Nonequilibrium orbital transitions via applied electrical current in calcium ruthenates

Abstract

Simultaneous control of structural and physical properties via applied electrical current poses a key, new research topic and technological significance. Studying the spin-orbit-coupled antiferromagnet Ca2RuO4, and its derivative with 3% Mn doping to alleviate the violent first-order transition at 357 K for more robust measurements, we find that a small applied electrical current couples to the lattice by significantly reducing its orthorhombicity and octahedral rotations, concurrently diminishing the 125 K-antiferromagnetic transition and inducing a new, orbital order below 80 K. Our effort to establish a phase diagram reveals a critical regime near a current density of 0.15 A/cm2 that separates the vanishing antiferromagnetic order and the new orbital order. Further increasing current density (> 1 A/cm2) enhances competitions between relevantinteractions in a metastable manner, leading to a peculiar glassy behavior above 80 K. The coupling between the lattice and nonequilibrium driven current is interpreted theoretically in terms of t2g orbital occupancies. The current-controlled lattice is the driving force of the observed novel phenomena. Finally, we note that current-induced diamagnetism is not discerned in pure and slightly doped Ca2RuO4. *

Polarity dependent heating at the phase interface in metal-insulator transitions

“…However, the behavior of the current-driven transition observed in Ca 2 RuO 4 appears to be inconsistent with these expectations [28][29][30][31][32][33][34][35]. In this material the insulating phase is destroyed at threshold fields (E th ∼ 40 V/cm) that are several orders of magnitude smaller than the fields required for a significant excitation of carriers, while it has been reported that the global temperature of the system remains below the equilibrium critical temperature [29].…”

Van Vleck excitons in Ca2RuO4

“…In case of the longitudinally polarized Higgs mode, its existence is precluded when no orbital degree of freedom is present, such as is the case for S = 5 2 in an undistorted octahedral weak ligand field. In this case, orthogonality ensures that matrix elements of the form m|J z |0 are zero, and thus by Eq.…”

“…The magnetism inherent to Hund's metals 1 and their parent multiorbital Mott states 2 is believed to play a crucial role in many of their anomalous electronic properties, 3 ranging from unconventional superconductivity, 4 to violations of Fermi liquid theory, to the recent unveiling of novel nonequilibrium states. 5 This is perhaps most clearly illustrated in the range of emergent phenomena that appear in 4d transition metal oxides-where an intermediate coupling regime manifests. In this regime, spin-orbit coupling, magnetic exchange, and crystalline electric field energies compete with one another on equal footing in determining a material's ground state properties.…”

Quest for quantum states via field-altering technology

“…1a), and this study demonstrates that it works particularly well for materials whose low energy physics is dictated by competing interactions [1][2][3][6][7][8][9][10][11][12][13][14][15] leading to multiple nearly degenerate states. Consequently, a resulting alteration in structural distortions (e.g., rotations/tilt of octahedra or Ir-O-Ir, and Ru-O-Ru bond angles) can be associated with disproportionately strong responses in physical properties, facilitating a different ground state to emerge 2,6,[16][17][18][19][20][21][22][23][24] . We would like to emphasize a crucial role of the bond angles that can significantly affect the physical properties via strong SOI and magnetoelastic coupling 2,6,17,18,20,22,24,25 .…”

“…Consequently, a resulting alteration in structural distortions (e.g., rotations/tilt of octahedra or Ir-O-Ir, and Ru-O-Ru bond angles) can be associated with disproportionately strong responses in physical properties, facilitating a different ground state to emerge 2,6,[16][17][18][19][20][21][22][23][24] . We would like to emphasize a crucial role of the bond angles that can significantly affect the physical properties via strong SOI and magnetoelastic coupling 2,6,17,18,20,22,24,25 . The field-altering technology allows the Zeeman effect to exert a torque on the spin, which couples to the orbit via SOI; the orbit in turn needs to readjust and tilts the octahedron, thus altering the bond angles.…”