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  • Open Access

Phase Separation in Doped Mott Insulators

Chuck-Hou Yee*

Leon Balents

  • Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA

  • Kavli Institute for Theoretical Physics, University of California, Santa Barbara, California 93106, USA

  • *chuckyee@physics.rutgers.edu
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Phys. Rev. X 5, 021007 – Published 15 April, 2015

DOI: https://doi.org/10.1103/PhysRevX.5.021007

Abstract

Motivated by the commonplace observation of Mott insulators away from integer filling, we construct a simple thermodynamic argument for phase separation in first-order doping-driven Mott transitions. We show how to compute the critical dopings required to drive the Mott transition using electronic structure calculations for the titanate family of perovskites, finding good agreement with experiment. The theory predicts that the transition is percolative and should exhibit Coulomb frustration.

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Popular Summary

Mott insulators have atomic lattices that enhance the mutual repulsion between their electrons so much that their movements are jammed, just like a crowd of people packed in shoulder to shoulder. The electrons cannot hop between atoms without encountering other negatively charged electrons, so the Mott insulator cannot conduct electricity. Using a process borrowed from the semiconductor industry called doping, we can chemically remove a small fraction of the electrons from a Mott insulator. Now, with a few holes in their midst, we expect that the electrons should be able to hop between atoms, just as an individual could move through a crowd by successively trading places with empty spots. In other words, we expect the doped material to become a metal.

Mott insulators are ubiquitous, especially among perovskites, a class of materials spanning thousands of compounds currently being studied. Surprisingly, Mott insulators resist our attempts to dope them. Because of simple thermodynamic reasons that we explain in this work, the holes prefer to cluster together, and the material separates into two types of regions: puddles of hole-rich areas that are metallic, and other areas that remain in the Mott insulating phase. A key aspect of our theory is the density of holes in the metallic phase. We show how to quantitatively compute this hole density in a well-studied class of titanium perovskites. Our hole densities follow experimental trends, although they are smaller by a factor of 2, a deviation that we attribute to chemical disorder present in experiments but ignored in our models. Knowledge of the metallic hole density allows us to predict the doping threshold where the metallic puddles grow sufficiently large to connect and produce metallic behavior across the Mott insulator.

It has long been unclear why doping a Mott insulator does not immediately transform it into a metal. Our work shows that phase separation is the culprit, paving the way towards a deeper understanding and control of the doping process in Mott insulators.

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Phys. Rev. X 5, 021007– Published 15 April, 2015
  • Received 3 July 2014
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This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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