Spin-1 chain material is considered. Recently, it was predicted that the spin nematic ordered phase can be realized due to the spin-elastic coupling. In this study we show that the appearance of the spin nematic ordering and phase transitions to such ordered states can be studied using magneto-acoustic experiments. One can observe such phases and phase transitions considering relative changes of the velocity of sound (related to the changes of the elastic modulus of the crystal) as a function of temperature and the external magnetic field. It is shown that those relative changes are proportional to the spin quadrupole susceptibility of the system. The results are obtained using the exact Bethe ansatz solution and Quantum Monte Carlo simulations for several typical values of the biquadratic spin-spin exchange interaction and for various values of the spin-elastic coupling and elastic modules.

We theoretically study the appearance of bound states around impurities in a superconducting bilayer. We focus our attention on $s$-wave pairing, which includes unconventional odd-parity states permitted by the layer degree of freedom. Utilizing numerical mean-field and analytical $T$-matrix methods, we survey the bound-state spectrum produced by momentum-independent impurity potentials in this model. For even-parity $s$-wave pairing, bound states are only found for impurities which break time-reversal symmetry. For odd-parity $s$-wave states, in contrast, bound states are generically found for all impurity potentials, and fall into six distinct categories. This categorization remains valid for nodal gaps. Our results are conveniently understood in terms of the “superconducting fitness” concept, and show an interplay between the pair-breaking effects of the impurity and the normal-state band structure.

Exceptional points (EPs) are singular points on a parameter space at which some eigenvalues (scattering poles) and their corresponding eigenmodes coalesce. This study shows the existence of second- and third-order EPs in cylindrical elastic systems with radiation loss. We consider multilayered cylindrical solids under the plane-strain condition placed in a background elastic or acoustic medium. Elastic and acoustic waves propagating in the background media experience radiation loss. We optimize the radii and the material constants of the multilayered solids, such that some scattering poles coalesce on the complex frequency plane. Some numerical experiments are performed to confirm that the coalescence originates from EPs. We expect that this study provides a new approach for enhancing mechanical sensors.

We present a theory of hydrodynamics for a vector U(1) charge in $2+1$ dimensions, whose rotational symmetry is broken to the point group of an equilateral triangle. We show that it is possible for this U(1) to have a chiral anomaly. The hydrodynamic consequence of this anomaly is the introduction of a ballistic contribution to the dispersion relation for the hydrodynamic modes. We simulate classical Markov chains and find compelling numerical evidence for the anomalous hydrodynamic universality class. Generalizations of our theory to other symmetry groups are also discussed.

We predict and examine various topological states on a two-dimensional triangular kagome lattice (TKL) using the tight-binding models and theory of topological quantum chemistry (TQC). First, on the basis of TQC, we diagnose band structures with fragile topology and calculate Wilson-loop spectra and Hofstadter butterfly spectra to confirm their nontrivial nature. Second, we examine the bulk-boundary correspondence and find that an obstructed atomic limit (OAL) insulator hosts fractional corner states without being accompanied by fragile topological band structures, which implies that the presence of OALs and corner states is not a sufficient condition to fragile topology. Last but not least, we predict a topological phase transition from a second-order topological phase to a first-order topological phase that can be realized in the TKL under the action of a magnetic field.

The properties of metallic systems with important and structured excitations at low energies, such as Cu, are challenging to describe with simple models like the plasmon pole approximation (PPA), and more accurate and sometimes prohibitive full frequency approaches are usually required. In this paper we propose a numerical approach to $GW$ calculations on metals that takes into account the frequency dependence of the screening via the multipole approximation (MPA), an accurate and efficient alternative to current full frequency methods that was recently developed and validated for semiconductors and overcomes several limitations of PPA. We now demonstrate that MPA can be successfully extended to metallic systems by optimizing the frequency sampling for this class of materials and introducing a simple method to include the long-wavelength limit of the intraband contributions. The good agreement between MPA and full frequency results for the calculations of quasiparticle energies, polarizability, self-energy, and spectral functions in different metallic systems confirms the accuracy and computational efficiency of the method. Finally, we discuss the physical interpretation of the MPA poles through a comparison with experimental electron energy loss spectra for Cu.

Anisotropy of light-matter interactions in materials gives remarkable information about phonons and their interactions with electrons. We report angle-resolved polarization Raman spectroscopy of single crystal of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ to obtain elements of Raman tensor for understanding strength of polarization along different crystallographic orientations. Intensity variation in polar plots corresponding to $E_g^1 \sim 37\mathrm{c}{\mathrm{m}}^{-1}, A_{1g}^1 \sim 71\mathrm{c}{\mathrm{m}}^{-1}, E_g^2 \sim 130\mathrm{c}{\mathrm{m}}^{-1}$, and $A_{1g}^2 \sim 173\mathrm{c}{\mathrm{m}}^{-1}$ suggests higher differential polarizability along the $bc$ plane. The polar plots and elements of Raman tensor provide evidence of fundamental electron-phonon (e-ph) and anisotropic light-matter interactions in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$.

In recent years, magnetic tunnel junctions (MTJs) have attracted strong research interest due to their potential use in nonvolatile memory technologies, such as magnetoresistive random access memory and magnetic logic applications. Half-metallic materials have been proposed as ideal electrode materials for MTJs to achieve large tunnel magnetoresistance (TMR) effects. Here, we design and investigate a spin-filter MTJ (sf-MTJ) consisting of a ferrimagnetic inverse Heusler alloy, ${\mathrm{Mn}}_2\mathrm{CoSi}$ as the electrode and CaS as the insulating tunnel barrier using ab initio quantum transport calculations. Our results demonstrate a high zero-bias voltage TMR ratio that initially oscillated before decreasing as the bias voltage increased. Despite the oscillatory TMR under bias voltage, the spin injection remains high and stable, highlighting the potential of sf-MTJs formed by ${\mathrm{Mn}}_2\mathrm{CoSi}$ electrodes for practical applications.

The quantum kinetic framework provides a versatile method for investigating the dynamical optical and transport currents of crystalline solids. In this paper, starting from the density-matrix equations of motion, we present a general theoretical path to obtain the nonlinear optical response in an elegant and transparent manner. We devise an extensive kinetic theory that can be applied to materials with arbitrary band structures and captures intraband and interband coherence effects, finite Fermi surfaces, and disorder effects. We present a classification of the nonlinear optical currents arising from the interference of the interband and intraband components of the density matrix with distinct symmetry and quantum geometrical origin for each contribution. In this context, we report the following four primary findings: (i) The Fermi golden rule approach is insufficient to derive the correct expression for the injection current, a shortcoming that we remedy in our theory while associating the injection current with the intraband-interband contribution to the second-order density matrix. (ii) The interband-intraband contribution yields a resonant current that survives irrespective of any symmetry constraint in addition to the well-known anomalous nonlinear current (nonresonant), which requires time-reversal symmetry. (iii) Quite generally, the nonlinear current is significantly enhanced by contributions arising from the finite Fermi surface. (iv) The finite Fermi surface and Fermi sea additionally lead to sizable novel nonlinear effects via contributions we term double resonant and higher-order pole. We investigate such effects in sum frequency and difference frequency generation. As an illustration, we compute the nonlinear response of the topological antiferromagnet CuMnAs and thin film tilted Weyl semimetals as model systems dominated by interband coherence contributions. We find that the nonlinear response of CuMnAs is responsive to the direction of the finite magnetization field and the response of Weyl semimetal to the tilt. In addition, the choice of the polarization angle of the beam is crucial to have a nonlinear current in CuMnAs, while it is not the case for Weyl semimetals.

Although the two-dimensional electron gas (2DEG) in (001) ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}/\mathrm{ZnO}$ heterostructures was discovered about 20 years ago, the origin of the 2DEG is still inconclusive. In the present paper, the formation mechanisms of 2DEG near the interfaces of (001) ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}/\mathrm{ZnO}$ heterostructures were investigated via the first-principles calculations method. It is found that the polarity discontinuity near the interface can neither lead to the formation of 2DEG in devices with thick ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ layers nor in devices with thin ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ layers. For a heterostructure with a thick ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ layer, the oxygen vacancies near the interface introduce a defect band in the band gap, and the top of the defect band overlaps with the bottom of the conduction band, leading to the formation of the 2DEG near the interface of the device. For a heterostructure with a thin ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ layer, the absorption of hydrogen atoms, oxygen atoms, or OH groups on the surface of the ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ film plays a key role for the formation of 2DEG in the device. Our results manifest the sources of 2DEGs in ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}/\mathrm{ZnO}$ heterostructures on the electronic structure level.

Comprehension of the electronic properties of nano-objects is a key to defining dedicated properties, which can be adjusted by changing their size. Beyond confinement effects, the presence of interfaces, i.e., places where there is an abrupt change of electronic density, should also play a role. Time-dependent density functional theory (TD-DFT) is a state-of-the-art ab initio formalism in which this effect is accounted for through the so-called local field effects. In an earlier paper [S. Mazzei and C. Giorgetti, Phys. Rev. B 106, 035431 (2022)], we showed that the framework inherited from three-dimensional crystals could not provide reliable absorption spectra. In the present paper, we propose to calculate the macroscopic average of the dielectric tensor of a quasi-two-dimensional (2D) object from the response function of the density to the total macroscopic potential in order to avoid use of the so-called Adler and Wiser formula. We evidence that the inclusion of interfaces in the thickness of the slab causes the response function for the out-of-plane component to move sharply from the bulk absorption resonance to the plasmon one. This shows that the longitudinal-longitudinal contraction of the dielectric tensor is no longer equal to the transverse-transverse one in a quasi-2D object for out-of-plane perturbation. Nevertheless, we also show that the macroscopic average of the dielectric tensor of an ultrathin slab calculated within the longitudinal formalism of TD-DFT depicts the properties of the transverse reflectance and transmittance spectra of a thin slab.

We propose a quantum-mechanical model to calculate the current through a single molecular junction immersed in a solvent and surrounded by a thin shell of bound water under an applied ac voltage. The solvent plus hydration shell are captured by a dielectric continuum model for which the resulting spectral density is determined. Here the dielectric properties, e.g., the Debye relaxation time and the dielectric constant, of the bulk solvent and the hydration shell as well as the shell thickness directly enter. We determine the charge current through the molecular junction under an ac voltage in the sequential tunneling regime where we solve a quantum master equation by a real-time diagrammatic technique. Interestingly, the Fourier components of the charge current show an exponential-like decline when the hydration shell thickness increases. Finally, we apply our findings to binary solvent mixtures with varying volume fractions and find that the current is highly sensitive to both the hydration shell thickness as well as the volume fraction of the solvent mixture, giving rise to possible applications as shell and concentration sensors on the molecular scale.

The mobility edge (ME) that marks the energy separating extended and localized states is a most important concept in understanding the metal-insulator transition induced by disordered or quasiperiodic potentials. MEs have been extensively studied in three-dimensional disorder systems and one-dimensional quasiperiodic systems. However, the studies of MEs in two-dimensional (2D) systems are rare. Here, we propose a class of 2D vertex-decorated Lieb lattice models with quasiperiodic potentials only acting on the vertices of the Lieb lattice or extended Lieb lattices. By mapping these models to the 2D Aubry-André model, we obtain exact expressions of MEs and the localization lengths of localized states, and further demonstrate that the flat bands remain unaffected by the quasiperiodic potentials. Finally, we propose a highly feasible scheme to experimentally realize our model in a quantum dot array. Our results open the door to studying and realizing exact MEs and robust flat bands in 2D systems.

Exceptional points (EPs) are degeneracy of non-Hermitian Hamiltonians, at which the eigenvalues, along with their eigenvectors, coalesce. Their orders are given by the Jordan decomposition. Here, we focus on higher-order EPs arising in fermionic systems with a sublattice symmetry, which restricts the eigenvalues of the Hamitlonian to appear in pairs of $\{E,-E\}$. Thus a naive prediction might lead to only even-order EPs at zero energy. However, we show that odd-order EPs can exist and exhibit enhanced sensitivity in the behavior of eigenvector coalescence in their neighborhood, depending on how we approach the degenerate point. The odd-order EPs can be understood as a mixture of higher- and lower-valued even-order EPs. Such an anomalous behavior is related to the irregular topology of the EPs as the subspace of the Hamiltonians in question, which is a unique feature of the Jordan blocks. The enhanced eigenvector sensitivity can be described by observing how the quantum distance to the target eigenvector converges to zero. In order to capture the eigenvector coalescence, we provide an algebraic method to describe the conditions for the existence of these EPs. This complements previous studies based on resultants and discriminants, and unveils heretofore unexplored structures of higher-order exceptional degeneracy.

Experiments detect small electron polarons in rutile-${\mathrm{TiO}}_2$, but only large ones in anatase. By using an accurate hybrid functional, we show that the “large polaron” in anatase is actually of medium size and can be obtained in a static calculation. These states are two-dimensional in the (001) plane and can get trapped at the (101) surface (where small polarons may exist too). The calculated binding energy and the simulated scanning tunneling microscopic image are in good agreement with the experimental observation. The fact that holes are trapped in small polaron states in the bulk while electrons are only at the surface means an efficient separation of photoexcited carriers, and is a factor in the excellent photocatalytic behavior of anatase.

Here, we present the Fermi surface properties of the kagome superconductor ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ using torque magnetometry at applied fields up to 45 T and temperatures down to that of liquid ${}^3\mathrm{He}$ (0.32 K). The torque signal shows clear de Haas–van Alphen (dHvA) oscillations with 14 major frequencies ranging from $\sim 33$ to 2149 T, nine of which are higher frequencies (above 500 T) that have never been reported in ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. Angular dependence measurements of the dHvA oscillations were carried out to investigate the dimensionality of the Fermi surface. Based on our analysis, several frequencies follow the $1/\cos \theta$ dependence, where $\theta$ is the tilt angle with respect to the applied field direction and oscillations disappear above $\theta ={60}^{\circ }$, which suggest that Fermi surfaces corresponding to these frequencies are quasitwo dimensional. The Berry phase (${\mathrm{\Phi }}_{\mathrm{B}}$), determined by constructing a Landau level fan diagram, was found to be ${\mathrm{\Phi }}_{\mathrm{B}}\sim \pi$, which strongly suggests the nontrivial topology of ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. To explain the experimental results, we carried out band-structure and Fermi-surface calculations using density functional theory (DFT) for both pristine and charge-density wave (CDW) phases. We found that the Fermi surface undergoes severe reconstruction in the CDW phase and, more importantly, our calculation results are in reasonable agreement with the experimentally measured Fermi-surface frequencies. The observation in this paper of very high quantum oscillation frequencies in ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and the determination of their detailed Fermi-surface properties, along with the analyses of corresponding DFT calculation results, are crucial for understanding CDW order, unconventional superconductivity, and nontrivial topology in $A{\mathrm{V}}_3{\mathrm{Sb}}_5(A=\mathrm{K},\mathrm{Rb},$ and Cs), as well as the interplay among them.

We propose theoretically a scheme to get a room-temperature two-dimensional topological exciton-polariton laser with propagating topological lasing modes. The structure uses guided modes in a photonic crystal slab. A ZnO layer provides strong excitonic resonances stable at room temperature. It is capped by a ${\mathrm{TiO}}_2$ layer pierced by a triangular lattice of air holes. The exciton-polariton modes of the three-dimensional structure are computed by solving numerically Maxwell's equations including the excitonic response. The designed triangular lattice of circular air holes shows a transverse electric band gap. The triangular lattice of air holes is shown to be well described by a staggered honeycomb tight-binding lattice, associated with valley Chern numbers defining the interface states. The interface between two shifted triangular lattices of air holes supports two counterpropagating modes lying in the gap of the bulk modes, analogous to quantum pseudospin Hall interface states. These modes show orthogonal polarizations. They can be selectively excited using polarized excitation and are well protected from backscattering. These modes can benefit from the exciton-polariton gain at room temperature because of their sufficiently large exciton fraction and favorable position in energy. The strong localization of these propagating modes makes them suitable to host topological lasing triggered by a nonresonant pump localized on the interface.

The electronic state at the interface between the Ag(111) surface and NaCl island has been studied. Due to the lattice mismatch, islands of NaCl grow on Ag(111) with various moiré patterns which lead to a weak periodic one-dimensional (1D) potential for the electronic state. In contrast to the well-known situation for a purely 1D system, the ideal two-dimensional system should cause a modulation of the local electronic density for energies only below and not above the mini band gap corresponding to the periodic potential. However, the experimental observation by scanning tunneling microscopy and spectroscopy of states reveals such a modulation also in the forbidden range of energy. Since the particular system allows us to disentangle the effects of local scattering and those caused by the periodic potential, it could be revealed that the loss of coherence and the mechanism of the detection by the STM are responsible for the experimentally observed modulation, which should be suppressed in the case of an ideal system.

We report on spin ratchet currents driven by terahertz radiation electric fields in a Co/Pt magnetic metamaterial formed by triangle-shaped holes forming an antidot lattice and subjected to an external magnetic field applied perpendicularly to the metal film plane. We show that for a radiation wavelength substantially larger than the period of the antidots array, the radiation causes a polarization-independent spin-polarized ratchet current. The current is generated by the periodic asymmetric radiation intensity distribution caused by the near-field diffraction at the edges of the antidots, which induces spatially inhomogeneous periodic electron gas heating, and a phase-shifted periodic asymmetric electrostatic force. The developed microscopic theory shows that the magnetization of the Co/Pt film results in a spin ratchet current caused by both the anomalous Hall and the anomalous Nernst effects. Additionally, we observed a polarization-dependent trigonal spin photocurrent, which is caused by the scattering of electrons at the antidot boundaries resulting in a spin-polarized current due to the magnetization. Microscopic theory of these effects reveals that the trigonal photocurrent is generated at the boundaries of the triangle antidots, whereas the spin ratchet is generated due to the spatially periodic temperature gradient over the whole film. This difference causes substantially different hysteresis widths of these two currents.