Motivated by the recent observation of the Kondo effect in graphene in transport experiments, we investigate the resistivity and dephasing rate in the Kondo regime due to magnetic impurities in graphene with different chemical potentials ($\mu$). The Kondo effect due to either carbon vacancies or magnetic adatoms in graphene is described by the single-orbital pseudogap asymmetric Anderson impurity model which is solved by the accurate numerical renormalization group method. We find that although the Anderson impurity model considered here is a mixed-valence system, it can be driven into either the Kondo [$\mu >{\mu }_c$ (critical value) $>0$], mixed-valency ($\mu \approx {\mu }_c$), or empty-orbital ($\mu <{\mu }_c$) regime by a gate voltage, giving rise to characteristic features in resistivity and dephasing rate in each regime. Specifically, in the case of $\mu <{\mu }_c$, the shapes of the resistivity (dephasing rate) curves for different $\mu$ are nearly identical. However, as temperature decreases, they start to increase to their maxima at a lower $T/T_K$, but more rapidly [as ${(T_K/T)}^{3/2}$] than in normal metals [here, $T$ ($T_K$) denotes the (Kondo) temperature]. As $T$ further decreases, after reaching the maximum, the dephasing rate drops more quickly than in normal metals, behaving as ${(T/T_K)}^3$ instead of ${(T/T_K)}^2$. Furthermore, the resistivity has a distinct peak above the saturation value near $T_K$. In the case of $\mu >{\mu }_c$, in contrast, the resistivity curve has an additional broad shoulder above 10$T_K$ and the dephasing rate exhibits an interesting shoulder-peak shape. In the narrow boundary region ($\mu \approx {\mu }_c$), both the resistivity and dephasing rate curves are similar to the corresponding ones in normal metals. This explains the conventional Kondo-like resistivity from recent experiments on graphene with defects, although the distinct features in the resistivity in the other cases ($\mu <{\mu }_c$ or $\mu >{\mu }_c$) were not seen in the experiments. The interesting features in the resistivity and dephasing rate are analyzed in terms of the calculated $T$-dependent spectral function, correlation self-energy, and renormalized impurity level.

In this work, we present a combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study of chlorine adsorption on Au(111) at low coverages. Our STM study of Cl/Au(111) system has shown that at submonolayer coverages ($\theta <$ 0.1 ML) chlorine atoms form chainlike structures with abnormally short distances of 3.8 Å between them. Our DFT calculations have shown that chlorine atoms can interact with each other through distortion of the substrate and this indirect elastic interaction is strong enough to affect their arrangement in the chainlike structures.

We study the effect of interactions on two-dimensional fermionic symmetry-protected topological (SPT) phases using the recently proposed braiding statistics approach. We focus on a simple class of examples: superconductors with a $Z_2$ Ising symmetry. Although these systems are classified by $\mathbb{Z}$ in the noninteracting limit, our results suggest that the classification collapses to ${\mathbb{Z}}_8$ in the presence of interactions—consistent with previous work that analyzed the stability of the edge. Specifically, we show that there are at least eight different types of Ising superconductors that cannot be adiabatically connected to one another, even in the presence of strong interactions. In addition, we prove that each of the seven nontrivial superconductors have protected edge modes.

We explore the effect of single-particle level fluctuations on the Stoner instability in a quantum dot with a strong spin-orbit coupling in the framework of the universal Hamiltonian with the Ising exchange interaction. We reduce the problem to studying the statistics of extrema of a certain Gaussian process and demonstrate that, in spite of the randomness of the single-particle levels, the longitudinal spin susceptibility and all its moments diverge simultaneously at the point of the Stoner instability which is determined by the standard criterion involving the mean level spacing only.

Devising ways of opening a band gap in graphene to make charge-carrier masses finite is essential for many applications. Recent experiments with graphene on hexagonal boron nitride ($h$-BN) offer tantalizing hints that the weak interaction with the substrate is sufficient to open a gap, in contradiction of earlier findings. Using many-body perturbation theory, we find that the small observed gap is what remains after a much larger underlying quasiparticle gap is suppressed by incommensurability. The sensitivity of this suppression to a small modulation of the distance separating graphene from the substrate suggests ways of exposing the larger underlying gap.

Symmetry-protected topological (SPT) phases are gapped quantum phases which host symmetry-protected gapless edge excitations. On the other hand, the edge states can be gapped by spontaneously breaking symmetry. We show that topological defects on the symmetry-broken edge cannot proliferate due to their fractional statistics. A gapped symmetric boundary, however, can be achieved between an SPT phase and certain fractionalized phases by condensing the bound state of a topological defect and an anyon. We demonstrate this by two examples in two dimensions: an exactly solvable model for the boundary between a topological Ising paramagnet and the double-semion model, and a fermionic example about the quantum spin Hall edge. Such a hybrid structure containing both SPT phase and fractionalized phase generally support ground-state degeneracy on a torus.

(Ga,Mn)As is a paradigm of a diluted magnetic semiconductor which shows ferromagnetism induced by doped hole carriers. With a few controversial models emerging from numerous experimental and theoretical studies, the mechanism of the ferromagnetism in (Ga,Mn)As still remains a puzzling enigma. In this article, we use soft x-ray angle-resolved photoemission spectroscopy to positively identify the ferromagnetic Mn $3d$-derived impurity band (IB) in (Ga,Mn)As. The band appears dispersionless and hybridized with the light-hole band of the host GaAs. These findings conclude the picture of the valence-band structure of (Ga,Mn)As disputed for more than a decade. The nondispersive character of the IB and its location in vicinity of the valence-band maximum indicate that the Mn $3d$-derived IB is formed as a split-off Mn-impurity state predicted by the Anderson impurity model. Responsible for the ferromagnetism is predominantly the transport of hole carriers in the IB.

Waiting time is an important transport quantity that is complementary to average current and its fluctuation. So far all the studies of waiting time distribution (WTD) are limited to steady-state transport (either dc or ac). The existing theory cannot deal with WTD in the transient regime. In this regard, we develop a theoretical formalism based on Keldysh nonequilibrium Green's-function formalism to study WTD. This theory is suitable for dc, ac, and transient transport and can be used for first-principles calculation on realistic systems. We apply this theory to a quantum dot system with an upward bias pulse and calculate cumulants of transferred charge as well as WTD in the transient regime. The oscillatory behavior of WTD is found in the transient regime. We give a general relation between WTD and experimental measured quantity and demonstrate its feasibility for a quantum dot system in the transient regime.

The atomic and electronic structures and stability of native defects in a single-layer ${\mathrm{MoS}}_2$ are investigated, based on density-functional theory calculations. Native defects such as a S vacancy ($V_{\mathrm{S}}$), a S interstitial (${\mathrm{S}}_i$), a Mo vacancy ($V_{\mathrm{Mo}}$), and a Mo interstitial (${\mathrm{Mo}}_i$) are considered. The ${\mathrm{S}}_i$ is found to have S-adatom configuration on top of a host S atom, and the ${\mathrm{Mo}}_i$ has Mo-${\mathrm{Mo}}_i$ split interstitial configuration along the $c$ direction. The formation energies of the native defects in neutral and charged states are calculated. For the charged states, the artificial electrostatic interactions between image charges in supercells are eliminated by a supercell size scaling scheme and a correction scheme that uses a Gaussian model charge. It is found that the $V_{\mathrm{S}}$ has a low formation energy of 1.3–1.5 eV in the Mo-rich limit condition, and the ${\mathrm{S}}_i$ has 1.0 eV in the S-rich limit condition. The $V_{\mathrm{S}}$ is found to be a deep single acceptor with the (0/$-$) transition level at 1.7 eV above the valence-band maximum (VBM). The ${\mathrm{S}}_i$ is found to be an electrically neutral defect. The Mo-related native defects of $V_{\mathrm{Mo}}$ and ${\mathrm{Mo}}_i$ are found to be high in formation energy above 4 eV. The $V_{\mathrm{Mo}}$ is a deep single acceptor and the ${\mathrm{Mo}}_i$ is a deep single donor, of which the (0/$-$) acceptor and (+/0) donor transition levels are found at 1.1 and 0.3 eV above the VBM, respectively.

We discuss the thermoelectrical properties of nanowires hosting Majorana states. For a Majorana nanowire directly coupled to two normal reservoirs the thermopower always vanishes regardless of the value of the Majorana overlap parameter ${\varepsilon }_M$, due to the particle-hole symmetric nature of the Majorana states. This situation changes drastically if a quantum dot (QD) is inserted to break the particle-hole symmetry. Then, the Majorana-side-coupled QD system exhibits a different behavior for the thermopower depending on the value of ${\varepsilon }_M$. The thermopower globally reverses its sign when the half-fermionic nature of the Majorana state is relevant, i.e., for ${\varepsilon }_M\lesssim k_B{T}_b$, where $T_b$ is the background temperature. For large overlap, ${\varepsilon }_M\gtrsim k_B{T}_b$, on the other hand, the Seebeck coefficient behaves similarly to that in a resonant level system. The sign change of the thermopower and the fact that both the electrical and thermal conductances reach their half-fermionic value when Majorana physics are maximal could serve as a proof of the existence of Majorana states in nanowires.

The effect that dipole-dipole interactions have on the magneto-optical (MO) properties of magnetoplasmonic dimers is theoretically studied. The specific plasmonic versus magnetoplasmonic nature of the dimer's metallic components and their specific location within the dimer play a crucial role in the determination of these properties. We find that it is possible to generate an induced MO activity in a purely plasmonic component, even larger than that of the MO one, therefore dominating the overall MO spectral dependence of the system. Adequate stacking of these components may allow one to obtain, for specific spectral regions, larger MO activities in systems with a reduced amount of MO metal and therefore with lower optical losses. Theoretical results are contrasted and confirmed with experiments for selected structures.

The structural, magnetic, and vibrational properties of ${\mathrm{PbFe}}_{0.5}$${\mathrm{Nb}}_{0.5}$${\mathrm{O}}_3$ relaxor multiferroic have been studied by means of x-ray, neutron powder diffraction, and Raman spectroscopy at pressures up to 30 GPa. Two successive structural phase transitions from the initial $R$3$m$ polar phase to Cm and Pm monoclinic polar phases were observed at $P$ = 5.5 and 8.5 GPa. Both transitions are associated with anomalies in pressure behavior of several stretching and bending modes of oxygen octahedra as well as Fe/Nb localized vibrational modes. The $G$-type antiferromagnetic order remains stable upon compression up to 6.4 GPa, assuming possible multiferroic properties of pressure-induced phases. The Néel temperature increases with a pressure coefficient $\left(1\text{/}{T}_N\right)d{T}_N\text{/}dP=0.012$ ${\mathrm{GPa}}^{-1}$. The observed pressure-induced phenomena in ${\mathrm{PbFe}}_{0.5}$${\mathrm{Nb}}_{0.5}$${\mathrm{O}}_3$ are in drastic contrast with conventional multiferroics, exhibiting a general tendency towards a suppression of polar phases and/or magnetoelectric coupling under pressure.

${\mathrm{C}}_{60}$ fullerenes are interesting molecular semiconductors for spintronics since they exhibit weak spin-orbit and hyperfine interactions, which is a prerequisite for long spin lifetimes. We report spin-polarized transport in spin-valve-like structures containing ultrathin (<10 nm) ${\mathrm{C}}_{60}$ layers, ferromagnetic (FM) epitaxial face-centered-cubic (fcc) Co (111) contacts, ${\mathrm{AlO}}_x$ tunnel barriers, and nonmagnetic Al counter electrodes. Even though genuine spin-valve behavior cannot occur for only one FM contact, we find significant tunneling anisotropic magnetoresistance (TAMR) upon rotating the in-plane magnetization, originating from spin-orbit interaction (SOI) induced anisotropy of the fcc (111) Co bands. The uniaxial magnetocrystalline anisotropy of the Co electrodes results in a predominantly twofold symmetric in-plane TAMR effect. We investigated the TAMR effect in the direct tunneling regime (2 nm ${\mathrm{C}}_{60}$), at the transition point to two-step tunneling (4 nm ${\mathrm{C}}_{60}$), and in the multistep regime (8 nm ${\mathrm{C}}_{60}$). A sizable TAMR of 4.5% is found at 5 K under application of a 500-mT in-plane magnetic field for ${\mathrm{C}}_{60}$ layers of 2 nm, which is strongly suppressed at 8 nm thickness, indicating that TAMR may strongly contribute to the “spin-valve” signal for direct tunneling, but not for multistep tunneling. The TAMR effect is proposed to be due to a combination of SOI induced modulation of the tunneling DOS upon rotating the in-plane magnetization of the fcc epitaxial Co thin film, resonant tunneling processes involving interfacial states, and different Bychkov-Rashba SOI at the different interfaces.

Sodium niobate (${\mathrm{NaNbO}}_3$) exhibits an extremely complex sequence of structural phase transitions in the perovskite family and therefore provides an excellent model system for understanding the mechanism of structural phase transitions. We report temperature dependence of inelastic neutron scattering measurements of phonon densities of states in sodium niobate. The measurements are carried out in various crystallographic phases of this material at various temperatures from 300 to 1048 K. The phonon spectra exhibit peaks centered on 19, 37, 51, 70, and 105 meV. Interestingly, the peak near 70 meV shifts significantly towards lower energy with increasing temperature, while the other peaks do not exhibit any appreciable shift. The phonon spectra at 783 K show prominent change and become more diffusive as compared to those at 303 K. In order to better analyze these features, we have performed first-principles lattice dynamics calculations based on the density functional theory. The computed phonon density of states is found to be in good agreement with the experimental data. Based on our calculation we are able to assign the characteristic Raman modes in the antiferroelectric phase, which are due to the folding of the $T$ (ω = 95 ${\mathrm{cm}}^{-1}$) and Δ (ω = 129 ${\mathrm{cm}}^{-1}$) points of the cubic Brillouin zone, to the $A_{1g}$ symmetry.

We report ${}^{63}\mathrm{Cu}$ nuclear magnetic resonance and muon spin rotation measurements on the $S=1/2$ antiferromagnetic Heisenberg spin chain compound ${\mathrm{Sr}}_{1.9}$${\mathrm{Ca}}_{0.1}$${\mathrm{CuO}}_3$. An exponentially decreasing spin-lattice relaxation rate $T_1^{-1}$ indicates the opening of a spin gap. This behavior is very similar to what has been observed for the cognate zigzag spin chain compound ${\mathrm{Sr}}_{0.9}$${\mathrm{Ca}}_{0.1}$${\mathrm{CuO}}_2$, and it confirms that the occurrence of a spin gap upon Ca doping is independent of the interchain exchange coupling $J^{\prime }$. Our results therefore suggest that the appearance of a spin gap in an antiferromagnetic Heisenberg spin chain is induced by a local bond disorder of the intrachain exchange coupling $J$. A low-temperature upturn of $T_1^{-1}$ evidences growing magnetic correlations. However, zero-field muon spin rotation measurements down to 1.5 K confirm the absence of magnetic order in this compound, which is most likely suppressed by the opening of the spin gap.

The interface layers of ${\mathrm{La}}_{2-x}$${\mathrm{Sr}}_x$${\mathrm{CuO}}_4$ (LSCO) thin films epitaxially grown on ${\mathrm{LaSrAlO}}_4$ substrates by molecular beam epitaxy were investigated using transmission electron microscopy (TEM). In single-phase LSCO film, we observed an irregular layering sequence near the interface between the film and the substrate, as well as an abundance of oxygen vacancies in ${\mathrm{CuO}}_2$ layers. A multilayer LSCO film with a high critical temperature ($T_c$ = 44.5 K) showed perfect interfaces between the sublayers. Furthermore, by combining scanning TEM, electron energy-loss spectroscopy, and electron holography, we show that there is little or no interdiffusion between the sublayers. The interfacial defects and oxygen vacancies reduce $T_c$, while the compressive strain in high-quality multilayers enhances $T_c$.

It is known as a purely quantum effect that a magnetic flux affects the real physics of a particle, such as the energy spectrum, even if the flux does not interfere with the particle's path—the Aharonov-Bohm effect. Here we examine an Aharonov-Bohm effect on a many-body wave function. Specifically, we study this many-body effect on the gapless edge states of a bulk gapped phase protected by a global symmetry (such as ${\mathbb{Z}}_N$)—the symmetry-protected topological (SPT) states. The many-body analog of spectral shifts, the twisted wave function, and the twisted boundary realization are identified in this SPT state. An explicit lattice construction of SPT edge states is derived, and a challenge of gauging its non-onsite symmetry is overcome. Agreement is found in the twisted spectrum between a numerical lattice calculation and a conformal field theory prediction.

We present a theoretical study of the detection of the decay time scales for a single-level quantum dot by means of a capacitively coupled sensor dot, which acts as an electrometer. We investigate the measurement back-action on the quantum-dot decay rates and elucidate its mechanism. We explicitly show that the setup can be used to measure the bare quantum-dot relaxation rates by choosing gate pulses that minimize the back-action. Interestingly, we find that besides the charge relaxation rate, also the rate associated to the fermion parity in the dot can be accessed with this setup.