We experimentally investigate thickness $\left(t\right)$-dependent evolution of structural and magnetic properties in $\mathrm{C}{\mathrm{o}}_2\mathrm{F}{\mathrm{e}}_{0.4}\mathrm{M}{\mathrm{n}}_{0.6}\mathrm{Si}$ (CFMS) thin films and correlate them with ultrafast demagnetization time $\left({\tau }_{\mathrm{d}}\right)$ and relaxation time $\left({\tau }_1\right)$ as well as the Gilbert damping coefficient ($\alpha$). Structural ordering and magnetic parameters, including $\alpha$, exhibit a nonmonotonic variation with increasing $t$. A remarkably low value of $\alpha$ of 0.009 is obtained for the CFMS film with $t=20\mathrm{nm}$ without any buffer layers, which helps to avoid possible diffusion of the buffer layer into CFMS. Highest saturation magnetization, lowest coercivity, and the $\alpha$ value imply CFMS film with $t=20\mathrm{nm}$ is most suitable for integrated spintronics devices, viz. low-current switched spin transfer torque, and magnetic tunnel junction with a high tunnel magnetoresistance ratio at room temperature. Despite the presence of strain, a lower degree of chemical ordering in the low-$t$ regime, and increased defect density in the high-$t$ regime, we obtained a reasonably low value of damping. In addition to the intrinsic fourfold magnetocrystalline anisotropy, an induced uniaxial anisotropy is found, which also varies nonmonotonically with $t$. Finally, unique band structure controlled demagnetization and fast relaxation in half-metallic CFMS is correlated to $\alpha$.

We performed high-resolution polarized optical transmission spectroscopy and theoretical studies of multiferroic $\mathrm{EuF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ single crystals in the three phases: paramagnetic $R32(T>T_{\mathrm{s}}=84\mathrm{K})$ and $P{3}_{1}21(T_{\mathrm{s}}>T>T_{\mathrm{N}}=34\mathrm{K})$, and antiferromagnetic $(T<T_{\mathrm{N}})$ ones. The analysis of the spectra reveals interesting details of the magnetic structure, namely, a collinear arrangement of the iron magnetic moments along the $C_2$ symmetry axis in the $ab$ crystallographic plane of $\mathrm{EuF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ below $T_{\mathrm{N}}$. Spectral signatures of the phase transitions and the spin-phonon interaction are observed and discussed. Reliable crystal-field and exchange-interaction parameters are obtained and used to model the magnetic susceptibility of the compound. The results of detailed calculations of the electric polarization of $\mathrm{EuF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ in the $R32$ phase are presented, and mechanisms of the magnetoelectric response are discussed. We detect a strong effect of impurities (that enter the crystal from a flux in the course of the crystal growth) on the structural phase-transition temperature and demonstrate a coexistence of both $R32$ and $P{3}_{1}21$ phases down to the lowest temperatures in a $\mathrm{EuF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ crystal grown with the $\mathrm{B}{\mathrm{i}}_2\mathrm{M}{\mathrm{o}}_3{\mathrm{O}}_{12}$ based flux, due to inhomogeneous distribution of impurity $\mathrm{B}{\mathrm{i}}^{3+}$ ions. Our study can be considered as a demonstration of the abilities of optical spectroscopy in delivering new information on a magnetic compound, even in the case when other methods fail.

We study inelastic interactions of particles with quantized vortices in superfluids by using a semiclassical matter wave theory that is analogous to the Landau two-fluid equations, but allows for the vortex dynamics. The research is motivated by recent experiments on xenon-doped helium nanodroplets that show clustering of the impurities along the vortex cores. We numerically simulate the dynamics of trapping and interactions of xenon atoms by quantized vortices in superfluid helium and the obtained results can be extended to scattering of other impurities by quantized vortices. Different energies and impact parameters of incident particles are considered. We show that inelastic scattering is closely linked to the generation of Kelvin waves along a quantized vortex during the interaction even if there is no capture. The capture criterion of an impurity is formulated in terms of the binding energy.

We present a theory describing the local electronic properties of the perovskite rare-earth nickelates—materials which have negative charge transfer energies, strong O $2p$ – Ni $3d$ covalence, and breathing-mode lattice distortions at the origin of highly studied metal-insulator and antiferromagnetic ordering transitions. Utilizing a full-orbital, full-correlation double-cluster approach, we find strong charge fluctuations, in agreement with a bond disproportionation interpretation. The double-cluster formulation permits the inclusion of necessary orbital degeneracies and Coulomb interactions to calculate resonant x-ray spectral responses, with which we find excellent agreement with well-established experimental results. This previously absent, crucial link between theory and experiment provides validation of the recently proposed bond disproportionation theory, and provides an analysis methodology for spectroscopic studies of engineered phases of nickelates and other high-valence transition-metal compounds.

$\mathrm{Dy}{\mathrm{B}}_4$ has a two-dimensional Shastry-Sutherland (Sh-S) lattice with strong Ising character of the Dy ions. Despite the intrinsic frustrations, it undergoes two successive transitions: a magnetic ordering at $T_{\mathrm{N}}=20\mathrm{K}$ and a quadrupole ordering at $T_Q=12.5\mathrm{K}$. From high-resolution neutron and synchrotron x-ray powder diffraction studies, we have obtained full structural information on this material in all phases and demonstrate that structural modifications occurring at quadrupolar transition lead to the lifting of frustrations inherent in the Sh-S model. Our paper thus provides a complete experimental picture of how the intrinsic frustration of the Sh-S lattice can be lifted by the coupling to quadrupole moments. We show that two other factors, i.e., strong spin-orbit coupling and long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in metallic $\mathrm{Dy}{\mathrm{B}}_4$, play an important role in this behavior.

Dimensionless ratios of physical properties can characterize low-temperature phases in a wide variety of materials. As such, the Wilson ratio (WR), the Kadowaki-Woods ratio, and the Wiedemann-Franz law capture essential features of Fermi liquids in metals, heavy fermions, etc. Here we prove that the phases of many-body interacting multicomponent quantum liquids in one dimension (1D) can be described by WRs based on the compressibility, susceptibility, and specific heat associated with each component. These WRs arise due to additivity rules within subsystems reminiscent of the rules for multiresistor networks in series and parallel—a novel and useful characteristic of multicomponent Tomonaga-Luttinger liquids (TLL) independent of microscopic details of the systems. Using experimentally realized multispecies cold atomic gases as examples, we prove that the Wilson ratios uniquely identify phases of TLL, while providing universal scaling relations at the boundaries between phases. Their values within a phase are solely determined by the stiffnesses and sound velocities of subsystems and identify the internal degrees of freedom of said phase such as its spin degeneracy. This finding can be directly applied to a wide range of 1D many-body systems and reveals deep physical insights into recent experimental measurements of the universal thermodynamics in ultracold atoms and spins.

By means of first-principles calculations, we predict a partially hydrogenated graphene system, ${\mathrm{C}}_6{\mathrm{H}}_2$, and find the one in $AB$-trans configuration is a Dirac material with a tilted anisotropic Dirac cone electronic structure. Different from graphene, in which the Dirac points are located at $K$ and $K^{\prime }$ and the Fermi surfaces are circular with doping, the $AB$-trans ${\mathrm{C}}_6{\mathrm{H}}_2$ exhibits Dirac points located on the lines from $\mathrm{\Gamma }$ to $M$ with quasielliptical Fermi surfaces when doped. Around the Dirac point, the Fermi velocity varies along different directions. Therefore, the propagation of charge carriers in this system is highly anisotropic, creating a new tunability for novel transport properties.

Using the properties of the transfer matrix of one-dimensional quantum mechanical systems, we derive an exact formula for the persistent current across a quantum mechanical ring pierced by a magnetic flux $\mathrm{\Phi }$ as a single integral of a known function of the system's parameters. Our approach provides exact results at zero temperature, which can be readily extended to a finite temperature $T$. We apply our technique to exactly compute the persistent current through $p$-wave and $s$-wave superconducting-normal hybrid rings, deriving full plots of the current as a function of the applied flux at various system's scales. Doing so, we recover at once a number of effects such as the crossover in the current periodicity on increasing the size of the ring and the signature of the topological phase transition in the $p$-wave case. In the limit of a large ring size, resorting to a systematic expansion in inverse powers of the ring length, we derive exact analytic closed-form formulas, applicable to a number of cases of physical interest.

We have conducted high-pressure x-ray diffraction and Raman spectroscopic studies on the $\mathrm{CdC}{\mathrm{r}}_2\mathrm{S}{\mathrm{e}}_4$ spinel at room temperature up to 42 GPa. We have resolved three structural transitions up to 42 GPa, i.e., the starting $Fd\overline{3}m$ phase transforms at $\sim 11\mathrm{GPa}$ into a tetragonal $I{4}_1/amd$ structure, an orthorhombic distortion was observed at $\sim 15\mathrm{GPa}$, whereas structural disorder initiates beyond 25 GPa. Our ab initio density functional theory studies successfully reproduced the observed crystalline-to-crystalline structural transitions. In addition, our calculations propose an antiferromagnetic ordering as a potential magnetic ground state for the high-pressure tetragonal and orthorhombic modifications, compared with the starting ferromagnetic phase. Furthermore, the computational results indicate that all phases remain insulating in their stability pressure range, with a direct-to-indirect band gap transition for the $Fd\overline{3}m$ phase taking place at 5 GPa. We attempted also to offer an explanation behind the peculiar first-order character of the $Fd\overline{3}m\left(\mathrm{cubic}\right)\to I{4}_1/amd$ (tetragonal) transition observed for several relevant Cr spinels, i.e., the sizeable volume change at the transition point, which is not expected from space group symmetry considerations. We detected a clear correlation between the cubic-tetragonal transition pressures and the next-nearest-neighbor magnetic exchange interactions for the Cr-bearing sulfide and selenide members, a strong indication that the cubic-tetragonal transitions in these systems are principally governed by magnetic effects.

We measured the temperature dependence of the magnetic susceptibility and specific heat and the magnetic-field dependence of the magnetization of ${\mathrm{CuInVO}}_5$. An antiferromagnetically ordered state appears below $T_{\mathrm{N}}=2.7$ K. We observed a $\frac{1}{2}$ quantum magnetization plateau above 30 T at 1.3 K. We consider that the probable spin model for ${\mathrm{CuInVO}}_5$ is an interacting spin-$\frac{1}{2}$ tetramer model. We evaluated values of the intratetramer interactions as $J_1=240\pm 20$ K (antiferromagnetic) and $J_2=-142\pm 10$ K (ferromagnetic). The ground state of the isolated spin tetramer with the $J_1$ and $J_2$ values is spin singlet. The shrinkage of ordered magnetic moments by quantum fluctuation can be expected. Detectable low-energy longitudinal-mode magnetic excitations may exist in ${\mathrm{CuInVO}}_5$.

The origin of magnetic phase transition from in-plane to perpendicular magnetic anisotropy (PMA) of Pt/Co/Pt thin film by ${\mathrm{Ga}}^{+}$ ion irradiation at fluences of $1–5\times {10}^{15}\mathrm{ions}/{\mathrm{cm}}^2$ is investigated by means of x-ray magnetic circular dichroism (XMCD) and extended x-ray absorption fine structure (EXAFS) analyses. We find that Pt and Co atoms are mixed with each other and that Co is oxidized near the surface due to removal of the Pt overlayer. Furthermore, polarization-dependent EXAFS analysis shows that Co is firstly dispersed as separated single-atom-thick sheets in a Pt matrix at $1\times {10}^{15}\mathrm{ions}/{\mathrm{cm}}^2$, then the Co sheets are divided into a few $\AA$ clusters at $5\times {10}^{15}\mathrm{ions}/{\mathrm{cm}}^2$, which are regarded as nanodiscs parallel to the film plane. This process is accompanied by the appearance of an out-of-plane magnetization component and a remanence peak is observed. Because we do not observe an enhancement in anisotropy of Co orbital moment which leads to change in magnetic anisotropy through the transition at about $5\times {10}^{15}\mathrm{ions}/{\mathrm{cm}}^2$, it might be possible that such nanodisc formation induces increase of magnetic anisotropy via a shape effect. By comparing with the phase transition observed at lower fluence [Phys. Rev. B 86, 024418 (2012)], we find that the mechanism of two transitions is different, i.e., the transition at lower fluence is caused by anisotropy of orbital moment due to structural strain, while the present transition is possibly by shape effect due to nanodisc formation.

Na-intercalated $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ was prepared using the liquid $\mathrm{N}{\mathrm{H}}_3$ technique, and a superconducting phase exhibiting a superconducting transition temperature $\left(T_{\mathrm{c}}\right)$ as high as 27 K was discovered. This can be called the high-$T_{\mathrm{c}}$ phase since a 21 K superconducting phase was previously obtained in ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_y\mathrm{N}{\mathrm{a}}_x\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$. The chemical composition of the high-$T_{\mathrm{c}}$ phase was determined to be ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_{0.61\left(4\right)}\mathrm{N}{\mathrm{a}}_{0.63\left(5\right)}\mathrm{F}{\mathrm{e}}_{0.85}\mathrm{S}{\mathrm{e}}_{0.55\left(3\right)}\mathrm{T}{\mathrm{e}}_{0.44\left(2\right)}$. The x-ray diffraction patterns of both phases show that a larger lattice constant $c$ (i.e., $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ plane spacing) produces a higher $T_{\mathrm{c}}$. This behavior is the same as that of metal-doped FeSe, suggesting that improved Fermi-surface nesting produces the higher $T_{\mathrm{c}}$. The high-$T_{\mathrm{c}}$ phase converted to the low-$T_{\mathrm{c}}$ phase within several days, indicating that it is a metastable phase. The temperature dependence of resistance for both phases was recorded at different magnetic fields, and the critical fields were determined for both phases. Finally, the $T_{\mathrm{c}}$ versus $c$ phase diagram was prepared for the metal-doped $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, which is similar to that of metal-doped FeSe, although the $T_{\mathrm{c}}$ is lower.

We develop a theory of the upper critical field in a BCS superconductor with a nonlocal interaction between electrons. We have shown that the nonlocal interaction is characterized by the parameter $k_F{\rho }_0$, where $k_F$ is the Fermi momentum and ${\rho }_0$ is the radius of electron-electron interaction. The presence of the external magnetic field leads to the generation of additional components of the order parameter with different angular momenta. This effect leads to the enhancement of the upper critical field above the orbital limiting field. In addition the upward curvature in the temperature dependence of $H_{c2}\left(T\right)$ in the clean limit is predicted. The impurity scattering suppresses the effect in the dirty limit.

We provide analytical and numerical evidence of spin-triplet Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity in the itinerant Kitaev-Heisenberg model (antiferromagnetic Kitaev coupling and ferromagnetic Heisenberg coupling) on the honeycomb lattice around quarter filling. The strong spin-orbit coupling in our model leads to the emergence of six inversion symmetry centers for the Fermi surface at nonzero momenta in the first Brillouin zone. We show how the Cooper pairs condense into these nontrivial momenta, causing spatial modulation of the superconducting order parameter. Applying a Ginzburg-Landau expansion analysis, we find that the superconductivity has three separated degenerate ground states with three different spin-triplet pairings. Exact diagonalizations on finite clusters support this picture while ruling out a spin (charge) density wave.

${\mathrm{U}}_2{\mathrm{Pd}}_2\mathrm{In}$ is the material where the elements of the geometrical frustration of the lattice coexist with strong spin-orbit coupling (SOC). The ground state of the system is a noncollinear planar magnetic structure with orthogonal atomic magnetic moments. There are three possible physical mechanisms that can lead to this nontrivial magnetic structure: frustrated isotropic exchange interaction, Dzyaloshinskii-Morija interaction (DMI), and magnetic anisotropy. Our first-principles calculations show that in the case where the SOC is neglected, and therefore the DMI and magnetic anisotropy are absent, the ground state structure is the collinear ferromagnetic one. The leading contribution to the stabilization of the magnetically compensated configuration of orthogonal atomic moments is provided by the local magnetic anisotropy of the U moments. A weaker DMI leads to the lifting of the degeneracy between the magnetic states with different local chirality. The established hierarchy of the interactions allows us to explain the metamagnetic phase transition in the in-plane external magnetic field. The analysis of the noncollinearity of the spin and orbital moments of the same U atom appearing in the applied external field show that the trend to the antiparallel orientation of the two atomic moments following from the third Hund's rule is much stronger than the trend to the parallel orientation of the moments due to the applied external magnetic field.

Simulations of magnetization dynamics in a multiscale environment enable the rapid evaluation of the Landau-Lifshitz-Gilbert equation in a mesoscopic sample with nanoscopic accuracy in areas where such accuracy is required. We have developed a multiscale magnetization dynamics simulation approach that can be applied to large systems with spin structures that vary locally on small length scales. To implement this, the conventional micromagnetic simulation framework has been expanded to include a multiscale solving routine. The software selectively simulates different regions of a ferromagnetic sample according to the spin structures located within in order to employ a suitable discretization and use either a micromagnetic or an atomistic model. To demonstrate the validity of the multiscale approach, we simulate the spin wave transmission across the regions simulated with the two different models and different discretizations. We find that the interface between the regions is fully transparent for spin waves with frequency lower than a certain threshold set by the coarse scale micromagnetic model with no noticeable attenuation due to the interface between the models. As a comparison to exact analytical theory, we show that in a system with a Dzyaloshinskii-Moriya interaction leading to spin spirals, the simulated multiscale result is in good quantitative agreement with the analytical calculation.

We report on the growth and magnetoresistance of MnSi films grown on SiC(0001) by molecular beam epitaxy. The growth resulted in a textured MnSi(111) film with a predominantly $\left[1\overline{1}0\right]\mathrm{MnSi}\left(111\right)\parallel \left[11\overline{2}0\right]$ SiC(0001) epitaxial relationship, as demonstrated by transmission electron microscopy, reflection high energy electron diffraction, and atomic force microscopy. The 500 ${}^{\circ }\mathrm{C}$ temperature required to crystallize the film leads to a dewetting of the MnSi layer. Although the sign of the lattice mismatch suggested the films would be under compressive stress, the films acquire an in-plane tensile strain likely driven by the difference in thermal expansion coefficients between the film and substrate during annealing. As a result, the magnetoresistive response demonstrates that the films possess a hard-axis out-of-plane magnetocrystalline anisotropy.

Magnetic susceptibility of the topological Weyl, type-II Weyl, Dirac, and line node semimetals is theoretically investigated. Dependences of this susceptibility on the chemical potential, temperature, direction, and magnitude of the magnetic field are found. The obtained results show that magnetic measurements can be very useful in investigating these semimetals. As an example, we calculate magnetic susceptibility of ${\mathrm{Cd}}_3{\mathrm{As}}_2,{\mathrm{Na}}_3\mathrm{Bi}$, and ${\mathrm{Ca}}_3{\mathrm{P}}_2$.