Temperature ͑5-250 K͒ and magnetic-field ͑0-70 kOe͒ variations of the low-energy ͑1-10 meV͒ electrodynamics of spin excitations have been investigated for a complete set of light-polarization configurations for a ferroelectric magnet DyMnO 3 by using terahertz time-domain spectroscopy. We identify the pronounced absorption continuum ͑1-8 meV͒ with a peak feature around 2 meV, which is electric-dipole active only for the light E vector along the a axis. This absorption band grows in intensity with lowering temperature from the spin-collinear paraelectric phase above the ferroelectric transition but is independent of the orientation of spiral spin plane ͑bc or ab͒, as shown on the original P s ͑ferroelectric polarization͒ ʈc phase as well as the magneticfield induced P s ʈ a phase. The possible origin of this electric-dipole active band is argued in terms of the large fluctuations of spins and spin current.

Optical Hall conductivity σxy(ω) is measured from the Faraday rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz frequency regime. The Faraday rotation angle (∼ fine structure constant ∼ mrad) is found to significantly deviate from the Drude-like behavior to exhibit a plateau-like structure around the Landau-level filling ν = 2. The result, which fits with the behavior expected from the carrier localization effect in the ac regime, indicates that the plateau structure, although not quantized, still exists in the terahertz regime.The quantum Hall effect (QHE), a highlight in the twodimensional electron gas (2DEG) system in strong magnetic fields [1][2][3][4], still harbors, despite its long history, a wealth of important physics. While static properties of the integer QHE have been well understood, we are still some way from a full understanding of dynamical responses in the QHE in the ac or even optical regime. In the static case, the states localized due to disorder with the localization length smaller than the sample size or the inelastic scattering length are crucial in realizing the quantum plateaus for the dc Hall current in a dc electric field [5][6][7][8][9][10][11]. On the other hand, the conventional wisdom for the dynamical response would be that an ac field will delocalize wave functions to make QHE disappear.For relatively low frequencies, the breakdown of QHE in ac fields has a long history of investigation [12]. One issue was whether the delocalization occurs for lowfrequencies (∼ 10 MHz), but the results were not conclusive. Subsequently, experimental study was extended to the microwave regime in the 1980s, where the delocalization as seen in the Hall conductivity σ xy was shown to be absent in the microwave (i.e., gigahertz) regime [13], while the gigahertz responses of the longitudinal conductivity σ xx [9,14,15] were explained with the scaling theory of localization [16]. Thus, a fundamental problem remains as to whether and how QHE is affected in the much higher, terahertz (closer to the optical) frequency regime (ω ∼ 10 12 Hz ∼ 10 −2 eV/h). This is an essential question, since the frequency is exactly the energy scale of interest (i.e., the cyclotron energyhω c ∼ 10 −2 eV for a magnetic field ∼ 10 T, which is the spacing between Landau levels, a prerequisite for QHE).Theoretically, the accurate quantization in QHE is firmly established as a topological (Chern) number [17] in the static case. However, such a picture may not be extended to the ac regime where the topological 'protection' no longer exists. Recently, Morimoto et al. [18] have theoretically examined the ac response of the disordered QHE systems based on the exact diagonalization method, and showed that a plateau-like behavior still exists in σ xy even in the terahertz energy range. This has motivated us to experimentally examine QHE by going beyond the microwave regime, which has so far remained a challenge. An essential experimental ingredient that enables the measurement is a recent development in t...

We have investigated the effect of vortex dynamics on high-frequency conductivity of a superconducting NbN film by using terahertz time-domain spectroscopy. The complex conductivity of the mixed state up to 7 T is determined without using Kramers-Kronig analysis. The experimentally obtained conductivity spectra are analyzed by considering the contribution from quasiparticles in vortices and also from the vortex dynamics. To include the local-field effect in the high-frequency electromagnetic responses of mixed-state superconductors, we combined the Maxwell-Garnett theory with a self-consistent two-fluid model and obtained the expression for the effective conductivity. The volume fraction of the vortices shows the linear dependence on the applied magnetic field, as expected for s-wave superconductors, by taking into account the flux-flow resistivity.

The magnetoresistance across interfaces in the itinerant ferromagnetic oxide SrRuO 3 have been studied. To define appropriately the interfaces, epitaxial thin films have been grown on bicrystalline and laser-patterned SrTiO 3 substrates. Comparison is made with results obtained on similar experiments using the double-exchange ferromagnetic oxide La 2/3 Sr 1/3 MnO 3. It is found that in SrRuO 3 , interfaces induce a substantial negative magnetoresistance, although no traces of the low-field spin tunneling magnetoresistance are found. We discuss these results on the basis of the distinct degree of spin polarization in ruthenates and manganites and the different nature of the surface magnetic layer formed at interfaces. S0163-18299905134-6 Half-metallic ferromagnetic oxides such as chromium dioxide or the celebrated manganese perovskites are predicted to be almost fully spin-polarized systems. This is a requisite of ideal electrodes for the newcoming generation of magne-toelectronic devices. These materials also share the notable characteristic of being double-exchange DE ferromagnets, i.e., systems with localized atomic moments coexisting with itinerant ones and where the ferromagnetic coupling is transmitted via the mobile charge carriers. Owing to their 100% polarization, spin-polarized tunnel experiments in sandwiched FM/I/FM heterostructures FM is a ferromagnet and I an insulating tunnel barrier have indeed revealed a very large resistivity change upon reversing the relative magnetization of the FM electrodes. Resistivity changes of about 85% at 5 K have been reported for La 2/3 Sr 1/3 MnO 3 /SrTiO 3 /La 2/3 Sr 1/3 MnO 3 junctions. 1 Large magnetoresistance ratios have also been reported for artificial grain boundaries either in bicrystals 2 or even in granular materials. 3-6 However, a serious drawback has appeared: there is a sharp decay of the magnetoresistance MR of the junction when rising the temperature, and it becomes vanishingly small well below the Curie temperature T C when the magnetization M b of the sample is still almost saturated. This appears to be a common trend, observed in all DE materials either manganites 2-4 or CrO 2. 5 In contrast, the tunnel magnetoresistance observed in ceramic superexchange ferromagnets, such as Tl 2 Mn 2 O 7 , displays a weaker temperature dependence and the MR follows basically the magnetization. 7 It thus appears that the mechanism of magnetic interaction has profound effects on the temperature dependence of the magnetoresistance. It has been suggested that DE materials are prone to display carrier depolarization at interfaces, which reduces the observed magnetoresistance at low fields below its ideal 100% value and promotes the sharp decay with temperature. Experiments and theoretical models indicate that this may arise from the existence of a nonferromagnetic surface layer at interfaces. 4,8-10 In superexchange ferromagnets the strength of the magnetic interaction is independent of any charge transfer, and thus ferromagnetism at interfaces is expected to be more...

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