Two-dimensional Character Research Articles

Ultrathin Limit on the Anisotropic Superconductivity of Single-Layered Cuprate Films

Exploring dimensionality effects on cuprates is important for understanding the nature of high-temperature superconductivity. By atomically layer-by-layer growth with oxide molecular beam epitaxy, we demonstrate that La2–x Sr x CuO4 (x = 0.15) thin films remain superconducting down to 2 unit cells of thickness but quickly reach the maximum superconducting transition temperature at and above 4 unit cells. By fitting the critical magnetic field (μ 0 H c2), we show that the anisotropy of the film’s superconductivity increases with decreasing film thickness, indicating that the superconductivity of the film gradually evolves from weak three- to two-dimensional character. These results are helpful to gain more insight into the nature of high-temperature superconductivity with dimensionality.

Intrinsic surface p-wave superconductivity in layered AuSn4

The search for topological superconductivity (TSC) is currently an exciting pursuit, since non-trivial topological superconducting phases could host exotic Majorana modes. However, the difficulty in fabricating proximity-induced TSC heterostructures, the sensitivity to disorder and stringent topological restrictions of intrinsic TSC place serious limitations and formidable challenges on the materials and related applications. Here, we report a new type of intrinsic TSC, namely intrinsic surface topological superconductivity (IS-TSC) and demonstrate it in layered AuSn4 with Tc of 2.4 K. Different in-plane and out-of-plane upper critical fields reflect a two-dimensional (2D) character of superconductivity. The two-fold symmetric angular dependences of both magneto-transport and the zero-bias conductance peak (ZBCP) in point-contact spectroscopy (PCS) in the superconducting regime indicate an unconventional pairing symmetry of AuSn4. The superconducting gap and surface multi-bands with Rashba splitting at the Fermi level (EF), in conjunction with first-principle calculations, strongly suggest that 2D unconventional SC in AuSn4 originates from the mixture of p-wave surface and s-wave bulk contributions, which leads to a two-fold symmetric superconductivity. Our results provide an exciting paradigm to realize TSC via Rashba effect on surface superconducting bands in layered materials.

Increasing the Magnetic Order Temperature in Co3O2BO3:In Ludwigite.

Below 42 K, the homometallic Co3O2BO3 ludwigite forms magnetic planes separated by nonmagnetic low-spin Co3+ ions. The substitution of Co3+ by other nonmagnetic ions enhances the magnetic interactions, raising the magnetic ordering temperature. However, depending on the nonmagnetic dopant ion, the remaining Co3+ ions could adopt a high-spin state, creating magnetic frustration and lowering the magnetic transition temperature. Doping Co3O2BO3 with nonmagnetic In3+ ions favors the appearance of both high-spin Co2+ and Co3+. The In3+ ions preferentially occupy sites 4 and are randomly distributed in each site. The two-dimensional magnetic character of the parent compound, Co3O2BO3, is preserved, and the magnetic transition temperature increases to 47.8 K. Measurements of magnetization, which show metamagnetic transitions at low temperatures, and specific heat are consistent with ferrimagnetic ordering in this system. Thus, using these results and those reported in the literature, the effects caused by doping of Co3O2BO3 with different nonmagnetic +3 ions are discussed in terms of the presence of high-spin Co2+ and Co3+ in the compounds.

Exciton absorption spectrum of thin films of ternary compounds in the CsCl–CuCl system

The absorption spectra of thin films (CsCl)1–x(CuCl)x were studied at T = 90 K in the spectral range 2–6 eV. From the analysis of the spectra, the formation of CsCu2Cl3 and Cs2CuCl3 compounds in the CsCl–CuCl system was created. The Cs2CuCl3 compound is stable only in a vacuum; in air, it apparently decomposes into CsCu2Cl3 with a disordered crystal lattice and CsCl. The exciton spectrum of both compounds is interpreted on the basis of transitions in the Cu+ ion. The one-dimensional character of excitons in CsCu2Cl3 and the two-dimensional character in Cs2CuCl3 were established.

Comparative study of Na0.67Ni0.25Co0.17Mn0.58O2 P-Type cathode materials for sodium-ion batteries: Combustion synthesis and combined analysis of structural, electrical, and dielectric properties

In this study, we present a promising sodium-ion cathode material, Na0.67Ni0.25Co0.17Mn0.58O2, which was synthesized by combustion method, and subsequent heat treatment at various calcination temperatures (700 °C, 800 °C, and 900 °C). The samples underwent analysis using various techniques, including Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-ray Analysis (EDS), Specific Surface Area Measurements (BET), and Impedance Spectroscopy Measurements (EIS). The XRD pattern of the sample prepared at 700 °C reveals that the lower annealing temperature induces the formation of a new phase, P3. Indeed, the phase transition from the rhombohedral P3-phase to hexagonal P2-phase at 900 °C via mixed phases (P3/P2) yielded at 800 °C. Furthermore, Rietveld refinement using the hexagonal unit cell leads the calculation of the network parameters and the structural stacking, showing that the P2-phase exhibits a more pronounced two-dimensional character compared to the P3-phase. FT-IR spectra reveal that the lattice structures of the samples consist of NaO6 and MeO6 (Me = Ni, Co, Mn) octahedra. SEM-EDS analysis shows a homogeneous surface morphology in good agreement with the theoretical and experimental proportions of the elements constituting the three P-type phases. Subsequently, we extensively investigated the effect of our P-type materials on the electrical and dielectric properties of the cathodes using EIS measurements. Our findings suggest that the low-cost and eco-friendly P2–Na0.67Ni0.25Co0.17Mn0.58O2 phase shows promise as a cathode material for sodium-ion batteries and represents a competitive alternative to the P3/P2- and P3–Na0.67Ni0.25Co0.17Mn0.58O2 phases.

Artificial Intelligence (AI)-Based Self-Deciding Character Development Application in Two-Dimensional Video Games

Artificial Intelligence (AI) technologies, which have started to be used in almost all areas of life, are now also used in games. Especially for bot systems that play the game automatically in many popular video games recently, it has become possible for these bots to defeat the grandmasters of the game with advanced Deep Learning (DL) methods. The purpose of this study is to develop an AI-based two-dimensional video game in Unity 2D Game Engine, which can decide what the game characters will do within their abilities. In the game application, the two-dimensional game character is controlled by the user. This character moves in a two-dimensional game plane that is created using procedural plane generation algorithms. In addition, statistics such as the number of enemies defeated and the level reached in the game are recorded. The game is based on the Unity2D game engine and is written in C#. In the study, 8 different experiments are performed. In these experiments, the player's success time in the game with different equipment defined for the character is measured. This time ranges from 0.54 seconds to 1.88 seconds. Today, AI algorithms are developed to play computer games better or to design better games, and new games are designed to improve AI algorithms. In this context, it is evaluated that this study will contribute to the literature on the use of AI in computer games.

Influence of Cation Vacancies on Li Conductivity of La1/2Li1/2-2x Sr x TiO3 Perovskites (0 < x ≤ 0.25): The Role of Nominal and Effective Vacancies.

The Li1/2-2x Sr x La1/2TiO3 series (0 ≤ x ≤ 0.25) is investigated with X-ray diffraction, nuclear magnetic resonance, and impedance spectroscopy techniques. The substitution of two Li+ by one Sr2+ in Li1/2La1/2TiO3 perovskite generates cation vacancies that, when ordered in alternating planes along the c-axis, confer a two-dimensional character to Li mobility. In previous works, it was shown that Li+ ions partially occupy the center of the six faces of the cubic perovskite, resulting in the associated A-sites to participate like a vacancy in the definition of the percolation vacancy threshold. The results obtained in the Li1/2-2x Sr x La1/2TiO3 series are compared with those obtained in the Li3x La2/3-x TiO3 series, and other Sr-doped solid solutions (Li1/2-x Sr2x La1/2-x TiO3 and Li x Sr x La2/3-x TiO3), to highlight the importance of the effective vacancies with respect to the nominal ones in conductivity. The analysis of four series, belonging to the ternary SrTiO3-La2/3TiO3-Li2TiO3 phase diagram, permits a better understanding of the ionic conduction mechanism in perovskites. The results show that the vacancy percolation model is more adequate to explain Li conductivity than the conventional hopping probability model. In the analyzed series, Li conductivity is maximum when a small amount of Sr is incorporated into the pseudo-cubic La1/2Li1/2TiO3 end member, while it decreases as the amount of strontium increases.

Prediction of time-reversal-symmetry breaking fermionic quadrupling condensate in twisted bilayer graphene

Recent mean-field calculations suggest that the superconducting state of twisted bilayer graphene exhibits either a nematic order or a spontaneous breakdown of the time-reversal symmetry. The two-dimensional character of the material and the large critical temperature relative to the Fermi energy dictate that the material should have significant fluctuations. We study the effects of these fluctuations using Monte Carlo simulations. We show that in a model proposed earlier for twisted bilayer graphene there is a fluctuation-induced phase with quadrupling fermionic order for all considered parameters. This four-electron condensate, instead of superconductivity, shows a spontaneous breaking of time-reversal symmetry. Our results suggest that twisted bilayer graphene is an especially promising platform to study different types of condensates, beyond the pair-condensate paradigm.

Two-dimensional effects on electrostatic instabilities in Hall thrusters. I. Insights from particle-in-cell simulations and two-point power spectral density reconstruction techniques

Using 2D particle-in-cell (PIC) simulations coupled to a fluid description of the gas dynamics, we study the electrostatic instabilities developing in the axial–azimuthal plane of a Hall thruster, during several periods of a low-frequency oscillation (the so-called breathing mode at 10 kHz). As done in experiments, the 2D PIC-MCC (Monte Carlo collision) code is coupled to an electrical circuit in order to partially damp the (otherwise large) discharge current fluctuations at the breathing mode frequency. The different electrostatic higher frequency modes that develop in the plasma are analyzed using a two-point power spectral density reconstruction method, which allows us to generate the dispersion diagrams (in the frequency-wavenumber space) along the axial and azimuthal directions and at different times during the low-frequency breathing mode oscillations. This technique allows us to distinguish between different well-identified instabilities: the electron cyclotron drift instability and its evolution toward an ion acoustic wave and the ion transit time instability. These instabilities are usually considered unidirectional (either axial or azimuthal); however, it is shown here that they exist in both directions. This two-dimensional character is instrumental in understanding where these instabilities grow and how they propagate in the thruster channel and plume. A theoretical discussion of this aspect is proposed in Paper II. The effects of (i) the azimuthal length of the simulation box and (ii) the electron temperature injection at the cathode are also discussed.

Superconductivity in single-crystalline ZrTe3-x (x ≤ 0.5) nanoplates.

Superconductivity with an unusual filamented character below 2 K has been reported in bulk ZrTe3 crystals, a well-known charge density wave (CDW) material, but still lacks in its nanostructures. Here, we systemically investigated the transport properties of controllable chemical vapor transport synthesized ZrTe3-x nanoplates. Intriguingly, superconducting behavior is found at T c = 3.4 K and can be understood by the suppression of CDW due to the atomic disorder formed by Te vacancies. Magnetic field and angle dependent upper critical field revealed that the superconductivity in the nanoplates exhibits a large anisotropy and two-dimensional character. This two-dimensional nature of superconductivity was further satisfactorily described using the Berezinsky-Kosterlitz-Thouless transition. Our results not only demonstrate the critical role of Te vacancies for superconductivity in ZrTe3-x nanoplates, but also provide a promising platform to explore the exotic physics in the nanostructure devices.

EFFECT OF ROTOR MISALIGNMENT ON MAGNETOELECTRIC VALVE MOTOR PERFORMANCE

A mathematical model of the magnetic field in the working clearance of a magnetoelectric valve motor is presented when the rotor is misaligned relative to the stator axis, which occurs during manufacture, for example, due to defects in bearing panels or during operation due to bearing wear. Using the conformal transformation of an uneven air gap into a uniform (circular ring), a study of the magnetic field in the air gap of the engine was carried out. A case is considered when the section of the air gap with the minimum size is stationary and is associated with a specific place on the surface of the stator boring. The serrature of the stator was taken into account on average using the Carter coefficient. The magnetic field in the inhomogeneous air gap created by the rotor magnets and the current of the stator winding was assumed to be plane-parallel, having a two-dimensional character.&#x0D; &#x0D; It is obtained that the misalignment of the rotor associated with the rotational motion of the air gap section with a minimum size leads to the appearance of non-sinusoidal EMF and pulsations of electromagnetic moment with a frequency exceeding 3p times the rotational speed of the rotor (p is the number of pairs of poles). With fixed eccentricity, an alternating EMF is induced along the rotor shaft, causing alternating current in the circuit: shaft – bearings – bearing panels – stator housing.&#x0D; &#x0D; Pulsations of the moment of the electromagnetic moment of the engine are caused by the appearance of mainly the 3rd and 9th harmonics, the values of which increase with increasing misalignment according to a law close to linear. The constant component of the electromagnetic moment increases somewhat with increasing misalignment.

Dynamic magnetic crossover at the origin of the hidden-order in van der Waals antiferromagnet CrSBr

The van-der-Waals material CrSBr stands out as a promising two-dimensional magnet. Here, we report on its detailed magnetic and structural characteristics. We evidence that it undergoes a transition to an A-type antiferromagnetic state below TN ≈ 140 K with a pronounced two-dimensional character, preceded by ferromagnetic correlations within the monolayers. Furthermore, we unravel the low-temperature hidden-order within the long-range magnetically-ordered state. We find that it is associated to a slowing down of the magnetic fluctuations, accompanied by a continuous reorientation of the internal field. These take place upon cooling below Ts ≈ 100 K, until a spin freezing process occurs at T* ≈ 40 K. We argue this complex behavior to reflect a crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by its mixed-anion character. Our findings reinforce CrSBr as an important candidate for devices in the emergent field of two-dimensional magnetic materials.

Epitaxial Growth of Crystalline CaF2 on Silicene.

Silicene is one of the most promising two-dimensional (2D) materials for the realization of next-generation electronic devices, owing to its high carrier mobility and band gap tunability. To fully control its electronic properties, an external electric field needs to be applied perpendicularly to the 2D lattice, thus requiring the deposition of an insulating layer that directly interfaces silicene, without perturbing its bidimensional nature. A promising material candidate is CaF2, which is known to form a quasi van der Waals interface with 2D materials as well as to maintain its insulating properties even at ultrathin scales. Here we investigate the epitaxial growth of thin CaF2 layers on different silicene phases by means of molecular beam epitaxy. Through electron diffraction images, we clearly show that CaF2 can be grown epitaxially on silicene even at low temperatures, with its domains fully aligned to the lattice of the underlying 2D structure. Moreover, in situ X-ray photoelectron spectroscopy data evidence that, upon CaF2 deposition, no changes in the chemical state of the silicon atoms can be detected, proving that no Si–Ca or Si–F bonds are formed. This clearly shows that the 2D layer is pristinely preserved underneath the insulating layer. Polarized Raman experiments show that silicene undergoes a structural change upon interaction with CaF2; however, it retains its two-dimensional character without transitioning to a sp3-hybridized silicon. For the first time, we have shown that CaF2 and silicene can be successfully interfaced, paving the way for the integration of silicon-based 2D materials in functional devices.

Fluorescent Zn(II)-Based Metal-Organic Framework: Interaction with Organic Solvents and CO2 and Methane Capture.

Adsorption of carbon dioxide (CO2), as well as many other kinds of small molecules, is of importance for industrial and sensing applications. Metal-organic framework (MOF)-based adsorbents are spotlighted for such applications. An essential for MOF adsorbent application is a simple and easy fabrication process, preferably from a cheap, sustainable, and environmentally friendly ligand. Herein, we fabricated a novel structural, thermally stable MOF with fluorescence properties, namely Zn [5-oxo-2,3-dihydro-5H-[1,3]-thiazolo [3,2-a]pyridine-3,7-dicarboxylic acid (TPDCA)] • dimethylformamide (DMF) •0.25 H2O (coded as QUF-001 MOF), in solvothermal conditions by using zinc nitrate as a source of metal ion and TPDCA as a ligand easy accessible from citric acid and cysteine. Single crystal X-ray diffraction analysis and microscopic examination revealed the two-dimensional character of the formed MOF. Upon treatment of QUF-001 with organic solvents (such as methanol, isopropanol, chloroform, dimethylformamide, tetrahydrofuran, hexane), interactions were observed and changes in fluorescence maxima as well as in the powder diffraction patterns were noticed, indicating the inclusion and intercalation of the solvents into the interlamellar space of the crystal structure of QUF-001. Furthermore, CO2 and CH4 molecule sorption properties for QUF-001 reached up to 1.6 mmol/g and 8.1 mmol/g, respectively, at 298 K and a pressure of 50 bars.

A Two-Dimensional Superconducting Electron Gas in Freestanding LaAlO3/SrTiO3 Micromembranes

Freestanding oxide membranes constitute an intriguing material platform for new functionalities and allow integration of oxide electronics with technologically important platforms such as silicon. Sambri et al. recently reported a method to fabricate freestanding LaAlO3/SrTiO3 (LAO/STO) membranes by spalling of strained heterostructures. Here, we first develop a scheme for the high-yield fabrication of membrane devices on silicon. Second, we show that the membranes exhibit metallic conductivity and a superconducting phase below ∼200 mK. Using anisotropic magnetotransport we extract the superconducting phase coherence length ξ ≈ 36-80 nm and establish an upper bound on the thickness of the superconducting electron gas d ≈ 17-33 nm, thus confirming its two-dimensional character. Finally, we show that the critical current can be modulated using a silicon-based backgate. The ability to form superconducting nanostructures of LAO/STO membranes, with electronic properties similar to those of the bulk counterpart, opens opportunities for integrating oxide nanoelectronics with silicon-based architectures.

Thermopower and magnetotransport properties of Bi100−xSbx topological insulator thin films prepared by flash evaporation

We have measured the temperature dependence of resistance R(T), thermopower S(T), magnetoresistance (MR) and the Hall effect (HE) of Bi80Sb20, Bi85Sb15 and Bi90Sb10 topological insulator thin films. Samples were prepared by sequential flash-evaporation at room temperature and annealing at T = 350 K. The R(T) of the three investigated samples show metallic-like behavior at temperatures less than T = 75 K, while at higher temperatures, R(T) curves show a semiconducting-like behavior. The thermopower S(T) of the three investigated samples is negative in the entire temperature range measured in this work, with a linear behavior from 5 K up to ≈100 K. The magnetoresistance of all samples is positive with a small temperature dependence. The highest MR(B = 7 T) was observed in Bi85Sb15 with a ≈600% and ≈125% change at 5 K and 300 K, respectively. Clear evidence of weak antilocalization contribution to the MR was observed only in sample Bi85Sb15 at temperatures T < 75 K. Quantum oscillations in the MR originating from the Fermi surface, which has a clear two-dimensional character, were observed in sample Bi85Sb15 up to ≈21 K. Carrier mobility information of sample Bi85Sb15 was extracted from low field HE data, showing a remarkably high value of μ ≈ 2.8 × 104 cm2/Vs at 5 K, with a small decrease for increasing temperature.

Polymorphism and sodium-ion conductivity of NaTa2PO8 synthesized via the Li+/Na+ ion-exchange reaction of LiTa2PO8

Ion exchange reaction is a promising method to explore metastable compounds that could not be synthesized by conventional high-temperature solid-phase reactions. Herein, a new sodium tantalum phosphate, NaTa2PO8 was synthesized via the Li+/Na+ ion-exchange reaction of the parent compound, LiTa2PO8 in molten NaNO3 medium. NaTa2PO8 underwent an irreversible phase transition from the low- (LT-NaTa2PO8) to the high-temperature polymorph (HT-NaTa2PO8) at approximately 1000 °C. The crystal structures were solved using an ab initio structural determination method based on synchrotron X-ray powder diffraction data. The LT-NaTa2PO8 presented an orthorhombic structure, closely related to that of the parent LiTa2PO8 structure. In contrast, the HT-NaTa2PO8 was found to adopt a monoclinic structure, belonging to a family of monophosphate tungsten bronzes with pentagonal tunnels. The ionic conductivities of LT-NaTa2PO8 (σ = 5 × 10−5 S/cm at 309 °C) and HT-NaTa2PO8 (σ = 2 × 10−7 S/cm at 300 °C) exhibited Arrhenius behavior with activation energies of 0.49 and 0.79 eV, respectively. Bond valence energy landscape (BVEL) calculations indicated that a three-dimensional (3D) conduction pathway is formed in LT-NaTa2PO8 structure, while the conduction pathway in HT-NaTa2PO8 shows a two-dimensional (2D) character.

DETERMINANTS OF THE DEGREE OF INNOVATION IN THE INDUSTRIES LISTED ON THE B3

The study aims to identify the determinants of the degree of innovation of industries listed on the Brazilian Stock Exchange - B3. To this end, it used as sample 129 industries that trade their shares on the B3. From the principal component analysis, it was found that the innovation indicators are combined into two distinct factors. Therefore, it can be said that the degree of innovation of the firms takes into account the combination of indicators referring to the innovation strategies of exploitation and exploration, and, by encompassing two distinct factors, it is valid to mention that the degree of innovation has a two-dimensional character, suggesting the existence of a synergy between the innovation strategies.

‘Funerary Culture Wars’ in Late 19th- and Early 20th-Century Europe and the Case of the Brussels’ Freethought Movement

Abstract This article explores how the control of religious authority over funerary culture became a contentious issue on a pan-European level during the course of the second half of the 19th century and the early 20th century. A comparative analysis of the conflicts around burial and cremation in various European nation-states highlights the two-dimensional character of these ‘funerary culture wars’. Freethinkers were radical players in these institutional and cultural conflicts as they challenged the tight grip of churches on the ‘death systems’ in their respective cities and countries. We will show how the institutional and cultural secularisation of funerary culture was not a universal and inevitable developmental trend but rather a historically variable and contingent outcome of social and political struggles around concrete change. We zoom in on the city of Brussels where freethinkers made new secular funerary practices very visible from the middle of the 19th century onwards.

Active control of micrometer plasmon propagation in suspended graphene

Due to the two-dimensional character of graphene, the plasmons sustained by this material have been invariably studied in supported samples so far. The substrate provides stability for graphene but often causes undesired interactions (such as dielectric losses, phonon hybridization, and impurity scattering) that compromise the quality and limit the intrinsic flexibility of graphene plasmons. Here, we demonstrate the visualization of plasmons in suspended graphene at room temperature, exhibiting high-quality factor Q~33 and long propagation length > 3 μm. We introduce the graphene suspension height as an effective plasmonic tuning knob that enables in situ change of the dielectric environment and substantially modulates the plasmon wavelength, propagation length, and group velocity. Such active control of micrometer plasmon propagation facilitates near-unity-order modulation of nanoscale energy flow that serves as a plasmonic switch with an on-off ratio above 14. The suspended graphene plasmons possess long propagation length, high tunability, and controllable energy transmission simultaneously, opening up broad horizons for application in nano-photonic devices.