Magnetoplasmons in gapped graphene in a periodically modulated magnetic field

Motivated by recent experiments on long‐lived magnetoplasmons in the presence of a perpendicular magnetic field, we investigate the dynamical dielectric response function of graphene in contact with a substrate using the random phase approximation. We add a periodically modulated magnetic field within the graphene plane and address both the inter and intra Landau band magnetoplasmons. Verification of the predicted magnetic modulation effects is possible by experiments analogous to those for the zero gap limit.     

Magnetothermoelectric transport in modulated and unmodulated graphene

We draw motivation from recent experimental studies and present a comprehensive study of magnetothermoelectric transport in a graphene monolayer within the linear response regime. We employ the modified Kubo formalism developed for thermal transport in a magnetic field. Thermopower as well as thermal conductivity as a function of the gate voltage of a graphene monolayer in the presence of a magnetic field perpendicular to the graphene plane is determined for low magnetic fields (～1 T) as well as high fields (～8 T). We include the effects of screened charged impurities on thermal transport. We find good qualitative and quantitative agreement with recent experimental work on the subject. In addition, in order to analyze the effects of modulation, which can be induced by various means, on the thermal transport in graphene, we evaluate the thermal transport coefficients for a graphene monolayer subjected to a periodic electric modulation in a magnetic field. The results are presented as a function of the magnetic field and the gate voltage.     

Collective modes and the far-infrared absorption of the two-dimensional electron gas in a periodic quantizing magnetic field

We investigate the far-infrared (FIR) absorption of a two-dimensional electron gas in a periodically modulated quantizing magnetic field. The magnetic field varies along only one spatial direction and the external time-dependent electric field is linearly polarized along that axis. The mutual Coulomb interaction of the electrons is treated self-consistently in the ground state and in the absorption calculation within the Hartree approximation. The effects of the magnetic material on top of the heterostructure as a grating coupler is included in the time-dependent incident FIR electric field. We show that, similar to an electric modulation, the absorption can be directly correlated to the underlying electronic energy bands. In addition, the magnetic modulation leads to absorption spectra with a richer structure due to the quite different static response of the electron density to the modulation.     

Effect of the anode-current magnetic field on the motion of electrons in a triode with a virtual cathode

The effect of the anode-current magnetic field on the electron motion in a triode with a virtual cathode is considered. It is shown that the anode-current magnetic field influences the oscillation period and trajectories of electrons. The condition of self-isolation of the electron beam is investigated as a function of the diode parameter. It is shown that the displacement of the beam electrons under the action of the anode-current magnetic field leads to a decrease in the electron phase modulation and an increase in the spread in the electron oscillation amplitude; as a result, the generation efficiency of microwave radiation decreases.     

Intrinsic pinning of vortices as a direct probe of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell state in layered superconductors

Previously the search for the modulated superconducting Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state was performed by means of measurements which do not give direct information on spatial modulation of the superconducting state. We propose to measure interlayer conductivity in Josephson-coupled layered superconductors as a function of the strength and the orientation of the parallel magnetic field. We show that interlayer critical current and the conductivity have peaks when the magnetic field is perpendicular to the in-plane wave vector of the LOFF state and when the period of the Josephson vortex lattice induced by the magnetic field is commensurate with the LOFF period.     

Tunable magnetoben-optic modulation based on magnetically responsive nanostructured magnetic fluid

Magnetic fluid is a kind of functional composite material with nanosized structure and unique optical properties. The tunable magneto-optic modulation of magnetic fluid under external magnetic field, achieved by adjusting the polarization direction of incident light, is investigated theoretically and experimentally in this work. The corresponding modulation depth and response time are obtained. The accompanying mechanisms are clarified by using the theory of dichroism of magnetic fluid and the aggregation/disintegration processes of magnetic particles within magnetic fluid when the external magnetic field turns on/off.     

Electron transport across a quantum wire embedding a saw-tooth superlattice

By developing the recursive Green function method, the transport properties through a quantum wire embedding a finite-length saw-tooth superlattice are studied in the presence of magnetic field. The effects of magnetic modulation and the geometric structures of the superlattice on transmission coefficient are discussed. It is shown that resonant peak splitting of this kind of structure is different from that of ‘magnetic' and ‘electric' superlattices in two-dimensional electron gas. The transmission spectrum can be tailored to match requirements through adjusting the size of saw-tooth quantum dot and field strength.     

Kondo effect in a many-electron quantum ring

The Kondo effect is investigated in a many-electron quantum ring as a function of the magnetic field. For fields applied perpendicular to the plane of the ring a modulation of the Kondo effect with the Aharonov-Bohm period is observed. This effect is discussed in terms of the energy spectrum of the ring and the parametrically changing tunnel coupling. In addition, we use gate voltages to modify the ground-state spin of the ring. The observed splitting of the Kondo-related zero-bias anomaly in this configuration is tuned with an in-plane magnetic field.     

Singularities in critical properties of superconductive superlattices as a function of magnetic field

The critical temperature singularities as a function of both magnetic field direction and value in periodic superconductive structure in metals have been predicted. The different types of lattices, modulation value (weak or strong), relation between characteristic magnetic length and lattice periods have been investigated on the basis of microscopic equations.     

Optical relaxation properties of magnetic fluids under externally magnetic fields

The relaxation behavior (including the rising and falling relaxation processes) of the transmitted light after the magnetic fluid thin films under longitudinal and transverse magnetic fields is investigated, respectively. The physical mechanisms of the two different relaxation processes are discussed. The experimental data of the rising and falling relaxation processes are fitted by using two exponential functions to achieve the rising and falling response times. The relationship between the response time and the strength of applied magnetic field, the concentration of magnetic fluid is studied experimentally. The modulation depth of the transmitted light is researched quantificationally and the deepest modulation depth is obtained with Sample 3 (with volume fraction of 5.62%) in our experiments.     

Magnetoquantum transport in a modulated 2D electron gas with spin-orbit interaction

We investigate the effects of spin-orbit interaction (SOI) and plane-perpendicular magnetic field on the conductivity of a two-dimensional electron system in the presence of one-dimensional electrostatic modulation. The calculations are performed when a low-intensity, low-frequency external electric field is applied. The Kubo formula for the conductivity is employed in the calculation. The single-particle eigenstates which depend on the strengths of the magnetic field, the SOI and modulation potential, are calculated and then used to determine the conductivity. We present numerical results for the conductivity along the channels as well as the tunneling conductivity perpendicular to the constrictions as functions of the modulation potential, the SOI and the magnetic field. We demonstrate that the effect of finite frequency is to related to the reduction of both the longitudinal and transverse conductivities.     

Magnetotransport in a dual waveguide coupled by a finite barrier： Energy filter and directional coupler

We propose in this paper that a dual waveguide coupled by a finite barrier be able to serve as an energy filter under a perpendicular magnetic field. In the waveguide direction, the conductance exhibits a periodic square-wave pattern in which the miniband is controlled by the magnetic and potential modulation. The electrons with energies in the miniband can completely transfer along one waveguide while the other electrons undergo filtration. Compared with the coupled waveguide without magnetic modulation, the structure under magnetic field is found to be a good directional coupler. By adjusting the potential barrier and magnetic field, the electrons input from one port of waveguide can transfer to any other ports.     

Spin filtering in a nonuniform quantum wire with Rashba spin-orbit interaction

We investigate theoretically the spin-polarized electron transport for a wide-narrow-wide (WNW) quantum wire under the modulation of Rashba spin-orbit interaction (SOI). The influence of both the structure of the quantum wire and the interference between different pairs of subbands on the spin-polarized electron transport is taken into account simultaneously via the spin-resolved lattice Green function method. It is found that a very large vertical spin-polarized current can be generated by the SOI-induced effective magnetic field at the structure-induced Fano resonance even in the presence of strong disorder. Furthermore, the magnitude of the spin polarization can be tuned by the Rashba SOI strength and structural parameters. Those results may provide an effective way to design a spin filter device without containing any magnetic materials or applying a magnetic field.     

Thermodynamic properties of a magnetically modulated graphene monolayer

The effect of magnetic modulation on thermodynamic properties of a graphene monolayer in the presence of a constant perpendicular magnetic field is reported here. One-dimensional spatial electric or magnetic modulation lifts the degeneracy of the Landau levels and converts into bands and their bandwidth oscillates with magnetic field, leading to Weiss-type oscillations in the thermodynamic properties. The effect of magnetic modulation on the thermodynamic properties of a graphene sheet is studied and then compared with electrically modulated graphene and magnetically modulated conventional two-dimensional electron gas (2DEG). We observe Weiss-type and de Haas-van Alphen oscillations at low and high magnetic fields, respectively. There is a definite phase difference in Weiss-type oscillations in thermodynamic quantities of magnetically modulated graphene compared to electrically modulated graphene. On the other hand, the phase remains the same and the amplitude of the oscillation is large when compared with the magnetically modulated two-dimensional electron gas (2DEG). Explicit asymptotic expressions of the density of states and the Helmholtz free energy are provided to understand the phase and amplitude of the Weiss-type oscillations qualitatively. We also study thermodynamic properties when both electric and magnetic modulations are present. The Weiss-type oscillations still exist when the modulations are out-of-phase.     

Combining a Magnetic Field Modulation Coil with a Surface-Coil-Type EPR Resonator

It is thought that the design of magnetic field modulation coils is one of the factors limiting enlargement of the sample size in electron paramagnetic resonance (EPR) measurements. In this study, we miniaturized the magnetic field modulation coil and combined it with a surface-coil-type resonator (SCR). The inductor of the SCR was a circular single-turn one-loop coil (diameter, 1 mm), and the magnetic field modulation coil was a twin-loop coil consisting of two solenoid coils each made of 15 turns of copper wire on a cylindrical bobbin with an axial length of 3 mm and an elliptical cross section (major axis, 7 mm; minor axis, 3 mm). The former was located on the latter via a spacer (thickness, 3 mm) in such a way that the directions of their axes coincided. Their combined size was about 10 mm wide, 10 mm deep, and 6 mm high. The transmission lines of the SCR were set on resonance at about 700 MHz. EPR measurements of a phantom (comprising agar that included a nitroxide radical and physiological saline solution), made with a miniaturized modulation coil combined with the SCR, exhibited a sensitivity similar to that for the conventional method. Authors' address: Hidekatsu Yokoyama, Department of Pharmaceutical Sciences, International University of Health and Welfare, 2600-1 Kitakanemaru, Ohtawara 324-8501, Japan     

1.1-GHz continuous-wave EPR spectroscopy with a frequency modulation method

Continuous-wave EPR spectroscopy using a frequency modulation (FM) scheme was developed. An electronically tunable resonator and an automatic tuning control (ATC) system were used. Using the FM scheme instead of magnetic field modulation, we detected EPR absorption at the first derivative mode. We used a microwave frequency of 1.1 GHz in the present experiment. Similar signal-to-noise ratios were obtained with conventional field modulation and the FM method, and a low-quality factor EPR resonator was not necessary to suppress the significant microwave reflection from the resonator. The FM method with a tunable resonator may be an alternative solution to achieving phase-sensitive detection, when the side-effects of magnetic field modulation, such as microphonic noise and mechanical vibration, are detrimental for EPR detection.     

Spin dynamics of magnetic resonance with parametric modulation in a potassium vapor cell

A typical magnetic-resonance scheme employs a static bias magnetic field and an orthogonal driving magnetic field oscillating at the Larmor frequency, at which the atomic polarization precesses around the static magnetic field. Here we demonstrate both theoretically and experimentally the variations of the resonance condition and the spin precession dynamics resulting from the parametric modulation of the bias field. We show that the driving magnetic field with the frequency detuned by different harmonics of the parametric modulation frequency can lead to resonance as well. Also, a series of frequency sidebands centered at the driving frequency and spaced by the parametric modulation frequency can be observed in the precession of the atomic polarization. We further show that the resonant amplitudes of the sidebands can be controlled by varying the ratio between the amplitude and the frequency of the parametric modulation. These effects could be used in different atomic magnetometry applications.     

Orientational phase transition into a modulated magnetic state in the weak ferromagnet FeBO<Subscript>3</Subscript>: Mg

The transition from a homogeneous into a modulated magnetic state in the easy-plane weak ferromagnet FeBO₃: Mg is studied by a magnetooptic method. At T < 135 K, the application of a magnetic field in the basal plane of the crystal is shown to excite the modulation of its magnetic order parameter, which manifests itself in a periodic deviation of the local ferromagnetism vector from the magnetization direction. The conditions for the existence of a modulated magnetic superstructure in FeBO₃: Mg are studied, and its preferred orientation in the basal plane of the crystal is analyzed. A magnetic H-T phase diagram that shows the boundaries between the homogeneous and inhomogeneous magnetic states of this weak ferromagnet is constructed. The modulation period and the azimuthal angle specifying the local ferromagnetism vector direction in the structure are studied as a function of temperature and magnetic field. The results obtained are discussed in terms of the theory of magnetic ripple using the model of anisotropic magnetic centers appearing in a crystal doped by magnesium ions.     

Magnetically induced chessboard pattern in the conductance of a Kondo quantum dot

We quantitatively describe the main features of the magnetically induced conductance modulation of a Kondo quantum dot-or chessboard pattern-in terms of a constant-interaction double quantum dot model. We show that the analogy with a double dot holds down to remarkably low magnetic fields. The analysis is extended by full 3D spin density functional calculations. Introducing an effective Kondo coupling parameter, the chessboard pattern is self-consistently computed as a function of magnetic field and electron number, which enables us to explain our experimental data quantitatively.     

Magnetic response of a two-dimensional degenerate electron gas in nanostructures with cylindrical symmetry

An investigation is made of the magnetic response of nanostructures with cylindrical symmetry located in a longitudinal magnetic field. Analytic expressions are obtained for the magnetic moment of the nanostructures, cylinders and bracelets. It is shown that the magnetic moment describes Aharonov-Bohm oscillations. The profile of the oscillations and the position of the oscillation maxima are studied. In the limit T→0 the curves of the magnetic response as a function of the magnetic field flux contain “beak”-shaped kinks, and the positions of the points at which these kinks occur are determined. The temperature dependence of the magnetic response is studied and the influence of the spin-magnetic interaction on the magnetic response of the nanostructures is examined. It is shown that this interaction destroys the periodicity of the magnetic response with respect to flux and gives rise to a monotonic term in the response. Zh. éksp. Teor. Fiz. 115, 1450–1462 (April 1999)