…land is one of the few places within EU, where tax-free sales onboard are still allowed. The right to tax-free was enabled by Ã…land's permanent exemption from the EU tax rules written in the Finland's EU-accession treaty. Another important factor for shipping is the EU-sanctioned system of subsidies for crewing costs that decreases the manning cost of domestic seafarers. Apart from measuring the size and effects of the Ã…land shipping cluster, we analyse the outcome of different scenarios for the shipping industries and the society of Ã…land using a dynamic one-region computable general equilibrium model. We the following scenarios are studied: i) Increasing number of passengers in the Baltic Sea. ii) Growth in the demand for shipping of goods. ii) The EU-sanctioned system of subsidies for crewing costs is abolished iv) Increased bunker costs (IMO-decision on low sulphur fuel) v) The Ã…land tax exemption within EU and, therefore, tax-free sales onboard are abolished It is shown that the manning subsidies mainly benefit the seafarers, but seen from the point of the macro economy, shipping companies or all households, abolition of manning subsidies could have positive impacts as well, depending on the manner of adjustment of shipping to new conditions. We show that from the point of view of the Ã…land economy and society, the first two scenarios are positive for almost everybody. The negative consequences of iv) are widespread, but the abolition of the Ã…land tax exemption would have disastrous consequences for the passenger ferry industry and for the economy as whole.

Strongly interacting Fermi gases with density imbalance

We consider density-imbalanced Fermi gases of atoms in the strongly interacting, i.e. unitarity, regime. The Bogoliubov-deGennes equations for a trapped superfluid are solved. They take into account the finite size of the system, as well as give rise to both phase separation and FFLO type oscillations in the order parameter. We show how radio-frequency spectroscopy reflects the phase separation, and can provide direct evidence of the FFLO-type oscillations via observing the nodes of the order parameter.Comment: Added one reference. Published in PR

Signatures of superfluidity for Feshbach-resonant Fermi gases

We consider atomic Fermi gases where Feshbach resonances can be used to continuously tune the system from weak to strong interaction regime, allowing to scan the whole BCS-BEC crossover. We show how a probing field transferring atoms out of the superfluid can be used to detect the onset of the superfluid transition in the high-$T_c$ and BCS regimes. The number of transferred atoms, as a function of the energy given by the probing field, peaks at the gap energy. The shape of the peak is asymmetric due to the single particle excitation gap. Since the excitation gap includes also a pseudogap contribution, the asymmetry alone is not a signature of superfluidity. Incoherent nature of the non-condensed pairs leads to broadening of the peak. The pseudogap and therefore the broadening decay below the critical temperature, causing a drastic increase in the asymmetry. This provides a signature of the transition.Comment: Revised version, accepted to Phys. Rev. Letters. Figures changed, explanations adde

Harnack's Inequality for Parabolic De Giorgi Classes in Metric Spaces

In this paper we study problems related to parabolic partial differential equations in metric measure spaces equipped with a doubling measure and supporting a Poincare' inequality. We give a definition of parabolic De Giorgi classes and compare this notion with that of parabolic quasiminimizers. The main result, after proving the local boundedness, is a scale and location invariant Harnack inequality for functions belonging to parabolic De Giorgi classes. In particular, the results hold true for parabolic quasiminimizers

Evaluating and improving web performance using free-to-use tools

Abstract. Fast website loading speeds can increase conversion rates and search engine rankings as well as encourage users to explore the site further, among other positive things. The purpose of the study was to find and compare free-to-use tools that can both evaluate the performance (loading and rendering speed) of a website and give suggestions how the performance could be improved. In addition, three tools were used to evaluate the performance of an existing WordPress site. Some of the performance improvement suggestions given by the tools were then acted upon, and the performance of the website was re-evaluated using the same tools. The research method used in the study was experimental research, and the research question was “How to evaluate and improve web performance using free-to-use tools?” There were also five sub-questions, of which the first two related to the tools and their features, and the last three to the case website. Eight free-to-use web performance evaluation tools were compared focusing on what performance metrics they evaluate, what performance improvement suggestions they can give, and six other features that can be useful to know in practice. In alphabetical order, the tools were: GTmetrix, Lighthouse, PageSpeed Insights, Pingdom Tools, Test My Site, WebPageTest, Website Speed Test (by Dotcom-Tools) and Website Speed Test (by Uptrends). The amounts of metrics evaluated by the tools ranged from one to fifteen. The performance improvement suggestions given by the tools could be put into three categories, meaning that the suggestions largely overlapped between the tools. All tools except Lighthouse were web-based tools. The performance of the case website was evaluated using GTmetrix, PageSpeed Insights and WebPageTest. On desktop, the performance was in the high-end range though varying between the three tools, and on mobile, the performance was noticeably slower due to the challenges of mobile devices (e.g. lower processing power compared to desktop computers) and mobile networks (e.g. higher latency compared to broadband connections). The common bottlenecks based on the suggestions given by the three tools seemed to be lack of using a CDN (Content Delivery Network), serving unoptimized images and serving large amounts of JavaScript. The results of the performance re-evaluation were mixed, highlighting the importance of carefully considering each performance improvement suggestion. The main takeaways of the study for practitioners are to use multiple tools to get a wide variety of performance metrics and suggestions, and to regard the suggestions and relative performance scores given by the tools only as guidelines with the main goal being to improve the time-based performance metrics

Pairing based cooling of Fermi gases

We propose a pairing-based method for cooling an atomic Fermi gas. A three component (labels 1, 2, 3) mixture of Fermions is considered where the components 1 and 2 interact and, for instance, form pairs whereas the component 3 is in the normal state. For cooling, the components 2 and 3 are coupled by an electromagnetic field. Since the quasiparticle distributions in the paired and in the normal states are different, the coupling leads to cooling of the normal state even when initially $T_{paired}\geq T_{normal}$ (notation $T_S\geq T_N$). The cooling efficiency is given by the pairing energy and by the linewidth of the coupling field. No superfluidity is required: any type of pairing, or other phenomenon that produces a suitable spectral density, is sufficient. In principle, the paired state could be cooled as well but this requires $T_N<T_S$. The method has a conceptual analogy to cooling based on superconductor -- normal metal (SN) tunneling junctions. Main differences arise from the exact momentum conservation in the case of the field-matter coupling vs. non-conservation of momentum in the solid state tunneling process. Moreover, the role of processes that relax the energy conservation requirement in the tunneling, e.g. thermal fluctuations of an external reservoir, is now played by the linewidth of the field. The proposed method should be experimentally feasible due to its close connection to RF-spectroscopy of ultracold gases which is already in use.Comment: Journal version 4 pages, 4 figure

Effect of a thin AlO_x layer on transition-edge sensor properties

We have studied the physics of transition-edge sensor (TES) devices with an insulating AlOx layer on top of the device to allow implementation of more complex detector geometries. By comparing devices with and without the insulating film, we have observed significant additional noise apparently caused by the insulator layer. In addition, AlOx was found to be a relatively good thermal conductor. This adds an unforeseen internal thermal feature to the system.Comment: 6 pages, 5 figures, Low Temperature Detectors 14 conferenc