arXiv subjects

K. W. Kim

Publications and source records attributed to K. W. Kim.

138 records · Page 8Linked to original sources

arXiv2002

T-shaped spin filter with a ring resonator

A planar ballistic structure is predicted to be highly effective in filtering electron spin from an unpolarized source into two output fluxes with the opposite and practically pure spin polarizations. The operability of the proposed device relies on the peculiar spin-dependent transmission properties of the T-shaped connector in the presence of the Rashba spin-orbit interaction as well as the difference in the dynamic phase gains of the two alternative paths around the ring resonator through upper and lower branches for even and odd eigenmodes.

cond-mat.mes-hall↗

arXiv2001

Spin relaxation of conduction electrons in bulk III-V semiconductors

Spin relaxation time of conduction electrons through the Elliot-Yafet, D'yakonov-Perel and Bir-Aronov-Pikus mechanisms is calculated theoretically for bulk GaAs, GaSb, InAs and InSb of both $n$- and $p$-type. Relative importance of each spin relaxation mechanism is compared and the diagrams showing the dominant mechanism are constructed as a function of temperature and impurity concentrations. Our approach is based upon theoretical calculation of the momentum relaxation rate and allows understanding of the interplay between various factors affecting the spin relaxation over a broad range of temperature and impurity concentration.

cond-mat.mtrl-sci↗

arXiv2001

Pseudogap formation of four-layer BaRuO$_3$ and its electrodynamic response changes

We investiaged the optical properties of four-layer BaRuO$_{3}$, which shows a fermi-liquid-like behavior at low temperature. Its optical conductivity spectra clearly displayed the formation of a pseudogap and the development of a coherent peak with decreasing temperature. Temperature-dependences of the density $n$ and the scattering rate $1/τ$ of the coherent component were also derived. As the temperature decreases, both $n$ and $1/τ$ decrease for four-layer BaRuO$_{3}$. These electrodynamic responses were compared with those of nine-layer BaRuO$_{3}$, which also shows a pseudogap formation but has an insulator-like state at low temperature. It was found that the relative rates of change of both $n$ and $1/τ$ determine either metallic or insulator-like responses in the ruthenates. The optical properties of the four-layer ruthenate were also compared with those of other pseudogap systems, such as high $T_{c}$ cuprates and heavy electron systems.

cond-mat.str-el↗

arXiv2001

Orchestrating an NMR quantum computation: the N=3 Deutsch-Jozsa algorithm

A detailed description of the development of a three qubit NMR realization of the Deutsch-Jozsa algorithm [Collins et.al., Phys. Rev. A 62, 022304 (2000)] is provided. The theoretical and experimental techniques used for the reduction of the algorithm's evolution steps into a sequence of NMR pulses are discussed at length. This includes the description of general pulse sequence compilation techniques, various schemes for indirectly coupled gate realizations, experimental pulse parameterization techniques and bookkeeping methods for pulse phases.

quant-ph↗

arXiv2001

Far-infrared transmission studies of c-axis oriented superconducting MgB2 thin film

We reported far-infrared transmission measurements on a c-axis oriented superconducting MgB$_{2}$ thin film in the frequency range of 30 $\sim$ 250 cm$^{-1}$. We found that these measurements were sensitive to values of scattering rate $1/τ$ and superconducting gap $2Δ$. By fitting the experimental transmission spectra at 40 K and below, we obtained $1/τ=$ (700 $\sim$ 1000) cm$^{-1}$ and $2Δ(0)\cong$ 42 cm$^{-1}$. These two quantities suggested that MgB$_{2}$ belong to the dirty limit.

cond-mat.supr-con↗

arXiv2000

Applicability of Fermi golden rule and possibility of low-field runaway transport in nitrides

In order to justify applicability of the standard approach of perturbation theory for the description of transport phenomena in wide-band polar semiconductors with strong electron-phonon interactions, we have compared dependences of energy losses to the lattice on the electron drift velocity obtained for different materials in the frameworks of (a) a perturbative approach based on calculation of the scattering rates from Fermi's golden rule and (b) a non-perturbative approach based on the path-integral formalism of Thornber and Feynman. Our results reveal that despite strong electron-phonon coupling in GaN and AlN such that intercollision times become of the order of the period of phonon oscillation, standard perturbative treatment can still be applied successfully for this type of material. Our findings also indicate possibility for unique long-distance runaway transport in nitrides which may occur at the pre-threshold electric fields. Polaron ground state energy and effective masses are calculated for GaN and AlN as well as for GaAs and Al_2 O_3.

cond-mat↗

arXiv2000

A scalable solid-state quantum computer based on quantum dot pillar structures

We investigate an optically driven quantum computer based on electric dipole transitions within coupled single-electron quantum dots. Our quantum register consists of a freestanding n-type pillar containing a series of pair wise coupled asymmetric quantum dots, each with a slightly different energy structure, and with grounding leads at the top and bottom of the pillar. Asymmetric quantum wells confine electrons along the pillar axis and a negatively biased gate wrapped around the center of the pillar allows for electrostatic confinement in the radial direction. We self-consistently solve coupled Schrodinger and Poisson equations and develop a design for a three-qubit quantum register. Our results indicate that a single gate electrode can be used to localize a single electron in each of the quantum dots. Adjacent dots are strongly coupled by electric dipole-dipole interactions arising from the dot asymmetry, thus enabling rapid computation rates. The dots are tailored to minimize dephasing due to spontaneous emission and phonon scattering and to maximize the number of computation cycles. The design is scalable to a large number of qubits.

cond-mat↗

arXiv1999

Cerenkov generation of high-frequency confined acoustic phonons in quantum wells

We analyze the Cerenkov emission of high-frequency confined acoustic phonons by drifting electrons in a quantum well. We find that the electron drift can cause strong phonon amplification (generation). A general formula for the gain coefficient, alpha, is obtained as a function of the phonon frequency and the structure parameters. The gain coefficient increases sharply in the short-wave region. For the example of a Si/SiGe/Si device it is shown that the amplification coefficients of the order of hundreds of 1/cm can be achieved in the sub-THz frequency range.

cond-mat.mes-hall↗

arXiv1999

Progressive suppression of spin relaxation in 2D channels of finite width

We have investigated spatio-temporal kinetics of electron spin polarization in semiconductor narrow 2D strip and explored the ability to manipulate spin relaxation. Information about spin of the conduction electrons and mechanisms of spin rotation is incorporated into transport Monte Carlo simulation program. A model problem, involving linear-in-k splitting of the conduction band, responsible for the D'yakonov-Perel' mechanism of spin relaxation in the zinc-blende semiconductors and heterostructures, is solved numerically to yield the decay of spin polarization of an ensemble of electrons in the 2D channel of finite width. For very wide channels, a conventional 2D value of spin relaxation is obtained. With decreasing channel width the relaxation time soares rapidly by orders of magnitude. Surprisingly, the cross-over point between 2D and quasi-1D behavior is found to be at tens of electron mean-free paths. Thus, classically wide channels can effectively suppress electron spin relaxation.

cond-mat.mes-hall↗

arXiv1999

NMR quantum computation with indirectly coupled gates

An NMR realization of a two-qubit quantum gate which processes quantum information indirectly via couplings to a spectator qubit is presented in the context of the Deutsch-Jozsa algorithm. This enables a successful comprehensive NMR implementation of the Deutsch-Jozsa algorithm for functions with three argument bits and demonstrates a technique essential for multi-qubit quantum computation.

quant-ph↗

arXiv1999

An optically driven quantum dot quantum computer

We propose a quantum computer structure based on coupled asymmetric single-electron quantum dots. Adjacent dots are strongly coupled by means of electric dipole-dipole interactions enabling rapid computation rates. Further, the asymmetric structures can be tailored for a long coherence time. The result maximizes the number of computation cycles prior to loss of coherence.

quant-ph↗

arXiv1998

Deutsch-Jozsa algorithm as a test of quantum computation

A redundancy in the existing Deutsch-Jozsa quantum algorithm is removed and a refined algorithm, which reduces the size of the register and simplifies the function evaluation, is proposed. The refined version allows a simpler analysis of the use of entanglement between the qubits in the algorithm and provides criteria for deciding when the Deutsch-Jozsa algorithm constitutes a meaningful test of quantum computation.

quant-ph↗