In history of development and applications of metrology, we are now facing substantially different domains in modernized metrology, which are quite challenging. The international system of units (SI) such as kelvin (K), kilogram (kg), mole (mol) and ampere (A) were redefined in 2019, based on the physical constants such as Boltzmann constant k, Plank constant h, Avogadro Number N_A and elementary charge e, respectively. In this talk, I will review basic principles of redefinitions of the four units, and deal with the realization of the unit of ampere in terms of “quantum metrology triangle” based on the quantum Hall resistance, Josephson voltage and quantized current with single electron source.

그동안 우리나라에서는 고온초전도 응용분야에 대한 많은 연구가 진행되어 왔다. 특히, 21C프론티어연구개발사업의 일환으로 2001년부터 2010년까지 진행된 차세대초전도응용기술개발사업을 통해 고온초전도 응용분야에 대한 다양한 연구개발이 수행되었으며, 그 결과 중 하나로 세계 최초로 초전도 전력케이블을 상용화 하였고, 2011년 국내 연구진에 의해 개발된 고온초전도 자석의 무절연 권선법의 등장 후, 초고자기장 고온초전도 자석의 가능성을 확인하는 다양한 연구개발 사례가 보고되었다. 무절연 고온초전도 자석의 가능성을 바탕으로 해외 선진국의 유수 연구기관들은 전기기기 제조산업 분야의 혁신을 목표로 다양한 형태의 고온초전도 응용기기 개발에 막대한 자본과 연구인력을 투입함에 따라 세계적인 기술 경쟁이 촉발되고 있다.

이러한 세계적인 흐름에 따라 2020년부터 2021년까지 과학기술정보통신부의 정책연구용역 과제로 “고온초전도 마그넷 기술 기반 국가 제조산업 혁신을 위한 중장기 전략 수립 기획연구”가 진행되었다. 앞서 언급한 차세대초전도응용기술개발사업 이후, 국내에서는 대규모 기술개발 사업이 아닌 소규모의 연구만이 진행되어, 체계적인 기술개발이 어려워졌으며 기술의 축적이 이루어지지 못하였다는 문제점이 있었다. 따라서 해당 기획연구를 바탕으로 고온초전도 자석의 중장기 전략을 수립하여, 체계적인 기술 개발을 진행하고자 하였다. 국내 고온초전도 자석 관련 산/학/연 전문가로 이루어진 기획위원회를 구성하였으며, 위원장으로 서울대학교 한승용 교수를 위촉하였다. 기획연구 과정에서 응용 분야에 따라 Material/Life/Energy 분과로 나누어 국내외 시장 및 기술 현황 분석, 필요한 기술개발의 수요조사를 수행하였다. 해당 기획연구를 통해 향후 국내 고온초전도 자석 연구개발의 중장기 전략 및 방향성을 도출하였으며, 이는 본 연구단의 비전 및 체계 수립을 위한 토대가 되었다.

중장기 전략 및 도출된 기술개발 방향을 토대로, 2022년 1월 18일 고온초전도마그넷기술개발사업의 RFP 사전공고가 진행되었으며, 수정사항을 반영하여 2022년 2월 9일부터 신규과제 공모가 진행되었다. 이후 국내 30개 산/학/연을 아우르는 기관이 컨소시엄을 구성하여 과제계획서를 동년 3월 25일에 제출하였으며, 4월 11일에 선정 평가가 진행되었고, 4월 22일 사업의 수행주체로 본 연구단(고온초전도자석 원천기술연구단)이 선정되었다.

국내 고온초전도 자석 기술이 세계적인 기술 경쟁의 우위를 유지하기 위하여, 본 연구단에서는 응용기기 중심의 기존의 연구개발 방법을 탈피하고 모든 응용분야를 망라하는 4대 형상별 자석 형태(솔레노이드, 토로이드, 레이스트랙, 새들)에 대하여 7대 기반기술(설계, 권선, 접합, 리드, 구조, 냉각, 운전/평가) 중심의 연구개발을 진행하고자 한다. 특히, 그 과정에서 고온초전도 자석의 고성능을 일정하게 유지할 수 있도록 “명품화”와 “양산화”가 가능한 기반기술 개발을 목표로 연구를 진행하고자 한다.

본 연구단은 초전도 기술의 “명품화” 및 “양산화”를 위해 다음과 같이 세 가지 핵심 역할을 수행하고자: 1) 원천기술 개발; 2) 연구 인프라 구축; 3) 인력 양성. 먼저 원천기술 개발을 위해 연구단을 3개의 기반기술 개발 과제(공통, 설계, 선재)와 4개의 형상별 핵심기술 개발 과제(솔레노이드, 토로이드, 레이스트랙, 새들)로 구성하여 가능한 모든 원천기술을 포괄할 수 있도록 연구 개발을 수행하며, 이를 위해 기반기술 개발 과제와 핵심기술 개발 과제 간의 유기적 협력체계를 구축하고 산/학/연과 물리/전기/기계/재료 등 다양한 전공분야를 통합하는 체계를 구추하였다. 마지막으로, 연구단 내 인력양성 플랫폼을 구성하여 해당 연구개발 사업 진행에 있어서 뿐만 아니라 향후 지속적인 기술의 유지·발전을 위해 인적자원의 발전을 도모하고, 이를 통해 궁극적으로는 초전도 기술의 국가 브랜드화를 위한 초석을 마련하고자 한다.

keywords : 고온초전도 자석, 무절연 권선, 원천기술

* 이 논문은 정부(과학기술정보통신부)의 재원으로 한국연구재단-고온초전도마그넷기술개발사업의 지원을 받아 수행된 연구임(2022M3I9A1073924).

Motivated by the recent discovery of superconductivity in layered nickelates [1,2], we derived effective low-energy models for NdNiO₂ from first principles [3]. We found that the electronic structure of NdNiO₂ shares a characteristic commonality with high-T_c cuprates in that the 3d_x2-y2 band forms a large two-dimensional Fermi surface. On the other hand, nickelates and cuprates have several distinct differences: The charge transfer from the oxygen p orbitals to the Ni 3d orbitals is much smaller than that in the cuprates. The 3d_x2-y2 band is not half-filled due to the charge transfer from the Nd layer to the NiO₂ layer. We then performed an extensive materials search by replacing the Nd block layer with other block layers. We found several dynamically stable Ni- and Pd-compounds whose low-energy electronic structure better mimics the electronic structure of high-T_c cuprates [4,5]. By solving the resulting low-energy models by means of the dynamical vertex approximation [6], we found that the ratio between the Coulomb interaction and the kinetic energy of Pd compounds is better for realizing high T_c superconductivity than Ni compounds [7].

keywords : Nickelate superconductors, ab initio downfolding, dynamical vertex approximation, Hubbard model

[1] D. Li et al., Nature 572, 624 (2019).

[2] Y. Nomura and R. Arita, Rep. Prog. Phys. 85 052501 (2022).

[3] Y. Nomura, M. Hirayama, T. Tadano, Y. Yoshimoto, K. Nakamura, and R. Arita, Phys. Rev. B 100, 205138 (2019).

[4] M. Hirayama, T. Tadano, Y. Nomura, and R. Arita, Phys. Rev. B 101, 075107 (2020).

[5] Y. Nomura, T. Nomoto, M. Hirayama, and R. Arita, Phys. Rev. Research 2, 043144 (2020).

[6] G. Rohringer, H. Hafermann, A. Toschi, A. A. Katanin, A. E. Antipov, M. I. Katsnelson, A. I. Lichtenstein, A. N. Rubtsov, and K. Held, Rev. Mod. Phys. 90, 025003 (2018).

[7] M. Kitatani, L. Si, P. Worm, J.M. Tomczak, R. Arita and K. Held, in prep.

* This work was supported by Grant-in-Aids for Scientific Re-search (JSPS KAKENHI) Grant No. JP19H05825.

Charge order is one of the most actively studied topics in recent cuprate superconductor research. Including the stripe order in La-based cuprates, charge orders or charge density wave modulations have been observed in most of the cuprate superconductor families, even in electron-doped cuprates. While the charge orders show ubiquitous existence, they display diverse characters among different families in same time [1]. Most of the charge orders have been characterized by X-ray scattering, in particular resonant soft X-ray scattering (RSXS) at Cu L₃-edge.

Recent technical progresses enable another challenges. High resolution resonant inelastic X-ray scattering at Cu L₃-edge has observed universal charge density fluctuation, which extends higher temperature and wider doping range. X-ray scattering with an external magnetic field revealed three-dimensional form (3D) of charge order in Y-based cuprate, YBa₂Cu₃O_6+x (YBCO), which is also driven by axial strain on the sample. Moreover, recent time-resolved RSXS also showed the transient precursor form of the 3D charge order.

In this talk, I will briefly review the recent X-ray scattering studies of charge order [1] and charge density fluctuation [2] in cuprate superconductors. Then, I will give the experimental and analysis details about magnetic-field-induced 3D charge order in YBCO [3,4]. Finally, recent time-resolved RSXS studies on YBCO will be presented [4,5].

keywords : cuprate, charge order, charge density fluctuation, X-ray scattering, 3D charge order

[1] R. Comin and A. Damascelli, Annu. Rev. Condens. Matter Phys. 7, 369 (2016).

[2] R. Arpaia and G. Ghiringhelli, J. Phys. Soc. Jpn. 90, 111005 (2021).

[3] S. Gerber, H. Jang et al. Science 350, 949 (2015).

[4] H. Jang et al. Proc. Natl. Acad. Sci. U.S.A. 113, 14649 (2016).

[5] H. Jang et al. Sci. Adv. 8, eabk0832 (2022).

* Part of this work was supported by the National Research Foundation grant funded by the Korea government (MSIT).

We performed temperature- and doping-dependent high-resolution Raman spectroscopy experiments on YBa₂Cu₃O_7-δ to study B_1g phonons. The temperature dependence of the real part of the phonon self-energy shows a distinct kink at T = T_B1g above T_c due to softening, in addition to the one due to the onset of the superconductivity. T_B1g is clearly different from the pseudogap temperature with a maximum in the underdoped region and resembles charge density wave onset temperature, T_CDW. We attribute the B_1g-phonon softening to an energy gap on the Fermi surface induced by a charge density wave order, which is consistent with the results of a recent electronic Raman scattering study. Our work demonstrates a way to investigate Fermi surface instabilities above T_c via phonon Raman studies.

keywords : Raman spectroscopy, copper oxide superconductors, charge density wave, B_1g phonon

* This work was supported by the Institute for Basic Science in Korea (Grant No. IBS-R009_G2), NRF (Grant No. 2020R1A2C1011439), NRF (Grants No. NRF-2019R1I1A1A01057393, No. NRF-2020-R1A2C2-007930). Sample preparation was supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grants No. 19204038 and No. 24340083).

Transition metal dichalcogenides like 2H-NbSe₂ in their two-dimensional (2D) form exhibit Ising superconductivity with the quasiparticle spins are firmly pinned in the direction perpendicular to the basal plane. This enables them to withstand exceptionally high magnetic fields beyond the Pauli limit for superconductivity. Using field-angle-resolved magnetoresistance experiments for fields rotated in the basal plane we investigate the field-angle dependence of the upper critical field (H_c2), which directly reflects the symmetry of the superconducting order parameter. We observe a six-fold nodal symmetry superposed on a two-fold symmetry. This agrees with theoretical predictions of a nodal topological superconducting phase near H_c2, together with a nematic superconducting state. We demonstrate that in NbSe₂ such unconventional superconducting states can arise from the presence of several competing superconducting channels.

keywords : Superconducting order parameter, superconducting phase transition, topological superconductors

We theoretically studied the Andreev states in nanowire Josephson junctions and their coupling to a microwave. Based on a simple theoretical model taking into account a finite length, multichannel structure and Rashba spin-orbit coupling of the nanowire, we show the spin-orbit splitting of Andreev states. In collaboration with an experimental group, we observed the features of the splitting and the response of the microwave resonator to the Josephson junction in microwave spectroscopy [1]. We also find additional experimental spectra, which evidence the presence of Coulomb interactions. By combining experimental measurements and model calculations including Coulomb interactions, we provide a qualitative understanding of the Andreev spectra [2].

keywords : Josephson junction, Spin-orbit coupling, Microwave spectroscopy

[1] L. Tosi, C. Metzger, M. F. Goffman, C. Urbina, H. Pothier, S. Park, A. Levy Yeyati, J. Nygard, and P. Krogstrup, "Spin-orbit splitting of Andreev states revealed by microwave spectroscopy", Phys. Rev. X 9, 011010 (2019).

[2] F. J. Matute-Canadas, C. Metzger, S. Park, L. Tosi, P. Krogstrup, J. Nygard, M. F. Goffman, C. Urbina, H. Pothier, and A. Levy Yeyati, J. Nygard, and P. Krogstrup, "Signatures of interactions in the Andreev spectrum of nanowire Josephson junctions", Phys. Rev. Lett. 128, 197702 (2022).

* This work was supported by the research program through ‘Convenio Banco Santander UAM-IFIMAC’.

Majorana zero modes (MZM) at the boundary of a topological superconductor could provide a platform for topological quantum computation utilizing its non-abelian exchange statistics. This motivated numerous attempts to generate and observe MZMs in nanowire-based devices, but the evidences from quantum transport experiments have been inconclusive yet. Here, we propose a distinct approach that employs superconducting nanoelectromechanical resonators to detect and control MZMs. Thin aluminum film deposited on InAs two dimensional electron gas (2DEG) induces topological superconductivity to the 2DEG. When a narrow stripe of aluminum film is removed, a pair of MZM is expected to appear at the ends of the exposed area. We position a nanomechanical resonator in a way that the change in local density of states from MZMs modifies the nanomechanical resonant frequency. Subsequently the nanomechanical frequency shift probes the evolution of MZMs under magnetic field and chemical potentials that could reveal the topologically distinct states. The current status and recent results of proposed experiments are discussed.

keywords : topological superconductivity, Majorana zero mode, NEMS

* This work was supported by Samsung Science & Technology Foundation.

Advances in ab-initio crystal structure prediction algorithms and methods for calculating electron-phonon coupling interactions have made it possible to discover and rationally design conventional superconductors with superior behavior. Here, we report theoretical studies of various light-element superconductors discovered using the XtalOpt evolutionary algorithm, or via calculations on prototype structures. Recent experiments have reported high-temperature superconductivity in SH₃ doped with a third element under pressure. To shed light on ternaries derived from SH₃ we carry out a systematic investigation of the effect of doping H₃S by carbon and phosphorous. Moreover, we show how a chemical pressure analysis can be used to design ternary or quaternary clathrate superhydride superconductors that can be stabilized to lower pressures.

keywords : density functional theory calculations, hydrides, pressure, superconductivity

* This work was supported the National Science Foundation (DMR-1827815), the US Department of Energy (DOE), Office of Science, Fusion Energy Sciences Award No DE-SC0020340 and the DOE National Nuclear Security Administration through the Capital-DOE Alliance Center under Cooperative Agreement DE-NA0003975.

Superconductivity exceeding 200 K was recently reported in the highly compressed hydrides [1-3]. Here we report the investigation of the superconductivity of them by several synthesis methods. An originally designed transport preparation in the gasket was used in a laser-heating diamond-anvil cell (DAC). In the case of sulfur hydride which was synthesized by a low-temperature pathway (compression at low temperature ~ 200 K) and showed superconductivity with the transition temperature of 200 K at around 150 GPa [1]. We found that the superconductive hydride was synthesized through the chemical reaction of 3H₂S → 2H₃S + S, by IR laser heating. The in-situ structural confirmation was conducted by using the synchrotron x-ray in SPring-8 [4]. This method, so-called “direct synthesis” opened the new pathway for the synthesis and was widely used for the synthesis of superconducting hydrides, for example, lanthanum-hydrides with elemental hydrogen [2] and hydrogen source material [3]. Our current experimental detail of the synthesis pathway will be reviewed.

keywords : superconductivity, hydride, laser heating, low temperature, DAC

[1] A. P. Drozdov et al., “Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system”, Nature 525, 73 (2015).

[2] A. P. Drozdov et al., “Superconductivity at 250 K in lanthanum hydride under high pressures”, Nature 569, 528 (2019).

[3] M. Somayazulu et al., “Evidence for Superconductivity above 260 K in Lanthanum Superhydride at Megabar Pressures”, Phys. Rev. Lett. 122, 027001 (2019).

[4] H. Nakao et al., “Superconductivity of Pure H3S Synthesized from Elemental Sulfur and Hydrogen”, J. Phys. Soc. Jpn. 88, 123701 (2019).

* This work was supported by JSPS KAKENHI Grant Number 20H05644.

The charge density wave (CDW) is often competing with superconductivity because they originate commonly from the electron-phonon coupling. In typical metallic materials showing the CDW property, the pressure induces the phase transition from CDW to superconducting (SC) state due to the suppression of nesting feature. In this work, we have found the occurrence of intriguing phase transition from SC to CDW state in pressurized cubic-Heusler compound LuPd₂In, which is quite unusual in view of that the pressure is detrimental to the CDW state in usual systems. Based on ab initio density functional theory, we have demonstrated that this abnormal transition originates from the extraordinary softened-phonon mode, which first enhances the SC transition temperature, but eventually yields the phonon softening instability so as to bring about the CDW transition. This extraordinary transition originates from the occurrence of phonon softening instability at a special q = M in the Brillouin zone. We have thus proposed that LuPd2In is a quite unique material, in which the CDW quantum critical point is realized under the SC dome by applying the pressure.

keywords : Superconductor, High pressure, CDW

[1] Heejung Kim, J. H. Shim, Sooran Kim, Jae-Hoon Park, Kyoo Kim, and B. I. Min "Unusual Pressure-Induced Quantum Phase Transition from Superconducting to Charge-Density Wave State in Rare-Earth-Based Heusler LuPd2In Compound", Phys. Rev. Letters. 125, 157001 (2020).

* This work was supported by the National Research Foundation (NRF) Korea, Max-Planck POSTECH/KOREA Research Initiative, Study for Nano Scale Optomaterials and Complex Phase Materials, Internal R&D program at KAERI funded by the MSIT of Korea and KISTI.

Electromagnetic responses in superconductors provide information on the pairing symmetry as well as physical quantities such as the superfluid density. However, at the superconducting gap energy scale, optical excitations of the Bogoliugov quasiparticles are forbidden in conventional Bardeen-Cooper-Schrieffer superconductors when momentum is conserved. Accordingly, far-infrared optical responses have been understood in the framework of a “dirty-limit” theory by Mattis and Bardeen for over 60 years. In this talk, we establish the conditions for nonzero intrinsic optical transitions in clean superconductors. We find that multi-band effects are essential, additionally, one of the following three conditions is required: (i) inversion symmetry breaking, (ii) the emergence of the Bogoliubov Fermi surfaces, or (iii) simply finite spin-orbit coupling. We apply our theory to optical responses in FeSe.

keywords : Optical transitions, Symmetry, Multi-band effects, Mattis-Bardeen theory

* J.A. was supported by the RIKEN Special Postdoctoral Researcher Program, the funding via Ashvin Vishwanath from the Center for Advancement of Topological Semimetals, an Energy Frontier Research Center funded by the US Department of Energy Office of Science, Office of Basic Energy Sciences, through the Ames Laboratory under contract No. DE-AC02-07CH11358, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant no. 2020R1A6A3A03037129). N.N. was supported by JST CREST Grant nos. JPMJCR1874 and JPMJCR16F1, Japan, and JSPS KAKENHI Grant no. 18H03676.

Topological superconductivity has attracted wide interest as it is expected to lead Majorana quasiparticles. A superconductor with topologically nontrivial phase is believed to be a prerequisite for the topological superconductor that several superconducting materials intrinsically or proximately combined with topological surface state have been intensively investigated in searching for the unique superconductivity. Despite the effort, so far, the previous studies have been mostly carried out on systems coupled with a strong topological insulator. We report the demonstration of the strain-driven topological phase transition from weak to strong topological insulator phase. Using angle-resolved photoemission spectroscopy and first-principles calculation, we identified that quasi-one-dimensional (quasi-1D) superconductor TaSe₃ is in weak topological insulator phase of Z₂ topological invariants (0;101) by observing band inversion gap without any signature of topological surface state on the naturally cleaved plane where is topologically dark surface. In addition, we revealed that topological surface state appears on the very surface of originally forbidden plane accompanied with Z₂ invariant change to (1;010), by applying an uniaxial tensile strain along the chain direction. This indicates the phase transition from weak to strong topological insulator in the quasi-1D superconductor TaSe₃. These results provide a new platform to study the superconductivity with two different topological insulator phases in a single material which can lead further understanding of the topological superconductivity.

keywords : topological insulator, superconductor, ARPES, strain-application

Pairing of two electrons into Cooper pair is a key ingredient of superconductivity. While phonon mediates the pair in the conventional superconductivity of Bardeen-Cooper-Schrieffer (BCS) theory, the pairing glue other than the phonon is anticipated to be required for unconventional superconductivity that normally comes after the suppression of symmetry-broken phases, so called competing behavior. Similarly, an intricate competition between the charge density wave (CDW) order and superconductivity is exhibited in numerous CDW systems, suggesting CDW-originated bosonic mode as a possible pairing glue. However, the role of the CDW-originated bosonic modes to superconductivity remains elusive.

In this talk, I will present the spectroscopic evidence of coupling between electron and CDW fluctuation, investigated by a temperature- and intercalation-dependent kink in the angle-resolved photoemission spectra of 2H-Pd_xTaSe₂. Kinks are observed only when the CDW phase exists, regardless of whether a long- or short-range order is established. Notably, the coupling strength is enhanced upon long-range CDW suppression. Interestingly, estimation of the superconducting critical temperature by incorporating the observed coupling characteristics into McMillan’s equation yields results closely resembling the known values of superconducting dome, suggesting a compelling possibility that this new coupling mediates Cooper pairs in 2H-TaSe₂.

keywords : Charge density wave, superconductivity, angle-resolved photoemission spectroscopy

Quantum spin liquid (QSL) is an exotic state possibly having quantum entanglement and fractionalization in excitations. Further intriguing is superconductivity that possibly emerges by doping QSL. The organic triangular-lattice material, k-(BEDT-TTF)₄Hg_2.89Br₈, is such a candidate. In this presentation, I talk about our recent experimental studies on the natures of the normal metal and superconductivity in this system under pressure control.

At ambient pressure, the ¹³C NMR measurements suggest spin-singlet nodal pairing. The superfluid density evaluated by the penetration depth measurements is considerably suppressed to 15-25% of the carrier number expected from the band structure, indicating that the substantial part of the spectral weight is incoherent [1]; we note that there has been observed no competing or coexisting orders unlike in the cuprates. The transport and Nernst effect measurements under pressure variation suggest that the superconductivity is a BEC-like condensate from a non-Fermi liquid at low pressures and crosses over to a BCS condensate from a Fermi liquid at high pressures, where the Coulombic interactions among electrons are weakened [2]. The thermoelectric measurements indicate that the low-pressure phase is in a quantum critical state and it may pertain to the superconductivity [3]. Finally, I also show the variation of the electronic state under uniaxial distortion of the triangular lattice [4].

The present work is a collaboration with Y. Suzuki, K. Wakamatsu, Y. Ueno, J. Ibuka, H. Oike, T. Fujii, K. Miyagawa and H. Taniguchi.

keywords : quantum spin liquid, BEC-BCS crossover, quantum criticality, superfluid density

[1] K. Wakamatsu et al., “Reduced superfluid density in a doped spin liquid candidate” arXiv: 2205.03682.

[2] Y. Suzuki et al., “Mott-Driven BEC-BCS Crossover in a Doped Spin Liquid Candidate k-(BEDT-TTF)₄Hg_2.89Br₈”, Phys. Rev. X 12, 011016 (2022).

[3] K. Wakamatsu et al., “Thermoelectric signature of quantum critical phase”, arXiv: 2201.10714.

[4] H. Oike et al., “Metal-insulator transition via control of spin liquidity in a doped Mott insulator”, arXiv: 2202.06032

High-entropy alloys (HEAs) are new class of functional materials owing to their superior physical and mechanical properties to conventional alloys [1]. HEAs are typically composed of five or more different metallic elements with an atomic fraction of each element between 5 and 35%, whereas conventional alloys have small amounts of additional elements in one principal element. Since the discovery of superconductivity of Ta–Nb–Hf–Zr–Ti HEAs in 2014, various kinds of HEA superconductors have been reported and garnered great interest for considerable potential use in a variety of extreme conditions due to their high mechanical hardness and excellent irradiation tolerance [2, 3]. However, the superconducting (SC) critical properties for practical applications, especially critical current density (J_c), have not yet been adequately characterized in HEA superconductors. In this talk, we present a high J_c of HEA superconductors Ta_1/6Nb_2/6Hf_1/6Zr_1/6Ti_1/6. Both bulk and thin-film HEA superconductors show a large J_c, and the film deposited at 520 ^oC exhibits the highest J_c (> 1 MA/cm² at 4.2 K), indicating promise for various technological applications, such as SC devices as well as high-field SC magnets. When subjected to the low-energy ion irradiations, the superconductivity of HEA superconductors is suppressed, but is more than 1000 times resistant to irradiation-induced disorder compared to other representative superconductors such as MgB₂, Nb₃Sn, Fe-based superconductors, and high-T_c cuprates. Under extreme dose of irradiation, in stark contrast, T_c of HEAs bounces back, showing a self-healing ability against the irradiation-induced disorder. These results indicate that HEA superconductors are promising new SC materials with significant potential for technological applications in various extreme areas, such as aerospace, nuclear reactors, and high-field SC magnets.

keywords : High-entropy alloy superconductor, bulk, thin film, critical current density, irradiation

[1] X. Wang, W. Guo, and Y. Fu, High-entropy alloys: emerging materials for advanced functional applications. J. Mater. Chem. A 9, 663-701 (2021).

[2] L. Sun, and R. J. Cava, High-entropy alloy superconductors: Status, opportunities, and challenges. Phys. Rev. Mater. 3, 090301 (2019).
[3] S. –G. Jung, Y. Han, J. H. Kim, R. Hidayati, J. –S. Rhyee, J. M. Lee, W. N. Kang, W. S. Choi, H. –R. Jeon, J. Suk and T. Park, High critical current density and high-tolerance superconductivity in high-entropy alloy thin films. Nat.Commun. 13, 3373 (2022).

* We wish to acknowledge the outstanding support of the accelerator group and operators of KOMAC, KAERI. This study was supported by the National Research Foundation (NRF) of Korea through a grant funded by the Korean Ministry of Science and ICT (No. 2021R1A2C2010925 and 2021R1A2C2011340) and by the Basic Science Research Program through the NRF of Korea funded by the Ministry of Education (NRF-2019R1F1A1055284, NRF-2020R1I1A1A01067677, and NRF-2021R1I1A1A01043885).

Vortices in topological superconductors are predicted to host Majorana bound states (MBSs) as exotic quasiparticles. In recent experiments, the spatially non-split zero-energy vortex bound states in topological superconductors has been regarded as an essential spectroscopic signature for the observation of MBSs. Here, we report the observation of anisotropic non-split zero-energy vortex bound states in a conventional elemental superconductor with a topologically trivial band structure using scanning tunneling microscopy and spectroscopy. The experimental results, corroborated by quasi-classical theoretical calculations, indicate that the non-split states directly reflect the quasiparticle trajectories governed by the surface electronic structure, especially due to the quasiparticle focusing through the Fermi surface [1, 2]. Additionally, we demonstrate the splitting of the vortex bound states at the artificially-introduced dislocation line on the surface due to the formation of a minigap at the fermi level. The observed minigap can be explained in terms of the contribution of normal reflections of quasiparticles at the line defect resulting in the breaking of the chiral spectral branch of the vortex bound states in the defect-free. The interplay between vortices and the surface defects can be utilized to distinguish the MBSs from the topologically trivial bound states.

keywords : Superconducting vortices, vortex bound states, Anisotropic Fermi surface, Andreev reflection, normal reflection

[1] H. Kim et al, "Long-range focusing of magnetic bound states in superconducting lanthanum", Nat Comm 11, 4573 (2020).

[2] H. Kim et al, "Anisotropic non-split zero-energy vortex bound states in a conventional superconductor", Appl Phys Rev 8, 031417 (2021).

* This work was supported by the European Research Council via Project No. 786020 (ERC Advanced Grant ADMIRE) at the University of Hamburg.

The interplay among magnetism, electronic nematicity, and superconductivity is the key issue in strongly correlated materials including iron-based, cuprate, and heavy-fermion superconductors. Magnetic fluctuations have been widely discussed as a pairing mechanism of unconventional superconductivity, but recent theory predicts that quantum fluctuations of electronic nematicity, which is characterized by rotational symmetry breaking, may also promote high-temperature superconductivity. FeSe-based superconductors are suitable to study this issue [1], because FeSe exhibits a nonmagnetic nematic order that can be suppressed by S or Te substitution for Se. I will review recent studies of FeSe-based superconductors, which show quite exotic superconducting states. In FeSe_1-xS_x superconductors, the nematic order can be completely suppressed at x=0.17, above which the superconducting properties change drastically with a significantly reduced critical temperature T_c [2,3]. From recent muon spin rotation (µSR) measurements [4], we find evidence for a novel ultranodal pair state with broken time reversal symmetry [5]. In the Te substitution case, however, we find quite different behavior; the suppression of nematic order leads to an enhancement of T_c, which is likely associated with quantum critical fluctuations of nematicity [6-8].

keywords : time-reversal symmetry breaking, nematicity, quantum critical point

[1] See, for a review, T. Shibauchi, T. Hanaguri, and Y. Matsuda, J. Phys. Soc. Jpn. 89, 102002 (2020).

[2] Y. Sato et al., Proc. Natl. Acad. Sci. USA 115, 1227-1231 (2018).

[3] T. Hanaguri et al., Sci. Adv. 4, eaar6419 (2018).

[4] K. Matsuura et al., preprint (2022).

[5] C. Setty, S. Bhattacharyya, Y. Cao, A. Kreisel, and P. J. Hirschfeld, Nat. Commun. 11, 523 (2020).

[6] K. Mukasa et al., Nat. Commun. 12, 381 (2021).

[7] K. Ishida et al., Proc. Natl. Acad. Sci. USA 119, e2110501119 (2022).

[8] K. Mukasa et al., arXiv:2202.11657 (2022).

We investigated CeCoIn₅ and LaCoIn₅ single crystals, which have the same HoCoGa₅-type tetragonal crystal structure, using infrared spectroscopy. However, while CeCoIn₅ has 4f electrons, LaCoIn₅ does not. By comparing these two material systems, we can extract the electronic properties of these f electrons. We observed that the differences caused by the f electrons are more obvious in low-energy optical spectra at low temperatures. We introduced a complex optical resistivity and found that the difference in optical resistivity between the two material systems is a clear indicator of the f electron response, which is intimately associated with the Kondo (f-electron) phenomena. From the temperature-related difference in optical resistivity, we also found that the onset temperature of the Kondo effect is much higher than what was previously reported. A large effective mass of 4f electrons for CeCoIn₅ is obtained by applying the extended Drude model formalism. The effective masses of the two material systems are sharply contrasted. From this approach using optical resistivity, we demonstrate that the temperature-dependent optical properties of f electron can be extracted via a comparative study between CeCoIn₅ and LaCoIn₅.

keywords : infrared spectroscopy, optical resistivity, f electrons, heavy fermions

* This work was supported by the Global Partnership Program from the National Research Foundation (Grant No. 2021R1A2C101109811).

Strongly correlated systems show interesting physical phenomena including superconductivity, magnetism, and metal-insulator transitions. In order to study the origin and nature of electronic correlations in a strongly correlated system, first principle calculations including density-functional theory (DFT) and DFT plus dynamical mean-field theory (DFT+DMFT) were performed. In the this talk, we will discuss the electronic structure of the infinite-layer nickelate, which was recently reported to be a superconductor. In the infinite-layer nickelate, the optical conductivity calculations enable us to identify the origin and nature of electronic correlations. Besides, we found a crossover from Mott to Hund region in the nickelate systems, where Hund’s physics is hidden at low energies but noticeable at intermediate energies. This kind of hidden Hund’s physics is quite new and thus opening a new research area in the theory of correlated systems.

keywords : Density-functional theory plus dynamical mean-field theory, the infinite-layer nickelate superconductors, hidden Hund’s physics

[1] Chang-Jong Kang and Gabriel Kotliar, "Optical Properties of the Infinite-Layer La_1-xSr_xNiO₂ and Hidden Hund’s Physics", Phys. Rev. Lett. 126, 127401 (2021).