We propose that bulk free carriers in topologically trivial multivalley insulators with nonvanishing Berry curvature can give rise to low-dissipation edge currents, which are squeezed within a distance on the order of the valley diffusion length from the edge. This happens even in the absence of edge states [topological (gapless) or otherwise], and when the bulk equilibrium carrier concentration is thermally activated across the gap. Physically, the squeezed edge current arises from the spatially inhomogeneous valley orbital magnetization that develops from valley-density accumulation near the edge. While this current possesses neither topology nor symmetry protection and, as a result, is not immune to dissipation, in clean enough devices it can mimic low-loss ballistic transport.

We study the electronic band structure and the topological properties of the twisted double bilayer graphene, or a pair of AB-stacked bilayer graphenes rotationally stacked on top of each other. We consider two different arrangements, AB-AB and AB-BA, which differ in the relative orientation. For each system, we calculate the energy band and the valley Chern number using the continuum Hamiltonian. We show that the AB-AB and the AB-BA have similar band structures, while the Chern numbers associated with the corresponding bands are completely different. In the absence of the perpendicular electric field, in particular, the AB-AB system is a trivial insulator when the Fermi energy is in a gap, while the AB-BA is a valley Hall insulator. Also, the lowest electron and hole bands of the AB-AB are entangled by the symmetry protected band touching points, while they are separated in the AB-BA. In both cases, the perpendicular electric field immediately opens an energy gap at the charge neutral point, where the electron branch becomes much narrower than the hole branch, due to the significant electron-hole asymmetry.

Combining graphene with plasmonic nanostructures has attracted great interest for enhancing graphene-light interactions. Here, we report an intensive experimental study on the strong interaction of graphene with plasmon-induced magnetic resonances, i.e., magnetic plasmon (MP) supported by simple one-dimensional (1D) metallic nanogroove arrays. We demonstrate that the absorption of graphene, which covers only 10% of the groove surface, is dramatically enhanced by one order of magnitude compared with the 2.3% light absorption for the pristine graphene monolayer. The strong graphene-MP interaction also enables a considerable suppression of Ohmic loss in the metallic groove array. Importantly, we demonstrate that limited metal-graphene charge transfer at the directly contacting interface leads to a reduced Fermi level of graphene, thus giving no contribution to the shift of magnetic resonances, which is in contrast to other reported graphene-plasmonic systems with considerable resonance shift. We also evaluate the influence of local dielectric change on graphene-plasmon interaction, confirming that the observed considerable MP resonance shift results from the small variation of the dielectric environment inside the grooves, which is further supported by numerical simulations. Such a hybrid nanostructure with a 1D flat nature and efficient enhancement of graphene-light interaction is expected to open exciting perspectives within highly integrable plasmonics for light-harvesting, photodetection, and sensing applications.

Periodically driven quantum many-body systems support anomalous topological phases of matter, which cannot be realized by static systems. In many cases, these anomalous phases can be many-body localized, which implies that they are stable and do not heat up as a result of the driving. What types of anomalous topological phenomena can be stabilized in driven systems, and in particular, can an anomalous phase exhibiting non-Abelian anyons be stabilized? We address this question using an exactly solvable, stroboscopically driven 2D Kitaev spin model, in which anisotropic exchange couplings are boosted at consecutive time intervals. The model shows a rich phase diagram which contains anomalous topological phases. We characterize these phases using weak and strong scattering-matrix invariants defined for the fermionic degrees of freedom. Of particular importance is an anomalous phase whose zero flux sector exhibits fermionic bands with zero Chern numbers, while a vortex binds a pair of Majorana modes, which as we show support non-Abelian braiding statistics. We further show that upon adding disorder, the zero flux sector of the model becomes localized. However, the model does not remain localized for a finite density of vortices. Hybridization of Majorana modes bound to vortices form “vortex bands,” which delocalize by either forming Chern bands or a thermal metal phase. We conclude that while the model cannot be many-body localized, it may still exhibit long thermalization times, owing to the necessity to create a finite density of vortices for delocalization to occur.

This study investigates spectral changes in the absorption cross sections of single strong coupling systems composed of single silver nanoparticle dimers and a few dye molecules during the quenching of surface-enhanced resonant Raman scattering (SERRS). The absorption cross section is obtained by subtracting the scattering cross section from an extinction cross section. The spectral changes in these cross sections are evaluated using a classical hybridization model composed of a plasmon and a molecular exciton including a molecular multilevel property. The changes in the scattering and extinction cross sections exhibit blueshifts in peak energy and increases in peak intensities, respectively, during SERRS quenching. These properties are effectively reproduced in the model by decreasing the coupling energy. In particular, the peaks in the scattering and extinction cross sections appear as peaks or dip-like structures in the absorption cross sections depending on the degree of scattering loss, which reflects the dimer sizes. These results are useful for optimizing photophysical and photochemical effects mediated by the electronic excited states of strong coupling systems.

We study second-order finite-temperature phase transitions of the two-dimensional quantum Ising and interacting honeycomb fermion models using infinite projected entangled pair states (iPEPS). We obtain an iPEPS thermal state representation by variational tensor network renormalization. We find that at the critical temperature $T_c$ the iPEPS correlation length is finite for the computationally accessible values of the iPEPS bond dimension $D$. Motivated by this observation, we investigate the application of finite correlation length scaling (FCLS), which has been previously used for iPEPS simulations of quantum critical points at $T=0$, to obtain precise values of $T_c$ and the universal critical exponents. We find that in the vicinity of $T_c$ the behavior of observables follows well the one predicted by FCLS. Using FCLS we obtain $T_c$ and the critical exponents in agreement with quantum Monte Carlo results except for couplings close to the quantum critical points where larger bond dimensions are required.

The nature and the very existence of the resonant plaquette valence bond state that separates the classical columnar phase and the Rokhsar-Kivelson point in the quantum dimer model remains unsettled. Here we take a different line of attack on this model, and on the closely related six-vertex model, by exploiting the global conservation law of the number of electric field lines. This allows us to study a single fluctuating electric field line which we show maps exactly onto a one-dimensional spin chain. In the case of the six-vertex model, the electric field line maps onto the celebrated spin 1/2 XXZ model which can be solved exactly. In the quantum dimer model, the electric field line is mapped onto a two-leg spin 1/2 ladder, which we study using numerical exact diagonalization. Our findings are consistent with the existence of three distinct phases including a Luttinger liquid phase, the one-dimensional precursor to the two-dimensional plaquette valence bond solid. The uncanny resemblance of our quasi-one-dimensional electric field line problem to the full two-dimensional problem suggests that much of the behavior of the latter might be understood by thinking of it as a closely packed array of field lines which themselves are undergoing nontrivial phase transitions.

Magnetoelectric properties displayed in multiferroics are generally associated with complex magnetic orders. This complexity often results in a delicate balance between several geometrically frustrated magnetic exchange interactions. Applying pressure will thus unbalance this equilibrium and strongly affect the multiferroic properties. In this paper, we study the effect of pressure on magnetism in three particular members of the ${R\mathrm{Mn}}_2{\mathrm{O}}_5$ multiferroics ($R$ = Gd, Sm, and Nd) with interesting magnetic orders. Using powder neutron diffraction, we studied the evolution of their magnetic structures as a function of pressure. Despite their singular properties with respect to the other compositions, we demonstrate that these three members present the same pressure-induced commensurate phase (PCM) with the propagation wave vector [q${}_{\mathrm{PCM}}=\left(\frac{1}{2},0,\frac{1}{2}\right)]$. Furthermore, the stabilization of the PCM phase under pressure can be explained by a similar mechanism. We ultimately conclude that the different origin of the CM and PCM phases of these three compounds are related to the competition of only two superexchange interactions, namely $J_1$ and $J_6$.

Reaching a target quantum state from an initial state within a finite temporal window is a challenging problem due to nonadiabaticity. We study the optimal protocol for switching on interactions to reach the ground state of a weakly interacting Luttinger liquid within a finite time $\tau$, starting from the noninteracting ground state. The protocol is optimized by minimizing the excess energy at the end of the quench, or by maximizing the overlap with the interacting ground state. We find that the optimal protocol is symmetric with respect to $\tau /2$, and can be expressed as a functional of the occupation numbers of the bosonic modes in the final state. For short quench durations, the optimal protocol exhibits fast oscillation and excites high-energy modes. In the limit of large $\tau$, minimizing energy requires a smooth protocol while maximizing overlap requires a linear quench protocol. In this limit, the minimal energy and maximal overlap are both universal functions of the system size and the duration of the protocol.

Manipulating the phase of electromagnetic radiation is of importance for applications ranging from communication to imaging. Here, real-time reconfigurable phase response and group delay of a tunable terahertz metamaterial consisting of dual-layer broadside coupled split-ring resonators is demonstrated. Utilizing electrostatic comb-drive actuators, the metamaterial resonant frequency is tuned by changing the lateral distance between the two layers which modifies the transmission amplitude and phase spectrum. The phase modulation is approximately 180˚ in the vicinity of the resonant frequency. In addition, remarkable modulation in the group delay of transmitted pulses (from −7 to 3 ps) is evaluated based on the measured frequency response using the convolution method when the lateral distance is changed from 0 to $24\mu \mathrm{m}$. A two-port resonator model, derived from coupled-mode theory and supported by finite-element full-wave simulations, reveals the underlying physics of the modulation. Specifically, the coupling factor between the two layers plays a critical role, the tuning of which provides a route for structure design and optimization. The capability of tuning the phase response and group delay enables applications, such as phase compensation and group-delay equalization at terahertz frequencies.

We reconsider a key point in semiconductor physics, the splitting of the valence band states induced by the spin-orbit interaction, through a novel approach which uses neither the group theory formalism, nor the usual $\mathbf{L}·\mathbf{S}$ formulation valid for atoms but conceptually incorrect for periodic lattices; the angular momenta $\mathbf{L}$ and $\mathbf{J}$ have no meaning due to the absence of spherical symmetry. We show that for zinc-blende structures, the valence band eigenstates resulting from spin-orbit coupling are uniquely determined by: (i) the equivalence of the ($x,y,z$) crystal axes, and (ii) the threefold degeneracy of the valence band. The fact that these two conditions are also fulfilled by atomic $p$ states allows us to understand why the spin-orbit eigenstates for threefold atomic and valence electrons have exactly the same structure, albeit with drastic differences in the potential and electronic symmetries. We also come back to the commonly accepted understanding of the exciton-photon interaction in terms of bright and dark excitons having total angular momenta $J=(1,2)$, respectively, and present a simple derivation of this interaction which only relies on spin conservation.

In the search for more efficient thermoelectric materials, scientists have placed high hopes in the possibility of enhancing the power factor using resonant states. In this paper, we investigate theoretically the effects of randomly distributed resonant impurities on the power factor. Using the Chebyshev polynomial Green's-function method, we compute the electron transport properties for very large systems ($\sim {10}^7\mathrm{atoms}$) with an exact treatment of disorder. The introduction of resonant defects can lead to a large enhancement of the power factor together with a sign inversion in the Seebeck coefficient. This boost depends crucially on the position of the resonant peak, and on the interplay between elastic impurity scattering and inelastic processes. Strong electron-phonon and electron-electron scatterings are found detrimental. Finally, the robustness of our results is examined in the case of anisotropic orbitals and two-dimensional confinement. Our findings are promising for the prospect of thermoelectric power generation.

A canonical quantization procedure is applied to elastic waves interacting with pinned dislocation segments (“strings”) of length $L$ via the Peach-Koehler force. The interaction Hamiltonian, derived from an action principle that classically generates the Peach-Koehler force, is a power series of creation and annihilation operators. The leading term is quadratic, and keeping only this term the observable quantities of scattering processes are computed to all orders in perturbation theory. The resulting theory is characterized by the magnitude of $kL$, with $k$ the wave number of an incident phonon. The theory is solved for arbitrary $kL$, and different limits are explored. A significant result at this level is the scattering cross section for phonons by dislocation segments. As a function of frequency, this cross section has a much richer structure than the linear-in-frequency behavior that is inferred from scattering by an infinite, static, dislocation. The rate of spontaneous phonon emission by an excited dislocation is computed as well. When many dislocations are present, an effective mass operator is computed in the weak and independent scattering approximation. The contribution of the cubic terms is computed to leading order in perturbation theory. They allow for a comparison of the scattering of a phonon by a string and the three-phonon scattering, as well as studying the dependence of scattering amplitudes on the temperature of the solid. It is concluded that the effect of dislocations of Burgers vector 0.5 nm and length 50 nm will dominate for relatively modest dislocation densities: ${10}^8{\mathrm{cm}}^{-2}$. Finally, the full power series of the interaction Hamiltonian is considered. The effects of quantum corrections, i.e., contributions proportional to Planck's constant, are estimated, and found to be controlled by another wave-number-dependent parameter $k{d}_q$, where $d_q$ is a length proportional to $\sqrt{\hslash }$. The possibility of using the results of this paper in the study of the phononic thermal properties of two- and three-dimensional materials is noted and discussed.

Research on magnetic systems with broken inversion symmetry has been stimulated by the experimental proof of particlelike configurations known as skyrmions, whose nontrivial topological properties make them ideal candidates for spintronic technology. In this class of materials, Dzyaloshinskii-Moriya interactions (DMI) are present, which favor the stabilization of chiral configurations. Recent advances in material engineering have shown that in confined geometries it is possible to stabilize skyrmionic configurations at zero field. Moreover, it has been shown that in systems based on Pd/Fe bilayers on top of Ir(111) surfaces skyrmions can be as small as a few nanometres in diameter. In this work, we present scanning tunneling microscopy measurements of small Pd/Fe and ${\mathrm{Pd}}_2/\mathrm{Fe}$ islands on Ir(111) that exhibit a variety of different spin textures, which can be reproduced using discrete spin simulations. These configurations include skyrmions and skyrmionlike states with extra spin rotations such as the target state, which have been of interest due to their promising dynamic properties. Furthermore, using simulations we analyze the stability of these skyrmionic textures as a function of island size, applied field and boundary conditions of the system. An understanding of the parameters and conditions affecting the stability of these magnetic structures in confined geometries is crucial for the development of energetically efficient and optimally sized skyrmion-based devices.

In many quantum magnets, especially the rare-earth ones, the low-lying crystal-field states are not well separated from the excited ones and thus they are insufficient to describe the low-temperature magnetic properties. Inspired by this simple observation, we develop a microscopic theory to describe the magnetic physics due to the collapses of the weak crystal-field states. We find two cases in which the excited crystal-field states should be seriously included in the theory. One case is when the bandwidth of the excited crystal-field states is comparable to the crystal-field gap. The other case is when the exchange-energy gain between the low-lying and the excited crystal-field states overcomes the crystal-field gap. Both cases could drive a phase transition and result in magnetic orders by involving the excited crystal-field states. We dub the above physics “upper branch magnetism and phase transition.” We discuss the multitude of magnetic phases and the emergent selection rules for the detection of the underlying excitations. We expect that our results will help improve the understanding of many rare-earth magnets with weak crystal-field gaps such as ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, and will also provide a complementary perspective to the prevailing local “$J$” physics in $4d/5d$ magnets.

We propose thermally induced generation of monochromatic microwave radiation in magnon-polaritons. The mechanism of thermal to microwave energy transformation is based on intrinsic energy loss compensation of coupled magnon and microwave cavity oscillators by thermally induced “negative damping.” A singularity at an exceptional point is achieved when at the critical value of negative damping, the damping of the system is fully compensated. At the exceptional point, the input energy is equally distributed between the magnon and photon subsystems of the magnon-polariton. The efficiency of transformation of thermal energy into useful microwave radiation is estimated to be as large as $17\%$ due to magnon-photon coupling mediated direct conversion of spin current into microwave photons.

We present an ab initio theory for superconductors, based on a unique mapping between the statistical density operator at equilibrium, on the one hand, and the corresponding one-body reduced density matrix $\gamma$ and the anomalous density $\chi$, on the other. This formalism for superconductivity yields the existence of a universal functional ${\mathfrak{F}}_{\beta }[\gamma ,\chi ]$ for the superconductor ground state, whose unique properties we derive. We then prove the existence of a Kohn-Sham system at finite temperature and derive the corresponding Bogoliubov–de Gennes–type single-particle equations. By adapting the decoupling approximation from density functional theory for superconductors we bring these equations into a computationally feasible form. Finally, we use the existence of the Kohn-Sham system to extend the Sham-Schlüter connection and derive a first exchange-correlation functional for our theory. This reduced density matrix functional theory for superconductors has the potential of overcoming some of the shortcomings and fundamental limitations of density functional theory of superconductivity.

We report measurements of the energy gap of granular aluminum films by THz spectroscopy. We find that as the grains progressively decouple, the coupling ratio $2\mathrm{\Delta }\left(0\right)/k_B{T}_c$ increases above the BCS weak-coupling ratio $3.53$, and reaches values consistent with an approach to BCS-BEC crossover for the high-resistivity samples, expected from the short coherence length. The Mattis-Bardeen theory describes remarkably well the behavior of ${\sigma }_{1,s}/{\sigma }_{1,n}$ for all samples up to very high normal-state resistivities.

We study the low-energy electronic structure of heterostructures formed by one sheet of graphene placed on a monolayer of ${\mathrm{NbSe}}_2$. We build a continuous low-energy effective model that takes into account the presence of a twist angle between graphene and ${\mathrm{NbSe}}_2$, and of spin-orbit coupling and superconducting pairing in ${\mathrm{NbSe}}_2$. We obtain the parameters entering the continuous model via ab initio calculations. We show that despite the large mismatch between the graphene's and ${\mathrm{NbSe}}_2$'s lattice constants, due to the large size of the ${\mathrm{NbSe}}_2$'s Fermi pockets, there is a large range of values of twist angles for which a superconducting pairing can be induced into the graphene layer. In addition, we show that the superconducting gap induced into the graphene is extremely robust to an external in-plane magnetic field. Our results show that the size of the induced superconducting gap, and its robustness against in-plane magnetic fields, can be significantly tuned by varying the twist angle.