Observation of superconductivity in H2S by nuclear resonant scattering

18-03-2016

The Meissner effect is used as proof of the superconducting nature of a material and typically demonstrated by the levitation of a magnet as the material reaches its superconducting state. A new approach was needed to study a superconductor created inside a high pressure cell. Scientists have used synchrotron Mössbauer spectroscopy at the ESRF to monitor the expulsion of the magnetic field by superconducting hydrogen sulfide at 150 GPa using 119Sn as a sensor.

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High temperature superconductivity at high pressures in hydrogen-rich systems has been predicted in numerous ab initio calculations. Recent resistivity and magnetic susceptibility measurements indicated that a superconducting transition occurs in H2S compressed to 150-190 GPa with a high onset temperature of 203 K [1]. For an unambiguous identification of the superconducting state, a direct observation of the Meissner effect in an external magnetic field is desirable.  However, the small size of the sample, contained within a diamond-anvil high-pressure cell (DAC), is the main problem for the study of the superconducting properties of samples at such high pressures.

We have developed a new method to follow the expulsion of the magnetic flux by a superconducting sample at high pressure by employing nuclear resonance scattering (NRS) at beamline ID18 [2]. A thin foil of tin enriched with the 119Sn isotope to 95% was used as the sensor of the external magnetic field. The tin foil sensor is immersed in the superconductor sample if it is a gas or a liquid at normal pressure. If the sample is solid, then the sensor could be surrounded by a powder of the superconducting sample. The sensor monitors the magnetic field via the magnetic interaction at the 119Sn nucleus using NRS of synchrotron radiation. The resonant character of NRS ensures that we acquire data only from the 119Sn sensor, with zero background from the sample environment.

We used this method to determine the superconducting properties of hydrogen sulfide at 150 GPa. The experimental setup is shown in Figure 1 for an external magnetic field applied perpendicular to the sample. When H2S is not in the superconducting state, or the state has been partially destroyed, the magnetic field penetrates into the sample volume and the 119Sn nuclear levels (nuclear spin Ig = 1/2) and exited (Iex = 3/2) are split by the magnetic field producing quantum beats in the time spectra (Figure 1a). In contrast, when H2S is in the superconducting state, the magnetic field does not penetrate to the tin foil, the 119Sn nuclear levels are not split, and the corresponding time spectra show an exponential decay (Figure 1b).

Layout of the experiment

Figure 1. Layout of the experiment. The tin foil, surrounded by compressed hydrogen sulfide, is located in a diamond anvil cell at a pressure of about 153 GPa. Pulsed synchrotron radiation excites the nuclei of the tin Mössbauer isotope 119Sn. The detection system measures the time evolution of radiation emitted by the tin nuclei in the forward direction.

To verify that superconductivity occurs in the H2S sample and not in the sensor 119Sn foil, the measurements were conducted simultaneously with two DACs.  One contained the H2S sample and the 119Sn sensor foil, and the other contained the reference 119Sn foil loaded with hydrogen (H2) as a pressure transmitting medium. The samples in both DACs were maintained under the same magnetic field and temperature conditions. To alternate between the two samples, the DACs were translated through the beam path by vertical motion of the sample stage. To crosscheck data reliability, the measurements were performed using two cryomagnet systems with different directions of the external magnetic field: one for the horizontal field directed along the X-ray beam and another one for the vertical field perpendicular to the X-ray beam. The measuring procedure was as follows: the sample was cooled in zero magnetic field down to the lowest temperature of 5 K and a magnetic field of about 0.7 T was applied. The exact values of the external field were derived from the measurements with the reference foil as 0.68 T and 0.65 T for the magnetic field perpendicular and parallel to the sample plane, respectively. The NRS spectra were then recorded at each temperature point while the temperature was increased.

Figures 2, a and b show the results obtained with the external magnetic field of 0.68 T applied along the X-ray beam, i.e., perpendicular to the sample plane. In the range of 4.7-59 K, the NRS spectra show an exponential decay, which demonstrates that the magnetic field is completely expelled from the sensor. The screening of the magnetic field at the 119Sn sensor is due to the expulsion of the field by superconducting H2S.  At 100 K, quantum beats appear indicating that the external magnetic field starts penetrating into the sensor foil and it then increases gradually above 100 K.  However, even for the data point at 120 K, the magnetic field at the sensor still does not reach the value of the external magnetic field (Figure 2a). This demonstrates that partial screening still remains up to at least 120 K.  The sample is then in the mixed superconducting state and the amount of the sample in the normal state increases with temperature. A similar trend was observed when an external magnetic field of 0.65 T was applied vertically, i.e., parallel to the sample plane (Figure 2, c and d). In this case, the partial screening of the magnetic field still remains at least up to 145 K.

Time NRS spectra

Figure 2. Time NRS spectra from 119Sn in H2S at 153 GPa (left panels) and in H2 at 150 GPa (right panels) in an external magnetic field in the horizontal (upper panels) and vertical (bottom panels) field geometry.  Symbols (blue) are the experimental data and solid lines (red) are the fits by the MOTIF software. Temperatures of the samples and the values of magnetic fields at the 119Sn nuclear site were obtained from the fits and are shown in parentheses next to the corresponding NRS spectra.

These results demonstrate that the superconducting H2S sample effectively shields the sensor from strong magnetic fields of about 0.7 T up to temperatures of 90-100 K.  The proposed method should allow the range of pressure for the study of superconductivity to be expanded up to 300 GPa.

 

Principal publication and authors
Observation of superconductivity in H2S from nuclear resonant scattering,  I. Troyan (a,b), A. Gavriliuk (b,c), R. Rüffer  (d), A. Chumakov (d,e),  A. Mironovich (c), I. Lyubutin (b),  D. Perekalin  (f), A.P. Drozdov (a), M.I. Eremets (a), Science 351, 1303-1306 (2016); doi: 10.1126/science.aac8176.
(a) Max-Planck Institute fur Chemie, Mainz (Germany)
(b) Shubnikov Institute of Crystallography of Russian Academy of Sciences, Moscow (Russia)
(c) Institute for Nuclear Research, Troitsk (Russia)
(d) ESRF
(e) National Research Center “Kurchatov Institute”, Moscow (Russia)
(f) A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, Moscow (Russia)

 

References
[1] A.P. Drozdov, M.I. Eremets, I.A. Troyan, V. Ksenofontov & S.I. Shylin, Nature 525, 73–76 (2015).
[2] R. Rüffer, A.I. Chumakov, Hyperfine interact. 97-98, 589 (1996).

 

Top image: Hydrogen sulfide at 153 GPa inside a diamond-anvil cell. The superconducting phase is the reflective central region.