The spin ice materials ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ are experimental and theoretical exemplars of highly frustrated magnetic materials. However, the effects of applied uniaxial pressure are not well studied, and here we report magnetization measurements of ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ under uniaxial pressure applied in the [001], [111], and [110] crystalline directions. The basic features are captured by an extension of the dipolar spin ice model. We find a good match between our model and measurements with pressures applied along two of the three directions, and we extend the framework to discuss the influence of crystal misalignment for the third direction. The parameters determined from the magnetization measurements reproduce neutron scattering measurements that we perform under uniaxial pressure applied along the [110] crystalline direction. In the detailed analysis, we include the recently verified susceptibility dependence of the demagnetizing factor. Our work demonstrates the application of a moderate applied pressure to modify the magnetic interaction parameters. The knowledge can be used to predict critical pressures needed to induce new phases and transitions in frustrated materials, and in the case of ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ we expect a transition to a ferromagnetic ground state for uniaxial pressures above $3.3\text{GPa}$.

We present spin pumping experiments in Co/Pt bilayers grown on ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) and on ferroelectric $Y$-cut ${\mathrm{LiNbO}}_3$ substrates. Spin pumping is triggered by resonant ferromagnetic resonance induced by surface acoustic waves. We observe that spin pumping efficiency varies when both the magnetization vector and the acoustic wave vector are reversed in Pt/Co/${\mathrm{LiNbO}}_3$. This phenomenon is not observed in Pt/Co/${\mathrm{Al}}_2{\mathrm{O}}_3$. We propose that the in-plane electric polarization of ${\mathrm{LiNbO}}_3$ is at the origin of the observed phenomenon. These observations open up the perspective of an electric field control of spin pumping efficiency.

Using first-principles density functional theory calculations, combined with a topological analysis, we have investigated the electronic properties of ${\text{Cd}}_3{\text{As}}_2$ and ${\text{Na}}_3\text{Bi}$ Dirac topological semimetals doped with nonmagnetic and magnetic impurities. Our systematic analysis shows that the selective breaking of the inversion, rotational, and time-reversal symmetry, controlled by specific choices of the impurity doping, induces phase transitions from the original Dirac semimetal to a variety of topological phases such as topological insulator, trivial semimetal, nonmagnetic and magnetic Weyl semimetal, and Chern insulator. The Dirac semimetal phase can exist only if the rotational symmetry $C_n$ with $n>2$ is maintained. One particularly interesting phase emerging in doped ${\text{Cd}}_3{\text{As}}_2$ is a coexisting Dirac-Weyl phase, which occurs when only inversion symmetry is broken while time-reversal symmetry and rotational symmetry are both preserved. To further characterize the low-energy excitations of this phase, we have complemented our density functional results with a continuum four-band $\mathit{k}·\mathit{p}$ model, which indeed displays nodal points of both Dirac and Weyl types. The coexisting phase appears as a transition point between two topologically distinct Dirac phases but may also survive in a small region of parameter space controlled by external strain.

Anderson's orthogonality catastrophe (AOC) theorem establishes that the ground state of the many-body fermion system is asymptotically orthogonal to the ground state of the same system perturbed by a scattering potential so that the overlap between the original and new ground states decays to zero with the system size. We adopt the AOC for a description of heat production in a complementary metal-oxide-semiconductor transistor. We find that the heat released in the transistor comprises two distinct components, contribution from the dissipation accompanying electron transmission under the applied voltage and purely quantum-mechanical AOC part due to the change in scattering matrix for electrons upon switching between high and low conductance regimes. We calculate the AOC-induced heat production, which we call switching heat.

We study the topological nature of a class of lattice systems whose Bloch vector can be expressed as the difference of two independent periodic vector functions (knots) in an auxiliary space. We show exactly that each loop as a degeneracy line generates a $\mathbf{P}$ field, which is defined as the gradient of the Zak phase in parameter space, obeying the Biot-Savart law: The degeneracy line acts as a current-carrying wire, while the $\mathbf{P}$ field corresponds to the generated magnetic field. Applying the Ampère's circuital law on a nontrivial topological system, we find that two Bloch knots entangle with each other, forming a link with the linking number being the value of Chern number of the energy band. In addition, two lattice models, an extended Qi-Wu-Zhang (QWZ) model and a quasi-one-dimensional model with magnetic flux, are proposed to exemplify the application of our approach. In the aid of the Biot-Savart law, the pumping charge as a dynamic measure of Chern number is obtained numerically from quasi-adiabatic processes.

The occurrence of the Kondo effect in the ultimate two-dimensional (2D) case, when the thickness of the conducting film is reduced down to the atomic-scale limit, was explored taking the Tl-adsorbed Au/Si(111)$\sqrt{3}\times \sqrt{3}$ reconstruction with embedded Cr atoms as a prototype system. The reconstruction is essentially an atomic-layer film, which resides on a semiconductor Si(111) substrate and demonstrates conductivity of a metallic type, at least down to 1.8 K. The tight binding of Cr atoms in the interstitial sites surrounded by six Si atoms prevents them from clustering. The Kondo effect in this system was investigated experimentally using low-temperature scanning tunneling spectroscopy and electronic transport measurements. Both techniques yield a Kondo temperature of $\sim 70–80\mathrm{K}$. A substantial increase of the sample resistivity caused by adding Cr atoms is believed to be a specific feature of the extremely thin metallic films, where small amounts of magnetic impurities can effectively block off the narrow conduction channel.

Motivated by the recent successful formation of boron and nitrogen-doped graphene, using density-functional-theory calculations, we investigate the atomic and electronic structure of bilayers of a graphene-like borocarbonitride ($\mathrm{g}\text{−}{\mathrm{BC}}_6\mathrm{N}@2\mathrm{L}$). Although pristine graphene is a semimetal, the single layer of $\mathrm{g}\text{−}{\mathrm{BC}}_6\mathrm{N}$ is a semiconductor with a direct band gap of 1.3 eV, as is the bilayer $\mathrm{g}\text{−}{\mathrm{BC}}_6\mathrm{N}@2\mathrm{L}$, which has a slightly smaller band gap of 1.2 eV. Systematic calculations for the bilayer structure, to investigate the effect of an electric field (E field), reveal that with increase of E-field, from 0.1 to 0.6 (V/Å), the band gap decreases linearly for both parallel and antiparallel E-field directions. For larger E fields ($>0.6$ V/Å), we find the formation of dual narrow band gaps of 50 meV (for parallel) and 0.4 eV (for antiparallel). The shape of energy bands is preserved and the value of band gaps are constant in both E-field directions. Our results indicate that monolayer and bilayer graphene-like borocarbonitride ($\mathrm{g}\text{−}{\mathrm{BC}}_6\mathrm{N}$) is an interesting system which may find applications in future low-dissipation, high-speed nanoelectronic devices.

The coherent quantum effect has become increasingly important in the heat dissipation bottleneck of semiconductor nanoelectronics with the characteristic size recently shrinking down to a few nanometers scale. However, the quantum mechanical model remains elusive for anharmonic phonon-phonon scattering in extremely small nanostructures with broken translational symmetry. It is a long-term challenging task to correctly simulate quantum heat transport including anharmonic scattering at a scale relevant to practical applications. In this article, we present a clarified theoretical formulation of anharmonic phonon nonequilibrium Green's function (NEGF) formalism for both one- and three-dimensional nanostructures, through a diagrammatic perturbation expansion and an introduction of Fourier's representation to both harmonic and anharmonic terms. A parallelized computational framework with first-principle force constants input is developed for large-scale quantum heat transport simulation. Some crucial approximations in numerical implementation are investigated to ensure the balance between numerical accuracy and efficiency. A quantitative validation is demonstrated for the anharmonic phonon NEGF formalism and computational framework by modeling cross-plane heat transport through a silicon thin film. The phonon-phonon scattering is shown to appreciably enhance the thermal resistance or conductance in extremely small homogeneous or heterogeneous thin films. The present methodology provides a robust platform for the device quantum thermal modeling, as well as a study on the transition from coherent to incoherent heat transport in nanophononic crystals. This work thus paves the way to understand and to manipulate heat conduction via the wave nature of phonons.

${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ is known as a unique example of insulating multiferroic compounds with a skyrmion spin texture, which is characterized by a chiral cubic crystal structure at ambient pressure. Recently, it has been reported that this compound shows a pressure-induced structural transition with a large enhancement of magnetic ordering temperature. In the present Rapid Communication, we have investigated the detailed crystal structure in the high-pressure phase, by combining a synchrotron x-ray diffraction experiment with a diamond anvil cell and an analysis based on the genetic algorithm. Our results suggest that the original pyrochlore Cu network is sustained even after the structural transition, while the orientation of the ${\mathrm{SeO}}_3$ molecule as well as the position of oxygen in the middle of the Cu tetrahedra are significantly modified. The latter features may be the key for the reported enhancement of $T_c$ and the associated stabilization of the skyrmion phase at room temperature.

The low-energy bands of twisted bilayer graphene form Dirac cones with approximate electron-hole symmetry at small rotation angles. These crossings are protected by the emergent symmetries of moiré patterns, conferring a topological character to the bands. Strain accumulated between layers (heterostrain) shifts the Dirac points both in energy and momentum. The overlap of conduction and valence bands favors an excitonic instability of the Fermi surface close to the neutrality point. The spontaneous condensation of electron-hole pairs breaks time-reversal symmetry and the separate conservation of charge within each valley sector. The order parameter describes interlayer circulating currents in a Kekulé-like orbital magnetization density wave. Vortices in this order parameter carry fermion numbers owing to the underlying topology of the bands. This mechanism may explain the occurrence of insulating states at neutrality in the most homogeneous samples, where uniform strain fields contribute to both stabilizing the relative orientation between layers and to the formation of an excitonic gap.

The manifestation of chiral anomaly in Weyl semimetals typically relies on the observation of longitudinal magnetoconductance (LMC) along with the planar Hall effect, with a specific magnetic field and angle dependence. Here we solve the Boltzmann equation in the semiclassical regime for a prototype of a Weyl semimetal, allowing for both intravalley and intervalley scattering, along with including effects from the orbital magnetic moment (OMM), in a geometry where the electric and magnetic fields are not necessarily parallel to each other. We construct the phase diagram in the relevant parameter space that describes the shift from positive to negative LMC in the presence of OMM and sufficiently strong intervalley scattering, as has been recently pointed out for only parallel electric and magnetic fields. On the other hand, we find that the chiral anomaly contribution to the planar Hall effect always remains positive (unlike the LMC) irrespective of the inclusion or exclusion of OMM, or the strength of the intervalley scattering. Our predictions can be directly tested in experiments, and may be employed as new diagnostic procedures to verify chiral anomaly in Weyl systems.

This work is a study of dynamical conductivity of a quasi-two-dimensional heterostructure $\mathrm{Li}{\left(\mathrm{BN}\right)}_8$. The conducting electrons have a free-electron-like parabolic dispersion and are assumed to scatter only on acoustic phonons. The approach used to derive the generalized Drude relation with frequency and temperature dependent relaxation consists of defining the induced current density as a function of the electron-hole operator. This element is proportional to the perturbating electric field and is decomposed into the zeroth and second order of the electron-phonon interaction. The electron-hole operator is then evaluated using the Heisenberg equation. The connection between the electron-hole operator and the memory function is shown. The analytical properties of the imaginary and real part of the frequency and temperature dependent memory function, that describe electron scattering on acoustic and optical phonons, are examined in detail. Finally, the dynamical conductivity of $\mathrm{Li}{\left(\mathrm{BN}\right)}_8$ is calculated, with the explanation of all relevant features originating from the memory function.

The lamellar cuprate superconductors exhibit the highest ambient-pressure superconducting transition temperatures ($T_c$), and after more than three decades of extraordinary research activity, continue to pose formidable scientific challenges. A major experimental obstacle has been to distinguish universal phenomena from materials- or technique-dependent ones. Angle-resolved photoemission spectroscopy (ARPES) measures momentum-dependent single-particle electronic excitations and has been invaluable in the endeavor to determine the anisotropic momentum-space properties of the cuprates. $\mathrm{Hg}{\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_{4+\delta }$ (Hg1201) is a single-$\mathrm{Cu}{\mathrm{O}}_2$-layer cuprate with a particularly high optimal $T_c$ and a simple crystal structure, yet there exists little information from ARPES about the electronic properties of this model system. Here we present an ARPES study of doping-, temperature-, and momentum-dependent systematics of near-nodal dispersion anomalies in Hg1201. The data reveal a hierarchy of three distinct energy scales: a subgap low-energy kink, an intermediate-energy kink near 55 meV, and a peak-dip-hump structure. The first two features are attributed to the coupling of electrons to Ba-derived optical phonons and in-plane bond-stretching phonons, respectively. The nodal peak-dip-hump structure appears to have a common doping dependence in several single-layer cuprates and is interpreted as a manifestation of pseudogap physics at the node. These results establish several universal phenomena, both in terms of connecting multiple experimental techniques for a single material and in terms of connecting comparable spectral features in multiple structurally similar cuprates.

Atomic scale defects in semiconductors enable their technological applications and realization of different quantum states. Using scanning tunneling microscopy and spectroscopy complemented by ab initio calculations we determine the nature of defects in the anisotropic van der Waals layered semiconductor $\mathrm{Re}{\mathrm{S}}_2$. We demonstrate the in-plane anisotropy of the lattice by directly visualizing chains of rhenium atoms forming diamond-shaped clusters. Using scanning tunneling spectroscopy we measure the semiconducting gap in the density of states. We reveal the presence of lattice defects and by comparison of their topographic and spectroscopic signatures with ab initio calculations we determine their origin as oxygen atoms absorbed at lattice point defect sites. These results provide an atomic-scale view into the semiconducting transition metal dichalcogenides, paving the way toward understanding and engineering their properties.

We present a microscopic model for exciton-induced transparency (ExIT) in a hybrid system comprised of an emitter resonantly coupled to a surface plasmon in a metal-dielectric structure. We obtain an effective optical polarizability of such a system with coupling between the system components expressed in terms of energy transfer rates. We demonstrate that, in the weak coupling regime, the underlying mechanism of ExIT is the energy exchange imbalance between the plasmon and the emitter in a narrow frequency region. We derive in analytic form a frequency-dependent function that accurately describes the shape and amplitude of the transparency window in scattering spectra, supported by numerical calculations.

The cooperation between non-Hermiticity and interaction brings about a lot of counterintuitive behaviors, which are impossible to exist in the framework of the Hermitian system. We study the effect of a non-Hermitian impurity on the Hubbard model in the context of $\eta$ symmetry. We show that the non-Hermitian Hubbard Hamiltonian can respect a full real spectrum even if a local non-Hermitian impurity is applied to. The balance between dissipation of single fermion and on-site pair fluctuation results in a highest-order coalescing state with off-diagonal long-range order (ODLRO). Based on the characteristic of high-order EP, the critical non-Hermitian Hubbard model allows the generation of such a steady superconducting-like state through the time evolution from an arbitrary initial state, including the vacuum state. Remarkably, this dynamic scheme is insensitive to the on-site interaction and entirely independent of the locations of particle dissipation and pair fluctuation. Our results lay the groundwork for the dynamical generation of a steady ODLRO state through the critical non-Hermitian strongly correlated system.

The beryllide ${\mathrm{GdBe}}_{13}$ with the ${\mathrm{NaZn}}_{13}$-type face-centered-cubic structure has been known to undergo a proper helical-magnet ordering from experimental studies using polycrystalline samples. In the present study, we carried out electrical resistivity, specific heat, and magnetization measurements of single-crystalline ${\mathrm{GdBe}}_{13}$ in order to investigate a mechanism of its helical ordering. These measurements reveal that the present compound is a metallic system exhibiting the magnetic ordering of local ${\mathrm{Gd}}^{3+}$ moments at $T_{\mathrm{N}}$ = 24.8 K accompanied with strong magnetic fluctuations extending to temperatures well above $T_{\mathrm{N}}$. Furthermore, we constructed a magnetic field–temperature ($B$–$T$) phase diagram for $B\parallel$ [001]. It consists of a multidomain state, which is composed of magnetic structures with $B$ applied parallel and perpendicular to the helical plane, in the lower-magnetic-field region below $\sim 0.45\mathrm{T}$ and a possible single-domain conical one in the higher-field region in the ordering state. The helical structure of ${\mathrm{GdBe}}_{13}$ characterized by an incommensurate ordering vector ${\mathit{q}}_0$ of (0, 0, 0.285) is discussed on the basis of a competition of Heisenberg exchange interactions between the ${\mathrm{Gd}}^{3+}$ moments assuming an one-dimensional layer crystal. The sequential change in the exchange interactions determined by a mean-field (MF) calculation can be essentially understood by the Ruderman-Kittel-Kasuya-Yosida interaction via anisotropic Fermi surfaces, whereas the orientation of the magnetic moments will be determined by the dipole-dipole interaction. On the other hand, the MF theory predicts a much smaller critical field $B_{\mathrm{c}}$ than the experimentally obtained one. To discuss the deviation of $B_{\mathrm{c}}$ from the MF calculation, we show a possibility of a fluctuation-induced first-order transition.

Ultrathin ferromagnets with frustrated exchange and the Dzyaloshinskii-Moriya interaction can support topological solitons such as skyrmions and antiskyrmions, which are metastable and can be considered particle-antiparticle counterparts. When spin-orbit torques are applied, the motion of an isolated antiskyrmion driven beyond its Walker limit can generate skyrmion-antiskyrmion pairs. Here, we use atomistic spin dynamics simulations to shed light on the scattering processes involved in this pair generation. Under certain conditions a proliferation of these particles and antiparticles can appear with a growth rate and production asymmetry that depend on the strength of the chiral interactions and the dissipative component of the spin-orbit torques. These features are largely determined by scattering processes between antiskyrmions, which can be elastic or result in bound states or annihilation.

The magnetic phase diagram at zero external field of an ensemble of dipoles with uniaxial anisotropy on an fcc lattice is investigated from tempered Monte Carlo simulations. The uniaxial anisotropy is characterized by a random distribution of easy axes and its magnitude ${\lambda }_u$ is the driving force of disorder and consequently frustration. The phase diagram, separating the paramagnetic, ferromagnetic, quasi-long-range ordered ferromagnetic, and spin-glass regions is thus considered in the temperature ${\lambda }_u$ plane. This system is aimed at modeling the magnetic phase diagram of supracrystals of magnetic nanoparticles.