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1. Asymptotic Pomeranchuk instability of Fermi liquids in half-filled Landau levels

   https://doi.org/10.1038/s41598-023-28614-z  

   Quintanilla, Jorge; Ciftja, Orion (January 2023, Scientific Reports)

   Abstract We present a theory of spontaneous Fermi surface deformations for half-filled Landau levels (filling factors of the form$$\nu =2 \, n+1/2$$ $\nu =2n+1/2$). We assume the half-filled level to be in a compressible, Fermi liquid state with a circular Fermi surface. The Landau level projection is incorporated via a modified effective electron-electron interaction and the resulting band structure is described within the Hartree-Fock approximation. We regulate the infrared divergences in the theory and probe the intrinsic tendency of the Fermi surface to deform through Pomeranchuk instabilities. We find that the corresponding susceptibility never diverges, though the system is asymptotically unstable in the$$n \rightarrow \infty $$ $n\to \infty$limit. 

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2. Structural phase purification of bulk HfO ₂ :Y through pressure cycling

   https://doi.org/10.1073/pnas.2312571121  

   Musfeldt, J. L.; Singh, Sobhit; Fan, Shiyu; Gu, Yanhong; Xu, Xianghan; Cheong, S.-W.; Liu, Z.; Vanderbilt, David; Rabe, Karin M. (January 2024, Proceedings of the National Academy of Sciences)

   We combine synchrotron-based infrared absorption and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of hafnia under compression. We find that pressure drives HfO ${}_2$:7%Y from the mixed monoclinic ( $P{2}_1/c$) $+$antipolar orthorhombic ( $\mathit{Pbca}$) phase to pure antipolar orthorhombic ( $\mathit{Pbca}$) phase at approximately 6.3 GPa. This transformation is irreversible, meaning that upon release, the material is kinetically trapped in the $\mathit{Pbca}$metastable state at 300 K. Compression also drives polar orthorhombic ( $Pca{2}_1$) hafnia into the tetragonal ( $P{4}_2/nmc$) phase, although the latter is not metastable upon release. These results are unified by an analysis of the energy landscape. The fact that pressure allows us to stabilize targeted metastable structures with less Y stabilizer is important to preserving the flat phonon band physics of pure HfO ${}_2$. 

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3. Thermally generated spin current in the topological insulator Bi ₂ Se ₃

   https://doi.org/10.1126/sciadv.adi4540  

   Jain, Rakshit; Stanley, Max; Bose, Arnab; Richardella, Anthony R.; Zhang, Xiyue S.; Pillsbury, Timothy; Muller, David A.; Samarth, Nitin; Ralph, Daniel C. (December 2023, Science Advances)

   We present measurements of thermally generated transverse spin currents in the topological insulator Bi₂Se₃, thereby completing measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnetoresistance for bilayers of Bi₂Se₃/CoFeB. We find that Bi₂Se₃does generate substantial thermally driven spin currents. A lower bound for the ratio of spin current density to thermal gradient is $\frac{J_s}{{\mathbf{\nabla }}_{\mathit{x}}\mathit{T}}$= (4.9 ± 0.9) × 10⁶ $\left(\frac{\hslash }{2e}\right)\frac{\mathbf{A}{\mathbf{m}}^{-2}}{\mathbf{K}\text{μ}{\mathbf{m}}^{-1}}$, and a lower bound for the magnitude of the spin Nernst ratio is −0.61 ± 0.11. The spin Nernst ratio for Bi₂Se₃is the largest among all materials measured to date, two to three times larger compared to previous measurements for the heavy metals Pt and W. Strong thermally generated spin currents in Bi₂Se₃can be understood via Mott relations to be due to an overall large spin Hall conductivity and its dependence on electron energy. 

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4. Partons as unique ground states of quantum Hall parent Hamiltonians: The case of Fibonacci anyons

   https://doi.org/10.21468/SciPostPhys.15.2.043  

   Tanhayi Ahari, Mostafa; Bandyopadhyay, Sumanta; Nussinov, Zohar; Seidel, Alexander; Ortiz, Gerardo (January 2023, SciPost Physics)

   We present microscopic, multiple Landau level, (frustration-free and positive semi-definite) parent Hamiltonians whose ground states, realizing different quantum Hall fluids, are parton-like and whose excitations display either Abelian or non-Abelian braiding statistics. We prove ground state energy monotonicity theorems for systems with different particle numbers in multiple Landau levels, demonstrate S-duality in the case of toroidal geometry, and establish complete sets of zero modes of special Hamiltonians stabilizing parton-like states, specifically at filling factor\nu=2/3 $\nu =2/3$. The emergent Entangled Pauli Principle (EPP), introduced in [Phys. Rev. B 98, 161118(R) (2018)] and which defines the “DNA” of the quantum Hall fluid, is behind the exact determination of the topological characteristics of the fluid, including charge and braiding statistics of excitations, and effective edge theory descriptions. When the closed-shell condition is satisfied, the densest (i.e., the highest density and lowest total angular momentum) zero-energy mode is a unique parton state. We conjecture that parton-like states generally span the subspace of many-body wave functions with the two-bodyM $M$-clustering property within any given number of Landau levels, that is, wave functions withM $M$th-order coincidence plane zeroes and both holomorphic and anti-holomorphic dependence on variables. General arguments are supplemented by rigorous considerations for theM=3 $M=3$case of fermions in four Landau levels. For this case, we establish that the zero mode counting can be done by enumerating certain patterns consistent with an underlying EPP. We apply the coherent state approach of [Phys. Rev. X 1, 021015 (2011)] to show that the elementary (localized) bulk excitations are Fibonacci anyons. This demonstrates that the DNA associated with fractional quantum Hall states encodes all universal properties. Specifically, for parton-like states, we establish a link with tensor network structures of finite bond dimension that emerge via root level entanglement. 

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5. Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces

   https://doi.org/10.1073/pnas.2307151120  

   Wang, Xiaoyu; Finney, Joe; Sharpe, Aaron_L; Rodenbach, Linsey_K; Hsueh, Connie_L; Watanabe, Kenji; Taniguchi, Takashi; Kastner, M_A; Vafek, Oskar; Goldhaber-Gordon, David (August 2023, Proceedings of the National Academy of Sciences)

   Anisotropic hopping in a toy Hofstadter model was recently invoked to explain a rich and surprising Landau spectrum measured in twisted bilayer graphene away from the magic angle. Suspecting that such anisotropy could arise from unintended uniaxial strain, we extend the Bistritzer–MacDonald model to include uniaxial heterostrain and present a detailed analysis of its impact on band structure and magnetotransport. We find that such strain strongly influences band structure, shifting the three otherwise-degenerate van Hove points to different energies. Coupled to a Boltzmann magnetotransport calculation, this reproduces previously unexplained nonsaturating $B^2$magnetoresistance over broad ranges of density near filling $\nu =\pm 2$and predicts subtler features that had not been noticed in the experimental data. In contrast to these distinctive signatures in longitudinal resistivity, the Hall coefficient is barely influenced by strain, to the extent that it still shows a single sign change on each side of the charge neutrality point—surprisingly, this sign change no longer occurs at a van Hove point. The theory also predicts a marked rotation of the electrical transport principal axes as a function of filling even for fixed strain and for rigid bands. More careful examination of interaction-induced nematic order versus strain effects in twisted bilayer graphene could thus be in order. 

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