arXiv subjects

Ma Luo

Publications and source records attributed to Ma Luo.

At least 19 recordsLinked to original sources

arXiv2026

Built-In Electric Field Modulation of Spontaneous Magnetism and Thermospin Transport in Boron-Nitrogen Doped Zigzag Graphene Nanoribbons

Zigzag-edged graphene nanoribbons (ZGNRs) host large magnetic moments at two zigzag edges due to spontaneous magnetism, whose ground state is antiferromagnetically coupled spin-order edge state (AF state). In this paper, the spontaneous magnetism and thermoelectricity of ZGNRs with periodic substituted doping of boron and nitrogen atoms along the axial direction are investigated by first principle calculation. The doping induces an internal electric field, which modulates the magnetic moments at the two zigzag edges and the band structure with spin splitting. For varying doping configuration, the ground state could be AF state, ferromagnetically coupled spin-order edge state (FM state), or degenerated pair of AF and FM states. External transversal static electric field can further modulate the magnetic moments and the band structure. By designing the doping configuration and the external field, pure thermal spin current with the absence of thermal charge current can be generated at room temperature, which can be switched on and off by flipping the magnetic configuration between AF and FM states. Thus, the doped ZGNRs can be applied as prototype devices for spin-caloritronic.

cond-mat.mes-hall↗

arXiv2026

Optical Amplification with Large Goos-H$\ddot{a}$nchen Shift Driven by Non-Hermitian Bilayer Meta-Grating

Optical Goos-H$\ddot{a}$nchen shifts can be enhanced by resonant mode with high quality factor, such as quasi-bound states in the continuum in meta-grating. Coexistence of gain and loss in bilayer meta-grating with parity-time symmetry could transfer bound states in the continuum into lasing threshold modes with real resonant frequencies and non-zero far-field radiation. When the incident frequency approaches the resonant frequency of a lasing threshold mode, the reflected and transmitted beams are strongly amplified and undergo large Goos-H$\ddot{a}$nchen shifts. The amplitude of the Goos-H$\ddot{a}$nchen shifts, including the magnitude and sign, are proportional to the reciprocal of the imaginary part of the resonant frequencies. As the incident frequency scan across the resonant frequency of a lasing threshold mode, the imaginary part flip sign, so that the Goos-H$\ddot{a}$nchen shifts diverge as well as flip sign. Simulations of optical responses under incident of Gaussian beams with finite beam width exhibit the sign flipping of the Goos-H$\ddot{a}$nchen shift with large magnitude by fine tuning the incident frequency across the resonant frequency of a lasing threshold mode.

physics.optics↗

arXiv2026

On the universal curve with unordered marked points in positive characteristic

We study the relative pro-$\ell$ and continuous relative completions of the algebraic fundamental groups of universal curves over the moduli stack of curves with unordered marked points in positive characteristic. Using specialization and homotopy exact sequences, we compare the ordered and unordered settings and prove that the natural projection from the relative completion of the universal curve over the unordered moduli stack admits no section in positive characteristic. This yields a non-splitting result for the corresponding projection on algebraic fundamental groups. The present paper is a sequel to our earlier work in characteristic zero.

math.AG↗

arXiv2025

On the Universal Curves with Unordered Marked Points

Over any field of characteristic $0$, we prove that the homotopy exact sequence of algebraic fundamental groups for the universal curve with unordered marked points does not split. The same nonsplitting holds for the universal hyperelliptic curve. Our approach extends Chen's topological result to the profinite setting and relies on the use of relative and continuous relative completions to detect the nonexistence of algebraic sections.

math.AG↗

arXiv2025

Goos-H$\ddot{a}$nchen shifts of bilayer meta-grating with unidirectional guide resonance

Bilayer meta-gratings with asymmetric structural parameters could host unidirectional guide resonances. The distribution of unidirectional guide resonances in the space of structural parameters and synthetic parameters is identified. As the incident optical beam being resonant with the unidirectional guide resonance, the Goos-H$\ddot{a}$nchen shifts of the scattered beams exhibit two anomalous behaviors: the resonant peak of the Goos-H$\ddot{a}$nchen shift is accompanied by constant transmittance and reflectance; the magnitude of the Goos-H$\ddot{a}$nchen shift is not always proportional to the quality factor of the unidirectional guide resonance. The temporal coupled mode theory analysis reveals that the first anomalous behavior is due to interference between direct scattering and radiation from the unidirectional guide resonance; the Goos-H$\ddot{a}$nchen shifts are proportional to the group velocity as well as the quality factor of the unidirectional guide resonance. Numerical simulations of incidence of Gaussian beam with finite beam width provide intuitive visualization of the Goos-H$\ddot{a}$nchen shift.

physics.optics↗

arXiv2025

Goos-H$\ddot{a}$nchen shift of normally incident beam on magneto-optical meta-grating

Goos-H$\ddot{a}$nchen shift of normally incident optical beam to meta-grating consisted of magneto-optical material are studied. The meta-grating is consisted of compound grating of magneto-optical rod on top of a dielectric slab, which induces zone-folding quasi-bound states in the continuum at the $\Gamma$ point with non-zero group velocity. By applying stationary-phase theory, the Goos-H$\ddot{a}$nchen shift of normally incident quasi-plane wave are found to be enhanced at the reflection peak. Simulations of normally incident Gaussian beam with finite beam width show that the Goos-H$\ddot{a}$nchen shift is dependent on the beam width.

physics.optics↗

arXiv2025

Coherent Goos-H$\ddot{a}$nchen shifts of meta-grating with radiation asymmetry

The coherent Goos-H$\ddot{a}$nchen shifts of meta-grating are proposed, which is the Goos-H$\ddot{a}$nchen shifts of the two outgoing beams under the simultaneous incidence of two coherent optical beams from opposite sides of the grating with the same lateral wave number. As both of the frequency and lateral wave number are resonant with a topological state of the meta-grating, such as unidirectionally guided resonance or circular polarized states, the energy flux and Goos-H$\ddot{a}$nchen shifts of the two outgoing beams are coherently controlled by the relative phase difference between the two incident beams. By applying stationary-phase method, it is found that the enhancement of coherent Goos-H$\ddot{a}$nchen shifts by the unidirectionally guided resonance and circular polarized states is accompanied by constant and peak transmittance, respectively. Analysis with temporal coupled mode theory shows that the different features are due to difference mechanism of interference between direction scattering and resonant radiation. The coherent Goos-H$\ddot{a}$nchen shifts with incident Gaussian beams are sensitive to the relative phase between the two beams, which can be applied in refractive index sensor.

physics.optics↗

arXiv2025

On the Birman exact sequence of the subgroups of the mapping class group of genus three

We prove that for any finite index subgroup of the mapping class group containing the Johnson subgroup, the profinite Birman exact sequence does not split in genus $g\ge 3$, extending prior results of Hain and the second author for $g\ge 4$. For the Torelli group, we prove that the graded Lie algebra version of the Birman exact sequence admits no section with symplectic equivariance, extending Hain's result from $g\ge 4$ to $g=3$. These results are deduced by our main tool, relative completion, with the help of Hodge theory and representation theory of symplectic groups, along with explicit structural obstructions coming from hyperelliptic mapping class groups.

math.AT↗

arXiv2024

Light storage in wavy dielectric grating with Kerr nonlinearity

Periodical corrugation in dielectric slab transfers the two waveguide modes at zero Bloch wave number into a leaky resonant mode and a symmetry protected bound states in the continuum (BIC) with small frequency detune. The leaky resonant mode can be directly excited by weak linearly polarized normally incident optical field. In the presence of Kerr nonlinearity, the BIC can be indirectly excited by an optical bistable response. Two types of bistable operations are considered. For the first type, the intensity of the incident field gradually increases to exceed a critical value, and then decreases to zero. For the second type, the intensity is fixed, while the linear polarization angle of the incident field gradually increases to exceed a critical value, and then decreases to 0$^{o}$. Theoretically, the indirectly excited BIC can store the optical energy without loss, even though the intensity of the incident field decreases to zero. Incidence of an optical field with double frequency or orthogonal linear polarization can erase the stored optical field by destroying the BIC. The proposed optical system could function as optical storage and switching device.

physics.optics↗

arXiv2024

Refractive index sensing based on large negative Goos-H$\ddot{a}$nchen shifts of wavy dielectric grating

Wavy dielectric grating hosts bound states in the continuum (BICs) at nonzero Bloch wave number. For oblique incident optical field with parameters near to the BICs, the reflectance spectrum exhibits ultra-sharp Fano line shape, and the reflected beam has large negative Goos-H$\ddot{a}$nchen shift, due to excitation of the corresponding quasi-BIC with negative group velocity. Under incidence of Gaussian beam with sizable beam width, the excited quasi-BIC could travel a long distance along the direction of the Goos-H$\ddot{a}$nchen shift, designated as $L_{GH}$, before the energy is completely radiated. If the length between the termination of the wavy shape and the focus of the incident Gaussian beam is smaller than $L_{GH}$, sizable energy flux can be coupled into the waveguide mode of the flat dielectric slab that is connected to the wavy dielectric grating. Measurement of the energy flux of the waveguide mode can sense the variation of the refractive index of the background medium. The proposed sensing scheme can be integrated with waveguide in optical circuit.

physics.optics↗

arXiv2023

Localized Floquet states in gated bilayer graphene induced by a focused optical beam with orbital angular momentum

We theoretically studied the Floquet state of gated bilayer graphene, which is irradiated by normally incident focused Gaussian beam with orbital angular momentum (OAM). According to the Floquet theory, in-plane and out-of-plane electric field of the OAM beam periodically perturbs the intralayer and interlayer hopping, which are equivalent to the presence of effective staggered sublattice potential and next-nearest neighboring interlayer hopping within the light spot region, respectively. The combination of the effective terms and the gated voltage form an effective trapping potential, which hosts localized quantum states with energy level being within the energy gap of the non-irradiated gated bilayer graphene. The energy spectrum can be tuned by the amplitude of the optical beam and additional static magnetic field. By engineering the parameters, valley-polarized two-fold degenerated zero energy Floquet states can be induced.

cond-mat.mes-hall↗

arXiv2023

Localized Refractive index sensing by integrated photonic crystal waveguide with edge-cavity

We have theoretically proposed a highly compact refractive-index sensor consisted of edge-cavity and line-defect waveguide in two-dimensional photonic crystal. The sensing object is completely outside of the single enclosed surface of the sensor. The edge-cavity is designed by engineering the spatial distribution of the cutoff frequency of edge modes. The coupling between the edge-cavity and the waveguide is maximized by optimizing the radius of the rods between them, so that the transmittance spectrum through the waveguide has a sharp anti-peak. As the refractive index of the sensing object changes, the resonant wavelength of the edge-cavity is changed, which in turn changes the wavelength of the anti-peak. The sensitivity of the sensor is up to 40 nm/RIU, and the footprint of the sensor is only 40 $\mu m^{2}$. Because the transmittance spectrum is determined by the overlap between the sensing object and the highly localized resonant mode, the sensor can also perceive spatial distribution of refractive index in the sensing object.

physics.optics↗

arXiv2023

Remarks on Collino cycles and hyperelliptic Johnson homomorphisms

A Collino cycle is a higher cycle on the Jacobian of a hyperelliptic curve. The universal family of Collino cycles naturally gives rise to a normal function, whose induced monodromy relates to the hyperelliptic Johnson homomorphism. Colombo computed this monodromy explicitly and made this relation precise. We recast this in the perspective of relative completion. In particular, we use Colombo's result to construct Collino classes, which are cohomology classes of hyperelliptic mapping class groups with coefficients in a certain symplectic representation. We also determine the dimension of their span in the case of the level two hyperelliptic mapping class group.

math.AG↗

arXiv2022

Wavy optical grating: wideband reflector and Fabry-Perot BICs

In this study, we theoretically and numerically investigate the resonant modes and reflectance of an optical grating consisting of a wavy dielectric slab by applying the spectral element method. The presence of the wavy shape transforms the waveguide modes into leaky resonant modes. A few resonant modes with specific longitudinal wave number have infinitely large Q factor, while the other resonant modes have finite Q factor. For the leaky resonant mode with zero longitudinal wave number, the Q factor is inversely proportional to the amplitude of the wavy shape. An array of multiple low-Q wavy gratings has a high reflectance in a large bandwidth. A double-layer wavy grating forms a Fabry-Perot cavity, which hosts Fabry-Perot bound states in the continuum (BICs) at the resonant frequency. The Q-factor of the Fabry-Perot cavity can be tuned by adjusting the distance between the two wavy slabs. The wavy shape could be generated by a vibrational wave in a flat dielectric slab so that the BICs mode and wideband reflectance could be controlled on-demand.

physics.optics↗

arXiv2022

Tuning the magnetic configuration of bilayer graphene quantum dot by twisting

Twistronic has recently attracted tremendous attention because the twisting can engineer the bilayer graphene-like materials into varying types of strongly correlated phases. In this paper, we study the twisting of bilayer graphene (BLG) quantum dots (QDs) with hexagonal shape and zigzag edges. In the untwisted BLG-QDs, the zigzag edges of graphene host spontaneous magnetism with varying magnetic configurations. As a BLG-QD being adiabatically twisted, the quantum state evolves as a function of the twisting angle. If the twisting angle changes across certain critical value, the magnetic configuration of the quantum state sharply changes. For the twisting process with increasing or decreasing twisting angle, the number and value of the critical twisting angles are different. Thus, the twisting process with the twisting angle increasing and decreasing back and forth could enter a hysteresis loop. The twisting of BLG QDs with adatom is also investigated. The tuning features of the magnetic configuration of the twisted BLG-QDs could be applied for graphene-based quantum memory devices.

cond-mat.mes-hall↗

arXiv2022

Engineering Majorana corner modes from two-dimensional hexagonal crystals

Second order topological insulator can be engineered from two-dimensional materials with strong spin-orbit coupling and in-plane Zeeman field. In proximity to superconductor, topological superconducting phase could be induced in the two-dimensional materials, which host Majorana corner modes at the intersection between two zigzag edges. Two types of tight binding models in hexagonal lattice, which include $p_{z}$ or $p_{x,y}$ orbit(s) in each lattice site, are applied to engineer two-dimensional materials in topological superconducting phase. In both models, the condition that induces the second order topological superconductor requires nonuniform value of either in-plane Zeeman fields or superconductor pairing parameters in two sublattices. The finite size effect of the second model is weaker than that of the first model.

cond-mat.supr-con↗

arXiv2022

Chiral Majorana Fermions in two dimensional square lattice antiferromagnet with proximity-induced superconductivity

Combination of proximity-induced superconductivity and ferromagnetic exchange field in a two-dimensional square-lattice antiferromagnet with spin-orbit coupling and nonsymmorphic symmetry can induce a topological superconductor phase with chiral Majorana edge states. The lattice model of the Bogoliubov-de Gennes (BdG) Hamiltonian was applied to study the phase diagram of bulks and chiral Majorana edge states in nanoribbons. By numerically studying the phase diagram, we found that the non-uniformity of either the superconducting pairing parameters or the exchange field at the two sublattices is necessary to induce a topological superconductor phase with chiral Majorana edge states. The BdG Chern number of certain topological superconductor phases is $\pm1$ or $\pm3$, such that the corresponding nanoribbons have one or three pairs of chiral Majorana edge states, respectively.

cond-mat.supr-con↗

arXiv2021

Floquet Engineering of Two Dimensional Photonic Waveguide Arrays with $\pi$ or $\pm2\pi/3$ Corner states

In this paper, we theoretically study the Floquet engineering of two dimensional photonic waveguide arrays in three types of lattices: honeycomb lattice with Kekule distortion, breathing square lattice and breathing Kagome lattice. The Kekule distortion factor or the breathing factor in the corresponding lattice is periodically changed along the axial direction of the photonic waveguide with frequency $\omega$. Within certain ranges of $\omega$, the Floquet corner states in the Floquet band gap of quasi-energy spectrum are found, which are localized at the corner of the finite two-dimensional arrays. Due to particle-hole symmetric in the model of honeycomb and square lattice, the quasi-energy level of the Floquet $\pi$ corner states is $\pm\omega/2$. On the other hand, Kagome lattice does not have particle-hole symmetric, so that the quasi-energy level of the Floquet $\pm2\pi/3$ corner states is near to $\pm1\omega/3$. The corner states are either protected by crystalline symmetry or reflection symmetry. The finding of Floquet fractional-$\pi$ corner states could provide more options for engineering of on-chip photonic devices.

physics.optics↗