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We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm-1 at temperatures between 4 and 300 K. A symmetry allowed on-site Mn $d$-$d$ transition near 1.7 eV is observed to blue shift (~0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in temperature dependence of the quasi-static dielectric constant epsilon_0 below the T_N ~90 K is overwhelmingly dominated by the phonon contributions.
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SrTiO$_3$ is a model perovskite compound with unique properties and technological relevance. At 105 K it undergoes a transition from a cubic to a tetragonal phase with characteristic antiferrodistortive rotations of the TiO$_6$ octahedra. Here we study systematically the effect of different exchange correlation functionals on the structural, electronic and optical properties of cubic and tetragonal STO by comparing the recently implemented strongly constrained and appropriately normed (SCAN) meta-GGA functional with the generalized gradient approximation (PBE96 and PBEsol) and the hybrid functional (HSE06). SCAN is found to significantly improve the description of the structural properties, in particular the rotational angle of the tetragonal phase, comparable to HSE06 at a computational cost similar to GGA. The addition of a Hubbard $U$-term (SCAN+$U$, $U=7.45$ eV) allows to achieve the experimental band gap of 3.25 eV with a moderate increase in the lattice constant, whereas within GGA+$U$ the gap is underestimated even for high $U$ values. The effect of the exchange-correlation functional on the optical properties is progressively reduced from 1.5 eV variance in the onset of the spectrum in the independent particle picture to 0.3 eV upon inclusion of many-body effects within the framework of the $GW$ approximation (single-shot $G_0W_0$) and excitonic corrections by solving the Bethe-Salpeter equation (BSE). Moreover, a model BSE approach is shown to reproduce the main features of the optical spectrum at a lower cost compared to $G_0W_0$+BSE. Strong excitonic effects are found in agreement with previous results and their origin is analyzed based on the contributing interband transitions. Last but not least, the effect of the tetragonal distortion on the optical spectrum is discussed and compared to available experimental data.
The (111) surface of noble metals is usually treated as an isolated two dimensional (2D) triangular lattice completely decoupled from the bulk. However, unlike topological insulators, other bulk bands cross the Fermi level. We here introduce an effective tight-binding model that accurately reproduces results from first principles calculations, accounting for both surface and bulk states. We numerically solve the many-body problem of two quantum impurities sitting on the surface by means of the density matrix renormalization group. By performing simulations in a star geometry, we are able to study the non-perturbative problem in the thermodynamic limit with machine precision accuracy. We find that there is a non-trivial competition between Kondo and RKKY physics and as a consequence, ferromagnetism is never developed, except at short distances. The bulk introduces a variation in the period of the RKKY interactions, and therefore the problem departs considerably from the simpler 2D case. In addition, screening, and the magnitude of the effective indirect exchange is enhanced by the contributions from the bulk states.
The optical conductivity of the undoped PrMnO$_3$ manganite has been investigated in details at various temperatures between 300 and 4 K. Its low energy spectrum exhibits an optical gap, and is characterized by a single broad peak centered at $sim$ 2 eV. This peak is interpreted in terms of an indirect interband transition between the split bands $e_text{g}$ caused by a strong electron Jahn-Teller phonon coupling. The spectral weight of this transition is found to be related to the magnetic ordering, which consists of ferromagnetic planes coupled antiferromagnetically. We show that such a 2D ferromagnetism plays, via the double exchange interaction, an essential role in the electronic properties of PrMnO$_3$, which is a 3D antiferromagnetic compound. Finally, an excess of optical spectral weight is found above the Neel temperature, and is attributed to ferromagnetic fluctuations. A signature of such fluctuations is equally found from electron spin resonance experiments.
We investigate the influence of biquadratic exchange interactions on the low-lying excitations of a S=1/2-ladder using perturbation theory, numerical diagonalization of finite systems and exact results for ladders with matrix product ground states. We consider in particular the combination of biquadratic exchange interactions corresponding to ring exchange on the basic ladder plaquette. We find that a moderate amount of ring exchange reduces the spin gap substantially and makes equal bilinear exchange on legs and rungs consistent with experimentally observed spectra.
In recent years, the optical control of exchange interactions has emerged as an exciting new direction in the study of the ultrafast optical control of magnetic order. Here we review recent theoretical works on antiferromagnetic systems, devoted to i) simulating the ultrafast control of exchange interactions, ii) modeling the strongly nonequilibrium response of the magnetic order and iii) the relation with relevant experimental works developed in parallel. In addition to the excitation of spin precession, we discuss examples of rapid cooling and the control of ultrafast coherent longitudinal spin dynamics in response to femtosecond optically induced perturbations of exchange interactions. These elucidate the potential for exploiting the control of exchange interactions to find new scenarios for both faster and more energy-efficient manipulation of magnetism.
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