Geometrical Resonances Research Articles

Signature of Fermi arc surface states in Andreev reflection at the WTe2 Weyl semimetal surface

We experimentally investigate charge transport through the interface between a niobium superconductor and a three-dimensional WTe2 Weyl semimetal. In addition to classical Andreev reflection, we observe sharp non-periodic subgap resistance resonances. From an analysis of their positions, magnetic field and temperature dependences, we can interpret them as an analog of Tomasch oscillations for transport along the topological surface state across the region of proximity-induced superconductivity at the Nb-WTe2 interface. The observation of distinct geometrical resonances implies a specific transmission direction for carriers, which is a hallmark of the Fermi arc surface states.

Optimum resonance control knobs for sextupoles

We discuss the placement of extra sextupoles in a magnet lattice that allows to correct third-order geometric resonances, driven by the chromaticity-compensating sextupoles, in a way that requires the least excitation of the correction sextupoles. We consider a simplified case, without momentum-dependent effects or other imperfections, where suitably chosen phase advances between the correction sextupoles leads to orthogonal knobs with equal treatment of the different resonance driving terms.

A Tunable Multigate Indium‐Tin‐Oxide‐Assisted All‐Dielectric Metasurface

AbstractIn this paper, an optically active reflective metasurface with tunable phase performance is designed which operates based on the excitation of both electric and magnetic resonances in a silicon‐made nanoantenna element and the use of layered indium tin oxide (ITO)–alumina under the influence of external multigate biasing. The capability of achieving robust control over the reflection characteristics of two closely spaced electric and magnetic resonances is investigated. Adopting the multigate biasing enables relatively high reflection amplitude of 0.4 over the entire phase‐change coverage (≈180°) in the near‐infrared regime by exploiting the constructive interference between the geometrical resonances. The physical mechanism behind this ITO‐assisted metal–insulator–semiconductor building block can be described as the strong confinement of electromagnetic fields at the vicinity of ITO layers and the possibility of modulating the carrier concentrations of the integrated active materials by electrically varying the bias voltage. The uniformity of reflection magnitude for all reflection phase coverage, relatively high reflectivity, and electro‐optical tunability is leveraged to design three multifunctional devices with step‐by‐step increase in complexity of biasing network including ultrathin reconfigurable linear/circular polarizer (simple identical biasing), dynamical beam steering platform (two‐state biasing), and tunable metalens with controllable on‐ and off‐axes focusing pattern in real‐time (advanced element‐by‐element biasing).

Cyclotron Orbits of Composite Fermions in the Fractional Quantum Hall Regime.

We study a bilayer GaAs hole system that hosts two distinct many-body phases at low temperatures and high perpendicular magnetic fields. The higher-density (top) layer develops a Fermi sea of composite fermions (CFs) in its half-filled lowest Landau level, while the lower-density (bottom) layer forms a Wigner crystal (WC) as its filling becomes very small. Owing to the interlayer interaction, the CFs in the top layer feel the periodic Coulomb potential of the WC in the bottom layer. We measure the magnetoresistance of the top layer while changing the bottom-layer density. As the WC layer density increases, the resistance peaks separating the adjacent fractional quantum Hall states in the top layer change nonmonotonically and attain maximum values when the cyclotron orbit of the CFs encloses one WC lattice point. These features disappear at T=275 mK when the WC melts. The observation of such geometric resonance features is unprecedented and surprising as it implies that the CFs retain a well-defined cyclotron orbit and Fermi wave vector even deep in the fractional quantum Hall regime, far from half-filling.

Antimatched Electromagnetic Metasurfaces for Broadband Arbitrary Phase Manipulation in Reflection.

Metasurfaces impart phase discontinuities on impinging electromagnetic waves that are typically limited to 0–2π. Here, we demonstrate that multiresonant metasurfaces can break free from this limitation and supply arbitrarily large, tunable time delays over ultrawide bandwidths. As such, ultrathin metasurfaces can act as the equivalent of thick bulk structures by emulating the multiple geometric resonances of three-dimensional systems that originate from phase accumulation with effective material resonances implemented on the surface itself via suitable subwavelength meta-atoms. We describe a constructive procedure for defining the required sheet admittivities of such metasurfaces. Importantly, the proposed approach provides an exactly linear phase response so that broadband pulses can experience the desired group delay without any distortion of the pulse shape. We focus on operation in reflection by exploiting an antimatching condition, satisfied by interleaved electric and magnetic Lorentzian resonances in the surface admittivities, which completely zeroes out transmission through the metasurface. As a result, the proposed metasurfaces can perfectly reflect a broadband pulse imparting a prescribed group delay. The group delay can be tuned by modifying the implemented resonances, thus opening up diverse possibilities in the temporal applications of metasurfaces.

Orienting proteins by nanostructured surfaces: evidence of a curvature-driven geometrical resonance.

Experimental and theoretical reports have shown that nanostructured surfaces have a dramatic effect on the amount of protein adsorbed and the conformational state and, in turn, on the performances of the related devices in tissue engineering strategies. Here we report an innovative method to prepare silica-based nanostructured surfaces with a reproducible, well-defined local curvature, consisting of ordered hexagonally packed arrays of curved hemispheres, from nanoparticles of different diameters (respectively 147 nm, 235 nm and 403 nm). The nanostructured surfaces have been made chemically homogeneous by partially embedding silica nanoparticles in poly(hydroxymethylsiloxane) films, further modified by means of UV-O3 treatments. This paper has been focused on the experimental and theoretical study of laminin, taken as a model protein, to study the nanocurvature effects on the protein configuration at nanostructured surfaces. A simple model, based on the interplay of electrostatic interactions between the charged terminal domains of laminin and the nanocurved charged surfaces, closely reproduces the experimental findings. In particular, the model suggests that nanocurvature drives the orientation of rigid proteins by means of a "geometrical resonance" effect, involving the matching of dimensions, charge distribution and spatial arrangement of both adsorbed molecules and adsorbent nanostructures. Overall, the results pave the way to unravel the nanostructured surface effects on the intra- and inter-molecular organization processes of proteins.

Ballistic geometric resistance resonances in a single surface of a topological insulator

Transport in topological matter has shown a variety of novel phenomena over the past decade. Although numerous transport studies have been conducted on three-dimensional topological insulators (TIs), study of ballistic motion and thus exploration of potential landscapes on a hundred nanometer scale is for the prevalent TI materials almost impossible due to their low carrier mobility. Therefore, it is unknown whether helical Dirac electrons in TIs, bound to interfaces between topologically distinct materials, can be manipulated on the nanometer scale by local gates or locally etched regions. Here we impose a submicron periodic potential onto a single surface of Dirac electrons in high-mobility strained mercury telluride (HgTe), which is a strong TI. Pronounced geometric resistance resonances constitute the clear-cut observation of a ballistic effect in three-dimensional TIs.

Spatial geometric and magnetic resonance signal intensity changes with advancing stages of nucleus pulposus degeneration

BackgroundWith advancing stages of degeneration, denaturation and degradation of proteoglycans in the nucleus pulposus (NP) lead to tissue dehydration and signal intensity loss on T2-weighted MR images. Pfirrmann grading is widely used for grading degeneration of intervertebral discs (IVDs). The criterion to differentiate IVDs of Pfirrmann Grade I from the other grades is NP homogeneity. Pfirrmann grading is qualitative and its assessment may be subjective. Therefore, assessment of quantitative objective measures correlating with early disc degeneration may complement the grading. This study aimed to evaluate the applicability of the distance between the center weighted by signal intensity (weighted center) and the geometric center as a parameter of NP homogeneity. Other phenomena related to advancing stages of degeneration were also investigated.MethodsMR images of 65 asymptomatic volunteers with a total of 288 lumbar IVDs with clearly identifiable nucleus pulposus boundary (Pfirrmann Grade I, II and III) were included in this study. A custom-written program was developed to determine the IVD longitudinal axis, define the NP boundary, and to locate the coordinates of geometric and weighted NP centers on the mid-sagittal image of each studied IVD. The distances between the weighted and geometric centers on the longitudinal axis and the perpendicular axis of each IVD were calculated.ResultsThe weighted center located posterior to the geometric center, which indicated the signal intensity was lower at the anterior portion of the NP, in 85.8% of studied IVDs. The distance between the weighted and geometric center on the longitudinal axis was significantly shorter in homogeneous (Pfirrmann Grade I) than in inhomogeneous (Grade II) IVDs. The distance on the perpendicular axis in Grade III IVDs was significantly larger than that in Grade I and Grade II IVDs.ConclusionThe relationship between the weighted and geometric centers can serve as an indicator for NP homogeneity. The distance between both centers through advancing stages of degeneration demonstrated decrease of signal intensity progressing along the longitudinal axis initially and then along the cranio-caudal direction at later stages. These findings could provide insights of initiation and subsequent progression of degenerative changes in IVDs.

Terahertz Nanofocusing with Cantilevered Terahertz-Resonant Antenna Tips

We developed THz-resonant scanning probe tips, yielding strongly enhanced and nanoscale confined THz near fields at their tip apex. The tips with length in the order of the THz wavelength (λ = 96.5 μm) were fabricated by focused ion beam (FIB) machining and attached to standard atomic force microscopy (AFM) cantilevers. Measurements of the near-field intensity at the very tip apex (25 nm radius) as a function of tip length, via graphene-based (thermoelectric) near-field detection, indicate their first and second order geometrical antenna resonances for tip length of 33 and 78 μm, respectively. On resonance, we find that the near-field intensity is enhanced by one order of magnitude compared to tips of 17 μm length (standard AFM tip length), which is corroborated by numerical simulations that further predict remarkable intensity enhancements of about 107 relative to the incident field. Because of the strong field enhancement and standard AFM operation of our tips, we envision manifold and straightforward future application in scattering-type THz near-field nanoscopy and THz photocurrent nanoimaging, nanoscale nonlinear THz imaging, or nanoscale control and manipulation of matter employing ultrastrong and ultrashort THz pulses.

Tidal dynamics and their influence on the climate system from the Cretaceous to present day

Global numerical ocean models used for paleo-climate reconstructions commonly only consider the ocean's general circulation but neglect tidal dynamics. However, tidal dynamics affect the ocean's mean general circulation, in particular by vertical mixing and tidal residual mean currents. Through feedback loops the whole climate system is affected.Plate tectonics modify geometric resonance conditions in ocean basins and thereby tidal dynamics. We study the influence of ocean tides on the ocean general circulation and atmospheric parameters by forcing the coupled atmosphere-ocean model ECHAM5/MPIOM with the complete lunisolar tidal potential. Simulations have been performed for five tectonically important time-slices: the Early Albian (ca. 110millionyears ago, Ma), the Cenomanian-Turonian Boundary (ca. 93Ma, CTB), the Early Eocene (ca. 55Ma), the Early Pliocene (ca. 3.5Ma), and a pre-industrial period (ca. 1850CE). The model results suggest that the global mean tidal potential energy in the Early Eocene is almost three times larger than in the CTB. The large potential energy input in the Early Eocene leads to a tripling of current velocities in 10% of the deep ocean. Although the effect of tides on the general ocean circulation is less pronounced in the other time-slices, horizontal velocities are modified by more than 20% in 55% of the deep ocean.The tidally induced shifts of ocean currents and vertical mixing also have an effect on the three-dimensional temperature distribution in the ocean. The impact of tidal dynamics on atmospheric temperatures is particularly strong in the Southern Ocean of the Early Pliocene and the pre-industrial period. By a feedback loop with the atmosphere, tidal forcing locally reduces sea-ice concentration by up to 30% and local atmospheric 2m temperatures by up to 4°C.Although uncertainties in bathymetry reconstructions limit the significance of quantitative analysis, the qualitative conclusions suggest that the impacts of ocean tides on climate reach an order of magnitude, which require its consideration as additional uncertainty in the interpretation of paleo-climate studies. However, the impact of ocean tides has specific characteristics in each time-slice hampering its simple parametrization.

Seismic estimates of turbulent diffusivity and evidence of nonlinear internal wave forcing by geometric resonance in the South China Sea

AbstractThe Luzon Passage generates some of the largest amplitude internal waves in the global ocean as the result of coupling between strong tides, strong stratification, and topography. These internal waves propagate into the South China Sea (SCS) and develop into soliton‐like internal wave pulses that are observed by moored instruments and satellite backscatter data. Despite the observation of these waves, little is known of the mechanisms related to their evolution into nonlinear wave pulses. Using seismic data, we find evidence that the geometry of bathymetric conditions between the Heng‐Chun and Lan‐Yu ridges drive nonlinear internal wave pulse generation. We produce three seismic images and associated maps of turbulent diffusivity to investigate structure around the two ridges and into the SCS. We do not observe large amplitude soliton‐like internal waves between the ridges, but do observe one outside the ridges, a finding in accord with the interpretation that wave pulses form due to geometrical resonance. Additionally, we find no evidence for lee wave activity above the ridges in either the seismic images or associated turbulence maps, suggesting an unlikelihood of hydraulic jump driven generation around the ridges. Our results show increased levels of turbulent diffusivity (1) in deep water below 1000 m, (2) associated with internal tide pulses, and (3) near the steep slopes of the Heng‐Chun and Lan‐Yu ridges as explored in this paper.

Aerosol manipulation through modulated multiple acoustic wavepackets with a pair of resonators

In general, the aerosols mixed in air medium can be regularly manipulated by means of the interactions between the aerosols and the acoustic field. The manipulable properties of aerosols can be engineered through geometric parameter and acoustic resonance condition of the underlying device. The aerosols manipulated by modulated multiple acoustic wavepackets (MAWP) in acoustic resonance condition is proposed and demonstrated in this paper, whose application belongs to an efficient aerosol removal technique. The experimental results indicated that the removal efficiency of aerosols was mainly influenced by the different harmonic order. The technique and process proposed in this paper was feasible for industrial application. The MAWP is modulated by means of the synthetic standing wave field at the resonant frequency 1.268kHz. The aerosol manipulation processes through MAWP consist of the aerosol shift, collision aggregation and deposition between the dot and the anti-neck of a single wavepacket. As a visual inspection confirming standing wave, the processes may also be applied in modulating the MAWP simultaneously. The manipulation efficiency can be increased due to the increase of wavepacket amount under the same operating conditions. The acoustic radiation force causes shift and accumulation of aerosol in waveguide and the secondary radiation force enhances collision aggregation and deposition thereof. The initial number concentration of particles <2.5μm (resp. larger than 2.5μm) was about 517,488 1/cf3 (resp. 51,918 1/cf3). Under the action of these forces, the aerosols in waveguide were aggregated and deposited onto the inner wall of waveguide. At the end of experiment, the final number concentration of particles <2.5μm (resp. larger than 2.5μm) is about 59,096 1/cf3 (resp. 392 1/cf3). The PM2.5 content of aerosols used in experiment is about 91%. The removal coefficients are larger than 85%. The deposition stripe distribution shows the specific pattern associated with a wavepacket. The surface of the stripe particles is covered with the uniform small hollows (~105.7–177.7nm), and plenty of nanoscale particles (~13.66–16.55nm) are grown around the surface of small hollows. It may be possible to develop a new way for large scale and batch aerosol processing to apply to the prospective emission control of industrial aerosols.

Remote sensing of a low pressure plasma in the radio near field

A novel approach to remotely monitor low pressure non-equilibrium plasmas is reported. A magnetic field antenna is positioned in the near field of a capacitively coupled plasma. Magnetic flux from plasma currents, present near the viewport, is intercepted by a calibrated loop antenna placed outside the chamber. The induced signal current is correlated to bulk plasma currents. The comparison of relative harmonic amplitudes shows resonance features for lower operating pressures. The geometric resonance and electron–neutral collision frequencies are evaluated from resonant harmonic features. This approach advances remote, noninvasive, and installation-free plasma monitoring, which is of particular interest to industrial scenarios.

Berry phase and anomalous transport of the composite fermions at the half-filled Landau level

Experimental signatures of a Berry phase for composite fermions in the fractional quantum Hall effect provide support for the predictions that these composite fermions are Dirac particles. The fractional quantum Hall effect (FQHE)1,2 in two-dimensional electron systems is an exotic, superfluid-like matter with an emergent topological order. From the consideration of the Aharonov–Bohm interaction between electrons and magnetic field, the ground state of a half-filled lowest Landau level is mathematically transformed to a Fermi sea of composite objects of electrons bound to two flux quanta, termed composite fermions (CFs)3,4,5. A strong support for the CF theories comes from experimental confirmation of the predicted Fermi surface at ν = 1/2 (where ν is the Landau level filling factor) from the detection of the Fermi wavevector in semi-classical geometrical resonance experiments2,6,7,8,9. Recent developments in the theory of CFs10,11,12,13,14,15,16,17,18,19,20,21 have led to the prediction of a π Berry phase for the CF circling around the Fermi surface at half-filling10,14,17,18,19,20. In this paper we provide experimental evidence for the detection of the Berry phase of CFs in the fractional quantum Hall effect. Our measurements of the Shubnikov–de Haas oscillations of CFs as a function carrier density at a fixed magnetic field provide strong support for the existence of a π Berry phase at ν = 1/2. We also discover that the conductivity of composite fermions at ν = 1/2 displays an anomalous linear density dependence, whose origin remains mysterious yet tantalizing.

Evaluation of blackbody radiation emitted by arbitrarily shaped bodies using the source model technique.

Planck's famous blackbody radiation law was derived under the assumption that the dimensions of the radiating body are significantly larger than the radiated wavelengths. What is unique about Planck's formula is the fact that it is independent of the exact loss mechanism and the geometry. Therefore, for a long period of time, it was regarded as a fundamental property of all materials. Deviations from its predictions were attributed to imperfections and referred to as the emissivity of the specific body, a quantity which was always assumed to be smaller than unity. Recent studies showed that the emission spectrum is affected by the geometry of the body and in fact, in a limited frequency range, the emitted spectrum may exceed Planck's prediction provided the typical size of the body is of the same order of magnitude as the emitted wavelength. For the investigation of the blackbody radiation from an arbitrarily shaped body, we developed a code which incorporates the fluctuation-dissipation theorem (FDT) and the source model technique (SMT). The former determines the correlation between the quasi-microscopic current densities in the body and the latter is used to solve the electromagnetic problem numerically. In this study we present the essence of combining the two concepts. We verify the validity of our code by comparing its results obtained for the case of a sphere against analytic results and discuss how the accuracy of the solution is assessed in the general case. Finally, we illustrate several configurations in which the emitted spectrum exceeds Planck's prediction as well as cases in which the geometrical resonances of the body are revealed.

All‐Dielectric C‐Shaped Nanoantennas for Light Manipulation: Tailoring Both Magnetic and Electric Resonances to the Desire

A systematic study is carried out on the interaction of electromagnetic waves with a nonsymmetric all‐dielectric C‐shaped nanoantenna to achieve an ultimate control over the independent and relative spectral positions of the excited magnetic and electric dipolar modes. By properly determining the structural parameters (inner and outer radii and the opening angle) and tailoring the interference of geometrical resonances, an ultrathin C‐element metasurface is designed, which can highly transmit the linearly polarized incident beam (&gt;80%) with full phase‐agility (2π) in the red light of visible regime. The investigation on the physical mechanism behind this nanoantenna reveals the confinement of magnetic and electric fields corresponding to the magnetic and electric dipolar modes mainly inside the hollow and opening angle, i.e., outside the constituent high‐index material of C‐element. This feature allows making building blocks less dependent on the material loss and dispersion. Three different approaches based on only varying the inner/outer radius or the opening angle are proposed which can facilitate the design complexity, the encoding of the desired phase distribution for specific functionality, and easing the fabrication procedure. The proposed approaches are leveraged to design several highly transparent graded‐pattern metasurfaces with the capability of beam steering, focusing, flat‐top generation, and holography.

Analysis of the technical possibility of submarine camouflage from direction finding means

The submarines studied in this work have an indisputable advantage, consisting in increased concealment, which allows them to concentrate in strategically important areas of the water area with attack means containing a combat unit of various composition and power. Concealment of the submarines significantly increases the danger of hitting critical infrastructures of states of medium and small area, even without their own water area. Submarines, in addition to the marked advantage, have obvious disadvantages, which include «blindness» and «noisiness». Therefore, when solving tactical and strategic tasks of defense significance, it is necessary to take into account the need to eliminate these disadvantages. One of the variants of the technical solution is presented for providing the submarine camouflage from the direction finding equipment with the help of hydroacoustic stations – underkeel, drop, towed. An analytical model of the technical effect is constructed to achieve limited visibility of the submarine outside the zone of geometric resonance. The effectiveness degree of submarine camouflage is analyzed in the case of the onset of a resonant situation in the enclosing module - wave coincidence. The preconditions for the onset of resonance are formulated under the condition that a large wave size of the outer shell of the enclosing module is provided. The content of aberration phenomenon irradiated by the outer shell of sound waves into the fluid of the inter-shell space is revealed. Semi-detailed laboratory studies of the submarine model allow using ultrasonic technologies to ensure the concealment of the submarine in the absence of its own propulsion by means of a two-shell module separated by a liquid and artificially irradiated with ultrasound. At the same time, the theoretical prerequisites for increasing the effectiveness of submarine camouflage using resonant appearance in the form of wave coincidence in a laboratory unit confirms the correctness of the initial technical solutions. This allows to evaluate step-by-step the requirements of high camouflage efficiency: – large wave size of the shell module; – creation of caustic zones in the inter-shell space, which acts as a enclosing surface from the direction finding equipment; – ensuring of the geometric resonance of the outer shell to form its «acoustic transparency»; – creation of a zone of increased energy in the form of turbulence of the inter-shell liquid.

Search for composite fermions at filling factor 5/2: Role of Landau level and subband index

The pairing of composite fermions (CFs), electron-flux quasi-particles, is commonly proposed to explain the even-denominator fractional quantum Hall state observed at $\nu=5/2$ in the first excited ($N=1$) Landau level (LL) of a two-dimensional electron system (2DES). While well-established to exist in the lowest ($N=0$) LL, much is unknown about CFs in the $N=1$ LL. Here we carry out geometric resonance measurements to detect CFs at $\nu=5/2$ by subjecting the 2DES to a one-dimensional density modulation. Our data, taken at a temperature of 0.3 K, reveal no geometric resonances for CFs in the $N=1$ LL. In stark contrast, we observe clear signatures of such resonances when $\nu=5/2$ is placed in the $N=0$ LL of the anti-symmetric subband by varying the 2DES width. This finding implies that the CFs' mean-free-path is significantly smaller in the $N=1$ LL compared to the $N=0$ LL. Our additional data as a function of in-plane magnetic field highlight the role of subband index and establish that CFs at $\nu=5/2$ in the $N=0$ LL are more anisotropic in the symmetric subband than in the anti-symmetric subband.

Electronic properties of superlattices on quantum rings

We present a theoretical study of the one-electron states of a semiconductor-made quantum ring (QR) containing a series of piecewise-constant wells and barriers distributed along the ring circumference. The single quantum well and the superlattice cases are considered in detail. We also investigate how such confining potentials affect the Aharonov–Bohm like oscillations of the energy spectrum and current in the presence of a magnetic field. The model is simple enough so as to allow obtaining various analytical or quasi-analytical results. We show that the well-in-a-ring structure presents enhanced localization features, as well as specific geometrical resonances in its above-barrier spectrum. We stress that the superlattice-in-a-ring structure allows giving a physical meaning to the often used but usually artificial Born–von-Karman periodic conditions, and discuss in detail the formation of energy minibands and minigaps for the circumferential motion, as well as several properties of the superlattice eigenstates in the presence of the magnetic field. We obtain that the Aharonov–Bohm oscillations of below-barrier miniband states are reinforced, owing to the important tunnel coupling between neighbour wells of the superlattice, which permits the electron to move in the ring. Additionally, we analysis a superlattice-like structure made of a regular distribution of ionized impurities placed around the QR, a system that may implement the superlattice in a ring idea. Finally, we consider several random disorder models, in order to study roughness disorder and to tackle the robustness of some results against deviations from the ideally nanostructured ring system.

Geometric Resonance of Composite Fermions near Bilayer Quantum Hall States.

Via the application of a parallel magnetic field, we induce a single-layer to bilayer transition in two-dimensional electron systems confined to wide GaAs quantum wells and study the geometric resonance of composite fermions (CFs) with a periodic density modulation in our samples. The measurements reveal that CFs exist close to bilayer quantum Hall states, formed at Landau level filling factors ν=1 and 1/2. Near ν=1, the geometric resonance features are consistent with half the total electron density in the bilayer system, implying that CFs prefer to stay in separate layers and exhibit a two-component behavior. In contrast, close to ν=1/2, CFs appear single-layer-like (single component) as their resonance features correspond to the total density.