More Like this

1. Correlated states controlled by a tunable van Hove singularity in moiré WSe2 bilayers

   https://doi.org/10.1038/s41467-025-57235-5  

   Knüppel, Patrick; Zhu, Jiacheng; Xia, Yiyu; Xia, Zhengchao; Han, Zhongdong; Zeng, Yihang; Watanabe, Kenji; Taniguchi, Takashi; Shan, Jie; Mak, Kin Fai (December 2025, Nature Communications)

   Abstract Twisted transition metal dichalcogenide (TMD) bilayers have enabled the discovery of superconductivity, ferromagnetism, correlated insulators, and a series of new topological phases of matter. However, the connection between these electronic phases of matter and the underlying band structure singularities has remained largely unexplored. Here, combining magnetic circular dichroism and exciton sensing measurements, we investigate the influence of a van Hove singularity (vHS) on the correlated phases in bilayer WSe₂with twist angle between 2 and 3 degrees. By tuning the vHS across the Fermi level using electric and magnetic fields, we observe Stoner ferromagnetism below moiré lattice filling one and Chern insulators at filling one. The experimental observations are supported by the continuum model band structure calculations. Our results highlight the prospect of engineering electronic phases of matter in moiré materials by tunable van Hove singularities. 

   more » « less
2. Prediction of Van Hove singularity systems in ternary borides

   https://doi.org/10.1038/s41524-023-01156-8  

   Sun, Yang; Zhang, Zhen; Porter, Andrew P.; Kovnir, Kirill; Ho, Kai-Ming; Antropov, Vladimir (December 2023, npj Computational Materials)

   Abstract A computational search for stable structures among both α and β phases of ternary ATB₄borides (A= Mg, Ca, Sr, Ba, Al, Ga, and Zn,Tis3dor4dtransition elements) has been performed. We found that α-ATB₄compounds withA= Mg, Ca, Al, andT = V, Cr, Mn, Fe, Ni, and Co form a family of structurally stable or almost stable materials. These systems are metallic in non-magnetic states and characterized by the formation of the localized molecular-like state of3dtransition metal atom dimers, which leads to the appearance of numerous Van Hove singularities in the electronic spectrum. The closeness of such singularities to the Fermi level can be easily tuned by electron doping. For the atoms in the middle of the3drow (Cr, Mn, and Fe), these singularities led to magnetic instabilities and magnetic ground states with a weakly metallic or semiconducting nature. Such states appear as non-trivial coexistence of the different spin ladders formed by magnetic dimers of3delements. These magnetic states can be characterized as an analog of the spin glass state. Experimental attempts to produce MgFeB₄and associated challenges are discussed, and promising directions for further synthetic studies are formulated. 

   more » « less
3. Electronic structure of the kagome metal ${\mathrm{YbTi}}_3{\mathrm{Bi}}_4$ studied using torque magnetometry

   https://doi.org/10.1103/PhysRevB.111.035145  

   Shtefiienko, Kyryl; Phillips, Cole; Ortiz, Brenden R; Graf, David E; Shrestha, Keshav (January 2025, Physical Review B)

   NA (Ed.)

   This study investigates the electronic structure of the kagome metal YbTi3Bi4 using high-field torque magnetometry. The torque signal measured at a maximum field of 41.5 T reveals clear de Haas–van Alphen (dHvA) oscillations with a major frequency peak at Fδ ∼ 130 T. By rotating the sample at various tilt angles θ, we observed that Fδ exhibits a nearly 1/cosθ dependence, indicating the presence of a quasi-two-dimensional (2D) Fermi surface (FS) in YbTi3Bi4. This argument is further supported by the detection of a forward-leaning, sawtoothlike waveform in the dHvA effect, a hallmark of 2D FS characteristics. Notably, we identified two high-frequency peaks near Fχ ∼ 1900 T and Fλ ∼ 5600 T; however, these peaks quickly disappear at θ greater than 21◦. To better understand experimental observations, we computed the electronic band structure and FS using ab initio density-functional theory (DFT). The electronic bands reveal the presence of several Dirac points, flat bands, and van Hove singularities near the Fermi level. Five bands cross the Fermi level and contribute to the FS of this material. The FS comprises cylindrical sheets, with theoretical frequencies from the FS pockets aligning well with the experimental dHvA frequencies. Several FS parameters characterizing Fδ were determined by analyzing the temperature and field dependence of the dHvA oscillations using the Lifshitz-Kosevich theory. The detailed electronic properties presented in this work provide critical insights into the electronic structure of YbTi3Bi4 and other titanium-based kagome compounds. 

   more » « less
4. Diverse electronic landscape of the kagome metal YbTi3Bi4

   https://doi.org/10.1038/s43246-024-00681-3  

   Sakhya, Anup Pradhan; Ortiz, Brenden R; Ghosh, Barun; Sprague, Milo; Mondal, Mazharul Islam; Matzelle, Matthew; Bin_Elius, Iftakhar; Valadez, Nathan; Mandrus, David G; Bansil, Arun; et al (December 2024, Communications Materials)

   Kagome lattices have emerged as an ideal platform for exploring exotic quantum phenomena in materials. Here, we report the discovery of Ti-based kagome metal YbTi3Bi4 which we characterize using angle-resolved photoemission spectroscopy (ARPES) and magneto-transport, in combination with density functional theory calculations. Our ARPES results reveal the complex fermiology of YbTi3Bi4 and provide spectroscopic evidence of four flat bands. Our measurements also show the presence of multiple van Hove singularities originating from Ti 3d orbitals and a linearly-dispersing gapped Dirac-like bulk state at the point in accord with our theoretical calculations. Our study establishes YbTi3Bi4 as a platform for exploring exotic phases in the wider LnTi3Bi4 (Ln = lanthanide) family of materials. 

   more » « less
5. Real Space and Time Imaging of Collective Headgroup Dipole Motions in Zwitterionic Lipid Bilayers

   https://doi.org/10.3390/membranes13040442  

   Bolmatov, Dima; Collier, C Patrick; Zav’yalov, Dmitry; Egami, Takeshi; Katsaras, John (April 2023, Membranes)

   Lipid bilayers are supramolecular structures responsible for a range of processes, such as transmembrane transport of ions and solutes, and sorting and replication of genetic materials, to name just a few. Some of these processes are transient and currently, cannot be visualized in real space and time. Here, we developed an approach using 1D, 2D, and 3D Van Hove correlation functions to image collective headgroup dipole motions in zwitterionic phospholipid bilayers. We show that both 2D and 3D spatiotemporal images of headgroup dipoles are consistent with commonly understood dynamic features of fluids. However, analysis of the 1D Van Hove function reveals lateral transient and re-emergent collective dynamics of the headgroup dipoles—occurring at picosecond time scales—that transmit and dissipate heat at longer times, due to relaxation processes. At the same time, the headgroup dipoles also generate membrane surface undulations due a collective tilting of the headgroup dipoles. A continuous intensity band of headgroup dipole spatiotemporal correlations—at nanometer length and nanosecond time scales—indicates that dipoles undergo stretching and squeezing elastic deformations. Importantly, the above mentioned intrinsic headgroup dipole motions can be externally stimulated at GHz-frequency scale, enhancing their flexoelectric and piezoelectric capabilities (i.e., increased conversion efficiency of mechanical energy into electric energy). In conclusion, we discuss how lipid membranes can provide molecular-level insights about biological learning and memory, and as platforms for the development of the next generation of neuromorphic computers. 

   more » « less