The Equation of State of a Low-Temperature Fermi Gas with Tunable Interactions

Interacting fermions are ubiquitous in nature and understanding their thermodynamics is an important problem. We measure the equation of state of a two-component ultracold Fermi gas for a wide range of interaction strengths at low temperature. A detailed comparison with theories including Monte-Carlo calculations and the Lee-Huang-Yang corrections for low-density bosonic and fermionic superfluids is presented. The low-temperature phase diagram of the spin imbalanced gas reveals Fermi liquid behavior of the partially polarized normal phase for all but the weakest interactions. Our results provide a benchmark for many-body theories and are relevant to other fermionic systems such as the crust of neutron stars.Comment: 28 pages, 7 figure

Numerical Model of a Reinforced Concrete Building: Earthquake Analysis and Experimental Validation

Shaking-table experiments of relatively large-scale specimens play a fundamental role in deepening our understanding of seismic response of existing structures and verification of numerical models. However, and in apparent contradiction, the preparation of such a dynamic laboratory experiment requires a-priori advanced numerical simulations, necessary to both fine-tune the test specimen properties and calibrate the input motion, as a function of the objectives of the test and capabilities and characteristics of the shaking table. This research thus concerns the development of a fibre-based finite elements model of a halfscale 3D reinforced concrete frame tested under dynamic conditions at the European Centre for Training and Research in Earthquake Engineering (EUCENTRE, Pavia, Italy). Since this reduced-scale specimen is very much based on a full-scale counterpart previously tested under pseudo-dynamic conditions at the European Laboratory for Structural Assessment (ELSA) of the Joint Research Centre (JRC, Ispra, Italy), the first part of the work consisted in verifying that the software tool employed in the numerical simulations was capable of duplicating the pseudo-dynamic real test results. Having successfully met the latter objective, the second part of the work consisted in the attempted numerical simulation of the shaking table tests, with a view to ascertain that the response of the model will be within the envisaged response targets and that the necessary input motion is compatible with the shaking-table characteristics

Probing quantum criticality and symmetry breaking at the microscopic level

We report on an experimental study of the Lipkin-Meshkov-Glick model of quantum spins interacting at infinite range in a transverse magnetic field, which exhibits a ferromagnetic phase transition in the thermodynamic limit. We use Dysprosium atoms of electronic spin $J=8$, subjected to a quadratic Zeeman light shift, to simulate $2J=16$ interacting spins $1/2$. We probe the system microscopically using single magnetic sublevel resolution, giving access to the spin projection parity, which is the collective observable characterizing the underlying $\mathbb{Z}_2$ symmetry. We measure the thermodynamic properties and dynamical response of the system, and study the quantum critical behavior around the transition point. In the ferromagnetic phase, we achieve coherent tunneling between symmetry-broken states, and test the link between symmetry breaking and the appearance of a finite order parameter.Comment: 13 pages, 13 figure

Optical trapping of ultracold dysprosium atoms: transition probabilities, dynamic dipole polarizabilities and van der Waals C6C_6 coefficients

The efficiency of optical trapping of ultracold atoms depend on the atomic dynamic dipole polarizability governing the atom-field interaction. In this article, we have calculated the real and imaginary parts of the dynamic dipole polarizability of dysprosium in the ground and first excited level. Due to the high electronic angular momentum of those two states, the polarizabilities possess scalar, vector and tensor contributions that we have computed, on a wide range of trapping wavelengths, using the sum-over-state formula. Using the same formalism, we have also calculated the $C_6$ coefficients characterizing the van der Waals interaction between two dysprosium atoms in the two lowest levels. We have computed the energies of excited states and the transition probabilities appearing in the sums, using a combination of \textit{ab initio} and least-square-fitting techniques provided by the Cowan codes and extended in our group. Regarding the real part of the polarizability, for field frequencies far from atomic resonances, the vector and tensor contributions are two-order-of-magnitude smaller than the scalar contribution, whereas for the imaginary part, the vector and tensor contributions represent a noticeable fraction of the scalar contribution. This offers the possibility to control the decoherence and trap losses due to spontaneous emission

Optical cooling and trapping of highly magnetic atoms: The benefits of a spontaneous spin polarization

From the study of long-range-interacting systems to the simulation of gauge fields, open-shell Lanthanide atoms with their large magnetic moment and narrow optical transitions open novel directions in the field of ultracold quantum gases. As for other atomic species, the magneto-optical trap (MOT) is the working horse of experiments but its operation is challenging, due to the large electronic spin of the atoms. Here we present an experimental study of narrow-line Dysprosium MOTs. We show that the combination of radiation pressure and gravitational forces leads to a spontaneous polarization of the electronic spin. The spin composition is measured using a Stern-Gerlach separation of spin levels, revealing that the gas becomes almost fully spin-polarized for large laser frequency detunings. In this regime, we reach the optimal operation of the MOT, with samples of typically $3\times 10^8$ atoms at a temperature of 15\,$\mu$K. The spin polarization reduces the complexity of the radiative cooling description, which allows for a simple model accounting for our measurements. We also measure the rate of density-dependent atom losses, finding good agreement with a model based on light-induced Van der Waals forces. A minimal two-body loss rate $\beta\sim 2\times10^{-11}\,$cm$^{3}$/s is reached in the spin-polarized regime. Our results constitute a benchmark for the experimental study of ultracold gases of magnetic Lanthanide atoms.Comment: 21 pages, 9 figure

Numerical Modeling and Seismic Analysis of Tall Steel Buildings with Braced Frame Systems

The following paper presents the design and verification steps of tall steel frame structures with braced core, belt and outrigger trusses. The two case study structures refer to sky scrapers of 180m and 300m respectively, located in Istanbul, Turkey. The main objectives of this study are two: firstly designing the buildings through multilevel structural analysis, secondly to compare the results, in terms of seismic response, between response spectrum analysis (RSA) and nonlinear time history analysis (NLTHA). Such comparison has been made with the intention of investigating the relationship between the structure height and the accuracy of RSA predictions, considering that the latter approach tends to underestimate the influence of higher mode effects. In conclusion the capacity curves of the two structures, developed using incremental nonlinear dynamic analysis, are presented as an ulterior way to assess the seismic capacity of such type of high-rise structural systems

performance based seismic design framework for rc floor diaphragms in dual systems

Abstract Floor diaphragms play several roles in the seismic response of dual systems: support vertically spanning components, transfer lateral forces to walls and frames, provide restraint to columns and walls, tie the structure together, and enable redundant load paths for lateral forces. In many buildings after events as recent as the 2010-2011 Christchurch earthquakes, floor diaphragms were unable to perform one or more of these functions, leading to extensive damage and collapse. Research consistently highlights three underlying causes: a failure to ensure the integrity of the load path, underestimation of in-plane forces, and poorly understood interactions with walls, supporting beams and RC moment frames. Most recent and ongoing research has focused on modifying the prescriptive code provisions, and indeed many codes–but not all- have been consequently updated. Such prescriptive rules, however, are not helpful for assessing existing buildings, comparing alternate means and methods, or in displacement-based design (DBD) for which a performance-based design (PBD) framework is necessary. This paper proposes a new performance-based framework for the seismic design of reinforced-concrete (RC) floor diaphragms with or without precast elements. The floor diaphragm performance limit states (LS) are re-defined in terms of the observed failure modes (FM). Results from prior research on these failure modes are used to select damage measures (DM, e.g. 'crack width') for pairs of FM and LS. Expressions for DMs in terms of engineering demand parameters (EDP, e.g. , 'strain') are derived from experimental results or from first principles. EDPs are the basic output from numerical analysis in PBD; this paper comments on the suitability of different analytical approaches

An integrated system for producing user-specific keys on demand: an application to Italian lichens

The identification of lichens is important in several applied fields, such as the biological monitoring of air pollution and the restoration of openair stone monuments. This often creates relevant problems for non-specialists and technicians which are in charge of routinely applying lichen monitoring techniques. The coupling of a complex information system (ITALIC), together with a new software which can automatically produce identification keys for any subset of species included in a database (FRIDA), is an innovative approach in the field of identifying biodiversity. ITALIC is able to produce a list of species which potentially occur under a set of ecological and distributional conditions specified by the user. The list is automatically transferred to FRIDA, which generates a user-oriented interactive identification key limited to the species present in the “virtual habitat” created by the user. The new system has relevant applications, since it effectively supports the technical personnel of Environmental Agencies, Nature Parks, Cultural Heritage Conservation Agencies involved in lichen monitoring throughout the Country

Contrasting multi-taxa diversity patterns between abandoned and non-intensively managed forests in the southern Dolomites

The abandonment of silvicultural activities can lead to changes in species richness and composition of biological communities, when compared to those found in managed forests. The aim of this study was to compare the multi-taxonomical diversity of two mature silver fir-beech-spruce forests in the southern Dolomites (Italy), corresponding to the European Union habitat type 9130. The two sites share similar ecological and structural characteristics, but differ in their recent management histories. In the last 50 years, one site underwent non-intensive management, while the other was left unmanaged and was included in a forest reserve. The species richness and composition of eight taxa were surveyed in the two sites between 2009 and 2011. The difference in mean species richness between the two forest management types was tested through permutation tests, while differences in species composition were tested by principal coordinates analysis and the permutational multivariate analysis of variance. Mean species richness of soil macrofungi, deadwood lichens, bark beetles, and longhorn beetles were significantly higher in the abandoned than in the non-intensively managed forests. Deadwood fungi and epiphytic lichens did not differ in mean species richness between the two study sites, while mean species richness of ground beetles and birds were higher in the non-intensively managed than in the abandoned forest. Significant differences in species composition between the two sites were found for all the taxa, except for longhorn beetles. These results indicate that improving forest landscape heterogeneity through the creation of a mosaic of abandoned and extensively managed forests should better fulfill the requirements of ecologically different taxa