Hasil untuk "Atomic physics. Constitution and properties of matter"

Saksham Mahajan, Ravi Kumar, Aferdita Xhameni et al.

We study $V_{\mathrm B}^-$ centres generated by 25 keV helium focused ion beam (FIB) irradiation in thin (∼70 nm) hexagonal boron nitride (hBN) nanoflakes, to investigate the effect of implantation conditions on the key parameters that influence the magnetic field sensitivity of $V_{\mathrm B}^-$ quantum sensors. Using a combination of photoluminescence, optically detected magnetic resonance, and Raman spectroscopy we examine the competing factors of maximising signal intensity through larger $V_{\mathrm B}^-$ concentration against the degradation in spin coherence and lattice quality at high ion fluences. Both the $V_{\mathrm B}^-$ spin properties and hBN lattice parameters are largely preserved up to an ion fluence of 10 ^14 ions cm ^−2 , and beyond this significant degradation occurs in both. Our measurements give an inferred AC magnetic field sensitivity of ${\sim}1\,\unicode{x03BC}\text{T}/\sqrt{\text{Hz}}$ at the optimal implantation dose. Using the patterned implantation enabled by the FIB, the $V_{\mathrm B}^-$ centres and the associated lattice damage are well localised to the implanted regions. This work demonstrates how careful selection of fabrication parameters optimises the properties of $V_{\mathrm B}^-$ centres in hBN, supporting their application as quantum sensors based in 2D materials.

T. Farajollahpour, R. Ganesh, K. V. Samokhin

Abstract Berry curvature can manifest itself in current responses. It has recently been shown that a Berry-curvature quadrupole induces a third-order ac Hall response in systems that break time reversal (K) and a fourfold rotational (C 4) symmetries, while remaining invariant under their combination (C 4 K). In this work, we demonstrate that incident light can induce a dc Hall current in such systems. We consider a combination of static and light-induced electric fields. We calculate the current perpendicular to both the static field and the fourfold axis. Remarkably, the induced current is generically spin-polarized. A net charge current appears for light that is linearly or elliptically polarized, but not for circular polarization. In contrast, the spin current remains unchanged when the polarization is varied. This allows for rich possibilities such as generating a pure spin current using circularly polarized light. We demonstrate this physics using a two-dimensional toy model with C 4 K symmetry.

Evan Dowling, Gerald Baumgartner, Richard Brewster et al.

In a future quantum network, two spatially separated individuals sharing a polarization entanglement source may need to assess the entanglement quality of the source without the presence of classical auxiliary signals. When the two are separated by a single-mode fiber, automated methods are necessary for the validation and estimation of polarization entanglement. We experimentally examine real-time iterative methods to search for maximal Bell violations, specifically the Clauser–Horne–Shimony–Holt (CHSH) inequality, between two observers sharing polarization entangled photons to evaluate the entanglement quality of the source. Our source allows us to tune the degree of entanglement of their shared photon pair by changing the temporal overlap of the two photons at an entangling 50/50 beam splitter so that we can compare the CHSH parameter these methods find to the theoretical values our source produces. The iterative methods used within our experiment are the Nelder–Mead optimization method, stochastic gradient descent, and Bayesian optimization. This is the first feedback experiment to study automated Bell violations in fiber and the first to compare all three of these iterative methods to one another in a quantum polarization control experiment. In our experiment, all methods are able to find Bell violations, but the Nelder–Mead method performed the best in terms of the speed and accuracy in finding the maximal violation.

Eric Sabo, Arun B. Aloshious, Kenneth R. Brown

Trellis decoders are a general decoding technique first applied to qubit-based quantum error correction codes by Ollivier and Tillich in 2006. Here, we improve the scalability and practicality of their theory, show that it has strong structure, extend the results using classical coding theory as a guide, and demonstrate a canonical form from which the structural properties of the decoding graph may be computed. The resulting formalism is valid for any prime-dimensional quantum system. The modified decoder works for any stabilizer code <inline-formula><tex-math notation="LaTeX">$S$</tex-math></inline-formula> and separates into two parts: 1) a one-time offline computation that builds a compact graphical representation of the normalizer of the code, <inline-formula><tex-math notation="LaTeX">$\mathcal {S}^{\perp}$</tex-math></inline-formula> and 2) a quick, parallel, online query of the resulting vertices using the Viterbi algorithm. We show the utility of trellis decoding by applying it to four high-density length-20 stabilizer codes for depolarizing noise and the well-studied Steane, rotated surface, and 4.8.8/6.6.6 color codes for <inline-formula><tex-math notation="LaTeX">$Z$</tex-math></inline-formula> only noise. Numerical simulations demonstrate a 20&#x0025; improvement in the code-capacity threshold for color codes with boundaries by avoiding the mapping from color codes to surface codes. We identify trellis edge number as a key metric of difficulty of decoding, allowing us to quantify the advantage of single-axis (<inline-formula><tex-math notation="LaTeX">$X$</tex-math></inline-formula> or <inline-formula><tex-math notation="LaTeX">$Z$</tex-math></inline-formula>) decoding for Calderbank&#x2013;Steane&#x2013;Shor codes and block decoding for concatenated codes.

V. О. Zheltonozhskyi, D. E. Myznikov, A. M. Savrasov et al.

The γ-spectra were measured of the radioactive material of the ZNPP which was irradiated by bremsstrahlung with end-point energy 18.5 MeV. Using the ratio of the 47Ca and 58Co activities, the cobalt and calcium mass ratio was determined. Using the obtained data and the measured 60Co activity in the studied sample, a method for determining the 41Ca activity was developed.

Maria Teresa Mercaldo, Canio Noce, Andrea D. Caviglia et al.

Abstract The Berry curvature (BC)—a quantity encoding the geometric properties of the electronic wavefunctions in a solid—is at the heart of different Hall-like transport phenomena, including the anomalous Hall and the non-linear Hall and Nernst effects. In non-magnetic quantum materials with acentric crystalline arrangements, local concentrations of BC are generally linked to single-particle wavefunctions that are a quantum superposition of electron and hole excitations. BC-mediated effects are consequently observed in two-dimensional systems with pairs of massive Dirac cones and three-dimensional bulk crystals with quartets of Weyl cones. Here, we demonstrate that in materials equipped with orbital degrees of freedom local BC concentrations can arise even in the complete absence of hole excitations. In these solids, the crystals fields appearing in very low-symmetric structures trigger BCs characterized by hot-spots and singular pinch points. These characteristics naturally yield giant BC dipoles and large non-linear transport responses in time-reversal symmetric conditions.

Moritz A. Goerzen, Stephan von Malottki, Sebastian Meyer et al.

Abstract Magnetic skyrmions have raised high hopes for future spintronic devices. For many applications, it would be of great advantage to have more than one metastable particle-like texture available. The coexistence of skyrmions and antiskyrmions has been proposed in inversion-symmetric magnets with exchange frustration. However, so far only model systems have been studied and the lifetime of coexisting metastable topological spin structures has not been obtained. Here, we predict that skyrmions and antiskyrmions with diameters below 10 nm can coexist at zero magnetic field in a Rh/Co bilayer on the Ir(111) surface—an experimentally feasible system. We show that the lifetimes of metastable skyrmions and antiskyrmions in the ferromagnetic ground state are above one hour for temperatures up to 75 and 48 K, respectively. The entropic contribution to the nucleation and annihilation rates differs for skyrmions and antiskyrmions. This opens the route to the thermally activated creation of coexisting skyrmions and antiskyrmions in frustrated magnets with Dzyaloshinskii–Moriya interaction.

O. V. Mykhailov, M. V. Saveliev, R. L. Godun et al.

A brief overview of data of automated temperature monitoring systems at the periphery of nuclearly hazardous clusters of fuel-containing materials (FCM NHC) and analytical materials regarding the study of its dynamics at various monitoring points before and after installation of New Safe Confinement (NSC) into its design position, is presented. The characterization of revealed dominant trends in temperature time characteristics underway in the observation period from 1991 to 2015 and the reasons influencing their formation at various distances from FCM NHC boundaries in room 305/2, are addressed. The importance of the work of an expert research system (ERS), which functionally supplemented the existing nuclear safety monitoring system (NSMS) of CHNPP "Shelter" Object (SO) installed instead of decommissioned information and measuring system (IMS "Finish") and other autonomous FCM monitoring systems, is highlighted. A critical analysis of the state of current temperature monitoring around the FCM clusters is provided, and a conclusion about the need to improve the existing monitoring network is made.

Sharu Theresa Jose, Osvaldo Simeone

Variational quantum algorithms (VQAs) offer the most promising path to obtaining quantum advantages via noisy intermediate-scale quantum (NISQ) processors. Such systems leverage classical optimization to tune the parameters of a parameterized quantum circuit (PQC). The goal is minimizing a cost function that depends on measurement outputs obtained from the PQC. Optimization is typically implemented via stochastic gradient descent (SGD). On NISQ computers, gate noise due to imperfections and decoherence affects the stochastic gradient estimates by introducing a bias. Quantum error mitigation (QEM) techniques can reduce the estimation bias without requiring any increase in the number of qubits, but they in turn cause an increase in the variance of the gradient estimates. This work studies the impact of quantum gate noise on the convergence of SGD for the variational eigensolver (VQE), a fundamental instance of VQAs. The main goal is ascertaining conditions under which QEM can enhance the performance of SGD for VQEs. It is shown that quantum gate noise induces a nonzero error-floor on the convergence error of SGD (evaluated with respect to a reference noiseless PQC), which depends on the number of noisy gates, the strength of the noise, as well as the eigenspectrum of the observable being measured and minimized. In contrast, with QEM, any arbitrarily small error can be obtained. Furthermore, for error levels attainable with or without QEM, QEM can reduce the number of required iterations, but only as long as the quantum noise level is sufficiently large, and a sufficiently large number of measurements is allowed at each SGD iteration. Numerical examples for a max-cut problem corroborate the main theoretical findings.

Kelly J. Neubauer, Feng Ye, Yue Shi et al.

Abstract The anomalous Hall effect (AHE), typically observed in ferromagnetic (FM) metals with broken time-reversal symmetry, depends on electronic and magnetic properties. In Co3Sn2-x In x S2, a giant AHE has been attributed to Berry curvature associated with the FM Weyl semimetal phase, yet recent studies report complicated magnetism. We use neutron scattering to determine the spin dynamics and structures as a function of x and provide a microscopic understanding of the AHE and magnetism interplay. Spin gap and stiffness indicate a contribution from Weyl fermions consistent with the AHE. The magnetic structure evolves from c-axis ferromagnetism at $$x = 0$$ x = 0 to a canted antiferromagnetic (AFM) structure with reduced c-axis moment and in-plane AFM order at $$x = 0.12$$ x = 0.12 and further reduced c-axis FM moment at $$x = 0.3$$ x = 0.3 . Since noncollinear spins can induce non-zero Berry curvature in real space acting as a fictitious magnetic field, our results revealed another AHE contribution, establishing the impact of magnetism on transport.

G Terrasanta, M Müller, T Sommer et al.

Aluminum nitride (AlN) is an emerging material for integrated quantum photonics with its excellent linear and nonlinear optical properties. In particular, its second-order nonlinear susceptibility χ ^(2) allows single-photon generation. We have grown AlN thin films on silicon nitride (Si _3 N _4 ) via reactive DC magnetron sputtering. The thin films have been characterized using x-ray diffraction (XRD), optical reflectometry, atomic force microscopy (AFM), and scanning electron microscopy. The crystalline properties of the thin films have been improved by optimizing the nitrogen to argon ratio and the magnetron DC power of the deposition process. XRD measurements confirm the fabrication of high-quality c -axis oriented AlN films with a full width at half maximum of the rocking curves of 3.9° for 300 nm-thick films. AFM measurements reveal a root mean square surface roughness below 1 nm. The AlN deposition on SiN allows us to fabricate hybrid photonic circuits with a new approach that avoids the challenging patterning of AlN.

S. V. Litvinov, N. M. Rashydov

Effective repair of radiation-induced DNA lesions is an important factor in the radioresistance of plants. Taking into account the role of the DNA mismatch repair (MMR) in the systemic reaction of plants to the action of sublethal doses of ionizing radiation has not yet been sufficiently clear enough. The study of the effect of low LET ionizing radiation in doses up to 21 Gy had been performed, including measurement of the biomass accumulation in Arabidopsis thaliana Atmsh2-/- plants, defective in one of the key components of MMR-repair, MSH2 protein. It has been established that the relative radiosensitivity of Atmsh2 SALK_002708 mutants to the action of sublethal doses of radiation depends on the dose and mode of irradiation. Changes in the radiosensitivity of mutant plants at different doses and due to the fractionation of the dose can be related to the radiation-induced transcriptional response of genes, coding DNA repair proteins, which compensate the lack of MMR-repair or along with MSH2 participate in the mutagenic pathways of DNA repair.

S. N. Pelykh, Huiyu Zhou, O. B. Maksymova

The physics and regularities of the neutron-thermoacoustic instability (NTAI) in nuclear channels with subcooled nucleate boiling flows are explained. The method for obtaining the characteristic equation of NTAI in a VVER-type reactor is given. The influence of steam reactivity coefficient ∂k/∂φ on NTAI boundaries is studied and the effect of a negative ∂k/∂φ favouring the oscillatory instability of neutron flux and pressure is shown.

Yan Song, Xiaocha Wang, Wenbo Mi

One-dimensional nanoribbons: A platform for probing exotic magnetoresistance Electrically-tunable electron-hole pockets could make antimonene nanoribbons a platform for probing unusual magnetoresistance effects. Some material changes their resistance when a magnetic field is applied—an effect known as magnetoresistance. This phenomenon is harnessed for a range of technological applications but the origin of the recently observed large, non-saturating magnetoresistance in certain materials is somewhat puzzling. Using first-principles calculations, Wenbo Mi and Yan Song from Tianjin University, along with Xiaocha Wang from Tianjin University of Technology, predict that one-dimensional antimonene nanoribbons with a zigzag asymmetric washboard structure could have electron-hole pockets, which are the key features necessary for observing a non-saturating magnetoresistance. As these pockets could be modulated using in-plane electric fields, these nanoribbons could be a tunable platform for probing this exotic phenomenon.

A. I. Lypska, V. I. Nikolaev, V. A. Shytiuk et al.

Distribution of radionuclides in the vertical soil profile on the nearest Chernobyl NPP zone of alienation was investigated. It is showed experimentally that the main activity of radionuclides is concentrated in the topsoil (10 сm). Coefficients of accumulation of 137Cs and 90Sr radionuclides by plants are estimated. The physico-chemical forms of radionuclides in soil and plants were defined using the method of sequential chemical extraction. It was established that the main contents of 137Cs and 90Sr in soils are represented in non-exchange and fixed forms, in plants - mainly in exchange-adsorption and organic forms.

A. M. Berlizov, I. A. Maliuk, A. D. Sazheniuk et al.

Method of obtaining test samples for quantitative determination of radionuclide activity in Zr-Nb alloy, which is used in NPP fuel channel tubes manufacturing was described.

T. V. Kovalinska, I. A. Ostapenko, V. I. Sakhno et al.

Ways of the improvement of technical base of the INR of NAS of Ukraine for functional researches and new technologies of control over the state of the equipment on NPPs are discussed. The scientific work is completed in the department of radiation technologies within the national program of the enhancement of the reliability of nuclear energetic and the prolongation of operation terms of nuclear power installations.

Piotr Jaranowski, Andrzej Królak

The article reviews the statistical theory of signal detection in application to analysis of deterministic gravitational-wave signals in the noise of a detector. Statistical foundations for the theory of signal detection and parameter estimation are presented. Several tools needed for both theoretical evaluation of the optimal data analysis methods and for their practical implementation are introduced. They include optimal signal-to-noise ratio, Fisher matrix, false alarm and detection probabilities, ℱ-statistic, template placement, and fitting factor. These tools apply to the case of signals buried in a stationary and Gaussian noise. Algorithms to efficiently implement the optimal data analysis techniques are discussed. Formulas are given for a general gravitational-wave signal that includes as special cases most of the deterministic signals of interest.

N. P. Atamaniuk, L. P. Derevianko, V. V. Tal'ko et al.

Influence of single total body irradiation and local head irradiation of female rats in doses 2,0 and 6,0 Gy on malonic dialdehyde (MDA) concentration, on catalase and on superoxiddismutase (SOD) activities in blood was studied in dynamics (7, 14, 30, 90 days after irradiation). Irradiation was fulfilled on X-ray-installation «РУМ-17» (Russia), the power of exposition dose 2,09 · 10-4 C/(kg · sec). Indices changes, which characterize the state of prooxidantiveantioxidantive equilibrium were noted both in the total and local irradiation of the head. Increase of MDA concentration in the blood serum irradiated rats and decrease of catalase and of SОD activities were found. The degree of changes of these indices depends from the type of irradiation (total, local), from the dose and from the term of observation. Changes of antioxidative system fermentale activities were less expressed after single local irradiation of the head.

Lahav Ofer, Suto Yasushi

Galaxy redshift surveys have achieved significant progress over the last couple of decades. Those surveys tell us in the most straightforward way what our local Universe looks like. While the galaxy distribution traces the bright side of the Universe, detailed quantitative analyses of the data have even revealed the dark side of the Universe dominated by non-baryonic dark matter as well as more mysterious dark energy (or Einstein's cosmological constant). We describe several methodologies of using galaxy redshift surveys as cosmological probes, and then summarize the recent results from the existing surveys. Finally we present our views on the future of redshift surveys in the era of precision cosmology.