RAON is a heavy ion accelerator facility in Korea that focuses on the rare isotope science by producing and accelerating Rare Isotope Beams (RIBs). The facility utilizes both In-Flight (IF) Fragmentation and Isotope Separation On-Line (ISOL) methods to produce various RIBs. A distinctive feature of RAON is its ability to combine the ISOL and IF methods to produce more exotic RIBs. The IF system is driven by a superconducting linear accelerator for accelerating heavy ions. On the other hand, the ISOL system is driven by a 70 MeV proton cyclotron. The ultimate goal of the superconducting Linac is to accelerate uranium and proton beams up to 200 MeV/u and 600 MeV, with maximum beam currents of 8.3 pμA and 660 pμA, respectively. After more than 10 years of construction, the low-energy superconducting Linac, consisting of Quater Wave Resonator (QWR) and Half Wave Resonator (HWR) type superconducting cavities, has been installed [1] and successfully commissioned. Additionally, the ISOL system for RIB production has also been commissioned. As part of the facility’s progress, prototyping of the Single Spoke Resonator (SSR) type superconducting cavities for the high-energy Linac section is under way.  

   In this talk, we report on the status of the RAON heavy ion accelerator, including the beam commissioning results of the low-energy superconducting Linac and the RIB production of the ISOL system. That will presumably provide insights on the facility's achievements and ongoing developments.

keywords : Accelerator, Superconducting Linac, Beam commissioning

[1] Yeonsei Chung, Hyungjin Kim, Myeun Kwon “Current status of the RAONlow-energy heavy ion accelerator”, J. Korean Phys. Soc. 80, 693 (2022)

* This work was supported by the Rare Isotope Science Project of the Institute for Basic Science funded by the Ministry of Science and ICT (MSIT), Republic of Korea, and by the National Research Foundation (NRF) of the Republic of Korea under Contract 2013M7A1A1075764

   The iron-based superconductor, FeSe1-xTex (FST), obtained significant attention due to two emergent phenomena of the material. The first is the topological superconductivity hosts Majorana Fermion in its boundary as a candidate of the topologically protected quantum bit [1,2]. The second is the orbital selective Mott transition, which is a selective localization of the Fe(dxy) orbital while other orbitals, including Fe(dxz/yz), remain as itinerant [3]. This talk shows that the topological superconductivity and the orbital selective Mott transition in the FST material are intimately connected [4]. We use the state-of-the-art linearized quasiparticle self-consistent GW plus dynamical mean-field theory framework with spin-orbit coupling (LQSGW+DMFT+SOC), which enables the quantitative description of the topological Dirac surface state of the FST material. We show that the topologically non-trivial band, the Fe(dxy) orbital origin, experiences a localization from the orbital selective Mott transition. This identification shows that the non-trivial Z2 topology for the topological superconductivity could be realized only for the physical regime that is not too far but not too close to the orbital selective Mott transition. This observation enables understanding and manipulation of the topological superconductivity of iron-based superconductors. Also, the strong electronic correlation at the topological surface state dominantly from the Fe(dxy) orbital can be the origin of the experimentally observed time-reversal symmetry breaking at the surface of the FST material [5].

keywords : Iron-based superconductor, topological superconductor, orbital selective Mott transition, dynamical mean-field theory, GW 

[1] G. Xu et al., Phys. Rev. Lett. 117, 047001 (2016), 

[2] P. Zhang et al., Science 360, 182 (2018)

[3] M. Yi et al., Nat. Comm. 6, 1 (2015)

[4] Minjae Kim et al., https://arxiv.org/abs/2304.05002 (2023).

[5] C. Farhang et al., Phys. Rev. Lett. 130, 046702 (2023) 

* This work was supported by the U.S Department of Energy, Office of Science, Basic Energy Sciences as a part of the Computational Materials Science Program. MK was supported Individual Grant (CG083501) at Korea Institute for Advanced Study. SC was supported by a KIAS Individual Grant (CG090601) at Korea Institute for Advanced Study.

   Layered transition metal chalcogenide FeSe nanostructures reveal unprecedented electronic and optical properties. We investigated the strange optical properties through the electronic structure calculations based on the DFT. Since a Fe has 5 up-spin (majority) electrons and 1 down- spin (minority) electron, the net magnetization was calculated to be 4 μB per Fe atom. It is found that two kinds of electronic structures are available, coupled with the lattice. The band structures reveals that the FeSe can be a Mott insulator with the band gap value of 1.82 eV by local spin density approximation+U (LSDA+U). For the Mott insulator state, the anti-ferromagnetic spin-coupling state is found to be the ground state of the spin-configuration in FeSe, which is more stable by 0.043 eV/Fe-atom than the spin-parallel ferromagnetic state. We examined the change of the electronic structure by changing the separation between 2D layers and found that the electronic structure is little changed by that mechanical strain change, indicating that the electrons in FeSe are confined in each layer. The more detailed physics of two Fe-eg-levels by considering that the band gap is formed by the strongly Coulomb interaction U between eg levels. This strong correlation provides the strange optical properties.

   The discovery of superconducting nickelates reignited hope for elucidating the high-Tc superconductivity mechanism in isostruc- tural cuprates. While the superconducting gap opens up on a single band of the quasi-2D Fermi surface in the cuprates, the nickelates are known to have a 3D nature of an electronic structure with a multi-band. This raises a serious question about the role of the 2D nature for the high-Tc superconductivity. Here, employing GW + dynamical mean field theory (DMFT), we discuss the Kondo effect driven by the strong correlation of Nd-4f and Ni-3d electrons emerging at low temperature. The Kondo effect modifies the topology of the Fermi surface, leading to a 3D multi-band nature. Remark- ably, the Kondo effect is easily destroyed by lattice modulation, leading to the quasi-2D nature. Our findings could provide a new perspective for explaining the inconsistent occurrence of superconductivity and distinct electrical resistivity behavior between NdNiO2 bulk and films, calling for an experimental measure of the Fermi surface of bulk NdNiO2.

[1] J. Kwon et al., Nano Today 43, 101424 (2022).

[2] B. Kang et al., Cell Reports Physical Science 4, 101325 (2023).

* This work was supported by the National Research Foundation of Korea through a grant (grant no. NRF- 2022R1A2C1005548).

   Cuprate-like superconductors have long been sought in condensed matter physics. The search for nickelate superconductors was one of the approaches. Recently, two members (n= and 5) of the layered nickelate family (Rn+1NinO2n+2, R=rare earth, n=1-) have been discovered as superconductors with Tc~15K [1,2]. These have n-NiO2 layers along the c-axis and d9-1/n nominal filling as in cuprates. Despite the similarities between the nickelates and the cuprates, there is a striking distinction between them. While the parent cuprates are antiferromagnet insulators, the parent layered nickelates are metallic and show no signature of long-range magnetic ordering. 

     We provide a set of computational experiments based on ab initio calculations to clarify whether a cuprate-like insulating antiferromagnet can be emergent in the phase diagram of the low-valence layered nickelate family near half-filling. We show that for the Ruddlesden-Popper (RP) reduced phases of the series (finite n) an antiferromagnet insulating ground state can be naturally obtained instead at d9 filling, due to the spacer RO2 fluorite plates present in their structure which block the c-axis dispersion. In the n =∞ nickelate, the same type of solution can be derived if the off-plane R-Ni coupling is suppressed. We show how this can be achieved by introducing a structural element that cuts off the c-axis dispersion (i.e. a vacuum in a monolayer of RNiO2, or a blocking layer in multilayers formed by (RNiO2)1/(RNaO2)1)[3].

Acknowledgments: This work was done in collaboration with Prof. Pardo at USC, and Prof. Botana and Harry at ASU.

Keywords: first-principles calculations, nickelates, antiferromagnetic insulating phase

[1] D. Li, K. Lee, B. Y. Wang, M. Osada, S. Crossley, H. R. Lee, Y. Cui, Y. Hikita, and H. Y. Hwang, Nature 572, 624 (2019).

[2] G. A. Pan, D. Ferenc Segedin, H. LaBollita, Q. Song, E. M. Nica, B. H. Goodge, A. T. Pierce, S. Doyle, S. Novakov, D. Córdova Carrizales, A. T. N’Diaye, P. Shafer, H. J. Paik, J. T. Heron, J. A. Mason, A. Yacoby, L. F. Kourkoutis, O. Erten, C. M. Brooks, A. S. Botana, and J. A. Mundy.

Nature Materials, 21, 160 (2022).

[3] M.-C. Jung, H. LaBollita, V. Pardo, and A. S. Botana, Scientific Reports 12, 17864 (2022).

    Anyons are quasiparticles in two dimensions. They do not belong to the two classes of elementary particles, bosons and fermions. Instead, they obey Abelian or non-Abelian fractional statistics. There have been efforts to find signatures of anyons in systems of topological order such as fractional quantum Hall systems and topological superconductors. I talk about a phenomenon of anyon braiding at a beam splitter, which was recently observed in fractional quantum Hall experiments [1,2]. I will also suggest a scheme for manipulating Majorana bound states and detecting their braiding and fusion. 

keywords : Anyons, Majorana fermions, braiding, fractional statistics 

[1] June-Young M. Lee and H.-S. Sim, Nature Communications 13, 6660 (2022).

[2] June-Young M. Lee, Changki Hong, Tomer Alkalay, Noam Schiller, Vladimir Umansky, Moty Heiblum, Yuval Oreg, and H.-S. Sim, Nature 617, 277 (2023). 

   The quest for topological superconductors (TSCs) and their enigmatic Majorana fermions has emerged as a thrilling domain within condensed matter physics. TSCs possess a complete pairing gap in the bulk and exhibit gapless surface states with Majorana fermions, which have promising applications in quantum computing. This study delves into TSCs and investigates three distinct types of Majorana fermions. First, we explore Majorana fermions in doped topological Dirac semimetals with lattice distortion. Analyzing lattice distortions and electron density-density interactions, we observe unconventional superconductivity with gapless surface Andreev bound states. These distortions enhance the critical temperature, supporting the potential for pressure-induced topological superconductivity. Next, we focus on Majorana fermions in higher-order topological insulators using orthorhombic Td-MoTe¬2. We identify a zero-bias conductance peak in the pressure-induced monoclinic 1T' phase through soft-point-contact spectroscopy, suggesting a connection between Majorana fermions and p-wave surface superconductivity. This finding prompts discussions on the relationship with topological zero-energy hinge states. Lastly, we investigate the emergence of Majorana fermions originating from the extended one-dimensional Kitaev chain. Departing from the conventional wisdom that zero-energy Majorana fermions exclusively reside in the BDI and DIII classes, our groundbreaking work demonstrates their existence even in the C and CI classes. Such newfound Majorana fermions can be elegantly conceptualized as extended counterparts within the framework of quantum field theory. Furthermore, we scrutinize their stability in the presence of disorder, shedding light on their robustness in realistic scenarios.

keywords: Topological Superconductor, Majorana Fermion, Dirac semimetal, Higher order topological insulator, Kitaev Chain

[1] J. Alicea, New directions in the pursuit of Majorana fermions in solid-state systems, Rep. Prog. Phys. 75, 076501 (2012). 

[2] M. Sato and Y. Ando, "Majorana fermions and topology in superconductors," Rep. Prog. Phys. 80, 076501 (2017).

[3] S. Cheon, K. H. Lee, S. B. Chung, and B. J. Yang, Emergence of topological superconductivity in doped topological Dirac semimetals under symmetry-lowering lattice distortions, Sci. Rep. 11, 1-25 (2021).

[4] S. Lee, M. Kang, D. Kim, D. Y., J. Kim, S. Cho, S. Cheon, T. Park, (in preparation).

[5] M. Kang, SH Han, MJ Park, S. Cheon, (in preparation)

   Impurities can significantly affect superconductivity, leading to the emergence of low-lying excitations within the superconducting gaps. As impurity concentration increases, these impurity-bound states can evolve into continuum states [1]. Here, we investigate the unconventional superconductor Fe(Se,Te), which exhibits spatially dispersive in-gap states that can be modulated by electric fields from a scanning tunneling microscope (STM) tip [2]. Interestingly, we also observe similar spectral features at energy levels higher than the superconducting gap. Our findings indicate that these distinctive spectral features are not solely attributed to the multiband electronic structure commonly observed in Fe-based superconductors. Instead, we propose that they arise from an impurity band formed through orbital-selective pair breaking induced by disorder potentials. These results contribute to the understanding of amorphous impurity bands, commonly called Shiba glass, and their topological properties [3].

keywords : STM, Fe-based superconductor, Shiba glass 

[1] H. Shiba, Progress of Theoretical Physics 40, 435-451 (1968).

[2] D. Chatzopoulos, et al. Nature Communications 12, 298 (2021).

[3] K. Pöyhönen, et al. Nature Communications 9, 2103 (2018).

   When a magnetic moment is embedded in a metal, it captures itinerant electrons to form the Kondo cloud with a size of a few micrometres [1]. For a metal where magnetic impurities are reasonably dense that the spacing of their magnetic moments is comparable to the size of the Kondo clouds, the Kondo clouds overlap each other to form a correlated ground state [2]. Here, I present electrical transport and tunneling DOS spectroscopy measurements in a crystalline silicon metal where localized magnetic moments exist. We detect the Kondo effect in the resistivity of the Si metal and an exotic BCS-like pseudogap in the DOS near the Fermi energy. The BCS-like DOS structure is tuned by applying an external magnetic field and transformed into a paramagnetic Coulomb gap. This phenomenon is interpreted as the formation of a correlated ground state of overlapping Kondo clouds (Kondo cloud condensate). I also discuss the interplay between the Kondo condensate and BCS-superconductor in the DOS spectrum of a Si:P-SiO2-Al tunnel junction where some interesting features are observed (i.e., Andreev reflection and particle-hole asymmetry like behavior). Our experimental findings demonstrate the observation of the magnetic version of BCS pair condensation and will be useful for understanding complex Kondo systems.

keywords : Kondo cloud, condensation, strongly correlated quantum matter 

[1] I. Borzenets et al., Nature 579, 210 (2020).

[2] H. Im et al., Nature Physics 19, 676 (2023).

   The investigation of complex atomic geometries in materials has attracted significant attention due to their ability to exhibit exotic electronic states. One of the most prominent examples is the electronic flat band arising from the suppressed electron kinetic energy in such systems. So far, the experimental realization of electronic flat bands has been limited to (quasi) two-dimensional (2D) crystalline materials such as twisted bilayer graphene [1] and the kagome lattice [2,3]. Consequently, a question that naturally arises is whether the flat band can exist in the three-dimensional (3D) system while remaining dispersionless along all momentum directions kx, ky, and kz. To answer these questions, we conducted angle-resolved photoemission spectroscopy (ARPES) experiments on the C15 Laves phase CaNi2, which consists of a Ca diamond network and a Ni pyrochlore network. We observe a partially flat band and a 3D flat band below the Fermi level. Moreover, we find that the energy of the flat bands and their dispersion can be modulated through chemical substitution, specifically by replacing Ni with Rh, ultimately resulting in the flat band aligning with the Fermi level. Interestingly, the emergence of superconductivity is observed in CaRh2, where the flat band is located at the Fermi level. Although direct experimental evidence demonstrating flat-band induced superconductivity is still elusive, our results offer a promising material platform for investigating novel emergent phenomena originating from the divergent density of states associated with 3D flat bands in complex lattice systems. 

keywords : geometrical frustration, flat band, pyrochlore lattice, superconductivity 

[1]. S. Lisi et al., Nat. Phys 17, 189-193 (2021).

[2]. M. Kang et al., Nat. Mater 19, 163-169 (2020)  

[3]. M. Kang et al., Nat. Commun 11, 4004 (2020) 

   The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is a prominent example of novel superconducting states in unconventional superconductors. The FFLO state is an unusual superconducting state that helps a superconductor overcome the ultimate magnetic-field limit for superconductivity, known as the Pauli limit. It was theoretically predicted in the middle of the last century, but has been confirmed in very few superconductors due to the stringent conditions for its formation. In this presentation, I will first give a brief history of the FFLO state and present where we are today with the recent findings for Fe-based & transition metal dichalcogenide superconducting systems [1-3]. Finally, I will also comment on the possible connection between the FFLO state and the newly discovered nodal topological superconductors [4]. 

Keywords : Fulde-Ferrell-Larkin-Ovchinnikov state, Fe-based superconductor, transition metal dichalcogenide superconductor

Reference

[1] Thermodynamic Evidence for the Fulde-Ferrell-Larkin-Ovchinnikov State in the KFe2As2 Superconductor, Chang-woo Cho et. al., Phys. Rev. Lett. 119 217002 (2017).

[2] Evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state in bulk NbS2, Chang-woo Cho et. al., Nat. Commun. 12 3676 (2021).

[3] Competition between orbital effects, Pauli limiting, and Fulde-Ferrell-Larkin-Ovchinnikov states in 2D transition metal dichalcogenide superconductors, Chang-woo Cho et. al., New J. Phys. 24 083001 (2022).

[4] Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in an Ising superconductor, Puhua Wan et al., Nature 619 46 (2023).

In my talk I plan to discuss the effects of quantum fluctuations on the transport properties of multiband superconductors near a pair-breaking quantum critical point. I will describe how one can use the microscopic diagrammatic technique to derive electrical conductivity in a normal state due to superconducting fluctuations in the entire low-temperature quantum regime. I will demonstrate that the sign of the conductivity correction depends on how the quantum critical point is approached in the phase diagram. For the last part of the presentation, I contrast my results to existing approaches to this problem based on renormalization group, time-dependent Ginzburg-Landau phenomenology, and effective bosonic action field theories.  

   I will present our discovery of universal scaling laws in the transverse transport of hole-doped cuprates, including the thermal Hall conductivity, the anomalous Hall coefficient, and the quasiparticle Nernst coefficient. I will show that they all decrease exponentially with increasing temperature over wide doping and temperature ranges, and this can be understood within a single unified framework where the carriers have nonzero Berry curvatures in a finite energy window associated with the pseudogap, possibly due to interaction with the spins. A simple schematic prove will be given based solely on the mathematical structure of the conductivity formula, which reveals why these scaling laws were previously never found in low-energy effective theories. The disparity in determining the pseudogap temperature in different measurements is then clarified.

If time allows, I will also briefly discuss our calculations on the recently discovered high-temperature superconductor.

keywords : cuprates, scaling law, Hall, Nernst 

[1] Y.-F. Yang, arXiv:2307.00993 (2023).

[2] Y.-F. Yang, arXiv:2304.08428 (2023).

[3] Y.-F. Yang et al., Phys. Rev. Lett. 124, 186602 (2020).

* This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.

   Revealing abundant quantum phases, the flat bands in moiré superlattices heterostructures have opened up new opportunities for exploring strongly correlated phenomena in two-dimensional systems. Especially, due to the absence of intrinsic scatters such as local defects or dopants, magic angle twisted bilayer graphene allows for the study of pristine quantum behaviors in superconducting phases to better understand electron pairing mechanism in unconventional superconductors like high-Tc superconductors. 

To unveil the pairing mechanism of the ground state of unconventional superconductors, it is essential to identify the exact ground state wavefunction of the correlated insulator phase, as unconventional superconductor phases emerge from doping these insulators. However, low bulk conductivity of insulating phase impedes the real space electronic structure measurement for decerning ground state wavefunction so far.

   In this talk, I will discuss about the spectroscopic experiments providing multiple key signatures of unconventional superconductivity of the MATBG by using density-tuned scanning tunneling spectroscopy combined with Andreev reflection spectroscopy [1]. Moreover, I will discuss the recent remarkable achievement of distinguishing the ground state of correlated insulating states [2] by introducing novel techniques to analyze atomic-scale electronic texture. Our study may pave the way for a deeper understanding of the microscopic mechanisms behind unconventional superconductors, including high-Tc superconductors.

keywords : STM, unconventional superconductivity, twisted bilayer graphene, flat bands

[1] Oh, M.*, Nuckolls, K. P.*, Wong, D.*, Lee, R. L., Liu, X., Watanabe, K., Taniguchi, T. & Yazdani, A. Evidence for unconventional superconductivity in twisted bilayer graphene. Nature 600, 240–245 (2021). 

[2] Nuckolls, K. P.*, Lee, R. L.*, Oh, M.*, Wong, D.*, Soejima T.*, Hong, J. P., Călugăru, D., Arbeitman, J. H., Bernevig, B. A., Watanabe, K., Taniguchi, T., Regnault, N., Zaletel M. P. , Yazdani, A. Nature (Accepted, May 16 2023)

* These authors are equally contributed.

   During recent years, pressure-stabilized hydrides, such as SH3 (203 K) and LaH10 (250-260 K), have continuously refreshed the superconducting critical temperature record, bringing the hope for room superconductivity. This talk includes the investigation of high superconductivity in hydrides at high pressure, by employing a state-of-the-art technique, a heuristic algorithm based on particle swarm optimization CALYPSO. Moreover, the recent findings also show that a plethora of unprecedented ternary hydrides at moderate pressure offer a new opportunity to search for high-temperature superconductors. These results may be helpful to design and discovery of high-temperature superconductors at moderate pressures in the near future.

keywords : High pressure, crystal structure prediction, hydrides

   The discovery of cuprate superconductors in 1986 immediately spawned a huge wave of hundreds and thousands of works on various cuprates, which can be described as a "cuprate" revolution in superconductivity. The echoes of this revolution are still felt in the ongoing research on the mechanism of superconductivity in cuprates. In 2015, the discovery of superconductivity at 200 K in the high-pressure sulfur hydride H3S [1] led to a surge in theoretical and experimental research in the field of polyhydrides. After the discovery of superconductivity in LaH10 (TC = 250 K [2]) in 2018, it was expected that room-temperature superconductivity would be found very soon [3]. However, despite the past 5 years, this has not happened. The "hydride" revolution, with its 5–10 experimental works per year, turned out to be much slower than the cuprate revolution in 1986.

In the report, I will focus on our studies of the superconducting properties of compounds in Sc-H, Mg-H, and La-Sc-H systems under high pressure. We will also talk about the anomalous behavior of superhydrides in the non-superconducting state and the negative magnetoresistance and dR/dT observed in SnH4, CeH10, and other hydrides. Finally, I will speak on the prospects for achieving room-temperature superconductivity and the practical applications of hydride superconductors.

keywords : hydrides, superconductivity, high-pressure

[1] A. Drozdov et al. Nature 525, 73–76 (2015).

[2] A. Drozdov et al. Nature 569, 528–531 (2019).

[3] L. Boeri et al. J. Phys.: Condens. Matter 34, 183002 (2022).

   In the period since the discovery of LaH10 in 2018 [1], significant efforts have been made to overcome the barrier at the critical temperature of superconductivity TC = 250 K. The main hopes were associated with the introduction of additional metal atoms into the La-H system in order to increase TC. Both the doping [2] and the synthesis of truly ternary polyhydrides (e.g., LaBeH8 [3]) were proposed. However, despite intensive studies of the La-Y-H [4], La-Nd-H [5], La-Ce-H [6], La-Al-H [7] systems, it was not possible exceed the critical temperature of LaH10. On the contrary, it turns out that superhydrides can be one of the most striking examples of the fulfillment of the Anderson-Gor’kov theorem [8] proposed for the Bardeen-Cooper-Schrieffer superconductors.

   In the report, I will talk about our studies of the Sc-H and La-Sc-H systems at pressures up to 200 GPa. High-pressure synchrotron X-ray diffraction studies and the results of transport measurements will be discussed. Despite the formation of higher scandium hydrides, they do not exhibit pronounced superconducting properties: the found TC in the Sc-H system is below 100 K, which contradicts theoretical expectations [9]. However, the use of a LaSc (1:1) alloy and ammonia borane as starting materials for the high-pressure synthesis leads to an unexpectedly high TC = 245 K at a pressure of 196 GPa. We investigated the behavior of electrical resistance of LaScHx in magnetic fields up to 16 T, and studied the current-voltage characteristics of this superhydride.

keywords : lanthanum hydrides, polyhydrides, superconductivity, high-pressure

[1] A. Drozdov et al. Nature 569, 528–531 (2019).

[2] T. Wang et al. Phys. Rev. B 105, 174516 (2022). 

[3] Y. Song et al. Phys. Rev. Lett. 130, 266001 (2023).

[4] D. Semenok et al. Materials Today, 48, 18-28 (2021).

[5] D. Semenok et al. Advanced Materials 2204038 (2022).

[6] W. Chen et al. Nat Commun 14, 2660 (2023).

[7] Su Chen et al. National Science Review, nwad107 (2023). 

[8] P. W. Anderson, J. Phys. Chem. Solids 11, 26–30 (1959).

[9] Xiaoqiu Ye et al. J. Phys. Chem. C 122, 11, 629