Phase diagram and a possible unified description of intercalated iron selenide superconductors.

We propose a theoretical description of the phase diagram and physical properties in A(2)Fe(4)Se(5)-type (A=K, Tl) compounds based on a coexistent local moment and itinerant electron picture. Using neutron scattering and angle-resolved photoemission spectroscopy measurements to fix the general structure of the local moment and itinerant Fermi pockets, we find a superconducting phase with s-wave pairing at the M pockets and an incipient sign-change s wave near the Γ point, which is adjacent to the insulating phases. The uniform susceptibility and resistivity are found to be consistent with the experiment. The main distinction with iron pnictide superconductors is also discussed.

Possible anomalous spin dynamics of the Hubbard model on a honeycomb lattice.

In this paper, the Hubbard model on a honeycomb lattice is investigated by using an O(3) nonlinear σ model. A possible candidate for a quantum non-magnetic insulator in a narrow parameter region is found near the metal-insulator transition. After studying the magnetic properties of the quantum non-magnetic insulator, anomalous spin dynamics is shown. In addition, we find that this region could be widened by hole doping.

Parity anomaly of lattice Maxwell fermions in two spatial dimensions.

Unconventional lattice fermions with high degeneracies that are not Weyl or Dirac fermions have attracted increased attention in recent years. In this paper, we consider pseudospin-1 Maxwell fermions and the (2 + 1)-dimensional parity anomaly, which are not constrained by the fermion doubling theorem. We derive the Hall conductivity of a single Maxwell fermion and explain how each Maxwell fermion has a quantized Hall conductance of e 2/h. Parity is spontaneously broken in the effective theory of lattice Maxwell fermions interacting with an (auxiliary) U(1) gauge field, leading to an effective anomaly-induced Chern-Simons theory. An interesting observation about the parity anomaly is that the lattice Maxwell fermions are not constrained by the fermion doubling theorem, so a single Maxwell fermion can exist in a lattice. In addition, our work considers the quantum anomaly in odd-dimensional spinor space.

Majorana corner modes in an s-wave second order topological superfluid.

In sharp contrast to conventional topological superfluids, higher order (order r > 1) topological superfluids in n dimensions do not host n - 1 dimensional Majorana boundary states, instead host n - r dimensional Majorana excitations. In this paper, we propose Majorana corner modes can emerge in a second order superfluid with s-wave pairing, instead of unconventional pairings such as d-wave and [Formula: see text]-wave pairings in most of previous proposals. There are three key ingredients in this scheme consisting of a topological insulator, an in-plane Zeeman field, and an s-wave pairing. Based on the low energy theory for edge states, where the effective Dirac mass sign changes at the corner, we unveil the emergence of Majorana corner modes. Our proposal provides a promising platform for implementing 2D second order topological superfluids and Majorana corner modes.

Experimental observation of classical analogy of topological entanglement entropy.

Long-range entanglement is an important aspect of the topological orders, so efficient methods to characterize the long-range entanglement are often needed. In this regard, topological entanglement entropy (TEE) is often used for such a purpose but the experimental observation of TEE in a topological order remains a challenge. Here, we propose a scheme to observe TEE in the topological order by constructing specific minimum entropy states (MESs). We then experimentally construct the classical microwave analogs of the MESs and simulate the nontrivial topological order with the TEE in Kitaev toric code, which is in agreement with theoretical predictions. We also experimentally simulate the transition from Z2 topologically ordered state to topologically trivial state.

Chiral quantum spin liquid on the repulsive Haldane-Hubbard model in square lattices.

In this paper, we present a study on the repulsive Haldane-Hubbard model with spin-rotation symmetry in square lattices by deriving non-linear σ model for magnetic states. It is found that a chiral spin liquid state as the ground state of the correlated system exists in the [Formula: see text] topological spin-density wave proposed by Wu et al (2016 J. Phys.: Condens. Matter 28 115602), of which the low energy physics can be determined by the Chern-Simons-Hopf gauge field theory.

Antiferromagnetic order driven chiral topological spin density waves on the repulsive Haldane-Hubbard model on square lattices.

In this paper, based on mean-field approach and random-phase-approximation, we study the magnetic properties of the repulsive Haldane-Hubbard model on a square lattice. We find antiferromagnetic order driven topological spin density waves beyond Landau's symmetry-breaking paradigm, for which the effective low energy physics is determined by Chern-Simons-Hopf gauge field theories with different K matrices.

Theory of ferrimagnetism in the Hubbard model on bipartite lattices with spectral symmetry.

The Hubbard model is one of the most important models in condensed matter physics. In this paper, we developed a theory of ferrimagnetism in the Hubbard model on bipartite lattices with spectral symmetry. By taking three models as examples, we studied the ferrimagnetic orders that emerge from three typical fermionic systems--metal, semi-metal and (Chern) insulator. In particular, we found that there may exist various ferrimagnetic orders and explored the universal features.

Topological edge states in the spin 1 bilinear-biquadratic model.

The spin 1 bilinear-biquadratic model H = ∑(ij)[cosφS(i)·S(j) + sinφ(S(i)·S(j))(2)] on a square lattice in the region 0 < φ < π/4 is studied in a fermion representation with a p-wave pairing Bardeen-Cooper-Schrieffer type of mean-field theory. Our results show there may exist a non-trivial gapped spin liquid with time-reversal symmetry spontaneously breaking. This exotic state manifests its topological nature by forming chiral states at the edges. To show this more clearly, we set up and solved a ribbon system. We got a gapless dispersion representing the edge modes beneath the bulk modes. The edge modes with nonzero longitudinal momentum (k(x) ≠ 0) convect in opposite directions at the two edges, which leads to a twofold degeneracy, while the modes with zero longitudinal momentum (k(x) = 0) turn out to be Majorana fermion states. The edge spin correlation functions are found to decay following a power law with increasing distance. We also calculated the contribution of the edge modes to the specific heat and obtained a linear law at low temperatures.

Topological quantum computation via the quantum tunneling effect.

Quantum computers are predicted to utilize quantum states to process tasks far faster than those of conventional classical computers. In this Letter we show an alternative approach towards building topological quantum computers by tuning the quantum tunneling effect of degenerate quantum states in topological order, instead of braiding anyons. Using a designer Hamiltonian-the Wen-Plaquette model as an example, we study its quantum tunneling effect of the toric codes and show how to control the toric codes to realize topological quantum computation. In particular, we give a proposal to the measurement of the toric codes from Aharonov-Bohm interferences of quasiparticles.

Topological gauge structure and phase diagram for weakly doped antiferromagnets.

We show that a topological gauge structure in an effective description of the t-J model gives rise to a global phase diagram of antiferromagnetic and superconducting phases in a weakly doped regime. Dual confinement and deconfinement of holons and spinons play essential roles here, with a quantum critical point at a doping concentration x(c) approximately equal to 0.043. The complex experimental phase diagram at low doping is well described within such a framework.