Cone Phase Research Articles

Crossed Andreev reflection in normal-superconductor-normal junction based on Kekulé-Y patterned graphene

We theoretically study the crossed Andreev reflection (CAR) of the normal metal-superconductor-normal metal (NSN) heterojunction based on Kekulé-Y patterned graphene with two doping types, i.e. nSn and nSp configurations. It is found that the enhanced CAR is more likely to occur in the nSp junction rather than the nSn junction. To be concrete, the almost perfect CAR occurs in a large range of incident angle in the single Dirac cone phase when the incident energy is inside the gap of the nonlinear band. Furthermore, the roles of the length of superconductor and pseudospin-valley coupling on conductance are also evaluated.

Skyrmion clusters and chains in bulk and thin-layered cubic helimagnets

Here, the author overviews the properties and the internal structure of isolated skyrmions within a cone phase of cubic helimagnets and chiral liquid crystals. The nascent conical state is shown to substantiate the attracting skyrmion-skyrmion interaction, which leads to the formation of skyrmion clusters and/or skyrmions chains. Whereas in bulk helimagnets skyrmion clusters exhibit the hexagonal arrangement of constituent skyrmions, in thin layers skyrmions align into polar skyrmion chains. Such behavior is thickness dependent and depends on the characteristic energy profiles.

Modelling of the pedestal growth of silicon crystals

The pedestal method is an alternative to the well-known floating zone method, both of which are performed with high-frequency electromagnetic heating. Unlike the floating zone method, in the pedestal method a single crystal is pulled upwards from the melt. It allows one to lower feed rod quality requirements and simplify the process control due to the absence of open melting front. As the pedestal method has not been widely used in industry for silicon crystals, its development requires extensive numerical modelling. The present work describes application of the previously created mathematical model for crystals with diameters higher than it is currently possible in the experimental setup, as well as for the cone growth phase. Supplementary free surface heating, that prevents melt centre freezing during the seeding phase, has been added at the beginning of cone phase. After multiple sets of simulations, an optimal scheme of heating control for cone growth was proposed.

Enhanced Andreev reflection in Kekulé-Y patterned graphene

We theoretically investigate the electron-hole conversions in normal metal-superconductor junctions based on the Kekul\'e-Y (Kek-Y) patterned graphene. It is shown that, in contrast to the Klein tunneling in the pristine graphene, the perfect Andreev reflection with almost unit efficiency can be extended to a large range of incident angles in the Kek-Y patterned graphene in single-Dirac cone phase with the bias voltage in the gap. The normalized zero-bias conductance is also obtained.

Hyperbolic plasmon modes in tilted Dirac cone phases of borophene

Hyperbolic materials are receiving significant attention due to their ability to support electromagnetic fields with arbitrarily high momenta and, hence, to achieve very strong light confinement. Here, based on first-principles calculations and many-body perturbation theory, we explore the characteristic of two-dimensional plasmon modes and its hyperbolic properties for two phases of single-layer boron hosting tilted Dirac cone, namely, the $hr\text{\ensuremath{-}}sB$ and 8Pmmn borophene. In-plane anisotropy in borophene is manifested in the structural, electronic, vibrational, and optical properties. We find two hyperbolic regimes for both phases of borophene, where the high-energy one is located in the visible range. The $hr\text{\ensuremath{-}}sB$ borophene is characterized with an intrinsic high carrier density and it supports strong hyperbolic plasmon modes in the visible part of the spectrum. The 8Pmmn borophene, on the other hand, resembles the prototypical Dirac material graphene and on carrier doping acquires anisotropic Dirac plasmons in the mid-infrared. We have also investigated the impact of the electron-phonon coupling and Landau damping on these hyperbolic plasmon modes. Our results show that borophene, having high anisotropy, intrinsic high carrier concentration, low-loss hyperbolic Dirac plasmon modes, and high confinement can represent a promising candidate for low-loss broadband surface plasmon polariton devices.

Strain-modulated helimagnetism and emergent magnetic phase diagrams in highly crystalline MnP nanorod films

We explore strain-modulated helimagnetism in highly crystalline MnP nanorod films grown on Si(100) substrates using molecular beam epitaxy. The strained MnP film exhibits a paramagnetic to ferromagnetic (FM) phase transition at ${T}_{C}\ensuremath{\sim}279\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the FM to helical phase transition at ${T}_{N}\ensuremath{\sim}110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The value of ${T}_{N}$ is greater than ${T}_{N}\ensuremath{\sim}47\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for the MnP single crystal, indicating strong strain-modulated helimagnetic states in the MnP nanorod film. The presence of significant thermal hysteresis in the helical phase indicates the coexistence of competing magnetic interactions, leading to the first-order metamagnetic transition. Similar to its single-crystal counterpart, an anisotropic magnetic effect is observed in the MnP film, which is independently confirmed by magnetic hysteresis loop and radio-frequency transverse susceptibility (TS) measurements. The evolution of the screw to the cone and fan phases is precisely tracked from magnetization vs magnetic field/temperature measurements. The temperature dependence of the anisotropy fields, extracted from the TS spectra, yields further insight into the competing nature of the magnetic phases. Unfolding of the different helical phases at $T<120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ $(\ensuremath{\sim}{T}_{\mathrm{N}})$ is analyzed by the temperature- and field-dependent magnetic entropy change. Based on these findings, the comprehensive magnetic phase diagrams of the MnP nanorod film are constructed for both the in-plane and out of plane magnetic field directions, revealing emergent strain/dimensionality-driven helical magnetic features that are absent in the magnetic phase diagram of the MnP single crystal.

Evaluation of the Performance of Published Point Defect Parameter Sets in Cone and Body Phase of a 300 mm Czochralski Silicon Crystal

Prediction and adjustment of point defect (vacancies and self-interstitials) distribution in silicon crystals is of utmost importance for microelectronic applications. The simulation of growth processes is widely applied for process development and quite a few different sets of point defect parameters have been proposed. In this paper the transient temperature, thermal stress and point defect distributions are simulated for 300 mm Czochralski growth of the whole crystal including cone and cylindrical growth phases. Simulations with 12 different published point defect parameter sets are compared to the experimentally measured interstitial–vacancy boundary. The results are evaluated for standard and adjusted parameter sets and generally the best agreement in the whole crystal is found for models considering the effect of thermal stress on the equilibrium point defect concentration.

Tunable tilted anisotropy of massless Dirac fermion in magnetic Kronig-Penney-type graphene

Modulation effects of alternating periodic electrostatic scalar and magnetic vector potentials on the anisotropic and the tilt characteristics of Dirac cone in a semi-infinite graphene was theoretically investigated using the transfer matrix technique. To study its anisotropic behavior, we characterized the geometry deformation of superlattice Dirac cone by introducing the tilt parameter, eccentricity, and group velocity. We found that the Dirac cone anisotropy and tilt effect depend on the amplitude of the periodically modulated scalar and/or vector potentials. As the results of tuning the coupled periodic scalar and vector potentials, four different phases of Dirac fermions emerged and were categorized through a phase diagram of the tilt parameter. For a strong coupled modulation, the tilt parameter becomes larger than one and the type-II Dirac cone phase emerged. We also found that pairs of extra Dirac points can be induced by a very high periodic scalar modulation coupled with a low periodic vector modulation. Moreover, strong modulation caused the Fermi contour to deform from a closed contour into an opened contour, introducing an electron-hole pocket on the graphene. Our results demonstrate the possibility to control the anisotropic behavior of the electronic structure of graphene, which might lead to an artificial tilted Dirac cone material and direction-dependent quantum devices.

The effect of pruning practice and term on a leaf area of apple-trees

The article is devoted to the study of morphological changes of the apple leaf apparatus in the intensive plantations depending on different methods and terms of pruning. The article scientifically substantiates and proves that changes in the leaf apparatus primarily depend on the timing and methods of pruning the crown. It was found out that the contour pruning of apple trees is 14% larger than the area of the leaf blade, and after pruning in the pink cone phase - 11%, the implementation of this agricultural measure in the pink cone phase increases the total leaf surface by 20%, thickens the leaf blade by 8% and reduces the number of leaves of apple trees by 17%. Key words: apple tree, pruning method, pruning period, mechanical (contour) pruning, leaf surface area, leaf blade thickness.

Magnon Modes for a Magnetic Disc in a Cone Vortex State

A cone phase appears as the ground state of uniaxial magnets with comparable second-order and fourth-order anisotropy constants. In such systems, specific vortex states can exist. The authors have studied magnetization oscillations in small cylindrical particles in a cone vortex state. The study has determined the frequencies of magnon excitations in the presence of a vortex in the cone phase of a ferromagnet. It has been established that there is a significant splitting of the doublets of modes with opposite values ​​of the azimuthal number m, including modes with |m| > 1.

Soft modes of the dielectric response in the twist-bend nematic phase and identification of the transition to a nematic splay bend phase in the CBC7CB dimer.

The dielectric spectra of the twist-bend nematic phase (NTB) of (the bent-shaped) achiral liquid-crystal dimer 1''-,7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB) are studied for the determination of the different relaxation modes. Two molecular processes and one collective process were observed in the megahertz frequency range. Two molecular processes were assigned: one to the precessional rotation of the longitudinal components of the cyanobiphenyl groups and the second one to the spinning rotation of the transverse component of the CB7CB dimer. The peak, at a frequency of about 1 MHz, shows a peculiar temperature behavior at the NTB to N transition, reminiscent of the soft mode at the transition from the SmA to the SmC phase. This peak can be assigned to a collective fluctuation of the tilt angle of the coarse grained director N with respect to the pseudo-layer normal. This corresponds well with the electro-clinic effect observed as a response to an electric field in electro-optic experiments. The low frequency relaxation process, observed in the frequency range of 1 Hz-102 Hz can be identified as a Goldstone mode, related to long-scale fluctuation of the cone phase. The frequency drop of the mode on increasing the bias field is interpreted as unwinding of a helix and an indication of the formation of a field induced nematic splay bend phase (NSB). This finding is also confirmed by birefringence data in the presence of strong bias fields.

Electric-Field-Driven Topological Phase Switching and Skyrmion-Lattice Metastability in Magnetoelectric Cu2OSeO3

Due to their topological protection and nanometric size, magnetic skyrmions are anticipated to form components of new high-density memory technologies. In metallic systems skyrmion manipulation is achieved easily under a low density electric current flow, although the inevitable thermal dissipation ultimately limits the energy efficacy of potential applications. On the other hand, a near dissipation-free skyrmion and skyrmion phase manipulation is expected by using electric \emph{fields}, thus meeting better the demands of an energy-conscious society. In this work on an insulating chiral magnet Cu$_{2}$OSeO$_{3}$ with magnetoelectric coupling, we use neutron scattering to demonstrate directly i) the creation of metastable skyrmion states over an extended range in magnetic field and temperature, and ii) the in-situ electric field-driven switching between topologically distinct phases; the skyrmion phase and a competing non-topological cone phase. For our accessible electric field range, the phase switching is achieved in a high temperature regime, and the remnant (E=0) metastable skyrmion state is thermally volatile with an exponential lifetime on hour timescales. Nevertheless, by taking advantage of the demonstrably longer-lived metastable skyrmion states at lower temperatures, a truly non-volatile and near dissipation-free topological phase change memory function is promised in magnetoelectric chiral magnets.

Chiral bobbers and skyrmions in epitaxial FeGe/Si(111) films

We report experimental and theoretical evidence for the formation of chiral bobbers - an interfacial topological spin texture - in FeGe films grown by molecular beam epitaxy (MBE). After establishing the presence of skyrmions in FeGe/Si(111) thin film samples through Lorentz transmission electron microscopy and topological Hall effect, we perform magnetization measurements that reveal an inverse relationship between film thickness and the slope of the susceptibility (d\c{hi}/dH). We present evidence for the evolution as a function of film thickness, L, from a skyrmion phase for L < LD/2 to a cone phase with chiral bobbers at the interface for L > LD/2, where LD ~ 70 nm is the FeGe pitch length. We show using micromagnetic simulations that chiral bobbers, earlier predicted to be metastable, are in fact the stable ground state in the presence of an additional interfacial Rashba Dzyaloshinskii-Moriya interaction (DMI).

Direct observation of attractive skyrmions and skyrmion clusters in the cubic helimagnet Cu2OSeO3

We report the discovery of attractive magnetic skyrmions and their clusters in non-centrosymmetric ferromagnets. These three-dimensional solitons have been predicted to exist in the cone phase of chiral ferromagnets (J. Phys: Condens. Matter 28 (2016) 35LT01) and are fundamentally different from the more common repulsive axisymmetric skyrmions that occur in the magnetically saturated state. We present real-space images of these skyrmion clusters in thin (~70 nm) single-crystal samples of Cu$_2$OSeO$_3$ taken using transmission electron microscopy and develop a phenomenological theory describing this type of skyrmion.

Slowed dark adaptation in older eyes; effect of location

PurposeThe rate of rod sensitivity recovery following a photobleach is a basic measure of the integrity of the outer retina. Rods are selectively impaired in aging and many disorders of the retina, notably Age-Related Macular Degeneration (AMD). It is not known for certain whether the age-related deficit is a pan-retinal effect or if there are localised regions of impaired rod function. To address this important issue a dual arc stimulus was developed that samples sensitivity recovery in two retinal locations. MethodsArc-shaped stimuli were presented on a black CRT screen at two locations, in the inferior visual field. Following a bleach, which was localised to the stimuli, recovery of sensitivity was measured using a modified method of adjustment technique. Neutral density filters were used to extend the luminance range of the CRT. Sensitivity recovery functions were fitted by non-linear regression to a seven-parameter model. ResultsPairs of sensitivity recovery functions were generated from the stimuli. The cone phases of these functions were identical. The slopes of the S2 sections of the curves were steeper for the outer stimuli for both young (p < 0.001) and older (p = 0.003) observers. The difference between the two was the same for the two groups. The α point was reached slightly earlier for the young observers and with the outer stimulus but neither of these effects reached statistical significance. The β point occurred earlier for the outer stimuli and this effect was statistically significant only for the older group. ConclusionsThe method places minimal demands on observers. The fact that rod sensitivity recovery is slowed in the older normal eye to the same extent in the two locations suggests that this deficit may be uniform across the retina. As there are localised losses in scotopic function in AMD, the technique is ideally suited to distinguishing impaired recovery dynamics due to normal ageing from those caused by disease.

Three-dimensional chiral skyrmions with attractive interparticle interactions

We introduce a new class of isolated three-dimensional skyrmion that can occur within the cone phase of chiral magnetic materials. These novel solitonic states consist of an axisymmetric core separated from the host phase by an asymmetric shell. These skyrmions attract one another. We derive regular solutions for isolated skyrmions arising in the cone phase of cubic helimagnets and investigate their bound states.

Umbrella-coplanar transition in the triangular XXZ model with arbitrary spin

The quantum triangular XXZ model has recently enjoyed a wealth of new theoretical results, especially in relation to the modeling of the Ba$_3$CoSb$_2$O$_9$ compound. In particular, it has been understood that in a longitudinal magnetic field the umbrella (cone) phase, classically stable in all the easy-plane region of the ground-state phase diagram, is considerably reduced by the effect of quantum fluctuations. We provide more quantitative information for this phenomenon at arbitrary value of the site spin $S$, by employing the dilute Bose gas expansion, valid in the high-field regime; our results improve the available estimates of the $1/S$ expansion. We quantify the extent to which a higher spin suppresses the effect of quantum fluctuations. Besides, we show how in three-dimensional layered systems a relatively small antiferromagnetic interlayer coupling has a similar consequence of bringing back the umbrella phase in some part of the phase diagram.

Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around $T\approx 40$ K and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields.

SU‐E‐T‐253: Development of a GDPM Monte Carlo Based Quality Assurance Tool for Cyberknife

Purpose:To develop a 3D quality assurance (QA) tool for Cyberknife plans using a GPU‐based Monte Carlo dose calculation package, gDPM. The developed QA tool will be used as a secondary check for Cyberknife treatment planning software (TPS) reported dose distributions.Methods:Phase space files of the 5, 7.5, 10, 15, and 60 mm iris cones were downloaded from IAEA phase‐space database. From the 60mm iris cone phase space file, we were able to generate raw phase space files for all other cones using a particle rejection method. The particles in the raw phase space files were binned with respect to the radial position and energy for commissioning. During the commissioning process, gDPM calculated percent depth dose and off‐center ratios which were compared to those measured in the Blue Phantom. The off‐axis data was measured with an SAD setup at depths of 15, 50, 100, 200, and 300 mm. At the commissioning stage, an optimization problem was solved to adjust the binned particles weights to minimize the difference between the calculated and measured ones. Commissioning validations will be performed by measuring point dose data of patient specific plans delivered to water phantoms. With commissioned phase space files, 3D patient‐specific dose distributions will be calculated and compared against TPS reported dose.Results:Beyond the initial buildup region, the root mean square difference between the calculated and measured percent depth dose differences was less than 0.5%. The full‐width half‐max data from the off‐axis ratio calculations was found to be within 0.1 mm of the measured data.Conclusion:Our current work showed excellent agreement between the gDPM calculated dose and the measured data for all cone sizes and types for the Cyberknife system. Implementation of a fast and accurate QA tool for patient specific plans will be feasible with this tool.

Hall effect and transmission electron microscopy of epitaxial MnSi thin films

We present Hall-effect measurements on MnSi/Si(111) epilayers and find an anomalous Hall contribution that is significantly smaller than that in bulk crystals, which enables the observation of an additional contribution to the anomalous signal previously overlooked in MnSi. Our measurements indicate the signal is not due to skyrmions in MnSi thin films, which are absent in out-of-plane fields, but rather are the result of scattering from the cone phase. The absence of magnetic contrast in the transmission electron microscopy (TEM) measurements is consistent with this interpretation [T. L. Monchesky et al., Phys. Rev. Lett. 112, 059701 (2014)]. We provide a method to model TEM images of skyrmion lattices to determine the conditions necessary for their observation in other B20 epilayers with an anisotropy that is favorable to their formation.