We analyze the dislocation content of grain boundary (GB) phase junctions, i.e., line defects separating two different GB phases coexisting on the same GB plane. While regular GB disconnections have been characterized for a variety of interfaces, GB phase junctions formed by GBs with different structures and different numbers of excess atoms have not been previously studied. We apply a general Burgers circuit analysis to calculate the Burgers vectors $\mathbf{b}$ of junctions in two $\mathrm{\Sigma }5$ Cu boundaries previously simulated with molecular dynamics. The Burgers vectors of these junctions cannot be described by the displacement shift complete (DSC) lattice alone. We show that, in general, the normal component of $\mathbf{b}$ is not equal to the difference in the GB excess volumes but contains another contribution from the numbers of GB atoms per unit area $\mathrm{\Delta }{N}^{*}$ required to transform one GB phase into another. In the boundaries studied, the latter component dominates and even changes the sign of $\mathbf{b}$ . We derive expressions for the normal and tangential components of b in terms of the DSC lattice vectors and the non-DSC part due to $\mathrm{\Delta }{N}^{*}$ and additional GB excess properties, including excess volume and shears. These expressions provide a connection between GB phase transformations driven by the GB free energy difference and the motion of GB junctions under applied normal and shear stresses. The proposed analysis quantifies b and therefore makes it possible to calculate the elastic part of the energy of these defects, evaluate their contribution to the nucleation barrier during GB phase transformations, and treat elastic interactions with other defects.

The electric field control of $d$-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic devices. Here, we report an extensive study of the $3d$ magnetism in magnetoelectric ${\mathrm{Ga}}_{0.6}{\mathrm{Fe}}_{1.4}{\mathrm{O}}_3$ (GFO) epitaxial films by polarization-dependent x-ray absorption spectroscopy. We found a nonzero integral of the x-ray magnetic circular dichroism (XMCD), with the sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. This finding translates into a sign reversal between the average Fe magnetic orbital and spin moments. Large Fe displacements, among inequivalent octahedral sites, lower the symmetry of the system producing anisotropic paths for the Fe-O bondings giving rise to a large orbital-lattice interaction akin to a preferential crystallographic direction for the uncompensated, or averaged among different sites, magnetic orbital moment. The latter may lead to a partial reorientation of the magnetic orbital moment under an external magnetic field that, combined with the ferrimagnetic nature of the GFO, can qualitatively explain the observed sign reversal of the XMCD integral. The results suggest that control over the local symmetry of the oxygen octahedra in transition metal oxides can offer a suitable leverage over the manipulation of the effective orbital and spin moments in magnetoelectric systems.

Weyl superconductors feature Weyl points at zero energy in the three-dimensional Brillouin zone and arc states that connect the projections of these Weyl points on the surface. We report that higher-order Weyl superconductors can be realized in odd-parity topological superconductors with time-reversal symmetry being broken by periodic driving. Different from conventional Weyl points, the higher-order Weyl points in the bulk separate 2D first- and second-order topological phases, while on the surface, their projections are connected not only by conventional surface Majorana arcs but also by hinge Majorana arcs. Strikingly, without the protection by a Chern number, the hinge Majorana arcs are anisotropic with respect to surface orientations, forcing a different connectivity of Weyl points for a rotated surface. We identify such anisotropic Weyl-point connectivity as a characteristic feature of higher-order Weyl materials. Moreover, with time-reversal symmetry being broken, the higher-order hinge Majorana arcs are spin polarized, which offers a promising opportunity to observe the anisotropic Weyl-point connectivity with spin-sensitive probes. Besides condensed-matter systems, we provide a feasible experimental setup for realizing our predictions in cold-atom systems.

For high-critical-temperature (high-$T_{\text{c}}$) superconductors, the normal state transport properties at low temperatures are of great importance to understand the Fermi surface structure and carrier profile. In this work we studied the doped ultrathin ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{\mathrm{Fe}}_2{\mathrm{As}}_2$ system, a member of the optimally doped 122-type multigapped iron-based superconductors with complex superconducting gap structure. To explore the normal state resistivity below the zero-field superconducting $T_{\text{c}}$, both Co and Zn dopants were substituted into the superconducting ${\mathrm{Fe}}_2{\mathrm{As}}_2$ layer to suppress the superconductivity. The temperature dependence of in-plane resistivity below $T_{\text{c}}$ reveals a typical metallic behavior with the magnetic fields up to 45 T along $c$ axis. A linear dependence of Hall resistivity $\left({\rho }_{xy}\right)$ is observed for ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{\mathrm{Fe}}_2{\mathrm{As}}_2$ and ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{\left({\mathrm{Fe}}_{0.96}{\mathrm{Zn}}_{0.04}\right)}_2{\mathrm{As}}_2$ at high magnetic field, indicating a conventional metal profile. Nevertheless, the Co dopants lead to a decreasing $d{\rho }_{xy}/dH$ when the field is above 20 T. Such transformation of carrier transport properties may be contributed by band shift within a two-carrier model.

We have measured $R(T,B)$ of a TiN thin film very close to the disorder-driven superconductor-insulator transition but still superconducting at zero field and low temperatures. In a magnetic field we find that three distinct crossing points of the magnetoresistance isotherms occur at magnetic fields $B_{cX}$ in three different temperature regions. Each crossing point in $R(T,B)$ corresponds to a plateau in $R(T,B_{cX})$. We systematically study the evolution of these crossing points near the disorder-induced superconductor/insulator transition, identify the most promising candidate for a quantum phase transition, and provide estimates for the two critical exponents $z$ and $\nu$.

Composite quantum compounds (CQC) are a classic example of quantum materials, which host more than one apparently distinct quantum phenomenon in physics. Magnetism, topological superconductivity, Rashba physics, etc. are a few such quantum phenomenon, which are ubiquitously observed in several functional materials and can coexist in CQCs. In this paper, we use ab initio calculations to predict the coexistence of two incompatible phenomena, namely topologically nontrivial Weyl semimetal and spin-gapless semiconducting (SGS) behavior, in a single crystalline system. SGS belongs to a special class of spintronics material, which exhibits a unique band structure involving a semiconducting state for one spin channel and a gapless state for the other. We report such a SGS behavior in conjunction with the topologically nontrivial multi-Weyl fermions in ${\mathrm{MnPO}}_4$. Interestingly, these Weyl nodes are located very close to the Fermi level with the minimal trivial band density. A drumhead-like surface state originating from a nodal loop around $Y$ point in the Brillouin zone is observed. A large value of the simulated anomalous Hall conductivity (1265 ${\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$) indirectly reflects the topological nontrivial behavior of this compound. Such co-existent quantum phenomena are not common in condensed matter systems and hence it opens up a fertile ground to explore and achieve newer functional materials.

Using angle-resolved photoemission spectroscopy and band structure calculation, we have investigated the three-dimensional nature and the orbital character of the low-energy electronic structure in the layered ternary boride ${\mathrm{AlMn}}_2{\mathrm{B}}_2$, in which ferromagnetic Mn $ab$ layers couple antiferromagnetically along the $c$ axis (Néel temperature ${\mathrm{T}}_{\mathrm{N}}\sim 320\mathrm{K}$). The calculation indicates that electronic bands in the vicinity of the Fermi energy $E_{\mathrm{F}}$ are dominated by the Mn $3d$ orbitals. The $e_g$ orbitals especially contribute high density of states (DOS) with large effective electron masses near $E_{\mathrm{F}}$. The calculated bands are renormalized by a factor of $\sim 1.5$ to match the overall experimental observations, indicating moderate electronic correlation. Considering that the neighbor compound ${\mathrm{AlFe}}_2{\mathrm{B}}_2$, with one more $3d$ electron, is ferromagnetic, the high DOS near $E_{\mathrm{F}}$ from the different $3d$ orbitals should be associated with the different magnetic orders along the special directions in these layered ternary borides. On the other hand, our studies of electronic structure of ${\mathrm{AlMn}}_2{\mathrm{B}}_2$ would further prompt its potential applications as a layered room-temperature antiferromagnetic material.

The study of topological band structures have sparked prominent research interest the past decade, culminating in the recent formulation of rather prolific classification schemes that encapsulate a large fraction of phases and features. Within this context we recently reported on a class of unexplored topological structures that thrive on the concept of subdimensional topology. Although such phases have trivial indicators and band representations when evaluated over the complete Brillouin zone, they have stable or fragile topologies within subdimensional spaces, such as planes or lines. This perspective does not just refine classification pursuits, but can result in observable features in the full dimensional sense. In three spatial dimensions (3D), for example, subdimensional topologies can be characterized by nontrivial planes, having general topological invariants that coexist with Weyl nodes away from these planes. As a result, such phases have 3D stable characteristics such as Weyl nodes, Fermi arcs and edge states that can be systematically predicted by subdimensional analysis. Within this work we further elaborate on these concepts. We present refined representation counting schemes and address distinctive bulk-boundary effects, that include momentum depended (higher order) edge states that have a signature dependence on the perpendicular momentum. As such, we hope that these insights might spur on new activities to further deepen the understanding of these unexplored phases.

We show that Floquet engineering with circularly polarized light (CPL) can selectively split the valley degeneracy of a twisted multilayer graphene (TMG), and thus generate a controlled valley-polarized Floquet Chern flat band with a tunable large Chern number. It offers a feasible optical way to manipulate the valley degree of freedom in moiré flat bands, and hence opens new opportunities to study the valleytronics of moiré flat band systems. We thus expect that many of the valley-related properties of TMG, e.g., orbital ferromagnetism, can be switched by CPL with proper doping. We reveal a Chern number hierarchy rule for the Floquet flat bands in a generic $(M+N)$-layer TMG. We also illustrate that the CPL effects on TMG strongly rely on the stacking chirality, which is a unique feature of TMG. All these phenomena could be tested in the twisted double bilayer graphene systems, which is the simplest example of TMG and has already been realized in experiments.

We describe a controllable and unbiased strong-coupling diagrammatic Monte Carlo technique that is applicable to a wide range of fermionic systems and spin models. Unlike previous strong coupling methods that generally rely on the Grassmannian Hubbard-Stratonovich transformation, our construction is based on Wick's theorem and a recursive procedure to group contractions into effective connected vertices that are nonperturbative in all local physics and can be calculated exactly. The resulting expansion is described by simple diagrammatic rules that make it suitable for systematic treatment via stochastic sampling. Benchmarks against numerical linked cluster expansion display excellent agreement.

Using the Keldysh technique, we derive a set of quasiclassical equations for Bloch electrons in noncentrosymmetric crystals upon excitation with quasimonochromatic radiation in the presence of external electrical and magnetic fields. These equations are the analog to the semiconductor-Bloch equations for the dynamics of electrons including the photogalvanic effect (PGE), in particular the shift mechanism. The shift PGE was recently identified as showing promise for the development of new photovoltaic materials. In addition, our theory may be useful to investigate the interplay between breaking time-reversal symmetry and topological properties as well as the analysis of recent local excitation experiments in nanophotonics. Explicit results for the photogalvanic tensors are presented for linear and circular polarized light and a magnetic field. In addition, we disprove existing statements that the shift-photogalvanic effect does not contribute to the photo-Hall current.

Performing a leading-order renormalization group analysis, here we compute the effects of generic local or short-range electronic interactions in monolayer and the Bernal bilayer graphene. Respectively in these two systems, gapless chiral quasiparticles display linear and biquadratic band touching, leading to linearly vanishing and constant density of states. Consequently, the former system remains stable for weak enough local interactions and supports a variety of ordered phases only beyond a critical strength of interactions. By contrast, ordered phases can nucleate for sufficiently weak interactions in bilayer graphene. By tuning the strength of all symmetry allowed local interactions, we construct various cuts of the phase diagram at zero and finite temperature and chemical doping. Typically, at zero doping, insulating phases (such as charge density wave, antiferromagnet, quantum anomalous, and spin Hall insulators) prevail at the lowest temperature, while gapless nematic or smectic liquids stabilize at higher temperatures. On the other hand, at finite doping, the lowest temperature ordered phase is occupied by a superconductor. Besides anchoring such an organizing principle among the candidate ordered phases, we also establish a selection rule between them and the interaction channel responsible for the breakdown of linear or biquadratic chiral nodal Fermi liquid. In addition, we also demonstrate the role of the normal state band structure in selecting the pattern of symmetry breaking from a soup of preselected incipient competing orders. As a direct consequence of the selection rule, while an antiferromagnetic phase develops in undoped monolayer and bilayer graphene, the linear (biquadratic) band dispersion favors condensation of a spin-singlet nematic (translational symmetry breaking Kekulé) superconductor in doped monolayer (bilayer) graphene, when the on-site Hubbard repulsion dominates in these systems. On the other hand, nearest-neighbor (next-nearest-neighbor) repulsion accommodates charge density wave (quantum spin Hall insulator) and $s+if$ ($s$-wave) pairing at zero and finite chemical doping in both systems, respectively.

The expectation value of the edge Majorana operator is calculated for the one-dimensional model of spinless fermions with nearest-neighbor interactions and with open boundary conditions. The consideration is performed for the regime of the topological insulator (gapped bulk excitations and edge modes inside the gap), and for the normal metal regime (gapless bulk excitations). We show that the expectation value of the edge Majorana operator can be formed by two contributions, from the bulk states, and from the localized edge modes.

Tensor network algorithms have proven to be very powerful tools for studying one- and two-dimensional quantum many-body systems. However, their application to three-dimensional (3D) quantum systems has so far been limited, mostly because the efficient contraction of a 3D tensor network is very challenging. In this paper, we develop and benchmark two contraction approaches for infinite projected entangled-pair states (iPEPS) in 3D. The first approach is based on a contraction of a finite cluster of tensors including an effective environment to approximate the full 3D network. The second approach performs a full contraction of the network by first iteratively contracting layers of the network with a boundary iPEPS, followed by a contraction of the resulting quasi-2D network using the corner transfer matrix renormalization group. Benchmark data for the Heisenberg and Bose-Hubbard models on the cubic lattice show that the algorithms provide competitive results compared to other approaches, making iPEPS a promising tool to study challenging open problems in 3D.

Here we report an ultrafast optical spectroscopic study of the nodal-line semimetal ZrSiTe. Our measurements reveal that, converse to other compounds of the family, the sudden injection of electronic excitations results in a strongly coherent response of an $A_{1g}$ phonon mode that dynamically modifies the distance between Zr and Te atoms and Si layers. “Frozen phonon” density functional theory calculations, in which band structures are calculated as a function of nuclear position along the phonon mode coordinate, show that large displacements along this mode alter the material's electronic structure significantly, forcing bands to approach and even cross the Fermi energy. The incoherent part of the time-domain response reveals that a delayed electronic response at low fluence discontinuously evolves into an instantaneous one for excitation densities larger than $3.43\times {10}^{17}{\mathrm{cm}}^{-3}$. This sudden change of the dissipative channels for electronic excitations is indicative of an ultrafast Lifshitz transition, which we tentatively associate with a change in topology of the Fermi surface driven by a symmetry-preserving $A_{1g}$ phonon mode.

Strain is a powerful experimental tool to explore new electronic states and understand unconventional superconductivity. Here, we investigate the effect of uniaxial strain on the nematic and superconducting phase of single crystal FeSe using magnetotransport measurements. We find that the resistivity response to the strain is strongly temperature dependent and it correlates with the sign change in the Hall coefficient being driven by scattering, coupling with the lattice and multiband phenomena. Band-structure calculations suggest that under strain the electron pockets develop a large in-plane anisotropy as compared with the hole pocket. Magnetotransport studies at low temperatures indicate that the mobility of the dominant carriers increases with tensile strain. Close to the critical temperature, all resistivity curves at constant strain cross in a single point, indicating a universal critical exponent linked to a strain-induced phase transition. Our results indicate that the superconducting state is enhanced under compressive strain and suppressed under tensile strain, in agreement with the trends observed in FeSe thin films and overdoped pnictides, whereas the nematic phase seems to be affected in the opposite way by the uniaxial strain. By comparing the enhanced superconductivity under strain of different systems, our results suggest that strain on its own cannot account for the enhanced high $T_{\mathrm{c}}$ superconductivity of FeSe systems.

We present here the magnetotransport experiment performed with a magnetic field up to 30 T on a 10 nm-wide $p$-type PbTe quantum well. Pronounced Shubnikov–de Haas oscillations and the integer quantum Hall effect are clearly observed, demonstrating the transport of a confined two-dimensional hole gas with negligible parallel conduction at low temperatures. The sequence of filling factors can be explained by considering the magnetic field evolution of Landau levels derived from two longitudinal and one oblique occupied subbands and taking Zeeman spin splitting into account. During illumination, the total carrier density of the system is raised by 15%, which leads to a plateau sequence in the integer quantum Hall effect with higher filling factors in comparison to the dark condition.

Photonic nanostructures can improve the efficiency and functionality of optoelectronic devices by controlling propagation and extraction of light in waveguides. The paradigmatic example is the extraction of light by matching the grating and waveguide mode momenta. A higher functionality can be achieved, however, via strong coupling between waveguide and plasmon modes. Here, we show that such hybridized systems provide a convenient strategy to tailor the dispersion band of the waveguide. By controlling the metallic grating height, the bands evolve from a Dirac cone to a flatband and a parabolic profile. We show an application of this system to light emitting waveguides, where a flatband leads to a sharply defined emission cone, with a 1.8 fold gain in emission efficiency and a Purcell factor of 2.4. We show that 72.2% of all radiated power is contained within the flatband region and is eight times larger than the total emission by the dipole in a homogenous medium. The system thus combines efficiency gain with the opportunity of beam shaping.

The recent discovery of magnetic topological insulators has opened new avenues to explore exotic states of matter that can emerge from the interplay between topological electronic states and magnetic degrees of freedom, be it ordered or strongly fluctuating. Motivated by the effects that the dynamics of the magnetic moments can have on the topological surface states, we investigate the magnetic fluctuations across the $\left({\mathrm{MnBi}}_2{\mathrm{Te}}_4\right){\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_{\mathrm{n}}$ family. Our paramagnetic electron spin resonance experiments reveal contrasting Mn spin dynamics in different compounds, which manifests in a strongly anisotropic Mn spin relaxation in ${\mathrm{MnBi}}_{\text{2}}{\mathrm{Te}}_{\text{4}}$ while being almost isotropic in ${\mathrm{MnBi}}_{\text{4}}{\mathrm{Te}}_{\text{7}}$. Our density-functional calculations explain these striking observations in terms of the sensitivity of the local electronic structure to the Mn spin orientation, and indicate that the anisotropy of the magnetic fluctuations can be controlled by the carrier density, which may directly affect the electronic topological surface states.

We demonstrate the realization of the resonant spin amplification (RSA) effect in Faraday geometry where a magnetic field is applied parallel to the optically induced spin polarization so that no RSA is expected. However, model considerations predict that it can be realized for a central spin interacting with a fluctuating spin environment. As a demonstrator, we choose an ensemble of singly-charged (In,Ga)As/GaAs quantum dots, where the resident electron spins interact with the surrounding nuclear spins. The observation of RSA in Faraday geometry requires intense pump pulses with a high repetition rate and can be enhanced by means of the spin-inertia effect. Potentially, it provides the most direct and reliable tool to measure the longitudinal $g$ factor of the charge carriers.