We show how tuning the proximity to the soft optical mode phase transition via chemical composition affects the lattice thermal conductivity $\kappa$ of ${\mathrm{Pb}}_{1-x}{\mathrm{Ge}}_x\mathrm{Te}$ alloys. Using first-principles virtual-crystal simulations, we find that the anharmonic contribution to $\kappa$ is minimized at the phase transition due to the maximized acoustic-optical anharmonic interaction. Mass disorder significantly lowers and flattens the dip in the anharmonic $\kappa$ over a wide composition range, thus shifting the $\kappa$ minimum away from the phase transition. The total $\kappa$ and its anharmonic contribution vary continuously between the rocksalt and rhombohedral phases as expected for the second-order phase transition. The actual phase and its strength of resonant bonding play a less prominent role in reducing the $\kappa$ of ${\mathrm{Pb}}_{1-x}{\mathrm{Ge}}_x\mathrm{Te}$ alloys than the proximity to the phase transition and the atomic mass. Our results show that alloys with soft optical mode transitions are promising materials for achieving low thermal conductivity and possibly high thermoelectric efficiency.

We investigate the nature of the electronic states in a variety of nonsymmorphic collinear antiferromagnets with glide reflection symmetry, a combination of mirror and half-lattice translation. In particular, the study refers to a class of systems with two-band itinerant electrons that are spin-orbit coupled and interacting with a magnetic background having a zigzag pattern. We describe the symmetry properties of the model system by focusing on the role of nonsymmorphic transformations arising from the antiferromagnetic structure of the spin ordering. Gapless phases with Dirac points having different types of symmetry-protection as well as electronic structures with triple and quadruple band-crossing points are obtained. A glide semimetal is shown to be converted into a gapless phase with Dirac points protected by inversion and time-inversion symmetry combination. Interestingly, we find a relation between the states in the glide sectors that provides a general mechanism to get multiple band touching points. The split of the multiple Fermi points drives the transition from a point node to a line node semimetal or to a metal with nontrivial winding around the Fermi pockets and an electronic structure that is tied to the presence of glide symmetric Dirac points. Besides a new perspective of ordered states in complex materials, our findings indicate relevant paths to topological gapless phases and edge states in a wide class of magnetic systems.

We apply Fermi hypernetted-chain theory to study the spin polarization of higher-order fractional quantum Hall (FQH) states at filling factors in between the primary FQH sequences, $\nu =p/(q_{e}p\pm 1)$, where $q_e$ is an even integer and $p$ is a nonzero integer. The filling factors related to the higher-order FQH states include $\nu =3/8$, $4/11$, $5/13$, $5/17$, $4/13$, $6/17$, $7/11$, and so on. We use a model of strongly interacting fermions with different spin degrees of freedom to explain the states beyond primary FQH sequences. We calculate the correlation energy, the radial distribution function, as well as the static structure function associated with the Halperin wave function adopted for the mixture states of fermions with different spins. The results are comparable with those from the residual interaction between composite fermions.

Carbon nanotubes (CNTs) have recently attracted attention as materials for flexible thermoelectric devices. To provide a theoretical guideline of how defects influence the thermoelectric performance of CNTs, we theoretically studied the effects of defects (vacancies and Stone-Wales defects) on their thermoelectric properties; thermal conductance, electrical conductance, and Seebeck coefficient. The results revealed that the defects most strongly suppress the electron conductance, and deteriorate the thermoelectric performance of a CNT. By plugging in the results and the intertube-junction properties into the network model, we further show that the defects with realistic concentrations can significantly degrade the thermoelectric performance of CNT-based networks. Our findings indicate the importance of the improvement of crystallinity of CNTs for improving CNT-based thermoelectrics.

We developed a six-band $k·p$ model that describes the electronic states of monolayer transition metal dichalcogenides (TMDCs) in $\mathit{K}$ valleys. The set of parameters for the $k·p$ model is uniquely determined by decomposing tight-binding (TB) models in the vicinity of ${\mathit{K}}^{\pm }$ points. First, we used TB models existing in literature to derive systematic parametrizations for different materials, including ${\mathrm{MoS}}_2,{\mathrm{WS}}_2,{\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$. Then, by using the derived six-band $k·p$ Hamiltonian we calculated effective masses, Landau levels, and the effective exciton $g$-factor $g_{X^0}$ in different TMDCs. We showed that TB parametrizations existing in literature result in small absolute values of $g_{X^0}$, which are far from the experimentally measured $g_{X^0}\approx -4$. To further investigate this issue we derived additional sets of $k·p$ parameters for different TMDCs by developing our own TB parametrizations based on simultaneous fitting of ab initio calculated, within the density functional theory (DFT) and $GW$ approaches, energy dispersion, and the value of $g_{X^0}$. We showed that the change in TB parameters, which only slightly affects the dispersion of higher conduction and deep valence bands, may result in a significant increase of $|g_{X^0}|$, yielding close-to-experiment values of $g_{X^0}$. Such a high parameter sensitivity of $g_{X^0}$ opens a way to further improvement of the description of TMDCs electronic structures by DFT and/or TB models.

We study a periodically driven nanowire with Rashba-like conduction and valence bands in the presence of a magnetic field. We identify topological regimes in which the noninteracting system hosts zero-energy bound states. We further investigate the effect of strong electron-electron interactions that give rise to parafermion zero energy modes hosted at the nanowire ends. The first setup we consider allows for topological phases by applying only static magnetic fields without the need of superconductivity. The second setup involves both superconductivity and time-dependent magnetic fields and supports topological phases without fine tuning of the chemical potential. Promising candidate materials are graphene nanoribbons due to their intrinsic particle-hole symmetry.

We present the results of a coincidence experiment aimed at studying the role of plasmons and electronic band structure in the emission of secondary electrons from an Al(100) sample. We measured the spectrum of secondary electrons in coincidence with photoelectrons that have scattered inelastically inside the solid and have excited a bulk plasmon. In this paper we put in evidence that the coincidence energy spectrum of the emitted electron is modulated by the density of occupied states of the sample under investigation. This interpretation is supported by comparing the coincidence energy spectrum with the calculated band structure of the Al(100) surface. The comparison suggests that the coincidence spectrum is dominated by the emission of electrons coming from the Al surface state; it also suggests that, even in the case of bulk plasmon excitation, secondary electron generation is dominated by decay that happens at the immediate surface of the solid.

Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holes in lightly hole-doped $\mathrm{L}{\mathrm{a}}_{1.95}\mathrm{S}{\mathrm{r}}_{0.05}\mathrm{C}{\mathrm{u}}_{0.95}\mathrm{Z}{\mathrm{n}}_{0.05}{\mathrm{O}}_4$. The changes of the local spin polarization manifest strongly in the temperature-dependent optical conductivity at $\sim 7.2\mathrm{eV}$, with an anomaly at the magnetic stripe phase $(\sim 25\mathrm{K})$, accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoretical calculations, we also assign high-energy optical transitions and their corresponding temperature dependence, particularly at $\sim 2.5,\sim 8.7,\sim 9.7,\sim 11.3$, and $\sim 21.8\mathrm{eV}$. Our result shows the importance of a strong mixture of spin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magnetic correlations using high-energy optical conductivity in correlated electron systems.

In view of the ultrashort spatial and temporal scales involved, carrier capture processes in nanostructures are genuine quantum phenomena. To describe such processes, methods with different levels of approximations have been developed. By properly tailoring the Lindblad-based nonlinear single-particle density matrix equation provided by an alternative Markov approach, in this work we present a Lindblad superoperator to describe how the phonon-induced carrier capture affects the spatiotemporal quantum dynamics of a flying wave packet impinging on a quantum dot. We compare the results with non-Markovian quantum kinetics calculations and discuss the advantages and drawbacks of the two approaches.

We extend Laughlin's magnetic-flux-threading argument to the quantized thermal Hall effect. A proper analog of Laughlin's adiabatic magnetic-flux threading process for the case of the thermal Hall effect is given in terms of an external gravitational field. From the perspective of the edge theories of quantum Hall systems, the quantized thermal Hall effect is closely tied to the breakdown of large diffeomorphism invariance, that is, a global gravitational anomaly. In addition, we also give an argument from the bulk perspective in which a free energy, decomposed into its Fourier modes, is adiabatically transferred under an adiabatic process involving external gravitational perturbations.

We report observation of strong and anisotropic third-harmonic generation (THG) in monolayer and multilayer ${\mathrm{ReS}}_2$. The third-order nonlinear optical susceptibility of monolayer ${\mathrm{ReS}}_2,|{\chi }^{\left(3\right)}|$, is on the order of ${10}^{-18}$ ${\mathrm{m}}^2/{\mathrm{V}}^2$, which is about one order of magnitude higher than reported results for hexagonal-lattice transition metal dichalcogenides such as ${\mathrm{MoS}}_2$. A similar magnitude for the third-order nonlinear optical susceptibility was also obtained for a multilayer sample. The intensity of the THG field was found to be dependent on the direction of the incident light polarization for both monolayer and multilayer samples. A point group symmetry analysis shows that such anisotropy is not expected from a perfect $1T$ lattice and must arise from the distortions in the ${\mathrm{ReS}}_2$ lattice. Our results show that THG measurements can be used to characterize lattice distortions of two-dimensional materials and that lattice distortions are important for the nonlinear optical properties of such materials.

Evidence in support of a shear driven anomaly in zirconium at elevated temperatures and pressures has been determined through the combined use of ultrasonic, diffractive, and radiographic techniques. Implications that these have on the phase diagram are explored through thermoacoustic parameters associated with the elasticity and thermal characteristics. In particular, our results illustrate a deviating phase boundary between the $\alpha$ and $\omega$ phases, referred to as a kink, at elevated temperatures and pressures. Further, pair distribution studies of this material at more extreme temperatures and pressures illustrate the scale on which diffusion takes place in this material. Possible interpretation of these can be made through inspection of shear-driven anomalies in other systems.

The thermodynamics of coupled frustrated ferromagnetic chains is studied within a spin-rotation-invariant Green's-function approach. We consider an isotropic Heisenberg spin-half system with a ferromagnetic in-chain coupling $J_1<0$ between nearest neighbors and a frustrating antiferromagnetic next-nearest-neighbor in-chain coupling $J_2>0$. We focus on the moderate strength of frustration $J_2<\left|J_1\right|/4$ such that the in-chain spin-spin correlations are predominantly ferromagnetic. We consider two interchain couplings (ICs) $J_{\perp ,y}$ and $J_{\perp ,z}$, corresponding to the two axes perpendicular to the chain, where ferromagnetic as well as antiferromagnetic ICs are taken into account. We discuss the influence of frustration on the ground-state properties for antiferromagnetic ICs, where the ground state is of a quantum nature. The major part of our study is devoted to the finite-temperature properties. We calculate the critical temperature $T_c$ as a function of the competing exchange couplings $J_2,J_{\perp ,y},J_{\perp ,z}$. We find that for fixed ICs, $T_c$ decreases monotonically with increasing frustration $J_2$, where as $J_2\to \left|J_1\right|/4$ the $T_c\left(J_2\right)$ curve drops down rapidly. To characterize the magnetic ordering below and above $T_c$, we calculate the spin-spin correlation functions $\langle {\mathbf{S}}_0{\mathbf{S}}_{\mathbf{R}}\rangle$, the magnetic order parameter $M$, the uniform static susceptibility ${\chi }_0$, as well as the correlation length $\xi$. Moreover, we discuss the specific heat $C_V$ and the temperature dependence of the excitation spectrum ${\omega }_{\mathbf{q}}$. As $J_2\to \left|J_1\right|/4$, some unusual frustration-induced features were found, such as an increase of the in-chain spin stiffness (in the case of ferromagnetic ICs) or of the in-chain spin-wave velocity (in the case of antiferromagnetic ICs) with growing temperature.

We computed spin-wave dispersions of surface-reconstructed Co films on the W(110) surface in the adiabatic approximation. The magnetic exchange interactions are obtained via first-principles electronic-structure calculations using the Korringa-Kohn-Rostoker Green-function method. We analyze the strength and oscillatory behavior of the intralayer and interlayer magnetic interactions and investigate the resulting spin-wave dispersions as a function of the thickness of Co films. In particular, we highlight and explain the strong impact of hybridization of the electronic states at the Co-W interface on the magnetic exchange interactions and on the spin-wave dispersions. We compare our results to recent measurements based on electron-energy-loss spectroscopy [E. Michel, H. Ibach, and C. M. Schneider, Phys. Rev. B 92, 024407 (2015)]. Good overall agreement with experimental findings can be obtained by considering the possible overestimation of the spin splitting, stemming from the local-spin-density approximation, and adopting an appropriate correction.

Properties of the vortex state in helical $p$-wave superconductor are studied by the quasiclassical Eilenberger theory. We confirm the instability of the helical $p$-wave state at high fields and that the spin-polarized local density of states $M(E,\mathit{r})$ appears even when Knight shift does not change. This is because the vorticity couples to the chirality of up-spin pair or down-spin pair of the helical state. In order to identify the helical $p$-wave state at low fields, we investigate the structure of the zero-energy $M(E=0,\mathit{r})$ in the vortex states, and also the energy spectra of $M(E,\mathit{r})$.

A thorough structural exploration has been made on the quasi-one-dimensional $S=1/2$ compound $\mathrm{L}{\mathrm{i}}_2\mathrm{C}{\mathrm{u}}_2\mathrm{O}{\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$ by neutron and synchrotron x-ray diffraction. It reveals the occurrence of a structural transition at 125 K, characterized by a lowering of symmetry from $P{4}_2/m$ to $P\overline{1}$, which is possibly driven by an exchange striction mechanism. This transition involves a dimerization of some Cu in the edge-sharing tetrahedral Cu chains. A symmetry mode analysis indicates that one representation, ${\mathrm{\Gamma }}_3{}^{+}{\mathrm{\Gamma }}_4{}^{+}$, dominates the structural transition. Interestingly, no intermediate structure with $P112/m$ symmetry is observed experimentally. Lastly, temperature dependent magnetic susceptibility measurements and neutron diffraction reveal that the magnetic ground state of this compound is a spin-singlet with a spin gap, characterized by the absence of long-range magnetic order down to 1.7 K.