What if pressure could create a superconductor, but the material could keep the useful high-pressure structure after the pressure vanished? In barium, severe twisting produces an ambient-pressure transition at twenty-four kelvin. Pressure-induced superconductivity often occurs following structural transition under hydrostatic pressure, but it disappears after the pressure is released.

In barium, superconductivity appears after transformation from body-centered cubic structure to hexagonal-close-packed structure at five gigapascals, and the transition temperature reaches a maximum of five kelvin at eighteen gigapascals. By stabilizing the low-temperature phase at about thirty gigapascals, the transition temperature reached eight kelvin, but this work demonstrates significantly higher superconductivity in barium even at ambient pressure.

High-pressure torsion makes this possible: the processed barium shows transitions at three kelvin and twenty-four kelvin in quasi-stabilized hcp and orthorhombic structures. The twenty-four-kelvin transition is the highest achieved at ambient pressure among single-element superconducting metals, including intermetallics, and is attributed to a strained high-pressure phase stabilized by residual strains.

High-pressure torsion introduces both shear and compressive strains into the material. The magnitude of shear strain increases with distance from the disk center and with the number of revolutions. Compressive stress supplies only diagonal strain-tensor components, while shear strain supplies nondiagonal components that can generate crystal structures realizing a strain-induced superconducting state.

Earlier high-pressure-torsion processing of rhenium and niobium stabilized superconductivity, with increases in transition temperature associated with increases in the density of states at the Fermi level. Barium has several high-pressure phases, and the orthorhombic Ba-VI phase, stabilized between twelve and thirty gigapascals below one hundred fifty kelvin, appears to be the best candidate for high transition temperature.

The high-pressure-torsion process is used to stabilize hcp and orthorhombic phases even at ambient pressure. These resulting high-pressure phases maintain their stability after the stress is released, and their structures differ significantly from those produced by normal hydrostatic compression.

The block identifies strain at the unit cell as a crucial factor for attaining superconductivity and anticipates unexpectedly high transition temperatures from appropriately strained structures. The first ambient-pressure signal appears at three kelvin: HPT-processed barium shows an unambiguous superconducting response there.

A second, smaller but visible diamagnetic signal appears at fourteen kelvin, and neither signal comes from the aluminum-magnesium-scandium cells used during processing. Applying a direct-current magnetic field of one hundred oersted shifts the diamagnetic signals toward lower temperature.

Figure one shows magnetic signatures from barium processed by high-pressure torsion at six gigapascals and ten revolutions. In panel a, the in-phase AC magnetization shows diamagnetic anomalies near three kelvin and fourteen kelvin, while annealing changes the observed response.

Panel b presents DC susceptibility transitions measured under fields from ten to three hundred oersted; its inset shows that the transition temperature depends on the applied field, supporting the interpretation of superconducting-like behavior in the pressure-stabilized material.

For the cryogenic experiments, high-pressure torsion used operating pressures from twelve to twenty-four gigapascals, ten revolutions, and a processing temperature of about one hundred kelvin in liquid nitrogen. Cryogenic processing generated a high-pressure orthorhombic phase as a quasi-stable state.

At an operating pressure of twenty-one gigapascals, a magnetic shielding signal appears at approximately twenty-four kelvin with no applied magnetic field and disappears at three hundred oersted. Across the tested pressures, the twenty-one-gigapascal sample gives the highest transition temperature, twenty-four kelvin, the highest achieved at ambient pressure for pure metals including intermetallics.

In panel a, the zero-field red curve shows the largest negative magnetic response, while increasing the applied field progressively suppresses that response and shifts the curves toward zero. In panel b, the field–temperature points and fitted lines form descending trends for every processing pressure, with the six-gigapascal data clustered near the lowest transition temperatures.

The transition temperature is measured at ambient pressure, while the horizontal axis records the operating pressure used during high-pressure torsion, not hydrostatic pressure. Transition temperatures exceed ten kelvin above twelve gigapascals and reach twenty to twenty-five kelvin between approximately ten and twenty gigapascals.

The largest superconducting volume fraction occurs at six gigapascals, consistent with high-temperature processing generating larger grains than lower-temperature processing. The block infers that the actual superconducting volume fraction for liquid-nitrogen processing at twenty-one gigapascals would approach that of room-temperature processing at six gigapascals.

In panel (b), the red-square data rise to a peak near the middle of the plotted pressure range, then drop sharply toward the rightmost measured point. In panel (c), the blue hatched bars are concentrated at the lower-pressure points, while the red squares for coercive field remain substantial across the measured range.

Ab initio density functional calculations examine barium under normal hydrostatic compression, comparing the enthalpies of the bcc and orthorhombic phases relative to hcp. Below four gigapascals, bcc is stable; above four gigapascals, hcp becomes stable, while between eighteen and thirty-one gigapascals the orthorhombic phase becomes stable before hcp returns above thirty-one gigapascals.

The calculated transition temperature gradually increases with pressure and reaches ten to eleven kelvin near thirty gigapascals, close to the experimental eight kelvin under hydrostatic pressure. Normal hydrostatic compression therefore predicts a maximum of only about ten kelvin, clearly below the approximately twenty-to-twenty-five-kelvin maximum reached in HPT-processed barium.

The reason for this difference remains unclear; if barium is a phonon-based superconductor, high-pressure torsion may strengthen effective electron-lattice interactions. In panel (b), the calculated transition-temperature points span roughly zero to twelve kelvin across the pressure range.

The green hcp markers stay near the bottom at lower pressures, whereas the blue orthorhombic markers rise into the upper part of the panel through the intermediate-pressure region. High-pressure torsion causes severe plastic deformation by compressing a disk between opposite tungsten-carbide anvils with a fifty-ton press.

The lower anvil is then rotated for a chosen number of revolutions, accumulating severe shear strain. That shear strain increases with both distance from the disk center and number of revolutions. Early processing reduces grain size, followed by recrystallization, producing a balance between these effects.

At cryogenic temperatures, the processing pressures range from twelve to twenty-four gigapascals, conditions suitable for stabilizing the orthorhombic Ba-VI phase. Approximately half of the sample volume belongs to bcc phases corresponding to hydrostatic pressures at or below five gigapascals, suggesting that about half recovered to the bcc Ba-I state after stress release.

The hcp phase remains after stress release, with an atomic-volume pressure corresponding to approximately twelve gigapascals, near the boundary between Ba-II hcp and Ba-VI orthorhombic phases. Two orthorhombic structures have space group Pnma, while ortho-prime number one has space group Cmcm and is created by shear strain under severe cryogenic compression.

The two shear-created states correspond to about zero gigapascals and cannot be created by typical hydrostatic compression, while ortho number two corresponds to twenty-two to twenty-six gigapascals. High-pressure torsion successfully stabilizes high-pressure hcp and orthorhombic barium phases at ambient temperature and pressure.

Compared with the original bcc phase, the hcp and orthorhombic phases are more conducive to superconductivity, giving barium new superconducting properties at ambient pressure. Room-temperature processing produced the Ba-II hcp phase with a transition near three kelvin, while cryogenic liquid-nitrogen processing increased the volume fraction of strained Ba-VI orthorhombic phase and produced a transition near twenty-four kelvin.

The relationship between strain in the strained phase and high transition temperature still requires more careful microscopic investigation. Structural analysis reveals coexistence of multiple strained phases, making their electronic states crucial for understanding the origin of the high transition temperature.

High-pressure torsion stabilizes strained hcp and orthorhombic phases of barium at ambient pressure, producing superconducting transitions up to twenty-four kelvin and exceeding normal hydrostatic-compression predictions.