Here we report a larger-scale atomic-level molecular dynamics (MD) simulation for the self-assembly of sodium dodecyl sulfate (SDS) surfactant on single-walled carbon nanotube (SWNT) surfaces and the interaction between supramolecular SDS/SWNT aggregates. We make an effort to address several important problems in regard to carbon nanotube dispersion/separation. At first, the simulation provides comprehensive direct evidence for SDS self-assembly structures on carbon nanotube surfaces, which can help to clarify the relevant debate over the exact adsorption structure. We also, for the first time, simulated the potential of mean force (PMF) between two SWNTs embedded in SDS surfactant micelles. A novel unified PMF approach has been applied to reveal various cooperative interactions between the SDS/SWNT aggregates, which is different from the previous electrostatic repulsion explanation. The unique role of sodium ions revealed here provides a new microscopic understanding of the recent experiments in the electrolyte tuning of the interfacial forces on the selective fractionation of SDS surrounding SWNTs.

The low-temperature magnetic phases in the layered honeycomb lattice material α-RuCl3 have been studied as a function of in-plane magnetic field. In zero field this material orders magnetically below 7 K with so-called zigzag order within the honeycomb planes. Neutron diffraction data show that a relatively small applied field of 2 T is sufficient to suppress the population of the magnetic domain in which the zigzag chains run along the field direction. We found that the intensity of the magnetic peaks due to zigzag order is continuously suppressed with increasing field until their disappearance at µoHc=8 T. At still higher fields (above 8 T) the zigzag order is destroyed, while bulk magnetization and heat capacity measurements suggest that the material enters a state with gapped magnetic excitations. We discuss the magnetic phase diagram obtained in our study in the context of a quantum phase transition.

Using bulk magnetization along with elastic and inelastic neutron scattering techniques, we have investigated the phase diagram of Fe 1þy Se x Te 1Àx and the nature of magnetic correlations in three nonsuperconducting samples of Fe 1:01 Se 0:1 Te 0:9 , Fe 1:01 Se 0:15 Te 0:85 , and Fe 1:02 Se 0:3 Te 0:7 . A cusp and hysteresis in the temperature dependence of the magnetization for the x ¼ 0:15 and 0.3 samples indicates spin-glass (SG) ordering below T sg ¼ 23 K. Neutron scattering measurements indicate that the spin-glass behavior is associated with short-range spin density wave (SDW) ordering characterized by a static component and a low-energy dynamic component with a characteristic incommensurate wave vector of Q m ¼ ð0:46; 0; 0:50Þ and an anisotropy gap of $2:5 meV. Our high Q-resolution data also show that the systems undergo a glassy structural distortion that coincides with the short-range SDW order.Following the discovery of superconductivity in Febased pnictides, 1) a resurgence of interest in the field of high temperature superconductivity ensued. 2-8) There has been particular interest in the possible connection between magnetism and superconductivity. In the iron pnictides, an antiferromagnetically ordered phase is in close proximity to optimal superconductivity. 9) In some cases, such as SmFeAsO 1Àx F x and Ba(Fe 1Àx Co x ) 2 As 2 , there is evidence for coexisting antiferromagnetic order and superconductivity. [10][11][12][13][14] The situation is somewhat different in the chalcogenide system, Fe 1þy Se x Te 1Àx . Here the details are sensitive to the Fe as well as the Se concentration, and we will focus on the situation for minimized excess Fe (i.e., y % 0). The Néel temperature drops rapidly for x 0:1, but our measurements indicate that bulk superconductivity only appears for x ! 0:4.One reason for a difference between the pnictides and chalcogenides concerns the nature of the antiferromagnetic order. To discuss that order, we first have to consider the crystal structure. In the -PbO structure of Fe 1þy Se x Te 1Àx (FST), the Fe layers have a square lattice structure; however, the positions of the Se/Te atoms above and below those planes break the translational symmetry. Thus, it is crystallographically appropriate to choose a unit cell with two Fe atoms per layer, such that the lattice parameter is a % 3:8 Å . We will specify reciprocal lattice vectors, Q ¼ ðh; k; lÞ, in reciprocal lattice units (rlu) of ð2=a; 2=b; 2=cÞ. In Fe 1þy Te, the long-range SDW state is accompanied by a tetragonal-to-monoclinic (or orthorhombic, depending on y) structural transition. 15,16) The spin arrangement is ferromagnetic along the b-direction and alternates in a þþÀÀ fashion along the a-direction, leading to a characteristic wave vector of ð0:5; 0; 0:5Þ. For larger y (e.g., y ¼ 0:14), the in-plane component of the magnetic wave vector becomes slightly incommensurate. 15) In Fe 1þy Se x Te 1Àx with 0:25x 0:33, static, but short-range, incommensurate magnetic order with Q m ¼ ð0:5 À ; 0; 0:5Þ is observed. 15,17,18) ...