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.

Fermi level tuning and a large activation gap achieved in the topological insulator Bi_{2}Te_{2}Se by Sn doping

We report the effect of Sn doping on the transport properties of the topological insulator Bi_{2}Te_{2}Se studied in a series of Bi_{2-x}Sn_{x}Te_{2}Se crystals with 0 \leq x \leq 0.02. The undoped stoichiometric compound (x = 0) shows an n-type metallic behavior with its Fermi level pinned to the conduction band. In the doped compound, it is found that Sn acts as an acceptor and leads to a downshift of the Fermi level. For x \geq 0.004, the Fermi level is lowered into the bulk forbidden gap and the crystals present a resistivity considerably larger than 1 Ohmcm at low temperatures. In those crystals, the high-temperature transport properties are essentially governed by thermally-activated carriers whose activation energy is 95-125 meV, which probably signifies the formation of a Sn-related impurity band. In addition, the surface conductance directly obtained from the Shubnikov-de Haas oscillations indicates that a surface-dominated transport can be achieved in samples with several um thickness.Comment: 6 pages, 4 figures, accepted for publication in PR

Oscillatory angular dependence of the magnetoresistance in a topological insulator Bi_{1-x}Sb_{x}

The angular-dependent magnetoresistance and the Shubnikov-de Haas oscillations are studied in a topological insulator Bi_{0.91}Sb_{0.09}, where the two-dimensional (2D) surface states coexist with a three-dimensional (3D) bulk Fermi surface (FS). Two distinct types of oscillatory phenomena are discovered in the angular-dependence: The one observed at lower fields is shown to originate from the surface state, which resides on the (2\bar{1}\bar{1}) plane, giving a new way to distinguish the 2D surface state from the 3D FS. The other one, which becomes prominent at higher fields, probably comes from the (111) plane and is obviously of unknown origin, pointing to new physics in transport properties of topological insulators.Comment: 4 pages, 5 figures, revised version with improved data and analysi

The neural correlates of speech motor sequence learning

Speech is perhaps the most sophisticated example of a species-wide movement capability in the animal kingdom, requiring split-second sequencing of approximately 100 muscles in the respiratory, laryngeal, and oral movement systems. Despite the unique role speech plays in human interaction and the debilitating impact of its disruption, little is known about the neural mechanisms underlying speech motor learning. Here, we studied the behavioral and neural correlates of learning new speech motor sequences. Participants repeatedly produced novel, meaningless syllables comprising illegal consonant clusters (e.g., GVAZF) over 2 days of practice. Following practice, participants produced the sequences with fewer errors and shorter durations, indicative of motor learning. Using fMRI, we compared brain activity during production of the learned illegal sequences and novel illegal sequences. Greater activity was noted during production of novel sequences in brain regions linked to non-speech motor sequence learning, including the BG and pre-SMA. Activity during novel sequence production was also greater in brain regions associated with learning and maintaining speech motor programs, including lateral premotor cortex, frontal operculum, and posterior superior temporal cortex. Measures of learning success correlated positively with activity in left frontal operculum and white matter integrity under left posterior superior temporal sulcus. These findings indicate speech motor sequence learning relies not only on brain areas involved generally in motor sequencing learning but also those associated with feedback-based speech motor learning. Furthermore, learning success is modulated by the integrity of structural connectivity between these motor and sensory brain regions.R01 DC007683 - NIDCD NIH HHS; R01DC007683 - NIDCD NIH HH

Chemical potential jump between hole- and electron-doped sides of ambipolar high-Tc cuprate

In order to study an intrinsic chemical potential jump between the hole- and electron-doped high-Tc superconductors, we have performed core-level X-ray photoemission spectroscopy (XPS) measurements of Y0.38La0.62Ba1.74La0.26Cu3Oy (YLBLCO), into which one can dope both holes and electrons with maintaining the same crystal structure. Unlike the case between the hole-doped system La_2-xSrxCuO4 and the electron-doped system Nd_2-xCexCuO4, we have estimated the true chemical potential jump between the hole- and electron-doped YLBLCO to be ~0.8 eV, which is much smaller than the optical gaps of 1.4-1.7 eV reported for the parent insulating compounds. We attribute the reduced jump to the indirect nature of the charge-excitation gap as well as to the polaronic nature of the doped carriers.Comment: 4 pages, 3 figure

Spectral shape analysis of ultraviolet luminescence in \textit{n}-type ZnO:Ga

Thin films of laser molecular-beam epitaxy grown \textit{n}-type Ga-doped ZnO were investigated with respect to their optical properties. Intense room-temperature photoluminescence (PL) in the near-band edge (NBE) region was observed. Moreover, its broadening of PL band was significantly larger than predicted by theoretical results modeled in terms of potential fluctuations caused by the random distribution of donor impurities. In addition, the lineshape was rather asymmetrical. To explain these features of the NBE bands, a vibronic model was developed accounting for contributions from a series of phonon replicas.Comment: 5 pages, 3 figures, 1 table, to appear in the Nov. 1st issue of J. Appl. Phys. (Scheduled Issue

Precise calibration of Mg concentration in MgxZn1-xO thin films grown on ZnO substrates

The growth techniques for MgxZn1-xO thin films have advanced at a rapid pace in recent years, enabling the application of this material to a wide range of optical and electrical applications. In designing structures and optimizing device performances, it is crucial that the Mg content of the alloy be controllable and precisely determined. In this study, we have established laboratory-based methods to determine the Mg content of MgxZn1-xO thin films grown on ZnO substrates, ranging from the solubility limit of x ~ 0.4 to the dilute limit of x < 0.01. For the absolute determination of Mg content, Rutherford backscattering spectroscopy is used for the high Mg region above x = 0.14, while secondary ion mass spectroscopy is employed to quantify low Mg content. As a lab-based method to determine the Mg content, c-axis length is measured by X-ray diffraction and is well associated with Mg content. The interpolation enables the determination of Mg content to x = 0.023, where the peak from the ZnO substrate overlaps the MgxZn1-xO peak in standard laboratory equipment, and thus quantitative determination. At dilute Mg contents below x = 0.023, the localized exciton peak energy of the MgxZn1-xO films as measured by photoluminescence is found to show a linear Mg content dependence, which is well resolved from the free exciton peak of ZnO substrate down to x = 0.0043. Our results demonstrate that X-ray diffraction and photoluminescence in combination are appropriate methods to determine Mg content in a wide Mg range from x = 0.004 to 0.40 in a laboratory environment.Comment: 21 pages, 1 table, 7 figure