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Register a new userTemperature dependent optical conductivity of undoped cuprates with weak exchange
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The optical conductivity sigma(omega) is calculated at finite temperature T for CuO_2 chain clusters within a pd-Hubbard model. Data at T = 300 K for Li_2CuO_2 are reanalyzed within this approach. The relative weights of Zhang-Rice singlet and triplet charge excitations near 2.5 and 4 eV, respectively, depend strongly on T, and a rather dramatic dependence of sigma(omega) on the ratio of the first to second neighbor exchange integrals is predicted. On the basis of these results, information about exchange interactionsfor frustrated edge-shared cuprates can be obtained from T-dependent optical spectra. Our results are also relevant for magnetically weakly coupled wide-gap insulators in general.
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Compounds with intermediate-size transition metals such as Fe or Mn are close to the transition between charge-transfer systems and Mott-Hubbard systems. We study the optical conductivity sigma(omega) of insulating layered LaSrFeO_4 in the energy range 0.5 - 5.5 eV from 15 K to 250 K by the use of spectroscopic ellipsometry in combination with transmittance measurements. A multipeak structure is observed in both sigma^a(omega) and sigma^c(omega). The layered structure gives rise to a pronounced anisotropy, thereby offering a means to disentangle Mott-Hubbard and charge-transfer absorption bands. We find strong evidence that the lowest dipole-allowed excitation in LaSrFeO_4 is of Mott-Hubbard type. This rather unexpected result can be attributed to Fe 3d - O 2p hybridization and in particular to the layered structure with the associated splitting of the e_g level. In general, Mott-Hubbard absorption bands may show a strong dependence on temperature. This is not the case in LaSrFeO_4, in agreement with the fact that spin-spin and orbital-orbital correlations between nearest neighbors do not vary strongly below room temperature in this compound with a high-spin 3d^5 configuration and a Neel temperature of T_N = 366 K.
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.
We study experimentally the Raman response of the undoped high-Tc parent compound $YBa_2Cu_3O_6$, and give a unified theory of the two-magnon Raman peak and optical conductivity based on the Hubbard-Holstein model with electron-phonon coupling (EPC). The Hubbard model without EPC can qualitatively account for the experimentally observed resonance of the Raman response, but only the Hubbard-Holstein model (i) reproduces asymmetry of the Raman spectrum, (ii) validates experimental visibility of the two-magnon peak, and (iii) predicts the correct shape and energy of the lower edge of the charge transfer gap in optical conductivity. Comparison of experiments with the theory gives the EPC strength $lambda$ = 0.6. This result convincingly indicates the vital role of EPC in high-Tc cuprates providing a clue to the mechanism of high-Tc.
The charge dynamics in weakly hole doped high temperature superconductors is studied in terms of the accurate numerical solution to a model of a single hole interacting with a quantum lattice in an antiferromagnetic background, and accurate far-infrared ellipsometry measurements. The experimentally observed two electronic bands in the infrared spectrum can be identified in terms of the interplay between the electron correlation and electron-phonon interaction resolving the long standing mystery of the mid-infrared band.
Using low-energy projection of the one-band t-t-t-Hubbard model we derive an effective spin-Hamiltonian and its spin-wave expansion to order 1/S. We fit the spin-wave dispersion of several parent compounds to the high-temperature superconducting cuprates: La2CuO4, Sr2CuO2Cl2 and Bi2Sr2YCu2O8. Our accurate quantitative determination of the one-band Hubbard model parameters allows prediction and comparison to experimental results of measurable quantities such as staggered moment, double occupancy density, spin-wave velocity and bimagnon excitation spectrum and density of states, which is discussed in relation to K-edge RIXS and Raman experiments.
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