What is it about?
Synchronization phenomena seen in our daily life can also occur in the small-scale world. One example is the synchronization between an oscillating external field and the collective motion of a charge-density wave (CDW). Since the CDW is an electron condensate, it can be driven to slide by a DC electric field, carrying an electric current with an oscillating CDW mode. When the system is exposed to an AC field with a specific frequency, it is known that Shapiro steps appear as quantized plateaus in the current–voltage characteristics. This phenomenon occurs due to the synchronization between the AC field frequency and the current oscillation frequency. The width of the Shapiro steps is a manifestation of the synchronization strength. In this study, we investigate the effects of simultaneously applied AC fields with two frequencies on the formation of the Shapiro steps. We find that mixing two different frequencies induces new interference effects, resulting in the appearance of additional steps and their unexpected dependence on the external field strength.
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Why is it important?
A recent experimental study, in which two AC fields with different frequencies were applied simultaneously to a CDW conductor, demonstrated that additional Shapiro steps were induced by the frequency mixing, but the mechanism of this non-trivial synchronization phenomenon has not yet been fully understood. In this study, we theoretically investigate how the motion of the CDW is synchronized to the external fields and successfully provide an explanation for the experimentally observed phenomenon, namely, the appearance of the additional steps. We also find that the width of the Shapiro steps exhibits unexpected behavior with increasing the external field strength. Our results are important from the viewpoints not only of understanding the nature of the CDW but also of controlling the CDW motion that carries the current. Since the Shapiro steps appear in superconducting systems with Josephson junctions as well, the same frequency mixing effects presented in this work may be applied to superconductors and other related synchronization phenomena.
Perspectives
Our theoretical results suggest that the Shapiro steps under two frequencies can exhibit more flexible behavior than those under one frequency. We believe that these differences will be further explored in future experimental studies, potentially leading to a deeper understanding and better control of the formation of the Shapiro steps. This is important not only from the academic viewpoint but also from the viewpoint of possible applications. Indeed, Shapiro steps in Josephson junctions play an important role in applications such as voltage standards and superconducting devices. We hope that this study will contribute to technological developments in these areas.
Yu Funami
Read the Original
This page is a summary of: Effects of frequency mixing on Shapiro-step formations in sliding charge-density-waves, Frontiers in Human Neuroscience, October 2024, American Institute of Physics,
DOI: 10.1063/5.0237536.
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