Enhanced magnetism has recently been reported for the topological-insulator/ferromagnet interface ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/EuS with Curie temperatures claimed to be raised above room temperature from the bulk EuS value of 16 K. Here we investigate the analogous interface ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/EuSe. EuSe is a low-temperature layered ferrimagnet that is particularly sensitive to external perturbations. We find that superconducting quantum interference device (SQUID) magnetometry of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/EuSe heterostructures reveals precisely the magnetic phase diagram known from EuSe, including the ferrimagnetic phase below 5 K, without any apparent changes from the bulk behavior. Choosing a temperature of 10 K to search for magnetic enhancement, we determine an upper limit for a possible magnetic coercive field of 3 mT. Using interface sensitive x-ray absorption spectroscopy we verify the magnetic divalent configuration of the Eu at the interface without contamination by ${\mathrm{Eu}}^{3+}$, and by x-ray magnetic circular dichroism (XMCD) we confirm at the interface the magnetic hysteresis obtained by SQUID. XMCD data obtained at 10 K in a magnetic field of 6 T indicate a magnetic spin moment of $m_{z,\mathrm{spin}}=7{\mu }_B/{\mathrm{Eu}}^{2+}$, in good agreement with the SQUID data and the expected theoretical moment of ${\mathrm{Eu}}^{2+}$. Subsequent XMCD measurements in zero field show, however, that sizable remanent magnetization is absent at the interface for temperatures down to about 10 K.

We study the optical response of bilayer graphene with a kink potential composed of a domain wall separating two AB regions with opposite interlayer biases. The band structure and the local optical conductivity in real space are investigated in detail based on a continuum model. We find that the one-dimensional chiral states localized at the domain wall contribute significantly to the local optical conductivity, which shows a clear distinction in different regions. The effects of domain wall states on optical conductivity can be detected by spatially and frequency-resolved spectroscopic features. From the spectrum at various Fermi energies, important features in the band structure such as the energy separation between two chiral states can be directly measured. When the domain wall region is broad, the spatial distribution of local optical conductivity can provide important information on the bound states as well as the topological domain wall states.

Because of the unique physical properties and potential applications, the exploration of quantum materials with diverse symmetry-protected topological states has attracted considerable interest in the condensed matter community in recent years. Most of the topologically nontirvial materials identified thus far have two-dimensional or three-dimensional structural characteristics, while the quasi-one-dimensional (quasi-1D) analogs are rare. Here, we report on anisotropic magnetoresistance, Hall effect, and quantum de Haas–van Alphen (dHvA) oscillations in $\mathrm{Ta}\mathrm{Pt}{\mathrm{Te}}_5$ single crystals, which possess a layered crystal structure with quasi-1D $\mathrm{Pt}{\mathrm{Te}}_2$ chains. $\mathrm{Ta}\mathrm{Pt}{\mathrm{Te}}_5$ manifests an anisotropic magnetoresistance and a nonlinear Hall effect at low temperatures. The analysis of the dHvA oscillations reveals two major oscillation frequencies (63.5 T and 95.2 T). The corresponding light effective masses and the nonzero Berry phases suggest the nontrivial band topology in $\mathrm{Ta}\mathrm{Pt}{\mathrm{Te}}_5$, which is further corroborated by the first-principles calculations. Our results suggest that $\mathrm{Ta}\mathrm{Pt}{\mathrm{Te}}_5$, in analogy with its sister compounds $\mathrm{Ta}\mathrm{Pd}{\mathrm{Te}}_5$ and $\mathrm{Ta}\mathrm{Ni}{\mathrm{Te}}_5$, is another quasi-1D material hosting topological Dirac fermions.

The extended Bose-Hubbard model with nearest-neighbor and next-nearest-neighbor repulsive interactions on a square lattice is investigated by using the quantum Monte Carlo method. We find that for the cases of weak next-nearest-neighbor interactions and small hoppings, incommensurate solids of fractional densities varying from 1/4 to 1/2 can be stabilized in the thermodynamic limit. We further show that the continuous changes of ordering wave vectors from $(\pi ,\pi /2)$ $[\mathrm{or} (\pi /2,\pi )]$ to $(\pi ,\pi )$ within the incommensurate solid phase can be understood by the mechanism of domain wall formation. The related ground-state phase diagram and thermal phase transitions are also discussed.

We study a quantum quench in which a magnetic impurity is suddenly coupled to Hubbard chains, whose low-energy physics is described by Tomonaga-Luttinger liquid theory. Using the time-dependent density-matrix renormalization-group (tDMRG) technique, we analyze the propagation of charge, spin, and entanglement in the chains after the quench and relate the light-cone velocities to the dispersion of holons and spinons. We find that the local magnetization at the impurity site decays faster if we increase the interaction in the chains, even though the spin velocity decreases. We derive an analytical expression for the relaxation of the impurity magnetization which is in good agreement with the tDMRG results at intermediate timescales, providing valuable insight into the time evolution of the Kondo screening cloud in interacting systems.

Lattice Monte Carlo calculations of interacting systems on nonbipartite lattices exhibit an oscillatory imaginary phase known as the phase or sign problem, even at zero chemical potential. One method to alleviate the sign problem is to analytically continue the integration region of the state variables into the complex plane via holomorphic flow equations. For asymptotically large flow times, the state variables approach manifolds of constant imaginary phase known as Lefschetz thimbles. However, flowing such variables and calculating the ensuing Jacobian is a computationally demanding procedure. In this paper, we demonstrate that neural networks can be trained to parametrize suitable manifolds for this class of sign problem and drastically reduce the computational cost for different severely afflicted small volume systems. In particular, we apply our method to the Hubbard model on the triangle and tetrahedron, both of which are nonbipartite. At strong interaction strengths and modest temperatures, the tetrahedron suffers from a severe sign problem that cannot be overcome with standard reweighting techniques, while it quickly yields to our method. We benchmark our results with exact calculations and comment on future directions of this work.

Sapphire $\left(\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\right)$ has been investigated by the muon spin rotation $\left(\mu \mathrm{SR}\right)$ method in several experiments in the past. The main $\mu \mathrm{SR}$ component is a diamagnetic-like signal with a fast relaxation. Because of this diamagnetic-like behavior, the signal was assigned to either positively charged muonium $\left({\mathrm{Mu}}^{+}\right)$ or negatively charged muonium $\left({\mathrm{Mu}}^{-}\right)$, but neither of the two assignments was satisfactory (the so-called “sapphire puzzle”). We have proposed that the signal is due to a weakly paramagnetic muonium configuration (transition state) which is formed during the reaction of muonium with the host lattice. In the present paper, we report new experimental data on ${\mathrm{Al}}_2{\mathrm{O}}_3$ and discuss these and earlier data in the ${\mathrm{Mu}}^{-}$ and in the transition state model. Calculations based on density functional theory were also performed with detailed findings on the energetics of the different muonium configurations and their migration energies. We conclude that the transition state model is more plausible than the ${\mathrm{Mu}}^{-}$ model, but the ${\mathrm{Mu}}^{-}$ interpretation cannot be excluded completely. In addition, the evidence is presented that the bare muon performs local motion but no long-range diffusion below room temperature in the microsecond time range.

A novel strategy of band structure engineering of van der Waals heterostructure is proposed using a ferroelectric (FE) clamped sandwich structure from first principles. The validity of the strategy is demonstrated in the sandwich structure of ${\mathrm{In}}_2{\mathrm{Se}}_3$/bilayer-${\mathrm{CrI}}_3/{\mathrm{In}}_2{\mathrm{Se}}_3$ (${\mathrm{In}}_2{\mathrm{Se}}_3/\mathrm{bi}-{\mathrm{CrI}}_3/{\mathrm{In}}_2{\mathrm{Se}}_3$) made by ferroelectric ${\mathrm{In}}_2{\mathrm{Se}}_3$ layers and semiconducting bilayer (SB) ${\mathrm{CrI}}_3$. Four states with different band structures in the FE/SB/FE sandwich structure are obtained by switching the FE polarization in the top and bottom ${\mathrm{In}}_2{\mathrm{Se}}_3$ layers. Two of the states possess spin-splitting semiconducting band structures with opposite spin channel in conduction bands which are generated from a spin-degenerated band structure of the ${\mathrm{CrI}}_3$ bilayer, resulting in an electric field controllable and nonvolatile four states spin-field effect transistor. The strategy of using FE layers to engineer band structures and generate spin-splitting semiconducting band structure in van der Waals heterostructure opens a new route in two-dimensional electronics and spintronics.

We study the effect of twisting on bilayer $h\text{−}\mathrm{BN}$. The effect of lattice relaxation is included; we look at the electronic structure, piezoelectric charges, and spontaneous polarization. We show that the electronic structure without lattice relaxation shows a set of extremely flat in-gap states similar to Landau levels, where the spacing scales with twist angle. With lattice relaxation we still have flat bands, but now the spectrum becomes independent of twist angle for sufficiently small angles. We describe in detail the nature of the bands and we study appropriate continuum models, at the same time explaining the structure of the in-gap states. We find that even though the spectra for both parallel and antiparallel alignment are very similar, the spontaneous polarization effects only occur for parallel alignment. We argue that this suggests a large interlayer hopping between boron and nitrogen.

High-harmonic generation (HHG) in solids is an emerging method to probe ultrafast electron dynamics in solids at attosecond time scale. In this work, we study HHG from monolayer and bilayer graphene. Bilayer graphenes with AA and AB stacking are considered in this work. It is found that the monolayer and bilayer graphenes exhibit significantly different harmonic spectra. The difference in the spectra is attributed to the interlayer coupling between the two layers. Also, the intraband and interband contributions to the total harmonic spectrum play a significant role. Moreover, interesting polarization and ellipticity dependence are noticed in total harmonic spectrum for monolayer and bilayer graphene.

We present a methodology based on the Néel model to build a classical spin-lattice Hamiltonian for cubic crystals capable of describing magnetic properties induced by the spin-orbit coupling like magnetocrystalline anisotropy and anisotropic magnetostriction, as well as exchange magnetostriction. Taking advantage of the analytical solutions of the Néel model, we derive theoretical expressions for the parametrization of the exchange integrals and Néel dipole and quadrupole terms that link them to the magnetic properties of the material. This approach allows us to build accurate spin-lattice models with the desired magnetoelastic properties. We also explore a possible way to model the volume dependence of magnetic moment based on the Landau energy. This feature allows us to consider the effects of hydrostatic pressure on the saturation magnetization. We apply this method to develop a spin-lattice model for BCC Fe and FCC Ni, and we show that it accurately reproduces the experimental elastic tensor, magnetocrystalline anisotropy under pressure, anisotropic magnetostrictive coefficients, volume magnetostriction, and saturation magnetization under pressure at zero temperature. This work could constitute a step towards large-scale modeling of magnetoelastic phenomena.

We present investigations of the frequency and magnetic field dependent properties of ordered magnetic nanoparticles (MNPs) arrangements consisting of magnetite (${\mathrm{Fe}}_3{\mathrm{O}}_4$) nanoparticles with an average diameter of 20 nm by employing micro Brillouin light scattering microscopy. We utilized electron beam lithography to prepare hexagonally arranged, circularly shaped MNP assemblies consisting of a single layer of MNPs using a variant of the Langmuir-Blodgett technique. By comparing the results with nonstructured, layered superlattices of MNPs, further insight into the influence of size and geometry of the arrangement on the collective magnetic properties is obtained. We show that at low static external field strengths, two signals occur in frequency dependent measurements for both nonstructured and structured assemblies. Increasing the static external field strength results in a sharpening of the main signal, while the satellite signal decreases in its intensity and increases in its linewidth. The occurrence of multiple signals at low external field strengths is also confirmed by sweeping the static external field and keeping the excitation frequency constant. Furthermore, two-dimensional spatial mapping of the resonances reveals that the main and the satellite signal originate from the center and the edge, respectively, of a single circular MNP assembly. Micromagnetic simulations confirm these assignments and the dependence of the two signals on the static external field strength, justifying an interpretation of the observed characteristics in terms of different local environments of MNPs forming the MNP assembly.

We have fabricated and studied a system of two tunable and coupled nonlinear superconducting resonators. The nonlinearity is introduced by galvanically coupled dc superconducting quantum interference devices. We simulate the system response by means of a circuit model, which includes an additional signal path introduced by the electromagnetic environment. Furthermore, we present two methods allowing us to experimentally determine the nonlinearity. First, we fit the measured frequency and flux dependence of the transmission data to simulations based on the equivalent circuit model. Second, we fit the power dependence of the transmission data to a model that is predicted by the nonlinear equation of motion describing the system. Our results show that we are able to tune the nonlinearity of the resonators by almost two orders of magnitude via an external coil and two on-chip antennas. The studied system represents a basic building block for larger systems, allowing for quantum simulations of bosonic many-body systems with a larger number of lattice sites.

Using small-angle neutron scattering we have studied the superconducting vortex lattice (VL) phase diagram in ${\mathrm{MgB}}_2$ as the applied magnetic field is rotated away from the $c$ axis and towards the basal plane. The field rotation gradually suppresses the intermediate VL phase which exists between end states aligned with two high-symmetry directions in the hexagonal basal plane for $\mathit{H}\parallel \mathit{c}$. Above a critical angle, the intermediate state disappears, and the previously continuous transition becomes discontinuous. The evolution towards the discontinuous transition can be parametrized by a vanishing twelvefold anisotropy term in the VL free energy.

In this work, we perform first-principles calculations, coupled with synchrotron small-angle x-ray scattering (SAXS) to investigate electronic origins of the chemical short-range ordering (CSRO) and to reveal the relationship between CSROs and mechanical properties in a typical bcc (body-centered-cubic) TiZrHfNb high-entropy alloys (HEAs). It was found that existence of an appropriate degree of CSROs not only stabilizes the HEA lattice structure, but also enhances the hardness, elastic modulus, and ideal strength. Comprehensive electronic structure analyses uncover that the reduction of $d$ electrons at the Fermi level due to the development of (Ti, Zr)-based CSROs stabilizes the bcc HEA, and the CSRO-induced strengthening is attributed to the local lattice distortion and the $d$-electron transfer from high-energy to low-energy states under applied strains. This finding not only gives insight into understanding the nature of CSRO strengthening in bcc HEAs, but also provides a paradigm for achieving desired mechanical properties via tailoring CSROs in HEAs.

We present an ab initio study of the thermodynamic properties of cubic $\mathrm{Ca}\mathrm{Si}{\mathrm{O}}_3$ perovskite (CaPv) over the pressure and temperature range of the Earth's lower mantle. We compute the anharmonic phonon dispersions throughout the Brillouin zone by utilizing the phonon quasiparticle approach, which characterizes the intrinsic temperature dependence of phonon frequencies and, in principle, captures full anharmonicity. Such temperature-dependent phonon dispersions are used to calculate ab initio free energy in the thermodynamic limit ($N\to \infty$) within the framework of the phonon gas model. Accurate free energy calculations enable us to investigate cubic CaPv's thermodynamic properties, e.g., thermal expansivity, Grüneisen parameter, bulk modulus, heat capacity and thermal equation of state, where anharmonic effects are demonstrated. The present methodology provides an important theoretical approach for exploring phase boundaries, thermodynamic, and thermoelastic properties of strongly anharmonic materials at high pressures and temperatures.

The interaction of a magnetic insulator with the helical electronic edge of a two-dimensional topological insulator has been shown to lead to many interesting phenomena. One of these is that for a suitable orientation of the magnetic anisotropy axis, the exchange coupling to an easy-plane magnet has no effect on DC electrical transport through a helical edge, despite the fact that it opens a gap in the spectrum of the helical edge [Meng et al., Phys. Rev. B 90, 205403 (2014)]. Here, we theoretically consider such a magnet embedded in an interferometer, consisting of a pair of helical edge states connected by two tunneling contacts, at which electrons can tunnel between the two edges. Using a scattering matrix approach, we show that the presence of the magnet in one of the interferometer arms gives rise to AC currents in response to an applied DC voltage. On the other hand, the DC Aharonov-Bohm effect is absent at zero temperature and small DC voltages, and only appears if the applied voltage or the temperature exceeds the magnet-induced excitation gap.

We calculated the magnetic Compton profiles (MCPs) of Ni using density functional theory supplemented by electronic correlations treated within dynamical mean-field theory (DMFT). We present comparisons between the theoretical and experimental MCPs. The theoretical MCPs were calculated using the Korringa-Kohn-Rostoker method with the perturbative spin-polarized $T$-matrix fluctuation exchange approximation DMFT solver, as well as with the full potential linear augmented plane-wave method with the numerically exact continuous-time quantum Monte Carlo DMFT solver. We show that the total magnetic moment decreases with the intra-atomic Coulomb repulsion $U$, which is also reflected in the corresponding MCPs. The total magnetic moment obtained in experimental measurements can be reproduced by intermediate values of $U$. The spectral function reveals that the minority $X_2$ Fermi-surface pocket shrinks and gets shallower with respect to the density functional theory calculations.