Novel metastable metallic and semiconducting germaniums (original) (raw)

β-tin→Imma→sh Phase Transitions of Germanium

Physical Review Letters, 2011

New paths were designed for the investigations of the-tin ! Imma ! sh phase transitions in nanocrystalline Ge under conditions of hydrostatic stress. A second-order transition between the-tin and Imma phases was identified at 66 GPa, and a first-order transition between the Imma and sh phases was determined at 90 GPa. Superconductivity was obtained up to 190 GPa using the acquired structural data in first-principles calculations. This provides evidence that the standard electron-phonon coupling mechanism is responsible for superconductivity in Ge, as evidenced by the good agreement between the calculations and existing experiments.

High-pressure and high-temperature study of phase transitions in solid germanium

physica status solidi (b), 2003

A detailed investigation of the pressure and temperature dependence of the structure of stable and metastable solid Ge phases in the 0-15 GPa and 295-500 K ranges is presented. Structural results including the equation of state, characteristic of coexistence regions of diamond and b-tin structures, and appearance of metastable phases are discussed and compared with the results presently available in the literature. The isothermal bulk modulus and its derivative are evaluated using temperature-dependent equations of state for Ge I (cubic) and Ge II (b-Sn), and for the Ge III (ST12) metastable polymorph at 400 K.

High-temperature phase transitions in dense germanium

The Journal of Chemical Physics, 2021

Through a series of high-pressure x-ray diffraction experiments combined with in situ laser heating, we explore the pressure-temperature phase diagram of germanium (Ge) at pressures up to 110 GPa and temperatures exceeding 3000 K. In the pressure range of 64-90 GPa, we observe orthorhombic Ge-IV transforming above 1500 K to a previously unobserved high-temperature phase, which we denote as Ge-VIII. This high-temperature phase is characterized by a tetragonal crystal structure, space group I4/mmm. Density functional theory simulations confirm that Ge-IV becomes unstable at high temperatures and that Ge-VIII is highly competitive and dynamically stable at these conditions. The existence of Ge-VIII has profound implications for the pressure-temperature phase diagram, with melting conditions increasing to much higher temperatures than previous extrapolations would imply.

Superconducting High Pressure Phase of Germane

Physical Review Letters, 2008

High-pressure structures of germane (GeH 4 ) are explored through ab initio evolutionary methodology to reveal a metallic monoclinic structure of C2=c (4 molecules=cell). The C2=c structure consists of layerlike motifs containing novel ''H 2 '' units. Enthalpy calculations suggest a remarkably wide decomposition (Ge þ H 2 ) pressure range of 0-196 GPa, above which C2=c structure is stable. Perturbative linear-response calculations for C2=c GeH 4 at 220 GPa predict a large electron-phonon coupling parameter of 1.12 and the resulting superconducting critical temperature reaches 64 K.

Superconductivity in multiple phases of compressed GeSb2Te4

Physical Review B, 2017

Here we report the discovery of superconductivity in multiple phases of the compressed GeSb2Te4 (GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous (a-GST) and for the crystalline (c-GST) phases. The superconducting critical temperature, TC, continuously increases with the applied pressure reaching a maximum Tc =6K at P=20 GPa for a-GST, whereas the critical temperature of the cubic phase reaches a maximum Tc =8 K at 30 GPa. This new material system, exhibiting a superconductor-insulator quantum phase transition (SIT) has an advantage over disordered metals since it has a continuous control of the crystal structure and the electronic properties using pressure as an external stimulus, which was lacking in SIT studies until today.

Superconductor-insulator transition in fcc GeSb2Te4 at elevated pressures

Physical Review B, 2018

We show that polycrystalline GeSb 2 Te 4 in the fcc phase (f-GST), which is an insulator at low temperature at ambient pressure, becomes a superconductor at elevated pressures. Our study of the superconductor-insulator transition versus pressure at low temperatures reveals a second order quantum phase transition with linear dependence of the transition temperature on the pressure above the critical zero-temperature pressure. In addition, we demonstrate that at higher pressures the f-GST goes through a structural phase transition via amorphization to bcc GST (b-GST), which also become superconducting. We also find that the pressure regime where an inhomogeneous mixture of amorphous and b-GST exists, there is an anomalous peak in magnetoresistance, and suggest an explanation for this anomaly.

Absence of superconductivity in the high-pressure polymorph of MgB2

Physical Review B, 2009

We report a high-pressure orthorhombic KHg 2 -type polymorph ͑space group Imma, 4 f.u./cell͒ of MgB 2 stable above 190 GPa predicted through ab-initio evolutionary simulations. The formation of this new phase results from the strong out-of-plane distortions of the two-dimensional honeycomb boron sublattice of the low-pressure AlB 2 -type structure creating a peculiar tetrahedrally bonded three-dimensional boron network. This high-pressure phase is a weak metal and not superconducting, re-highlighting the key role of the planar boron sublattice in forming the superconducting state and clear structure-property relations that can enable design of new superconductors.

Superconductivity in thin-film germanium in the temperature regime around 1 K

Superconductor Science and Technology, 2010

We report recent discoveries of superconductivity in p-type-doped germanium which has been fabricated by implantation of gallium ions into near-intrinsic cubic Ge. Depending on the detailed preparation and annealing conditions, we demonstrate that superconductivity can be generated and tailored in thin p-doped layers of the Ge host. By carefully adjusting the annealing parameters, we have been able to raise the onset temperature of superconductivity to about 1.4 K at a Ga peak concentration of ∼10 at.%. This progress and the large in-plane critical magnetic field of about the size of the Chandrasekhar-Clogston limit makes thin-film Ga-doped Ge (Ge:Ga) even more attractive for technological applications. There might be particular interest to utilize on-chip thin-film superconductivity in a semiconducting environment as our preparation method of Ge:Ga is fully compatible with state-of-the-art semiconductor processing used nowadays for the mass production of logic circuits. After its finding in Si and diamond, our work adds another unexpected observation of superconductivity in doped elemental semiconductors and in one of the few remaining 'islands of the periodic table of elements' on which superconductivity has not been found so far.

Evidence for the R8 Phase of Germanium

2013

The formation of R8 germanium is reported. The-Sn phase is first induced by the indentation of amorphous germanium (a-Ge) and the resultant phases on pressure release are characterized by Raman scattering. The expected Raman line frequencies for the various phases of Ge are determined from firstprinciples calculations using density functional perturbation theory of the zone-center phonons in the diamond, ST12, BC8, and R8 Ge phases. In addition to the R8 phase, traces of BC8 may also be present following pressure release.

Superconducting State in a Gallium-Doped Germanium Layer at Low Temperatures

Physical Review Letters, 2009

We demonstrate that the third elemental group-IV semiconductor, germanium, exhibits superconductivity at ambient pressure. Using advanced doping and annealing techniques of state-of-the-art semiconductor processing, we have fabricated a highly Ga-doped Ge (Ge:Ga) layer in near-intrinsic Ge. Depending on the detailed annealing conditions, we demonstrate that superconductivity can be generated and tailored in the doped semiconducting Ge host at temperatures as high as 0.5 K. Critical-field measurements reveal the quasi-two-dimensional character of superconductivity in the $60 nm thick Ge:Ga layer. The Cooper-pair density in Ge:Ga appears to be exceptionally low.