Blueshift in MgxZn1-xO alloys: nature of bandgap bowing

A Mg composition-dependent blueshift has been studied in MgxZn1−xO alloys deposited on 6H-SiC(0001) substrates. The localized exciton energy in MgxZn1−xO alloys for x ∼ 0.3 was blueshifted in the range 212–248 meV. The large negative bowing parameter was estimated in MgxZn1−xO alloys to be 4.72±0.84 eV. This large bandgap bowing emphasizes the Stokes shift, which has been attributed to the existence of spontaneous polarization effects due to the polar growth of MgxZn1−xO/SiC heterostructure and local compositional inhomogeneity

Doping n-type carriers by La-substitution for Ba in YBa_2Cu_3O_y system

Thus far, there is no cuprate system where both n-type and p-type charge carriers can be doped without changing the crystallographic structure. For studying the electron-hole symmetry in an identical structure, we try to dope n-type carriers to YBa2Cu3Oy system by reducing oxygen content and substituting La3+ ions for Ba2+. Single crystals of La-doped YBa2Cu3Oy are grown by a flux method with Y2O3 crucibles and it is confirmed that La actually substitutes \~13% of Ba. The oxygen content y can be varied between 6.21 and 6.95 by annealing the crystals in an atmosphere with controlled oxygen partial pressure. The in-plane resistivity rho_ab at room temperature was found to increase with decreasing oxygen content y down to 6.32, but interestingly further decrease in y results in a decrease in rho_ab. The most reduced samples with y = 6.21 show rho_ab of ~30 mOhm cm at room temperature, which is as much as seven orders-of-magnitude smaller than the maximum value at y = 6.32. Furthermore, both the Hall coefficient and the Seebeck coefficient of the y = 6.21 samples are found to be negative at room temperatures. The present results demonstrate that the non-doped Mott-insulating state has been crossed upon reducing y and n-type carriers are successfully doped in this material.Comment: 4 pages, 4 figures, 1 table, accepted for publication in Phys. Rev.

A note on the spectral mapping theorem of quantum walk models

We discuss the description of eigenspace of a quantum walk model $U$ with an associating linear operator $T$ in abstract settings of quantum walk including the Szegedy walk on graphs. In particular, we provide the spectral mapping theorem of $U$ without the spectral decomposition of $T$. Arguments in this direction reveal the eigenspaces of $U$ characterized by the generalized kernels of linear operators given by $T$.Comment: 17 page