A semiclassical phase-space model is used to study the ultrafast charge and spin dynamics in thin ferromagnetic films. Both itinerant and localized magnetism are taken into account. It is shown that an oscillating spin current can be generated in the film via the application of a femtosecond laser pulse in the visible range.

The magnetic domains in two-dimensional layered material $\mathrm{F}{\mathrm{e}}_3\mathrm{GeT}{\mathrm{e}}_2$ are studied by using a variable-temperature scanning tunneling microscope with a magnetic tip after in situ cleaving of single crystals. A stripy domain structure is revealed in a zero-field-cooled sample below the ferromagnetic transition temperature of 205 K, which is replaced by separate double-walled domains and bubble domains when cooling the sample under a magnetic field of a ferromagnetic Ni tip. The Ni tip can further convert the double-walled domain to a bubble domain pattern as well as move the Neel-type chiral bubble in submicrometer distance. The temperature-dependent evolutions of both zero-field-cooled and field-cooled domain structures correlate well with the bulk magnetization from magnetometry measurements. Atomic resolution scanning tunneling images and spectroscopy are acquired to understand the atomic and electronic structures of the material, which are further corroborated by first-principles calculations.

We have synthesized the cation ordered rutile-type $\mathrm{C}{\mathrm{r}}_2\mathrm{Re}{\mathrm{O}}_6$ and $\mathrm{CrRe}{\mathrm{O}}_4$ under high-pressure and high-temperature conditions and performed detailed characterizations on their structural, transport, and magnetic properties via x-ray and neutron powder diffraction, resistivity, magnetic susceptibility, and specific heat measurements. $\mathrm{C}{\mathrm{r}}_2\mathrm{Re}{\mathrm{O}}_6$ crystallizes in the inverse trirutile structure with lattice parameters $a=b=4.55221\left(5\right)\AA$ and $c=8.8934\left(1\right)\AA$ in the space group $P{4}_2/mnm$. It undergoes a second-order antiferromagnetic transition at $T_{\mathrm{N}}\approx 67\mathrm{K}$ with the magnetic structure described by the propagation vector $\mathbf{k}=\left[0.5,0,0.5\right]$. For $\mathrm{CrRe}{\mathrm{O}}_4$, it adopts a monoclinically distorted rutile-like structure with unit-cell parameters $a=9.3393\left(2\right)\AA$, $b=5.6869\left(1\right)\AA$, $c=4.6243\left(1\right)\AA$, and $\beta =92.043{\left(8\right)}^{\circ }$ in the space group $C2/m$. It also exhibits an antiferromagnetic order at $T_{\mathrm{N}}=98\mathrm{K}$ with the magnetic structure described by the propagation vector $\mathbf{k}=\left[1,0,0.5\right]$. Interestingly, a large paramagnetic Weiss temperature of ${\theta }_{\mathrm{CW}}=-478\mathrm{K}$ is evidenced from the Curie-Weiss fitting to the inverse magnetic susceptibility. The temperature dependence of resistivity ρ($T$) for both compounds can be described with Mott's variable range hopping mechanism in the one-dimensional model for $\mathrm{C}{\mathrm{r}}_2\mathrm{Re}{\mathrm{O}}_6$ and the three-dimensional model for $\mathrm{CrRe}{\mathrm{O}}_4$, respectively. For both compounds, a weak resistivity anomaly can be discerned around $T_{\mathrm{N}}$ from the temperature derivative curves, signaling the interplay of charge and spin degrees of freedom for these $3d$(Cr)-$5d$(Re) coupled electron systems.

We theoretically study the spin current and its dc noise generated between two spin-$1/2$ spin chains weakly coupled at a single site in the presence of an over-population of spin excitations and a temperature elevation in one subsystem relative to the other, and we compare the corresponding transport quantities across two weakly coupled magnetic insulators hosting magnons. In the spin chain scenario, we find that applying a temperature bias exclusively leads to a vanishing spin current and a concomitant divergence in the spin Fano factor, defined as the spin current noise-to-signal ratio. This divergence is shown to have an exact analogy to the physics of electron scattering between fractional quantum Hall edge states and not to arise in the magnon scenario. We also reveal a suppression in the spin current noise that exclusively arises in the spin chain scenario due to the fermion nature of the spin-1/2 operators. We discuss how the spin Fano factor may be extracted experimentally via the inverse spin Hall effect used extensively in spintronics.

We study the Lorentz nonreciprocal transmission and nonreciprocal complex band structures supported in a $\mathcal{P}\mathcal{T}$-symmetric waveguide sandwiched by magnetized layers. We found that, near the emergency and coalescence of exceptional points, nonreciprocal transmission can be significantly enhanced by the non-Hermitian properties. With the help of gain-loss balance, one-way light transmission could be achieved even in the presence of a weak magnetic field. The results may offer us insight into the interplay between Lorentz nonreciprocity and non-Hermiticity.

The tunneling density of states in dirty $s$-wave superconductors is often well described by the phenomenological Dynes formula. Recently we have shown that this formula can be derived, within the coherent potential approximation, for superconductors with simultaneously present pair-conserving and pair-breaking impurity scattering. Here we demonstrate that the theory of such so-called Dynes superconductors is thermodynamically consistent. We calculate the specific heat and critical field of the Dynes superconductors, and we show that their gap parameter, specific heat, critical field, and penetration depth exhibit power-law scaling with temperature in the low-temperature limit. We also show that in the vicinity of a coupling-constant-controlled superconductor to normal metal transition, the Homes law is replaced by a different, pair-breaking-dominated scaling law.

In relativistic quantum field theories, compact objects of interacting bosons can become stable owing to conservation of an additive quantum number $Q$. Discovering such $Q$ balls propagating in the universe would confirm supersymmetric extensions of the standard model and may shed light on the mysteries of dark matter, but no unambiguous experimental evidence exists. We have created long-lived $Q$-ball solitons in superfluid ${}^3\mathrm{He}$, where the role of the $Q$ ball is played by a Bose-Einstein condensate of magnon quasiparticles. The principal qualitative attribute of a $Q$ ball is observed experimentally: its propagation in space together with the self-created potential trap. Additionally, we show that this system allows for a quantitatively accurate representation of the $Q$-ball Hamiltonian. Our $Q$ ball belongs to the class of the Friedberg-Lee-Sirlin $Q$ balls with an additional neutral field $\zeta$, which is provided by the orbital part of the Nambu-Goldstone mode. Multiple $Q$ balls can be created in the experiment, and we have observed collisions between them. This set of features makes the magnon condensates in superfluid ${}^3\mathrm{He}$ a versatile platform for studies of $Q$-ball dynamics and interactions in three spatial dimensions.

Recently it has been shown that interparticle interactions generically destroy dynamical localization in periodically driven systems, resulting in diffusive transport and heating. In this Rapid Communication we rigorously construct a family of interacting driven systems which are dynamically localized and effectively decoupled from the external driving potential. We show that these systems exhibit tunable periodic or quasiperiodic revivals of the many-body wave function and thus of all physical observables. By numerically examining spinless fermions on a one-dimensional lattice we show that the analytically obtained revivals of such systems remain stable for finite systems with open boundary conditions while having a finite lifetime in the presence of static spatial disorder. We find this lifetime to be inversely proportional to the disorder strength.

Recent experimental realizations of superfluid mixtures of Bose and Fermi quantum gases provide a unique platform for exploring diverse superfluid phenomena. We study dipole oscillations of a double superfluid in a cigar-shaped optical dipole trap, consisting of ${}^{41}\mathrm{K}$ and ${}^6\mathrm{Li}$ atoms with a large mass imbalance, where the oscillations of the bosonic and fermionic components are coupled via the Bose-Fermi interaction. In our high-precision measurements, the frequencies of both components are observed to be shifted from the single-species ones, and exhibit unusual features. The frequency shifts of the ${}^{41}\mathrm{K}$ component are upward (downward) in the radial (axial) direction, whereas the ${}^6\mathrm{Li}$ component has downshifted frequencies in both directions. Most strikingly, as the interaction strength is varied, the frequency shifts display a resonantlike behavior in both directions, for both species, and around a similar location at the BCS side of the fermionic superfluid. These rich phenomena challenge the theoretical understanding of superfluids.

We investigate the critical Berezinskii-Kosterlitz-Thouless (BKT) behavior of disordered two-dimensional Josephson-junction arrays (JJA) on the insulating side of the superconductor-insulator transition (SIT) taking into account the effect of hitherto ignored residual random dipole moments of the superconducting grains. We show that for weak Josephson coupling the model is equivalent to a Coulomb gas subjected to a disorder potential with logarithmic correlations. We demonstrate that strong enough disorder transforms the BKT divergence of the correlation length, ${\xi }_{\mathrm{BKT}}\propto exp(\mathrm{const}/\sqrt{T-T_{\mathrm{BKT}}})$, characterizing the average distance between the unbound topological excitations of the opposite signs, into a more singular Vogel-Fulcher-Tamman (VFT) behavior, ${\xi }_{\mathrm{VFT}}\propto exp[\mathrm{const}/(T-T_{\mathrm{VFT}})]$, which is viewed as a hallmark of glass transitions in glass-forming materials. We further show that the VFT criticality is a precursor of the transition into a nonergodic superinsulating state, while the BKT critical behavior implies freezing into an ergodic confined BKT state. Our finding sheds light on the yet unresolved problem of the origin of the VFT criticality.

We report a high-pressure study on the heavily electron doped $\mathrm{L}{\mathrm{i}}_{0.36}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_y\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2$ single crystal by using a cubic anvil cell apparatus. The superconducting transition temperature $T_c\approx 44\mathrm{K}$ at ambient pressure is first suppressed to below 20 K upon increasing pressure to $P_c\approx 2\mathrm{GPa}$, above which the pressure dependence of $T_c\left(P\right)$ reverses and $T_c$ increases steadily to ca. 55 K at 11 GPa. These results thus evidence a pressure-induced second high-$T_c$ superconducting (SC-II) phase in $\mathrm{L}{\mathrm{i}}_{0.36}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_y\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2$ with the highest $T_c^{\max }\approx 55\mathrm{K}$ among the FeSe-based bulk materials. Hall data confirms that in the emergent SC-II phase the dominant electron-type carrier density undergoes a fourfold enhancement and tracks the same trend as $T_c\left(P\right)$. Interestingly, we find a nearly parallel scaling behavior between $T_c$ and the inverse Hall coefficient for the SC-II phases of both $\mathrm{L}{\mathrm{i}}_{0.36}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_y\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2$ and (Li,Fe)OHFeSe. The present study demonstrates that high pressure offers a distinctive means to further raise the maximum $T_c$ of heavily electron doped FeSe-based materials by increasing the effective charge-carrier concentration via a possible Fermi-surface reconstruction at $P_c$.

Coordination-related, two-dimensional (2D) structural phase transitions are a fascinating facet of two-dimensional materials with structural degeneracies. Nevertheless, a unified theoretical account of these transitions remains absent, and the following points are established through ab initio molecular dynamics and 2D discrete clock models here: Group-IV monochalcogenide (GeSe, SnSe, SnTe,...) monolayers have four degenerate structural ground states, and a phase transition from a threefold coordinated onto a fivefold coordinated structure takes place at finite temperature. On unstrained samples, this phase transition requires lattice parameters to evolve freely. A fundamental energy scale $J$ permits understanding this transition, and numerical results indicate a transition temperature $T_c$ of about $1.41J$. Numerical data provides a relation among the experimental (rhombic) parameter $\langle \mathrm{\Delta }\alpha \rangle$ [Chang et al., Science 353, 274 (2016)] and $T$ of the form $\langle \mathrm{\Delta }\alpha \rangle =\mathrm{\Delta }\alpha (T=0){\left(1-T/T_c\right)}^{\beta }$, with a critical exponent $\beta \simeq 1/3$ that coincides with experiment. It is also shown that $\langle \mathrm{\Delta }\alpha \rangle$ is temperature independent in another theoretical work [Fei et al., Phys. Rev. Lett. 117, 097601 (2016)], and thus incompatible with experiment. $T_c$ and the orientation of the in-plane intrinsic electric dipole can be controlled by moderate uniaxial tensile strain, and a modified discrete clock model describes the transition on strained samples qualitatively. An analysis of out-of-plane fluctuations and a discussion of the need for van der Waals corrections to describe these materials are given too. These results provide an experimentally compatible framework to understand structural phase transitions in 2D materials and their effects on material properties.

Direct measurements of the spin glass correlation function $G\left(R\right)$ for Gaussian and bimodal Ising spin glasses in dimension two have been carried out in the temperature region $T\sim 1$. In the Gaussian case the data are consistent with the known anomalous dimension value $\eta \equiv 0$. For the bimodal spin glass in this temperature region $T>T^{*}\left(L\right)$, well above the crossover $T^{*}\left(L\right)$ to the ground-state-dominated regime, the effective exponent $\eta$ is clearly nonzero and the data are consistent with the estimate $\eta \sim 0.28\left(4\right)$ given by McMillan in 1983 from similar measurements. Measurements of the temperature dependence of the Binder cumulant $U_4(T,L)$ and the normalized correlation length $\xi (T,L)/L$ for the two models confirms the conclusion that the two-dimensional (2D) bimodal model has a nonzero effective $\eta$ both below and above $T^{*}\left(L\right)$. The 2D bimodal and Gaussian interaction distribution Ising spin glasses are not in the same universality class.

We have studied the free energy migration barriers $\mathrm{\Delta }{F}^{‡}$ for oxygen diffusion in pure ceria and Sm-doped ceria for the temperatures 300, 700, and 1000 K. We used the density functional theory in the generalized gradient approximation and an additional Hubbard $U$ parameter for the Ce $4f$ electronic states. We compare the results for the free energy deduced from three different methods. First, a static harmonic approach is applied in which the temperature dependent vibrational contributions to energy and entropy are deduced from the phonon frequencies of supercells with a fixed volume. Second, a static quasiharmonic approach is used in which a part of the anharmonicity effect is introduced via an implicit dependence of the harmonic frequencies on the thermally expanding cell volume. Third, the free energy barriers are calculated using metadynamics and molecular dynamics in which anharmonicity effects are naturally taken into account. The three methods examined in this study lead to distinctly different results. According to the harmonic approximation, the migration free energy difference $\mathrm{\Delta }{F}^{‡}$ increases with increasing temperature due to an increasing entropic contribution. According to the quasiharmonic approximation, the migration free energy is independent of temperature. Finally, molecular dynamics predicts a thermally induced increase in the migration free energy. We conclude that temperature dependent experimental lattice constants cancel out the increasing entropic contribution with increasing temperature in the static quasiharmonic approach. The full consideration of anharmonicity effects in the metadynamics method again leads to a temperature dependent migration free energy.

We consider a one-dimensional photonic crystal composed of alternating layers of isotropic and anisotropic dielectric materials. Such a system has different band structures for different polarizations of light. We demonstrate that if a defect layer is inserted into the structure, the crystal can support a polarized optical defect mode that is a bound state in the continuum (BIC). In the case of an anisotropic defect layer, by tilting the principle dielectric axes of the defect layer relative to those of the photonic crystal the BIC is transformed to a quasi-BIC. We derive an analytical expression for the decay rate of the resonance. An experimental setup with a liquid crystal defect layer is proposed to tune the $Q$ factor of the quasi-BIC through applying an external electric field. We speculate that the setup provides a simple and robust platform for observing optical bound states in the continuum in the form of resonances with tunable $Q$ factor.

The magnetic properties of ${\mathrm{CoCl}}_2·2{\mathrm{H}}_2\mathrm{O}$ are analyzed in the mean-field/random-phase approximation using a basis of clusters with four spins along the $c$-axis chains of Co ions. The model gives a unifying account of the bulk properties, the spin waves, and the higher-order cluster-spin excitations. The theory describes accurately the neutron scattering measurements of the excited doublet of the $S=3/2{\mathrm{Co}}^{++}$ ions in both the antiferromagnetic and the paramagnetic phases. The theory has been applied by Larsen et al. [Phys. Rev. B 96, 174424 (2017)] for analyzing the quantum phase transition at a transverse field of 160 kOe and is found to agree closely with their observations.

The Bloch-Bloembergen equation is extended to study the magnetization precession in a spin-valve structure by taking into account the short-wavelength magnon excitation and spin-transfer torque. A stable magnetization precession is achieved when the transverse relaxation time $T_2$ is longer than the longitudinal relaxation time $T_1$. A wide current-density window of stable magnetization precession is found for large $T_2$ and it broadens with increasing $T_2$ for fixed $T_1$. Our simulation results also reveal that short-wavelength magnon excitation can reduce the amplitude of the magnetization precession, which is different from the prediction of the macro-spin Landau-Lifshitz-Gilbert-Slonczewski equation.

We present a study of nonlinear NMR and Bose-Einstein condensation (BEC) of nuclear spin waves in antiferromagnetic ${\mathrm{MnCO}}_3$ with coupled electron and nuclear spins. In particular, we show that the observed behavior of NMR signals strongly contradicts the conventional description of paramagnetic ensembles of noninteracting spins based on the phenomenological Bloch equations. We present a theoretical description of the coupled electron-nuclear spin precession, which takes into account an indirect relaxation of nuclear spins via the electron subsystem. We show that the magnitude of the nuclear magnetization is conserved for arbitrary large excitation powers, which is drastically different from the conventional heating scenario derived from the Bloch equations. This provides strong evidence that the coherent precession of macroscopic nuclear magnetization observed experimentally can be identified with the BEC of nuclear spin waves with $\mathbf{k}=0$.

At zero temperature, a two-dimensional lattice of Majorana zero modes on mesoscopic superconducting islands has a topologically ordered toric-code phase. Recently, a Landau field theory has been proposed for the system that captures its different phases and the associated phase transitions. It was shown that with the increase of Josephson tunneling between the islands, a continuous symmetry-breaking three-dimensional (3D)-XY transition gets transformed into a discrete symmetry-breaking 3D-Ising transition through a couple of tricritical points and first-order transitions. Using the proposed field theory, we analyze the charge response of the system at the different continuous phase transitions. We calculate the universal conductivity at the 3D-XY transitions and the change in the superconducting density at the Ising transition using 1/$N$ expansion. Furthermore, by computing a one-loop correction to the field theory, we show that an additional tricritical point is likely to be present in the phase diagram. Finally, we provide a mean-field calculation that supports the earlier proposed field theory.