Flat bands in the energy spectrum have attracted a lot of attention in recent years because of their unique properties and promising applications. Special arrangement of impurities on monolayer graphene are proposed to generate multiflat bands in the electronic band structure. In addition to the single midgap states in the spectrum of graphene with low hydrogen density, we found closely spaced bands around the Fermi level with increasing impurity density, which are similar to discrete lines in the spectrum of quantum dots, as well as the unusual Landau-level energy spectrum of graphene in the presence of a strong magnetic field. The presence of flat bands crucially depends on whether or not there are odd or even electrons of H(F) atoms bound to graphene. Interestingly, we found that a fully hydrogenated (fluoridated) of a hexagon of graphene sheet with six hydrogen (fluorine) atoms sitting on top and bottom in consecutive order exhibits Dirac cones in the electronic band structure with a $20\%$ smaller Fermi velocity as compared to the pristine graphene. Functionalizing graphene introduces various C-C bond lengths resulting in nonuniform strains. Such a nonuniform strain may induce a giant pseudomagnetic field in the system, resulting in quantum Hall effect.

We analyze a well-known experimental work [J. Crossno et al., Science 351, 1058 (2016)] which reported on the failure of the Wiedemann-Franz law in graphene at $T\sim 10\text{–}100\mathrm{K}$, attributing this failure to the non-Fermi liquid nature of the Dirac fluid associated with undoped intrinsic graphene. In spite of serious theoretical efforts, the reported observations remain unexplained. Our detailed quantitative analysis based on Fermi liquid considerations, which apply to extrinsic doped graphene, establishes that one possible explanation for the reported observations is the opening of a gap at the Dirac point, induced perhaps by the boron nitride substrate. We suggest that more experiments are necessary to resolve the issue, and we believe that the experiment may not actually have anything to do with Dirac fluid hydrodynamics but relates to finite-temperature low-density bipolar diffusive transport by electrons and holes in the presence of short- and long-range disorder, and phonons.

The negative piezoelectric coefficient in ferroelectric polymers has been widely studied. However, the same in the two-dimensional (2D) materials is scarcely reported, which stresses the need for its thorough investigation. Here, the 2D van der Waals (vdW) heterobilayers (vdWHs) of the dialkali metal monochalcogenide $M_{2}X$ ($M=\mathrm{Na}$, K, Rb, or Cs; and $X=\mathrm{O}$, S, Se, or Te) monolayers show negative out-of-plane piezoelectricity. Out of all the explored heterobilayers of the family, the maximum piezoelectric strain coefficient in the out-of-plane direction $d_{33}=-39\mathrm{pm}{\mathrm{V}}^{-1}$ is found in the ${\mathrm{Na}}_2\mathrm{Te}$/${\mathrm{Cs}}_2\mathrm{S}$ system. This strong piezoelectric coefficient arises from the notable electrostatic potential energy difference and band offset between the constituent monolayers. The small out-of-plane Young's modulus ($Y\sim 11\mathrm{N}{\mathrm{m}}^{-\mathbf{1}}$), originating from the weak vdW interlayer bonding relative to the stronger intralayer bonding, accounts for the anomalous negative piezoelectricity in the heterostructure. The combination of minute out-of-plane ($Y=\sim 11\mathrm{N}{\mathrm{m}}^{-\mathbf{1}}$) and in-plane ($Y=\sim 23\mathrm{N}{\mathrm{m}}^{-\mathbf{1}}$) Young's moduli with small bending modulus (8.83 eV) and negative piezoelectric coefficient underlines the suitability of the proposed vdWH for stretchable and flexible piezotronic devices. Moreover, external perturbations are found to modify the properties of the system. The vertical compressive strain widens the band gap by elevating the conduction band minimum in the heterostructure. An externally applied electric field of low strength ($-0.33$ and $+0.31\mathrm{V}/\AA$) is found to change the semiconducting heterostructure to metallic upon the closure of the band gap, which is attributed to the dropping of nearly free electron gas (NFEG) states to the Fermi level. A significant enhancement in electrical conductivity has been brought up by the NFEG states upon reaching the Fermi level, which inspires the use of the heterostructure in a nonresistive charge carrier transport application in electronics.

Spin superfluids enable long-distance spin transport through classical ferromagnets by developing topologically stable magnetic textures. For small spins at low dimensions, however, the topological protection suffers from strong quantum fluctuations. We study the remanence of spin superfluidity inherited from the classical magnet by considering the two-terminal spin transport through a finite spin-1/2 magnetic chain with planar exchange. By fermionizing the system, we recast the spin-transport problem in terms of quasiparticle transmission through a superconducting region. We show that the topological underpinnings of a semiclassical spin superfluid relate to the topological superconductivity in the fermionic representation. In particular, we find an efficient spin transmission through the magnetic region of a characteristic resonant length, which can be related to the properties of the boundary Majorana zero modes.

The extreme pressures required to stabilize the recently discovered superhydrides represent a major obstacle to their practical application. In this Letter, we propose and substantiate a route to attain high-temperature superconductivity in hydrides at ambient pressure, by doping commercial metal borohydrides. Using first-principles calculations based on density functional theory and Migdal-Eliashberg theory, we demonstrate that in $\mathrm{Ca}{\left({\mathrm{BH}}_4\right)}_2$ a moderate hole doping of 0.03 holes per formula unit, obtained through a partial replacement of Ca with monovalent K, is sufficient to achieve $T_{\text{c}}$'s as high as 110 K. The high $T_{\text{c}}$ arises because of the strong electron-phonon coupling between the B-H $\sigma$ molecular orbitals and bond-stretching phonons. Using a random sampling of large supercells to estimate the local effects of doping, we show that the required doping can be achieved without significant disruption of the electronic structure and at moderate energetic cost. Given the wide commercial availability of metal borohydrides, the ideas presented here can find prompt experimental confirmation. If successful, the synthesis of high-$T_{\text{c}}$ doped borohydrides will represent a formidable advancement towards the technological exploitation of conventional superconductors.

We propose an alternative mechanism for the quantum anomalous Hall (QAH) effect in the $AB$ stacked $\mathrm{Mo}{\mathrm{Te}}_2/\mathrm{W}{\mathrm{Se}}_2$ system. Based on the observation that interlayer tunneling is suppressed in $AB$ stacking, we consider a model with two layers coupled through the Coulomb interaction. The moiré lattices of the two layers are shifted to form a honeycomb lattice. Initially, the system is in a layer-polarized Mott insulator at ${\nu }_T=1$. But with a displacement field, an equal number of holes and electrons are doped into the two layers, forming interlayer exciton condensation. Through mean field theory, we find $p\pm ip$ exciton condensation in a certain parameter regime, which leads to a Chern insulator with Chern number $\text{C}=\pm 1$. The valleys are polarized due to the kinetic energy instead of interaction, but the polarization in the two layers can be either the same or opposite. In particular, an intervalley-coherent (IVC) Chern insulator phase is possible, in agreement with the recent magnetic circular dichroism (MCD) measurement. Our work opens another direction in the search for a topologically nontrivial excitonic insulator with high angular momentum exciton pairing symmetry.

In this work, we show how novel topological effects appear when considering arrangements of increasing complexity of quantum emitters coupled to two-dimensional bosonic topological insulators. For a single emitter coupled to the Haldane model, we find a “fragile” quasibound state that makes the emitter dynamics very sensitive to the model's parameters and gives rise to effective long-range interactions that break time-reversal symmetry. We then discuss one-dimensional arrangements of emitters, emitter line defects, and how the topology of the bath affects the effective polariton models that appear in the weak-coupling regime when the emitters are spectrally tuned to a band gap. In the Harper-Hofstadter model, we link the nonmonotonic character of the effective interactions to the Chern numbers of the surrounding energy bands, while in the Haldane model, we show that the effective models are either gapless or not depending on the topology of the bath. Last, we discuss how the presence of emitters forming an ordered array, an emitter superlattice, can produce polariton models with nontrivial Chern numbers, and also modify the topology of the photonic states in the bath.

We investigate the proximity effect in an $s$-wave superconductor/ferromagnetic metal with a Rashba spin-orbit coupling/diffusive normal metal junction and an $s$-wave superconductor/noncollinear magnetic metal/diffusive normal metal junction. We show the generation of polarized spin-triplet pairings in the diffusive normal metal due to coherent spin rotation in the intermediate magnetic regions. The emergence of the spin-triplet odd-frequency Cooper pairings can generate a zero-energy peak in the quasiparticle density of states in the diffusive normal metal.

We study topological features of a bipartite chain of magnetic spheres embedded in a rectangular metallic waveguide. The coherently coupled chain via the waveguide mode can host two topologically distinct phases identified by the Zak phase. A finite chain supports a pair of doubly degenerate topological edge states within the gap when the intercoupling dominates over the intracoupling, demonstrating that despite the strong coupling to the waveguide mode, the bulk-edge correspondence still holds. The nontrivial (trivial) topological phase is determined by the intracell and intercell coupling of magnetic spheres which depend on the separation, the external magnetic inductions, and the waveguide dimensions. Magnetically tunable topological magnon modes may enable unprecedented topological photonic devices.

The rapidly evolving utilization of spin Hall effect (SHE) arising from spin-orbit coupling in $5d$ transition metals and alloys has made giant strides in the development of designing low-power, robust, and nonvolatile magnetic memory. Recent studies, on incorporating nonmetallic lighter elements such as oxygen, nitrogen, and sulfur into $5d$ transition metals, have shown an enhancement in dampinglike torque efficiency ${\theta }_{\mathrm{DL}}$ due to the modified SHE, but the mechanism behind this enhancement is not clear. In this paper, we study ${\theta }_{\mathrm{DL}}$ at different temperatures (100–293 K) to disentangle the intrinsic and extrinsic side-jump scattering induced SHE in N-implanted Pt. We observe a crossover of intrinsic to extrinsic side-jump mechanism as the implantation dose increases from $2\times {10}^{16}$ to $1\times {10}^{17}\mathrm{ions}/\mathrm{c}{\mathrm{m}}^2$. A sudden decrease in the intrinsic spin Hall conductivity is counterbalanced by the increase in the extrinsic side-jump induced SHE efficiency. These results conclude that studying ${\theta }_{\mathrm{DL}}$ as a function of implantation dose, and as a function of temperature, is important to understand the physical mechanism contributing to SHE.

We focused on the transverse AC magnetoconductance of a high-mobility $p$-GaAs/AlGaAs quantum well $(p=1.2\times {10}^{11}{\mathrm{cm}}^{-2})$ in the vicinity of two values of the Landau-level filling factor $\nu$: $\nu =1$ (integer quantum Hall effect) and $\nu =1/3$ (fractional quantum Hall effect). The complex transverse AC conductance ${\sigma }_{xx}^{\mathrm{AC}}\left(\omega \right)$ was found from simultaneous measurements of attenuation and velocity of surface acoustic waves propagating along the interface between a piezoelectric crystal and the two-dimensional hole system under investigation. We analyzed both the real and imaginary parts of the hole conductance and compared the similarities and differences between the results for filling factor 1 and filling factor 1/3. Both to the left and to the right of these values maxima of a specific shape, “wings,” arose in the $\sigma \left(\nu \right)$ dependences at those two $\nu$. Analysis of the results of our acoustic measurements at different temperatures and surface acoustic wave frequencies allowed us to attribute these wings to the formation of collective localized states, namely, the domains of a pinned Wigner crystal, i.e., a Wigner solid. While the Wigner solid has been observed in 2D hole systems previously, we were able to detect it at the highest hole density and, therefore, the lowest hole-hole interaction reported.

Recent computational studies have successfully predicted the dramatic uptake of hydrogen by metals under pressure leading to the formation of superhydrides, now ubiquitously observed. ${\mathrm{LaH}}_{10}$ exemplifies the properties of these novel H-rich compounds, some of which form a novel class of superconducting materials. We show here another remarkable property for superhydrides, namely, ${\text{H}}^{\delta -}$ superionicity. By means of ab initio molecular dynamics simulations in ${\mathrm{LaH}}_{10}$, an exceptionally high hydride $\left({\mathrm{H}}^{\delta -}\right)$ diffusion coefficient is calculated at high temperature, with $D=1.7\times {10}^{-4}\mathrm{cm} {}^2$/s at 170 GPa and 1500 K, corresponding to an ionic conductivity of $\sigma =0.9{\left(\mathrm{\Omega }\mathrm{cm}\right)}^{-1}$ and so indicating a superionic phase. The superionic phase is surprisingly stable up to 2500 K and its melting temperature is remarkably high, similar to that of pure La. The connected path for the hydride ionic diffusion is disclosed, with the H sublattice keeping its clathrate structure. The conductivity properties of ${\mathrm{LaH}}_{10}$ are discussed in relation to the recently discovered family of compounds showing fast pure hydride ions transport.

It is widely believed that many-body localization in one dimension is fragile and can be easily destroyed by thermal inclusions; however, there are still many open questions regarding the stability of the localized phase and under what conditions it breaks down. Here I construct models with dilute disorder, which interpolate between translationally invariant and fully random models, in order to study the breakdown of localization. This opens up the possibility to controllably increase the density of thermal regions and examine the breakdown of localization as this density is increased. At strong disorder, the numerical results are consistent with commonly used diagnostics for localization even when the concentration of thermalizing regions is high. At moderate disorder, I present evidence for slow dynamics and subdiffusive transport across a large region of the phase diagram, suggestive of a “bad metal” phase. This suggests that dilute disorder may be a useful effective model for studying Griffiths effects in many-body localization, and perhaps also in a wider class of disordered systems.

We experimentally investigate the displacement of oxygen vacancies at high current densities in highly untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ films grown on top of MgO substrates. Transport bridges oriented along the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ crystallographic directions [100] ($a$ axis), [010] ($b$ axis), and [110] $({45}^{\circ }$ from principal axes) reveal that the onset of vacancy migration is mainly determined by the local temperature (or equivalently by the dissipated power) rather than the associated activation energy. Exceeding this threshold value, a clear directional migration proceeds as evidenced by optical microscopy. Concomitant electrotransport measurements show that an intermediate phase, characterized by a decrease in resistivity, precedes long-range migration of vacancies. We numerically demonstrate that this intermediate phase consists of a homogenization of the oxygen distribution along the transport bridge, a phenomenon strongly dependent on the activation energy and the initial degree of disorder. These findings provide some important clues to determine the level of order/disorder in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ films based on electric transport measurements.

A nonperturbative lattice regularization of chiral fermions and bosons with anomaly-free chiral symmetry $G$ in $1+1\mathrm{D}$ spacetime is proposed. More precisely, we ask “whether there is a local short-range quantum Hamiltonian with a finite Hilbert space for a finite system realizing on-site symmetry $G$ defined on a 1D spatial lattice with continuous time, such that its low-energy physics produces a $1+1\mathrm{D}$ anomaly-free chiral matter theory of symmetry $G$?” In particular, we propose that the chiral fermion theory with chiral U(1) $3_L\text{−}{5}_R\text{−}{4}_L\text{−}{0}_R$ symmetry, with two left-moving fermions of charge 3 and 4, and two right-moving fermions of charge 5 and 0 at IR low energy, can emerge from a 1D UV spatial lattice with a chiral U(1) symmetry, if we include properly designed multi-fermion interactions with intermediate strength (i.e., the dimensionless coupling constant is naturally order 1). In general, we propose that any $1+1\mathrm{D}$ U(1)-anomaly-free chiral matter theory can be defined as a finite system on a 1D lattice with on-site symmetry by using a quantum Hamiltonian with continuous time, but without suffering from Nielsen-Ninomiya theorem's fermion doubling, if we include properly designed interactions between matter fields. We propose how to design such interactions by looking for extra anomaly-free symmetries via bosonization/fermionization. We comment on the new ingredients and the differences of ours compared to Ginsparg-Wilson fermion, Eichten-Preskill and Chen-Giedt-Poppitz (CGP) models, and suggest modifying CGP model to have successful mirror decoupling. Since a lattice on-site internal symmetry can be gauged, we thus can further define a nonperturbative regularization of any anomaly-free U(1) chiral gauge theory in $1+1\mathrm{D}$. As an additional remark, we show a topological nonperturbative proof of the equivalence relation between the $G$-symmetric 't Hooft anomaly cancellation conditions and the $G$-symmetry-preserving gapping rules (e.g. Haldane's stability conditions for Luttinger liquid) for multiple U(1) symmetries. We expect that our result holds universally regardless of spatial Hamiltonian or spacetime Lagrangian/path integral formulation of quantum theory. Numerical tests on our proposal are demanding tasks but highly desirable for future work.

Magnetic adatoms coupled to an $s$-wave superconductor give rise to local bound states, so-called Yu-Shiba-Rusinov states. Focusing on the ultimate goal of tailoring chains of such adatoms into a topologically superconducting phase, we investigate basic building blocks—single Fe and Mn adatoms and Mn dimers on clean superconducting Ta(110)—using scanning tunneling microscopy and spectroscopy. We perform a systematic study of the hybridizations and splittings in dimers, and their dependence on the crystallographic directions and interatomic spacings, in order to identify potentially interesting chain geometries for this sample type. Subsequently, we study the spin structure as well as the length dependent Shiba band structure in Mn chains of those geometries using spin-resolved scanning tunneling spectroscopy. All results are compared to the according properties of structurally identical dimers and chains on the previously studied Nb(110), which has almost identical surface structure and electronic properties, but an about three times smaller spin-orbit interaction.

We report on the spin-wave propagation along a magnonic waveguide with a local area of decreased magnetization, which is induced by heating produced with a focused laser spot. A phase-sensitive Brillouin light scattering technique is used to image how the spin wave propagates along the waveguide with a local heat landscape. Frequency-selective signal propagation along the waveguide is demonstrated. Micromagnetic simulations reveal intermodal interference variation in the region after the heated area. The proposed way to reconfigure the magnetization landscape can be used in magnonic devices with frequency-selective spin-wave transport.