Recently, topological aspects of magnon band structure have attracted much interest, and especially, the Dirac magnons in ${\mathrm{Cu}}_3{\mathrm{TeO}}_6$ have been observed experimentally. In this work, we calculate the magnetic exchange interactions $J$'s using the first-principles linear-response approach and find that these $J$'s are short range and negligible for the Cu-Cu atomic pair longer than 7 Å. Moreover, there are only five sizable magnetic exchange interactions, and according to their signs and strengths, modest magnetic frustration is expected. Based on the obtained magnetic exchange couplings, we successfully reproduce the experimental spin-wave dispersions. The calculated neutron scattering cross section also agrees very well with the experiments. We also calculate Dzyaloshinskii-Moriya interactions (DMIs) and estimate the canting angle ($\sim 1.3{}^{\circ }$) of the magnetic noncollinearity based on the competition between DMIs and $J$'s, which is consistent with the experiment. The small canting angle agrees with that current experiments cannot distinguish the DMI-induced nodal line from a Dirac point in the spin-wave spectrum. Finally, we analytically prove that the “sum rule” conjectured in W. Yao et al. [Nat. Phys. 14, 1011 (2018)] holds but only up to the 11th-nearest neighbor.

Identification of the electronic state that appears upon doping a Mott insulator is important to understand the physics of cuprate high-temperature superconductors. Recent scanning tunneling microscopy of cuprates provides evidence that a charge-ordered state emerges before the superconducting state upon doping the parent compound. We study this phenomenon by computing the charge response function of the Hubbard model including frequency-dependent local vertex corrections that satisfy the compressibility sum rule. We find that upon approaching the Mott phase from the overdoped side, the charge fluctuations at wave vectors connecting hot spots are suppressed much faster than at the other wave vectors. It leads to a momentum dependence of the dressed charge susceptibility that is very different from either the bare susceptibility or from the susceptibility obtained from the random phase approximation. We also find that the paramagnetic lightly hole-doped Mott phase at finite temperature is unstable to charge ordering only at zero wave vector, confirming the results previously obtained from the compressibility. Charge order is driven by the frequency-dependent scattering processes that induce an attractive particle-hole interaction at large interaction strength and small doping.

It is known that the low-energy physics of the Josephson effect in the presence of Majorana zero modes exhibits a $4\pi$ periodicity as the Aharonov-Bohm flux varies in contrast to the $2\pi$ Josephson periodicity in usual superconducting junctions. We study this fractional Josephson effect in one-dimensional topological superconductors in Majorana nanowire systems by focusing on the features of the phase-energy relations in a superconducting semiconductor nanowire with spin-orbital coupling by including different factors operational in experimental systems, such as short wire length, suppression of superconducting gap, and the presence of an Andreev bound state. We show that even in the absence of the Majorana zero modes, some nontopological physical effects can manifest a $4\pi$ periodicity of the phase-energy relation in the Josephson junction, thus providing an alternative physics for fractional Josephson effect with no underlying Majorana zero modes. Furthermore, we consider several scenarios of inhomogeneous chemical potential distributions in the superconducting nanowire leading to four Majorana bound states and construct the effective four-Majorana model to correctly describe the low-energy theory of the Josephson effect. In this setup, multiple Majorana zero modes can also have the $4\pi$ fractional Josephson effect, although the underlying physics arises from Andreev bound states since two close-by Majorana bound states effectively form Andreev bound states. Our work demonstrates that the mere observation of a fractional Josephson effect simulating $4\pi$ periodicity might not, by itself, be taken as the definitive evidence for topological superconductivity. This finding has important implications for the ongoing search for non-Abelian Majorana zero modes and efforts for developing topological qubits.

Monolayer graphene and bilayer graphene have strikingly different properties. One such difference is the shape of the Fermi surface. Although anisotropic band structures can be detected in optical measurements, they have so far been difficult to detect in transport experiments on two-dimensional materials. Here we describe a ballistic transport experiment using high-quality graphene that revealed Fermi surface anisotropy in the magnetoresistance. The shape of the Fermi surface is closely related with the cyclotron orbit in real space. Electron trajectories in samples with triangular lattices of holes depend on the anisotropy of the Fermi surface. We found that this results in magnetoresistance that is dependent on crystallographic orientation of the antidot lattice, which indicates the anisotropic Fermi surface of bilayer graphene that is a trigonally warped circle in shape. While in the monolayer, the shape of magnetoresistance was approximately independent of the orientation of antidot lattice, which indicates that the Fermi surface is a circle in shape. The ballistic transport experiment is a different method of detecting anisotropic electronic band structures in two-dimensional electron systems.

We have performed density functional theory calculations of graphene decorated with carbon adatoms, which bind at the bridge site of a C–C bond. Earlier studies have shown that the C adatoms have magnetic moments and have suggested the possibility of ferromagnetism with high Curie temperature. Here we propose to use a gate voltage to fine tune the magnetic moments from zero to $1{\mu }_{\mathrm{B}}$ while changing the magnetic coupling from antiferromagnetism to ferromagnetism and again to antiferromagnetism. These results are rationalized within the Stoner and Ruderman-Kittel-Kasuya-Yosida (RKKY) models. When the SCAN meta-GGA (generalized gradient approximation) correction is used, the magnetic moments for zero gate voltage are reduced and the Stoner band ferromagnetism is slightly weakened in the ferromagnetic region.

A model for two-dimensional electronic, photonic, and mechanical metamaterial systems is presented, which has flat one-dimensional zero-mode energy bands and stable localized states of a topological origin confined within twin boundaries, antiphase boundaries, and at open edges. Topological origins of these flatbands are analyzed for an electronic system as a specific example, using a two-dimensional extension of the Su-Schrieffer-Heeger Hamiltonian with alternating shift of the chains. It is demonstrated that the slow group velocities of the localized flat band states are sensitively controlled by the distance between the boundaries and the propagation can be guided through designed paths of these boundaries. We also discuss how to realize this model in metamaterials.

The ionization potential and the other quasiparticle energies of electrons in molecules or solids are often evaluated within the $GW$ approximation. Though this approximation can be considered as a fantastic tradeoff between accuracy and simplicity, there exist ionization potentials of simple molecules that are unacceptably wrong. We derive here a working approximation to the density matrix that we name the $GW$ density matrix. This relatively light approximation improves several physical properties and, in particular, it yields excellent electronic dipoles for the examined molecules. As a direct consequence, the ionization potentials are affected through the change in the electrostatic potential and in the exchange operator. Benchmarking on a set of 34 molecules, we demonstrate that most of the error in the $GW$ ionization potentials is indeed eliminated thanks to this simple addition. This contribution is not a vertex correction, but nevertheless it is crucial when going beyond the standard $GW$.

We investigate the steady-state Rényi entanglement entropies after a quench from a piecewise homogeneous initial state in integrable models. In the quench protocol, two macroscopically different chains (leads) are joined together at the initial time, and the subsequent dynamics is studied. We study the entropies of a finite subsystem at the interface between the two leads. The density of Rényi entropies coincides with that of the entropies of the generalized Gibbs ensemble that describes the interface between the chains. By combining the generalized hydrodynamics treatment of the quench with the Bethe ansatz approach for the Rényi entropies, we provide exact results for quenches from several initial states in the anisotropic Heisenberg chain (XXZ chain), although the approach is applicable, in principle, to any low-entangled initial state and any integrable model. An interesting protocol that we consider is the expansion quench, in which one of the two leads is prepared in the vacuum of the model excitations. An intriguing feature is that for moderately large anisotropy the transport of bound state is not allowed. Moreover, we show that there is a “critical” anisotropy, below which bound-state transport is permitted. This is reflected in the steady-state entropies, which for large enough anisotropy do not contain information about the bound states. Finally, we benchmark our results against time-dependent density matrix renormalization group simulations.

High-pressure synthesis of denser glass has been a longstanding interest in condensed-matter physics and materials science because of its potentially broad industrial application. Nevertheless, understanding its nature under extreme pressures has yet to be clarified due to experimental and theoretical challenges. Here we reveal the formation of $\mathrm{OS}{\mathrm{i}}_4$ tetraclusters associated with that of $\mathrm{Si}{\mathrm{O}}_7$ polyhedra in $\mathrm{Si}{\mathrm{O}}_2$ glass under ultrahigh pressures to 200 gigapascal confirmed both experimentally and theoretically. Persistent homology analyses with molecular dynamics simulations found increased packing fraction of atoms whose topological diagram at ultrahigh pressures is similar to a pyrite-type crystalline phase, although the formation of tetraclusters is prohibited in the crystalline phase. This critical difference would be caused by the potential structural tolerance in the glass for distortion of oxygen clusters. Furthermore, an expanded electronic band gap demonstrates that chemical bonds survive at ultrahigh pressure. This opens up the synthesis of topologically disordered dense oxide glasses.

Second-order topological insulators (SOTI) exhibit protected gapless boundary states at their hinges or corners. In this paper, we propose a generic means to construct SOTIs in static and Floquet systems by coupling one-dimensional topological insulator wires along a second dimension through dimerized hopping amplitudes. The Hamiltonian of such SOTIs admits a Kronecker sum structure, making it possible for obtaining its features by analyzing two constituent one-dimensional lattice Hamiltonians defined separately in two orthogonal dimensions. The resulting topological corner states do not rely on any delicate spatial symmetries, but are solely protected by the chiral symmetry of the system. We further utilize our idea to construct Floquet SOTIs, whose number of topological corner states is arbitrarily tunable via changing the hopping amplitudes of the system. Finally, we propose to detect the topological invariants of static and Floquet SOTIs constructed with our approach in experiments by measuring the mean chiral displacements of wavepackets.

The search for and understanding of low-dimensional magnetic materials is essential for both fundamental and technological purposes. Here we propose a combined experimental and theoretical investigation of such a system, namely the monoclinic phase of ${\mathrm{SeCuO}}_3$. This low-dimensional spin-$\frac{1}{2}$ antiferromagnet appears to be based on two decoupled magnetic subsystems which respond differently to applied magnetic field in the antiferromagnetic phase. From our results we are able to propose a zero-field magnetic structure as well as a more exotic finite magnetic field structure, to be tested by future experiments. This finding is based on torque magnetometry measurements on the one side, and the use of a refined phenomenological model and state-of-the-art density functional theory calculations on the other. The existence of such systems opens a way to very exciting physics with the possibility to control separately two magnetic subsystems in one material.

Using a set of wave-vector-specific multielement antennas, we have characterized the dispersion of spin waves in an yttrium iron garnet film at submicron lengths and resolved the dispersion relations of multiple backward volume modes, particularly in the region of their minima. The techniques developed now facilitate the characterization of spin waves at length scales limited only by available lithography and at a spectral resolution that generally exceeds that of Brillouin scattering. The data obtained are in excellent agreement with theoretical predictions based on a model Hamiltonian.

Nonreciprocal devices effectively mimic the breaking of time-reversal symmetry for the subspace of dynamical variables that they couple, and can be used to create chiral information processing networks. We study the systematic inclusion of ideal gyrators and circulators into Lagrangian and Hamiltonian descriptions of lumped-element electrical networks. The proposed theory is of wide applicability in general nonreciprocal networks on the quantum regime. We apply it to pedagogical and pathological examples of circuits containing Josephson junctions and ideal nonreciprocal elements described by admittance matrices, and compare it with the more involved treatment of circuits based on nonreciprocal devices characterized by impedance or scattering matrices. Finally, we discuss the dual quantization of circuits containing phase-slip junctions and nonreciprocal devices.

An accurate modeling of a Josephson junction that is embedded in an arbitrary environment is of crucial importance for qubit design. We present a formalism to obtain a Lindblad master equation that describes the evolution of the system. As the qubit degrees of freedom oscillate with a well-defined frequency ${\omega }_q$, the environment only has to be modeled close to this frequency. We show that this goal, differently from alternative approaches, can be achieved by modeling the environment with only a few degrees of freedom. We treat the example of a transmon qubit coupled to a stripline resonator. We derive the parameters of a dissipative single-mode Jaynes-Cummings model starting from first principles. We show that the leading contribution of the off-resonant modes is a correlated decay process involving both the qubit and the resonator mode. In particular, our results show that the effect of the off-resonant modes in the multimode Jaynes-Cummings model is perturbative in $1/{\omega }_q$.

Layered molybdenum dichalchogenides differ from the classic example of bilayer graphene with their unique electronic properties: the application of pressure can continuously tune electronic structure since the band gap is controlled by delicate interlayer interaction. Here, we have performed measurements of Raman scattering, synchrotron x-ray diffraction, electrical conductivity, and Hall coefficient combined with density functional theory calculations to synthetically study the pressure effect on $2H-{\mathrm{MoTe}}_2$. Both the experiments and calculations consistently demonstrate that ${\mathrm{MoTe}}_2$ undergoes a semiconductor-to-metallic (S-M) transition above 10 GPa. Unlike ${\mathrm{MoS}}_2$, the S-M transition is driven by the gradual tunability of electric structure and band gap without structural transition. The applied pressure also effectively enhances conductivity and carrier concentration while reducing the mobility, which makes ${\mathrm{MoTe}}_2$ more suitable for applications than most other transition-metal dichalchogenides and allows it to be applied in strain-modulated optoelectronic devices.

We present measurements and ab initio molecular dynamic simulations of interdiffusion in aluminum-rich Al-Cu melts for concentrations up to 30 at. % copper. To obtain accurate data, a combination of x-ray radiography and shear-cell technique is used, including an experiment in microgravity. Interdiffusion coefficients between 5.3 and $3.2\times {10}^{-9}{\mathrm{m}}^2/\mathrm{s}$ at 973 K are found experimentally with a slight decrease with increasing Cu concentration. Results from ab initio molecular dynamic simulation at 1000 K show a decrease of interdiffusion coefficients with the same slope at slightly higher values. Using Darken's equation to discuss the relation between self-diffusion and interdiffusion, we find the cross-correlation term $S$ to be around unity by simulation and experiment.

We study the temperature-filling phase diagram of the single-band Holstein model in two dimensions using the self-consistent Migdal approximation, where both the electron and phonon self-energies are treated on an equal footing. By employing an efficient numerical algorithm utilizing fast Fourier transforms to evaluate momentum and Matsubara frequency summations, we determine the charge-density-wave (CDW) and superconducting transition temperatures in the thermodynamic limit using lattice sizes that are sufficient to eliminate significant finite-size effects present at lower temperatures. We obtain the temperature-filling phase diagrams for a range of coupling strengths and phonon frequencies for the model defined on a square lattice with and without next-nearest-neighbor hopping. We find the appearance of a superconducting dome with a critical temperature that decreases before reaching the ${\mathbf{q}}_{\text{max}}=(\pi ,\pi )$ CDW phase boundary. For very low phonon frequencies, we also find an incommensurate CDW phase with the ordering vector ${\mathbf{q}}_{\text{max}}\approx (\pi ,\pi )$ appearing between the commensurate CDW and superconducting phases. Our numerical implementation can be easily extended to treat momentum-dependent electron-phonon coupling, as well as dispersive phonon branches, and has been made available to the public.

Self-duality or matching between the magnetic and the condensate coherence lengths is a fundamental property of isotropic superconductors at the critical Bogomolnyi point (B point). The self-dual state of the condensate is infinitely degenerate, which is the core reason for the sharp transition between the superconductivity types in the nearest vicinity of the critical temperature $T_c$. Below $T_c$ nonlocal interactions in the condensate remove the degeneracy, which leads to the appearance of a finite intertype (IT) domain between types I and II. This domain exhibits the mixed state with exotic field-condensate configurations and nonstandard magnetic response, which cannot be understood within the dichotomy of the conventional superconductivity types. At a first glance, this picture does not apply to an anisotropic system because no spatial matching between the condensate and magnetic field can be generally expected for direction-dependent characteristic lengths. However, contrary to these expectations, here we demonstrate that anisotropic superconductors follow the same scenario of the interchange between types I and II. In anisotropic materials the IT domain is governed by the B point of the effective isotropic model obtained by the appropriate scaling transformation of the initial anisotropic formalism. This transformation depends on the direction of the applied magnetic field, and thus the superconductivity type of strongly anisotropic materials can be dependent on this direction.

Ordered states in itinerant magnets may be related to magnetic moments displaying some weak local moment characteristics, as in intermetallic compounds hosting transition metal coordination complexes. In this paper, we report on the Fe $K$-edge x-ray absorption spectroscopy (XAS) of the itinerant magnets $R{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ $(R=\mathrm{Na}, \mathrm{K}, \mathrm{Ca}, \mathrm{Sr}, \mathrm{Ba})$, aiming at exploring the electronic and structural properties of the octahedral building block formed by Fe and the Sb ligands. We find evidence for strong hybridization between the Fe $3d$ and Sb $5p$ states at the Fermi level, giving experimental support to previous electronic structure calculations of the $R{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ skutterudites. The electronic states derived from Fe $3d$–Sb $5p$ mixing are shown to be more occupied and/or less localized in the cases of the magnetically ordered systems, for which $R=\mathrm{Na}$ or K, connecting the local Fe electronic structure to the itinerant magnetic properties. Moreover, the analysis of the extended region of the XAS spectra (EXAFS) suggests that bond disorder may be a more relevant parameter to explain the suppression of the ferromagnetically ordered state in ${\mathrm{CaFe}}_4{\mathrm{Sb}}_{12}$ than the decrease of the density of states.