Youngseok Kim

Probing unconventional superconductivity in inversion-symmetric doped Weyl semimetal

Unconventional superconductivity has been predicted to arise in the topologically nontrivial Fermi surface of doped inversion-symmetric Weyl semimetals (WSMs). In particular, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and nodal BCS states are theoretically predicted to be possible superconductor pairing states in inversion-symmetric doped WSMs. In an effort to resolve the preferred pairing state, we theoretically study two separate four-terminal quantum transport methods that each exhibit a unique electrical signature in the presence of FFLO and nodal BCS states in doped WSMs. We first introduce a Josephson junction that consists of a doped WSM and an

Pseudospin Transfer Torques in Semiconductor Electron Bilayers

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Electronic transport in a two-dimensional superlattice engineered via self-assembled nanostructures

-wave superconductor in which we show that the application of a transverse uniform current in

Fulde-Ferrell states in inverse proximity-coupled magnetically doped topological heterostructures

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Topological Excitonic Superfluids in Three Dimensions

-wave superconductors effectively cancels the momentum carried by FFLO states in doped WSMs. From our numerical analysis, we find a peak in Josephson current amplitude at finite uniform current in

Modeling of black phosphorus vertical TFETs without chemical doping for drain

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Interlayer transport in disordered semiconductor electron bilayers

-wave superconductors that serves as an indicator of FFLO states in doped WSMs. Furthermore, we show using a four-terminal measurement configuration that the nodal points may be shifted by an application of transverse uniform current in doped WSMs. We analyze the topological phase transitions induced by nodal pair annihilation in nonequilibrium by constructing the phase diagram and we find a characteristic decrease in the density of states that serves as a signature of the quantum critical point in the topological phase transition, thereby identifying nodal BCS states in doped WSMs.

OPTICAL PROPERTIES OF ZINC-BLENDE CDSE AND ZNXCD1-XSE FILMS GROWN ON GAAS

In this paper we address interlayer transport in separately contacted nanometer length scale semiconductor bilayers, with a view toward the identification of possible interaction-induced collective transport effects. When interaction effects are neglected a nanoscale conductor always reaches a steady state 1 in which current increases smoothly with bias voltage. Nanoscale transport theory has at its heart the evaluation of the density-matrix of non-interacting electrons in contact with two or more reservoirs whose chemical potentials differ. This problem is efficiently solved using Green’s function techniques, for example by using the non-equilibrium Green’s function (NEGF) method 1 . Real electrons interact, of course, and the free-fermion degrees of freedom which appear in this type of theory should always be thought of as Fermi liquid theory 2–4 quasiparticles. The effective singleparticle Hamiltonian therefore depends on the microscopic configuration of the system. In practice the quasiparticle Hamiltonian is often 5;6 calculated from a selfconsistent mean-field theory like Kohn-Sham densityfunctional-theory (DFT). DFT, spin-density functional theory, current-density functional theory, Hartree theory, and Hartree-Fock theory all function as useful fermion self-consistent-field theories. Since a bias voltage changes the system density matrix, it inevitably changes the quasiparticle Hamiltonian. Determination of the steady state density matrix therefore requires a self-consistent calculation. Self-consistency is included routinely in NEGF simulations 7–9 of transport at the Hartree theory level in

Spectroscopic ellipsometry study of the diluted magnetic semiconductor system Zn(Mn,Fe,Co)Se.

Nanoscience offers a unique opportunity to design modern materials from the bottom up via low-cost, solution processed assembly of nanoscale building blocks. These systems promise electronic band structure engineering using not only the nanoscale structural modulation, but also the mesoscale spatial patterning, although experimental realization of the latter has been challenging. Here, we design and fabricate a new type of artificial solid by stacking graphene on a self-assembled, nearly periodic array of nanospheres, and experimentally observe superlattice miniband effects. We find conductance dips at commensurate fillings of charge carriers per superlattice unit cell, which are key features of minibands that are induced by the quasi-periodic deformation of the graphene lattice. These dips become stronger when the lattice strain is larger. Using a tight-binding model, we simulate the effect of lattice deformation as a parameter affecting the inter-atomic hopping integral, and confirm the superlattice transport behavior. This 2D material-nanoparticle heterostructure enables facile band structure engineering via self-assembly, promising for large-area electronics and optoelectronics applications.Electronic transport: a 2D artificial solid shows superlattice miniband effectsA hybrid heterostructure of self-assembled nanoparticles on graphene displays conductance dips due to superlattice miniband effects. A team led by Yingjie Zhang and Nadya Mason at the University of Illinois designed and fabricated a large-area artificial solid combining self-assembled dielectric nanospheres and graphene grown by chemical vapor deposition. Taking advantage of the structural versatility of SiO2 nanoparticle assembly and the high mobility of graphene, the resulting heterostructure displays miniband effects in the electronic transport characteristics as a result of superlattice formation. Due to the size variability and imperfect ordering of the nanospheres, a broadening was observed in the miniband density of states. This effect can find an application in optoelectronic devices, such as optical modulators and sensor, requiring broadband modulation of their optical spectrum.

Spectroscopic ellipsometry study of Zn1-xCoxSe alloys grown on GaAs.

We study the superconducting properties of the thin film BCS superconductor proximity coupled to a magnetically doped time-reversal invariant topological insulator (TI). Using mean-field theory, we show that Fulde-Ferrell (FF) pairing can be induced in the conventional superconductor through the inverse proximity effect (IPE). This occurs when the IPE of the TI to the superconductor is large enough that the normal bands of the superconductor possess a proximity induced spin-orbit coupling and magnetization. We find that the energetics of the different pairings are dependent on the coupling strength between the TI and the BCS superconductor and the thickness of the superconductor film. As the thickness of the superconductor film is increased, we find a crossover from the FF pairing to the BCS pairing phase. This is a consequence of the increased number of the superconducting bands, which favor the BCS pairing, implying that the FF phase can only be observed in the thin film limit. In addition, we also propose transport experiments that show distinct signatures of the FF phase.