The two-dimensional ferroelectrics GeS, GeSe, SnS and SnSe are expected to have large spontaneous in-plane electric polarization and enhanced shift-current response. Using density functional methods, we show that these materials also exhibit the largest effective second harmonic generation reported so far. It can reach magnitudes up to [Formula: see text] which is about an order of magnitude larger than that of prototypical GaAs. To rationalize this result we model the optical response with a simple one-dimensional two-band model along the spontaneous polarization direction. Within this model the second-harmonic generation tensor is proportional to the shift-current response tensor. The large shift current and second harmonic responses of GeS, GeSe, SnS and SnSe make them promising non-linear materials for optoelectronic applications.
We study the injection current susceptibility tensor (also known as the circular photogalvanic effect) of ferrolectric single-layer GeS, GeSe, SnS, and SnSe. We find that the injection current is perpendicular to the spontaneous in-plane polarization, can reach effective values of the order of 10 10 A/V 2 s, and peaks at photon energies in the visible spectrum. The magnitude is the largest reported in the literature so far. To rationalize our results, we construct a simple two-band model of injection current. Analysis of the model suggests that two-dimensions, in-plane polarization, and covalent bonding are important factors determining the magnitude and direction of the injection current. Our results also suggest strain as a control knob of injection current in optoelectronic applications of these materials. arXiv:1811.06474v3 [cond-mat.mes-hall]
Cerium-based ternary compounds CeNi2Cd20 and CePd2Cd20 do not exhibit long-range order down to millikelvin temperature range. Given the large separation between Ce ions which significantly reduces the super-exchange interactions and vanishingly small RKKY interaction, here we show that nodal superconductivity mediated by the valence fluctuations must be a ground state in these materials. We propose that the critical temperature for the superconducting transition can be significantly increased by applying hydrostatic pressure. We employ an extended periodic Anderson lattice model which includes the long-range Coulomb interactions between the itinerant electrons as well as the local Coulomb interaction between the predominantly localized and itinerant electrons to compute a critical temperature of the superconducting transition. Using the slave-boson approach we show that fluctuations mediated by the repulsive electron-electron interactions lead to the emergence of d-wave superconductivity.
Highly unconventional behavior of the thermodynamic response functions has been experimentally observed in a narrow gap semiconductor samarium hexaboride. Motivated by these observations, we use renormalization group technique to investigate many-body instabilities in the f-orbital narrow gap semiconductors with band inversion in the limit of weak coupling. By projecting out the double occupancy of the f-states we formulate a low-energy theory describing the interacting particles in two hybridized electron- and hole-like bands. The interactions are assumed to be weak and short-ranged. We take into account the difference between the effective masses of the quasiparticles in each band. Upon carrying out the renormalization group analysis we find that there is only one stable fixed point corresponding to the excitonic instability with time-reversal symmetry breaking for small enough mismatch between the effective masses.
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