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We compare the ultrafast dynamics of the spin reorientation transition in the orthoferrite ${\mathrm{Sm}}_{0.7}{\mathrm{Er}}_{0.3}{\mathrm{FeO}}_{3}$ following two different pumping mechanisms. Intense few-cycle pulses in the midinfrared selectively excite either the $f\text{\ensuremath{-}}f$ electronic transition of ${\mathrm{Sm}}^{3+}$ or optical phonons. With phonon pumping, a finite time delay exists for the spin reorientation, reflecting the energy transfer between the lattice and $4f$ system. In contrast, an instantaneous response is found for resonant $f\text{\ensuremath{-}}f$ excitation. This suggests that $4f$ electronic pumping can directly alter the magnetic anisotropy due to the modification of $4f\text{\ensuremath{-}}3d$ exchange at femtosecond timescales, without involving lattice thermalization.

We report comprehensive temperature and doping dependences of the Raman scattering spectra for ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$ ($x=0$, 0.07, 0.24, 0.32, and 0.38), focusing on the nematic fluctuation and the superconducting responses. With increasing $x$, the bare nematic transition temperature estimated from the Raman spectra reaches $T=0$ K at the optimal doping, which indicates a quantum critical point (QCP) at this composition. In the superconducting compositions, in addition to the pair-breaking peaks observed in the ${A}_{1g}$ and ${B}_{1g}$ spectra, another strong ${B}_{1g}$ peak appears below the superconducting transition temperature, which is ascribed to the nematic resonance peak. The observation of this peak indicates significant nematic correlations in the superconducting state near the QCP in this compound.

$\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ has an exotic physical property, i.e., the ${C}_{2}$ spin-density-wave phase suddenly changes to the ${C}_{4}$ magnetic phase, and superconductivity is suppressed within a narrow composition range around $x=0.25$. We have investigated the doping dependence of the electronic structure of $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ using angle-resolved photoemission spectroscopy (ARPES), and we found an anomaly in the superconducting (SC) gap structure only for $x=0.25$ in both the hole and electron Fermi surfaces. We also propose that the mechanism of a newly observed double-resonance peak by inelastic neutron scattering is understandable because of the SC gap obtained by ARPES. Our discovery reveals the important relationship between the ${C}_{4}$-magnetic phase and superconductivity, and it provides an opportunity to survey the underlying relationship between the SC gap and the resonance mode in the hole-doped iron-based superconductor.

In this paper, we investigated the effects of annealing on the physical properties of $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$ and $\mathrm{BaF}{\mathrm{e}}_{2}{(\mathrm{A}{\mathrm{s}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$ single crystals systematically. In both systems, the effects of annealing on the phase transition temperatures and the critical current density ${J}_{\mathrm{c}}$ are contrasting; the superconducting and magnetostructural transition temperatures ${T}_{\mathrm{c}}$ and ${T}_{\mathrm{sN}}$, respectively, increased for all doping levels after annealing, whereas the critical current density ${J}_{\mathrm{c}}$ only increased around the optimally doped region while it decreased in the other regions. These facts indicate that the changes in ${T}_{\mathrm{c}}$ and ${J}_{\mathrm{c}}$ caused by annealing are governed by different mechanisms and that the mechanism for ${J}_{\mathrm{c}}$ varies depending on the doping level. The results demonstrate that the enhancement of ${T}_{\mathrm{c}}$ and ${T}_{\mathrm{sN}}$ as well as the decrease of ${J}_{\mathrm{c}}$ can be attributed to the reduction of disorder, while the enhancement of ${J}_{\mathrm{c}}$ is associated with the enhanced pinning arising from the antiferromagnetic-orthorhombic phase mixed into the superconducting phase.

We exploit an intense terahertz magnetic near field combined with femtosecond laser excitation to break the symmetry of photoinduced spin reorientation paths in ErFeO_{3}. We succeed in aligning macroscopic magnetization reaching up to 80% of total magnetization in the sample to selectable orientations by adjusting the time delay between terahertz and optical pump pulses. The spin dynamics are well reproduced by equations of motion, including time-dependent magnetic potential. We show that the direction of the generated magnetization is determined by the transient direction of spin tilting and the magnetic field at the moment of photoexcitation.

The phase diagram of iron-based superconductors exhibits structural transitions, electronic nematicity, and magnetic ordering, which are often accompanied by an electronic in-plane anisotropy and a sharp maximum of the superconducting critical current density (${J}_{\text{c}}$) near the phase boundary of the tetragonal and the antiferromagnetic-orthorhombic phase. We utilized scanning Hall-probe microscopy to visualize the ${J}_{\text{c}}$ of twinned and detwinned $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ ($x=5%--8%$) crystals to compare the electronic normal state properties with superconducting properties. We find that the electronic in-plane anisotropy continues into the superconducting state. The observed correlation between the electronic and the ${J}_{\text{c}}$ anisotropy agrees qualitatively with basic models, however, the ${J}_{\text{c}}$ anisotropy is larger than predicted from the resistivity data. Furthermore, our measurements show that the maximum of ${J}_{\text{c}}$ at the phase boundary does not vanish when the crystals are detwinned. This shows that twin boundaries are not responsible for the large ${J}_{\text{c}}$, suggesting an exotic pinning mechanism.

The phase diagram of LaFeAs$_{1-x}$P$_x$O system has been extensively studied through hole- and electron-doping as well as As/P-substitution. It has been revealed that there are three different superconducting phases with different Fermi surface (FS) topologies and thus with possibly different pairing glues. One of them is well understood as spin fluctuation-mediated superconductivity within a FS nesting scenario. Another one with the FSs in a bad nesting condition must be explained in a different context such as orbital or spin fluctuation in strongly correlated electronic system. In both phases, $T$-linear resistivity was commonly observed when the superconducting transition temperature $T_{\rm c}$ becomes the highest value, indicating that the strength of bosonic fluctuation determines $T_{\rm c}$. In the last superconducting phase, the nesting condition of FSs and the related bosonic fluctuation are moderate. Variety of phase diagram characterizes the multiple orbital nature of the iron-based superconductors which are just near the boundary between weak and strong correlation regimes., 18 pages, 9 figures, accepted in PRB

The charge dynamics of $\mathrm{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{TM}}_{x}{)}_{2}{\mathrm{As}}_{2}$ (TM = Mn and Cr) has been investigated by optical spectroscopy. It was found that in addition to the strong suppression of the coherent charge transport, the magnetic impurity induces a novel spectral feature related to the carrier localization in the far-infrared region above the antiferromagnetic transition temperature. We attribute it to the cooperative effect between conduction electrons and local magnetic order induced by magnetic impurities. The present results demonstrate that Mn and Cr are not conventional magnetic pair breakers in iron pnictides.

Phonons in iron pnictide materials were predicted to show large effects from magnetic ordering that had not been observed, and, indeed, the overall phonon dispersion has been in poor agreement with \textit{ab initio} calculations. By carefully detwinning a sample of SrFe${}_{2}$As${}_{2}$, clear phonon splitting was observed below ${T}_{N}$, allowing the authors to suggest an improved model for the phonon response for both the antiferromagnetically ordered phase below ${T}_{N}$ and the paramagnetic phase above ${T}_{N}$.

We performed a laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of the ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ (BaK) iron-based superconductor. We observed a robust low-binding energy (BE) kink structure (kink1) in the dispersion which is doping dependent whereby its energy peaks at the optimally doped level $(x\ensuremath{\sim}0.4)$ and decreases towards the underdoped and overdoped sides. We attribute this kink to electron-mode coupling in good agreement with the inelastic neutron scattering (INS) and scanning tunneling microscopy (STM) results on the same compound where a similar bosonic mode associated with spin excitations was observed. The relation between the mode energy $(\ensuremath{\Omega})$ and the SC transition temperature $({T}_{c})$ deduced from our laser ARPES data follow the universal relation deduced from INS and STM. In addition, we could resolve another kink at higher BE (kink2) showing less doping and temperature dependence compared to kink1 and which thus may be of different origin.

We demonstrate enhancement of the spin precession of orthoferrite ${\mathrm{ErFeO}}_{3}$ using the magnetic near-field produced by a split-ring resonator (SRR), using the terahertz pump-optical Faraday probe measurement. The precession amplitude was enhanced by \ensuremath{\sim}8 times when the resonance frequency of spin precession was close to the magnetic resonance of SRR. The time evolution of spin precession was successfully reproduced by a coupled spin- and SRR-resonance model mediated by the magnetic near-field. It is suggested that optimization of the metamaterial structure would further increase the enhancement factor, leading to the nonlinear control of spin dynamics using terahertz radiation.

Ultrafast dynamics of holes in a solid is usually hard to observe directly because the optical transition includes both electron and hole dynamics. In this paper we show an approach combining femtosecond luminescence and photoemission spectroscopy, successfully applied to graphite. Ultrafast infrared luminescence is observed in graphite and ascribed to recombination luminescence, though graphite is a semimetal. The dynamics of holes is deduced by taking a ratio between the time-resolved luminescence and photoemission data recorded on the same sample, and shown to be very close to that of electrons.

The structural, electronic, and optical properties of pyrochlore-type Pb(2)Ir(2)O(6)O(0.55)('), which is a metal without spatial inversion symmetry at room temperature, were investigated. Structural analysis revealed that the structural distortion relevant to the breakdown of the inversion symmetry is dominated by the Pb-O' network but is very small in the Ir-O network. At the same time, gigantic second-harmonic generation signals were observed, which can only occur if the local environment of the Ir 5d electrons features broken inversion symmetry. First-principles electronic structure calculations reveal that the underlying mechanism for this phenomenon is the induction of the noncentrosymmetricity in the Ir 5d bands by the strong hybridization with O' 2p orbitals. Our results stimulate theoretical study of inversion-broken iridates, where exotic quantum states such as a topological insulator and Dirac semimetal are anticipated.

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.

Photoinduced phase switching dynamics between LTP (low-temperature phase: Fe${}^{2+}$-CN-Mn${}^{3+}$) and PIHTP (photoinduced high-temperature phase: Fe${}^{3+}$-CN-Mn${}^{2+}$) in rubidium manganese hexacyanoferrate at 4 K was investigated by visible pump midinfrared probe transient absorption spectroscopy. By monitoring the CN stretching vibration modes, which are sensitive to the valence states of the adjacent metal ions, we could qualify not only LTP and PIHTP but also the phase boundary configurations (Fe${}^{2+}$-CN-Mn${}^{2+}$ and Fe${}^{3+}$-CN-Mn${}^{3+}$). In addition, by irradiating with another light inducing the reverse process to the pump light, we were able to apply a pump-probe measurement to the persistent phase transitions. The electronic and structural dynamics in the picosecond region were understood with a phenomenological spectral fitting model. In particular, in both directions of these phase transitions, the instantaneous generation of the boundary configuration was observed. This observation suggests that relatively small domains and/or low-dimensional fjord-like domains are created at the early stage of the transition.

We performed a laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba${}_{1\ensuremath{-}x}$K${}_{x}$Fe${}_{2}$As${}_{2}$ (BaK) and precisely determined the doping evolution of the superconducting gaps in this compound. The gap size of the outer hole Fermi-surface (FS) sheet around the Brillouin zone (BZ) center shows an abrupt drop with overdoping (for $x\ensuremath{\gtrsim}$ 0.6) while the inner and middle FS gaps roughly scale with ${T}_{c}$. This is accompanied by the simultaneous disappearance of the electron FS sheet with similar orbital character at the BZ corner. These results indicate the different contributions of ${X}^{2}\ensuremath{-}{Y}^{2}$ and $XZ/YZ$ orbitals to superconductivity in BaK and can hardly be completely reproduced by the available theories on iron-based superconductors.

We introduced a weak prepulse 0--2 ns before the main pulse in the formation of a terahertz (THz) wave in air plasma by laser pulses composed of fundamental and second-harmonic waves. The prepulse suppressed the THz wave generated by the main pulse. We analyzed the suppression factor as a function of the time interval between the prepulse and the main pulse and found that the THz wave from the main pulse decreases immediately after the prepulse, recovers partially within several hundred picoseconds, and then recovers slowly. This suppression is attributed to the dense plasma, which absorbs and screens the generated THz wave, since the relaxation time is close to the lifetime of the fluorescence from the plasma. The results support the idea that the major mechanism of THz wave generation is explained by the plasma current model.

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.

Subpicosecond dynamics of resonant secondary emission from the $F$ center in KCl at 5 K is investigated by frequency upconversion method. Detected photon energy range is from 0.65 to 1.55 eV and covers almost all the stationary emission band. The decay time of the luminescence intensity is longer in the low-energy side of emission band than in the high-energy side. A series of transient luminescence spectra shows relatively high intensities in the low-energy side. These two features indicate the existence of the second $2s\text{\ensuremath{-}}2p$ level crossing near the relaxed excited state.

Nuclear wave-packet dynamics in the excited state of KI $F$ centers is investigated by using time-resolved luminescence spectroscopy based on the up-conversion technique. The observed transient spectrum involves an oscillatory and a nonoscillatory component. The origins of these components are interrogated by comparing the results with a model calculation based on the resonant secondary radiation theory. The oscillatory part reflects the coupling of the $2s$-like electron mainly to the totally symmetric local mode around the $F$ center. The nonoscillatory part reflects the incoherent population of the vibrational levels in the $2p$-like state, which couples to many bulk phonon modes with a broad density of state. The model calculation suggests that the transition probability to the $2p$-like state is approximately 0.05.

Generation of high-order coherent confined LO phonon modes by 20-fs ultrashort laser pulse irradiation was observed in a new class of nonpolar semiconductors; Ge/Ge isotope superlattices. The phonon oscillations were Fourier transformed and compared with a theoretical calculation based on a planar force-constant model and a bond polarizability approach. The comparison between the calculated and Fourier transformed spectra shows clearly that the amplitudes of coherent phonons are determined solely by the degree of the atomic displacement and that only the Raman active odd-number-order modes are observable. The spectra taken with variety of polarization of pump beam show clearly that the generation mechanism of coherent phonons in nonpolar semiconductor like Ge is stimulated Raman scattering.