Phonons and Anisotropic Thermal Expansion Behaviour of NiX (X = S, Se, Te)

Metal Chalcogenides have been known for important technological applications and have attracted continuous interest in their structure, electronic, thermal and transport properties. Here we present first principles calculations of the vibrational and thermodynamic properties of NiX (X = S, Se, Te) compounds along with inelastic neutron scattering measurements of the phonon spectrum in NiSe. The measured phonon spectrum is in very good agreement with the computed result. We also report the measurement of thermal expansion behavior of NiSe using X-ray diffraction from 13 K to 300 K. The change in the hexagonal c lattice parameter in NiSe is considerably greater as compared to a parameter. The ab-initio calculated anisotropic Gr\"uneisen parameters of the different phonon modes in all the chalcogenides along with the elastic constants are used to compute anisotropic thermal expansion behviour, which is found in good agreement with experiments. The displacement pattern of phonons indicate that difference in amplitudes of Ni and X atoms follow the anisotropy of thermal expansion behavior along c- and a-axis.Comment: 21 Pages, 9 Figure

First-principles study of the optoelectronic properties and photovoltaic absorber layer efficiency of Cu-based chalcogenides

Cu-based chalcogenides are promising materials for thin-film solar cells with more than 20% measured cell efficiency. Using first-principles calculations based on density functional theory, the optoelectronic properties of a group of Cu-based chalcogenides Cu$_2$-II-IV-VI$_4$ is studied. They are then screened with the aim of identifying potential absorber materials for photovoltaic applications. The spectroscopic limited maximum efficiency (SLME) introduced by Yu and Zunger is used as a metric for the screening. After constructing the current-voltage curve, the maximum spectroscopy dependent power conversion efficiency is calculated from the maximum power output. The role of the nature of the band gap, direct or indirect, and also of the absorptivity of the studied materials on the maximum theoretical power conversion efficiency is studied. Our results show that Cu$_2$-II-GeSe$_4$ with II=Cd and Hg, and Cu$_2$-II-SnS$_4$ with II=Cd and Zn have a higher theoretical efficiency compared to the materials currently used as absorber layer

Effect of d-f hybridization on the Josephson current through Eu-chalcogenides

A superconducting ring with a pi junction made from superconductor/ferromagnetic-metal/superconductor (S-FM-S) exhibits a spontaneous current without an external magnetic field in the ground state. Such pi ring provides so-called quiet qubit that can be efficiently decoupled from the fluctuation of the external field. However, the usage of the FM gives rise to strong Ohmic dissipation. Therefore, the realization of pi junctions without FM is expected for qubit applications. We theoretically consider the possibility of the pi coupling for S/Eu-chalcogenides/S junctions based on the d-f Hamiltonian. By use of the Green's function method we found that pi junction can be formed in the case of the finite d-f hybridization between the conduction d and the localized f electrons.Comment: 4 pages, 4 figure

Non-saturating magnetoresistance of inhomogeneous conductors: comparison of experiment and simulation

The silver chalcogenides provide a striking example of the benefits of imperfection. Nanothreads of excess silver cause distortions in the current flow that yield a linear and non-saturating transverse magnetoresistance (MR). Associated with the large and positive MR is a negative longitudinal MR. The longitudinal MR only occurs in the three-dimensional limit and thereby permits the determination of a characteristic length scale set by the spatial inhomogeneity. We find that this fundamental inhomogeneity length can be as large as ten microns. Systematic measurements of the diagonal and off-diagonal components of the resistivity tensor in various sample geometries show clear evidence of the distorted current paths posited in theoretical simulations. We use a random resistor network model to fit the linear MR, and expand it from two to three dimensions to depict current distortions in the third (thickness) dimension. When compared directly to experiments on Ag$_{2\pm\delta}$Se and Ag$_{2\pm\delta}$Te, in magnetic fields up to 55 T, the model identifies conductivity fluctuations due to macroscopic inhomogeneities as the underlying physical mechanism. It also accounts reasonably quantitatively for the various components of the resistivity tensor observed in the experiments.Comment: 10 pages, 7 figure

Robustness of s-wave Pairing in Electron-Overdoped A1−yFe2−xSe2\text{A}_{1-y}\text{Fe}_{2-x}\text{Se}_2

Using self consistent mean field and functional renormalization group approaches we show that s-wave pairing symmetry is robust in the heavily electron-doped iron chalcogenides $(\text{K, Cs}) \text{Fe}_{2-x}\text{Se}_2$. This is because in these materials the leading antiferromagnetic (AFM) exchange coupling is between next-nearest-neighbor (NNN) sites while the nearest neighbor (NN) magnetic exchange coupling is ferromagnetic (FM). This is different from the iron pnictides, where the NN magnetic exchange coupling is AFM and leads to strong competition between s-wave and d-wave pairing in the electron overdoped region. Our finding of a robust s-wave pairing in $(\text{K, Cs}) \text{Fe}_{2-x}\text{Se}_2$ differs from the d-wave pairing result obtained by other theories where non-local bare interaction terms and the NNN $J_2$ term are underestimated. Detecting the pairing symmetry in $(\text{K, Cs}) \text{Fe}_{2-x}\text{Se}_2$ may hence provide important insights regarding the mechanism of superconducting pairing in iron based superconductors.Comment: 10 pages, 16 figure

A Short Review of the S4S_4 Symmetric Microscopic Model for Iron-Based High Temperature Superconductors

We briefly review the recently constructed two orbital microscopic model for iron-based superconductors based on $S_4$ symmetry (PRX 2 021009(2012)). With this faithful representation of the kinematics of the tri-layer FeAs or FeSe structure, the model provides answers and physical pictures to fundamental questions related to the robustness of superconductivity and pairing symmetry, unifies different families of iron-based superconductors, casts new insight into the connections to the other high $T_c$ superconductors, cuprates, and reveals why an s-wave pairing can be stabilized by repulsive interactions. Further progresses include that the model provides a clean understanding of band reconstruction observed in magnetically ordered states, which is a strong support to the kinematics of the $S_4$ model, and captures the essential low energy physics of iron-based superconductors based on numerical results from unbiased quantum Monte Carlo simulation.Comment: 15 pages, invited submission to Journal of Physics (Conference series) for M2s conferenc