The pursuit of the non-Hermitian skin effect (NHSE) in various physical systems is of great research interest. Compared with recent progress in nonelectronic systems, the implementation of the NHSE in condensed matter physics remains elusive. Here, we show that the NHSE can be engineered in the mesoscopic heterojunctions (system plus reservoir) in which electrons in two channels of the system moving towards each other have asymmetric coupling to those of the reservoir. This makes electrons in the system move forward and in the opposite direction have unequal lifetimes, and so gives rise to a point-gap spectral topology. Accordingly, the electron eigenstates exhibit NHSE under the open boundary condition, consistent with the description of the generalized Brillouin zone. Such a reservoir-engineered NHSE visibly manifests itself as the nonreciprocal charge current that can be probed by the standard transport measurements. Further, we generalize the scenario to the spin-resolved NHSE, which can be probed by the nonreciprocal spin transport. Our work opens a research avenue for implementing and detecting the NHSE in electronic mesoscopic systems, which will lead to interesting device applications.

We theoretically study the coherent nonlinear response of electrons confined in semiconductor quantum wells under the effect of an electromagnetic radiation close to resonance with an intersubband transition. Our approach is based on the time-dependent Schrodinger-Poisson equation stemming from a Hartree description of Coulomb-interacting electrons. This equation is solved by standard numerical tools and the results are interpreted in terms of approximated analytical formulas. For growing intensity, we observe a redshift of the effective resonance frequency due to the reduction of the electric dipole moment and the corresponding suppression of the depolarization shift. The competition between coherent nonlinearities and incoherent saturation effects is discussed. The strength of the resulting optical nonlinearity is estimated across different frequency ranges from mid-IR to THz with an eye to ongoing experiments on Bose-Einstein condensation of intersubband polaritons and to the speculative exploration of quantum optical phenomena such as single-photon emission in the mid-IR and THz windows.

A Josephson junction laser is realized when a microwave cavity is driven by a voltage-biased Josephson junction. Through the ac Josephson effect, a dc voltage generates a periodic drive that acts on the cavity and generates interactions between its modes. A sufficiently strong drive enables processes that downconvert a drive resonant with a high harmonic into photons at the cavity fundamental frequency, breaking the discrete time translation symmetry set by the Josephson frequency. Using a classical model, we determine when and how this transition occurs as a function of the bias voltage and the number of cavity modes. We find that certain combinations of mode number and voltage tend to facilitate the transition which emerges via an instability within a subset of the modes. Despite the complexity of the system, there are cases in which the critical drive strength can be obtained analytically.

The intersection of topology, symmetry, and magnetism yields a rich structure of possible phases. In this work, we study theoretically the consequences of magnetism on IrF4, which was recently identified as a possible candidate topological nodal chain semimetal in the absence of magnetic order. We show that the spin-orbital nature of the Ir moments gives rise to strongly anisotropic magnetic couplings resembling a tetrahedral compass model on a diamond lattice. The predicted magnetic ground state preserves key symmetries protecting the nodal lines, such that they persist into the ordered phase at the mean-field level. The consequences for other symmetry reductions are also discussed.

We study the effects of thermal fluctuations and pinned boundaries in graphene membranes by using a phasefield crystal model with out-of-plane deformations. For sufficiently long times, the linear diffusive behavior of height fluctuations in systems with free boundaries becomes a saturation regime, while at intermediate times the behavior is still subdiffusive as observed experimentally. Under compression, we find mirror buckling fluctuations where the average height changes from above to below the pinned boundaries, with the average time between fluctuations diverging below a critical temperature corresponding to a thermally induced buckling transition. Near the transition, we find a nonlinear height response in agreement with recent renormalizationgroup calculations and observed in experiments on graphene membranes under an external transverse force with clamped boundaries.

Excitonic bound states are characterized by a binding energy eb and a single-particle band gap Ab. This work provides a theoretical description for both strong (eb - Ab) and weak (eb << Ab) excitonic bound states, with particular application to biased bilayer graphene. Standard description of excitons is based on a wave function that is determined by a Schrodinger-like equation with screened attractive potential. The wave function approach is valid only in the weak-binding regime eb << Ab. The screening depends on frequency, i.e., dynamical screening, and this implies retardation. In the case of strong binding, eb - Ab, a wave function description is not possible due to the retardation. Instead we appeal to the Bethe-Salpeter equation, written in terms of the electron-hole Green's function, to solve the problem. So far only the weak-binding regime has been achieved experimentally. Our analysis demonstrates that the strong-binding regime is also possible and we specify conditions in which it can be achieved for the prototypical example of biased bilayer graphene. The conditions concern the bias, the configuration of gates, and the substrate material. To verify the accuracy of our analysis we compare with available data for the weak-binding regime. We anticipate applying the developed dynamical screening Bethe-Salpeter techniques to various 2D materials with strong binding.