Pressure Squeezes Material to Potentially Unlock Superconductivity and Enhanced Properties

Researchers investigated the pressure-dependent properties of tantalum diselenide (Ta2Se), a layered chalcogenide with a unique structural arrangement. Tauhidur Rahman, Jubair Hossan Abir, and Sourav Kumar Sutradhar, all from the Department of Physics at the University of Rajshahi, alongside Sutradhar et al., present a comprehensive first-principles study examining the behaviour of Ta2Se under hydrostatic pressure from 0 to 10 GPa. This work is significant because it connects structural, mechanical, thermophysical, electronic, optical, and superconducting characteristics within a unified framework, revealing how pressure can be used to tune the material’s properties and potentially enhance its superconductivity. The findings demonstrate a pressure-stiffened lattice, persistent metallicity, and a calculated critical temperature consistent with experimental observations, establishing pressure as a key control parameter for this promising material.

Pressure induced structural, mechanical and electronic evolution in tantalum diselenide

Researchers have unveiled a comprehensive understanding of how pressure dramatically alters the properties of tetragonal Ta2Se, a layered, tantalum-rich chalcogenide. This work presents a unified investigation, from 0 to 10 GPa, connecting structural changes with mechanical stability, thermophysical behaviour, bonding characteristics, electronic structure, optical response, lattice dynamics and ultimately, superconductivity.
The study reveals a pressure-induced lattice contraction of approximately 9.9% at 10 GPa, with enhanced interlayer coupling along the c-axis, demonstrating a robust response to external stress. All independent elastic constants remained stable throughout the pressure range, indicating enhanced bulk, shear and Young’s moduli and a corresponding increase in hardness.

Pugh’s ratio and Poisson’s ratio consistently suggest ductile, metallic bonding, while maintaining weak to moderate elastic anisotropy and confirming structural integrity under compression. Detailed analysis of thermophysical descriptors corroborates a pressure-stiffened lattice, evidenced by increases in density, Debye temperature, melting temperature and minimum thermal conductivity, without any anomalous anharmonic softening.
Investigation of bonding through bond population metrics and electron-density-difference analysis revealed a unique mixed metallic-covalent character, dominated by a strong tantalum-tantalum metallic backbone and strengthened tantalum-selenium hybridization. Electronic structure calculations confirm persistent metallicity under compression, with pressure broadening bands and reshaping the Fermi surface, potentially leading to a Lifshitz-type reconstruction without symmetry breaking.

The optical response remained metallic, exhibiting Drude-like low-energy behaviour and tunable spectral features, while phonon dispersions confirmed dynamical stability with no imaginary modes. Electron-phonon coupling calculations classify Ta2Se as a weak-coupling, phonon-mediated superconductor with a critical temperature of approximately 3.9 K, consistent with existing experimental data. This establishes pressure as a viable control parameter for tuning stability and superconductivity-relevant descriptors within this metal-rich layered material, opening avenues for advanced materials design and potential applications in superconducting circuitry and high-field conductors.

Computational parameters for Ta2Se structural prediction

Density functional theory calculations underpinned this work, employing both the Cambridge Serial Total Energy Package (CASTEP) and the Vienna Ab initio Simulation Package (VASP) to comprehensively investigate the behaviour of Ta2Se under pressure. Vanderbilt-type ultrasoft pseudopotentials and the projector augmented wave (PAW) method were utilised to describe the interactions between valence electrons and ion cores, with valence configurations of 5s2 5p6 5d3 6s2 for tantalum and 4s2 4p4 for selenium.

Geometry optimisation was performed using the BFGS minimisation scheme, converging total energy and residual forces to ensure accurate structural determination. A plane-wave cutoff energy of 550 eV was consistently applied across all pressures, guaranteeing convergence of the calculations. Brillouin-zone sampling was achieved with a 12 × 12 × 4 Monkhorst-Pack k-point grid, providing adequate convergence for total energy.

Stringent calculation tolerances were maintained, including energy convergence below 10-5 eV/atom, maximum displacement less than 10-3 Å, maximum ionic force below 0.03 eV Å-1, and maximum stress below 0.05 GPa, alongside a 0.1 eV smearing width with finite basis set corrections. A denser k-point mesh of 32 × 32 × 11 was employed for precise Fermi-surface construction.

The elastic constants were determined using the stress-strain method, calculating six independent second-order coefficients for the tetragonal crystal system. These constants were then used with the Voigt-Reuss-Hill averaging scheme to compute bulk, shear, and Young’s moduli. Optical properties were derived from the complex dielectric function, ε(ω) = ε1(ω) + iε2(ω), where the imaginary part, ε2(ω), was calculated directly from the electronic band structure via a summation over occupied and unoccupied bands.

The real part, ε1(ω), was subsequently obtained using the Kramers-Kronig relation, enabling the determination of refractive index, absorption coefficient, energy loss function, reflectivity, and optical conductivity. Bonding characteristics were examined through Mulliken bond population analysis, projecting plane-wave states onto a linear combination of atomic orbitals to quantify bond populations and net atomic charges.

The energy-volume data was fitted with the third-order Birch-Murnaghan equation of state to assess thermodynamic and structural behaviours. Superconducting properties were investigated using Quantum ESPRESSO, employing PAW-type pseudopotentials with a PBE exchange-correlation framework, 80 Ry and 640 Ry cutoffs for wavefunctions and charge density respectively, and density functional perturbation theory to obtain phonon dispersion relations. A 19 × 19 × 7 Monkhorst-Pack k-point mesh and a 3 × 3 × 3 q-point grid were used for accurate electron-phonon coupling calculations, estimating a critical temperature of approximately 3.9 K.

Tetragonal Ta2Se exhibits a decrease in lattice constants and unit-cell volume with increasing hydrostatic pressure. The optimised volume contracts from 111.787 Å3 at 0 GPa to 102.285 Å3 at 10 GPa, representing a reduction of approximately 9.86 percent. Normalization reveals that the a/a0 and c/c0 parameters both contract under pressure.

Pressure-induced evolution of structural, electronic and thermophysical behaviour in Ta2Se

Researchers have comprehensively investigated the behaviour of tetragonal Ta2Se under hydrostatic pressure up to 10 GPa, establishing a connection between its structural, mechanical, thermophysical, electronic, optical, and superconducting properties. This unified first-principles study reveals that increasing pressure stiffens the lattice of Ta2Se, evidenced by increases in density, Debye temperature, melting temperature, and minimum thermal conductivity, while the Grüneisen parameter remains relatively stable.

The bonding within the material is described as a mixture of metallic and covalent character, with a robust metallic Ta-Ta network strengthened by Ta-Se hybridization under compression. Electronic structure calculations demonstrate persistent metallicity throughout the pressure range, with modifications to the band structure, density of states, and Fermi surface potentially indicating a Lifshitz transition without symmetry changes.

Optical properties remain metallic, exhibiting Drude-like behaviour at low energies and tunable spectral features with applied pressure. Dynamical stability is confirmed by the absence of imaginary modes in phonon dispersions, and electron-phonon coupling calculations suggest a weak-coupling, phonon-mediated superconductivity with a critical temperature around 3.9 K, aligning with experimental observations. Furthermore, mechanical properties such as Poisson’s ratio and machinability index indicate that Ta2Se becomes more ductile and easier to machine under pressure.