Multipolar Orders and Interplay with Hybridization in non-Kramers Kondo Lattice Materials

William Marsh Rice University, Houston TX

Investigators

Abstract

I. Nontechnical Summary Over the past decades, intensive research in condensed matter physics has focused on materials with strong electron correlations. These materials behave differently from ordinary metals such as aluminum or copper because the electrically charged particles of which they consist do not always behave like “ordinary” electrons. For instance, when an electric current or thermal heat flow travels through an ordinary metal, the electrons move independently and interact only very weakly with each other. However when a current travels through a metal that has strong electron correlations, the electrons lose their individuality and form collective excitations, compelling researchers to call them “strange metals.” A classical example of a collective behavior is the complex pattern of a flying flock of birds, seemingly behaving as one unit despite comprising thousands of individual birds. A fascinating class of “strange metals” is realized in the Kondo lattice materials, so named in honor of the Japanese physicist Jun Kondo who first explained the mechanism behind the unusual electric current flow in materials of this type. While the theoretical underpinnings of the Kondo effect, in its simplest form, were explained in the 1970s, a recent discovery of a novel class of Kondo materials based on the chemical element praseodymium has puzzled scientists. The electrons in these materials are forced to choose between two kinds of collective behavior — as if birds in the above example were forced to choose between two different “flocks” traveling in different directions. In the materials that are subject of this research, one direction leads to an emergent order in the shape of praseodymium orbitals, while the other results in electrons forming a “strange metal” whose properties are not yet well understood. The proposed research will elucidate the nature of these collective behaviors, including superconductivity, using a toolbox of innovative theoretical and computational methods. If successful, the proposed research has the potential to significantly advance the state of the art in the field and improve our understanding of the fundamental aspects of the “strange metal” behavior in these materials. This project also seeks to integrate educational and outreach activities with the fundamental research. Recognizing the national need for high-quality, well-informed career mentoring for graduate students in the physical sciences, the principal investigator will organize a series of workshops to train faculty in how best to provide this kind of mentoring. These biennial COMPASS (Career and Occupational Mentoring for the Professional Advancement of Science Students) workshops will introduce faculty to career planning and mentoring resources and prepare them to implement effective graduate student mentoring at their institutions. It is hoped that such robust mentoring will help adequately prepare science graduate students to translate their technical knowledge into skills needed across the full range of STEM-related careers. The need for such efforts is recognized nationwide, and with the NSF’s support, we hope to make this series of workshops available to faculty at as many institutions as possible. II. Technical Summary Over the past decades, intensive research in condensed matter physics has focused on materials with strong electron correlations. Kondo lattice materials containing f-shell elements are a class of strongly correlated electron systems with a rich history, and the key challenge is to understand the interplay between magnetic ordering of the local f-moments and their hybridization with the conduction electrons. In this research, we focus on elucidating the solution for a specific type of Kondo lattice problem whose ions, such as praseodymium 3+, contain an even number of f-electrons, with a ground state that is a non-Kramers doublet. A recent discovery of a family of such materials, in particular PrV2Al20 and PrTi2Al20, has shown very rich behavior, including unusual quadrupolar magnetism, a non-Fermi liquid metal phase, and unconventional superconductivity. Intellectual Merit: To date, most of the theoretical research on non-Kramers f-ions has focused on either the nature of the quadrupolar ordering, or on the effectively mean-field description of the multi-channel Kondo effect, which occurs when the non-Kramers doublet is screened by conduction electrons. The principal intellectual merit of this proposal is to combine these two aspects in an integral way, aiming to answer the questions of how the magnetic multipolar order gets suppressed by the Kondo hybridization; what the nature of the quantum critical point is and how the observed non-Fermi liquid behavior arises; and what the nature of the superconductivity is. Importantly, the proposed research will go beyond the mean-field description of the multipolar order to include the effect of quantum fluctuations, and will employ advanced analytical and numerical techniques to elucidate the effect of Kondo hybridization beyond what has been done previously. If successful, the proposed research has the potential to significantly advance the state of the art in the field and improve our understanding of the fundamental aspects of strong electron interactions in these materials. Broader Impacts: Recognizing the national need for high-quality, well-informed career mentoring for graduate students in the physical sciences, we propose to organize a series of biennial COMPASS (Career and Occupational Mentoring for the Professional Advancement of Science Students) workshops that will introduce faculty to career planning and mentoring resources, provide them with descriptions of how to use these resources, and prepare them to formulate concrete action plans to implement graduate student mentoring at their institutions. It is hoped that such robust mentoring will help adequately prepare science graduate students to translate their technical knowledge into skills needed across the full range of STEM-related careers. The need for such efforts is recognized nationwide, and with the NSF’s support, we hope to make this series of workshops available to faculty at as many institutions as possible. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.