One of the long-standing challenges in experimental physics is the observation of room-temperature superconductivity1,2. Recently, high-temperature conventional superconductivity in hydrogen-rich materials has been reported in several systems under high pressure3–5. An  important discovery leading to room-temperature superconductivity is the pressure-driven disproportionation of hydrogen sulfide (H2S) to H3S, with a confirmed transition temperature of 203 kelvin at 155 gigapascals3,6. Both H2S and CH4 readily mix with hydrogen to form guest–host structures at lower pressures7, and are of  comparable size at 4 gigapascals. By introducing methane at low pressures into the H2S + H2 precursor mixture for H3S, molecular exchange is allowed within a large assemblage of van der Waals solids that are hydrogen-rich with H2 inclusions; these guest–host structures become the building blocks of superconducting compounds at extreme conditions. Here we report superconductivity in a photochemically transformed carbonaceous sulfur hydride system, starting from elemental precursors, with a maximum superconducting transition temperature of 287.7 ± 1.2 kelvin (about 15 degrees Celsius) achieved at 267 ± 10 gigapascals. The superconducting state is observed over a broad pressure range in the diamond anvil cell, from 140 to 275 gigapascals, with a sharp upturn in transition temperature above 220 gigapascals. Superconductivity is established by the observation of zero resistance, a magnetic susceptibility of up to 190 gigapascals, and reduction of the transition temperature under an external magnetic field of up to 9 tesla, with an upper critical magnetic field of about 62 tesla according to the Ginzburg–Landau model at zero temperature. The light, quantum nature of hydrogen limits the structural and stoichiometric determination of the system by X-ray scattering techniques, but Raman spectroscopy is used to probe the chemical and structural transformations before metallization. The introduction of chemical tuning within our ternary system could enable the preservation of the properties of room-temperature superconductivity at lower pressures. Room-temperature superconductivity is observed in a photochemically synthesized ternary carbonaceous sulfur hydride system at 15 °C and 267 GPa.

Room-temperature superconductivity in a carbonaceous sulfur hydride

An experimental setup for Fourier transform infrared (FTIR) studies in condensed matter at high pressure and low temperatures is described. We have adapted a close-cycle cryostat (T=20–300 K) to the sample compartment, which is used as a cryo chamber, of a FTIR spectrometer (frequency range 10–15 000 cm−1). A Cassegrain-type beam condenser is assembled to measure infrared absorptions of samples contained in a membrane diamond anvil cell (P up to 100 GPa). The tuning of the pressure and the cell alignment is performed from outside the evacuated instrument. An additional light path allows visual observation and in situ pressure calibration. The advantages of this system, demonstrated by its application to CH4 and Ar–(H2)2 crystals, are high radiation throughput, long time stability, visual observation of the sample, remote measurement and variation of the local pressure, and remote alignment of the cell with the IR beam.

Experimental setup for Fourier transform infrared spectroscopy studies in condensed matter at high pressure and low temperatures

We investigate CZTSe films by polarization dependent Raman spectroscopy. The main peaks at 170 cm(-1), and 195 cm(-1) are found to have A symmetry. The Raman signal at 170 cm(-1) is found to be composed of two modes at 168 cm(-1) and 172 cm(-1). We attribute these three Raman peaks to the three A symmetry modes predicted for kesterite ordered Cu(2)ZnSnSe(4). The main Raman peak is asymmetrically broadened towards lower energies. Possible sources of the broadening are tested through temperature and depth dependent measurements. The broadening is attributed to phonon confinement effects related to the presence of lattice defects.

The three A symmetry Raman modes of kesterite in Cu2ZnSnSe4.

We have measured frequencies of all internal and external modes in natural and 32S isotopically pure oriented orthorhombic single sulfur crystals using high-resolution Raman and FTIR spectroscopy. Polarized spectra at low temperatures deliver mode frequencies and allow mode assignment to vibrational, librational, and translational character. Experimental data, such as temperature dependent frequencies at constant volume, are analyzed in terms of anharmonic processes. In addition, frequency shifts at the lowest temperature due to isotopic impurities are qualitatively explained by a relative mass defect.

Mode Assignment of Sulfur α-S8 by Polarized Raman and FTIR Studies at Low Temperatures

Synthesis of sulfur films from S2 gas: spectroscopic evidence for the formation of Sn species

We have studied the temperature and pressure dependence of the vibrational energy levels of orthorhombic sulfur. Spectroscopic measurements were performed using standard Raman equipment; high pressure experiments made use of a diamond anvil cell (DAC). In general all external modes are sensitive to temperature variation in the range 50−300 K; internal modes exhibit a different behavior. It was found that mode energies were more influenced by pressure (0−5 GPa) than by temperature (12−300 K). In addition it was found that the intensities of prominent features of the Raman spectrum were pressure dependent.

Raman Studies of Sulfur Crystal (α-S8) at High Pressures and Low Temperatures

Molecular crystals1–3 are in general characterized by strong intramolecular forces and much weaker intermolecular forces. The molecular structure remains unaltered and the internal vibrations split off under the influence of the lattice field. One important property of such crystals is their large compressibility, a result of the weak external forces. Therefore pressure dependent investigations on vibrational modes are a good tool to test theoretical models, e.g. forms of potentials, approaches in lattice dynamics. As can be seen later temperature variation gives additional information about anharmonicity.

Sulphur at high pressure and low temperatures

We have investigated by means of conventional Raman spectroscopy the temperature and pressure dependence of internal and external vibrations of crystalline S6 For the low temperature measurements we used chemically prepared S6, for the high pressure experiments we produced S6, from p-S in a high pressure diamond anvil cell We systematically measured the temperature and pressure dependence of frequencies and intensities of internal and external modes and improved the mode assignment

Low temperature and high pressure Raman studies on crystalline S6

The phase transition of orthorhombic sulphur α-S8 to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S8 molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S8. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm2 at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S6 can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

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Raman Studies of Sulfur Crystal (α-S8) at High Pressures and Low Temperatures

Sulphur at high pressure and low temperatures

Low temperature and high pressure Raman studies on crystalline S6

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy