Antiferromagnetic insulators can become active spintronics components by controlling and detecting their dynamics via spin currents in adjacent metals. This cross talk occurs via spin transfer and spin pumping, phenomena that have been predicted to be as strong in antiferromagnets as in ferromagnets. Here, we demonstrate that a temperature gradient drives a significant heat flow from magnons in antiferromagnetic insulators to electrons in adjacent normal metals. The same coefficients as in the spin-transfer and spin-pumping processes also determine the thermal conductance. However, in contrast to ferromagnets, the heat is not transferred via a spin Seebeck effect which is absent in antiferromagnetic insulator-normal metal systems. Instead, the heat is proportional to a large staggered spin Seebeck effect.

YbBiPt is a heavy-fermion compound possessing significant short-range antiferromagnetic correlations below a temperature of $T^{\text{*}}=0.7\mathrm{K}$, fragile antiferromagnetic order below $T_{\mathrm{N}}=0.4\mathrm{K}$, a Kondo temperature of $T_{\mathrm{K}}\approx 1\mathrm{K}$, and crystalline-electric-field splitting on the order of $E/k_{\mathrm{B}}=1\text{–}10\mathrm{K}$. Whereas the compound has a face-centered-cubic lattice at ambient temperature, certain experimental data, particularly those from studies aimed at determining its crystalline-electric-field scheme, suggest that the lattice distorts at lower temperature. Here, we present results from high-resolution, high-energy x-ray diffraction experiments which show that, within our experimental resolution of $\approx 6\text{–}10\times {10}^{-5}\AA$, no structural phase transition occurs between $T=1.5$ and $50\mathrm{K}$. In combination with results from dilatometry measurements, we further show that the compound's thermal expansion has a minimum at $\approx 18\mathrm{K}$ and a region of negative thermal expansion for $9\lesssim T\lesssim 18\mathrm{K}$. Despite diffraction patterns taken at $1.6\mathrm{K}$ which indicate that the lattice is face-centered cubic and that the Yb resides on a crystallographic site with cubic point symmetry, we demonstrate that the linear thermal expansion may be modeled using crystalline-electric-field level schemes appropriate for ${\mathrm{Yb}}^{3+}$ residing on a site with either cubic or less than cubic point symmetry.

Discovering materials that display a linear magnetoelectric (ME) effect at room temperature is a challenge. Such materials could facilitate devices based on the electric field control of magnetism. Here we present simple, chemically intuitive design rules to identify a class of bulk magnetoelectric materials based on the “bicolor” layering of $Pbnm$ ferrite perovskites, e.g., ${\mathrm{LaFeO}}_3/{\mathrm{LnFeO}}_3$ superlattices, Ln = lanthanide cation. We use first-principles density functional theory calculations to confirm these ideas. We elucidate the origin of this effect and show it is a general consequence of the layering of any bicolor $Pbnm$ perovskite superlattice in which the number of constituent layers are odd (leading to a form of hybrid improper ferroelectricity). Our calculations suggest that the ME effect in these superlattices is larger than that observed in the prototypical magnetoelectric materials ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and ${\mathrm{BiFeO}}_3$. Furthermore, in these proposed materials, the strength of the linear ME coupling increases with the magnitude of the induced spontaneous polarization which is controlled by the La/Ln cation radius mismatch. We use a simple mean field model to show that the proposed materials order magnetically above room temperature.

We demonstrate how the technique of ultrafast resonant x-ray scattering can be applied to imaging dynamics of electronic wave packets in crystals. We study scattering patterns from crystals with electron dynamics in valence bands taking into account that inelastic and elastic scattering events induced by a broadband probe pulse cannot be separated through the spectroscopy of the scattered photon. As a result, scattering patterns are not determined by the structure factor at the time of measurement, but can encode the instantaneous electron current between scattering atoms. We provide examples of how the interatomic electron current in a periodic structure can be extracted from a single scattering pattern by considering valence electron-hole motion in ${\left(\mathrm{KBr}\right)}_{108}$ and ${\mathrm{Ge}}_{83}$ clusters.

We illustrate the magnetoelectric effect conditioned by the Katsura-Nagaosa-Balatsky (KNB) mechanism within the frames of exactly solvable spin-$1/2 XY$ chains. Due to three-spin interactions which are present in our consideration, the magnetization (polarization) is influenced by the electric (magnetic) field even in the absence of the magnetic (electric) field. We also discuss a magnetoelectrocaloric effect examining the entropy changes under the isothermal varying of the magnetic and/or electric field.

Inelastic neutron scattering experiments have been carried out on a powder sample of ${\mathrm{Cu}}_2{\mathrm{PO}}_4\mathrm{OH}$, which consists of diamond-shaped tetramer spin units with $S=1/2$. We have observed two nearly dispersionless magnetic excitations at $E_1\sim 12$ and $E_2\sim 20$ meV whose energy widths are broader than the instrumental resolution. The simplest square tetramer model with one dominant interaction, which predicts two sharp excitation peaks at $E_1$ and $E_2 (=2E_1)$, does not explain the experimental result. We found that two diagonal intratetramer interactions compete with the main interaction, and weak intertetramer interactions connect the tetramers. The main intratetramer interaction is found to split into two inequivalent ones due to a structural distortion below 160 K. ${\mathrm{Cu}}_2{\mathrm{PO}}_4\mathrm{OH}$ is considered to be a good material to study the $S=1/2$ Heisenberg tetramer system.

We performed ultrafast time-resolved near-infrared pump, resonant soft x-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic $\mathrm{TbMn}{\mathrm{O}}_3$ at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of $22.3\pm 1.1\mathrm{ps}$, which is much slower than the ∼1 ps melting times previously observed in other systems. To explain the data, we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wave vector $q$ of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wave vector on these time scales.

An in-depth study of the metal-organic magnet $\mathrm{Ni}{\left(\mathrm{TCNQ}\right)}_2$ was conducted where the deuterated form was synthesised both to attempt to alter the magnetic properties of the material and to be advantageous in techniques such as neutron scattering and muon spectroscopy. Deuteration saw a 3 K increase in $T_C$ with magnetization and heat capacity measurements demonstrating a spin wave contribution at low temperatures confirming the 3D nature of the ferromagnetic state shown by $\mathrm{Ni}{(\mathrm{TCNQ}-{\mathrm{D}}_4)}_2$. AC susceptibility results suggest there is a glassy component associated with the magnetically ordered state, though muon spectroscopy measurements did not support the presence of a spin glass state. Instead muon spectroscopy at zero magnetic field indicated the presence of two magnetic transitions, one at 20 K and another below 6 K; the latter is likely due to the system entering a quasistatic regime, similar to what one might expect of a superspin or cluster glass. Neutron diffraction measurements further supported this by revealing very weak magnetic Bragg peaks suggesting that the magnetism may have a short coherence length and be confined to small grains or clusters. The separation of the ferromagnetic and glassy magnetic components of the material's properties suggest that this system may show promise as a metal-organic magnet which is easily modified to change its magnetic properties, providing larger grain sizes can be synthesized.

Mixed valence spinels provide a fertile playground for the interplay between charge, spin, and orbital degrees of freedom in strongly correlated electrons on a geometrically frustrated lattice. Among them, ${\mathrm{AlV}}_2{\mathrm{O}}_4$ and ${\mathrm{LiV}}_2{\mathrm{O}}_4$ exhibit contrasting and puzzling behavior: self-organization of seven-site clusters and heavy fermion behavior. We theoretically perform a comparative study of charge-spin-orbital fluctuations in these two compounds, on the basis of the multiband Hubbard models constructed by using the maximally localized Wannier functions obtained from the ab initio band calculations. Performing the eigenmode analysis of the generalized susceptibility, we find that, in ${\mathrm{AlV}}_2{\mathrm{O}}_4$, the relevant fluctuation appears in the charge sector in $\sigma$-bonding type orbitals. In contrast, in ${\mathrm{LiV}}_2{\mathrm{O}}_4$, optical-type spin fluctuations in the $a_{1\mathrm{g}}$ orbital are enhanced at an incommensurate wave number at low temperature. Implications from the comparative study are discussed for the contrasting behavior, including the metal-insulator transition under pressure in ${\mathrm{LiV}}_2{\mathrm{O}}_4$.

We investigate the Kondo effect in a two-dimensional electron system with a nontrivial quadratic energy band crossing point. We show that the Kondo effect can induce a hybrid topological insulator phase, which is a coexistence state of the quantum anomalous Hall effect and the time-reversal-symmetry-broken quantum spin Hall effect. This hybrid topological insulator exhibits not only a quantized charge Hall current but also a net spin current, which are localized at the edge boundaries. This peculiar topological state arises due to the interplay of two marginally relevant operators, i.e., the Kondo coupling between the electrons and the local magnetic moment and the electron-electron interaction in the two-dimensional system.

We examine the interplay of interaction and disorder for a Heisenberg spin-1/2 ladder system with random fields. We identify many-body localized states based on the entanglement entropy scaling, where delocalized and localized states have volume and area laws, respectively. We first establish the dynamic phase transition at a critical random field strength $h_c\sim 8.5\pm 0.5$, where all energy eigenstates are localized beyond that value. Interestingly, the entanglement entropy and fluctuations of the bipartite magnetization show distinct probability distributions which characterize different phases. Furthermore, we show that for weaker $h$, energy eigenstates with higher-energy density are delocalized while states at lower-energy density are localized, which defines a mobility edge separating these two phases. With increasing disorder strength, the mobility edge moves towards higher-energy density, which drives the system to the phase of the full many-body localization.

We consider an efficient scheme to simulate fermionic Hubbard models with nonlocal density-density interactions in two dimensions, based on bond-centered auxiliary-field quantum Monte Carlo. The simulations are shown to be sign-problem-free within a finite, restricted parameter range. Using this approach, we first study the Hubbard model on the half-filled square lattice bilayer, including an interlayer repulsion term in addition to the local repulsion, and present the ground-state phase diagram within the accessible parameter region. Starting from the antiferromagnetically ordered state in the absence of interlayer repulsion, the interlayer interactions are found to destabilize the antiferromagnetic order, leading to a band insulator state. Moreover, for sufficiently strong interlayer tunneling, we also observe the emergence of a direct dimer product state of mixed D-Mott and S-Mott character along the equal coupling line. We discuss the stability range of this state within strong-coupling perturbation theory. Furthermore, we consider the Hubbard model on the honeycomb lattice with next-nearest-neighbor interactions. Such an interaction is found to enhance both charge density and spin-current correlations within the semimetallic region. However, inside the accessible parameter region, they do not stabilize long-ranged charge density wave order nor a quantum spin Hall state, and the only insulating state that we observe exhibits long-range antiferromagnetism.

We investigate theoretically the Andreev transport through a quantum dot strongly coupled with a normal metal/ferromagnet and a superconductor (N/F-QD-S), in which the interplay between the Kondo resonance and the Andreev bound states (ABSs) has not been clearly clarified yet. Here we show that the interference between the Kondo resonance and the ABSs modifies seriously the line shape of the Kondo resonance, which manifests as a Fano resonance. The ferromagnetic lead with spin polarization induces an effective field, which leads to splitting of both the Kondo resonance and the ABSs. The spin polarization together with the magnetic field applied provides an alternative way to tune the line shape of the Kondo resonances, which is dependent of the relative positions of the Kondo resonance and the ABSs. These results indicate that the interplay between the Kondo resonance and the ABSs can significantly affect the Andreev transport, which could be tested by experiments.

In two-dimensional hole systems confined to wide GaAs quantum wells, where the heavy- and light-hole states are close in energy, we observe a very unusual crossing of the lowest two Landau levels as the sample is tilted in a magnetic field. At a magic tilt angle $\theta \simeq {34}^{\circ }$, which surprisingly is independent of the well width or hole density, in a large filling factor range near $\nu =1$, the lowest two levels are nearly degenerate as evidenced by the presence of two-component quantum Hall states. Remarkably, a quantum Hall state is seen at $\nu =1$, consistent with a correlated ${\mathrm{\Psi }}_{111}$ state.

The electron and nuclear spins of donor impurities in enriched ${}^{28}\mathrm{Si}$ have great potential as long-lived qubits for a silicon-based quantum information technology. The ability to resolve the hyperfine-split neutral donor ground-state levels in the near-infrared donor bound exciton transitions of the ubiquitous phosphorus impurity in highly isotopically enriched ${}^{28}\mathrm{Si}$ has led to new methods of hyperpolarizing and measuring the donor electron and nuclear spins. This has resulted in optically assisted magnetic resonance methods that have permitted the measurement of remarkably long nuclear coherence times for both the neutral and ionized phosphorus donor in very lightly doped and highly enriched ${}^{28}\mathrm{Si}$. Other shallow donors such as arsenic, antimony, and bismuth offer the potential of larger hyperfine couplings and nuclear spins as compared to phosphorus. Here, we investigate whether donor bound exciton transitions can be used to initialize and read out the nuclear spins of arsenic and antimony in ${}^{28}\mathrm{Si}$. The projective readout of the electron and nuclear spins is demonstrated for both ${}^{75}\mathrm{As}$ and ${}^{121}\mathrm{Sb}$, and these optical transitions can strongly hyperpolarize the nuclear spin of ${}^{75}\mathrm{As}$. Only a small nuclear hyperpolarization is achieved for ${}^{121}\mathrm{Sb}$, likely due to the relative weakness of the no-phonon transition of the Sb donor bound exciton. Optically assisted EPR and NMR is demonstrated for ${}^{75}\mathrm{As}$, including Hahn echo coherence time measurements of the six NMR transitions.

Probing detailed spectral dependence of phonon transport properties in bulk materials is critical to improve the function and performance of structures and devices in a diverse spectrum of technologies. Currently, such information can only be provided by the phonon spectral energy density (SED) or equivalently, time domain normal mode analysis (TDNMA) methods in the framework of equilibrium molecular dynamics simulations (EMD), but has not been realized in nonequilibrium molecular dynamics simulations (NEMD) so far. In this paper we generate a scheme directly based on NEMD and lattice dynamics theory, called the time domain direct decomposition method (TDDDM), to predict the phonon mode specific thermal conductivity. Two benchmark cases of Lennard-Jones (LJ) argon and Stillinger-Weber (SW) Si are studied by TDDDM to characterize contributions of individual phonon modes to overall thermal conductivity and the results are compared with that predicted using SED and TDNMA. Similar trends are found for both cases, which indicate that our TDDDM approach captures the major phonon properties in NEMD run. The biggest advantage of TDDDM is that it can be used to investigate the size effect of individual phonon modes in NEMD simulations, which cannot be tackled by SED and TDNMA in EMD simulations currently. We found that the phonon modes with mean free path larger than the system size are truncated in NEMD and contribute little to the overall thermal conductivity. The TDDDM provides direct physical origin for the well-known strong size effects in thermal conductivity prediction by NEMD. Moreover, the well-known common sense of the zero thermal conductivity contribution from the Γ point is rigorously proved by TDDDM. Since TDDDM inherently possesses the nature of both NEMD simulations and lattice dynamics, we anticipate that TDDDM is particularly useful for offering a deep understanding of phonon behaviors in nanostructures or under strong confinement, especially when the structure size is significantly smaller than the characteristic mean free path of the dominant phonons.