The compound GdPtBi is known as a material where the nontrivial topology of electronic bands interplays with an antiferromagnetic order, which leads to the emergence of many interesting magnetotransport phenomena. Although the magnetic structure of the compound was previously reliably determined, the magnetic interactions responsible for this type of order have remained controversial. In the present study, we employed time-of-flight inelastic neutron scattering to map out the low-temperature spectrum of spin excitations in single-crystalline GdPtBi. The observed spectra reveal two spectrally sharp dispersive spin-wave modes, which reflects the multidomain state of the $\mathbf{k}=\left(\frac{1}{2}\frac{1}{2}\frac{1}{2}\right)$ fcc antiferromagnet in the absence of a symmetry-breaking magnetic field. The magnon dispersion reaches an energy of $\sim 1.1\mathrm{meV}$ and features a gap of $\sim 0.15\mathrm{meV}$. Using linear spin-wave theory, we determine the main magnetic microscopic parameters of the compound that provide good agreement between the simulated spectra and the experimental data. We show that GdPtBi is well within the $\left(\frac{1}{2}\frac{1}{2}\frac{1}{2}\right)$ phase and is dominated by second-neighbor interactions, thus featuring low frustration.

The breathing modes of a skyrmion, corresponding to coupled oscillations of its size and chirality angle, are studied numerically for a conservative classical-spin system on a $500\times 500$ lattice. The dependence of the oscillation frequency on the magnetic field is computed for a model with Dzyaloshinskii-Moriya interaction. In accordance with previous works, it is linear at small fields, reaches maximum on increasing the field, then sharply tends to zero as the field approaches the threshold above which the skyrmion loses stability and collapses. Physically transparent analytical model is developed that explains the results qualitatively and provides the field dependence of the oscillation frequency that is close to the one computed numerically. Dissipation of a breathing motion in which the skyrmion chirality angle $\gamma$ is rotating in one direction depends on the initial amplitude. Below a certain threshold the mode is stable, while above that threshold it becomes strongly damped by the reservoir of spin waves and quickly ends with the skyrmion collapse. To the contrary, smaller-amplitude breathing motion in which $\gamma$ oscillates is undamped in the absence of other interactions. Adding perpendicular anisotropy and removing the applied field makes the breathing mode of any amplitude very slow and undamped.

Magnon-photon coupling inside a cavity has been experimentally realized and has attracted significant attention for its potential docking with quantum information science. Whether this coupling implies the steady entanglement of photons and magnons is crucial for its usage in quantum information but is still an open question. Here we study the entanglement properties among magnons and photons in an antiferromagnet-light system and find that the entanglement between a magnon and a photon is nearly zero, while the magnon-magnon entanglement is very strong and can be even further enhanced through the coupling with the cavity photons. The maximum enhancement occurs when the antiferromagnet is resonant with the light. The essential physics can be well understood within the picture of cavity-induced cooling of the magnon-magnon state near its joint vacuum with stronger entanglement. Our findings can be used to cool magnetic magnons toward their ground state and may also be significant to extend the cavity spintronics to quantum manipulation. Furthermore, the hybrid antiferromagnet-light system provides a natural platform to manipulate the deep strong correlations of continuous modes with a generic stable condition and easy tunability.

We illustrate an approach based on itinerant magnetism for finding ferromagnetic semiconductors that can be gate tuned to study quantum critical points. We show that $p$-type ${\mathrm{FeAs}}_2$ is an example. The complex nonparabolic band structure of this material leads to a ferromagnetic instability when doped, while at the same time allowing for a modest transport effective mass. This leads to an analogy between magnetic semiconductors and thermoelectric materials.

Iron hydrides attract significant interest as candidates for the main constituents of the Earth's core in geophysics and planetary science. However, their basic physical properties are still not well known. Here, we combined high-pressure transport, synchrotron radiation Mössbauer, and Fe $K\beta$ x-ray emission spectroscopy measurements on ${\varepsilon }^{\prime }$-FeH to map out a detailed magnetic phase diagram of this hydride phase of iron. In contrast to our original expectations, we found two magnetic phase transitions at high pressure due to two inequivalent iron sites existing in the ${\varepsilon }^{\prime }$-FeH structure. Our results account for the previous large pressure difference on the loss of ferromagnetism between experiment and theoretical calculations. The discovery of an unexpected complex magnetic phase diagram in ${\varepsilon }^{\prime }$-FeH has implications for a better understanding of the magnetic and physical properties of the iron-hydrogen compounds, important for the conditions of planetary interiors.

We observe that space- and time-crystallization effects in multicomponent superfluids—while having the same physical origin and mathematical description as in the single-component case—are conceptually much more straightforward. Specifically, the values of the temporal and spatial periods are absolute rather than relative, and the broken translation symmetry in space and/or time can be revealed with experiments involving only one equilibrium sample. We discuss two realistic setups—one with cold atoms and another one with bilayer superconductors—for the observation of space and time crystallization in two-component counterflow superfluids.

An inverted hysteresis loop (IHL) exhibits negative coercivity and remanence, and has been ascribed to various mechanisms ranging from exchange coupling to competing anisotropies and even experimental artifacts. Here, we investigate the IHL behavior of monolithic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ (LSMO) film using magnetization and ferromagnetic resonance (FMR) studies at 300 K. FMR measurements reveal the presence of fourfold ($K_4=5.7\times {10}^2\mathrm{erg}{\mathrm{cm}}^{-3}$), uniaxial ($K_u=1.9\times {10}^2\mathrm{erg}{\mathrm{cm}}^{-3}$), and exchange ($K_{A/F}=-1.15\times {10}^3\mathrm{erg}{\mathrm{cm}}^{-3}$) anisotropy. It is known that the competition of fourfold and uniaxial anisotropy can lead to an IHL. Furthermore, magnetic measurements exhibit positive exchange bias (EB), which could also lead to an IHL. The appearance of positive EB in monolithic LSMO film indicates the presence of secondary phases and an antiferromagnetic (AFM) coupling between the phases. We show unambiguously that the observed IHL is a result of interfacial exchange coupling in the LSMO film. Using FMR, we verify the AFM coupling at the interface and, in conjunction with the results of an earlier report, we develop an intuitive picture for the spin arrangement at the interface. Our study reveals a complex interface physics in LSMO/${\mathrm{SrTiO}}_3$ (STO), whose understanding may generate new pathways for the development of novel functionalities in LSMO.

Understanding the dynamical properties of magnetic skyrmions is of prime importance for realization of skyrmion based devices. Here we report on the dynamics of a confined magnetic skyrmion under application of circular magnetic fields (CMFs). We provide the complex map of the dynamical states and demonstrate that under some conditions CMFs can change the skyrmion number and lead to the switching of the core polarity. Combing the results of micromagnetic simulations with those of analytical calculations, based on the extended Thiele equation, we unravel the physical mechanism behind this dynamical switching. The core switching is associated with the dynamical motions of the skyrmion driven by a CMF. The formation of domain walls and their movement from the border towards the center leads to shrinking of the skyrmion and the creation of a skyrmion with opposite core polarity. The results provide a guideline for checking the skyrmion's stability as well as an efficient way for switching the core polarity using CMFs.

Recent experiments have established previously predicted ${\mathrm{LaH}}_{10}$ as the highest-temperature superconductor, with $T_C$ up to 250–260 K [Drozdov et al., Nature (London) 569, 528 (2019); Somayazulu et al., Phys. Rev. Lett. 122, 027001 (2019)]. In this work we explore the high-pressure phase stability and superconductivity of lanthanum hydrides ${\mathrm{LaH}}_m$. We predict the stability of the hitherto unreported polyhydride $P6/mmm-{\mathrm{LaH}}_{16}$ at pressures above 150 GPa; at 200 GPa, its predicted superconducting $T_C$ is 156 K, the critical field ${\mu }_0{H}_C\left(0\right)$ is approximately 35 T, and the superconducting gap is up to 35 meV. We revisit the superconductivity of ${\mathrm{LaH}}_{10}$ and find its $T_C$ to be up to 259 K at 170 GPa from solving the Eliashberg equation and 271 K from solving the gap equation within the superconducting density functional theory, which also allows us to compute the Coulomb pseudopotential ${\mu }^{*}$ for ${\mathrm{LaH}}_{10}$ and ${\mathrm{LaH}}_{16}$.

In two recent papers [Phys. Rev. B 99, 214438 (2019); Phys. Rev. B 96, 104405 (2017)], Johnston employs his unified molecular field theory to study the phase diagram of a classical one-dimensional magnet with exchange frustration in dependence of the anisotropy and magnetic field. Here, it is argued that the assumptions made about the stable configurations are too restrictive and not well justified.

The Comment criticizes the assumptions of the model I used. I was aware of the limitations of my model. However, my model is useful for fitting high-field magnetization versus transverse-field data for real helical Heisenberg antiferromagnets at temperatures much lower than their Néel temperatures and estimating their critical fields. To my knowledge, there existed no prior theory that could fit such experimental data over the entire field range of experiments. I welcome that the Comment calls for more accurate theory.

Application of pressure is known to be an effective tool for tuning structural and electronic properties of transition metal dichalcogenides. In this work we present evolution of PdPS with pressure using Raman spectroscopy and synchrotron x-ray diffraction up to 26 GPa, complemented with first-principles theoretical analysis of PdPS under pressure up to 36 GPa. Raman spectra reveal changes in the pressure derivatives of Raman frequencies at $P\sim 2$, 11, and 21 GPa, suggesting three isostructural electronic phase transitions in PdPS. The pressure-dependent x-ray diffraction shows a sudden rise in the bulk modulus from $90\pm 3$ to $123\pm 7$ GPa occurring at $P\sim 11$ GPa. Using first-principles density functional theory calculations, we demonstrate that the low-pressure phase transitions are associated with changes in direct band gap at $\mathrm{\Gamma }$ point while the high-pressure (at $P=21$ GPa) transition is associated with semiconductor to semimetal transition. From analysis of PdPS at higher pressures, we predict a structural phase transition in PdPS at $P\sim 32$ GPa from orthorhombic to monoclinic structure.

We have studied the strongly interacting, two-valley two-dimensional (2D) electron system in ultrahigh mobility SiGe/Si/SiGe quantum wells in parallel magnetic fields strong enough to completely polarize the electron spins thus making the electron system “spinless.” It occurs that the metallic temperature dependence of the resistivity, although weaker than that in the absence of magnetic field, still remains strong even when the spin degree of freedom is removed. Several independent methods have been used to establish the existence of the genuine MIT in the spinless two-valley 2D system. This is in contrast to the previous results obtained on more disordered silicon samples, where the polarizing magnetic field causes a complete quench of the metallic temperature behavior.

We present a theoretical study of compact magnetic skyrmions in ferromagnetic films with perpendicular magnetic anisotropy that accounts for the full stray field energy in the thin film and low interfacial Dzyaloshinskii-Moriya interaction (DMI) regime. In this regime, the skyrmion profile is close to a Belavin-Polyakov profile, which yields analytical expressions for the equilibrium skyrmion radius and energy. The obtained formulas provide a clear identification of DMI and long-range dipolar interaction as two physical mechanisms determining skyrmion size and stability.

Unlike the edge state of a topological insulator where its energy level lives in the bulk energy gap, the edge state of a topological semimetal hides in the bulk spectrum and is difficult to be identified by the energy. We investigate the sensitivity of bulk and edge states of the gapless phase for a topological semimetal to the disordered perturbation via a concrete two-dimensional chiral symmetric lattice model. The topological gapless phase is characterized by two opposite vortices in the momentum space and nonzero winding numbers, which correspond to the edge flatband when the open boundary condition is applied. For this system, numerical results reveal that a distinguishing feature is that the robustness of the edge states against weak disorder and the flatband edge modes remain locked at zero energy in the presence of weak chiral-symmetry-preserving disorder. We employ the Loschmidt echo (LE) for both bulk and edge states to study the dynamic effect of disordered perturbation. We show that, for an initial bulk state, the LE decays exponentially, whereas it converges to a constant for an initial edge state in the presence of weak disorder. Furthermore, the convergent LE can be utilized to identify the positions of vortices as well as the phase diagram. We discuss the realization of such dynamic investigations in a topological photonic system.

We investigate the magnetic structure and the electric properties of $\mathrm{YC}{\mathrm{r}}_{0.5}\mathrm{F}{\mathrm{e}}_{0.5}{\mathrm{O}}_3$, which was earlier reported to feature ferroelectricity from nonequivalent spins due to the cation disorder. We find a centrosymmetric magnetic order at all temperatures below $T_N$, which is incompatible with magnetically induced ferroelectricity. Our dielectric and pyroelectric studies also do not support intrinsic ferroelectricity in this system.

Superconductivity in two dimensions is nontrivial. One way to achieve global superconductivity is via the Berezinskii-Kosterlitz-Thouless (BKT) transition due to proliferation of vortex-antivortex pairs. This transition is expected to have a clear signature on the specific heat. The singularity at the transition temperature $T_{\mathrm{BKT}}$ is predicted to be immeasurable, and a broad nonuniversal peak is expected at $T>T_{\mathrm{BKT}}$. Up to date, this has not been observed in two-dimensional superconductors; this work is then dedicated to investigate $c_p$ signatures in the limit of ultrathin 2d superconductors. We use a unique highly sensitive technique to measure the specific heat of quench condensed ultrathin Pb films. We find that thick films exhibit a specific heat jump at $T_C$ that is consistent with BCS theory. As the film thickness is reduced below the superconducting coherence length and the systems enter the 2D limit, the specific heat reveals BKT-like behavior in what can appear as to be a continuous BCS-BKT crossover as a function of film thickness. However, a number of problems arise with this interpretation. We discuss the experimental results and the possible significance of various scenarios involving BKT physics.

In the superconducting regime of ${\mathrm{FeTe}}_{(1-x)}{\mathrm{Se}}_x$, there exist two types of vortices which are distinguished by the presence or absence of zero-energy states in their core. To understand their origin, we examine the interplay of Zeeman coupling and superconducting pairings in three-dimensional metals with band inversion. Weak Zeeman fields are found to suppress intraorbital spin-singlet pairing, known to localize the states at the ends of the vortices on the surface. On the other hand, an orbital-triplet pairing is shown to be stable against Zeeman interactions, but leads to delocalized zero-energy Majorana modes which extend through the vortex. In contrast, the finite-energy vortex modes remain localized at the vortex ends even when the pairing is of orbital-triplet form. Phenomenologically, this manifests as an observed disappearance of zero-bias peaks within the cores of topological vortices upon an increase of the applied magnetic field. The presence of magnetic impurities in ${\mathrm{FeTe}}_{(1-x)}{\mathrm{Se}}_x$, which are attracted to the vortices, would lead to such Zeeman-induced delocalization of Majorana modes in a fraction of vortices that capture a large enough number of magnetic impurities. Our results provide an explanation for the dichotomy between topological and nontopological vortices recently observed in ${\mathrm{FeTe}}_{(1-x)}{\mathrm{Se}}_x$.

We present ab initio calculations of electron capture and release coefficients as a function of temperature for Tl dopants in the widely used scintillator NaI, which is a soft ionic crystal. The modeled capture and release events occur by transitions mediated by multiple phonon absorption and emission between states with significantly different local geometries around the trapping site. We demonstrate that such transitions are not well-described by the normal harmonic approximation to the nuclear dynamics. We go beyond the harmonic approximation by numerically solving the vibrational Schrödinger equation for the motion along/of a single effective phonon coordinate. The localized trapped state is not correctly described by semilocal density functionals, so we employ a global hybrid functional tuned to reproduce the band gap and lattice constant of the host material. Our calculations, which combine plane-wave and embedded cluster electronic structure methods are in reasonable agreement with available experimental data for detrapping and predict an unusual temperature dependence for trapping rates.