We describe the nonlocal effective properties of a two-dimensional dissipative phononic crystal made by periodic arrays of rigid and motionless cylinders embedded in a viscothermal fluid such as air. The description is based on a nonlocal theory of sound propagation in stationary random fluid/rigid media that was proposed by Lafarge and Nemati [Wave Motion 50, 1016 (2013)]. This scheme arises from a deep analogy with electromagnetism and a set of physics-based postulates including, particularly, the action-response procedures, whereby the effective density and bulk modulus are determined. Here, we revisit this approach, and clarify further its founding physical principles through presenting it in a unified formulation together with the two-scale asymptotic homogenization theory that is interpreted as the local limit. Strong evidence is provided to show that the validity of the principles and postulates within the nonlocal theory extends to high-frequency bands, well beyond the long-wavelength regime. In particular, we demonstrate that up to the third Brillouin zone including the Bragg scattering, the complex and dispersive phase velocity of the least-attenuated wave in the phononic crystal which is generated by our nonlocal scheme agrees exactly with that reproduced by a direct approach based on the Bloch theorem and multiple scattering method. In high frequencies, the effective wave and its associated parameters are analyzed by treating the phononic crystal as a random medium.

The rich physics associated with magnetism often centers around directional effects. Here we demonstrate how spin-transfer torques in general result in unidirectional ferromagnetic resonance dynamics upon field reversal. The unidirectionality is a direct consequence of both field-like and damping-like dynamic torques simultaneously driving the motion. This directional effect arises from the field-like torque being odd and the damping-like torque being even under field reversal. The directional effect is observed when the magnetization has both an in-plane and out-of-plane component, since then the linear combination of the torques rotates with a different handedness around the magnetization as the magnetization is tipped out-of-plane. The effect is experimentally investigated via spin-torque ferromagnetic resonance measurements with the field applied at arbitrary directions away from the interface normal. The measured asymmetry of the voltage spectra are well explained within a phenomenological torque model.

The electronic structure of ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ has been investigated using a combination of first-principles calculations, x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption. The full range of compositions, $x=0,1/2,1$, and a variety of magnetic orders have been covered. Jahn-Teller as well as Zener polaron orders are considered. The free parameters of the local hybrid density functionals used in this study have been determined by comparison with measured XPS spectra. A model Hamiltonian, valid for the entire doping range, has been extracted. A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided. The comparison of theoretical calculations and different experimental spectra provide a detailed and consistent picture of the electronic structure. The large variations of measured optical absorption spectra are traced back to the co-existence of magnetic orders (respectively, to the occupation of local orbitals). A consistent treatment of the Coulomb interaction indicates a partial cancellation of Coulomb parameters and supports the dominance of the electron-phonon coupling.

Electrons in periodic potentials exhibit oscillatory motion in the presence of an electric field. Such oscillations are known as Bloch oscillations. In this article we theoretically investigate the emergence of Bloch oscillations for systems where the electric field is confined to a finite region, like in typical electronic devices. We use a one-dimensional tight-binding model within the single-band approximation to numerically study the dynamics of electrons after a sudden switching-on of the electric field. We find a transition from a regime with direct current to Bloch oscillations when increasing the system size or decreasing the field strength. We propose a pump-probe scheme to observe the oscillations by measuring the accumulated charge as a function of the pulse length.

By using density-functional theory and many-body perturbation theory based first-principles calculations, we have systematically investigated the electronic and optical properties of monolayer group-IV monochalcogenides $MX$ ($M=\mathrm{Ge},\mathrm{Sn}$; $X=\mathrm{S},\mathrm{Se},\mathrm{Te}$). All $MX$ monolayers are predicted to be indirect gap semiconductors, except the GeSe monolayer, which has a direct gap of 1.66 eV. The carrier mobilities of $MX$ monolayers are estimated to be on the order of ${10}^3$ to ${10}^5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, which is comparable to, and in some cases higher than, that of phosphorene using a phonon-limited scattering model. Moreover, the optical spectra of $MX$ monolayers obtained from GW-Bethe-Salpeter equation calculations are highly orientation dependent, especially for the GeS monolayer, suggesting their potential application as a linear polarizing filter. Our results reveal that the GeSe monolayer is an attractive candidate for optoelectronic applications as it is a semiconductor with a direct band gap, a relatively high carrier mobility, and an onset optical absorption energy in the visible light range. Finally, based on an effective-mass model with nonlocal Coulomb interaction included, we find that the excitonic effects of the GeSe monolayer can be effectively tuned by the presence of dielectric substrates. Our studies provide an improved understanding of electronic, optical, and excitonic properties of group-IV monochalcogenides monolayers and might shed light on their potential electronic and optoelectronic applications.

We study anisotropies of the helicity modulus, excitation spectrum, sound velocity, and angle-resolved luminescence spectrum in a two-dimensional system of interacting excitons in a periodic potential. Analytical expressions for anisotropic corrections to the quantities characterizing superfluidity are obtained. We consider particularly the case of dipolar excitons in quantum wells. For GaAs/AlGaAs heterostructures as well as ${\mathrm{MoS}}_2$/hBN/${\mathrm{MoS}}_2$ and ${\mathrm{MoSe}}_2$/hBN/${\mathrm{WSe}}_2$ transition-metal dichalcogenide bilayers estimates of the magnitude of the predicted effects are given. We also present a method to control superfluid motion and to determine the helicity modulus in generic dipolar systems.

Deformations of single-wall carbon nanotubes are investigated within the tight-binding model with deformation-dependent hopping energies. We show that the nanotubes tend to twist and shrink spontaneously at zero temperature. The explicit values of the deformation parameters are computed for a wide range of nanotubes with varying diameter and chirality. The changes of the spectral gap associated with the spontaneous deformation are shown to depend on the chirality of the nanotubes.

We study the thermoelectric properties and heat-to-work conversion performance of an interacting, multilevel quantum dot (QD) weakly coupled to electronic reservoirs. We focus on the sequential tunneling regime. The dynamics of the charge in the QD is studied by means of master equations for the probabilities of occupation. From here we compute the charge and heat currents in the linear response regime. Assuming a generic multiterminal setup, and for low temperatures (quantum limit), we obtain analytical expressions for the transport coefficients which account for the interplay between interactions (charging energy) and level quantization. In the case of systems with two and three terminals we derive formulas for the power factor $Q$ and the figure of merit $ZT$ for a QD-based heat engine, identifying optimal working conditions which maximize output power and efficiency of heat-to-work conversion. Beyond the linear response we concentrate on the two-terminal setup. We first study the thermoelectric nonlinear coefficients assessing the consequences of large temperature and voltage biases, focusing on the breakdown of the Onsager reciprocal relation between thermopower and Peltier coefficient. We then investigate the conditions which optimize the performance of a heat engine, finding that in the quantum limit output power and efficiency at maximum power can almost be simultaneously maximized by choosing appropriate values of electrochemical potential and bias voltage. At last we study how energy level degeneracy can increase the output power.

This paper presents an extensive study of the structural, magnetic, and thermodynamic properties of the heterometallic ludwigite ${\mathrm{Co}}_{4.76}{\mathrm{Al}}_{1.24}{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$. This material orders ferrimagnetically at 57 K. Despite the fact that, in ${\mathrm{Co}}_{4.76}{\mathrm{Al}}_{1.24}{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$, one quarter of the Co ions of the parent compound ${\mathrm{Co}}_3{\mathrm{O}}_2{\mathrm{BO}}_3$ is replaced by nonmagnetic ${\mathrm{Al}}^{3+}$ ions, the $T_C$ of the former (57 K) is larger than that of the latter (42 K). We obtain the phase diagram of ${\mathrm{Co}}_{4.76}{\mathrm{Al}}_{1.24}{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$ in an external magnetic field. We find a line of second-order phase transitions where the system presents a metamagnetic transition (from 57 to 25 K). This line ends at a tricritical point at 25 K and 4.3 T, below which the transition to the paramagnetic phase is first order (from 25 to 3 K). In the magnetic phase at low temperatures specific heat measurements in zero and finite magnetic fields show that the low-energy excitations are magnetic (magnons) and elastic (phonons) excitations with two-dimensional character. These measurements also reveal the magnetocaloric properties of this material.

We present a mean-field theory describing the influence of long-range dipolar forces on the temperature transition from the paramagnetic to ordered phases in frustrated Heisenberg spiral magnets. It is shown that the dipolar interaction produces a cascade of first- and second-order phase transitions between the paramagnetic and the spiral states upon temperature decreasing. Depending on system parameters, the following intermediate phases can arise: an incommensurate and a commensurate sinusoidally modulated state, spiral phases in which perpendicular spin components have different amplitudes and are modulated with the same and with different wave vectors. We distinguish six possible sequences of phase transitions upon temperature decreasing at least four of which were observed before experimentally in specific compounds. It is found that the action of dipolar forces cannot always be modeled even qualitatively by small one-ion anisotropic spin interactions. We demonstrate that the dipolar interaction is responsible for successive phase transitions in the triangular-lattice multiferroic ${\mathrm{MnI}}_2$: Almost all available experimental findings are described quantitatively within the mean-field theory by taking into account the exchange, the dipolar and small symmetry-allowed anisotropic spin interactions.

The longitudinal spin Seebeck effect (LSSE) consists in the generation of a spin current parallel to a temperature gradient in a magnetic material. The LSSE has only been measured unequivocally in magnetic insulators because in metallic films it is contaminated by the anomalous Nernst effect (ANE). Here we report theoretical and experimental studies of the LSSE in the metallic ferromagnet ${\mathrm{N}}_{81}\mathrm{F}{\mathrm{e}}_{19}$ (permalloy-Py) separated from the ANE. We have used trilayer samples of Py/NiO/NM (NM is a normal metal, Pt or Ta) under a temperature gradient perpendicular to the plane to generate a spin current in Py that is transported across the NiO layer and reaches the NM layer, where it is converted into a charge current by the inverse spin Hall effect. The LSSE is detected by a voltage signal in the NM layer while the ANE is measured by the voltage induced in the Py layer. The separation of the two effects is made possible because the antiferromagnetic insulator NiO layer transports spin current while providing electrical insulation between the Py and NM layers. The measured spin Seebeck coefficient for Py has a value similar to the one for the ferrimagnetic insulator yttrium iron garnet, with the same sign, and is in good agreement with the value calculated with a thermoelectric spin drift-diffusion model.

We consider the situation when a femtosecond laser pulse creates a hot electron state in a half-metallic ferromagnet (e.g., ferromagnetic semiconductor) on a picosecond time scale but does not act directly on the localized spin system. We show that the energy and magnetic moment transfer from hot itinerant electrons to localized spins is facilitated by so-called nonquasiparticle states, which are the scattering states of a magnon and spin-majority electron. The magnon distribution is described by a quantum kinetic equation that we derive using the Keldysh diagram technique. In a typical ferromagnetic semiconductor such as EuO, magnons remain essentially in nonequilibrium on a scale of the order of microseconds after the laser pulse.

The magnetic interactions between rare-earth and Fe ions in hexagonal rare-earth ferrites $\left(h\text{−}R{\mathrm{FeO}}_3\right)$, may amplify the weak ferromagnetic moment on Fe, making these materials more appealing as multiferroics. To elucidate the interaction strength between the rare-earth and Fe ions as well as the magnetic moment of the rare-earth ions, element-specific magnetic characterization is needed. Using x-ray magnetic circular dichroism, we have studied the ferrimagnetism in $h\text{−}{\mathrm{YbFeO}}_3$ by measuring the magnetization of Fe and Yb separately. The results directly show antialignment of magnetization of Yb and Fe ions in $h\text{−}{\mathrm{YbFeO}}_3$ at low temperature, with an exchange field on Yb of about 17 kOe. The magnetic moment of Yb is about $1.6{\mu }_B$ at low temperature, significantly reduced compared with the $4.5{\mu }_B$ moment of a free ${\mathrm{Yb}}^{3+}$. In addition, the saturation magnetization of Fe in $h\text{−}{\mathrm{YbFeO}}_3$ has a sizable enhancement compared with that in $h\text{−}{\mathrm{LuFeO}}_3$. These findings directly demonstrate that ferrimagnetic order exists in $h\text{−}{\mathrm{YbFeO}}_3$; they also account for the enhancement of magnetization and the reduction of coercivity in $h\text{−}{\mathrm{YbFeO}}_3$ compared with those in $h\text{−}{\mathrm{LuFeO}}_3$ at low temperature, suggesting an important role for the rare-earth ions in tuning the multiferroic properties of $h\text{−}R{\mathrm{FeO}}_3$.

The low energy magnetic excitations spectra of the pristine and doped quasi-one-dimensional spin chains cuprates $\mathrm{SrCu}{\mathrm{O}}_2$ have been investigated by inelastic neutron scattering. The momentum-integrated magnetic dynamical structure factor yields a constant integrated intensity with regard to energy in the pure compound, while it shows a strong decay, at low energies, in the compounds doped with nonmagnetic impurities, namely, $\mathrm{SrC}{\mathrm{u}}_{0.99}{\mathrm{M}}_{0.01}{\mathrm{O}}_2$ (with $M=\mathrm{Zn}$ or Mg) and $\mathrm{S}{\mathrm{r}}_{0.99}\mathrm{L}{\mathrm{a}}_{0.01}\mathrm{Cu}{\mathrm{O}}_2$ ($\mathrm{C}{\mathrm{u}}^{+}$ carrying $S=0$ created within the chains). These results evidence the opening of a spin pseudogap in the two-spinon continuum of $\mathrm{SrCu}{\mathrm{O}}_2$ upon doping, stemming from disruptions of the spin chains by quantum impurities.

Spin current plays a central role in spintronics. In particular, finding more efficient ways to generate spin current has been an important issue and has been studied actively. For example, representative methods of spin-current generation include spin-polarized current injections from ferromagnetic metals, the spin Hall effect, and the spin battery. Here, we theoretically propose a mechanism of spin-current generation based on nonlinear phenomena. By using Boltzmann transport theory, we show that a simple application of the electric field $\mathit{E}$ induces spin current proportional to ${\mathit{E}}^{\mathit{2}}$ in noncentrosymmetric spin-orbit coupled systems. We demonstrate that the nonlinear spin current of the proposed mechanism is supported in the surface state of three-dimensional topological insulators and two-dimensional semiconductors with the Rashba and/or Dresselhaus interaction. In the latter case, the angular dependence of the nonlinear spin current can be manipulated by the direction of the electric field and by the ratio of the Rashba and Dresselhaus interactions. We find that the magnitude of the spin current largely exceeds those in the previous methods for a reasonable magnitude of the electric field. Furthermore, we show that application of ac electric fields (e.g., terahertz light) leads to the rectifying effect of the spin current, where dc spin current is generated. These findings will pave a route to manipulate the spin current in noncentrosymmetric crystals.

Three-dimensional quantum electrodynamics exhibits a number of interesting properties, such as dynamical chiral symmetry breaking, weak confinement, and non-Fermi-liquid behavior, and also has wide applications in condensed-matter physics. We study the effects of random potentials, which exist in almost all realistic condensed-matter systems, on the low-energy behaviors of massless Dirac fermions by means of renormalization-group method, and show that the role of random mass is significantly enhanced by the gauge interaction, whereas random scalar and vector potentials are insusceptible to the gauge interaction at the one-loop order. The static random potential breaks the Lorentz invariance, and as such induces unusual renormalization of fermion velocity. We then consider the case in which three types of random potentials coexist in the system. The random scalar potential is found to play a dominant role in the low-energy region, and drives the system to undergo a quantum phase transition.

Metallic phases have been observed in several disordered two-dimensional (2D) systems, including thin films near superconductor-insulator transitions and quantum Hall systems near plateau transitions. The existence of 2D metallic phases at zero temperature generally requires an interplay of disorder and interaction effects. Consequently, experimental observations of 2D metallic behavior have largely defied explanation. We formulate a general stability criterion for strongly interacting, massless Dirac fermions against disorder, which describe metallic ground states with vanishing density of states. We show that (2+1)-dimensional quantum electrodynamics (${\mathrm{QED}}_3$) with a large, even number of fermion flavors remains metallic in the presence of weak scalar potential disorder due to the dynamic screening of disorder by gauge fluctuations. We also show that ${\mathrm{QED}}_3$ with weak mass disorder exhibits a stable, dirty metallic phase in which both interactions and disorder play important roles.

Quantum electrodynamics in 2+1 dimensions (${\mathrm{QED}}_3$) is a strongly coupled conformal field theory (CFT) of a U(1) gauge field coupled to $2N$ two-component massless fermions. The $N=2$ CFT has been proposed as a ground state of the spin-1/2 kagome Heisenberg antiferromagnet. We study ${\mathrm{QED}}_3$ in the presence of weak quenched disorder in its two spatial directions. When the disorder explicitly breaks the fermion flavor symmetry from $\mathrm{SU}\left(2N\right)$ $\to$ U(1) $\times$ $\mathrm{SU}\left(N\right)$ but preserves time-reversal symmetry, we find that the theory flows to a nontrivial fixed line at nonzero disorder with a continuously varying dynamical critical exponent $z>1$. We determine the zero-temperature flavor (spin) conductivity along the critical line. Our calculations are performed in the large-$N$ limit, and the disorder is handled using the replica method.