Transverse Geometry Research Articles

Efficient treatment of low-frequency response and decoherence in a real-time evolution scheme.

In this paper, we present an improved real-time current-based approach for calculating the frequency-dependent dielectric function of a bulk periodic system, which can achieve a unified treatment of longitudinal and transverse macroscopic geometries on the same footing, and an improvement to make the approach of calculating dielectric function more robust for the avoidance of numerical divergencies at low frequency near zero in some specific cases. The validity of the improved approach implementation is verified by calculating the dielectric function of bulk periodic system in the ground in the longitudinal geometry, enabling the improved approach to be extended to excited bulk periodic systems in the transverse geometry. Further, a phenomenological description of decoherence has been incorporated within the framework of time-dependent density-functional theory (TDDFT). It is concluded that the decoherence model can suppress the numerical divergence of low frequency and grows the excitonic feature of silicon, although it adopts the approximate time-dependent exchange-correlation potential. Thus, the use of the decoherence TDDFT model opens pathways for handling the decoherence effects within the framework of TDDFT.

Effects of transverse geometry on the thermal conductivity of Si and Ge nanowires

We explore the effects of geometry on the thermal conductivity (κ) of silicon and germanium nanowires, with lengths between 10–120 nm and diameters up to 5–6 nm. To this end we perform molecular dynamics simulations with the LAMMPS software, using Tersoff interatomic potentials. We consider nanowires with polygonal cross section and we discuss the effect of the transverse geometry on the thermal conductivity. We also consider tubular (hollow) nanowires and core/shell combinations of Si/Ge and Ge/Si, and we compare the heat transport of the core/shell structure with that of the separated core and shell components.

Boosting the SiN nonlinear photonic platform with transition metal dichalcogenide monolayers.

In the past few years, we have witnessed increased interest in the use of 2D materials to produce hybrid photonic nonlinear waveguides. Although graphene has attracted most of the attention, other families of 2D materials such as transition metal dichalcogenides have also shown promising nonlinear performance. In this work, we propose a strategy for designing silicon nitride waveguiding structures with embedded MoS2 for nonlinear applications. The transverse geometry of the hybrid waveguide is optimized for high third-order nonlinear effects using optogeometrical engineering and multiple layers of MoS2. Stacking multiple monolayers results in an improvement of two orders of magnitude compared to standard silicon nitride waveguides. The hybrid waveguide performance is then investigated in terms of four-wave mixing enhancement in micro-ring resonator configurations. A signal/idler conversion efficiency of -6.3 dB is reached for a wavelength of around 1.55 µm with a 5 mW pumping level.

Lie groupoids and semi-local models of singular Riemannian foliations

We describe a local model for any singular Riemannian foliation in a neighborhood of a closed saturated submanifold of a singular stratum. Moreover, we construct a Lie groupoid that controls the transverse geometry of the linear approximation of the singular Riemannian foliation around these submanifolds. We also discuss the closure of this Lie groupoid and its Lie algebroid.

Giant anomalous Nernst signal in the antiferromagnet YbMnBi2

A large anomalous Nernst effect (ANE) is crucial for thermoelectric energy conversion applications because the associated unique transverse geometry facilitates module fabrication. Topological ferromagnets with large Berry curvatures show large ANEs; however, they face drawbacks such as strong magnetic disturbances and low mobility due to high magnetization. Herein, we demonstrate that YbMnBi2, a canted antiferromagnet, has a large ANE conductivity of ~10 A m−1 K−1 that surpasses large values observed in other ferromagnets (3–5 A m−1 K−1). The canted spin structure of Mn guarantees a non-zero Berry curvature, but generates only a weak magnetization three orders of magnitude lower than that of general ferromagnets. The heavy Bi with a large spin–orbit coupling enables a large ANE and low thermal conductivity, whereas its highly dispersive px/y orbitals ensure low resistivity. The high anomalous transverse thermoelectric performance and extremely small magnetization make YbMnBi2 an excellent candidate for transverse thermoelectrics.

Berry curvature generation detected by Nernst responses in ferroelectric Weyl semimetal

The quest for nonmagnetic Weyl semimetals with high tunability of phase has remained a demanding challenge. As the symmetry-breaking control parameter, the ferroelectric order can be steered to turn on/off the Weyl semimetals phase, adjust the band structures around the Fermi level, and enlarge/shrink the momentum separation of Weyl nodes which generate the Berry curvature as the emergent magnetic field. Here, we report the realization of a ferroelectric nonmagnetic Weyl semimetal based on indium-doped Pb1- x Sn x Te alloy in which the underlying inversion symmetry as well as mirror symmetry are broken with the strength of ferroelectricity adjustable via tuning the indium doping level and Sn/Pb ratio. The transverse thermoelectric effect (i.e., Nernst effect), both for out-of-plane and in-plane magnetic field geometry, is exploited as a Berry curvature-sensitive experimental probe to manifest the generation of Berry curvature via the redistribution of Weyl nodes under magnetic fields. The results demonstrate a clean, nonmagnetic Weyl semimetal coupled with highly tunable ferroelectric order, providing an ideal platform for manipulating the Weyl fermions in nonmagnetic systems.

In-plane magnetic field induced helicity dependent photogalvanic effect on the surface states of topological insulators (BixSb1−x)2Te3

A hallmark signature of the three-dimensional (3D) topological insulator (TI) is that the spin-momentum locked massless Dirac fermions populate its surface states, where the carrier spins are locked to their momentum. Here, we report on the magnetic-field induced helicity dependent photogalvanic effect (MHPGE) of 3D TI thin films Bi2Te3 or (BixSb1−x)2Te3 of different thicknesses excited by near-infrared (1064 nm) under an in-plane magnetic field. It is found that the MHPGE current Jcx under the longitudinal geometry, i.e., Jcx∥Bx, is induced by the Larmor procession, while that under the transverse geometry, i.e., Jcx∥By, is mainly introduced by the hexagonal warping, which can be enhanced by the in-plane magnetic field. Our work demonstrates the possibility to tune the spin-polarized photocurrent of the surface states in 3D TIs via a magnetic field, which may be utilized to design new kinds of opto-spintronic devices.

Description of longitudinal space charge effects in beams and plasma through dielectric permittivity

We develop a universal framework that allows for quickly solving a wide class of problems for longitudinal space charge effects in beams and plasmas in cylindrical geometry. We introduce the longitudinal dielectric permittivity for the beam of charged particles, which describes its collective space charge response. The analysis yields an effective plasma frequency, which depends on the transverse geometry of the system. This dielectric permittivity mirrors the dielectric permittivity of plasma and matches the one dimensional expression once the transverse size of the beam is large. Several particle species can be included as additive terms describing susceptibility of each specie. The developed approach allows to study stability criteria for collective beam–beam and beam–plasma instabilities for arbitrary transverse distributions in particle densities.

Effect of Welding Sequence and the Transverse Geometry of the Weld Overlay on the Distribution of Residual Stress in the Weld Overlay Repair of T23 Tubes

Water-wall tubes are important components in power plants and are used to absorb the heat from the boilers, and they often fail prematurely due to corrosion, erosion, and fatigue during service. To repair the defects, weld overlay repair is often adopted by forming a compressive stress area around the susceptible area to prevent the cracks from propagating. In order to obtain reasonable process parameters of weld overlay repair, a study combining experiments and numerical simulations was performed to investigate the welding residual stress distribution on the repaired area with different welding repair methods. The results reveal that a compressive stress area is generated on the repaired area after the repair, and a center-to-outside welding sequence is better than either a right-to-left welding sequence or an outside-to-center welding sequence when overlaying a one-layer weld; with an increase in the number of weld layers, the compressive stress area of the repaired area is expanded, and the stress level is increased, which results from the upward movement of the compressive stress area of the subsequent-overlaying layer and the superposition of the compression areas of the subsequent-overlaying layer and previous-overlaying layer, respectively. In addition, the number of the weld passes of each layer should be not less than four with the center-to-outside welding sequence.

Sense current dependent coercivity and magnetization relaxation in Gd-Fe-Co Hall bar

The understanding of the characteristics of a magnetic layer in a different environment is crucial for any spintronics application. Before practical applications, thorough scrutiny of such devices is compulsory. Here we study such a potential Hall device of MgO-capped Hf/GdFeCo bilayer (FeCo-rich) for magnetization relaxation around nucleation fields at different voltage probe line widths and dc sensing currents. The device is characterized by anomalous Hall measurements in transverse and longitudinal Hall geometries for two different probe widths A (5 µm) and B (1 µm). The coercivities of the Hall loops (ρxy-H and Rxx-H) drop with increasing the sense current for both the probes. For probe B, the sharp and large drop in coercivity (ρxy-H loops) at comparatively lower sensing currents is observed, which is attributed to the negligible current shunting and presence of pinning site at B caused by the patterning process. The average domain wall velocities at various sensing currents for probe B are found to be smaller than probe A, from the transverse and longitudinal Hall geometry magnetization relaxation measurements, which agrees with pinning sites and Joule heating effect at probe B. The notch position in the pattern and the longitudinal Hall resistance curve peak shape suggest the domain wall propagation direction from probe B to probe A in the current channel. This study highlights the domain wall propagation at different nucleation fields, sensing currents, and the Hall probe aspect ratios.

Momentum-dependent flow fluctuations as a hydrodynamic response to initial geometry

We propose a redefinition of the principal component analysis (PCA) of anisotropic flow that makes it more directly connected to fluctuations of the initial geometry of the system. Then, using state-of-the-art hydrodynamic simulations, we make an explicit connection between flow fluctuations and a cumulant expansion of the initial transverse geometry. In particular, we show that the second principal component of elliptic flow is generated by higher-order cumulants, and therefore probes smaller length scales of the initial state. With this information, it will be possible to put new constraints on properties of the early-time dynamics of a heavy-ion collision, including small-scale structure, as well as properties of the quark-gluon plasma.

Hydrodynamization in systems with detailed transverse profiles

The observation of fluid-like behavior in nucleus-nucleus (AA), proton-nucleus (pA) and high-multiplicity proton-proton (pp) collisions motivates systematic studies of how different measurements approach their fluid-dynamic limit. We have developed numerical methods to solve the ultra-relativistic Boltzmann equation for systems of arbitrary size and transverse geometry. Here, we apply these techniques for the first time to the study of azimuthal flow coefficients vn including non-linear mode-mode coupling and to an initial condition with realistic event-by-event fluctuations. We show how both linear and non-linear response coefficients extracted from vn develop as a function of opacity from free streaming to perfect fluidity. We note in particular that away from the fluid-dynamic limit, the signal strength of linear and non-linear response coefficients does not reduce uniformly, but that their hierarchy and relative size shows characteristic differences.

Core‐type transformer short‐circuit voltage calculation using conformal mapping

This study describes a method for calculation of transformer short-circuit voltage using conformal mappings, which map the transformer longitudinal and transverse cross-section geometry to a unit disk where an analytical solution for magnetic flux density is obtained and then transformed back to the original domain. The short-circuit voltage is calculated by the integration of the magnetic energy of the leakage field in the windings and the space around windings, excluding the core. This method takes into account windings of different height, the fringing effect at the ends of the windings and any relative position of windings located on the same or different limbs. Also, it is possible to calculate the short-circuit voltage for multiple windings.

Sensitivity enhancement of a fiber-based interferometric optofluidic sensor.

Optofluidic sensors, which tightly bridge photonics and micro/nanofluidics, are superior candidates in point-of-care testing. A fiber-based interferometric optofluidic (FIO) sensor can detect molecular biomarkers by fusing an optical microfiber and a microfluidic tube in parallel. Light from the microfiber side coupled to the microtube leads to lateral localized light-fluid evanescent interaction with analytes, facilitating sensitive detection of biomolecules with good stability and excellent portability. The determination of the sensitivity with respect to the interplay between light and fluidics, however, still needs to be understood quantitatively. Here, we theoretically and experimentally investigate the relationship between refractive index (RI) sensitivity and individual geometrical parameters to determine the lateral localized light-fluid evanescent interaction. Theoretical analysis predicted a sensitive maximum, which could be realized by synergically tuning the fiber diameter d and the tube wall thickness t at an abrupt dispersion transition region. As a result, an extremely high RI sensitivity of 1.6×104 nm/RIU (σ=4074 nm/RIU), an order of magnitude higher than our previous results, with detection limit of 3.0×10-6 RIU, is recorded by precisely governing the transverse geometry of the setup. The scientific findings will guide future exploration of both new light-fluid interaction devices and biomedical sensors.

Iron-based binary ferromagnets for transverse thermoelectric conversion.

Thermoelectric generation using the anomalous Nernst effect (ANE) has great potential for application in energy harvesting technology because the transverse geometry of the Nernst effect should enable efficient, large-area and flexible coverage of a heat source. For such applications to be viable, substantial improvements will be necessary not only for their performance but also for the associated material costs, safety and stability. In terms of the electronic structure, the anomalous Nernst effect (ANE) originates from the Berry curvature of the conduction electrons near the Fermi energy1,2. To design a large Berry curvature, several approaches have been considered using nodal points and lines in momentum space3-10. Here we perform a high-throughput computational search and find that 25 percent doping of aluminium and gallium in alpha iron, a naturally abundant and low-cost element, dramatically enhances the ANE by a factor of more than ten, reaching about 4 and 6 microvolts per kelvin at room temperature, respectively, close to the highest value reported so far. The comparison between experiment and theory indicates that the Fermi energy tuning to the nodal web-a flat band structure made of interconnected nodal lines-is the key for the strong enhancement in the transverse thermoelectric coefficient, reaching a value of about 5 amperes per kelvin per metre with a logarithmic temperature dependence. We have also succeeded in fabricating thin films that exhibit a large ANE at zero field, which could be suitable for designing low-cost, flexible microelectronic thermoelectric generators11-13.

Correlation between the structural state and magnetoresistive properties of granular CoxAg100-x alloy thin films

Extensive investigations into structural, magnetotransport and magnetic properties of granular thin films with a thickness of 20 nm based on Co and Ag in a wide range of component concentrations were carried out: 17 ≤ x ≤ 89 where x is at.% of Co. The samples were obtained by the method of electron beam co-evaporation using two independent electron guns at room temperature. The correlation between the structural-phase state and magnetotransport and magnetic properties was established by TEM method. The threshold of structural percolation in granular films was observed for x ≈ 60%. At lower concentration x < 64%, the films had a phase composition of Ag (fcc) + Co (hcp) and were characterized by an isotropic giant magnetoresistance (GMR). The maximum GMR values of 7.2% and 7.5% in transverse and longitudinal geometry, respectively, were observed at x = 55 at.%. For higher concentrations (x > 70%) the films demonstrated anisotropic magnetoresistance (AMR), which was assumed to be associated with the appearance of ferromagnetic interactions between the cobalt granules.

Visualization of relativistic laser pulses in underdense plasma

We present experimental evidence of relativistic electron-cyclotron resonances (RECRs) in the vicinity of the relativistically intense pump laser of a laser wakefield accelerator (LWFA). The effects of the RECRs are visualized by imaging the driven plasma wave with a few-cycle, optical probe in transverse geometry. The probe experiences strong, spectrally dependent and relativistically modified birefringence in the vicinity of the pump that arises due to the plasma electrons' relativistic motion in the pump's electromagnetic fields. The spectral birefringence is strongly dependent on the local magnetic field distribution of the pump laser. Analysis and comparison to both 2D and 3D particle-in-cell simulations confirm the origin of the RECR effect and its appearance in experimental and simulated shadowgrams of the laser-plasma interaction. The RECR effect is relevant for any relativistic, magnetized plasma and in the case of LWFA could provide a nondestructive, in situ diagnostic for tracking the evolution of the pump's intensity distribution with propagation through tenuous plasma.

Spin-isospin correlated configurations in complex nuclei and neutron skin effect in W± production in high-energy proton-lead collisions

We extend our Monte Carlo algorithm for generating global configurations in nuclei to include different spatial distributions of protons and neutrons in heavy nuclei, taking into account the difference of spatial correlations between two protons, two neutrons, and proton-neutron pairs. We generate configurations for $^{48}\mathrm{Ca}$ and $^{208}\mathrm{Pb}$ neutron-rich nuclei, which can be used in general-purpose high-energy $A(e,{e}^{\ensuremath{'}}p)$, $p\text{\ensuremath{-}}A$, and $A\text{\ensuremath{-}}A$ event generators. As an application of lead configurations, we developed an algorithm for proton-heavy nucleus collisions at the CERN Large Hadron Collider for final states with a hard interaction in the channels where cross sections for $p\text{\ensuremath{-}}p$ and $p\text{\ensuremath{-}}n$ scattering differ. Soft interactions are taken into account in the color fluctuation extension of the Glauber algorithm, taking into account the inherently different transverse geometry of soft and hard $p\text{\ensuremath{-}}N$ collisions. We use the new event generator to test an interesting observation of Paukkunen [Phys. Lett. B 745, 73 (2015)] that the ratio of ${\mathrm{W}}^{\ifmmode\pm\else\textpm\fi{}}$ production rates in $p$-Pb collisions should significantly deviate from the inclusive value for peripheral collisions due to the presence of a neutron skin. We qualitatively confirm expectation of Paukkunen, although, for a realistic centrality trigger, we find the effect to be a factor of 2 smaller than the original estimate.

Design, Manufacturing and Characterization of Linear Fresnel Reflector’s Facets

This paper presents a procedure for making facetted mirrors to use in linear Fresnel reflectors, considering the design of the transversal geometry, materials, and structure configuration. Four different assemblies of the structure that supports and shapes the mirror are documented and evaluated. An assembly that implies a curved, pleated aluminum rectangular plate with a thin silvered-glass mirror vacuum glued to the plate is defined as the optimal. The geometrical quality of the chosen mirror facet’s configuration is accomplished by photogrammetry.

Revealing minijet dynamics via centrality dependence of double parton interactions in proton\u2013nucleus collisions

One of the main challenges hampering an accurate measurement of the double parton scattering (DPS) cross sections is the difficulty in separating the DPS from the leading twist (LT) contributions. We argue that such a separation can be achieved, and cross section of DPS measured, in proton–nucleus scattering by exploiting the different centrality dependence of DPS and LT processes. We developed a Monte Carlo implementation of the DPS processes which includes realistic nucleon–nucleon (NN) correlations in nuclei, an accurate description of transverse geometry of both hard and soft NN collisions as well as fluctuations of the strength of interaction of nucleon with nucleus (color fluctuation effects). Our method allows the calculation of probability distributions of single and double dijet events as a function of centrality, also distinguishing double hard scatterings originating from a single target nucleon and from two different nucleons. We present numerical results for the rate of DPS as a function of centrality, following the model developed by the ATLAS collaboration which relates the distribution over the number of wounded nucleons to the distribution over the sum of transverse energies of hadrons produced at large negative (along the nucleus direction) rapidities, which is experimentally measurable. We suggest a new quantity which allows to test the geometry of DPS and we argue that it is a universal function of centrality for different DPS processes. This quantity can be tested by analyzing existing LHC data. The method developed in this work can be extended to the search for triple parton interactions.