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High Temperature Superconductivity Reported at -23°C (250K) and 170 GPa

The discovery of superconductivity at room temperature has been a long-standing goal of physicists and material scientists for decades. However, scientifically verifying superconductivity is a difficult task. So much so, that the history of the search is littered with unverified claims of superconductivity somewhat euphemistically called Unidentified Superconducting Objects (U.S.O.’s).

Last month, researchers as the Max Planck Institute of Chemistry reported superconductivity with a record Tc of about 250K within the Fm3m structure of LaH10 at a pressure P of about 170 GPa. The above image shows a device from the Max Planck Institute used to put samples at high pressure. Diamond is the only material that can resist the pressure, hence it uses industrial diamonds. (Image by © Thomas Hartmann).

These results are still somewhat tentative and have not yet been published in peer-reviewedwed journal or confirmed by others in the field, but they did publish a preprint (non-peer reviewed article) in a well-known preprint (e-print) archive site arXiv.org (pronounced “archive” where X represents the Greek letter chi.)1

Observation of superconducting in LaH10. Superconducting transitions in lanthanum superhydride LaH10 measured in different samples synthesized from a La+H2 mixture: red curve corresponds to the sample heated up under 145 GPa displaying Tc of about 244 K, which shifts to about 249 K when the pressure is increased up to 151 GPa (orange curve); dark yellow curve corresponds to the sample heated under 135 GPa with a Tc of about 245K; blue curve corresponds to a sample heated under 150 GPa with Tc about 249 K. Red, orange and dark yellow curves show the sharpest transitions to zero-resistance upon cooling. Blue curve, as well as many others samples, shows onsets of the superconductive transition around the same temperatures but the sharp superconducting step being distorted by the presence of an impurity phase and/or inhomogeneity in the sample. The resistance of the samples was divided by the shown coefficients for the sake of clarity. A vertical line drawn at 273 K marks the RTSC limit. Inset: pressure dependence of Tc for the 6 different samples.

For superconductivity to be verified, a potential superconducting material must confirm its behaviors through several tests, that then must be repeated by other labs.

First, the material has to demonstrate zero resistance. Then, the material has to show that as the temperature increases that the lattice starts to vibrate more strongly until above a critical temperature the pairs of electrons that are bound together at low temperatures known as Cooper pairs breakup, and the superconductivity stops.

BCS theory also forecasts that superconducting materials expel all magnetic fields via the Meisner effect. So, the second test to confirm superconductivity is demonstrating the Meisner effect.

The BCS theory further implies that the Meisner effect continues until the strength of the magnetic field (in Teslas), exceeds a certain point, beyond which the material turns back into a regular conductor. Again, the tests must confirm this.

In another BCS suggested outcome, if atoms within the superconducting lattice molecules are replaced with a lighter or heavier isotope, the superconducting temperature should show a corresponding increase or decrease. This is the final test to prove superconductivity.

The hunt for superconductors went through a long period of relative stagnation until till just four years ago a breakthrough was made that revived the pursuit of the Holy Grail of power electronics, a Room Temperature Superconductor (RTSC). After a period of nearly 25 years with few significant breakthoughs, in 2015, researchers at the Max Planck Institute for Chemistry achieved superconductivity at 203K(-70 degrees Celsius) in H3S.2

This discovery of superconductivity at a higher temperature than ever before revived the hunt for conventional high-temperature superconductors whose properties can be described by the Migdal-Eliashberg and Bardeen-Cooper-Schrieffer (BCS) theories.

These theories predict that high, and even room temperature superconductivity (RTSC) is possible in metals with specific favorable parameters including lattice vibrations at high frequencies. However, these general theories are known to not be sufficient to predict real-world superconductors, according to the researchers involved in the most recent breakthrough. However, these researchers noted that new superconducting materials can now be predicted with the help of calculations based on Density Functional Theory (DFT).

In particular, the calculations implied a potential new family of hydrides with a clathrate structure, where the host atom (Ca, Y, La) resides at the center of the cage that the hydrogen atoms produce.3-5 For LaH10 and YH10 superconductivity, with critical temperatures, Tc from between 240K and 320 K are predicted at megabar pressures 4-7.

The most recent breakthrough that brought the temperature up to about 250K (-23 degrees Celsius) proved the existence of superconductivity at this temperature through the observation of zero-resistance, isotope effect, and the decrease of Tc under an external magnetic field.

The results suggest an upper critical magnetic field of about 120T at zero temperature. So, it suggests that when exposed to a magnetic field at 120T or above, the superconductivity stops. This is one test that apparently was not completed due to the small size of the sample and the requirements for such a test.

They found that the pressure dependence of the transition temperatures Tc (P) has a maximum of 250K to 252K at the pressure of about 170GPa.

According to the researchers this jump, by about 50K, from the previous Tc record of 203K indicates the real possibility of achieving RTSC (that is at 273 K) in the near future at high pressures and the perspective of conventional superconductivity at ambient pressure.

“We found a record Tc = 250 K for LaH10 belonging to the lanthanum – hydrogen system, thus confirming the prediction of high temperature superconductivity in superhydrides with the sodalite-like clathrate structure…” 1

The researchers further stated, “Our study makes a leap forward on the road to the room- temperature superconductivity, and also provides an evidence that the art-of-state methods for crystal structure prediction can be very useful for the search of high temperature superconductors.” 1

They also noted that “the current theoretical predictions for RTSC in yttrium superhydrides3,5 motivate further experiments.” 1

And hopefully, other labs will soon confirm their results.

References

  1. Drozdov, A. P., Kong, P. P., Minkov, V. S., Besedin, S. P., et al. Superconductivity at 250 K in lanthanum hydride under high pressures. arXiv 1812.01561 [Preprint] Cited Jan. 10, 2019. https://arxiv.org/pdf/1812.01561.pdf.
  2. Drozdov, A. P., Eremets, M. I., Troyan, I. A., Ksenofontov, V. & Shylin, S. I. Conventional superconductivity at 203 K at high pressures. Nature 525, 73 (2015).
  3. Wang, H., Tse, J. S., Tanaka, K., Iitaka, T. i. & Ma, Y. Superconductive sodalite-like clathrate calcium hydride at high pressures. PNAS 109, 6463-6466 (2012).
  4. Peng, F. et al. Hydrogen Clathrate Structures in Rare Earth Hydrides at High Pressures: Possible Route to Room-Temperature Superconductivity. Phys. Rev. Lett. 119 107001 (2017).
  5. Liu, H. et al. Dynamics and superconductivity in compressed lanthanum superhydride. Phys. Rev. B 98, 100102(R) (2018).
  6. Liu, H., Naumov, I. I., Hoffmann, R., Ashcroft, N. W. & Hemley, R. J. Potential high Tc superconducting lanthanum and yttrium hydrides at high pressure. PNAS 114, 6990 (2017).
  7. Kruglov, I. A. et al. Superconductivity in LaH10: a new twist of the story. arXiv:1810.01113 (2018).