Superconductivity at 161 K in Thorium Hydride $ThH_{10}$: Synthesis and Properties (1902.10206v4)

Abstract: Here we report targeted high-pressure synthesis of two novel high-$T_C$ hydride superconductors, $P6_3/mmc$-$ThH_9$ and $Fm\bar{3}m$-$ThH_{10}$, with the experimental critical temperatures ($T_C$) of 146 K and 159-161 K and upper critical magnetic fields ($\mu$$H_C$) 38 and 45 Tesla at pressures 170-175 Gigapascals, respectively. Superconductivity was evidenced by the observation of zero resistance and a decrease of $T_C$ under external magnetic field up to 16 Tesla. This is one of the highest critical temperatures that has been achieved experimentally in any compounds, along with such materials as $LaH_{10}$, $H_3S$ and $HgBa_2Ca_xCu_2O_{6+z}$. Our experiments show that $fcc$-$ThH_{10}$ has stabilization pressure of 85 GPa, making this material unique among all known high-$T_C$ metal polyhydrides. Two recently predicted Th-H compounds, $I4/mmm$-$ThH_4$ (> 86 GPa) and $Cmc2_1$-$ThH_6$ (86-104 GPa), were also synthesized. Equations of state of obtained thorium polyhydrides were measured and found to perfectly agree with the theoretical calculations. New phases were examined theoretically and their electronic, phonon, and superconducting properties were calculated.

• The paper reports the synthesis of thorium hydrides exhibiting superconductivity at 159–161 K with critical fields around 45 Tesla.
• The study employs diamond anvil cells, laser heating, and XRD to synthesize fcc ThH10 and hcp ThH9 at pressures as low as 85–175 GPa.
• DFT calculations and electron-phonon coupling analyses provide a theoretical basis supporting the observed high-TC superconductivity in these compounds.

Superconductivity in High-Pressure Synthesized Thorium Hydrides

The paper of superconductivity in hydrides, synthesized under extreme pressure conditions, represents an exciting frontier in condensed matter physics. The paper under analysis discusses the high-pressure synthesis and characterization of novel thorium hydrides, notably the face-centered cubic (fcc) thorium decahydride (ThH$_{10}$) and hexagonal close-packed (hcp) ThH$_9$. These materials exhibit superconductivity at critical temperatures ($T_C$) of 159-161 K and 146 K, respectively, under pressures ranging from 170 to 175 GPa. The work confirms the stability and superconducting properties of these hydrides, underscoring their relevance in the quest for high-critical-temperature materials.

Methodology and Synthesis

The synthesis involved subjecting thorium and ammonia borane mixtures to high pressures within diamond anvil cells (DACs), followed by laser heating. This method led to the formation of polyhydrides, including ThH$_{10}$ and ThH$_9$, validated by X-ray diffraction (XRD) analysis. ThH$_{10}$ was found stable at pressures as low as 85 GPa, contrasting with other high-$T_C$ hydrides that require higher stabilization pressures. This low stabilization threshold enhances its potential for practical application.

Superconducting Properties

Experiments revealed that ThH$_{10}$ transitions to a superconducting state at $T_C$ approximately 159-161 K with an upper critical magnetic field ($\mu_0H_{C2}(0)$) of 45 Tesla, ascertained through resistivity measurements. These values place ThH$_{10}$ alongside other materials exhibiting high-$T_C$ superconductivity, evidencing its competitivity in the field of superconducting materials.

Similarly, calculations of the electron-phonon interaction in ThH$_{10}$ provided insights into its superconducting mechanism, with the computed electron-phonon coupling constant $\lambda$ and logarithmic frequency $\omega_{log}$ supporting the experimentally observed superconducting behavior. On the theoretical front, density functional theory (DFT) was used to comprehend the stability and electronic structures of these novel phases.

Theoretical and Practical Implications

The synthesis and characterization of ThH$_{10}$ and ThH$_9$ extend theoretical predictions, previously made using computational tools like the USPEX evolutionary algorithm, which predicted their possible high-$T_C$ superconductivity. The observed superconducting temperatures are significant steps towards understanding superconductivity in hydrogen-rich compounds and moving closer to ambient pressure room-temperature superconductors.

The paper highlights the role of high-pressure synthesis in discovering and stabilizing new phases, which may also possess unique electronic properties, such as superconductivity. These thorium hydrides are particularly notable for their relatively low-pressure stabilization compared to other known superconducting hydrides, offering a potentially more accessible platform for future experimental investigations.

Future Directions

Future research will likely focus on decreasing the synthesis pressure of these materials while maintaining their superconducting properties. Furthermore, exploring ternary and more complex chemical systems could lead to compounds with even more desirable superconducting characteristics. Theoretical advancements will also be pivotal in guiding experimentalists toward synthesizing new materials with tailored properties.

In conclusion, this research significantly contributes to the understanding of superconductivity in hydrides under high pressure, with ThH$_{10}$ and ThH$_9$ emerging as promising candidates for next-generation superconductors. Further investigations into their properties and those of related systems might yield accessible pathways to room-temperature superconductivity, meeting the long-standing goal in the field.