Quantum dimer models typically arise in various low energy theories like those of frustrated antiferromagnets. We introduce a quantum dimer model on the kagome lattice which stabilizes an alternative ${\mathbb{Z}}_2$ topological order, namely the so-called “double semion” order. For a particular set of parameters, the model is exactly solvable, allowing us to access the ground state as well as the excited states. We show that the double semion phase is stable over a wide range of parameters using numerical exact diagonalization. Furthermore, we propose a simple microscopic spin Hamiltonian for which the low-energy physics is described by the derived quantum dimer model.

We present a rigorous formulation of the time-dependent density-functional theory for interacting lattice electrons strongly coupled to cavity photons. We start with an example of one particle on a Hubbard dimer coupled to a single photonic mode, which is equivalent to the single mode spin-boson model or the quantum Rabi model. For this system we prove that the electron-photon wave function is a unique functional of the electronic density and the expectation value of the photonic coordinate, provided the initial state and the density satisfy a set of well defined conditions. Then we generalize the formalism to many interacting electrons on a lattice coupled to multiple photonic modes and prove the general mapping theorem. We also show that for a system evolving from the ground state of a lattice Hamiltonian any density with a continuous second time derivative is locally $v$ representable.

We utilize a glasslike structural transition in order to induce a Mott metal-insulator transition in the quasi-two-dimensional organic charge-transfer salt $\kappa -{\text{(BEDT-TTF)}}_2\mathrm{Cu}\left[\mathrm{N}{\text{(CN)}}_2\mathrm{Br}\right]$. In this material, the terminal ethylene groups of the BEDT-TTF molecules can adopt two different structural orientations within the crystal structure, namely eclipsed (E) and staggered (S) with the relative orientation of the outer C–C bonds being parallel and canted, respectively. These two conformations are thermally disordered at room temperature and undergo a glasslike ordering transition at $T_g\sim 75$ K. When cooling through $T_g$, a small fraction that depends on the cooling rate remains frozen in the S configuration, which is of slightly higher energy, corresponding to a controllable degree of structural disorder. We demonstrate that, when thermally coupled to a low-temperature heat bath, a pulsed heating current through the sample causes a very fast relaxation with cooling rates at $T_g$ of the order of several $1000 \mathrm{K}/\min$. The freezing of the structural degrees of freedom causes a decrease of the electronic bandwidth $W$ with increasing cooling rate, and hence a Mott metal-insulator transition as the system crosses the critical ratio ${(W/U)}_c$ of bandwidth to on-site Coulomb repulsion $U$. Due to the glassy character of the transition, the effect is persistent below $T_g$ and can be reversibly repeated by melting the frozen configuration upon warming above $T_g$. Both by exploiting the characteristics of slowly changing relaxation times close to this temperature and by controlling the heating power, the materials can be fine-tuned across the Mott transition. A simple model allows for an estimate of the energy difference between the E and S state as well as the accompanying degree of frozen disorder in the population of the two orientations.

Semiconductor nanomembranes can exhibit strain gradients that lead to quantum confinement effects similar to the well known quantum-confined Stark effect (QCSE) in semiconductor quantum wells. The deformation of square well into triangular well potential leads to modifications of the exciton resonance, but important differences between the quantum-confined strain gradient effect (QCsgE) and the QCSE include (i) the versatility of the QCsgE in which conduction and valence bands can have different slopes (even reverse slopes are possible), and (ii) the fact that in the QCsgE exciton shifts are determined by the gradients in the heavy-hole and light-hole energies as well as a gradient in the heavy-hole and light-hole coupling.

Using nonequilibrium molecular dynamics and lattice dynamics, we investigate phonon conduction on two-dimensional graphene/boron nitride superlattices with varying periods and interface structures. As the period of superlattice increases to a critical value near 5 nm the lattice thermal conductivity drops sharply to a minimum, and beyond that it smoothly increases with the period. We show that the minimum in the thermal conductivity arises from a competition between lattice dispersion and anharmonic effects such as interface scattering. The initial reduction of thermal conductivity can partially be accounted for by harmonic wave effects induced by interfacial modulation, such as the opening of phononic band gaps and reduction of group velocity. Beyond the minimum, reduced inelastic interface scattering is responsible for the recovery. The overall range of thermal conductivity exhibited by the superlattices is substantially reduced with respect to the parent materials. A universal scaling of the thermal conductivity with total superlattice length is found, suggesting that the critical period is independent of total length and that long-wavelength phonons are dominant carriers. Furthermore, we demonstrate the ultrasensitivity of thermal conductivity to interfacial defects and superlattice periodicity disorder.

The Rashba effect is interesting for thermoelectrics because of the unique spin-splitting band structure. By using bulk BiTeI with a giant Rashba effect as an example, we prove that the spin-splitting-induced constant density of states leads to a two-dimensional thermopower. The thermopower is higher as compared with that in spin-degenerate bands, primarily due to the lower Fermi level in the Rashba spin-splitting bands at given carrier concentrations. A quantitative relation between thermopower and the Rashba parameter is established. Furthermore, the internal electric field in the Rashba system can be beneficial to bond anharmonicity and low lattice thermal conductivity. We suggest that bulk materials with large Rashba effect may become potential candidates for efficient thermoelectricity.

Photoemission, from core levels and valence band, and low-energy electron diffraction (LEED) have been employed to investigate the electronic and structural properties of graphene-ferromagnetic (G-FM) systems, obtained by intercalation of one monolayer (1 ML) and several layers (4 ML) of Co on G grown on Ir(111). Upon intercalation of 1 ML of Co, the Co lattice is resized to match the Ir-Ir lattice parameter, resulting in a mismatched G/Co/Ir(111) system. The intercalation of further Co layers leads to a relaxation of the Co lattice and a progressive formation of a commensurate G layer lying on top. We show the C $1s$ line shape and the band structure of G in the two artificial phases, mismatched and commensurate G/Co, through a comparison with the electronic structure of G grown directly on a Co thick film. Our results show that while the G valence band mainly reflects the hybridization with the $d$ states of Co, regardless of the structural phase, the C $1s$ line shape is very sensitive to the rumpling of the G layer and the coordination of carbon atoms with the underlying Co. Even in the commensurate ($1\times 1$) G/Co phase, where graphene is in register with the Co film, from the angular dependence of the C $1s$ core level we infer the presence of more than a single component, due to inequivalent adsorption sites of carbon sublattices.

Detailed study of nuclear spin order in semiconducting ${}^{13}\mathrm{C}$ nanotubes reveals subtle interplay of the chemical potential, length, diameter, and chirality, resulting in the complex four-dimensional phase diagram of the helical ground states. Increase of the chemical potential causes abrupt transitions between different helical spin orderings of three regimes which are interpolated by a smooth change of helical angle within each regime: in the middle one the helimagnet is a deviation from the commensurate order and fully characterizes the geometry of the nanotube, while outside it the ground state is an incommensurate helical deviation from the ferromagnet. This behavior of the ${}^{13}\mathrm{C}$ nanotubes manifests the long-range of the RKKY interaction and quasi-one-dimensional geometry, thus being universal for all RKKY interaction governed nanowires. Short enough nanotubes are ferromagnetic; the critical length when frustration arises decreases with the chemical potential but increases with diameter and chiral angle. The results, verified numerically, show that with nanotubes of the different but realistic lengths, various scenarios of the helical order response to the gate voltage can be achieved.

A longstanding puzzle for the lowest-Landau-level crystal phase has been an order of magnitude discrepancy between the theoretically calculated energy of the defects and the measured activation gap. We perform an extensive study of various kinds of defects in the correlated composite fermion crystal and find that the lowest energy defect is a sixfold symmetric “hypercorrelated bubble interstitial,” in which an interstitial particle forms a strongly correlated bound state with a particle of the crystal. The energy of the bubble defect is a factor of $\sim 3$ smaller than that of the lowest energy defect in a Hartree-Fock crystal. The anomalously low activation energies measured in transport experiments are thus a signature of the unusual quantum nature of the crystal and its defects.

Potential steps exceeding 1 eV are regularly formed at metal|insulator interfaces, even when the interaction between the materials at the interface is weak physisorption. From first-principles calculations on metal|$h\text{−}\mathrm{BN}$ interfaces we show that these potential steps are only indirectly sensitive to the interface bonding through the dependence of the binding energy curves on the van der Waals interaction. Exchange repulsion forms the main contribution to the interface potential step in the weakly interacting regime, which we show with a simple model based upon a symmetrized product of metal and $h\text{−}\mathrm{BN}$ wave functions. In the strongly interacting regime, the interface potential step is reduced by chemical bonding.

While the topological classification of insulators, semimetals, and superconductors in terms of nonspatial symmetries is well understood, less is known about topological states protected by crystalline symmetries, such as mirror reflections and rotations. In this work, we systematically classify topological semimetals and nodal superconductors that are protected, not only by nonspatial (i.e., global) symmetries, but also by a crystal reflection symmetry. We find that the classification crucially depends on (i) the codimension of the Fermi surface (nodal line or point) of the semimetal (superconductor), (ii) whether the mirror symmetry commutes or anticommutes with the nonspatial symmetries, and (iii) how the Fermi surfaces (nodal lines or points) transform under the mirror reflection and nonspatial symmetries. The classification is derived by examining all possible symmetry-allowed mass terms that can be added to the Bloch or Bogoliubov–de Gennes Hamiltonian in a given symmetry class and by explicitly deriving topological invariants. We discuss several examples of reflection-symmetry-protected topological semimetals and nodal superconductors, including topological crystalline semimetals with mirror ${\mathbb{Z}}_2$ numbers and topological crystalline nodal superconductors with mirror winding numbers.

We investigate the optical response of quantum dot molecules coherently driven by polarized laser light. Our description includes the splitting in excitonic levels caused by isotropic and anisotropic exchange interactions. We consider interdot transitions mediated by hole tunneling between states with the same total angular momentum and between bright and dark exciton states as allowed by spin-flip hopping between the dots in the molecule. Using realistic experimental parameters we demonstrate that the excitonic states coupled by tunneling exhibit a rich and controllable optical response. We show that through the appropriate control of an external electric field and light polarization, the tunneling coupling establishes an efficient destructive quantum interference path that creates a transparency window in the absorption spectra whenever states of appropriate symmetry are mixed by the carrier tunneling. We explore the relevant parameter space that allows probing this phenomenon in experiments. Controlled variation in applied field and laser detuning would allow the optical characterization of spin-preserving and spin-flip hopping amplitudes in such systems by measuring the width of the tunneling-induced transparency windows.

In this paper, we examine the conditions under which the nonlinear transport theory is inescapable, when a correlated quantum dot is symmetrically coupled to two leads submitted to temperature and voltage biases. By detailed numerical comparisons between nonlinear and linear currents, we show that the claimed nonlinear behavior in a temperature gradient for the electric current is not so genuine, and the linear theory made at the operating temperature $\overline{T}=(T_H+T_C)/2$ is unexpectedly robust. This is demonstrated for the single impurity Anderson model, in different regimes: resonant tunneling, Coulomb blockade, and Kondo regimes.

We study the time evolution of the entanglement entropy in the short- and long-range-coupled harmonic oscillators that have well-defined continuum limit field theories. We first introduce a method to calculate the entanglement evolution in generic coupled harmonic oscillators after quantum quench. Then we study the entanglement evolution after quantum quench in harmonic systems in which the couplings decay effectively as $1/r^{d+\alpha }$ with the distance $r$. After quenching the mass from a nonzero value to zero we calculate numerically the time evolution of von Neumann and Rényi entropies. We show that for $1<\alpha <2$ we have a linear growth of entanglement and then saturation independent of the initial state. For $0<\alpha <1$ depending on the initial state we can have logarithmic growth or just fluctuation of entanglement. We also calculate the mutual information dynamics of two separated individual harmonic oscillators. Our findings suggest that in our system there is no particular connection between having a linear growth of entanglement after quantum quench and having a maximum group velocity or generalized Lieb-Robinson bound.

A combination of experimental and computational evaluations of disorder level and lattice swelling in ion-irradiated materials is presented. Information obtained from x-ray diffraction experiments is compared to x-ray diffraction data generated using atomic-scale simulations. The proposed methodology, which can be applied to a wide range of crystalline materials, is used to study the amorphization process in irradiated SiC. Results show that this process can be divided into two steps. In the first step, point defects and small defect clusters are produced and generate both large lattice swelling and high elastic energy. In the second step, enhanced coalescence of defects and defect clusters occurs to limit this increase in energy, which rapidly leads to complete amorphization.

Contrary to the widespread belief that Majorana zero-energy modes, existing as bound edge states in two-dimensional topological insulator (TI) -superconductor (SC) hybrid structures, are unaffected by nonmagnetic static disorder by virtue of time-reversal symmetry, we show that such a protection against disorder does not exist in realistic multichannel TI/SC/ferromagnetic insulator (FI) sandwich structures of experimental relevance since the time-reversal symmetry is explicitly broken locally at the SC/FI interface where the end Majorana mode (MM) resides. We find that although the MM itself and the bulk topological superconducting phase inside the TI are indeed universally protected against disorder, extra fermionic subgap states are generically introduced at the TI edge due to the presence of the FI/SC interface as long as multiple edge channels are occupied. We discuss the important implications of our finding for the detection and manipulation of the edge MM in TI/SC/FI experimental systems of current interest.

It is of significant technological interest to have an understanding of cooperativity in spin-crossover phenomena observed in metalorganic polymeric complexes. Microscopic origin of this cooperativity has been discussed in literature primarily in terms of elastic interactions. Through Monte Carlo simulation of the model Hamiltonian we show that the superexchange interactions between localized magnetic moments at spin-crossover sites can play a crucial role in cooperativity, depending on the nature of the elastic interactions. While in the case of the effective elastic interaction of ferroelastic nature, inclusion of the above-mentioned exchange interaction produces quantitative changes in the hysteresis loop, for effective elastic interaction of antiferroelastic nature, the existence of hysteresis is found to be dictated entirely by the magnetic exchange interaction. Considering the example of a real material, namely Fe-triazole, employing the material-specific density functional theory calculation, we show the later scenario to be true for these systems. Our study should motivate further investigation on the role of magnetic exchanges in metalorganic polymers.

Skyrmions recently discovered in chiral magnets are a promising candidate for magnetic storage devices because of their topological stability, small size ($\sim 3–100$ nm), and ultralow threshold current density ($\sim {10}^6$ ${A/m}^2$) to drive their motion. However, the time-dependent dynamics has hitherto been largely unexplored. Here, we show, by combining the numerical solution of the Landau-Lifshitz-Gilbert equation and the analysis of a generalized Thiele's equation, that inertial effects are almost completely absent in skyrmion dynamics driven by a time-dependent current. In contrast, the response to time-dependent magnetic forces and thermal fluctuations depends strongly on frequency and is described by a large effective mass and a (anti-) damping depending on the acceleration of the skyrmion. Thermal diffusion is strongly suppressed by the cyclotron motion and is proportional to the Gilbert damping coefficient $\alpha$. This indicates that the skyrmion position is stable, and its motion responds to the time-dependent current without delay or retardation even if it is fast. These findings demonstrate the advantages of skyrmions as information carriers.

We develop a simple theory for the spin-dependent Seebeck effect in $n$-doped InSb in an external magnetic field. We consider spin-$1/2$ electrons in the conduction band of InSb with a temperature gradient parallel to the applied magnetic field. In the absence of spin-orbit interactions, a Boltzmann equation approach leads to a spin current parallel to the field and proportional to the temperature gradient. The calculated spin-dependent Seebeck coefficient oscillates as a function of magnetic field $B$; the peak positions are approximately periodic in $1/B$. The oscillations arise when the Fermi energy crosses the bottom of a Landau band.