Annalen der Physik最新文献

This contribution describes two episodes from the history of the Lennard-Jones (LJ) potential. The first, located in the 1920s and 1930s, is about a computational approach that aimed at pragmatics rather than truth and that remained remarkably robust when quantum theory arrived. The second episode covers the birth of the LJ substance in 1964. Due to increasing interest in computer methods, simulated model substances became objects on their own, the prime targets of investigation. The history of the LJ potential and substance exemplifies the dynamic relationship between prediction, theory, mathematization, and computer instrumentation.

Diffusion Model for Relativistic Heavy-Ion Collisions

Relativistic heavy-ion collisions are a versatile tool to study the partial approach of a quantum many-body system towards statistical equilibrium. In article number 2300307, Johannes Hölck and Georg Wolschin present an explicit and rigorous derivation of the stochastic Fokker–Planck equation for the momentum distribution function of produced charged hadrons in longitudinal and transverse rapidities, thus placing the relativistic diffusion model on a firm statistical foundation. The model is used to analyse Pb–Pb collisions at energies reached at the Large Hadron Collider LHC. Detailed comparisons with data from the ATLAS and ALICE collaborations in transverse-momentum and pseudorapidity space are given.

A theoretical scheme is proposed for improving the performance of cooling and entanglement, where the physical model is based on a double cavity optomechanical system assisted by the field-mediated coherent feedback. The cooling performance is evaluated by calculating the final mean phonon number. The steady-state bipartite entanglement between the optical mode and mechanical mode is measured by the logarithmic negativity. The result manifests that with assistance of the quantum coherent feedback, the mechanical resonator (MR) can be cooled close to its quantum ground state and the steady-state optomechanical entanglement is simultaneously created, all of which are obtained under the condition beyond the resolved sideband. The presented feedback strategy is measurement-independent, which can effectively preserve the quantum coherence of system. The scheme is conducive to relaxing the current experimental condition and it may provide a new path for the optomechanical manipulation involving the low-frequency MR.

Conceptually, the Matsubara formalism (MF), using imaginary frequencies, and the Keldysh formalism (KF), formulated in real frequencies, give equivalent results for systems in thermal equilibrium. The MF has less complexity and is thus more convenient than the KF. However, computing dynamical observables in the MF requires the analytic continuation from imaginary to real frequencies. The analytic continuation is well-known for two-point correlation functions (having one frequency argument), but, for multipoint correlators, a straightforward recipe for deducing all Keldysh components from the MF correlator had not been formulated yet. Recently, a representation of MF and KF correlators in terms of formalism-independent partial spectral functions and formalism-specific kernels was introduced by Kugler, Lee, and von Delft [Phys. Rev. X 11, 041006 (2021)]. This representation is used to formally elucidate the connection between both formalisms. How a multipoint MF correlator can be analytically continued to recover all partial spectral functions and yield all Keldysh components of its KF counterpart is shown. The procedure is illustrated for various correlators of the Hubbard atom.

By combining general relativity and CPT symmetry, the theory of CPT gravity predicts gravitational repulsion between matter and CPT-transformed matter, i.e., antimatter inhabiting an inverted space-time. Such repulsive gravity turned out to be an excellent candidate for explaining the accelerated expansion of the Universe, without the need for dark energy. The recent results of the ALPHA-g experiment, which show gravitational attraction between antihydrogen atoms and the Earth, seem to undermine this success in the cosmological field. Analyzing the above theory, two solutions are found that can be consistent with the experimental results, while preserving the large-scale gravitational repulsion. The first highlights how repulsive gravity can be the result of the interaction with an inverted space-time, but occupied by matter and not antimatter, and therefore the antimatter present in our space-time has no reason to exhibit gravitational repulsion. The second retains the original CPT transformation, resulting in repulsive gravity between matter and antimatter, but with the caveat that antimatter immersed in our space-time cannot exhibit the PT transformation which is the cause of the repulsion. Finally, it is shown that, in a Newtonian approximation of the geodesic equation, time reversal is not a necessary operation for repulsive gravity, therefore opening the possibility of an expanding cosmos with a single time direction.

The concept of attention has proven to be very relevant in artificial intelligence. Relative entropy (RE, aka Kullback-Leibler divergence) plays a central role in communication theory. Here, these concepts, attention, and RE are combined. RE guides optimal encoding of messages in bandwidth-limited communication as well as optimal message decoding via the maximum entropy principle. In the coding scenario, RE can be derived from four requirements, namely being analytical, local, proper, and calibrated. Weighted RE, used for attention steering in communications, turns out to be improper. To see how proper attention communication can emerge, a scenario of a message sender who wants to ensure that the receiver of the message can perform well-informed actions is analyzed. In case only the curvature of the utility function maxima are known, it becomes desirable to accurately communicate an attention function, in this case a by this curvature weighted and re-normalized probability function. Entropic attention communication is here proposed as the desired generalization of entropic communication that permits weighting while being proper, thereby aiding the design of optimal communication protocols in technical applications and helping to understand human communication. It provides the level of cooperation expected under misaligned interests of otherwise honest communication partners.

In optomechanics, the interaction between light and matter is enhanced by engineering cavities where the electromagnetic field and the mechanical displacement are confined simultaneously within the same volume. This leads to a wide range of interesting phenomena, such as optomechanically induced transparency and the cooling of macroscopic objects to their lowest possible motion state. In this manuscript, the focus is on designed optomechanical cavities exploiting heterostructures in air-slot photonic-crystal waveguides, incorporating different hole shapes and dimensions to engineer and control their optomechanical properties. The aim is to maximize the optical quality factor of the optical cavity, while ensuring optical mode volumes below the diffraction limit. These optimized optical modes interact with in-plane motional degrees of freedom of the structures achieving high optomechanical coupling rates, thus opening up the possibility of mechanical amplification, nonlinear dynamics and chaos through the optomechanical back-action.

In this paper, the introduction of a novel type of electron beams known as electron odd-Pearcey Gaussian beams (EOPGBs) is presented. For the first time, the dynamics of EOPGBs are explored propagating in both free space and a constant magnetic field by employing the Schrödinger equation. The investigation reveals that the electron beams exhibit remarkable auto-focusing characteristics in these two media, and the focal length can be controlled by adjusting certain parameters. Additionally, in a constant magnetic field, EOPGBs exhibit intriguing properties, notably the dual auto-focusing property, which sets them apart from other known electron beams. The probability currents and distribution factors are used to provide a theoretical explanation for the aforementioned features of EOPGBs. Furthermore, the conclusions are consistent with the results obtained from numerical simulations.

Brownian Motion

The statistics of the position of an optically-trapped colloid immersed in a bacterial bath is prominently non-Gaussian. Costantino Di Bello, Rita Majumdar, Édgar Roldán, and coworkers (article number 2300427) develop an exactly-solvable theory for the fluctuations of a colloid in a diluted bacterial bath and explore its thermodynamic consequences. Fruits of their theory, they establish how can experimentalists infer the statistics of the forces exerted by the bacteria (often unaccessible experimentally) from the non-Gaussian features of the statistics of the colloid's position.