The upper critical field Hc2 anisotropy of Ca10(PtnAs8)(Fe2-xPtxAs2)5 (n = 3, 4) single crystals with long FeAs interlayer distance (d) was studied by angular dependent resistivity measurements. A scaling of the angular dependent resistivity was real
ized for both single crystals using the anisotropic Ginzburg-Landau (AGL) approach with an appropriate anisotropy parameter {gamma}. The AGL scaling parameter {gamma} increases with decreasing temperature and reaches a value of about 10 at 0.8Tc for both single crystals. These values are much larger than those of other iron-based superconductors (FeSCs). Remarkably, the values of {gamma}2 show an almost linear increase with the FeAs/FeSe interlayer distance d for FeSCs. Compared to cuprates, FeSCs are less anisotropic, indicating that two dimensionality of the superconductivity is intrinsically weak.
In high magnetic fields, the exciton absorption spectrum of a semiconducting single-walled carbon nanotube splits as a result of Aharonov-Bohm magnetic flux. A magnetic field of 370 T, generated by the electro-magnetic flux compression destructive pu
lsed magnet-coil technique, was applied to single-chirality semiconducting carbon nanotubes. Using streak spectroscopy, we demonstrated the separation of the independent band-edge exciton states at the K and K points of the Brillouin zone after the mixing of the dark and bright states above 150 T. These results enable a quantitative discussion of the whole picture of the Aharonov-Bohm effect in single-walled carbon nanotubes.
We report a systematic polarization-dependent angle-resolved photoemission spectroscopy study of the three-dimensional electronic structure of the recently discovered 112-type iron-based superconductor Ca1-xLaxFeAs2 (x = 0.1). Besides the commonly re
ported three hole-like and two electron-like bands in iron-based superconductors, we resolve one additional hole-like band around the zone center and one more fast-dispersing band near the X point in the vicinity of Fermi level. By tuning the polarization and the energy of incident photons,we are able to identify the specific orbital characters and the kz dependence of all bands. Combining with band calculations, we find As 4pz and 4px (4py) orbitals contribute significantly to the additional three-dimensional hole-like band and the narrow band, respectively. Also, there are considerable hybridization between the As 4p zand Fe 3d orbitals in the additional hole-like band, which suggests the strong coupling between the unique arsenic zigzag bond layers and the FeAs layers therein. Our findings provide a comprehensive picture of the orbital characters of the low-lying band structure of 112-type iron-based superconductors, which can be a starting point for the further understanding of their unconventional superconductivity.
High-quality superconducting KxFeySe2 single crystals were synthesized using an easy one-step method. Detailed annealing studies were performed to make clear the phase formation process in KxFeySe2. Compatible observations were found in temperature-d
ependent X-ray diffraction patterns, back-scattered electron images and corresponding electromagnetic properties, which proved that good superconductivity performance was close related to the microstructure of superconducting component. Analysis based on the scaling behavior of flux pinning force indicated that the dominant pinning mechanism was delta(Tc) pinning and independent of connectivity. The annealing dynamics studies were also performed, which manifested that the humps in temperature-dependent resistance (RT) curves were induced by competition between the metallic/superconducting and the semiconducting/insulating phases.
Single crystals of Ca1-xLaxFe2As2 for x ranging from 0 to 0.25 have been grown and characterized by structural, transport and magnetic measurements. Coexistence of two superconducting phases is observed, in which the low superconducting transition te
mperature (Tc) phase has Tc ~ 20 K, and the high Tc phase has Tc higher than 40 K. These data also delineate an x - T phase diagram in which the single magnetic/structural phase transition in undoped CaFe2As2 appears to split into two distinct phase transitions, both of which are suppressed with increasing La substitution. Superconductivity emerges when x is about 0.06 and coexists with the structural/magnetic transition until x is ~ 0.13. With increasing concentration of La, the structural/magnetic transition is totally suppressed, and Tc reaches its maximum value of about 45 K for 0.15 < x < 0.19. A domelike superconducting region is not observed in the phase diagram, however, because no obvious over-doping region can be found. Two superconducting phases coexist in the x - T phase diagram of Ca1-xLaxFe2As2. The formation of the two separate phases, as well as the origin of the high Tc in Ca1-xLaxFe2As2 is studied and discussed in detail.
Superconductivity of Ca1-xLaxFe2As2 single crystals with various doping level were investigated via electromagnetic measurements for out-plane (H//c) and in-plane (H//ab) directions. Obvious double superconducting transitions, which can survive in ma
gnetic fields up to several Tesla, were observed in the medium-doped (x = 0.13) sample. Two kinds of distinct Hc2 phase diagrams were established for the low superconducting phase with Tc lower than 15 K and the high superconducting phase with Tc of over 40 K, respectively. Both the two kinds of phase diagrams exist in the medium-doped sample. Unusual upward curvature near Tc was observed in Hc2 phase diagrams and analyzed in detail. Temperature dependences of anisotropy for different doping concentrations were obtained and compared. Both superconducting phases manifest extremely large anisotropies, which may originate from the interface or intercalation superconductivity.
Significant differences exist among literature for thermal conductivity of various systems computed using molecular dynamics simulation. In some cases, unphysical results, for example, negative thermal conductivity, have been found. Using GaN as an e
xample case and the direct non-equilibrium method, extensive molecular dynamics simulations and Monte Carlo analysis of the results have been carried out to quantify the uncertainty level of the molecular dynamics methods and to identify the conditions that can yield sufficiently accurate calculations of thermal conductivity. We found that the errors of the calculations are mainly due to the statistical thermal fluctuations. Extrapolating results to the limit of an infinite-size system tend to magnify the errors and occasionally lead to unphysical results. The error in bulk estimates can be reduced by performing longer time averages using properly selected systems over a range of sample lengths. If the errors in the conductivity estimates associated with each of the sample lengths are kept below a certain threshold, the likelihood of obtaining unphysical bulk values becomes insignificant. Using a Monte-Carlo approach developed here, we have determined the probability distributions for the bulk thermal conductivities obtained using the direct method. We also have observed a nonlinear effect that can become a source of significant errors. For the extremely accurate results presented here, we predict a [0001] GaN thermal conductivity of 185 $rm{W/K cdot m}$ at 300 K, 102 $rm{W/K cdot m}$ at 500 K, and 74 $rm{W/K cdot m}$ at 800 K. Using the insights obtained in the work, we have achieved a corresponding error level (standard deviation) for the bulk (infinite sample length) GaN thermal conductivity of less than 10 $rm{W/K cdot m}$, 5 $rm{W/K cdot m}$, and 15 $rm{W/K cdot m}$ at 300 K, 500 K, and 800 K respectively.
We use a new, quantum-mechanics-based bond-order potential (BOP) to reveal melt-growth dynamics and fine-scale defect formation mechanisms in CdTe crystals. Previous molecular dynamics simulations of semiconductors have shown qualitatively incorrect
behavior due to the lack of an interatomic potential capable of predicting both crystalline growth and property trends of many transitional structures encountered during the melt $rightarrow$ crystal transformation. Here we demonstrate successful molecular dynamics simulations of melt-growth in CdTe using a BOP that significantly improves over other potentials on property trends of different phases. Our simulations result in a detailed understanding of defect formation during the melt-growth process. Equally important, we show that the new BOP enables defect formation mechanisms to be studied at a scale level comparable to empirical molecular dynamics simulation methods with a fidelity level approaching quantum-mechanical methods
