날짜 : 04월 30일 화요일

시간 : 오후 4시 30분

장소 : R404

발표자 : 윤규석 (김현정 교수님)

제목 : Coherence and pulse duration characterization of the PAL-XFEL in the hard X-ray regime


초록 : We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL, Pohang, Korea). The measurement of the single-shot speckle contrast, together with the introduction of corrections considering experimental conditions, allows obtaining an intrinsic degree of transverse coherence of 0.85 ± 0.06. In the Self-Amplified Spontaneous Emission regime, the analysis of the intensity distribution of X-ray pulses also provides an estimate for the number of longitudinal modes. For monochromatic and pink (i.e. natural bandwidth provided by the first harmonic of the undulator) beams, we observe that the number of temporal modes is 6.0 ± 0.4 and 90.0 ± 7.2, respectively. Assuming a coherence time of 2.06 fs and 0.14 fs for the monochromatic and pink beam respectively, we estimate an average X-ray pulse duration of 12.6 ± 1.0 fs.

발표자 : 조용기 (손원민 교수님)

제목 : Enhancement of security in quantum key distribution using high-dimensional quantum systems


  In this presentation, efficient protocols for quantum key distribution which have enhanced security are investigated. These protocols exploit high-dimensional quantum states to improve security of quantum key distribution, in comparison to the conventional quantum key distribution protocols using 2-dimensional quantum states. The protocols investigated in this work provide an improved secret key rate by exploiting high-dimensional quantum systems as an information carrier, and they guarantee semi-device-independent security, which means that they can prevent a certain type of side channel attack. It is shown that these protocols allow enhanced security, not only theoretically, but also under the consideration of various experimental conditions, compared with the existing semi-device-independent quantum key distribution protocols using 2-dimensional quantum states


발표자 : 김강원 (정현식 교수님)

제목 : Suppression of magnetic ordering in XXZ-type antiferromagnetic monolayer NiPS 3

Kangwon Kim 1 , Soo Yeon Lim 1 , Jae-Ung Lee 1,2 , Sungmin Lee 3 , Tae Yun Kim 3 , Kisoo Park 3 , Gun Sang

Jeon 4 , Cheol-Hwan Park 3,* , Je-Geun Park 3,* , and Hyeonsik Cheong 1,*

Department of Physics, Sogang University, Seoul 04107, Korea
Department of Physics, Ajou University, Suwon 16499, Korea

Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea
Department of Physics, Ewha Womans University, Seoul 03760, Korea

 초록 : Magnetism in low dimension is important for both in fundamental science and application.
Transition metal phosphorus trisulfides (TMPS 3 ) are a new class of magnetic van der Waals materials
and show different magnetic ground states depending on the TM element: FePS 3 [1], NiPS 3 , and
MnPS 3 have Ising-, XXZ-, and Heisenberg-type antiferromagnetic ordering, respectively. We
investigated XXZ-type antiferromagnetic NiPS 3 in the 2D limit by Raman spectroscopy. Raman
spectroscopy is a powerful technique for studying 2D magnetic materials, as it can investigate phonon
scattering as well as magnetic scattering. Below the Néel temperature, several Raman signatures due
to antiferromagnetic phase transition are observed in the Raman spectrum of bulk NiPS 3 : 2-magnon
scattering, Fano resonance [2], suppression of the quasi-elastic scattering, and splitting of a phonon
mode. The Néel temperature (T N ) of NiPS 3 can be estimated by analyzing the temperature dependence
of these signatures. We synthesized bulk single-crystal NiPS 3 by the vapour transport method and
prepared atomically thin samples down to the monolayer by using mechanical exfoliation. We
measured temperature dependent Raman spectra of few-layer NiPS 3 samples and found that the Néel
temperature shows slightly difference from bulk for the thickness down to bilayer, but seems to be
suppressed significantly for monolayer [3]. We note that all these experimental observations are in
good agreement with the theoretical predictions of the XY model.

[1] J.-U. Lee et al., Nano Letters 16, 7433 (2016).
[2] S. Rosenblum et al., Phys. Rev. B 49, 4352 (1994).
[3] K. Kim et al., Nature Communications 10(1), 345 (2019).

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