The concept of quasiparticles in solid-state physics is an extremely powerful tool for describing complex many-body phenomena in terms of single-particle excitations. Introducing a simple particle, such as an electron, hole or phonon, deforms a many-body system through its interactions with other particles. In this way, the added particle is 'dressed' or 'renormalized' by a self-energy cloud that describes the response of the many-body system, so forming a new entity--the quasiparticle. Using ultrafast laser techniques, it is possible to impulsively generate bare particles and observe their subsequent dressing by the many-body interactions (that is, quasiparticle formation) on the time and energy scales governed by the Heisenberg uncertainty principle. Here we describe the coherent response of silicon to excitation with a 10-femtosecond (10(-14) s) laser pulse. The optical pulse interacts with the sample by way of the complex second-order nonlinear susceptibility to generate a force on the lattice driving coherent phonon excitation. Transforming the transient reflectivity signal into frequency-time space reveals interference effects leading to the coherent phonon generation and subsequent dressing of the phonon by electron-hole pair excitations.

We have investigated the dynamical properties of the coherent anharmonic phonons generated in Bi under high density excitation. The time-resolved reflectivity in the intensely photoexcited Bi film is modulated by the coherent A(1g) phonon oscillation with a time-dependent oscillation period. As the pump power density is increased, the line shape of the A(1g) mode in the Fourier transformed spectra becomes asymmetric, and the redshift of the phonon frequency is observed. Analysis of the transient redshift with a wavelet transform reveals that the frequency of the A(1g) mode depends on the squared amplitude of the oscillation, which is attributed to an anharmonicity of the lattice potential.

Hase et al. Reply: The Comment from Fahy and Reis [1] presents calculations of the time-dependent A 1g phonon frequency in photoexcited Bi using the simple model of electronic softening [2], including carrier recombination and carrier diffusion effects but without anharmonicity. The calculated variation in the A 1g frequency looks similar to that observed in Fig. 4 in our Letter [3], regarding to the amplitude in arbitrary units for the 7:6 mJ=cm 2 pulse fluence. They claimed that recombination and diffusion of the electron-hole (e-h) plasma is rather more important than anharmonicity of the phonon.First of all, we have recently observed, using highdensity photoexcitation, the 2nd order and the higher harmonics of the coherent A 1g phonon in Bi at 7 K [4]. Formation of such higher harmonics of the fundamental phonon mode provides evidence of lattice anharmonicity as confirmed in ferroelectric materials [5]. Thus our data clearly show that the coherent A 1g phonon in Bi under high-density photoexcitation that we observe is really anharmonic oscillation.There are some points they could consider in addition, when comparing with the experimental data. Our observation of the time-dependent frequency of the coherent A 1g phonon in Bi includes the contribution from the electronic softening due to photoexcited e-h plasma, in addition to the anharmonicity, as discussed in our Letter [3], and the calculation by Fahy and Reis for that in photoexcited Bi using the electronic softening model is important. However, the calculation seems to be not sufficient for complete discussion on the anharmonic phonon dynamics.They use the slow e-h recombination time of 5 ps to evaluate the variation of the frequency of the coherent A 1g phonon versus the square of the oscillation amplitude. However, the e-h recombination time is much less under the high-density excitation (10 21 cm ÿ3 ). Actually it was obtained from the pump and probe measurement to be only 1 ps for Bi at 300 K. Therefore, the carrier density in the conduction bands decays within the first few cycles of the coherent phonon oscillation, which is much faster than they expect.The ambipolar diffusion constant D 5 cm 2 =s should also be corrected for the optically excited carriers in Bi. The carrier diffusion constant depends on the carrier density [6], particularly for the case of high-density excitation. They estimated D 10 cm 2 =s by considering this effect for photoexcited Te, which was significantly smaller than its equilibrium value D 40 cm 2 =s [2], but do not consider this effect for Bi in the Comment. The value of D in photoexcited Bi becomes 1:25 cm 2 =s if taking the same ratio of D excited =D equilibrium 0:25 as in Te. In this case, the e-h plasma density at the surface falls to 0.434 (1=e) of its initial value in a time l 2 =D 2:3 ps, indicating the carrier diffusion is less efficient than they expected (l 2 =D 0:6 ps), where l 17 nm is the optical penetration depth for the pump light with 800 nm.In addition to the arguments for the e-h recombination time and the...

Interference of impulsively excited coherent phonons in semimetals has been studied by using a double-pulse pump–probe technique. Enhancement of the oscillation amplitude of an A1g mode is observed when the separation time of the double-pulse is matched to the period of the phonon oscillation, and a cancellation is observed when the separation time is adjusted to half the period of the phonon oscillation. The amplitude after the second pulse shows a sinusoidal dependence as a function of the separation time, and this dependence is explained in terms of a superposition of two coherent phonon oscillations. In addition, not only the A1g mode but also an Eg mode have been observed by electro-optic sampling.

We report the ultrafast dynamics of the 47.4 THz coherent phonons of graphite interacting with a photoinduced nonequilibrium electron-hole plasma. Unlike conventional materials, upon photoexcitation the phonon frequency of graphite upshifts, and within a few picoseconds relaxes to the stationary value. Our first-principles density functional calculations demonstrate that the phonon stiffening stems from the light-induced decoupling of the nonadiabatic electron-phonon interaction by creating a nonequilibrium electron-hole plasma. Timeresolved vibrational spectroscopy provides a window on the ultrafast nonquilibrium electron dynamics. Graphite possesses highly anisotropic crystal structure, with strong covalent bonding of atoms within and weak van der Waals bonding between the hexagonal symmetry graphene sheets. The layered lattice structure translates to a quasi-two-dimensional ͑2D͒ electronic structure, in which the electronic bands disperse linearly near the Fermi level ͑E F ͒ and form pointlike Fermi surfaces. The discovery of massless relativistic behavior of quasiparticles at E F of graphene and graphite has aroused great interest in the nature of carrier transport in these materials.1-3 Because of the linear dispersion of the electronic bands in graphene, the quasiparticle mass associated with the charge carrier interaction with the periodic crystalline lattice nearly vanishes, leading to extremely high electron mobilities and unusual halfinteger quantum Hall effect.1,2 Since graphite has a quasi-2D band structure very similar to that of graphene, these electronic properties may be expressed also in graphite.The electron-phonon ͑e-p͒ interaction contributes to the carrier mass near E F and limits the high-field transport through the carrier scattering. The strong e-p interaction in graphite is a distinctive characteristic of ineffective screening of the Coulomb interaction in semimetals. 4,5 It is expressed in the phonon frequency shift by carrier doping 6 and the strong electronic renormalization of the phonon bands ͑Kohn anomalies͒.7 Time-resolved measurements on the optically generated nonthermal electron-hole ͑e-h͒ plasma in graphite provide evidence for the carrier thermalization within 0.5 ps both through electron-electron ͑e-e͒ scattering and optical phonon emission. 8 The nonthermal carriers decay nonuniformly in phase space because of the anisotropic band structure of graphite. 5,9 Quasiparticle correlations in nonthermal plasmas can also be probed from the perspective of the coherent optical phonons. In the present work, through the time-dependent complex self-energy ͑frequency and lifetime͒ of the 47 THz E 2g2 phonon of graphite, we study the transient changes in the e-p coupling induced by the optical perturbation of the nonadiabatic Kohn anomaly.To probe the ultrafast response of the coherent phonons, we perform transient anisotropic reflectivity measurements on a natural single crystal and highly oriented pyrolytic graphite ͑HOPG͒ samples. Because the phonon properties were identical, we repor...

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Multiferroics, materials in which both magnetic and electric fields can induce each other, resulting in a magnetoelectric response, have been attracting increasing attention, although the induced magnetic susceptibility and dielectric constant are usually small and have typically been reported for low temperatures. The magnetoelectric response usually depends on d-electrons of transition metals. Here we report that in [(GeTe)2(Sb2Te3)l]m superlattice films (where l and m are integers) with topological phase transition, strong magnetoelectric response may be induced at temperatures above room temperature when the external fields are applied normal to the film surface. By ab initio computer simulations, it is revealed that the multiferroic properties are induced due to the breaking of spatial inversion symmetry when the p-electrons of Ge atoms change their bonding geometry from octahedral to tetrahedral. Finally, we demonstrate the existence in such structures of spin memory, which paves the way for a future hybrid device combining nonvolatile phase-change memory and magnetic spin memory.

We have studied the A(1g) coherent phonons in bismuth generated by high fluence ultrashort laser pulses. We observed that the nonlinear regime, where the phonons' oscillation parameters depend on fluence, consists of subregimes with distinct dynamics. Just after entering the nonlinear regime, the phonons become chirped. Increasing the fluence further leads to the emergence of a collapse and revival, which next turns into multiple collapses and revivals. This is explained by the dynamics of a wave packet in an anharmonic potential, where the packet periodically breaks up and reconstitutes in its original form, giving convincing evidence that the phonons are in a quantum state, with no classical analog.