This article provides an overview and illustrative examples of how the electric and magnetic fields of intense terahertz transients can be used to resonantly, and even nonresonantly, control matter and light. It discusses the fundamental interaction mechanisms of intense terahertz radiation with matter.

Resonant and nonresonant control over matter and light by intense terahertz transients

Although the vast majority of high-$T_c$ cuprate superconductors are hole-doped, a small family of electron-doped compounds exists. Under investigated until recently, there has been tremendous recent progress in their characterization. A consistent view is being reached on a number of formerly contentious issues, such as their order parameter symmetry, phase diagram, and normal state electronic structure. Many other aspects have been revealed exhibiting both their similarities and differences with the hole-doped compounds. This review summarizes the current experimental status of these materials, with a goal to providing a snapshot of our current understanding of electron-doped cuprates. When possible we put our results in the context of the hole-doped compounds. We attempt to synthesize this information into a consistent view on a number of topics important to both this material class as well as the overall cuprate phenomenology including the phase diagram, the superconducting order parameter symmetry, phase separation, pseudogap effects, the role of competing orders, the spin-density wave mean-field description of the normal state, and electron-phonon coupling.

Progress and perspectives on electron-doped cuprates

A theoretical description of magneto-optical effects in metallic transition metal systems on the basis of an itinerant description for the underlying electronic structure is presented and applications to the magneto-optical Kerr effect and the circular dichroism in x-ray absorption are discussed Simple arguments based on symmetry considerations as well as electronic excitation schemes are given to show that magneto-optical effects are caused by the interplay of magnetic ordering and spin - orbit coupling The various band structure techniques developed to deal with this situation are reviewed with an emphasize on fully relativistic methods based on the Dirac equation The theoretical framework to calculate magneto-optical spectra on the basis of the underlying band structure is outlined in some detail In addition, a number of sum rules derived by various authors are presented that allow one to estimate spin and orbital magnetic moments of an absorbing atom from the magnetic x-ray dichroism spectra For the magneto-optical Kerr effect, as well as the magnetic x-ray dichroism, application of the theoretical formalism is presented for a great variety of systems, ie pure elements, compounds, alloys and multilayer systems For both kinds of spectroscopies it is demonstrated that model calculations which allow one to manipulate the exchange splitting and the spin - orbit coupling strength give valuable information for a better understanding of the rather complex spectra Concerning the sum rules, this technique together with the direct calculation of the dichroism spectra allow for a stringent test of the various assumptions on which these rules are based

https://iopscience.iop.org/article/10.1088/0034-4885/59/12/003/pdf

Magneto-optical effects in transition metal systems

Control and detection of spin order in ferromagnetic materials is the main principle enabling magnetic information to be stored and read in current technologies. Antiferromagnetic materials, on the other hand, are far less utilized, despite having some appealing features. For instance, the absence of net magnetization and stray fields eliminates crosstalk between neighbouring devices, and the absence of a primary macroscopic magnetization makes spin manipulation in antiferromagnets inherently faster than in ferromagnets. However, control of spins in antiferromagnets requires exceedingly high magnetic fields, and antiferromagnetic order cannot be detected with conventional magnetometry. Here we provide an overview and illustrative examples of how electromagnetic radiation can be used for probing and modification of the magnetic order in antiferromagnets. We also discuss possible research directions that are anticipated to be among the main topics defining the future of this rapidly developing field. An overview of how electromagnetic radiation can be used for probing and modification of the magnetic order in antiferromagnets, and possible future research directions.

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Antiferromagnetic opto-spintronics

Control and detection of spin order in ferromagnets is the main principle allowing storing and reading of magnetic information in nowadays technology. The large class of antiferromagnets, on the other hand, is less utilized, despite its very appealing features for spintronics applications. For instance, the absence of net magnetization and stray fields eliminates crosstalk between neighbouring devices and the absence of a primary macroscopic magnetization makes spin manipulation in antiferromagnets inherently faster than in ferromagnets. However, control of spins in antiferromagnets requires exceedingly high magnetic fields, and antiferromagnetic order cannot be detected with conventional magnetometry. Here we provide an overview and illustrative examples of how electromagnetic radiation can be used for probing and modification of the magnetic order in antiferromagnets. Spin pumping from antiferromagnets, propagation of terahertz spin excitations, and tracing the reversal of the antiferromagnetic and ferroelectric order parameter in multiferroics are anticipated to be among the main topics defining the future of this field.

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Antiferromagnetic opto-spintronics: Part of a collection of reviews on antiferromagnetic spintronics

Coherent control of the precession motion of magnetizations in a single crystal ${\mathrm{YFeO}}_{3}$ with double half-cycle pulse terahertz waves was demonstrated. Quasiferromagnetic (0.299 THz) and quasiantiferromagnetic (0.527 THz) precession modes were selectively excited by choosing an appropriate interval of two pulses and were observed as free induction decay (FID) signals from the spin system. By observing the circularly polarized FID signals due to ferromagnetic resonance, we also succeeded in confirming directly the energy storage in the spin system and recovery from that to the electromagnetic radiation.

Coherent Control of Spin Precession Motion with Impulsive Magnetic Fields of Half-Cycle Terahertz Radiation

Terahertz time domain spectroscopy was performed on orthoferrite ${\mathrm{ErFeO}}_{3}$. Through the emission from the two magnetic resonance modes, we succeeded in observing the spin reorientation transition. Depending on the orientation of the single crystal, the reorientation can be detected as either mode switching between the two modes or polarization change of the emission. This method enables picosecond resolved observation of the reorientation without disturbances such as electronic excitation and heating, and it is expected to open the doorway to observe ultrafast reorientation with the terahertz pulse.

Terahertz Time-Domain Observation of Spin Reorientation in Orthoferrite ErFeO 3 through Magnetic Free Induction Decay

We introduce a simple and efficient method of enhancing the terahertz field in an air plasma produced by two-color laser pulses, by inserting a specially designed dual-wavelength wave plate between the non-linear optical crystal and the plasma. Adjusting the polarization of the two laser pulses yielded an electric field of 1.4 MV/cm, which was 1.7 times as intense as that obtained from the unmodified system. Additionally, taking a dispersion of the group velocities of the two-color laser pulses into account, we discussed the validity of the enhancement factor.

High-power THz wave generation in plasma induced by polarization adjusted two-color laser pulses

Magnetic-circular-dichroism (MCD) spectra have been obtained at and around the ${\mathit{M}}_{2,3}$ core absorption edges in single crystals of ferromagnetic Ni and ferrimagnetic ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$. Furthermore, strong MCD signals were observed for the weak multiplet structures of ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ in the prethreshold region. The experiments were performed using a bulk-sensitive reflection method by utilizing directly characterized, circularly polarized synchrotron radiation in conjunction with an ultrahigh-vacuum-compatible, i.e., bakable, superconducting magnet of interesting design. The present observation of a strong MCD not only provides a sensitive method for probing the local magnetic states of magnetic substances, but also could provide valuable information concerning the 3d electronic states of these materials.

Strong magnetic circular dichroism at the M 2 , 3 edges in ferromagnetic Ni and ferrimagnetic Fe 3 O 4

We have studied the photoemission spectra of ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (0.05\ensuremath{\le}x\ensuremath{\le}0.2) using synchrotron radiation in the Cu 3p\ensuremath{\rightarrow}3d, Nd 4d\ensuremath{\rightarrow}4f, and Ce 4d\ensuremath{\rightarrow}4f core absorption regions. The valence-band satellite corresponding to ${\mathrm{Cu}}^{2+}$ shows a resonance behavior. The Nd 4f--derived photoemission spectrum shows a double-peak structure, which is characteristic of the localized 4${\mathit{f}}^{3}$ ground state and strong 4f--O 2p hybridization. The Ce 4f--derived photoemission is distributed predominantly near the top of the O 2p band. Comparing the results with those of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$, we find that the Fermi level is located only \ensuremath{\sim}0.7 eV above the top of the valence band in spite of the large (1.5--eV) band gap of ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, indicating that the Fermi level of ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ lies within the band gap of ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$.

Keita Yamaguchi

Papers

Coherent Control of Spin Precession Motion with Impulsive Magnetic Fields of Half-Cycle Terahertz Radiation

Terahertz Time-Domain Observation of Spin Reorientation in Orthoferrite ErFeO 3 through Magnetic Free Induction Decay

High-power THz wave generation in plasma induced by polarization adjusted two-color laser pulses

Strong magnetic circular dichroism at the M 2 , 3 edges in ferromagnetic Ni and ferrimagnetic Fe 3 O 4

Resonant-photoemission study of Nd2-xCexCuO4.