We introduce a simple model for an engine based on the Nernst effect. In the presence of a magnetic field, a vertical heat current can drive a horizontal particle current against a chemical potential. For a microscopic model invoking classical particle trajectories subject to the Lorentz force, we prove a universal bound 3 − 2 √ 2 ≃ 0.172 for the ratio between maximum efficiency and Carnot efficiency. This bound, as the slightly lower one 1/6 for efficiency at maximum power, can indeed be saturated for large magnetic field and small fugacity irrespective of the aspect ratio.PACS numbers: 05.60. Cd, 05.70.Ln, Introduction.-The Nernst effect describes the emergence of an electrical voltage perpendicular to a heat current transversing an isotropic conductor in the presence of a constant magnetic field [1]. However, while Seebeck-based devices, for which the heat and the particle current are coupled without a magnetic field, have been the subject of intensive research efforts during the last decades [2][3][4][5], only a few attempts were made to utilize the Nernst effect for power generation [6][7][8][9]. This lack of interest may have been caused by the uncompetitive net efficiency of Nernst-based devices, which is inevitably suppressed by the energetic cost of the strong magnetic fields they require. New discoveries in the phenomenological theory of thermoelectric effects as well as recent experiments showing the accessibility of magnetic field effects in nanostructures even at low and moderate field strengths [10-13], however, cast new light on the topic of Nernst engines.Benenti and co-workers showed by a quite general analysis within the framework of linear irreversible thermodynamics that breaking the microscopic time-reversal symmetry by a magnetic field could, in principle, increase thermoelectric efficiency such that even devices operating reversibly at finite power seem to be achievable [14]. Such an intriguing suggestion asks for a better understanding of coupled heat and particle transport in magnetic fields. First progress in this direction was recently achieved within the paradigmatic class of multi-terminal models, for which it turned out that current conservation implies much stronger bounds on the efficiency than the standard rules of linear irreversible thermodynamics [15, 16]. For the minimal case of three terminals, these bounds were even shown to be tight [17]. Since these models were based on general particle transmission probabilities without reference to any specific microscopic dynamics, they leave the necessary conditions for saturating these bounds open.Simple mechanical models have led to remarkable insight into the microscopic mechanisms underlying heat and matter transport [18, 19], especially in the context of thermoelectric efficiency [20][21][22]. So far, the effect of a

In-vacuo cryogenic environments are ideal for applications requiring both low temperatures and extremely low particle densities. This enables reaching long storage and coherence times for example in ion traps, essential requirements for experiments with highly charged ions, quantum computation, and optical clocks. We have developed a novel cryostat continuously refrigerated with a pulse-tube cryocooler and providing the lowest vibration level reported for such a closed-cycle system with 1 W cooling power for a < 5 K experiment. A decoupling system suppresses vibrations from the cryocooler by three orders of magnitude down to a level of 10 nm peak amplitudes in the horizontal plane. Heat loads of about 40 W (at 45 K) and 1W (at 4 K) are transferred from an experimental chamber, mounted on an optical table, to the cryocooler through a vacuum-insulated massive 120 kg inertial copper pendulum. The 1.4 m long pendulum allows installation of the cryocooler in a separate, acoustically isolated machine room. In the laser laboratory, we measured the residual vibrations using an interferometric setup. The positioning of the 4 K elements is reproduced to better than a few µm after a full thermal cycle to room temperature. Extreme high vacuum on the 10 −15 mbar level is achieved. In collaboration with the Max-Planck-Intitut für Kernphysik (MPIK), such a setup is now in operation at the Physikalisch-Technische Bundesanstalt (PTB) for a next-generation optical clock experiment using highly charged ions.

Intershell, resonant electronic recombination is studied experimentally in an electron-beam ion trap for O-like Si⁶⁺ to He-like Si¹²⁺ ions at plasma temperatures in the megakelvin range similar to those found in the solar radiative zone and is compared to extended multiconfiguration Dirac-Fock and relativistic configuration-interaction predictions. For this low-Z ion, the higher-order electronic recombination processes are comparable in strength to the first-order one. The ratio of trielectronic to dielectric recombination for B-like species agrees well with predictions, whereas for C-like ions the measured value is only half as large. This difference is explained by the influence of metastable states populated in the recombining plasma

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