Tianxing Ma

A gauge invariant dressed holon and spinon description of the normal state of underdoped cuprates

A partial charge?spin separation fermion-spin theory is developed to study the normal-state properties of the underdoped cuprates. In this approach, the physical electron is decoupled as a gauge invariant dressed holon and spinon, with the dressed holon behaving like a spinful fermion, representing the charge degree of freedom together with the phase part of the spin degree of freedom, while the dressed spinon is a hard-core boson, representing the amplitude part of the spin degree of freedom. The local electron constraint for single occupancy is satisfied. Within this approach, the charge and spin dynamics of the underdoped cuprates are studied based on the model. It is shown that the charge dynamics is mainly governed by the scattering from the dressed holons due to the dressed spinon fluctuation, while the scattering from the dressed spinons due to the dressed holon fluctuation dominates the spin dynamics.

Electronic band gaps and transport in aperiodic graphene superlattices of Thue-Morse sequence

We have studied the electronic properties in aperiodic graphene superlattices of Thue-Morse sequence. Although the structure is aperiodic, an unusual Dirac point (DP) does exist and its location is exactly at the energy corresponding to the zero-averaged wave number (zero-k¯). Furthermore, the zero-k¯ gap associated with the DP is robust against the lattice constants and the incident angles, and multi-DPs may appear under the suitable conditions. A resultant controllability of electronic transport in Thue-Morse sequence is predicted, which may facilitate the development of many graphene-based electronics.

Magnetic nature of superconductivity in doped cuprates

Within the kinetic energy driven superconducting mechanism, the magnetic nature of cuprate superconductors is discussed. It is shown that the superconducting state is controlled by both charge carrier gap function and quasiparticle coherent weight. This quasiparticle coherent weight grows linearly with the hole doping concentration in the underdoped and optimally doped regimes, and then decreases with doping in the overdoped regime, which leads to that the maximal superconducting transition temperature occurs around the optimal doping, and then decreases in both underdoped and overdoped regimes. Within this framework, we calculate the dynamical spin structure factor of cuprate superconductors, and reproduce all main features of inelastic neutron scattering experiments, including the energy dependence of the incommensurate magnetic scattering at both low and high energies and commensurate resonance at intermediate energy.

Magnetic Impurities In Graphene

We used a quantum Monte Carlo method to study the magnetic impurity adatoms on graphene. We found that by tuning the chemical potential we could switch the values of the impurity local magnet moment between relatively large and small values. Our computations of the impurity spectral density found its behavior to differ significantly from that of an impurity in a normal metal and our computations of the charge-charge and spin-spin correlations between the impurity and the conduction-band electrons found them to be strongly suppressed. In general, our results are consistent with those from poor mans scaling and numerical renormalization group methods.

GAUGE INVARIANT DRESSED HOLON AND SPINON IN DOPED CUPRATES

We develop a partial charge-spin separation fermion-spin theory implemented by the gauge invariant dressed holon and spinon. In this novel approach, the physical electron is decoupled as the gauge invariant dressed holon and spinon, with the dressed holon behaviors like a spinful fermion, and represents the charge degree of freedom together with the phase part of the spin degree of freedom, while the dressed spinon is a hard-core boson, and represents the amplitude part of the spin degree of freedom, then the electron single occupancy local constraint is satisfied. Within this approach, the charge transport and spin response of the underdoped cuprates is studied. It is shown that the charge transport is mainly governed by the scattering from the dressed holons due to the dressed spinon fluctuation, while the scattering from the dressed spinons due to the dressed holon fluctuation dominates the spin response.

Controllability of ferromagnetism in graphene

We systematically study magnetic correlations in graphene within Hubbard model on a honeycomb lattice by using quantum Monte Carlo simulations. In the filling region below the Van Hove singularity, the system shows a short-range ferromagnetic correlation, which is slightly strengthened by the on-site Coulomb interaction and markedly by the next-nearest-neighbor hopping integral. The ferromagnetic properties depend on the electron filling strongly, which may be manipulated by the electric gate. Due to its resultant controllability of ferromagnetism, graphene-based samples may facilitate the development of many applications.

Enhancement of superconducting transition temperature by the additional second neighbor hopping t′ in the t–J model

Abstract Within the kinetic energy driven superconducting mechanism, the effect of the additional second neighbor hopping t ′ on the superconducting state of the t – J model is discussed. It is shown that t ′ plays an important role in enhancing the superconducting transition temperature of the t – J model. It is also shown that the superconducting-state of cuprate superconductors is the conventional Bardeen–Cooper–Schrieffer like, so that the basic Bardeen–Cooper–Schrieffer formalism is still valid in quantitatively reproducing the doping dependence of the superconducting gap parameter and superconducting transition temperature, and electron spectral function at [ π , 0 ] point, although the pairing mechanism is driven by the kinetic energy by exchanging dressed spin excitations.

Electronic band gaps and transport properties in aperiodic bilayer graphene superlattices of Thue-Morse sequence

We investigate electronic band structure and transport properties in bilayer graphene superlattices of Thue-Morse sequence. It is interesting to find that the zero-k¯ gap center is sensitive to interlayer coupling t′, and the centers of all gaps shift versus t′ at a linear way. Extra Dirac points may emerge at ky≠0, and when the extra Dirac points are generated in pairs, the electronic conductance obeys a diffusive law, and the Fano factor tends to be 1/3 as the order of Thue-Morse sequence increases. Our results provide a flexible and effective way to control the transport properties in graphene.

Strain-induced edge magnetism at the zigzag edge of a graphene quantum dot

We study the temperature dependent magnetic susceptibility of a strained graphene quantum dot by using the determinant quantum Monte Carlo method. Within the Hubbard model on a honeycomb lattice, our unbiased numerical results show that a relative small interaction U may lead to a edge ferromagnetic like behavior in the strained graphene quantum dot, and a possible room temperature transition is suggested. Around half filling, the ferromagnetic fluctuations at the zigzag edge is strengthened both markedly by the on-site Coulomb interaction and the strain, especially in low temperature region. The resultant strongly enhanced ferromagnetic like behavior may be important for the development of many applications.

Electronic Bloch oscillation in bilayer graphene gradient superlattices

We investigate the electronic Bloch oscillation in bilayer graphene gradient superlattices using transfer matrix method. By introducing two kinds of gradient potentials of square barriers along electrons propagation direction, we find that Bloch oscillations up to terahertz can occur. Wannier-Stark ladders, as the counterpart of Bloch oscillation, are obtained as a series of equidistant transmission peaks, and the localization of the electronic wave function is also signature of Bloch oscillation. Forthermore, the period of Bloch oscillation decreases linearly with increasing gradient of barrier potentials.