Semiconductor quantum dots embedded in photonic-crystal nanostructures have been the subject of intense study. In this context, quantum dots are often considered to be simple two-level emitters, i.e., the complexities arising from the internal fine structure are neglected. We show that due to the intricate spatial variations of the electric field polarization found in photonic crystal, the two orthogonal fine-structure states of quantum dots in general both couple significantly to a cavity mode, implying that the two-level description is not sufficient. As a consequence the emission dynamics and spectra, which are often recorded in experiments, are modified both in the weak- and strong-coupling regimes. The proposed effects are found to be significant for system parameters of current state-of-the-art photonic-crystal cavities.

In this paper, we theoretically demonstrate the strong mechanical response of graphene sheets actuated by near-field optical forces. We study single-layer graphene and a two-layer graphene stack with large separation and show that tunable attractive and repulsive forces can be generated. A large nonlinear mechanical response can be obtained by driving the sheets through external radiation and guided modes. We report formation of graphene bubbles of several nanometers in height. Our study points towards new routes for mechanical actuation of graphene, providing new platforms for straintronics and flexible optoelectronics.

We measured Drude absorption of gated CVD graphene using far-infrared transmission spectroscopy and determined the carrier scattering rate ($\gamma$) as a function of the varied carrier density ($n$). The $n$-dependent $\gamma \left(n\right)$ was obtained for a series of conditions systematically changed as (10 K, vacuum) $\to$ (300 K, vacuum) $\to$ (300 K, ambient pressure), which reveals that (1) at low-T, charged impurity ($=A/\sqrt{n}$) and short-range defect ($=B\sqrt{n}$) are the major scattering sources which constitute the total scattering $\gamma =A/\sqrt{n}+B\sqrt{n}$, (2) among various kinds of phonons populated at room-$T$, surface polar phonon of the ${\mathrm{SiO}}_2$ substrate is the dominantly scattering source, and (3) in air, the gas molecules adsorbed on graphene play a dual role in carrier scattering as charged impurity center and resonant scattering center. We present the absolute scattering strengths of those individual scattering sources, which provides the complete map of scattering mechanism of CVD graphene. This scattering map allows us to find out practical measures to suppress the individual scatterings, the mobility gains accompanied by them, and finally the ultimate attainable carrier mobility for CVD graphene.

Measuring the local temperature of nanoscale systems out of equilibrium has emerged as a new tool to study local heating effects and other local thermal properties of systems driven by external fields. Although various experimental protocols and theoretical definitions have been proposed to determine the local temperature, the thermodynamic meaning of the measured or defined quantities remains unclear. By performing analytical and numerical analysis of bias-driven quantum dot systems both in the noninteracting and strongly-correlated regimes, we elucidate the underlying physical meaning of local temperature as determined by two definitions: the zero-current condition that is widely used but not measurable and the minimal-perturbation condition that is experimentally realizable. We show that, unlike the zero-current condition, the local temperature determined by the minimal-perturbation protocol establishes a quantitative correspondence between the nonequilibrium system of interest and a reference equilibrium system, provided the probed system observable and the related electronic excitations are fully local. The quantitative correspondence thus allows the well-established thermodynamic concept to be extended to nonequilibrium situations.

The tunability of the exciton $g$ factor in InGaAs quantum dots using compressive biaxial stress applied by piezoelectric actuators is investigated. We find a clear relation between the exciton $g$ factor and the applied stress. A linear decrease of the $g$ factor with compressive biaxial strain is observed consistently in all investigated dots. A connection is established between the response of the exciton $g$ factor to the voltage applied to the piezoelectric actuator and the response of the quantum dot emission energy. We employ a numerical model based on eight-band $\mathbf{k}·\mathbf{p}$ theory to calculate the exciton $g$ factor of a typical dot as a function of strain and a good agreement with our experiments is found. Our calculations reveal that the change in exciton $g$ factor is dominated by the contribution of the valence band and originates from increased heavy hole light hole splitting when applying external stress.

We have surveyed the in-plane transport properties of the graphene twist bilayer using (i) a low-energy effective Hamiltonian for the underlying electronic structure, (ii) an isotropic elastic phonon model, and (iii) the linear Boltzmann equation for elastic electron-phonon scattering. We find that transport in the twist bilayer is profoundly sensitive to the rotation angle of the constituent layers. Similar to the electronic structure of the twist bilayer, the transport is qualitatively different in three distinct angle regimes. At large angles ($\theta >\approx {10}^{\circ }$) and at temperatures below an interlayer Bloch-Grüneisen temperature of $\approx 10$ K, the conductivity is independent of the twist angle, i.e., the layers are fully decoupled. Above this temperature the layers, even though decoupled in the ground state, are recoupled by electron-phonon scattering and the transport is different both from single-layer graphene as well as the Bernal bilayer. In the small-angle regime $\theta <\approx 2^{\circ }$, the conductivity drops by two orders of magnitude and develops a rich energy dependence, reflecting the complexity of the underlying topological changes (Lifshitz transitions) of the Fermi surface. At intermediate angles, the conductivity decreases continuously as the twist angle is reduced, while the energy dependence of the conductivity presents two sharp transitions, that occur at specific angle-dependent energies, and that may be related to (i) the well-studied van Hove singularity of the twist bilayer and (ii) a Lifshitz transition that occurs when trigonally placed electron pockets decorate the strongly warped Dirac cone. Interestingly, we find that, while the electron-phonon scattering is dominated by layer symmetric flexural phonons in the small-angle limit, at large angles, in contrast, it is the layer antisymmetric flexural mode that is most important. We examine the role of a layer perpendicular electric field finding that it affects the conductivity strongly at low temperatures, whereas this effect is washed out by Fermi smearing at room temperatures.

Weak localization was observed in a black phosphorus field-effect transistor 65 nm thick. The weak localization behavior was found to be in excellent agreement with the Hikami-Larkin-Nagaoka model for fields up to 1 T, from which characteristic scattering lengths could be inferred. The temperature dependence of the phase coherence length $L_{\phi }$ was investigated, and above 1 K, it was found to decrease weaker than the $L_{\phi }\propto T^{-1/2}$ dependence characteristic of electron-electron scattering in the presence of elastic scattering in two dimensions. Rather, the observed power law was found to be close to that observed previously in quasi-one-dimensional systems such as metallic nanowires and carbon nanotubes.

Two-dimensional silicene, with an observable intrinsic spin-orbit coupling, has a great potential to perform fascinating physics and new types of applications in spintronics and valleytronics. By introducing an electromotive force from a temperature difference in ferromagnetic silicene, we discover that a longitudinal spin Seebeck effect can be driven even near room temperature, with spin-up and spin-down currents flowing in opposite directions, originating from the asymmetric electron-hole spin band structures. We further propose a silicene field-effect transistor constructed of two ferromagnetic electrodes and a central dual-gated region, and find that a valley Seebeck effect appears, with currents from two different valleys flowing in opposite directions. The forbidden transport channels are determined by either spin-valley dependent band gaps or spin mismatch. By tuning the electric field in the central region, the transport gaps depending on spin and valley vary correspondingly, and a transition from valley Seebeck effect to spin Seebeck effect is observed. These spin-valley caloritronic results near room temperature are robust against many real perturbations, and thus suggest silicene to be an excellent candidate for future energy-saving technologies and bidirectional information processing in solid-state circuits.

The charge-spin-orbital state plays a vital role in correlated oxides with mixed transition-metal elements, and it is often a matter of debate. Here, we address this issue for the newly synthesized unusual high-valence perovskite ${\mathrm{SrFe}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_3$, using an analysis of crystal-field level diagrams, density functional calculations, and Monte Carlo simulations. We have identified the formal high-spin ${\mathrm{Fe}}^{5+}(t_{2g}^3, S=3/2)$ and high-spin ${\mathrm{Ni}}^{3+}(t_{2g}^5{e}_g^2, S=3/2)$ state for ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$, rather than the previously suggested high-spin ${\mathrm{Fe}}^{4+}(t_{2g}^3{e}_g^1, S=2)$ and low-spin ${\mathrm{Ni}}^{4+}$ $(t_{2g}^6, S=0)$ state. Moreover, our model and the present results well explain the observed small lattice variation of ${\mathrm{SrFe}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_3$ $(0\le x\le 0.5)$ and the above room temperature ferromagnetic order in ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$ regardless of a Fe-Ni atomic order or disorder.

To explore whether a flat-band system can accommodate superconductivity, we consider repulsively interacting fermions on the diamond chain, a simplest possible quasi-one-dimensional system that contains a flat band. Exact diagonalization and the density-matrix renormalization group are used to show that we have a significant binding energy of a Cooper pair with a long-tailed pair-pair correlation in real space when the total band filling is slightly below 1/3, where a filled dispersive band interacts with the flat band that is empty but close to $E_F$. Pairs selectively formed across the outer sites of the diamond chain are responsible for the pairing correlation. At exactly 1/3-filling an insulating phase emerges, where the entanglement spectrum indicates the particles on the outer sites are highly entangled and topological. These come from a peculiarity of the flat band in which “Wannier orbits” are not orthogonalizable.

A number of experiments on the hyperkagome iridate, ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$, suggest existence of a gapless quantum spin liquid state at low temperature. Circumventing the slave-particle approach commonly used in theoretical analyses of frustrated magnets, we provide a more intuitive, albeit more phenomenological, construction of a quantum spin liquid state for the hyperkagome Heisenberg model. An effective monomer-dimer model on the hyperkagome lattice is proposed à la Hao and Tchernyshyov's approach cultivated from the Husimi cactus model. Employing an arrow representation for the monomer-dimer model, we obtain a compact $U\left(1\right)$ gauge theory with a finite density of fermionic spinons on the hyperoctagon lattice. The resulting theory and its mean-field treatment are consistent with previous slave-particle construction of a quantum spin liquid state on the hyperkagome lattice. Our results offer novel insights into the emergence of spinon Fermi surfaces and useful predictions for future experiments.

Majorana zero modes (MZMs) have been predicted to exist in a topological insulator (TI)/superconductor (SC) heterostructure. A recent spin-polarized scanning tunneling microscope (STM) experiment [Sun et al., Phys. Rev. Lett. 116, 257003 (2016)] has observed a spin-polarization dependence of the zero bias differential tunneling conductance at the center of a vortex core. Here, we consider a helical electron system described by a Rashba spin-orbit coupling Hamiltonian on a spherical surface with an $s$-wave superconducting pairing due to proximity effect. We examine the in-gap excitations of a pair of vortices with one at the north pole and the other at the south pole. While the MZM is not a spin eigenstate, the spin wave function of the MZM at the center of the vortex core, $r=0$, is parallel to the magnetic field, and the local Andreev reflection of the MZM is spin selective, namely, occurs only when the STM tip has the spin polarization parallel to the magnetic field, similar to the case in a one-dimensional nanowire [He et al., Phys. Rev. Lett. 112, 037001 (2014)]. The total local differential tunneling conductance consists of the normal term proportional to the local density of states and an additional term arising from the Andreev reflection. We also discuss the finite size effect, for which the MZM at the north pole is hybridized with the MZM at the south pole. We apply our theory to examine the recently reported spin-polarized STM experiments and show good agreement with the experiments.

We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high-quality single crystals of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ of $ac\text{−}$ and $ab\text{-plane}$ surfaces. Our data demonstrate the itinerant character of the narrow $5f$ bands, becoming progressively coherent as the temperature is lowered below a crossover temperature $T^{*}\sim 75$ K. $T^{*}$ is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density-response (energy-loss) function of this compound, and determine the energies of the heavy-fermion plasmons with $a\text{−}$ and $c\text{-axis}$ polarization. Our observation of a suppression of optical conductivity below 50 meV along both the $a$ and $c$ axes, along with a heavy-fermion plasmon at 18 meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature $T_{\mathrm{HO}}=17.5$ K. In the hidden order phase the low-frequency optical conductivity shows a single gap at $\sim 6.5$ meV, which closes at $T_{\mathrm{HO}}$.

Bismuth chalcogenides are the most studied 3D topological insulators. As a rule, at low temperatures, thin films of these materials demonstrate positive magnetoresistance due to weak antilocalization. Weak antilocalization should lead to resistivity decrease at low temperatures; in experiments, however, resistivity grows as temperature decreases. From transport measurements for several thin films (with various carrier density, thickness, and carrier mobility), and by using a purely phenomenological approach, with no microscopic theory, we show that the low-temperature growth of the resistivity is accompanied by growth of the Hall coefficient, in agreement with the diffusive electron-electron interaction correction mechanism. Our data reasonably explain the low-temperature resistivity upturn.

A density-matrix formalism within the length gauge is developed for the purpose of calculating the nonlinear response of intrinsic bilayer graphene at terahertz frequencies. Employing a tight-binding model, we find that the interplay between the interband and intraband dynamics leads to strong harmonic generation at moderate field amplitudes. Specifically, we find that at low temperature (10 K), the reflected field of undoped suspended bilayer graphene exhibits a third harmonic amplitude that is 0.06% of the fundamental of the incident field, which corresponds to 30% of the fundamental in the reflected field for an incident 1-THz single-cycle pulse with a field amplitude of 1.5 kV/cm. More interestingly, we find that as the central frequency of the incident field is increased, the third harmonic amplitude also increases; reaching a maximum of 53% of the fundamental in the reflected field (0.11% of the fundamental in the incident field) for an incident frequency of 2 THz and amplitude of 2.5 kV/cm.

Weyl fermions play a major role in quantum field theory but have been quite elusive as fundamental particles. Materials based on quasi-two-dimensional bismuth layers were recently designed and provide an arena for studying the interplay between anisotropic Dirac fermions, magnetism, and structural changes, allowing the formation of Weyl fermions in condensed matter. Here, we perform an optical investigation of ${\mathrm{YbMnBi}}_2$, a representative type-II Weyl semimetal, and contrast its excitation spectrum with the optical response of the more conventional semimetal ${\mathrm{EuMnBi}}_2$. Our comparative study allows us to disentangle the optical fingerprints of type-II Weyl fermions, but also challenges the present theoretical understanding of their electrodynamic response.

We study scalar and chiral fermionic models in next-to-leading order with the help of the functional renormalization group. Their critical behavior is of special interest in condensed-matter systems, in particular graphene. To derive the $\beta$ functions, we make extensive use of computer algebra. The resulting flow equations were solved with pseudospectral methods to guarantee high accuracy. New estimates of critical quantities for both the Ising and the Gross-Neveu model are provided. For the Ising model, the estimates agree with earlier renormalization-group studies of the same level of approximation. By contrast, the approximation for the Gross-Neveu model retains many more operators than all earlier studies. For two Dirac fermions, the results agree with both lattice and large-$N_f$ calculations, but for a single flavor, different methods disagree quantitatively, and further studies are necessary.