Undoped Cuprates Research Articles

Competing incommensurate spin fluctuations and magnetic excitations in infinite-layer nickelate superconductors

The recently discovered infinite-layer nickelates show great promise in helping to disentangle the various cooperative mechanisms responsible for high-temperature superconductivity. However, lack of antiferromagnetic order in the pristine nickelates presents a challenge for connecting the physics of the cuprates and nickelates. Here, by using a quantum many-body Green’s function-based approach to treat the electronic and magnetic structures, we unveil the presence of many two- and three-dimensional magnetic stripe instabilities that are shown to persist across the phase diagram of LaNiO2. Our analysis indicates that the magnetic properties of the infinite-layer nickelates are closer to those of the doped cuprates, which host a stripe ground state, rather than the undoped cuprates. The computed longitudinal-spin, transverse-spin, and charge spectra of LaNiO2 are found to contain an admixture of contributions from localized and itinerant carriers. Theoretically obtained dispersion of magnetic excitations (spin-flip) is found to be in good accord with the results of recent resonant inelastic X-ray scattering experiments. Our study gives insight into the origin of strong magnetic competition in the infinite-layer nickelates and their relationship with the cuprates.

Thermal conductivity and theory of inelastic scattering of phonons by collective fluctuations

Understanding thermal Hall conductivity poses a theoretical challenge in insulators, where low-energy excitations carry no charge. In undoped cuprates, where the signal is large, we expect phonons contribute significantly to the effect. While there is no obvious universal mechanism whereby phonons break time-reversal symmetry and acquire chirality, this paper provides a general treatment of the phonon Hall effect from the inelastic scattering of phonons off of other excitations in quantum materials. As an application, magnetoelastic coupling in square-lattice N\'eel antiferromagnets is investigated thoroughly using the formalism developed in the first part of the paper.

Direct visualization and control of antiferromagnetic domains and spin reorientation in a parent cuprate

We report magnetic optical second-harmonic generation (SHG) polarimetry and imaging on Sr$_2$Cu$_3$O$_4$Cl$_2$, which allows direct visualization of the mesoscopic antiferromagnetic (AFM) structure of a parent cuprate. Temperature- and magnetic-field-dependent SHG reveals large domains with 90$^{\circ}$ relative orientations that are stabilized by a combination of uniaxial magnetic anisotropy and the Earth's magnetic field. Below a temperature $T_R$ $\sim$ 97 K, we observe an unusual 90$^{\circ}$ spin reorientation transition, possibly driven by competing magnetic anisotropies of the two copper sublattices, which swaps the AFM domain states while preserving the domain structure. This allows deterministic switching of the AFM states by thermal or laser heating. Near $T_R$, the domain walls become exceptionally responsive to an applied magnetic field, with the Earth's field sufficient to completely expel them from the crystal. Our findings unlock opportunities to study the mesoscopic AFM behavior of parent cuprates and explore their potential for AFM technologies.

Local transformation of the electronic structure and generation of free carriers in cuprates and ferropnictides under heterovalent and isovalent doping

We have previously shown that most of the anomalies in the superconducting characteristics of cuprates and ferropnictides observed at dopant concentrations within the superconducting dome, as well as the position of the domes in the phase diagrams, do not require knowledge of the details of their electronic structure for explanation, but can be understood and calculated with high accuracy within the framework of a simple model describing the cluster structure of the superconducting phase. This fact suggests a change in the paradigm that forms our understanding of HTSC. In this paper, we propose a unified view on the transformation of the electronic structure of cuprates and ferropnictides upon heterovalent and isovalent doping, based on the assumption of self-localization of doped carriers. In this representation, in undoped cuprates and ferropnictides, which initially have different electronic structures (Mott insulator and semimetal), local doping forms percolation clusters with the same electronic structure of a self-doped excitonic insulator where a specific mechanism of superconducting pairing is implemented, which is genetically inherent in such a system. The proposed model includes a mechanism for generating additional free carriers under heterovalent and isovalent doping and makes it possible to predict their sign, which, in the general case, does not coincide with the sign of doped carriers.

Effect of the electronic charge gap on LO bond-stretching phonons in undoped La2CuO4 calculated using LDA+U

Typical density-functional theory calculations that wrongly predict undoped cuprates to be metallic also predict Cu-O half- and full-breathing phonon energies that are significantly softer than observed, presumably because of weak on-site Coulomb repulsion on the Cu 3d orbitals. We used DFT+U calculations with antiferromagnetic supercells of La$_2$CuO$_4$ to establish correlation between the on-site repulsion strength, tuned via adjusting the value of U, and phonon dispersions. We find that breathing and half-breathing phonons reach experimental values when U is tuned to obtain the correct optical gap and magnetic moments. We demonstrate that using distorted supercells within DFT+U is a promising framework to model phonons in undoped cuprates and other perovskite oxides with complex, interrelated structural and electronic degrees of freedom.

Implications of second harmonic generation for hidden order in Sr2CuO2Cl2

${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ (SCOC) is a model undoped cuprate with $I4/mmm$ crystallographic symmetry, and a simple magnetic space group ${C}_{A}mca$ with associated magnetic point group $mmm{1}^{\ensuremath{'}}$. However, recent second harmonic spectroscopy in the antiferromagnetic phase has challenged this picture, suggesting instead a magnetic point group $4/m{m}^{\ensuremath{'}}{m}^{\ensuremath{'}}$ that coexists with the antiferromagnetism and breaks the two orthogonal mirror planes containing the tetragonal $c$ axis. Here, we analyze the symmetry of SCOC in light of the second harmonic results, and discuss possible ground states that are consistent with the data.

Effect of strain-induced orbital splitting on the magnetic excitations in undoped cuprates

We investigate the magnetic excitations in view of the recent reports suggesting that the spin-wave energy may exhibit a significant dependence on the in-plane strain of a thin film of ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$. The nature of dependence, as we find, can be explained naturally within a two-orbital model based on the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals. In particular, as the orbital-splitting energy between the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals increases with compressive strain, the zone-boundary spin-wave energy hardens. However, the hardening persists only until the orbital splitting reaches $\ensuremath{\sim}2$ eV, beyond which there is no significant change. The behavior of zone-boundary spin-wave energy is explained in terms of the extent of hybridization between one of the exchange-split ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ bands which is nearly half-filled and the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ band. The role of second order antiferromagnetic superexchange process involving the interorbital hopping is also discussed.

Electron–phonon-driven three-dimensional metallicity in an insulating cuprate

The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron-electron and the electron-phonon interaction and its relevance to the formation of the ordered phases have also been emphasized. Here, we combine polarization-resolved ultrafast optical spectroscopy and state-of-the-art dynamical mean-field theory to show the importance of the crystal lattice in the breakdown of the correlated insulating state in an archetypal undoped cuprate. We identify signatures of electron-phonon coupling to specific fully symmetric optical modes during the buildup of a three-dimensional (3D) metallic state that follows charge photodoping. Calculations for coherently displaced crystal structures along the relevant phonon coordinates indicate that the insulating state is remarkably unstable toward metallization despite the seemingly large charge-transfer energy scale. This hitherto unobserved insulator-to-metal transition mediated by fully symmetric lattice modes can find extensive application in a plethora of correlated solids.

Photoemission spectrum of Ca2RuO4 : Spin polaron physics in an S=1 antiferromagnet with anisotropies

We derive an S=1 spin polaron model which describes the motion of a single hole introduced into the S=1 spin antiferromagnetic ground state of Ca2RuO4. We solve the model using the self-consistent Born approximation and show that its hole spectral function qualitatively agrees with the experimentally observed high-binding energy part of the Ca2RuO4 photoemission spectrum. We explain the observed peculiarities of the photoemission spectrum by linking them to two anisotropies present in the employed model---the spin anisotropy and the hopping anisotropy. We verify that these anisotropies, and not the possible differences between the ruthenate (S=1) and the cuprate (S=1/2) spin polaron models, are responsible for the strong qualitative differences between the photoemission spectrum of Ca2RuO4 and of the undoped cuprates.

Quantum Correlated Plasmons and Their Tunability in Undoped and Doped Mott-Insulator Cuprates

Plasmons, collective excitations of charge density, have attracted much interest in fundamental science and applications. While plasmons have been observed in metal-like phase of high-Tc supercondu...

Doublon-holon pairing mechanism via exchange interaction in two-dimensional cuprate Mott insulators.

Coupling of charge and spin degrees of freedom is a critical feature of correlated electron oxides, as represented by the spin-related mechanism of a Cooper pair under high-T c superconductivity. A doublon-holon pair generated on an antiferromagnetic spin background is also predicted to attract each other via the spin-spin interaction J, similar to a Cooper pair, while its evidence is difficult to obtain experimentally. Here, we investigate such an excitonic effect by electroreflectance spectroscopy using terahertz electric field pulses in undoped cuprates: Nd2CuO4, Sr2CuO2Cl2, and La2CuO4. Analyses of the spectral changes of reflectivity under electric fields reveal that the splitting of odd-parity and even-parity excitons, a measure of doublon-holon binding energy, increases with J. This trend is reproduced by t-J-type model calculations, providing strong evidence of the spin-related doublon-holon pairing. Agreement with the calculations supports the s-wave symmetry of the doublon-holon pair in contrast to the d-wave Cooper pair in doped cuprates.

Consideration of thermal Hall effect in undoped cuprates

A recent observation of thermal Hall effect of magnetic origin in underdoped cuprates calls for critical re-examination of low-energy magnetic dynamics in undoped antiferromagnetic compound on square lattice, where traditional, renormalized spin-wave theory was believed to work well. Using Holstein-Primakoff boson formalism, we find that magnon-based theories can lead to finite Berry curvature in the magnon band once the Dzyaloshinskii-Moriya spin interaction is taken into account explicitly, but fail to produce non-zero thermal Hall conductivity. Assuming accidental doping by impurities and magnon scattering off of such impurity sites fails to predict skew scattering at the level of Born approximation. Local formation of skyrmion defects is also found incapable of generating magnon thermal Hall effect. Turning to spinon-based scenario, we write down a simple model by adding spin-dependent diagonal hopping to the well-known {\pi}-flux model of spinons. The resulting two-band model has Chern number in the band structure, and generates thermal Hall conductivity whose magnetic field and temperature dependences mimic closely the observed thermal Hall signals. In disclaimer, there is no firm microscopic basis of this model and we do not claim to have found an explanation of the data, but given the unexpected nature of the experimental observation, it is hoped this work could serve as a first step towards reaching some level of understanding.

Influence of the Diagonal and Off-Diagonal Electron–Phonon Interactions on the Formation of Local Polarons and Their Band Structure in Materials with Strong Electron Correlations

For systems with strong electron correlations and strong electron–phonon interaction, we analyze the electron–phonon interaction in local variables. The effects of the mutual influence of electron–electron and electron–phonon interactions that determine the structure of local Hubbard polarons are described. Using a system containing copper–oxygen layers as an example, we consider the competition between the diagonal and off-diagonal interactions of electrons with the breathing mode as the polaron band structure is formed within a corrected formulation of the polaron version of the generalized tight-binding method. The band structure of Hubbard polarons is shown to depend strongly on the temperature due to the excitation of Franck–Condon resonances. For an undoped La2CuO4 compound we have described the evolution of the band structure and the spectral function from the hole dispersion in an antiferromagnetic insulator at low temperatures with the valence band maximum at point (π/2, π/2) to the spectrum with the maximum at point (π, π) typical for the paramagnetic phase. The polaron line width at the valence band top and its temperature dependence agree qualitatively with angle-resolved photoemission spectroscopy for undoped cuprates.

The Renaissance of High-Tc Superconductivity―Discovery of Undoped Cuprate Superconductors and Revise of the Electronic Phase Diagram

The Renaissance of High-<i>T</i>_c Superconductivity―Discovery of Undoped Cuprate Superconductors and Revise of the Electronic Phase Diagram

Undoped high-Tc superconductivity in T’-La1.8Eu0.2CuO[formula omitted] revealed by 63,65Cu and 139La NMR: Bulk superconductivity and antiferromagnetic fluctuations

We performed 63,65Cu and 139La NMR measurements of T’-La1.8Eu0.2CuO4+δ (T’-LECO) with the Nd2CuO4-type structure (so-called T’-structure). As a result, we detected the 63,65Cu NMR signal under finite magnetic fields and found superconductivity without antiferromagnetic (AF) order only in the reduced T’-LECO, where excess apical oxygen atoms are properly removed. This indicates that the intrinsic ground state of the ideal T’-LECO is a paramagnetic and superconducting (SC) state. Below Tc, the Knight shift was found to rapidly decrease, which indicates the emergence of bulk superconductivity due to spin-singlet Cooper pairs in the reduced T’-LECO. In the SC state of the reduced T’-LECO, moreover, a characteristic temperature dependence of the spin-lattice relaxation rate 1/T1 was observed, which implies the existence of nodal lines in the SC gap. These findings suggest that the superconductivity in the reduced T’-LECO probably has d-wave symmetry. In the normal state of the reduced T’-LECO, on the other hand, AF fluctuations were found to exist from the temperature dependence of 1/T1T, though no clear pseudogap behavior was observed. This suggests that the AF correlation plays a key role in the superconductivity of undoped high-Tc cuprate superconductors with the T’-structure.

Resonant inelastic x-ray scattering study of spin-wave excitations in the cuprate parent compound Ca2CuO2Cl2

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.

Coexistence of Midgap Antiferromagnetic and Mott States in Undoped, Hole- and Electron-Doped Ambipolar Cuprates

We report the first observation of the coexistence of a distinct midgap state and a Mott state in undoped and their evolution in electron and hole-doped ambipolar Y_{0.38}La_{0.62}(Ba_{0.82}La_{0.18})_{2}Cu_{3}O_{y} films using spectroscopic ellipsometry and x-ray absorption spectroscopies at the O K and Cu L_{3,2} edges. Supported by theoretical calculations, the midgap state is shown to originate from antiferromagnetic correlation. Surprisingly, while the magnetic state collapses and its correlation strength weakens with dopings, the Mott state in contrast moves toward a higher energy and its correlation strength increases. Our result provides important clues to the mechanism of electronic correlation strengths and superconductivity in cuprates.

Higgs mode and magnon interactions in 2D quantum antiferromagnets from Raman scattering

We present a theory for Raman scattering on 2D quantum antiferromagnets. The microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective O(3)-model. Well within the Néel ordered phase, the Raman spectrum contains a two-magnon and a two-Higgs contribution, which are calculated diagramatically. The vertex functions for both the Higgs and magnon contributions are determined from a numerical solution of the corresponding Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in the relevant B 1g + E 2g symmetry, the contribution from the Higgs mode is strongly suppressed. Except for intermediate values of the Higgs mass, it does not show up as separate peak in the spectrum but gives rise to a broad continuum above the dominant contribution from two-magnon excitations. The latter give rise to a broad, asymmetric peak at ω ≃ 2.44 J, which is a result of magnon-magnon interactions mediated by the Higgs mode. The full Raman spectrum is determined completely by the antiferromagnetic exchange coupling J and a dimensionless Higgs mass. Experimental Raman spectra of undoped cuprates turn out to be in very good agreement with the theory only with inclusion of the Higgs contribution. They thus provide a clear signature of the presence of a Higgs mode in spin one-half 2D quantum antiferromagnets.

Effect of broken symmetry on resonant inelastic x-ray scattering from undoped cuprates

We study the magnetic excitation spectra of resonant inelastic x-ray scattering (RIXS) at the L-edge from undoped cuprates beyond the fast collision approximation. We analyse the effect of the symmetry breaking ground state on the RIXS process of the Heisenberg model by using a projection procedure. We derive the expressions of the scattering amplitude in both one-magnon and two-magnon excitation channels. Each of them consists of the isotropic and anisotropic contributions. The latter is a new finding and attributed to the long range order of the ground state. The presence of anisotropic terms is supported by numerical calculations on a two-dimensional spin cluster. We express the RIXS spectra in the form of spin-correlation functions with the coefficients evaluated on the cluster, and calculate the function in a two dimensional system within the 1/S expansion. Due to the anisotropic terms, the spectral intensities are considerably enhanced around momentum transfer q = 0 in both one-magnon and two-magnon excitation channels. This finding may be experimentally confirmed by examining carefully the q-dependence of the spectra.

Charge recombination in undoped cuprates

We theoretically analyse the process of charge recombination in the planar Mott-Hubbard insulators with the aim to explain short picosecond-range lifetime of photoexcited carriers, experimentally studied via pump-probe experiments on the undoped cuprates. The recombination mechanism consists of two essential ingredients: the formation of a metastable s-type bound holon-doublon pair, i.e. the Mott exciton, and the decay of such an excitonic state via the multimagnon emission. In spite of the large gap that requires many bosons to be emitted, latter process is fast due to large exchange scale and strong charge-spin coupling in planar systems. As the starting microscopic model we consider the single-band Hubbard model, and then more realistic three-band model for cuprates, both leading to the same minimal one. The decay rate of the exciton is evaluated numerically via the Fermi golden rule, having consistency also with the direct time-evolution calculation. The decay rate reveals exponential dependence on the ratio of the Mott-Hubbard gap and the exchange coupling - the result qualitatively reproduced also within a toy exciton-boson model.