Abstract-Terahertz electrodynamics and superconducting energy gap of NbN

Kyung Ik Sim, Young Chan Jo, Taewoo Ha, Jong Hyeon Kim, Jae Hoon Kim, Hirotake Yamamori

https://link.springer.com/article/10.3938%2Fjkps.71.571

We have measured the transmission spectra of the conventional Bardeen-Cooper-Schrieffer (BCS) superconductor niobium nitride (NbN) thin films (Tc = 11 K) using terahertz time-domain spectroscopy (THz-TDS) over the spectral range of 10 - 110 cm −1 and the temperature range of 3.9 - 295 K. We extracted both the real part, σ1, and the imaginary part, σ2, of the optical conductivity, σ̃ = σ1 + iσ2, independently and simultaneously, without a Kramers-Kronig analysis. The superconducting gap Δ(T) was observed in the real part of the conductivity, σ1, below Tc = 11 K with a maximum value of 2Δ(0) = 30 cm −1 and the gap ratio 2Δ(0)/kBTc = 3.92.

Abstract-Terahertz single conductance quantum and topological phase transitions in topological insulator Bi2Se3 ultrathin films.

Byung Cheol Park, Tae-Hyeon Kim, Kyung Ik Sim, Boyoun Kang, Jeong Won Kim, Beongki Cho,Kwang-Ho Jeong, Mann-Ho Cho, Jae Hoon Kim

http://www.pubfacts.com/detail/25775141/Terahertz-single-conductance-quantum-and-topological-phase-transitions-in-topological-insulator-Bi2S

Strong spin-orbit interaction and time-reversal symmetry in topological insulators generate novel quantum states called topological surface states. Their study provides unique opportunities to explore exotic phenomena such as spin Hall effects and topological phase transitions, relevant to the development of quantum devices for spintronics and quantum computation. Although ultrahigh-vacuum surface probes can identify individual topological surface states, standard electrical and optical experiments have so far been hampered by the interference of bulk and quantum well states. Here, with terahertz time-domain spectroscopy of ultrathin Bi2Se3 films, we give evidence for topological phase transitions, a single conductance quantum per topological surface state, and a quantized terahertz absorbance of 2.9% (four times the fine structure constant). Our experiment demonstrates the feasibility to isolate, detect and manipulate topological surface states in the ambient at room temperature for future fundamental research on the novel physics of topological insulators and their practical applications.

Affiliation

Department of Physics, Yonsei University, Seoul 120-749, Republic of Korea.

Abstract-Double Fano resonances in a composite metamaterial possessing tripod plasmonic resonances

Y U Lee1, E Y Choi1, E S Kim1, J H Woo1, B Kang1, J Kim1, Byung Cheol Park2, T Y Hong2, Jae Hoon Kim2 and J W Wu1

http://iopscience.iop.org/2040-8986/17/2/025103

Hide affiliations

jwwu@ewha.ac.kr

1 Department of Physics and Quantum Metamaterials Research Center, Ewha Womans University, Seoul 120-750, Korea
2 Department of Physics, Yonsei University, Seoul 120-749, Korea 

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By embedding four-rod resonators inside a double-split ring resonator superlattice, a planar composite metamaterial possessing tripod plasmonic resonances is fabricated. Double Fano resonances are observed where a common subradiant driven oscillator is coupled with two superradiant oscillators. As a classical analogue of a four-level tripod atomic system, the extinction spectrum of the composite metamaterial exhibits a coherent effect based on double Fano resonances. Transfer of the absorbed power between two orthogonal superradiant oscillators is shown to be mediated by the common subradiant oscillator.

Abstract-Terahertz and optical study of monolayer graphene processed by plasma oxidation

Choi¹, Juhwan Lim², J. R. Rani², Hyong Seo Yoon², Juyeong Oh², Taeyoon Hong¹, Taewoo Ha¹, Byung Cheol Park¹, Kyung Ik Sim¹, Seong Chan Jun², and Jae Hoon Kim¹

¹Department of Physics and Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea
²Department of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea 

http://apl.aip.org/resource/1/applab/v102/i13/p131901_s1?isAuthorized=no

We report on our terahertz and optical study of monolayer graphene grown by chemical vapor deposition and processed by plasma oxidation. The plasma oxidation induces oxygen-related defects, and the resulting disorder increases the sheet resistance of graphene as measured via terahertz spectroscopy. The excitonic absorption peak weakens considerably and blue shifts upon plasma oxidation, resulting in higher transmittance in both the visible and ultraviolet regions. Our oxygen plasma-treated graphene also exhibits a free-carrier doping effect as confirmed by the blue shift of the Raman G band.

© 2013 American Institute of Physics