Bulletin of the American Physical Society
Mid-Atlantic Section Meeting 2021
Volume 66, Number 18
Friday–Sunday, December 3–5, 2021; Rutgers University, New Brunswick, New Jersey
Session H01: Quantum Matter: Superconductivity II |
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Chair: Kristjan Haule, Rutgers University Room: 201A |
Sunday, December 5, 2021 11:15AM - 11:51AM |
H01.00001: Superconductivity in Dilute Quantum Critical Polar Metals Invited Speaker: Premala Chandra Superconductivity in low carrier density metals challenges the conventional electron-phonon theory. More specifically the traditional approach to overcoming Coulomb repulsion is not appropriate here. Here we show that superconducting pairing mediated by energy fluctuations, ubiquitously present close to continuous phase transitions, occurs in dilute quantum critical polar metals. It 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 optic 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 neutral 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. The presented material is based on work in the preprint P.A. Volkov, P. Chandra and P. Coleman, "Superconductivity from Energy Fluctuations in Dilute Quantum Critical Polar Metals," arXiv:2106.11295. P. A. V. is supported by a Rutgers Center for Materials Theory Fellowship, P. Chandra is supported by DOE Basic Energy Sciences grant DE-SC0020353 and P. Coleman is supported by NSF grant DMR-1830707. [Preview Abstract] |
Sunday, December 5, 2021 11:51AM - 12:27PM |
H01.00002: Monopole Superconducting Order and Phase Sensitive Characterizations Invited Speaker: Yi Li Monopole superconductors, which can potentially be realized in doped magnetic Weyl semimetal materials, are characterized by exotic monopole harmonic symmetry in the superconducting pairing order. Because of the non-trivial Berry phase of the Cooper pair, its pairing order is topologically frustrated with the superconducting phase cannot be well defined over the entire Fermi surface. We further studied phase sensitive characterizations of the monopole pairing to distinguish it from known unconventional pairing symmetry. [Preview Abstract] |
Sunday, December 5, 2021 12:27PM - 12:39PM |
H01.00003: Josephson Effects of the Monopole Superconductors Junjia Zhang, Xinyu Sun, Yi Li As an exotic type of three-dimensional topological superconductors, monopole superconductors have unconventional pairing symmetry characterized by monopole harmonic functions and can be potentially realized in doped magnetic Weyl semimetals. Phase sensitive transport measurements have been crucial to obtaining a clear understanding of pairing properties of superconductors. We study the Josephson effects between two monopole superconductors and between a monopole superconductor and an s-wave superconductor. We find that a Josephson junction coupling two identical monopole superconductors exhibits similar behaviors to that coupling two identical chiral p-wave superconductors, while a junction coupling two monopole superconductors related by time-reversal symmetry behaves similarly to that coupling two chiral p-wave superconductors with opposite chirality. For a Josephson junction coupling a monopole superconductor and an s-wave superconductor, however, its current phase relation exhibits $2\pi$-periodicity, which is a distinctly different feature from the $\pi$-periodicity of the current phase relation for a junction between chiral p-wave and s-wave superconductors. Combining the results above provides a distinguishing feature to identify monopole superconductors in experiments. [Preview Abstract] |
Sunday, December 5, 2021 12:39PM - 12:51PM |
H01.00004: Monopole Superconductivity in Magnetically Doped Cd$_3$As$_2$ Eric Bobrow, Yi Li When magnetically doped, the Dirac semimetal Cd$_3$As$_2$ features Fermi pockets enclosing Weyl points with chiralities $\pm 1$ and $\pm 2$. The superconducting pairing between parity related Fermi pockets with opposite Chern numbers exhibits exotic pairing gap functions, which have nodes enforced by nonzero Cooper pair Berry phase. We show that for pairing between Fermi pockets with Chern numbers $\pm 1$ or $\pm 2$ in magnetically doped Cd$_3$As$_2$, the gap function exhibits monopole harmonic symmetry with monopole charge $1$ or $2$, depending on the chemical potential. The total vorticity of the gap function, i.e., the winding of the pairing phase over a Fermi surface, is independent of the form of the proximity pairing and is equal to twice the monopole charge of the Cooper pair. [Preview Abstract] |
Sunday, December 5, 2021 12:51PM - 1:03PM |
H01.00005: Anomalously small superconducting gap in the strong spin-orbit coupled superconductor: $ \beta- $Tungsten Prashant Chauhan, Ramesh Budhani, Peter Armitage Thin films of $ \beta- $tungsten host superconductivity in the presence of strong spin-orbit coupling. This non-equilibrium crystalline phase of tungsten has attracted considerable attention in recent years due to its giant spin Hall effect and the potential promise of exotic superconductivity. However, more than 60 years after its discovery, superconductivity in this material is still not well understood. Using time-domain THz spectroscopy, we measure the frequency response of the complex optical conductivity of $ \beta- $tungsten thin film with a T$_c$ of 3.7 K in its superconducting state. At temperatures down to 1.6 K, we find that both the superconducting gap and the superfluid spectral weight are much smaller than that expected for a weakly coupled superconductor given the T$_c$. The conclusion of a small gap holds up even when accounting for possible inhomogeneities in the system, which could come from other crystalline forms of tungsten (that are not superconducting at these temperatures) or surface states on $ \beta- $tungsten grains. Using detailed X-ray diffraction measurements, we preclude the possibility of a significant amount of other tungsten allotropes, strongly suggesting the topological surface states of $ \beta- $tungsten play the role of inhomogeneity in th [Preview Abstract] |
Sunday, December 5, 2021 1:03PM - 1:15PM |
H01.00006: BCS $d$-wave behavior in the THz electrodynamic response of electron-doped cuprate superconductors Zhenisbek Tagay, Fahad Mahmood, Anaelle Legros, Tarapada Sarkar, Richard L. Greene, N. Peter Armitage Although cuprate superconductors have been intensively studied for the past decades, there is no consensus regarding the microscopic origin of their superconductivity. In this work, we measure the low-energy electrodynamic response of slightly underdoped and overdoped La$_{2-x}$Ce$_{x}$CuO$_{4}$ thin films using time-domain terahertz (THz) spectroscopy to determine the temperature and field dependence of the superfluid spectral weight. We show that the temperature dependence obeys the relation $n_{s\, }$\textasciitilde 1-($T/T_{c})^{2}$, typical for dirty limit BCS-like $d$-wave superconductors. Furthermore, the magnetic field dependence was found to follow a sublinear $B^{1/2}$ form, which supports predictions based on a $d$-wave symmetry for the superconducting gap. These observations imply that the superconducting order in these electron-doped cuprates can be well described in terms of a disordered BCS $d$-wave formalism. [Preview Abstract] |
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