We examine the Bose-Hubbard model in the Penrose lattice based on inhomogeneous mean-field theory. Since the averaged coordination number in the Penrose lattice is four, the mean-field phase diagram consisting of the Mott insulator (MI) and superfluid (SF) phase is similar to that of the square lattice. However, the spatial distribution of the Bose condensate in the SF phase is significantly different from uniform distribution in the square lattice. We find a fractal structure in its distribution near the MI-SF phase boundary. The emergence of the fractal structure is a consequence of the cooperative effect between quasiperiodicity in the Penrose lattice and criticality at the phase transition.

We have theoretically studied the supercurrent profiles in three-dimensional normal metal and ferromagnetic Josephson configurations, where the magnitude of the superconducting gaps in the superconducting leads are unequal, i.e., ${\mathrm{\Delta }}_1\ne {\mathrm{\Delta }}_2$, creating asymmetric ${\mathrm{S}}_1{\mathrm{NS}}_2$ and ${\mathrm{S}}_1{\mathrm{FS}}_2$ systems. Our results reveal that by increasing the ratio of the superconducting gaps ${\mathrm{\Delta }}_2/{\mathrm{\Delta }}_1$, the critical supercurrent in a ballistic ${\mathrm{S}}_1{\mathrm{NS}}_2$ system can be enhanced by more than $100\%$ and reaches a saturation point, or decays away, depending on the junction thickness, magnetization strength, and chemical potential. The total critical current in a diffusive ${\mathrm{S}}_1{\mathrm{NS}}_2$ system was found to be enhanced by more than $50\%$ parabolically and reaches saturation by increasing one of the superconducting gaps. In a uniform ferromagnetic junction, the supercurrent undergoes reversal by increasing ${\mathrm{\Delta }}_2/{\mathrm{\Delta }}_1 >1$. Through decomposing the total supercurrent into its supergap and subgap components, our results illustrate their crucial relative contributions to the Josephson current flow. It was found that the competition of subgap and supergap currents in a ${\mathrm{S}}_1{\mathrm{FS}}_2$ junction results in the emergence of second harmonics in the current-phase relation. In contrast to a diffusive asymmetric Josephson configuration, the behavior of the supercurrent in a ballistic system with ${\mathrm{\Delta }}_2/{\mathrm{\Delta }}_1 =1$ can be properly described by the subgap current component only, in a wide range of parameter sets, including Fermi level mismatch, magnetization strength, and junction thickness. Interestingly, when ${\mathrm{\Delta }}_2/{\mathrm{\Delta }}_1 >1$, our results have found multiple parameter sets where the total supercurrent is driven by the supergap component. Therefore, our comprehensive study highlights the importance of subgap and supergap supercurrent components in both the ballistic and diffusive regimes. We focus on experimentally accessible material and geometric parameters that can lead to advancements in cryogenic devices based on Josephson junction architectures that utilize supergap currents, which are less sensitive to temperature compared to the subgap current.

We introduce the topological mirror excitonic insulator as a new type of interacting topological crystalline phase in one dimension. Its mirror-symmetry-protected topological properties are driven by exciton physics, and it manifests in the quantized bulk polarization and half-charge modes on the boundary. And the bosonization analysis is performed to demonstrate its robustness against strong correlation effects in one dimension. Besides, we also show that Rashba nanowires and Dirac semimetal nanowires could provide ideal experimental platforms to realize this new topological mirror excitonic insulating state. Its experimental consequences, such as quantized tunneling conductance in the tunneling measurement, are also discussed.

Motivated by the insulating behavior of $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TSeF}\right)}_2{\mathrm{I}}_3$ at low temperatures ($T$'s), we first performed first-principles calculations based on the crystal structural data at 30 K under ambient pressure, and we constructed a two-dimensional effective model using maximally localized Wannier functions. As possible causes of the insulating behavior, we studied the effects of the on-site Coulomb interaction $U$ and spin-orbit interaction (SOI) by investigating the electronic state and the transport coefficient using the Hartree approximation and the $T$-matrix approximation. The calculations at a finite $T$ demonstrated that a spin-ordered massive Dirac electron (SMD) appeared due to the on-site Coulomb interaction. We had an interest in the anomalous competitive effect with $U$ and SOI when the SMD phase is present in $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$, and we investigated these contributions to the electronic state and conductivity. The SMD is not a conventional spin order, but it exhibits the spin-valley Hall effect. Direct current resistivity in the presence of a spin order gap increased divergently and exhibited negative magnetoresistance in the low $T$ region with decreasing $T$. The charge density hardly changed below and above the $T$ at which this insulating behavior appeared. However, when considering the SOI alone, the state changed to a topological insulator phase, and the electrical resistivity is saturated by edge conduction at quite low $T$. When considering both the SMD and the SOI, the spin order gap was suppressed by the SOI, and gaps with different sizes opened in the left and right Dirac cones. This phase transition leads to distinct changes in microwave conductivity, such as a discontinuous jump and a peak structure.

The relativistic Doppler reflection of terahertz (THz) light from a photoinduced moving plasma front in a silicon wafer irradiated by an optical pump light was investigated. The intensity and the phase shift spectra of the reflected THz light were investigated by THz time-domain spectroscopy with broadband electro-optic sampling in a thin GaP crystal. The frequency up-shift factor, which is a ratio of averaged frequencies from after to before Doppler reflection, was measured to be $\mathrm{\Gamma }=2.02$ with a pump energy density of $3.3\mu \mathrm{J}/\mathrm{m}{\mathrm{m}}^2$ at a wavelength of 800 nm. The frequency up-shift increased as optical pump energy density increased. The frequency up-shift was reproduced by evaluating the moving plasma front whose spatial carrier density distribution depended on the pump energy density.

A recently proposed two-dimensional (2D) carbon allotrope, tetrahex-carbon composed of tetragonal and hexagonal rings in a buckled plane, draws scientific research interests due to its remarkable mechanical and electronic properties, including ultrahigh strength, negative Poisson's ratio, finite direct band gap, and high carrier mobility. In this work, a series of carbon nanotubes rolled from this new 2D tetrahex-C nanosheet with various size and chirality are studied through first-principles density-functional theory calculations. The tube diameter is in the range of 4.5–18 Å. It is found that the smallest thermodynamically stable nanotubes have the diameter of 4.5 Å for zigzag tetrahex-C nanotubes (z-TH-CNTs) and 6.0 Å for armchair tetrahex-C nanotubes (a-TH-CNTs). The z-TH-CNTs are energetically more favorable than the a-TH-CNTs given similar size. The single-wall nanotubes have a particular wall thickness which shrinks with the decrease of tube size. It is also found that all explored nanotubes show semiconducting behavior and HSE predicted band gaps can be effectively tuned in the range of 1.84–2.69 eV with size and chirality. Large curvature in small z-TH-CNTs alters the band structures, resulting in a significantly reduced effective mass of electron thus indicating potentially enhanced carrier mobility. Work function of the tubes can be manipulated within the range of 5.25–5.43 eV dependent on the tube size and chirality. These remarkable structural and electronic properties in the tetrahex-C nanotubes may have potential applications in nanoelectronics.

Strain-inducing deformations in graphene alter charge distributions and provide a new method to design specific features in the band structure and transport properties. Novel approaches implement engineered substrates to induce specifically targeted strain profiles. Motivated by this technique, we study the evolution of charge distributions with an increasing number of out-of-plane deformations as an example of a finite size periodic substrate. We first analyze a system of two overlapping deformations and determine the quantitative relation between geometrical parameters and features in the local density of states. We extend the study to sets of three and four deformations in linear and two-dimensional arrays and observe the emergence of moiré patterns that are more pronounced for a hexagonal cell composed of seven deformations. A comparison between the induced strain profile and spatial maps of the local density of states at different energies provides evidence for the existence of states confined by the pseudomagnetic field in bounded regions, reminiscent of quantum dots structures. Due to the presence of these states, the energy level scaling to be observed by local probes should exhibit a linear dependence with the pseudofield, in contrast to the expected scaling of pseudo-Landau levels.

Topological phases and their topological features are enriched by the fundamental time-reversal, particle-hole, and chiral as well as crystalline symmetries. While one-dimensional (1D) generalized Su-Schrieffer-Heeger (SSH) systems show various topological phenomena such as topological solitons and topological charge pumping, it remains unclear how such symmetry protects and relates such topological phenomena. Here we show that the generalized time-reversal, particle-hole, and chiral symmetry operators consistently explain not only the symmetry transformation properties between the ground states but also the topological features of the topological solitons in prototypical quasi-1D systems such as the SSH, Rice-Mele, and double-chain models. As a consequence, we classify generalized essential operators into three groups: Class I and class II operators connect ground states in between after spontaneous symmetry breaking while class III operators give the generalized particle-hole and chiral symmetries to ground states. Furthermore, class I operators endow the equivalence relation between topological solitons while class II and III operators do the particle-hole relations. Finally, we demonstrate three distinct types of topological charge pumping and soliton chirality from the viewpoint of class I, II, and III operators. We build a general framework to explore the topological features of the generalized 1D electronic system, which can be easily applied in various condensed matter systems as well as photonic crystal and cold atomic systems.

We study Berry curvature driven and Zeeman magnetic field dependent electric current responses of two-dimensional electron system with spin-orbit coupling. A nondissipative component of the electric current occurring in the applied in-plane magnetic field is described. This component is transverse to the electric field, odd in the magnetic field, and depends only on one particular direction of the magnetic field defined by the spin-orbit coupling. We show that the effect can be observed in a number of systems with $C_{3v}$ symmetry.

Ultraflat bands that have been theoretically and experimentally detected in a bunch of van der Waals stacked materials show some peculiar properties, for instance, highly localized electronic states and enhanced electron-electron interactions. In this Rapid Communication, using an accurate tight-binding model, we study the formation and evolution of ultraflat bands in transition metal dichalcogenides (TMDCs) under low rotation angles. We find that, unlike in twisted bilayer graphene, ultraflat bands exist in TMDCs for almost any small twist angles and their wave function becomes more localized when the rotation angle decreases. Lattice relaxation, pressure, and local deformation can tune the width of the flat bands, as well as their localization. Furthermore, we investigate the effect of spin-orbit coupling on the flat bands and discover spin/orbital/valley locking at the minimum of the conduction band at the $K$ point of the Brillouin zone. Ultraflat bands found in TMDCs with a range of rotation angle below $7^{\circ }$ may provide an ideal platform to study strongly correlated states.

Materials with the structural motif of a square net have in recent years attracted attention for their often unexpected electronic properties. Here, we present low-temperature magnetotransport experiments on high-quality ${\mathrm{NdTe}}_3$ single crystals with a carrier mobility of $6\times {10}^4$ ${\mathrm{cm}}^2$/V s at 2.25 K; an increase of 58% is observed below the Néel temperature. Shubnikov-de Haas oscillations up to 9 T exhibit anomalies in their amplitude which deviates from the conventional Lifshitz-Kosevich behavior below the magnetic ordering temperature and around 20 K. The latter, which has so far not been observed in any tritelluride, suggests a field- and temperature-induced electronic structure transition. We provide a thorough analysis of three different samples, highlighting the importance of sample quality for elucidating details in the transport behavior. Our results on ${\mathrm{NdTe}}_3$ suggest that rare-earth tritellurides may display so-far obscured exotic electronic properties in addition to the well-studied high-temperature charge density wave anomalies.

We adapted a recently proposed framework to characterize the optical response of interacting electrons in solids in order to expedite its computation without compromise in accuracy at the microscopic level. Our formulation is based on reliable parametrizations of Hamiltonians and Coulomb interactions, which allows economy and flexibility in obtaining response functions. It is suited to computing the optical response to fields of arbitrary temporal shape and strength, to arbitrary order in the field, and natively accounts for excitonic effects. We demonstrate the approach by computing the frequency-dependent susceptibilities of $\mathrm{Mo}{\mathrm{S}}_2$ and hexagonal BN monolayers up to the third harmonic. Grounded on a generic nonequilibrium many-body perturbation theory, this framework allows extensions to handle generic interaction models or to describe electronic processes taking place at ultrafast time scales.

Spherical high-index nanoparticles with low material losses support sharp high-$Q$ electric and magnetic resonances and exhibit a number of interesting optical phenomena. Developments in fabrication techniques have enabled further study of their properties and the investigation of related optical effects. After deposition on a substrate, the optical properties of a particle change dramatically due to mutual interaction. Here, we consider a silicon spherical nanoparticle on a dielectric one-layered substrate. At normal incidence of light, the layer thickness controls the contribution of the nanoparticle's electric and magnetic multipoles to the subsequent optical response. We show that changing the polarization of incident light at a specific excitation angle and layer thickness leads to switching between the multipoles. We further observe a related polarization-driven control over the direction of the scattered radiation.

We consider a collection of two level systems, such as qubits, embedded into a microwave cavity as a promising candidate for the realization of high power quantum batteries. In this perspective, the possibility to design devices where the conventional single-photon coupling is suppressed and the dominant interaction is mediated by two-photon processes is investigated, opening the way to an even further enhancement of the charging performance. By solving a Dicke model with both single- and two-photon coupling we determine the range of parameters where the latter unconventional interaction dominates the dynamics of the system leading to better performances both in the charging times and average charging power of the QB compared to the single-photon case. In addition, the scaling of the maximum stored energy, fluctuations and charging power with the finite number of qubits $N$ is inspected. While the energy and fluctuations scale linearly with $N$, the quadratic growth of the average power leads to a relevant improvement of the charging performance of quantum batteries based on this scheme with respect to the purely single-photon coupling case. Moreover, it is shown that the charging process is progressively faster by increasing the coupling from the weak to the ultra-strong regime.

Realizing individual control on single qubits in a spin-based quantum register is an ever-increasing challenge due to the close proximity of the qubits’ resonance frequencies. Current schemes typically suffer from an inherent trade-off between fidelity and qubits selectivity. Here, we report on a scheme which combines noise protection by dynamical decoupling and magnetic gradient based selectivity, to enhance both the fidelity and the selectivity. With a single nitrogen-vacancy center in diamond, we experimentally demonstrate quantum gates with fidelity $=0.9\pm 0.02$ and a $50-\mathrm{kHz}$ spectral bandwidth, which is almost an order of magnitude narrower than the unprotected bandwidth. Our scheme will enable selective control of an individual nitrogen-vacancy qubit in an interacting qubits array using relatively moderate gradients of about $1\mathrm{mG}/\mathrm{nm}$.

In 2015, experiments were performed with superconducting microwave photonic crystals emulating artificial graphene [B. Dietz et al., Phys. Rev. B 91, 035411 (2015)]. The associated density of states comprises two Dirac points with adjacent bands including van Hove singularities, thus exhibiting the characteristic features originating from the extraordinary electronic band structure of graphene. They are separated by a narrow region of particularly high resonance density corresponding to a nearly flatband in the band structure, which is reminiscent of that of a honome lattice—a combination of two sublattices: honeycomb and kagome. We demonstrate that, indeed, the density of states, and also the eigenmode properties and the fluctuations in the resonance-frequency spectra are well reproduced by a tight-binding model based on the honome lattice. A good description was achieved by means of the reverse Monte Carlo approach, thereby confirming our interpretation of the microwave photonic crystal as an experimental realization of a honome lattice and providing an answer to longstanding problem, namely the understanding of the origin of the flatband bordered by two Dirac points, generally observed in microwave photonic crystals of different shapes.

We study superconductivity in a normal metal, arising from effective electron-electron interactions mediated by spin fluctuations in a neighboring antiferromagnetic insulator. Introducing a frustrating next-nearest neighbor interaction in a Néel antiferromagnet with an uncompensated interface, the superconducting critical temperature is found to be enhanced as the frustration is increased. Further, for sufficiently large next-nearest neighbor interaction, the antiferromagnet is driven into a stripe phase, which can also give rise to attractive electron-electron interactions. For the stripe phase, as previously reported for the Néel phase, the superconducting critical temperature is found to be amplified for an uncompensated interface where the normal metal conduction electrons are coupled to only one of the two sublattices of the magnet. The superconducting critical temperature arising from fluctuations in the stripe phase antiferromagnet can be further enhanced by approaching the transition back to the Néel phase.

We present a theory for quantum impurity relaxometry of magnons in thin films, exhibiting quantitative agreement with recent experiments without needing arbitrary scale factors used in theoretical models thus far. Our theory reveals that chiral coupling between prototypical $\text{spin}>1/2$ quantum impurities and magnons plays a central role in determining impurity relaxation, which is further corroborated by our experiments on nickel films interfaced with nitrogen-vacancy centers. Along with advancing magnonics and understanding decoherence in hybrid quantum platforms with magnets, the ability of a quantum impurity spin to sense chiral magnetic noise presents an opportunity to probe chiral phenomena in condensed matter.

We investigate the effects of dissipation on the quantum dynamics of many-body systems at quantum transitions, especially considering those of the first order. This issue is studied within the paradigmatic one-dimensional quantum Ising model. We analyze the out-of-equilibrium dynamics arising from quenches of the Hamiltonian parameters and dissipative mechanisms modeled by a Lindblad master equation, with either local or global spin operators acting as dissipative operators. Analogously to what happens at continuous quantum transitions, we observe a regime where the system develops a nontrivial dynamic scaling behavior, which is realized when the dissipation parameter $u$ (globally controlling the decay rate of the dissipation within the Lindblad framework) scales as the energy difference $\mathrm{\Delta }$ of the lowest levels of the Hamiltonian, i.e., $u\sim \mathrm{\Delta }$. However, unlike continuous quantum transitions where $\mathrm{\Delta }$ is power-law suppressed, at first-order quantum transitions $\mathrm{\Delta }$ is exponentially suppressed with increasing the system size (provided the boundary conditions do not favor any particular phase).

Magnetic excitations in layered electride ${\mathrm{Y}}_2\mathrm{C}$ have been found by inelastic neutron scattering. We have observed weak but clear magnetic scattering around the wave number $Q=0$, but no magnetic order down to the lowest temperature measured (7 K). The imaginary part of the dynamical susceptibility deduced is well described by the Lorentz function of energy $E$ for each momentum $Q$. The width $\mathrm{\Gamma }\left(Q\right)$ of the Lorentzian is proportional to $Q(Q^2+{\kappa }^2)$ with ${\kappa }^{-1}\sim 4\AA$ at $T=7$ K. We have also found that with increasing $Q$ the magnetic form factor decays faster than that of a $4d$ electron in a single Y atom, which indicates a more extended magnetic moment in ${\mathrm{Y}}_2\mathrm{C}$. These results provide experimental evidence that the itinerant magnetism in ${\mathrm{Y}}_2\mathrm{C}$ originates from the anionic electrons that reside in the interlayers. The Curie-Weiss-like behavior of the magnetic susceptibility reported in ${\mathrm{Y}}_2\mathrm{C}$ is ascribed to the mode coupling effects of spin fluctuations.