New mechanism and exact theory of superconductivity from strong repulsive interaction

We introduce a general mechanism for superconductivity in Fermi systems with strong repulsive interaction. Because kinetic terms are small compared to the bare repulsion, the dynamics of charge carriers is constrained by the presence of other nearby carriers. By treating kinetic terms as a perturbation around the atomic limit, we show that pairing can be induced by correlated multiparticle tunneling processes that favor two itinerant carriers to be close together. Our analytically controlled theory provides a quantitative formula relating Tc to microscopic parameters, with maximum Tc reaching about 10% of the Fermi temperature. Our work demonstrates a powerful method for studying strong coupling superconductivity with unconventional pairing symmetry. It also offers a realistic new route to realizing finite angular momentum superfluidity of spin-polarized fermions in optical lattice.

Gate-controlled BCS-BEC crossover in a two-dimensional superconductor

Bardeen-Cooper-Schrieffer (BCS) superfluidity and Bose-Einstein condensation (BEC) are the two extreme limits of the ground state of the paired fermion systems. We report crossover behavior from the BCS limit to the BEC limit realized by varying carrier density in a two-dimensional superconductor, electron-doped zirconium nitride chloride. The phase diagram, established by simultaneous measurements of resistivity and tunneling spectra under ionic gating, demonstrates a pseudogap phase in the low-doping regime. The ratio of the superconducting transition temperature and Fermi temperature in the low–carrier density limit is consistent with the theoretical upper bound expected in the BCS-BEC crossover regime. These results indicate that the gate-doped semiconductor provides an ideal platform for the two-dimensional BCS-BEC crossover without added complexities present in other solid-state systems.

Point defects in tight binding models for insulators

We consider atomistic geometry relaxation in the context of linear tight binding models for point defects. A limiting model as Fermi-temperature is sent to zero is formulated, and an exponential rate of convergence for the nuclei configuration is established. We also formulate the thermodynamic limit model at zero Fermi-temperature, extending the results of [H. Chen, J. Lu and C. Ortner, Thermodynamic limit of crystal defects with finite temperature tight binding, Arch. Ration. Mech. Anal. 230 (2018) 701–733]. We discuss the non-trivial relationship between taking zero temperature and thermodynamic limits in the finite Fermi-temperature models.

Locality of interatomic forces in tight binding models for insulators

The tight binding model is a minimalistic electronic structure model for predicting properties of materials and molecules. For insulators at zero Fermi-temperature we show that the potential energy surface of this model can be decomposed into exponentially localised site energy contributions, thus providing qualitatively sharp estimates on the interatomic interaction range which justifies a range of multi-scale models. For insulators at finite Fermi-temperature we obtain locality estimates that are uniform in the zero-temperature limit. A particular feature of all our results is that they depend only weakly on the point spectrum. Numerical tests confirm our analytical results. This work extends Chen and Ortner [Multiscale Model. Simul. 14 (2016) 232–264] and Chen et al. [Arch. Ration. Mech. Anal. 230 (2018) 701–733] to the case of zero Fermi-temperature as well as strengthening the results proved therein.

Characterization of Near-Room-Temperature Superconductivity in Yttrium Superhydrides

Recently, Troyan et al (2019 arXiv:1908.01534) and Kong et al (2019 arXiv:1909.10482) extended near-room-temperature superconductors family by new yttrium superhydride polymorphs, YHn (n = 4,6,7,9), which exhibit superconducting transition temperatures in the range of Tc = 210-243 K at pressure of P = 160-255 GPa. In this paper, temperature dependent upper critical field data, Bc2(T), for highly-compressed mixture of YH4+YH6 phases (reported by Kong et al 2019 arXiv:1909.10482) is analysed to deduce the ratio of Tc to the Fermi temperature, TF. Our analysis shows that in all considered scenarios the YH4+YH6 mixture has the ratio 0.01 ≤ Tc/TF ≤ 0.04. As the result, YH4+YH6 falls in the unconventional superconductors band in the Uemura plot. It is also found that the characteristic temperature of the order parameter amplitude fluctuations, Tfluc, in the YH4+YH6 mixture is only several percent above observed Tc, and thus the superconducting transition in yttrium superhydride polymorphs is fundamentally limited by thermodynamics fluctuations.

Classifying Superconductivity in ThH-ThD Superhydrides

Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) discovered that Th4H15-Th4D15 superhydrides exhibit superconductivity and have no isotope effect. The latter is fundamental contradiction with the concept of electron-phonon mediated superconductivity of Bardeen-Cooper-Schrieffer (BCS) theory. Soon after this work, Stritzker and Buckel (1972 Zeitschrift für Physik A Hadrons and nuclei 257 1-8) reported that superconductors in PdHx-PdDx system exhibit reverse isotope effect. Yussouff et al (1995 Solid State Communications 94 549) extended this finding on PdHx-PdDx-PdTx system. Recent interest to hydrogen- and deuterium-rich superconductors is based on the discovery of near-room-temperature superconductivity in highly-compressed H3S (Drozdov et al. 2015 Nature 525 73) and LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001). To date, there is no clarity about isotope effect in H3S-D3S system, because thorough examination of available experimental data reported by Drozdov et al (2015 Nature 525 73) shows that H3S-D3S system perhaps has reverse isotope effect. In attempt to reaffirm/disprove our primary idea that the mechanism for near-room-temperature superconductivity in hydrogen-rich superconductors is not BCS electron-phonon interaction, we analyse the upper critical field data, Bc2(T), in Th4H15-Th4D15 phases (Satterthwaite and Toepke 1970 Phys. Rev. Lett. 25 741) and two recently discovered high-pressure hydrogen-rich phases of ThH9 and ThH10 (Semenok et al 2019 arXiv:1902.10206). As the result, it is found that all known to date thorium super-hydrides/deuterides are unconventional superconductors which have Tc/TF ratios within a range of 0.008 < Tc/TF < 0.120, where Tc is the superconducting transition temperature and TF is the Fermi temperature.

Classifying superconductivity in compressed H3S

The discovery of high-temperature superconductivity in compressed [Formula: see text] by Drozdov and co-workers [A. Drozdov et al., Nature 525 (2015) 73] heralded a new era in superconductivity. To date, the record transition temperature of [Formula: see text] stands with another hydrogen-rich compound, [Formula: see text] [M. Somayazulu et al., Phys. Rev. Lett. 122 (2019) 027001] which becomes superconducting at pressure of [Formula: see text]. Despite very intensive first-principle theoretical studies of hydrogen-rich compounds compressed to megabar level pressure, there is a very limited experimental dataset available for such materials. In this paper, we analyze the upper critical field, [Formula: see text], data of highly compressed [Formula: see text] reported by Mozaffari and co-workers [S. Mozaffari et al., LA-UR-18-30460. https://doi.org/10.2172/1481108 ] by utilizing four different models of [Formula: see text]. As the result, we find that the ratio of superconducting energy gap, [Formula: see text], to the Fermi energy, [Formula: see text], in all considered scenarios is [Formula: see text], with respective ratio of [Formula: see text] to the Fermi temperature, [Formula: see text], [Formula: see text]. These characterize [Formula: see text] as unconventional superconductor and places it on the same trend line in [Formula: see text] versus [Formula: see text] plot, where all unconventional superconductors are located.

Subsonic Potentials in Ultradense Plasmas

The existence of the subsonic dynamic potential for a test charge in extremely dense quantum plasmas is pointed out for the first time. The dispersion equation of ion acoustic wave in relativistic plasmas is derived by using the quantum hydrodynamic model. The relativistic electrons obey Fermi statistics, whereas the ions are taken classically. The standard model of wake potential is hereafter applied for the derivation of dynamic potential of the test particle. A usual supersonic potential is found suppressed. However, the oscillatory subsonic wake potential does exist in small length scales. The analytical results are applied in different regions by taking the range of magnetic field as well as the electron number density. It is found that the dynamic potential exists only when vt < Cs, showing the presence of subsonic wake potential contrary to the usual supersonic condition vt > Cs. Here vt is the test particle speed and Cs is the acoustic speed defined by the Fermi temperature of the electrons. This work is significant in order to describe the structure formation in the astrophysical environment and laboratory dense plasmas.