Nonthermal phase transitions in metals

Abstract

It is well known that sufficiently thick metals irradiated with ultrafast laser pulses exhibit phonon hardening, in contrast to ultrafast nonthermal melting in covalently bonded materials. It is still an open question how finite size metals react to irradiation. We show theoretically that generally metals, under high electronic excitation, undergo nonthermal phase transitions if material expansion is allowed (e.g. in finite samples). The nonthermal phase transitions are induced via an increase of the electronic pressure which leads to metal expansion. This, in turn, destabilizes the lattice triggering a phase transition without a thermal electron-ion coupling mechanism involved. We find that hexagonal close-packed metals exhibit a diffusionless transition into a cubic phase, whereas metals with a cubic lattice melt. In contrast to covalent solids, nonthermal phase transitions in metals are not ultrafast, predicative on the lattice expansion. Ultrafast deposition of high energy density into a solid target creates far-from-equilibrium states of matter with unusual properties that are not achievable in equilibrium states 1-3. Such a material with dynamically changing properties poses fundamental difficulties for a theoretical description, as it occupies the gap in standard methods between the solid state, plasma, chemical physics, and other disciplines 4,5. Such energy deposition may be realized via irradiation of the target with intense femtosecond laser pulses, in particular from X-ray free-electron lasers (XFEL) such as e.g. LCLS 6 , SACLA 7 , EuXFEL 8 , etc. In an irradiated solid, energy deposition by photons excites primarily the electronic system 9. Excited electrons transfer their energy to the ions, which may result in material modifications 10. Upon ultrafast energy deposition into an electronic system of a material, there are two typical scenarios of material damage. Energy transfer via electronphonon (or, more generally, electron-ion) coupling leads to heating of the atomic system, which, when exceeding the melting threshold, leads to atomic disordering. Alternatively, electronic excitation may affect the interatomic potential and destabilize the lattice without any electron-ion coupling involved. Such a mechanism is known as nonthermal melting, and has been demonstrated to take place in highly excited covalently bonded materials 1,11,12. Since the original work by Recoules et al. 13 , it is generally accepted in the laser-mater interaction community that metals do not melt nonthermally. Indeed, it was shown that bulk metals typically exhibit hardening of phonon modes under intense electronic excitation, in contrast to covalently bonded materials 12-14 , materials with mixed ionic-covalent bonds 15,16 , or with Peierls distortions 17. Absence of nonthermal melting in metals was confirmed in numerous subsequent ab-initio simulations for various metals, see e.g. Ref. 18. Experimental evidence supported this conclusion 19,20. On the other hand, softening of phonon modes in gold nanospheres was reported in R...

Electron-phonon coupling in metals at high electronic temperatures

“…At volumes exceeding ∼1.1 V 0 , lattice instabilities occur, especially at elevated electronic temperatures, as was studied in detail in our previous work [92]. This in turn leads to a change of the slope of the coupling curve seen in Fig.…”

Structural instability of transition metals upon ultrafast laser irradiation

“…For example, interatomic potentials revealing a loss of equilibrium position can lead to the lattice destabilization if the change of volume is allowed [46,48,49]. Solid expansion leads systematically to the phonon softening [43,50]. Until now, homogenous relaxation has been considered in the study of lattice dynamics with phonon dispersion curves calculated for the equilibrium volume at given electron temperature V eq (T e ) [43,45].…”

Monte Carlo simulation of ultrafast nonequilibrium spin and charge transport in iron

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