We investigate the magnetic-field effects on the Mott transition in the kagome lattice Hubbard model using the cellular dynamical mean-field theory combined with the continuous-time quantum Monte Carlo simulations. By calculating the magnetization and the momentum-resolved one-particle spectrum, we clarify that an applied magnetic field drives the first-order metal-insulator transition in a metallic phase around the zero-field Mott transition point. It is found that near the field-induced transition, the quasiparticle weight (effective mass) is increased (decreased) with increasing field, showing a sharp contrast to the results of the single-site dynamical mean-field study. The calculation of spin correlation functions elucidates that the nearest-neighbor antiferromagnetic correlations get enhanced with increasing field toward the transition point, indicating that the magnetic field stabilizes the insulating state with strong antiferromagnetic correlations.

We predict that a combined system of Ir dimer situated on the single-vacancy (SV) graphene is extremely stable and has high magnetic anisotropy energy (MAE), promising to be a potential candidate for high density magnetic storage. Our theoretical results show that high energy gradient in the SV graphene makes it possible to move the Ir dimer from the perfect graphene area into the center of SV (CSV), leading to the formation of the highly stable system with the Ir-Ir bond axis exactly perpendicular to the graphene plane. The calculated MAE value is as large as 25.7 meV per Ir atom. The origin of giant MAE in such a system is attributed to the difference of local densities of electronic states magnetized along different directions caused by special symmetry of Ir${}_2$ at CSV. We also propose a feasible avenue to assemble the Ir dimers on the graphene with the pattern of SV defects for actual applications.

Guided by the analogy to Mie scattering of light on small particles, we show that the propagation of a Dirac-electron wave in graphene can be manipulated by a circular gated region acting as a quantum dot. Large dots enable electron lensing, while for smaller dots resonant scattering entails electron confinement in quasibound states. Forward scattering and Klein tunneling can be almost switched off for small dots by a Fano resonance arising from the interference between resonant scattering and the background partition.

We study the thermoelectric transport coefficients of a one-dimensional (1D) electron waveguide connected to one and then two off-channel cavities, in the presence of dephasing phonons. The model system is that of a linear chain described as a 1D lattice. For simplicity we consider single-mode cavities, which may be tuned with external gates. While the presence of only one off-channel cavity yields a nearly symmetric transmission profile, which is canceled around the cavity mode, an additional cavity modifies this profile strongly and results in an asymmetric shape characterized by oscillations. In both cases, we consider electron-phonon interactions in our calculations and analyze their effects on the transmission function around the Fermi energy. Knowledge of the energy-dependent transmission function allows the numerical computation of thermoelectric transport coefficients, including the thermopower. In the presence of a second off-channel cavity, the sign of the thermopower depends on the relative position of this cavity energy level with respect to the Fermi energy: the thermopower is positive when low-energy electrons in the vicinity of the Fermi level are not transmitted and becomes negative when the higher-energy electrons are not transmitted.

Using finite-difference time domain and band structure computer simulations, we show that it is possible to construct optical cavities and waveguide architectures in hyperuniform disordered photonic solids that are unattainable in photonic crystals. The cavity modes can be classified according to the symmetry (monopole, dipole, quadrupole, etc.) of the confined electromagnetic wave pattern. Owing to the isotropy of the band-gap characteristics of hyperuniform disordered solids, high-quality waveguides with free-form geometries (e.g., arbitrary bending angles) can be constructed that are unprecedented in periodic or quasiperiodic solids. These capabilities have implications for many photonic applications.

We investigate thermoelectric efficiency of systems with broken time-reversal symmetry under a three-terminal transport. Using a model of Aharonov-Bohm interferometer formed with three noninteracting quantum dots, we show that Carnot efficiency ${\eta }_C$ can be achieved when the thermopower is a symmetric function of the applied magnetic field. On the other hand, the maximal value of the efficiency at maximum power is obtained for asymmetric thermopower. Indeed, we show that the Curzon-Ahlborn limit is exceeded within the linear response regime in our model. Moreover, we investigate thermoelectric efficiency for random Hamiltonians drawn from the Gaussian unitary ensemble and for a more abstract transmission model. In this latter model, we find that the efficiency is improved using sharp energy-dependent transmission functions.

We predict a tunable and nonadiabatic Berry phase effect of Dirac fermions, which is an electronic analog of the Pancharatnam phase of polarized light. The Berry phase occurs as a scattering phase shift in a single scattering event of transmission or reflection of Dirac fermions at a junction with a spatially nonuniform mass gap, unveiling the topological aspects of scattering of chiral Dirac fermions. This geometric phase plays different roles in solids as compared with the Pancharatnam phase in optics. It provides a unique approach of detecting the Chern number of the insulator side in a metal-insulator junction of Dirac fermions, implying a different type of bulk-edge correspondence at the boundary between a metal and an insulator. This phase also modifies the quantization rule of Dirac fermions, suggesting geometric-phase devices with nontrivial charge and spin transport such as a topological waveguide and a topological transistor.

Using a stable random matrix approach, we consider the diffusion of vibrations in $3d$ harmonic lattices with strong force-constant disorder. Above some frequency ${\omega }_{\mathrm{IR}}$, corresponding to the Ioffe-Regel crossover, the notion of phonons becomes ill defined. They cannot propagate through the system and transfer energy. Nevertheless, most of the vibrations in this range are not localized. We show that they are similar to diffusons introduced by Allen et al. [P. B. Allen, J. L. Feldman, J. Fabian, and F. Wooten, Phil. Mag. B 79, 1715 (1999)] to describe heat transport in glasses. The crossover frequency ${\omega }_{\mathrm{IR}}$ is close to the position of the boson peak. By changing the strength of disorder we can vary ${\omega }_{\mathrm{IR}}$ from zero value (when the rigidity is zero and there are no phonons in the lattice) up to a typical frequency in the system. Above ${\omega }_{\mathrm{IR}}$, the energy in the lattice is transferred by means of diffusion of vibrational excitations. We calculated the diffusivity of the modes $D\left(\omega \right)$ using both the direct numerical solution of the Newton equations and the formula of Edwards and Thouless. It is nearly a constant above ${\omega }_{\mathrm{IR}}$ and goes to zero at the localization threshold. We show that apart from the diffusion of energy, a diffusion of particle displacements in the lattice takes place as well. Above ${\omega }_{\mathrm{IR}}$, a displacement structure factor $S(\mathbf{q},\omega )$ coincides well with a structure factor of random walk on the lattice. As a result, the vibrational line width is $\Gamma \left(q\right)=D_u{q}^2$, where $D_u$ is a diffusion coefficient of particle displacements. These findings may be important for the interpretation of experimental data on inelastic x-ray scattering and mechanisms of heat transfer in glasses. We also show that the scaling with model parameters in our model maps directly onto the scaling observed in the jamming transition model.

We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ for $x=0.039$, 0.022, and 0.021 as-grown single crystals. Our investigations probe the behavior near the magnetic tricritical point in the $(x,T)$ plane, $x_{\text{tr}}\approx 0.022$, as well as systematically exploring the character of the magnetic phase transition across a range of doping values. All samples show long-range antiferromagnetic order that may be described near the transition by simple power laws, with $\beta =0.306\pm 0.060$ for $x=0.039$, $\beta =0.208\pm 0.005$ for $x=0.022$, and $\beta =0.198\pm 0.009$ for $x=0.021$. For the $x=0.039$ sample, the data are reasonably well described by the order parameter exponent $\beta =0.326$ expected for a three-dimensional Ising model while the $x=0.022$ and $x=0.021$ samples are near the $\beta =0.25$ value for a tricritical system in the mean-field approximation. These results are discussed in the context of existing experimental work and theoretical predictions.

Building on the generalization of the exactly dimerized Majumdar-Ghosh ground state to arbitrary spin $S$ for the Heisenberg chain with a three-site term $({\mathbf{S}}_{i-1}·{\mathbf{S}}_i)({\mathbf{S}}_i·{\mathbf{S}}_{i+1})+\text{H.c.}$, we use density-matrix renormalization group simulations and exact diagonalizations to determine the nature of the dimerization transition for $S=1$, $3/2$, and 2. The resulting central charge and critical exponent are in good agreement with the $\text{SU}{\left(2\right)}_{k=2S}$ Wess-Zumino-Witten values $c=3k/(2+k)$ and $\eta =3/(2+k)$. Since the three-site term that induces dimerization appears naturally if exchange interactions are calculated beyond second order, these results suggest that $\text{SU}{\left(2\right)}_{k>1}$ Wess-Zumino-Witten models might finally be realized in actual spin chains.

Surface superconductivity in rhombohedral graphite is a robust phenomenon which can exist even when higher order hoppings between the layers lift the topological protection of the surface flat band and introduce a quadratic dispersion of electrons with a heavy effective mass. We show that for weak pairing interaction, the flat-band character of the surface superconductivity transforms into a BCS-like relation with high critical temperature characterized by a higher coupling constant due to a much larger density of states than in the bulk. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high transition temperature.

We show that anti-phase synchronization can emerge in a pair of electrically coupled micromechanical beams. Under impulsive perturbation, desynchronization occurs, distorting the output of each beam. We derive a formula for the relaxation rate and verify it numerically. We also find that the difference between the displacements of the two beams, or the differential signal, is robustly immune to impulsive perturbation, implying that the system can effectively counter external disturbances. This can have significant applications in the development of various microscale devices, which we elaborate on using microelectromechanical resonators.

We consider the thermal phase transition from a paramagnetic to a stripe-antiferromagnetic phase in the frustrated two-dimensional square-lattice Ising model with competing interactions $J_1<0$ (nearest neighbor, ferromagnetic) and $J_2>0$ (second neighbor, antiferromagnetic). The striped phase breaks a $Z_4$ symmetry and is stabilized at low temperatures for $g=J_2/\left|J_1\right|>1/2$. Despite the simplicity of the model, it has proved difficult to precisely determine the order and the universality class of the phase transitions. This was done convincingly only recently by Jin et al. [Phys. Rev. Lett. 108, 045702 (2012)]. Here, we further elucidate the nature of these transitions and their anomalies by employing a combination of cluster mean-field theory, Monte Carlo simulations, and transfer-matrix calculations. The $J_1$-$J_2$ model has a line of very weak first-order phase transitions in the whole region $1/2<g<g^{*}$, where $g^{*}=0.67\pm 0.01$. Thereafter, the transitions from $g=g^{*}$ to $g\to \infty$ are continuous and can be fully mapped, using universality arguments, to the critical line of the well-known Ashkin-Teller model from its four-state Potts point to the decoupled Ising limit. We also comment on the pseudo-first-order behavior at the Potts point and its neighborhood in the Ashkin-Teller model on finite lattices, which in turn leads to the appearance of similar effects in the vicinity of the multicritical point $g^{*}$ in the $J_1$-$J_2$ model. The continuous transitions near $g^{*}$ can therefore be mistaken for first-order transitions, and this realization was the key to understanding the paramagnetic-striped transition for the full range of $g>1/2$. Most of our conclusions are based on Monte Carlo calculations, while the cluster mean-field and transfer-matrix results provide useful methodological benchmarks for weakly first-order behaviors and Ashkin-Teller criticality.

SrRuO${}_3$ is an itinerant ferromagnet with a Curie temperature of $150\mathrm{K}$ and a uniaxial magnetocrystalline anisotropy. Using high-quality epitaxial thin films, we measure at $2\mathrm{K}$ the anisotropic magnetoresistance and the planar Hall effect as a function of the in-plane current direction and the magnetization orientation. We identify three types of contributions related to (a) the orientation of the magnetization relative to the crystal axes, (b) the direction of the current relative to the crystal axes, and (c) the orientation of the magnetization relative to the current.

Neutron diffraction and nuclear quadrupole resonance (NQR) measurements were employed to investigate magnetic order in the nonferroelectric phase preceding the low-temperature multiferroic state in FeTe${}_2$O${}_5$Br. Refinement of the neutron diffraction data and simulations of ${}^{79,81}$Br NQR spectra reveal that the incommensurate magnetic ordering in the nonferroelectric state comprises amplitude-modulated magnetic moments, similar to that seen in the multiferroic state. The two ordered states differ in the orientation of the magnetic moments and phase shifts between modulation waves. Surprisingly, all symmetry restrictions for the electric polarization are absent in both states. The different ferroelectric responses of the two states are thus argued to arise from the differences in the phase shifts between certain modulation waves, which cancel out in the nonferrolectric state.

We investigate the dependence of the photoluminescence intensity of degenerately doped ($6\times {10}^{17}\text{−}\mathrm{to}1\times {10}^{20}\text{−}{\mathrm{cm}}^{-3}$) InN films on their threading dislocation density and background doping level. The photoluminescence intensity is found to be not determined by the structural quality of the film but by its doping density. The inverse relationship between the photoluminescence intensity and the electron density suggests that Auger recombination is the mechanism which actually limits the internal quantum efficiency of these films.

The quasiparticle (QP) band structures of both strainless and strained monolayer MoS${}_2$ are investigated using more accurate many-body perturbation $\mathit{GW}$ theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent $\mathit{GW}$${}_0$ (sc$\mathit{GW}$${}_0$) calculation, the predicted optical gap magnitude is in good agreement with available experimental data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The sc$\mathit{GW}$${}_0$ and BSE calculations are also performed on monolayer WS${}_2$, similar characteristics are predicted and WS${}_2$ possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calculation results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.

We calculate shot noise and the corresponding Fano factors in magnetic double-barrier tunnel junctions. Two situations are analyzed: (i) the central metallic layer is nonmagnetic while the external ones are ferromagnetic, and (ii) all of the metallic layers are ferromagnetic. In the latter case, the number of various magnetic configurations of the junctions is larger, which improves the functionality of such devices. The corresponding shot noise and Fano factor are shown to depend on the magnetic configuration of the junctions. The effect of spin relaxation in the central layer is also taken into account. The theoretical results are compared with experimental data on the shot noise in Fe/MgO/Fe/MgO/Fe structures.

We analyze the response of a nanomechanical resonator to an external drive when it is also coupled to a single-electron transistor (SET). The interaction between the SET electrons and the mechanical resonator depends on the amplitude of the mechanical motion leading to a strongly nonlinear response to the drive which is similar to that of a Duffing oscillator. We show that the average dynamics of the resonator is well described by a simple effective model which incorporates damping and frequency renormalization terms which are amplitude dependent. We also find that for a certain range of parameters the system displays interesting bistable dynamics in which noise arising from charge fluctuations causes the resonator to switch slowly between different dynamical states.