2022, Ji, Ran, Zhang, Zongbao, Hofstetter, Yvonne J., Buschbeck, Robin, Hänisch, Christian, Paulus, Fabian, Vaynzof, Yana

Modern photovoltaic devices are often based on a heterojunction structure where two components with different optoelectronic properties are interfaced. The properties of each side of the junction can be tuned by either utilizing different materials (for example, donor/acceptor) or doping (for example, p–n junction) or even varying their dimensionality (for example, 3D/2D). Here we demonstrate the concept of phase heterojunction (PHJ) solar cells by utilizing two polymorphs of the same material. We demonstrate the approach by forming γ-CsPbI3/β-CsPbI3 perovskite PHJ solar cells. We find that all of the photovoltaic parameters of the PHJ device significantly surpass those of each of the single-phase devices, resulting in a maximum power conversion efficiency of 20.1%. These improvements originate from the efficient passivation of the β-CsPbI3 by the larger bandgap γ-CsPbI3, the increase in the built-in potential of the PHJ devices enabled by the energetic alignment between the two phases and the enhanced absorption of light by the PHJ structure. The approach demonstrated here offers new possibilities for the development of photovoltaic devices based on polymorphic materials.

2021, Liu, Yannan, Chen, Chuanshuang, Valdez, Jesus, Meira, Debora Motta, He, Wanting, Wang, Yong, Harnagea, Catalin, Lu, Qiongquiong, Guner, Tugrul, Wang, Hao, Liu, Cheng-Hao, Zhang, Qingzhe, Huang, Shengyun, Yurtsever, Aycan, Chaker, Mohamed, Ma, Dongling

Conversion of clean solar energy to chemical fuels is one of the promising and up-and-coming applications of metal–organic frameworks. However, fast recombination of photogenerated charge carriers in these frameworks remains the most significant limitation for their photocatalytic application. Although the construction of homojunctions is a promising solution, it remains very challenging to synthesize them. Herein, we report a well-defined hierarchical homojunction based on metal–organic frameworks via a facile one-pot synthesis route directed by hollow transition metal nanoparticles. The homojunction is enabled by two concentric stacked nanoplates with slightly different crystal phases. The enhanced charge separation in the homojunction was visualized by in-situ surface photovoltage microscopy. Moreover, the as-prepared nanostacks displayed a visible-light-driven carbon dioxide reduction with very high carbon monooxide selectivity, and excellent stability. Our work provides a powerful platform to synthesize capable metal–organic framework complexes and sheds light on the hierarchical structure-function relationships of metal–organic frameworks.

2021, Freelon, B., Sarkar, R., Kamusella, S., Brückner, F., Grinenko, V., Acharya, Swagata, Laad, Mukul, Craco, Luis, Yamani, Zahra, Flacau, Roxana, Swainson, Ian, Frandsen, Benjamin, Birgeneau, Robert, Liu, Yuhao, Karki, Bhupendra, Alfailakawi, Alaa, Neuefeind, Joerg C., Everett, Michelle, Wang, Hangdong, Xu, Binjie, Fang, Minghu, Klauss, H.-H.

Nematic fluctuations occur in a wide range physical systems from biological molecules to cuprates and iron pnictide high-Tc superconductors. It is unclear whether nematicity in pnictides arises from electronic spin or orbital degrees of freedom. We studied the iron-based Mott insulators La2O2Fe2OM2M = (S, Se), which are structurally similar to pnictides. Nuclear magnetic resonance revealed a critical slowing down of nematic fluctuations and complementary Mössbauerr spectroscopy data showed a change of electrical field gradient. The neutron pair distribution function technique detected local C2 fluctuations while neutron diffraction indicates that global C4 symmetry is preserved. A geometrically frustrated Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of the low temperature magnetic fluctuations. The nematicity is not due to spontaneous orbital order, instead it is linked to geometrically frustrated magnetism based on orbital selectivity. This study highlights the interplay between orbital order and spin fluctuations in nematicity.

2020, Chikina, A., Fedorov, A., Bhoi, D., Voroshnin, V., Haubold, E., Kushnirenko, Y., Kim, K.H., Borisenko, S.

The relationship between charge-density waves (CDWs) and superconductivity is a long-standing debate. Often observed as neighbors in phase diagrams, it is still unclear whether they cooperate, compete, or simply coexist. Using angle-resolved photoemission spectroscopy, we demonstrate here that by tuning the energy position of the van Hove singularity in Pd-doped 2H-TaSe2, one is able to suppress CDW and enhance superconductivity by more than an order of magnitude. We argue that it is particular fermiology of the material that is responsible for each phenomenon, thus explaining their persistent proximity as phases.

2021, Nikitin, S., Nishimoto, S., Fan, Y., Wu, J., Wu, L., Sukhanov, A., Brando, M., Pavlovskii, N., Xu, J., Vasylechko, L., Yu, R., Podlesnyak, A.

The Heisenberg antiferromagnetic spin-1/2 chain, originally introduced almost a century ago, is one of the best studied models in quantum mechanics due to its exact solution, but nevertheless it continues to present new discoveries. Its low-energy physics is described by the Tomonaga-Luttinger liquid of spinless fermions, similar to the conduction electrons in one-dimensional metals. In this work we investigate the Heisenberg spin-chain compound YbAlO3 and show that the weak interchain coupling causes Umklapp scattering between the left- and right-moving fermions and stabilizes an incommensurate spin-density wave order at q = 2kF under finite magnetic fields. These Umklapp processes open a route to multiple coherent scattering of fermions, which results in the formation of satellites at integer multiples of the incommensurate fundamental wavevector Q = nq. Our work provides surprising and profound insight into bandstructure control for emergent fermions in quantum materials, and shows how neutron diffraction can be applied to investigate the phenomenon of coherent multiple scattering in metals through the proxy of quantum magnetic systems.

2020, Yao, M., Manna, K., Yang, Q., Fedorov, A., Voroshnin, V., Valentin Schwarze, B., Hornung, J., Chattopadhyay, S., Sun, Z., Guin, S.N., Wosnitza, J., Borrmann, H., Shekhar, C., Kumar, N., Fink, J., Sun, Y., Felser, C.

Non-symmorphic chiral topological crystals host exotic multifold fermions, and their associated Fermi arcs helically wrap around and expand throughout the Brillouin zone between the high-symmetry center and surface-corner momenta. However, Fermi-arc splitting and realization of the theoretically proposed maximal Chern number rely heavily on the spin-orbit coupling (SOC) strength. In the present work, we investigate the topological states of a new chiral crystal, PtGa, which has the strongest SOC among all chiral crystals reported to date. With a comprehensive investigation using high-resolution angle-resolved photoemission spectroscopy, quantum-oscillation measurements, and state-of-the-art ab initio calculations, we report a giant SOC-induced splitting of both Fermi arcs and bulk states. Consequently, this study experimentally confirms the realization of a maximal Chern number equal to ±4 in multifold fermionic systems, thereby providing a platform to observe large-quantized photogalvanic currents in optical experiments.

2021, Yang, Xiaoqin, Liu, Yu, Ta, Huy Q., Rezvani, Ehsan, Zhang, Yue, Zeng, Mengqi, Fu, Lei, Bachmatiuk, Alicja, Luo, Jinping, Liu, Lijun, Rümmeli, Mark H.

Anchored Single-atom catalysts have emerged as a cutting-edge research field holding tremendous appeal for applications in the fields of chemicals, energy and the environment. However, single-atom-catalysts for crystal growth is a nascent field. Of the few studies available, all of them are based on state-of-the-art in situ microscopy investigations and computational studies, and they all look at the growth of monolayer graphene from a single-atom catalyst. Despite the limited number of studies, they do, collectively, represent a new sub-field of single-atom catalysis, namely single-atom catalytic growth of crystalline solids. In this review, we examine them on substrate-supported and as freestanding graphene fabrication, as well as rolled-up graphene, viz., single-walled carbon nanotubes (SWCNT), grown from a single atom. We also briefly discuss the catalytic etching of graphene and SWCNT’s and conclude by outlining the future directions we envision this nascent field to take.

2021, Senkovskiy, B., Nenashev, A., Alavi, S., Falke, Y., Hell, M., Bampoulis, P., Rybkovskiy, D., Usachov, D., Fedorov, A., Chernov, A., Gebhard, F., Meerholz, K., Hertel, D., Arita, M., Okuda, T., Miyamoto, K., Shimada, K., Fischer, F., Michely, T., Baranovskii, S., Lindfors, K., Szkopek, T., Grüneis, A.

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.

2020, Baek, S.-H., Ok, J.M., Kim, J.S., Aswartham, S., Morozov, I., Chareev, D., Urata, T., Tanigaki, K., Tanabe, Y., Büchner, B., Efremov, D.V.

The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc.

2021, Anh, Mai Lê, Potapov, Pavel, Lubk, Axel, Doert, Thomas, Ruck, Michael

The emergence of topological insulators (TIs) raised high expectations for their application in quantum computers and spintronics. Being bulk semiconductors, their nontrivial topology at the electronic bandgap enables dissipation-free charge and spin transport in protected metallic surface states. For application, crystalline thin films are requested in sufficient quantity. A suitable approach is the liquid phase exfoliation (LPE) of TI crystals that have layered structures. Bi2TeI is a weak 3D TI, which leads to protected edge states at the side facets of a crystal, as well as a topological crystalline insulator, which is responsible for protected states at the top and bottom faces. We developed an effective, scalable protocol for LPE of freestanding nanoflakes from Bi2TeI crystals. By heat treatment and sonication in isopropyl alcohol and poly(vinylpyrrolidone), crystalline Bi2TeI sheets with a thickness of ~50 nm were obtained and can therefore be considered for further processing toward microelectronic applications.