Emergence of a metallic metastable phase induced by electrical current in Ca2RuO4

Abstract

A comprehensive study of the behavior of the Mott insulator Ca2RuO4 under electrical current drive is performed by combining two experimental probes: the macroscopic electrical transport and the microscopic X-Ray diffraction. The resistivity, ρ, vs electric current density, J, and temperature, T , ρ(J,T), resistivity map is drawn. In particular, the meta-stable state, induced between the insulating and the metallic thermodynamic states by current biasing Ca2RuO4 single crystals, is investigated. Such an analysis, combined with the study of the resulting RuO6 octahedra energy levels, reveals that a metallic crystal phase emerges in the meta-stable regime. The peculiar properties of such a phase, coexisting with the well-established orthorhombic insulating and tetragonal metallic phases, allow to explain some of the unconventional and puzzling behaviors observed in the experiments, as a negative differential resistivity. arXiv:1912.01690v1 [cond-mat.str-el]

Stable and Tunable Current-Induced Phase Transition in Epitaxial Thin Films of Ca2RuO4

“…Layered perovskite Ca2RuO4 (CRO) is one such material, where the metal-insulator transition is induced by currents or fields through electron redistributions in the 4d t2g orbitals. 9,10,[13][14][15][16] In CRO, novel electronic properties that do not appear in the thermally induced phases have been observed in the electrically induced phases, including strong diamagnetism under a high current density. 13 This indicates the particular importance of the electricity induced transitions of CRO because the electrically driven transitions in other materials generally induces only the same phases with the thermally driven transitions.…”

Universal size-dependent nonlinear charge transport in single crystals of the Mott insulator Ca2RuO4

“…Furthermore, current density as a function of electric field is hysteretic which complicates the study of size effects carried out here. We have conducted electric field versus current density (E(J)-characteristic) and resistivity measurements as a function of temperature and found that the microscopic samples qualitatively show similar behavior as the mm-sized bulk samples 14 . Typical results from microscopic samples are summarized in Fig.…”

“…A prime example of this, is the family of ruthenates witch exhibits a rich and diverse phase diagram that includes Mott insulators 6,7 , unconventional superconductivity 8,9 , and magnetism 10,11 . The 4d electron Mott insulator Ca 2 RuO 4 6 (hereafter Ca214) has become the subject of intense research in recent years due to its intriguing electrical [12][13][14][15][16][17][18][19][20][21][22][23][24] and magnetic properties 25,26 . Specifically, at room temperature it undergoes a current-driven insulator to metal transition (IMT) which occurs at unusually low electric (E-)field or current density (J) thresholds (~40 V/cm or few A/cm 2 respectively) 27,28 .…”

“…Specifically, in a recent work, it was found that the IMT is always accompanied by a local temperature increase to the transition temperature 33 . Moreover, as a precursor to the metallic phase, a third and non-equilibrium (also called metastable) phase has been detected by X-ray and neutron diffraction experiments, and additionally with Raman spectroscopy [14][15][16] . This non-equilibrium phase seems to be induced by low current densities, but its role in the current-driven IMT remains elusive.…”

“…This non-equilibrium phase seems to be induced by low current densities, but its role in the current-driven IMT remains elusive. Another major hurdle is the pronounced structural transition accompanying the IMT, in which the RuO 2 octahedra are elongated, and the unit cell is transformed from orthorhombic to tetragonal [13][14][15]17,27,38,39 . This transition leads to a strong temperature dependence of the unit cell volume (~1% between 100 K and 400 K), introducing large internal strains in the crystal, often resulting in the formation of cracks or even shattering the crystal upon reentering the insulating phase 38 .…”