Condensed Matter

The rise and fall of the superconducting transition temperature T c upon tuning carrier density or external parameters, such as pressure or magnetic field, is ubiquitously observed in a wide range of quantum materials. In order to investigate such domes of T c , we go beyond the prototypical attractive Hubbard model, and consider a lattice model of electrons coupled via instantaneous, spatially extended, attractive interactions. By numerically solving the mean-field equations, as well as going beyond mean field theory using a functional renormalization group approach, we find that for a characteristic interaction range ℓ , there exists a dome in T c around k F ℓ∼O(1) . For multiband systems, our mean field theory shows the presence of additional domes in the vicinity of Lifshitz transitions. Our results hold in both two and three dimensions and can be intuitively understood from the geometric relation between the Fermi surface and the interaction range. Our model may be relevant for domes of T c in dilute weakly coupled superconductors or in engineered cold atom systems.

The interplay of time-reversal and n -fold rotation symmetries ( n=2,4,6 ) is known to bring a new class of topological crystalline insulators (TCIs) having n surface Dirac cones due to surface rotation anomaly. We show that the proximity-induced s -wave superconductivity on the surface of these TCIs yields a topological superconducting phase in which two Majorana zero modes are bound to a vortex, and that n -fold rotation symmetry ( n=2,4,6 ) enriches the topological classification of a superconducting vortex from Z 2 to Z 2 × Z 2 . Using a model of a three-dimensional high-spin topological insulator with s -wave superconductivity and two-fold rotation symmetry, we show that, with increasing chemical potential, the number of Majorana zero modes at one end of a vortex changes as 2→1→0 through two topological vortex phase transitions. In addition, we show that additional magnetic-mirror symmetry further enhances the topological classification to Z×Z

CsV3Sb5 is a newly discovered Z2 topological kagome metal showing the coexistence of a charge density wave (CDW)-like order at T* = 94 K and superconductivity (SC) at Tc = 2.5 K at ambient pressure. Here we study the interplay between CDW and SC in CsV3Sb5 via measurements of resistivity and magnetic susceptibility under hydrostatic pressures. We find that the CDW transition decreases with pressure and experience a subtle modification at Pc1 = 0.6-0.9 GPa before it vanishes completely at Pc2 = 2 GPa. Correspondingly, Tc(P) displays an unusual M-shaped double dome character with two maxima around Pc1 and Pc2, respectively, leading to a tripled enhancement of Tc to about 8 K at 2 GPa. The obtained temperature-pressure phase diagram resembles those of many unconventional superconductors, illustrating an intimated competition between CDW-like order and SC. The competition is found to be particularly strong for the intermediate pressure range Pc1 <= P <= Pc2 as evidenced by the broad superconducting transition and reduced superconducting volume fraction. This work not only demonstrates the potential to raise the Tc of the V-based kagome superconductors, but also offers more insights into the rich physics related to the electronic correlations in this novel family of topological kagome metals.

The properties of a two-dimensional low density (n<<1) electron system with strong onsite Hubbard attraction U>W (W is the bandwidth) in the presence of a strong random potential V uniformly distributed in the range from -V to +V are considered. Electronic hoppings only at neighboring sites on the square lattice are taken into account, thus W=8t. The calculations were carried out for a lattice of 24x24 sites with periodic boundary conditions. In the framework of the Bogoliubov - de Gennes approach we observed an appearance of inhomogeneous states of spatially separated Fermi-Bose mixture of Cooper pairs and unpaired electrons with the formation of bosonic droplets of different size in the matrix of the unpaired normal states We observed a decrease in the droplet size (from larger droplets to individual bielectronic pairs) when we decrease the electron density at fixed values of the Hubbard attraction and random potential. The obtained results are important for the construction of the gross phase diagram and understanding of the nature of the phase transition between superconducting, normal metallic and localized states in quasi-2D (thin) film of a dirty metal. In a more practical sense it is interesting also for the experimental implementation of superconducting qubits on quantum circuits with high impedances in granular superconductors.

Topological superconductors have long been predicted to host Majorana zero modes which obey non-Abelian statistics and have potential for realizing non-decoherence topological quantum computation. However, material realization of topological superconductors is still a challenge in condensed matter physics. Utilizing high-resolution angle-resolved photoemission spectroscopy and first-principles calculations, we predict and then unveil the coexistence of topological Dirac semimetal and topological insulator states in the vicinity of Fermi energy ( E F ) in the titanium-based oxypnictide superconductor BaTi 2 Sb 2 O. Further spin-resolved measurements confirm its spin-helical surface states around E F , which are topologically protected and give an opportunity for realization of Majorana zero modes and Majorana flat bands in one material. Hosting dual topological superconducting states, the intrinsic superconductor BaTi 2 Sb 2 O is expected to be a promising platform for further investigation of topological superconductivity.

In strongly correlated systems the strength of Coulomb interactions between electrons, relative to their kinetic energy, plays a central role in determining their emergent quantum mechanical phases. We perform resonant x-ray scattering on Bi 2 Sr 2 CaCu 2 O 8+δ , a prototypical cuprate superconductor, to probe electronic correlations within the CuO 2 plane. We discover a dynamic quasi-circular pattern in the x - y scattering plane with a radius that matches the wave vector magnitude of the well-known static charge order. Along with doping- and temperature-dependent measurements, our experiments reveal a picture of charge order competing with superconductivity where short-range domains along x and y can dynamically rotate into any other in-plane direction. This quasi-circular spectrum, a hallmark of Brazovskii-type fluctuations, has immediate consequences to our understanding of rotational and translational symmetry breaking in the cuprates. We discuss how the combination of short- and long-range Coulomb interactions results in an effective non-monotonic potential that may determine the quasi-circular pattern.

We report numerical calculations of a dynamic pairbreaking current density J d and a critical superfluid velocity v d in a nonequilibrium superconductor carrying a uniform, large-amplitude ac current density J(t)= J a sinΩt with Ω well below the gap frequency Ω≪ Δ 0 /ℏ . The dependencies J d (Ω,T) and v d (Ω,T) near the critical temperature T c were calculated from either the full time-dependent nonequilibrium equations for a dirty s-wave superconductor and the time-dependent Ginzburg-Landau (TDGL) equations for a gapped superconductor, taking into account the GL relaxation time of the order parameter τ GL and the inelastic electron-phonon relaxation time of quasiparticles τ E . We show that both approaches give similar frequency dependencies of J d (Ω) and v d (Ω) which gradually increase from their static pairbreaking GL values J c and v c at Ω τ E ≪1 to 2 – √ J c and 2 – √ v c at Ω τ E ≫1 . Here J d , v d and a dynamic superheating field at which the Meissner state becomes unstable were calculated in two different regimes of a fixed ac current and a fixed ac superfluid velocity induced by the applied ac magnetic field H= H a sinΩt in a thin superconducting filament or a type-II superconductor with a large GL parameter. We also calculated a nonlinear electromagnetic response of a nonequilibrium superconducting state, particularly a dynamic kinetic inductance and a dissipative quasiparticle conductivity, taking into account the oscillatory dynamics of superconducting condensate and the kinetics of quasiparticles driven by a strong ac current. It is shown that an ac current density produces multiple harmonics of the electric field, the amplitudes of the higher-order harmonics diminishing as τ E increases.

Dynamical states offer extended possibilities to control the properties of quantum matter. Recent efforts are focused on studying the ordered states which appear exclusively under the time-dependent drives. Here we demonstrate a class of systems which feature dynamical spin-triplet superconducting order stimulated by the alternating electric field. The effect is based on the interplay of ferromagnetism, interfacial spin-orbital coupling and the oscillating motion of Cooper pairs. We demonstrate that the critical current of Josephson junctions hosting these states is proportional to the electromagnetic power, supplied either by the external irradiation or by the ac current source. Based on these unusual properties we propose the scheme of a Josephson transistor which can be switched by the ac voltage and demonstrates an even-numbered sequence of Shapiro steps. Combining the photo-active Josephson junctions with recently discovered Josephson phase batteries we find photo-magnetic SQUID devices which can generate spontaneous magnetic fields while being exposed to the irradiation.

BaNi 2 As 2 is a non-magnetic analogue of BaFe 2 As 2 , the parent compound of a prototype ferro-pnictide high-temperature superconductor. Recent diffraction studies on BaNi 2 As 2 demonstrate the existence of two types of periodic lattice distortions above and below the tetragonal to triclinic phase transition, suggesting charge-density-wave (CDW) order to compete with superconductivity. We apply time-resolved optical spectroscopy and demonstrate the existence of collective CDW amplitude modes. The smooth evolution of these modes through the structural phase transition implies the CDW order in the triclinic phase smoothly evolves from the unidirectional CDW in the tetragonal phase and suggests that the CDW order drives the structural phase transition.

The effect of annealing in high pressure oxygen atmosphere on superconducting transition temperatures for ceramic samples of magnetic superconductor europium based 1222 ruthenocuprate was studied. It was shown that the properties of the samples are consistent with the behavior of granular superconducting system. As a result of oxygen saturation the superconducting transition temperatures become higher. Particularly, the shift of superconducting transition temperature for the intergranular medium is 9.2 K and for the matter within the granules 6.8 K. This difference is supposed due to the mechanism of oxygen diffusion along the grain boundaries. In the temperature range between 135 K and 350 K the behavior of resistance obeyed the Mott law of variable range hopping for three dimensional case.