Electroinductive waves have emerged as an attractive solution for designing metamaterials that support backward propagating waves. Stacked metasurfaces etched with complementary split-ring resonators (CSRRs) have also been shown to exhibit a broadband negative dispersion. We demonstrate, through experiment and numerical modeling, that the operational bandwidth of a CSRR metamaterial waveguide can be improved by restricting cross-polarization effects in the constituent meta-atoms. We report a fractional bandwidth of $>56\%$, which, to the best of our knowledge, is broader than any previously reported value for an electroinductive metamaterial. We present a traditional coupled-dipole toy model as a tool to understand the field interactions in CSRR-based metamaterials, and to explain the origin of their negative dispersion response.

We study the relation between quantum pumping of charge and the work exchanged with the driving potentials in a strongly interacting ac-driven quantum dot. We work in the large-interaction limit and in the adiabatic pumping regime, and we develop a treatment that combines the time-dependent slave-boson approximation with linear response in the rate of change in the ac potentials. We find that the time evolution of the system can be described in terms of equilibrium solutions at every time. We analyze the effect of the electronic interactions on the performance of the dot when operating as a quantum motor. The main two effects of the interactions are a shift of the resonance and an enhancement of the efficiency with respect to a noninteracting dot. This is due to the appearance of additional ac parameters accounting for the interactions that increase the pumping of particles while decreasing the conductance.

A variety of quantum systems exhibit Weyl points in their spectra where two bands cross in a point of three-dimensional parameter space with conical dispersion in the vicinity of the point. We consider theoretically the soft constraint regime where the parameters are dynamical quantum variables. We have shown that in general the soft constraints, in the quasiclassical limit, result in Weyl disks where two states are (almost) degenerate in a finite two-dimensional region of the three-dimensional parameter space. We provide concrete calculations for two setups: Weyl point in a four-terminal superconducting structure and a Weyl exciton, i.e., a bound state of Weyl electron and a massive hole.

Using electrically detected magnetic resonance spectroscopy, we demonstrate that doping the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) with ethylene glycol allows for the control of effective local charge carrier hyperfine fields through motional narrowing. These results suggest that doping of organic semiconductors could enable the tuning of macroscopic material properties dependent on hyperfine fields such as magnetoresistance, magneto-optical responses, and spin diffusion.

Freestanding single bilayer Bi(111) is a two-dimensional topological insulator with edge states propagating along its perimeter. Given the interlayer coupling experimentally, the topological nature of Bi(111) thin films and the impact of the supporting substrate on the topmost Bi bilayer are still under debate. Here, combined with scanning tunneling microscopy and first-principles calculations, we systematically study the electronic properties of Bi(111) thin films grown on a $\mathrm{NbS}{\mathrm{e}}_2$ substrate. Two types of nonmagnetic edge structures, i.e., a conventional zigzag edge and a 2 × 1 reconstructed edge, coexist alternately at the boundaries of single bilayer islands, the topological edge states of which exhibit remarkably different energy and spatial distributions. Prominent edge states are persistently visualized at the edges of both single and double bilayer Bi islands, regardless of the underlying thickness of Bi(111) thin films. We provide an explanation for the topological origin of the observed edge states that is verified with first-principles calculations. Our paper clarifies the long-standing controversy regarding the topology of Bi(111) thin films and reveals the tunability of topological edge states via edge modifications.

Atomically thin materials hold mutative electronic transition properties by virtue of the inevitable strains adding to the lattice distribution. A quantitative approach and clear systematical verification have seldom been presented to effectively illustrate the substrate-induced strain along the lattice distribution, although the substrate perturbations for monolayers have been repeatedly mentioned. Here, a model of difference analysis is introduced to reveal the structural strain of monolayers from the substrate, which is derived from the mismatch effect based on the variation of the thermal expansion coefficient between the substrate and monolayers. A coupling coefficient is proposed to quantitatively manifest the substrate-induced strain and mismatch effect. Furthermore, combined with the coupling coefficient, the lattice disorder from the mismatch effect is demonstrated, experimentally using the ${\mathrm{WS}}_2$ monolayer as reference. This study could promote broad investigations of some fundamental issues from preparation to electrical structural performances in atomically thin materials and further practical applications.

We study the fractional boundary charges (FBCs) occurring in nanowires in the presence of periodically modulated chemical potentials and connect them to the FBCs occurring in a two-dimensional electron gas in the presence of a perpendicular magnetic field in the integer quantum Hall effect (QHE) regime. First, we show that in nanowires the FBCs take fractional values and change linearly as a function of phase offset of the modulated chemical potential. This linear slope takes quantized values determined by the period of the modulation and depends only on the number of the filled bands. Next, we establish a mapping from the one-dimensional system to the QHE setup, where we again focus on the properties of the FBCs. By considering a cylinder topology with an external flux similar to the Laughlin construction, we find that the slope of the FBCs as function of flux is linear and assumes universal quantized values, also in the presence of arbitrary disorder. We establish that the quantized slopes give rise to the quantization of the Hall conductance. Importantly, the approach via FBCs is valid for arbitrary flux values and disorder. The slope of the FBCs plays the role of a topological invariant for clean and disordered QHE systems. Our predictions for the FBCs can be tested experimentally in nanowires and in Corbino disk geometries in the integer QHE regime.

We consider a Josephson contact mediated by 1D chiral modes on a surface of a 3D topological insulator with superimposed superconducting and magnetic layers. The system represents an interferometer in which 1D chiral Majorana modes on the boundaries of superconducting electrodes are linked by ballistic chiral Dirac channels. This model may be realized also in recently fabricated heterostructures based on quantum anomalous Hall insulators. We investigate the noise of the Josephson current as a function of the dc phase bias and the Aharonov-Bohm flux. Starting from the scattering formalism, a Majorana representation of the Keldysh generating action for cumulants of the transmitted charge is found. At temperatures higher than the Thouless energy $E_{\mathrm{Th}}$, we obtain the usual Johnson-Nyquist noise, $4G_0{k}_{\mathrm{B}}T$, characteristic for a single-channel wire with $G_0\equiv e^2/\left(2\pi \hslash \right)$. At lower temperatures, the behavior is much richer. In particular, the equilibrium noise is strongly enhanced to a temperature-independent value $\sim G_0{E}_{\mathrm{Th}}$ if the Aharonov-Bohm and superconducting phases are both close to $2\pi n$, which are points of emergent degeneracy in the ground state of the junction. The equilibrium noise is related to the Josephson junction's impedance via the fluctuation-dissipation theorem. In a striking contrast to usual Josephson junctions (tunnel junctions between two $s$-wave superconductors), the real part of the impedance does not vanish, reflecting the gapless character of Majorana modes in the leads.

In spite of more than 60 years of research, hundreds of publications, and dramatic influence on properties, the driving force of the formation of the polar nanoregion (PNR) in the paraelectric phase of perovskite ferroelectrics and consequent relaxor properties remains uncertain. We show that these peculiar features follow directly from the vibronic, pseudo-Jahn-Teller (PJT) theory of ferroelectricity. Due to the higher disorder (and entropy) in the paraelectric phase (created by the local PJT dynamics), as compared with the polarized phase (where the PJT dynamics is partially quenched), a small PNR of the latter with $n$ unit cells is formed in a dipole-alignment self-assembly process. It emerges encapsulated by a border layer with intermedium ordering that produces “surface tension” and limits its size. The thermodynamic equilibrium between the PNR and the bulk cubic phase at temperatures $T_n$, well above the phase transition $T_{\mathrm{C}}$, is reached by compensation of the excessive entropy contribution $T_n\mathrm{\Delta }S$ with ordering energy and the work against this surface tension. The calculations based on the vibronic theory, including the PJT induced local dipolar dynamics and intercell interactions, yield the size of PNR as a function of the temperature increments $T_n-T_{\mathrm{C}}$ and some crystal parameters. In accordance with experimental data, the size of the emerging PNR decreases with temperature, $n\sim {\left(T_n-T_{\mathrm{C}}\right)}^{-3}$, becoming undetectable at the Burns temperature $T_{\mathrm{B}}$. At temperatures $T_f$, nearer to the phase transition, the sizes of PNRs grow rapidly, and their interaction leads to the formation of the nonergodic (glasslike) phase.

Nanomechanical resonators have demonstrated great potential for use as versatile tools in a number of emerging quantum technologies. For such applications, the performance of these systems is restricted by the decoherence of their fragile quantum states, necessitating a thorough understanding of their dissipative coupling to the surrounding environment. In bulk amorphous solids, these dissipation channels are dominated at low temperatures by parasitic coupling to intrinsic two-level system (TLS) defects; however, there remains a disconnect between theory and experiment on how this damping manifests in dimensionally reduced nanomechanical resonators. Here, we present an optomechanically mediated thermal ringdown technique, which we use to perform simultaneous measurements of the dissipation in four mechanical modes of a cryogenically cooled silicon nanoresonator, with resonant frequencies ranging from 3–19 MHz. Analyzing the device's mechanical damping rate at fridge temperatures between 10 mK and 10 K, we demonstrate quantitative agreement with the standard tunneling model for TLS ensembles confined to one dimension. From these fits, we extract the defect density of states $\left(P_0\sim 1-4\times {10}^{44}{\mathrm{J}}^{-1}{\mathrm{m}}^{-3}\right)$ and deformation potentials $(\gamma \sim 1–2$ eV), showing that each mechanical mode couples on average to less than a single thermally active defect at 10 mK.

Polar magnets are promising materials for applications such as multiferroics or in spintronics. In double-corundum-related oxides, the cation ordering imposes a polar structure and the use of high pressure facilitates the insertion of magnetic cations into the compounds. Here we present the high-pressure synthesis of a polar and ferrimagnetic corundum derivative of ${\mathrm{Mn}}_2{\mathrm{InSbO}}_6$, which adopts the ordered-ilmenite-type structure. Neutron powder diffraction reveals that the high-temperature nearly collinear ferrimagnetic phase evolves to an incommensurate helical structure with $k_{\delta }=\left[00k_z\right]$ propagation vector, which then locks to the commensurate value of $k_z=1/8$. This complex magnetic behavior is likely to be related to magnetic frustration and the polar nature of the ordered double-corundum structure.

We present results on the entropy and heat capacity of spin-$S$ honeycomb-lattice Kitaev models using high-temperature series expansions and thermal pure quantum state methods. We study models with anisotropic couplings $J_z=1\ge J_x=J_y$ for spin values 1/2, 1, 3/2, and 2. We show that for $S>1/2$, any anisotropy leads to well-developed plateaus in the entropy function at an entropy value of $\frac{1}{2}ln2$, independent of $S$. However, in the absence of anisotropy, there is an incipient entropy plateau at $S_{\text{max}}/2$, where $S_{\text{max}}$ is the infinite temperature entropy of the system. We discuss the possible underlying microscopic reasons for the origin and implications of these entropy plateaus.

A recent paper [D. Go et al., Phys. Rev. Lett. 121, 086602 (2018)] proposed that the intrinsic orbital Hall effect (OHE) can emerge from momentum-space orbital texture in centrosymmetric materials. In searching for real materials with strong OHE, we investigate the intrinsic OHE in metals with small spin-orbit coupling (SOC) in face-centered cubic and body-centered cubic structures (Li, Al, V, Cr, Mn, Ni, and Cu). We find that orbital Hall conductivities (OHCs) in these materials are gigantic $\sim {10}^3-{10}^4(\hslash /e){(\mathrm{\Omega }·\mathrm{cm})}^{-1}$, which are comparable or larger than spin Hall conductivity (SHC) of Pt. Although SHCs in these materials are smaller than OHCs due to small SOC, we found that SHCs are still sizable and the spin Hall angles may be of the order of 0.1. We discuss implications on recent spin-charge interconversion experiments on materials having small SOC.

A quantum Maxwell demon is a device that can lower the entropy of a quantum system by providing it with purity. The functionality of such a quantum demon is rooted in a quantum-mechanical SWAP operation exchanging mixed and pure states. We describe the setup and performance of a quantum Maxwell demon that purifies an energy-isolated system from a distance. Our cQED-based design involves two transmon qubits, where the mixed-state target qubit is purified by a pure-state demon qubit connected via an off-resonant transmission line; this configuration naturally generates an iSWAP gate. Although less powerful than a full SWAP gate, we show that assuming the present-day performance characteristics of a cQED implementation, such an extended quantum Maxwell demon can purify the target qubit over macroscopic distances on the order of meters and tolerates elevated temperatures of the order of a few Kelvin in the transmission line.