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1. Data-driven studies of magnetic two-dimensional materials

   https://doi.org/10.1038/s41598-020-72811-z  

   Rhone, Trevor David ; Chen, Wei ; Desai, Shaan ; Torrisi, Steven B. ; Larson, Daniel T. ; Yacoby, Amir ; Kaxiras, Efthimios ( September 2020 , Scientific Reports)

   We use a data-driven approach to study the magnetic and thermodynamic properties of van der Waals (vdW) layered materials. We investigate monolayers of the form$$\hbox {A}_2\hbox {B}_2\hbox {X}_6$$${\text{A}}_2{\text{B}}_2{\text{X}}_6$, based on the known material$$\hbox {Cr}_2\hbox {Ge}_2\hbox {Te}_6$$${\text{Cr}}_2{\text{Ge}}_2{\text{Te}}_6$, using density functional theory (DFT) calculations and machine learning methods to determine their magnetic properties, such as magnetic order and magnetic moment. We also examine formation energies and use them as a proxy for chemical stability. We show that machine learning tools, combined with DFT calculations, can provide a computationally efficient means to predict properties of such two-dimensional (2D) magnetic materials. Our datamore »analytics approach provides insights into the microscopic origins of magnetic ordering in these systems. For instance, we find that the X site strongly affects the magnetic coupling between neighboring A sites, which drives the magnetic ordering. Our approach opens new ways for rapid discovery of chemically stable vdW materials that exhibit magnetic behavior.

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2. Magnetic field-tuned Fermi liquid in a Kondo insulator

   https://doi.org/10.1038/s41467-019-13421-w  

   Kushwaha, Satya K. ; Chan, Mun K. ; Park, Joonbum ; Thomas, S. M. ; Bauer, Eric D. ; Thompson, J. D. ; Ronning, F. ; Rosa, Priscila F. S. ; Harrison, Neil ( December 2019 , Nature Communications)

   Kondo insulators are expected to transform into metals under a sufficiently strong magnetic field. The closure of the insulating gap stems from the coupling of a magnetic field to the electron spin, yet the required strength of the magnetic field–typically of order 100 T–means that very little is known about this insulator-metal transition. Here we show that Ce$${}_{3}$$${}_3$Bi$${}_{4}$$${}_4$Pd$${}_{3}$$${}_3$, owing to its fortuitously small gap, provides an ideal Kondo insulator for this investigation. A metallic Fermi liquid state is established above a critical magnetic field of only$${B}_{{\rm{c}}}\approx$$$B_c\approx$11 T. A peak in the strength of electronic correlations near$${B}_{{\rm{c}}}$$$B_c$, which is evidentmore »in transport and susceptibility measurements, suggests that Ce$${}_{3}$$${}_3$Bi$${}_{4}$$${}_4$Pd$${}_{3}$$${}_3$may exhibit quantum criticality analogous to that reported in Kondo insulators under pressure. Metamagnetism and the breakdown of the Kondo coupling are also discussed.

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3. Exchange biased anomalous Hall effect driven by frustration in a magnetic kagome lattice

   https://doi.org/10.1038/s41467-020-14326-9  

   Lachman, Ella ; Murphy, Ryan A. ; Maksimovic, Nikola ; Kealhofer, Robert ; Haley, Shannon ; McDonald, Ross D. ; Long, Jeffrey R. ; Analytis, James G. ( January 2020 , Nature Communications)

   Co$$_{3}$$${}_3$Sn$$_{2}$$${}_2$S$$_{2}$$${}_2$is a ferromagnetic Weyl semimetal that has been the subject of intense scientific interest due to its large anomalous Hall effect. We show that the coupling of this material’s topological properties to its magnetic texture leads to a strongly exchange biased anomalous Hall effect. We argue that this is likely caused by the coexistence of ferromagnetism and geometric frustration intrinsic to the kagome network of magnetic ions, giving rise to spin-glass behavior and an exchange bias.
4. Roaming pathways and survival probability in real-time collisional dynamics of cold and controlled bialkali molecules

   https://doi.org/10.1038/s41598-021-90004-0  

   Kłos, Jacek ; Guan, Qingze ; Li, Hui ; Li, Ming ; Tiesinga, Eite ; Kotochigova, Svetlana ( May 2021 , Scientific Reports)

   Perfectly controlled molecules are at the forefront of the quest to explore chemical reactivity at ultra low temperatures. Here, we investigate for the first time the formation of the long-lived intermediates in the time-dependent scattering of cold bialkali$$^{23}\hbox {Na}^{87}$$${}^{23}{\text{Na}}^{87}$Rb molecules with and without the presence of infrared trapping light. During the nearly 50 nanoseconds mean collision time of the intermediate complex, we observe unconventional roaming when for a few tens of picoseconds either NaRb or$$\hbox {Na}_2$$${\text{Na}}_2$and$$\hbox {Rb}_2$$${\text{Rb}}_2$molecules with large relative separation are formed before returning to the four-atom complex. We also determine the likelihood of molecular loss when themore »trapping laser is present during the collision. We find that at a wavelength of 1064 nm the$$\hbox {Na}_2\hbox {Rb}_2$$${\text{Na}}_2{\text{Rb}}_2$complex is quickly destroyed and thus that the$$^{23}\hbox {Na}^{87}$$${}^{23}{\text{Na}}^{87}$Rb molecules are rapidly lost.

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5. Melting of charge order in the low-temperature state of an electronic ferroelectric-like system

   https://doi.org/10.1038/s41535-020-0217-5  

   Hassan, Nora M. ; Thirunavukkuarasu, Komalavalli ; Lu, Zhengguang ; Smirnov, Dmitry ; Zhilyaeva, Elena I. ; Torunova, Svetlana ; Lyubovskaya, Rimma N. ; Drichko, Natalia ( March 2020 , npj Quantum Materials)

   Strong electronic interactions can drive a system into a state with a symmetry breaking. Lattice frustration or competing interactions tend to prevent symmetry breaking, leading to quantum disordered phases. In spin systems frustration can produce a spin liquid state. Frustration of a charge degree of freedom also can result in various exotic states, however, experimental data on these effects is scarce. In this work we demonstrate how in a Mott insulator on a weakly anisotropic triangular lattice a charge ordered state melts on cooling down to low temperatures. Raman scattering spectroscopy finds that$$\kappa$$$\kappa$-(BEDT-TTF)$${}_{2}$$${}_2$Hg(SCN)$${}_{2}$$${}_2$Cl enters an insulating “dipole solid” statemore »at$$T=30\,{\mathrm{K}}$$$T=30K$, but below$$T=15\,{\mathrm{K}}$$$T=15K$the order melts, while preserving the insulating energy gap. Based on these observations, we suggest a phase diagram relevant to other quantum paraelectric materials.

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