M.A. Subramanian

Bio: M.A. Subramanian is an academic researcher from DuPont. The author has contributed to research in topics: Crystal structure & Tetragonal crystal system. The author has an hindex of 25, co-authored 83 publications receiving 2494 citations. Previous affiliations of M.A. Subramanian include University of California, Santa Barbara.

Structures of the superconducting oxides Tl2Ba2CuO6 and Bi2Sr2CuO6.

Abstract: The structures of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Cu${\mathrm{O}}_{6}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Cu${\mathrm{O}}_{6}$ have been solved and refined from single-crystal x-ray diffraction data. The structures are essentially the same and have single Cu-O sheets separated by either Tl-O or Bi-O double layers. The ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Cu${\mathrm{O}}_{6}$ structure is tetragonal with $a=3.87$ \AA{} and $c=23.24$ \AA{}, and there are strictly flat Cu-O sheets. ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Cu${\mathrm{O}}_{6}$ has a lower-symmetry structure which may be approximated with an orthorhombic cell with $a=5.36$ \AA{}, $b=5.37$ \AA{}, and $c=24.62$ \AA{}; however, this ignores superstructure reflections along both the $a$ and $c$ axes. The Tl-O layers are much more strongly bound to each other than are the Bi-O layers; thus, better conduction along the $c$ axis is expected for ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Cu${\mathrm{O}}_{6}$ relative to ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Cu${\mathrm{O}}_{6}$. Superconducting transition temperatures of 9 and 90 K were observed for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Cu${\mathrm{O}}_{6}$ and ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Cu${\mathrm{O}}_{6}$, respectively.

Superconductivity near liquid nitrogen temperature in the PbSrRCaCuO system (R=Y or rare earth)

Abstract: Superconductivity close to the boiling point of liquid nitrogen has been discovered in the P-Sr-R-Ca-Cu-O system where R is Y or rare earth. The superconducting compound has been identified as Pb 2 Sr 2 (R,Ca) 1 Cu 3 O 8+ y and the structure of the R=Y compound has been solved by single-crystal X-ray diffraction methods. The structure consists of double CuO 2 sheets interleaved by (Ca,R), a unit which is common to most of the high- T c copper oxide superconductors. A new structural feature found in this system is a double PbO layer separated by a sheet of copper atoms. A non-superconducting oxide of approximate composition (Y,Ca)Sr 2 (Cu,Pb) 3 O 7− y formed as a second phase in some preparations has been also characterized; its structure is related YBa 2 Cu 3 O 7 .

Mn3+ in trigonal bipyramidal coordination: a new blue chromophore.

Abstract: We show that trivalent manganese, Mn3+, imparts an intense blue color to oxides when it is introduced at dilution in trigonal bipyramidal coordination. Our optical measurements and first-principles density functional theory calculations indicate that the blue color results from an intense absorption in the red/green region. This absorption is due in turn to a symmetry-allowed optical transition between the valence-band maximum, composed of Mn 3dx2−y2,xy states strongly hybridized with O 2px,y states, and the narrow Mn 3dz2-based conduction-band minimum. We begin by demonstrating and explaining the effect using a well-defined prototype system: the hexagonal YMnO3−YInO3 solid solution. We then show that the behavior is a general feature of diluted Mn3+ in this coordination environment.

Structure refinements of superconducting Tl2Ba2CaCu2O8 and Tl2Ba2Cu3O10 from neutron diffraction data.

Abstract: The structures of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Ca${\mathrm{Cu}}_{2}$ ${\mathrm{O}}_{8}$ and ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{2}$ ${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ were refined from neutron-powder-diffraction data. The data for ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Ca${\mathrm{Cu}}_{2}$ ${\mathrm{O}}_{8}$ were taken at 298 K, and the data for ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{2}$ ${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ were obtained at both 150 and 13 K. The results are essentially in agreement with the structural refinements based on single-crystal x-ray-diffraction data and confirm the positional disorder of the oxygen atoms in the Tl-O planes. The low-temperature refinements for ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{2}$ ${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ indicate that the symmetry remains tetragonal down to 13 K and that there is no significant structural change or discontinuity in the cell parameters through the superconducting critical temperature near 125 K.

Dielectric polarizabilities of ions in oxides and fluorides

Abstract: A set of 61 ion polarizabilities has been derived from the dielectric constants of 129 oxides and 25 fluorides using a least squares refinement technique in conjunction with the Clausius–Mosotti equation. These polarizabilities can be used to estimate mean dielectric constants of ‘‘well‐behaved’’ compounds. They should be particularly useful in calculation of mean dielectric constants of compounds whose dielectric constants have not been determined. They can also be used as a framework for distinguishing unusual dielectric behavior from normal dielectric behavior where deviations can frequently be attributed to piezo‐ or ferroelectricity, conductivity (ionic or electronic), ‘‘rattling’’ or ‘‘compressed’’ cations with correspondingly high or low polarizabilities, or the presence of dipolar impurities. Deviations observed from calculated dielectric constants can be used to search for unusual physical behavior.

Electronic structure of the high-temperature oxide superconductors

Abstract: Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Magnetic Pyrochlore Oxides

Abstract: Within the past 20 years or so, there has occurred an explosion of interest in the magnetic behavior of pyrochlore oxides of the type $A_{2}^{3+}$$B_{2}^{4+}$O$_{7}$ where $A$ is a rare-earth ion and $B$ is usually a transition metal. Both the $A$ and $B$ sites form a network of corner-sharing tetrahedra which is the quintessential framework for a geometrically frustrated magnet. In these systems the natural tendency to form long range ordered ground states in accord with the Third Law is frustrated, resulting in some novel short range ordered alternatives such as spin glasses, spin ices and spin liquids and much new physics. This article attempts to review the myriad of properties found in pyrochlore oxides, mainly from a materials perspective, but with an appropriate theoretical context.

Superconductivity at 94 K in HgBa 2 Cu0 4+δ

Abstract: FOLLOWING the discovery1 of high-transition-temperature (high-Tc) superconductivity in doped La2CuO4, several families of related compounds have been discovered which have layers of CuO2 as the essential requirement for superconductivity: the highest transition temperatures so far have been found for thallium-bearing compounds2. Recently the mercury-bearing compound HgBa2Rcu2O6+δ (Hg-1212) was synthesized3 (where R is a rare-earth element), with a structure similar to the thallium-bearing superconductor TlBa2CaCu2O7 (Tl-1212), which has one T1O layer and two CuO2 layers per unit cell, and a Tc of 85 K (ref. 2). But in spite of its resemblance to Tl-1212, Hg-1212 was found not to be superconducting. Here we report the synthesis of the related compound HgBa2CuO4+δ (Hg-1201), with only one CuO2 layer per unit cell, and show that it is superconducting below 94 K. Its structure is similar to that of Tl-1201 (which has a Tc of < 10 K)4, but its transition temperature is considerably higher. The availability of a material with high Tc but only a single metal oxide (HgO) layer may be important for technological applications, as it seems that a smaller spacing between CuO2 planes leads to better superconducting properties in a magnetic field5.

Scanning tunneling spectroscopy of high-temperature superconductors

Abstract: Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.