We develop a Boltzmann transport theory of coupled magnon-phonon transport in ferromagnetic insulators. The explicit treatment of the magnon-phonon coupling within the Boltzmann approach allows us to calculate the low-temperature magnetic-field dependence of the spin-Seebeck voltage. Within the Boltzmann theory we find that this magnetic field dependence shows similar features as found by Flebus et al. [Phys. Rev. B 95, 144420 (2017)] for a strongly coupled magnon phonon system that forms magnon-polarons, and consistent with experimental findings in yttrium iron garnet by Kikkawa et al. [Phys. Rev. Lett. 117, 207203 (2016)]. In addition to the anomalous magnetic-field dependence of the spin Seebeck effect, we also predict a dependence on the system size.

In the circular photogalvanic effect, circularly polarized light can produce a direct electron photocurrent in metals, and the direction of the current depends on the polarization. We suggest that an analogous nonlinear effect exists for antiferromagnetic insulators wherein the total spin of light and spin waves is conserved. In consequence, a spin angular momentum is expected to be transferred from photons to magnons so that a circularly polarized electromagnetic field will generate a direct magnon spin current. The direction of the current is determined by the helicity of the light. We show that this resonant effect appears as a second-order light-matter interaction. We find also a geometric contribution to the spin photocurrent, which appears for materials with complex lattice structures and Dzyaloshinskii-Moriya interactions.

Low-field magnetization curves for an assembly of polydisperse magnetic nanoparticles described as double-well systems (DWS) with randomly distributed easy axes are generated by means of a rate-equation approach applied to states obtained by cooling with and without a magnetic field. The model has notable advantages over the existing steady-state approaches, allowing one to accurately determine the magnetization frozen in a sample after cooling under field, and to study the effect of the magnitude of the cooling field on the shape of the field-cooled (FC) curve. The DWS scheme keeps validity well above the blocking temperature, making it possible to apply the model over all the region of interest (i.e., when the system is out of equilibrium). A linearized expression for the magnetization curves is obtained and compared with the existing formulas. It is shown that the linearized expression for the zero-field-cooled (ZFC) curve is more satisfactory, while no linearized expression for the FC curve is able to accurately reproduce the behavior derived from the rate equations. A method to obtain the size distribution function is developed; this makes use of the linearized model in the DWS scheme and is based on the analysis of the experimental ZFC curve only. Explicit expressions for the FC/ZFC curves of DWS characterized by an average blocking temperature higher than the starting temperature of the measurement cycle are proposed and shown to naturally explain “anomalous” experimental curves found in the literature.

We use transverse field muon spin rotation to probe the low field dynamics of $\mathrm{H}{\mathrm{o}}^{3+}$ ions imbedded in a single crystal host matrix of $\mathrm{CaW}{\mathrm{O}}_4$. The $\mathrm{H}{\mathrm{o}}^{3+}$ ions at sites with the full tetragonal symmetry of the crystal have avoided level crossings of hyperfine sublevels of the ground crystal-field doublet, resulting in measurable changes in the both the AC susceptibility and transverse field muon depolarization rate. The host material has primarily nonmagnetic nuclear species and no magnetic ions, and so the muon is shown to be a direct nonresonant probe of pronounced changes of relaxation rates of the coupled electron-nuclear excitations of $\mathrm{H}{\mathrm{o}}^{3+}$ at avoided level crossings.

Three decades ago, Ioffe and Larkin pointed out a generic mechanism for the formation of a gapped spin liquid. In the case when a classical two-dimensional (2D) frustrated Heisenberg magnet undergoes a Lifshitz transition between a collinear Néel phase and a spin-spiral phase, quantum effects usually lead to the development of a spin-liquid phase sandwiched between the Néel and spin-spiral phases. In this work, using field theory techniques, we study properties of this universal spin-liquid phase. We examine the phase diagram near the Lifshitz point and calculate the positions of critical points, excitation spectra, and spin-spin correlation functions. We argue that the spin liquid in the vicinity of 2D Lifshitz point (LP) is similar to the gapped Haldane phase in integer-spin one-dimensional chains. We also consider a specific example of a frustrated system with the spiral-Néel LP, the $J_1\text{−}{J}_3$ antiferromagnet on the square lattice that manifests the spin-liquid behavior. We present numerical series expansion calculations for this model and compare results of the calculations with predictions of the developed field theory.

The measurements of the magnetic and nematic correlation lengths in a generalization of the two-dimensional XY model on the square lattice are presented using classical Monte Carlo simulation. The full phase diagram is reexamined based on these correlation lengths, demonstrating their power in studying generalized XY models. The ratio between the correlation length and the lattice size has distinctive behaviors which can be used to distinguish different types of phase transition. More importantly, the magnetic correlation length give more insights into the tricritical region where the paramagnetic, nematic, and quasi-long-range phases meet. It shows signatures for the intermediate region starting from the tricritical point, where the transition line is neither of the same physics as the Ising transition below nor the Berezinskii-Kosterlitz-Thouless transition far above the tricritical point.

The control of magnetic domain walls is essential for magnetic-based memory and logic applications. As an elementary excitation of magnetic order, a spin wave is capable of moving magnetic domain walls just like a conducting electric current. Ferromagnetic spin waves can only be right-circularly polarized. In contrast, antiferromagnetic spin waves have full polarization degrees of freedom, including both left- and right-circular polarizations, as well as all possible linear or elliptical ones. Here we demonstrate that, due to the Dzyaloshinskii-Moriya interaction, the spin wave driven domain-wall motion in antiferromagnets strongly depends on the linear polarization direction of the injected spin waves. Steering domain-wall motion by simply tuning the polarization of spin waves offers new design principles for domain-wall based information processing devices.

Fe-Ag multilayers were prepared by vacuum evaporation in a wide range of Fe and Ag thicknesses ($t_{\mathrm{Fe}}$, $t_{\mathrm{Ag}}$) and bilayer numbers $\left(n\right)$, while the magnetic properties as a function of these parameters were measured. Samples with discontinuous Fe layers are superparamagnetic (SPM) and our paper shows that $t_{\mathrm{Ag}}$ and $n$ affect the superparamagnetic blocking temperature ($T_{\mathrm{B}}$) not only by affecting the interactions between the Fe grains but also through influencing the growth process of Fe and thereby modifying the magnetic-grain-size distribution. A magnetic “phase diagram” of our Fe-Ag multilayers is constructed in the $t_{\mathrm{Fe}}$, $t_{\mathrm{Ag}}$, $n$ parameter space, where the SPM and ferromagnetic behaviors are separated by a mixed zone showing the characteristics of both. The measured trend of the susceptibilities attained at the blocking temperature ($T_{\mathrm{B}}$), increasing with increasing $T_{\mathrm{B}}$, was explained by the volume distribution of the magnetic particles, as illustrated by calculations for an ensemble of interaction-free magnetic particles.

We study the anomalous Josephson effect, as well as the dependence on the direction of the critical Josephson current, in an S/N/S junction, where the normal part is realized by alternating spin-orbit coupled and ferromagnetic layers. We show that to observe these effects it is sufficient to break spin rotation and time-reversal symmetry in spatially separated regions of the junction. Moreover, we discuss how to further improve these effects by engineering multilayer structures with more that one couple of alternating layers.

We show theoretically that in the generic finite chemical potential situation, the clean superconducting spin-orbit-coupled nanowire has two distinct nontopological regimes as a function of Zeeman splitting (below the topological quantum phase transition): one is characterized by finite-energy in-gap Andreev bound states, while the other has only extended bulk states. The Andreev bound state regime is characterized by strong features in the tunneling spectra creating a “gap closure” signature, but no “gap reopening” signature should be apparent above the topological quantum phase transition, in agreement with most recent experimental observations. The gap closure feature is actually the coming together of the Andreev bound states at high chemical potential rather than a simple trivial gap of extended bulk states closing at the transition. Our theoretical finding establishes the generic intrinsic Andreev bound states on the trivial side of the topological quantum phase transition as the main contributors to the tunneling conductance spectra, providing a generic interpretation of existing experiments in clean Majorana nanowires. Our work also explains why experimental tunnel conductance spectra generically have gap closing features below the topological quantum phase transition, but no gap opening features above it.

Local excitations in fractional quantum Hall systems are amongst the most intriguing objects in condensed matter, as they behave like particles of fractional charge and fractional statistics. In order to experimentally reveal these exotic properties and, further, to use such excitations for quantum computations, microscopic control over the excitations is necessary. Here, we discuss different optical strategies to achieve such control. First, we propose that the application of a light field with nonzero orbital angular momentum can pump orbital angular momenta to electrons in a quantum Hall droplet. In analogy to Laughlin's argument, we show that this field can generate a quasihole or a quasielectron in such systems. Second, we consider an optical potential that can trap a quasihole, by repelling electrons from the region of the light beam. We simulate a moving optical field, which is able to control the position of the quasihole. This allows for imprinting the characteristic Berry phase, which reflects the fractional charge of the quasihole.

Nodal-line semimetals are topological semimetals characterized by one-dimensional band-touching loops protected by the combined symmetry of inversion $\mathcal{P}$ and time-reversal $\mathcal{T}$ in absence of spin-orbit coupling. These nodal loops can be understood as a one-parameter family of Dirac points exhibiting the parity anomaly associated to $\mathcal{P}\mathcal{T}$ symmetry. We find that the parity anomaly also appears in the nonlinear optical response of these systems in an analogous way to the linear response transport. We analyze the presence of a tilting term in the Hamiltonian as an element that does not spoil $\mathcal{P}\mathcal{T}$ symmetry: while it is $\mathcal{P}\mathcal{T}$ symmetric, it breaks separately both $\mathcal{P}$ and $\mathcal{T}$ symmetries, allowing for the potential experimental observability of the linear and nonlinear Hall conductivities in appropriate nodal-line semimetals. We also propose effective actions for both the linear and nonlinear electromagnetic responses of tilted nodal-line semimetals. We find that the linear Hall-like response of tilted nodal-line semimetals is an axion response due to the parity anomaly, extending the class of systems that display such electromagnetic response.

An approximation scheme determining the excitations of an electronic model on the lattice with pairwise interactions is proposed. A systematic truncation of the set of Dyson-Schwinger equations for correlators, supplemented by a “covariant” calculation of correlators lead to a converging series of approximates. The covariance preserves all the Ward identities among correlators describing various condensed matter probes. It is shown that the third-order approximant of this kind beyond classical and Gaussian (Hartree-Fock) is precise enough and due to several fortunate features the complexity of calculation is surprisingly low so that a realistic material computation might be feasible. Focus here is on the electron field correlator describing the electron (hole) excitations measured in photoemission and other probes. The scheme is tested on several solvable benchmark models.

The successful use of graphene nanoribbons (GNRs) in a variety of applications in nanoelectronics depends not only on reliable control of their forbidden gaps, but also on the understanding of the effects that contacts to leads may have on their conductance $G$. By combining Landauer's formalism and a simplified version of the embedded cluster method, $G$ through suspended 9-AGNR has been calculated as a function of energy (sample bias) and the strength of the contact between the ribbon and leads attached to both zigzag edges. Green's functions of contacted ribbons have been derived from HF nonspin polarized solutions of the Pariser-Parr-Pople Hamiltonian. It is shown that the G associated with the two quasidegenerate states around the Fermi level, which are strongly localized at the zigzag edges, equals a conductance quantum $G_0$ for very weak leads-ribbon coupling, decreasing to zero as that coupling increases. At the Fermi level $G$ is zero for small coupling, increasing up to $G_0$ for a value of coupling that depends on the GNR length, and, finally, decreasing to zero for large coupling. Conductance through other energies, starting at $G=0$ for no coupling, increases with coupling to the electrodes up to near one quantum at a pace that may appreciably depend on the particular molecular orbital. These results illustrate the difficulties that may be found in exploring practical uses of GNRs.

The formation of hydrogen overlayers on the Zn-terminated $\mathrm{ZnO}\left(0001\right)$ surface has been reexamined by angle-resolved photoemission spectroscopy (ARPES). While low-energy electron diffraction patterns display the same (1 × 1) symmetry for different surface preparations, the electronic structure feature close to the Fermi level shows the formation of electron pockets, compatible with hydrogen-induced metallic states. Using ARPES and density functional theory (DFT) calculations, we show that hydrogen adspecies can also lead to metallization of this zinc-oxide surface in a similar manner as observed previously on $\mathrm{ZnO}\left(10\overline{1}0\right)$ and O-terminated $\mathrm{ZnO}\left(000\overline{1}\right)$. Importantly, our DFT calculations indicate that these electron pockets are formed by $sp$ hybridized states and therefore the angular distribution of the emitted photoelectron is significantly suppressed at the normal emission.

We study spectra of long-wavelength plasma oscillations in a system of two-energy split one-dimensional (1D) massless Dirac fermion subbands coupled by spin-orbit interaction. Such a system may be formed by edge subbands in semiconducting transition-metal dichalcogenide monolayers. Intrasubband transitions of massless Dirac fermions give rise to optical and acoustic gapless branches of intrasubband 1D plasmons. We reveal that the optical branch is of quantum character with group velocity being inversely proportional to the square root of the Planck constant, whereas the acoustic branch is a classical one with group velocity proportional to the geometric mean of the edge subband velocities. Spin-orbit interaction, allowing intersubband transitions in the system, results in the emergence of two branches of intersubband 1D plasmons: upper and lower ones. The upper and lower branches are gapped at small wave vectors and evolve with positive and negative group velocities, respectively, from energy splitting of the edge subbands at the Fermi level. Both intersubband branches adjoin the intersubband single-particle excitation continuum from above, while in the case of the edge subbands with unequal velocities the lower one experiences Landau damping at small wave vectors. In addition, the lower branch, attaining zero frequency at a nonzero wave vector, alters its group velocity from a negative to positive one.

Phonon surface scattering is of great importance for understanding thermal transport in nanostructured materials and has been widely utilized to tailor thermal properties. However, the current understanding of phonon surface scattering is largely based on the Ziman's formula which was derived at the continuum limit of the scalar wave equation and, thus, ignoring the atomistic information of the surface. In this work, we applied the atomistic phonon wave-packet simulations to study the impact of surface amorphization and surface roughness on phonon reflection. We found that for both types of surfaces, the energies of the specularly reflected components decrease with increasing wavelength. However, the underlying reflection mechanisms are different. For the amorphous surface, the energy of the incident wave packet will first enter the amorphous layer before being partially and diffusely scattered in this layer. The local density and elastic moduli fluctuations as well as the localized vibrational modes are reasons for the diffuse scattering in the amorphous region, which affects the short wavelength phonons more severely. For the rough surface, the diffuse reflection is the combined effect from surface irregularity, surface Rayleigh waves, and spatially localized modes induced by the surface roughness. The extent of diffuse reflection at the rough surface can be tuned by engineering the surface boundary condition.

Dirac particles have been notoriously difficult to confine. Implementing a curved-space Dirac equation solver based on the quantum lattice Boltzmann method, we show that curvature in a two-dimensional space can confine a portion of a charged, massless Dirac fermion wave packet. This is equivalent to a finite probability of confining the Dirac fermion within a curved-space region. We propose a general power-law expression for the probability of confinement with respect to average spatial curvature for the studied geometry.

We consider a small $p$-type island defined within an $n$-type graphene nanoribbon induced by the potential of a floating electrode. In the quantum Hall conditions the island supports persistent currents localized at the $n-p$ junction. When coupled to the graphene edge, the island acts as an Aharonov-Bohm interferometer. We evaluate the electrostatic potential induced by the floating gate within the ribbon near the charge neutrality point and consider equilibration of the currents at both sides of the junction. The incoherent equilibration is introduced by the virtual-probes technique. We describe the evolution of the coherent Aharonov-Bohm conductance oscillations to the quantum Hall fractional plateaus due to the current equilibration.