We propose an SU(3) symmetric Hamiltonian with short-range interactions on the kagome lattice and show that it hosts an Abelian chiral spin liquid (CSL) state. We provide numerical evidence based on exact diagonalization to show that this CSL state is stabilized in an extended region of the parameter space and can be viewed as a lattice version of the Halperin 221 fractional quantum Hall state of two-component bosons. We also construct a parton wave function for this CSL state and demonstrate that its variational energies are in good agreement with exact diagonalization results. The parton description further supports that the CSL is characterized by a chiral edge conformal field theory of the $\mathrm{SU}{\left(3\right)}_1$ Wess-Zumino-Witten type.

Motivated by the recent development in strong spin-orbit-coupled materials, we consider the dipole-octupole doublets on the triangular lattice. We propose the most general interaction between these unusual local moments. Due to the spin-orbit entanglement and the special form of its wave function, the dipole-octupole doublet has a rather peculiar property under the lattice symmetry operation. As a result, the interaction is highly anisotropic in the pseudospin space, but remarkably, is uniform spatially. We analyze the ground state properties of this generic model and emphasize the hidden multipolar orders that emerge from the dipolar and octupolar interactions. We clarify the quantum mutual modulations between the dipolar and octupolar orders. We predict the experimental consequences of the multipolar orders and propose the rare-earth triangular materials as candidate systems for these unusual properties.

Motivated by recently discovered quasi-one-dimensional superconductor ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ with $D_{3h}$ lattice symmetry, we study a one-dimensional three-orbital Hubbard model with generic electron repulsive interaction described by intraorbital repulsion $U$, interorbital repulsion $U^{\prime }$, and Hund's coupling $J$. As extracted from density functional theory calculation, two of the three atomic orbitals are degenerate ($E^{\prime }$ states) and the third one is nondegenerate ($A_1^{\prime }$), and the system is at incommensurate filling. With the help of bosonization, the normal state is described by a three-band Tomonaga-Luttinger liquid. Possible charge density wave (CDW), spin density wave (SDW), and superconducting (SC) instabilities are analyzed by renormalization group method. The ground state depends on the ratio $J/U$ and is sensitive to the degeneracy of $E^{\prime }$ bands. The spin-singlet SC state is favored at $0<J<U/3$, and the spin-triplet SC state is favored in the region $U/3<J<U/2$. The SDW state has the lowest energy only in the unphysical parameter region $J>U/2$. When the twofold degeneracy of $E^{\prime }$ bands is lifted, the SDW instability has the tendency to dominate over the spin-singlet SC state at $0<J<U/3$, while the order parameter of the spin-triplet SC state will be modulated by a phase factor $2\mathrm{\Delta }{k}_{F}x$ at $U/3<J<U/2$. Possible experimental consequences and applications to ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ are discussed.

We investigate the electronic structure, dielectric, and optical properties of bismuth tellurohalides $\mathrm{BiTe}X$ ($X=\text{I}$, Cl, Br) by means of all-electron density functional theory. In particular, we present the ab initio conductivities and dielectric tensors calculated over a wide frequency range, and compare our results with the recent measurements by Akrap et al. [Phys. Rev. B 90, 035201 (2014)], Makhnev et al. [Opt. Spectrosc. 117, 764 (2014)], and Rusinov et al. [JETP Lett. 101, 507 (2015)]. We show how the low-frequency branch of the optical conductivity can be used to identify characteristic intra- and interband transitions between the Rashba spin-split bands in all three bismuth tellurohalides. We further calculate the refractive indices and dielectric constants, which in turn are systematically compared to previous predictions and measurements. We expect that our quantitative analysis will contribute to the general assessment of bulk Rashba materials for their potential use in spintronics devices.

We study the magnetoconductance of topological insulator nanowires in a longitudinal magnetic field, including Aharonov-Bohm, Altshuler-Aronov-Spivak, perfectly conducting channel, and universal conductance fluctuation effects. Our focus is on predicting experimental behavior in single wires in the quantum limit where temperature is reduced to zero. We show that changing the Fermi energy $E_F$ can tune a wire from from ballistic to diffusive conduction and to localization. In both ballistic and diffusive single wires we find both Aharonov-Bohm and Altshuler-Aronov-Spivak oscillations with similar strengths, accompanied by quite strong universal conductance fluctuations, all with amplitudes between $0.3G_0$ and $1G_0$. This contrasts strongly with the average behavior of many wires, which shows Aharonov-Bohm oscillations in the ballistic regime and Altshuler-Aronov-Spivak oscillations in the diffusive regime, with both oscillations substantially larger than the conductance fluctuations. In single wires the ballistic and diffusive regimes can be distinguished by varying $E_F$ and studying the sign of the Aharonov-Bohm signal, which depends periodically on $E_F$ in ballistic wires and randomly on $E_F$ in diffusive wires. We also show that in long wires the perfectly conducting channel is visible at a wide range of energies within the bulk gap. We present typical conductance profiles at several wire lengths, showing that conductance fluctuations can dominate the average signal. Similar behavior will be found in carbon nanotubes.

We study theoretically the transport properties of a three-dimensional spin texture made from three orthogonal helices, which is essentially a lattice of monopole-antimonopole pairs connected by skyrmion strings. This spin structure is proposed for MnGe based on neutron scattering experiments as well as Lorentz transmission electron microscopy observations. Equipped with a sophisticated spectral analysis method, we adopt the finite temperature Green's function technique to calculate the longitudinal dc electric transport in such a system. We consider conduction electrons interacting with spin waves of the topologically nontrivial spin texture, wherein fluctuations of monopolar emergent magnetic fields enter. We study in detail the behavior of electric resistivity under the influence of temperature, external magnetic field, and a characteristic monopole motion, especially a novel magnetoresistivity effect describing the latest experimental observations in MnGe, wherein a topological phase transition signifying strong correlations is identified.

We report a theoretical study of the ac response of superconducting quantum metamaterials (SQMs), i.e., an array of qubits (two-level systems) embedded in a low-dissipative resonator. By making use of a particular example of a SQM, namely the array of charge qubits capacitively coupled to the resonator, we obtain a second-order phase transition between an incoherent (the high-temperature phase) and coherent (the low-temperatures phase) state of photons. This phase transition in many aspects resembles the paramagnetic-ferromagnetic phase transition. The critical temperature of the phase transition, $T^{☆}$, is determined by the energy splitting of two-level systems $\delta$, number of qubits in the array $N$, and the strength of the interaction $\eta$ between qubits and photons in the cavity. We obtain that the photon states manifest themselves by resonant drops in the frequency-dependent transmission $D\left(\omega \right)$ of electromagnetic waves propagating through a transmission line weakly coupled to the SQM. At high temperatures the $D\left(\omega \right)$ displays a single resonant drop, and at low temperatures a peculiar double-resonance response has to be observed. The physical origin of such a resonant splitting is the quantum oscillations between two coherent states of photons of different polarizations.

We investigate analytically and numerically eigenfunction statistics in a disordered system on a finite Bethe lattice (Cayley tree). We show that the wave-function amplitude at the root of a tree is distributed fractally in a large part of the delocalized phase. The fractal exponents are expressed in terms of the decay rate and the velocity in a problem of propagation of a front between unstable and stable phases. We demonstrate a crucial difference between a loopless Cayley tree and a locally treelike structure without a boundary (random regular graph) where extended wave functions are ergodic.

We report the ultrafast pump-probe study of $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$, Sb, Bi, and Te aimed to check the two-tensor model predictions for the creation of lattice coherence. The dependence of coherent ultrafast response on phonon frequency was measured for topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$, the spectrum of which possesses two fully symmetric phonons. The effect of the pump pulse duration and power on the magnitude of coherent amplitude was evaluated in the model opaque crystals, such as two semimetals, bismuth and antimony, and semiconducting tellurium. In our analysis of the pump-probe data, we separated the transient total reflectivity into the sum of two contributions: one due to the photogenerated carriers and the second due to the coherent phonons. All fully symmetric phonons exhibit a cosinelike dependence and grow linearly with increasing average pump power provided the pulse duration remains unchanged. Varying the pump pulse duration, we observed a monotonic decrease of coherent amplitude for longer pulses, whereas the electronic contribution was almost unchanged. This lack of the correlation between the carriers and the coherent amplitude was further supported by coherent control experiments on Te. Based on the comparison of theoretical predictions with experimental observations, we conjecture that the lattice coherence creation in opaque crystals can be linked to a Raman-like process.

In the last 10 years, it has been shown in various types of experiments that it is possible to induce biological effects in cells using the torque generated by magnetic nanoparticles submitted to an alternating or a rotating magnetic field. In biological systems, particles are generally found under the form of assemblies because they accumulate at the cell membrane, are internalized inside lysosomes, or are synthesized under the form of beads containing several particles. The torque undergone by assemblies of single-domain magnetic nanoparticles has not been addressed theoretically so far and is the subject of the present article. The results shown in the present article have been obtained using kinetic Monte Carlo simulations, in which thermal activation is taken into account, so the torque undergone by ferromagnetic and superparamagnetic nanoparticles could both be simulated. The first system under study is a single ferromagnetic particle with its easy axis in the plane of the rotating magnetic field. Then, elements adding complexity to the problem are introduced progressively and the properties of the resulting system presented and analyzed: random anisotropy axes, thermal activation, assemblies, and finally magnetic interactions. The most complex studied systems are particularly relevant for applications and are assemblies of interacting superparamagnetic nanoparticles with randomly oriented anisotropy axes. Whenever it is possible, analytical equations describing the torque properties are provided, as well as their domain of validity. Although the properties of an assembly naturally derive from those of single particles, it is shown here that several of them were unexpected and are particularly interesting with regard to the maximization of torque amplitude in biological applications. In particular, it is shown that, in a given range of parameters, the torque of an assembly increases dramatically in the direction perpendicular to the plane of the rotating magnetic field. This effect results from a breaking of time reversal symmetry when the field is rotated and is comprehensively explained. This strong enhancement occurs only if the magnetic field rotates, not if it oscillates. When this enhancement does not occur, the total torque of an assembly scales with the square root of the number of particles in the assembly. In the enhancement regime, the total torque scales with a power exponent larger than 1/2. It is also found that, in superparamagnetic nanoparticles, this enhancement is induced by the presence of magnetic interactions so that, in a rather large range of parameters, interacting superparamagnetic particles display a much larger torque than otherwise identical ferromagnetic particles. In all cases studied, the conditions required to obtain this enhancement are provided. The concepts presented in this article should help chemists and biologists in synthesizing nano-objects with optimized torque properties. For physicists, it would be interesting to test experimentally the results described in this article. For this purpose, torque measurements on well-characterized assemblies of nanoparticles should be performed and compared with numerical simulations.

Magnetization excitation in micron sized FeNi disks with different diameters is studied by broadband ferromagnetic resonance (FMR) measurement. Except the main FMR peak, additional adsorption peaks with lower energies are observed. Both micromagnetic simulation and quantum spin wave calculation confirm that the low-energy excitation states are attributed to backward volume magnetostatic (BVM) spin waves. The size dependence of the low-energy states is systematically studied in 50-nm-thick Py disks with diameters larger than 500 nm, and the linewidth of the first BVM state is found to be obviously smaller than that of the FMR absorption peak. Through a quantitative comparison with experimental results, the quantum spin wave calculation is proven to be a reliable method to get the susceptibility and is much faster than the classical micromagnetic simulations.

How to control the formation of native defects is crucial to obtaining high-quality samples and realizing the balance between electrons and holes for achieving high carrier mobilities in noncentrosymmetric Weyl fermion semimetals (WSMs). Using first-principles calculations, we explore the formation mechanisms of native defects in the family of $TX$ ($T=\text{Ta}$, Nb; $X=\text{As}$, P), and find that the spin-orbit coupling (SOC) is not only intrinsic to these semimetals but also plays a significant role in dictating the formation of native defects. The calculated defect formation energies with the SOC are lower than those without the SOC. The detailed analyses of partial density of states reveal that the valence shells of $T$–$d$ and $X$–$p$ hybridization states contribute to the antibonding states in $TX$ compounds. The broadness of $T$–$d$ and $X$–$4p$ hybridization states with the SOC inclusion increases by about 1 eV compared with the corresponding $TX$ without the SOC consideration. The more delocalized $T$–$d$ and $X$–$p$ hybridization states increase the energy of antibonding states and further attribute to the stabilization of native defects with the reduced formation energies in $TX$ compounds. We also estimate the defect concentrations based on our accurately calculated formation energies of native defects, and propose practical strategies to control their concentrations to grow high-quality samples. Our results provide insights to the defect behavior under the effect of the SOC in WSMs.

We study the generation of terahertz (THz) radiation from the acceleration of ultrafast photoexcited charge carriers in graphene in the presence of a dc electric field. Our model is based on calculating the transient current density from the time-dependent distribution function which is determined using the Boltzmann transport equation (BTE) within a relaxation time approximation. We include the time-dependent generation of carriers by the pump pulse by solving for the carrier generation rate using the optical Bloch equations in the rotating wave approximation (RWA). The linearly polarized pump pulse generates an anisotropic distribution of photoexcited carriers in the $k_x-k_y$ plane. The collision integral in the Boltzmann equation includes a term that leads to the thermalization of carriers via carrier-carrier scattering to an effective temperature above the lattice temperature, as well as a cooling term, which leads to energy relaxation via inelastic carrier-phonon scattering. The radiated signal is proportional to the time derivative of the transient current density. In spite of the fact that the magnitude of the velocity is the same for all the carriers in graphene, there is still emitted radiation from the photoexcited charge carriers with frequency components in the THz range due to a change in the direction of velocity of the photoexcited carriers in the external electric field as well as cooling of the photoexcited carriers on a subpicosecond time scale.

It is well established that formation energies of point defects depend on the chemical potential (μ) and Fermi level position $\left(E_F\right)$, which is widely used when modeling diffusion phenomena in semiconductors. In return, Arrhenius analysis of self-diffusion can be exploited for the investigation of point defect energetics since self-diffusion is mediated by intrinsic point defects. Specifically, the energetics of Zn vacancies ($V_{\mathrm{Zn}}$) and/or Zn interstitials in ZnO can be potentially revealed via Zn self-diffusion measurements. In this study we have measured Zn self-diffusion varying μ (by shifting from Zn- to O-rich conditions during the sample synthesis) and $E_F$ (by Ga, F, and Cu in situ doping). Corresponding diffusion activation energies were deduced and are discussed in terms of the vacancy diffusion mechanism. This results in an upper limit estimate for the $V_{\mathrm{Zn}}$ migration energy of $\sim 1.5\mathrm{eV}$, and prominent trends for the $V_{\mathrm{Zn}}$ formation energy as a function of μ and $E_F$ are revealed. Concurrently, it is argued that dopant-$V_{\mathrm{Zn}}$ clustering and $E_F$ pinning at deep donor traps should be taken into account when generalizing the interpretation of diffusion data for impurities in ZnO.

Photonic topological insulators are an interesting class of materials whose photonic band structure can have a band gap in the bulk while supporting topologically protected unidirectional edge modes. Recent studies on bianisotropic metamaterials that emulate the electronic quantum spin Hall effect using its electromagnetic analog are examples of such systems with a relatively simple and elegant design. In this paper, we present a rotating magnetic dipole antenna, composed of two perpendicularly oriented coils, that can efficiently excite the unidirectional topologically protected surface waves in the bianisotropic metawaveguide (BMW) structure recently realized by T. Ma et al. [Phys. Rev. Lett. 114, 127401 (2015)] despite the fact that the BMW medium does not break time-reversal invariance. In addition to achieving a high directivity, the antenna can be tuned continuously to excite reflectionless edge modes in the two opposite directions at various amplitude ratios. We demonstrate its performance through experiments and compare the results to simulation results.

Numerous experimental and theoretical studies have focused on low-dimensional systems locally perturbed by the biased tip of a scanning force microscope. In all cases either open or closed weakly gate-tunable nanostructures have been investigated, such as quantum point contacts, open or closed quantum dots, etc. We study the behavior of the conductance of a quantum point contact with a gradually forming adjacent cavity in series under the influence of a scanning gate. Here, an initially open quantum point contact system gradually turns into a closed cavity system. We observe branches and interference fringes known from quantum point contacts coexisting with irregular conductance fluctuations. Unlike the branches, the fluctuations cover the entire area of the cavity. In contrast to previous studies, we observe and investigate branches under the influence of the confining stadium potential, which is gradually built up. We find that the branches exist only in the area surrounded by cavity top gates. As the stadium shrinks, regular fringes originate from tip-induced constrictions leading to quantized conduction. In addition, we observe arclike areas reminiscent of classical electron trajectories in a chaotic cavity. We also argue that electrons emanating from the quantum point contact spread out like a fan leaving branchlike regions of enhanced backscattering.

We show that the highly frustrated transverse-field Ising model on the three-dimensional pyrochlore lattice realizes a first-order phase transition without symmetry breaking between the low-field Coulomb quantum spin liquid and the high-field polarized phase. The quantum phase transition is located quantitatively by comparing low- and high-field series expansions. Furthermore, the intriguing properties of the elementary excitations in the polarized phase are investigated. We argue that this model can be achieved experimentally by applying mechanical strain to a classical spin-ice material composed of non-Kramers spins such as ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Taken together with our results, this provides an experimental platform to study quantum spin liquid physics.

We studied the magnetic ground state of Co/Rh-disordered ${\mathrm{LaCo}}_{1-x}{\mathrm{Rh}}_x{\mathrm{O}}_3$ ($x=0.4$, 0.5, and 0.6) by means of powder neutron diffraction (PND), muon spin relaxation ($\mu \mathrm{SR}$), ac susceptibility, isothermal magnetic relaxation, and heat capacity measurements. The dc magnetic susceptibility measurements exhibit a cusp around the freezing temperature $T_f$ of all studied compounds. PND measurements show no indications of long-range magnetic ordering down to 4 K in all studied samples. $\mu \mathrm{SR}$ experiments on these compounds show a peak in the muon spin relaxation rate around $T_f$. However, no spontaneous muon spin precession can be observed evidencing the short-range nature of the magnetic ordering. For the $x=0.5$ sample ac susceptibility measurements show that $T_f$ shifts to higher temperatures with increasing frequency and becomes suppressed by a dc biasing field. All these observations indicate that the Co/Rh-disordered ${\mathrm{LaCo}}_{1-x}{\mathrm{Rh}}_x{\mathrm{O}}_3$ system is a spin glass for $x=0.4$, 0.5, and 0.6.

We report a multiscale modeling of electronic structure of a conducting polymer poly(3,4-ethylenedioxythiopehene) (PEDOT) based on a realistic model of its morphology. We show that when the charge carrier concentration increases, the character of the density of states (DOS) gradually evolves from the insulating to the semimetallic, exhibiting a collapse of the gap between the bipolaron and valence bands with the drastic increase of the DOS between the bands. The origin of the observed behavior is attributed to the effect of randomly located counterions giving rise to the states in the gap. These results are discussed in light of recent experiments. The method developed in this work is general and can be applied to study the electronic structure of other conducting polymers.