Raman scattering from current-stabilized nonequilibrium phases in ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$

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Raman scattering from current-stabilized nonequilibrium phases in ${Ca}_{2} {RuO}_{4}$

K. Fürsich, J. Bertinshaw, P. Butler, M. Krautloher, M. Minola, and B. Keimer

Phys. Rev. B 100, 081101(R) – Published 1 August 2019

Abstract

We used Raman light scattering to study the current-stabilized nonequilibrium semimetallic and metallic phases in ${Ca}_{2} {RuO}_{4}$ . 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.

Received 7 June 2019
Revised 17 July 2019

DOI:https://doi.org/10.1103/PhysRevB.100.081101

Physics Subject Headings (PhySH)

Electron-phonon coupling Magnons Phonons

Mott insulators Nonequilibrium systems Strongly correlated systems

Inelastic light scattering Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

K. Fürsich , J. Bertinshaw, P. Butler, M. Krautloher, M. Minola^*, and B. Keimer^†

Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany

^*M.Minola@fkf.mpg.de
^†B.Keimer@fkf.mpg.de

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Vol. 100, Iss. 8 — 15 August 2019

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Figure 1
Current-induced IMT in ${Ca}_{2} {RuO}_{4}$ . (a) Transport measurement. $R_{ins}$ ( $R_{m}$ ) indicates regions with high (low) resistance, which corresponds to the insulating (metallic) phase. The dotted line is an extrapolation of the $R_{ins}$ region. (b) Raman spectra in $B_{1 g}$ geometry. (c) and (d) Raman spectra in $A_{g}$ geometry for anti-Stokes and Stokes parts of the spectrum, respectively. The symbols in panels (b), (c), and (d) represent experimental Raman data, taken at the current values indicated with the corresponding marker in panel (a). The lines in panels (b), (c), and (d) are fits to experimental data. The inset in panels (b) and (d) illustrate the scattering geometry of the Raman experiment with respect to the Ru lattice (dotted lines: Ru-O-Ru bonds; $a, b$ : crystallographic axes). The shaded area in panel (d) illustrates the increased electronic response, as discussed in the text.
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Figure 2
Temperature driven IMT in ${Ca}_{2} {RuO}_{4}$ . (a) Transport measurement. (b) Raman spectra in $B_{1 g}$ geometry. (c) and (d) Raman spectra in $A_{g}$ geometry for anti-Stokes and Stokes parts of the spectrum, respectively. Temperatures are given in Table 1. In analogy to Fig. 1, the current values for the Raman spectra (experimental data: symbols; fit: lines) are indicated with the corresponding symbol in panel (a). In panel (d) the shaded area illustrates the increased electronic response.
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Figure 3
Raman scattering from metallic phases in ${Ca}_{2} {RuO}_{4}$ . (a), (b) Raman spectra in $B_{1 g}$ and $A_{g}$ geometry, respectively. Symbols (lines) represent experimental (fitted) data.
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Figure 4
Current-stabilized semimetallic diamagnetic phase in comparison to the equilibrium AFM insulating state. Raman spectra in (a) crossed $B_{1 g}$ geometry and (b) $A_{g}$ geometry. The lines are superpositions of Fano peaks fitted to the data (small symbols). The phonon modes marked with large symbols are analyzed in Fig. 5.
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Figure 5
Asymmetry parameter $1 / q$ for (a) equilibrium and (b) nonequilibrium phases of ${Ca}_{2} {RuO}_{4}$ . Data for panel (b) were taken at $T = 10 K$ . Magenta (blue) symbols correspond to $1 / q$ values for parallel (crossed) polarization. For the $B_{1 g}$ geometry, we analyze the phonons at 211, 388, and $458 {cm}^{- 1}$ and for the $A_{g}$ channel at 252, 321, and $392 {cm}^{- 1}$ , as indicated in Fig. 4 with the corresponding open symbol. Filled symbols summarize all phonons for a specific geometry. The increased asymmetry in the insulating paramagnetic state is in qualitative agreement with Ref. [24]. The dark blue star marks the asymmetry value for Sr-doped ${Ca}_{2} {RuO}_{4}$ at $456 {cm}^{- 1}$ ( $B_{1 g}$ channel), taken from Ref. [33].
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Physical Review B

covering condensed matter and materials physics

Raman scattering from current-stabilized nonequilibrium phases in ${Ca}_{2} {RuO}_{4}$

K. Fürsich, J. Bertinshaw, P. Butler, M. Krautloher, M. Minola, and B. Keimer

Phys. Rev. B 100, 081101(R) – Published 1 August 2019

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Physical Review B

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Raman scattering from current-stabilized nonequilibrium phases in Ca2RuO4

K. Fürsich, J. Bertinshaw, P. Butler, M. Krautloher, M. Minola, and B. Keimer

Phys. Rev. B 100, 081101(R) – Published 1 August 2019

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Raman scattering from current-stabilized nonequilibrium phases in ${Ca}_{2} {RuO}_{4}$