Cooperative interactions govern the fermiology of the polar metal Ca3Ru2O7

Danilo Puggioni,M Horio,J Chang,James M Rondinelli

doi:10.1103/physrevresearch.2.023141

Danilo Puggioni, M Horio + Show 2 more

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https://doi.org/10.1103/physrevresearch.2.023141

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Abstract

The antiferromagnetic Ruddlesden-Popper ruthenate Ca$_3$Ru$_2$O$_7$ is a model polar metal, combining inversion symmetry breaking with metallic conductivity; however, its low temperature ($T < 48$ K) crystal structure and Fermi surface topology remain ambiguous despite numerous measurements and theoretical studies. Here we perform both first principles calculations with static correlations and angle resolved photoelectron spectroscopy experiments to construct a complete model of Ca$_3$Ru$_2$O$_7$, reconciling inconsistencies among interpretations of electrical transport, thermopower measurements, and momentum- and energy-resolved band dispersions. The solution relies on treating the interplay among Coulomb repulsion, magnetic ordering, spin-orbit interactions, and the RuO$_6$ octahedral degrees-of-freedom on equal footing. For temperatures $30<T < 48$ K, we propose weak electron-electron interactions produce a symmetry-preserving metal-semimetal transition with Weyl nodes in proximity to the Fermi level, whereas a new orthorhombic $Pn2_1a$ structure emerges for $T<30$ K, exhibiting charge and spin density waves from enhanced Coulombic interactions.

Highlights

Ruddlesden-Popper Ca3Ru2O7 is a unique polar metal exhibiting a rich phase diagram (Fig. 1), colossal magnetoresistance [2,3], highly anisotropic electrical resistivity [4], and polar domain structures [5]
Upon varying U and performing density-functional theory (DFT) + U + spin-orbit interaction (SOI) atomic relaxations, we find that Ca3Ru2O7 undergoes a structural transition for U > 1.3 eV characterized by a change in translational symmetry
Using DFT calculations combined with Angle-resolved photoelectron spectroscopy (ARPES) measurements, we reconciled existing experiments on the lowtemperature fermiology of Ca3Ru2O7

Summary

INTRODUCTION

Ruddlesden-Popper Ca3Ru2O7 is a unique polar metal exhibiting a rich phase diagram (Fig. 1), colossal magnetoresistance [2,3], highly anisotropic electrical resistivity [4], and polar domain structures [5]. The electronlike band about the point is removed, and asymmetric FS changes occur at the zone boundary [11]: A temperaturedependent electron pocket at k = M(π /a, 0) shrinks upon cooling through T = 30 K, while a temperature-independent hole pocket is found at M (0, π /b) These data suggest either a FS reconstruction [11] or the appearance of a charge density wave at Ts [12], respectively; there remains no direct evidence for its presence in Ca3Ru2O7. For all values of the Coulomb interaction U , we find a Van Hove singularity (VHS) within 20 meV of the Fermi level owing to static Coulomb interactions that enhance broken band degeneracies near k = X (π /a, π /b) split by the spin-orbit interaction (SOI) The filling of these states occurs proximate to Weyl points in momentum space that arise from broken inversion and time-reversal symmetries.

MODEL AND METHODS

Crystal structure

Electronic structure

Phase transition assignments

CONCLUSIONS