Recently, anomalies in the temperature dependences of the carrier density and/or mobility derived from analysis of the magnetoresistivities using the conventional two-band model have been used to unveil intriguing temperature-induced Lifshitz transitions in various materials. For instance, two temperature-driven Lifshitz transitions were inferred to exist in the Dirac nodal-line semimetal ZrSiSe, based on two-band model analysis of the Hall magnetoconductivities where the second band exhibits a change in the carrier type from holes to electrons when the temperature decreases below $T=106\mathrm{K}$ and a dip is observed in the mobility vs temperature curve at $T=80\mathrm{K}$. Here, we revisit the experiments and two-band model analysis on ZrSiSe. We show that the anomalies in the second band may be spurious because the first band dominates the Hall magnetoconductivities at $T>80\mathrm{K}$, making the carrier type and mobility obtained for the second band from the two-band model analysis unreliable. That is, care must be taken in interpreting these anomalies as evidence for temperature-driven Lifshitz transitions. Our skepticism on the existence of such phase transitions in ZrSiSe is further supported by the validation of Kohler's rule for magnetoresistances for $T\le 180\mathrm{K}$. In this paper, we showcase potential issues in interpreting anomalies in the temperature dependence of the carrier density and mobility derived from the analysis of magnetoconductivities or magnetoresistivities using the conventional two-band model.

It is a widely accepted view that the interplay of spin and charge degrees of freedom in doped antiferromagnets (AFMs) gives rise to the rich physics of high-temperature superconductors. Nevertheless, it remains unclear how effective low-energy degrees of freedom and the corresponding field theories emerge from microscopic models, including $t-J$ and Hubbard Hamiltonians. A promising view comprises that the charge carriers have a rich internal parton structure on intermediate scales, but the interplay of the emergent partons with collective magnon excitations of the surrounding AFM remains unexplored. Here we study a doped one-dimensional spin chain in a staggered magnetic field and demonstrate that it supports a zoo of various long-lived excitations. These include magnons, mesonic pairs of spinons and chargons along with their rovibrational excitations, and tetra-parton bound states of mesons and magnons. We identify these types of quasiparticles in various spectra using density-matrix renormalization group simulations. Moreover, we introduce a strong-coupling theory describing the polaronic dressing and molecular binding of mesons to collective magnon excitations. The effective theory can be solved by standard tools developed for polaronic problems and can be extended to study similar physics in two-dimensional doped AFMs in the future. Experimentally, the doped spin-chain in a staggered field can be directly realized in quantum gas microscopes.

In the frame of a nonretarded local density hydrodynamic approach nonlocal effects in the scattering of a fast probe interacting with a metallic interface are studied. In addition to quantal effects in the electron-gas response, effects derived from the interface density profile are taken into account. The relevance in the excitation spectrum of both collective and single electron excitations is studied. For probes at grazing incidence the decay of collective excitations into single electron ones is found. Around the surface plasmon frequency the momentum carried by these excitations allows identifying them as secondary electrons emerging from the target. The recoil associated with them leads to a repulsive force with qualitative agreement with experimental observations. Effects of the momentum transfer in the deflection angle of the beam are also studied.

We revisit the magnetic properties of the antiferromagnetic Kondo lattice CeNiAsO by ${}^{75}\mathrm{As}$ nuclear magnetic resonance measurements. Our results confirm two successive antiferromagnetic transitions of Ce moments at $T_{N1}=9.0\left(3\right)\mathrm{K}$ and $T_{N2}=7.0\left(3\right)\mathrm{K}$. Incommensurate and commensurate antiferromagnetic orders are suggested for $T_{N2}<T<T_{N1}$ and $T<T_{N2}$, respectively, consistent with previous neutron and muon experiments. A Knight shift anomaly, characterized by the failure of $K\left(T\right)\text{−}\chi \left(T\right)$ scaling, is observed below $T^{*}\sim 15\mathrm{K}$, which gives a measure of the onset of coherent $c\text{−}f$ correlations. This energy scale is further confirmed by the spin-lattice relaxation rate ($1/T_1$). The analysis of spin dynamics also reveals a quasi-two-dimensional character of spin fluctuations in CeNiAsO. This work paves the way for further ${}^{75}\mathrm{As}$ nuclear magnetic resonance studies under pressure.

Topological quantum computation relies on a protected degenerate subspace enabling complicated operations in a noise-resilient way. To this end, hybrid platforms based on non-Abelian quasiparticles such as parafermions hold great promise. These are predicted to emerge at the interface between fractional quantum Hall states and superconductors and therefore naturally couple to superconducting qubits. Here, we study a parafermionic fluxonium circuit and show that the presence of topological states yields a striking periodicity in the qubit spectrum. In addition, peculiar and marked signatures of different parafermion coupling, associated with multiple tunneling of fractional quasiparticles, can be detected in the qubit microwave spectrum. Finite parafermion coupling can reduce the full degeneracy of the non-Abelian manifold, and we show that this configuration can be used to assess the remaining degree of protection of the system.

The Fisher-Kolmogorov-Petrovsky-Piskunov (FKPP) equation provides a mean-field theory of out-of-time-ordered commutators in locally interacting quantum chaotic systems at high energy density. In systems with power-law interactions, the corresponding fractional-derivative FKPP equation provides an analogous mean-field theory. However, the fractional FKPP description is potentially subject to strong quantum fluctuation effects, so it is not clear a priori if it provides a suitable effective description for generic chaotic systems with power-law interactions. Here we study this problem using a model of coupled quantum dots with interactions decaying as $1/r^{\alpha }$, where each dot hosts $N$ degrees of freedom. The large-$N$ limit corresponds to the mean-field description, while quantum fluctuations contributing to the OTOC can be modeled by $1/N$ corrections consisting of a cutoff function and noise. Within this framework, we show that the parameters of the effective theory can be chosen to reproduce the butterfly light cone scalings previously found for $N=1$ and generic finite $N$. In order to reproduce these scalings, the fractional index $\mu$ in the FKPP equation needs to be shifted from the naïve value of $\mu =2\alpha -1$ to a renormalized value $\mu =2\alpha -2$. We provide supporting analytic evidence for the cutoff model and numerical confirmation for the full fractional FKPP equation with cutoff and noise.

We report on the magnetic, thermodynamic, and electronic properties of the Gd-doped nearly ferromagnetic Fermi liquid (NFFL) system ${\mathrm{YFe}}_2{\mathrm{Zn}}_{20}$ by means of dc magnetic susceptibility, specific heat, and electron spin resonance (ESR) measurements. As Gd is incorporated (${\mathrm{Y}}_{1-x}{\mathrm{Gd}}_x{\mathrm{Fe}}_2{\mathrm{Zn}}_{20}$; $x$ = 0.005, 0.01, and 0.05), the magnetization measurements present evidence of ferromagnetic correlations with unusual results in temperature-dependent ESR experiments for all samples. Strong evidence of an induced conduction ESR (CESR) is found due to the presence of ${\mathrm{Gd}}^{3+}$ ions for all of the doped samples. The usual local magnetic moment resonance associated with ${\mathrm{Gd}}^{3+}4f$ localized electrons were only present for $x$ = 0.05, coexisting with the CESR. Although ${\mathrm{YFe}}_2{\mathrm{Zn}}_{20}$ is a NFFL, no CESR was detected for the undoped sample, demonstrating the role of ${\mathrm{Gd}}^{3+}$ as a CESR activator. These results are discussed in terms of a highly polarized, enhanced Pauli paramagnetism of $d$-type electrons and a Dyakonov-Perel-like relaxation mechanism involving spin-flip due to magnetic impurities.

Among the numerous two-dimensional (2D) systems that can be prepared via exfoliation, iron phosphorus trisulfide $\left({\mathrm{FePS}}_3\right)$ attracted a lot of attention recently due to its broad-range photoresponse, its unusual Ising-type magnetic order and possible applications in spintronic nanodevices. Despite various experimental and theoretical-computational reports, there are still uncertainties in identifying its magnetic ground state. In this paper, we investigate the structural and magnetic properties of single-layer ${\mathrm{FePS}}_3$ by using density functional theory. Our findings show that orbital ordering leads to a variation in distance between pairs of iron atoms by 0.14 Å. These lattice distortions, albeit small, trigger different (ferromagnetic and antiferromagnetic) exchange couplings so that the ground state consists of ferromagnetically aligned zigzag chains along the long Fe-Fe bonds which couple antiferromagnetically along the shorter Fe-Fe bonds. Within the $\mathrm{DFT}+U$ framework, we parametrize a spin Hamiltonian including Heisenberg, single-ion anisotropy, Dzyaloshinskii-Moriya, and biquadratic interactions. Using $U=2.22$ eV gives a consistent description of both the electronic band gap and the Néel temperature in 2D ${\mathrm{FePS}}_3$.

Two-dimensional (2D) magnetic van der Waals materials provide a fertile platform for the design and control of topological spin textures such as skyrmions. However, despite studies reporting skyrmions in many 2D magnetic systems, the hosting van der Waals materials are still limited. Here, via first-principles calculations and Monte Carlo simulations, we propose a series of 2D magnetic heterostructures as a different materials’ family hosting skyrmions. Unlike previously studied van der Waals heterostructures, a strong interface coupling generates here a significant inversion symmetry breaking, which then results in an inherently large Dzyaloshinskii-Moriya interaction (DMI). Furthermore, upon applying a moderate magnetic field, isolated skyrmions and skyrmions lattices emerge in various types of these heterostructures, and are robust within a relatively wide temperature range. Our work thus suggests a general method to induce DMI and paves the way toward topological magnetism in 2D materials through interfacial engineering.

Non-Hermitian Chern insulators differ from their Hermitian cousins in one key aspect: their edge spectra are incredibly rich and confounding. For example, even in the simple case where the bulk spectrum consists of two bands with Chern number $\pm 1$, the edge spectrum in the slab geometry may have one or two edge states on both edges or only at one of the edges, depending on the model parameters. This blatant violation of the familiar bulk-edge correspondence casts doubt on whether the bulk Chern number can still be a useful topological invariant and demands a working theory that can predict and explain the myriad of edge spectra from the bulk Hamiltonian to restore the bulk-edge correspondence. We outline how such a theory can be set up to yield a thorough understanding of the edge phase diagram based on the notion of the generalized Brillouin zone (GBZ) and the asymptotic properties of block Toeplitz matrices. The procedure is illustrated by solving and comparing three non-Hermitian generalizations of the Qi-Wu-Zhang model, a canonical example of two-band Chern insulators. We find that, surprisingly, in many cases the phase boundaries and the number and location of the edge states can be obtained analytically. Our analysis also reveals a non-Hermitian semimetal phase whose energy-momentum spectrum forms a continuous membrane with the edge modes transversing the hole, or genus, of the membrane. Subtleties in defining the Chern number over GBZ, which in general is not a smooth manifold and may have singularities, are demonstrated using examples. The approach presented here can be generalized to more complicated models of non-Hermitian insulators or semimetals in two or three dimensions.

When being compressed by a single shock, most materials show a maximum compression ratio beyond the ideal-gas limit of four. In most cases, this phenomenon has long been conjectured to originate from the ionization of core electrons, but a clear physical picture is still lacking. Here we present a simple illustration from the energy-band picture of dense plasmas, where the total energy includes only the contributions from ionization of the bound electrons in addition to those from ideal-gas-like free particles. This additional energy term (ionization energy) directly leads to an additional term in the compression ratio $\eta$, which gives a peak on the Hugoniot adiabat. Furthermore, peaks given by different bound states are shown to add linearly ($\eta \approx 4+{\sum }_i\mathrm{\Delta }{\eta }_i$), yielding multipeaked Hugoniots. Quantitative predictions regarding those Hugoniot peaks can be provided systematically by using this minimum model, showing a good agreement with first-principles calculations and experimental data.

We formulate a construction of type-I fracton models based on gauging planar subsystem symmetries of topologically ordered two-dimensional layers that have been stacked in three ambient spatial dimensions. Via our construction, any defect of an Abelian symmetry group in a two-dimensional symmetry-enriched topological order can be promoted to a fracton. This allows us to construct fracton models supporting chiral boundaries and fractons of noninteger quantum dimension. We also find a lineon model supporting non-Abelian surface fractons on its boundary.

We study the electronic transport properties in a tilted $n\text{−}p\text{−}n$ junction of monolayer $1T^{\prime }\text{−}{\mathrm{MoS}}_2$ under a vertical electric field. The analytical derivations and numerical results show that, since the spin degeneracy of the anisotropic bands of monolayer $1T^{\prime }\text{−}{\mathrm{MoS}}_2$ is lifted by the electric field $E_z$, electrically tunable valley- and spin-dependent electron retroreflection occurs in the tilted $n\text{−}p\text{−}n$ junction. It is also found that when the electric field $E_z$ is adjusted to a critical value $E_c$, valley- and spin-dependent Klein tunneling happens in the $n\text{−}p\text{−}n$ junction, and the incident angle of Klein tunneling depends on the tilt angle of the junction. Especially, for a certain specific tilt angle, in the case of normal incidence, only a spin-up electron from the $K$ valley can undergo Klein tunneling. Our work offers an efficient mechanism to modulate valley- and spin-dependent electron retroreflection and Klein tunneling in anisotropic tilted Dirac systems.

Defect-free $\mathrm{Ca}\mathrm{Ti}{\mathrm{O}}_3$ is a band insulator, but angle-resolved photoemission spectroscopy experiments have demonstrated the existence of a nanometer-thin two-dimensional electron system (2DES) at the (001)-oriented surface of this compound. Our ab initio study finds that while oxygen defects are the driving mechanism for the metallicity, tiltings and rotations of the oxygen octahedra significantly impact the electronic structure of the 2DES and its response to externally applied strain deformations. The low-energy conduction subbands have a mixed $t_{2g}\text{−}{e}_g$ orbital character close to the center of the Brillouin zone. Good agreement with the experimental spectra is found for $\mathrm{Ti}{\mathrm{O}}_2$ surface divacancy configurations.

The pillars of quantum theory include entanglement and operators' failure to commute. The Page curve quantifies the bipartite entanglement of a many-body system in a random pure state. This entanglement is known to decrease if one constrains extensive observables that commute with each other (Abelian “charges”). Non-Abelian charges, which fail to commute with each other, are of current interest in quantum thermodynamics. For example, noncommuting charges were shown to reduce entropy-production rates and may enhance finite-size deviations from eigenstate thermalization. Bridging quantum thermodynamics to many-body physics, we quantify the effects of charges' noncommutation—of a symmetry's non-Abelian nature—on Page curves. First, we construct two models that are closely analogous but differ in whether their charges commute. We show analytically and numerically that the noncommuting-charge case has more entanglement. Hence charges' noncommutation can promote entanglement.

Measurements of magnetotransport in SnTe/PbTe heterojunctions grown by the molecular beam epitaxy technique on (001) undoped CdTe substrates were performed. At low magnetic fields, quantum corrections to conductivity were observed that may be attributed to the presence of topological states at the junction interface. For a sample with a 5-nm-thick SnTe layer, the data analysis suggests that midgap states are actually gapped. However, the phase coherence effects in 10 and 20 nm SnTe/PbTe samples are fully explained assuming the existence of gapless Dirac cones. Magnetotransport at higher magnetic fields is described in the framework of mobility spectrum analysis (MSA). We demonstrate that the electron- and holelike peaks observed simultaneously for all SnTe/PbTe heterojunctions may originate from the concave and convex parts of the energy isosurface for topological states—and not from the existence of quasiparticles both carrying negative and positive charges. This interpretation is supported by numerical calculations of conductivity tensor components for gapless (100) Dirac cones, performed within a classical model and based on the solutions of the Boltzmann transport equation. Our approach shows the feasibility of MSA in application to magnetotransport measurements on topological matter.

Within the framework of independent particle approximation, the optical activity tensor of solids is formulated as from different contributions: the magnetic dipole, electric quadrupole, and band dispersion terms. The first two terms have similar counterparts in the theory of finite systems, whereas the last term is unique for crystals. The magnetic dipole and electric quadrupole transition moments are calculated with a sum-over-states formulation. We apply the formulation to calculate and analyze the optical rotation of elemental tellurium and the circular dichroism of the (6,4) carbon nanotube. Decomposed optical activity into different contributions is discussed. The calculated spectra agree well with experiments. As a showcase of achiral crystals, we calculate the optical activity of wurtzite GaN.

When normal metals (NMs) are attached to topological insulators or topological superconductors, it is conceivable that the quantum states in these finite adjacent materials can intermix. In this case—and because the NM usually does not possess the same symmetry as the topological material—it is pertinent to ask whether zero-energy edge states in the topological layer are affected by the presence of the NM. To address this issue, we consider three prototype systems simulated by tight-binding models, namely a Su-Schrieffer-Heeger/NM, a Kitaev/NM, and a Chern insulator/NM. For all junctions investigated, we find that there exist trivial “fine-tuned” zero-energy states in the NM layer that can percolate into the topological region, thus mimicking a topological state. These zero-energy states are created by fine-tuning the NM chemical potential such that some of the NM states cross zero energy; they can occur even when the topological material is in the topologically trivial phase, and exist over a large region of the topological phase diagram. Interestingly, the true Majorana end modes of the Kitaev/NM model cannot be crossed by any NM state, as the NM metal layer in this case does not break particle-hole symmetry. On the other hand, when the chiral symmetry of the Su-Schrieffer-Heeger chain is broken by the attached NM, crossings are allowed. In addition, even in Chern insulators that do not preserve nonspatial symmetries, but the topological edge state self-generates a symmetry eigenvalue, such a fine-tuned zero-energy state can still occur. Our results indicate that when a topological material is attached to a metallic layer, one has to be cautious as to identify true topological edge states merely from their energy spectra and wave function profiles near the interface.

Based on ab initio calculations, we study the electronic structure of the $\mathrm{BiTeI}/\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ heterostructure interface composed of the antiferromagnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and the polar semiconductor trilayer BiTeI. We found a significant difference in the electronic properties of the different contacts between the substrate and overlayer. While the case of a Te-Te interface forms a natural expansion of the substrate, when the Dirac cone state locates mostly in the polar overlayer region and undergoes a slight exchange splitting, the Te-I contact is the source of a four-band state contributed by the substrate Dirac cone and Rashba-type state of the polar trilayer. Owing to magnetic proximity, the pair of Kramers degeneracies for this state is lifted, which produces a Hall response in the transport regime. We believe our findings provide new opportunities to construct novel spintronic devices.