Raman fingerprint of high-temperature superconductivity in compressed hydrides

Abstract

The discovery of high-temperature superconductivity in hydrogen-rich compounds under extreme pressures has prompted great excitement, intense research, but also debate over the past decade. Electrical transport has been the primary diagnostic tool for identifying superconductivity in these systems, whereas complementary probes, including magnetic, spectroscopic, tunnelling and ultrafast methods, remain mostly qualitative due to experimental constraints and sample heterogeneity. Recent concerns over their reliability have fuelled controversy, leading to scepticism and pointing out the need for alternative, quantitative approaches. In this study, we acquired unprecedented high-quality Raman spectra of hexagonal LaH10 at approximately 145 GPa and low temperatures, in conjunction with electrical transport measurements. Upon cooling, we observe a drop of resistivity and simultaneous remarkable variations of phonon frequencies and linewidths. These effects are interpreted and perfectly reproduced by the Migdal-Eliashberg theory, providing a definitive proof of phonon-mediated superconductivity and enabling a quantitative determination of the superconducting energy gap. Our results establish Raman spectroscopy as a robust, contact-free probe with micrometric resolution for studying high temperature superconductivity, opening a powerful route to its discovery and characterization.

Figures (12)

• Figure Fig. 1: High-pressure setup and schematic evolution of Raman spectra across the SC transition.a, Schematic illustration (not to scale) of diamond anvil cell (DAC) for combined Raman and transport measurements. The hydride sample is compressed between two opposing diamond anvils, with four electrodes for resistance measurements and ammonia borane (NH$_{3}$BH$_{3}$) as pressure medium. The sample is confined by concentric gaskets: an insulating inner ring (MgO or BN), and an outer Rhenium gasket. b, c Theoretical temperature evolution of the electronic (b) and vibrational (c) Raman spectra across the SC transition at $T_{\rm c}$ predicted by the ME approach, see Methods. Upon cooling, the SC gap energy $2\Delta(T)$ opens and saturates at $2\Delta_0 \sim 4k_BT_c$. The electronic Raman spectrum develops a pair-breaking peak that blue-shifts and gains intensity below $T_c$devereaux_inelastic_2007. Phonon modes with strong electron–phonon coupling and energies near the gap edge ($\omega_{\text{ph}} \lesssim 2\Delta_0$) exhibit, upon cooling, a redshift and linewidth narrowing when $\omega_{\text{ph}} < 2\Delta(T)$, preceded by a transient broadening around $T_c$zeyher_zpb90zeyher_phonon_1988thomsen_phonon_1990FriedlCardona_PhysRevLett_1990cardona_raman_1991hardy_prb93tajima_prb00hardy_prb04kim_prl21. The redshift magnitude decreases for lower $\omega_{\text{ph}} / (2\Delta_0)$ ratios, while linewidth narrowing persists. These behaviours reflect the modification of electron–phonon scattering induced by the opening of the SC gap in the electronic density of states.
• Figure Fig. 2: Experimental investigation of La hydride.a, Synchrotron X-ray diffraction of sample 1 collected at 140 GPa on beamline ID27 (ESRF) using 0.3738 Å radiation focused to a 0.5 $\mu$m spot. The Le Bail fit (solid line) reveals a dominant hexagonal phase (P6$_3$/mmc) with minor cubic and monoclinic contributions, consistent with diffraction mapping. The refined volume of 34.45 Å$^3$ per La atom agrees with previous reports laniel_high-pressure_2022chen_high-temperature_2024, confirming LaH$_{10}$ stoichiometry. b, Electrical transport of sample 2 measured in a four-lead van der Pauw configuration Pauw_1958. The resistance drops sharply by nearly five orders of magnitude below the SC transition, reaching values indistinguishable from zero ($2 \pm 2~\mu\Omega$). The absence of hysteresis confirms a second-order phase transition, as expected for superconductivity, while a shoulder near 140 K indicates phase coexistence. The green line shows a two-$T_{\rm c}$ percolation model fit with components at $159 \pm 4\,\mathrm{K}$ and $115 \pm 15\,\mathrm{K}$ (Methods), the higher $T_{\rm c}$ matching previous reports for the hexagonal phase chen_high-temperature_2024. The temperature evolution of the resistivity of ideal Copper (magenta curve from matula_electrical_1979) is presented to underline the incompatibility of the abrupt discontinuity measured in LaH$_{10}$ with a normal metal. c, d, Raman spectra of samples 1 without leads and 2 with leads at selected temperatures. Spectra were corrected for the Bose population factors (Methods). The low-frequency sharp peaks (A-C), attributed to La vibrations, redshift and narrow on cooling. Higher frequency modes (D-E), assigned to H vibrations, also redshift but show a non-monotonic linewidth evolution, broadening just below $T_{\rm c}$ before narrowing at lower temperatures, with mode D evolving into a prominent peak below $T_{\rm c}$. All features are reproducible across thermal cycles, with pressure rising from 140 to 150 GPa (sample 1) and from 135 to 145 GPa (sample 2) during cooling. Spectra presented in c and d where obtained probing the system with laser of different wavelengths $\lambda_L$ and have different frequency resolution $\Delta\omega$. Frequency regions within $\pm$ 7 cm$^{-1}$ (c) and $\pm$ 12 cm$^{-1}$ (d) are omitted due to laser and filters artifacts.
• Figure Fig. 3: Raman signatures of superconductivity. Positions (a-c) and linewidths (d-f) of the most pronounced peaks A, D, E. Scatter points illustrate the experimental data as extracted by the multipeaks procedure detailed in the Methods, whose error bars report the fitting uncertainties. The grey vertical line at 174 K marks the critical gap temperature $T^{\Delta}_{\rm c}$ where all phonons exhibit an abrupt change in their temperature evolution. Solid lines represent the theoretical model (with $r_{\omega}=1$), illustrating its ability to capture the characteristic temperature evolution observed in the experimental spectra. Dashed lines indicate the expected behaviour of the normal metallic state, evaluated within the same theoretical framework. g, h depict the interplay between the SC condensate (hatched region up to the pair-breaking energy $2\Delta(T)$) and key spectral features for sample 1 and 2, respectively. Experimental peak positions (circles) overlay theoretical frequencies (solid lines). Theoretical linewidths are represented by coloured shaded areas. In h, the dashed purple tick labelled $T_{\rm c}$ marks the critical temperature obtained from the transport measurement in sample 2. Red dashed vertical lines in a-c, g-h indicate the temperatures at which the calculated phonon energy intersects the optical gap energy $2\Delta(T)$, nearly coinciding with the temperature where the phonon position is most strongly renormalized by electron–phonon interactions. Consequently, for each phonon mode, the superconducting gap magnitude at the crossing temperature can be experimentally inferred from the phonon peak position at its maximum redshift.
• Figure Extended Data Fig. 1: Photograph of the gasket and sample heterogeneity.a, optical top view of the DAC before sample loading. The outer ring (barely visible) marks the edge of the diamond bevel. A composite gasket, consisting of an external rhenium (Re) support and an internal MgO ring, is positioned on the bevel. The gasket core is filled with ammonia borane (NH$_3$BH$_3$), which serves as the pressure-transmitting medium and is in direct contact with the sample (see inset of Fig. \ref{['fig1:abstract']}a). b, top-view image of sample 1 inside the DAC at 150 GPa. Small circles indicate the laser spots corresponding to selected measurement positions used for Raman mapping. The iridescent background arises from the MgO layer in the gasket. Each position is labeled by a number, and the continuous yellow curve represents the corresponding Raman spectrum acquired at 75 K. Variations in crystallinity and local purity are reflected in the sharpness of the low-energy Raman peaks: positions 1, 2, 3, and 7 exhibit broader, less defined features indicative of lower crystalline quality, whereas positions 4, 5, 6, and 8 display sharper phonon modes consistent with higher crystallinity. Spectra from positions 6 and 8 were primarily used for the analysis presented in \ref{['fig3:raman_analysis']}.
• Figure Extended Data Fig. 2: Temperature evolution waterfall of Raman spectra.a,b, experimental Raman spectra for samples 1 and 2, respectively. For graphical clarity only (and not for any quantitative analysis), the spectra have been processed to reduce noise and highlight the temperature evolution of the most prominent features. Specifically, the baseline was subtracted, and the data from sample 1 (sample 2) were smoothed using an 8-point (4-point) moving average. c-e, theoretical calculations performed within the Migdal–Eliashberg (ME) framework and the parameters used for modelling sample 2 (see Methods). The temperature evolution of the vibrational (c) and electronic (d,e) contributions to the Raman signal exhibits distinct signatures governed by the evolution of the pair-breaking energy $2\Delta(T)$ (indicated by green bars and the shaded grey region). The calculated vibrational Raman spectra (c) show that peaks D and E remain visible across all temperatures. Their positions evolve discontinuously around the resonant condition $2\Delta(T)=\omega_{\rm ph}$, marking the transition between high- and low-temperature limits. The corresponding linewidths display a similar trend: at low temperature, the peaks are sharpest, while at higher temperatures they broaden as $2\Delta(T)<\omega_{\rm ph}$, reflecting enhanced electron–phonon scattering as spectral weight is released from the condensate. The electronic contribution (d,e for two impurity levels $\eta_{imp}$) features a distinct peak emerging at the pair-breaking energy $2\Delta(T)$. As temperature increases, the peak shifts to lower energy and vanishes smoothly once the superconducting gap closes above $T^{\Delta}_{\rm c}$, merging into the smooth background. This contribution displays a broad temperature dispersion near the gap-closing regime $T\sim T^{\Delta}_{\rm c}$, similar to that observed in cuprates bohm_balancing_2014, and becomes sharper at lower temperatures. The impurity level $\eta_{imp}$ acts primarily as a spectral broadening factor, as illustrated by comparing d ($\eta_{imp}=226$) and e ($\eta_{imp}=887$ cm$^{-1}$).