Showing papers by "Taner Yildirim published in 2018"

Mechanical control of crystal symmetry and superconductivity in Weyl semimetal MoTe 2

Abstract: Semimetalic MoTe${}_{2}$ is an exciting material exhibiting both type-II Weyl nodes and superconductivity. Broken inversion symmetry is required for the Weyl semimetal phase, and this material is complicated by a structural phase transition between inversion symmetric (1T') and nonsymmetric phases (T${}_{d}$). Further, pressure suppresses the T${}_{d}$ phase and strongly enhances the superconducting transition temperature. The authors combined pressure-dependent neutron scattering, transport measurements, and first-principles calculations to deconvolve the structural phase transformation from the superconducting transition. Unexpectedly, both structural phases support superconductivity, and the authors show that anisotropic strain can be used to control which structure accommodates this pressure-enhanced superconductivity.

Mechanical control of crystal symmetry and superconductivity in Weyl semimetal MoTe$_2$

Abstract: The non-centrosymmetric Weyl semimetal candidate, MoTe$_2$ was investigated through neutron diffraction and transport measurements at pressures up to 1.5 GPa and at temperatures down to 40 mK. Centrosymmetric and non-centrosymmetric structural phases were found to coexist in the superconducting state. Density Functional Theory (DFT) calculations reveal that the strength of the electron-phonon coupling is similar for both crystal structures. Furthermore, it was found that by controlling non-hydrostatic components of stress, it is possible to mechanically control the ground state crystal structure. This allows for the tuning of crystal symmetry in the superconducting phase from centrosymmetric to non-centrosymmetric. DFT calculations support this strain control of crystal structure. This mechanical control of crystal symmetry gives a route to tuning the band topology of MoTe$_2$ and possibly the topology of the superconducting state.

A non-invasive method to directly quantify surface heterogeneity of porous materials.

Abstract: It is extremely challenging to measure the variation of pore surface properties in complex porous systems even though many porous materials have widely differing pore surface properties at microscopic levels. The surface heterogeneity results in different adsorption/desorption behaviors and storage capacity of guest molecules in pores. Built upon the conventional Porod's law scattering theory applicable mainly to porous materials with relatively homogeneous matrices, here we develop a generalized Porod's scattering law method (GPSLM) to study heterogeneous porous materials and directly obtain the variation of scattering length density (SLD) of pore surfaces. As SLD is a function of the chemical formula and density of the matrix, the non-invasive GPSLM provides a way to probe surface compositional heterogeneity, and can be applied to a wide range of heterogeneous materials especially, but not limited to, porous media and colloids, using either neutron or X-ray scattering techniques.

First-principles study of magnetism, lattice dynamics, and superconductivity in LaFeSiH x

Abstract: The structural, electronic, magnetic, and vibrational properties of LaFeSiH$_x$ for $x$ between 0 and 1 are investigated using density functional calculations. We find that the electronic and magnetic properties are strongly controlled by the hydrogen concentration $x$ in LaFeSiH$_x$. While fully hydrogenated LaFeSiH has a striped antiferromagnetic ground state, the underdoped LaFeSiH$_x$ for $x\leq0.75$ is not magnetic within the virtual crystal approximation or with explicit doping of supercells. The antiferromagnetic configuration breaks the symmetry of Fe $d$ orbitals and increases electron-phonon coupling up to $50\%$, especially for modes in the 20-50 meV range that are associated with Fe atomic movement. We find competing nearest and next-nearest neighbor exchange interactions and significant spin-phonon coupling, qualitatively similar but smaller in magnitude compared those found in LaOFeAs superconductors. The superconducting $T_c$ for antiferromagnetic LaFeSiH$_x$, assuming conventional superconductivity via McMillan's equation, therefore is computed to be 2-10 K, in contrast to $T_c\approx0$ for the nonmagnetic material. We also predict that the LaFeSiH$_x$ could be a good proton conductor due to phase stability with a wide range of hydrogen concentration $x < 1$.