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Personne# Chunyu Guo

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Jonas De Jesus Diaz Gomez, Chunyu Guo, Xiangwei Huang, Philip Johannes Walter Moll, Matthias Carsten Putzke, Yi-Chiang Sun

The concept of quasi-symmetry-a perturbatively small deviation from exact symmetry-is introduced and leads to topological materials with strong resilience to perturbations. The crystal symmetry of a material dictates the type of topological band structure it may host, and therefore, symmetry is the guiding principle to find topological materials. Here we introduce an alternative guiding principle, which we call 'quasi-symmetry'. This is the situation where a Hamiltonian has exact symmetry at a lower order that is broken by higher-order perturbation terms. This enforces finite but parametrically small gaps at some low-symmetry points in momentum space. Untethered from the restraints of symmetry, quasi-symmetries eliminate the need for fine tuning as they enforce that sources of large Berry curvature occur at arbitrary chemical potentials. We demonstrate that quasi-symmetry in the semi-metal CoSi stabilizes gaps below 2 meV over a large near-degenerate plane that can be measured in the quantum oscillation spectrum. The application of in-plane strain breaks the crystal symmetry and gaps the degenerate point, observable by new magnetic breakdown orbits. The quasi-symmetry, however, does not depend on spatial symmetries and hence transmission remains fully coherent. These results demonstrate a class of topological materials with increased resilience to perturbations such as strain-induced crystalline symmetry breaking, which may lead to robust topological applications as well as unexpected topology beyond the usual space group classifications.

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Superconductor/metal interfaces are usually fabricated in heterostructures that join these dissimilar materials. A conceptually different approach has recently exploited the strain sensitivity of heavy-fermion superconductors, selectively transforming regions of the crystal into the metallic state by strain gradients. The strain is generated by differential thermal contraction between the sample and the substrate. Here, we present an improved finite-element model that reliably predicts the superconducting transition temperature in CeIrIn5 even in complex structures. Different substrates are employed to tailor the strain field into the desired shapes. Using this approach, both highly complex and strained as well as strain-free microstructures are fabricated to validate the model. This enables a high degree of control over the microscopic strain fields and forms the basis for more advanced structuring of superconductors as in Josephson junctions yet also finds natural use cases in any material class in which a modulation of the physical properties on a chip is desirable.& nbsp; (C)& nbsp;2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/).

Chunyu Guo, Philip Johannes Walter Moll, Yi-Chiang Sun

In materials, certain approximated symmetry operations can exist in a lower-order approximation of the effective model but are good enough to influence the physical responses of the system, and these approximated symmetries were recently dubbed "quasisymmetries" [Nat. Phys. 18, 813 (2022)]. In this paper, we reveal a hierarchy structure of the quasisymmetries and the corresponding nodal structures that they enforce via two different approaches of the perturbation expansions for the effective model in the chiral crystal material CoSi. In the first approach, we treat the spin-independent linear momentum (k) term as the zero-order Hamiltonian. Its energy bands are fourfold degenerate due to an SU(2) x SU(2) quasisymmetry. We next consider both the k-independent spin-orbit coupling (SOC) and full quadratic k terms as the perturbation terms and find that the first-order perturbation leads to a model described by a self-commuting "stabilizer code" Hamiltonian with a U(1) quasisymmetry that can protect nodal planes. In the second approach, we treat the SOC-free linear k term and k-independent SOC term as the zero order. They exhibit an SU(2) quasisymmetry, which can be reduced to U(1) quasisymmetry by a choice of quadratic terms. Correspondingly, a twofold degeneracy for all the bands due to the SU(2) quasisymmetry is reduced to twofold nodal planes that are protected by the U(1) quasisymmetry. For both approaches, including higher-order perturbation will break the U(1) quasisymmetry and induce a small gap similar to 1 meV for the nodal planes. These quasisymmetry protected near degeneracies play an essential role in understanding recent quantum oscillation experiments in CoSi.