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Direct observation of cross-polarized excitons in aligned single-chirality single-wall carbon nanotubes
摘要： Optical properties of single-wall carbon nanotubes (SWCNTs) for light polarized parallel to the nanotube axis have been studied extensively, whereas their response to light polarized perpendicular to the nanotube axis has not been well explored. Here, by using a macroscopic film of highly aligned single-chirality (6,5) SWCNTs, we performed a systematic polarization-dependent optical absorption spectroscopy study. In addition to the commonly observed angular-momentum-conserving interband absorption of parallel-polarized light, which generates E11 and E22 excitons, we observed a small but unambiguous absorption peak whose intensity is maximum for perpendicular-polarized light. We attribute this feature to the lowest-energy cross-polarized interband absorption processes that change the angular momentum along the nanotube axis by ±1, generating E12 and E21 excitons. Unlike previous observations of cross-polarized excitons in polarization-dependent photoluminescence and circular dichroism spectroscopy experiments, our direct observation using absorption spectroscopy allowed us to quantitatively analyze this resonance. Specifically, we determined the energy and oscillator strength of this resonance to be 1.54 and 0.05, respectively, compared with the values for the E11 exciton peak. These values, in combination with a comparison with theoretical calculations, in turn led to an assessment of the environmental effect on the strength of Coulomb interactions in this aligned single-chirality SWCNT film.
关键词： polarization-dependent,single-wall carbon nanotubes,cross-polarized excitons,aligned film,optical absorption spectroscopy,Coulomb interactions
更新于2025-09-23 15:22:29
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-electron materials: Sn monolayer on a SiC(0001) surface
摘要： Materials with sp magnetism demonstrate strongly nonlocal Coulomb interactions, which opens a way to probe correlations in the regimes not achievable in transition metal compounds. By the example of Sn monolayer on a SiC(0001) surface, we show that such systems exhibit unusual but intriguing magnetic properties at the nanoscale. Physically, this is attributed to the presence of a significant ferromagnetic coupling, the so-called direct exchange, which fully compensates ubiquitous antiferromagnetic interactions of the superexchange origin. Having a nonlocal nature, the direct exchange was previously ignored because it cannot be captured within the conventional density functional methods and significantly challenges ground state models earlier proposed for Sn/SiC(0001). Furthermore, heavy adatoms induce strong spin-orbit coupling, which leads to a highly anisotropic form of the spin Hamiltonian, in which the Dzyaloshinskii-Moriya interaction (DMI) is dominant. The latter is suggested to be responsible for the formation of a nanoskyrmion state at realistic magnetic fields and temperatures.
关键词： Dzyaloshinskii-Moriya interaction,SiC(0001) surface,direct exchange,nonlocal Coulomb interactions,nanoskyrmion state,sp magnetism,Sn monolayer
更新于2025-09-23 15:21:21
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Ferromagnetism in quantum dot plaquettes
摘要： Following recent experimental progress concerning Nagaoka ferromagnetism in finite-size quantum dot plaquettes, a general theoretical analysis is warranted in order to ascertain in rather generic terms which arrangements of a small number of quantum dots can produce saturated ferromagnetic ground states and under which constraints on interaction and interdot tunneling in the plaquette. This is particularly necessary since Nagaoka ferromagnetism is fragile and arises only under rather special conditions. We test the robustness of ground state ferromagnetism in the presence of a long-range Coulomb interaction and long-range as well as short-range interdot hopping by modeling a wide range of different plaquette geometries accessible by arranging a few (～4) quantum dots in a controlled manner. We find that ferromagnetism is robust to the presence of long-range Coulomb interactions, and we develop conditions constraining the tunneling strength such that the ground state is ferromagnetic. Additionally, we predict the presence of a partially spin-polarized ferromagnetic state for 4 electrons in a Y-shaped 4-quantum-dot plaquette. Finally, we consider 4 electrons in a ring of 5 dots. This does not satisfy the Nagaoka condition; however, we show that the ground state is spin 1 for strong, but not infinite, on-site interaction. Thus, even though Nagaoka’s theorem does not apply, the ground state for the finite system with one hole in a ring of 5 dots is partially ferromagnetic. We provide detailed fully analytical results for the existence or not of ferromagnetic ground states in several quantum dot geometries which can be studied in currently available coupled quantum dot systems.
关键词： long-range Coulomb interactions,Nagaoka ferromagnetism,interdot tunneling,ferromagnetic ground states,quantum dot plaquettes
更新于2025-09-12 10:27:22
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Strongly interacting Weyl semimetals: Stability of the semimetallic phase and emergence of almost free fermions
摘要： Using a combination of analytical arguments and state-of-the-art diagrammatic Monte Carlo simulations, we show that the corrections to the dispersion in interacting Weyl semimetals are determined by the ultraviolet cutoff and the inverse screening length. If both of these are finite, then the diagrammatic series is convergent even in the low-temperature limit, which implies that the semimetallic phase remains stable. Meanwhile, the absence of a UV cutoff or screening results in logarithmic divergences at zero temperature. These results highlight the crucial impact of Coulomb interactions and screening, mediated, e.g., through the presence of parasitic bands, which are ubiquitous effects in real-world materials. Also, despite sizable corrections from Coulomb forces, the contribution from the frequency-dependent part of the self-energy remains extremely small, thus giving rise to a system of effectively almost free fermions with a strongly renormalized dispersion.
关键词： Weyl semimetals,Coulomb interactions,diagrammatic Monte Carlo,ultraviolet cutoff,screening
更新于2025-09-09 09:28:46