Brain tissue is one of the softest tissues in the human body and the quantification of its mechanical properties has challenged scientists over the past decades. Associated experimental results in the literature have been contradictory as characterizing the mechanical response of brain tissue not only requires well-designed experimental setups that can record the ultrasoft response, but also appropriate approaches to analyze the corresponding data. Due to the extreme complexity of brain tissue behavior, nonlinear continuum mechanics has proven an expedient tool to analyze testing data and predict the mechanical response using a combination of hyper-, visco-, or poro-elastic models. Such models can not only allow for personalized predictions through finite element simulations, but also help to comprehensively understand the physical mechanisms underlying the tissue response. Here, we use a nonlinear poro-viscoelastic computational model to evaluate the effect of different intrinsic material properties (permeability, shear moduli, nonlinearity, viscosity) on the tissue response during different quasi-static biomechanical measurements, i.e., large-strain compression and tension as well as indentation experiments. We show that not only the permeability but also the properties of the viscoelastic solid largely control the fluid flow within and out of the sample. This reveals the close coupling between viscous and porous effects in brain tissue behavior. Strikingly, our simulations can explain why indentation experiments yield that white matter tissue in the human brain is stiffer than gray matter, while large-strain compression experiments show the opposite trend. These observations can be attributed to different experimental loading and boundary conditions as well as assumptions made during data analysis. The present study provides an important step to better understand experimental data previously published in the literature and can help to improve experimental setups and data analysis for biomechanical testing of brain tissue in the future.

We report the observation of a very high flux of ultra-cold bosonic chromium atoms in a magnetic guide. The beam is created by operating a magneto-optical trap/moving optical molasses within the magnetic field of the guide. A relative detuning between two pairs of the cooling lasers cools the atoms into a frame moving along the axes of the guide. When the atoms are cooled into a moving frame with a velocity of 6 m s−1 we observe a maximum of the flux of 6 × 109 atoms s−1. For these parameters the transversal temperature of the atoms after a 25 fold increase of the confining magnetic potential is about 1.2 mK. The longitudinal temperature is 400 µK.

-Atom manipulation (scanning probe microscopy, laser cooling, etc.). PACS. 34.50.-s -Scattering of atoms and molecules. PACS. 07.20.Pe -Heat engines; heat pumps; heat pipes.Abstract. -We propose a cooling scheme based on depolarisation of a polarised cloud of trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the coupling between the internal spin reservoir of the cloud and the external kinetic reservoir via dipolar relaxation to reduce the temperature of the cloud. By optical pumping one can cool the spin reservoir and force the cooling process. In case of a trapped gas of dipolar chromium atoms, we show that this cooling technique can be performed continuously and used to approach the critical phase space density for BEC.Introduction. -Adiabatic demagnetisation [1, 2] is a well established and very efficient cooling scheme which enables researchers in solid state physics to cool their samples by several orders of magnitude in a single cooling step [3,4]. However, depolarisation processes have not yet led to a cooling concept in atomic physics. Instead, evaporative cooling which can be observed in many fields of physics is typically applied to obtain temperatures in the nK regime. This cooling mechanism was proposed and demonstrated for magnetically trapped atoms by Hess [5]. Meanwhile, it has been studied intensively [6,7] and could also successfully be applied to atoms [8,9] and molecules trapped in optical dipole traps [10]. Up to now, this scheme is essential to obtain degenerate atomic or molecular quantum gases and allows nowadays to generate gases with temperatures below T = 500 pK [11]. By means of a controllable finite trap depth U 0 , high energetic particles carrying more than the mean energy of a trapped particle are allowed to escape from the trap. Rethermalisation of the remaining particles via elastic collisions reduces the temperature of the trapped atomic cloud and at the same time produces particles which have sufficient energy to leave the trap again. In typical experiments, this technique allows one to increase the phase space density ρ = n 0 (2πh 2 /(mk B T )) 3/2 by several orders of magnitude, where n 0 and m denote the peak density and the atomic mass, respectively. The ratio η ev = U 0 /(k B T ) between the trap depth and the thermal energy of the cloud is commonly referred as cutoff parameter. The higher this ratio is chosen, the more energy can be carried away by a single atom and the less atoms are lost to achieve the final temperature. However, in this case the particles need more time to rethermalise, so that trap losses become more significant and finally limit the cooling process. Thus, the efficiency χ of the cooling process is defined by the gain in phase space density per atom loss

We have realized a magnetic guide for ultracold chromium atoms by continuously loading atoms directly from a Zeeman slower into a horizontal guide. We observe an atomic flux of 2 · 10 7 atoms/s and are able to control the mean velocity of the guided atoms between 0 m/s and 3 m/s. We present our experimental results on loading and controlling the mean velocity of the guided atoms and discuss the experimental techniques that are used.

We report on the first fabrication of nanostructures with exactly resonant light revealing the quantum character of the atom-light interaction. Classically the formation of nanostructures is not expected; thus, the observed formation of complex periodic line patterns can be explained only by treating atom-light interaction and propagation of the atoms quantum mechanically. Our numerical quantum calculations are in quantitative agreement with this experimental finding. Moreover, the theory predicts that for small detunings nanostructures with lambda/4 period can be produced, which beats the standard nanofabrication limit of lambda/2. Our experiments confirm this prediction.