Semiconductor quantum dots driven by the broadband radiation fields of nearby quantum point contacts provide an interesting setting for probing dynamics in driven quantum systems at the nanoscale. We report on real-time charge-sensing measurements of the dot occupation, which reveal sharp resonances in the ionization rate as a function of gate voltage and applied magnetic field. Despite the broadband nature of excitation, the resonance widths are much smaller than the scale of thermal broadening. We show that such resonant enhancement of ionization is not accounted for by conventional approaches relying on elastic scattering processes, but can be explained via a mechanism based on a bottleneck process that is relieved near excited state level crossings. The experiment thus reveals a regime of a strongly driven quantum dynamics in few-electron systems. The theoretical results are in good agreement with observations.

The Andreev transport through a quantum dot coupled to two external ferromagnetic leads and one superconducting lead is studied theoretically by means of the real-time diagrammatic technique in sequential and cotunneling regimes. We show that the tunnel magnetoresistance (TMR) of the Andreev current displays a nontrivial dependence on the bias voltage and the level detuning and can be described by analytical formulas in the zero-temperature limit. The cotunneling processes lead to a strong modification of the TMR, which is most visible in the Coulomb blockade regime. We find a zero-bias anomaly of the Andreev differential conductance in the parallel configuration, which is associated with a nonequilibrium spin accumulation in the dot triggered by Andreev processes.

The recombination and spin dynamics of excitons in colloidal CdTe nanocrystals (NCs) are studied by time-resolved photoluminescence in high magnetic fields up to 15 T and at cryogenic temperatures. The recombination decay shows a nonexponential temporal behavior, with the longest component corresponding to the dark excitons having 260-ns decay time at zero magnetic field and 4.2-K temperature. This long component shortens to 150 ns at 15 T due to the magnetic-field-induced mixing of the bright- and dark-exciton states. The spin dynamics, assessed through the evolution of the magnetic-field-induced circular polarization degree of the photoluminescence, has a fast component shorter than 1 ns related to the bright excitons and a slow component of 5–10 ns associated with the dark excitons. The latter shortens with increasing magnetic field, which is characteristic for a phonon-assisted spin-relaxation mechanism. The relatively low saturation level of the associated magnetic-field-induced circular polarization degree of $-30\%$ is explained by a model that suggests the CdTe NCs to constitute an ensemble of prolate and oblate NCs, both having a structural quantization axis. The exciton $g$ factor of 2.4–2.9 evaluated from fitting the experimental data in the frame of the suggested approach is in good agreement with the expected value for the dark excitons in CdTe NCs.

Two-dimensional electron gas (2DEG) coexists with topological states on the surface of topological insulators (TIs), while the origin of the 2DEG remains elusive. In this work, electron density in TI thin films (${\text{Bi}}_2$${\text{Se}}_3$, ${\text{Bi}}_2$${\text{Te}}_3$, and their alloys) were manipulated by controlling the density of electronically active native defects with particle irradiation. The measured electron concentration increases with irradiation dose but saturates at different levels for ${\text{Bi}}_2$${\text{Se}}_3$ and ${\text{Bi}}_2$${\text{Te}}_3$. The results are in quantitative agreement with the amphoteric defect model, which predicts that electronically active native defects shift the Fermi energy ($E_{\mathrm{F}}$) toward a Fermi stabilization level ($E_{\mathrm{FS}}$) located universally at ∼4.9 eV below the vacuum level. Combined with thickness-dependent data, it is demonstrated that regardless of the bulk doping, the surface $E_{\mathrm{F}}$ is always pinned at $E_{\mathrm{FS}}$, producing a band bending and 2DEG on TI film surfaces. Our work elucidates native defect physics of TIs with a model universally applicable to other semiconductors and has critical implications for potential device applications of TIs.

We present numerical simulations of the capacitive coupling between graphene nanoribbons of various widths and gate electrodes in different configurations. We compare the influence of lateral metallic or graphene side gate structures on the overall back gate capacitive coupling. Most interestingly, we find a complex interplay between quantum capacitance effects in the graphene nanoribbon and the lateral graphene side gates, giving rise to an unconventional negative quantum capacitance. The emerging nonlinear capacitive couplings are investigated in detail. The experimentally relevant relative lever arm, the ratio between the coupling of the different gate structures, is discussed.

We calculate the spin-polarized electronic transport through a molecular bilayer spin valve from first principles, and establish the link between the magnetoresistance and the spin-dependent interactions at the metal-molecule interfaces. The magnetoresistance of a Fe$|$bilayer-C${}_{70}|$Fe spin valve attains a high value of 70% in the linear-response regime, but it drops sharply as a function of the applied bias. The current polarization has a value of 80% in linear response and also decreases as a function of bias. Both these trends can be modeled in terms of prominent spin-dependent Fe$|$${\text{C}}_{70}$ interface states close to the Fermi level, unfolding the potential of spinterface science to control and optimize spin currents.

We use the density functional theory to demonstrate that the chemical reactivity of nitrogen atoms with a propagating zigzag edge during the nucleation of graphene grains can give rise to persistent sublattice symmetry breaking phenomena. Their effect on the electronic structure of a formed two-dimensional graphene sheet is studied by unfolding the bands obtained from large supercell calculations. We argue that the loss of inversion symmetry enhances the creation of a band gap when assisted by dopant agglomeration. At higher concentrations of graphitic nitrogen the conduction band gets strongly suppressed, paving the way for the use of nitrogen-doped graphene as a valley-filter component.

Nonequilibrium charge dynamics, such as cooling-rate-dependent charge vitrification and physical aging, have been demonstrated for a charge-cluster glass in $\theta$-(BEDT-TTF)${}_2$CsZn(SCN)${}_4$ using electron transport measurements. The temperature evolution of the relaxation time obeys the Arrhenius law, indicating that the glass-forming charge liquid can be classified as a “strong” liquid in the scheme of canonical structural-glass formers. X-ray diffuse scattering further reveals that the spatial growth of charge clusters in the charge liquid is frozen below the glass transition temperature, indicating an intrinsic relationship between dynamics and structure in the charge-cluster glass.

I propose a van der Waals density functional (vdW-DF) that improves upon the description of energetics and geometries of molecules, solids, and adsorption systems over the original vdW-DF. The functional is based on the nonlocal correlation for the second version of the vdW-DF [Lee et al., Phys. Rev. B 82, 081101(R) (2010)] and an exchange functional that recovers the second-order gradient expansion approximation in the slowly varying limit, while reproducing the large density gradient behavior proposed by Becke [J. Chem. Phys. 85, 7184 (1986)]. A systematic assessment of the proposed functional is presented, which demonstrates the applicability of the proposed vdW-DF to a wide range of systems.

Recent experiments have revealed the possibility of optically orienting electron spins in bulk germanium via indirect optical transitions. However, the temporal limitations to both the spin lifetime and the coherence of photogenerated electrons have remained unexplored so far. Here we demonstrate resonant spin amplification in intrinsic bulk germanium using a 90 MHz femtosecond pulse train at 0.8 eV central photon energy. Most importantly, we find remarkably long spin lifetimes exceeding 50 ns at temperatures of up to 60 K, limited by Elliott-Yafet type processes. Consistent with model simulations we also find pronounced signatures of the $g$-factor anisotropy in germanium in the resonant spin amplification data.

We report on multiterminal electrical transport measurements performed on a bilayer graphene sheet enclosed by two hexagonal boron nitride flakes. We characterize the temperature dependence of electrical resistivity from 300 mK to 50 K, varying the carrier densities with a back gate. The resistivity curves clearly show a temperature-independent crossing point at density $n=n_c\approx 2.5\times {10}^{11}$ cm${}^{-2}$ for both positive and negative carriers, separating two distinct regions with $d\rho /dT<0$ and $d\rho /dT>0$, respectively. Our analysis rules out the possibility of a zero-T quantum phase transition, revealing instead the onset of robust ballistic transport for $n>n_c$, while the T dependence close to the neutrality point is the one expected from the parabolic energy-momentum relation. At low temperature ($T\ll 10$ K), the data are compatible with transport via variable range hopping mediated by localized impurity sites, with a characteristic exponent 1/3 that is renormalized to 1/2 by Coulomb interaction in the high-density regime.

Accurate experimental data of adsorbate potential energy landscapes are crucial as benchmarks for the evaluation of first-principles calculations. Here, we present a Bayesian method, analyzing the difference in forward and backward hopping rate in helium spin-echo measurements, that allows us to determine the binding-energy difference between two sites with unprecedented accuracy. Demonstrating the power of the method on the model system cyclopentadienyl/Cu(111), we find an energy difference between fcc and hcp hollow sites of ($10.6\pm 1.7$) meV.

We describe the evolution of the SU(4) Kondo effect as the number of magnetic centers increases from one impurity to the two-dimensional (2D) lattice. We derive a Hubbard-Anderson model which describes a 2D array of atoms or molecules with twofold orbital degeneracy, acting as magnetic impurities and interacting with a metallic host. We calculate the differential conductance, observed typically in experiments of scanning tunneling spectroscopy, for different arrangements of impurities on a metallic surface: a single impurity, a periodic square lattice, and several sites of a rectangular cluster. Our results point towards the crucial importance of the orbital degeneracy and agree well with recent experiments in different systems of iron(II) phtalocyanine molecules deposited on top of Au(111) [N. Tsukahara et al., Phys. Rev. Lett. 106, 187201 (2011)], indicating an experimental realization of an artificial 2D SU(4) Kondo-lattice system.

In a recent experimental paper [Bid et al., Nature 466, 585 (2010)] a qualitative confirmation of the existence of upstream neutral modes at the $\nu =\frac{2}{3}$ quantum Hall edge was reported. Using the chiral Luttinger liquid theory of the quantum Hall edge we develop a quantitative model of the experiment of Bid et al. A good quantitative agreement of our theory with the experimental data reinforces the conclusion of the existence of the upstream neutral mode. Our model also enables us to extract important quantitative information about nonequilibrium processes in Ohmic and tunneling contacts from the experimental data. In particular, for $\nu =\frac{2}{3}$, we find a power-law dependence of the neutral mode temperature on the charge current injected from the Ohmic contact.

We study theoretically and experimentally a type of metamaterial with hybrid elements composed of twisted pairs of cross-shaped meta-atoms and their complements. We reveal that such two-layer metasurfaces demonstrate large, dispersionless optical activity at the transmission resonance accompanied by very low ellipticity. We develop a retrieval procedure to determine the effective material parameters for this structure, which has symmetry (${\mathrm{C}}_4$) of a lower order than other commonly studied chiral structures. We verify our theoretical approach by reproducing numerical and experimental scattering parameters.

We address low-density two-dimensional circular quantum dots with spin-restricted Kohn-Sham density functional theory. By using an exchange-correlation functional that encodes the effects of the strongly correlated regime (and that becomes exact in the limit of infinite correlation), we are able to reproduce characteristic phenomena such as the formation of ring structures in the electronic total density, preserving the fundamental circular symmetry of the system. The observation of this and other well-known effects in Wigner-localized quantum dots such as the flattening of the addition energy spectra has until now only been within the scope of other, numerically more demanding theoretical approaches.

We report pressure-dependent reflection and transmission measurements on ZnCr${}_2$Se${}_4$, HgCr${}_2$S${}_4$, and CdCr${}_2$O${}_4$ single crystals at room temperature over a broad spectral range 200–24 000 cm${}^{-1}$. The pressure dependence of the phonon modes and the high-frequency electronic excitations indicate that all three compounds undergo a pressure-induced structural phase transition with critical pressures of 15, 12, and 10 GPa for CdCr${}_2$O${}_4$, HgCr${}_2$S${}_4$, and ZnCr${}_2$Se${}_4$, respectively. The eigenfrequencies of the electronic transitions are very close to the expected values for chromium crystal-field transitions. In the case of the chalcogenides, pressure induces a redshift of the electronic excitations, which indicates a strong hybridization of the Cr $d$ bands with the chalcogenide bands.

Using classical and ab initio calculations we demonstrate that extra electrons can be trapped in pure crystalline and amorphous SiO${}_2$ (a-SiO${}_2$) in deep band gap states. The structure of trapped electron sites in pure a-SiO${}_2$ is similar to that of Ge electron centers and so-called [SiO${}_4$/Li]${}^0$ centers in $\alpha$ quartz. Classical potentials were used to generate amorphous silica models and density functional theory to characterize the geometrical and electronic structures of trapped electrons in crystalline and amorphous silica. The calculations demonstrate that an extra electron can be trapped at a Ge impurity in $\alpha$ quartz in six different configurations. An electron in the [SiO${}_4$/Li]${}^0$ center is trapped on a regular Si ion with the Li ion residing nearby. Extra electrons can trap spontaneously on pre-existing structural precursors in amorphous SiO${}_2$, while the electron self-trapping in $\alpha$ quartz requires overcoming a barrier of about 0.6 eV. The precursors for electron trapping in amorphous SiO${}_2$ comprise wide ($\ge$132${}^{\circ }$) O–Si–O angles and elongated Si–O bonds at the tails of corresponding distributions. Using this criterion, we estimate the concentration of these electron trapping sites at $\approx 4\times {10}^{19}$ cm${}^{-3}$.

Fractional Chern insulators (FCIs), having properties similar to those of the fractional quantum Hall effect, have been established numerically in various toy models. To fully explore their fundamental physics and to develop practical applications, material realization is indispensable. Here we theoretically predict a realization of FCI in a two-dimensional organometallic material, which is known to have the prerequisite topological flat bands. Using numerical exact diagonalization we demonstrate that the presence of strong electronic correlations and fractional filling of such a system could lead to a rich phase diagram, including Abelian fractional quantum Hall (FQH), Fermi-liquid, and Wigner crystal states. In particular, the FQH state has been confirmed systematically by calculating the topological ground-state degeneracies, topological Chern number, and the quasihole excitation spectrum as well as the particle entanglement spectrum. Future experimental realization of the FQH state in such material may provide a route for developing practical applications of FCI with no need of the extreme conditions.

Using low-temperature scanning tunneling spectroscopy we study quantum well states in the topmost copper layer of a Cu/Co/Cu(100) system above the Fermi energy. The emergence and the energetic positions of QWSs within this layer crucially depend on the interface quality tailored by the sample preparation method. Samples deposited at room temperature show a rough interface and lead to the well-known QWSs with only a momentum perpendicular to the interface. Atomically smooth interfaces for samples grown at $80\mathrm{K}$ exhibit states caused by stationary points with a large nonvanishing parallel momentum. Simulations taking into account the different band structures allow QWSs to be modeled from both stationary points and an identification of a crossover between bound and resonance states.