G. Ihm

Large Current Modulation and Spin-Dependent Tunneling of Vertical Graphene/MoS2 Heterostructures

Vertical graphene heterostructures have been introduced as an alternative architecture for electronic devices by using quantum tunneling. Here, we present that the current on/off ratio of vertical graphene field-effect transistors is enhanced by using an armchair graphene nanoribbon as an electrode. Moreover, we report spin-dependent tunneling current of the graphene/MoS2 heterostructures. When an atomically thin MoS2 layer sandwiched between graphene electrodes becomes magnetic, Dirac fermions with different spins feel different heights of the tunnel barrier, leading to spin-dependent tunneling. Our finding will develop the present graphene heterostructures for electronic devices by improving the device performance and by adding the possibility of spintronics based on graphene.

Spin filtering in an electromagnetic structure

We investigate possible spin-dependent resonant tunneling through double-barrier electromagnetic structure. We point out that previous related studies contain serious errors in the calculation and the correct ones show much reduced spin polarization in the tunneling. In this study, we show that the significant amount of spin polarization with nice resonant structures can be achieved by applying a proper electric field subject to the given magnetic barriers. The electric potential U, which is crucial in our model, is chosen to exactly cancel the A2 term originated from the applied magnetic field. In this way, we can remove unwanted obstacles and expedite the tunneling of electrons.

MANY-BODY EFFECTS ON MODULATION-DOPED INAS/GAAS QUANTUM DOTS

The excitation intensity dependent photoluminescence spectra of various modulation-doped InAs/GaAs quantum dots exhibit the band filling and band-gap renormalization. From the time-resolved photoluminescence spectra, in addition to the interaction of the carriers in quantum dots with the remote ionized impurities in a GaAs barrier, the screening due to the two-dimensional charges is found to mainly affect the carrier lifetime in the modulation-doped quantum dots.

Electronic subband studies in In0.52Al0.48As/InxGa1−xAs one‐side‐modulation‐doped asymmetric coupled double quantum wells

Shubnikov–de Haas (SdH) and van der Pauw Hall effect measurements on In0.52Al0.48As/InxGa1−xAs coupled double quantum wells grown by metalorganic chemical vapor deposition have been carried out to investigate the magnetotransport properties of an electron gas and to determine the subband energies and wave functions in the coupled quantum wells. Transmission electron microscopy measurements showed that In0.8Ga0.2As and In0.53Ga0.47As quantum wells were separated by an In0.25Ga0.75As potential barrier in an active region. The SdH measurements at 1.5 K demonstrated clearly the existence of a quasi‐two‐dimensional electron gas in the quantum wells. The fast Fourier transformation results for the SdH data clearly indicate the occupation of three subbands in the In0.52Al0.48As/InxGa1−xAs coupled quantum wells. Electron subband energies and wave functions in the quantum wells were calculated by a self‐consistent method taking into account exchange‐correlation effects. The first and second excited subband wave fu...

Spin-dependent transport and spin-switching effect in graphene with magnetoelectric modulations

We investigate spin transport through magnetic potential structures in single graphene layer with the consideration of Zeeman effect. We find that the magnetic step leads to the spin-dependent transport, and the spin-polarized conductance has the oscillatory behavior as a function of fermion energy. Furthermore, the electrostatic delta-potential added to the magnetic step causes dramatic changes in the spin transport. The spin polarization can be reversed by the application of the electrostatic delta potential, leading to so called spin-switching effect. We expect that the spin-switching effect in this study gives rise to great convenience for experimental investigations on graphene spintronics.

Spin-polarized tunneling in an electromagnetic structure

We have shown that highly spin-polarized electrons can be obtained through the resonant tunneling of electromagnetic structure, which consists of triple-magnetic barriers and correspondingly designed electrostatic potential distributions. The spin dependence of the tunneling comes from the Zeeman term and thus is expected to be better observed in materials with higher g* and m*. The huge difference between spin-up and spin-down electrons in the characteristics of transmission probability is found. The proposed structure is expected to be utilized as spin-switching device as well as spin-filtering device.

Edge-channel transport through quantum wires with a magnetic quantum dot

We investigate the scattering of edge channels by a magnetic quantum dot, which is formed by two different magnetic fields B∗ and B0 inside and outside the dot, respectively, in a quantum wire. We calculate the two-terminal conductance (G) of the wire and find that for γ(=B∗/B0)>0, G is quantized, while for γ<0 it is not quantized. This feature results from the different magnetic confinements, caused by nonuniform magnetic fields, between the two cases.

Edge state formation in magnetic quantum structures

The formation of magnetic edge states is investigated for magnetic quantum structures with inhomogeneous magnetic fields such as a magnetic quantum dot (B=0 for r<r0 and B≠0 elsewhere) and a magnetic quantum ring (B=0 for r1<r<r2 and B≠0 elsewhere). It is found that the eigenenergy spectra for these structures critically depend on the number of missing flux quanta and eigenstates which are composed of bulk Landau levels and magnetic edge states. The magnetic edge states correspond well with the classical particle trajectories which circulate either clockwise or counterclockwise along the boundary between magnetic domains.

Anisotropy effects on the orbital magnetism of electrons in a spheroidal quantum dot

Anisotropy effects in the orbital magnetic susceptibility of electrons in a spheroidal quantum dot are investigated by applying tilted magnetic field with parabolic confinement potential. The results show dramatic phase change; magnetic susceptibility as a function of the electron concentration changes from para- or diamagnetic in the case of anisotropic quantum dot to diamagnetic only in the isotropic case. Temperature, field and tilted angle dependences are also extensively studied.

Time-resolved spectroscopy of InAs quantum dots using one-side modulation-doping technique: renormalization and screening

We present the optical properties of modulation-doped InAs/GaAs quantum dots (QDs). Bandgap renormalization is clearly observed from a red shift of the emission peaks on the QDs with various modulation-doping concentrations, demonstrating that exchange interactions of electrons within single states have no effect on renormalization. Screening due to the electrons at modulation-doped layer as well as the photo-generated carriers in GaAs matrix plays an important role in reducing the radiative recombination rate.