@article {ahn_anisotropic_2021,
title = {Anisotropic fermionic quasiparticles},
journal = {Phys. Rev. B},
volume = {103},
number = {4},
year = {2021},
note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article},
month = {jan},
abstract = {We have carried out a comprehensive investigation of the quasiparticle properties of a two-dimensional electron gas, interacting via the long-range Coulomb interaction in the presence of bare mass anisotropy (i.e., with an elliptic noninteracting Fermi surface) by calculating the self-energy, the spectral function, the scattering rate, and the effective mass within the leading-order dynamical self-energy approximation. Our theory is exact in the high-density limit. We find anisotropic features of quasiparticle properties that are not captured by the commonly used isotropic approximation where the anisotropic effective mass is replaced by the isotropic averaged density-of-states mass. Some of these interesting results are as follows: (1) The many-body renormalization of the quasiparticle spectrum becomes highly anisotropic as the quasiparticle energy increases away from the Fermi energy; (2) the interaction-induced inelastic-scattering rate features a strong anisotropy, exhibiting an abrupt jump at different injected energies depending on the momentum direction of the injected electron; (3) the effective-mass enhancement is larger (smaller) for the light (heavy) mass, showing that the anisotropy is reduced by interactions. Our results and analysis show that the unjustified neglect of the mass anisotropy can lead to an incorrect description of quasiparticle properties of the anisotropic system and inaccurate estimates of physical quantities of interest although the use of an equivalent isotropic approximation using the density-of-states effective mass as is commonly and uncritically performed in the literature, works as a reasonable approximation in many situations. In addition to the complete random phase approximation theory for the anisotropic quasiparticles, we also provide a theory using the simpler plasmon-pole approximation, commenting on its validity for anisotropic self-energy calculations. We comment also on the interaction effect on the Fermi-surface topology, finding that the elliptic shape of the bare Fermi surface is preserved with suppressed ellipticity in the interacting system to a high degree of accuracy except in the very strongly interacting limit (and for very high bare mass anisotropy). Our theory provides a complete generalization of the existing isotropic many-body theory of interacting electrons to the corresponding anisotropic systems.},
issn = {2469-9950},
doi = {10.1103/PhysRevB.103.045303},
author = {Ahn, Seongjin and Das Sarma, S.}
}
@article {ahn_microscopic_2021,
title = {Microscopic bath effects on noise spectra in semiconductor quantum dot qubits},
journal = {Phys. Rev. B},
volume = {103},
number = {4},
year = {2021},
note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article},
month = {jan},
abstract = {When a system is thermally coupled to only a small part of a larger bath, statistical fluctuations of the temperature (more precisely, the internal energy) of this {\textquotedblleft}sub-bath{\textquotedblright} around the mean temperature defined by the larger bath can become significant. We show that these temperature fluctuations generally give rise to 1/f-like noise power spectral density from even a single two-level system. We extend these results to a distribution of fluctuators, finding the corresponding modification to the Dutta-Horn relation. Then we consider the specific situation of charge noise in silicon quantum dot qubits and show that recent experimental data [E. J. Connors et al., Phys. Rev. B 100, 165305 (2019)] can be modeled as arising from as few as two two-level fluctuators, and accounting for sub-bath size improves the quality of the fit.},
issn = {2469-9950},
doi = {10.1103/PhysRevB.103.L041304},
author = {Ahn, Seongjin and Das Sarma, S. and Kestner, J. P.}
}
@article {ahn_theory_2021,
title = {Theory of anisotropic plasmons},
journal = {Phys. Rev. B},
volume = {103},
number = {4},
year = {2021},
note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article},
month = {jan},
abstract = {We develop the complete theory for the collective plasmon modes of an interacting electron system in the presence of explicit mass (or velocity) anisotropy in the corresponding noninteracting situation, with the effective Fermi velocity being different along different axes. Such effective mass anisotropy is common in solid state materials (e.g., silicon or germanium), where the Fermi surface is often not spherical. We find that the plasmon dispersion itself develops significant anisotropy in such systems, and the commonly used isotropic approximation of using a density of states or optical effective mass does not work for the anisotropic system. We predict a qualitatively new phenomenon in anisotropic systems with no corresponding isotropic analog, where the plasmon mode along one direction decays into electron-hole pairs through Landau damping while the mode remains undamped and stable along a different direction.

},
issn = {2469-9950},
doi = {10.1103/PhysRevB.103.L041303},
author = {Ahn, Seongjin and Sankar Das Sarma}
}
@article { ISI:000579337200002,
title = {Fermi-surface topology and renormalization of bare ellipticity in an interacting anisotropic electron gas},
journal = {Phys. Rev. B},
volume = {102},
number = {16},
year = {2020},
month = {OCT 19},
pages = {161114},
publisher = {AMER PHYSICAL SOC},
type = {Article},
abstract = {We investigate effects of electron-electron interactions on the shape of the Fermi surface in an anisotropic two-dimensional electron gas using the {\textquoteleft}{\textquoteleft}RPA-GW{{\textquoteright}{\textquoteright}} self-energy approximation. We find that the interacting Fermi surface deviates from an ellipse but not in an arbitrary way. The interacting Fermi surface has only two qualitatively distinct shapes for most values of r(s). The Fermi surface undergoes two distinct transitions between these two shapes as r(s) increases. For larger r(s), the degree of the deviation from an ellipse rapidly increases, but, in general, our theory provides a justification for the widely used elliptical Fermi-surface approximation, even for the interacting system, since the nonelliptic corrections are quantitatively rather small except for very large r(s).},
issn = {2469-9950},
doi = {10.1103/PhysRevB.102.161114},
author = {Ahn, Seongjin and Das Sarma, S.}
}