arXiv subjects

Ki Wook Kim

Publications and source records attributed to Ki Wook Kim.

At least 19 recordsLinked to original sources

arXiv2023

Voltage Control of Electromagnetic Properties in Antiferromagnetic Materials

Dynamic modulation of electromagnetic responses is theoretically examined in dielectric antiferromagnets. While both magneto-electric and magneto-elastic coupling can achieve robust electrical control of magnetic anisotropy, the latter is considered in a bilayer structure with a piezoelectric material. Numerical calculations based on the frequency-dependent permeability tensor clearly illustrate that the anisotropy profile in the typical uniaxial or biaxial antiferromagnets such as NiO and Cr2O3 can be modified sufficiently to induce a shift in the resonance frequency by as much as tens of percent in the sub-mm wavelength range (thus, an electrically tunable bandwidth over 10's of GHz). The polarization of the electromagnetic response is also affected due to the anisotropic nature of the effect, offering a possibility to encode the signal. The intrinsic delay in switching may be minimized to the ns level by using a sufficiently thin antiferromagnets. Application to specific devices such as a band-pass filter further illustrates the validity of the concept.

cond-mat.mtrl-sci↗

arXiv2023

Electronic properties of c-BN/diamond heterostructures for high-frequency high-power applications

Using first principles calculations, this work investigates the suitability of diamond/c-BN heterojunctions for high frequency, high power device applications. The key quantities of band offsets and interface charge polarization are examined for different crystallographic orientations [(110), (111), or (100)], bond terminations (C-B or C-N), and substrates (diamond or c-BN). The results indicate that both the (111) and (100) structures with polar interfaces are likely to be a type-I alignment with the diamond conduction and valence band extrema nested within the c-BN bandgap, whereas the non-polar (110) counterpart may form type II as the valence band of c-BN is shifted down substantially lower. The (111) and (100) structures also show net charge polarization in a narrow region at the interface. The electron-deficient and electron-rich nature of the C-B and C-N bonding are found to induce charge redistribution leading to an essentially 2D sheet of negative and positive polarization. With the predicted band alignments suitable for carrier confinement as well as the possibility of the modulation and polarization doping, the diamond/c-BN heterostructures are a promising candidate for high-performance electronic devices with a highly conductive 2D channel. Both p-type and n-type devices appear possible with a judicious choice of the heterojunction configuration.

cond-mat.mtrl-sci↗

arXiv2020

Spin Wave Generation via Localized Spin-Orbit Torque in an Antiferromagnet-Topological Insulator Heterostructure

The spin-orbit torque induced by a topological insulator (TI) is theoretically examined for spin wave generation in a neighboring antiferromagnetic thin film. The investigation is based on the micromagnetic simulation of Néel vector dynamics and the analysis of transport properties in the TI. The results clearly illustrate that propagating spin waves can be achieved in the antiferromagnetic thin-film strip through localized excitation, traveling over a long distance. The oscillation amplitude gradually decays due to the non-zero damping as the Néel vector precesses around the magnetic easy axis with a fixed frequency. The frequency is also found to be tunable via the strength of the driving electrical current density. While both the bulk and the surface states of the TI contribute to induce the effective torque, the calculation indicates that the surface current plays a dominant role over the bulk counterpart except in the heavily degenerate cases. Compared to the more commonly applied heavy metals, the use of a TI can substantially reduce the threshold current density to overcome the magnetic anisotropy, making it an efficient choice for spin wave generation. The Néel vector dynamics in the nano-oscillator geometry are examined as well.

cond-mat.mtrl-sci↗

arXiv2019

Electrical generation and propagation of spin waves in antiferromagnetic thin films

Electrical generation of THz spin waves is theoretically explored in an antiferromangetic nanostrip via the current-induced spin-orbit torque. The analysis based on micromagnetic simulations clearly illustrates that the Neel-vector oscillations excited at one end of the magnetic strip can propagate in the form of a traveling wave when the nanostrip axis aligns with the magnetic easy-axis. A sizable threshold is observed in the driving current density or the torque to overcome the unfavorable anisotropy as expected. The generated spin waves are found to travel over a long distance while the angle of rotation undergoes continuous decay in the presence of non-zero damping. The oscillation frequency is tunable via the strength of the spin-orbit torque, reaching the THz regime. Other key characteristics of the spin waves such as the phase and the chirality can also be modulated actively. The simulation results further indicate the possibility of wave-like superposition between the excited spin oscillations, illustrating its application as an efficient source of spin-wave signals for information processing.

cond-mat.mes-hall↗

arXiv2019

Controllable dispersion of domain wall movement in antiferromagnetic thin films at finite temperatures

The dynamics of a 90$^{\circ }$ domain wall in an antiferromagnetic nanostrip driven by the current-induced spin-orbital torque are theoretically examined in the presence of random thermal fluctuations. A soliton-type equation of motion is developed on the basis of energy balance between the driving forces and dissipative processes in terms of the domain wall velocity. Comparison with micromagnetic simulations in the deterministic conditions shows good agreement in both the transient and steady-state transport. When the effects of random thermal fluctuations are included via a stochastic treatment, the results clearly indicate that the dispersion in the domain wall position can be controlled electrically by tailoring the strength and duration of the driving current mediating the spin orbital torque in the antiferromagnet. More specifically, the standard deviation of the probability distribution function for the domain wall movement can be tuned widely while maintaining the average position unaffected. Potential applications of this unusual functionality include the probabilistic computing such as Bayesian learning.

cond-mat.mtrl-sci↗

arXiv2019

Thermal fluctuations in antiferromagnetic nanostructures

A theoretical model is developed that can accurately analyze the effects of thermal fluctuations in antiferromagnetic (AFM) nano-particles. The approach is based on Fourier series representation of the random effective field with cut-off frequencies of physical origin at low and high limits while satisfying the fluctuation-dissipation theorem at the same time. When coupled with the formalism of a Langevin dynamical equation, it can describe the stochastic Néel vector dynamics with the AFM parameters, circumventing the arbitrariness of the commonly used treatments in the micro-magnetic simulations. Subsequent application of the model to spontaneous Néel vector switching provides a thermal stability analysis of the AFM states. The numerical simulation shows that the AFM states are much less prone to the thermally induced accidental flips than the ferromagnetic counterparts, suggesting a longer retention time for the former.

cond-mat.mes-hall↗

arXiv2019

Diamond/c-BN HEMTs for power applications: A theoretical feasibility analysis

Diamond is a promising material for high-power electronic applications in both the dc and rf domains. However, the predicted advantages are yet to be realized for a number of technical challenges. In particular, n-type devices have not been feasible due to the large ionization energies and low thermodynamic solubility limits of n-dopants. Motivated by the recent advances in nonequilibrium processing, we propose and theoretically examine a diamond/c-BN HEMT that can circumvent the critical limitations. A first-principles calculation suggests the desired type-I alignment at the heterojunction of these two nearly lattice matched semiconductors. The investigation also illustrates that a large sheet carrier density in excess of $5\times10^{12}~\mathrm{cm^{-2}}$ can be induced in the undoped diamond channel by the gate bias. A subsequent analysis of a simple prototype design indicates that the proposed device can achieve large current drive (~10 A/cm), low $R_{on}$ ($\sim 0.05 ~ \mathrm{m}Ω\cdot \mathrm{cm}^2$), and high $f_T$ (~300 GHz) simultaneously.

physics.app-ph↗

arXiv2016

Currentless reversal of Néel vector in antiferromagnets

The bias driven perpendicular magnetic anisotropy is a magneto-electric effect that can realize 90$^\circ$ magnetization rotation and even 180$% ^\circ $ flip along the easy axis in the ferromagnets with a minimal energy consumption. This study theoretically demonstrates a similar phenomenon of the Néel vector reversal via a short electrical pulse that can mediate perpendicular magnetic anisotropy in the antiferromagnets. The analysis based on the dynamical equations as well as the micro-magnetic simulations reveals the important role of the inertial behavior in the antiferromagnets that facilitates the Néel vector to overcome the barrier between two free-energy minima of the bistable states along the easy axis. In contrast to the ferromagnets, this Néel vector reversal does not accompany angular moment transfer to the environment, leading to acceleration in the dynamical response by a few orders of magnitude. Further, a small switching energy requirement of a few attojoules illustrates an added advantage of the phenomenon in low-power spintronic applications.

cond-mat.mes-hall↗

arXiv2016

Highly anisotropic electronic transport properties of monolayer and bilayer phosphorene from first principles

The intrinsic carrier transport dynamics in phosphorene is theoretically examined. Utilizing a density functional theory treatment, the low-field mobility and the saturation velocity are characterized for both electrons and holes in the monolayer and bilayer structures. The analysis clearly elucidates the crystal orientation dependence manifested through the anisotropic band structure and the carrier-phonon scattering rates. In the monolayer, the hole mobility in the armchair direction is estimated to be approximately five times larger than in the zigzag direction at room temperature (460 cm$^2$/Vs vs. 90 cm$^2$/Vs). The bilayer transport, on the other hand, exhibits a more modest anisotropy with substantially higher mobilities (1610 cm$^2$/Vs and 760 cm$^2$/Vs, respectively). The calculations on the conduction-band electrons indicate a comparable dependence while the characteristic values are generally smaller by about a factor of two. The variation in the saturation velocity is found to be less pronounced. With the anticipated superior performance and the diminished anisotropy, few-layer phosphorene offers a promising opportunity particularly in p-type applications.

cond-mat.mes-hall↗

arXiv2015

Conductance Discontinuity on the Surface of a Topological Insulator with Magnetic Electrodes

Asymmetric electrical conductance is theoretically demonstrated on the surface of a topological insulator (TI) in the limit of infinitesimally small forward and reverse biases between two spin selective electrodes. The discontinuous behavior relies on the spin-momentum interlocked nature of TI surface electrons together with the resulting imbalance in the coupling coefficients between the electrodes and TI surface states. The analysis is based on a transmission matrix model that, in combination with a phenomenological treatment for the diffusive limit, accounts for both ballistic and scattered paths simultaneously. With the estimated conductance asymmetry over a factor of 10, implementation in the ratchet-like applications and low-voltage rectification circuits appears practicable.

cond-mat.mes-hall↗

arXiv2015

Efficient Interlayer Relaxation and Transition of Excitons in Epitaxial and Non-epitaxial MoS2/WS2 Heterostructures

Semiconductor heterostructures provide a powerful platform for the engineering of excitons. Here we report the excitonic properties of two-dimensional (2D) heterostructures that consist of monolayer MoS2 and WS2 stacked epitaxially or non-epitaxially in the vertical direction. We find similarly efficient interlayer relaxation and transition of excitons in both the epitaxial and nonepitaxial heterostructures. This is manifested by a two orders of magnitude decrease in the photoluminescence and the appearance of an extra absorption peak at low energy region. The MoS2/WS2 heterostructures show weak interlayer coupling and can essentially act as atomicscale heterojunctions with the intrinsic bandstructures of the two monolayers largely preserved. They are particularly promising for the applications that request efficient dissociation of excitons and strong light absorption, including photovoltaics, solar fuels, photodetectors, and optical modulators. Our results also indicate that 2D heterostructures promise unprecedented capabilities to engineer excitons from the atomic level without concerns of interfacial imperfection.

cond-mat.mtrl-sci↗

arXiv2014

Spin Logic via Controlled Correlation in Nanomagnet-Dirac Fermion Heterostructures

A hybrid structure combining the advantages of topological insulator (TI), dielectric ferromagnet (FM), and graphene is investigated to realize the electrically controlled correlation between electronic and magnetic subsystems for low-power, high-functional applications. Two-dimensional Dirac fermion states provide an ideal environment to facilitate strong coupling through the surface interactions with proximate materials. The unique properties of FM-TI and FM-graphene interfaces make it possible for active "manipulation" and "propagation", respectively, of the information state variable based solely on the spin logic platform through electrical gate biases. Our theoretical analysis verifies the feasibility of the concept for logic application with both current-driven and current-less interconnect approaches. The device/circuit characteristics are also examined in realistic conditions, suggesting the desired low-power performance with the estimated energy consumption for COPY/NOT as low as the \textit{attojoule} level.

cond-mat.mes-hall↗

arXiv2014

Thermal transport properties of metal/MoS2 interfaces from first principles

Thermal transport properties at the metal/MoS2 interfaces are analyzed by using an atomistic phonon transport model based on the Landauer formalism and first-principles calculations. The considered structures include chemisorbed Sc(0001)/MoS2 and Ru(0001)/MoS2, physisorbed Au(111)/MoS2, as well as Pd(111)/MoS2 with intermediate characteristics. Calculated results illustrate a distinctive dependence of thermal transfer on the details of interfacial microstructures. More specifically, the chemisorbed case with a stronger bonding exhibits a generally smaller interfacial thermal resistance than the physisorbed. Comparison between metal/MoS2 and metal/graphene systems suggests that metal/MoS2 is significantly more resistive. Further examination of lattice dynamics identifies the presence of multiple distinct atomic planes and bonding patterns at the interface as the key origin of the observed large thermal resistance.

cond-mat.mes-hall↗

arXiv2014

Intrinsic Transport Properties of Electrons and Holes in Monolayer Transition Metal Dichalcogenides

Intrinsic electron- and hole-phonon interactions are investigated in monolayer transition metal dichalcogenides MX$_2$ (M=Mo,W; X=S,Se) based on a density functional theory formalism. Due to their structural similarities, all four materials exhibit qualitatively comparable scattering characteristics with the acoustic phonons playing a dominant role near the conduction and valence band extrema at the K point. However, substantial differences are observed quantitatively leading to disparate results in the transport properties. Of the considered, WS$_2$ provides the best performance for both electrons and holes with high mobilities and saturation velocities in the full-band Monte Carlo analysis of the Boltzmann transport equation. It is also found that monolayer MX$_2$ crystals with an exception of MoSe$_2$ generally show hole mobilities comparable to or even larger than the value for bulk silicon at room temperature, suggesting a potential opportunity in p-type devices. The analysis is extended to estimate the effective deformation potential constants for a simplified treatment as well.

cond-mat.mes-hall↗

arXiv2014

Temperature Dependent Valley Relaxation Dynamics in Single Layer WS2 Measured Using Ultrafast Spectroscopy

We measured the lifetime of optically created valley polarization in single layer WS2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS2. One process involves direct scattering of excitons from K to K' valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Gamma valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Gamma and K valleys.

cond-mat.mtrl-sci↗

arXiv2013

Controlling electron propagation on a topological insulator surface via proximity interactions

The possibility of electron beam guiding is theoretically explored on the surface of a topological insulator through the proximity interaction with a magnetic material. The electronic band modification induced by the exchange coupling at the interface defines the path of electron propagation in analogy to the optical fiber for photons. Numerical simulations indicate the guiding efficiency much higher than that in the "waveguide" formed by an electrostatic potential barrier such as p-n junctions. Further, the results illustrate effective flux control and beam steering that can be realized by altering the magnetization/spin texture of the adjacent magnetic materials. Specifically, the feasibility to switch on/off and make a large-angle turn is demonstrated under realistic conditions. Potential implementation to logic and interconnect applications is also examined in connection with electrically controlled magnetization switching.

cond-mat.mes-hall↗

arXiv2013

Intrinsic Electrical Transport Properties of Monolayer Silicene and MoS2 from First Principles

The electron-phonon interaction and related transport properties are investigated in monolayer silicene and MoS2 by using a density functional theory calculation combined with a full-band Monte Carlo analysis. In the case of silicene, the results illustrate that the out-of-plane acoustic phonon mode may play the dominant role unlike its close relative - graphene. The small energy of this phonon mode, originating from the weak sp2 bonding between Si atoms, contributes to the high scattering rate and significant degradation in electron transport. In MoS2, the longitudinal acoustic phonons show the strongest interaction with electrons. The key factor in this material appears to be the Q valleys located between the Γ and K points in the first Brillouin zone as they introduce additional intervalley scattering. The analysis also reveals the potential impact of extrinsic screening by other carriers and/or adjacent materials. Subsequent decrease in the actual scattering rate can be drastic, warranting careful consideration. Finally, the effective deformation potential constants are extracted for all relevant intrinsic electron-phonon scattering processes in both materials.

cond-mat.mes-hall↗

arXiv2012

Tunable photo-galvanic effect on topological insulator surfaces via proximity interactions

An unusual photo-galvanic effect is predicted on the topological insulator surface when its semi-metallic electronic spectrum is modified by an adjacent ferromagnet. The effect is correlated with light absorption in a wide frequency range (from a few to hundreds of meV) and produces a pronounced response that is not only resonant to the photon energy but also tunable by an external electrical bias. The exceptionally strong peak photocurrent of the order of $μ$A/cm may be achieved at elevated temperatures with the illumination power of 1 W/cm$^2$ in the THz range on Bi$_2$Se$_3$. These advantages could enable room-temperature detection of far-infrared radiation.

cond-mat.mes-hall↗