Browsing by Author "Nica, Emilian M."
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Item Fully gapped d-wave superconductivity in CeCu2Si2(National Academy of Sciences, 2018) Pang, Guiming; Smidman, Michael; Zhang, Jinglei; Jiao, Lin; Weng, Zongfa; Nica, Emilian M.; Chen, Ye; Jiang, Wenbing; Zhang, Yongjun; Xie, Wu; Jeevan, Hirale S.; Lee, Hanoh; Gegenwart, Philipp; Steglich, Frank; Si, Qimiao; Yuan, HuiqiuThe nature of the pairing symmetry of the first heavy fermion superconductor CeCu2Si2 has recently become the subject of controversy. While CeCu2Si2 was generally believed to be a d-wave superconductor, recent low-temperature specific heat measurements showed evidence for fully gapped superconductivity, contrary to the nodal behavior inferred from earlier results. Here, we report London penetration depth measurements, which also reveal fully gapped behavior at very low temperatures. To explain these seemingly conflicting results, we propose a fully gapped d+d band-mixing pairing state for CeCu2Si2, which yields very good fits to both the superfluid density and specific heat, as well as accounting for a sign change of the superconducting order parameter, as previously concluded from inelastic neutron scattering results.Item Glide reflection symmetry, Brillouin zone folding, and superconducting pairing for the P4/nmm space group(American Physical Society, 2015) Nica, Emilian M.; Yu, Rong; Si, QimiaoMotivated by the studies of the superconducting pairing states in the iron-based superconductors, we analyze the effects of Brillouin zone folding procedure from a space-group symmetry perspective for a general class of materials with the P4/nmm space group. The Brillouin zone folding amounts to working with an effective 1-Fe unit cell, instead of the crystallographic 2-Fe unit cell. We show that the folding procedure can be justified by the validity of a glide reflection symmetry throughout the crystallographic Brillouin zone and by the existence of a minimal double degeneracy along the edges of the latter. We also demonstrate how the folding procedure fails when a local spin-orbit coupling is included although the latter does not break any of the space-group symmetries of the bare Hamiltonian. In light of these general symmetry considerations, we further discuss the implications of the glide reflection symmetry for the superconducting pairing in an effective multiorbital t−J1−J2 model. We find that, for spin-singlet pairing states, the P4/nmm space-group symmetry allows only even parity under the glide reflection and zero total momentum.Item Multiorbital singlet pairing and d + d superconductivity(Springer Nature, 2021) Nica, Emilian M.; Si, Qimiao; Rice Center for Quantum MaterialsRecent experiments in multiband Fe-based and heavy-fermion superconductors have challenged the long-held dichotomy between simple s- and d-wave spin-singlet pairing states. Here, we advance several time-reversal-invariant irreducible pairings that go beyond the standard singlet functions through a matrix structure in the band/orbital space, and elucidate their naturalness in multiband systems. We consider the sτ3 multiorbital superconducting state for Fe-chalcogenide superconductors. This state, corresponding to a d + d intra- and inter-band pairing, is shown to contrast with the more familiar d + id state in a way analogous to how the B- triplet pairing phase of 3He superfluid differs from its A- phase counterpart. In addition, we construct an analog of the sτ3 pairing for the heavy-fermion superconductor CeCu2Si2, using degrees-of-freedom that incorporate spin-orbit coupling. Our results lead to the proposition that d-wave superconductors in correlated multiband systems will generically have a fully-gapped Fermi surface when they are examined at sufficiently low energies.Item Orbital Selectivity in Electron Correlations and Superconducting Pairing of Iron-Based Superconductors(Frontiers Media S.A., 2021) Yu, Rong; Hu, Haoyu; Nica, Emilian M.; Zhu, Jian-Xin; Si, Qimiao; Center for Quantum MaterialsElectron correlations play a central role in iron-based superconductors. In these systems, multiple Fe $3d$-orbitals are active in the low-energy physics, and they are not all degenerate. For these reasons, the role of orbital-selective correlations has been an active topic in the study of the iron-based systems. In this paper, we survey the recent developments on the subject. For the normal state, we emphasize the orbital-selective Mott physics that has been extensively studied, especially in the iron chalcogenides, in the case of electron filling $n \sim 6$. In addition, the interplay between orbital selectivity and electronic nematicity is addressed. For the superconducting state, we summarize the initial ideas for orbital-selective pairing, and discuss the recent explosive activities along this direction. We close with some perspectives on several emerging topics. These include the evolution of the orbital-selective correlations, magnetic and nematic orders and superconductivity as the electron filling factor is reduced from $6$ to $5$, as well as the interplay between electron correlations and topological bandstructure in iron-based superconductors.Item Orbital-selective superconductivity in the nematic phase of FeSe(American Physical Society, 2018) Hu, Haoyu; Yu, Rong; Nica, Emilian M.; Zhu, Jian-Xin; Si, QimiaoThe interplay between electronic orders and superconductivity is central to the physics of unconventional superconductors, and is particularly pronounced in the iron-based superconductors. Motivated by recent experiments on FeSe, we study the superconducting pairing in its nematic phase in a multiorbital model with frustrated spin-exchange interactions. Electron correlations in the presence of nematic order give rise to an enhanced orbital selectivity in the superconducting pairing amplitudes. This orbital-selective pairing produces a large gap anisotropy on the Fermi surface. Our results naturally explain the striking experimental observations, and shed light on the unconventional superconductivity of correlated electron systems in general.Item Sequential localization of a complex electron fluid(National Academy of Sciences, 2019) Martelli, Valentina; Cai, Ang; Nica, Emilian M.; Taupin, Mathieu; Prokofiev, Andrey; Liu, Chia-Chuan; Lai, Hsin-Hua; Yu, Rong; Ingersent, Kevin; Küchler, Robert; Strydom, André M.; Geiger, Diana; Haenel, Jonathan; Larrea, Julio; Si, Qimiao; Paschen, SilkeComplex and correlated quantum systems with promise for new functionality often involve entwined electronic degrees of freedom. In such materials, highly unusual properties emerge and could be the result of electron localization. Here, a cubic heavy fermion metal governed by spins and orbitals is chosen as a model system for this physics. Its properties are found to originate from surprisingly simple low-energy behavior, with 2 distinct localization transitions driven by a single degree of freedom at a time. This result is unexpected, but we are able to understand it by advancing the notion of sequential destruction of an SU(4) spin–orbital-coupled Kondo entanglement. Our results implicate electron localization as a unified framework for strongly correlated materials and suggest ways to exploit multiple degrees of freedom for quantum engineering.Item Theoretical investigation of superconductivity in trilayer square-planar nickelates(American Physical Society, 2020) Nica, Emilian M.; Krishna, Jyoti; Yu, Rong; Si, Qimiao; Botana, Antia S.; Erten, Onur; Rice Center for Quantum MaterialsThe discovery of superconductivity in Sr-doped NdNiO2 is a crucial breakthrough in the long pursuit for nickel oxide materials with electronic and magnetic properties similar to those of the cuprates. NdNiO2 is the infinite-layer member of a family of square-planar nickelates with general chemical formula Rn+1NinO2n+2 (R = La, Pr, Nd, n=2,3,...∞). In this Rapid Communication, we investigate superconductivity in the trilayer member of this series (R4Ni3O8) using a combination of first-principles and t−J model calculations. R4Ni3O8 compounds resemble cuprates more than RNiO2 materials in that only Ni-dx2−y2 bands cross the Fermi level, they exhibit a largely reduced charge transfer energy, and as a consequence superexchange interactions are significantly enhanced. We find that the superconducting instability in doped R4Ni3O8 compounds is considerably stronger with a maximum gap about four times larger than that in Sr0.2Nd0.8NiO2.