Strong Dimensional and Structural Dependencies of Hot Carrier Effects in InGaAs Nanowires: Implications for Photovoltaic Solar Cells

III-V nanowire structures are among the promising material systems with applications in hot carrier solar cells. These nanostructures can meet the requirements for such photovoltaic devices, i.e., the suppression of thermalization loss, an efficient hot carrier transport, and enhanced photoabsorption thanks to their unique one-dimensional (1D) geometry and density-of-states. Here, we investigate the effects of spatial confinement of photogenerated hot carriers in InGaAs-InAlAs core-shell nanowires, which presents an ideal class of hot carrier solar cell materials due to its suitable electronic properties. Using steady-state photoluminescence spectroscopy, our study reveals that by increasing the degree of spatial confinement and Auger recombination, the effects of hot carriers increase, which is in good agreement with theoretical modeling. However, for thin nanowires, the temperature of hot carriers decreases as the effects of crystal disorder increase. This observation is confirmed by probing the extent of the disorder-induced Urbach tail and linked to the presence of a higher density of stacking defects in the limit of thin nanowires. These findings expand our knowledge of hot carrier thermalization in nanowires, which can be applied for designing efficient 1D hot carrier absorbers for advanced-concept photovoltaic solar cells.

Effective field theory of chiral spin liquid between ordered phases in a kagome antiferromagnet

We propose in this work an effective field theory description of the chiral spin liquid state in a Heisenberg spin system on a kagome lattice. To this end, we derive the low-energy effective theory of a kagome (isotropic) Heisenberg antiferromagnet around its ordered ground states found numerically and show that quantum fluctuations induced by further neighbor spin exchanges are equally strong as those from the first neighbor. We use a chiral order parameter theory to argue for the occurrence of finite temperature chiral symmetry breaking transition into a chiral ordered state in a kagome antiferromagnet with further neighbor spin exchange interactions. We compute the chiral symmetry breaking term in the effective ground state energy and show that chiral spin liquid necessarily occurs in the ground state of a kagome antiferromagnet with the first three nearest-neighbor spin exchange interactions. Finally, we consider the quantum criticality of the kagome antiferromagnet and show that a Chern-Simons term emerges naturally at the transition between the two ordered states that satisfy an appropriate 'matching condition' that we derive, providing an explanation for why chiral spin liquid could occur at the transition between appropriate ordered states. This emergent Chern-Simons term is the low energy effective theory of the chiral spin liquid state, where the chirality is the immediate consequence of the breaking of discrete symmetries by this topological field theory.

Hole properties on and off magnetization plateaus in 2D antiferromagnets

The phenomenon of magnetization plateaus in antiferromagnets under a magnetic field has always been an important topic in magnetism. We propose to probe the elusive physics of plateaus in 2D by considering a hole-doped antiferromagnet and studying the signatures of magnetization plateaus in terms of the properties of holes, coupled to an effective gauge field generated by the spin sector. The latter mediates interaction between the holes, found to be algebraically decaying and long ranged with both Coulombic and dipolar forms outside the plateau and short ranged (local) inside the plateau. The resulting hole spectral weight is significantly broadened off plateau, while it remains sharply peaked on plateau. We also extend the result obtained for a 1D system where finite hole doping gives rise to a shift in the magnetization value of the plateaus.

Inertia and chiral edge modes of a Skyrmion magnetic bubble

The dynamics of a vortex in a thin-film ferromagnet resembles the motion of a charged massless particle in a uniform magnetic field. Similar dynamics is expected for other magnetic textures with a nonzero Skyrmion number. However, recent numerical simulations reveal that Skyrmion magnetic bubbles show significant deviations from this model. We show that a Skyrmion bubble possesses inertia and derive its mass from the standard theory of a thin-film ferromagnet. In addition to center-of-mass motion, other low energy modes are waves on the edge of the bubble traveling with different speeds in opposite directions.