Record high (Tc) element superconductivity achieved in titanium

Titanium (Ti) is a lightweight, high-strength, corrosion-resistant metal widely used in aerospace, biomedicine, and extreme conditions. Pressure profoundly modifies its crystal structures (ambient hcp α phase; pressure-induced ω, γ/δ-like distorted bcc, and bcc phases) and can induce new functionalities. Prior studies reported superconductivity in Ti under pressure with a maximum measured T_c ~3.5 K around 56 GPa, while conventional electron-phonon calculations predicted T_c ~5 K in primary high-pressure phases. Compression affects 4s and 3d bands differently, promoting s→d electron transfer; narrower d bands can enhance the density of states near the Fermi level, potentially favoring superconductivity. The present work investigates whether extreme compression can drive significantly enhanced superconductivity in elemental Ti and elucidates the underlying mechanisms, including possible roles of s–d transfer and electron correlations beyond conventional phonon-mediated pairing.

Previous experimental work found superconductivity in high-pressure Ti phases with T_c up to ~3.5 K near 56 GPa, aligning with early electron-phonon predictions of ~5 K for main high-pressure structures. The α→ω and further transitions occur with increasing pressure, altering electronic structure. Conventional theory often anticipates decreasing T_c at very high pressures due to phonon stiffening/softening effects that reduce electron-phonon coupling efficacy. In the broader context of elemental superconductors under pressure, only a few elements (e.g., Li, Ca, Y) have shown substantial T_c enhancements at megabar pressures. These backgrounds frame the surprising observation reported here of robust, much higher T_c in compressed Ti, suggesting mechanisms beyond straightforward electron-phonon coupling.

High-pressure transport: Four-probe Van der Pauw measurements were performed on micrometer-scale Ti specimens in diamond anvil cells (DACs). Pressures were calibrated via the shift of the first-order Raman edge of the diamond culet. Applied current was 100 µA. Anvils with beveled edges (e.g., 300 µm, 340/400–300 µm, or 300/140–300 µm) were used. A cBN powder/epoxy mixture served as insulating gasket; a ~15–30 µm hole was drilled on the culet side. Four ~0.5 µm-thick Pt foils deposited on the culet acted as inner electrodes. Cross-shaped Ti specimens (~10 µm × 10 µm lateral size, ~1 µm thickness) were placed on the electrodes. Measurements were conducted in a cryogenic magnet system (1.5–300 K, up to 9 T). Hall effect measurements were performed at room temperature across pressures. High-pressure synchrotron XRD: Angle-dispersive XRD at room temperature was conducted at GSECARS (APS, Argonne). X-rays (λ = 0.3344 Å) were focused to ~3 µm. A symmetric DAC with beveled anvils (50/300 µm) and a rhenium gasket pre-pressed to ~20 µm (sample chamber drilled at center and filled with Ti powder mixed with Pt) was used. Pressure was calibrated using the equations of state of Re and Pt. Diffraction images were integrated to 2D patterns using Dioptas/related tools for analysis. First-principles calculations: Using QUANTUM ESPRESSO, total energies, lattice dynamics, and electron-phonon coupling were computed to assess structural and superconducting properties under pressure. T_c was estimated via the McMillan-Allen-Dynes formalism with Coulomb pseudopotential µ* ≈ 0.13 (typical range near 0.1), providing a baseline phonon-mediated expectation for comparison with experiment. Band structures at representative pressures were analyzed to assess s–d hybridization and the evolution of d-band dominance near the Fermi level.

- Elemental Ti becomes superconducting under pressure with T_c > 2 K by ~18 GPa and exhibits dramatic enhancement at higher pressures. - T_c shows a steep rise from ~10.2 K at 99 GPa to ~20.3 K at 108 GPa; further increases lead to robust high T_c. - Maximum T_c(onset) ≈ 26–26.2 K (observed near ~248 GPa) with T_c(zero) ≈ 21 K; T_c remains ≥20 K across a wide pressure window (~180–240 GPa). Measurements extended up to 310 GPa. - Upper critical field estimated via Ginzburg–Landau fit: µ0H_c2(0) ≈ 32 T, implying a coherence length ξ ≈ 3.2 Å from H_c2(0) = Φ0/(2πξ^2). At other pressures with T_c > 20 K, H_c2(0) stays near 30 T, exceeding that of NbTi (~10 T). - Hall effect indicates electron-dominated conduction with negative Hall slope. Carrier density is ~2.5 × 10^22 cm^-3 at low pressures, increases by over an order of magnitude to ~3.1 × 10^23 cm^-3 at ~137 GPa, then drops to ~4.5 × 10^21 cm^-3 at ~144 GPa, signaling electronic phase transitions. - Evidence of pressure-induced phase transitions near ~108 GPa and ~144 GPa is seen in carrier density and resistance vs pressure at fixed temperature. - Superconducting-structural phase diagram up to 310 GPa indicates five structural regions. Ti phase (P ~0–2 GPa) hosts T_c < 2 K; a Ti-O phase (P ~9–108 GPa) shows monotonically increasing T_c up to ~12 K; high-pressure Ti-V (108–144 GPa) and Ti-B (144–240 GPa) phases sustain T_c > 20 K, with maximum T_c ~26 K near the Ti-B/Ti-B4 phase boundary. - Calculated band structures reveal strong s–d overlap near E_F at lower pressures and progressive d-band dominance at higher pressures, consistent with enhanced s–d scattering and suggesting correlation-driven DOS enhancement near E_F. - The robust, high T_c over a wide pressure range conflicts with expectations from simple phonon-mediated mechanisms, pointing to significant electronic correlation and s–d interaction contributions.

The study demonstrates that elemental Ti exhibits unexpectedly high and robust superconductivity under extreme compression, addressing the central question of whether simple transition metals can achieve elevated T_c via pressure. The pronounced enhancement of T_c and its persistence above 20 K over a broad pressure range, together with H_c2(0) ~ 32 T, surpass standard phonon-mediated predictions for Ti. Hall measurements and resistance anomalies identify pressure-driven electronic/structural transitions (near ~108 and ~144 GPa) that correlate with T_c jumps. First-principles band analyses show s–d hybridization at moderate pressures and d-state dominance at higher pressures, implying increased DOS at the Fermi level and suggesting many-body correlation effects beyond conventional electron-phonon coupling. This behavior parallels, in qualitative spirit, robust high-T_c regimes in correlated materials (e.g., cuprates), indicating that unconventional mechanisms tied to s–d transfer and correlations may underlie high-T_c superconductivity in compressed Ti. These insights broaden the conceptual framework for designing high-T_c superconductors among simple metals by leveraging pressure-tuned electronic structure and correlation effects.

This work reports record-high T_c for an elemental superconductor in titanium under megabar pressures, achieving T_c(onset) ~26 K (T_c(zero) ~21 K) and H_c2(0) ~32 T with robust superconductivity above 20 K across a wide pressure window. Transport, Hall, and structural data delineate pressure-induced phase transitions and a superconducting phase diagram up to 310 GPa. First-principles analyses indicate strong s–d mixing and eventual d-band dominance near E_F at high pressures, pointing to correlation-enhanced DOS and possible non-phonon-mediated contributions to pairing. These findings suggest new routes to high-T_c superconductivity in simple materials via pressure-driven electronic mechanisms and motivate future efforts to stabilize such phases at lower pressures using mechanical strain or chemical pressure, as well as to develop advanced many-body theoretical treatments to elucidate the pairing mechanism. An independent contemporaneous study reported T_c up to 23.6 K at ~145 GPa, consistent with the robustness of high-T_c superconductivity in compressed Ti.

- The superconducting mechanism is not definitively established; evidence suggests roles for electron correlations and s–d interactions, but direct many-body characterization is pending. - Upper critical fields are estimated by GL fits based on measurements up to 9 T and extrapolated to 0 K; direct high-field measurements would further validate H_c2(0). - Structural phase assignments at ultrahigh pressures rely on combining in situ XRD with prior reports; some phase boundaries and labels may require further confirmation with higher-resolution diffraction and complementary probes. - Superconductivity is realized at extreme pressures (up to 310 GPa), which limits immediate practical applicability; stabilization at lower pressures via strain or chemical substitution remains a subject for future work.