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In the search for MgB2-like phonon-mediated superconductors we have carried out a systematic density functional theory study of the Ca-B system, isoelectronic to Mg-B, at ambient and gigapascal pressures. A remarkable variety of candidate…
We report micro-Raman measurements that reveal a low-energy soft mode at 16-19 meV in MgB_2 which becomes sharper in the superconducting state. With a boson spectrum based on this mode we reproduce, within Eliashberg theory, the T_c, the…
The Raman spectra of the parent compound NaxCoO2 (x=0.75) and the superconducting oxyhydrates NaxCoO2.yH2O with different superconducting temperatures (Tc) have been measured. Five Raman active phonons around 195 cm-1 (E1g), 482 cm-1, 522…
We present grazing incidence reflectivity measurements in the far infrared region at temperatures above and below Tc for a series of thallium (Tl2Ba2CuO6, Tl2Ba2CaCu2O8) and bismuth (Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and Bi(2-x)Pb(x)Sr2CaCu2O8)…
We analyze superonductivity in MgB$_2$ observed below $T_c=39$ K resulting from electron-phonon coupling involving a mode at $\hbar \omega_1 = 24$ meV and most importantly the in-plane B-B $E_{2g}$ vibration at $\hbar \omega_2=67$ meV. The…
Multi-center (molecular) structures can play an important role in light nuclei. The highly deformed rotational band in 10Be with band head at 6.179 MeV has been observed recently and suggested to have an exotic alpha:2n:alpha configuration.…
The optical properties of the superconducting single crystal Rb$_{0.8}$Fe$_{1.68}$Se$_2$ with $T_{c}$ $\simeq$ 31 K have been measured over a wide frequency range in the $ab$ plane. We found that the optical conductivity is dominated by a…
Weak localization has a strong influence on both the normal and superconducting properties of metals. In particular, since weak localization leads to the decoupling of electrons and phonons, the temperature dependence of resistance (i.e.,…
We report the boron isotope effect on the E2g phonon mode by micro-Raman spectroscopy on the ternary Mg1-xAlxB2 system, synthesized with pure isotopes 10B and 11B. The isotope coefficient on the phonon frequency is near 0.5 in the full…
Two-component BECs subject to laser-induced coupling exhibit a complicated spectrum of excitations, which can be viewed as two interacting phonon modes. We study the conditions required to make these two phonon modes decouple. Once…
Two-dimensional (2D) superconductors have attracted great attention in recent years due to the possibility of new phenomena in lower dimensions. With many bulk transition metal carbides being well-known conventional superconductors, here we…
Through first-principles calculations, the phonon-limited transport properties of cubic boron-V compounds (BP, BAs and BSb) are studied. We find that the high optical phonon frequency in these compounds leads to the substantial suppression…
New candidate ground states at 1:4, 1:2, and 1:1 compositions are identified in the well-known Fe-B system via a combination of ab initio high-throughput and evolutionary searches. We show that the proposed oP12-FeB2 stabilizes by a break…
The (Cd, Be)Te semiconductor alloy that exhibits a dramatic mismatch in bond covalency and stiffness clarifying its vibrational and mechanical properties is used as a benchmark to test the limits of the percolation model (PM) worked out to…
Unveiling the nature of the bosonic excitations that mediate the formation of Cooper pairs is a key issue for understanding unconventional superconductivity. A fundamen- tal step toward this goal would be to identify the relative weight of…
La-doped and Y-doped Bi2Sr2CaCu2O8+d (Bi2212) compounds Bi2Sr2-xLax(Ca,Y)Cu2O8+d, which range from the insulator to the deeply underdoped superconductor, have been studied by angle-resolved photoemission spectroscopy. We have observed that…
We use density functional theory to calculate the electronic band structures, cohesive energies, phonon dispersions, and optical absorption spectra of two-dimensional In$_2$X$_2$ crystals, where X is S, Se, or Te. We identify two…
Most functional materials possess one single outstanding property and are limited to be used for a particular purpose. Instead of integrating materials with different functions into one module, designing materials with controllable…
Measurements of the superconducting upper critical field, H_{c2}, and critical current density, J_c, have been carried out for MgB_2 doped with Ti and/or C in order to explore the problems encountered if these dopants are used to enhance…
We measured polarized far-infrared reflectivity and Raman scattering spectra of SrCuO$_2$ single crystals. The frequencies for infrared-active modes were determined using an oscillator-fitting procedure of reflectivity data. The Raman…