Space groups and crystal structures of $A$-site ordered ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ perovskite have been investigated by a combination of selected-area electron diffraction (SAED), convergent-beam electron diffraction (CBED), and first-principles methods at high-temperature (HT $\approx$ 450 K), room-temperature (RT $\approx$ 293 K), and low-temperature (LT $\approx$ 95 K) phases. By symmetry analysis of the CBED patterns, space groups of ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ at HT, RT, and LT phases have been determined to be $P4/mmm$ (No. 123), $C2mm$ (No. 38), and $P{2}_{1}am$ (No. 26), respectively. The space groups and the optimized structures using first-principle calculations at the HT and RT phases allow a single crystallographic site for Mn atoms, indicating no charge ordering state. At the LT phase, the observed SAED and CBED patterns revealed the existence of fourfold periodicity along the $c$ axis compared with the pseudocubic unit cell. The optimized structure of the LT phase suggested four inequivalent Mn sites classified into two crystallographically different ${\mathrm{MnO}}_2$ layers. Moreover, the structure showed that the four distinct ${\mathrm{MnO}}_6$ octahedra are similarly compressed in the apical direction.

We report a ${}^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) local spectroscopic study of the NMR lineshape and Knight shift of a six-component ${\mathrm{Al}}_{0.5}\mathrm{TiZrPdCuNi}$ metallic alloy that can be prepared either as a crystalline high-entropy alloy (HEA) or as an amorphous metallic glass (MG) at the same chemical composition. For both structural modifications of the material (HEA and MG), we have determined the distribution of electric-field-gradient (EFG) tensors and the local electronic density of states (DOS) $g\left({\varepsilon }_{\mathrm{F}}\right)$ at the Fermi level at the position of ${}^{27}\mathrm{Al}$ nuclei. A theoretical $I=\frac{5}{2}$ quadrupole-perturbed NMR spectrum, pertinent to both cubic HEAs and amorphous MGs, has been derived using the Gaussian isotropic model of the EFG tensor distribution, and excellent fits of the experimental spectra were obtained. The EFG distribution function of the MG state is about twice broader than that of the HEA state, reflecting the existence of a (distorted) crystal lattice in the latter and its absence in the former. The $T^2$ dependence of the Knight shift indicates that the DOS is changing rapidly with energy within the Fermi level region for both structural modifications. The local DOS at the ${}^{27}\mathrm{Al}$ sites of the HEA sample is $\sim 10\%$ larger than that of the MG state, indicating comparable degrees of disorder.

Inelastic neutron scattering measurements were performed with a time-of-flight chopper spectrometer to observe phonons in all parts of the Brillouin zone of a single crystal of cuprite ${\mathrm{Cu}}_2\mathrm{O}$. We reduced the experimental data to phonon dispersions in the high-symmetry directions, and changes between 10 and 300 K are reported. In this paper, we show ab initio quasiharmonic (QH) and anharmonic (AH) calculations of phonon dispersions. We performed all AH calculations with a temperature-dependent effective potential method. Both QH and AH calculations account for the small negative thermal expansion of cuprite at low temperatures. However, the measured temperature-dependent phonon behavior was predicted more accurately with the AH calculations than the QH ones. Nevertheless, at 300 K, the cubic AH used in this paper did not entirely account for the experimental phonon dispersions in cuprite.

Exploring unique topological states in condensed-matter systems has attracted great interest especially for the topological phonons recently. Based on the unbiased structure prediction approach combined with first-principles calculations, the long-sought crystal structure of ${\mathrm{Th}}_2{\mathrm{BC}}_2$ is determined. Most importantly, we show by the symmetry analysis and the phonon tight-binding Hamiltonian that ${\mathrm{Th}}_2{\mathrm{BC}}_2$ hosts nodal surface phonons on the $q_z=\pm \pi$ plane, coexisting with nodal line phonons on the $q_y=0$ and $q_y=\pm \pi$ planes, consequently, forming cagelike phonons. The nodal surface phonons are protected by the screw axis ${\tilde{C}}_{2z}$, and the nodal line phonons are enforced by inversion and time-reversal symmetries, demonstrated by the codimension argument and the effective model analysis. In addition, we also investigate the phonon surface states and the isofrequency arc on the (100) surface, which benefit the confirmation of the nodal cage phonons in experiments. Our paper not only determines the long-sought crystal structure of ${\mathrm{Th}}_2{\mathrm{BC}}_2$, but also provides an ideal candidate to realize the exotic topological phonon excitations.

We calculate the intermediate scattering function of warm dense aluminum by using density functional theory molecular dynamics simulations. From this data set, we derive the static and dynamic ion-ion structure factors. By applying a generalized collective modes model, we can fit the excitation spectra of the ion system and thereby extract the dispersion for the ion acoustic modes, as well as the decay coefficients for the diffusive and collective modes. The results are discussed and compared with experimental data if available. We show that computational limitations prevent sufficient access to the hydrodynamic limit and demonstrate that this can be circumvented using high-dimensional neural network potentials. We extract the ionic thermal conductivity of aluminum in the hydrodynamic limit and compare to values computed using a Green-Kubo relation. We highlight the importance of partitioning the heat capacity into electronic and ionic contributions and only using the ionic contribution to compute the thermal conductivity of the ions in the hydrodynamic limit.

The low-temperature photoinduced effects of the spin-crossover $\left[\right\{\mathrm{Fe}{\left(2\text{−}\mathrm{pytrz}\right)}_2[\mathrm{Pd}{\left(\mathrm{CN}\right)}_4]\}].3{\mathrm{H}}_2\mathrm{O}$ single crystal have been investigated by means of a cryogenic optical microscopy technique down to 10 K from which the imaging and quantitative analysis of the spatiotemporal transformation are derived. The magnetic investigations revealed that this compound exhibits an incomplete spin transition between a full high-spin (HS) state at high temperature and an intermediate HS and low-spin (LS) state, where HS and LS species coexist, as a result of the existence of an elastic frustration at the molecular scale, most likely caused by the rigidity of the interconnected $[\mathrm{Pd}{\left(\mathrm{CN}\right)}_4$] [Fe(II)/Pd(II)] two-dimensional network. At low temperature (10 K), thanks to reverse light-induced excited spin-state trapping effect, we could switch the system from the intermediate HS-LS state to the fully photoinduced LS state by irradiating the sample in near-infrared region, revealed by photomagnetic and optical microscopy studies. Optical microscopy images showed monotonous and homogeneous transformation of the crystal color along this process, corresponding to a gradual change of the spin state under light. In contrast, the thermal relaxation in the dark of this photoinduced LS-LS state shows a transition to the intermediate HS-LS state at $\sim 90$ K with domain formation, which is characteristic of a first-order transition at equilibrium. Interestingly, the same behavior is also obtained during the heating process of the reverse light-induced thermal hysteresis cycle with a heating branch almost unchanged, confirming that light does not act on this transition. It is then concluded that the transition from the full LS to the intermediate HS-LS transition is of first order and therefore the LS state reached by light is a hidden stable state of the system until $\sim 90$ K. These experimental results are modeled using an adapted version of the electroelastic model including photoexcitation effects and solved by Monte Carlo method. Both thermal equilibrium and light-induced thermal hysteresis are reproduced, in fair qualitative agreement with the experimental data of photomagnetism and optical microscopy.

We report our theoretical results on the emergence of a rectangular- and square-shaped magnetic skyrmion crystals on a centrosymmetric square lattice with magnetic anisotropy. By performing the simulated annealing for a frustrated spin model with an easy-axis single-ion anisotropy on a square lattice, we find that a competition of the interactions in momentum space gives rise to the formation of two skyrmion crystals, where the skyrmion cores are aligned in a rectangle- or square-lattice way. Especially, the interactions at the higher-harmonic wave vectors play an important role in stabilizing the square/rectangular skyrmion crystals. Our results provide a reference to explore further skyrmion-hosting materials in centrosymmetric tetragonal systems.

In order to investigate the effects leading to the strong magnetoelectric coupling in the type II multiferroic $\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_3$ we have studied the thermal properties and temperature dependence of the lattice vibrations of ${\mathrm{TbMn}}^{16}{\mathrm{O}}_3$ and its isotopically substituted counterpart ${\mathrm{TbMn}}^{18}{\mathrm{O}}_3$. Heat capacity measurements on powder samples revealed no significant change in the ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ magnetic phase transition temperatures, as well as the multiferroic ordering temperature upon isotope substitution, indicating that a change in the dynamical modulation of the $\mathrm{Mn}{\mathrm{O}}_6$ octahedral distortions and rotations altering the Mn-O-Mn bond angles has no influence on the magnetic properties of $\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_3$. Raman light scattering experiments have been performed on isotopically substituted single crystals to determine the temperature induced changes in phonon energies and linewidths at the sinusoidal and multiferroic phase transitions. A detailed modeling indicates that the spin-phonon coupling can be accounted for the pronounced anomalies in the temperature dependence of the phonon behaviors at the transition to the sinusoidal spin phase at $T_{\mathrm{N}}^{\mathrm{Mn}}=41$ K and to the multiferroic spin-spiral phase at $T_{\mathrm{FE}}=28$ K. No further effects such as the appearance of the electric polarization or the electromagnon were required to explain the data, especially below the multiferroic phase transition.

In the absence of dissipation a nonrotating magnetic nanoparticle can be stably levitated in a static magnetic field as a consequence of the spin origin of its magnetization. Here we study the effects of dissipation on the stability of the system, considering the interaction with the background gas and the intrinsic Gilbert damping of magnetization dynamics. At large applied magnetic fields we identify magnetization switching induced by Gilbert damping as the key limiting factor for stable levitation. At low applied magnetic fields and for small particle dimensions, magnetization switching is prevented due to the strong coupling of rotation and magnetization dynamics, and the stability is mainly limited by the gas-induced dissipation. In the latter case, high vacuum should be sufficient to extend stable levitation over experimentally relevant timescales. Our results demonstrate the possibility to experimentally observe the phenomenon of quantum spin stabilized magnetic levitation.

Magnetic, specific heat, and electrical transport measurements of ${\mathrm{Gd}}_4{\mathrm{ScGe}}_4$ single crystals reveal sharp, discontinuous, nearly anhysteretic first-order magnetostructural transformation at $T_{\mathrm{C}}=63\mathrm{K}$. The electrical resistivity exhibits two distinct regions where it increases with decreasing temperature: between $T_{\mathrm{C}}$ and 120 K, as well as below 3 K; electronic transport remains conventionally metallic at all other measured temperatures, up to 325 K. The dispersion of charge carriers due to electron-paramagnon scattering is the likely reason for the observed anomalous transport above $T_{\mathrm{C}}$. Additionally, the existence of intermediate lattice states near the transition recognized by the spike in the interslab Ge-Ge distances is expected to reduce the mean free path of the electrons contributing to the unusual behavior of the electrical resistivity between $T_{\mathrm{C}}$ and 120 K. Beyond conventional electronic and lattice terms, the third component of likely magnetic origin contributes to the low-temperature heat capacity; the presence of spin waves may be responsible for the increased electron-magnon scattering below 3 K. Minor magnetocrystalline anisotropy is observed with the $b$ axis as the easy magnetization axis in ${\mathrm{Gd}}_4{\mathrm{ScGe}}_4$. A negative deviation from linearity in the temperature dependence of the inverse magnetic susceptibility is detected below 150 K.

We extend to the longitudinal component of the magnetization the spintronics idea that a magnet near equilibrium can be described by two magnetic variables. One is the usual magnetization $\vec{M}$. The other is the nonequilibrium quantity $\vec{m}$, called the spin accumulation, by which the nonequilibrium spin current can be transported. $\vec{M}$ represents a correlated distribution of a very large number of degrees of freedom, as expressed in some equilibrium distribution function for the excitations; we therefore forbid $\vec{M}$ to diffuse, but we permit $\vec{M}$ to decay. On the other hand, we permit $\vec{m}$, due to spin excitations, to both diffuse and decay. For this physical picture, diffusion from a given region occurs by decay of $\vec{M}$ to $\vec{m}$, then by diffusion of $\vec{m}$, and finally by decay of $\vec{m}$ to $\vec{M}$ in another region. This somewhat slows down the diffusion process. Restricting ourselves to the longitudinal variables $M$ and $m$ with equilibrium properties $M_{eq}=M_0+{\chi }_{M\parallel }H$ and $m_{eq}=0$, we argue that the effective energy density must include a new, thermodynamically required exchange constant ${\lambda }_M=-1/{\chi }_{M\parallel }$. We then develop the appropriate macroscopic equations by applying Onsager's irreversible thermodynamics and use the resulting equations to study the space and time response. At fixed real frequency $\omega$ there is, as usual, a single pair of complex wave vectors $\pm k$ but with an unusual dependence on $\omega$. At fixed real wave vector, there are two decay constants, as opposed to one in the usual case. Extending the idea that nonequilibrium diffusion in other ordered systems involves a nonequilibrium quantity, this work suggests that, in a superconductor, the order parameter $\mathrm{\Delta }$ can decay but not diffuse, but a nonequilibrium gap-like $\delta$, due to pair excitations, can both decay and diffuse.

We combine a muon spin relaxation ($\mu \mathrm{SR}$) technique with thermodynamic measurements to explore the spin dynamics of one-dimensional (1D) $S=\frac{1}{2}$ antiferromagnetic double chain ${\mathrm{KNaCuP}}_2{\mathrm{O}}_7$. Static magnetic susceptibility and specific heat are well described by a uniform 1D spin chain model with the intrachain interaction $J/k_B\approx 55\mathrm{K}$ and small interchain interactions. Spin excitations probed by zero-field $\mu \mathrm{SR}$ evince that high-temperature diffusive spin transport turns into ballistic behavior with decreasing temperature below 30 K. In addition, we observe that longitudinal-field $\mu \mathrm{SR}$ varies hardly with an external magnetic field. The field-independent dynamical spin susceptibility disagrees with diffusive or ballistic behaviors.

${\text{UTe}}_2$ is a leading candidate for chiral $p$-wave superconductivity and for hosting exotic Majorana fermion quasiparticles. Motivated by recent STM experiments in this system, we study particle-hole symmetry breaking in chiral $p$-wave superconductors. We compute the local density of states from Majorana fermion surface states in the presence of Rashba surface spin-orbit coupling, which is expected to be sizable in heavy-fermion materials like ${\mathrm{UTe}}_2$. We show that time-reversal and surface reflection symmetry breaking lead to a natural pairing tendency toward a triplet pair density wave state, which naturally can account for broken particle-hole symmetry. The relevance of our results to the recent STM measurements requires further investigation.

Convergent-beam electron diffraction (CBED), recently demonstrated on two-dimensional (2D) materials, offers a number of interesting applications such as imaging atomic in- and out-of plane shifts, interlayer distances, and individual adsorbates. In this study, we show how CBED allows for atomic-precision imaging of individual defects in 2D materials using one single-shot intensity measurement. In combination with structural calculations using density-functional theory, we present simulated CBED patterns for various defects in graphene, each of which exhibits a unique fingerprint distribution. We also show how atomic positions, including the individual atomic defects in graphene, can be reconstructed by iterative phase retrieval from a single CBED pattern.

Density functional theory (DFT) is widely used to study defects in monolayer graphene with a view to applications ranging from water filtration to electronics to investigations of radiation damage in graphite moderators. To assess the accuracy of DFT in such applications, we report diffusion quantum Monte Carlo (DMC) calculations of the formation energies of some common and important point defects in monolayer graphene: monovacancies, Stone-Wales defects, and silicon substitutions. We find that standard DFT methods underestimate monovacancy formation energies by around 1 eV. The disagreement between DFT and DMC is somewhat smaller for Stone-Wales defects and silicon substitutions. We examine vibrational contributions to the free energies of formation for these defects, finding that vibrational effects are non-negligible. Finally, we compare the DMC atomization energies of monolayer graphene, monolayer silicene, and bulk silicon, finding that bulk silicon is significantly more stable than monolayer silicene by 0.7522(5) eV per atom.

Recent progress in the realization of high-quality optical resonators and waveguides along with the possibility to incorporate rare-earth ions, make lithium niobate a promising material to build integrated platforms for quantum information processing. ${}^{171}\mathrm{Yb}^{3+}$ is a particularly attractive system because it can show long coherence lifetimes at zero magnetic field thanks to transitions insensitive to magnetic field fluctuations. In this paper, we investigate the optical and spin properties of ${}^{171}\mathrm{Yb}^{3+}$ ions in ${\mathrm{LiNbO}}_3$ bulk crystals as well as implanted waveguides. Using hole-burning spectroscopy and optically detected magnetic resonance, we studied ground and excited state hyperfine structures and probed optical and spin spectral holes. Importantly, the hole linewidths suggest that part of the ions in the waveguides are in a similar environment as in the bulk sample. We furthermore characterized spin population relaxation and coherence lifetimes of ${}^{171}\mathrm{Yb}^{3+}$ ions in the bulk crystal at temperatures between 50 mK and 9 K. At low temperatures, $T_2$ up to 9.5 $\mu \mathrm{s}$ (34 kHz homogeneous linewidth) and spin relaxation rates as long as $\approx 100$ ms were measured. Our results show that ${}^{171}\mathrm{Yb}^{3+}$:$\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ is a system that exhibits narrow optical homogeneous linewidths over a 50 GHz bandwidth together with an electron spin degree of freedom. This is of interest for a variety of applications in integrated quantum photonics such as quantum memories or quantum processors.

Simultaneous conservation of charge (spin projection to the $z$ axis) and dipole moment can partially suppress ergodicity (thermalization) by means of shattering the phase space. This shattering results in many-body localization of some states even in the absence of disordering, while other states remain delocalized. Here we show for the minimalist one-dimensional spin-$\frac{1}{2}$ model how to distinguish localized and delocalized states, based on any representative product state with given projections of spins to the $z$ axis, separating states into four groups characterized by distinguishable behaviors. These include two groups of delocalized states with translationally invariant Krylov subspaces with integrable (group I) or ergodic (group II) dynamics, and the other two groups with confined spin transport having either all mobile spins (group III) or some immobile (frozen) spins (group IV). The states of the first two groups are delocalized, while the states of the last two groups are mostly localized. The theory is used to interpret recent experiments [Q. Guo et al., Phys. Rev. Lett. 127, 240502 (2021); W. Morong et al., Nature (London) 599, 393 (2021)], and suggests their extension necessary to observe both localized and delocalized behaviors in a dipole moment conserving regime.

We study theoretical transitions between the localized and chaotic many-body regimes in one-dimensional quantum lattice systems with long-range couplings between particles and linear external potential. In terms of established criteria characterizing localization, we construct effective phase diagrams for several types of lattice systems with variable amplitude of the external linear tilt and interaction strength. By means of exact diagonalization and time-dependent variational principle numerical approaches we analyze system dynamics after quenches. Our results reveal that the Stark localization without any artificial source of disorder remains stable upon inclusion of long-range interactions.

We derive the microscopic spin Hamiltonian for rhombohedral ${\mathrm{CrI}}_3$ using extensive first-principles density functional theory calculations that incorporate spin-orbit coupling and Hubbard $U$. Our calculations indicate a dominant nearest-neighbor ferromagnetic Heisenberg exchange with weaker further-neighbor Heisenberg terms. In addition, we find a Dzyaloshinskii-Moriya interaction that primarily drives a topological gap in the spin-wave spectrum at the Dirac point, and we uncover a non-negligible antiferromagnetic Kitaev coupling between the $S=3/2$ Cr moments. The out-of-plane magnetic moment is stabilized by weak symmetric bond-dependent terms and a local single-ion anisotropy. Using linear spin-wave theory, we find that our exchange parameters are in reasonably good agreement with inelastic neutron scattering (INS) experiments. Employing classical Monte Carlo simulations, we study the magnetic phase transition temperature $T_c$ and its evolution with an applied in-plane magnetic field. We further demonstrate how future high-resolution INS experiments on the magnon dispersion of single crystals in an in-plane magnetic field may be used to quantitatively extract the strength of the antiferromagnetic Kitaev exchange coupling.

The honeycomb antiferromagnet ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, in which small in-plane magnetic fields ($H1\approx 0.26$ T and $H2\approx 0.52$ T at $T=1.8$ $\mathrm{K}<T_N\approx 5.4$ K) induce two magnetic phase transitions, has attracted attention as a possible candidate material for the realization of Kitaev physics based on the $3d$ element ${\mathrm{Co}}^{2+}$. Here, we report on the change in the transition temperature $T_N$ and the critical fields $H1$ and $H2$ of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$ with hydrostatic pressure up to $\sim 20$ kbar, as determined from magnetization and specific heat measurements. Within this pressure range, a marginal increase in the magnetic ordering temperature is observed. At the same time, the critical fields are changed significantly (up to $\sim 25$–35%). Specifically, we find that $H1$ is increased with hydrostatic pressure, i.e., the antiferromagnetic state is stabilized, whereas $H2$, which was previously associated with a transition into a proposed Kitaev spin-liquid state, decreases with increasing pressure. We discuss to what extent these results are compatible with suggested models with sizable third-nearest-neighbor exchange. Overall, the results put constraints on the magnetic models that are used to describe the low-temperature magnetic properties of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$.