Small Magnetic Field Research Articles

Edge photocurrent driven by terahertz electric field in bilayer graphene

We report on the observation of edge electric currents excited in bi-layer graphene by terahertz laser radiation. We show that the current generation belongs to the class of second order in electric field phenomena and is controlled by the orientation of the THz electric field polarization plane. Additionally, applying a small magnetic field normal to the graphene plane leads to a phase shift in the polarization dependence. Increasing the magnetic field strength, the current starts to exhibit 1/B-magnetooscillations with a period consistent with that of the Shubnikov-de-Haas effect and amplitude by an order of magnitude larger as compared to the current at zero magnetic field measured under the same conditions. The microscopic theory developed shows that the current is formed in the edges vicinity limited by the mean-free path of carriers and the screening length of the high-frequency electric field. The current originates from the alignment of the free carrier momenta and dynamic accumulation of charge at the edges, where the P-symmetry is naturally broken. The observed magnetooscillations of the photocurrent are attributed to the formation of Landau levels.

Application of Magnetosomes in Magnetic Hyperthermia.

Nanoparticles, specifically magnetosomes, synthesized in nature by magnetotactic bacteria, are very promising to be usedin magnetic hyperthermia in cancer treatment. In this work, using the solution of the stochastic Landau–Lifshitz equation, we calculate the specific absorption rate (SAR) in an alternating (AC) magnetic field of assemblies of magnetosome chains depending on the particle size D, the distance between particles in a chain a, and the angle of the applied magnetic field with respect to the chain axis. The dependence of SAR on the a/D ratio is shown to have a bell-shaped form with a pronounced maximum. For a dilute oriented chain assembly with optimally chosen a/D ratio, a strong magneto-dipole interaction between the chain particles leads to an almost rectangular hysteresis loop, and to large SAR values in the order of 400–450 W/g at moderate frequencies f = 300 kHz and small magnetic field amplitudes H0 = 50–100 Oe. The maximum SAR value only weakly depends on the diameter of the nanoparticles and the length of the chain. However, a significant decrease in SAR occurs in a dense chain assembly due to the strong magneto-dipole interaction of nanoparticles of different chains.

Magnetoresistance in Quasi-One-Dimensional Weyl Semimetal (TaSe4)2I

Magnetic field effect on linear and nonlinear conductivity in a quasi-one-dimensional Weyl semimetal with a charge density wave (CDW) (TaSe$_4$)$_2$I is studied. Longitudinal magnetoresistance in all known regimes of CDW motion (linear conduction, creep, sliding, "Fr\"ohlich superconductivity") is small, positive and do not exceed a fraction of per cent. Similar magnetotransport measurements were performed in samples profiled by focused ion beams is such a way that motion of the CDW in them is accompanied by phase slip of the CDW. In such samples, a peak-like non-parabolic negative magnetoresistance is observed in relatively small magnetic fields $B \lesssim 4$ T in the nonlinear conduction regime in both longitudinal and transverse geometries. Our results differ significantly from ones obtained earlier and raise the question concerning conditions for observing the axion anomaly in Weyl semimetals in the Peierls state.

Aharonov–Bohm oscillations and equilibrium current distributions in open quantum dot and in ring interferometer

Magnetotransport in two submicron-sized devices formed on the basis of GaAs/AlGaAs structures has been simulated using nonequilibrium Green functions. The effect of a perpendicular magnetic field on quantum transport in a quasi-one-dimensional quantum dot and in an Aharonov–Bohm interferometer has been analyzed in a single-particle approximation. Magnetic field oscillations of two-terminal conductance of the devices, equilibrium (persistent) current distributions and magnetic moment generated in the devices by persistent currents have been determined using numerical methods. Correlations between the magnetic moment, magnetic field oscillations of conductance and energy resonance in a specific magnetic field have been traced. Sufficiently regular conductance oscillations similar to Aharonov–Bohm ones have been revealed for a quasi-one-dimensional quantum dot at small magnetic fields (0.05–0.4 T). For a ring interferometer the contribution to the total equilibrium current and magnetic moment at a specific energy may change abruptly both in magnitude and in sign as a result of changing magnetic field within one Aharonov–Bohm oscillation. We show that the conductance of an interferometer is determined not by the number of modes propagating in the ring but rather by the effect of triangular quantum dots at the ring entrance that produce a strong reflection. The period of the calculated Aharonov–Bohm oscillations is in agreement with the measurement results for these devices.

Ultrasensitive Magnetic Tuning of Optical Properties of Films of Cholesteric Cellulose Nanocrystals.

Chiral photonic crystals derived from the self-assembly of cellulose nanocrystals (CNCs) have found important applications in optical devices due to the capacity to adjust the chiral nematic phase under external stimulus, in particular an applied magnetic field. To date, strong magnetic fields have been required to induce an optical response in CNC films. In this work, the self-assembly of films of CNCs can be tuned by applying an ultrasmall magnetic field. The CNCs, decorated with Fe3O4 nanoparticles (Fe3O4/CNCs), were dispersed in suspensions of neat CNCs so as to alter the magnetic response of the CNCs. A subsequent process of dispersion not only prevents the clumping of the magnetic nanoparticles but also enhances the sensitivity to an applied magnetic field. A small magnetic field of 7 mT can tune the self-assembly and the microstructure of the CNCs. The pitch of the chiral structure decreased with an increase in applied magnetic field, from 302 to 206 nm, for fields from 7 to 15 mT. This phenomenon is opposite that observed for neat CNCs, in which the pitch is observed to increase with an increase in the external magnetic strength. The optical response under application of an ultrasmall magnetic field could help with theoretical research and enable more applications, such as sensors or nanotemplating agents.

Magnetic field effect on the dielectric properties of rare earth doped multiferroic BiFeO3

The temperature, doping concentration and magnetic field dependence of the dielectric constant in lanthanum doped barium titanate are studied using a microscopic model and the Green’s function technique. A kink in the temperature dependence of the dielectric constant is observed near the Neel temperature. The Neel temperature and the ferroelectric transition temperature decrease with increasing doping concentration. The doping dependence of the dielectric constant shows a maximum value at 0.15. For small magnetic fields the dielectric constant increases strongly, whereas for stronger ones it begins to decrease slightly, i.e. there is a critical field for the transition from spiral cycloidal to antiferromagnetic spin arrangement.

Single state variational calculations with applications to large and small magnetic fields

Variational techniques to calculate estimates of bound-state energies and wave functions for single states are applied to simple electronic systems in magnetic fields. Explicit constraints to other state functions are not required. Our numerical energies compare favourably with results of other calculations, and the speed of our calculations is greatly enhanced by using a separable basis set and analytical integration.

Nonlocal Thermoelectricity in a Superconductor-Topological-Insulator-Superconductor Junction in Contact with a Normal-Metal Probe: Evidence for Helical Edge States.

We consider a Josephson junction hosting a Kramers pair of helical edge states of a quantum spin Hall bar in contact with a normal-metal probe. In this hybrid system, the orbital phase, induced by a small magnetic field threading the junction known as a Doppler shift, combines with the conventional Josephson phase difference and originates an effect akin to a Zeeman field in the spectrum. As a consequence, when a temperature bias is applied to the superconducting terminals, a thermoelectric current is established in the normal probe. We argue that this purely nonlocal thermoelectric effect is a unique signature of the helical nature of the edge states coupled to superconducting leads and it can constitute a useful tool for probing the helical nature of the edge states in systems where the Hall bar configuration is difficult to achieve. We fully characterize thermoelectric response and performance of this hybrid junction in a wide range of parameters, demonstrating that the external magnetic flux inducing the Doppler shift can be used as a knob to control the thermoelectric response and the heat flow in a novel device based on topological junctions.

Topological superconductivity in carbon nanotubes with a small magnetic flux

We show that a one-dimensional topological superconductor can be realized in carbon nanotubes, using a relatively small magnetic field. Our analysis relies on the intrinsic curvature-enhanced spin-orbit coupling of the nanotubes, as well as on the orbital effect of a magnetic flux threaded through the nanotube. Tuning experimental parameters, we show that a half-metallic state may be induced in the nanotube. Coupling the system to an Ising superconductor, with an appreciable spin-triplet component, can then drive the nanotube into a topological superconducting phase. The proposed scheme is investigated by means of real-space tight-binding simulations, accompanied by an effective continuum low-energy theory, which allows us to gain some insight on the roles of different terms in the Hamiltonian. We calculate the topological phase diagram and ascertain the existence of localized Majorana zero modes near the edges. Moreover, we find that in the absence of a magnetic field, a regime exists where sufficiently strong interactions drive the system into a time-reversal-invariant topological superconducting phase.

Vortical configurations in mesoscopic superconducting square loop with mixed pairing orders

Based on the microscopic Bogoliubov-de Gennes theory, we study vortical configurations in a mesoscopic superconducting square loop with mixed pairing orders. By choosing appropriate interaction strengths and chemical potential, the spin-triplet p-wave and subdominant spin-singlet d-wave pairing symmetries can be stabilized. In a small external magnetic field, unusual elliptical-like vortices in chiral p-wave orders as well as elongated vortices in the d-wave order tend to form in sample diagonals. For an enlarged flux, we find a two-quanta skyrmionic mode for the p-wave components, while the closed domain wall does not appear compared to the previous studies in superconducting systems with a pure p-wave state. Particularly, a novel multi-skyrmionic pattern containing four single-quanta skyrmions takes place with increasing flux. Such a single-quanta vortex structure consisting of two spatially separated half-quantum vortices is analogous to the nematic skyrmion with unit topological charge. In addition, several complex hybrid or multiple skyrmionic configurations are revealed in the strong flux range.

The promise of optical NMR spectroscopy for experimental aqueous geochemistry

New quantum technologies are being adapted to detect extraordinarily small magnetic fields. The signals are derived from the stimulated emission of light from single-atom defects in diamonds. The intensity of light reports the spin state of the electrons in the color center. The physics is relevant to geochemists because it points to a means of conducting NMR experiments on solutions in diamond-anvil cells. Because the method is so sensitive, it can detect signals in subnanoliter volumes, making work at elevated temperatures and pressures possible, and also on solutions that are dangerous in larger volumes.

Laser-induced magnetization dynamics in a van der Waals ferromagnetic Cr2Ge2Te6 nanoflake

Laser-induced magnetization dynamics is quantitatively investigated in a van der Waals ferromagnetic Cr2Ge2Te6 nanoflake by means of time-resolved Faraday rotation. Under ferromagnetic resonance conditions, the angular dependence of spin precession dynamics gives rise to a perpendicular magnetic anisotropy with an effective field of 125 ± 8 mT. We further determine the field dependence of the effective damping coefficient, which is dominated by the inhomogeneous broadening of magnetic anisotropy in the regime of a small magnetic field while it diminishes to an intrinsic value of 0.006 ± 0.002 at high fields.

Multi-channel whole-head OPM-MEG: Helmet design and a comparison with a conventional system

Magnetoencephalography (MEG) is a powerful technique for functional neuroimaging, offering a non-invasive window on brain electrophysiology. MEG systems have traditionally been based on cryogenic sensors which detect the small extracranial magnetic fields generated by synchronised current in neuronal assemblies, however, such systems have fundamental limitations. In recent years, non-cryogenic quantum-enabled sensors, called optically-pumped magnetometers (OPMs), in combination with novel techniques for accurate background magnetic field control, have promised to lift those restrictions offering an adaptable, motion-robust MEG system, with improved data quality, at reduced cost. However, OPM-MEG remains a nascent technology, and whilst viable systems exist, most employ small numbers of sensors sited above targeted brain regions. Here, building on previous work, we construct a wearable OPM-MEG system with ‘whole-head’ coverage based upon commercially available OPMs, and test its capabilities to measure alpha, beta and gamma oscillations. We design two methods for OPM mounting; a flexible (EEG-like) cap and rigid (additively-manufactured) helmet. Whilst both designs allow for high quality data to be collected, we argue that the rigid helmet offers a more robust option with significant advantages for reconstruction of field data into 3D images of changes in neuronal current. Using repeat measurements in two participants, we show signal detection for our device to be highly robust. Moreover, via application of source-space modelling, we show that, despite having 5 times fewer sensors, our system exhibits comparable performance to an established cryogenic MEG device. While significant challenges still remain, these developments provide further evidence that OPM-MEG is likely to facilitate a step change for functional neuroimaging.

Nanoscale Coordination Polymer of Dimanganese(II) as Infinite, Flexible Nanosheets with Photo‐Switchable Morphology

An original co‐ligand association, consisting of 4,4'‐azopyridine and 1,3‐bis(carboxypropyl)tetramethyldisiloxane, is used for complexing MnII, to obtain a 2D dinuclear coordination array having infinite horizontal dimensions and nanometric thickness. Due to the shielding effect of the tetramethyldisiloxane units facing outwards on both sides, the sheets are free of any chemical or physical intermolecular interactions, as crystallographic analysis reveals. The compound shows two glass transitions, at –51 and 33 °C, and begins to decompose at 292 °C. The co‐existence in the structure of the highly hydrophobic tetramethyldisiloxane spacers and the polar metal complexed units confers amphiphilic character to the compound, making it capable of reducing the surface tension of the DMF, from 35.8 to 29.4 mN/m, at a critical micellar concentration around 1 wt.‐% complex. The presence of aggregates in solution is evidenced by dynamic light scattering. The light sensitive azopyridine units make the compound photo‐responsive. This behavior occurs both in solution as well as in the film, where the compound self‐assembles in micellar morphology when UV‐irradiated. Both conductivity and dielectric permittivity of the film increase as a result of trans–cis configuration. Magnetic measurement data indicate two antiferromagnetic coupled spins, S = 5/2, that are able to interact with small external magnetic field, i.e., during MFM scanning.

Novel Chiral Quantum Spin Liquids in Kitaev Magnets.

Quantum magnets with pure Kitaev spin exchange interactions can host a gapped quantum spin liquid with a single Majorana edge mode propagating in the counterclockwise direction when a small positive magnetic field is applied. Here, we show how under a sufficiently strong positive magnetic field a topological transition into a gapped quantum spin liquid with two Majorana edge modes propagating in the clockwise direction occurs. The Dzyaloshinskii-Moriya interaction is found to turn the nonchiral Kitaev's gapless quantum spin liquid into a chiral one with equal Berry phases at the two Dirac points. Thermal Hall conductance experiments can provide evidence of the novel topologically gapped quantum spin liquid states predicted.

Lateral Electric‐Field‐Controlled Perpendicular Magnetic Anisotropy and Current‐Induced Magnetization Switching in Multiferroic Heterostructures

AbstractA strain‐mediated perpendicular magnetic anisotropy (PMA) and current‐induced magnetization switching via spin–orbit torque (SOT) in PbMg1/3Nb2/3O3‐PbTiO3 (PMN‐PT)/Ta/Pt/Co/Pt ferromagnetic heterostructures are reported. It is found that the PMA changes regularly with the preloaded lateral electric field. The SOT‐based current‐induced magnetization switching also shows a reversible trend via applying a lateral electric field. Domain wall propagation driven by electric field is observed directly under a fixed perpendicular magnetic field by magnetic optical Kerr (MOKE) microscope. These behaviors can be attributed to the strain from the PMN‐PT substrates induced by the piezoelectric effect under electric field. Basing on the domain wall motion mechanism, repeatable resistance states of the Hall bar can be controlled by external electric field under a small auxiliary magnetic field, which enables the information recorded in the device can be programmed by voltage. This study provides a potential method to design the electric field controlled spintronic devices.

Mitigation of multispecies Weibel instability in a finite non-neutral magnetized beam-plasma system.

Compressing and charge neutralization of the heavy-ion beam in an inertial fusion reactor are considered as pivotal processes for increasing the energy gain. For this purpose, the ion beam is usually transported through the plasma channel so that multispecies Weibel instability can grow in this system. Recently, a small solenoidal magnetic field has been added to this method to have additional control of these processes. On the other hand, charge and current may not be completely neutralized in this transport; as a result, the fractional charge and current neutralization can affect the growth rate of this instability. In this work, the dispersion equation has been obtained in cylindrical, cold, magnetized, non-neutral plasma in the macroscopic fluid frame. Numerical results show that if the electron cyclotron frequency of the background plasma normalized by plasma frequency is larger than the ion beam velocity normalized by the speed of light, as well as the current fraction being smaller than 1/2, the eigenmodes of multispecies Weibel instability can be completely stabilized. Moreover, these results are valid when the percent deviation from the charge-neutral state is positive. In the negative regime of percent deviation, the instability increases drastically so that the system can be completely unstable only for more than 2%. Therefore, selecting the radius of the ion beam smaller than the electron skin depth of the plasma and the beam pulse duration much longer than the plasma oscillation time are proposed for quenching of this instability.

Linear nonsaturating magnetoresistance in the Nowotny chimney ladder compound Ru2Sn3

We present magnetoresistivity measurements in high-quality single crystals of the Nowotny chimney ladder compound Ru$_2$Sn$_3$. We find a linear and nonsaturating magnetoresistance up to 20 T. The magnetoresistance changes with the magnetic field orientation at small magnetic fields, from a positive to a negative curvature. Above 5 T, the magnetoresistance shows no sign of saturation up to 20 T for any measured angle. The shape of the anisotropy in the magnetoresistance remains when increasing temperature and Kohler's rule is obeyed. We associate the linear and nonsaturating magnetoresistance to a small Fermi surface with hot spots, possibly formed as a consequence of the structural transition. We discuss the relevance of electron-electron interactions under magnetic fields and aspects of the topologically nontrivial properties expected in Ru$_2$Sn$_3$.

Controlling domain wall oscillations in bent cylindrical magnetic wires

Magnetic cylindrical nanowires are promising candidates for future three-dimensional nanotechnology. Domain walls (DWs) in magnetic nanowires play the role of information carriers, and the development of applications requires proper description of their dynamics. Here we perform a detailed analytical and numerical analysis of the DW motion along a bent magnetic nanowire under the action of tangential magnetic fields. Our results show that the DW velocity, precession, and oscillation frequencies can be controlled by the interplay between the curvature and the external magnetic field. Small magnetic fields induce a DW motion without precession and oscillatory behavior, while higher magnetic fields yield a Walker breakdown regime, in which an oscillatory forward and backward DW motion is observed. Controlled DW motion under the Walker breakdown regime makes magnetic nanowires potential candidates for nanoscale microwave generation and sensing.

Unveiling the Physics of the Mutual Interactions in Paramagnets

In real paramagnets, there is always a subtle many-body contribution to the system’s energy, which can be regarded as a small effective local magnetic field (Bloc). Usually, it is neglected, since it is very small when compared with thermal fluctuations and/or external magnetic fields (B). Nevertheless, as both the temperature (T) → 0 K and B → 0 T, such many-body contributions become ubiquitous. Here, employing the magnetic Grüneisen parameter (Γmag) and entropy arguments, we report on the pivotal role played by the mutual interactions in the regime of ultra-low-T and vanishing B. Our key results are: i) absence of a genuine zero-field quantum phase transition due to the presence of Bloc; ii) connection between the canonical definition of temperature and Γmag; and iii) possibility of performing adiabatic magnetization by only manipulating the mutual interactions. Our findings unveil unprecedented aspects emerging from the mutual interactions.