We present the measurements and theoretical analysis of the deformation and fragmentation of spherical liquid-metal drops by picosecond and subpicosecond laser pulses. In the experiments, 60 μm droplets of Sn-In alloy were irradiated by Ti:Sa laser pulses with a peak energy fluence of ∼100 J cm −2 . The observed evolution of the droplet shape dramatically differs from that previously reported for nanosecond pulses. Invoking 2D hydrodynamic simulations, we explain how, due to the specifics of matter dynamics in the liquid-vapor phase coexistence region, a liquid droplet is transformed into a characteristic acorn-like expanding shell with two inner cavities. High sensitivity of the measured shell parameters to the details of the equation of state and metastable dynamics suggests that such experiments offer new possibilities in exploration of thermophysical properties of metals in the region of liquidvapor phase transition.

The deformation and fragmentation of liquid metal microdroplets by intense subpicosecond Ti:sapphire laser pulses is experimentally studied with stroboscopic shadow photography. The experiments are performed at a peak intensity of 10^{14}W/cm^{2} at the target's surface, which produces shock waves with pressures in the Mbar range. As a result of such a strong impact, the droplet is transformed into a complex-shaped hollow structure that undergoes asymmetrical expansion and eventually fragments. The hollow structure of the expanding target is explained by the effects of cavitation and spallation that follow the propagation of the laser-induced shock wave.

Institute of Physics ⌽ DEUTSCHE PHYSIKALISCHE GESELLSCHAFT the presence of the SRO phase. An SRO phase is found to have lower energy than either the FM or AF phases for 0.26 p < 1. Phase separation (PS) disappears as J H → 0 but appears for any nonzero coupling. For fillings near p = 1, PS occurs between an AF with p = 1 and either an SRO or a FM phase. The stability of an SRO state at T = 0 can be understood by examining the interacting density-ofstates, which is gapped for any nonzero J H in an AF but only when J H exceeds a critical value in an SRO state. Institute of Physics ⌽ DEUTSCHE PHYSIKALISCHE GESELLSCHAFT Figure 1. A Bethe lattice with z c = 4 nearest neighbours. As shown, the Bethe lattice may be partitioned into A and B sublattices, denoted by the blue and red dots. and antiferromagnetic (AF) phases are magnetically frustrated by a Ruderman-Kittel-Kasuya-Yosida (RKKY)-like interaction between the local moments [7]. This paper uses dynamical mean-field theory (DMFT) to evaluate the magnetic instabilities and T = 0 phase diagram of the DE model. Developed in the late 1980s by Müller-Hartmann [8] and Metzner and Vollhardt [9], DMFT exploits the momentum-independence of the self-energy in infinite dimensions, where DMFT becomes formally exact. Even in three dimensions, DMFT is believed to capture the physics of correlated systems including the narrowing of electron bands and the Mott-Hubbard transition [10]. Although DMFT has been widely applied to the DE model [4, 5], [11]-[18], until now there has been no complete treatment of the phase instabilities and T = 0 phase diagram of the DE model for arbitrary J H and p.We shall study a system with a bare semicircular density-of-states (DOS) given by N 0 (ω) = (8/πW 2 )Re W 2 /4 − ω 2 . In real space, this DOS belongs to an infinite-dimensional Bethe lattice or an infinite Cayley tree with no closed loops [19]. A finite-dimensional Bethe lattice with coordination number z c = 4 is sketched in figure 1. Although the Bethe lattice lacks translational symmetry, it is quite convenient for calculations. As shown in figure 1, the Bethe lattice can be partitioned into A and B sublattices so that both FM and AF long-range orders are possible. Due to the bounds ±W/2, the DOS of the Bethe lattice more closely resembles the DOS of two-and three-dimensional systems than does the unbounded DOS of the hypercubic lattice. Indeed, pathological results have been obtained on a hypercubic lattice due to the asymptotic freedom of the quasiparticles in the tails of the Gaussian DOS [16]. As we shall see, the Bethe lattice also has the advantage that analytic results are possible in the limit of small J H , precisely the regime where controversies persist.The high-temperature, non-magnetic (NM) phases of the Heisenberg and DE models have a correlation length ξ that vanishes as z c → ∞. By contrast, the short-range ordered (SRO) states introduced in an earlier paper [20] possess some of the same characteristics as spin glasses: local magnetic order and exponentially decaying magnet...

We report an experimental and numerical investigation of the fragmentation mechanisms of micrometer-sized metal droplet irradiated by ultrashort laser pulses. The results of the experiment show that the fast one-side heating of such a droplet may lead to either symmetric or asymmetric expansion followed by different fragmentation scenarios. To unveil the underlying processes leading to fragmentation we perform simulation of liquid-tin droplet expansion produced by the initial conditions similar to those in experiment using the smoothed particle hydrodynamics (SPH) method. Simulation demonstrates that a thin heated surface layer generates a ultrashort shock wave propagating from the frontal side to rear side of the droplet. Convergence of such shock wave followed by a rarefaction tale to the droplet center results in the cavitation of material inside the central region by the strong tensile stress. Reflection of the shock wave from the rear side of droplet produces another region of highly stretched material where the spallation may occur producing a thin spallation layer moving with a velocity higher than expansion of the central shell after cavitation. It is shown both experimentally and numerically that the threshold laser intensity necessary for the spallation is higher than the threshold required to induce cavitation in the central region of droplet. Thus, the regime of asymmetrical expansion is realized if the laser intensity exceeds the spallation threshold. The transverse and longitudinal expansion velocities obtained in SPH simulations of different regimes of expansion are agreed well with our experimental data. * grigorev@phystech.edu †