Manipulation of a two-photon pump in superconductor - semiconductor heterostructures

We investigate the photon statistics, entanglement and squeezing of a pn-junction sandwiched between two superconducting leads, and show that such an electrically-driven photon pump generates correlated and entangled pairs of photons. In particular, we demonstrate that the squeezing of the fluctuations in the quadrature amplitudes of the emitted light can be manipulated by changing the relative phase of the order parameters of the superconductors. This reveals how macroscopic coherence of the superconducting state can be used to tailor the properties of a two-photon state.Comment: 4+ pages, 3 figures; includes Supplemental Material (9 pages, 1 figure). Published versio

Andreev-Bragg reflection from an Amperian superconductor

We show how an electrical measurement can detect the pairing of electrons on the same side of the Fermi surface (Amperian pairing), recently proposed by Patrick Lee for the pseudogap phase of high-$T_c$ cuprate superconductors. Bragg scattering from the pair-density wave introduces odd multiples of $2k_{\rm F}$ momentum shifts when an electron incident from a normal metal is Andreev-reflected as a hole. These Andreev-Bragg reflections can be detected in a three-terminal device, containing a ballistic Y-junction between normal leads $(1,2)$ and the superconductor. The cross-conductance $dI_1/dV_2$ has the opposite sign for Amperian pairing than it has either in the normal state or for the usual BCS pairing.Comment: 5 pages, 6 figures; v2 includes study of disorder and interface barrie

Scattering theory of the chiral magnetic effect in a Weyl semimetal: Interplay of bulk Weyl cones and surface Fermi arcs

We formulate a linear response theory of the chiral magnetic effect in a finite Weyl semimetal, expressing the electrical current density $j$ induced by a slowly oscillating magnetic field $B$ or chiral chemical potential $\mu$ in terms of the scattering matrix of Weyl fermions at the Fermi level. Surface conduction can be neglected in the infinite-system limit for $\delta j/\delta \mu$, but not for $\delta j/\delta B$: The chirally circulating surface Fermi arcs give a comparable contribution to the bulk Weyl cones no matter how large the system is, because their smaller number is compensated by an increased flux sensitivity. The Fermi arc contribution to $\mu^{-1}\delta j/\delta B$ has the universal value $(e/h)^2$, protected by chirality against impurity scattering --- unlike the bulk contribution of opposite sign.Comment: 8 pages, 8 figures; V2: added references with discussion; V3: To be published in the Focus Issue on "Topological semimetals" of New Journal of Physic

Weyl-Majorana solenoid

A Weyl semimetal wire with an axial magnetization has metallic surface states (Fermi arcs) winding along its perimeter, connecting bulk Weyl cones of opposite topological charge (Berry curvature). We investigate what happens to this "Weyl solenoid" if the wire is covered with a superconductor, by determining the dispersion relation of the surface modes propagating along the wire. Coupling to the superconductor breaks up the Fermi arc into a pair of Majorana modes, separated by an energy gap. Upon variation of the coupling strength along the wire there is a gap inversion that traps the Majorana fermions.Comment: 6 pages, 6 figures; V2: added discussion of charge operator, updated figures; V3: added a section on analytical mode-matching calculations, an appendix, and three new figures. To be published in the Focus Issue on "Topological semimetals" of New Journal of Physic

On transport properties of Weyl semimetals

The main topics of this thesis are the surface Fermi arcs and transport properties of Weyl semimetals. The thesis begins with a general introduction in chapter one. Chapter two addresses transport properties of bulk Weyl cones. In chapter three, the chiral magnetic effect is studied, with particular focus on the role of the surface Fermi arcs. The fourth chapter deals with how the Fermi arcs are affected by the proximity of a superconductor. The final chapter explores Fermi arcs and transport properties of a very different system: the pseudo-gap phase of high-temperature cuprate superconductors.Theoretical Physic

Identifying Chern numbers of superconductors from local measurements

Fascination in topological materials originates from their remarkable response properties and exotic quasiparticles which can be utilized in quantum technologies. In particular, large-scale efforts are currently focused on realizing topological superconductors and their Majorana excitations. However, determining the topological nature of superconductors with current experimental probes is an outstanding challenge. This shortcoming has become increasingly pressing due to rapidly developing designer platforms which are theorized to display very rich topology and are better accessed by local probes rather than transport experiments. We introduce a robust machine-learning protocol for classifying the topological states of two-dimensional (2D) chiral superconductors and insulators from local density of states (LDOS) data. Since the LDOS can be measured with standard experimental techniques, our protocol overcomes the almost three decades standing problem of identifying the topology of 2D superconductors with broken time-reversal symmetry.Comment: 11 pages, 10 figure

Chirality blockade of Andreev reflection in a magnetic Weyl semimetal

A Weyl semimetal with broken time-reversal symmetry has a minimum of two species of Weyl fermions, distinguished by their opposite chirality, in a pair of Weyl cones at opposite momenta $\pm K$ that are displaced in the direction of the magnetization. Andreev reflection at the interface between a Weyl semimetal in the normal state (N) and a superconductor (S) that pairs $\pm K$ must involve a switch of chirality, otherwise it is blocked. We show that this "chirality blockade" suppresses the superconducting proximity effect when the magnetization lies in the plane of the NS interface. A Zeeman field at the interface can provide the necessary chirality switch and activate Andreev reflection.Comment: 15 pages, 9 figures. V2: added investigation of the dependence of the chirality blockade on the direction of the magnetization and (Appendix C) calculations of the Fermi-arc mediated Josephson effec