Conductive Dense Hydrogen M

LETTERS PUBLISHED ONLINE: 13 NOVEMBER 2011 | DOI: 10.1038/NMAT3175

Conductive dense hydrogen M. I. Eremets* and I. A. Troyan*

Molecular hydrogen is expected to exhibit metallic properties resistance during loading at 4 K was ascribed to the transition to under megabar pressures. This metal is predicted to be metallic hydrogen, but a direct contact of the anvils could not superconducting with a very high critical temperature, Tc, of be fully excluded. The opaque anvils also precluded additional 200–400 K (ref. 1), and it may acquire a new quantum state as proof from optical measurements. Electrical measurements in a a metallic superfluid and a superconducting superfluid2. It may DAC under applied pressures up to 210 GPa at temperatures of potentially be recovered metastably at ambient pressures3. 80–295 K showed that the hydrogen sample remained insulating13. However, experiments carried out at low temperatures, Conductive hydrogen was observed in shock-wave experiments T < 100 K (refs 4,5), showed that at record pressures of at T = 3,000 K and pressures of 140 GPa (ref. 14), as well as 300 GPa, hydrogen remains in the molecular insulating state. under isentropic compressions at 400 K and 200 GPa (refs 15,16) Here we report on the transformation of normal molecular or 200 K and 300 GPa (ref. 17). However, to prove the existence hydrogen at room temperature (295 K) to a conductive of the metallic state, electrical measurements down to the lowest and metallic state. At 200 GPa the Raman frequency of temperatures are required. For such tests, static experiments with a the molecular vibron strongly decreased and the spectral DAC are indispensable. width increased, evidencing a strong interaction between Theoretical predictions of hydrogen metallization and calcula- molecules. Deuterium behaved similarly. Above 220 GPa, tions of hydrogen properties require structural knowledge. Exper- hydrogen became opaque and electrically conductive. At imentally, the structure of hydrogen at pressures up to 5.4 GPa at 260–270 GPa, hydrogen transformed into a metal as the 300 K has been determined as P63/mmc (ref. 8). This structure conductance of hydrogen sharply increased and changed (phase I) persists at least to 109 GPa at 300 K as shown by X-ray little on further pressurizing up to 300 GPa or cooling to diffraction studies18. Studies carried out at lower temperatures at least 30 K; and the sample reflected light well. The mostly with the aid of vibrational spectroscopy revealed a complex metallic phase transformed back at 295 K into molecular phase diagram. At T < 150 K and 110 GPa, phase I transforms hydrogen at ∼200 GPa. This significant hysteresis indicates to broken-symmetry phase II, which then transforms to phase III that the transformation of molecular hydrogen into a metal is at 150 GPa, which persists at least up to 300 GPa (ref. 8). X-ray accompanied by a first-order structural transition presumably diffraction studies of this phase, measured up to 183 GPa at 100 K into a monatomic liquid state. Our findings open an avenue for (ref. 19), showed that the molecules remained in the vicinity of detailed and comprehensive studies of metallic hydrogen. the hexagonal close-packed lattice point, but the structure was not Hydrogen is one of the basic materials in science, and many determined. From the theoretical side, recent ab initio random major discoveries have been made from studies of atomic and structure searches at 0 K (ref. 20) found a new C2/c structure that molecular hydrogen. Dense hydrogen has been of great interest is a good candidate for phase III because its vibrational properties since 1935, when it was predicted that molecular hydrogen could agree with the available experimental data. At P > 240 GPa and 0 K form a monatomic crystal at 25 GPa (ref. 6). Metallic hydrogen (ref. 20), other structures were predicted that have not yet been ob- has not been found at these pressures, which are now routinely served experimentally: Cmca-12, followed by Cmca (385–490 GPa) achieved in the laboratory, and the metallization pressure has been and then monatomic I41/amd. The pressure of metallization was revised upward to much higher pressures4,5,7. Data obtained at estimated to be in the 350–410 GPa range. Thus, the 400 GPa range the available megabar pressures, which may be achieved using is the most likely value judging from most experimental4,5 and diamond anvil cells (DAC), are inconsistent. Optical experiments theoretical studies7,20. This value is at the limits of experimentally showed that hydrogen darkened above 200 GPa at 77 K (ref. 8). achievable pressures using existing techniques for generating static Furthermore, absorption and reflectivity measurements at 295 K pressures using diamonds. and pressures up to 177 GPa showed that the low-frequency Our present attempts aimed at producing metallic hydrogen reflectivity of hydrogen samples increased at pressures above at pressures beyond the present limit of 300 GPa (corresponding =∼150 GPa (ref. 9). Such an increase can be interpreted as to a 13-fold compression of solid hydrogen), at which pressures an indicator of metallization. However, the absorption spectra a darkening has been observed4,5. We conducted experiments at measured up to 230 GPa at temperatures from 77 to 295 K low temperatures, T < 100 K, to avoid the well-known breaking of were incompatible with metallization10. In another experiment, the diamonds due to hydrogen diffusing into the anvils at higher hydrogen was reported to be transparent at pressures up to temperatures and destroying the diamond material. However, soon 342 GPa (ref. 11), but the sample was poorly characterized. Recent we realized that pressurizing at room temperature can be more experiments have reported the reproducible darkening of hydrogen promising. We noticed that the molecular vibron frequencies in the at pressures of 300 GPa (refs 4,5) and T =∼ 100 K, indicating that the Raman spectra decreased significantly at pressures above 200 GPa optical gap decreases to ∼2 eV. before the diamonds broke (as expected) at 220 GPa and 295 K. In early static-conductivity experiments12, solid hydrogen was Later, we covered the diamond culets with thin semi-transparent compressed between conducting diamond anvils. A drop in the layers of Cu, Au, alumina or other materials to hamper the

Max Planck Institute for Chemistry, Biogeochemistry Department, PO Box 3060, 55020 Mainz, Germany. *e-mail: [email protected]; [email protected].

LETTERS NATURE MATERIALS DOI: 10.1038/NMAT3175

a a 4,200 Hydrogen 4,000 250 GPa 3,800 b )

¬1 H2 3,600 400 )

¬1 H 3,400 300 2

3,200 220 GPa D2 Raman shift (cm 200 218 GPa Raman intensity (arb. units) 3,000 207 GPa

D2 Half-width (cm 2,800 100 173 GPa 2,600 2,000 4,000 0 0 50 100 150 200 250 300 0 100 200 300 Raman shift (cm¬1) Pressure (GPa) Pressure (GPa) bc )

248 GPa ¬1 Figure 1 | Pressure shift and broadening of the Raman H2 and D2 vibrons 4,000 at 295 K. Different red points correspond to our different pressure runs with hydrogen. a, At pressures above 200 GPa, the frequency of the hydrogen 230 GPa vibrons decreased strongly. This trend contrasted with the trend observed (arb. units) 3,500 at low temperatures (100 K; ref. 5; blue lines and squares). Deuterium Raman intensity 220 GPa exhibited similar behaviour but the bending point is at =∼220 GPa. Our data 200 400 600

(green points) collected at 295 K are shown in comparison to the available ¬1 Frequency of vibron (cm 234 GPa Raman shift (cm ) 3,000 data gathered at 77 K (ref. 8) (blue lines) and at 295 K (ref. 29) (red line). 100 150 200 250 b, The width of the hydrogen and deuterium Raman lines increased Pressure (GPa) simultaneously with the decrease in the vibron frequency. Figure 2 | Raman spectra of hydrogen at high pressures and room diffusion of hydrogen into the anvils. As a result, we were able to temperature. The spectra were gathered using a 676.4 nm excitation laser achieve pressures of up to 300 GPa at room temperature and carry line. a, New Raman lines appeared above 220 GPa, with the strongest peak out Raman and electrical conductivity measurements. We studied at 304 cm−1. b, This peak slightly shifts to lower frequencies at higher normal hydrogen, which has an equilibrium concentration of 3:1 pressures. The red vertical line helps to distinguish this shift. A large para/ortho molecules at room temperature8. intensity of the lattice Raman peaks can be a result of resonance Raman We found that at pressures above 200 GPa and at 295 K the excitation because the bandgap at 220 GPa is close to the energy of the vibron frequencies of molecular hydrogen decreased markedly and laser excitation as follows from the appearance of the photoconductivity monotonically, in contrast to low-temperature experiments, in (see Fig. 3). The Raman signal in the 1,300–1,800 cm−1 spectral range which a transition to phase III was evident from the sharp changes in originates from the stressed diamond anvils. c, The vibron (as well as the the vibron frequency4,5. At the same time, the linewidth strongly in- low-frequency peaks) shows no hysteresis at decreasing pressure from creased (Fig. 1). At pressures of 220–260 GPa, the hydrogen sample 260 GPa. The inset shows a darkening sample. The full set of photographs darkened and became completely opaque (Fig. 2 and Supplemen- on transformation of hydrogen from 170 GPa to 300 GPa is shown in tary Fig. S1). Deuterium behaved similarly (Supplementary Fig. S2): Supplementary Fig. S1. the molecular vibron frequency markedly decreased and widened at pressures =∼20 GPa higher than the corresponding pressures ob- increases because the absorption increases as the bandgap drops served with hydrogen (Fig. 1). Simultaneously with the darkening (Fig. 3a, inset). The bandgap decreased markedly because above above 220 GPa, three new Raman bands arose with a dominant peak 240 GPa hydrogen conducts without illumination, indicating that at 304 cm−1 (Fig. 2 and Supplementary Figs S3, S4). This can indi- the bandgap should be <0.7 eV; in this case a significant number of cate a phase transformation probably to the Cmca-12 phase because carriers can be excited at room temperature (kT ∼ 0.025 eV) as in of a good correspondence of some Raman peaks with the T = 0 Ge (bandgap Eg = 0.664 eV). The bandgap can be extrapolated to calculations20 (Supplementary Fig. S5). On the other hand, the zero at pressures of 250–260 GPa. The following phenomena indi- prominent 304 cm−1 mode should be infrared, not Raman active. cate that the bandgap of hydrogen indeed closed at 260–270 GPa: Therefore, the Raman peaks might relate to the P63/m structure: the photoconductivity disappeared, and instead an increase in this is the only phase in the calculations of ref. 20 (for T = 0) with resistance under illumination was observed; the Raman bands a Raman active mode for which the frequency notably decreases also disappeared with the concurrent appearance of the sample with pressure, that is in agreement with our observations (Fig. 2). reflection (Fig. 3 and Supplementary Figs S1, S7). This band closure Furthermore, the positions of these Raman peaks also reasonably is likely to be accompanied with a sharp phase transformation extrapolate the pressure dependence of rotons and phonons from because the resistance markedly dropped at a pressure of 260– phase I (ref. 21). Further studies are needed to identify the structure. 270 GPa and thereafter showed no significant change with pressure We monitored the electrical properties of the opaque state (Fig. 3a and Supplementary Fig. S6). An indication of a first-order (Fig. 3 and Supplementary Fig. S6). At a pressure above 220 GPa transition is a significant delay in the back transformation: the (the point at which the sample darkened), hydrogen conducted sample remained opaque, reflective and showed no Raman signals as soon as it was illuminated with a He–Ne laser. The observed down to =∼200 GPa (Supplementary Fig. S7). At this point, the photoconductivity indicated that the hydrogen at this pressure is sample became transparent and the molecular vibron appeared. a semiconductor with a bandgap of =∼1.96 eV (the energy of the Importantly, this represented a direct transformation: the new laser radiation)—in this case, the laser photons can excite carriers Raman peaks presumably related to the Cmca-12 or P63/m phases across the bandgap. With increasing pressure the photoconductivity did not reappear under decreasing pressure. We noticed also that

NATURE MATERIALS DOI: 10.1038/NMAT3175 LETTERS

a d.c. b 3,000 8 10 PC ) Photo-response

Ω 270 GPa

2,500 107 ) Ω Laser on off E = 8 meV Resistance ( 2,000 a

6 ) 40 10 Ω 222 GPa 228 226 231 50 100 150 200 250 30 Temperature (K) Resistance ( 231.6 105 20

Resistance (M 235 GPa 104 0 200 400 Time (s) 200 220 240 260 280 300 Pressure (GPa)

c d Electrodes: at front and back anvils

Alumina–epoxy Pt electrodes gasket Sputtered electrodes Re gasket Sample

IU 270 GPa Hydrogen U I

150 GPa 20 μm

Figure 3 | Electrical measurements of the hydrogen samples. a, Resistance of the hydrogen sample as a function of pressure and illumination. A measurable current appeared at pressures above 220 GPa every time the sample was illuminated with a He–Ne laser of ∼5 mW power: photoconductivity (PC) is shown by the dark cyan line. At P > 240 GPa the sample conducted without illumination (d.c. conductivity) while a strong photoconductive response persisted. The inset shows an increase of photoconductivity at the pressure rise in another run (hydrogen was illuminated only during the pressure measurements). The resistance decreased considerably at pressures above =∼270 GPa. b, Temperature dependence of the resistance measured after the transition at 270 GPa. The resistance increased under laser illumination. This photoresponse vanished on cooling below 130 K. In spite of apparent metallic conductivity at low temperatures (nearly zero activation energy), the measured value of resistance is high (the highest resistance of the sample is estimated as ∼300 , if taking a value of minimum conductivity for the well-studied silicon-doped phosphorus22 ∼2,000 −1 m−1) and can be explained by a significant input of contact resistance into the measurements as schematically shown in c. A cross-section of the diamond anvils for electric measurements. The sample contacts two strips of electrodes sputtered on the anvils (seen in the photographs in d)—one is shown in the plane of the drawing, another (at the opposite anvil) is in the perpendicular direction and is invisible in the drawing. The scheme (bottom) shows the directions of electrical current (I–I) and the probed electrical potential (U–U). In this quasi-four-probe arrangement, the electrical contacts (highlighted in yellow) also contribute to the measured potential. d, Photographs of the culet taken at 150 and 270 GPa through a diamond anvil in transmission mode (left) or (right) reflection mode. One electrode (Ta) was sputtered on the front anvil and another was sputtered on the opposite anvil (as seen through the transparent insulating gasket). The hydrogen sample is positioned in the centre of the culet. The sample was transparent at 150 GPa, and opaque and reflective at 270 GPa. One more run with conducting hydrogen is presented in Supplementary Fig. S7. after the transition to metal, hydrogen became softer judging from melted hydrogen (as discussed below). It should be stated that we the sharpening of the high-frequency edge of the Raman spectrum had a quasi-four-probe arrangement (Fig. 3 and Supplementary measured from the stressed diamond tip (Supplementary Fig. S7). Fig. S6) and therefore contacts between the sample and electrodes We verified whether the sample is in a metallic state by contribute to the measured resistance. Typically this is not measuring the temperature dependence of the resistance—a metal an obstacle for measurements of metallic or superconducting should conduct to the lowest temperatures—this is the only properties of many studied substances13. In the case of hydrogen, rigorous criterion to distinguish between a metal and a non-metal22. dissociation to a monatomic state is possible and the contribution A semiconductor, in contrast, has a bandgap in the electron from the contacts could be significant because protons can also energy spectrum and its conductivity exponentially decreases so participate in conductance. Unlike electron conduction, protons that it insulates at sufficiently low temperatures. We found that moving in an electric field cannot freely move through the hydrogen conducts well down to at least 30 K (at this point electrodes; instead, they accumulate at the edge of the sample the electrical contacts broke). During cooling, the resistance first or partly diffuse into the electrodes. This charged layer creates increased (1R/R =∼ 20%, but with an insignificant activation an electrical field that decreases or compensates the internal energy of 8 meV estimated from the temperature dependence of electric field in the sample. Thus, the measured potential between the resistance), then levelled, evidencing a metallic behaviour. the electrodes decreases, which manifests as an increase in the The increase in resistance at the beginning of cooling that is resistance of the d.c. measurements. To avoid interference from well known for disordered metals13 would be consisted with the boundaries and contacts we are developing more sophisticated

LETTERS NATURE MATERIALS DOI: 10.1038/NMAT3175

1,200 transforms into a quantum liquid. According to calculations25,26, the melting is accompanied by a sharp transformation from Molecular semiconducting to metallic behaviour due to the collapse of the 1,000 liquid molecular state with a volume discontinuity of =∼2% (ref. 25). This description for liquid hydrogen agrees with our experimental Metallic results on the transition at 295 K and pressure above 270 GPa, and 800 monatomic this point might represent an actual boundary between solid and liquid quantum liquid hydrogen. Melting curve 600 Methods We used type Ia diamond anvils with 20–30 µm culets, a 7.5◦ bevel and a

Temperature (K) 250–300 µm tip. We loaded 99.95% pure hydrogen into a DAC by condensation 400 at 16–20 K, or, at room temperature, in a gas loader at a pressure of 0.2 GPa. Altogether, more than 70 experimental runs with hydrogen were carried out. We observed photoconducting and conducting hydrogen in at least five pressure runs. A Raman spectrometer was equipped with a nitrogen-cooled CCD (charge-coupled 200 I III device), notch filters and edge filters. Several laser lines (671.2 nm, solid state laser, 632.8 nm line of a He–Ne laser, and the 647.1 nm and 676.4 nm lines of a krypton II laser) were used to excite the Raman spectra and to determine whether the signals 0 0 100 200 300 originated from Raman scattering or luminescence. The sputtered thin protection Pressure (GPa) layers on diamond anvils do not distort the Raman spectra as shown from a control experiment up to 240 GPa with uncovered anvils (Supplementary Fig. S4). Figure 4 | Phase diagram of hydrogen. The pressure path at room Low-temperature measurements (pressure, electric, Raman measurements) were carried out in an optical cryostat. The pressure was determined from the shift temperature in the experiments described here is indicated by the red line. in the high-frequency edge of the Raman spectrum gathered from the stressed The red circle indicates the pressure at which the sample transformed to a tip of the diamond anvil5. This scale cannot promise high accuracy because the new phase according to the Raman data, and the square indicates the stresses in the tip can vary depending on the particular geometry of the tip, the appearance of the metallic state. For lower temperatures, the boundaries gasket, the diameter of the sample and so on. We estimated the uncertainty of the ± between phases I–II–III were taken from ref. 8. The blue line indicates the pressure determination as 10 GPa above 200 GPa judging from the scattering of hydrogen vibron data measured from run to run. In addition, a highly hydrostatic low-temperature path from refs 4,5. In the higher temperature range, the sample, such as hydrogen, requires that this pressure scale be modified28. However, melting curve is plotted according to calculations in ref. 24 (blue bars). Our the experiments were carried out only below 180 GPa (ref. 28), so we used an experimental data27 for the melting of the hydrogen sample are indicated uncorrected scale5 that may overestimate the pressure by 10–20 GPa. by the green points. Above the melting line is drawn a calculated phase We used insulating gaskets based on a mixture of epoxy with different fillers, 13 boundary between the molecular and monatomic metallic liquids25 (black for example cubic boron nitride or alumina. The gasket is a good material for clamping of hydrogen, helium and other materials. Diffusion of hydrogen symbols, thin lines; the two lines were calculated using different methods). into the gasket was negligible, as we observed no Raman signal from hydrogen At P > 200 GPa the melting and the molecular–monatomic boundaries are when the focused light from the excitation laser was shifted from the sample to shown by the extrapolated lines. The black star represents conditions the gasket. The gasket continued to insulate at pressures up to at least 340 GPa, where hydrogen is in a liquid state according to the quantum Monte Carlo as verified in two runs carried out without a sample, and other runs at lower pressures. Furthermore, the diamond surface did not conduct, which we confirmed calculations30. with two electrodes sputtered at the same anvil (Supplementary Fig. S6). The gasket was supported on the outside with a metallic belt (Re or T301 steel). We techniques (for example, four-probe d.c., a.c. and impedance had an additional electrical contact with the metallic belt and thus permanently spectroscopy). These will allow us to measure the true conductivity, controlled that it did not contact with the four platinum electrical probes going to which is expected to be low, to separate the electron and proton the sample. Electrical measurements were carried out using the quasi-four-probe method, in which two leads contacted the sample (Fig. 3 and Supplementary contributions, and to search for superconductivity. Fig. S6). The current passed through two ends of the leads, and the voltage was The observed transformation to a metallic state agrees very measured at the two remaining ends. Thus, the contact resistance between the well with a simple Goldhammer–Herzfeld criterion of ‘dielectric sample and the electrodes was included in the measurements. Typically, Au, Cu catastrophe’ giving the pressure of metallization =∼230 GPa (ref. 22), or Ta electrodes 5–20 µm wide and 100 nm thick were prepared by sputtering. whereas the state-of-the-art calculations7,20,23 predict metallization Platinum foil leads contacted these electrodes outside the culet and provided at pressures much higher than 270 GPa. However, all of these contact outside the cell. = calculations were carried out for T 0 but the temperature effect for Received 27 May 2011; accepted 18 October 2011; published online hydrogen might be high already at room temperature as observed 13 November 2011 for vibrons (Fig. 1). Therefore, we consider our data within the framework of the hydrogen phase diagram (Fig. 4). Our room- References temperature run starts in phase I and lies above the domain of the 1. Ashcroft, N. W. Metallic hydrogen: A high-temperature superconductor? low-temperature phase II as well as above the critical point at the Phys. Rev. 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