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Register a new userPhonon anomalies and lattice dynamics in superconducting oxychlorides Ca$_{2-x}$CuO$_2$Cl
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Matteo DAstuto
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We present a comprehensive study of the phonon dispersion in an underdoped, superconducting Ca$_{2-x}$CuO$_2$Cl$_2$ crystal. We interpret the results using lattice dynamical calculations based on a shell model, and we compare the results, to other hole-doped cuprates, in particular to the ones isomorphic to La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). We found that an anomalous dip in the Cu-O bond stretching dispersion develops in oxychlorides with a simultaneous marked broadening of the mode. The broadening is maximum at $approx (pi / (2a) ~ 0 ~ 0)$ that corresponds to the charge-modulations propagation vector. Our analysis also suggests that screening effects in calculations may cause an apparent cosine-shaped bending of the Cu-O bond-stretching dispersion along both the ($q$ 0 0) and ($q$ $q$ 0) directions, that is not observed on the data close to optimal doping. This observation suggests that the discrepancy between experimental data and textit{ab-initio} calculations on this mode originates from an overestimation of the doping effects on the mode.
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By means of resonant inelastic x-ray scattering at the Cu L$_3$ edge, we measured the spin wave dispersion along $langle$100$rangle$ and $langle$110$rangle$ in the undoped cuprate Ca$_2$CuO$_2$Cl$_2$. The data yields a reliable estimate of the superexchange parameter $J$ = 135 $pm$ 4 meV using a classical spin-1/2 2D Heisenberg model with nearest-neighbor interactions and including quantum fluctuations. Including further exchange interactions increases the estimate to $J$ = 141 meV. The 40 meV dispersion between the magnetic Brillouin zone boundary points (1/2,,0) and (1/4,,1/4) indicates that next-nearest neighbor interactions in this compound are intermediate between the values found in La$_{2}$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$. Owing to the low-$Z$ elements composing Ca$_2$CuO$_2$Cl$_2$, the present results may enable a reliable comparison with the predictions of quantum many-body calculations, which would improve our understanding of the role of magnetic excitations and of electronic correlations in cuprates.
We present angle resolved photoemission (ARPES) data on Na-doped Ca$_2$CuO$_2$Cl$_2$. We demonstrate that the chemical potential shifts upon doping the system across the insulator to metal transition. The resulting low energy spectra reveal a gap structure which appears to deviate from the canonical $d_{x2-y2} ~ |cos(k_x a)-cos(k_y a)|$ form. To reconcile the measured gap structure with d-wave superconductivity one can understand the data in terms of two gaps, a very small one contributing to the nodal region and a very large one dominating the anti-nodal region. The latter is a result of the electronic structure observed in the undoped antiferromagnetic insulator. Furthermore, the low energy electronic structure of the metallic sample contains a two component structure in the nodal direction, and a change in velocity of the dispersion in the nodal direction at roughly 50 meV. We discuss these results in connection with photoemission data on other cuprate systems.
The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La$_2$CuO$_4$ were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures and magnetic orders - Nd$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$ - and show that two potential mechanisms can be excluded - the scattering of phonons by rare-earth impurities and by structural domains. Our comparative study further reveals that orthorhombicity, apical oxygens, the tilting of oxygen octahedra and the canting of spins out of the CuO$_2$ planes are not essential to the mechanism of chirality. Our findings point to a chiral mechanism coming from a coupling of acoustic phonons to the intrinsic excitations of the CuO$_2$ planes.
Understanding the complex phase diagram of cuprate superconductors is an outstanding challenge. The most actively studied questions surround the nature of the pseudogap and strange metal states and their relationship to superconductivity. In contrast, there is general agreement that the low energy physics of the Mott insulating parent state is well captured by a two-dimensional spin $S$ = 1/2 antiferromagnetic (AFM) Heisenberg model. However, recent observations of a large thermal Hall conductivity in several parent cuprates appear to defy this simple model and suggest proximity to a magneto-chiral state that breaks all mirror planes perpendicular to the CuO$_2$ layers. Here we use optical second harmonic generation to directly resolve the point group symmetries of the model parent cuprate Sr$_2$CuO$_2$Cl$_2$. We report evidence of an order parameter $Phi$ that breaks all perpendicular mirror planes and is consistent with a magneto-chiral state in zero magnetic field. Although $Phi$ is clearly coupled to the AFM order parameter, we are unable to realize its time-reversed partner ($-Phi$) by thermal cycling through the AFM transition temperature ($T_{textrm{N}}$ $approx$ 260 K) or by sampling different spatial locations. This suggests that $Phi$ onsets above $T_{textrm{N}}$ and may be relevant to the mechanism of pseudogap formation.
The lattice dynamics in Sr$_2$RuO$_4$ has been studied by inelastic neutron scattering combined with shell-model calculations. The in-plane bond-stretching modes in Sr$_2$RuO$_4$ exhibit a normal dispersion in contrast to all electronically doped perovskites studied so far. Evidence for strong electron phonon coupling is found for c-polarized phonons suggesting a close connection with the anomalous c-axis charge transport in Sr$_2$RuO$_4$.
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