Sum-frequency ionic Raman scattering

In a recent report sum-frequency excitation of a Raman-active phonon was experimentally demonstrated for the first time. This mechanism is the sibling of impulsive stimulated Raman scattering, in which difference-frequency components of a light field excite a Raman-active mode. Here we propose that ionic Raman scattering analogously has a sum-frequency counterpart. We compare the four Raman mechanisms, photonic and ionic difference- and sum-frequency excitation, for three different example materials using a generalized oscillator model for which we calculate the parameters with density functional theory. Sum-frequency ionic Raman scattering completes the toolkit for controlling materials properties by means of selective excitation of lattice vibrations

Light-induced weak ferromagnetism through nonlinear magnonic rectification

Rectification describes the generation of a quasistatic component from an oscillating field, such as an electric polarization in optical rectification, or a structural distortion in nonlinear phononic rectification. Here, we present a third fundamental process for magnetization, in which spin precession is rectified along the coordinates of a nonlinearly driven magnon mode in an antiferromagnet. We demonstrate theoretically that a quasistatic magnetization can be induced by transient spin canting in response to the coherent excitation of a chiral phonon mode that produces an effective magnetic field for the spins. This mechanism, which we call nonlinear magnonic rectification, is generally applicable to magnetic systems that exhibit degenerate chiral phonon modes. Our result serves as an example of light-induced weak ferromagnetism and provides a promising avenue to creating nonequilibrium spin configurations

Giant phonon-induced effective magnetic fields in 4f4f paramagnets

We present a mechanism by which circularly driven phonon modes in the rare-earth trihalides generate giant effective magnetic fields acting on the paramagnetic $4f$ spins. With cerium trichloride (CeCl$_3$) as our example system, we calculate the coherent phonon dynamics in response to the excitation by an ultrashort terahertz pulse using a combination of phenomenological modeling and first-principles calculations. We find that effective magnetic fields of over 100 tesla can possibly be generated that polarize the spins for experimentally accessible pulse energies. The direction of induced magnetization can be inverted by reversing the polarization of the laser pulse. The underlying mechanism is a phonon analog to the inverse Faraday effect in optics and enables novel ways of achieving control over and switching of magnetic order at terahertz frequencies

Phono-magnetic analogs to opto-magnetic effects

The magneto-optical and opto-magnetic effects describe the interaction of light with a magnetic medium. The most prominent examples are the Faraday and Cotton-Mouton effects that modify the transmission of light through a medium, and the inverse Faraday and inverse Cotton-Mouton effects that can be used to coherently excite spin waves. Here, we introduce the phenomenology of the analog magneto-phononic and phono-magnetic effects, in which coherently excited vibrational quanta take the place of the light quanta. We show, using a combination of density functional theory and phenomenological modeling, that the effective magnetic fields exerted by these phono-magnetic effects on the spins of antiferromagnetic nickel oxide yield magnitudes comparable or larger than those of the opto-magnetic effects

Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope

Electron paramagnetic resonance (EPR) spectroscopy is widely employed to characterize paramagnetic complexes. Recently, EPR combined with scanning tunneling microscopy (STM) achieved single-spin sensitivity with sub-angstrom spatial resolution. The excitation mechanism of EPR in STM, however, is broadly debated, raising concerns about widespread application of this technique. Here, we present an extensive experimental study and modelling of EPR-STM of Fe and hydrogenated Ti atoms on an MgO surface. Our results support a piezoelectric coupling mechanism, in which the EPR species oscillate adiabatically in the inhomogeneous magnetic field of the STM tip. An analysis based on Bloch equations combined with atomic-multiplet calculations identifies different EPR driving forces. Specifically, transverse magnetic-field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic-field gradients drive the spin-2 Fe. Additionally, our results highlight the potential of piezoelectric coupling to induce electric dipole moments, thereby broadening the scope of EPR-STM to nonpolar species and nonlinear excitation schemes