Raman scattering from current-stabilized nonequilibrium phases in Ca2RuO4

Abstract

We used Raman light scattering to study the current-stabilized nonequilibrium semimetallic and metallic phases in Ca2RuO4. By determining the local temperature through careful analysis of the Stokes and anti-Stokes intensities, we find that Joule heating can be completely avoided by supplying sufficient cooling power in a helium-flow cryostat, and that the current induces the semimetallic state without inducing any significant heating. We further investigate the current-induced semimetallic state as a function of temperature and current. We confirm the absence of long-range antiferromagnetic order and identify a substantial Fano broadening of several phonons, which suggests coupling to charge and orbital fluctuations. Our results demonstrate that the semimetallic state is a genuine effect of the applied electrical current and that the current-induced phases have characteristics distinct from the equilibrium ones.Controlling and stabilizing electronic phases in correlated-electron materials are pivotal objectives of modern solid-state science 1,2 . In this context, compounds with 4d valence electrons have attracted a lot of attention as a plethora of ground states can be observed 3 . In particular, the fine balance between electron correlations, spin-orbit coupling and structural distortions in 4d metal oxides manifests itself as a high susceptibility to external perturbations. One prominent example is the layered perovskite system (Ca 1−x Sr x ) 2 RuO 4 whose end members, Ca 2 RuO 4 and Sr 2 RuO 4 , are a Mott insulator and an unconventional superconductor, respectively 4,5 .Ca 2 RuO 4 shows a temperature-driven insulator-tometal transition (IMT) at T IMT = 357 K 6-8 . The concomitant structural phase transition preserves the lattice symmetry (space group Pbca), but yields elongated RuO 6 octahedra with a reduced tilt and an enlarged c lattice parameter in the high-temperature metallic state. The insulating and metallic phases are therefore named "S"-Pbca and "L"-Pbca phase, respectively, where S and L refer to short and long c axes. In addition to temperature, other parameters, such as Sr substitution 5 , epitaxial strain 9 and hydrostatic pressure 10 , can turn Ca 2 RuO 4 metallic.The key role of octahedral rotation, compression and tilts is further exemplified in the magnetic state of Ca 2 RuO 4 , which sets in at T N = 110 K and is characterized by an A-type antiferromagnetic (AFM) ordering 6 . Several theoretical 11-13 and experimental 14,15 studies point towards an unconventional mechanism for magnetic ordering in Ca 2 RuO 4 , resulting from the condensation of spin-orbit excitons. The "excitonic magnetism" model further predicts that Ca 2 RuO 4 lies close to a quantum critical point separating antiferromagnetic and spin-orbit singlet states, which was confirmed by the experimental observation of a soft longitudinal amplitude mode of the spin-orbit condensate.Recent research has revealed new perspectives for insitu control of nonequilibrium phases in Ca 2 RuO 4 by electrical currents. In their pioneering wo...

Role of local temperature in the current-driven metal-insulator transition of Ca2RuO4

“…S5). This voltage-current characteristics is consistent with the one observed in several previous experiments on Ca 2 RuO 4 under current flow [14,17,18,20,23,24]. With increasing current, the supplied electrical power increases (Fig.…”

“…3e. This indicates that the temperature sensors placed on the sample holder largely underestimate the actual temperature of Ca 2 RuO 4 which is heated up by the direct current, as also reported by a recent study by K. Fürsich et al [24]. This observation remarks the need for monitoring the local sample temperature as allowed by our imaging technique.…”

“…However, the considerable amount of power supplied by a flowing current requires one to carefully take into account the unavoidable Joule heating, which makes it often difficult to measure the actual sample temperature. Recent reports by Fürsich et al indicate that heating by direct current can lead to a large temperature rise in Ca 2 RuO 4 , strongly depending on the cooling technique * mattoni@scphys.kyoto-u.ac.jp [24]. Moreover, Joule heating can cause less trivial effects in measurements.…”

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

“…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.…”