234 research outputs found
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
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
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
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