Electronic reconstruction forming a C2-symmetric Dirac semimetal in Ca3Ru2O7

Abstract

Electronic band structures in solids stem from a periodic potential reflecting the structure of either the crystal lattice or electronic order. In the stoichiometric ruthenate Ca3Ru2O7, numerous Fermi surface-sensitive probes indicate a low-temperature electronic reconstruction. Yet, the causality and the reconstructed band structure remain unsolved. Here, we show by angle-resolved photoemission spectroscopy, how in Ca3Ru2O7 a C2-symmetric massive Dirac semimetal is realized through a Brillouin-zone preserving electronic reconstruction. This Dirac semimetal emerges in a two-stage transition upon cooling. The Dirac point and band velocities are consistent with constraints set by quantum oscillation, thermodynamic, and transport experiments, suggesting that the complete Fermi surface is resolved. The reconstructed structure—incompatible with translational-symmetry-breaking density waves—serves as an important test for band structure calculations of correlated electron systems.

Spontaneous cycloidal order mediating a spin-reorientation transition in a polar metal

“…In both phases the spins are aligned ferromagnetically within each bilayer and antiferromagnetically between the bilayers, with propagation vector (0,0,1) [8,11,12]. Due to the interplay of electronic correlations and spin-orbit coupling, the SRT is intricately coupled to the crystal structure and fermiology, coinciding with an isostructural change in the lattice parameters [8] and an increase in the resistivity caused by a gapping of most of the Fermi surface [13][14][15].…”

Cooperative Interactions Govern the Fermiology of the Polar Metal Ca$_3$Ru$_2$O$_7$

“…Fig. 4 also allows us to deduce that the electron pockets centered near M(π/a, 0) originate from a Dirac-like feature, located at ≈ 70 meV below E F , which has been recently detected by ARPES experiments [11]. These features shift away from M(π/a, 0) owing to SOI and the broken parity and time reversal symmetries in Ca 3 Ru 2 O 7 .…”

“…Below T = 30 K, however, negative and positive thermopower responses were measured along the crystallographic a and b axes, respectively, indicating predominately electron-and hole-like bands along these directions on the FS-features more consistent with the ARPES. Interestingly, all previous band-structure calculations based on density functional theory (DFT) [14,15] are also inconsistent with the ARPES [8,9,11] and thermoelectric transport measurements [13]. This is unusual given that DFT methods typically provide accurate descriptions of weakly correlated metals [16][17][18].…”

“…Angle resolved photoelectron spectroscopy (ARPES) and optical-pump/optical-probe experiments propose an electronic instability underlies the transport anisotropy at T s by removal of well-defined electron-and hole-like pockets on the Fermi surface (FS) upon cooling [8][9][10][11].…”

“…The electron-like band about the Γ point is removed and asymmetric FS changes occur at the zone boundary [11]: A temperature-dependent 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 T s [12], respectively; however, there remains no direct evidence for its presence in Ca 3 Ru 2 O 7 . Below 30 K, quasi-2D electron and hole pockets (areas ≈0.3 % of the Brillouin zone) survive and form the low-temperature FS as determined from quantum oscillations [8].…”