We report the properties of a triangular lattice iron-chalcogenide antiferromagnet ${\mathrm{FeAl}}_2{\mathrm{Se}}_4$.The spin susceptibility reveals a significant antiferromagnetic interaction with a Curie-Weiss temperature ${\mathrm{\Theta }}_{\text{CW}}\simeq -200\text{K}$ and a spin-2 local moment. Despite a large spin and a large $|{\mathrm{\Theta }}_{\text{CW}}|$, the low-temperature behaviors are incompatible with conventional classical magnets. No long-range order is detected down to 0.4 K. Similar to the well-known spin-1 magnet ${\mathrm{NiGa}}_2{\mathrm{S}}_4$, the specific heat of ${\mathrm{FeAl}}_2{\mathrm{Se}}_4$ exhibits a double-peak structure and a $T^2$ power law at low temperatures, which are attributed to the underlying quadrupolar spin correlations and the Halperin-Saslow modes, respectively. The spin freezing occurs at $\sim 14\mathrm{K}$, below which the relaxation dynamics is probed by the ac susceptibility. Our results are consistent with the early theory for the spin-1 system with Heisenberg and biquadratic spin interactions. We argue that the early proposal of the quadrupolar correlation and gauge glass dynamics may be well extended to ${\mathrm{FeAl}}_2{\mathrm{Se}}_4$. Our results provide useful insights about the magnetic properties of frustrated quantum magnets with high spins.

We study the effect of a parallel magnetic field in a thin and small superconductor. The field suppresses superconductivity through Zeeman coupling while stabilizing the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state at high fields before superconductivity is destroyed. When the spatial period of FFLO state is comparable to the size of the superconductor, there is a strong commensuration effect, which modifies the superconducting phase diagram. We investigate the FFLO state and the phase diagram in the presence of strong commensuration effect both for the $s$- and $d$-wave superconductors using the Bogoliubov–de Gennes equation, Green function approach, and Ginzburg-Landau theory. We found that the superconducting phase diagram is strongly modulated. Interestingly, there is reentrance of superconductivity upon increasing the magnetic field. The commensuration effect of the FFLO state can be used to detect the FFLO state in experiments.

Superfluid ${}^3\mathrm{He}$ has a rich spectrum of collective modes with both massive and massless excitations. The masses of these modes can be precisely measured using acoustic spectroscopy and fit to theoretical models. Prior comparisons of the experimental results with theory did not include strong-coupling effects beyond the weak-coupling-plus BCS model, so-called nontrivial strong-coupling corrections. In this paper we utilize recent strong-coupling calculations to determine the Higgs masses and find consistency between experiments that relate them to a subdominant $f$-wave pairing strength.

We present a unified analysis of the drag forces acting on oscillating bodies submerged in superfluid helium such as a vibrating wire resonator, tuning forks, a double-paddle oscillator, and a torsionally oscillating disk. We find that for high-Stokes-number oscillatory flows, the drag force originating from the normal component of superfluid helium exhibits a clearly defined universal scaling. Following classical fluid dynamics, we derive the universal scaling law and define relevant dimensionless parameters such as the Donnelly number. We verify this scaling experimentally using all of our oscillators in superfluid ${}^4\mathrm{He}$ and validate the results by direct comparison with classical fluids. We use this approach to illustrate the transition from laminar to turbulent drag regime in superfluid oscillatory flows and compare the critical velocities associated to the production of quantized vortices in the superfluid component with the critical velocities for the classical instabilities occurring in the normal component. We show that depending on the temperature and geometry of the flow, either type of instability may occur first and we demonstrate their crossover due to the temperature dependence of the viscosity of the normal fluid. Our results have direct bearing on present investigations of superfluids using nanomechanical devices [Bradley et al., Sci. Rep. 7, 4876 (2017)].

We investigate charge densities around the vortex cores of an $s$-wave and a chiral $p$-wave superconductor (SC) in two dimensions within quasiclassical theory. We consider contributions of particle-hole asymmetry through gradient expansions in first order of a quasiclassical parameter using augmented quasiclassical theory. The chiral $p$-wave SC has two inequivalent vortices: One is charged and the other is uncharged on the basis of Bogoliubov-de Gennes (BdG) equation. We explain this qualitative difference in vortex charges also by the augmented quasiclassical theory. In addition, we find that much larger charge is induced for the charged vortex of the chiral $p$-wave SC compared with the vortex of the $s$-wave SC in the clean system. We also confirm this enhancement through quantitative comparison with results based on the BdG theory. We also study effects of Born-type impurities using self energy formalism and find that the larger charge density is rather suppressed in contrast to the $s$-wave's one. Furthermore we study the local density of states as another equilibrium property of particle-hole asymmetry.

We investigated the superconducting fluctuation in FeSe, which is assumed to be located in the BCS-BEC crossover region, via magnetic torque measurements. In our method, the absolute cantilever displacement is measured by detecting the interference intensity of the Fabry-Pérot cavity formed between the cantilever and optical fiber. Our findings are different from the results of previous torque magnetometry studies using a piezoresistive cantilever; the “giant” fluctuation diamagnetism related to the BCS-BEC crossover does not exist. Instead, a considerably smaller fluctuation signal originating from the vortex liquid was observed that showed qualitatively similar behavior to that in cuprate superconductors. We also discuss the inconsistency between our torque data and the existence of a pseudogap proposed by an NMR experiment.

The properties of stress-free and biaxially strained stoichiometric $\mathrm{SrTi}{\mathrm{O}}_3$ in the absence and presence of antiferrodistortive (AFD) distortion were calculated ab initio. To obtain reliable results, multiple exchange correlation (XC) functionals, including the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, were used. HSE was the primary XC functional, while another functional provided a good lower bound of ferroelectricity (FE). The reliability of the calculations was further reinforced by the calculations of the strain and AFD dependence. In contrast with previous works, we show that the ferroelectric phase ($C_{4\mathrm{v}}^1$, $C_{4\mathrm{v}}^{10}$) is more stable in the absence of quantum and thermal fluctuations than the paraelectric phase ($O_{\mathrm{h}}^1$, $D_{4\mathrm{h}}^{18}$), even for the stress-free case, and clarify the properties of these FE phases. The energy gain of stress-free FE, in comparison with the thermal and quantum fluctuation energy, indicates that a paraelectric phase emerges at room temperature by thermal fluctuations, but is near 0 K marginally close to the FE phase, which aligns with the experimental incipient FE. This implies that the paraelectricity of stress-free $\mathrm{SrTi}{\mathrm{O}}_3$ in experiments contains incoherent atomic-scale FE regions. These results are consistent with the FE microregions (FMR), signatures of polar disorders, and the emergence of FE in $\mathrm{SrTi}{\mathrm{O}}_3$ due to impurities and defects. The value of spontaneous polarization $P_{\mathrm{S}}$ could reach $10\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ in the absence of fluctuations, even for the stress-free case. In view of the earlier theory of the carrier layer at polar discontinuities, the present results may explain the conduction at the interfaces of the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$. In addition, an “enhancement of FE due to symmetry constriction” is proposed as an additional mechanism to the strain-enhanced FE in the epitaxial effect. For large compressive strain, e.g., 2%, the Perdew-Burke-Ernzerhof density functional for solids (PBEsol) yielded properties with $P_{\mathrm{S}}>20\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$, agreeing with HSE, and therefore is usable as a practical substitute of HSE for $\mathrm{SrTi}{\mathrm{O}}_3$.

Density functional alloy theory is used to accurately describe the three core effects controlling the thermodynamics of random Cu-Au alloys. These three core effects are exchange correlation (XC), local lattice relaxations (LLRs), and short-range order (SRO). Within the real-space grid-based projector augmented-wave (GPAW) method based on density functional theory (DFT), we adopt the quasinonuniform XC approximation (QNA), and take into account the LLR and the SRO effects. Our approach allows us to study the importance of all three core effects in a unified way within one DFT code. The results demonstrate the importance of the LLR term and show that going from the classical gradient level approximations to QNA leads to accurate formation energies at various degrees of ordering. The order-disorder transition temperatures for the 25%, 50%, and 75% alloys reach quantitative agreement with the experimental values only when also the SRO effects are considered.

We numerically investigate the stationary and nonequilibrium critical dynamics in three-dimensional isotropic Heisenberg antiferromagnets. Since the nonconserved staggered magnetization couples dynamically to the conserved magnetization density, we employ a hybrid simulation algorithm that combines reversible spin precession with relaxational Kawasaki spin-exchange processes. We measure the dynamic critical exponent and identify a suitable intermediate time window to obtain the aging scaling exponents. Our results support an earlier renormalization-group prediction: While the critical aging collapse exponent assumes a universal value, the associated temporal decay exponent in the two-time spin autocorrelation function depends on the initial distribution of the conserved fields; here, specifically on the width of the initial spin orientation distribution.

We have studied the impact of lattice geometry on the dynamic properties of close-spaced arrays of circular nanomagnets, also known as magnonic crystals. To this end, we prepared 2D nanomagnet arrays with both square and hexagonal lattice symmetries (300-nm disk diameter, 400-nm center-to-center distance) and performed broadband ferromagnetic resonance (FMR) measurements. Micromagnetic simulations were used to interpret distinct features of the measured resonance spectra. The FMR bias field was applied along two distinct principal directions for each lattice, and a sample with well-separated, decoupled disks was measured for reference. We found that the interdisk dipolar coupling has a strong impact on the FMR for these 2D magnonic crystals. Distinctly different oscillation modes were found for the individual nanomagnets, dependent on lattice symmetry and direction of the bias field. Moreover, we find that spectral peak splitting from excitation of edge and center modes, as well as the damping, depends on the lattice symmetry and the orientation of the bias field. These findings demonstrate that lattice geometry has a strong influence on the excited spin-wave spectrum and is a relevant design parameter for spintronic devices.

We present a detailed investigation of the physical properties of ${\mathrm{Ce}}_2\mathrm{Sb}$ and ${\mathrm{Ce}}_2\mathrm{Bi}$ single crystals, which undergo antiferromagnetic transitions at around 8.2 and 10 K, respectively. When magnetic fields are applied parallel to the $c$ axis, metamagnetic transitions are observed at low temperatures, corresponding to a magnetic field induced phase transition. It is found that the field induced transition changes from second order at higher temperatures, to first order at low temperatures, suggesting the existence of tricritical points (TCPs) in both compounds. Since replacing Bi with Sb suppresses the TCP to lower temperatures and corresponds to a positive chemical pressure, these results suggest that applying pressure to ${\mathrm{Ce}}_2\mathrm{Sb}$ may suppress the TCP to lower temperatures, potentially to zero temperature at a quantum tricritical point.

Sharp magnetization switching and large magnetoresistance (MR) were previously discovered in single crystals of 2H-${\mathrm{Fe}}_x{\mathrm{TaS}}_2$ and attributed to the Fe superstructure and its defects. We report similar sharp switching and large MR in 1T-${\mathrm{Fe}}_x{\mathrm{TiS}}_2$ ($0.086\le x\le 0.703$) while providing a side-by-side comparison of the only two such ferromagnetic transition-metal dichalcogenides. The switching field $H_s$ and MR values are similar in both 1T-${\mathrm{Fe}}_x{\mathrm{TiS}}_2$ and 2H-${\mathrm{Fe}}_x{\mathrm{TaS}}_2$, with a larger than expected bowtie $\rho \left(H\right)$ and a sharper hysteresis loop for $H\parallel c$ in the former. The Curie and Weiss temperatures remain roughly constant below $x\sim 1/3$ in the $T=\text{Ti}$ single crystals before increasing monotonically for higher $x$, while $H_s$ and MR reach maxima where defects in the superstructure exist, or a minimum near superstructure compositions, and they remain constant above $x\sim 0.4$. Despite previous reports, electron diffraction shows only the $\sqrt{3}\times \sqrt{3}$ superstructure in 1T-${\mathrm{Fe}}_x{\mathrm{TiS}}_2$. Glassy behavior is shown to coexist within the ferromagnetic state in 1T-${\mathrm{Fe}}_x{\mathrm{TiS}}_2$ for compositions between 0.1 and 0.703. A simple theoretical model considering first-, second- and third-neighbor interactions yields a phase diagram that accounts for both spin glass behavior and different superstructures.

We present a theoretical study of excitations formed by hybridization between magnons and phonons—magnon-polarons—in antiferromagnets. We first outline a general approach to determining which magnon and phonon modes can and cannot hybridize in a system thereby addressing the qualitative questions concerning magnon-polaron formation. As a specific and experimentally relevant case, we study nickel oxide quantitatively and find perfect agreement with the qualitative analysis, thereby highlighting the strength of the former. We find that there are two distinct features of antiferromagnetic magnon-polarons which differ from the ferromagnetic ones. First, hybridization between magnons and the longitudinal phonon modes is expected in many cubic antiferromagnetic structures. Second, we find that the very existence of certain hybridizations can be controlled via an external magnetic field, an effect which comes in addition to the ability to move the magnon modes relative to the phonons modes.

We report all phases and corresponding critical lines of the quantum anisotropic transverse XY model with uniform and alternating transverse magnetic fields (ATXY) in the presence of the Dzyaloshinskii-Moriya (DM) interaction by using appropriately chosen order parameters. We prove that when DM interaction is weaker than the anisotropy parameter, it has no effect at all on the zero-temperature states of the XY model with uniform transverse magnetic field (UXY), which is not the case for the ATXY model. However, when DM interaction is stronger than the anisotropy parameter, we show the appearance of a new gapless chiral phase—in the XY model with uniform as well as alternating field. We further observe that first derivatives of nearest neighbor two-site entanglement with respect to magnetic fields can detect all the critical lines present in the system. We also find that the factorization surface at zero temperature present in this model without DM interaction becomes a volume on the introduction of the latter. Moreover, DM interaction turns out to be good to generate bipartite entanglement sustainable at large times, leading to a proof of ergodic nature of bipartite entanglement in this system.

We probe the local magnetic properties of interfaces between the insulating ferromagnet EuS and the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ using low energy muon spin rotation ($\mathrm{LE}-\mu \mathrm{SR}$). We compare these to the interface between EuS and the topologically trivial metal, titanium. Below the magnetic transition of EuS, we detect strong local magnetic fields which extend several nm into the adjacent layer and cause a complete depolarization of the muons. However, in both ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and titanium we measure similar local magnetic fields, implying that their origin is mostly independent of the topological properties of the interface electronic states. In addition, we use resonant soft x-ray angle resolved photoemission spectroscopy (SX-ARPES) to probe the electronic band structure at the interface between EuS and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. By tuning the photon energy to the Eu antiresonance at the Eu $M_5$ pre-edge we are able to detect the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ conduction band, through a protective ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping layer and the EuS layer. Moreover, we observe a signature of an interface-induced modification of the buried ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ wave functions and/or the presence of interface states.

The electrical detection of spin-torque ferromagnetic resonance (st-FMR) is becoming a popular method for measuring the spin-Hall angle of heavy metals (HM). However, various sensible analysis on the same material with either the same or different experimental setups yielded different spin-Hall angles with large discrepancy, indicating some missing ingredients in our current understanding of st-FMR. Here we carry out a careful analysis of electrical signals of the st-FMR in a HM/ferromagnet (HM/FM) bilayer with an arbitrary magnetic anisotropy. The FM magnetization is driven by two radio-frequency (rf) forces: the rf Oersted field generated by an applied rf electric current and the so called rf spin-orbit torque from the spin current flowing perpendicularly from the HM to the FM due to the spin-Hall effect. By using the universal form of the dynamic susceptibility matrix of magnetic materials at the st-FMR, the electrical signals, originated from the anisotropic magnetoresistance, anomalous Hall effect, and inverse spin-Hall effect are analyzed and dc-voltage line shapes near the st-FMR are obtained. Angular dependence of dc voltage is given for two setups. A way of experimentally extracting the spin-Hall angle of a HM is proposed.

Compounds of transition metal ions with strong spin-orbit coupling recently attracted attention due to the possibility to host frustrated bond-dependent anisotropic magnetic interactions. In general, such interactions lead to complex phase diagrams that may include exotic phases, e.g., the Kitaev spin liquid. Here we report on our comprehensive analysis of the global phase diagram of the extended Kitaev-Heisenberg model relevant to honeycomb lattice compounds ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and $\alpha -{\mathrm{RuCl}}_3$. We have utilized recently developed method based on spin coherent states that enabled us to resolve arbitrary spin patterns in the cluster ground states obtained by exact diagonalization. Global trends in the phase diagram are understood in combination with the analytical mappings of the Hamiltonian that uncover peculiar links to known models—Heisenberg, Ising, Kitaev, or compass models on the honeycomb lattice—or reveal entire manifolds of exact fluctuation-free ground states. Finally, our study can serve as a methodological example that can be applied to other spin models with complex bond-dependent non-Heisenberg interactions.

Magnetic skyrmions are topologically protected excitations of the magnetization vector field with promising applications in spintronics and spin-caloritronics, particularly due to their high mobility. Skyrmions can be steered by a spin-polarized charge current or by exposure to a magnonic spin current. Here, we propose a further method for driving skyrmions by applying an inhomogeneous electric field and a homogeneous thermal bias. We show that the inhomogeneous electric torque leads to an efficient skyrmionic drag which can be thermally assisted as to enhance the skyrmion velocity. The calculations and analysis are limited to insulating samples; for conducting materials the influence of the inhomogeneous electric field on the charge carriers needs to be taken also into account.

Since Ginzburg and Landau's seminal work in 1950, superconducting states have been classified by the hierarchy of the fundamental length scales of the theory, the magnetic-field penetration lengths and coherence lengths. In the simplest single-component case they form a dimensionless ratio $\kappa$. The model was generalized by Ginzburg for anisotropic materials in 1952. In this paper we expand the above length-scale analysis to anisotropic multicomponent superconductors that can have multiple coherence lengths as well as multiple magnetic-field penetration lengths, leading to unconventional length-scale hierarchies. We demonstrate that the anisotropies in multiband superconductors lead to new regimes with various mixed hierarchies in different directions. For example, a regime is possible, where for a field applied in a certain direction coherence lengths are smaller than the magnetic-field penetration lengths in one of the perpendicular directions, whereas the penetration lengths are larger in the other direction. Focusing on a model of a clean anisotropic multiband $s$-wave supercocoductors we show exampes of a new regime where vortex cores overlap in one direction, resulting in attractive core-core interaction, while in the orthogonal direction the magnetic-field penetration length exceeds the coherence lengths, leading to dominance of repulsive current-current interaction, resulting in an unconventional magnetic response.

On the basis of the microscopic quasiclassical Eilenberger theory, we analyze the recent angle-resolved specific heat experiment carried out at low temperature for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ to identify the superconducting gap symmetry, comprising either horizontal or vertical line nodes relative to the tetragonal crystal symmetry. Several characteristics, in particular, the landscape of the in-plane oscillation amplitude $A_4(B,T)$ with a definite sign for almost the entire $B\text{−}T$ plane are best explained by the horizontal line node symmetry, especially when the multiband and Pauli paramagnetic effects are taken into account. The present analysis of $A_4(B,T)$ with definite sign points to the presence of an anomalous field region at a lower temperature in the experimental data, whose origin is investigated. Our theory demonstrates the application and uniqueness of the field-rotating thermodynamic measurements in uncovering the precise gap structure for target materials.