Comment on “Evidence of a first-order phase transition to metallic hydrogen”

Abstract

A recent paper by Zaghoo et al. 1 presents optical data at high-pressure and high-temperature and interprets the data as evidence for a first-order phase transition to metallic hydrogen during heating. Here we argue that the presented data are contradictory with these claims. Elucidating this issue is important for building a coherent picture that is emerging as the results of theoretical calculations of various levels 2-4 and experimental investigations employing static and dynamic compression techniques 5,6 . In this context, the use of adequate probes of the electronic and chemical state is crucial. Optical probes [1] that do not address the energy dependent conductivity while making multiple references to the Drude model are highly speculative. Indeed, the available dynamic and static compression data and theoretical modeling show that study of energy dependent conductivity is important for understanding the nature of hot dense hydrogen (see Ref. 6 and Refs. therein). Moreover, the most recent investigations [3][4][5] , suggest that the critical point, which demarcates the regimes of crossover between insulating and plasma state at low densities to the first order liquidliquid transition, is above 200 GPa. Below we concentrate on inconsistencies in the interpretation of data in Ref. 1, which call for careful examination of their claims and further detailed investigations. We analyze their optical data and use finite element calculations and argue that the high-temperature state studied in Zaghoo et al. is not metallic and that the data cannot discriminate between a firstorder phase transition and a continuous phase transition, or even a bandgap drop within one phase.Observations of plateaus in temperature versus heating power are the purported evidence for a firstorder phase transition. In particular, Zaghoo et al. suggests that a 0.1 eV/molecule latent heat of dissociation of H 2 molecules is plausible and could explain the plateau. The latter argument is based on an energy balance estimate, with limited analysis of heat transport from the tungsten laserabsorber into the hydrogen medium. Our finite-element (FE) calculations of heat transport, on the other hand, show that latent heat must be orders of magnitude larger than 0.1 eV/molecule to cause plateaus similar in magnitude to those shown in the measurements of Zaghoo et al. In particular, we assume the physical properties and sample chamber geometry of Table 1, a laser pulse with a shape of Ref. 1 shown in Fig. 1, and the finite element solver of Ref. 7. We find that the latent heat of a transition at 1140 K and 170 GPa must be ~3.8 eV/molecule to reproduce the measurements of Zaghoo (Fig. 2). A latent heat of ~ 3.8 eV/molecule is implausibly high because the molecular binding energy (4.75 eV at ambient pressure), has been shown (both experimentally and theoretically) to decrease with pressure to much lower values 8,9 . We conclude that the observed plateaus are due to something besides latent heat. Fig. 2 shows that an onset of absorption at T c =1140 ...

A Spectroscopic Study of the Insulator–Metal Transition in Liquid Hydrogen and Deuterium

“…The results presented here clearly demonstrate the existence of two transformation boundaries corresponding to the formation of absorptive and reflecting hydrogen (Figure ). The one at lower P–T conditions has been established in dynamic and DAC experiments . It has been suggested that this boundary is related to a bandgap closure, rather than the plasma transition.…”

“…Transient reflectance signal in this regime shows a small, spectrally independent increase (Figure S3, Supporting Information) which can be explained by a small change in the refractive index of H 2 (D 2 ) correlated with bandgap reduction . In this regime, peak temperature measured radiometrically (Figure S4, Supporting Information) tends to increase slowly with laser power, while the duration at which the sample remains hot (and thus emits) increases (Figure S5, Supporting Information); temperature increases more rapidly at higher laser power.…”

“…The difficulty of interpreting these optical DAC experiments is due to indirect probing of the state of hydrogen, or detection of reflectance signals superimposed with those of other materials in the DAC cavity and interpreted assuming a priori a direct transformation from insulator to metal . The latter results, reporting transient reflectance and transmission at a few laser wavelengths, have been found inconsistent with the proposed IMT, while an indirect transformation via intermediate‐conductivity states is a plausible alternative . One of the major drawbacks of the majority of preceding dynamic and static experiments is an extreme paucity of robust spectroscopic observations, which are critical for assessing the material electronic properties.…”

Finite-element simulation of the liquid-liquid transition to metallic hydrogen

“…In this paper we have used the proper geometry and thermodynamic properties for the experimental demonstration of LMH [16,17], whereas earlier simulations are not relevant to this experiment [19][20][21]. We have found that the results of the simulation are quite sensitive to material properties such as thermal conductivity (TC) and latent heat, but also the optical properties of all materials in the DAC, as these play a large role in determining the absorbed laser power.…”

“…Additionally, the heating curve relating temperature to applied laser power is accompanied by the appearance of a plateau, indicating the presence of latent heat of transition. Several authors [15,[18][19][20] interpreted the plateau differently, instead relating it to a continuous change in optical parameters due to temperature dependent bandgap closure.…”