Zero-energy Modes Research Articles

Robust localized zero-energy modes from locally embedded PT -symmetric defects

We demonstrate the creation of robust localized zero-energy states that are induced into topologically trivial systems by insertion of a PT-symmetric defect with local gain and loss. A pair of robust localized states induced by the defect turns into zero-energy modes when the gain-loss contrast exceeds a threshold, at which the defect states encounter an exceptional point. Our approach can be used to obtain robust lasing or perfectly absorbing modes in any part of the system.

An implicit non-ordinary state-based peridynamics with stabilised correspondence material model for finite deformation analysis

This paper is devoted to the development of a stabilised implicit non-ordinary state-based peridynamics approach. We propose a geometrically nonlinear implicit approach focusing on quasi-static analyses. Since the construction of the Jacobian matrix is the most time-consuming step in conducting this nonlinear analysis, we formulate an analytical expression based on the equation of motion of non-ordinary state-based peridynamics to ensure optimum convergence of the global residual force. The implicit formulation can adopt fairly large time increments, making it a good choice for analyses of finite deformation. Another important extension presented in this paper is the modification of the correspondence material model to remove zero-energy mode instabilities and reduce the spurious oscillations, as proposed by Silling (2017). The derivative of the additional stabilisation term with respect to displacement is included in the formulation of the Jacobian for the first time. Computational examples of 2D finite deformation problems with a stabilised correspondence model are presented. We assess the effectiveness of different values of the stabilisation parameter, G in terms of the particles’ spacings and horizon sizes for different problems. This allows the non-ordinary state-based peridynamics approach to model material behaviour with greater accuracy where correspondence materials have previously failed due to instabilities. In this paper, a damage model is also proposed, which provides for the first time an implicit approach for the static solution of crack propagation problems for non-ordinary state-based peridynamics. This paper lays the groundwork for non-ordinary state-based peridynamics to be used for a much greater variety of solid mechanics problems than is currently possible and at the same time satisfying the stability condition.

Lasing in para-Fermi class-B microring resonator arrays

We demonstrate lasing in arrays of microring resonators with underlying para-Fermi symmetry. The properties of the algebra allow the analytic prediction of propagation constants and normal modes of an array of passive resonators that show the optical analogue of a zero-energy mode that is its own chiral pair. The rest of the normal modes in the linear model form chiral pairs that are evenly distributed around the pseudo zero-energy mode. We use this information to construct a class-B laser model where even and odd sites are driven with different strength in a pattern following para-Fermi algebra chirality to demonstrate lasing with enhancement or suppression of the zero-energy mode. Zero-energy mode lasing is stable to initial conditions for a large set of driving ratios. Lasing with zero-energy mode suppression is unstable and more susceptible to initial field configurations.

Engineering the topological state transfer and topological beam splitter in an even-sized Su-Schrieffer-Heeger chain

The usual Su-Schrieffer-Heeger model with an even number of lattice sites possesses two degenerate zero energy modes. The degeneracy of the zero energy modes leads to the mixing between the topological left and right edge states, which makes it difficult to implement the state transfer via topological edge channel. Here, enlightened by the Rice-Male topological pumping, we find that the staggered periodic next-nearest neighbor hoppings can also separate the initial mixed edge states, which ensures the state transfer between topological left and right edge states. Significantly, we construct an unique topological state transfer channel by introducing the staggered periodic on-site potentials and the periodic next-nearest neighbor hoppings added only on the odd sites simultaneously, and find that the state initially prepared at the last site can be transfered to the first two sites with the same probability distribution. This special topological state transfer channel is expected to realize a topological beam splitter, whose function is to make the initial photon at one position appear at two different positions with the same probability. Further, we demonstrate the feasibility of implementing the topological beam splitter based on the circuit quantum electrodynamic lattice. Our scheme opens up a new way for the realization of topological quantum information processing and provides a new path towards the engineering of new type of quantum optical device.

How to measure the Majorana polarization of a topological planar Josephson junction

We analyze the topological superconductivity and investigate the spectroscopic properties manifested by the zero-energy modes, induced in a metallic strip embedded into a Josephson-type junction. Focusing on the Majorana polarization of such quasiparticles, we propose feasible means for its empirical detection, using the spin-selective Andreev reflection method. Our study reveals a gradual development of a transverse gradient of the Majorana polarization across the metallic strip upon increasing its width. We also inspect the spatial profile and polarization of the Majorana quasiparticles in the presence of a strong electrostatic impurity. We show that, depending on its position, such a defect can lead to a substantial localization of the Majorana mode.

Magnetic field effect on trivial and topological bound states of superconducting quantum dot

We investigate the properties of a quantum dot embedded between the normal and superconducting leads which is additionally side-attached to the topological superconducting nanowire, hosting the Majorana modes. This setup enables formation of the trivial (finite-energy) bound states induced in the quantum dot through the superconducting proximity effect, coexisting/competing with the topological (zero-energy) mode transmitted from the topological superconductor. We analyze their interplay, focusing on a role played by the external magnetic field. To distinguish between these bound states we analyze the qualitative and quantitative features manifested in the subgap charge tunneling originating under nonequilibrium conditions from the Andreev (particle to hole) scattering processes.

Packings of frictionless spherocylinders.

We present simulation results on the properties of packings of frictionless spherocylindrical particles. Starting from a random distribution of particles in space, a packing is produced by minimizing the potential energy of interparticle contacts until a force-equilibrated state is reached. For different particle aspect ratios α=10⋯40, we calculate contacts z, pressure as well as bulk and shear modulus. Most important is the fraction f_{0}(α) of spherocylinders with contacts at both ends, as it governs the jamming threshold z_{c}(α)=8+2f_{0}(α). These results highlight the important role of the axial "sliding" degree of freedom of a spherocylinder, which is a zero-energy mode but only if no end contacts are present.

Leakage of Majorana bound states in an inhomogeneous topological nanowire

We present an exact solution of the continuum Bogolyubov-de-Gennes Hamiltonian for Majorana bound states (MBSs) generated in a superconductor–semiconductor hybrid topological nanowire. The full energy spectra that include the band states and in-gap states could be obtained. We show that for relatively short wire length, the zero energy mode could be induced even in the topological trivial regime, which also indicates oscillatory dependence on the chemical potential. With the increase of the Zeeman field, the MBSs are almost fully spin-polarized and do not localize at the wire ends gradually. We also extend our discussion to the property of Majorana modes in an inhomogeneous nanowire, in which a local gate voltage is applied to one end of the nanowire. It is found that the local potential barrier or well could modulate the Majorana energy splitting periodically. The leakage of MBSs to the potential region is exponentially suppressed for the barrier case. A potential well could induce near-zero-energy bound states and these states merge with MBSs, leading to the delocalization of MBSs. In the potential well region, both the spin-up and spin-down components of the trivial states account for a significant proportion, which can be detected experimentally.

Peridynamic simulation of finite elastic deformation and rupture in polymers

This study concerns the finite elastic deformation and rupture in styrene-butadiene rubber (SBR) and Polydimethylsiloxane (PDMS) sheets under quasi-static loading conditions. The weak form of peridynamic (PD) equilibrium equation is derived based on Anand’s model and Talamini-Mao-Anand (TMA) model for polymers with slight compressibility. The nonlocal deformation gradient tensor is computed in a bond-associated domain of interaction using the PD differential operator. This approach is free of oscillations and spurious zero energy modes that are commonly observed in the PD correspondence models. Also, it permits the direct imposition of natural and essential boundary conditions. The fidelity of this PD model for predicting large deformation, progressive damage and rupture is established by comparison with experimental measurements and previous simulations of SBR and PDMS sheets with defects under displacement controlled tensile loading.

Peridynamic correspondence model for finite elastic deformation and rupture in Neo-Hookean materials

This study considers finite elastic deformation and rupture in rubber-like materials under quasi-static loading conditions by employing the bond-associated weak form of peridynamics with nonuniform horizon. The weak form of peridynamic equilibrium equation is derived based on the Neo-Hookean material model with slight compressibility. The nonlocal deformation gradient tensor is computed in a bond-associated domain of interaction using the PD differential operator. This approach is free of oscillations and spurious zero energy modes that are commonly observed in the PD correspondence models. Also, it permits the direct imposition of natural and essential boundary conditions. Its fidelity for predicting large deformation is established by comparison with those of finite element analysis of a rubber sheet with a hole under stretch. Also, its validity for predicting damage is demonstrated through simulations of experiments concerning progressive damage growth and final rupture in polymers undergoing large elastic deformation.

Photon-Assisted Transport Through a Quantum Dot Side-Coupled to Majorana Bound States

Transmission function of a system composing of a quantum dot (QD) subjected to a photon field and side-coupled to a topological superconductor nanowire hosting a pair of Majorana bound states (MBSs) is calculated by using the nonequilibrium Green's function technique. We find that a series of photon-induced peaks emerge and are split by the coupling between the QD and the MBSs. Moreover, the peaks' height are suppressed to zero because the MBSs absorb (emit) the photon energy. Under this condition, the MBSs may be shifted to the non-zero energy mode, and thus provide another detection scheme for its existence which is quite different from the currently adopted ones depending on the zero-energy mode of the MBSs. In the presence of MBS-MBS overlaping, the central photon-assisted peaks in the transmission function reappear due to the fact that the photon absorbed (emitted) by one mode of the MBSs are subsequently emitted (absorbed) by another MBSs' mode. We also find that the positions of the additional peaks induced by the MBS-MBS overlaping in the presence of the photon field are quite different from the case of zero photon field.

Nonlinear displacement-based and hybrid-mixed quadrilaterals for three-dimensional stress analysis through sampling surfaces formulation

The finite deformation displacement-based and hybrid-mixed four-node quadrilateral elements using the sampling surfaces (SaS) technique are developed. The SaS formulation is based on choosing inside the plate N not equally spaced SaS parallel to the middle surface to introduce the displacements of these surfaces as basic plate unknowns. Such choice of unknowns with the consequent use of Lagrange polynomials of degree N –1 in the thickness direction permits the presentation of the plate formulation in a very compact form. The SaS are located at only Chebyshev polynomial nodes that allows one to minimize uniformly the error due to the Lagrange interpolation. To circumvent shear locking and have no spurious zero energy modes, the assumed transverse shear strain field is employed. Both plate quadrilaterals pass patch tests and exhibit superior performance in the case of coarse distorted mesh configurations. However, the hybrid-mixed element allows the use of load increments, which are much larger than possible with existing displacement-based elements. The developed finite elements can be useful for the 3D stress analysis of thin and thick plates, since the SaS solutions asymptotically approach the solutions of elasticity as N →∞. • The nonlinear displacement-based and hybrid-mixed quadrilaterals for the 3D stress analysis of plates are proposed. • The finite element formulation is based on the SaS method with the SaS located at only Chebyshev polynomial nodes. • The developed hybrid-mixed plate element demonstrates superior performance and allows the use of very large load increments.

Symmetry-protected topological phase for spin-tensor-momentum-coupled ultracold atoms

We propose a realizable experiment scheme to construct a one-dimensional synthetic magnetic flux lattice with spin-tensor-momentum coupled spin-1 atoms and explore its exotic topological states. Different from the Altland-Zirnbauer classification, we show that our system hosts a symmetryprotected phase protected by a magnetic group symmetry (M) and characterized by a Z2 topological invariant. In single-particle spectra, we show that the topological nontrivial phase supports two kinds of edge states, which include two (four) zero-energy edge modes in the absence (presence) of two-photon detuning. We further study the bulk-edge correspondence in a non-Hermitian model by taking into account the particle dissipation. It is shown that the non-Hermitian system preserves the bulk-edge correspondence under the M symmetry but exhibits the non-Hermitian skin effect with breaking the M symmetry at nonzero magnetic flux. This work provides insights in understanding the exotic topological quantum states of high-spin systems and facilitating their experimental explorations.

One-dimensional p-wave superconductivity with long-range hopping and pairing

In this paper, we have considered the extended version of the Kitaev model in one dimension, i.e., a long-range p-wave superconducting wire. In the long-range Kitaev chain, the superconducting hopping and pairing terms in the Hamiltonian decay, independently, in a power-law fashion , where l is the distance between the two sites and x is some positive constant. We have studied the appearance of Majorana zero-energy edge modes and also, massive Dirac edge modes by exact diagonalization, as well as analytical computations. Exact diagonalization indicates the existence of both kinds of massless and massive edge modes in the energy spectrum. Furthermore, we obtain the phase diagram and the topological phase transitions by calculating the winding number, which is the topological invariant.

Fermion zero-energy modes and fractional fermion numbers in a fractional vortex-fermion model

Topological materials have attracted much attention from both physicists and mathematicians recently. Topological properties are closely related to the fermion number (index) of Dirac fermions. The fermion number is given by the η invariant introduced by Atiyah, Padoti and Singer. We discuss a system of Dirac fermions interacting with a vortex and a kink. This system will be realized as a layered material of superconductors and topological insulators, where the Dirac fermion exists on the surface of the topological insulator. The fermion number is fractionalized and the fermion zero-energy excitation mode emerges when Dirac fermions interact with vortices and kinks. Our discussion includes the case where there is a half-flux quantum vortex associated with a kink in a magnetic field in a bilayer superconductor. A normalizable single-valued fermion zero-energy mode does not exist in the core of the half-flux quantum vortex.

Three-dimensional stress analysis of structures in instability conditions using nonlinear displacement-based and hybrid-mixed quadrilaterals based on SaS formulation

In this paper, the three-dimensional (3D) stress analysis of plate-type structures in instability conditions is presented. The displacement-based and hybrid-mixed four-node quadrilateral elements are developed taking the advantages of the sampling surfaces (SaS) method. The SaS formulation is based on considering inside the plate N not equally spaced SaS parallel to the middle surface to specify the displacements of these surfaces as primary plate unknowns. The displacements, strains and stresses are assumed to be distributed through the thickness using Lagrange polynomials of degree N–1 that lead to a well-set higher-order plate theory. The locations of SaS are based on the use of Chebyshev polynomial nodes that allow us to minimize uniformly the error due to Lagrange interpolation. To circumvent shear locking and have no spurious zero energy modes, the assumed transverse shear strains are employed. The nonlinear equilibrium equations are solved by the Newton–Raphson iterative method combined with the Crisfield arc-length algorithm. The accuracy and efficiency of both elements in different conditions such as coarse and distorted meshes are investigated. The developed assumed-natural strain (ANS) elements can be useful for the 3D stress analysis of thin and thick plates in whole states of equilibrium path involving bifurcation, snap-through, and/or snap-back phenomena.

Higher-order topological insulators and semimetals in generalized Aubry-André-Harper models

Higher-order topological phases of matter have been extensively studied in various areas of physics. While the Aubry-Andr\'e-Harper model provides a paradigmatic example to study topological phases, it has not been explored whether a generalized Aubry-Andr\'e-Harper model can exhibit a higher-order topological phenomenon. Here, we construct a two-dimensional higher-order topological insulator with chiral symmetry based on the Aubry-Andr\'e-Harper model. We find the coexistence of zero-energy and nonzero energy corner-localized modes. The former is protected by the quantized quadrupole moment, while the latter by the first Chern number of the Wannier band. The nonzero-energy mode can also be viewed as the consequence of a Chern insulator localized on a surface. More interestingly, the non-zero energy corner mode can lie in the continuum of extended bulk states and form a bound state in the continuum of higher-order topological systems. We finally propose an experimental scheme to realize our model in electric circuits. Our study opens a door to further study higher-order topological phases based on the Aubry-Andr\'e-Harper model.

Majorana-Kondo interplay in T-shaped double quantum dots

The transport behavior of a double quantum dot side-attached to a topological superconducting wire hosting Majorana zero-energy modes is studied theoretically in the strong correlation regime. It is shown that Majorana modes can leak to the whole nanostructure, giving rise to a subtle interplay between the two-stage Kondo screening and the half-fermionic nature of Majorana quasiparticles. In particular, the coupling to the topological wire is found to reduce the effective exchange interaction between the two quantum dots in the absence of normal leads. Interestingly, it also results in an enhancement of the second-stage Kondo temperature when the normal leads are attached. Moreover, it is shown that the second stage of the Kondo effect can become significantly modified in one of the spin channels due to the interference with the Majorana zero-energy mode, yielding the low-temperature conductance equal to $G=G_0/4$, where $G_0 = 2e^2/h$, instead of $G=0$ in the absence of the topological superconducting wire. We also identify a nontrivial spin-charge ${\rm SU}(2)$ symmetry present in the system at a particular point in parameter space, despite lack of the spin nor charge conservation. Finally, we discuss the consequences of a finite overlap between two Majorana modes, as relevant for short Majorana wires.

A higher-order stress point method for non-ordinary state-based peridynamics

A higher-order stress point method for non-ordinary state-based peridynamic (NOSB-PD) is presented. The stress point is interpolated by its adjacent original nodes, where the second-order peridynamic derivatives are determined according to the peridynamic differential operator. By introducing the higher-order derivatives, the proposed stress point method could efficiently suppress the zero-energy mode oscillations. In contrast to other available control methods, the stress point method has better accuracy and stability, and much fewer period elongations in the dynamic calculation. More importantly, the method does not require any additional parameters and it can mitigate the errors arising from the surface effect as well. Furthermore, the proposed method can easily solve the discontinuities problems since it leverages the non-local property of the NOSB-PD. As demonstrated in the last two examples, the phenomena of crack propagation and branching are successfully captured by the proposed stress point method.

Optical non-Hermitian para-Fermi oscillators

We present a proposal for the optical simulation of para-Fermi oscillators in arrays of coupled waveguides. We use a representation that arises as a deformation of the $su(2)$ algebra. This provides us with a set of chiral and a zero-energy-like normal modes. The latter is its own chiral pair and suggest the addition of controlled losses/gains following a pattern defined by parity. In these non-Hermitian para-Fermi oscillators, the analog of the zero-energy mode presents the largest effective loss/gain and it is possible to tune the system to show sequences of exceptional points and varying effective losses/gains. These arrays can be used for mode suppression or enhancement depending on the use of loss or gain, in that order. We compare our coupled mode theory predictions with finite element method simulations to good agreement.