Degrees of freedom like charge, spin, and valley in condensed matter have drawn much attention in the past decades. Recently, several kinds of nodal line semimetals have been discovered for which the conduction and valence bands cross along one-dimensional lines, usually forming loops. These distinct loops, similar to valleys, can be used as a possible carrier of information. Here we investigate the loop polarization during quantum transports, i.e., barrier tunneling as well as disorder scattering, in Hopf semimetals with two disconnected loops. We find that in both cases, the loop polarization is mostly dominated by the performance of normal incidence, due to the mirror symmetry of two loop planes and the finite size effect along one direction. After scattered by nonmagnetic disordered potentials, the opposite polarizations can intrinsically reveal the topological structure of two loops, linked or unlinked. The effects from smooth barriers and disorders are also investigated. We find that a long range smooth barrier leads to an even clearer and purer picture of scattering and loop polarization, due to the suppression of large momentum transfer. These effects can be used as a controllable loop filter and detector.

The combination of strong disorder and interactions in closed quantum systems can lead to many-body localization (MBL). However, this quantum phase is not stable when the system is coupled to a thermal environment. We investigate how MBL is destroyed in systems that are weakly coupled to a dephasive Markovian environment by focusing on their entanglement dynamics. We numerically study the third Rényi negativity $R_3$, a recently proposed entanglement proxy based on the negativity that captures the unbounded logarithmic growth in the closed case and that can be computed efficiently with tensor networks. We also show that the decay of $R_3$ follows a stretched exponential law, similarly to the imbalance, with, however, a smaller stretching exponent.

We report the physical properties of $R{\mathrm{Cd}}_3{\mathrm{As}}_3$ ($R$ = La and Ce) compounds, crystallized into a hexagonal ${\mathrm{ScAl}}_3{\mathrm{C}}_3$-type structure ($P{6}_3$/mmc) such that the $R$ sublattice forms a spin-orbit coupled triangular lattice. Magnetic susceptibility measurements indicate the $4f$ electrons of ${\mathrm{Ce}}^{3+}$ ions are well localized and reveal a large magnetic anisotropy. The electrical resistivity and specific heat measurements for $R{\mathrm{Cd}}_3{\mathrm{As}}_3$ exhibit an anomaly at high temperatures ($T_0\sim$ 63 K for $R$ = La and $T_0\sim$ 136 K for $R$ = Ce), most likely due to a structural transition. Specific heat measurements for ${\mathrm{CeCd}}_3{\mathrm{As}}_3$ clearly indicate a long-range magnetic order below $T_N$ = 0.42 K. Although the magnetic contribution to the specific heat $C_m/T$ increases significantly below $\sim 10$ K, the electrical resistivity for ${\mathrm{CeCd}}_3{\mathrm{As}}_3$ follows the typical, metallic behavior inconsistent with Kondo lattice systems. In ${\mathrm{CeCd}}_3{\mathrm{As}}_3$ only $\sim 40 \%$ of the $Rln\left(2\right)$ magnetic entropy is recovered by $T_N$ and the $R\ln$(2) entropy is fully achieved at about the Curie-Weiss temperature $|{\theta }_p|$. Unusually, based on our current investigations, the magnetic specific heat below $|{\theta }_p|$ is not attributed to a Kondo contribution, but rather associated with the magnetic ordering and frustration on the triangular lattice. Specific heat measurements in an applied magnetic field show a negligible variation of $T_N$ for $H\parallel c$, whereas a suppression of $T_N$ is observed above 40 kOe for $H\parallel ab$. Such behavior is consistent with the application of a magnetic field within the $ab$-plane breaking the triangular symmetry and partially relieving the magnetic frustration in this system.

Here, we report a magnetotransport study of high-quality $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ thin films with high residual resistivity ratios grown by reactive oxide molecular-beam epitaxy. The transverse resistivity exhibits clear anomalies which are typically believed to be signatures of the topological Hall effect and the presence of magnetic skyrmions. By systematically investigating these anomalies as a function of temperature, field, film thickness, and excitation currents we verify that these anomalies originate from a two-channel anomalous Hall effect arising from magnetic inhomogeneities in the samples and not from an intrinsic topological Hall effect. Using a combination of magnetic circular dichroism, scanning transmission electron microscopy, x-ray diffraction, and magnetotransport, we discover that strain relaxation effects in the films are the origin of these inhomogeneities. Our results shed light on the recently reported Hall effect anomalies in $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ thin films and heterostructures and provide an approach to determine whether such anomalies arise from an intrinsic topological Hall effect or from extrinsic mechanisms.

Although it is well accepted that the spin-$1/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) has a long-range ordered ground state, a thorough understanding of its spin dynamics is still missing. While the linear spin wave theory (LSWT) predicts three branches of magnon mode in the magnetic Brillouin zone (MBZ), the $1/S$ expansion at the next order is found to break down in a large portion of the MBZ centered around the $M$ point, leaving the fate of the magnon modes there undecided. Recent neutron scattering measurement on ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$, an ideal realization of the spin-$1/2$ TLHAF, provides a surprising answer to this issue. Two, rather than three branches, of magnon mode are observed around the $M$ point, whose dispersion is strongly renormalized with respect to the LSWT prediction and exhibit pronounced rotonlike minimum at the $M$ point. This is accompanied by a strong spin fluctuation continuum at higher energy, inside which two strong and broad spectral peaks of unknown origin are observed. In this work, we propose a simple picture for these spectral anomalies by invoking the resonating valence bond (RVB) physics in the description of the ground state of the system. We find that the rotonlike minimum in the magnon dispersion can be explained by the coupling between the collective spin fluctuation and the continuum of Dirac spinon excitation moving in a $\pi$-flux background. We also propose that the two broad peaks in the continuum can be understood respectively as the Landau damped third magnon mode and the Landau damped longitudinal mode. Such a picture can be verified by studying the polarization character of the various spectral features.

The study of vacancies in graphene is a topic of growing interest. A single vacancy induces a localized stable charge of order unity interacting with other charges of the conductor through an unscreened Coulomb potential. It also breaks the symmetry between the two triangular graphene sublattices hence inducing zero energy states at the Dirac points. Here we show the fractional and pseudoscalar nature of this vacancy charge. A continuous Dirac model is presented which relates zero modes to vacuum fractional charge and to a parity anomaly. This relation constitutes an index theorem and is achieved by using particular chiral boundary conditions, which map the vacancy problem onto edge state physics. Vacancies in graphene thus allow us to realize prominent features of $2+1$ quantum electrodynamics but without coupling to a gauge field. This essential difference makes vacancy physics relatively easy to implement and an interesting playground for topological state switching.

We propose an exactly solvable model for $j_{\text{eff}}=\frac{3}{2}$ local moments on the honeycomb lattice. Our construction is guided by a symmetry analysis and by the requirement of an exact solution in terms of a Majorana fermion representation for multipole operators. The main interaction in the model can be interpreted as a bond-dependent quadrupole-quadrupole interaction. When time reversal symmetry is explicitly broken, we obtain a gapped spin liquid with a single chiral Majorana edge mode. We also investigate another solvable model in which the time-reversal-invariant spin liquid is coupled to conduction electrons in a superconductor. In the presence of a Kondo-like coupling that involves the octupole moment of the localized spins, the itinerant electrons hybridize with the emergent Majorana fermions in the spin liquid. This leads to spontaneous time reversal symmetry breaking and generates odd-frequency pairing. Our results suggest that $j_{\text{eff}}=\frac{3}{2}$ systems with strong quadrupole-quadrupole interactions may provide a route towards non-Abelian quantum spin liquids and unconventional superconductivity.

An in-depth study of the Fermi surface and electronic structure of the ternary nanolamellar carbide ${\mathrm{Ti}}_3\mathrm{Si}{\mathrm{C}}_2$ is reported. The outputs of angle-resolved photoemission spectroscopy (ARPES) measured on single crystals and density functional theory calculations are systematically compared. The band structure is found to be dominated by Ti $d$ orbitals near the Fermi level. Band inversions near K points are responsible for the intricate shape of the ${\mathrm{Ti}}_3\mathrm{Si}{\mathrm{C}}_2$ pockets. Previous theory found that those pockets were giving opposite contributions leading to zero thermoelectric power in polycrystalline samples. Here we directly visualize them by ARPES, describe their precise shape, and examine the way they should behave regarding thermoelectric power. The presence of linear band crossings and the effect of spin-orbit coupling on the near Fermi level electronic structure are discussed, as well as potential consequences for magnetotransport and spin Hall effect.

Using a multiscale computational approach, we probe the origin and evolution of ultraflat bands in moiré superlattices of twisted bilayer $\mathrm{Mo}{\mathrm{S}}_2$, a prototypical transition metal dichalcogenide. Unlike twisted bilayer graphene, we find no unique magic angles in twisted bilayer $\mathrm{Mo}{\mathrm{S}}_2$ for flat-band formation. Ultraflat bands form at the valence band edge for twist angles ($\theta$) close to $0^{\circ }$ and at both the valence and conduction band edges for $\theta$ close to ${60}^{\circ }$, and have distinct origins. For $\theta$ close to $0^{\circ }$, inhomogeneous hybridization in the reconstructed moiré superlattice is sufficient to explain the formation of flat bands. For $\theta$ close to ${60}^{\circ }$, additionally, local strains cause the formation of modulating triangular potential wells such that electrons and holes are spatially separated. This leads to multiple energy-separated ultraflat bands at the band edges closely resembling eigenfunctions of a quantum particle in an equilateral triangle well. Twisted bilayer transition metal dichalcogenides are thus suitable candidates for the realization of ordered quantum dot array.

Two-dimensional graphene-based van der Waals heterostructures have received considerable interest because of their intriguing characteristics compared with the constituent single-layer two-dimensional materials. Here, we investigate the interfacial characteristics, Schottky contact, and optical performance of $\mathrm{graphene}/{\mathrm{Ga}}_2\mathrm{SSe}$ van der Waals (vdW) heterostructure using first-principles calculations. The effects of stacking patterns, electric gating, and interlayer coupling on the interfacial properties of $\mathrm{graphene}/{\mathrm{Ga}}_2\mathrm{SSe}$ heterostructures are also examined. Our results demonstrate that the Dirac cone of graphene is well preserved at the $\mathrm{\Gamma }$ point in all stacking patterns due to the weak vdW interactions, which keep the heterostructures feasible such that they can be obtained in further experiments. Moreover, depending on the stacking patterns, a small band gap of about 13–17 meV opens in graphene and has a high carrier mobility, indicating that the $\mathrm{graphene}/{\mathrm{Ga}}_2\mathrm{SSe}$ heterostructures are potential candidates for future high-speed nanoelectronic applications. In the ground state, the $\mathrm{graphene}/{\mathrm{Ga}}_2\mathrm{SSe}$ heterostructures form an $n$-type Schottky contact. The transformation from an $n$-type to a $p$-type Schottky contact or to an Ohmic contact can be forced by electric gating or by varying the interlayer coupling. Our findings could provide physical guidance for designing controllable Schottky nanodevices with high electronic and optical performances.

In this study, we present a rutile-related material, $\mathrm{Li}\mathrm{Mo}{\mathrm{O}}_2$, that becomes a cluster magnet and exhibits a spin singlet formation on a preformed molybdenum dimer upon cooling. Unlike ordinary cluster magnets, the atomic $d_{yz}$ orbital robustly survives despite the formation of molecular orbitals, thereby affecting the magnetic properties of the selected material. Such hybrid cluster magnets with the characters of molecular and atomic orbitals realize multiple independent spins on an isolated cluster, leading to an ideal platform to study the isolated spin dimers physics.

Based on the first-principles calculations and theoretical analysis, we investigate the electronic structures, topological phase transition (TPT), and topological properties of the layered magnetic compound ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$. We have synthesized a ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ sample and determined its crystal structure. It has a crystal similar to that of the magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ but has Mn and Sb site mixing. For the ideal case without such site mixing, our calculation indicates ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ is antiferromagnetic (AFM), and there is no band inversion at $\mathrm{\Gamma }$. The band inversion can be realized by increasing the spin-orbit coupling (SOC) of Sb by more than $30\%$, and this results in a TPT from a trivial AFM insulator to an AFM topological insulator or an axion insulator. The compressive uniaxial strain can also drive a similar TPT if the interlayer distance is shortened by more than $5\%$. For the ferromagnetic (FM) case without Mn and Sb site mixing, it is a normal FM insulator. The band inversion can happen when SOC is enhanced by $10\%$ or the interlayer distance is decreased by more than $1\%$. Thus, FM ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ can be tuned to be the simplest type-I Weyl semimetal with only one pair of Weyl nodes on the threefold rotational axis, which is different from the proposal that Mn and Sb site mixing can result in a ferrimagnetic state and a type-II Weyl semimetal state.

A novel Bethe ansatz scheme is proposed to calculate physical properties of quantum integrable systems without $U\left(1\right)$ symmetry. As an example, the antiperiodic XXZ spin chain, a typical correlated many-body system embedded in a topological manifold, is examined. Conserved “momentum” and “charge” operators are constructed despite the absence of translational invariance and $U\left(1\right)$ symmetry. The ground state energy and elementary excitations are derived exactly. It is found that two intrinsic fractional (one half) zero modes accounting for the double degeneracy exist in the eigenstates. The elementary excitations show quite a different picture from that of a periodic chain. This method can be applied to other quantum integrable models either with or without $U\left(1\right)$ symmetry.

Current-driven insulator-metal transitions are in many cases driven by Joule heating proportional to the square of the applied current. Recent nanoimaging experiments in $\mathrm{Ca}{}_2{\mathrm{RuO}}_4$ reveal a metal-insulator phase boundary that depends on the direction of an applied current, suggesting an important nonheating effect. Motivated by these results, we study the effects of an electric current in a system containing interfaces between metallic and insulating phases. Derivation of a heat balance equation from general macroscopic Onsager transport theory reveals a heating term proportional to the product of the current across the interface and the discontinuity in the Seebeck coefficient, so that heat can either be generated or removed at an interface, depending on the direction of the current relative to the change in material properties. For parameters appropriate to $\mathrm{Ca}{}_2{\mathrm{RuO}}_4$, this heating can be comparable to or larger than Joule heating. A simplified model of the relevant experimental geometry is shown to provide results consistent with the experiments. Extension of the results to other inhomogeneous metal-insulator transition systems is discussed.

Frictional forces acting during the relative motion of nanosurfaces are the cause of energy loss and wear which limit an efficient assembly and yield of atomic-scale devices. In this research, we investigate the microscopic origin of the dissipative processes as a result of the frictional response, with the aim to control them in a subtle way. We recast the study of friction in terms of phonon modes of the system at the equilibrium, with no need to resort to dynamics simulations. As a case study, we here consider layer sliding in transition metal dichalcogenides thin films. We find that the population of specific atomic orbitals and the relative contribution of the atomic type to selected system vibrations are the crucial quantities which determine the frictional response in tribological conditions. A reduced amount of energy dissipation is found when the bond character is more ionic and the layer sliding is realized by a faster motion of the chalcogen atoms. The individuated relevant parameters governing the energy dissipation can be used as descriptors in high-throughput calculations or machine learning engines to screen databases of frictional materials. The presented framework is general and can be promptly extended to the design of tribological materials with targeted frictional response, irrespective of the chemistry and atomic topology.

The ability to manipulate the absorption bandwidth has enabled a wide range of applications, from narrowband lasing emission to broadband photodetection. Applying the concept to emerging two-dimensional materials has the potential to enable breakthroughs in advanced compact photonic and optoelectronic devices. Here we present a general method for tailoring the absorption bandwidth of graphene via critical coupling in the near-infrared regime. In a simple two-port resonant structure, dissipative graphene is integrated with a lossless photonic crystal slab, achieving a significant bandwidth manipulation up to 100 times from ultra-narrowband (<0.50 nm) to broadband (>50 nm) with a maximum absorption efficiency of 0.5. The modulation mechanism lies in the intrinsic dependence of bandwidth on the relationship between radiation rate and dissipative loss rate, which is fulfilled by changing the structure parameters and graphene conductivity. This work offers an effective route to engineer light-graphene interactions and shows great prospects in designing high-performance graphene-based devices with enhanced efficiency and flexibility.

We analyze the topological superconductivity and investigate the spectroscopic properties manifested by the zero-energy modes, induced in a metallic strip embedded into a Josephson-type junction. Focusing on the Majorana polarization of such quasiparticles, we propose feasible means for its empirical detection, using the spin-selective Andreev reflection method. Our study reveals a gradual development of a transverse gradient of the Majorana polarization across the metallic strip upon increasing its width. We also inspect the spatial profile and polarization of the Majorana quasiparticles in the presence of a strong electrostatic impurity. We show that, depending on its position, such a defect can lead to a substantial localization of the Majorana mode.

We present a study of the Nernst effect in an iron-based superconductor with a nontrivial band topology ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$. A nonzero Nernst signal is observed in a narrow temperature region around the superconducting transition temperature $T_c$ at a zero field. This anomalous Nernst signal shows symmetric dependence on the external magnetic field and indicates an unconventional vortex contribution in an $s$-wave superconductor with a strong spin-orbit coupling, which is originated from the local magnetic moments of the interstitial Fe atoms. Our experiments also provide the first evidence of a locally broken time-reversal symmetry in bulk ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ single crystals.

The rise and fall of the superconducting transition temperature $T_c$ upon tuning carrier density or external parameters, such as pressure or magnetic field, is ubiquitously observed in a wide range of quantum materials. In order to investigate such domes of $T_c$, we go beyond the prototypical attractive Hubbard model and consider a lattice model of electrons coupled via instantaneous, spatially extended, attractive interactions. By numerically solving the mean-field equations, as well as going beyond mean field theory using a functional renormalization group approach, we find that for a characteristic interaction range $\ell$, there exists a dome in $T_c$ around $k_F\ell \sim \mathcal{O}\left(1\right)$. For multiband systems, our mean field theory shows the presence of additional domes in the vicinity of Lifshitz transitions. Our results hold in both two and three dimensions and can be intuitively understood from the geometric relation between the Fermi surface and the interaction range. Our model may be relevant for domes of $T_c$ in dilute weakly coupled superconductors or in engineered cold atom systems.