We report the growth of high quality Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ ultrathin nanoplates (BSTS-NPs) on an electrically insulating fluorophlogopite mica substrate using a catalyst-free vapor solid method. Under an optimized pressure and suitable Ar
gas flow rate, we control the thickness, the size and the composition of BSTS-NPs. Raman spectra showing systematic change indicate that the thicknesses and compositions of BSTS-NPs are indeed accurately controlled. Electrical transport demonstrates that a robust Dirac cone carrier transport in BSTS-NPs. Since BSTS-NPs provide superior dominant surface transport of the tunable Dirac cone surface states with negligible contribution of the conduction of the bulk states, BSTS-NPs provide an ideal platform to explore intrinsic physical phenomena as well as technological applications of 3-dimensional topological insulators in the future.
Longitudinal-field muon-spin-relaxation measurements have revealed inhomogeneous distribution of the internal magnetic field at temperatures above the bulk superconducting (SC) transition temperature, $T_{rm c}$, in slightly overdoped Bi$_2$Sr$_2$Ca$
_{1-x}$Y$_x$Cu$_2$O$_{8+delta}$. The distribution width of the internal magnetic field, $Delta$, evolves continuously with decreasing temperature toward $T_{rm c}$. The origin of the increase in $Delta$ is discussed in terms of the creation of SC domains in a sample.
Unconventional high temperature superconductivity as well as three-dimensional bulk Dirac cone quantum states arising from the unique d-orbital topology has been a recent priority research area in physics. In iron pnictide compounds, although transpo
rt phenomena arisen from this multiple band Fermi surface are intriguing and scientifically important, they still do not give an adequate matching to neither experimental observations on the band picture nor theoretical calculations and a debate continues. Here we describe a new analytical approach of mobility spectrum, in which the carrier number is conveniently described as a function of mobility without any hypothesis about the number of carriers, on both longitudinal and transverse transport of high quality single crystal Ba(FeAs)$_2$ in a wide range of magnetic field. We show that the major numbers of carriers reside in large parabolic hole and electron pockets with very different topology as well as remarkably different mobility spectra, while the minor number of Dirac carriers resides in both hole- and electron- Dirac quantum states with the largest mobility as high as 70,000 cm$^2$(Vs)$^{-1}$.
As a methodology for controlling the carrier transport of topological insulators (TIs), a flexible tuning in carrier number on the surface states (SSs) of three dimensional TIs by surface modifications using organic molecules is described. The princi
ple of the carrier tuning and its type conversion of TIs presented in this research are based on the charge transfer of holes or electrons at the TI/organic molecule interface. By employing 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as an electron acceptor or tetracyanoquinodimethane (TCNQ) as a donor for n- and p- Bi2-xSbxTe3-ySey (BSTS) single crystals, successful carrier conversion from n to p and its reverse mode is demonstrated depending on the electron affinities of the molecules. The present method provides a nondestructive and efficient method for local tuning in carrier density of TIs, and is useful for future applications.
Low-temperature specific heat is measured on the overdoped Ba(Fe_{1-x}Co_x)_2As_2 (x = 0.13) single crystal under magnetic fields along three different directions. A clear anisotropy is observed on the field dependent electronic specific heat coeffic
ient {gamma}(H). The value of {gamma}(H) is obviously larger with magnetic field along [001] (c-axis) than that within the ab-plane of the crystal lattice, which cannot be attributed to the effect by anisotropy of the upper critical field. Meanwhile, the data show a rather small difference when the direction of the field is rotated from [100] to [110] direction within the ab-plane. Our results suggest that a considerable part of the line nodes is not excited to contribute to the quasiparticle density of states by the field when the field is within the ab-plane. The constraints on the topology of the gap nodes are discussed based on our observations.
Low-temperature specific heat (SH) is measured on Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals in a wide doping region under different magnetic fields. For the overdoped sample, we find the clear evidence for the presence of $T^2$ term in the dat
a, which is absent both for the underdoped and optimal doped samples, suggesting the presence of line nodes in the energy gap of the overdoped samples. Moreover, the field induced electron specific heat coefficient $Deltagamma(H)$ increases more quickly with the field for the overdoped sample than the underdoped and optimal doped ones, giving another support to our arguments. Our results suggest that the superconducting gap(s) in the present system may have different structures strongly depending on the doping regions.
