Periodic Magnetic Modulation Research Articles

Evidence of striped electronic phases in a structurally modulated superlattice.

The electronic properties of crystals can be manipulated by superimposing spatially periodic electric, magnetic or structural modulations. Long-wavelength modulations incommensurate with the atomic lattice are particularly interesting1, exemplified by recent advances in two-dimensional (2D) moiré materials2,3. Bulk van der Waals (vdW) superlattices4-8 hosting 2D interfaces between minimally disordered layers represent scalable bulk analogues of artificial vdW heterostructures and present a complementary venue to explore incommensurately modulated 2D states. Here we report the bulk vdW superlattice SrTa2S5 realizing an incommensurate one-dimensional (1D) structural modulation of 2D transition metal dichalcogenide (TMD) H-TaS2 layers. High-quality electronic transport in the H-TaS2 layers, evidenced by quantum oscillations, is made anisotropic by the modulation and exhibits commensurability oscillations paralleling lithographically modulated 2D systems9-11. We also find unconventional, clean-limit superconductivity in SrTa2S5 with a pronounced suppression of interlayer relative to intralayer coherence. The in-plane magnetic field dependence of interlayer critical current, together with electron diffraction from the structural modulation, suggests superconductivity12-14 in SrTa2S5 is spatially modulated and mismatched between adjacent TMD layers. With phenomenology suggestive of pair-density wave superconductivity15-17, SrTa2S5 may present a pathway for microscopic evaluation of this unconventional order18-21. More broadly, SrTa2S5 establishes bulk vdW superlattices as versatile platforms to address long-standing predictions surrounding modulated electronic phases in the form of nanoscale vdW devices12,13 to macroscopic crystals22,23.

Periodic magnetic modulation enhanced electrochemical analysis for highly sensitive determination of genomic DNA methylation

DNA methylation aberrations have a strong correlation with cancer in early detection, diagnosis, and prognosis, which make them possible candidate biomarkers. Electrochemical biosensors offer rapid protocols for detecting DNA methylation status with minimal pretreatment of samples. However, the inevitable presence of background current in the time domain, including electrochemical noise and variations, limits the detection performance of these biosensors, especially for low concentration analytes. Here, we propose an ultrasensitive frequency-domain electrochemical analysis strategy to effectively separate the weak signals from background current. To achieve this, we employed periodic magnetic field modulation of magnetic beads (MBs) on and off the electrode surface to generate a periodic electrochemical signal for subsequent frequency-domain analysis. By capturing labeled MBs with as low as 0.5 pg of DNA, we successfully demonstrated a highly sensitive electrochemical method for determination of genome-wide DNA methylation levels. We also validated the effectiveness of this methodology using DNA samples extracted from three types of hepatocellular carcinoma (HCC) cell lines. The results revealed varying genomic methylation levels among different HCC cell lines, indicating the potential application of this approach for early-stage cancer detection in terms of DNA methylation status.

Tuning the magnetoresistance properties of phosphorene with periodic magnetic modulation

Periodic superlattices constitute ideal structures to modulate the transport properties of two-dimensional materials. In this paper, we show that the tunneling magnetoresistance (TMR) in phosphorene can be tuned effectively through periodic magnetic modulation. Deltaic magnetic barriers are arranged periodically along the phosphorene armchair direction in parallel (PM) and anti-parallel magnetization (AM) fashion. The theoretical treatment is based on a low-energy effective Hamiltonian, the transfer matrix method and the Landauer–Büttiker formalism. We find that the periodic modulation gives rise to oscillating transport characteristics for both PM and AM configurations. More importantly, by adjusting the electrostatic potential appropriately we find Fermi energy regions for which the AM conductance is reduced significantly while the PM conductance keeps considerable values, resulting in an effective TMR that increases with the magnetic field strength. These findings could be useful in the design of magnetoresistive devices based on magnetic phosphorene superlattices.

Hofstadter butterflies in magnetically modulated graphene bilayer: An algebraic approach

It has been shown that Bernal stacked bilayer graphene (BLG) in a uniform magnetic field demonstrates integer quantum Hall effect with a zero Landau-level anomaly \cite{Geimbilayer}. In this article we consider such system in a two dimensional periodic magnetic modulation with square lattice symmetry. It is shown algebraically that the resulting Hofstadter spectrum can be expressed in terms of the corresponding spectrum of monolayer graphene in a similar magnetic modulation. In the weak-field limit, using the tight-binding model, we also derive the Harper-Hofstadter equation for such BLG system in a periodic magnetic modulation. We further demonstrate the topological quantisation of Hall conductivity in such system and point out that the quantised Hall plateaus are equally spaced for all quantum numbers for the quantised Hall conductivity.

Higgs and Goldstone spin-wave modes in striped magnetic texture

Spontaneous symmetry breaking is ubiquitous in physics. Its spectroscopic signature consists in the softening of a specific mode upon approaching the transition from the high-symmetry side and its subsequent splitting into a zero-frequency Goldstone mode and a nonzero-frequency Higgs mode. Although they determine the whole system dynamics, these features are difficult to address in practice because of their vanishing coupling to most experimental probes and/or their strong interaction with other fluctuations. In this paper, we consider a periodic magnetic modulation occurring in a ferromagnetic film with perpendicular-to-plane magnetic anisotropy and observe its Goldstone and Higgs spin-wave modes at room temperature using microwave and optical techniques. This simple system constitutes a particularly convenient platform for further exploring the dynamics of symmetry breaking.

Improvement of tunneling magnetoresistance and spin-valley polarization in magnetic silicene superlattices induced by structural disorder

It is well known that the unavoidable variation of the heights and widths of the barriers (structural disorder) in semiconductor superlattices and superlattices based on two-dimensional materials degrades important physical properties such as the electronic transport and the thermoelectric response. Here, we show that the structural disorder contrary to what is expected improves the magnetoresistive and spin-valleytronic properties of magnetic silicene superlattices. We reach this conclusion by studying the impact of the random variations of the width and strength of the magnetic barriers on the tunneling magnetoresistance and spin-valley polarization. The theoretical treatment is based on a Dirac-like Hamiltonian, the transfer matrix method, and the Landauer-B\"uttiker formalism to describe silicene electrons, to obtain the transmittance, and to obtain the conductance, respectively. Our results indicate that structural disorder effects improve the magnetoresistance response and the spin-valley polarization by eliminating the conductance oscillations caused by the periodic magnetic modulation as well as by differentiating the response of the conductance for the parallel and antiparallel magnetization configuration. These results could be useful in designing versatile devices with magnetoresistive and spin-valleytronic capabilities. Particularly, magnetic silicene superlattices with moderate structural disorder are more convenient than perfect or nearly perfect ones.

Magnetic Hofstadter butterfly and its topologically quantized Hall conductance

The energy spectrum of massless Dirac fermions in graphene under two dimensional periodic magnetic modulation having square lattice symmetry is calculated. We show that the translation symmetry of the problem is similar to that of the Hofstadter or TKNN problem and in the weak field limit the tight binding energy eigenvalue equation is indeed given by Harper Hofstadter hamiltonian. We show that due to its magnetic translational symmetry the Hall conductivity can be identified as a topological invariant and hence quantized. We thus extend the idea of Quantum Hall Effect to magnetically modulated two dimensional electron system. Finally we indicate possible experimental systems where this may be verified.

Commensurability resonances in two-dimensional magnetoelectric lateral superlattices

Hybrid lateral superlattices composed of a square array of antidots and a periodic one-dimensional magnetic modulation are prepared in $\mathrm{Ga}[\mathrm{Al}]\mathrm{As}$ heterostructures. The two-dimensional electron gases exposed to these superlattices are characterized by magnetotransport experiments in vanishing average perpendicular magnetic fields. Despite the absence of closed orbits, the diagonal magnetoresistivity in the direction perpendicular to the magnetic modulation shows pronounced classical resonances. They are located at magnetic fields where snake trajectories exist which are quasicommensurate with the antidot lattice. The diagonal magnetoresistivity in the direction of the magnetic modulation increases sharply above a threshold magnetic field and shows no fine structure. The experimental results are interpreted with the help of numerical simulations based on the semiclassical Kubo model.

Influence of electrostatic field on the Weiss oscillations in graphene

Abstract Recent theoretical works have predicated the appearance of Weiss oscillations in the magnetoconductivity with a one-dimensional periodic electrical or magnetic modulation in graphene. This paper further explores the electrostatic field effect on the Weiss oscillations in the presence of crossed uniform in-plane electric field and perpendicular magnetic field that is weakly and periodically modulated along one direction. We find that the oscillation amplitude (OA) of Weiss oscillations and the value of conductivity are both shown to increase as the electric field E increases for a given magnetic field B. More interestingly, the electric field leads to an abrupt disappearance of the Weiss oscillations, when the value of electric to magnetic field ratio approaches a threshold value, i.e., γ e = E υ F B = 1 . These phenomena, not known in the conventional 2D electron gas, are a consequence of the anomalous spectrum of electron in graphene.

Electronic structure in a two-dimensional triangle-like periodic magnetic field

Electronic structures of 2DEG in a spatially periodic magnetic field modulation are investigated. For the consideration of reality, we employ the first two terms of Fourier series with an adjustable parameter to simulate the magnetic field profiles. The systemic Hamiltonian is expanded by the eigenstates of two-dimensional pure harmonic oscillator and then the systemic eigenvalues can be calculated. The results show the energy spectra are strongly dependent on the parameters of magnetic field, such as the magnetic field amplitude Bm, the period k and the phase φ. For a varying Bm, the Landau bands are disappeared and the levels group in pairs. On the other hand, with increasing k, the energy level splitting is dependent on the phase φ, which determines whether the zero point of magnetic field locates at the coordinate origin. A significant convergence in spectra structure is also observed for either low field amplitude or strong field amplitude.

Tunneling magnetoresistance on the surface of a topological insulator with periodic magnetic modulations

We investigate the band and transport features of Dirac electrons on the surface of a three-dimensional topological insulator under the modulation of a periodic exchange field provided by an array of ferromagnetic insulating (FI) stripes. The Dirac point is shifted (unchanged) by the superlattice when the magnetizations of adjacent FI stripes are parallel (antiparallel). For a finite magnetic superlattice, there exists a full transmission gap for both the parallel and antiparallel configurations. When the two kinds of transmission gaps have no overlap, a large magnetoresistance ratio with a tunable sign can be achieved.

Movement of diagonal resistivity in fractional quantum hall effect via periodic modulation of magnetic field strength

We examine the effect induced by periodic modulation of magnetic field strength in the fractional quantum Hall effect (FQHE). The classical Coulomb energy of the fractional quantum Hall (FQH) state is linearly dependent on 1/v, where v represents the fractional filling factor. This energy varies continuously with v. The residual Coulomb interactions produce quantum transitions. Then, the binding energy via the transitions yields an energy gap for specific fractional filling factors and no gap for the other fractional filling factors. The strength of the static magnetic field is fixed to yield an FQH state with gap energy. Then a periodic magnetic modulation is added to the system. Its resultant diagonal resistivity depends upon the oscillation frequency of the magnetic modulation. Examination of that dependence shows that the resistivity varies drastically at some frequency value, which can be evaluated for several fractional filling factors. These phenomena are strongly dependent upon the energy spectra of FQH states

Spin-dependent quantum transport in periodic magnetic modulations: Aharonov–Bohm ring structure as a spin filter

Quantum interference in Aharonov–Bohm (AB) ring structure provides additional control of spin at mesoscopic scale. We propose a scheme for spin filter by studying the coherent transport through the AB structure with lateral magnetic modulation on both arms of ring structure. Large spin polarized current can be obtained with many energy channels.

Spin-dependent transport in lateral periodic magnetic modulations: Scheme for spin filters

A scheme for spin filters is proposed by studying the coherent transport of electrons through quantum wires with lateral magnetic modulation. Unlike other schemes reported in the literature, the modulation in our scheme is much weaker than the Fermi energy. Large spin polarization through the filter is predicted. Further study suggests the robustness of this spin filter.

Density of states and Fermi level of a periodically modulated two-dimensionalelectron gas

Explicit analytic expressions are obtained for the density of statesD(E) and Fermienergy EFof a two-dimensional electron gas in the presence of a weak and periodic unidirectionalelectric or magnetic modulation and of a uniform perpendicular magnetic fieldB.The Landau levels broaden into bands and their width,proportional to the modulation strength, oscillates withB and gives rise to Weissoscillations in D(E),EFand the transport coefficients. When both electric and magnetic modulations arepresent the position of the resulting oscillations depends on the ratio δbetween the two modulation strengths. When the modulations are out ofphase there is no shift in the position of the oscillations when δ varies and for a particularvalue of δthe oscillations are suppressed.

Even-odd transition in the Shubnikov–de Haas oscillations in a two-dimensional electron gas subjected to periodic magnetic and electric modulations

We investigate low-temperature magnetotransport of high-mobility two-dimensional electron gases subjected to one-dimensional periodic magnetic and electric modulations. Our previous quantum perturbation theory is extended to lower temperatures and the energy broadening due to impurity scattering is incorporated. Numerical calculations are made for situations where several Landau bands overlap. We find that the Shubnikov--de Haas (SdH) oscillations are dominated by collisional resistance. The amplitudes of the SdH oscillations are strongly modulated and the positions of the SdH minima switch between even and odd Landau-level filling factors, in the resistance both parallel and perpendicular to the one-dimensional modulation. This is a consequence of the internal structure (i.e., smeared out van Hove singularities) of overlapping Landau bands. Our theoretical results are in good agreement with recent experiments.

Investigation of the periodic magnetic field modulation in LHC superconducting dipoles

The windings of high-field accelerator magnets are usually made of Rutherford-type superconducting cables. The magnetic field distribution along the axis of such magnets exhibits a periodic modulation with a wavelength equal to the twist pitch length of the cable used in the winding. This effect, resulting from quasipersistent currents, was investigated with a Hall probe array inserted inside the aperture of the LHC superconducting dipoles, both in short models and full-scale prototypes. The amplitude and the time dependence of this periodic field oscillation have been studied as a function of the magnet current history. The origin and, the impact on the LHC dipole stability of the nonuniform current redistribution producing such a field modulation are discussed.

Magnetoresistance oscillations in a periodic magnetic field due to internal Landau band structure

We present magneto-transport results for a two-dimensional electron gas subjected to a one-dimensional periodic magnetic modulation of varying amplitude. Commensurability and Shubnikov–de Haas oscillations are found to co-exist over a wide range of Landau level filling factors and modulation amplitudes. The Shubnikov–de Haas oscillations are strongly modulated in amplitude and show switches in phase. The observations are shown to be due to the internal structure of overlapping Landau bands.

Electrical transport of 2D electrons in non-uniform magnetic fields

We present a review of some of our recent experimental work on hybrid ferromagnet/semiconductor devices in which 2D electrons propagate through magnetic barriers, periodic magnetic modulations, and random magnetic fields.

Magnetoresistance oscillations due to internal Landau band structure of a two-dimensional electron system in a periodic magnetic field

We report alternative types of magnetoresistance oscillations in high mobility two-dimensional electron systems subjected to large amplitude one-dimensional periodic magnetic modulations, of period 500 nm to 1 \ensuremath{\mu}m. We observe Shubnikov--de Haas oscillations that are strongly modified in amplitude and phase, Hall resistance oscillations, and aperiodic magnetoresistance oscillations. These effects are shown to arise from the internal structure of overlapping Landau bands and are well accounted for by perturbation calculations.