Experimental evidence of the existence of a nonstationary coherent crystal state in bismuth
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Experimental Evidence of the Existence of a Nonstationary Coherent Crystal State in Bismuth O. V. Misochko Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia email: [email protected] Received July 7, 2013
Abstract—The excitation of a bismuth (Bi) crystal by intense ultrashort laser pulses at liquidhelium temper ature leads to consistent motion of atoms and variations in the electron density. At various time instants over an interval reaching several dosen picoseconds, atoms in the Bi lattice exhibit sequential pairing in the real and reciprocal spaces. This behavior may correspond to the formation of a coherent crystal—a special state of matter that combines the properties of a solid and quantum fluid. Experimental data showing evidence of the possible existence of this unusual state are presented and analyzed. DOI: 10.1134/S1063776114010129
1. INTRODUCTION Crystallization, i.e., spatial ordering of atoms accompanied by breakage of the translational symme try, can proceed in two ways [1, 2]. In the first case, which is typical of usual crystals, spontaneous viola tion of the translational symmetry is caused by repul sive forces driving each atom to stay far from its neigh bors. In the second, qualitatively different case, atoms are linked by longrange attractive forces driving atoms to form a coherent crystal—a special state of matter in which it possesses the properties of both a solid and quantum fluid. In a coherent crystal, the unit cell contains a noninteger number of atoms and a diagonal (crystalline) order (resulting from violation of spatial homogeneity) exists together with a offdiag onal order that is caused by violation of the gauge invariance [3]. As a result, the properties of usual and coherent crystals are significantly different, since peri odic variations in the distribution of the mass density and selfconsistent field, in which these masses are moving in the latter case, mutually maintain each other. This can be illustrated by comparing the charac ter of atomic motion in the two cases. In a usual crys tal, each atom moves such that it spends a minimum time at the lattice site (where its velocity reaches max imum). In contrast, the probability of finding an atom at the lattice site in a coherent crystal is at the maxi mum [2]. Thus, usual and coherent crystals are antip odes, i.e., basic forms between which all types of crys talline states occur [4]. According to the modern terminology, a coherent crystal is a quantum crystal or supersolid, i.e., a crystal possessing (under certain conditions) the properties of a superfluid [3]. The existence of this state, albeit the oretically predicted for helium more than four decades ago [5], has been only observed quite recently [6]. It
should especially be noted that the formation of a coherent crystal under equilibrium conditions was considered by some researchers as lowprobable and it was suggested that this state can be created in nonequi librium solid helium [7]
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