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Register a new userTopological crystalline superconductivity in locally noncentrosymmetric CeRh$_2$As$_2$
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Recent discovery of superconductivity in CeRh$_2$As$_2$ clarified an unusual $H$-$T$ phase diagram with two superconducting phases [Khim et al. arXiv:2101.09522]. The experimental observation has been interpreted based on the even-odd parity transition characteristic of locally noncentrosymmetric superconductors. Indeed, the inversion symmetry is locally broken at the Ce site, and CeRh$_2$As$_2$ molds a new class of exotic superconductors. The low-temperature and high-field superconducting phase is a candidate for the odd-parity pair-density-wave state, suggesting a possibility of topological superconductivity as spin-triplet superconductors are. In this paper, we first derive the formula expressing the $mathbb{Z}_2$ invariant of glide symmetric and time-reversal symmetry broken superconductors by the number of Fermi surfaces on a glide invariant line. Next, we conduct a first-principles calculation for the electronic structure of CeRh$_2$As$_2$. Combining the results, we show that the field-induced odd-parity superconducting phase of CeRh$_2$As$_2$ is a platform of topological crystalline superconductivity protected by the nonsymmorphic glide symmetry and accompanied by boundary Majorana fermions.
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We report the discovery of two-phase unconventional superconductivity in CeRh$_2$As$_2$. Using thermodynamic probes, we establish that the superconducting critical field of its high-field phase is as high as 14 T, remarkable in a material whose transition temperature is 0.26 K. Furthermore, a $c$-axis field drives a transition between two different superconducting phases. In spite of the fact that CeRh$_2$As$_2$ is globally centrosymmetric, we show that local inversion-symmetry breaking at the Ce sites enables Rashba spin-orbit coupling to play a key role in the underlying physics. More detailed analysis identifies the transition from the low- to high-field states to be associated with one between states of even and odd parity.
Recent experiments reported gate-induced superconductivity in the monolayer 1T$$-WTe$_2$ which is a two-dimensional topological insulator in its normal state [1, 2]. The in-plane upper critical field $B_{c2}$ is found to exceed the conventional Pauli paramagnetic limit $B_p$ by 1-3 times. The enhancement cannot be explained by conventional spin-orbit coupling which vanishes due to inversion symmetry. In this work, we unveil some distinctive superconducting properties of centrosymmetric 1T$$-WTe$_2$ which arise from the coupling of spin, momentum and band parity degrees of freedom. As a result of this spin-orbit-parity coupling: (i) there is a first-order superconductor-metal transition at $B_{c2}$ much higher than the Pauli paramagnetic limit $B_p$, (ii) spin-susceptibility is anisotropic with respect to in-plane directions and results in anisotropic $B_{c2}$ and (iii) the $B_{c2}$ exhibits a strong gate dependence as the spin-orbit-parity coupling is significant only near the topological band crossing points. The importance of SOPC on the topologically nontrivial inter-orbital pairing phase is also discussed. Our theory generally applies to centrosymmetric materials with topological band
We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor PdTe$_2$ by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe$_2$ with its sister compound PtSe$_2$, we demonstrate how enhanced inter-layer hopping in the Te-based material drives a band inversion within the anti-bonding p-orbital manifold well above the Fermi level. We show how this mediates spin-polarised topological surface states which form rich multi-valley Fermi surfaces with complex spin textures. Scanning tunneling spectroscopy reveals type-II superconductivity at the surface, and moreover shows no evidence for an unconventional component of its superconducting order parameter, despite the presence of topological surface states.
Recently, evidence has emerged for a field-induced even- to odd-parity superconducting phase transition in CeRh$_2$As$_2$ [S. Khim {it et al.}, arXiv:2101.09522]. Here we argue that the $P4/nmm$ non-symmorphic crystal structure of CeRh$_2$As$_2$ plays a central role in enabling this transition. Specifically, the non-symmorphic symmetries enforce an unusual spin structure near Brillouin zone boundaries that ensures large spin-orbit interactions in these regions of momentum space. This enables a high-temperature field-induced even- to odd-parity transition. We further provide an explicit illustration of the robustness of a field induced odd-parity state within a DFT-inspired model of the superconducting state that includes Fermi surfaces located about a Dirac line at the zone boundary and also about the zone center. Finally, we comment on the relevance of our results to superconducting FeSe, which also crystallizes in a $P4/nmm$ structure.
Superconductivity was first observed more than a century ago, but the search for new superconducting materials remains a challenge. The Cooper pairs in superconductors are ideal embodiments of quantum entanglement. Thus, novel superconductors can be critical for both learning about electronic systems in condensed matter and for possible application in future quantum technologies. Here two previously unreported materials, NbIr$_2$B$_2$ and TaIr$_2$B$_2$, are presented with superconducting transitions at 7.2 and 5.2 K, respectively. They display a unique noncentrosymmetric crystal structure, and for both compounds the magnetic field that destroys the superconductivity at 0 K exceeds one of the fundamental characteristics of conventional superconductors (the Pauli limit), suggesting that the superconductivity may be unconventional. Supporting this experimentally based deduction, first-principle calculations show a spin split Fermi surface due to the presence of strong spin-orbit coupling. These materials may thus provide an excellent platform for the study of non-BCS superconductivity in intermetallic compounds.
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