Absence Of External Magnetic Field Research Articles

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.

The effect of the electron–phonon interaction on the energy levelsand g-factor of electrons in nanowires

We analyze the effect of longitudinal optical phonons on the energy states of electrons in a nanowire in the presence of Rashba interaction and an in-plane magnetic field. Due to the electron–phonon interaction, an energy splitting appears at zero wave vector accompanied by a downward shift of the dispersion relation in the absence of external magnetic fields. The splitting increases linearly with the product of the Rashba parameter and the dimensionless constant characterizing the Fröhlich Hamiltonian. It also induces an enhancement in the [Formula: see text]-factor that is inversely proportional to the strength of the magnetic field. Moreover, we calculate the contribution of Rashba intersubband coupling (RIC) to the electron energy. This contribution does not influence the [Formula: see text]-factor for the case of parallel magnetic field to the nanowire, but it causes reduction in it by increasing the angle between the nanowire and the field.

Unified theory of the anomalous and topological Hall effects with phase-space Berry curvatures.

Spontaneously broken time-reversal symmetry in magnetic materials leads to a Hall response, with a nonzero voltage transverse to an applied current, even in the absence of external magnetic fields. It is common to analyze the Hall resistivity of chiral magnets as the sum of two terms: an anomalous Hall effect arising from spin-orbit coupling and a topological Hall signal coming from skyrmions, which are topologically nontrivial spin textures. The theoretical justification for such a decomposition has long remained an open problem. Using a controlled semiclassical approach that includes all phase-space Berry curvatures, we show that the solution of the Boltzmann equation leads to a Hall resistivity that is just the sum of an anomalous term arising from momentum-space curvature and a topological term related to the real-space curvature. We also present numerically exact results from a Kubo formalism that complement the semiclassical approach.

Long-lived qubit entanglement by surface plasmon polaritons in a Weyl semimetal

We investigate spontaneous entanglement of two qubits mediated by nonreciprocal surface plasmon polaritons (SPPs) in a Weyl semimetal. In the absence of external magnetic fields, the topology of the Weyl semimetal even gives rise to nonreciprocal SPPs that are topologically protected and reside inside the photonic gap. We utilize this nonreciprocal SPP as a mediator of entanglement of two spatially separated qubits. Our two main findings are (1) the nonreciprocal SPP gives better quantum entanglement than the reciprocal one, and (2) the entanglement achieved in the Weyl semimetal is sufficiently long lived compared to the entanglement using SPPs in conventional metals.

Transition from spiral to ferromagnetic structure in Fe[formula omitted]Co[formula omitted]Si compounds: Small-angle neutron scattering study

The cubic noncentrosymmetric structure of the B20 compounds produces the spin helix with the wave vector ks=D/J balanced by the competition of two interactions: the large ferromagnetic exchange interaction J and small antisymmetric Dzyaloshinskii–Moriya (DM) interaction D. The mixed Fe1−xCoxSi compounds demonstrate a switch of the sign of magnetic chirality in dependence of concentration x. The sign of magnetic chirality is dictated by the structural chirality and chemical elements (Fe, Co) as well. The switch of chirality is accompanied by transformation of the helix structure to the ferromagnet and is observed at xc=0.65. Moreover, this transformation (helimagnet–ferromagnet) occurs as a function of temperature. We observe the decrease of the helical wave vector ks with lowering temperature from Tc=17 K and its abrupt zeroing at Tf=7 K. The magnetic field applied not along the easy but along the hard anisotropic axis is able to restore the helical structure in the temperature range below Tf. The mechanism of the transformation is theoretically described by a competition between the cubic anisotropy and the DM interaction. We show that anisotropy-induced ferromagnet has nonreciprocal magnon spectrum due to DM interaction even in the absence of external magnetic field.

Dynamics of domain formation in a ferromagnetic fluid

Ferrofluids are colloidal suspensions of magnetic nanoparticles. Ferromagnetic ordering – with spontaneous net magnetization in the absence of external fields – has been described for a system of permanently magnetic nanoplatelets suspended in n-butanol, concentrated enough to undergo the transition to the nematic phase. Such material shows magnetic domain structures analogous to those of solid-state ferromagnets. Through optimization of fundamental interactions, the threshold concentration for nematic ordering was reduced. The optimized material allows reproducible and controllable analysis of domain structures and their formation dynamics, with domains forming on the time scale of the order of seconds. Finally, the effect of an external applied magnetic field on the nematic director fundamental fluctuations inside a domain has been studied with differential dynamic microscopy. Interestingly, the results provide an estimation of the ferrofluid's spontaneous magnetization in the absence of external magnetic fields.

Investigation of electron acceleration in vacuum by a radially polarized laser pulse in the presence of helical wiggler field and axial magnetic field guiding

In this paper, electron acceleration by a radially polarized laser pulse in the presence of helical wiggler magnetic field and axial magnetic field has been studied. The governing equations on electron dynamics in the presence of these electric and magnetic fields were solved by the fourth order Runge-Kuta method. The effect of axial magnetic field and electron injection angle on energy gained by the electron was investigated. Numerical studies have shown that the final energy of an electron depends on the value of the magnetic field and the injection angle. It was seen that by applying a magnetic field of about 0.2 and the injection angle of about 11 degrees, the electron can gain final energy of about 3.8 GeV, which is 40 percent more than the case of zero degrees injection angle and the absence of external magnetic field.

Acoustically Induced Giant Synthetic Hall Voltages in Graphene.

Any departure from graphene's flatness leads to the emergence of artificial gauge fields that act on the motion of the Dirac fermions through an associated pseudomagnetic field. Here, we demonstrate the tunability of strong gauge fields in nonlocal experiments using a large planar graphene sheet that conforms to the deformation of a piezoelectric layer by a surface acoustic wave. The acoustic wave induces a longitudinal and a giant synthetic Hall voltage in the absence of external magnetic fields. The superposition of a synthetic Hall potential and a conventional Hall voltage can annihilate the sample's transverse potential at large external magnetic fields. Surface acoustic waves thus provide a promising and facile avenue for the exploitation of gauge fields in large planar graphene systems.

Current-induced self-switching of perpendicular magnetization in CoPt single layer

All-electric switching of perpendicular magnetization is a prerequisite for the integration of fast, high-density, and low-power magnetic memories and magnetic logic devices into electric circuits. To date, the field-free spin-orbit torque (SOT) switching of perpendicular magnetization has been observed in SOT bilayer and trilayer systems through various asymmetric designs, which mainly aim to break the mirror symmetry. Here, we report that the perpendicular magnetization of CoxPt100-x single layers within a special composition range (20 < x < 56) can be deterministically switched by electrical current in the absence of external magnetic field. Specifically, the Co30Pt70 shows the largest out-of-plane effective field efficiency and best switching performance. We demonstrate that this unique property arises from the cooperation of two structural mechanisms: the low crystal symmetry property at the Co platelet/Pt interfaces and the composition gradient along the thickness direction. Compared with that in bilayers or trilayers, the field-free switching in CoxPt100-x single layer greatly simplifies the SOT structure and avoids additional asymmetric designs.

Controlling domain wall and field-free spin–orbit torque switching in synthetic antiferromagnets

Perpendicular magnetization switching driven by spin–orbit torques plays an increasingly important role for spintronic devices toward practical applications but is also hindered by the well-known technical challenge that an external in-plane magnetic field is required for deterministic switching. Here, we show that the deterministic switching can be achieved in synthetic antiferromagnets through the flexible domain control in the absence of external magnetic fields. Specifically, we have observed that the domain wall (DW) distorts under an applied electric current in contrast to the conventional rigid DW motion in a single ferromagnet. More importantly, the distorted DWs can be precisely controlled under zero magnetic field, leading to the deterministic switching. Our results indicate that the critical technical challenge may be addressed by employing a synthetic antiferromagnetic layer through the DW motion dominated field-free switching.

Theory of zero-field superconducting diode effect in twisted trilayer graphene

In a recent experiment (Lin et al 2021 arXiv:2112.07841 [cond-mat.str-el]), the superconducting phase hosted by a heterostructure of mirror-symmetric twisted trilayer graphene and WSe2 was shown to exhibit significantly different critical currents in opposite directions in the absence of external magnetic fields. We here develop a microscopic theory and analyze necessary conditions for this zero-field superconducting diode effect. Taking into account the spin–orbit coupling induced in trilayer graphene via the proximity effect, we classify the pairing instabilities and normal-state orders and derive which combinations are consistent with the observed diode effect, in particular, its field trainability. We perform explicit calculations of the diode effect in several different models, including the full continuum model for the system, and illuminate the relation between the diode effect and finite-momentum pairing. Our theory also provides a natural explanation of the observed sign change of the current asymmetry with doping, which can be related to an approximate chiral symmetry of the system, and of the enhanced transverse resistance above the superconducting transition. Our findings not only elucidate the rich physics of trilayer graphene on WSe2, but also establish a means to distinguish between various candidate interaction-induced orders in spin-orbit-coupled graphene moiré systems, and could therefore serve as a guide for future experiments as well.

Origin and properties of an unexpected exchange bias of Ta/Ni80Fe20/Ir20Mn80/Ta heterostructure in ultrathin limit: Impact of the oblique deposition and Ta/Ni80Fe20 alloying

Ultrathin Si(100)/Ta/Ni80Fe20/Ir20Mn80/Ta heterostructure deposited by oblique incidence in a confocal magnetron sputtering exhibits an unexpected exchange bias effect, despite the rotation of the sample holder and absence of external magnetic field during the deposition. Its magnetic loops have anomalous shifts along the field axis, whose features are non-uniform and strongly dependent on the orientation of the applied magnetic field. Structural, magnetic and thermal properties of the heterostructure were investigated using x-ray scattering and magnetic measurements to highlight the role of interfaces between ultrathin films and the related magnetic effects on the setting of the exchange bias. We show that the shadowing effect due to the oblique incidence of the sputtering flux combined to the atomic diffusion at the Ta/Ni80Fe20 interface favors the emergence of a uniaxial anisotropy in the ferromagnetic layer formed by a Ni80Fe20 sublayer at the top and a NiFeTa alloy at the bottom. The later increases the uniaxial anisotropy constant of the ferromagnet, which was strong enough to impose a well-defined direction for the spontaneous magnetization and, consequently, set the unidirectional anisotropy during the Ir20Mn80 layer formation. The thermal processes can reversibly be applied up to 460 K and, for higher temperatures, there were the vanishing of the exchange bias at 600 K and a thickening of the NiFeTa alloy which led to a reduction in the saturation magnetization of the heterostructure. Therefore, our findings suggest that the role of the interfacial NiFeTa alloy is only revealed in the ultrathin dimension and is imperceptible in thicker samples.

Plasmonic excitation for a tunable transmitter without magnetic field immune to backscattering

We predict a plasmonic excitation that exhibits chiral properties in the absence of external magnetic field. This excitation, that we call an interedge chiral Berry plasmon (IE CBP), is bound to an interface between two two-dimensional electron systems with different anomalous Hall conductivities. Crucially, the IE CBP is a unidirectional mode whose propagation direction and frequency are susceptible to external control, in sharp contrast to state-of-the-art magnetic field-free devices. We believe that the IE CBP represents a unique platform for tunable backscattering-immune signal transmission in the absence of magnetic field.

Large photonic spin Hall effect in two dimensional semi-Dirac materials

A strong photonic spin Hall effect (PSHE) in the absence of external magnetic field is important to control the spin states of photons and design next-generation photonic devices based on spinotpics. Herein, we theoretically study the PSHE on the surface of semi-Dirac materials. We established a general model, by changing various incident conditions, to describe the spin-orbit interaction of light in semi-Dirac materials. When we made these changes, we found that a large PSHE arises from the intrinsic anisotropy in the dispersion of semi-Dirac materials. The in-plane (transverse) spin Hall shift is dozens times of λ i , where λ i is the wavelength of the incident photon. Both of them can be effectively tuned by adjusting the optical axis angle and the photon energy. The maximum of the total in-plane and transverse spin Hall shift is 83.91λ i and 19.65λ i , respectively, which is larger than those in conventional anisotropic two-dimensional materials and isotropic Dirac materials. Our results shed light on the spin-orbit coupling of light in semi-Dirac materials and pave the way for designing spin-optical devices.

Localization of helical edge states in the absence of external magnetic field

Theoretically, the helical edge states of two-dimensional topological insulators are protected from coherent backscattering due to nonmagnetic disorder provided electron interactions are not too strong. Experimentally, the edges typically do not demonstrate the systematic and robust quantization, at the same time little is known about the sub-Kelvin temperature behavior. Here, we report the surprising localization of the edge states in an 8 nm HgTe quantum well in zero magnetic field at millikelvin temperatures. Additionally, the magnetoresistance data at 0.5 K for the edges few micrometers long suggests the field-dependent localization length $l_B\propto B^{-\alpha}$, with $\alpha$ ranging approximately from $1.6$ to $2.8$ at fields $B\lesssim0.1\,\text{T}$ and $\alpha\approx1.1$ at higher fields up to $0.5\,\text{T}$. In the frame of disordered interacting edge, these values of $\alpha$ correspond to the Luttinger liquid parameters $K\approx 0.9-1.1$ and $K\approx 0.6$, respectively. We discuss possible scenarios which could result in the zero magnetic field localization.

Spontaneous magnetization in unitary superconductors with time reversal symmetry breaking

We report the study of spontaneous magnetization (i.e., spin polarization) for time-reversal-symmetry-breaking (TRS-breaking) superconductors with unitary pairing potentials, in the absence of external magnetic fields or Zeeman fields. Spin-singlet (${\mathrm{\ensuremath{\Delta}}}_{s}$) and spin-triplet (${\mathrm{\ensuremath{\Delta}}}_{t}$) pairings can coexist in superconductors whose crystal structures lack inversion symmetry. The TRS can be spontaneously broken once a relative phase of $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2$ is developed, forming a TRS-breaking unitary pairing state (${\mathrm{\ensuremath{\Delta}}}_{s}\ifmmode\pm\else\textpm\fi{}i{\mathrm{\ensuremath{\Delta}}}_{t}$). We demonstrate that such unitary pairing could give rise to spontaneous spin polarization with the help of spin-orbit coupling. Our result provides an alternative explanation for the TRS breaking, beyond the current understanding of such phenomena in the noncentrosymmetric superconductors. The experimental results of ${\mathrm{Zr}}_{3}\mathrm{Ir}$ and CaPtAs are also discussed in view of our theory.

Voltage control of ferrimagnetic order and voltage-assisted writing of ferrimagnetic spin textures.

Voltage control of magnetic order is desirable for spintronic device applications, but 180° magnetization switching is not straightforward because electric fields do not break time-reversal symmetry. Ferrimagnets are promising candidates for 180° switching owing to a multi-sublattice configuration with opposing magnetic moments of different magnitudes. In this study we used solid-state hydrogen gating to control the ferrimagnetic order in rare earth-transition metal thin films dynamically. Electric field-induced hydrogen loading/unloading in GdCo can shift the magnetic compensation temperature by more than 100 K, which enables control of the dominant magnetic sublattice. X-ray magnetic circular dichroism measurements and ab initio calculations indicate that the magnetization control originates from the weakening of antiferromagnetic exchange coupling that reduces the magnetization of Gd more than that of Co upon hydrogenation. We observed reversible, gate voltage-induced net magnetization switching and full 180° Néel vector reversal in the absence of external magnetic fields. Furthermore, we generated ferrimagnetic spin textures, such as chiral domain walls and skyrmions, in racetrack devices through hydrogen gating. With gating times as short as 50 μs and endurance of more than 10,000 cycles, our method provides a powerful means to tune ferrimagnetic spin textures and dynamics, with broad applicability in the rapidly emerging field of ferrimagnetic spintronics.

Y-shaped beam splitter with topologically protected defect waveguide

The exotic properties of topological photonic crystals (PCs) have a series of application prospects in the field of optical communication. We present a Y-shaped beam splitter based on topologically protected waveguide, which consists of the topologically trivial PCs with line defect. The investigation results indicate it has the merits of unidirectional propagation, robustness, and excellent transmission performance. Compared with magnetic-optical PC beam splitter, the proposed configuration composed of rods with silicon material that can realize topological beam-splitting effect in the absence of external magnetic field and has a smaller size. It is suitable for miniaturization to implement in all-optical network as an optical communication device.

Spin coherence on the ferromagnetic spherical surface

Spintronics on flat surfaces has been studied over the years, and the scenario is relatively well-known; however, there is a lack of information when we consider non-flat surfaces. In this paper, we are concerned about the spin dynamics of the ferromagnetic model on the spherical surface. We use the Schwinger bosonic formalism for describing the thermodynamics of spin operators in terms of spinon operators. Opposite to the flat two-dimensional model, which is disordered at finite temperature, the curvature of the spherical surface provides non-zero critical temperature for Schwinger boson condensation, which characterizes order at finite temperature even in the absence of external magnetic fields. The thermodynamics is then analyzed in the low-temperature regime. In addition, we consider the presence of both static and oscillating magnetic fields, the necessary condition for inducing the ferromagnetic resonance, and we show systematically that the studied model is well-described by SU(2) coherent states, which provides the correct dynamics of the magnetization. The archived results can be applied for describing a diversity of experiments such as spin superfluidity, angular momentum injection by spin pumping and spin-transfer torque in non-conventional junctions, magnon dissipation, and magnetoelectronics on the spherical surface.

Cellular SPION Uptake and Toxicity in Various Head and Neck Cancer Cell Lines.

Superparamagnetic iron oxide nanoparticles (SPIONs) feature distinct magnetic properties that make them useful and effective tools for various diagnostic, therapeutic and theranostic applications. In particular, their use in magnetic drug targeting (MDT) promises to be an effective approach for the treatment of various diseases such as cancer. At the cellular level, SPION uptake, along with SPION-mediated toxicity, represents the most important prerequisite for successful application. Thus, the present study determines SPION uptake, toxicity and biocompatibility in human head and neck tumor cell lines of the tongue, pharynx and salivary gland. Using magnetic susceptibility measurements, microscopy, atomic emission spectroscopy, flow cytometry, and plasma coagulation, we analyzed the magnetic properties, cellular uptake and biocompatibility of two different SPION types in the presence and absence of external magnetic fields. Incubation of cells with lauric acid and human serum albumin-coated nanoparticles (SPIONLA-HSA) resulted in substantial particle uptake with low cytotoxicity. In contrast, uptake of lauric acid-coated nanoparticles (SPIONLA) was substantially increased but accompanied by higher toxicity. The presence of an external magnetic field significantly increased cellular uptake of both particles, although cytotoxicity was not significantly increased in any of the cell lines. SPIONs coated with lauric acid and/or human serum albumin show different patterns of uptake and toxicity in response to an external magnetic field. Consequently, the results indicate the potential use of SPIONs as vehicles for MDT in head and neck cancer.