K. Yamamoto

Observation of orbital waves as elementary excitations in a solid

A basic concept in solid-state physics is that when some kind of symmetry in a solid is spontaneously broken, collective excitations will arise. For example, phonons are the collective excitations corresponding to lattice vibrations in a crystal, and magnons correspond to spin waves in a magnetically ordered compound. Modulations in the relative shape of the electronic clouds in an orbitally ordered state could in principle give rise to orbital waves, or ‘orbitons’, but this type of elementary excitation has yet to be observed experimentally. Systems in which the electrons are strongly correlated—such as high-temperature superconductors and manganites exhibiting colossal magnetoresistivity—are promising candidates for supporting orbital waves, because they contain transition-metal ions in which the orbital degree of freedom is important. Orbitally ordered states have been found in several transition-metal compounds, and orbitons have been predicted theoretically for LaMnO3 (refs 4, 5). Here we report experimental evidence for orbitons in LaMnO3, using Raman scattering measurements. We perform a model calculation of orbiton resonances which provides a good fit to the experimental data.

Orbital physics (Communication arising): Experimental quest for orbital waves

We disagree with the assertion of Grüninger et al. that the Raman peaks from LaMnO3, which we interpret as being due to scattering from orbital excitation, are caused by multiphonon excitation.

Polarization selectivity of charge localization induced by a 7-fs nearly single-cycle light field in an organic metal

Polarization selectivity of light-field-induced charge localization was investigated in an organic metal alpha-(BEDT-TTF)2I3 with a triangular lattice. Dependences of transient reflectivity spectra on polarizations of the 7-fs pump and probe lights indicate that a short-range charge order (CO) is efficiently induced from the metallic phase for the pump polarization perpendicular to the 1010-type CO axis. Numerical solution of a time-dependent Schrodinger equation clarified that the 1010-CO is induced by field-induced re-distribution of charges cooperating with competing inter-site Coulomb repulsions in the triangular lattice.

Experimental quest for orbital waves

One challenge in condensed-matter physics is the experimental confirmation of a new kind of elementary excitation — orbital waves, or orbitons, which are predicted to exist in an orbitally ordered state. Saitoh et al. have observed three peaks at 160, 144 and 126 meV in the Raman scattering of orbitally ordered lanthanum manganate (LaMnO3), and interpret these as evidence of orbitons. However, we find similar peaks in the optical conductivity, σ(ω), of LaMnO3 and point out that the direct observation of orbitons in σ(ω) is prohibited by a selection rule. This suggests that the Raman peaks observed by Saitoh et al. arise from multiphonons, and that the existence of orbitons has yet to be experimentally confirmed.

Ultrafast response of plasma-like reflectivity edge in (TMTTF)2AsF6 driven by 7-fs, 1.5-cycle strong-light field

The strong light-field effect of (TMTTF)2AsF6 was investigated utilizing 1.5-cycle, 7-fs infrared pulses. The ultarfast (20 fs) and large (40%) response of the plasma-like reflectivity edge (0.7 eV) was analyzed by the changes in omega_p=sqrt(ne2/(epsilon_0*epsilon(infty)*m)} (n: number of charges in the 1/4 filled-band, m: mass of charge, epsilon(infty): dielectric constants for high-frequency and vacuum, e: elementary charge). The 3% reduction in omega_p is attributed to the 6% increase in m. Furthermore, 20 fs oscillation of omega_p in the time domain indicates that the plasma-like edge is affected by the charge gap (0.2 eV) nature. Theoretical calculations suggest that the Coulomb repulsion plays an important role in the increase in m.

Interference of the Bloch phase in layered materials with stacking shifts

In periodic systems, electronic wave functions of the eigenstates exhibit the periodically modulated Bloch phases and are characterized by their wave numbers

Capturing ultrafast magnetic dynamics by time-resolved soft x-ray magnetic circular dichroism

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Commensurate versus incommensurate charge ordering near the superconducting dome in Ir1-xPtxTe2 revealed by resonant x-ray scattering

. We theoretically address the effects of the Bloch phase in general layered materials with a stacking shift. When the interlayer shift and the Bloch wave vector

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Raman spectroscopy of the charge-orbital ordering in layered manganites

satisfy certain conditions, interlayer transitions of electrons are prohibited by the interference of the Bloch phase. We specify the manifolds in the