Tunability of mobility and conductivity over large ranges in poly(3,3'''-didodecylquaterthiophene)/insulating polymer composites.

Semiconducting polymers are currently being considered as active layers in field-effect transistors, in which high charge carrier mobility and low off conductivity are important. For other applications, such as certain spintronic mechanisms, the opposite characteristics are desirable. Blending such polymers with insulating polymers would be expected to lower the mobility. In this paper, we report that the use of hydrocarbon polymers such as polystyrene as insulators generally raises the mobility when the semiconducting polymer is poly(bisdodecylquaterthiophene). A high mobility value of nearly 0.1 cm(2)/V.s was obtained for an optimal blend. While this is counterintuitive, it is consistent with a few other recent reports. In order to lower the mobility significantly, a much more polar and irregular blending agent is needed. The further addition of tetrafluorotetracyanoquinodimethane as a dopant gave a rare low mobility/high conductivity combination of properties, with a charge carrier density on the order of 10(19) cm(-3). Thus, mobility and conductivity were tuned somewhat independently over 3 and 4 orders of magnitude, respectively.

Quantum criticality in an organic magnet.

Exchange interactions between S=1/2 sites in piperazinium hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground state. We have determined the field-temperature phase diagram by high field magnetization and neutron scattering experiments. There are two quantum critical points: Hc1=7.5 T separates a quantum paramagnet phase from a three dimensional, antiferromagnetically ordered state while Hc2=37 marks the onset of a fully polarized state. The ordered phase, which we describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum critical regime with short range correlations. A low temperature anomaly in the BEC phase boundary indicates that additional low energy features of the material become important near Hc1.

Elastic torque and the levitation of metal wires by a nematic liquid crystal.

Anisotropic particles suspended in a nematic liquid crystal disturb the alignment of the liquid crystal molecules and experience small forces that depend on the particles' orientation. We have measured these forces using magnetic nanowires. The torque on a wire and its orientation-dependent repulsion from a flat surface are quantitatively consistent with theoretical predictions based on the elastic properties of the liquid crystal. These forces can also be used to manipulate submicrometer-scale particles. We show that controlled spatial variations in the liquid crystal's alignment convert the torque on a wire to a translational force that levitates the wire to a specified height.

Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain.

Measurements are reported of the magnetic field dependence of excitations in the quantum critical state of the spin S=1/2 linear chain Heisenberg antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was measured at k(B)T/J< or =0.025 for H=0 and H=8.7 T, where the system is approximately 30% magnetized. At H=0, the results are in agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At H=8.7 T, there are multiple overlapping continua with incommensurate soft modes. The boundaries of these continua confirm long-standing predictions, and the intensities are consistent with exact diagonalization and Bethe ansatz calculations.