We present the results of neutron scattering experiments to study the crystal and magnetic structures of the Mott-insulating transition metal oxyselenides $\mathrm{P}{\mathrm{r}}_2{\mathrm{O}}_2{M}_2{\mathrm{OSe}}_2(M=\mathrm{Mn},\mathrm{Fe})$. The structural role of the non-Kramers $\mathrm{P}{\mathrm{r}}^{3+}$ ion was investigated, and analysis of $\mathrm{P}{\mathrm{r}}^{3+}$ crystal field excitations was performed. Long-range order of $\mathrm{P}{\mathrm{r}}^{3+}$ moments in $\mathrm{P}{\mathrm{r}}_2{\mathrm{O}}_2\mathrm{F}{\mathrm{e}}_2{\mathrm{OSe}}_2$ can be induced by an applied magnetic field.

The crystal and spin structure of ${\mathrm{Li}}_2{\mathrm{Cu}}_5{\mathrm{Si}}_4{\mathrm{O}}_{14}$ [chemical formula ${\mathrm{Li}}_2{\mathrm{Cu}}_5{\left({\mathrm{Si}}_2{\mathrm{O}}_7\right)}_2$] can be dissected into a copper spin chain system of alternating ${\mathrm{Cu}}_2{\mathrm{O}}_2$ dimers and ${\mathrm{Cu}}_3{\mathrm{O}}_3$ trimers linked with corner-shared oxygen and ${\mathrm{Si}}_2{\mathrm{O}}_7$ group in three dimensions (3D). The magnetic susceptibility ($\chi$) and specific heat ($C_P$) measurement results suggest the existence of a magnetic ground state of 3D antiferromagnetic (AF) ordering with $T_N\sim 22\mathrm{K}$. Density functional theory (DFT) calculations have been applied to extract the exchange coupling constants for the ground-state spin structure. The calculated dominant intratrimer AF coupling ($J_1\sim -9\mathrm{meV}$ ) and the intradimer ferromagnetic (FM) coupling ($J_3\sim 1.8\mathrm{meV}$ ) supports that the copper spin-1/2 system evolves from a paramagnetic spin chain composed of alternating spin dimer and trimer to the 3D AF ordered ground state on cooling, and a weak frustration is proposed along the chain direction below $T_N$.

Our polycrystalline sample study on the Yb-based chalcogenide spinels $A{\mathrm{Yb}}_2{X}_4$ ($A=$ Cd, Mg; $X=$ S, Se) has revealed frustrated magnetism due to the antiferromagnetically coupled Heisenberg spin on a pyrochlore lattice. The crystal electric field analysis indicates the Yb ground state has nearly Heisenberg spins with a strong quantum character of the ground doublet. All the materials exhibit an antiferromagnetic order at 1.4–1.8 K, much lower temperatures than the antiferromagnetic exchange coupling scale of $\sim 10\mathrm{K}$. The magnetic specific heat $C_{\mathrm{M}}$ shows a $T^3$ dependence, indicating the gapless feature in the Yb-based chalcogenide spinels. The magnetic entropy change much smaller than $R$ ln 2 below the Néel temperature and the small local magnetic field estimated from the $\mu \mathrm{SR}$ measurements strongly suggest the significantly reduced size of the ordered moment in comparison with the bare moment size 1.33 ${\mu }_{\mathrm{B}}/\mathrm{Yb}$, supporting strong fluctuations in the commensurate and incommensurate ordered states in ${\mathrm{CdYb}}_2{\mathrm{S}}_4$ and ${\mathrm{MgYb}}_2{\mathrm{S}}_4$, respectively.

High-resolution electron energy loss spectroscopy (HR-EELS) is utilized to probe the surface spin canting in nanoparticles of two technologically important magnetic materials, i.e. $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ and $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ (CFO). A soft experimental technique has been developed that is capable of extracting EELS spectra with single atomic plane resolution recorded in a single frame. The technique yields information at different depths of the nanoparticle from the surface to the core regions with high signal-to-noise ratio and without beam damage. This enables comparing the fine structures between the surface and core regions of the nanoparticles. The results confirm earlier observations of uniformly oriented spin canting structure for CFO and provide additional information regarding atom site-selective spin canting information. In the case of $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$, preferred canting orientation forming core and shell structure is deduced. Unlike earlier reports based on polarized spin-flip neutron scattering measurement, it is possible from the experimental spectra combined with the first principle-based calculations considering noncollinear magnetism to narrow down the canting angles for $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ ($T_d,O_h$ tilts 40°, 40°) and CFO ($T_d,O_h$ tilts 17°, 17°). In addition, the role of Dzyaloshinskii-Moriya interaction in stabilizing the spin canting at the nanoparticle surface is discussed. The results demonstrate that HR-EELS can be a powerful technique to probe the magnetic structure in nanodimensional systems and has advantages over neutron-based techniques in terms of superior spatial resolution, site-specific information, and ease of sample preparation.

The compounds $({\mathrm{Pr}}_{1-y}{\mathrm{Sm}}_y{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ belong to a class of Pr-based cobaltates presenting a unique case of simultaneous valence (charge transfer between Pr and Co ions) and spin-state transition (of the ${\mathrm{Co}}^{3+}$ ions), hereafter referred to as VSST. The present study sheds light on the debated issues of the ${\mathrm{Co}}^{4+}$ and ${\mathrm{Co}}^{3+}$ spin states, by combining x-ray absorption spectroscopy (XAS) at Co and Pr edges and x-ray magnetic circular dichroism (XMCD) at Co $L_{2,3}$ edges. XAS experiments at both $L_3$ and $M_{4,5}$ Pr edges attest to the appearance of ${\mathrm{Pr}}^{4+}$ below the VSST at ${\mathrm{T}}^{*}=106\mathrm{K}$, and allow a precise characterization of the evolution of the ${\mathrm{Co}}^{4+}$ content as a function of the temperature. XMCD at the Co $L_{2,3}$ edges at 5 K, and in magnetic field up to 9 T, directly tackles the issue of the ${\mathrm{Co}}^{4+}$ spin state. It is found that the ${\mathrm{Co}}^{4+}$ ions are most likely in a low spin state, and experience ferromagnetic interactions at $\mathrm{T}\ll {\mathrm{T}}^{*}$. On the basis of temperature dependent XMCD at 9 T, the fingerprint of the VSST on the Co moments is isolated, and found to be consistent with bulk magnetization data when accounting for the rare-earth contributions derived from reference samples. These temperature dependent XMCD data are used to characterize the evolutions of the various valence/spin state of the Co species involved in the VSST. It appears that the ${\mathrm{Co}}^{3+}$ moments above ${\mathrm{T}}^{*}$ are not consistent with a pure intermediate spin state, whereas they can be well reproduced by considering a low/high spin mixture. Finally, these XMCD results are compared to those derived from fitting of the XAS spectra recorded in zero field at various temperatures.

We report the observation of superconductivity at a temperature near 3.5 K for the previously unreported compound $\mathrm{TaI}{\mathrm{r}}_2\mathrm{G}{\mathrm{e}}_2$. In addition to being a superconductor, this material displays a new crystal structure type that contains endohedral clusters, as determined by single-crystal x-ray diffraction structure refinement; the structure is more complex than those of the commonly observed tetragonal 122 intermetallic phases. Despite the strong metal-metal interactions, electronic structure calculations show the presence of a relatively simple set of states at the Fermi level, consisting mostly of Ta atom orbital contributions. The superconducting transition is characterized by temperature-dependent resistivity, magnetic susceptibility and specific heat measurements, and is of the weak coupling BCS type with $\mathrm{\Delta }C/\gamma T_c=1.55$. The upper critical field at 0 K is estimated to be 2 T. Basic characterization of the superconductivity suggests that this material is similar to $\mathrm{PbTaS}{\mathrm{e}}_2$, for which the effects of spin-orbit coupling may be important.

We report evidence for a reentrant spin glass phase in electron-doped ${\text{EuFe}}_2{\text{As}}_2$ single crystals and first traces of the superconductivity reentrance in optics. In the close-to-optimal doped $\text{Eu}{\left({\text{Fe}}_{0.91}{\text{Ir}}_{0.09}\right)}_2{\text{As}}_2$ and $\text{Eu}{\left({\text{Fe}}_{0.93}{\text{Rh}}_{0.07}\right)}_2{\text{As}}_2$ samples, two magnetic transitions are observed below the superconducting critical temperature $T_c\approx 21$ K: the canted $A$-type antiferromagnetic order of the ${\text{Eu}}^{2+}$ ions sets in around $17\text{K}$ and the spin glass behavior occurs another $2\text{K}$ lower in temperature. In addition, strong evidence for an additional transition is found far below the spin glass temperature. Our extensive optical and magnetic investigations provide important insight into the interplay of local magnetism and superconductivity in these systems and elucidate the effect of the spin glass phase on the reentrant superconducting state.

The electronic structure of recently discovered $\beta \text{−}{\mathrm{Fe}}_4{\mathrm{Se}}_5$ with $\sqrt{5}\times \sqrt{5}$ ordered Fe vacancies is calculated using first-principles density functional theory. We find that the ground state is an antiferromagnetic (AFM) insulator in agreement with the experimental observation. In ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$, it is known that the ground state is $\sqrt{5}\times \sqrt{5}$-blocked-checkerboard AFM ordered. But for this material, we find that the ground state is $\sqrt{5}\times \sqrt{5}$-pair-checkerboard AFM ordered, in which the intrablock four Fe spins exhibit the collinear AFM order and the interblock spins on any two nearest-neighboring sites are antiparallel aligned. This state is about 130 meV/Fe lower in energy than the $\sqrt{5}\times \sqrt{5}$-blocked-checkerboard AFM one. Electron doping can lower the energy of the $\sqrt{5}\times \sqrt{5}$-blocked-checkerboard AFM state and introduce a transition between these two states, suggesting that there is strong AFM fluctuation in FeSe-based materials upon doping. This provides a unified picture to understand the AFM orders in $\beta \text{−}{\mathrm{Fe}}_4{\mathrm{Se}}_5$ and in alkali-metal intercalated FeSe materials.

Defect motion in solid helium has a unique quantum nature due to the large zero-point motion of helium atoms, which allows vacancies and isotopic impurities to tunnel and move ballistically. Recent shear modulus experiments showed that dislocations are also extraordinarily mobile in solid ${}^4\mathrm{He}$. The lighter isotope, ${}^3\mathrm{He}$, has even larger zero-point motion and an extra degree of freedom—nuclear spin—which can affect defect motion. We have measured the shear modulus of hcp solid ${}^3\mathrm{He}$ to probe the motion of dislocations and isotopic impurities. We observed a crossover between stiff and soft states, due to ${}^4\mathrm{He}$ impurities which immobilize dislocations at low temperatures. In contrast to solid ${}^4\mathrm{He}$, the impurities in hcp ${}^3\mathrm{He}$ act as static pinning sites because of the disordered configuration of ${}^3\mathrm{He}$ nuclear spins. In addition, we observed an unexpected dissipation that increased rapidly at low frequencies, indicating a strong interaction between nuclear spins and moving dislocations.

The application of transformation theory to underwater acoustics has been a challenging task because highly anisotropic density is unachievable in water. A possible strategy is to exploit the anisotropic modulus rather than density, although it has not been experimentally demonstrated. We present an annular underwater acoustic cloak designed from particular graded solid microstructures. The geometry tailored microstructures mimic metafluid with highly anisotropic modulus through substantially suppressed shear waves. Transient wave experiments are conducted with the cloak in a designed two-dimensional underwater waveguide system and proved excellent cloaking performance for an enclosed target over broadband frequencies 9–15 kHz. This finding paves the way for controlling underwater acoustics using the structured anisotropic modulus metafluid.

The structure of ${\mathrm{K}}_2{\mathrm{Ni}}_2{\left({\mathrm{MoO}}_4\right)}_3$ consists of $S=1$ tetramers formed by ${\mathrm{Ni}}^{2+}$ ions. The magnetic susceptibility $\chi \left(T\right)$ and specific heat $C_P\left(T\right)$ data on a single crystal show a broad maximum due to the low dimensionality of the system with short-range spin correlations. A sharp peak is seen in $\chi \left(T\right)$ and $C_P\left(T\right)$ at about 1.13 K, well below the broad maximum. This is an indication of magnetic long-range order, i.e., the absence of spin gap in the ground state. Interestingly, the application of a small magnetic field ($H>0.1$ T) induces magnetic behavior akin to the Bose-Einstein condensation (BEC) of triplon excitations observed in some spin-gap materials. Our results demonstrate that the temperature-field ($T\text{−}H$) phase boundary follows a power law $(T-T_N)\propto H{}^{1/\alpha }$ with the exponent $1/\alpha$ close to $\frac{2}{3}$, as predicted for the BEC scenario. The observation of BEC of triplon excitations in small $H$ infers that ${\mathrm{K}}_2{\mathrm{Ni}}_2{\left({\mathrm{MoO}}_4\right)}_3$ is located in the proximity of a quantum critical point, which separates the magnetically ordered and spin-gap regions of the phase diagram.

We study the annihilation of topological solitons in the simplest setting: a one-dimensional ferromagnet with an easy axis. We develop an effective theory of the annihilation process in terms of four collective coordinates: two zero modes of the translational and rotational symmetries $Z$ and $\mathrm{\Phi }$, representing the average position and azimuthal angle of the two solitons, and two conserved momenta $\zeta$ and $\phi$, representing the relative distance and twist. A comparison with micromagnetic simulations shows that our approach captures well the essential physics of the process.

We report the magnetic response of Pt/Au/GdFeCo trilayers to optical irradiation of the Pt surface. For trilayers with Au thickness greater than 50 nm, the great majority of energy is absorbed by the Pt layer, creating an initial temperature differential of thousands of kelvin between the Pt/Au layers and the GdFeCo layer. The resulting electronic heat current across the metal multilayer lasts for several picoseconds with energy flux in excess of $2\mathrm{TW}{\mathrm{m}}^{-2}$ and provides sufficient heating to the GdFeCo electrons to induce deterministic reversal of the magnetic moment.

The magnetic field–temperature ($H-T$) phase diagram of a lacunar spinel ${\mathrm{GaV}}_4{\mathrm{Se}}_8$ is determined by means of ac magnetic susceptibility and magnetoelectric measurements on single crystals and classical Monte Carlo simulation. ${\mathrm{GaV}}_4{\mathrm{Se}}_8$ is pyroelectric below the structural phase transition temperature $T_{\mathrm{S}}=41\mathrm{K}$ and magnetically ordered below $T_{\mathrm{C}}=17.5\mathrm{K}$. Ac magnetic susceptibility measurement has revealed that ${\mathrm{GaV}}_4{\mathrm{Se}}_8$ undergoes successive magnetic phase transitions with increasing applied magnetic field. Each phase is assigned to cycloidal, skyrmion lattice, and forced ferromagnetic phases. Both cycloidal and skyrmion-lattice magnetic orders induce electric polarization up to around $10\mu {\mathrm{C}/\mathrm{m}}^2$ compared with ferromagnetic order, suggesting a spin-driven magnetoelectric nature in ${\mathrm{GaV}}_4{\mathrm{Se}}_8$. The skyrmion lattice phase seems to be stable down to $T=2\mathrm{K}$ and up to ${\mu }_{0}H=370\mathrm{mT}$. This enhanced stability of skyrmion lattice in ${\mathrm{GaV}}_4{\mathrm{Se}}_8$ compared with ${\mathrm{GaV}}_4{\mathrm{S}}_8$ may provide a key to understanding the formation mechanism of the skyrmion lattice.

We propose an approach to perform the global search for low-lying crystal structures from first principles, by combining the artificial force induced reaction (AFIR) method and the periodic boundary conditions (PBCs). The AFIR method has been applied extensively to molecular systems to elucidate the mechanism of chemical reactions such as homogeneous catalysis. The present PBC/AFIR approach found 274 local minima for carbon crystals in the ${\mathrm{C}}_8$ unit cell described by the generalized gradient approximation–Perdew-Burke-Ernzerhof functional. Among many newly predicted structures, three low-lying structures, which exhibit somewhat higher energy compared with those previously predicted, such as $\mathrm{Cco}-{\mathrm{C}}_8$ ($Z$-carbon) and $M$-carbon, are further discussed with calculations of phonon and band dispersion curves. Furthermore, approaches to systematically explore two- or one-dimensional periodic structures are proposed and applied to the ${\mathrm{C}}_8$ unit cell with the slab model. These results suggest that the present approach is highly promising for predicting crystal structures.

We study the behavior of Fisher zeros (FZs) and dynamical quantum phase transitions (DQPTs) for a linearly ramped Haldane model occurring in the subsequent temporal evolution of the same, and we probe the intimate connection with the equilibrium topology of the model. Here, we investigate the temporal evolution of the final state of the Haldane Hamiltonian (evolving with the time-independent final Hamiltonian) reached following a linear ramping of the staggered (Semenoff) mass term from an initial to a final value, first selecting a specific protocol, so chosen that the system is ramped from one nontopological phase to the other through a topological phase. We establish the existence of three possible behaviors of areas of FZs corresponding to a given sector: (i) no-DQPT, (ii) one-DQPT (intermediate), and (iii) two-DQPTs (reentrant), depending on the inverse quenching rate $\tau$. Our study also reveals that the appearance of the areas of FZs is an artefact of the nonzero (quasi-momentum-dependent) Haldane mass ($M_H$), whose absence leads to an emergent one-dimensional behavior indicated by the shrinking of the area's FZs to lines and the nonanalyticity in the dynamical “free energy” itself. Moreover, the characteristic rates of crossover between the three behaviors of FZs are determined by the time-reversal-invariant quasimomentum points of the Brillouin zone where $M_H$ vanishes. Thus, we observe that through the presence or absence of $M_H$, there exists an intimate relation to the topological properties of the equilibrium model even when the ramp drives the system far away from equilibrium.

We report on zero-field muon spin rotation, electron-spin resonance, and polarized Raman scattering measurements of the coupled quantum spin ladder ${\mathrm{Ba}}_2{\mathrm{CuTeO}}_6$. Zero-field muon spin rotation and electron-spin resonance probes disclose a successive crossover from a paramagnetic through a spin-liquid-like into a magnetically ordered state with decreasing temperature. More significantly, the two-magnon Raman response obeys a $T$-linear scaling relation in its peak energy, linewidth, and intensity. This critical scaling behavior presents an experimental signature of proximity to a quantum-critical point from an ordered side in ${\mathrm{Ba}}_2{\mathrm{CuTeO}}_6$.

Binary random compacts with different proportions of small (volume $V$) and large (volume $2V$) essentially bare maghemite nanoparticles are used to investigate the effect of controllably broadening the particle size distribution on the magnetic properties of magnetic nanoparticle assemblies with strong dipolar interaction. A series of eight random mixtures of highly uniform 9.0- and 11.5-nm-diameter maghemite particles prepared by thermal decomposition is studied. In spite of the severely broadened size distributions in the mixed samples, well-defined superspin glass transition temperatures are observed across the series, their values increasing linearly with the weight fraction of large particles.

An extended atomistic spin model allowing for studies of the finite-temperature magnetic properties of alloys is proposed. The model is obtained by extending the Heisenberg Hamiltonian via a parametrization from a first-principles basis, interpolating from both the low-temperature ferromagnetic and the high-temperature paramagnetic reference states. This allows us to treat magnetic systems with varying degree of itinerant character within the model. Satisfactory agreement with both previous theoretical studies and experiments are obtained in terms of Curie temperatures and paramagnetic properties. The proposed model is not restricted to elements but is also applied to binary alloys, such as the technologically important material permalloy, where significant differences in the finite magnetic properties of Fe and Ni magnetic moments are found. The proposed model strives to find the right compromise between accuracy and computational feasibility for accurate modeling, even for complex magnetic alloys and compounds.

Analyzing temperature dependent photoemission (PE) data of the ferromagnetic Kondo-lattice (KL) system YbNiSn in the light of the periodic Anderson model (PAM) we show that the KL behavior is not limited to temperatures below a temperature ${\overline{T}}_K$, defined empirically from resistivity and specific heat measurements. As characteristic for weakly hybridized Ce and Yb systems, the PE spectra reveal a $4f$-derived Fermi level peak, which reflects contributions from the Kondo resonance and its crystal electric field (CEF) satellites. In YbNiSn this peak has an unusual temperature dependence: With decreasing temperature a steady linear increase of intensity is observed which extends over a large interval ranging from 100 K down to 1 K without showing any peculiarities in the region of ${\overline{T}}_K\sim T_C=5.6$ K. In the light of the single-impurity Anderson model (SIAM) this intensity variation reflects a linear increase of $4f$ occupancy with decreasing temperature, indicating an onset of Kondo screening at temperatures above 100 K. Within the PAM this phenomenon could be described by a non-Fermi-liquid-like $T$- linear damping of the self-energy which accounts phenomenologically for the feedback from the closely spaced CEF states.