More Like this

1. Superconductivity from energy fluctuations in dilute quantum critical polar metals

   https://doi.org/10.1038/s41467-022-32303-2  

   Volkov, Pavel A.; Chandra, Premala; Coleman, Piers (December 2022, Nature Communications)

   Abstract Superconductivity in low carrier density metals challenges the conventional electron-phonon theory due to the absence of retardation required to overcome Coulomb repulsion. Here we demonstrate that pairing mediated by energy fluctuations, ubiquitously present close to continuous phase transitions, occurs in dilute quantum critical polar metals and results in a dome-like dependence of the superconducting T c on carrier density, characteristic of non-BCS superconductors. In quantum critical polar metals, the Coulomb repulsion is heavily screened, while the critical transverse optical phonons decouple from the electron charge. In the resulting vacuum, long-range attractive interactions emerge from the energy fluctuations of the critical phonons, resembling the gravitational interactions of a chargeless dark matter universe. Our estimates show that this mechanism may explain the critical temperatures observed in doped SrTiO 3 . We provide predictions for the enhancement of superconductivity near polar quantum criticality in two- and three-dimensional materials that can be used to test our theory. 

   more » « less
2. A toy model for two-dimensional spin-fluctuation-induced unconventional superconductivity

   https://doi.org/10.1063/10.0034348  

   Cao, Tu M; Mazin, Igor I (December 2024, Low Temperature Physics)

   Superconductivity had been one of the most enigmatic phenomena in condensed matter physics, puzzling the best theorists for 45 years, since the original discovery by Kamerlingh-Onnes in 1911 till the final solution by Bardeen, Cooper, and Schrieffer (BCS) in 1957. The original BCS proposal assumed the highest-symmetry form for the superconducting order parameter Δ, namely, a constant, and a uniform pairing interaction due to attractive mediation of ionic vibration. While it was rather soon realized that generalizations onto k-dependent order parameters and anisotropic pairing interaction was straightforward, only thirty years later, upon the discovery of high-temperature superconductivity in cuprates, high-order angular dependence of Δ and repulsive interaction, mediated by spin fluctuations or Coulomb repulsion brought such “unconventional” into the spotlight. In 2008 yet another such system was discovered, and eventually the idea of repulsion-mediated unconventional superconductivity was generally accepted. Apart from the two specific systems mentioned above, a large number of various specific implementations of this idea have been proposed, and it is becoming increasingly clear that it is worth studying mathematically how unconventional superconductivity emerges, and with what properties, for a simple, but sufficiently general theoretical model. In our project, we study systematically unconventional superconductivity in an isotropic two-dimensional model system of electrons, subjected to repulsive interactions of a simple, but physically motivated form: a delta function peaked at a particular momentum (from 0 to twice the Fermi momentum), or Gaussian of varying widths. 

   more » « less
3. Anomalous quantum criticality in the electron-doped cuprates

   https://doi.org/10.1073/pnas.1817653116  

   Mandal, P. R.; Sarkar, Tarapada; Greene, Richard L. (March 2019, Proceedings of the National Academy of Sciences)

   In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (H C2 ) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high- T C superconductivity. Here we present measurements of the low-temperature normal-state thermopower ( S ) of the electron-doped cuprate superconductor La 2− x Ce x CuO 4 (LCCO) from x = 0.11–0.19. We observe quantum critical S / T versus l n ( 1 / T ) behavior over an unexpectedly wide doping range x = 0.15–0.17 above the QCP ( x = 0.14), with a slope that scales monotonically with the superconducting transition temperature ( T C with H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked with T C . Above the superconductivity dome, at x = 0.19, a conventional Fermi-liquid S ∝ T behavior is found for T ≤ 40 K. 

   more » « less
4. Migdal–Eliashberg superconductivity in a Kondo lattice

   https://doi.org/10.1088/1361-648X/ad43a5  

   Awelewa, Samuel; Dzero, Maxim (May 2024, Journal of Physics: Condensed Matter)

   We apply the Migdal–Eliashberg theory of superconductivity to heavy-fermion and mixed valence materials. Specifically, we extend the Anderson lattice model to a case when there exists a strong coupling between itinerant electrons and lattice vibrations. Using the saddle-point approximation, we derive a set of coupled nonlinear equations which describe competition between the crossover to a heavy-fermion or mixed-valence regimes and conventional superconductivity. We find that superconductivity at strong coupling emerges on par with the development of the many-body coherence in a Kondo lattice. Superconductivity is gradually suppressed with the onset of the Kondo screening and for strong electron-phonon coupling the Kondo screening exhibits a characteristic re-entrant behavior. Even though for both weak and strong coupling limits the suppression of superconductivity is weaker in the mixed-valence regime compared to the local moment one, superconducting critical temperature still remains nonzero. In the weak coupling limit the onset of the many body coherence develops gradually, in the strong coupling limit it emerges abruptly in the mixed valence regime while in the local moment regime the f-electrons remain effectively decoupled from the conduction electrons. Possibility of experimental realization of these effects in Ce-based compounds is also discussed. 

   more » « less
5. Does filling-dependent band renormalization aid pairing in twisted bilayer graphene?

   https://doi.org/10.1038/s41535-021-00379-6  

   Lewandowski, Cyprian; Nadj-Perge, Stevan; Chowdhury, Debanjan (December 2021, npj Quantum Materials)

   Abstract Magic-angle twisted bilayer graphene (MATBG) exhibits a panoply of many-body phenomena that are intimately tied to the appearance of narrow and well-isolated electronic bands. The microscopic ingredients that are responsible for the complex experimental phenomenology include electron–electron (phonon) interactions and nontrivial Bloch wavefunctions associated with the narrow bands. Inspired by recent experiments, we focus on two independent quantities that are considerably modified by Coulomb interaction-driven band renormalization, namely the density of states and the minimal spatial extent associated with the Wannier functions. First, we show that a filling-dependent enhancement of the density of states, caused by band flattening, in combination with phonon-mediated attraction due to electron-phonon umklapp processes, increases the tendency towards superconducting pairing in a range of angles around magic-angle. Second, we demonstrate that the minimal spatial extent associated with the Wannier functions, which contributes towards increasing the superconducting phase stiffness, also develops a nontrivial enhancement due to the interaction-induced renormalization of the Bloch wavefunctions. While our modeling of superconductivity (SC) assumes a weak electron-phonon coupling and does not consider many of the likely relevant correlation effects, it explains simply the experimentally observed robustness of SC in the wide range of angles that occurs in the relevant range of fillings. 

   more » « less