Electron beams in scanning transmission electron microscopes (STEMs) can be used as a tool to induce movement on nanoparticles. Employing a classical-electrodynamics approach, it has been reported that the linear momentum transfer from a STEM-beam electron to a metallic spherical nanoparticle can be either repulsive or attractive towards the swift electron trajectory. This is in qualitative agreement with experimental observations. The interaction time between a swift electron and a nanoparticle is typically on the order of attoseconds. Hence, the electromagnetic response of the nanoparticle at short times is of utmost importance. However, it has been reported that the dielectric function employed in previous studies presented a noncausal pre-echo at the attosecond timescale, which might have led to incorrect unphysical results. Therefore, the validity of these linear momentum transfer results should be revisited. In this theoretical work, we study the noncausality effects on the linear momentum transferred from a swift electron to a metallic nanoparticle, made of either aluminum or gold. Using an efficient numerical methodology, we found that noncausality, as well as deficient numerical convergence, may lead to incorrect repulsive linear momentum transfer results. Contrary to what previous theoretical studies have reported, our results show that the linear momentum transfer from a swift electron to spherical aluminum and gold nanoparticles, with radius 1 nm, is always attractive. Hence a theoretical description of the experimentally observed repulsive interaction is pending.

We apply the density-matrix renormalization group (DMRG) method to a one-dimensional Hubbard model that lacks Umklapp scattering and thus provides an ideal case to study the Mott-Hubbard transition analytically and numerically. The model has a linear dispersion and displays a metal-to-insulator transition when the Hubbard interaction $U$ equals the bandwidth $U_{\mathrm{c}}=W$, where the single-particle gap opens linearly, $\mathrm{\Delta }(U\ge W)=U-W$. The simple nature of the elementary excitations permits us to determine numerically with high accuracy the critical interaction strength and the gap function in the thermodynamic limit. The jump discontinuity of the momentum distribution $n_k$ at the Fermi wave number $k_{\mathrm{F}}=0$ cannot be used to locate accurately $U_{\mathrm{c}}$ from finite-size systems. However, the slope of $n_k$ at the band edges $k_{\mathrm{B}}=\pm \pi$, reveals the formation of a single-particle bound state which can be used to determine $U_{\mathrm{c}}$ reliably from $n_k$ using accurate finite-size data.

An analysis with a transferred hyperfine field has been performed on the spectra of the ${}^{115}\mathrm{In}$ nuclear quadrupole resonance experiments of heavy fermion superconductor ${\mathrm{Ce}}_3{\mathrm{PtIn}}_{11}$ exhibiting co-occurrence of two successive antiferromagnetic orderings ($T_{\mathrm{N}1}$ = 2.2 K and $T_{\mathrm{N}2}$ = 2.0 K) followed by a superconducting transition ($T_{\mathrm{c}}$ = 0.32 K). The spectral changes at magnetic transition temperatures $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$ indicate that the Ce(2) site has the dominant magnetic contribution, with a small magnetic moment, but not negligible, at the Ce(1) site. Our analysis using a transferred hyperfine field which overcomes the previous simple dipolar model evinces the magnetic structures of ${\mathrm{Ce}}_3{\mathrm{PtIn}}_{11}$. $\mathrm{For}{T}_{\mathrm{N}2}<T<T_{\mathrm{N}1}$, the propagation vectors at the Ce(1) and Ce(2) sublattices are ${\mathit{q}}_1={\mathit{q}}_2=(\frac{1}{2},\frac{1}{2},0\text{or}\frac{1}{2})$. On the other hand, for $T<T_{\mathrm{N}2}$, the propagation vectors are ${\mathit{q}}_1=(\frac{1}{2},\frac{1}{2},\frac{1}{6}\text{or}\frac{1}{3})$ and ${\mathit{q}}_2=(\frac{1}{2},\frac{1}{2},\frac{1}{2}\text{or}0)$, respectively.

Molecular-based quasi-two-dimensional charge-ordered Mott insulators can possess both spin $S=1/2$ and an electric dipole degree of freedom residing on each ${\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2$ lattice site. There exists a theoretical proposal for a magnetoelectric effect in such a material, which can manifest itself as a sensitivity of charge distribution on ${\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2$ dimers to the magnetic field. In this work, we used Raman scattering spectroscopy in the magnetic field up to 31 T applied perpendicular to the two-dimensional triangular lattice planes to probe a magnetoelectric effect in $\kappa \text{−}{\left(\mathrm{ET}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Cl}$. This is a model compound, which shows charge order and antiferromagnetic correlations at temperatures below 30 K. We discuss possible reasons for the absence of an observable magnetoelectric effect in the probed temperature and magnetic field ranges.

We investigate the effect of a high-frequency electromagnetic field with both circular and linear polarization on the emergence of quantum phases in thin topological insulators. Simultaneously, the influence of the system parameters (such as the magnetic impurity, thickness engineering, and structural inversion asymmetry of the potential) on the emergence of topological phases is studied. We focus our attention on the high-frequency regime in which it is possible to consider an expansion for the Floquet Hamiltonian in terms of orders of $1/\mathrm{\Omega }$. The topological invariants are determined and it is demonstrated that some phase transitions between the quantum anomalous Hall insulator, quantum pseudospin Hall insulator, quantum spin Hall insulator, and normal insulator can be induced by altering the aforementioned parameters of the system. To avoid the heating process, tuning of the system parameters gives us the opportunity to observe these phase transitions at low intensities of light.

In monolayers of semiconducting transition metal dichalcogenides, the electron-hole exchange interaction splits the exciton dispersion into a massive transverse branch and a longitudinal branch that has very light or even zero mass depending on the form of screened Coulomb interaction. The group velocity of the longitudinal branch is sensitive to the strength of electron-hole exchange, which can be engineered through the dielectric environment. Here we show that dielectric patterning of the substrate can be exploited to realize the waveguide of the exciton in the longitudinal branch in a homogeneous monolayer, leaving the massive transverse branch unaffected. At a lateral interface of different dielectric constants in the substrate, the transmission and reflection of the exciton in the longitudinal branch obey the Snell-Descartes law of optical systems, and total reflection can be exploited to realize an excitonic waveguide using two parallel interfaces. The same dielectric pattern of the substrate appears to be completely transparent for the massive transverse branch exciton, which has no interface scattering. When the monolayer is placed on a one-dimensional dielectric superlattice, the dispersion of the longitudinal branch is strongly renormalized, and the wave functions exhibit one-dimensional features, confined to either the low-dielectric or high-dielectric region. In contrast, the massive transverse branch excitons are not affected by the substrate dielectric pattern, exhibiting pristine properties as in a freestanding monolayer.

Thermal conductance of single molecular junctions at room temperature has been measured recently using picowatt-resolution scanning probes. However, fully understanding thermal transport in a much wider temperature range is needed for the exploration of energy transfer at single-molecular limit and the development of single-molecular devices. Here, employing a semiclassical Langevin molecular dynamics method, a comparative study is performed on the thermal transport of an alkane chain between Au and graphene electrodes, respectively. We illustrate the different roles of quantum statistics and anharmonic interaction in the two types of junctions. For a graphene junction, quantum statistics is essential at room temperature, while the anharmonic interaction is negligible. For a Au junction, it is the other way. Our study paves the way for theoretically understanding thermal transport of realistic single-molecular junctions in the full temperature range by including both quantum statistics and anharmonic interaction within one theoretical framework.

To explore the behaviors of valley degrees of freedom, we built a breathing kagome-honeycomb (BKH) lattice composed of a breathing kagome sublattice (BKL) and a breathing honeycomb sublattice (BHL). Many metal-organic frameworks (MOFs) without space-inversion symmetry (SIS) can be modeled as BKH lattices. The electronic states and the evolution of Dirac cones of the constructed BKH lattice under magnetic exchange and spin-orbit coupling interactions were investigated based on a tight-binding model and first-principles calculations. Coexistence states of anomalous valley Hall (AVH) and quantum anomalous Hall (QAH) effects, namely, the multiple Hall effects, were achieved in the BKH lattice with $p_z$ orbitals located at the BKL and $p_x$, $p_y$ (and also $d_{xy}$, $d_{x^2-y^2}$) orbitals located at the BHL. The properties of the coexisting state, including the valley polarized status (at $K$ or $K^{\prime }$) and the chirality of the QAH effect, were determined by the overlapping styles of the bands from the two sublattices. An MOF of ${\mathrm{C}}_6{\mathrm{N}}_3{\mathrm{H}}_3\mathrm{Au}$ with a BKH lattice is proposed. Given the coexistence of the AVH and QAH effects in this built MOF, the metallic edge state protected by the topology of the MOF is 100% valley polarized, different from the ordinary QAH effect. Our results provide insights into the electronic states of the MOFs without SIS and a material platform with potential applications in not only valleytronics but also electronics and spintronics.

Magnetic impurities create in-gap states on superconductors. Recent experiments explore the topological properties of one-dimensional arrays of magnetic impurities on superconductors, because in certain regimes $p$-wave pairing can be locally induced leading to new topological phases. A byproduct of the new accessible phases is the appearance of zero-energy edge states that have non-Abelian exchange properties and can be used for topological quantum computation. Despite the large amount of theory devoted to these systems, most treatments use approximations that render their applicability limited when comparing with usual experiments of 1D impurity arrays on wide-band superconductors. These approximations either involve tight-binding-like approximations where the impurity energy scales match the minute energy scale of the superconducting gap and are many times unrealistic, or they assume strongly-bound in-gap states. Here, we use a theory for $s$-wave superconductors based on a wide-band normal metal, with any possible energy scale for the magnetic impurities and develop an efficient way of computing the well-known topological invariants of infinite chains. We perform concrete calculations on ferromagnetic spin chains using BCS Green's function for the superconductor and including Rashba coupling to compare with recent experimental results. The infinite-chain properties can be analytically obtained, giving us a way to compare with finite-chain calculations. We show that it is possible to converge to the infinite limit by doing finite-size numerical calculation, paving the way for numerical calculations not based on analytical Green's functions. As an application, we show that energy oscillations around zero with increasing number of atoms in the spin chain does not reflect the topological origin of the low-energy state.

In the realm of two-dimensional materials, magnetic and transport properties of a unique representative ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ attract ever increasing attention. Here, we use a developed first-principles method for calculating laser-induced response to study the emergence of photoinduced currents of charge and spin in single-layer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$, which are of second order in the electric field. We provide a symmetry analysis of the emergent photocurrents in the system, finding it to be in excellent agreement with ab initio calculations. We analyze the magnitude and behavior of the charge photocurrents with respect to disorder strength, frequency, and band filling. Remarkably, not only do we find a large charge current response, but also predict that ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ can serve as a source of significant laser-induced spin currents, which makes this material as a promising platform for various applications in optospintronics.

We study the avalanche dynamics of shear-induced ferroelasticity by molecular dynamics simulations and statistical analysis. The dynamics of ferroelastic domain switching proceeds by avalanches which are power-law distributed. These avalanches can therefore be classified as wild with an energy exponent near 3. Wildness originates from the interaction between domain boundaries and defects, and jamming between domain boundaries. Concomitantly, mild events also arise but their distributions do not follow power-laws so that these mild energy releases are not scale invariant and exhibit a characteristic energy. We identify several mild domain switching events, namely the motion of single kinks and highly nonlinear relaxations of solitonic waves. The solitonic waves are reflected by domain boundaries, kinks, junctions, and free surfaces. Relaxations during domain switching have different characteristic energies from those created during creep. We observe the coexistence of mild and wild events depending on the external forces acting on the ferroelastic material.

We consider the random-field Heisenberg model, a paradigmatic model for many-body localization (MBL), and add a Markovian dephasing bath coupled to the Anderson orbitals of the model's noninteracting limit. We map this system to a classical facilitated hopping model that is computationally tractable for large system sizes and investigate its dynamics. The classical model exhibits a robust crossover between an ergodic (thermal) phase and a frozen (localized) phase. The frozen phase is destabilized by thermal subregions (bubbles), which thermalize surrounding sites by providing a fluctuating interaction energy and so enable off-resonance particle transport. Investigating steady-state transport, we observe that the interplay between thermal and frozen bubbles leads to a clear transition between diffusive and subdiffusive regimes. This phenomenology both describes the MBL system coupled to a bath and provides a classical analog for the MBL transition in the corresponding quantum model, in that the classical model displays long local memory times. It also highlights the importance of the details of the bath coupling in studies of MBL systems coupled to thermal environments.

Crystalline ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is one of the most explored three-dimensional (3D) topological insulators with a $0.3\mathrm{eV}$ energy gap making it promising for applications. Its amorphous counterpart could bring to light new possibilities for large scale synthesis and applications. Using ab initio molecular dynamics simulations, we have studied realistic amorphous ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ phases generated by different processes of melting, quenching, and annealing. Extensive structural and electronic characterizations show that the melting process induces an energy gap decrease ruled by growth of the defective local environments. This behavior dictates a weak stability of the topological phase to disorder, characterized by the spin Bott index. Interestingly, we identify the occurrence of topologically trivial surface states in amorphous ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ that show a strong resemblance to standard helical topological states. Our results and methods advance the search of topological phases in 3D amorphous solids.

We present the electron spin resonance study of the influence of an electric field on the low-field multiferroic magnetic state in $\mathrm{Li}\mathrm{Cu}{\mathrm{VO}}_4$. The shift of the magnetic resonance spectra in the electric field has been observed experimentally. Symmetry analysis has been performed in order to describe the static properties of the magnetic system. The low-frequency dynamics of $\mathrm{Li}\mathrm{Cu}{\mathrm{VO}}_4$ in magnetic and electric fields was considered in the framework of the hydrodynamic approach. It was shown that the application of the external electric field leads to the change of the configuration of the magnetic system before and after the spin-flop reorientation. A satisfactory agreement was obtained between the results of experimental studies and theoretical considerations.

We investigate the coupled dynamics of quantized vortices and normal fluid in superfluid ${}^4\mathrm{He}$ at finite temperatures using a numerical approach based on the vortex filament model and lattice Boltzmann method (LBM). The LBM allows us to simulate a fluid flow with only local operations, i.e., rather than solving the Navier–Stoles (NS) equations directly, a fluid flow is considered a convection of mesoscopic particles between sites on a lattice grid. Although the two-fluid nature of He II makes its flow complex, the particlelike treatment of the normal fluid in the LBM significantly reduces the complexity. We confirm, by comparing to results obtained with direct NS simulations, that the proposed numerical approach reproduces characteristic flow structures. We also demonstrate that the proposed computational approach is suitable for a thermal counterflow simulation with a solid boundary to elucidate a thermal boundary layer near a heater in a closed channel.

Spin-orbit torque enables the electrical control of the orientation of ferromagnets' or antiferromagnets' order parameter. In this work we consider antiferromagnets in which the magnetic sublattices are connected by $\text{inversion}+\text{time-reversal}$ symmetry, and in which the exchange and anisotropy energies are similar in magnitude. We identify the staggered dampinglike spin-orbit torque as the key mechanism for electrical excitation of the Néel vector for this case. To illustrate this scenario, we examine the two-dimensional van der Waals antiferromagnetic bilayer ${\mathrm{CrI}}_3$, in the $n$-doped regime. Using a combination of first-principles calculations of the spin-orbit torque and an analysis of the ensuing spin dynamics, we show that the deterministic electrical switching of the Néel vector is the result of dampinglike spin-orbit torque which is staggered on the magnetic sublattices.

We study the magnetism of a quantum spin-$\frac{1}{2}$ antiferromagnet on a maple-leaf lattice which is obtained by regularly depleting $\frac{1}{7}$ of the sites of a triangular lattice. Although the interactions are set to be spatially isotropic, the ground state shows a stripe Néel order and the temperature dependence of magnetic susceptibility follows that of the one-dimensional XXZ model with a finite spin gap. We examine the nature of frustration by mapping the low-energy degenerate manifold of states to the fully packed loop-string model on a dual cluster-depleted honeycomb lattice, finding that the order-by-disorder due to quantum fluctuation characteristic of highly frustrated magnets is responsible for the emergent stripes. The excited magnons split into two spinons and propagate in the one-dimensional direction along the stripe, which is reminiscent of the XXZ or Ising model in one dimension. Unlike most of the previously studied dimensional-reduction effects, our case is purely spontaneous as the interactions of the Hamiltonian retain a two-dimensional structure.

We study the effects of the Lifshitz transition from the closed to open Fermi surface in dirty topological insulators with nematic superconductivity near the critical temperature. We solve linearized Gor'kov equations and find that the nematic superconductor with an open Fermi surface has a lower critical temperature and is more susceptible to disorder than the superconductor with the closed Fermi surface. We propose that correspondence between the critical temperature and stability against the disorder is a general feature of the superconductivity. We investigate the effects of the Lifshitz transition on the competition between superconducting phases in a topological insulator. The open Fermi surface is beneficial for the nematic order parameter ${\mathrm{\Delta }}_4$ in competition with orbital-triplet ${\mathrm{\Delta }}_2$ and disfavors nematic state over the $s$-wave order parameter. We study Meissner currents in both clean and dirty limits. We found that transition from closed to open Fermi surface increases anisotropy of Meissner currents. Finite disorder suppresses superconducting density stronger than critical temperature. We compare our results with existing experimental data.

We show that a class of compounds with I4/mcm crystalline symmetry hosts three-dimensional semi-Dirac fermions. Unlike the known two-dimensional semi-Dirac points, the degeneracy of these three-dimensional semi-Dirac points is not lifted by spin-orbit coupling due to the protection by a nonsymmorphic symmetry—mirror reflection in the $a\text{−}b$ plane and a translation along the $c$ axis. This crystalline symmetry is found in tetragonal perovskite oxides, realizable in thin films by epitaxial strain that results in $a^0{a}^0{c}^{-}$-type octahedral rotation. Interestingly, with broken time-reversal symmetry, two pairs of Weyl points emerge from the semi-Dirac points within the Brillouin zone, and an additional lattice distortion leads to an enhanced intrinsic anomalous Hall effect. The ability to tune the Berry phase by epitaxial strain can be useful in novel oxide-based electronic devices.

One of the fundamental questions about high-temperature cuprate superconductors is the size of the Fermi surface underlying the superconducting state. By analyzing the single-particle spectral function for the Fermi-Hubbard model as a function of repulsion $U$ and chemical potential $\mu$, we find that the Fermi surface in the normal state undergoes a transition from a large Fermi surface matching the Luttinger volume as expected in a Fermi liquid, to a Fermi surface that encloses fewer electrons that we dub the “Luttinger breaking” phase, as the Mott insulator is approached. This transition into a non-Fermi-liquid phase that violates the Luttinger count occurs at a critical density in the absence of any other broken symmetry. We obtain the Fermi-surface contour from the spectral weight $A_{\mathbf{k}}(\omega =0)$ and from an analysis of the singularities of the Green's function $\mathrm{Re}{G}_{\mathbf{k}}(E=0)$, calculated using determinantal quantum Monte Carlo and analytic continuation methods. We discuss our numerical results in connection with experiments on Hall measurements, scanning tunneling spectroscopy, and angle-resolved photoemission spectroscopy.