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Like the twisting control in magic angle twisted bilayer graphenes, the stacking control is another mechanical approach to manipulate the fundamental properties of solids, especially the van der Waals materials. We explore the stacking-induced magnetic frustration and the spiral spin liquid on a multilayer triangular lattice antiferromagnet where the system is built from ABC stacking with competing intralayer and interlayers couplings. By combining the nematic bond theory and the self-consistent Gaussian approximation, we establish the phase diagram for this ABC-stacked multilayer magnet. It is shown that, the system supports a wide regime of spiral spin liquid with multiple degenerate spiral lines in the reciprocal space, separating the low-temperature spiral order and the high-temperature featureless paramagnet. The transition to the spiral order from the spiral spin liquid regime is first order. We further show that the spiral-spin-liquid behavior persists even with small perturbations such as further neighbor intralayer exchanges. The connection to the ABC-stacked magnets, the effects of Ising or planar spin anisotropy, and the outlook on the stacking-engineered quantum magnets are discussed., Comment: main text: 7 pages + 4 figures; supplemental materials: 15 pages + 5 figures. update: added discussions on the few-layer case

A quantum spin liquid (QSL) is an exotic state in which electron spins are highly entangled, yet keep fluctuating even at zero temperature. Experimental realization of model QSLs has been challenging due to imperfections, such as antisite disorder, strain, and extra or a lack of interactions in real materials compared to the model Hamiltonian. Here we report the magnetic susceptibility, thermodynamic, inelastic neutron scattering (INS), and muon-spin relaxation studies on a polycrystalline sample of PrZnAl$_{11}$O$_{19}$, where the Pr$^{3+}$ ions form an ideal two-dimensional triangular lattice. Our results demonstrate that this system does not order nor freeze, but keep fluctuating down to 50 mK despite large antiferromagnetic couplings ($\sim$ -10 K). Furthermore, the INS and specific-heat data suggest that PrZnAl$_{11}$O$_{19}$ is best described as a gapless QSL., 8 pages, 6 figures

Using scanning tunneling microscopy/spectroscopy (STM/STS), we investigate the local atomic and electronic structure of the archetype charge density wave (CDW) system ${\mathrm{TaSe}}_{2}$. Two structural phases with distinct CDW orders, namely $3\ifmmode\times\else\texttimes\fi{}3\text{\ensuremath{-}}2H$ and $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}\ensuremath{-}1T$ phases, coexist at low temperatures and an intermediate phase is discovered around the phase boundaries demonstrating energy-dependent $2H$ and $1T$ electronic wave-function textures. The existence of such an intermediate state and its dual electronic appearance indicate that the $2H\ensuremath{-}1T$ transition is not instantaneous. Along with the gradual recovery of $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}\ensuremath{-}1T$ lattice distortions towards the $2H$ phase, an insulator-metal transition occurs as evidenced by the collapse of the Mott insulating gap. Our results provide a direct visualization of the strong link between the Mott insulating state and the local lattice distortions.

This work demonstrates that Kirchhoff's law of radiation, stating that the spectral directional emissivity and absorptivity of a surface are equal at thermal equilibrium, can be violated in planar surfaces without an external magnetic field or structures such as gratings. Modeling a type-I magnetic Weyl semimetal with an antisymmetric dielectric tensor, we show an intrinsic violation of Kirchhoff's law due to nonreciprocal surface polaritons induced by the Berry curvature and anomalous Hall velocity. This work provides a simple way to physically realize the violation of Kirchhoff's law.

Most of the searches for Kitaev materials deal with $4d/5d$ magnets with spin-orbit-coupled $J=1/2$ local moments such as iridates and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$. Here we propose the monoclinic ${\mathrm{YbCl}}_{3}$ with a ${\mathrm{Yb}}^{3+}$ honeycomb lattice for the exploration of Kitaev physics. We perform thermodynamic, $ac$ susceptibility, angle-dependent magnetic torque, and neutron diffraction measurements on ${\mathrm{YbCl}}_{3}$ single crystal. We find that the ${\mathrm{Yb}}^{3+}$ ion exhibits a Kramers doublet ground state that gives rise to an effective spin ${J}_{\text{eff}}=1/2$ local moment. The compound exhibits short-range magnetic order below 1.20 K, followed by a long-range N\'eel-type antiferromagnetic order at 0.60 K, below which the ordered ${\mathrm{Yb}}^{3+}$ spins lie in the $ac$ plane with an angle of 16(11)${}^{\ensuremath{\circ}}$ away from the $a$ axis. These orders can be suppressed by in-plane and out-of-plane magnetic fields at around 6 and 10 T, respectively. Moreover, the N\'eel temperature varies nonmonotonically under the out-of-plane magnetic fields, suggesting a reduced spin dimensionality. Together with the strong in-plane magnetic anisotropy and the reduced order moment 0.8(1) ${\ensuremath{\mu}}_{B}$ at 0.25 K, all indicate that ${\mathrm{YbCl}}_{3}$ could be a two-dimensional spin system to proximate the Kitaev physics.

The characterization of quantum magnetism in a large spin ($\geq 1$) system naturally involves both spin-vectors and -tensors. While certain types of spin-vector (e.g., ferromagnetic, spiral) and spin-tensor (e.g., nematic in frustrated lattices) orders have been investigated separately, the coexistence and correlation between them have not been well explored. Here we propose a novel quantum spiral spin-tensor order on a spin-1 Heisenberg chain subject to a spiral spin-tensor Zeeman field, which can be experimentally realized using a Raman-dressed cold atom optical lattice. We develop a method to fully characterize quantum phases of such spiral tensor magnetism with the coexistence of spin-vector and spin-tensor orders as well as their correlations using eight geometric parameters. Our method provides a powerful tool for characterizing spin-1 quantum magnetism and opens an avenue for exploring novel magnetic orders and spin-tensor electronics/atomtronics in large-spin systems., Comment: 11 pages, 15 figures

The wide application of amorphous materials in solar cells, memristors, and optical sensors has stimulated interest in understanding heat conduction in amorphous systems owing to their thermal management issues. Thermal transport in amorphous materials fundamentally differs from their crystalline counterparts due to the lack of long-range order. Despite great progress in understanding the thermal transport in crystalline materials over the past few decades from both first-principles computations and thermal transport characterizations, details of heat conduction in amorphous systems remain largely unknown. Here, we quantify different types of heat carriers in amorphous silicon using mean free path spectroscopy, with characteristic sizes down to 50 nm. We show that despite its disordered nature, more than half of thermal conductivity is contributed by propagating vibrational waves, which have mean free paths mostly above 100 nm. This provides direct evidence supporting the diversity of heat carriers in amorphous systems; some modes transport heat as propagating waves, while others do not. Our results suggest mean free path spectroscopy is a versatile tool for understanding thermal transport in disordered systems.

Motivated by the recent activities on the diamond lattice antiferromagnet ${\mathrm{NiRh}}_{2}{\mathrm{O}}_{4}$ with ${\mathrm{Ni}}^{2+}\phantom{\rule{4pt}{0ex}}3{d}^{8}$ local moments, we theoretically explore on general grounds the unique spin and orbital physics for the diamond lattice antiferromagnet with $3{d}^{8}$ local moments. The superexchange interaction between the local moments usually favors magnetic orders. Due to the particular electron configuration of the $3{d}^{8}$ ion with a partially filled upper ${t}_{2g}$ level and a fully filled lower ${e}_{g}$ level, the atomic spin-orbit coupling becomes active at the linear order and would favor a spin-orbital-entangled singlet with quenched local moments in the single-ion limit. Thus, the spin-orbital entanglement competes with the superexchange and could drive the system to a quantum critical point that separates the spin-orbital singlet and the magnetic order. We further explore the effects of magnetic field and uniaxial pressure. The nontrivial response to the magnetic field is intimately tied to the underlying spin-orbital structure of the local moments. We discuss future experiments such as doping and pressure and point out the correspondence between different electron configurations.

We point out that the Kitaev materials may not necessarily support Kitaev spin liquid. It is well-known that having a Kitaev term in the spin interaction is not the sufficient condition for the Kitaev spin liquid ground state. Many other spin liquids may be stabilized by the competing spin interactions of the systems. We thus explore the possibilities of non-Kitaev spin liquids in the honeycomb Kitaev materials. We carry out a systematic classification of gapped $\mathbb{Z}_2$ spin liquids using the Schwinger boson representation for the spin variables. The presence of strong spin-orbit coupling in the Kitaev materials brings new ingredients into the projective symmetry group classification of the non-Kitaev spin liquid. We predict the spectroscopic properties of these gapped non-Kitaev spin liquids. Moreover, among the gapped spin liquids that we discover, we identify the spin liquid whose spinon condensation leads to the magnetic Bragg peak structure of the zig-zag magnetic order that was observed in Na$_2$IrO$_3$ and $\alpha$-RuCl$_3$. We further discuss the possibility of gapped $\mathbb{Z}_2$ spin liquid and the deconfined quantum criticality from the zig-zag magnetic order to spin dimerization in pressurized $\alpha$-RuCl$_3$., Comment: 13 pages, 6 figures, to appear in PRB. Minor change. https://meetings.aps.org/Meeting/MAR18/Session/F24.10

We report the properties of a triangular lattice iron-chalcogenide antiferromagnet ${\mathrm{FeAl}}_{2}{\mathrm{Se}}_{4}$.The spin susceptibility reveals a significant antiferromagnetic interaction with a Curie-Weiss temperature ${\mathrm{\ensuremath{\Theta}}}_{\text{CW}}\ensuremath{\simeq}\ensuremath{-}200\phantom{\rule{3.33333pt}{0ex}}\text{K}$ and a spin-2 local moment. Despite a large spin and a large $|{\mathrm{\ensuremath{\Theta}}}_{\text{CW}}|$, the low-temperature behaviors are incompatible with conventional classical magnets. No long-range order is detected down to 0.4 K. Similar to the well-known spin-1 magnet ${\mathrm{NiGa}}_{2}{\mathrm{S}}_{4}$, the specific heat of ${\mathrm{FeAl}}_{2}{\mathrm{Se}}_{4}$ exhibits a double-peak structure and a ${T}^{2}$ power law at low temperatures, which are attributed to the underlying quadrupolar spin correlations and the Halperin-Saslow modes, respectively. The spin freezing occurs at $\ensuremath{\sim}14\phantom{\rule{3.33333pt}{0ex}}\mathrm{K}$, below which the relaxation dynamics is probed by the ac susceptibility. Our results are consistent with the early theory for the spin-1 system with Heisenberg and biquadratic spin interactions. We argue that the early proposal of the quadrupolar correlation and gauge glass dynamics may be well extended to ${\mathrm{FeAl}}_{2}{\mathrm{Se}}_{4}$. Our results provide useful insights about the magnetic properties of frustrated quantum magnets with high spins.

Since Peierls's pioneering work, it is generally accepted that phonon-phonon scattering processes consist of momentum-conserving normal scatterings and momentum-destroying Umklapp scatterings, and that the latter always induce thermal resistance. We show in this work that Umklapp scatterings are not necessarily resistive---no thermal resistance is induced if the projected momentum is conserved in the direction of heat flow. This distinction is especially important in anisotropic materials such as graphite and black phosphorous. By introducing a direction-dependent definition of normal and Umklapp scattering, we can model thermal transport in anisotropic materials using the Callaway model accurately. This accuracy is physically rooted in the improved description of mode-specific phonon dynamics. With the new definition, we predict that second sound might persist over much longer distances than previously expected.

We report the thermodynamic and muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$ evidences for a possible gapless spin liquid in ${\mathrm{Tm}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$, with the rare-earth ions ${\mathrm{Tm}}^{3+}$ forming a two-dimensional kagom\'e lattice. We extract the magnetic specific heat of ${\mathrm{Tm}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$ by subtracting the phonon contribution of the nonmagnetic isostructural material ${\mathrm{La}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$ and obtain a clear linear-$T$ temperature dependence of magnetic specific heat at low temperatures. No long-range magnetic order was observed down to 0.35 K in the heat capacity measurements, and no signature of spin freezing down to 50 mK was observed in A.C. susceptibility measurements. The absence of magnetic order is further confirmed by the $\ensuremath{\mu}\mathrm{SR}$ measurements down to 20 mK. We find that the spin-lattice relaxation time remains constant down to the lowest temperature. We point out that the physics in ${\mathrm{Tm}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$ is fundamentally different from the Cu-based herbertsmithite and propose spin liquid ground states with non-Kramers doublets on the kagom\'e lattice to account for the experimental results. However, we cannot rule out that these exotic properties are induced by the Tm/Zn site-mixing disorder in ${\mathrm{Tm}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$.

Through first-principles calculations, the phonon-limited transport properties of cubic boron-V compounds (BP, BAs and BSb) are studied. We find that the high optical phonon frequency in these compounds leads to the substantial suppression of polar scattering and the reduction of inter-valley transition mediated by large-wavevector optical phonons, both of which significantly facilitate charge transport. We also discover that BAs simultaneously has a high hole mobility (2110 cm2/V-s) and electron mobility (1400 cm2/V-s) at room temperature, which is rare in semiconductors. Our findings present a new insight in searching high mobility polar semiconductors, and point to BAs as a promising material for electronic and photovoltaic devices in addition to its predicted high thermal conductivity., Comment: 18 pages, 4 figures

Most works on pyrochlore magnets deal with the interacting spin-1/2 local moments. We here study the spin-one local moments on the pyrochlore lattice and propose a generic interacting spin model on a pyrochlore lattice. Our spin model includes the antiferromagnetic Heisenberg interaction, the Dzyaloshinskii-Moriya interaction, and the single-ion spin anisotropy. We develop a flavor wave theory and combine with a mean-field approach to study the global phase diagram of this model and establish the relation between different phases in the phase diagram. We find the regime of the quantum paramagnetic phase where a degenerate line of the magnetic excitations emerges in the momentum space. We further predict the critical properties of the transition out of the quantum paramagnet to the proximate orders. The presence of quantum order by disorder in the parts of the ordered phases is then suggested. We point out the existence of degenerate and topological excitations in various phases. We discuss the relevance with fluoride pyrochlore material ${\mathrm{NaCaNi}}_{2}{\mathrm{F}}_{7}$ and explain the role of the spin-orbit coupling and the magnetic structures of the Ru-based pyrochlore ${\mathrm{A}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ and the Mo-based pyrochlore ${\mathrm{A}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$.

Motivated by the recent progress in the spin-orbit-coupled triangular lattice spin liquid candidate ${\mathrm{YbMgGaO}}_{4}$, we carry out a systematic projective symmetry group analysis and mean-field study of candidate $U(1)$ spin-liquid ground states. Due to the spin-orbital entanglement of the Yb moments, the space-group symmetry operation transforms both the position and the orientation of the local moments, and hence it brings different features for the projective realization of the lattice symmetries from the cases with spin-only moments. Among the eight $U(1)$ spin liquids that we find with the fermionic parton construction, only one spin-liquid state, which was proposed and analyzed by Yao Shen et al. [Nature (London) 540, 559 (2016)] and labeled as U1A00 in the present work, stands out and gives a large spinon Fermi surface and provides a consistent explanation for the spectroscopic results in ${\mathrm{YbMgGaO}}_{4}$. Further connection of this spinon Fermi surface $U(1)$ spin liquid with ${\mathrm{YbMgGaO}}_{4}$ and the future directions are discussed. Finally, our results may apply to other spin-orbit-coupled triangular lattice spin-liquid candidates, and more broadly, our general approach can be well extended to spin-orbit-coupled spin-liquid candidate materials.

Motivated by the rapid experimental progress of quantum spin ice materials, we study the dynamical properties of pyrochlore spin ice in the U(1) spin liquid phases. In particular, we focus on the spinon excitations that appear at high energies and show up as an excitation continuum in the dynamic spin structure factor. The keen connection between the crystal symmetry fractionalization of the spinons and the spectral periodicity of the spinon continuum is emphasized and explicitly demonstrated. When the spinon experiences a background $\ensuremath{\pi}$ flux and the spinon continuum exhibits an enhanced spectral periodicity with a folded Brillouin zone, this spectral property can then be used to detect the spin quantum number fractionalization and U(1) spin liquid. Our prediction can be immediately examined by inelastic neutron-scattering experiments among quantum spin ice materials with Kramers' doublets. Further application to the non-Kramers' doublets is discussed.

Using large-scale quantum Monte Carlo simulations we show that a spin-$1/2$ XXZ model on a two-dimensional anisotropic Kagome lattice exhibits a tripartite entangled plaquette state that preserves all of the Hamiltonian symmetries. It is connected via phase boundaries to a ferromagnet and a valence-bond solid that break U(1) and lattice translation symmetries, respectively. We study the phase diagram of the model in detail, in particular the transitions to the tripartite entangled plaquette state, which are consistent with conventional order-disorder transitions. Our results can be interpreted as a description of the charge sector dynamics of a Hubbard model applied to the description of the spin liquid candidate ${\mathrm{LiZn}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, as well as a model of strongly correlated bosonic atoms loaded onto highly tunable {\it trimerized} optical Kagome lattices., Comment: 10 pages, 10 figures

We present a first-principles framework to investigate the electron scattering channels and transport properties for polar materials by combining the exact solution of the linearized electron-phonon (e-ph) Boltzmann transport equation in its integral-differential form associated with the e-ph coupling matrices obtained from the polar Wannier interpolation scheme. No ad hoc parameter is required throughout this calculation, and GaAs, a well-studied polar material, is used as an example to demonstrate this method. In this work, the long-range and short-range contributions as well as the intravalley and intervalley transitions in the e-ph interactions (EPIs) have been quantitatively addressed. Promoted by such mode-by-mode analysis, we find that in GaAs, the piezoelectric scattering is comparable to deformation-potential scattering for electron scatterings by acoustic phonons in EPI even at room temperature, and it makes a significant contribution to mobility. Furthermore, we achieved good agreement with experimental data for the mobility, and we identified that electrons with mean free paths between 130 and 210 nm provide the dominant contribution to the electron transport at 300 K. Such information provides a deeper understanding of the electron transport in GaAs, and the presented framework can be readily applied to other polar materials.

Pyrochlore iridates and pyrochlore ices are two families of materials where novel quantum phenomena are intertwined with strong spin-orbit coupling, substantial electron correlation, and geometrical frustration. Motivated by the puzzling experiments on two pyrochlore systems ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, we study the proximate Ising orders and the quantum phase transition out of quantum spin ice U(1) quantum spin liquid (QSL). We apply the electromagnetic duality of the compact quantum electrodynamics to analyze the condensation of the ``magnetic monopoles'' in the U(1) QSL. The monopole condensation naturally and necessarily leads to the Ising orders that generically break the lattice translation symmetry. We demonstrate that the antiferromagnetic Ising order with the ordering wave vector $\mathbf{Q}=2\ensuremath{\pi}(001)$ is proximate to the U(1) QSL while the ferromagnetic Ising state with $\mathbf{Q}=(000)$ is not proximate to the U(1) QSL. This implies that if there exists a direct transition from the U(1) QSL to the ferromagnetic Ising order, the transition must be strongly first order. We apply the monopole condensation to explain the magnetic orders and the transitions in ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$.

The singlet ground-state magnetism in pyrochlore osmates ${\mathrm{Y}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Ho}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ is studied by dc and ac susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected nonmagnetic singlet in the strong spin-orbit coupling (SOC) limit for ${\mathrm{Os}}^{4+}$ ($5{d}^{4}$), ${\mathrm{Y}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ exhibits a spin-glass ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the nonmagnetic state in the strong SOC limit and a magnetic state in the strong superexchange limit. ${\mathrm{Ho}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ has the same structural distortion as in ${\mathrm{Y}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$; however, the Os sublattice in ${\mathrm{Ho}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ shows long-range magnetic ordering below 36 K. The sharp difference of the magnetic ground state between ${\mathrm{Y}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Ho}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ signals that the singlet ground-state magnetism in ${R}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ is fragile and can be disturbed by the weak $4f\ensuremath{-}5d$ interactions.

The evolution of the domain structure with the thickness of bcc Fe films deposited on the Ag(116) vicinal surface is studied by spin-polarized low-energy electron microscopy. We show that a spin reorientation transition proceeds via two mechanisms: continuous rotation of magnetization within the vertical plane perpendicular to the steps and discontinuous reorientation of the in-plane component of magnetization, leading to splitting of the domains. In contrast to previously investigated systems with stripe domains, we reveal that in the case of a vicinal ferromagnetic surface, the domain width increases while changing the orientation of the magnetization from a canted out-of-plane state into an in-plane state. A theoretical model developed in this work successfully describes the domain structure behavior observed in our experiments and can be equally applied to other ferromagnetic films grown on vicinal surfaces.

© 2015 American Physical Society. High-aspect-ratio Fe nanostrips are studied with real-space micromagnetic imaging methods. We experimentally demonstrate reversible switching from essentially homogeneous single-domain states at room temperature to multidomain diamond states at elevated temperature. This temperature-dependent magnetic bistability can be understood and modeled by accounting for the temperature dependence of the magnetocrystalline, shape, and magnetoelastic anisotropies. These results show how the transition temperature between two magnetic domain states can be tailored by controlling epitaxial strain and particle geometry, which may generate new opportunities for magnetic memory and logic device design.

The monolayer of black phosphorous, or phosphorene, has recently emerged as a new 2D semiconductor with intriguing highly anisotropic transport properties. Existing calculations of its intrinsic phonon-limited electronic transport properties so far rely on the deformation potential approximation, which is in general not directly applicable to anisotropic materials since the deformation along one specific direction can scatter electrons traveling in all directions. We perform a first-principles calculation of the electron-phonon interaction in phosphorene based on density functional perturbation theory and Wannier interpolation. Our calculation reveals that 1) the high anisotropy provides extra phase space for electron-phonon scattering, and 2) optical phonons have appreciable contributions. Both effects cannot be captured by the deformation potential calculations., Comment: 25 pages, 15 figures

In contrast to the extensive study of domain reversal in ferromagnetic materials, the domain switching process in antiferromagnets is much less studied due to the difficulty of probing antiferromagnetic spins. Using a combination of hysteresis loop, Kerr microscope, and x-ray magnetic linear dichroism measurements, we investigated the antiferromagnetic (AFM) domain switching process in single crystalline Fe/CoO bilayers on MgO(001). We demonstrate that the CoO AFM switching is a Kolmogorov-Avrami process in which the thermal activation energy creates AFM domain nucleation centers which further expand by domain wall propagation. From the temperature- and thickness-dependent measurements, we are able to retrieve quantitatively the important parameter of the CoO AFM activation energy, which is shown to increase linearly with CoO thickness.

Ge heteroepitaxy on Si (1 1 10) substrates induces the formation of prism-shaped in-plane nanowires bounded with ${1\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}5}$ facets. In this work, in-plane nanowires were fabricated via the growth of Ge onto rib-patterned Si (1 1 10) templates oriented in the $[55\overline{1}]$ direction. Atomic force microscopy (AFM) reveals that a self-elongation of the nanowires occurs, resembling the phenomena observed on rib-patterned Si (0 0 1) templates, which indicates that this is a universal effect for nanowires grown on rib patterns. Finite-element simulations, performed with input from the latest ab initio calculations, reveal that the mechanism behind these phenomena is the minimization of the total energy density of the epilayer under rib-dominated geometry. Ge surface diffusion leads to a broadening of the Ge nanowires at the rib shoulder sites, which is proved to be an effective route to reduce the total energy density. Our results provide a straightforward solution for the realization of a single or a few Ge nanowires for potential device applications.

Understanding and manipulating coherent phonon transport in solids is of interest both for enhancing the fundamental understanding of thermal transport as well as for many practical applications, including thermoelectrics. In this study, we investigate phonon transmission across Si/Ge superlattices using the Green's function method with first-principles force constants derived from ab initio density functional theory. By keeping the period thickness fixed while changing the number of periods, we show that interface roughness partially destroys coherent phonon transport, especially at high temperatures. The competition between the low-frequency coherent modes and high-frequency incoherent modes leads to an optimum period length for minimum thermal conductivity. To destroy coherence of the low-frequency modes, scattering length scale on the order of period length is required. This finding is useful to guide the design of superlattices to reach even lower thermal conductivity.

Using first principles, we calculate the lattice thermal conductivity of Bi, Sb, and Bi-Sb alloys, which are of great importance for thermoelectric and thermomagnetic cooling applications. Our calculation reveals that the ninth-neighbor harmonic and anharmonic force constants are significant; accordingly, they largely affect the lattice thermal conductivity. Several features of the thermal transport in these materials are studied: (1) the relative contributions from phonons and electrons to the total thermal conductivity as a function of temperature are estimated by comparing the calculated lattice thermal conductivity to the measured total thermal conductivity, (2) the anisotropy of the lattice thermal conductivity is calculated and compared to that of the electronic contribution in Bi, and (3) the phonon mean free path distributions, which are useful for developing nanostructures to reduce the lattice thermal conductivity, are calculated. The phonon mean free paths are found to range from 10 to 100 nm for Bi at 100 K.

We study the thermal transport properties of FeSb${}_{2}$, a promising thermoelectric material for cooling applications at cryogenic temperatures. A first-principles formalism based on density functional theory and ab initio lattice dynamics is applied. We calculate the electronic structure, the phonon dispersion relation, the bulk thermal expansion coefficient, and the thermal conductivity of FeSb${}_{2}$ and compare them with other calculations and experiments. Our calculation is found insufficient to fully explain the temperature dependence of the lattice thermal conductivity of FeSb${}_{2}$, suggesting new scattering mechanisms in this strongly correlated system. The mean free path distribution of different phonon modes is also calculated, which may provide valuable guidance in designing nanostructures for reducing the thermal conductivity of FeSb${}_{2}$ and improving the thermoelectric figure of merit zT.

We investigate the thermal conductivity of bulk silicon crystals based on molecular-dynamics (MD) simulations. If it is taken that the system size must be larger than the phonon mean free path, several hundreds of millions of atoms must be computed for crystals with large thermal conductivity values such as Si. We demonstrate in this work that the thermal conductivity of Si crystals can be simulated by MD techniques using several thousands of atoms with periodic boundary conditions. We identify that the key issues generating size artifacts in small molecular-dynamics systems are the frequency cutoff imposed by the simulation domain length and the correlation artifacts caused by the periodic boundary conditions. Our method relies on the spectral Green-Kubo formulation combined with a model-based extrapolation. The obtained thermal conductivity results are in good agreement with the reference data. Both the Green-Kubo formulation and the Boltzmann transport equation lead to the prediction that the thermal conductivities of bulk crystals depend on the frequency of the thermal disturbance. This result has important implications for high-frequency electronic devices.

Significant reductions in both the in-plane and cross-plane thermal conductivities of superlattices, in comparison to the values calculated from the Fourier heat conduction theory using bulk material properties, have been observed experimentally in recent years. Understanding the mechanisms controlling the thermal conductivities of superlattice structures is of considerable current interest for microelectronic and thermoelectric applications. In this work, models of the thermal conductivity and phonon transport in the direction perpendicular to the film plane of superlattices are established based on solving the phonon Boltzmann transport equation (BTE). Different phonon interface scattering mechanisms are considered, including elastic vs inelastic, and diffuse vs specular scattering of phonons. Numerical solution of the BTE yields the effective temperature distribution, thermal conductivity, and thermal boundary resistance (TBR) of the superlattices. The modeling results show that the effective thermal conductivity of superlattices in the perpendicular direction is generally controlled by phonon transport within each layer and the TBR between different layers. The TBR is no longer an intrinsic property of the interface, but depends on the layer thickness as well as the phonon mean free path. In the thin layer limit, phonon transport within each layer is ballistic, and the TBR dominates the effective thermal conductivity of superlattices. Approximate analytical solutions of the BTE are obtained for this thin-film limit. The modeling results based on partially specular and partially diffuse interface scattering processes are in reasonable agreement with recent experimental data on GaAs/AlAs and Si/Ge superlattices. From the modeling, it is concluded that the cross-plane thermal conductivity of these superlattices is controlled by diffuse and inelastic scattering processes at interfaces. Results of this work suggest that it is possible to make superlattice structures with thermal conductivity totally different from those of their constituting materials.

We propose and investigate a design for ``electron cloaks'' comprised of two electrodes, one top gate and one back gate, on either side of a graphene sheet arranged in a concentric disk configuration. Dirac electrons with specific energies can flow through these electron cloaks with negligible scattering, while electrons with different energies experience significant scattering. The scattering widths of the electron cloaks are analyzed using the partial wave formalism applied to the Dirac equation, and the contributions of the first two partial waves to the scattering widths are set to zero simultaneously via a proper combination of the potentials on the two electrodes. We show that this strategy is sufficient for reducing the total scattering widths to below 0.01$%$ of the physical widths of the cloaks. This new design differs from the well-known Klein tunneling phenomenon in that, in our case, the transparency is isotropic and energy selective. These characteristics, in tandem with tunable Fermi levels and/or the gate voltages on the electrodes, enable the electron cloaks to serve as core units in the designs of new sensors, switches, or transistors.

The thermal conductivity of silicon-germanium superlattices is computed from density-functional perturbation theory using relaxation times that include both anharmonic and interface roughness effects. A decrease in the group velocity of low-frequency phonons in addition to the interface-disorder-induced scattering of high-frequency phonons drives the superlattice thermal conductivity to below the alloy limit. At short periods, interplay between decrease in group velocity and increase in phonon lifetimes with increase in superlattice period leads to a minimum in the cross-plane thermal conductivity. Increasing the mass mismatch between the constituent materials in the superlattice further lowers the thermal conductivity below the alloy limit, pointing to avenues for higher efficiency thermoelectric materials.

We present a theory for the metal-insulator transition (MIT) in the quantum spin liquid candidate material Na${}_{4}$Ir${}_{3}$O${}_{8}$. We consider an extended Hubbard model on the hyperkagome lattice, which incorporates atomic spin-orbit coupling (SOC) and multiorbital interactions of iridium 5$d$ electrons. This model is analyzed using the slave-rotor mean-field theory, and thermodynamic properties across the MIT are studied. The ground state in the insulating side is a U(1) quantum spin liquid with spinon Fermi surfaces that consist of multiple particle-like and hole-like pockets. It is shown that the Wilson ratio in the quantum spin liquid phase is highly enhanced compared to the metallic state. This originates from the fact that the magnetic susceptibility in the quantum spin liquid phase acquires multiple enhancements due to the strong SOC, reduced bandwidth, and on-site spin-orbital exchange, while the heat capacity does not change much across the MIT. This explains the large Wilson ratio of the insulating phase observed in the previous experiment on Na${}_{4}$Ir${}_{3}$O${}_{8}$. Possible connections to other existing and future experiments, in particular on the metallic phase, are discussed.

We study theoretically the effects of $f$-$d$ magnetic exchange interaction in the R${}_{2}$Ir${}_{2}$O${}_{7}$ pyrochlore iridates. The R${}^{3+}$ $f$ electrons form localized Kramers or non-Kramers doublets, while the Ir${}^{4+}$$d$ electrons are more itinerant and feel a strong spin-orbit coupling. We construct and analyze a minimal model capturing this physics, treating the Ir subsystem using a Hubbard-type model. First neglecting the Hubbard interaction, we find Weyl semimetal and Axion insulator phases induced by the $f$-$d$ exchange. Next, we find that $f$-$d$ exchange can cooperate with the Hubbard interaction to stabilize the Weyl semimetal over a larger region of parameter space than when it is induced by $d$-electron correlations alone. Applications to experiments are discussed.

Knowledge on phonon transmittance as a function of phonon frequency and incidence angle at interfaces is vital for multiscale modeling of heat transport in nanostructured materials. Although thermal conductivity reduction in nanostructured materials can usually be described by phonon scattering due to interface roughness, we show how a Green's function method in conjunction with the Landauer formalism suggests that interface roughness induced by atomic mixing can increase phonon transmission and interfacial thermal conductance. This is an attempt to incorporate first-principles force constants derived from ab initio density-functional theory (DFT) into Green's function calculation for infinitely large three-dimensional crystal structure. We also demonstrate the importance of accurate force constants by comparing the phonon transmission and thermal conductance using force constants obtained from semiempirical Stillinger-Weber potential and first-principles DFT calculations., United States. Dept. of Energy. Office of Basic Energy Sciences (Solid-State Solar-Thermal Energy Conversion Center Award DE-FG02-09ER46577)

Nanostructuring has been shown to be an effective approach to reduce the lattice thermal conductivity and improve the thermoelectric figure of merit. Because the experimentally measured thermal conductivity includes contributions from both carriers and phonons, separating out the phonon contribution has been difficult and is mostly based on estimating the electronic contributions using the Wiedemann-Franz law. In this paper, an experimental method to directly measure electronic contributions to the thermal conductivity is presented and applied to Cu${}_{0.01}$Bi${}_{2}$Te${}_{2.7}$Se${}_{0.3}$, [Cu${}_{0.01}$Bi${}_{2}$Te${}_{2.7}$Se${}_{0.3}$]${}_{0.98}$Ni${}_{0.02}$, and Bi${}_{0.88}$Sb${}_{0.12}$. By measuring the thermal conductivity under magnetic field, electronic contributions to thermal conductivity can be extracted, leading to knowledge of the Lorenz number in thermoelectric materials.

International audience; We provide a derivation allowing the calculation of thermal conductance at interfaces by equilibrium molecular dynamics simulations and illustrate our approach by studying thermal conduction mechanisms in Si/Ge superlattices. Thermal conductance calculations of superlattices with period thicknesses ranging from 0.5 to 60 nm are presented as well as the temperature dependence. Results have been compared to complementary Green-Kubo thermal conductivity calculations demonstrating that thermal conductivity of perfect superlattices can be directly deduced from interfacial conductance in the investigated period range. This confirms the predominant role of interfaces in materials with large phonon mean free paths.

The microscopic physics behind low lattice thermal conductivity of single crystal rocksalt lead telluride (PbTe) is investigated. Mode-dependent phonon (normal and umklapp) scattering rates and their impact on thermal conductivity were quantified by the first-principles-based anharmonic lattice dynamics calculations that accurately reproduce thermal conductivity in a wide temperature range. The low thermal conductivity of PbTe is attributed to the scattering of longitudinal acoustic phonons by transverse optical phonons with large anharmonicity, and small group velocity of the soft transverse acoustic phonons. This results in enhancing the relative contribution of optical phonons, which are usually minor heat carrier in bulk materials., 18 pages, 4 figures, accepted for publication in Phys. Rev. B

We demonstrate successful application of first-principles-based thermal conductivity calculation on half-Heusler compounds that are promising, environmentally friendly thermoelectric materials. Taking the case of a $p$-type half-Heusler structure, the harmonic and anharmonic interatomic force constants were obtained from a set of force-displacement data calculated by the density functional theory. Thermal conductivity was obtained by two different methods: (1) Boltzmann-Peierls formula with phonon relaxation times calculated by either Fermi's golden rule of three-phonon scattering processes or spectral analysis of molecular dynamics phase space trajectories and (2) Green-Kubo formula for heat current obtained by equilibrium molecular dynamics simulations. The calculated temperature dependence of thermal conductivity is in reasonable agreement with experiments. The method was extended to alloy crystals assuming the transferability of interatomic force constants. By having access to accurate phonon-dependent transport properties, the contribution from an arbitral subset of phonon modes can be quantified. This helps understanding the influence of nanostructures on thermal conductivity.

Using harmonic and anharmonic force constants extracted from density-functional calculations within a supercell, we have developed a relatively simple but general method to compute thermodynamic and thermal properties of any crystal. First, from the harmonic, cubic, and quartic force constants we construct a force field for molecular dynamics (MD). It is exact in the limit of small atomic displacements and thus does not suffer from inaccuracies inherent in semi-empirical potentials such as Stillinger-Weber's. By using the Green-Kubo (GK) formula and molecular dynamics simulations, we extract the bulk thermal conductivity. This method is accurate at high temperatures where three-phonon processes need to be included to higher orders, but may suffer from size scaling issues. Next, we use perturbation theory (Fermi Golden rule) to extract the phonon lifetimes and compute the thermal conductivity $\kappa$ from the relaxation time approximation. This method is valid at most temperatures, but will overestimate $\kappa$ at very high temperatures, where higher order processes neglected in our calculations, also contribute. As a test, these methods are applied to bulk crystalline silicon, and the results are compared and differences discussed in more detail. The presented methodology paves the way for a systematic approach to model heat transport in solids using multiscale modeling, in which the relaxation time due to anharmonic 3-phonon processes is calculated quantitatively, in addition to the usual harmonic properties such as phonon frequencies and group velocities. It also allows the construction of accurate bulk interatomic potentials database., Comment: appear in PRB (2011)

Thermoelectric properties of dense bulk polycrystalline In4Se3-x (x = 0, 0.25, 0.5, 0.65, and 0.8) compounds are investigated. A peak dimensionless thermoelectric figure of merit (ZT) of about 1 is achieved for x = 0.65 and 0.8. The peak ZT is about 50% higher than the previously reported highest value for polycrystalline In4Se3-x} compounds. Our In4Se3-x samples were prepared by ball milling and hot pressing. We show that it is possible to effectively control the electrical conductivity and thermal conductivity by controlling selenium (Se) deficiency x. The ZT enhancement is mainly attributed to the thermal conductivity reduction due to the increased phonon scattering by Se deficiency, defects, and nanoscale inclusions in the ball-milled and hot-pressed dense bulk In4Se3-x samples.

The relations between the thermal conductivity of cagelike structures and their crystal parameters are investigated using a two-dimensional toy model. The model consists of host atoms on a rectangular lattice with fillers at the center of each rectangle. The effect of mass and size of the filler on thermal conductivity is investigated using equilibrium molecular-dynamics simulations. We show that the thermal conductivity decreases with increasing atomic displacement parameter while it has local minima versus the filler to host mass ratio. Similar trends were observed in experiments on filled skutterudites. The trends are explained by analyzing the effect of the filler on the phonon dispersion and relaxation times of the host material.

The thermal conductivity of nanostructures generally decreases with decreasing size because of classical size effects. The axial thermal conductivity of polymer chain lattices, however, can exhibit the opposite trend, because of reduced chain-chain anharmonic scattering. This unique feature gives rise to an interesting one-dimensional-to-three-dimensional transition in phonon transport. We study this transition by calculating the thermal conductivity of polyethylene with molecular dynamics simulations. The results are important for designing inexpensive high thermal-conductivity polymers.