Zhongbo Yan

Zhongbo Yan
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Zhongbo Yan

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Physics - Mesoscopic Systems and Quantum Hall Effect (9)
Physics - Superconductivity (8)
Physics - Strongly Correlated Electrons (8)
Physics - Other (3)
Physics - Materials Science (2)
Quantum Physics (1)
Physics - Optics (1)

Publications Authored By Zhongbo Yan

Majorana zero modes are usually attributed to topological superconductors. We study a class of two-dimensional topologically trivial superconductors without chiral edge modes, which nevertheless host robust Majorana zero modes in topological defects. The construction of the specific single-band model is facilitated by the Hopf map and the Hopf invariant. Read More

Backscattering-immune chiral modes arise along certain line defect in three-dimensional materials. We study Floquet chiral modes along Floquet defects, namely, the topological defects come entirely from spatial modulations of periodic driving. We define a precise topological invariant that counts the number of Floquet chiral modes, which is expressed as an integral on a five-dimensional torus parameterized by $(k_x,k_y,k_z,\theta,t)$. Read More

Weyl semimetals and nodal line semimetals are characterized by linear band touching at zero-dimensional points and one-dimensional lines, respectively. We predict that a circularly polarized light drives nodal line semimetals into Weyl semimetals. The Floquet Weyl points thus obtained are tunable by the incident light, which enables investigations of them in a highly controllable manner. Read More

Recently, the nodal line semimetals have attracted considerable interests in condensed matter physics. We show that their distinct band structure can be detected by measuring the collective modes. In particular, we find that the dependence of the plasmon frequency $\omega_p$ on the electron density $n$ follows a $\omega_p \sim n^{1/4}$ law in the long wavelength limit. Read More

Tunneling Magnetoresistance between two ferrromagnets is an issue of fundamental importance in spintronics. In this work, we show that tunneling magnetoresistance can also emerge in junctions composed of ferromagnets and time-reversal invariant topological superconductors without spin-rotation symmetry. Here the physical origin is that when the spin-polarization direction of injected electron from the ferromagnet lying in the same plane of the spin-polarization direction of Majorana zero modes, the electron will undergo a perfect spin-equal Andreev reflection, while injected electrons with other spin-polarization direction will be partially Andreev reflected and partially normal reflected, which consequently have a lower conductance, and therefore, the magnetoresistance effect emerges. Read More

Recently it has been shown that multicomponent spin-orbit-coupled fermions in one-dimensional optical lattices can be viewed as spinless fermions moving in two-dimensional synthetic lattices with synthetic magnetic flux. The quantum Hall edge states in these systems have been observed in recent experiments. In this paper we study the effect of an attractive Hubbard interaction. Read More

The spin polarization (SP) of the ferromagnet (FM) is a quantity of fundamental importance in spintronics. In this work, we propose a quasi-one-dimensional junction structure composed of a FM and a time-reversal invariant topological superconductor (TRITS) with un-spin-polarized pairing type to determine the SP of the FM. We find that due to the topological property of the TRITS, the zero-bias conductance (ZBC) of the FM/TRITS junction which is directly related to the SP is a non-quantized but topological quantity. Read More

Spin-orbit coupling related new physics and quantum magnetism are two branches of great interest both in condensed matter physics and in cold atomic physics. With the introduction of a Rashba-like SOC into a Bose-Einstein condensate (BEC) loaded in a two-dimensional bipartite optical square lattice, we find that the ground state of the BEC always favors a coherent condensate than a fragmented condensate and always exhibits very large degeneracy, and most importantly, an antiferromagnetic order of quantum nature emerges when parameters satisfy certain condition. This provides an ideal platform to study the interplay of antiferromagnetic phase and superfluid phase. Read More

For cold atomic systems, varying the optical lattice potential periodically provides a general and simple way to drive the system into phases with nontrivial topology. Besides its simplicity, this driving approach, compared to the usual driving approach by exerting an external electromagnetic field to the static system, has the merit that it does not break the original static system's time-reversal symmetry at any given time. Based on this approach, we find that a trivial insulator with time-reversal symmetry can be driven into a Floquet quantum spin Hall insulator. Read More

The Su-Schrieffer-Heeger (SSH) model describes a one-dimensional $Z_{2}$ topological insulator, which has two topological distinct phases corresponding to two different dimerizations. When spin-orbit coupling is introduced into the SSH model, we find the structure of the Bloch bands can be greatly changed, and most interestingly, a new topological phase with single zero-energy bound state which exhibits non-Abelian statistics at each end emerges, which suggests that a new topological invariant is needed to fully classify all phases. In a comparatively large range of parameters, we find that spin-orbit coupling induces completely flat band with nontrivial topology. Read More

We study the complete tunneling spectroscopy of a normal metal/$p$-wave superconductor junction ($N-pS$) and normal metal/heterostructure superconductor junction ($N-hS$) by Blonder-Tinkham-Klapwijk (BTK) method. We find that, for $p$-wave superconductor with non-trivial topology, there exists a quantized zero-bias conductance peak stably, for heterostructure superconductor with non-trivial topology, the emerging zero-bias conductance peak is non-quantized and usually has a considerable gap to the quantized value. Furthermore, it is sensitive to parameters, especially to spin-orbit coupling and the $s$-wave pairing potential. Read More

One-dimensional spinless Fermi gas with attractive dipole-dipole interaction is investigated. Results obtained show when the interaction is weak, the excitation spectrum is linear and the superconducting correlation function decays as power law, indicating the validity of the Tomonaga-Luttinger (TL) liquid picture. However, when the interaction reaches a critical value, the excitation spectrum is nonlinear and the superconducting correlation function keeps finite for infinity separation, indicating real long-range order established and the breakdown of the TL liquid picture. Read More

We study the Bose-Einstein condensate in a honeycomb optical lattice within Bogoliubov theory and find that for a ${\bf k} = 0$ condensate, the Dirac points appear in the Bogoliubov excitation spectrum when $0 < \beta < 2$, which illustrates that the bose-bose interaction does not change the Dirac point structure but only give a modification of the velocity of the Dirac cone. When the bosons are driven to condense at ${\bf k} = {\bf K}$, however, we find that the topology of the Dirac points will be altered by arbitrary weak interaction. Furthermore, we find that the next-nearest-neighbor hopping in an isotropic and an anisotropic lattice has different effects to the dynamics of the condensate and it should be taken into account when the lattice is not sufficiently deep. Read More

Two-component Bose condensates with repulsive interaction are stable when $g_{\rm \scriptscriptstyle 1} g_{\rm \scriptscriptstyle 2}Read More

We consider magnetic impurities in a two dimensional superfluid Fermi gas in the presence of spin-orbit coupling. By using the methods of t-matrix and Green's function, we find spin-orbit coupling has some dramatic impacts on the effects of magnetic impurities. For the single impurity problem, the number of bound states localized around the magnetic impurity is doubled. Read More

We propose a one-dimensional Hamiltonian $H_{1D}$ which supports Majorana fermions when $d_{x^{2}-y^{2}}$-wave superfluid appears in the ultracold atomic system and obtain the phase-separation diagrams both for the time-reversal-invariant case and time-reversal-symmetry-breaking case. From the phase-separation diagrams, we find that the single Majorana fermions exist in the topological superfluid region, and we can reach this region by tuning the chemical potential $\mu$ and spin-orbit coupling $\alpha_{R}$. Importantly, the spin-orbit coupling has realized in ultracold atoms by the recent experimental achievement of synthetic gauge field, therefore, our one-dimensional ultra-cold atomic system described by $H_{1D}$ is a promising platform to find the mysterious Majorana fermions. Read More

The Fano resonance of a single symmetry broken Ag nanodisk under a normal incidence was investigated by using finite-difference time-domain (FDTD) simulations. The asymmetry line shape of the Fano resonance was controlled by modifying the open angle of the nanodisk, and this Fano splitting was demonstrated as the result of the overlap between the broad dipolar and narrow quadrupolar modes, which could be strengthened by enlarging the radius of the nanodisk. A semi-analytical method was developed to calculate the plasmon hybridization, which was used to analyze the sub-process of the quadru Fano resonance. Read More