Common microscopic origin of the phase transitions in Ta₂NiS₅ and the excitonic insulator candidate Ta₂NiSe₅

The structural phase transition in Ta2NiSe5 has been envisioned as driven by the formation of an excitonic insulating phase. However, the role of structural and electronic instabilities on crystal symmetry breaking has yet to be disentangled. Meanwhile, the phase transition in its complementary material Ta2NiS5 does not show any experimental hints of an excitonic insulating phase. We present a microscopic investigation of the electronic and phononic effects involved in the structural phase transition in Ta2NiSe5 and Ta2NiS5 using extensive first-principles calculations. In both materials the crystal symmetries are broken by phonon instabilities, which in turn lead to changes in the electronic bandstructure also observed in the experiment. A total energy landscape analysis shows no tendency towards a purely electronic instability and we find that a sizeable lattice distortion is needed to open a bandgap. We conclude that an excitonic instability is not needed to explain the phase transition in both Ta2NiSe5 and Ta2NiS5

Extreme rejuvenation of a bulk metallic glass at the nanoscale by swift heavy ion irradiation

Swift heavy ions can be used as a tool to tune material properties by generating high aspect ratio, nanometric trails of defects, or new disordered phases. This work explores different aspects of using this tool for rejuvenating and enhancing plasticity in bulk metallic glasses. An amorphous alloy with a nominal composition of Pd40Ni40P20 was irradiated with GeV-accelerated Au ions. Irradiation-induced out-of-plane swelling steps up to approxi- mately 100 nm in height are measured at the boundary between irradiated and non-irradiated areas. Changes of the relaxation enthalpy have been investigated using differential scanning calorimetry. Low-temperature heat capacity measurements substantiate an irradiation-induced increase of the boson peak height with increasing fluences. Accompanying transport measurements using radioactive Ag atoms as tracer also revealed increased diffusion rates in the irradiated samples dependent on the total fluence. Nano-indentation measurements show enhanced plasticity in the ion-irradiated glass which can be correlated with an increased heterogeneity as indicated by variable resolution fluctuation electron microscopy. The whole volume of the derived data sub- stantiates a prominent enhancement of the excess volume in the solidified ion tracks and the irradiation-induced modifications are discussed and analyzed in the framework of strong glass rejuvenation within the nanometric ion tracks

Nanolithographic Top‐Down Patterning of Polyoxovanadate‐based Nanostructures with Switchable Electrical Resistivity

The top-down fabrication of ∼10 nm vanadium oxide nanostructures by electron beam lithography based on a molecular vanadium oxide resist material is reported. The new material enables the large-scale deposition of electrically switchable nanostructures which can be directly incorporated in established e-beam lithography. The findings could in future enable the top-down fabrication of functional metal oxide nanostructures in the < 10 nm domain. The top-down lithographic fabrication of functional metal oxide nanostructures enables technologically important applications such as catalysis and electronics. Here, we report the use of molecular vanadium oxides, polyoxovanadates, as molecular precursors for electron beam lithography to obtain functional vanadium oxide nanostructures. The new resist class described gives access to nanostructures with minimum dimensions close to 10 nm. The lithographically prepared structures exhibit temperature-dependent switching behaviour of their electrical resistivity. The work could lay the foundation for accessing functional vanadium oxide nanostructures in the sub-10-nm domain using industrially established nanolithographic methods

Hydrogen atom moving across a strong magnetic field: analytical approximations

Analytical approximations are constructed for binding energies, quantum-mechanical sizes and oscillator strengths of main radiative transitions of hydrogen atoms arbitrarily moving in magnetic fields 10^{12}-10^{13} G. Examples of using the obtained approximations for determination of maximum transverse velocity of an atom and for evaluation of absorption spectra in magnetic neutron star atmospheres are presented.Comment: 17 pages, 3 figures, 5 tables, LaTeX with IOP style files (included). In v.2, Fig.1 and Table 5 have been corrected. In v.3, a misprint in the fit for oscillator strengths, Eq.(21), has been correcte

The helium atom in a strong magnetic field

We investigate the electronic structure of the helium atom in a magnetic field b etween B=0 and B=100a.u. The atom is treated as a nonrelativistic system with two interactin g electrons and a fixed nucleus. Scaling laws are provided connecting the fixed-nucleus Hamiltonia n to the one for the case of finite nuclear mass. Respecting the symmetries of the electronic Ham iltonian in the presence of a magnetic field, we represent this Hamiltonian as a matrix with res pect to a two-particle basis composed of one-particle states of a Gaussian basis set. The corresponding generalized eigenvalue problem is solved numerically, providing in the present paper results for vanish ing magnetic quantum number M=0 and even or odd z-parity, each for both singlet and triplet spin symmetry. Total electronic energies of the ground state and the first few excitations in each su bspace as well as their one-electron ionization energies are presented as a function of the magnetic fie ld, and their behaviour is discussed. Energy values for electromagnetic transitions within the M=0 sub space are shown, and a complete table of wavelengths at all the detected stationary points with respect to their field dependence is given, thereby providing a basis for a comparison with observed ab sorption spectra of magnetic white dwarfs.Comment: 21 pages, 4 Figures, acc.f.publ.in J.Phys.

The ground state of the Lithium atom in strong magnetic fields

The ground and some excited states of the Li atom in external uniform magnetic fields are calculated by means of our 2D mesh Hartree-Fock method for field strengths ranging from zero up to 2.35 10^8 T. With increasing field strength the ground state undergoes two transitions involving three different electronic configurations: for weak fields the ground state configuration arises from the field-free 1s^22s configuration, for intermediate fields from the 1s^22p_{-1} configuration and in high fields the 1s2p_{-1}3d_{-2} electronic configuration is responsible for the properties of the atom. The transition field strengths are determined. Calculations on the ground state of the Li+ ion allow us to describe the field-dependent ionization energy of the Li atom. Some general arguments on the ground states of multi-electron atoms in strong magnetic fields are provided.Comment: 11 pages, 6 figures, submitted to Physical Review