摘要 :
We report on a systematic excitation-density-dependent all-optical femtosecond time-resolved study of the spin density wave state in iron-based superconductors. The destruction and recovery dynamics are measured by means of the st...
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We report on a systematic excitation-density-dependent all-optical femtosecond time-resolved study of the spin density wave state in iron-based superconductors. The destruction and recovery dynamics are measured by means of the standard and a multipulse pump-probe technique. The experimental data are analyzed and interpreted in the framework of an extended three-temperature model. The analysis suggests that the optical-phonon energy relaxation plays an important role in the recovery of almost exclusively electronically driven spin density wave order.
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We systematically investigate temperature- and spectrally dependent optical reflectivity dynamics in AAs_2Fe_2, (A = Ba, Sr, and Eu), iron-based superconductors parent spin-density wave (SDW) compounds. Two different relaxation pr...
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We systematically investigate temperature- and spectrally dependent optical reflectivity dynamics in AAs_2Fe_2, (A = Ba, Sr, and Eu), iron-based superconductors parent spin-density wave (SDW) compounds. Two different relaxation processes are identified. The behavior of the slower process, which is strongly sensitive to the magnetostructural transition, is analyzed in the framework of the relaxation-bottleneck model involving magnons. The results are compared to recent time-resolved angular photoemissiori results (TR-ARPES) and possible alternative assignment of the slower relaxation to the magnetostructural order parameter relaxation is discussed.
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Here we report that femtosecond laser pulses are able to trigger oscillations of the magneto-optical Faraday rotation in the ferromagnetic semiconductor CdCr_2Se_4 in the presence of an applied magnetic field. The frequency of the...
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Here we report that femtosecond laser pulses are able to trigger oscillations of the magneto-optical Faraday rotation in the ferromagnetic semiconductor CdCr_2Se_4 in the presence of an applied magnetic field. The frequency of these oscillations is a linear function of the magnetic field and corresponds to the ferromagnetic resonance (FMR). Tuning the photon energy of the pump pulses we reveal two different mechanisms, which induce FMR precession in this material. In the case of pumping from the valence band deep into the conduction band (photon energy 3.1 eV), the phase of the spin oscillations is not sensitive to the polarization of the pump, but can be reversed over 180 deg by changing the polarity of the applied magnetic field.We assign these oscillations to the coherent spin precession triggered by ultrafast laser-induced heating. This mechanism requires a strong optical absorption in the material and becomes inactive if the pump photon energy is below the band gap. Tuning the photon energy in a wide range from 0.88 to 2.1 eV reveals the second mechanism of optical excitation of coherent spin oscillations with a maximum around 1.2 eV, i.e., very close to the energy of the band gap in the semiconductor. Contrary to the laser-induced heating, this excitation mechanism is pump polarization dependent, being the most efficient if the pump is circularly polarized. The phase of the spin oscillations is independent of the polarity of the applied magnetic field, but changes by 180 deg under reversing the helicity of light.We suggest that the effect can be interpreted in terms of spin transfer torque experienced by the network of the ordered Cr~(3+) spins as a result of excitation of electrons from the top of the p-type valence band to the bottom of the s-type conduction band. In particular, a strong spin-orbit interaction experienced by the carriers in the valence band is responsible for the coupling of the spins of the photogenerated carriers and the polarization of li
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We investigated temperature- and fluence-dependent dynamics of the time-resolved optical reflectivity in undoped spin-density-wave (SDW) and doped superconducting (SC) EuFe_2(As, P)_2 with emphasis on the ordered Eu~(2+) spin temp...
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We investigated temperature- and fluence-dependent dynamics of the time-resolved optical reflectivity in undoped spin-density-wave (SDW) and doped superconducting (SC) EuFe_2(As, P)_2 with emphasis on the ordered Eu~(2+) spin temperature region. The data indicate that in EuFe_2(As,P)_2 the SDW order coexists at low temperature with the SC and Eu~(2+)-ferromagnetic order. Increasing the excitation fluence leads to a slow thermal suppression of the Eu~(2+) spin order due to the crystal-lattice heating on a nanosecond time scale while the SDW order is suppressed nonthermally on a subpicosecond time scale at a higher fluence.
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Ultrafast laser excitation of the ferromagnetic semiconductor InMnAs is shown to trigger spin precession with the largest amplitude reported for magnetic semiconductors so far. To reveal the electronic transitions mediating the co...
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Ultrafast laser excitation of the ferromagnetic semiconductor InMnAs is shown to trigger spin precession with the largest amplitude reported for magnetic semiconductors so far. To reveal the electronic transitions mediating the coupling between light and spins, we compared the spin dynamics triggered by short terahertz (photon energy 5 meV) and midinfrared (photon energy 500 meV) pulses. The experiments reveal that terahertz pump pulses excite qualitatively similar spin dynamics, but are 100 times more energy efficient than the mid-IR pulses. This finding shows that in a semiconductor with hole-mediated ferromagnetism intraband electronic transitions mediate ultrafast and the most efficient coupling between light and spins.
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Temperature and fluence dependence of the 1.55-eV optical transient reflectivity in BaFe_2(As_(1-x)P_x)_2 was measured and analyzed in the low and high excitation density limit. The effective magnitude of the superconducting gap o...
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Temperature and fluence dependence of the 1.55-eV optical transient reflectivity in BaFe_2(As_(1-x)P_x)_2 was measured and analyzed in the low and high excitation density limit. The effective magnitude of the superconducting gap of ～5 meV obtained from the low-fluence-data bottleneck model fit is consistent with the angle-resolved photoemission spectroscopy results for the γ- and β-hole Fermi surfaces. The superconducting state nonthermal optical destruction energy was determined from the fluence dependent data. The planar optical destruction energy density scales well with T_c~2 and is found to be similar in a number of different layered superconductors.
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Employing temperature dependent time-resolved optical femtosecond spectroscopy, we investigated the quasiparticle and Eu~(2+) spin relaxation dynamics in EuFe_2As_2 (EFA). As previously reported in other undoped iron-based pnictid...
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Employing temperature dependent time-resolved optical femtosecond spectroscopy, we investigated the quasiparticle and Eu~(2+) spin relaxation dynamics in EuFe_2As_2 (EFA). As previously reported in other undoped iron-based pnictides, we observe the quasiparticle relaxation bottleneck due to the charge gap opening in the spin density wave (SDW) state below T_(SDW) = 189 K. Below the Eu~(2+) antiferromagnetic (AFM) spin ordering temperature, T_(AFM) = 19 K, we observe another slower relaxation component, which we attribute to the Eu~(2+) AFM order dynamics. The slow dynamics of this component suggests a weak coupling between the Eu~(2+) spins and the carriers in the Fe-d derived bands.
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Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coup...
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Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coupling between the itinerant superconducting electrons and the localized ordered spins. In some iron based pnictide superconductors the coexistence of ferromagnetism and superconductivity has been clearly demonstrated. The nature of the coexistence, however, remains elusive since no clear understanding of the spin structure in the superconducting state has been reached and the reports on the coupling strength are controversial. We show, by a direct optical pump-probe experiment, that the coupling is weak, since the transfer of the excess energy from the itinerant electrons to ordered localized spins is much slower than the electron-phonon relaxation, implying the coexistence without the short-lengthscale ferromagnetic order parameter modulation. Remarkably, the polarization analysis of the coherently excited spin wave response points towards a simple ferromagnetic ordering of spins with two distinct types of ferromagnetic domains.
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