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 (August 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 superconductingT_con 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 SrTiO₃. 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. Investigation of pressure-driven superconductivity in TlInTe ₂ : an ab initio study

   https://doi.org/10.1088/1402-4896/ad59dc  

   Renskers, Christopher; Margine, Elena_R (July 2024, Physica Scripta)

   Abstract The Zintl compound TlInTe₂is an intriguing material because of its outstanding thermoelectric properties at ambient pressure. Interestingly, it has recently been found that TlInTe₂exhibits a V-shape dependence of the superconducting critical temperature (T_c) under increasing pressure, which has been linked to the reversed behavior of the Raman active A_gphonon mode and anharmonic effects. In this study, we have performed first-principles calculations of the electron-phonon interactions and the superconducting properties of TlInTe₂in order to understand this unusual pressure-induced response. In contrast to experiment, we find a dome-shaped pressure-induced dependence ofT_cwith a maximum value of 0.23 K at 18 GPa, significantly lower than the experimental results. Electron doping has the potential to adjust theT_cto fall within the experimental range, but it necessitates considerably high levels of doping. Furthermore, our analysis of the phonon spectra and phonon lifetimes, including anharmonic effects, show that anharmonicity is unlikely to influence the superconducting properties of TlInTe₂. It remains an open question whether there is indeed an unusual V-shapeT_cdependence with pressure or whether the phonon-mediated theory of superconductivity used here breaks down in this system. 

   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_Csuperconductivity. Here we present measurements of the low-temperature normal-state thermopower (S) of the electron-doped cuprate superconductor La_2−xCe_xCuO₄(LCCO) fromx= 0.11–0.19. We observe quantum critical $\mathit{S}/\mathit{T}$versus $\mathbf{l}\mathbf{n}\left(\mathbf{1}/\mathit{T}\right)$behavior over an unexpectedly wide doping rangex= 0.15–0.17 above the QCP (x= 0.14), with a slope that scales monotonically with the superconducting transition temperature (T_Cwith 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 withT_C. Above the superconductivity dome, atx= 0.19, a conventional Fermi-liquid $\mathit{S}\propto \mathit{T}$behavior is found for $\mathit{T}\le$40 K. 

   more » « less
4. Monte Carlo study of the pseudogap and superconductivity emerging from quantum magnetic fluctuations

   https://doi.org/10.1038/s41467-022-30302-x  

   Jiang, Weilun; Liu, Yuzhi; Klein, Avraham; Wang, Yuxuan; Sun, Kai; Chubukov, Andrey V.; Meng, Zi Yang (May 2022, Nature Communications)

   Abstract The origin of the pseudogap behavior, found in many high-T_csuperconductors, remains one of the greatest puzzles in condensed matter physics. One possible mechanism is fermionic incoherence, which near a quantum critical point allows pair formation but suppresses superconductivity. Employing quantum Monte Carlo simulations of a model of itinerant fermions coupled to ferromagnetic spin fluctuations, represented by a quantum rotor, we report numerical evidence of pseudogap behavior, emerging from pairing fluctuations in a quantum-critical non-Fermi liquid. Specifically, we observe enhanced pairing fluctuations and a partial gap opening in the fermionic spectrum. However, the system remains non-superconducting until reaching a much lower temperature. In the pseudogap regime the system displays a “gap-filling rather than “gap-closing behavior, similar to the one observed in cuprate superconductors. Our results present direct evidence of the pseudogap state, driven by superconducting fluctuations. 

   more » « less
5. Ferromagnetic order beyond the superconducting dome in a cuprate superconductor

   https://doi.org/10.1126/science.aax1581  

   Sarkar, Tarapada; Wei, D. S.; Zhang, J.; Poniatowski, N. R.; Mandal, P. R.; Kapitulnik, A.; Greene, Richard L. (April 2020, Science)

   According to conventional wisdom, the extraordinary properties of the cuprate high-temperature superconductors arise from doping a strongly correlated antiferromagnetic insulator. The highly overdoped cuprates—whose doping lies beyond the dome of superconductivity—are considered to be conventional Fermi liquid metals. We report the emergence of itinerant ferromagnetic order below 4 kelvin for doping beyond the superconducting dome in thin films of electron-doped La_2–xCe_xCuO₄(LCCO). The existence of this ferromagnetic order is evidenced by negative, anisotropic, and hysteretic magnetoresistance, hysteretic magnetization, and the polar Kerr effect, all of which are standard signatures of itinerant ferromagnetism in metals. This surprising result suggests that the overdoped cuprates are strongly influenced by electron correlations. 

   more » « less