The 342nd forum: Why holes are necessary for superconductivity
Department of Physics, UCSD
Prof. Jorge E. Hirsch is the Professor of Department of Physics, University of California, San Diego (UCSD). His research is in theoretical condensed matter physics, investigating the mechanisms of conventional and unconventional superconductivity. He received his Ph.D. in Physics from University of Chicago in 1980. He began at UCSD in 1987 and has been working there for more than 30 years. He is a Fellow of the American Physical Society. He has been recipient of many awards and other recognitions for his teaching and research. He has published 248 papers (16380+ citations; H-index = 57).
The empirical observation that superconductivity is favored in materials that have hole carriers was made already in the 1930's , but no explanation for it was found at the time. BCS theory, developed in 1957, makes no distinction between electrons and holes. The discovery of high Tc cuprates in 1986 highlighted the important role of hole carriers in many materials, yet it is still generally believed today that both electron and hole carriers can give rise to superconductivity. Instead, we have proposed since 1989 that superconductivity cannot exist without hole carriers . I will review the vast empirical evidence indicating that holes are necessary for superconductivity including the elements, transition metal alloys, A-15 materials, doped semiconductors, hole- and electron-doped cuprates, MgB2, iron pnictides and chalcogenides, and elements under high pressure . I will then discuss the many theoretical reasons why holes are different from electrons, contrary to what Ashcroft-Mermin and other solid state physics textbooks say, and explain the reasons for why superconductivity can only occur if the material has hole carriers . In particular why holes can form Cooper pairs and electrons cannot, and why the Meissner effect, exhibited by all superconductors, cannot occur without hole carriers. I will discuss the implications of this for the understanding of superconductivity in 'conventional' and 'unconventional' superconductors, and for guiding the search for new superconducting materials with higher transition temperatures.
 K. Kikoin, B. Lasarew, Nature 129, 57 (1932); I.M. Chapnik, Phys. Lett. A 72, 255 (1979).
 J. E. Hirsch, Phys. Lett. A134, 451 (1989).
 J. E. Hirsch, Physica C 472, 78 (2012).
 References in https://jorge.physics.ucsd.edu/hole.html.
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