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Register a new userCollective Electron Dynamics in Metallic and Semiconductor Nanostructures
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Understanding the electron dynamics and transport in metallic and semiconductor nanostructures -- such as metallic nanoparticles, thin films, quantum wells and quantum dots -- represents a considerable challenge for todays condensed matter physics, both fundamental and applied. In this review article, we will describe the collective electron dynamics in metallic and semiconductor nanostructures using different, but complementary, approaches. For small excitations (linear regime), the spectral properties can be investigated via quantum mean-field models of the TDLDA type (time-dependent local density approximation), generalized to account for a finite electron temperature. In order to explore the nonlinear regime (strong excitations), we will adopt a phase-space approach that relies on the resolution of kinetic equations in the classical phase space (Vlasov and Wigner equations). The phase-space approach provides a useful link between the classical and quantum dynamics and is well suited to model effects beyond the mean field approximation (electron-electron and electron-phonon collisions). We will also develop a quantum hydrodynamic model, based on velocity moments of the corresponding Wigner distribution function: this approach should lead to considerable gains in computing time in comparison with simulations based on conventional methods, such as density functional theory (DFT). Finally, the magnetization (spin) dynamics will also be addressed.
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We report a systematic study of the $5d$-electron-doped system Ce(Fe$_{1-x}$Ir$_x$)$_2$Al$_{10}$ ($0 leq x leq 0.15$). With increasing $x$, the orthorhombic $b$~axis decreases slightly while accompanying changes in $a$ and $c$ leave the unit cell volume almost unchanged. Inelastic neutron scattering, along with thermal and transport measurements, reveal that for the Kondo semiconductor CeFe$_2$Al$_{10}$, the low-temperature energy gap which is proposed to be a consequence of strong $c mhyphen f$ hybridization, is suppressed by a small amount of Ir substitution for Fe, and that the system adopts a metallic ground state with an increase in the density of states at the Fermi level. The charge or transport gap collapses (at $x=$~0.04) faster than the spin gap with Ir substitution. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements demonstrate that the system undergoes long-range antiferromagnetic order below a Neel temperature, $T_{mathrm{N}}$, of 3.1(2)~K for $x = 0.15$. The ordered moment is estimated to be smaller than 0.07(1)~$mu_mathrm{B}$/Ce although the trivalent state of Ce is confirmed by Ce L$_3$-edge x-ray absorption near edge spectroscopy. It is suggested that the $c mhyphen f$ hybridization gap, which plays an important role in the unusually high ordering temperatures observed in Ce$T_2$Al$_{10}$ ($T$ = Ru and Os), may not be necessary for the onset of magnetic order with a low $T_{mathrm{N}}$ seen here in Ce(Fe$_{1-x}$Ir$_x$)$_2$Al$_{10}$.
We theoretically study the effect of electron-electron interactions on the metallic state of quasicrystals. To address the problem, we introduce the extended Hubbard model on the Ammann-Beenker tiling as a simple theoretical model. The model is numerically solved within an inhomogeneous mean-field theory. Because of the lack of periodicity, the metallic state is nonuniform in the electron density even in the noninteracting limit. We clarify how this charge distribution pattern changes with electron-electron interactions. We find that the intersite interactions change the distribution substantially and in an electron-hole asymmetric way. We clarify the origin of these changes through the analyses in the real and perpendicular spaces. Our results offer a fundamental basis to understand the electronic states in quasicrystalline metals.
We present experimental coherent two-dimensional Fourier transform spectra of the exciton resonances in semiconductor quantum wells for a pulse sequence that isolates two-quantum coherences. By measuring the real part of the spectra, we can determine that the spectra are dominated by two quantum coherences due to many-body interactions, not bound biexcitons. Simulations performed using dynamics controlled truncation agree well with the experiments.
We performed optical-pump terahertz-probe measurements of a Mott insulator YTiO$_{3}$ and a band semiconductor Si using a laser diode (1.47 eV) and a femtosecond pulse laser (1.55 eV). Both samples possess long energy-relaxation times (1.5 ms for YTiO$_{3}$ and 15 $mu$s for Si); therefore, it is possible to extract terahertz complex conductivities of photoinduced carriers under equilibrium. We observed highly contrasting behavior - Drude conductivity in Si and localized conductivity possibly obeying the Jonscher law in YTiO$_{3}$. The carrier number at the highest carrier-concentration layer in YTiO$_{3}$ is estimated to be 0.015 per Ti site. Anisotropic conductivity of YTiO$_{3}$ is determined. Our study indicates that localized carriers might play an important role in the incipient formation of photoinduced metallic phases in Mott insulators. In addition, this study shows that the transfer-matrix method is effective for extracting an optical constant of a sample with a spatially inhomogeneous carrier distribution.
In order to exploit the intriguing optical properties of graphene it is essential to gain a better understanding of the light-matter interaction in the material on ultrashort timescales. Exciting the Dirac fermions with intense ultrafast laser pulses triggers a series of processes involving interactions between electrons, phonons and impurities. Here we study these interactions in epitaxial graphene supported on silicon carbide (semiconducting) and iridium (metallic) substrates using ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) based on high harmonic generation. For the semiconducting substrate we reveal a complex hot carrier dynamics that manifests itself in an elevated electronic temperature and an increase in linewidth of the $pi$ band. By analyzing these effects we are able to disentangle electron relaxation channels in graphene. On the metal substrate this hot carrier dynamics is found to be severely perturbed by the presence of the metal, and we find that the electronic system is much harder to heat up than on the semiconductor due to screening of the laser field by the metal.
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