The ripplocation is a crystallographic defect which is unique to layered materials, combining nanoscale delamination with the crystallographic slip of a basal dislocation. Here, we have studied basal dislocations and ripplocations, in single and multiple van der Waals layers, using analytical and computational techniques. Expressions for the energetic and structural scaling factors of surface ripplocations are derived, which are in close correspondence to the physics of a classical carpet ruck. Our simulations demonstrate that the lowest-energy structure of dislocation pileups in layered materials is the ripplocation, while large dislocation pileups in bulk graphite demonstrate multilayer delamination, curvature, and voids. This can provide a concise explanation for the large volumetric expansion seen in irradiated graphite.

We study the effects of the Zeeman field on the Kane-Mele model with a next-nearest-neighbor Rashba spin-orbit coupling (SOC) and find that two different types of quantum anomalous Hall (QAH) insulators appear and that there is a gap oscillation region with relatively small Rashba SOC in nanoribbon systems. Calculations of the total and spin Chern numbers show that the oscillation appears in the two QAH phases and near the phase transition between a metal and the two QAH insulators. Interestingly, the closing and opening of the energy gap just corresponds to a metal-insulator transition. Therefore, the gap oscillation is also a kind of quantum critical phenomenon, which is caused by the competition between the Zeeman field and the Rashba SOC. Besides, through investigating the Zeeman-field dependence of the bulk gap in real materials, we predict that silicene, as a quantum spin Hall insulator, can become a QAH insulator with a sufficiently large Zeeman field.

We present a detailed study of the Bose-Hubbard model in a $p$-band triangular lattice by focusing on the evolution of orbital order across the superfluid-Mott insulator transition. Two distinct phases are found in the superfluid regime. One of these phases adiabatically connects the weak interacting limit. This phase is characterized by the intertwining of axial $p_{\pm }=p_x\pm i{p}_y$ and in-plane $p_{\theta }=cos\theta p_x+sin\theta p_y$ orbital orders, which break the time-reversal symmetry and lattice symmetries simultaneously. In addition, the calculated Bogoliubov excitation spectrum gaps the original Dirac points in the single-particle spectrum but exhibits emergent Dirac points. The other superfluid phase in close proximity to the Mott insulator with unit boson filling shows a detwined in-plane ferro-orbital order. Finally, an orbital exchange model is constructed for the Mott insulator phase. Its classical ground state has an emergent $\mathrm{S}\mathrm{O}\left(2\right)$ rotational symmetry in the in-plane orbital space and therefore enjoys an infinite degeneracy, which is ultimately lifted by the orbital fluctuation via the order by disorder mechanism. Our systematic analysis suggests that the in-plane ferro-orbital order in the Mott insulator phase agrees with and likely evolves from the latter superfluid phase.

The bulk-edge correspondence in topological phases is extended to systems with the generalized chiral symmetry, where the conventional chiral symmetry is broken. In such systems, we find that the edge state exhibits an unconventional behavior in the presence of the symmetry breaking by the mass, which is explored explicitly in the case of a deformed Su-Schrieffer-Heeger model. The localization length of the edge states diverges at a certain critical mass, where the edge state touches the bulk band. The edge state is specified by an imaginary wave vector that becomes real at the touching energy.

The spectral and dynamical properties of dissipative quantum systems, as modeled by a damped oscillator in the Fock space, are investigated from a topological point of view. Unlike a physical lattice system that is naturally under the open boundary condition, the bounded-from-below nature of the Fock space offers a unique setting for understanding and verifying non-Hermitian skin modes under semi-infinity boundary conditions that are elusive in actual physical lattices. A topological characterization based on the complex spectra of the Liouvillian superoperator is proposed and the associated complete set of topologically protected skin modes can be identified, thus reflecting the complete bulk-boundary correspondence of point-gap topology generally absent in realistic materials. Moreover, we discover anomalous skin modes with exponential amplification even though the quantum system is purely dissipative, a counterintuitive dynamical phenomenon that can be accessed in actual experiments. Our results indicate that current studies of non-Hermitian topological matter can greatly benefit research on quantum open systems and vice versa.

Out-of-equilibrium states in glasses and crystals have been a major topic of research in condensed-matter physics for many years, and the idea of time crystals has triggered a flurry of new research. Here, we provide a description for the recently conjectured time glasses using fractional calculus methods. An exactly solvable effective theory is introduced with a continuous parameter describing the transition from liquid through normal glass and time glass into the Gardner phase. The phenomenological description with a fractional Langevin equation is connected to a microscopic model of a particle in a sub-Ohmic bath in the framework of a generalized Caldeira-Leggett model.

We theoretically investigate the anomalous Hall effect (AHE) that requires neither a net magnetization nor an external magnetic field in collinear antiferromagnets. We show that such an emergent AHE is essentially caused by a ferroic ordering of the anisotropic magnetic dipole (AMD), which provides an effective coupling between ordered magnetic moments and electronic motion in the crystal. We demonstrate that the AMD is naturally induced by the antiferromagnetic ordering, in which the magnetic moments have a quadrupole spatial distribution. In view of the ferroic AMD ordering, we analyze the behavior of the AHE in the orthorhombic lattice system, where the AHE is largely enhanced by the large coupling between the AMD and the spin-orbit interaction. From these findings, the AMD can be used as a descriptor in general to investigate the ferromagnetic-related physical quantities in antiferromagnets including noncollinear ones, which are detectable by using the x-ray magneto-circular dichroism.

The elastic description of planar quasicrystals can be formulated as an interplay between two Goldstone fields corresponding to phonon and phason degrees of freedom. We reformulate this description as a gauge theory with one gauge field that is symmetric under an exchange of indices and one that is not. We also show that topological defects in quasicrystals can be succinctly incorporated in the dual description and interpret them as fractonic excitations. Finally, we calculate the static interaction potential between defects in a quasicrystal with fivefold symmetry. This is done in the limit of a small coupling between phonon and phason stresses.

The random $q$-state quantum Potts model is studied on hypercubic lattices in dimensions 2 and 3 using the numerical implementation of the strong disorder renormalization group introduced by Kovacs and Iglói [Phys. Rev. B 82, 054437 (2010)]. Critical exponents $\nu ,$ $d_f,$ and $\psi$ at the infinite disorder fixed point are estimated by finite-size scaling for several numbers of states $q$ between 2 and 50. When scaling corrections are not taken into account, the estimates of both $d_f$ and $\psi$ systematically increase with $q$. It is shown, however, that $q$-dependent scaling corrections are present and that the exponents are compatible within error bars, or close to each other, when these corrections are taken into account. This provides evidence of the existence of a superuniversality of all two- and three-dimensional random Potts models.

The understanding and modeling of inelastic scattering of thermal phonons at a solid/solid interface remain an open question. We present a fully quantum theoretical scheme to quantify the effect of anharmonic phonon-phonon scattering at an interface via nonequilibrium Green's function (NEGF) formalism. Based on the real-space scattering rate matrix, a decomposition of the interfacial spectral energy exchange is made into contributions from local and nonlocal anharmonic interactions, of which the former is shown to be predominant for high-frequency phonons whereas both are important for low-frequency phonons. The anharmonic decay of interfacial phonon modes is revealed to play a crucial role in bridging the bulk modes across the interface. The overall quantitative contribution of anharmonicity to thermal boundary conductance is found to be moderate. The present work promotes a deeper understanding of heat transport at the interface and an intuitive interpretation of anharmonic phonon NEGF formalism.

We study the effect of discrete symmetries on charge and heat pumping through noninteracting Floquet systems when spatial symmetry (SS) is broken. Particle-hole symmetry (PHS) implies that the pumping of charge (heat) is an odd (even) function of the chemical potential. If PHS is broken, the product of PHS and SS produces even (odd) charge (heat) pumping. Simultaneous breaking of PHS and SS can be due to the coupling to the leads, even if the latter is spatially symmetric. This provides a very simple criterion for reversing (or maintaining) the direction of the flow. We illustrate these results by considering two variants of the Su-Schrieffer-Heeger model under a time-periodic perturbation.

There is a growing family of rare-earth kagome materials with dominant nearest-neighbor interactions and strong spin-orbit coupling. The low symmetry of these materials makes theoretical description complicated, with six distinct nearest-neighbor coupling parameters allowed. In this article, we ask what kinds of classical, ordered, ground states can be expected to occur in these materials, assuming generic (i.e., non-fine-tuned) sets of exchange parameters. We use symmetry analysis to show that there are only five distinct classical ground state phases occurring for generic parameters. The five phases are: (i) a coplanar, twofold degenerate state with vanishing magnetization $\left({\mathsf{A}}_{\mathsf{1}}\right)$, (ii) a noncoplanar, twofold degenerate state with magnetization perpendicular to the kagome plane $\left({\mathsf{A}}_{\mathsf{2}}\right)$, (iii) a coplanar, sixfold degenerate state with magnetization lying within the kagome plane ($\mathsf{E}$-coplanar), (iv) a noncoplanar, sixfold degenerate state with magnetization lying within a mirror plane of the lattice $\left(\mathsf{E}\text{−}{\mathrm{noncoplanar}}_6\right)$, (v) a noncoplanar, twelvefold degenerate state with magnetization in an arbitrary direction $\left(\mathsf{E}\text{−}{\mathrm{noncoplanar}}_{12}\right)$. All five are translation invariant $(\mathbf{q}=0)$ states. Having found the set of possible ground states, the ground state phase diagram is obtained by comparing numerically optimized energies for each possibility as a function of the coupling parameters. The state $\mathsf{E}\text{−}{\mathrm{noncoplanar}}_{12}$ is extremely rare, occupying $<1\%$ of the full phase diagram, so for practical purposes there are four main ordered states likely to occur in anisotropic kagome magnets with dominant nearest neighbor interactions. These results can aid in interpreting recent experiments on “tripod kagome” systems ${\mathrm{R}}_3{\mathrm{A}}_2{\mathrm{Sb}}_3{\mathrm{O}}_{14}$, as well as materials closer to the isotropic limit such as Cr and Fe jarosites.

Recently it was reported experimentally that the monolayer graphene can be heavily overdoped to beyond the Van Hove regime. We study theoretically the possible superconductivity and the corresponding spin excitations of the monolayer graphene in this doping region. A static spin-density-wave state is favorable due to the nested Fermi surface as the Fermi level is doped to the Van Hove singularity point. Superconductivity may be realized upon further doping. The spin excitations in the superconducting state are studied theoretically based on the random phase approximation. The overall features are qualitatively the same with those in cuprate superconductors due to similar Fermi surface topologies of these two families of materials. Thus we have proposed an exciting possibility, namely, the recently realized beyond–Van Hove graphene is a cuprate analog and can become a novel platform to study the unconventional superconductivity.

We study a submonolayer ${}^4\mathrm{He}$ adsorbed on fluorographene (GF) and on hexagonal boron nitride (hBN) at low coverage. The adsorption potentials have been computed ab initio with a suitable density functional theory including dispersion forces. The properties of the adsorbed ${}^4\mathrm{He}$ atoms have been computed at finite temperature with path integral Monte Carlo and at $T=0\mathrm{K}$ with variational path integral. From both methods we find that the lowest energy state of ${}^4\mathrm{He}$ on GF is a superfluid. Due to the very large corrugation of the adsorption potential this superfluid has a very strong spatial anisotropy, the ratio between the largest and smallest areal density being about 6, the superfluid fraction at the lowest $T$ is about 55%, and the temperature of the transition to the normal state is in the range 0.5–1 K. Thus, GF offers a platform for studying the properties of a strongly interacting highly anisotropic bosonic superfluid. At a larger coverage ${}^4\mathrm{He}$ has a transition to an ordered commensurate state with occupation of 1/6 of the adsorption sites. This phase is stable up to a transition temperature located between 0.5 and 1 K. The system has a triangular order similar to that of ${}^4\mathrm{He}$ on graphite but each ${}^4\mathrm{He}$ atom is not confined to a single adsorption site and the atom visits also the nearest neighboring sites giving rise to a novel three-lobed density distribution. The lowest energy state of ${}^4\mathrm{He}$ on hBN is an ordered commensurate state with occupation of 1/3 of the adsorption sites and triangular symmetry. A disordered state is present at lower coverage as a metastable state. In the presence of an electric field the corrugation of the adsorption potential is slightly increased but up to a magnitude of 1 V/Å the effect is small and does not change the stability of the phases of ${}^4\mathrm{He}$ on GF and hBN. We have verified that also in the case of graphene such electric field does not modify the stability of the commensurate $\sqrt{3}\times \sqrt{3}R{30}^{\circ }$ phase.

Quantum dynamics on quasiperiodic geometries has recently gathered significant attention in ultracold atom experiments where nontrivial localized phases have been observed. One such quasiperiodic model is the so-called Fibonacci model. In this tight-binding model, noninteracting particles are subject to on-site energies generated by a Fibonacci sequence. This is known to induce critical states, with a continuously varying dynamical exponent, leading to anomalous transport. In this work, we investigate whether anomalous diffusion present in the noninteracting system survives in the presence of interactions and establish connections to a possible transition towards a localized phase. We investigate the dynamics of the interacting Fibonacci model by studying real-time spread of density-density correlations at infinite temperature using the dynamical typicality approach. We also corroborate our findings by calculating the participation entropy in configuration space and investigating the expectation value of local observables in the diagonal ensemble.

We study the localization-delocalization transition of Floquet eigenstates in a driven fermionic chain with an incommensurate Aubry-André potential and a hopping amplitude which is varied periodically in time. Our analysis shows the presence of a mobility edge separating single-particle delocalized states from localized and multifractal states in the Floquet spectrum. Such a mobility edge does not have any counterpart in the static Aubry-André model and exists for a range of drive frequencies near the critical frequency at which the transition occurs. The presence of the mobility edge is shown to leave a distinct imprint on fermion transport in the driven chain; it also influences the Shannon entropy and the survival probability of the fermions at long times. In addition, we find the presence of CAT (linear superposition of localized single particle states) states in the Floquet spectrum with weights centered around a few nearby sites of the chain. This is shown to be tied to the flattening of Floquet bands over a range of quasienergies. We support our numerical studies with a semianalytic expression for the Floquet Hamiltonian ($H_F$) computed within a Floquet perturbation theory. The eigenspectra of the perturbative $H_F$ so obtained exhibit qualitatively identical properties to the exact eigenstates of $H_F$ obtained numerically. Our results thus constitute an analytic expression of a $H_F$ whose spectrum supports multifractal and cat states. We suggest experiments which can test our theory.

We explore strain-modulated helimagnetism in highly crystalline MnP nanorod films grown on Si(100) substrates using molecular beam epitaxy. The strained MnP film exhibits a paramagnetic to ferromagnetic (FM) phase transition at $T_C\sim 279\mathrm{K}$, and the FM to helical phase transition at $T_N\sim 110\mathrm{K}$. The value of $T_N$ is greater than $T_N\sim 47\mathrm{K}$ for the MnP single crystal, indicating strong strain-modulated helimagnetic states in the MnP nanorod film. The presence of significant thermal hysteresis in the helical phase indicates the coexistence of competing magnetic interactions, leading to the first-order metamagnetic transition. Similar to its single-crystal counterpart, an anisotropic magnetic effect is observed in the MnP film, which is independently confirmed by magnetic hysteresis loop and radio-frequency transverse susceptibility (TS) measurements. The evolution of the screw to the cone and fan phases is precisely tracked from magnetization vs magnetic field/temperature measurements. The temperature dependence of the anisotropy fields, extracted from the TS spectra, yields further insight into the competing nature of the magnetic phases. Unfolding of the different helical phases at $T<120\mathrm{K}$ $(\sim T_{\mathrm{N}})$ is analyzed by the temperature- and field-dependent magnetic entropy change. Based on these findings, the comprehensive magnetic phase diagrams of the MnP nanorod film are constructed for both the in-plane and out of plane magnetic field directions, revealing emergent strain/dimensionality-driven helical magnetic features that are absent in the magnetic phase diagram of the MnP single crystal.

We report detailed thermodynamic studies on high-quality single crystals of the centrosymmetric skyrmion-hosting intermetallic ${\mathrm{Gd}}_2{\mathrm{PdSi}}_3$ by means of high-resolution capacitance dilatometry in fields up to 15 T which are complemented by specific heat and magnetization studies. Our dilatometric measurements show magnetoelastic effects associated with antiferromagnetic order at $T_{\mathrm{N}1}=22.3$ K and $T_{\mathrm{N}2}=19.7$ K, as well as strong field effects in an applied magnetic field of 15 T up to 200 K (150 K) for $B\parallel c$ ($B\parallel a$*, i.e., $B\perp c$). The data allow us to complete the magnetic phase diagram, including a feature at $T^{*}\approx 13\mathrm{K}$ below which an additional degree of freedom becomes relevant. The magnetic B vs T phase diagram for the $a$* axis is also reported. Grüneisen analysis shows the onset of magnetic contributions around 60 K, i.e., well above $T_{\mathrm{N}1}$. Uniaxial pressure dependencies of opposite signs, $-1.3$ and 0.3 K/GPa, are extracted for the out-of-plane and in-plane directions at $T_{\mathrm{N}1}$. For $T^{*}$ we obtain $\partial T^{*}/\partial p_c=1.4$ K/GPa. In particular we elucidate thermodynamic properties of the recently discovered skyrmion lattice phase and show that it is strongly enhanced by uniaxial pressure.

Berg, Metlitski, and Sachdev [Science 338, 1606 (2012)] have shown that the exchange of hidden spin fluctuations by conduction electrons with two orbitals can result in high-temperature superconductivity in copper-oxide materials. We introduce a similar model for high-temperature iron-selenide superconductors that are electron-doped. Conduction electrons carry the minimal $3d_{xz}$ and $3d_{yz}$ iron-atom orbitals. Low-energy hidden spin fluctuations at the checkerboard wave vector ${\mathit{Q}}_{\mathrm{AF}}$ result from nested Fermi surfaces at the center and at the corner of the unfolded (one-iron) Brillouin zone. Magnetic frustration from superexchange interactions via the selenium atoms stabilize hidden spin fluctuations at ${\mathit{Q}}_{\mathrm{AF}}$ versus true spin fluctuations. At half-filling, Eliashberg theory based purely on the exchange of hidden spin fluctuations reveals a Lifshitz transition to electron/hole Fermi-surface pockets at the corner of the folded (two-iron) Brillouin zone, but with vanishing spectral weights. The underlying hidden spin-density-wave ground state is therefore a Mott insulator. Upon electron doping, Eliashberg theory finds that the spectral weights of the hole Fermi-surface pockets remain vanishingly small, while the spectral weights of the larger electron Fermi-surface pockets become appreciable. This prediction is therefore consistent with the observation of electron Fermi-surface pockets alone in electron-doped iron selenide by angle-resolved photoemission spectroscopy (ARPES). Eliashberg theory also finds an instability to $S^{+-}$ superconductivity at electron doping, with isotropic Cooper pairs that alternate in sign between the visible electron Fermi-surface pockets and the faint hole Fermi-surface pockets. Comparison with the isotropic energy gaps observed in electron-doped iron selenide by ARPES and by scanning tunneling microscopy is consistent with short-range hidden magnetic order.