The promise posed by half-Heusler compounds as thermoelectric materials depends on their thermal conductivity, which is strongly affected by doping. Here we elucidate the effect of $p$ dopants on the lattice thermal conductivity (${\kappa }_{\text{ph}}$) of seven selected half-Heusler compounds and for twelve different substitutional defects. We unveil a strong reduction in ${\kappa }_{\text{ph}}$ even for low concentrations of transition-metal substitutional atoms. Furthermore, we quantify the strength of the bond perturbation induced by substitutional impurities and interpret it in terms of the changes in the local electronic density of states. In several cases we find a significant destructive interference between the mass difference and bond perturbations which reduces the phonon scattering rates below the value expected if the two effects were treated independently. We compare our first-principles calculations with the available experimental measurements on the thermal conductivity of (${\mathrm{Zr},\mathrm{Hf})}_{\text{Nb}}$-doped NbFeSb and ${\mathrm{Sn}}_{\text{Sb}}$-doped ZrCoSb. For the latter, including the effect of independent Co vacancies and interstitials yields an almost perfect agreement with experiment.

We study the reaction-diffusion dynamics of Fibonacci anyons in a one-dimensional lattice. Due to their non-Abelian nature, besides the position degree of freedom (DOF), these anyons also have a nonlocal internal DOF, which can be characterized by a fusion tree. We first consider a pure-reaction dynamics associated with the internal DOF, which is of intrinsically quantum origin, with either an “all-$\tau$” or “completely random” initial fusion tree. These two fusion trees are unstable and likely stable steady states for the internal DOF, respectively. We obtain the decay rate of the anyon number for these two cases exactly. Still using these two initial fusion trees, we study the full reaction-diffusion dynamics and find an interesting interplay between classical and quantum behaviors: These two fusion trees are still respectively unstable and likely stable steady states of the internal DOF, while the dynamics of the position DOF can be mapped to a hybrid classical $A+A\to 0$ and $A+A\to A$ reaction-diffuson dynamics, with the relative reaction rates of these two classical dynamics determined by the state of the nonlocal internal DOF. In particular, the anyon density at late times are given by $\rho \left(t\right)=\frac{c}{\sqrt{8\pi }}{\left(Dt\right)}^{-\mathrm{\Delta }}$, where $D$ is a nonuniversal diffusion constant, $\mathrm{\Delta }=1/2$ is superuniversal, and $c$ is universal and can be obtained exactly in terms of the fusion tree structure. Specifically, $c=\frac{2\phi }{\phi +1}$ and $c=\frac{2(4\phi +3)}{5(\phi +1)}$ for the all-$\tau$ and completely random configuration respectively, where $\phi =\frac{\sqrt{5}+1}{2}$ is the golden ratio. We also study the two-point correlation functions.

We report the signatures of dynamic spin fluctuations in the layered honeycomb ${\mathrm{Li}}_3{\mathrm{Cu}}_2\mathrm{Sb}{\mathrm{O}}_6$ compound, with a $3d S=1/2 d^9{\mathrm{Cu}}^{2+}$ configuration, through muon spin rotation and relaxation ($\mu \mathrm{SR}$) and neutron scattering studies. Our zero-field (ZF) and longitudinal-field (LF) $\mu \mathrm{SR}$ results demonstrate the slowing down of the ${\mathrm{Cu}}^{2+}$ spin fluctuations below 4.0 K. The saturation of the ZF relaxation rate at low temperature, together with its weak dependence on the longitudinal field between 0 and 3.2 kG, indicates the presence of dynamic spin fluctuations persisting even at 80 mK without static order. Neutron scattering study reveals the gapped magnetic excitations with three modes at 7.7, 13.5, and 33 meV. Our density functional theory calculations reveal that the next-nearest-neighbor (NNN) antiferromagnetic (AFM) exchange ($J_{\text{AFM}}=31$ meV) is stronger than the NN ferromagnetic (FM) exchange ($J_{\text{FM}}=-21$ meV), indicating the importance of the orbital degrees of freedom. Our results suggest that the physics of ${\mathrm{Li}}_3{\mathrm{Cu}}_2\mathrm{Sb}{\mathrm{O}}_6$ can be explained by an alternating AFM chain rather than the honeycomb lattice.

We study the onset of spin superfluidity, namely, coherent spin transport mediated by a topological spin texture, in frustrated exchange-dominated magnetic systems, engendered by an external magnetic field. We show that for typical device geometries used in nonlocal magnetotransport experiments, the magnetic field stabilizes a spin superflow against fluctuations up to a critical current. For a given current, the critical field depends on the precessional frequency of the texture, which can be separately controlled. We contrast such dynamic stabilization of a spin superfluid to the conventional approaches based on topological stabilization.

Boundary time crystals (BTC's) are nonequilibrium phases of matter occurring in quantum systems in contact to an environment, for which a macroscopic fraction of the many-body system breaks time translation symmetry. We study BTC's in collective $d$-level systems, focusing on the cases with $d=2$, 3, and 4. We find that BTC's appear in different forms for the different cases. We first consider the model with collective $d=2$-level systems [Phys. Rev. Lett. 121, 035301 (2018)], whose dynamics is described by a Gorini-Kossakowski-Sudarshan-Lindblad master equation, and perform a throughout analysis of its phase diagram and Jacobian stability for different interacting terms in the coherent Hamiltonian. In particular, using perturbation theory for general (non-Hermitian) matrices, we obtain analytically how a specific ${\mathbb{Z}}_2$ symmetry-breaking Hamiltonian term destroys the BTC phase in the model. Based on these results we define a $d=4$ model composed of a pair of collective two-level systems interacting with each other. We show that this model support richer dynamical phases, ranging from limit cycles, period-doubling bifurcations, and a route to chaotic dynamics. The BTC phase is more robust in this case, not annihilated by the former symmetry-breaking Hamiltonian terms. The model with collective $d=3$-level systems is defined similarly, as competing pairs of levels, but sharing a common collective level. The dynamics can deviate significantly from the previous cases, supporting phases with the coexistence of multiple limit cycles, closed orbits and a full degeneracy of zero Lyapunov exponents.

A superposition of spin helices can yield topological spin textures, such as skyrmion and hedgehog lattices. Based on the analogy with the moiré in optics, we study the magnetic and topological properties of such superpositions in a comprehensive way by modulating the interference pattern continuously. We find that the control of the angles between the superposed helices and the net magnetization yields successive topological transitions associated with pair annihilation of hedgehogs and antihedgehogs. Accordingly, emergent electromagnetic fields, magnetic monopoles and antimonopoles, and Dirac strings arising from the noncoplanar spin textures show systematic evolution. In addition, we also show how the system undergoes the magnetic transitions with dimensional reduction from the three-dimensional hedgehog lattice to a two-dimensional skyrmion lattice or a one-dimensional conical state. The results indicate that the concept of “spin moiré” provides an efficient way of engineering the emergent electromagnetism and topological nature in magnets.

We report a comprehensive study on the magnetic ground state of ${\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}\mathrm{Co}{\mathrm{O}}_4$ combining single crystal neutron diffraction and resonant magnetic x-ray scattering at the $\mathrm{Co}{L}_{2,3}$ edges. Three single-crystal samples obtained from the same boule were investigated exhibiting magnetic phase transitions from a high-temperature paramagnetic phase to an antiferromagnetic phase at $T_{\mathrm{N}}\approx 52\mathrm{K}$. Single crystal neutron diffraction reveals that the crystal structure at room temperature shows an orthorhombic $A$-centered lattice but with $a$ and $b$ axes almost equal in length. The structural phase transition (charge-ordering-like) from the parent tetragonal cell takes place above 523 K into the space group $A2mm$ where two nonequivalent compressed and expanded $\mathrm{Co}{\mathrm{O}}_6$ octahedra are ordered showing a checkerboard pattern in the $ab$ plane. The charge segregation between the nonequivalent Co sites is about 0.4(1) electrons. Resonant magnetic x-ray reflections indexed as ${\left(1/4,1/4,0\right)}_{\mathrm{t}}$, ${\left(1/4,1/4,1\right)}_{\mathrm{t}}$, and ${\left(1/4,1/4,1/2\right)}_{\mathrm{t}}$ in the parent tetragonal cell were observed at low temperature at the $\mathrm{Co}{L}_{2,3}$-edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the $\mathrm{Co}{L}_2$ edge, indicates that only ${\mathrm{Co}}^{2+}$-like ions participate in the magnetic ordering. The polarization analysis discloses that the orientation of Co magnetic moments is the same for the three magnetic orders and they are long-range ordered along the diagonal in the $ab$ plane of the parent tetragonal cell with a slight tilt in the $c$ axis. Despite the onset temperatures for the three resonant magnetic reflections being the same, $\approx 55\mathrm{K}$, different thermal behavior is observed between ${\left(1/4,1/4,1/2\right)}_{\mathrm{t}}$ and ${\left(1/4,1/4,L\right)}_{\mathrm{t}}$ ($L=\mathrm{integer}$) reflections whose intensities maximize at different temperatures, suggesting the coexistence of two magnetic arrangements. Moreover, the intensity of the ${\left(1/4,1/4,1/2\right)}_{\mathrm{t}}$ magnetic reflection is at least ten times larger than that of the ${\left(1/4,1/4,L\right)}_{\mathrm{t}}$ $(L=\mathrm{integer})$ ones. On the other hand\, neutron diffraction measurements only detect a single type of antiferromagnetic ordering following the propagation vector $\mathit{k}={\left(1/4,1/4,1/2\right)}_{\mathrm{t}}$ that involves half of the Co atoms in the unit cell. We conclude that the bulk magnetic order in ${\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}\mathrm{Co}{\mathrm{O}}_4$ corresponds then to this propagation vector $\mathit{k}={\left(1/4,1/4,1/2\right)}_{\mathrm{t}}$ while ${\left(1/4,1/4,0\right)}_{\mathrm{t}}$ and ${\left(1/4,1/4,1\right)}_{\mathrm{t}}$ magnetic reflections correspond to a minority magnetic phase that must be due to changes in the oxygen stoichiometry near the surface.

X-ray photon correlation spectroscopy (XPCS) is used to investigate the fluctuation dynamics during self-organized nanopatterning of silicon by ${\mathrm{Ar}}^{+}$ bombardment at ${65}^{\circ }$ polar angle. Rich structure is observed in the development of the correlation dynamics as seen in the evolving correlation time $\tau \left(q_{\left|\right|}\right)$ and fluctuation relaxation exponent $n\left(q_{\left|\right|}\right)$. On length scales of the ripple structure, local structure becomes ever more long lived as coarsening progresses. In addition, $\tau \left(q_{\left|\right|}\right)$ develops a peak on length scales corresponding to the ripple wavelength. As patterning progresses, correlation times become asymmetric between the positive and negative directions, suggesting the possibility of different dynamics on the slopes facing toward and away from the ion beam. Relaxation exponents show evolution from linear dynamics at early times to compressed exponential relaxation at low wave numbers and stretched exponential relaxation at high wave numbers. Compressed exponential behavior is reminiscent of stress relaxation processes observed in glasses.

Understanding the self-organized ion beam nanopatterning of elemental semiconductors, particularly silicon, is of intrinsic scientific and technological interest. This is the second component of a two-part coherent x-ray scattering and x-ray photon correlation spectroscopy (XPCS) investigation of the kinetics and fluctuation dynamics of nanoscale ripple development on silicon during 1 keV ${\mathrm{Ar}}^{+}$ (part I) and ${\mathrm{Kr}}^{+}$ bombardment at ${65}^{\circ }$ polar angle. Here it is found that the ion-enhanced viscous flow relaxation is essentially equal for ${\mathrm{Kr}}^{+}$-induced patterning as previously found for ${\mathrm{Ar}}^{+}$ patterning despite the difference in ion masses. However, the magnitude of the surface curvature-dependent roughening rate in the early-stage kinetics is larger for ${\mathrm{Kr}}^{+}$ than for ${\mathrm{Ar}}^{+}$, consistent with expectations that the heavier ion gives an increased mass redistributive contribution to the initial surface instability. As with the ${\mathrm{Ar}}^{+}$ case, fluctuation dynamics in the late stage show a peak in correlation times at the length scale corresponding to the dominant structural feature on the surface—the ripples. Finally, it is shown that speckle motion during the surface evolution can be analyzed to determine spatial inhomogeneities in erosion rate and ripple velocity. This allows the direction and speed of ripple motion to be measured in a real time experiment. In the present case, ripple motion is found to be into the projected direction of the ion source, in contrast to expectations from an existing sputter erosion driven model with parameters derived from binary collision approximation simulations.

Nonlocal constitutive relations promise to homogenize metamaterials even though the ratio of period over operational wavelength is not much smaller than unity. However, this ability has not yet been verified, as frequently only discrete structures were considered. This denies a systematic variation of the relevant ratio. Here, we explore, using the example of an electric dipolar lattice, the superiority of the nonlocal over local constitutive relation to homogenize metamaterials when the period tends to be comparable to the wavelength. Moreover, we observe a breakdown of the ability to homogenize the metamaterial at shorter lattice constants. This surprising failure occurs when energy is transported across the lattice thanks to a well-pronounced near-field interaction among the particles forming the lattice. Contrary to common wisdom, this suggests that the period should not just be much smaller than the operational wavelength to homogenize a metamaterial, but, for a given size of the inclusion, there is an optimal period.

The first-principles computation of the surfaces of metals is typically accomplished through slab calculations of finite thickness. The extraction of a convergent surface formation energy from slab calculations is dependent upon defining an appropriate bulk reference energy. I describe a method for an independently computed, slab-consistent bulk reference that leads to convergent surface formation energies from slab calculations that also provides realistic uncertainties for the magnitude of unavoidable nonlinear divergence in the surface formation energy with slab thickness. The accuracy is demonstrated on relaxed, unreconstructed low-index aluminum surfaces with slabs with up to 35 layers.

The stability of low-index platinum surfaces and their electronic properties is investigated with density functional theory, toward the goal of understanding the surface structure and electron emission, and identifying precursors to electrical breakdown, on nonideal platinum surfaces. Propensity for electron emission can be related to a local work function, which, in turn, is intimately dependent on the local surface structure. The $(1\times N)$ missing row reconstruction of the Pt(110) surface is systematically examined. The $(1\times 3)$ missing row reconstruction is found to be the lowest in energy, with the $(1\times 2)$ and $(1\times 4)$ slightly less stable. In the limit of large $(1\times N)$ with wider (111) nanoterraces, the energy accurately approaches the asymptotic limit of the infinite Pt(111) surface. This suggests a local energetic stability of narrow (111) nanoterraces on free Pt surfaces that could be a common structural feature in the complex surface morphologies, leading to work functions consistent with those on thermally grown Pt substrates.

The temperature dependence of the surface free energy of the industrially important TiN(001) system has been investigated by means of an extended two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) methodology, to include the fully anharmonic vibrational contribution, as obtained from ab initio molecular dynamics (AIMD). Inclusion of the fully anharmonic behavior is crucial, since the standard low-temperature quasiharmonic approximation exhibits a severe divergence in the surface free energy due to a high-temperature dynamical instability. The anharmonic vibrations compensate for the quasiharmonic divergence and lead to a modest overall temperature effect on the TiN(001) surface free energy, changing it from around $78\mathrm{meV}{\AA }^{-2}$ at 0 K to $73\mathrm{meV}{\AA }^{-2}$ at 3000 K. The statistical convergence of the molecular dynamics is facilitated by the use of machine-learning potentials, specifically moment tensor potentials, fitted for TiN(001) at finite temperature. The surface free energy obtained directly from the fitted machine-learning potentials is close to that obtained from the full AIMD simulations.

The exact study of small systems can guide us toward relevant measures for extracting information about many-body physics as we move to larger and more complex systems capable of quantum information processing or quantum analog simulation. We use exact diagonalization to study many electrons in short one-dimensional atom chains represented by long-range extended Hubbard-like models. We introduce a measure, the single-particle excitation content (SPEC) of an eigenstate and show that the functional dependence of the SPEC on the eigenstate number reveals the nature of the ground state (ordered phases), and the onset and saturation of correlation between the electrons as Coulomb interaction strength increases. We use this SPEC behavior to identify five regimes as interaction is increased: A noninteracting single-particle regime, a regime of perturbative Coulomb interaction in which the SPEC is a nearly universal function of eigenstate number, the onset and saturation of correlation, a regime of fully correlated states in which hopping is a perturbation, and the SPEC is a different universal function of state number and the regime of no hopping. In particular, the behavior of the SPEC shows that when electron-electron correlation plays a minor role, all of the lowest-energy eigenstates are made up primarily of single-particle excitations of the ground state, and as the Coulomb interaction increases, the lowest-energy eigenstates increasingly contain many-particle excitations. In addition, the SPEC highlights a fundamental distinct difference between a noninteracting system and one with minute very weak interactions. Although the SPEC is a quantity that can be calculated for small exactly diagonalizable systems, it guides our intuition for larger systems, suggesting the nature of excitations and their distribution in the spectrum. Thus, this function, such as correlation functions or order parameters, provides us with a window of intuition about the behavior of a physical system.

The possibility of excitation and amplification of terahertz plasmons in a structure containing a layer of graphene with charge-carrier drift and a layer of graphene without carrier drift is theoretically investigated. The plasmon excitation in the graphene structure by an incident terahertz electromagnetic wave is calculated in the attenuated total reflection geometry. It is shown that, in graphene with charge-carrier drift, it is possible to achieve negative values of the real part of graphene conductivity for the phase velocity of plasmons exceeding the charge carrier's drift velocities, which indicates a non-Cherenkov amplification of terahertz plasmons in the double-layer graphene structure for practically achievable direct electric current values. Terahertz amplification originates due to the variation of the carrier mass density in graphene with terahertz electric field. In the case of weak deceleration of terahertz wave incident onto graphene structure, the value of the negative conductivity of graphene does not depend on the direction of the direct current in graphene (only the codirectional and counterdirectional currents with respect to the direction of plasmon propagation are considered). This is due to the insignificant spatial dispersion of the hydrodynamic conductivity of graphene in the case of weak deceleration of terahertz waves. The results of this work can be used to create compact terahertz radiation amplifiers operating at room temperature.

We report on the Shubnikov–de Haas (SdH) oscillations in the quasi-two-dimensional molecular conductor $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ [BETS: bis(ethylenedithio)tetraselenafulvalene] laminated on polyimide films at 1.7 K. From the SdH phase factor we verified experimentally that the material is in the Dirac fermion phase under pressure. $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ is in the vicinity of the phase transition between strongly correlated insulating and Dirac fermion phases, and is a possible candidate for an ambient-pressure molecular Dirac fermion system. However, the SdH oscillations indicate that the Berry phase is zero at ambient pressure. Under pressure, a $\pi$ Berry phase emerges when the metal-insulator crossover is almost suppressed at $\sim 0.5$ GPa. The results contrast with those for the pioneering molecular Dirac fermion system $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] in which Dirac fermions and semiconducting behavior are simultaneously observed.

We present extensive new ab initio path integral Monte Carlo results for the momentum distribution function $n\left(\mathbf{k}\right)$ of the uniform electron gas in the warm dense matter regime over a broad range of densities and temperatures. This allows us to study the nontrivial exchange-correlation-induced increase of low-momentum states around the Fermi temperature, and to investigate its connection to the related lowering of the kinetic energy compared to the ideal Fermi gas. In addition, we investigate the impact of quantum statistics on both $n\left(\mathbf{k}\right)$ and the off-diagonal density matrix in coordinate space, and find that it cannot be neglected even in the strongly coupled electron liquid regime. Our results were derived without any nodal constraints, and thus constitute a benchmark for other methods and approximations.

We present a comprehensive study of single crystals of ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, a putative Kitaev honeycomb magnet, focusing on its low-temperature phase behaviors. A new thermal phase transition is identified at 31.0 K, below which the system develops a two-dimensional (2D) long-range magnetic order. This order precedes the well-known three-dimensional (3D) order below 26.7 K, and is likely driven by strongly anisotropic interactions. Surprisingly, excitations from the 3D order do not support the order's commonly accepted “zigzag” nature, and are instead consistent with a “triple-$\mathbf{q}$” description, which has remained hitherto unexplored in the study of Kitaev honeycomb magnets. The 3D order exerts a fundamental feedback on high-energy excitations that likely involve orbital degrees of freedom, and it features strongly scattered spin waves until at much lower temperatures, a sign for ground state near degeneracy and frustration. These findings constrain microscopic models and render ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ a spin-orbit entangled frustrated magnet that hosts very rich physics.

The electrical transport in the half-Heusler phases LuNiSb and YPdSb was measured in a temperature range 2–300 K. For both compounds, the electrical resistivity was found to decrease with increasing temperature, showing a linear-in-$T$ behavior over an extended temperature interval. In order to interpret the experimental data, a two-channel conductivity model was applied, which revealed that not only the semiconducting-like transport but also the metallic-like one exhibit negative temperature coefficients. The unusual behavior in the metallic channel was described within the Cote-Meisel formalism based on the diffraction model of strongly disordered metals. In addition, a weak localization scenario was considered including spin-orbit scattering and Coulomb interaction between conducting electrons. The electron-electron interaction was found most important at the lowest temperatures, where the semiconducting channel becomes ineffective, reminiscent of charge transport confined to a narrow yet finite-size metallic band located inside the semiconducting energy gap. The low-temperature resistivity of YPdSb appeared fully describable in terms of the Altshuler-Aronov quantum correction due to interacting electrons. In turn, the electronic transport in LuNiSb was found affected by the Kondo effect associated with a small amount of paramagnetic impurities present in the specimen investigated. The approach developed for LuNiSb and YPdSb can be applied to other half-Heusler compounds that exhibit atom disorder in their crystal structures.

We investigate the properties of the Kitaev-Kondo lattice model defined on a bilayer honeycomb lattice by means of the SO(3) Majorana representation for spin-$1/2$ moments. We first consider the pairing of neighboring sites for the parent Kitaev spin-liquid (KSL) Hamiltonian to render the Majorana and the spin-$1/2$ Hilbert spaces perfectly equivalent to each other. As a consequence, we demonstrate that this decoupling of the Kitaev interaction in terms of the SO(3) Majorana fermions reproduces exactly the spectrum of the KSL model alone. Then, by considering the effect of a local Kondo coupling $J_K$ in the model and decoupling it in terms of an order parameter that physically must have a finite staggering phase, we obtain that the system undergoes a quantum phase transition from a fractionalized Fermi liquid to a nematic triplet superconducting (SC) phase as $J_K$ is increased. Depending on the model parameters, this SC phase can exhibit either Dirac points, Bogoliubov-Fermi lines, or Bogoliubov-Fermi surfaces as nodal bulk manifolds. The surface states in this latter case are also characterized by topologically protected antichiral edge modes. The SC phase breaks time-reversal symmetry and exhibits a coexistence of a dominant odd-frequency pairing with a small even-frequency component for electronic excitations localized on sites of the same sublattice of the system. Finally, we show that this SC phase is in fact a pair-density-wave state, with Cooper pairs possessing a finite center-of-mass momentum in zero magnetic field.