Record high (Tc) element superconductivity achieved in titanium

Titanium (Ti), known for its lightweight, high strength, and corrosion resistance, finds wide applications in diverse fields. High pressure can significantly alter its crystal structure, potentially leading to new functionalities. At ambient pressure, Ti exists in a hexagonal close-packed (hcp) structure (α-Ti), undergoing phase transitions under pressure to β-Ti, ω-Ti, and other high-pressure phases. Previous studies reported superconductivity in Ti at high pressures, with a maximum Tc of 3.5 K at 56 GPa, aligning with theoretical predictions based on electron-phonon coupling. However, these predictions significantly underestimate the observed high-Tc superconductivity reported in this study. The research presented here investigates the effect of high pressure on the superconducting properties of Ti, exploring the possibility of achieving significantly higher Tc values than previously observed. This investigation is crucial because understanding the mechanisms behind high-Tc superconductivity in simple elements can offer valuable insights into the design and discovery of novel high-Tc superconductors.

Prior research on Ti under high pressure has focused on its structural transformations and the resulting effects on its mechanical and physical properties. Studies have established the existence of several high-pressure phases, including β-Ti, ω-Ti, and others, with transitions occurring at various pressures. Previous work on the superconductivity of Ti at high pressure reported a maximum Tc of 3.5 K, consistent with theoretical calculations based on the conventional electron-phonon coupling mechanism. These earlier theoretical calculations, however, did not anticipate the significantly higher Tc values observed in this study. The discrepancies between these earlier findings and the present results highlight the limitations of relying solely on conventional theoretical models for predicting high-Tc superconductivity in transition metals.

High-pressure resistance and Hall effect measurements were conducted using a diamond anvil cell (DAC) apparatus, employing the four-probe Van der Pauw method. Pressure calibration was achieved using the Raman shift of the diamond anvils. Samples of Ti metal were prepared and measured in the DAC, applying pressures up to 310 GPa. The temperature dependence of electrical resistance was measured to determine Tc. The influence of magnetic fields on the superconducting transition was also investigated. Additionally, high-pressure synchrotron X-ray diffraction experiments were performed at room temperature at GSECARS, using a symmetric DAC with a focused X-ray beam. The collected diffraction data were used to identify the various Ti crystal structures under different pressure conditions. First-principles theoretical calculations were carried out using the QUANTUM ESPRESSO package to determine the total energy, lattice dynamics, and electron-phonon coupling of Ti under pressure. The superconducting critical temperature was calculated based on Eliashberg theory, considering the electron-phonon interaction and Coulomb pseudopotential.

The study's primary finding is the observation of surprisingly high Tc superconductivity in Ti metal under high pressure. Tc values exceeding 20 K were achieved in a wide pressure range from 180 GPa to 240 GPa. The maximum Tc (onset) reached 26.2 K, significantly higher than previously reported values for Ti and representing a record high for an elemental superconductor. The zero-resistance Tc was 21 K. The upper critical field Hc2(0) was estimated to be ~32 Tesla, and the coherence length was calculated to be approximately 32 Å. Hall measurements revealed significant changes in carrier density above 100 GPa, indicating pressure-induced phase transitions correlated with the observed increase in Tc. The pressure-dependent superconductivity phase diagram shows that Tc remains robust above 20 K across a wide pressure range, contrasting with expectations from conventional electron-phonon coupling theory. The calculated electronic band structure suggests strong s-d electron transfer and band dominance, contributing to the high Tc. These observations strongly suggest the involvement of unconventional mechanisms beyond conventional electron-phonon coupling in driving the high-Tc superconductivity in Ti under high pressure.

The observed high Tc superconductivity in Ti at high pressure challenges conventional theoretical understanding of superconductivity in transition metals. The significant discrepancy between the experimental results and predictions based on solely electron-phonon coupling mechanisms indicates the importance of considering other contributing factors, such as strong s-d electron interactions and electron correlation effects. The robust nature of the high Tc over a wide pressure range is particularly striking and points toward novel many-body interactions that stabilize the superconducting state. These findings open up new possibilities for exploring unconventional superconductivity in simple elements, potentially leading to the discovery of new high-Tc superconductors. The unusually high Hc2(0) value is also significant, potentially opening up applications for high-field superconducting magnets.

This study demonstrates record-high superconductivity in compressed Ti metal, with Tc exceeding 20 K in a wide pressure range, reaching a maximum Tc (onset) above 26.2 K. This discovery challenges conventional BCS theory and points towards the importance of unconventional mechanisms, including strong s-d electron transfer and electron correlations. This work opens new avenues for exploring and designing high-Tc superconductors based on unconventional mechanisms in simple elements.

The study is limited by the high pressures required to induce superconductivity in Ti. The high-pressure environment poses experimental challenges, and the precise nature of the high-pressure phases responsible for the enhanced superconductivity requires further investigation. The current theoretical understanding of the observed phenomena remains incomplete, warranting more detailed theoretical studies to fully elucidate the underlying mechanisms.