E L E C T R O N I C S T R U C T U R E , M A G N E T I S M A N D D Y N A M I C S
S C I E N T I F I C H I G H L I G H T S
1 1 8 H I G H L I G H T S 2 0 2 1 I
Although MoxSy clusters are similar in chemical composition to conventional layered MoS2 catalysts, their catalytic hydrogenation performance is decidedly different. In a feed without sulfiding agents, both catalysts showed stable ethene hydrogenation rates over a period of ~150 h, whereas a traditional catalyst composed of MoS2 slabs supported on MFI zeolite deactivated significantly under the same conditions. Interestingly, the activities per cluster of Mo2S4 and Mo4S4 are the same, leading to the conclusion that the whole cluster, irrespective of the cluster s nuclearity, must be considered as the active hydrogenation
site. The role of unpaired electrons at the active centres on the catalytic stability and the hydrogenation mechanism are currently under further investigation.
In summary, combining state-of-the-art XAS and XES spectroscopy with DFT made it possible to determine the structure of two TMS clusters incorporated within the framework of a zeolite. The findings on structural and electronic similarities of these catalysts and the nitrogenase FeMo-cofactor open an intriguing pathway towards linking traditional heterogeneous and enzymatic catalysis.
PRINCIPAL PUBLICATION AND AUTHORS
Zeolite-stabilized di- and tetranuclear molybdenum sulfide clusters form stable catalytic hydrogenation sites, R. Weindl (a), R. Khare (a), L. Kovarik (b), A. Jentys (a), K. Reuter (a,c), H. Shi (a,d), J.A. Lercher (a,b), Angew. Chem. Int. Ed. 60, 9301-9305 (2021); https:/doi.org/10.1002/anie.202015769 (a) Department of Chemistry and Catalysis Research Center, Technical University of Munich, Garching (Germany) (b) Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland (USA) (c) Fritz Haber Institute of the Max Planck Society, Berlin (Germany) (d) School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou (China)
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Making the strange metal in high-critical-temperature superconductors even stranger
Mechanical strain in ultrathin films sheds new light on the understanding of high-temperature superconductors (HTS). Here, transport and RIXS measurements are combined to show that strain can be used to modify the material ground state and suppress charge density waves, which in turn restores the strange metal behaviour in a large portion of the HTS phase diagram.
Cuprate high-critical-temperature superconductors (HTS) belong to a class of materials where strong electron- electron correlations play a fundamental role . The strange metal phase of these superconductors is one of the most striking manifestations of the strong correlations. At optimal doping, this phase manifests as a linear temperature-dependence of the resistivity that persists to the lowest T when superconductivity is suppressed. This behaviour is fundamentally different from that observed in more conventional metals, where a T-linear dependence of the resistivity is found only at high temperatures where phonon scattering dominates the transport.
In cuprates, the T-linear resistivity is lost for doping below the optimal doping. Here, the deviation from T-linear behaviour happens at temperatures close to T*, known as the pseudogap temperature, where states are missing at
the Fermi energy. In the pseudogap region, the HTS phase diagram also hosts a plethora of intertwined symmetric- breaking ordering phenomena ; charge density wave (CDW) order  is the most prominent one. The association between the departure from the T-linear resistivity and the occurrence of the pseudogap has long been speculated. However, there is no consensus on the physics at play, nor on the causality hierarchy among pseudogap, local orders and strange metal phenomenology . The challenge is to disentangle the various possible mechanisms leading to the breakdown of the T-linear resistivity. One way to address this challenge is to tune the local properties of underdoped high-temperature superconductors.
To tune the ground state in thin films of the cuprate material YBa2Cu3O7 δ (YBCO), the geometric modification of the unit cell occurring under the strong strain induced by the substrate when the film thickness is reduced from 50 down to 10 nm was utilised. In particular, X-ray diffraction characterisation showed that by squeezing the thickness the total volume of the cell is unchanged, while the b-axis expands significantly. Such cell deformation implies an increase of orthorhombicity and, indirectly, the occurrence of a nematic state, which dramatically modifies the Fermi surfaces of YBCO and the CDW order. To investigate the transport and possibly determine its connection with the charge density wave order, resistivity vs. temperature ρ(T) and resonant inelastic X-ray scattering (RIXS) spectra were measured at beamline ID32 along both in-plane (a and b) directions of the 50- and 10-nm-thick films.