We report on theoretical studies of electronic transport in the archetypical molecular hybrid formed by DNA wrapped around single-walled carbon nanotubes (CNTs). Using a Green's function formalism in a π-orbital tight-binding representation, we investigate the role that spin-orbit interactions play on the CNT in the case of the helicoidal electric field induced by the polar nature of the adsorbed DNA molecule. We find that spin polarization of the current can take place in the absence of magnetic fields, depending strongly on the direction of the wrapping and length of the helicoidal field. These findings open new routes for using CNTs in spintronic devices.

We theoretically investigate the manipulation of the quantum anomalous Hall effect (QAHE) in graphene by means of the uniaxial strain. The values of Chern number and Hall conductance demonstrate that the strained graphene in presence of Rashba spin-orbit coupling and exchange field, for vanishing intrinsic spinorbit coupling, possesses non-trivial topological phase which is robust against the direction and modulus of the strain. Besides, we also find that the interplay between Rashba and intrinsic spin-orbit couplings results in a topological phase transition in the strained graphene. Remarkably, as the strain strength is increased beyond approximately 7%, the critical parameters of the exchange field for triggering the quantum anomalous Hall phase transition show distinct behaviors -decrease (increase) for strains along zigzag (armchair) direction. Our findings open up a new platform for manipulation of the QAHE by an experimentally accessible strain deformation of the graphene structure, with promising application on novel quantum electronic devices with high energy efficiency performance.

We propose a quantum electronic device based on a strained graphene nanoribbon. Mechanical strain, internal exchange field and spin-orbit couplings (SOCs) have been exploited as principle parameters to tune physical properties of the device. We predict a remarkable zero-field topological quantum phase transition between the time-reversal-symmetry broken quantum spin Hall (QSH) and quantum anomalous Hall (QAH) states, which was previously thought to take place only in the presence of finite magnetic field. We illustrate as intrinsic SOC is tuned, how two different helicity edge states located in the opposite edges of the nanoribbon exchange their locations. Our results indicates that the pseudomagnetic field induced by the strain could be coupled to the spin degrees of freedom through the SOC responsible for the stability of a QSH state. The controllability of this zero-field phase transition with strength and direction of the strain is also demonstrated. Our prediction offers a tempting prospect of strain, electric and magnetic manipulation of the QSH effect.

Using the Numerical Renormalization Group (NRG) and Anderson's poor man's scaling, we show that a system containing a quantum impurity (QI), strongly coupled to a semiconductor (with gap 2∆) and weakly coupled to a metal, displays a reentrant Kondo stage as one gradually lowers the temperature T. The NRG analysis of the corresponding Single Impurity Anderson Model (SIAM), through the impurity's thermodynamic and spectral properties, shows that the reentrant stage is characterized by a second sequence of SIAM fixed points, viz., free orbital (FO) → local moment (LM) → strong coupling (SC). In the higher temperature stage, the SC fixed point (with a Kondo temperature T K1 ) is unstable, while the lower temperature Kondo screening exhibits a much lower Kondo temperature T K2 , associated to a stable SC fixed point. The results clearly indicate that the reentrant Kondo screening is associated to an effective SIAM, with an effective Hubbard repulsion U eff , whose value is clearly identifiable in the impurity's local density of states. This low temperature effective SIAM, which we dub as reentrant SIAM, behaves as a replica of the high temperature (bare) SIAM. The second stage RG flow (obtained through NRG), whose FO fixed point emerges for T ≈ ∆ < T K1 , takes over once the RG flows away from the unstable first stage SC fixed point. The intuitive picture that emerges from our analysis is that the first Kondo state develops through impurity screening by semiconducting electrons, while the second Kondo state involves screening by metallic electrons, once the semiconducting electrons are out of reach to thermal excitations (T < ∆) and only the metallic (low) spectral weight inside the gap is available for impurity screening. This switch implies that the first Kondo cloud is much smaller than the second, since the NRG results show that, for all parameter ranges analyzed, T K2 T K1 . Last, but not least, we analyze a hybrid system formed by a QI 'sandwiched' between an armchair graphene nanoribbon (AGNR) and a scanning tunneling microscope (STM) tip (an AGNR+QI+STM system), with respective couplings set to reproduce the generic model described above. The energy gap (2∆) in the AGNR can be externally tuned by an electric-field-induced Rashba spin-orbit interaction. We analyzed this system for realistic parameter values, using NRG, and concluded that the reentrant SIAM, with its associated second stage Kondo, is worthy of experimental investigation.