# Bardeen, CDWs and Macroscopic Quantum Phenomena

There is a well-written 1990 Physics Today article by John Bardeen entitled Superconductivity and Other Macroscopic Quantum Phenomena (pdf!). For those who are unaware, Bardeen was a two-time Nobel Laureate in Physics for inventing the transistor and secondly for the BCS theory of superconductivity.

Later in his career, Bardeen focused on the theory of transport in quasi-1D charge density wave materials. Bardeen was vocal in advocating that the transport in these materials must be understood in a quantum mechanical manner whereas most other physicists working on the problem treated it as a classical one (see True Genius by Daitch and Hoddeson). In the Physics Today article, he describes why he believes that the CDW sliding in these quasi-1D materials must be viewed as a manifestation of a macroscopic quantum phenomenon similar to that in superconductors and superfluids.

While Bardeen seemed to have lost his battle against the mainstream condensed matter physics community on this point upon his death in 1991, some interesting work has taken place since his death that has started to provide evidence for his perspective. In 1997, Monceau and co-workers showed the presence of Aharonov-Bohm-like oscillations in the CDW compound NbSe$_3$, with an oscillation period of, interestingly, $hc/2e$. While his tunneling theory of CDW transport may have been incorrect, his view of CDW transport as a macroscopic quantum phenomenon may yet be vindicated.

A lot of interest in these problems dissipated as scientists shifted to work on the problem of high temperature superconductivity following the discovery of the cuprates in 1986. However, it seems to me that there are still many unresolved issues in these compounds that persist to the present day that were cast aside rather than figured out.

As Paul Valery once said:

A poem is never finished, only abandoned.

The same can aptly be said about scientific problems.