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1. Weyl Fermions and broken symmetry phases of laterally confined 3 He films

   https://doi.org/10.1088/1361-648X/acf42b  

   Wu, Hao ; Sauls, J. A. ( September 2023 , Journal of Physics: Condensed Matter)

   Broken symmetries in topological condensed matter systems have implications for the spectrum of Fermionic excitations confined on surfaces or topological defects. The Fermionic spectrum of confined (quasi-2D)³He-A consists of branches of chiral edge states. The negative energy states are related to the ground-state angular momentum,$L_z=(N/2)\hslash$, for$N/2$Cooper pairs. The power law suppression of the angular momentum,$L_z(T)\simeq (N/2)\hslash [1-\frac{2}{3}(\pi T/\mathrm{\Delta }{)}^2]$for$0⩽T\ll T_c$, in the fully gapped 2D chiral A-phase reflects the thermal excitation of the chiral edge Fermions. We discuss the effects of wave function overlap, and hybridization between edge states confined near opposing edge boundaries on the edge currents, ground-state angular momentum and ground-state order parameter of superfluid³He thin films. Under strong lateral confinement, the chiral A phase undergoes a sequence of phase transitions, first to a pair density wave (PDW) phase with broken translational symmetry at$D_{c2}\sim 16{\xi }_0$. The PDW phase is described by a periodic array of chiral domains with alternating chirality, separated by domain walls. The period of PDW phase diverges as the confinement length$D\to D_{c_2}$. The PDW phase breaks time-reversal symmetry, translation invariance, but is invariant under the combination of time-reversal and translation by a one-half period of the PDW. The mass current distribution of the PDW phase reflects this combined symmetry, and originates from the spectra of edge Fermions and the chiral branches bound to the domain walls. Under sufficiently strong confinement a second-order transition occurs to the non-chiral ‘polar phase’ at$D_{c1}\sim 9{\xi }_0$, in which a single p-wave orbital state of Cooper pairs is aligned along the channel.

    

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2. Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr 3 SnO

   https://doi.org/10.1002/adma.202000809  

   Ma, Yanjun ; Edgeton, Anthony ; Paik, Hanjong ; Faeth, Brendan D. ; Parzyck, Christopher T. ; Pamuk, Betül ; Shang, Shun‐Li ; Liu, Zi‐Kui ; Shen, Kyle M. ; Schlom, Darrell G. ; et al ( July 2020 , Advanced Materials)

   Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr₃SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations, a deposition approach is designed and implemented to achieve the adsorption‐controlled growth of epitaxial Sr₃SnO single‐crystal films by molecular‐beam epitaxy (MBE). In situ transport and angle‐resolved photoemission spectroscopy measurements reveal the metallic and electronic structure of the as‐grown samples. Compared with conventional MBE, the used synthesis route results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of Sr₃SnO opens opportunities to manipulate topological states by tuning symmetries via strain engineering and heterostructuring.

    

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3. Magnetochiral tunneling in paramagnetic Co 1/3 NbS 2

   https://doi.org/10.1073/pnas.2318443121  

   Lim, Seongjoon ; Singh, Sobhit ; Huang, Fei-Ting ; Pan, Shangke ; Wang, Kefeng ; Kim, Jaewook ; Kim, Jinwoong ; Vanderbilt, David ; Cheong, Sang-Wook ( March 2024 , Proceedings of the National Academy of Sciences)

   Electric currents have the intriguing ability to induce magnetization in nonmagnetic crystals with sufficiently low crystallographic symmetry. Some associated phenomena include the non-linear anomalous Hall effect in polar crystals and the nonreciprocal directional dichroism in chiral crystals when magnetic fields are applied. In this work, we demonstrate that the same underlying physics is also manifested in the electronic tunneling process between the surface of a nonmagnetic chiral material and a magnetized scanning probe. In the paramagnetic but chiral metallic compound Co_1/3NbS₂, the magnetization induced by the tunneling current is shown to become detectable by its coupling to the magnetization of the tip itself. This results in a contrast across different chiral domains, achieving atomic-scale spatial resolution of structural chirality. To support the proposed mechanism, we used first-principles theory to compute the chirality-dependent current-induced magnetization and Berry curvature in the bulk of the material. Our demonstration of this magnetochiral tunneling effect opens up an avenue for investigating atomic-scale variations in the local crystallographic symmetry and electronic structure across the structural domain boundaries of low-symmetry nonmagnetic crystals.

    

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4. Exploring the Pb 1− x Sr x HfO 3 System and Potential for High Capacitive Energy Storage Density and Efficiency

   https://doi.org/10.1002/adma.202105967  

   Acharya, Megha ; Banyas, Ella ; Ramesh, Maya ; Jiang, Yizhe ; Fernandez, Abel ; Dasgupta, Arvind ; Ling, Handong ; Hanrahan, Brendan ; Persson, Kristin ; Neaton, Jeffrey B. ; et al ( October 2021 , Advanced Materials)

   The hafnate perovskites PbHfO₃(antiferroelectric) and SrHfO₃(“potential” ferroelectric) are studied as epitaxial thin films on SrTiO₃(001) substrates with the added opportunity of observing a morphotropic phase boundary (MPB) in the Pb_1−xSr_xHfO₃system. The resulting (240)‐oriented PbHfO₃(Pba2) films exhibited antiferroelectric switching with a saturation polarization ≈53 µC cm⁻²at 1.6 MV cm⁻¹, weak‐field dielectric constant ≈186 at 298 K, and an antiferroelectric‐to‐paraelectric phase transition at ≈518 K. (002)‐oriented SrHfO₃films exhibited neither ferroelectric behavior nor evidence of a polarP4mmphase . Instead, the SrHfO₃films exhibited a weak‐field dielectric constant ≈25 at 298 K and no signs of a structural transition to a polar phase as a function of temperature (77–623 K) and electric field (–3 to 3 MV cm⁻¹). While the lack of ferroelectric order in SrHfO₃removes the potential for MPB, structural and property evolution of the Pb_1−xSr_xHfO₃(0 ≤x < 1) system is explored. Strontium alloying increased the electric‐breakdown strength (E_B) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (U_r) and efficiency (η). The composition, Pb_0.5Sr_0.5HfO₃produced the best combination ofE_B = 5.12 ± 0.5 MV cm⁻¹,U_r = 77 ± 5 J cm⁻³, and η = 97 ± 2%, well out‐performing PbHfO₃and other antiferroelectric oxides.

    

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5. Influence of Temperature and CO 2 On Plasma‐membrane Permeability to CO 2 and HCO 3 − in the Marine Haptophytes Emiliania huxleyi and Calcidiscus leptoporus (Prymnesiophyceae)

   https://doi.org/10.1111/jpy.13017  

   Blanco‐Ameijeiras, Sonia ; Stoll, Heather M. ; Zhang, Hongrui ; Hopkinson, Brian M. ; Raven, ed., J. ( June 2020 , Journal of Phycology)

   Membrane permeabilities to CO₂and HCO₃⁻constrain the function of CO₂concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO₂but effectively impermeable to HCO₃⁻. Here, CO₂and HCO₃⁻membrane permeabilities were measured using an¹⁸O‐exchange technique on two species of haptophyte algae,Emiliania huxleyiandCalcidiscus leptoporus, which showed that the plasma membranes of these species are also highly permeable to CO₂(0.006–0.02 cm · s⁻¹) but minimally permeable to HCO₃⁻. Increased temperature and CO₂generally increased CO₂membrane permeabilities in both species, possibly due to changes in lipid composition or CO₂channel proteins. Changes in CO₂membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma‐membrane permeabilities to CO₂were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret¹⁸O‐exchange data assumes that carbonic anhydrase, which catalyzes¹⁸O‐exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two‐compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate¹⁸O‐exchange data and then inferring plasma‐membrane CO₂permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO₂permeabilities are minimal estimates but should be quite accurate under most conditions.

    

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