Projection of Bloch states obtained from quantum-mechanical calculations onto atomic orbitals is the fastest scheme to construct ab initio tight-binding Hamiltonian matrices. However, the presence of spurious states and unphysical hybridizations of the tight-binding eigenstates has hindered the applicability of this construction. Here we demonstrate that those spurious effects are due to the inclusion of Bloch states with low projectability. The mechanism for the formation of those effects is derived analytically. We present an improved scheme for the removal of the spurious states which results in an efficient scheme for the construction of highly accurate ab initio tight-binding Hamiltonians.

We propose an H-shaped two-dimensional topological insulator (2DTI) as a persistent quantum resonance device. The helical edge states of 2DTI are robust against a nonmagnetic field. However, the helical edge states interfere with bound states created by a nonmagnetic impurity. Transmissions between leads shows two kinds of quantum resonance in this device, the Breit-Wigner resonance and a Fano-like resonance. These resonances can be realized in the device through modulating the on-site impurity potential. Resonances in 2DTI are persistent because the helical state has no backscattering that is protected by time-reversal-symmetry conservation. The finite-size effect in 2DTI leads to the phase transition between the Fano and the Breit-Wigner resonances through modulating the thickness of the 2DTI leads.

We study transport properties of a ${\text{MoS}}_2$ monolayer in the presence of a perpendicular magnetic field $B$. We derive and discuss its band structure and take into account spin and valley Zeeman effects. Compared to a conventional two-dimensional electron gas, these effects lead to new quantum Hall plateaus and new peaks in the longitudinal resistivity as functions of the magnetic field. The field $B$ leads to a significant enhancement of the spin splitting in the conduction band, to a beating of the Shubnikov–de Haas (SdH) oscillations in the low-field regime, and to their splitting in the high-field regime. The Zeeman fields suppress significantly the beating of the SdH oscillations in the low-field regime and strongly enhance their splitting at high fields. The spin and valley polarizations show a similar beating pattern at low fields and are clearly separated at high fields in which they attain a value higher than $90\%$.

Using a fully relativistic theory we study the quantum Vavilov-Cherenkov radiation and quantum friction occurring during relative sliding of two transparent dielectric plates with the refractive index $n$. These phenomena occur above the threshold velocity $v_c=2nc/(n^2+1)$. Close to the threshold velocity they are dominated by the contribution from $s$-polarized electromagnetic waves, which agrees with the approximate (relativistic) theory by Pendry [J. B. Pendry, J. Mod. Opt. 45, 2389 (1998)]. However, in the ultrarelativistic case $\left(v\to c\right)$, the contributions from both polarizations are strongly enhanced in comparison with the approximate theory, and a new contribution occurs from the mixing of the electromagnetic waves with different polarizations. The numerical results are supplemented by an analytical treatment close to the threshold velocity and the light velocity.

Nanostructured materials exhibit low thermal conductivity because of the additional scattering due to phonon-boundary interactions. As these interactions are highly sensitive to the mean free path (MFP) of phonons, MFP distributions in nanostructures can be dramatically distorted relative to bulk. Here we calculate the MFP distribution in periodic nanoporous Si for different temperatures, using the recently developed MFP-dependent Boltzmann transport equation. After analyzing the relative contribution of each phonon branch to thermal transport in nanoporous Si, we find that at room temperature optical phonons contribute $17\%$ to heat transport, compared to $5\%$ in bulk Si. Interestingly, we observe a constant thermal conductivity over the range $200\mathrm{K}<T<300\mathrm{K}$. We attribute this behavior to the ballistic transport of acoustic phonons with long intrinsic MFP and the temperature dependence of the heat capacity. Our findings, which are in qualitative agreement with the temperature trend of thermal conductivities measured in nanoporous Si-based systems, shed light on the origin of the reduction of thermal conductivity in nanostructured materials and demonstrate the necessity of multiscale heat transport engineering, in which the bulk material and geometry are optimized concurrently.

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide $\left(\mathrm{MoS}{\mathrm{e}}_2\right)$, using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond time scale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by $\sim 50\%$. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides.

We report anomalous quantum transport features in bilayer graphene in the presence of a random distribution of structural vacancies. By using an efficient real-space Kubo-Greenwood transport methodology, the impact of a varying density of dimer versus nondimer vacancies is investigated in very large scale disordered models. While nondimer vacancies are shown to induce localization regimes, dimer vacancies result in an unexpected ballistic regime whose energy window surprisingly enlarges with increasing impurity density. Such counterintuitive phenomenon is explained by the formation of an effective linear dispersion in the bilayer band structure, which roots in the symmetry breaking effects driven by dimer vacancies, and provides a realization of Dirac semimetals in high dimension.

The exploration of topological states is of significant fundamental and practical importance in contemporary condensed matter physics, for which the extension to two-dimensional (2D) organometallic systems is particularly attractive. Using first-principles calculations, we show that a 2D hexagonal triphenyl-lead lattice composed of only main group elements is susceptible to a magnetic instability, characterized by a considerably more stable antiferromagnetic (AFM) insulating state rather than the topologically nontrivial quantum spin Hall state proposed recently. Even though this AFM phase is topologically trivial, it possesses an intricate emergent degree of freedom, defined by the product of spin and valley indices, leading to Berry curvature-induced spin and valley currents under electron or hole doping. Furthermore, such a trivial band insulator can be tuned into a topologically nontrivial matter by the application of an out-of-plane electric field, which destroys the AFM order, favoring instead ferrimagnetic spin ordering and a quantum anomalous Hall state with a nonzero topological invariant. These findings further enrich our understanding of 2D hexagonal organometallic lattices for potential applications in spintronics and valleytronics.

Gold single crystals were deformed by uniaxial compression tests with the help of an experimental apparatus that allows one to characterize in situ, by UHV scanning tunneling microscopy, the evolution of surface structures under stress. It is demonstrated that the slip traces resulting from the emergence of moving dislocations at the free surface highly modify the organization of the vicinal steps. A model based on energetic considerations is proposed and discussed to explain the observed phenomenon.

We develop a general perturbative computation of finite-frequency quantum noise that applies, in particular, to both good or weakly transmitting strongly correlated conductors coupled to a generic environment. Under a minimal set of hypotheses, we show that the noise can be expressed through the nonequilibrium dc current only, generalizing a nonequilibrium fluctuation dissipation relation. We use this relation to derive explicit predictions for the nonequilibrium finite-frequency noise for a single channel conductor connected to an arbitrary Ohmic environment.

We investigate the electronic structure and photoexcitation dynamics of alkali atoms (Rb and Cs) chemisorbed on transition-metal Ru(0001) single-crystal surface by angle- and time-resolved multiphoton photoemission. Three- and four-photon photoemission (3PP and 4PP) spectroscopic features due to the $\sigma$ and $\pi$ resonances arising from the $ns$ and $np$ states of free alkali atoms are observed from $\sim 2$ eV below the vacuum level in the zero-coverage limit. As the alkali coverage is increased to a maximum of 0.02 monolayers, the resonances are stabilized by formation of a surface dipole layer, but in contrast to alkali chemisorption on noble metals, both resonances form dispersive bands with nearly free-electron mass. Density functional theory calculations attribute the band formation to substrate-mediated interaction involving hybridization with the unoccupied $d$ bands of the substrate. Time-resolved measurements quantify the phase and population relaxation times in the three-photon photoemission (3PP) process via the $\sigma$ and $\pi$ resonances. Differences between alkali-atom chemisorption on noble and transition metals are discussed.

We present first-principles density functional calculations that clarify the electronic properties of carbon defects in hexagonal boron nitride $(h$-BN) monolayers under biaxially applied strains. We find that strain can control the ionization energies of both donor and acceptor states. Furthermore, we also find that strain can lead to the dramatic change in conduction channel properties of donor states due to the interchange of the conduction-band-minimum state with the nearly-free-electron state. We also report the simulated scanning tunneling microscopy (STM) images of carbon defects in $h$-BN monolayers for experimental identification of those defects. We show that the STM images strongly reflect distinctive spatial distributions of local density of states around carbon defects depending on the substitution sites and thereby they could be identified by using STM experiments.

Inducing magnetism in graphene holds great promises, such as controlling the exchange interaction with a gate electrode, and generating exotic magnetic phases. Coating graphene with magnetic molecules or atoms has so far mostly led to decreased graphene mobility. In the present work, we show that Pt-porphyrin molecules adsorbed on graphene lead both to an enhanced mobility, and to gate-dependent magnetism. We report that porphyrins can act both as donor or acceptor molecules, depending on the initial doping of the graphene sheet. The porphyrins transfer charge and ionize around the charged impurities on graphene, and, consequently, the graphene doping is decreased and its mobility is enhanced. In addition, ionized porphyrin molecules carry a magnetic moment. Using the sensitivity of mesoscopic transport to magnetism, in particular, the superconducting proximity effect and conductance fluctuations, we explore the magnetic order induced in graphene by the interacting magnetic moments of the ionized porphyrin molecules. Among the signatures of magnetism, we find two-terminal-magnetoresistance fluctuations with an odd component, a tell-tale sign of time-reversal symmetry breaking at zero field, which does not exist in uncoated graphene samples. When graphene is connected to superconducting electrodes, the induced magnetism leads to a gate-voltage-dependent suppression of the supercurrent, modified magnetic interference patterns, and gate-voltage-dependent magnetic hysteresis. The magnetic signatures are greatest for long superconductor/graphene/superconductor junctions, and for samples with the highest initial doping, compatible with a greater number of ionized, and thus magnetic porphyrin molecules. Our findings suggest that long-range (of the order of the coherence length, or micrometers) magnetism is induced through graphene by the ionized porphyrins' magnetic moment. This magnetic interaction is controled by the density of carriers in graphene, a tunability that could be exploited in spintronic applications.

${}^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) experiments have been performed on a single crystal of ${\text{CeRhAl}}_4{\text{Si}}_2$, which is an antiferromagnetic Kondo-lattice compound with successive antiferromagnetic transitions of $T_{\mathrm{N}1}=14$ K and $T_{\mathrm{N}2}=9$ K at zero external field. In the paramagnetic state, the Knight shifts, quadrupolar frequency, and asymmetric parameter of electrical field gradient on the Al sites have been determined, which have local orthorhombic symmetry. The transferred hyperfine coupling constants are also determined. Analysis of the NMR spectra indicates that a commensurate antiferromagnetic structure exists below $T_{\mathrm{N}2}$, but an incommensurate modulation of antiferromagnetic moments is present in the antiferromagnetic state between $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$. The spin-lattice relaxation rate suggests that the $4f$ electrons behave as local moments at temperatures above $T_{\mathrm{N}1}$.

The dynamics of magnetic domain walls in ultrathin strip-patterned Pt/Co/AlOx samples with perpendicular easy axis has been studied experimentally under an easy-axis field, superposed to a hard-axis field oriented along the strip. The easy-axis field is large so that the domain walls move well beyond the creep regime. A chiral effect is observed where the domain wall velocity shows a monotonous and surprisingly large variation with an in-plane field. A micromagnetic analysis, combining analytic, one-dimensional, and two-dimensional simulations with structural disorder, shows that this behavior can be reproduced with a Dzyaloshinskii-Moriya interaction of the interfacial type, with due consideration of the dynamics of the tilt degree of freedom of the domain wall. The estimated effective value of this interaction ($D\approx -2.2\mathrm{mJ}/{\mathrm{m}}^2$ for a 0.6 nm Co thickness) is consistent with values obtained by other techniques. It is also shown, by micromagnetic analysis, that several modes and characteristic times occur in the dynamics of the tilt of such domain walls.

We consider the potential for novel forms of magnetism arising from the subtle interplay between electrons and spins in the underscreened kagome Kondo lattice model. At weak coupling, we show that incommensurate noncoplanar multiwave vector magnetic orders arise at nearly all fillings and that this results from Fermi surface effects that introduce competing interactions between the spins. At strong coupling, we find that such a complex order survives near half filling despite the presence of ferromagnetism at all other fillings. We show this arises due to state selection among a massive degeneracy of states at infinite coupling. Finally, we show that at intermediate filling only commensurate orders seem to survive, but these orders still include noncoplanar magnetism. So, the mere presence of both local moments and itinerant electrons enables complex orders to form unlike any currently observed in kagome materials.

Single crystals of ${\mathrm{Yb}}_3\left(\mathrm{Rh}{}_{1-x}{T}_x\right){}_4{\mathrm{Ge}}_{13}$ $(T=\mathrm{Co},\mathrm{Ir})$ have been grown using the self-flux method. Powder x-ray diffraction data on these compounds are consistent with the cubic structure with space group $Pm\overline{3}n$. Intermediate-valence behavior is observed in ${\mathrm{Yb}}_3\left(\mathrm{Rh}{}_{1-x}{T}_x\right){}_4{\mathrm{Ge}}_{13}$ upon $T$ = Co doping, while $T$ = Ir doping drives the system into a heavy-fermion state. Antiferromagnetic order is observed in the Ir-doped samples ${\mathrm{Yb}}_3\left(\mathrm{Rh}{}_{1-x}{T}_x\right){}_4{\mathrm{Ge}}_{13}$ for $0.5<x\le 1$ with $T_N=0.96$ K for ${\mathrm{Yb}}_3\mathrm{Ir}{}_4{\mathrm{Ge}}_{13}$. With decreasing $x$, the magnetic order is suppressed towards a quantum critical point around $x_c$ = 0.5, accompanied by non-Fermi-liquid behavior evidenced by logarithmic divergence of the specific heat and linear temperature dependence of the resistivity. The Fermi-liquid behavior is recovered with the application of large magnetic fields.

We propose a versatile strategy for numerical renormalization group (NRG) solution of general channel-mixing Kondo and Anderson impurity models beyond previous reach. We illustrate the strategy by investigating the quantum phase transitions in models of Anderson impurities coupled to $s$- and $d$-wave superconducting baths. We discuss the effects of nontrivial channel-mixing in such models. Our strategy opens the door toward broad applications of NRG as impurity solver in cluster dynamical mean field theory for strongly correlated electron systems.