Absence Of External Magnetic Field Research Articles

Particles with magnetic dipole moment orbiting magnetized Schwarzschild black holes: Applications to orbits of hot-spots around Sgr A*

We study the circular motion of particles with magnetic dipole moment around Schwarzschild black holes immersed in external asymptotically uniform and dipolar magnetic field configurations and calculate the orbital velocity of the particles. We assume that the flares observed around supermassive black hole Sagittarius (Sgr) A* detected by the GRAVITY ESO collaboration (on May 27, July 22, and July 28, 2018) are magnetized compact objects, in particular, neutron stars (NSs) and white dwarfs (WDs). Also, we apply orbital and luminosity data from (three) flares to explore the possible role of magnetic interaction on hot spots’ dynamics. First, we found the mass range of SgrA* using the orbital data in the absence of external magnetic fields as M=(3−6.3)M⊙. It is also found that, for magnetized particles that can be in the hot spot’s orbits, the magnetic coupling parameter β has to take values less than 0.13 and 0.25 in the cases of external dipolar and uniform magnetic fields, respectively. The critical values for the surface magnetic fields, mass, and radius of magnetized NSs and WDs are found to be candidates for the hot-spot objects. Finally, we found upper and lower limits for the surface magnetic field, radius, and rotating period of the NSs and WDs, using the luminosity data of the hot spots.

A perspective of recent advances in PECVD-grown graphene thin films for scientific research and technological applications

The purpose of this review is to provide a perspective on some of the recent advances in applying graphene thin films, including monolayer and multilayer graphene sheets synthesized by a scalable approach based on plasma-enhanced chemical vapor deposition (PECVD), to scientific research and technologies. The technological applications present here include the use of graphene in flexible electronics, for anticorrosion of metals, and for Si-based technologies including superlubricity and optoelectronics. In the case of fundamental scientific research, we demonstrate that periodic patterns of nanoscale strain distributions in monolayer graphene can lead to local giant pseudomagnetic fields as well as global modifications to the electronic properties of monolayer graphene, including strain-induced valley Hall signals and quantized Landau levels in the absence of external magnetic fields, edge states, as well as increasing electronic correlation with increasing strain. These findings suggest new approaches towards developing emerging quantum states with tunable electronic correlation based on graphene straintronics.

Correlation of density fluctuation in a magnetized QCD matter near the critical end point

The dynamical correlation of density fluctuation in quark gluon plasma with a critical end point has been investigated within the scope of the Müller–Israel–Stewart theory in the presence of static ultra-high external magnetic field. The dynamic structure factor of the density fluctuation exhibits three Lorentzian peaks in absence of external magnetic field – a central Rayleigh peak and two Brillouin peaks situated symmetrically on the opposite sides of the Rayleigh peak. The spectral structure displays five peaks in presence of the magnetic field due to the coupling of the magnetic field with the hydrodynamic fields in second-order hydrodynamics. The emergence of the extra peaks is due to the asymmetry in the pressure gradient caused by the external magnetic field in the system. Interestingly, it is observed that near the critical end point, all the Brillouin peaks disappear irrespective of the presence or absence of the external magnetic field.

Ferromagnetism on an atom-thick & extended 2D metal-organic coordination network

Ferromagnetism is the collective alignment of atomic spins that retain a net magnetic moment below the Curie temperature, even in the absence of external magnetic fields. Reducing this fundamental property into strictly two-dimensions was proposed in metal-organic coordination networks, but thus far has eluded experimental realization. In this work, we demonstrate that extended, cooperative ferromagnetism is feasible in an atomically thin two-dimensional metal-organic coordination network, despite only ≈ 5% of the monolayer being composed of Fe atoms. The resulting ferromagnetic state exhibits an out-of-plane easy-axis square-like hysteresis loop with large coercive fields over 2 Tesla, significant magnetic anisotropy, and persists up to TC ≈ 35 K. These properties are driven by exchange interactions mainly mediated by the molecular linkers. Our findings resolve a two decade search for ferromagnetism in two-dimensional metal-organic coordination networks.

Analytical calculation of beam profile and orbital angular momentum spectrum of Laguerre Gaussian beams reflected from a graphene plasmonic structure.

In this paper, Laguerre Gaussian (LG) beams with different topological charges are used for excitation of surface plasmon polaritons (SPPs) through a graphene layer inserted in the Otto-configuration. By utilizing the angular spectrum representation (ASR) and Lorenz-gauge vector potential, an explicit analytical expression is derived for the electromagnetic fields of the reflected beam. At the optimal excitation state of graphene SPPs, the reflected beam exhibits a distinctive field profile characterized by two identical crescent-shaped lobes separated by a vertical strip with null intensity. Furthermore, in the absence of external magnetic field, the orbital angular momentum (OAM) spectrum of the reflected beam at the optimal excitation of SPPs reveals the annihilation of central OAM mode and the generation of two equal OAM sidebands, regardless of the incident OAM topological charge. Furthermore, the phase distributions of electric field of the reflected beam confirm the existence of OAM sidebands in the vicinity of optimal SPPs excitation. As the system is taken away from the optimal excitation of SPPs by introduction of an external magnetic field or increasing the chemical potential or increasing the incident angle, both central and sideband modes appear in the OAM spectrum of the reflected beam. In this case, when the topological charge of the incident wave increases, the weight of central OAM mode decreases while the weight of sidebands increases. In contrast, in the presence of external magnetic field, at the optimal excitation of SPPs, both central OAM and sidebands modes exist in the reflected beam such that the weight of central modes increases with the external magnetic field. This effect is also confirmed by plotting the phase distributions of the reflected beam at different external magnetic fields and for different incident topological charges. Therefore, the manipulation of graphene plasmons characteristics leads to the control of OAM sideband generation.

Permanent-magnet-based transverse thermoelectric generator with high fill factor driven by anomalous Nernst effect

A transverse thermoelectric generator for magnetic-field-free and high-density power generation utilizing the anomalous Nernst effect is constructed and its performance is characterized. By alternately stacking two different permanent magnets with the large coercivity and anomalous Nernst coefficients of opposite sign, transverse thermoelectric voltage and power can be generated in the absence of external magnetic fields and enhanced owing to a thermopile structure without useless electrode layers. In the permanent-magnet-based stack, the magnetic attractive force enables easy construction of the thermopile structure with a high fill factor. In this study, we construct a bulk module consisting of 12 pairs of SmCo5- and Nd2Fe14B-type permanent magnets having positive and negative anomalous Nernst coefficients, respectively, whose fill factor reaches ∼80%, whereas that of conventional thermoelectric modules based on the Seebeck effect is typically 30%–60%. We demonstrate magnetic-field-free anomalous Nernst power generation up to 177 µW at a temperature difference of 75 K around room temperature, which corresponds to the largest anomalous Nernst power density of 65 µW/cm2. The presented module structure concept will provide a design guideline for high-performance transverse thermoelectric power generation.

Chirality-dependent spin transport and spin Seebeck effect in the spiral spin chains

The observation of spin Seebeck effect has triggered intense research in the spintronics. Especially, it is considered to be one alternative method to generate pure spin current in magnetic materials. Here, the spin Seebeck coefficient and the spin figure of merit are calculated by using Green's function method in the spiral spin chain with Dzyaloshinskii–Moriya interaction (DMI). The Zeeman-like effect and the similarity between the chirality and the spin in the spiral spin chain are revealed according to the dispersion relation. In addition, we find that the spin Seebeck coefficient and the spin figure of merit show peaks around the magnon spin transmission channel. In the absence of external magnetic field, the spin Seebeck peaks for opposite chirality are degenerated. However, the spin Seebeck peaks are chirality-dependent in the presence of magnetic field, and the spin Seebeck peaks with opposite chirality locate at different chemical potentials, leading to the energy difference between the spin Seebeck peaks with opposite chirality. Furthermore, the energy difference is determined by the magnetic field and the DMI, and the energy difference becomes larger for stronger DMI or higher magnetic field.

Field-free domain wall spin torque nano-oscillators with multimodal real-time modulation and high-quality factor

We report on a magnetic domain wall (DW)-based spin torque nano-oscillator (STNO) with real-time multimodal frequency modulation characteristics by engineering the synergistic effect of shape anisotropy, Dzyaloshinskii-Moriya interaction (DMI), and spin-transfer torque. The achieved manageable oscillation and precession of magnetic DW motivate us to implement such attributes into the developed STNO under a low current density of ∼107 A/cm2, which demonstrates outstanding functionality of multimodal real-time modulation ranging from few GHz and sub-THz with corresponding few tens and >104 quality factor (fSTNO/Δf) in absence of external magnetic field. Furthermore, the dynamic process of DW motion and magnetic moment precession under different modes has been revealed systematically, and the frequency dependence of various physical parameters including damping constant, uniaxial anisotropy constant, saturation magnetization, exchange stiffness, and DMI constant has also been summarized in detail. Such a DW-based device enriches the STNO family and promises great potential with a broad spectrum of applications in microwave generators and neuromorphic computing.

Exceptional points and lines and Dirac points and lines in magnetoactive cholesteric liquid crystals

We investigated the properties of cholesteric liquid crystals (CLCs) being in external static magnetic field directed along the helix axis. We have shown that in the case of the wavelength dependence of magneto-optic activity parameter, and in the presence of absorption new features appear in the optics of CLCs. We have shown that in this case new photonic band gaps (PBGs) appear. This new PBG is sensitive to the polarization of the incident light. But if the chirality sign of the polarization of the incident diffracting light for the basic PBG (which exist also at the absence of external magnetic field) is determined only by the chirality sign of the CLC helix, then for the second one it is determined by the external magnetic field direction (i.e., on whether the directions of the external magnetic field and the incident light are parallel, or they are antiparallel). We have shown that in this case besides Dirac points there appear also Dirac lines as well as exceptional points and exceptional lines. And moreover, near some of these points and lines the narrow bands or wide bands of magnetically induced transparency, and magnetically induced absorption appear, and near the others a wide coherent perfect absorption band appears that is insensitive to incident light polarization. And finally at some wavelength the same reflection, transmission and absorption takes place for any polarization of incident light, although it is not Dirac point nor exceptional point. This system can apply as tunable narrow-band or broad-band filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and other devices.

Pair density wave facilitated by Bloch quantum geometry in nearly flat band multiorbital superconductors

Bloch electrons in multiorbital systems carry quantum geometric information characteristic of their wavevector-dependent interorbital mixing. The geometric nature impacts electromagnetic responses, and this effect carries over to the superconducting state, which receives a geometric contribution to the superfluid weight. In this paper, we show that this contribution could become negative under certain appropriate circumstances. This may facilitate the stabilization of Cooper pairings with real space phase modulation, i.e. the pair density wave order, as we demonstrate through two-orbital model Bogoliubov de-Gennes mean-field calculations. The quantum geometric effect therefore constitutes an intrinsic mechanism for the formation of such a novel phase of matter in the absence of external magnetic field.

Magnetic flux trapping in hydrogen-rich high-temperature superconductors

Recent discoveries of superconductivity in various hydrides at high pressures have shown that a critical temperature of superconductivity can reach near-room-temperature values. However, experimental studies are limited by high-pressure conditions, and electrical transport measurements have been the primary technique for detecting superconductivity in hydrides. Here we implement a non-conventional protocol for the magnetic measurements of superconductors in a SQUID magnetometer and probe the trapped magnetic flux in two near-room-temperature superconductors H3S and LaH10 at high pressures. Contrary to traditional magnetic susceptibility measurements, the magnetic response from the trapped flux is almost unaffected by the background signal of the diamond anvil cell due to the absence of external magnetic fields. The behaviour of the trapped flux generated under zero-field-cooled and field-cooled conditions proves the existence of superconductivity in these materials. We reveal that the absence of a pronounced Meissner effect is associated with the very strong pinning of vortices inside the samples. This approach can also be a tool for studying multiphase samples or samples that have a low superconducting fraction at ambient pressure.

Spin noise in Er3+ single-ion magnets surrounded by ferromagnetic microparticles

Ferromagnetic microparticles significantly affect spin relaxation in the Er3+ single-ion magnet sandwiched in a composite material. The balance of thermal spin noise corresponding to Orbach, Raman, quantum tunneling, and direct relaxation channels is shifted in Er3+ complexes under the influence of surrounding ferromagnetic matrix. There are two competing sources of the electron spin noise controlled by ferromagnetic media. First, internal residual magnetic field delays spin relaxation in the Er3+ complexes due to the Zeeman interaction of the Er3+ spin even in the absence of external magnetic field. Second, chemical bonding between the Er 4d shell and the O 1s shell accelerates magnetic relaxation in the Er3+ ions on the surface of microparticles. Significance of these results is that composite media can be sliced into small elements with a variable frequency of spin noise depending on individual magnetization programmed within each element.

Versatile Ultrasoft Electromagnetic Actuators with Integrated Strain‐Sensing Cellulose Nanofibril Foams

As robots more frequently fraternize with humans in everyday life, aspects such as safety, flexibility of tasks, and appearance become increasingly important. Soft robotics is attractive for new human‐close applications, but soft actuators constitute a major challenge both in terms of actuation force and speed, and in terms of control and accuracy of the deformable soft actuator body. Herein, several of these challenges are addressed by developing versatile ultrasoft electromagnetic actuators that operate in absence of external magnetic fields, while simultaneously monitoring their states by internal strain sensors. The versatile actuators can compress to less than 50% of their initial length with strain‐independent contraction force and operate in both contraction and expansion modes up to 200 Hz frequency while conforming to curved surfaces. The soft multilayer conductive cellulose‐based foams are lightweight (3 mg cm−3) and provide internal strain‐sensing capability and structural support, thereby improving the monitoring and controllability of the actuators while maintaining an axial softness of 0.6 kPa. It is believed that the concept of soft versatile electromagnetic actuators with integrated lightweight strain‐sensing foams is promising for a wide range of applications within soft robotics.

Spin-orbit coupled superconductivity with spin-singlet nonunitary pairing

An unconventional superconductor is distinguished with two types of gap functions: unitary and non-unitary. This core subject has been concentrated on purely spin-triplet or singlet-triplet mixed superconductors. However, the generalization to a purely spin-singlet superconductor has remained primarily of theoretical interest, which requires at least a multi-orbital correlated electronic systems. In this work, we present a possible establishment of both unitary and non-unitary pairings for spin-singlet superconductors with two atomic orbitals. Then we investigate the effects of atomic spin-orbit coupling and find a new spin-orbit-coupled non-unitary superconductor that supports exotic phenomena. Remarkably, there are mainly three features. Firstly, the atomic spin-orbit coupling locks the electron spins to be out-of-plane, which could give birth to the Type II Ising superconductivity with a large in-plane upper critical field compared to the Pauli limit. Secondly, topological chiral or helical Majorana edge state could be realized even in the absence of external magnetic fields or Zeeman fields. In addition, a spin-polarized superconducting state could even be generated by spin-singlet non-unitary pairings with spontaneous time-reversal breaking, which severs as a smoking gun to detect this exotic state by measuring the spin-resolved density of states. Therefore, our work might pave a new avenue to spin-orbit-coupled superconductors with spin-singlet non-unitary pairing symmetries.

Sub-terahertz/terahertz electron resonances in hard ferrimagnets

Intensive development of ultrafast electronics requires materials with high-frequency spin dynamics. In this light, the insulators that possess the magnetization precession phenomenon due to magnetic anisotropy are dark horses. On the one hand, modern hard magnetic materials reveal relatively moderate resonance frequencies of the ferromagnetic mode (generally, dozens of GHz), which are lower than the frequencies of the antiferromagnetic resonances; on the other hand, the research in this area is quite scanty, which implies a room for a breakthrough. Here, an example of a hard ferrimagnetic insulator (cobalt ferrite CoFe2O4) was obtained in the form of nanoparticles and bulk ceramics via high-temperature methods. Due to high magnetic anisotropy fields, the samples in a single domain state show broad hysteresis loops. The materials also possess intensive resonance absorption at frequencies higher than 0.20 THz in zero external magnetic fields. For the first time, natural ferromagnetic resonance (NFMR) frequencies higher than 0.30 THz were registered. The ceramic sample demonstrates the highest-known NFMR frequency of 0.35 THz. The model based on the Landau-Lifshitz equation was developed to explain the demonstrated magnetodynamic properties and shed light on those of hard ferrimagnets in general. The practical application of the electron resonances in hard magnetic insulators, including cobalt ferrite, Al-doped M-type hexaferrite, and epsilon iron oxide, is discussed. Our findings reveal that these materials should provide several orders of magnitude more powerful spin pumping at sub-terahertz/terahertz frequencies compared to insulating antiferromagnets, even under unpolarized irradiation and even in the absence of external magnetic fields. This opens new horizons for the development of practical ultrafast electronics.

Numerical studies on holographic paramagnetic-ferromagnetic phase transition in Gauss–Bonnet gravity

Based on the shooting method, we numerically investigate the properties of holographic paramagnetism-ferromagnetism phase transition in the presence of higher-order Gauss–Bonnet (GB) correction terms on the gravity side. On the matter field side, however, we consider the effects of Power-Maxwell (PM) nonlinear electrodynamics on the phase transition of this system. For this purpose, we introduce a massive 2-form coupled to PM field, and neglect the effects of 2-form fields and gauge field on the background geometry. We observe that increasing the strength of both the power parameter [Formula: see text] and GB coupling constant [Formula: see text] decreases the critical temperature of holographic model, and leads to the harder formation of magnetic moment in the black hole background. Interestingly, we find out that at low temperatures, the spontaneous magnetization and ferromagnetic phase transition happen in the absence of external magnetic field. In this case, the critical exponent for magnetic moment has the mean field value, [Formula: see text], regardless of the values of [Formula: see text] and [Formula: see text]. In the presence of external magnetic field, however, the magnetic susceptibility satisfies the Curie–Weiss law.

The effects of spin-orbit coupling on optical properties of monolayer text {MoS}_{2} due to mechanical strains

We have studied the optical conductivity of a quasi two-dimensional text {MoS}_{2} in the presence of external magnetic field and spin-orbit coupling. Specially, we address the frequency dependence of optical conductivity due to spin-orbit interaction. Using linear response theory the behavior of optical conductivity has been obtained within Green’s function method. We have also considered the effects of uniaxial and biaxial in-plane strain on the optical absorption of text {MoS}_{2} layer. In the absence of external magnetic field with negative uniaxial strain parameter, optical conductivity includes Drude weight at zero frequency limit while Drude weight vanishes for text {MoS}_{2} layer under positive uniaxial strain. Our results show that the increase of uniaxial positive strain parameter causes to move the position peak to the higher frequencies. In contrast to uniaxial strain case, the Drude weight in optical conductivity appears at positive biaxial strain value 0.15. Also we have studied the effects of magnetic field, electron doping, hole doping in the presence of spin-orbit coupling on frequency dependence of optical conductivity of text {MoS}_{2} in details. The magnetic field dependence of optical absorption shows a monotonic decreasing behavior for each value of temperature in the absence of strain parameter.

Exciton insulator in a moiré lattice

Interlayer electron and hole can be paired up through coulomb interaction to form an exciton insulator when their kinetic energy is substantially smaller than the interaction energy. The traditional platform to realize such an interlayer interaction is the double quantum well with dielectric material between electron and hole, for which an external magnetic field is required to generate Landau level flat bands that can reduce the kinetic energy of charged carriers. When both quantum wells are at the half filling of the lowest landau level, the electron-electron repulsive interaction, by the particle-hole transformation in one well, will be equivalent to electron-hole attractive interaction, from which interlayer exciton and its condensation can emerge. In a two-dimensional twisted homostructure or an angle aligned heterostructure, there exists a moiré superlattice, in which bands are folded into the mini-Brillouin zone by the large moiré period. Gap opening at the boundary of mini-Brillouin zone can form the well-known moiré flat band. This review will discuss how to use the moiré flat bands to generate exciton insulator in the absence of external magnetic field in transitional metal dichalcogenide (TMD) moiré heterostructure. Unlike the double quantum well where symmetric well geometry is used, the moiré related sample can have multiple different geometries, including monolayer TMD-hexagonal boron nitride-moiré structure, moiré-moiré structure, and monolayer TMD-bilayer TMD structure. The carriers in those structures can be well tuned to locate equally in different layers, and particle-hole transformation in the moiré first Hubbard band can transform the interlayer repulsive coulomb interaction into attractive interaction, which is the same as that in quantum well under magnetic field. We will show that by using differential contrast reflection spectrum, interlayer photoluminescence, 2s exciton sensing, quantum capacitance and microwave impedance microscopy, the signature of exciton fluid can be identified. The excitonic coherence features in those structures will promise by using the coulomb drag technique and counter flow technique in future. In general, exciton in moiré lattice is a promising candidate for studying the Bose-Hubbard model in solids and can well realize exciton superfluidity, excitonic mott insulator as well as the crossover between them.

Theoretical Model of Self-Magnetic Flux Leakage and Its Application in Estimating the Depth Direction of a Fatigue Crack

In this study, theoretical models were proposed to explain the changes in self-magnetic flux density (SMFD) due to fatigue cracks in the presence and absence of external magnetic fields. Three theoretical models were proposed: rotation domain model (RDM), concentration domain model (CDM), and vertical domain model (VDM), considering the deformation and non-deformation possibilities. To prove the theoretical model, fatigue cracks with different depth angles were fabricated through fatigue testing and EDM processing on the CT specimens. In addition, tunnel magnetoresistance (TMR) sensors were used to evaluate the 3-axis distribution of SMFD. Comparing the simulation and experimental results, similar tendencies of the occurrence and depth angle of fatigue cracks and their effect on the distribution of SMFD were observed. According to the RDM, the distribution of SMFD occurs in the direction of the crack length (y-direction), while the CDM explains that the SMFD does not occur if the fatigue crack is in a direction perpendicular to the surface. In addition, the VDM shows that SMFDs occur in a direction perpendicular to the crack length (x-direction) and the specimen surface (z-direction). Interestingly, these trends agree with the experimental results, which confirms the validity of the theoretical model and thus can be used to estimate the depth direction of a fatigue crack.

Totally Spin-Polarized Currents in an Interferometer with Spin-Orbit Coupling and the Absence of Magnetic Field Effects.

The paper studies the electronic current in a one-dimensional lead under the effect of spin-orbit coupling and its injection into a metallic conductor through two contacts, forming a closed loop. When an external potential is applied, the time reversal symmetry is broken and the wave vector k of the circulating electrons that contribute to the current is spin-dependent. As the wave function phase depends upon the vector k, the closed path in the circuit produces spin-dependent current interference. This creates a physical scenario in which a spin-polarized current emerges, even in the absence of external magnetic fields or magnetic materials. It is possible to find points in the system's parameter space and, depending upon its geometry, the value of the Fermi energy and the spin-orbit intensities, for which the electronic states participating in the current have only one spin, creating a high and totally spin-polarized conductance. For a potential of a few tens of meV, it is possible to obtain a spin-polarized current of the order of μA. The properties of the obtained electronic current qualify the proposed device as a potentially important tool for spintronics applications.