• Open Access

Nano-Resolved Current-Induced Insulator-Metal Transition in the Mott Insulator Ca2RuO4

Jiawei Zhang, Alexander S. McLeod, Qiang Han, Xinzhong Chen, Hans A. Bechtel, Ziheng Yao, S. N. Gilbert Corder, Thomas Ciavatti, Tiger H. Tao, Meigan Aronson, G. L. Carr, Michael C. Martin, Chanchal Sow, Shingo Yonezawa, Fumihiko Nakamura, Ichiro Terasaki, D. N. Basov, Andrew J. Millis, Yoshiteru Maeno, and Mengkun Liu
Phys. Rev. X 9, 011032 – Published 15 February 2019
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Abstract

The Mott insulator Ca2RuO4 is the subject of much recent attention following reports of emergent nonequilibrium steady states driven by applied electric fields or currents. In this paper, we carry out infrared nano-imaging and optical-microscopy measurements on bulk single crystal Ca2RuO4 under conditions of steady current flow to obtain insight into the current-driven insulator-to-metal transition. We observe macroscopic growth of the current-induced metallic phase, with nucleation regions for metal and insulator phases determined by the polarity of the current flow. A remarkable metal-insulator-metal microstripe pattern is observed at the phase front separating metal and insulator phases. The microstripes have orientations tied uniquely to the crystallographic axes, implying a strong coupling of the electronic transition to lattice degrees of freedom. Theoretical modeling further illustrates the importance of the current density and confirms a submicron-thick surface metallic layer at the phase front of the bulk metallic phase. Our work confirms that the electrically induced metallic phase is nonfilamentary and is not driven by Joule heating, revealing remarkable new characteristics of electrically induced insulator-metal transitions occurring in functional correlated oxides.

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  • Received 29 July 2018
  • Revised 16 December 2018

DOI:https://doi.org/10.1103/PhysRevX.9.011032

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jiawei Zhang1,*, Alexander S. McLeod2,*, Qiang Han2,*, Xinzhong Chen1, Hans A. Bechtel3, Ziheng Yao1, S. N. Gilbert Corder1, Thomas Ciavatti1, Tiger H. Tao4, Meigan Aronson5, G. L. Carr6, Michael C. Martin3, Chanchal Sow7, Shingo Yonezawa7, Fumihiko Nakamura8, Ichiro Terasaki9, D. N. Basov2, Andrew J. Millis2,10,†, Yoshiteru Maeno7,‡, and Mengkun Liu1,¶

  • 1Department of Physics, Stony Brook University, Stony Brook, New York 11794, USA
  • 2Department of Physics, Columbia University, New York, New York 10027, USA
  • 3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
  • 4State Key Laboratory of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
  • 5Department of Physics, Texas A&M University, College Station, Texas 77843, USA
  • 6NSLS-II Photon Sciences, Brookhaven National Laboratory, Upton, New York 11973, USA
  • 7Department of Physics, Kyoto University, Kyoto 606-8502, Japan
  • 8Department of Education and Creation Engineering, Kurume Institute of Technology, Fukuoka 830-0052, Japan
  • 9Department of Physics, Nagoya University, Nagoya 464-8602, Japan
  • 10Center for Computational Quantum Physics, The Flatiron Institute, 162 5th Avenue, New York, New York 10010, USA

  • *These authors contributed equally to the present work.
  • Corresponding authors. ajm2010@columbia.edu
  • maeno@scphys.kyoto-u.ac.jp
  • mengkun.liu@stonybrook.edu

Popular Summary

Some materials have the ability to transform from an electrical insulator to a conductor. Triggering this transition via applied current or voltage is a daunting challenge that could enable new types of photonic and electrical devices based on emerging quantum phenomena. To establish purely electrical control of this transition and to understand its microscopic nature, researchers must clarify how the underlying electronic and atomic lattice structures evolve. Here, we experimentally clarify these electronically driven changes in the material Ca2RuO4.

This compound is a Mott insulator, a class of material that should conduct electricity given traditional theories of electron behavior but instead acts as an insulator. Using infrared nano-imaging and optical microscopy, we characterize changes in electron and phonon properties at each stage of the insulator-to-metal transition in a single crystal of Ca2RuO4 that is subject to a steady flow of current.

We find that a nonfilament metallic domain can be switched on electrically and propagates through the sample with increasing electric current. The metallic domain emerges strictly from the negative electrodes; switching the polarity of the electrodes can change the direction of the domain propagation. In the boundary region separating the metallic and insulating phases, we discover a peculiar submicron stripe pattern, consisting of alternating insulator and metal phases, and find it to be aligned to a particular crystalline axis.

These discoveries successfully resolve different stages of the electronically driven phase transition in Ca2RuO4 and demonstrate, for the first time, the nonequilibrium mesoscopic pattern formation sustained by electric current at a propagating insulator-metal phase front. This work should stimulate the effort for electronic control of a variety of Mott insulators.

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Vol. 9, Iss. 1 — January - March 2019

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