The family of rare-earth ($R$) tritellurides $R\mathrm{T}{\mathrm{e}}_3$ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [J. J. Hamlin et al., Phys. Rev. Lett. 102, 177002 (2009)]. What sets the heavier members of the $R\mathrm{T}{\mathrm{e}}_3$ series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wave vector $q_{\mathrm{CDW},2}\parallel \left[100\right]$ of almost the same magnitude but orthogonal to the ordering wave vector $q_{\mathrm{CDW},1}\parallel \left[001\right]$ observed at higher temperatures [N. Ru et al., Phys. Rev. B 77, 035114 (2008)]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of $\mathrm{DyT}{\mathrm{e}}_3$ In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at $T_{\mathrm{CDW},1}=308\mathrm{K}$. Surprisingly, these soft modes close to $q_{\mathrm{CDW},2}=\left(0.68,0,0\right)$ show strong softening near $T_{\mathrm{CDW},1}$ but do not exhibit any response to the lower-temperature transition at $T_{\mathrm{CDW},2}=68\mathrm{K}$. Our results indicate that the low-temperature CDW order is not just the 90° rotated analogue of the one appearing at high temperatures.

We use time-domain terahertz spectroscopy to measure the low energy conductivity and magnons in ${\mathrm{RuCl}}_3$ under external magnetic field. At zero field, an oscillation with a frequency of 0.62 THz is clearly observed in time-domain spectrum below ${\mathrm{T}}_N$, which is identified as a magnon excitation in the magnetic order state. The magnon excitation is not affected by the external magnetic field ${\mathbf{H}}_{DC}$ when it is applied along the $c$ axis, but is clearly suppressed when ${\mathbf{H}}_{DC}$ is applied within the $ab$ plane. More interestingly, when the magnetic component of the THz wave $\mathbf{h}\left(t\right)$ is perpendicular to the applied in-plane magnetic field, we observe another coherent oscillation at slightly higher energy scale at the field above 2 T, which is eventually suppressed for $H_{DC}>5$ T. The measurement seems to indicate that the in-plane magnetic field can lift the degeneracy of two branches of low energy magnons at the $\mathrm{\Gamma }$ point. The low energy optical conductivity calculated from the measured transmission spectrum is dominated by a broad continuum contribution, which is not affected by changing either temperature or external magnetic field.

Pinning and thermal creep determine the response of numerous systems containing superstructures, e.g., vortices in type II superconductors, domain walls in ferroics, or dislocations in metals. The combination of drive and thermal fluctuations leads to the superstructure's depinning, and its velocity $v$ determines the electric, magnetic, or mechanical response. It is commonly believed that pinning and creep collapse above the critical drive $F_c$, entailing a sharp rise in the velocity $v$. We challenge this perception by studying the effects of thermal fluctuations within the framework of strong vortex pinning in type II superconductors. In fact, we show that pinning and thermal creep persist far beyond the critical force. The resulting force-velocity characteristic largely maintains its zero-temperature shape, and thermal creep manifests itself by a downward renormalization of the critical drive. Such characteristics are in agreement with Coulomb's law of dry friction, and they have been often observed in experiments.

We study the differential resistivity transition of two-dimensional superconducting arrays induced by an external driving current, in the presence of thermal fluctuations and a magnetic field corresponding to $f$ flux quantum per plaquette. Recent experiments have identified this transition as a dynamic vortex Mott insulator transition at vortex densities near rational values of $f$. The critical behavior is determined from a scaling analysis of the current-voltage relation near the transition, obtained by Monte Carlo simulations of a Josephson-junction array model in the vortex representation. For a square-lattice array, the critical exponents obtained near $f=1/2$ are consistent with the experimental observations. The same scaling behavior is observed near $f=1/3$. For a honeycomb array, although similar results are obtained for $f=1/3$, the transition is absent for $f=1/2$, consistent with an incommensurate vortex phase.

The magnetic insulator $\alpha -{\mathrm{RuCl}}_3$ is a promising candidate to realize Kitaev interactions on a quasi-two-dimensional honeycomb lattice. We perform extensive susceptibility measurements on single crystals of $\alpha -{\mathrm{RuCl}}_3$, including angle dependence of the in-plane longitudinal and transverse susceptibilities, which reveal a unidirectional anisotropy within the honeycomb plane. By comparing the experimental results to a high-temperature expansion of a Kitaev-Heisenberg-$\mathrm{\Gamma }$ spin Hamiltonian with bond anisotropy, we find excellent agreement with the observed phase shift and periodicity of the angle-resolved susceptibilities. Within this model, we show that the pronounced difference between in-plane and out-of-plane susceptibilities as well as the finite transverse susceptibility are rooted in strong symmetric off-diagonal $\mathrm{\Gamma }$ spin exchange. The $\mathrm{\Gamma }$ couplings and relationships between other terms in the model Hamiltonian are quantified by extracting relevant Curie-Weiss intercepts from the experimental data.

The novel quantum magnet ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ was recently discovered to be a rare physical realization of a one-dimensional antiferromagnetic-ferromagnetic dimerized chain which displays strongly correlated phenomena at elevated temperatures [E. S. Klyushina et al., Phys. Rev. B 93, 241109(R) (2016)]. This paper presents an extended study of the Hamiltonian of ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ at base temperature. Static susceptibility and inelastic neutron scattering data are compared to several theoretical models. An analytical relation for the dynamic structure factor of the complex unit cell of ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ is derived and used to identify the intrachain exchange paths. Further analysis using the first moment of the dynamic structure factor was employed to determine the exchange path responsible for the intradimer interaction. This analysis reveals that the dimer chain is formed by a dominant antiferromagnetic exchange interaction $J_{\mathrm{intra}}=40.92\mathrm{meV}$ which is realized via the Cu-O-V(II)-O-Cu superexchange path and a weak ferromagnetic coupling $J_{\mathrm{inter}}=-11.97$ meV which arises within the copper-oxygen double plaquettes.

${\mathrm{Er}}_2{\mathrm{CoMnO}}_6$ is a ferrimagnetic oxide consisting of both $4f$ and $3d$ magnetic ions in a monoclinic double perovskite crystal structure (space group $P{2}_1/n$). The present work probes some novel magnetic features associated with its ferrimagnetic state on a polycrystalline sample of ${\mathrm{Er}}_2{\mathrm{CoMnO}}_6$. A clear low temperature compensation point followed by negative magnetization is observed in the field-cooled state of the sample. The sample shows a large field-induced change in entropy at very low temperature, which is related to the observed metamagnetism. Interestingly, ${\mathrm{Er}}_2{\mathrm{CoMnO}}_6$ is found to show remarkable feature of inverse hysteresis loop which is quite rare among bulk ferrimagnets. Presence of three different magnetic ions, with $4f$ moment of Er having larger uniaxial anisotropy, is likely to be responsible for the observed magnetization reversal and inverse hysteresis loop. The sample also shows inverse exchange bias effect, which is presumably related to the negative exchange interaction at the interface of ordered and antisite disordered ferrimagnetic phases. Such antisite disorder is quite common among the metal oxides having double perovskite structure.

We demonstrate that thick permalloy films exhibiting a weak growth-induced perpendicular magnetic anisotropy can be employed as an ideal test system for the investigation of gyration dynamics in topologically trivial and nontrivial magnetic states ranging from an isolated magnetic skyrmion to more complex $n\pi$ spin configurations.

Two multiferroic hexaferrites Y-type $\mathrm{B}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{1.5}\mathrm{Z}{\mathrm{n}}_2{\left(\mathrm{F}{\mathrm{e}}_{0.92}\mathrm{A}{\mathrm{l}}_{0.08}\right)}_{12}{\mathrm{O}}_{22}$ and Z-type $\mathrm{B}{\mathrm{a}}_{0.52}\mathrm{S}{\mathrm{r}}_{2.48}\mathrm{C}{\mathrm{o}}_2\mathrm{F}{\mathrm{e}}_{24}{\mathrm{O}}_{41}$ have been reported to show giant electric field control of magnetization and small magnetic field control of drastic change of polarization, i.e., giant magnetoelectric (ME) effects up to room temperature. However, it is still a mystery that their ME properties are quite different in many aspects even though they have very similar transverse cone spin configurations. Here, we report the comparative investigation of angle-dependent ME effects in two compounds. When the external $H$ is rotated clockwise by 360°, the in-plane $P$ vector is rotated clockwise by 360° in the Y-type hexaferrite and counterclockwise by 720° in the Z-type hexaferrite. These contrasting ME effects can only be explained with a new symmetry-based spin model and tensor analysis. Our new approach reveals that the faster and opposite rotation of the $P$ vector in the Z-type hexaferrite is associated with the lack of the space inversion in one of the magnetic blocks. Moreover, such a peculiar crystal symmetry also results in contrasting microscopic origins for the spin-driven ME behaviors; only the inverse DM interaction is responsible for the Y-type hexaferrite while additional p-d hybridization becomes more important in the Z-type hexaferrite. This work demonstrates the importance of the crystal symmetry in the determination of ME properties in the hexaferrites and provides a fundamental framework for understanding and applying the ME properties in other spin-driven multiferroics.

We study projected entangled pair states (PEPS) with continuous virtual symmetries, i.e., symmetries in the virtual degrees of freedom, through an elementary class of models with $\mathrm{SU}\left(2\right)$ symmetry. Discrete symmetries of that kind have previously allowed for a comprehensive explanation of topological order in the PEPS formalism. We construct local parent Hamiltonians whose ground space with open boundaries is exactly parametrized by the PEPS wave function, and show how the ground state can be made unique by a suitable choice of boundary conditions. We also find that these models exhibit a logarithmic correction to the entanglement entropy and an extensive ground space degeneracy on systems with periodic boundaries, which suggests that they do not describe conventional gapped topological phases, but either critical models or some other exotic phase.

To construct reliable material platforms and to uncover new rules to realize spin-Seebeck effect (SSE) and thermal spin-filtering effect (SFE) are core topics in spin caloritronics. Here we design several single-layer boron-nitrogen nanotubes (BNNTs) with $n$ boron (nitrogen) atoms substituted by carbons in every unit cell. We find that for $n=1$, the magnetic BNNTs generate a good SSE with nearly symmetric spin-up and spin-down currents, while as the carbon dopant concentration increases (c.f. $n\ge 2$), a high rotational symmetry of the carbons contributes to generate the SSE with more symmetric thermal spin-up and spin-down currents, otherwise towards the thermal SFE. Moreover, some metallic BNNTs can generate the SSE or the SFE with finite threshold temperatures, due to the compensation effect around the Fermi level. More importantly, we find that the compression strain engineering is an effective route to improve these effects and to realize the transition between them. These theoretical results about the SSE in nanotubes enrich the spin caloritronics and put forward material candidates to realize the SSE and other inspiring thermospin phenomena.

We report the observation of ferromagnetism in thin films of paramagnetic metal ${\mathrm{LaNiO}}_3$ grown on nonmagnetic insulating substrate ${\mathrm{LaAlO}}_3$(111) substrates. The films exhibit a large hysteresis loop on magnetoresistance and anomalous Hall effect as well as sixfold anisotropic magnetoresistance. Together with the ferroelectricity and inversion symmetry breaking reported in the same system, these results suggest that, by the geometric constraints by the substrate, paramagnetic metals can turn into ferromagnetic semiconductors. The ferromagnetic ground state is consistent with the predicted spin-split Fermi surfaces of spin-orbit-coupled correlated metals. Moreover, our results reveal a positive linear magnetoresistance and a sign reversal of hysteresis loop, which is consistent with the existence of a massive Dirac point as predicted by theories.

We investigate excitons in stacked transition-metal dichalcogenide layers under a perpendicularly applied electric field, herein ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ van der Waals heterostructures (vdWHs). Band structures are obtained with density functional theory (DFT), along with electron and hole wave functions in conduction and valence bands, respectively. A minimal continuum model, parametrized by the DFT results, is presented, allowing for the calculation of the excitonic states. Although the type-II nature of the heterostructure leads to a fully charge separated interlayer exciton on the ground states, our results show that moderate values of electric field produce more evenly distributed wave functions along the vdWH, namely, hybrid inter/intralayer exciton states, where both the interlayer exciton binding energy and, most notably, its oscillator strength are enhanced.

The anomalous $4\pi$-periodic ac Josephson effect, a hallmark of topological Josephson junctions, was experimentally observed in a quantum spin Hall insulator. This finding is unexpected due to time-reversal symmetry preventing the backscattering of the helical edge states and therefore suppressing the $4\pi$-periodic component of the Josephson current. Here, we analyze the two-particle inelastic scattering as a possible explanation for this experimental finding. We show that a sufficiently strong inelastic scattering restores the $4\pi$-periodic component of the current beyond the short Josephson junction regime. Its signature is an observable peak in the power spectrum of the junction at half the Josephson frequency. We propose to use the exponential dependence of the peak width on the applied bias and the magnitude of the dc current as means of verifying that the inelastic scattering is indeed the mechanism responsible for the $4\pi$-periodic signal.

Recent experiments on type-II Weyl semimetals such as ${\mathrm{WTe}}_2, {\mathrm{MoTe}}_2, {\mathrm{Mo}}_x{\mathrm{W}}_{1-x}{\mathrm{Te}}_2$, and ${\mathrm{WP}}_2$ reveal remarkable transport properties in the presence of a strong magnetic field, including an extremely large magnetoresistance and an unusual temperature dependence. Here, we investigate magnetotransport via the Kubo formula in a minimal model of a type-II Weyl semimetal taking into account the effect of a charge density wave (CDW) transition, which can arise even at weak coupling in the presence of a strong magnetic field because of the special Landau level dispersion of type-II Weyl systems. Consistent with experimental measurements we find an extremely large magnetoresistance with close to $B^2$ scaling at particle-hole compensation, while in the extreme quantum limit there is a transition to a qualitatively new scaling with approximately $B^{0.75}$. We also investigate the Shubnikov-de Haas effect and find that the amplitude of the resistivity quantum oscillations are greatly enhanced below the CDW transition temperature which is accompanied by an unusual nonmonotonous (non-Lifshitz-Kosevich) temperature dependence.

We predict dominating cooperative two-photon emission from two quantum dots coupled with a single-mode photonic crystal cavity. The cooperative two-photon emission occurs when excitons in two off-resonantly coupled quantum dots decay simultaneously. The interaction with a common cavity field leads to cavity-induced two-photon emission which is strongly inhibited by electron-phonon coupling. The interaction with a common phonon bath produces phonon-induced two-photon emission which increases on increasing temperature. For identical quantum dots, cavity-induced two-photon emission is negligible, but phonon-induced two-photon emission could be large.

In recent years, efficient interatomic potentials approaching the accuracy of density functional theory (DFT) calculations have been developed using rigorous atomic descriptors satisfying strict invariances, for example, for translation, rotation, permutation of homonuclear atoms, among others. In this paper, we generalize the spectral neighbor analysis potential (SNAP) model to bcc-fcc binary alloy systems. We demonstrate that machine-learned SNAP models can yield significant improvements even over the well-established high-performing embedded atom method (EAM) and modified EAM potentials for fcc Cu and Ni. We also report on the development of a SNAP model for the fcc Ni-bcc Mo binary system by machine learning a carefully constructed large computed data set of elemental and intermetallic compounds. We demonstrate that this binary Ni-Mo SNAP model can achieve excellent agreement with experiments in the prediction of a Ni-Mo phase diagram as well as near-DFT accuracy in the prediction of many key properties, such as elastic constants, formation energies, melting points, etc., across the entire binary composition range. In contrast, the existing Ni-Mo EAM has significant errors in the prediction of the phase diagram and completely fails in binary compounds. This paper provides a systematic model development process for multicomponent alloy systems, including an efficient procedure to optimize the hyperparameters in the model fitting, and paves the way for long-time large-scale simulations of such systems.

We present a systematic powder neutron diffraction study on Sr-based $M$-type hexaferrites ${\mathrm{SrSc}}_x{\mathrm{Fe}}_{12-x}{\mathrm{O}}_{19}$ using high-resolution and high-flux instruments. We have derived the magnetic configurations for the compounds with $x=1.4$, 1.6, and 1.8 at room temperature and at 2 K revealing a ferrimagnetic structure and a complex conical magnetic structure, respectively, where the latter is decomposed into the axial ferrimagnetic and the helicoidally modulated component in the hexagonal $a-b$ plane. Our temperature- and composition-dependent investigation reveals that doping nonmagnetic Sc into this system reinforces the frustration and pushes the transition between the two ordered phases toward higher temperatures. The continuous opening of the spin cone suggests a smooth transition from an easy-axis toward an easy-plane anisotropy with decreasing temperature, which can be reinforced by increasing Sc content.

In geometrically frustrated materials with low-dimensional and small spin moment, the quantum fluctuation can interfere with the complicated interplay of the spin, electron, lattice, and orbital interactions, and host exotic ground states such as the nematic spin state and chiral liquid phase. While the quantum phases of the one-dimensional chain and $S=\frac{1}{2}$ two-dimensional triangular-lattice antiferromagnet (TLAF) have been more thoroughly investigated by both theorists and experimentalists, the work on the $S=1$ TLAF has been limited. We induced the lattice distortion into the TLAFs $A_3\mathrm{NiN}{\mathrm{b}}_2{\mathrm{O}}_9$ ($A=\mathrm{Ba}$, Sr, and Ca) with $S\left(\mathrm{N}{\mathrm{i}}^{2+}\right)=1$, and applied thermodynamic, magnetic, and neutron scattering measurements. Although $A_3\mathrm{NiN}{\mathrm{b}}_2{\mathrm{O}}_9$ kept the noncollinear $120^{\circ }$ antiferromagnetic phase as the ground state, the $\mathrm{N}{\mathrm{i}}^{2+}$ lattice changed from an equilateral triangle ($A=\mathrm{Ba}$) into an isosceles triangle ($A=\mathrm{Sr}$ and Ca). The inelastic neutron scattering data were simulated by the linear spin-wave theory, and the competition between the single-ion anisotropy and the exchange anisotropy from the distorted lattice are discussed.

Density functional theory is a standard model for condensed-matter theory and computational material science. The accuracy of density functional theory is limited by the accuracy of the employed approximation to the exchange-correlation functional. Recently, the so-called strongly constrained appropriately normed (SCAN) [Sun, Ruzsinszky, and Perdew, Phys. Rev. Lett. 115, 036402 (2015)] functional has received a lot of attention due to promising results for covalent, metallic, ionic, as well as hydrogen- and van der Waals–bonded systems alike. In this work, we focus on assessing the performance of the SCAN functional for itinerant magnets by calculating basic structural and magnetic properties of the transition metals Fe, Co, and Ni. We find that although structural properties of bcc-Fe seem to be in good agreement with experiment, SCAN performs worse than standard local and semilocal functionals for fcc-Ni and hcp-Co. In all three cases, the magnetic moment is significantly overestimated by SCAN, and the $3d$ states are shifted to lower energies, as compared to experiments.