Bulk Polarization Research Articles

Acoustic higher-order topological states in kagome lattice with split-ring resonators

The next-nearest-neighbour (NNN) hoppings can induce many unique topological phenomena but are rarely considered in acoustics. This work investigates the higher-order topological states induced by the NNN hoppings in acoustic kagome lattice with split-ring resonators (SRRs). We build an extended tight-binding model (TBM) considering NNN hoppings to predict the topological corner modes (CMs) of the acoustic system and obtain the coupling strengths between the SRRs in different rotational angles. During rotating the SRRs, two nontrivial and one trivial systems characterized by bulk polarization can be obtained. In these two nontrivial systems, four novel CMs induced by the NNN hoppings can be observed in the first bandgap. Furthermore, the interactions between resonant and Bragg scattering effect of the SRRs are researched and the defect modes (DMs) are realized in the second bandgap. Finally, the CMs predicted by the extended TBM and DMs match well with the numerical and experimental ones. These discoveries unveil the intricate physics driven by the NNN hoppings, paving the way for innovative approaches to the design of tunable sound energy localization and multi-band metamaterials.

Observation of Topologically Distinct Corner States in “Bearded” Photonic Kagome Lattices

AbstractKagome lattices represent an archetype of intriguing physics, attracting a great deal of interest in different branches of natural sciences, recently in the context of topological crystalline insulators. Here, two distinct classes of corner states in breathing Kagome lattices (BKLs) with “bearded” edge truncation are demonstrated, unveiling their topological origin. The in‐phase corner states are found to exist only in the topologically nontrivial regime, characterized by a nonzero bulk polarization. In contrast, the out‐of‐phase corner states appear in both topologically trivial and nontrivial regimes, either as bound states in the continuum or as in‐gap states depending on the lattice dimerization conditions. Furthermore, the out‐of‐phase corner states are highly localized, akin to flat‐band compact localized states, and they manifest both real‐ and momentum‐space topology. Experimentally, both types of corner states are observed in laser‐written photonic bearded‐edge BKLs, corroborated by numerical simulations. The results not only deepen the current understanding of topological corner modes in BKLs but also provide new insight into their physical origins, which may be applied to other topological BKL platforms beyond optics.

Quantifying the Ground‐State Hydrogen‐Bond Formation of a Super‐Photoacid by Inspecting Its Excited‐State Dynamics

AbstractThe identification and quantification of hydrogen (H)‐bonded complexes form the cornerstone of reaction‐mechanism analysis in ultrafast proton‐transfer reactions. Traditionally, the Benesi–Hildebrand method has been employed to obtain the formation constants of H‐bonded complexes, given that H‐bonding additives induce an alteration in spectral features exclusively through H‐bond formation. However, if the additive introduction impacts the bulk polarity of the solution, inducing a spectral shift, the spectroscopic method‘s accuracy in analyzing the H‐bond formation becomes compromised. In this study, we scrutinize H‐bond formation under the influence of an H‐bond accepting solute in an aprotic solvent. This is achieved by quantifying the fractions of two concurrent pathways involved in the excited‐state proton transfer (ESPT) of a super‐photoacid: the ultrafast ESPT of an H‐bonded complex versus the diffusion‐controlled ESPT of the free acid. Our method offers improved accuracy compared to conventional steady‐state spectroscopic techniques, by directly quantifying the H‐bonded complexes using the time‐resolved spectroscopic method, thereby circumventing the aforementioned limitation.

Real higher-order Weyl photonic crystal

Higher-order Weyl semimetals are a family of recently predicted topological phases simultaneously showcasing unconventional properties derived from Weyl points, such as chiral anomaly, and multidimensional topological phenomena originating from higher-order topology. The higher-order Weyl semimetal phases, with their higher-order topology arising from quantized dipole or quadrupole bulk polarizations, have been demonstrated in phononics and circuits. Here, we experimentally discover a class of higher-order Weyl semimetal phase in a three-dimensional photonic crystal (PhC), exhibiting the concurrence of the surface and hinge Fermi arcs from the nonzero Chern number and the nontrivial generalized real Chern number, respectively, coined a real higher-order Weyl PhC. Notably, the projected two-dimensional subsystem with kz = 0 is a real Chern insulator, belonging to the Stiefel-Whitney class with real Bloch wavefunctions, which is distinguished fundamentally from the Chern class with complex Bloch wavefunctions. Our work offers an ideal photonic platform for exploring potential applications and material properties associated with the higher-order Weyl points and the Stiefel-Whitney class of topological phases.

Degeneration of topological corner, hinge, and surface states in three-dimensional photonic crystals.

The third-order topological insulators based on three-dimensional (3D) photonic crystals (PCs) have hardly been achieved because the nontrivial bandgap in 3D PCs is difficult to form. In this Letter, we elaborately construct 3D Su-Schrieffer-Heeger lattice in which the periodic modulation of refractive index is uniform in three axis directions. The high-order topological PCs are characterized by the nontrivial bulk polarizations and the mirror eigenvalues. Such a structure can achieve topological 1-codimensional surface states, 2-codimensional hinge states, and 3-codimensional corner states. More importantly, it is found for the first time, to the best of our knowledge, that the topological states exhibit a degeneration behavior, i. e., the corner, and hinge state, or corner and surface states coexist at nearly the same frequency, but maintain their own mode properties. The multiple topological states in 3D PCs as well as the degeneration of topological states will open a new window for the study of topological photonics.

Hydrocarbon residue in a Danish chalk reservoir and its effects on CO2 injectivity

Depleted oil and gas chalk reservoirs in the Danish North Sea are considered for Carbon Capture and Storage (CCS) as part of Denmark's strategy to reduce CO2 emissions. While these reservoirs are well understood after decades of hydrocarbon production, the interaction between CO2 and hydrocarbon residues (HCRs) is less understood and pose a major challenge. This study investigates the effects of supercritical CO2 (sc-CO2) injection on HCRs in a core plug from the Upper Cretaceous chalk reservoir of the Halfdan Field, Danish North Sea, after sc-CO2 flooding for nine days in near reservoir conditions (85 °C and 27.6 MPa). The HCRs from before and after the sc-CO2 flooding were analyzed using Extended Slow Heating (ESH®) pyrolysis, Thermal Desorption Pyrolysis Gas Chromatography-Mass Spectrometry (TD-Py-GC/MS), and organic petrography methods. Results indicate that sc-CO2 can effectively mobilize and put into solution all lighter hydrocarbon fractions, leaving behind heavy immiscible asphaltene-rich hydrocarbons and solid bitumen. In the post-CO2 flooding sample, the asphaltene-rich, immobile oil and solid bitumen aggregates were physically trapped at various points, though primarily at the outlet section due to a sudden change in pressure. The results indicate pressure differences cause up to 10.5% of total rock volume precipitation of pure solid bitumen, which can significantly alter the ongoing flow of sc-CO2 in the reservoir. The HCRs also show an increasing proportion the heaviest, solid bitumen fraction from the inlet of sc-CO2 injection to the outlet of the plug. The increase in solid bitumen content away from the sc-CO2 inlet point is suggested to be caused by selective solubility together with imbibition of hydrocarbons within the flowing sc-CO2 phase, thereby changing the bulk polarity and hence the solubility - here referred to as the Avalanche Effect.

Third-order square-root topological insulators on decorated diamond sonic crystals

The square-root operation can generate novel topological phases, whose nontrivial topological properties are inherited from the parent Hamiltonian. Here we report the acoustic realization of third-order square-root topological insulators by adding additional resonators between the site resonators of original diamond lattice. Due to the square-root operation, multiple acoustic localized modes appear in doubled bulk gaps. The bulk polarizations of the tight-binding models are employed to reveal the topological feature of the higher-order topological states. By tuning the coupling strength, we find the emergence of third-order topological corner states in doubled bulk gaps on tetrahedron-like and rhombohedron-like sonic crystals, respectively. The shape dependence of square-root corner states provides an extra degree of freedom for flexible manipulation on the sound localization. Furthermore, the robustness of the corner states in three-dimensional (3D) square-root topological insulator is well elucidated by introducing random disorders into the irrelevant bulk region of the proposed 3D lattices. This work extends square-root higher-order topological states into 3D system, and may find possible applications in selective acoustic sensors.

Enhanced Self-Stimulated Dissociation in Non-fullerene Layer-by-Layer Organic Solar Cells through Superimposed and Oriented Dipolar Polarization

A non-fullerene layer-by-layer (LBL) structure can establish effective donor/acceptor (D/A) interfaces and lead to remarkable photovoltaic efficiencies based on facile solution processing properties, as compared with non-fullerene bulk heterojunction devices. However, the operating mechanisms have been remaining as an unaddressed issue to understand the role of D/A interfaces toward developing photovoltaic processes in non-fullerene LBL where the high electrically polarizable non-fullerene molecules exist. Here, we address the operating mechanisms of photovoltaic actions by in situ monitoring the dynamic D/A interfaces upon selectively exciting the non-fullerene acceptor layer to address the effects of optically induced superimposed and oriented dipoles on electron–hole pair self-stimulated dissociation in non-fullerene LBL organic solar cells [ITO/PEDOT:PSS/PM6/Y6/PDIN/Al]. Essentially, the self-stimulated dissociation at the D/A interfaces can be monitored by magneto-photocurrent while separately exciting the donor and acceptor components with two different laser beams. We observed an interesting phenomenon that optically exciting the non-fullerene Y6 acceptor component leads to easier electron–hole dissociation at D/A interfaces and consequently an increase on electron–hole pair self-stimulated dissociation to generate photocurrent in non-fullerene LBL solar cells. This phenomenon presents a hypothesis that optically exciting the compactly stacked non-fullerene Y6 molecules induces superimposed and oriented electrical dipoles within A–D–A–D–A structures to increase the self-stimulated dissociation at D/A interfaces toward developing the photovoltaic actions in non-fullerene LBL solar cells. This hypothesis is supported by our photoinduced capacitance result: photoexcitation increases bulk polarization in the dipolar polarization zone in non-fullerene LBL solar cells. Furthermore, optically generated dipoles are verified by photoinduced magneto-capacitance, solely generated by photoinduced polarization rather than photogenerated carriers, in non-fullerene LBL PM6/Y6. Clearly, the optically generating superimposed and oriented electrical dipoles become a critical parameter to operate the photovoltaic actions at D/A interfaces in non-fullerene LBL solar cells.

Multiple higher-order topological phases with even and odd pairs of zero-energy corner modes in a C3symmetry broken model

Here we study emergent higher-order topological (HOTI) phases in the extended Haldane model without C 3 symmetry. For the inversion symmetric case, the QSHI and QAHI phases can embed the HOTI phases while the remaining QASHI phase does not yield any HOTI phases. Remarkably, four-fold degeneracy of zero-energy corner states can be reduced to two-fold under the application (withdrawal) of sub-lattice mass (Zeeman field) term. The sub-lattice mass and Zeeman field terms compete with each other to pin down the two mid-gap states at zero-energy. Interestingly, the bulk polarization can topologically characterize the second-order topological insulator phase with the mid-gap corner modes irrespective of their energies as long as inversion symmetry is preserved. Our study indicates that a hybrid symmetry can in principle protect the second-order topological insulator phases, however, spin-spectrum gap has to be essentially finite there.

Determination of birefringence of biological tissues using modified PS-OCT based on the quaternion approach

Introduction: Polarization-sensitive optical coherence tomography (PS-OCT) is a functional extension of standard OCT. PS-OCT systems can be generally categorized into two categories based on the number of input polarization states on the sample: multi-input polarization state (multi-IPS) and single IPS. In addition, each category includes two configurations: fiber-based system and bulk optics-based system. However, there are complex and time-consuming steps to calibrate the polarization states of light among the reference, the sample, and detection arms for fiber-based system. And it is not compact and robust enough for bulk optics-based system.Methods: In the modified SD PS-OCT system with structural symmetry in both arms of the reference and sample, there are no bulk polarization optical elements in both arms of the reference and the sample. A circularly polarized light was used to incident on sample, and Stokes vector of backscattered light was employed to characterize the birefringence of biological tissues based on the quaternion approach, which directly establishes the relationship between Stokes vectors of backscattered light and Jones matrix of the sample.Results and discussion: The new algorithm provides the analytic solution of retardance and fast-axis orientation. To evaluate the performance of the developed system, an eighth-wave plate is used. Then, the polarization properties of the myocardial tissue in vivo are quantitatively reconstructed based on the quaternion approach. The results demonstrated that the proposed method has an advantage over Jones formalism based on a single input state and two polarization input states. In the future, the modified SD PS-OCT could be improved as a common path SD PS-OCT for clinical applications.

Topological transitions with an imaginary Aubry-André-Harper potential

We study one-dimensional lattices with imaginary-valued Aubry-Andr\'e-Harper (AAH) potentials. Such lattices can host edge states with purely imaginary eigenenergies, which differ from the edge states of the Hermitian AAH model and are stabilized by a non-Hermitian particle-hole symmetry. The edge states arise when the period of the imaginary potential is a multiple of four lattice constants. They are topological in origin, and can manifest on domain walls between lattices with different modulation periods and phases, as predicted by a bulk polarization invariant. Interestingly, the edge states persist and remain localized even if the gap in the real spectrum closes. These features can be used in laser arrays to select topological lasing modes under spatially extended pumping.

Adsorption Tuning of Polarity and Magnetism in AgCr2S4 Monolayer.

As a recent successfully exfoliated non-van der Waals layered material, AgCrS2 has received a lot of attention. Motivated by its structure-related magnetic and ferroelectric behavior, a theoretical study on its exfoliated monolayer AgCr2S4 has been carried out in the present work. Based on density functional theory, the ground state and magnetic order of monolayer AgCr2S4 have been determined. The centrosymmetry emerges upon two-dimensional confinement and thus eliminates the bulk polarity. Moreover, two-dimensional ferromagnetism appears in the CrS2 layer of AgCr2S4 and can persist up to room temperature. The surface adsorption has also been taken into consideration, which shows a nonmonotonic effect on the ionic conductivity through ion displacement of the interlayer Ag, but has little impact on the layered magnetic structure.

Higher-order topological insulator in a spring-mass system

Higher-order topological insulators (HOTIs) have attracted much attention in various fields in the recent years. In this work, we propose a two-dimensional (2D) square lattice spring-mass model, where restoring forces between nearest neighbors can be induced by either longitudinal or transverse vibrations. Edge states exist on the interface between topological nontrivial and trivial lattices, and two different types of corner states are identified. The proposed spring-mass model has plenty of degrees of freedoms (DOFs), including both longitudinal and transverse spring constants as well as the mass of each particle. It turns out that edge and corner states with flexible spatial distributions can be realized by slightly tuning the elastic and inertial parameters. A lot of interesting topological patterns such as edge states along 4, 2, or only 1 edge, and second-order corner states around 4, 2, or 1 corner are unambiguously demonstrated. Our work provides an easy-to-tune platform to realize HOTIs, which is useful when multipole bulk polarizations or negative couplings are absent or difficult to realize. Classical spring-mass models governed by Newton's laws also have the advantage of superior controllability and feasible experimental implementability.

Tunable Second‐Order Topological States in Rhombic Photonic Crystals

Topological photonic devices with dynamically tunable functions are highly on demand in practice, but the majority of previously proposed photonic systems have been limited to fixed performances, once fabricated. Although several approaches have been proposed for obtaining the tunability in topological photonic systems, they are limited to first‐order topological states and require rather complicated structures. Herein, second‐order topological properties of rhombic photonic crystals (PCs) are revealed, for the first time, enabling to realize tunable photonic devices. For this purpose, the conventional square lattice PCs composed of four rigid dielectric rods are reshaped to rhomboid ones with preserved inversion symmetry, which exhibit well‐quantized bulk polarizations. Since the eigenfrequencies of topological edge and corner states depend on the angle between the neighboring sides of unit cells, the second‐order topological systems exhibit dynamic tunability, being useful for diverse applications such as optical switching and flexible beam control. Unlike the previous results for reconfigurable routing limited to special angles, this lattice‐reshaping mechanism has the ability to realize dynamically tunable routing, extending the realm of applications of topological photonics. For its simplicity and feasibility, this mechanical lattice‐reshaping approach paves the way toward higher‐order topological photonic devices with dynamically controlled functions.

Acoustic Higher-Order Weyl Semimetal with Bound Hinge States in the Continuum.

Higher-order topological phases have raised widespread interest in recent years with the occurrence of the topological boundary states of dimension two or more less than that of the system bulk. The higher-order topological states have been verified in gapped phases, in a wide variety of systems, such as photonic and acoustic systems, and recently also observed in gapless semimetal phase, such as Weyl and Dirac phases, in systems alike. The higher-order topology is signaled by the hinge states emerging in the common band gaps of the bulk states and the surface states. In this Letter, we report our first prediction and observation of a new type of hinge states, the bound hinge states in the continuum (BHICs) bulk band, in a higher-order Weyl semimetal implemented in phononic crystal. In contrast to the hinge state in gap, which is characterized by the bulk polarization, the BHIC is identified by the nontrivial surface polarization. The finding of the topological BHICs broadens our insight to the topological states, and may stimulate similar researches in other systems such as electronic, photonic, and cold atoms systems. Our Letter may pave the way toward high-Q acoustic devices in application.

Hexagonal photonic crystal with nontrivial quadrupole topology

Topological theory of multipole moments in condensed matter physics have recently been generalized to classical systems such as photonic and sonic crystals and several types of quadrupole topological photonic crystals (QTPCs) have already been investigated. However, preceding reports relevant to this issue dealt mainly with square lattice. In this work, we reveal that quadrupole topological phases can also be appeared in hexagonal photonic lattices with the C3v-crystalline symmetry. For this purpose, we extend the topological theory of bulk and Wannier-sector polarizations to the affine coordinate system for analyzing the topological characteristics of non-square lattice systems. The calculation results show that the hexagonal photonic crystals with C3-crystalline symmetry have the quadrupole moments quantized to 1/3, provided that all photonic bands below the gap in consideration are non-degenerate. The hexagonal QTPCs support topological edge and corner states, which are much stronger confined compared with the edge and corner modes originating from nontrivial bulk polarizations. The presented results have fundamental importance and pave the way towards the broader applications of higher-order photonic topological insulators.

Simultaneously achieving high permittivity and low loss in CuO/Nb2O5 substituted Li2O-TiO2-Nb2O5 low-temperature microwave dielectric ceramics

To cope with the growing challenges in device miniaturization increase, there is an urgent need to develop high K materials with excellent microwave properties. Here, novel (Cu1/3Nb2/3)4+ substituted LiNb0.6Ti0.5O3 (LNT) ceramics (LNT-xCN, 0≤x ≤ 0.20) were successfully fabricated via solid-state reaction method. The use of CuO as an effective burning agent enabled one to successfully enhance ceramic densification and reduce the sintering temperature to 950 °C. Outstanding microwave dielectric performance (εr∼65.21, Q × f∼9215 GHz, τf ∼ -24.06 ppm/°C) was achieved in LNT-0.16CN ceramic. In particular, the Q × f value was improved nearly 1.44 times relative to that of pure LNT ceramic (6385 GHz@1100 °C). More importantly, the variation of microwave dielectric parameters not only was related to bulk density, densification and polarization, but also to LN/[Ti2O3]2+layers and the squeezing pressure of oxygen octahedron [NbO6]. Encouragingly, the characteristics most compatible with those of silver were found in LNT-0.16CN ceramic, making a suitable substitute material for the next-generation 5G communication devices.

Evolution of the edge states and corner states in a multilayer honeycomb valley-Hall topological metamaterial

The valley-Hall effect provides topological protection to a broad class of defects in valley-Hall photonic topological metamaterials. Unveiling precisely how such protection is achieved and its implications in practical implementations is paramount to move from fundamental science to applications. To this end, we investigate a honeycomb valley-Hall topological metamaterial and monitor the evolution of the topological valley-Hall edge states and higher-order corner states under different perturbation $\ensuremath{\delta}R$. The evolutions of the edge states of the armchair and zigzag interfaces are demonstrated, respectively. By adjusting the geometric parameters and introducing disturbances to break the inversion symmetry, we achieve the edge states with different modes including the conventional crossed edge state and the specific gapped edge state. It is found that the edge states of topological valley kinking will gradually separate with the increase of $\ensuremath{\delta}R$, and finally a complete gap between the edge states appears. The gap has rarely been reported previously in topological materials fabricated by printed circuit board technology. In addition, the higher-order topological corner states can also be observed in the proposed topological metamaterial. The higher-order topological phase is theoretically characterized by nontrivial bulk polarization and the Wannier centers. Our results show that the corner state localization becomes stronger with the increase of $\ensuremath{\delta}R$. It is expected that our results will provide a platform for the realization of optical topological insulators.

Amplified Overhauser DNP with Selective Deuteration: Attenuation of Double-Quantum Cross-Relaxation

We recently used selective 2H labeling of BDPA to investigate the Overhauser Effect (OE) dynamic nuclear polarization (DNP) mechanism in insulating solids doped with 1,3-bis(diphenylene)-2-phenylallyl (BDPA), and established that the α and γ 1H spins on the fluorene rings are responsible for generating a zero quantum (ZQ) mediated positive bulk polarization. Here, we establish that the phenyl 1H spins relax via double-quantum (DQ) processes and therefore contribute negative enhancements which attenuate the OE-DNP. With measurements at different magnetic field strengths, we show that phenyl-d5-BDPA offers >50% improvement in OE-DNP enhancement compared to h21-BDPA attaining a maximum of ∼90 at 14.1 T and 5 kHz MAS, the highest observed OE-DNP enhancement to date under these conditions. The approach may be utilized to optimize other polarizing agents exhibiting an OE, an important DNP mechanism with a favorable field and spinning frequency dependence.

First-principles study on corrosion resistance of copper-bearing hypereutectic high chromium cast iron

Based on the Tafel formula, Butler-Volmer equation, and first-principles calculations, the bulk model, surface model, and polarization prediction curve of the Hypereutectic High Chromium Cast Iron (HHCCI) matrix were constructed. The results show that the predicted corrosion current density (2.19×10 -5 A·cm 2 ) of the copper-bearing HHCCI matrix is lower than the measured corrosion current density of the copper-bearing HHCCI (4.41×10 -5 A·cm 2 ) under the acidic condition of pH=1. In the neutral environment of pH=7, the corrosion resistance of HHCCI is better than that of HHCCI under acidic conditions. The predicted corrosion current density of the copper-bearing HHCCI matrix (4.39×10 -10 A·cm 2 ) is also smaller than that of copper-free HHCCI (7.98×10 -10 A·cm 2 ), which shows that the corrosion resistance of copper-containing HHCCI matrix is better than that without copper from the perspective of first-principles calculation. Combined with the electrochemical experimental results, it can be seen that the corrosion potential (-409.76 mV) of 1.5 wt.% Cu HHCCI is significantly higher than that of 0 wt.% Cu HHCCI (-450.34 mV). The trend of the electrochemical test results is consistent with the predicted trend of the calculated polarization curve, which indicates that copper alloying can improve the corrosion resistance of HHCCI. Predicted polarization curves on γ-Fe (100) surfaces (PH=1): (a) γ-Fe (Cu-Free); (b) γ-Fe (Cu-Bearing) • Predicting corrosion resistance using equations and first-principles. • Copper improves the predicted current density of HHCCI matrix at pH=1. • Copper improves the current density of HHCCI at electrochemical experiments. • Copper alloying improves HHCCI corrosion resistance.