Lattice thermal conductivities of $\beta \text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_4\mathrm{S}{\mathrm{i}}_5{\mathrm{O}}_{18}$ were predicted at various hydrostatic pressures based on some theoretical models. An abnormal decrement on lattice thermal conductivity is observed for compressed crystal structure. A rigorous analysis of structural stability, bonding characteristics, vibration modes, group velocities, and mode Grüneisen parameters helps us to recognize the origin of this anomalous behavior. We attribute the negative dependent trend to the softening of low frequency phonons and strengthening of anharmonicity at elevated pressure, both of which arise from the specific corner-linked tetrahedral framework in the crystal structure. To validate theoretical calculations, we synthesized pure and dense $\beta \text{−}\mathrm{M}{\mathrm{g}}_2\mathrm{A}{\mathrm{l}}_4\mathrm{S}{\mathrm{i}}_5{\mathrm{O}}_{18}$ ceramic by using a two-step processing method and determined its intrinsic lattice thermal conductivity by successfully eliminating the phonon scattering from defects and high-temperature thermal radiation. The experimental intrinsic values agreed quite well with the theoretical predictions. This paper reports an anomalous pressure-induced reduction of lattice thermal conductivity and also provides a key insight into the interesting phonon modification mechanism through tailoring the crystal structure of complex compounds.

We investigate the excitation of phonons in photoexcited antimony and demonstrate that the entire electron-lattice interactions, in particular coherent and incoherent electron-phonon coupling, can be probed simultaneously. Using femtosecond electron diffraction (FED) with high temporal resolution, we observe the coherent excitation of the fully symmetric ${\mathrm{A}}_{1g}$ optical phonon mode via the shift of the minimum of the atomic potential energy surface. Ab initio molecular dynamics simulations on laser excited potential energy surfaces are performed to quantify the change in lattice potential and the associated real-space amplitude of the coherent atomic oscillations. Good agreement is obtained between the parameter-free calculations and the experiment. In addition, our experimental configuration allows observing the energy transfer from electrons to phonons via incoherent electron-lattice scattering events. The electron-phonon coupling is determined as a function of electronic temperature from our DFT calculations and the data by applying different models for the energy transfer.

We report the observation of a Griffiths phase in the geometrically frustrated antiferromagnet $\mathrm{DyBaCo}{}_4\mathrm{O}{}_{7+\delta }$. Its onset is identified using measurements of the thermoremanent magnetization, which appears to be superior to conventional in-field measurement protocols for the characterization of the Griffiths phase. Within this phase, the temporal relaxation of magnetization exhibits a functional form which is expected for Heisenberg systems, reflecting the nature of spin interactions in this class of materials. Interestingly, the effective $\mathrm{Co}{}^{2+}/\mathrm{Co}{}^{3+}$ ratio tailored by varying the oxygen nonstoichiometry $\delta$ is only seen to influence the antiferromagnetic ordering temperature ($T{}_N$), leaving the Griffiths temperature ($T{}_G$) invariant.

We derive an effective time independent Hamiltonian for the transverse Ising model coupled to a spin bath, in the presence of a high frequency AC magnetic field. The spin blocking mechanism that removes the quantum phase transition can be suppressed by the AC field, allowing tunability of the quantum critical point. We calculate the phase diagram, including the nuclear spins, and apply the results to quantum Ising systems with long-range dipolar interactions; the example of ${\text{LiHoF}}_4$ is discussed in detail.

We investigate the zero-temperature quantum phase transitions of the disordered three-color quantum Ashkin-Teller spin chain by means of large-scale Monte Carlo simulations. We find that the first-order phase transitions of the clean system are rounded by the quenched disorder. For weak intercolor coupling, the resulting emergent quantum critical point between the paramagnetic phase and the magnetically ordered Baxter phase is of infinite-randomness type and belongs to the universality class of the random transverse-field Ising model, as predicted by recent strong-disorder renormalization group calculations. We also find evidence for unconventional critical behavior in the case of strong intercolor coupling, even though an unequivocal determination of the universality class is beyond our numerical capabilities. We compare our results to earlier simulations, and we discuss implications for the classification of phase transitions in the presence of disorder.

${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$ provides a material platform to study three-dimensional quantum spin liquids in the geometrically frustrated hyperkagome lattice of ${\mathrm{Ir}}^{4+}$ ions. In this work, we consider quantum spin liquids on a hyperkagome lattice for generic spin models, focusing on the effects of anisotropic spin interactions. In particular, we classify possible ${\mathbb{Z}}_2$ and $U\left(1\right)$ spin liquid states, following the projective symmetry group analysis in the slave-fermion representation. There are only three distinct ${\mathbb{Z}}_2$ spin liquids, together with 2 different $U\left(1\right)$ spin liquids. The nonsymmorphic space group symmetry of the hyperkagome lattice plays a vital role in simplifying the classification, forbidding “$\pi$-flux” or “staggered-flux” phases in contrast to symmorphic space groups. We further prove that both $U\left(1\right)$ states and one ${\mathbb{Z}}_2$ state among all 3 are symmetry-protected gapless spin liquids, robust against any symmetry-preserving perturbations. Motivated by the “spin-freezing” behavior recently observed in ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$ at low temperatures, we further investigate the nearest-neighbor spin model with the dominant Heisenberg interaction subject to all possible anisotropic perturbations from spin-orbit couplings. We find that a $U\left(1\right)$ spin liquid ground state with spinon Fermi surfaces is energetically favored over ${\mathbb{Z}}_2$ states. Among all spin-orbit coupling terms, we show that only the Dzyaloshinskii-Moriya interaction can induce spin anisotropy in the ground state when perturbing from the isotropic Heisenberg limit. Our work paves the way for a systematic study of quantum spin liquids in various materials with a hyperkagome crystal structure.

The classical Ising model on the frustrated three-dimensional (3D) swedenborgite lattice has disordered spin liquid ground states for all ratios of inter- and intraplanar couplings. Quantum fluctuations due to a transverse field give rise to several exotic phenomena. In the limit of weakly coupled kagome layers we find a 3D version of disorder by disorder degeneracy lifting. For large out-of-plane couplings one-dimensional macrospins are formed, which realize a disordered macrospin liquid phase on an emerging two-dimensional triangular lattice. We speculate about a possibly exotic version of quantum criticality that connects the polarized phase to the macrospin liquid.

Inelastic neutron scattering is used to study the low-energy magnetic excitations in the spin-1 triangular lattice of the $6H$-B phase of ${\mathrm{Ba}}_3{\mathrm{NiSb}}_2{\mathrm{O}}_9$. We study two powder samples: ${\mathrm{Ba}}_3{\mathrm{NiSb}}_2{\mathrm{O}}_9$ synthesized under high pressure and ${\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}{\mathrm{NiSb}}_2{\mathrm{O}}_9$ in which chemical pressure stabilizes the $6H$-B structure. The measured excitation spectra show broad gapless and nondispersive continua at characteristic wave vectors. Our data rules out most theoretical scenarios that have previously been proposed for this phase, and we find that it is well described by an exotic quantum spin liquid with three flavors of unpaired fermionic spinons, forming a large spinon Fermi surface.

The interplay between quantum fluctuations and disorder is investigated in a quantum spin-glass model, in the presence of a uniform transverse field $\mathrm{\Gamma }$, as well as of a longitudinal random field $h_i$, which follows a Gaussian distribution characterized by a width proportional to $\mathrm{\Delta }$. The interactions are infinite-ranged, and the model is studied through the replica formalism, within a one-step replica-symmetry-breaking procedure; in addition, the dependence of the Almeida-Thouless eigenvalue ${\lambda }_{\mathrm{AT}}$ (replicon) on the applied fields is analyzed. This study is motivated by experimental investigations on the ${\mathrm{LiHo}}_x{\mathrm{Y}}_{1-x}{\mathrm{F}}_4$ compound, where the application of a transverse magnetic field yields rather intriguing effects, particularly related to the behavior of the nonlinear magnetic susceptibility ${\chi }_3$, which have led to a considerable experimental and theoretical debate. We have analyzed two physically distinct situations, namely, $\mathrm{\Delta }$ and $\mathrm{\Gamma }$ considered as independent, as well as these two quantities related, as proposed recently by some authors. In both cases, a spin-glass phase transition is found at a temperature $T_f$, with such phase being characterized by a nontrivial ergodicity breaking; moreover, $T_f$ decreases by increasing $\mathrm{\Gamma }$ towards a quantum critical point at zero temperature. The situation where $\mathrm{\Delta }$ and $\mathrm{\Gamma }$ are related $[\mathrm{\Delta }\equiv \mathrm{\Delta }(\mathrm{\Gamma }\left)\right]$ appears to reproduce better the experimental observations on the ${\mathrm{LiHo}}_x{\mathrm{Y}}_{1-x}{\mathrm{F}}_4$ compound, with the theoretical results coinciding qualitatively with measurements of the nonlinear susceptibility ${\chi }_3$. In this later case, by increasing $\mathrm{\Gamma }$ gradually, ${\chi }_3$ becomes progressively rounded, presenting a maximum at a temperature $T^{*}$ $\left(T^{*}>T_f\right)$, with both the amplitude of the maximum and the value of $T^{*}$ decreasing gradually. Moreover, we also show that the random field is the main responsible for the smearing of the nonlinear susceptibility, acting significantly inside the paramagnetic phase, leading to two regimes delimited by the temperature $T^{*}$, one for $T_f<T<T^{*}$, and another one for $T>T^{*}$. It is argued that the conventional paramagnetic state corresponds to $T>T^{*}$, whereas the temperature region $T_f<T<T^{*}$ may be characterized by a rather unusual dynamics, possibly including Griffiths singularities.

A new modified embedded-atom method (MEAM) force field is developed for liquid tin. Starting from the Ravelo and Baskes force field [Phys. Rev. Lett. 79, 2482 (1997)], the parameters are adjusted using a simulated annealing optimization procedure in order to obtain better agreement with liquid-phase data. The predictive capabilities of the new model and the Ravelo and Baskes force field are evaluated using molecular dynamics by comparing to a wide range of first-principles and experimental data. The quantities studied include crystal properties (cohesive energy, bulk modulus, equilibrium density, and lattice constant of various crystal structures), melting temperature, liquid structure, liquid density, self-diffusivity, viscosity, and vapor-liquid surface tension. It is shown that although the Ravelo and Baskes force field generally gives better agreement with the properties related to the solid phases of tin, the new MEAM force field gives better agreement with liquid tin properties.

The atomic-scale structure of aerodynamically levitated and laser-heated liquid tricalcium aluminate (${\mathrm{Ca}}_3{\mathrm{Al}}_2{\mathrm{O}}_6$) was measured at 2073(30) K by using the method of neutron diffraction with Ca isotope substitution (NDIS). The results enable the detailed resolution of the local coordination environment around calcium and aluminum atoms, including the direct determination of the liquid partial structure factor, $S_{\mathrm{CaCa}}\left(Q\right)$, and partial pair distribution function, $g_{\mathrm{CaCa}}\left(r\right)$. Molecular dynamics (MD) simulation and reverse Monte Carlo (RMC) refinement methods were employed to obtain a detailed atomistic model of the liquid structure. The composition ${\mathrm{Ca}}_3{\mathrm{Al}}_2{\mathrm{O}}_6$ lies at the CaO-rich limit of the CaO:${\mathrm{Al}}_2{\mathrm{O}}_3$ glass-forming system. Our results show that, although significantly depolymerized, liquid ${\mathrm{Ca}}_3{\mathrm{Al}}_2{\mathrm{O}}_6$ is largely composed of ${\mathrm{AlO}}_4$ tetrahedra forming an infinite network with a slightly higher fraction of bridging oxygen atoms than expected for the composition. Calcium-centered polyhedra exhibit a wide distribution of four- to sevenfold coordinated sites, with higher coordinated calcium preferentially bonding to bridging oxygens. Analysis of the MD configuration reveals the presence of $\sim 10\%$ unconnected ${\mathrm{AlO}}_4$ monomers and ${\mathrm{Al}}_2{\mathrm{O}}_7$ dimers in the liquid. As the CaO concentration increases, the number of these isolated units increases, such that the upper value for the glass-forming composition of CaO:${\mathrm{Al}}_2{\mathrm{O}}_3$ liquids could be described in terms of a percolation threshold at which the glass can no longer support the formation of an infinitely connected ${\mathrm{AlO}}_4$ network.

An in situ optical pump and x-ray probe technique has been utilized to study photoinitiated solid-state diffusion in a Ni-Pt multilayer system. Hard x-ray diffraction has been used to follow the systematic growth of the NiPt alloy as a function of laser intensity and total energy deposited. It is observed that new phase growth can be driven in as little as one laser pulse, and that repeated photoexcitation can completely convert the entire multilayer structure into a single metallic alloy. The data suggest that lattice strain relaxation takes place prior to atomic diffusion and the formation of a NiPt alloy.

The conversion of charge current to spin current by the spin Hall effect is of considerable current interest from both fundamental and technological perspectives. Measurement of the spin Hall angle, especially for atomically thin systems with large magnetic anisotropies, is not straightforward. Here we demonstrate a hybrid phase-resolved optical-electrical ferromagnetic resonance method that we show can robustly determine the spin Hall angle in heavy-metal/ferromagnet bilayer systems with large perpendicular magnetic anisotropy. We present an analytical model of the ferromagnetic resonance spectrum in the presence of the spin Hall effect, in which the spin Hall angle can be directly determined from the changes in the amplitude response as a function of the spin current that is generated from a dc charge current passing through the heavy-metal layer. Increased sensitivity to the spin current is achieved by operation under conditions for which the magnetic potential is shallowest at the “Smit point.” Study of the phase response reveals that the spin Hall angle can be reliably extracted from a simplified measurement that does not require scanning over time or magnetic field but rather only on the dc current. The method is applied to the Pt-Co/Ni/Co system whose spin Hall angle was to date characterized only indirectly and that is especially relevant for spin-orbit torque devices.

Several materials in the regime of strong spin-orbit interaction such as HgTe, the pyrochlore iridate ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, and the half-Heusler compound LaPtBi, as well as various systems related to these three prototype materials, are believed to host a quadratic band touching point at the Fermi level. Recently, it has been proposed that such a three-dimensional gapless state is unstable to a Mott-insulating ground state at low temperatures when the number of band touching points $N$ at the Fermi level is smaller than a certain critical number $N_{\mathrm{c}}$. We further substantiate and quantify this scenario by various approaches. Using $\epsilon$ expansion near two spatial dimensions, we show that $N_{\mathrm{c}}=64/\left(25{\epsilon }^2\right)+O(1/\epsilon )$ and demonstrate that the instability for $N<N_{\mathrm{c}}$ is towards a nematic ground state that can be understood as if the system were under (dynamically generated) uniaxial strain. We also propose a truncation of the functional renormalization group equations in the dynamical bosonization scheme which we show to agree to one-loop order with the results from $\epsilon$ expansion both near two as well as near four dimensions, and which smoothly interpolates between these two perturbatively accessible limits for general $2<d<4$. Directly in $d=3$ we therewith find $N_{\mathrm{c}}=1.86$, and thus again above the physical $N=1$. All these results are consistent with the prediction that the interacting ground state of pure, unstrained HgTe, and possibly also ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, is a strong topological insulator with a dynamically generated gap—a topological Mott insulator.

We have studied the magnetotransport properties of the metallic, $p$-type ${\mathrm{Sb}}_2{\mathrm{Te}}_2\mathrm{Se}$, which is a topological insulator. Magnetoresistance shows Shubnikov–de Haas oscillations in fields above $B=15$ T. The maxima and minima positions of oscillations measured at different tilt angles with respect to the $B$ direction align with the normal component of field $B\cos \theta$, implying the existence of a two-dimensional Fermi surface in ${\mathrm{Sb}}_2{\mathrm{Te}}_2\mathrm{Se}$. The value of the Berry phase $\beta$ determined from a Landau level fan diagram is very close to 0.5, further suggesting that the oscillations result from topological surface states. From Lifshitz-Kosevich analyses, the position of the Fermi level is found to be $E_F=250$ meV, above the Dirac point. This value of $E_F$ is almost 3 times larger than that in our previous study on the ${\mathrm{Bi}}_2{\mathrm{Se}}_{2.1}{\mathrm{Te}}_{0.9}$ topological insulator; however, it still touches the tip of the bulk valence band. This explains the metallic behavior and holelike bulk charge carriers in the ${\mathrm{Sb}}_2{\mathrm{Te}}_2\mathrm{Se}$ compound.

We consider scaling of the entanglement entropy across a topological quantum phase transition for the Kitaev chain model. The change of the topology manifests itself in a subleading term, which scales as $L^{-1/\alpha }$ with the size of the subsystem $L$, here $\alpha$ is the Rényi index. This term reveals the scaling function $h_{\alpha }(L/\xi )$, where $\xi$ is the correlation length, which is sensitive to the topological index. The scaling function $h_{\alpha }(L/\xi )$ is independent of model parameters, suggesting some degree of its universality.

The ability to perform efficient electrical spin injection from ferromagnetic metals into two-dimensional semiconductor crystals based on transition metal dichalcogenide monolayers is a prerequisite for spintronic and valleytronic devices using these materials. Here, the hexagonal close-packed (hcp) $\mathrm{Co}\left(0001\right)/\mathrm{Mo}{\mathrm{S}}_2$ interface electronic structure is investigated by first-principles calculations based on the density functional theory. In the lowest energy configuration of the hybrid system after optimization of the atomic coordinates, we show that interface sulfur atoms are covalently bound to one, two, or three cobalt atoms. A decrease of the Co atom spin magnetic moment is observed at the interface, together with a small magnetization of S atoms. Mo atoms also hold small magnetic moments, which can take positive as well as negative values. The charge transfers due to covalent bonding between S and Co atoms at the interface have been calculated for majority and minority spin electrons, and the connections between these interface charge transfers and the induced magnetic properties of the $\mathrm{Mo}{\mathrm{S}}_2$ layer are discussed. Band structure and density of states of the hybrid system are calculated for minority and majority spin electrons, taking into account spin-orbit coupling. We demonstrate that $\mathrm{Mo}{\mathrm{S}}_2$ bound to the Co contact becomes metallic due to hybridization between Co $d$ and S $p$ orbitals. For this metallic phase of $\mathrm{Mo}{\mathrm{S}}_2$, a spin polarization at the Fermi level of 16% in absolute value is calculated, which could allow spin injection into the semiconducting $\mathrm{Mo}{\mathrm{S}}_2$ monolayer channel. Finally, the symmetry of the majority and minority spin electron wave functions at the Fermi level in the Co-bound metallic phase of $\mathrm{Mo}{\mathrm{S}}_2$ and the orientation of the border between the metallic and semiconducting phases of $\mathrm{Mo}{\mathrm{S}}_2$ are investigated, and their impact on spin injection into the $\mathrm{Mo}{\mathrm{S}}_2$ channel is discussed.

Femtosecond x-ray absorption spectroscopy with a laser-driven high-harmonic source is used to map ultrafast changes of x-ray absorption by femtometer-scale coherent phonon displacements. In ${\mathrm{LiBH}}_4$, displacements along an $A_g$ phonon mode at 10 THz are induced by impulsive Raman excitation and give rise to oscillatory changes of x-ray absorption at the Li $K$ edge. Electron density maps from femtosecond x-ray diffraction data show that the electric field of the pump pulse induces a charge transfer from the ${\mathrm{BH}}_4{}^{-}$ to neighboring ${\mathrm{Li}}^{+}$ ions, resulting in a differential Coulomb force that drives lattice vibrations in this virtual transition state.