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Feynman’s ratchet is a microscopic machine in contact with two heat reservoirs, at temperatures T _A and T _B , that was proposed by Richard Feynman to illustrate the second law of thermodynamics. In equilibrium ( T _A = T _B ), thermal fluctuations prevent the ratchet from generating directed motion. When the ratchet is maintained away from equilibrium by a temperature difference ( ${T}_{A}\ne {T}_{B}$ ), it can operate as a heat engine, rectifying thermal fluctuations to perform work. While it has attracted much interest, the operation of Feynman’s ratchet as a heat engine has not been realized experimentally, due to technical challenges. In this work, we realize Feynman’s ratchet with a colloidal particle in a one-dimensional optical trap in contact with two heat reservoirs: one is the surrounding water, while the effect of the other reservoir is generated by a novel feedback mechanism, using the Metropolis algorithm to impose detailed balance. We verify that the system does not produce work when T _A = T _B , and that it becomes a microscopic heat engine when ${T}_{A}\ne {T}_{B}$ . We analyze work, heat and entropy production as functions of the temperature difference and external load. Our experimental realization of Feynman’s ratchet and the Metropolis algorithm can also be used to study the thermodynamics of feedback control and information processing, the working mechanism of molecular motors, and controllable particle transportation.

A compressed terahertz imaging method using a terahertz time domain spectroscopy system (THz-TDSS) is suggested and demonstrated. In the method, a parallel THz wave with the beam diameter 4 cm from a usual THz-TDSS is used and a square shaped 2D echelon is placed in front of an imaged object. We confirm both in simulation and in experiment that only one terahertz time domain spectrum is needed to image the object. The image information is obtained from the compressed THz signal by deconvolution signal processing, and therefore the whole imaging time is greatly reduced in comparison with some other pulsed THz imaging methods. The present method will hopefully be used in real-time imaging. [ABSTRACT FROM AUTHOR]

The Goodenough–Kanamori–Anderson (GKA) rules have been widely applied for explaining the magnetic properties induced by super-exchange interaction. As conclusions of the super-exchange theory, they reveal the antiferromagnetic (ferromagnetic) ordering along with bond angle of 180° (90°) in the cation–anion–cation interaction path, in which the theory sets a pre-condition that the electronic states of cations in all paths are identical. We observed that the GKA rules are in fact not universal and even invalid to materials containing anions with different valence states, for example, the layered CrOCl crystal (with two valence states of anions: O ^2− and Cl ^− ). In this study, we propose an extended super-exchange theory (ESET) related to superposed electronic states of cation in a specific path. ESET is capable of predicting not only the sign and relative magnitude of magnetic exchange constants in different cation–anion–cation paths, but also the magnetic ground state. Through our proposed theory, we conclude that the magnetic ordering along with bond angle of 90° in Cr–Cl–Cr path is moderately antiferromagnetic and of 180° in Cr–O–Cr path is strongly ferromagnetic, which are opposite to the contents of GKA rules. Moreover, we clarify that monolayer CrOCl has antiferromagnetic ordering rather than ferromagnetic as reported recently. The reliability of ESET is verified via first-principles calculation and previous experimental report as well, and its universality is also demonstrated. Thus, our theory is powerful to predict the magnetic properties, which makes it possible to design new high Curie temperature two-dimensional semiconducting ferromagnets with polyvalent anion materials.

Nanostructures with giant magnetic anisotropy energies (MAEs) are desired in designing miniaturized magnetic storage and quantum computing devices. Previous works focused mainly on materials or elements with d electrons. Here, by taking Bi–X(X = In, Tl, Ge, Sn, Pb) adsorbed on nitrogenized divacancy of graphene and Bi atoms adsorbed on MgO(100) as examples, through ab initio and model calculations, we propose that special p -element dimers and single-adatoms on symmetry-matched substrates possess giant atomic MAEs of 72–200 meV, and has room temperature structural stability. The huge MAEs originate from the p -orbital degeneracy around the Fermi level in a symmetry-matched surface ligand field and the lifting of this degeneracy when spin–orbit interaction (SOI) is taken into account. Especially, we developed a simplified quantum mechanical model for the design principles of giant MAEs of supported magnetic adatoms and dimers. Thus, our discoveries and mechanisms provide a new paradigm to design giant atomic MAE of p electrons in supported nanostructures.

The exchange bias (EB) is an effective fundamental and applicational method to realize magnetic hysteresis loop shifting. However, further manipulation of EB unidirectional anisotropy is difficult after setup using either field deposition or post-annealing. In this work, we experimentally show a new approach to control the magnetic hysteresis loop bias in a [Co(0.2)/Pd(1)]5/CoFeB orthogonal exchange-spring (ES) coupling system, where the direction and strength of unidirectional anisotropy can be easily manipulated by applying an external magnetic field. [ABSTRACT FROM AUTHOR]