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Register a new userBand structure of tungsten oxide W$_{20}$O$_{58}$ with ideal octahedra
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Added by
Anatoly Slobodchikov
Publication date
2021
fields
Physics
and research's language is
English
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The band structure, density of states, and the Fermi surface of a tungsten oxide WO$_{2.9}$ with idealized crystal structure (ideal octahedra WO$_6$ creating a square lattice) is obtained within the density functional theory in the generalized gradient approximation. Because of the oxygen vacancies ordering this system is equivalent to the compound W$_{20}$O$_{58}$ (Magn{e}li phase), which has 78 atoms in unit cell. We show that 5$d$-orbitals of tungsten atoms located immediately around the voids in the zigzag chains of edge-sharing octahedra give the dominant contribution near the Fermi level. These particular tungsten atoms are responsible of a low-energy properties of the system.
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The band structure, density of states, and the Fermi surface of a recently discovered superconductor, oxygen-deficient tungsten oxide WO$_{2.9}$ that is equivalent to W$_{20}$O$_{58}$, studied within the density functional theory (DFT) in the generalized gradient approximation (GGA). Here we show that despite the extremely complicated structure containing 78 atoms in the unit cell, the low-energy band structure is quite feasible. Fermi level is crossed by no more than 10 bands per one spin projection (and even 9 bands per pseudospin projection when the spin-orbit coupling is considered) originating from the $d$-orbitals of tungsten atoms forming zigzag chains.
Electrical resistivity, magnetic susceptibility, and specific heat measurements on single crystals of La$Tr_2$Al$_{20}$ with $Tr$ = Mo and W revealed that these compounds exhibit superconductivity with transition temperatures $T_c$ = 3.22 and 1.81 K, respectively, achieving the highest values in the reported La$Tr_2$Al$_{20}$ compounds. There appears a positive correlation between $T_c$ and the electronic specific heat coefficient, which increases with increasing the number of $4d$- and $5d$-electrons. This finding indicates that filling of the upper $e_g$ orbitals in the $4d$ and $5d$ bands plays an essential role for the significant enhancement of the superconducting condensation energy. Possible roles played by the $d$ electrons in the strongly correlated electron phenomena appearing in $RTr_{2}$Al$_{20}$ are discussed.
Electrochromic devices, which dynamically change color under the applied potential, are widely studied because of its wide range of applications such as energy-efficient smart windows, rear view mirrors and display devices etc. In this study we are reporting four layer electrochromic device based on tungsten oxide as a electrochromic layer and nafion membrane as a ionic conducting layer. Nafion membranes are generally used in fuel cell applications because of its high ionic conductivity and high optical transparency which is suitable for electrochromic device to attain higher efficiencies. We have prepared an electrochromic device by sandwiching ITO coated glass and WO3 coated ITO thin film between nafion membrane. The overall structure of the device is Glass/ITO/WO3/Nafion/ITO/Glass. We deposited tungsten oxide thin films with different thickness on ITO coated glass substrate at room temperature by using reactive DC Magnetron sputtering and we studied the performance of the electrochromic device with the function of thickness. We have observed that electrochromic efficiency is increasing with increase in the tungsten oxide layer thickness. The efficiency of the device increased from 24.8 cm2/C to 184.3 cm2/C.
In multiorbital materials, superconductivity can exhibit new exotic forms that include several coupled condensates. In this context, quantum confinement in two-dimensional superconducting oxide interfaces offers new degrees of freedom to engineer the band structure and selectively control 3d-orbitals occupancy by electrostatic doping. However, the presence of multiple superconducting condensates in these systems has not yet been demonstrated. Here, we use resonant microwave transport to extract the superfluid stiffness of the (110)-oriented LaAlO3/SrTiO3 interface in the entire phase diagram. We evidence a transition from single-band to two-band superconductivity driven by electrostatic doping, which we relate to the filling of the different 3d-orbitals based on numerical simulations of the quantum well. Interestingly, the superconducting transition temperature decreases while the second band is populated, which challenges the Bardeen-Cooper-Schrieffer theory. To explain this behaviour, we propose that the superconducting order parameters associated with the two bands have opposite signs with respect to each other.
The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide interfaces becomes superconducting when the carrier density is tuned by gating. The measured resistance and superfluid density reveal an inhomogeneous superconductivity resulting from percolation of filamentary structures of superconducting puddles with randomly distributed critical temperatures, embedded in a non-superconducting matrix. Following the evidence that superconductivity is related to the appearance of high-mobility carriers, we model intra-puddle superconductivity by a multi-band system within a weak coupling BCS scheme. The microscopic parameters, extracted by fitting the transport data with a percolative model, yield a consistent description of the dependence of the average intra-puddle critical temperature and superfluid density on the carrier density.
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