Session LL02: V: Unconventional Superconductivity

Longitudinal magnetic fluctuations observed by the 77Se nuclear spin-spin relaxation measurement in FeSe

A relaxation rate of the transverse nuclear magnetization, so-called 1/T₂, can be induced by longitudinal magnetic fluctuations along the applied fields in addition to transverse one as the nuclear spin-lattice relaxation rate 1/T₁. The former can manifest itself when there is an ultra-slow spin dynamics[1-3]. Here, we performed the ⁷⁷Se nuclear magnetic resonance measurement on one of the typical iron-chalcogenide superconductors FeSe, which exhibits only orbital ordering at T_s,nem ≃ 90 K whereas iron-pnictide materials exhibit both spin and orbital ordering[4-7]. We measured 1/T₂ to study the spin dynamics of FeSe in magnetic fields of B||c. As a result, 1/T₂ increases on cooling below T_s,nem while 1/T₁ decreases gradually. This contrasting behavior between 1/T₂ and 1/T₁ indicates that the contribution of longitudinal fluctuations along the c-direction is dominant over that of 1/T₁ and thus ultra-slow spin dynamics appear in FeSe. The existence of longitudinal fluctuations along the c-direction at the Se sites can be understood by considering the striped correlations between iron electron-spin, which is consistent with the inelastic neutron scattering result[8]. The ultra-slow spin dynamics in FeSe may be ascribed to the magnetic frustration coming from local moments. The frustrated magnetism with the nearest and next-nearest exchange couplings between iron electron-spin may play a role in unconventional superconductivity in FeSe.   

NMR study on S-substituted FeSe: spin fluctuations from Bogoliubov Fermi surfaces

  The study of the iron-based superconductor, FeSe, has resulted in various topics, such as the interplay among superconductivity, nematicity, and magnetism, BCS-BEC crossover, and FFLO superconductivity. Recently, topologically protected nodal Fermi surfaces, referred to as Bogoliubov Fermi surfaces (BFSs), have garnered much attention. A theoretical model for the S-substituted FeSe system demonstrated that BFSs can manifest under the conditions of spin-orbit coupling, multi-band systems, and superconductivity with time-reversal symmetry breaking.

  To investigate electronic properties of the superconducting (SC) state on a microscopic level, we performed  ⁷⁷Se-NMR measurements on  FeSe_1-xS_x (x=0.05, 0.12, and 0.18) at very low temperatures to 100 mK. For x=0.18, we found  anomalous spin fluctuations originating from BFSs deep in the SC state.  The relaxation rate divided by temperature (1/T₁T), which provides a measure of low-energy spin fluctuations, exhibited an anomalous increase with decreasing temperature deep in the SC state despite that spin fluctuations should decrease due to the opening of the SC gap.  Such unusual behavior cannot be explained by an impurity effect and implies the presence of significant spin fluctuations of Bogoliubov quasiparticles, which are associated with possible nesting properties between BFSs. We will discuss how the anomalous spin fluctuations evolve upon S substitution in the meeting.   

Title: Spectroscopic evidence for topological band structure in FeTe0.55Se0.45

FeTe_0.55Se_0.45(FTS) occupies a special spot in modern condensed matter physics at the intersections of electron correlation, topology, and unconventional superconductivity. The bulk electronic structure of FTS is predicted to be topologically nontrivial thanks to the band inversion. between the d_xz and p_z bands along Γ-Z. This in turn would give rise to a Dirac surface state (DSS) hosting topological superconductivity below the bulk superconducting temperature. However, whether this prediction is indeed realized in FTS remains controversial in recent angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) studies. In ARPES experiments, a Dirac-cone-like feature has been identified, yet only at certain out-of-plane momenta (k_z), leading to speculations that it might originate from a bulk band instead of the DSS. Moreover, the measured band structure differs significantly from the density functional theory (DFT) predictions, with no direct observations of either the p_z band or the bulk band inversion. Here we resolve this debate through a comprehensive ARPES investigation. We first observe a persistent DSS independent of k_z. Then, by comparing FTS with FeSe which has no band inversion along Γ-Z, we identify the spectral weight fingerprint of both the presence of the p_z band and inversion between the d_xz and p_z bands. Our results highlight the impact of band renormalization and large spin-orbit coupling in FTS and make a strong case for the existence of topological band structure in this unconventional superconductor.   

The cumulant Green's functions method for a triangular lattice: Mott transition and superconductivity

We study the single-band Hubbard model for a triangular two-dimensional (2D) lattice using the cumulant Green's functions method (CGFM) [1,2]. The starting point of the method is to diagonalize a 2D cluster containing N correlated sites ("seed") and employ the cumulants calculated from the cluster solution to obtain the full Green's functions for the lattice. All calculations are done directly, and no self-consistent process is needed.

We apply the method to study the triangular lattice formed by the Sn adatoms on a Si(111) substrate with the 1/3 monolayer coverage. The system exhibits geometric frustration and strong electronic correlations. We use as a "seed" of the atomic cumulants closed clusters of three atoms (triangular), seven atoms (a hexagon with a central site), and nine atoms (a cluster formed by eight triangles), including high order hopping realistic process up to fifth order. The Sn triangular develops non-conventional superconductivity in the presence of heavily doped p-type Si(111) substrates [4]. We study the interplay of geometric frustration and correlation in the Mott transition and in the development of unconventional superconductivity.

[1] R N Lira et al. J. Phys.: Condens. Matter 35 (2023) 245601

[2] R N Lira Physics Letters A 474 (2023) 128818

[3] Gang Li et al. NATURE COMMUNICATIONS | 4:1620 | DOI: 10.1038/ncomms2617

[4] F Ming et al. Nature Physics | Volume 19 | April 2023 | 500–506   

Theory of Antiferromagnetic Mott States in the Hubbard Model

This work presents an analytical framework for antiferromagnetic Mott insulating states in the Hubbard model. We first derive an analytical solution for the single-particle Green's functions in the atomic limit. With a second-order perturbation theory, we compute the perturbation to the ground state energy and prove that the ground state is antiferromagnetically ordered. Then we derive an analytical solution for single-particle Green's functions in the presence of the hopping term for the Neel state. With the analytical solution, we compute and explain various properties of AFM Mott insulators observed both experimentally and numerically: i) magnetic blueshift of the Mott gap; ii) ARPES data on parental compounds of cuprate high Tc superconductors. This work not only enhances our understanding of electronic properties in AFM Mott states but also provides a solid foundation for future investigations of the doped AFM Mott insulators, which is believed to be the key to the mechanism of cuprates high-Tc superconductivity.   

Dressed Domain Wall Model of Cuprate Pseudogap

A dressed-domain-wall model of the cuprate pseudogap (arXiv:2303.11254, 2206.00077_v2) captures several features of cuprate physics in a material-specific manner. These include thermodynamic signatures of pseudogap collapse in terms of the collapse of an underlying AFM order, the Fermi arc phenomenon, and the presence of three branches of charge-stripe texture that dominate the intertwined order of the pseudogap.  Moreover, two branches of the electronic spectrum delineate a Mott-Slater crossover, where the q-vector of the low-doping branch is controlled by stripe repulsion independent of the Fermi surface (i.e., Mott-like behavior), while the q-vector of the high-doping branch corresponds to (Slater-like) Fermi surface nesting.  However, the nesting is not associated with the nonmagnetic Fermi surface, but with the AFM Fermi surface, indicating that the charge stripe order is secondary to the underlying AFM order. Work supported by the U. S. Department of Energy.   

New ReaxFF Reactive Force Field Optimized for Phonon and Thermal Properties of Molybdenum Disulfide

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have demonstrated immense potential in various applications, including nanoelectronics, optoelectronics, sensing, and energy storage, owing to their exceptional electronic, optoelectronic, and chemical properties. Accurately characterizing the thermal properties of TMDs for further implementation necessitates computationally expensive large-scale atomic simulations that are not feasible using first-principles methods. In this study, we present the development of a reliable ReaxFF reactive force field, guided by extensive first-principles calculations. The new empirical force field exhibits a significant improvement in describing the phonon dispersion curve of 2D Molybdenum Disulfide (MoS2) systems compared to the previous ReaxFF force field, which primarily focused on characterizing vacancies and ripple defects in this system. Utilizing the new ReaxFF force field, we calculate the phonon thermal conductivity (κ_∞) of MoS₂ to be 72.68 WK^-1m^-1, which agrees well with reference calculations using the Stillinger-Weber (SW) and moment tensor potential (MTP), yielding κ_∞ values of 66.09 and 63.45 WK-1m-1, respectively. In contrast, the previous ReaxFF force field yielded a κ_∞value of 77.23 WK-1m-1. These results highlight the enhanced reliability of the optimized ReaxFF force field in predicting the thermal properties of 2D MoS₂ system and its potential for studying the thermal properties of large-scale defective MoS₂ systems. Furthermore, the methods developed in this work for MoS2 can be extended to other TMDs and emerging materials.