Using ultracold atoms trapped in an optical lattice, we form a line-centered-square lattice in the condensedmatter physics, where a crossover from massive to massless Dirac fermion behavior can be easily achieved by tuning the laser intensities. The present Dirac fermions satisfy a three-component quantum equation for pseudospin-1 fermions, resulting in a single Dirac cone in the energy spectrum, a flat band touching at the Dirac point, and a vanishing Berry's phase. Interestingly, the massless Dirac fermions here may exhibit an all-angle Klein tunneling; i.e., the barrier is completely transparent for all incident angles.

We study the spin-dependent thermoelectric transport through a single-molecule-magnet junction in the sequential tunneling regime. It is found that the intrinsic magnetic anisotropy of the single-molecule magnet can lead to gate-voltage-dependent oscillations of charge thermopower and a large violation of the Wiedeman-Franz law. More interestingly, the spin-Seebeck coefficient is shown to be greater than the charge-Seebeck coefficient, and a pure spin thermopower or/and a pure spin current can be obtained by tuning only the gate voltage. It needs neither an external magnetic field or irradiation of circularly polarized light on the molecule nor ferromagnetic leads to realize these interesting effects, indicating the powerful prospect of single-molecule-magnet applications in spintronic devices.

We propose that a Floquet Weyl semimetal state can be induced in three-dimensional topological insulators, either nonmagnetic or magnetic, by the application of off-resonant light. The virtual photon processes play a critical role in renormalizing the Dirac mass and so resulting in a topological semimetal with vanishing gap at Weyl points. The present mechanism via off-resonant light is quite different from that via on-resonant light, the latter being recently suggested to give rise to a Floquet topological state in ordinary band insulators. [8][9][10] has partly gained success; however, candidate materials for TIs are still very limited. Inspiringly, an intriguing method was put forward to realize topologically non-trivial phases in nonequilibrium by applying time-dependent perturbations to trivial phases [11][12][13][14][15][16]. Typical examples are the optically-activated anomalous Hall effect and spin Hall effect in n-doped paramagnetic semiconductors [14], and the so-called Floquet topological insulator (FTI) suggested by Lindner, Refael, and Galitski [16], whose quasienergy spectrum exhibits a single pair of helical edge states due to the on-resonant-light-induced band inversion. In the bulk FTI spectrum there is an avoided crossing separating the reshuffled valence band from the conduction band.

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