arXiv subjects

Z. Y. Chen

Publications and source records attributed to Z. Y. Chen.

At least 145 records · Page 8Linked to original sources

arXiv2023

Chess-board Acoustic Crystals with Momentum-space Nonsymmorphic Symmetries

Spatial symmetries appearing in both real and momentum space are of fundamental significance to crystals. However, in the conventional framework, every space group in real space, either symmorphic or nonsymmorphic, corresponds to a symmorphic dual in momentum space. Our experiment breaks the framework by showing that in a 2D acoustic crystal with chess-board pattern of $π$ and 0 fluxes, mirror reflections are manifested nonsymmorphically as glide reflections in momentum space. These momentum-space nonsymmorphic symmetries stem from projective, rather than ordinary, representations of the real-space symmetries due to the peculiar flux pattern. Moreover, our experiment demonstrates that the glide reflection can reduce the topological type of the Brillouin zone from the torus to the Klein bottle, resulting in novel topological phases with new topological invariants. Since crystalline topologies are based on momentum-space symmetries, our work paves the way for utilizing engineerable gauge fluxes over artificial crystals to extend the current topological classifications into the broader regime of momentum-space nonsymmorphic symmetries.

cond-mat.mes-hall↗

arXiv2023

Stiefel-Whitney topological charges in a three-dimensional acoustic nodal-line crystal

Band topology of materials describes the extent Bloch wavefunctions are twisted in momentum space. Such descriptions rely on a set of topological invariants, generally referred to as topological charges, which form a characteristic class in the mathematical structure of fiber bundles associated with the Bloch wavefunctions. For example, the celebrated Chern number and its variants belong to the Chern class, characterizing topological charges for complex Bloch wavefunctions. Nevertheless, under the space-time inversion symmetry, Bloch wavefunctions can be purely real in the entire momentum space; consequently, their topological classification does not fall into the Chern class, but requires another characteristic class known as the Stiefel-Whitney class. Here, in a three-dimensional acoustic crystal, we demonstrate a topological nodal-line semimetal that is characterized by a doublet of topological charges, the first and second Stiefel-Whitney numbers, simultaneously. Such a doubly charged nodal line gives rise to a doubled bulk-boundary correspondence: while the first Stiefel-Whitney number induces ordinary drumhead states of the nodal line, the second Stiefel-Whitney number supports hinge Fermi arc states at odd inversion-related pairs of hinges. These results establish the Stiefel-Whitney topological charges as intrinsic topological invariants for topological materials, with their unique bulk-boundary correspondence beyond the conventional framework of topological band theory.

cond-mat.mes-hall↗

arXiv2023

Classification of time-reversal-invariant crystals with gauge structures

A peculiar feature of quantum states is that they may embody so-called projective representations of symmetries rather than ordinary representations. Projective representations of space groups-the defining symmetry of crystals-remain largely unexplored. Despite recent advances in artificial crystals, whose intrinsic gauge structures necessarily require a projective description, a unified theory is yet to be established. Here, we establish such a unified theory by exhaustively classifying and representing all 458 projective symmetry algebras of time-reversal-invariant crystals from 17 wallpaper groups in two dimensions-189 of which are algebraically non-equivalent. We discover three physical signatures resulting from projective symmetry algebras, including the shift of high-symmetry momenta, an enforced nontrivial Zak phase, and a spinless eight-fold nodal point. Our work offers a theoretical foundation for the field of artificial crystals and opens the door to a wealth of topological states and phenomena beyond the existing paradigms.

cond-mat.mes-hall↗

arXiv2022

Precision measurement of reactor antineutrino oscillation at kilometer-scale baselines by Daya Bay

We present a new determination of the smallest neutrino mixing angle $θ_{13}$ and the mass-squared difference $Δ{\rm m}^{2}_{32}$ using a final sample of $5.55 \times 10^{6}$ inverse beta-decay (IBD) candidates with the final-state neutron captured on gadolinium. This sample was selected from the complete data set obtained by the Daya Bay reactor neutrino experiment in 3158 days of operation. Compared to the previous Daya Bay results, selection of IBD candidates has been optimized, energy calibration refined, and treatment of backgrounds further improved. The resulting oscillation parameters are ${\rm sin}^{2}2θ_{13} = 0.0851 \pm 0.0024$, $Δ{\rm m}^{2}_{32} = (2.466 \pm 0.060) \times 10^{-3}{\rm eV}^{2}$ for the normal mass ordering or $Δ{\rm m}^{2}_{32} = -(2.571 \pm 0.060) \times 10^{-3} {\rm eV}^{2}$ for the inverted mass ordering.

hep-ex↗

arXiv2022

Periodic Clifford symmetry algebras on flux lattices

Real Clifford algebras play a fundamental role in the eight real Altland-Zirnbauer symmetry classes and the classification tables of topological phases. Here, we present another elegant realization of real Clifford algebras in the $d$-dimensional spinless rectangular lattices with $π$ flux per plaquette. Due to the $T$-invariant flux configuration, real Clifford algebras are realized as projective symmetry algebras of lattice symmetries. Remarkably, $d$ mod $8$ exactly corresponds to the eight Morita equivalence classes of real Clifford algebras with eightfold Bott periodicity, resembling the eight real Altland-Zirnbauer classes. The representation theory of Clifford algebras determines the degree of degeneracy of band structures, both at generic $k$ points and at high-symmetry points of the Brillouin zone. Particularly, we demonstrate that the large degeneracy at high-symmetry points offers a rich resource for forming novel topological states by various dimerization patterns, including a $3$D higher-order semimetal state with double-charged bulk nodal loops and hinge modes, a $4$D nodal surface semimetal with $3$D surface solid-ball zero modes, and $4$D Möbius topological insulators with a eightfold surface nodal point or a fourfold surface nodal ring. Our theory can be experimentally realized in artificial crystals by their engineerable $\mathbb{Z}_2$ gauge fields and capability to simulate higher dimensional systems.

cond-mat.mes-hall↗

arXiv2022

Spinless Mirror Chern Insulator from Projective Symmetry Algebra

It was commonly believed that a mirror Chern insulator (MCI) must require spin-orbital coupling, since time-reversal symmetry for spinless systems contradicts with the mirror Chern number. So MCI cannot be realized in spinless systems which include the large field of topological artificial crystals. Here, we disprove this common belief. The first point to clarify is that the fundamental constraint is not from spin-orbital coupling but the symmetry algebra of time reversal and mirror operations. Then, our theory is based on the conceptual transformation that the symmetry algebras will be projectively modified under gauge fields. Particularly, we show that the symmetry algebra of mirror reflection and time-reversal required for MCI can be achieved projectively in spinless systems with lattice $\mathbb{Z}_2$ gauge fields, i.e., by allowing real hopping amplitudes to take $\pm$ signs. Moreover, we propose the basic structure, the twisted $π$-flux blocks, to fulfill the projective symmetry algebra, and develop a general approach to construct spinless MCIs based on these building blocks. Two concrete spinless MCI models are presented, which can be readily realized in artificial systems such as acoustic crystals.

cond-mat.other↗

arXiv2022

Brillouin Klein Bottle From Artificial Gauge Fields

A Brillouin zone is the unit for the momentum space of a crystal. It is topologically a torus, and distinguishing whether a set of wave functions over the Brillouin torus can be smoothly deformed to another leads to the classification of various topological states of matter. Here, we show that under $\mathbb{Z}_2$ gauge fields, i.e., hopping amplitudes with phases $\pm 1$, the fundamental domain of momentum space can assume the topology of a Klein bottle. This drastic change of the Brillouin zone theory is due to the projective symmetry algebra enforced by the gauge field. Remarkably, the non-orientability of the Brillouin Klein bottle corresponds to the topological classification by a $\mathbb{Z}_2$ invariant, in contrast to the Chern number valued in $\mathbb{Z}$ for the usual Brillouin torus. The result is a novel Klein-bottle insulator featuring topological modes at two edges related by a nonlocal twist, radically distinct from all previous topological insulators. Our prediction can be readily achieved in various artificial crystals, and the discovery opens a new direction to explore topological physics by gauge-field-modified fundamental structures of physics.

cond-mat.mes-hall↗

arXiv2021

Evolution of valence-specific spin states and local distortions in La$_{2-x}$Sr$_x$CoO$_4$

We present X-ray spectroscopic evidence for the evolution of valence-specific spin states and tetragonal distortions in single-layer cobaltates. Measurements of Co $L_3$-edge resonant inelastic X-ray scattering reveal the $t_{2g}$ electronic structure of Co for hole-doped La$_{2-x}$Sr$_x$CoO$_4$ ($x$ = 0.5, 0.7 and 0.8). As the Sr-doping $x$ increases, the tetragonal splitting of the $t_{2g}$ states of high-spin Co$^{2+}$ decreases, whereas that of low-spin Co$^{3+}$ increases and the fraction of high-spin Co$^{3+}$ increases. The results enable us to clarify the origin of the change of magnetic anisotropy and in-plane resistivity in a mixed-valence cobaltate caused by the interplay of spin-orbit coupling and tetragonal distortion.

cond-mat.str-el↗

arXiv2021

Zr and Mo macrosegregation in Ti6246 titanium alloy industrial-scale ingot by vacuum arc remelting

Zr and Mo macrosegregations were investigated in Ti6246 titanium alloy industrial-scale ingotby vacuum arc remelting. The formation mechanism of Zr and Mo macrosegregations was studied during the solidification process. Zr macrosegregation was characterized by low content in the equiaxed grain zone and high content in the hot top zone. Mo exhibits an opposite trend with Zr. The macrosegregations of Zr and Mo were the most pronounced, with a statistic segregation degree higher than Al and Sn. It could be concluded that temperature gradient and solidification rate dominated the macrosegregation formations during the solidification process. The thermal buoyancy made the negative segregation Zr be continuously discharged to the front of the solid-liquid interface. Mo was enriched in the solid phase at the solid-liquid interface as the positive segregation.

cond-mat.mtrl-sci↗

arXiv2019

Rotating magnetic field driven antiferromagnetic domain wall motion: Role of Dzyaloshinskii-Moriya interaction

In this work, we study the rotating magnetic field driven domain wall (DW) motion in antiferromagnetic nanowires, using the micromagnetic simulations of the classical Heisenberg spin model. We show that in low frequency region, the rotating field alone could efficiently drive the DW motion even in the absence of Dzyaloshinskii-Moriya interaction (DMI). In this case, the DW rotates synchronously with the magnetic field, and a stable precession torque is available and drives the DW motion with a steady velocity. In large frequency region, the DW only oscillates around its equilibrium position and cannot propagate. The dependences of the velocity and critical frequency differentiating the two motion modes on several parameters are investigated in details, and the direction of the DW motion can be controlled by modulating the initial phase of the field. Interestingly, a unidirectional DW motion is predicted attributing to the bulk DMI, and the nonzero velocity for high frequency is well explained. Thus, this work does provide useful information for further antiferromagnetic spintronics applications.

physics.comp-ph↗

arXiv2019

Ultralow-loss domain wall motion driven by magnetocrystalline anisotropy gradient in antiferromagnetic nanowire

Searching for new methods controlling antiferromagnetic (AFM) domain wall is one of the most important issues for AFM spintronic device operation. In this work, we study theoretically the domain wall motion of an AFM nanowire, driven by the axial anisotropy gradient generated by external electric field, allowing the electro control of AFM domain wall motion in the merit of ultra-low energy loss. The domain wall velocity depending on the anisotropy gradient magnitude and intrinsic material properties is simulated based on the Landau-Lifshitz-Gilbert equation and also deduced using the energy dissipation theorem. It is found that the domain wall moves at a nearly constant velocity for small gradient, and accelerates for large gradient due to the enlarged domain wall width. The domain wall mobility is independent of lattice dimension and types of domain wall, while it is enhanced by the Dzyaloshinskii-Moriya interaction. In addition, the physical mechanism for much faster AFM wall dynamics than ferromagnetic wall dynamics is qualitatively explained. This work unveils a promising strategy for controlling the AFM domain walls, benefiting to future AFM spintronic applications.

physics.app-ph↗

arXiv2019

Ultrafast depinning of domain wall in notched antiferromagnetic nanostructures

The pinning and depinning of antiferromagnetic (AFM) domain wall is certainly the core issue of AFM spintronics. In this work, we study theoretically the Néel-type domain wall pinning and depinning at a notch in an antiferromagnetic (AFM) nano-ribbon. The depinning field depending on the notch dimension and intrinsic physical parameters are deduced and also numerically calculated. Contrary to conventional conception, it is revealed that the depinning field is remarkably dependent of the damping constant and the time-dependent oscillation of the domain wall position in the weakly damping regime benefits to the wall depinning, resulting in a gradual increase of the depinning field up to a saturation value with increasing damping constant. A one-dimensional model accounting of the internal dynamics of domain wall is used to explain perfectly the simulated results. It is demonstrated that the depinning mechanism of an AFM domain wall differs from ferromagnetic domain wall by exhibiting a depinning speed typically three orders of magnitude faster than the latter, suggesting the ultrafast dynamics of an AFM system.

physics.app-ph↗

arXiv2019

The Landau-Lifshitz-Bloch equation for domain wall motion in antiferromagnets

In this work, we derive the Landau-Lifshitz-Bloch equation accounting for the multi-domain antiferromagnetic (AFM) lattice at finite temperature, in order to investigate the domain wall (DW) motion, the core issue for AFM spintronics. The continuity equation of the staggered magnetization is obtained using the continuum approximation, allowing an analytical calculation on the domain wall dynamics. The influence of temperature on the static domain wall profile is investigated, and the analytical calculations reproduce well earlier numerical results on temperature gradient driven saturation velocity of the AFM domain wall, confirming the validity of this theory. Moreover, it is worth noting that this theory could be also applied to dynamics of various wall motions in an AFM system. The present theory represents a comprehensive approach to the domain wall dynamics in AFM materials, a crucial step toward the development of AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2018

Staggered field driven domain walls motion in antiferromagnetic heterojunctions

In this work, we study the antiferromagnetic (AFM) spin dynamics in heterostructures which consist of two kinds of AFM layers. Our micromagnetic simulations demonstrate that the AFM domain-wall (DW) can be driven by the other one (driven by field-like Neel spin-orbit torque, Phys. Rev. Lett. 117, 017202 (2016)) through the interface couplings. Furthermore, the two DWs detach from each other when the torque increases above a critical value. The critical field and the highest possible velocity of the DW depending on several factors are revealed and discussed. Bases on the calculated results, we propose a method to modulate efficiently the multi DWs in antiferromagnet, which definitely provides useful information for future AFM spintronics device design.

physics.app-ph↗

arXiv2018

Microwave fields driven domain wall motions in antiferromagnetic nanowires

In this work, we study the microwave field driven antiferromagnetic domain wall motion in an antiferromagnetic nanowire, using the numerical calculations based on a classical Heisenberg spin model. We show that a proper combination of a static magnetic field plus an oscillating field perpendicular to the nanowire axis is sufficient to drive the domain wall propagation along the nanowire with the axial magnetic anisotropy. More importantly, the drift velocity at the resonance frequency is comparable to that induced by temperature gradients, suggesting that microwave field can be a very promising tool to control domain wall motions in antiferromagnetic nanostructures. Furthermore, the dependences of resonance frequency and drift velocity on the static and oscillating fields, the axial anisotropy, and the damping constant are discussed in details. This work provides useful information for the spin dynamics in antiferromagnetic nanostructures for spintronics applications.

physics.app-ph↗

arXiv2017

A study on combined effects of stochastic magnetic fluctuations and synchrotron radiation on the production of runaway electrons

The dynamics of relativistic runaway electrons are analyzed using the relativistic Fokker-Planck equation including deceleration due to the synchrotron radiation and radial diffusion loss caused by stochastic magnetic fluctuations (SMFs). SMFs are treated as friction force in [J. Martin-Solis et al., Phys. Plasmas 6, 3925 (1999)]. However, we think SMFs act as a porter in configuration space, but not directly affect the runaway electrons (REs) in momentum space. Both critical electric fields for sustainment of the existing REs and for avalanche onset are enhanced, and the modified avalanche growth rate is reduced by the combined effects of SMFs and synchrotron radiation as compared to the case with only synchrotron radiation [P. Aleynikov et al., Phys. Rev. Lett. 114, 155001 (2015)].

physics.plasm-ph↗

arXiv2017

Coulomb correlations intertwined with spin and orbital excitations in LaCoO$_3$

We carried out temperature-dependent (20 - 550 K) measurements of resonant inelastic X-ray scattering on LaCoO$_3$ to investigate the evolution of its electronic structure across the spin-state crossover. In combination with charge-transfer multiplet calculations, we accurately quantized the renormalized crystal-field excitation energies and spin-state populations. We show that the screening of the on-site Coulomb interaction of 3d electrons is orbital selective and coupled to the spin-state crossover in LaCoO$_3$. The results establish that the gradual spin-state crossover is associated with a relative change of Coulomb energy versus bandwidth, leading to a Mott-type insulator-to-metal transition.

cond-mat.str-el↗

arXiv2017

Formation of soap bubbles by gas jet

Soap bubbles can be easily generated by varies methods, while their formation process is complicated and still worth study. A model about the bubble formation process was proposed in Phys. Rev. Lett. 116, 077801 recently, and it was reported that the bubbles were formed when the gas blowing velocity was above one threshold. However, after repeating these experiments, we found the bubbles could be generated in two velocities ranges which corresponded to laminar and turbulent gas jet respectively, and the predicted threshold was only effective for turbulent gas flow. The study revealed that the bubble formation was mainly influenced by the aerodynamics of the gas jet blowing to the film, and these results will help to further understand the formation mechanism of the soap bubble as well as the interaction between gas jet and thin liquid film.

physics.flu-dyn↗