We present a simple atomistic model for the description of noncollinear coupling in magnetic multilayers with hybrid spacer layers made of Ru alloyed to ferromagnetic atoms such as Fe. In contrast to previous analytical and micromagnetic models that explain the noncollinear coupling by means of lateral fluctuations in the coupling constant, the presented model accounts for atom-atom coupling in all three spatial dimensions within the spacer layer. The model is able to accurately predict the dependence of the macroscopic bilinear and biquadratic coupling constants on the spacer-layer composition and thickness, showing much better quantitative agreement than lateral-fluctuation models. Moreover, it predicts that the noncollinear coupling is virtually independent of the exchange stiffness in the ferromagnetic layers, which goes beyond the predictions of previous models. This prediction is validated by experimental measurements.

Determining the phase diagram of interacting quantum many-body systems is an important task for a wide range of problems such as the understanding and design of quantum materials. For classical equilibrium systems, the Lee-Yang formalism provides a rigorous foundation of phase transitions, and these ideas have also been extended to the quantum realm. Here, we develop a Lee-Yang theory of quantum phase transitions that can include thermal fluctuations caused by a finite temperature, and it thereby provides a link between the classical Lee-Yang formalism and recent theories of phase transitions at zero temperature. Our methodology exploits analytic properties of the moment generating function of the order parameter in systems of finite size, and it can be implemented in combination with tensor-network calculations. Specifically, the onset of a symmetry-broken phase is signaled by the zeros of the moment generating function approaching the origin in the complex plane of a counting field that couples to the order parameter. Moreover, the zeros can be obtained by measuring or calculating the high cumulants of the order parameter. We determine the phase diagram of the two-dimensional quantum Ising model and thereby demonstrate the potential of our method to predict the critical behavior of two-dimensional quantum systems at finite temperatures.

Magnetic symmetry is vital for characterizing the topological protections of magnons. Here, we propose a second-order topological magnon insulator by stacking the honeycomb ferromagnets with antiferromagnetic interlayer coupling. The system exhibits a magnonic three-dimensional $Z_2$ topological phase, broken by an easy-plane anisotropy. Nevertheless, it preserves various magnetic crystalline symmetries, which protect second-order topological corner and hinge modes. First, a magnetic twofold rotational symmetry protects the corner modes in the bilayer and hinge modes along the stacking direction. Second, a mirror symmetry along the stacking direction establishes a $Z_2\times Z$ topology, giving rise to gapped topological surface modes carrying nonzero Chern numbers, which leads to a surface thermal Hall effect of magnons and chiral hinge modes along the horizontal hinges. We further demonstrate that the corner and hinge modes are robust against the XYZ exchange anisotropy and Kitaev interaction.

The reversal of magnetic bubble helicity through topologically trivial transient states provides an additional degree of freedom that promises the development of multidimensional magnetic memories. A key requirement for this concept is the stabilization of bubble states at ambient conditions on application-compatible substrates. In the present work, we demonstrate a stabilization routine for remanent bubble states in high perpendicular magnetic anisotropy ${\text{[Co(0.44 nm)/Pt(0.7 nm)]}}_X,$ $X=48$, 100, and 150 multilayers on $\mathrm{Si}/{\mathrm{SiO}}_2$ substrates by exploring the effect of external magnetic fields $\left(H_{\mathrm{m}}\right)$ of different strength and angles $\left(\theta \right)$ with respect to the film surface normal. By systematic variation of these two parameters, we demonstrate that remanent bubble density and mean bubble diameter can be carefully tuned and optimized for each sample. Our protocol based on magnetometry only reveals the densest remanent bubble states at $H_{\mathrm{m}}=0.87H_{\mathrm{s}}$ ($H_{\mathrm{s}}$ is the magnetic saturation field) and $\theta ={60}^{\circ }$–${75}^{\circ }$ for all $X$ with a maximum of 3700 domains/100 $\mu {\mathrm{m}}^2$ for the $X=48$ sample. The experimental observations are supported by micromagnetic simulations, taking into account the nanoscale lateral grain structure of multilayers synthesized by magnetron sputter deposition, and thus helping to understand the different densities of the bubble states found in these systems.

Superfluid helium-4 (He II) has been widely utilized as a coolant in various scientific and engineering applications due to its superior heat transfer capability. An important parameter required in the design of many He II based cooling systems is the peak heat flux $q^{*}$, which refers to the threshold heat flux above which boiling spontaneously occurs in He II. Past experimental and numerical studies showed that $q^{*}$ increases when the heating time $t_h$ is reduced, which leads to an intuitive expectation that very high $q^{*}$ may be achievable at sufficiently small $t_h$. Knowledge on how $q^{*}$ actually behaves at small $t_h$ is important for applications such as laser ablation in He II. Here we present a numerical study on the evolution of the thermodynamic state of the He II in front of a planar heater by solving the He II two-fluid equations of motion. For an applied heat flux, we determine the heating time beyond which the He II near the heater transits to the vapor phase. As such, a curve correlating $q^{*}$ and $t_h$ can be obtained, which nicely reproduces some relevant experimental data. Surprisingly, we find that there exists a critical peak heat flux $q_c^{*}$, above which boiling occurs nearly instantaneously regardless of $t_h$. We reveal that the boiling in this regime is essentially cavitation caused by the combined effects of the first-sound and the second-sound waves in He II. Based on this physical picture, an analytical model for $q_c^{*}$ is developed, which reproduces the simulated $q_c^{*}$ values at various He II bath temperatures and hydrostatic head pressures. This work represents a major progress in our understanding of transient heat transfer in He II.

Topological phononic insulators are the counterpart of three-dimensional quantum spin Hall insulators in phononic systems and, as such, their topological surfaces are characterized by Dirac cone-shaped gapless edge states arising as a consequence of a bulk-boundary correspondence. We propose a theoretical framework for the possible superconducting phase in these materials, where the attractive interaction between electrons is mediated by topological phonons in nontrivial boundary modes. Within the BCS limit, we develop a self-consistent two-band gap equation, whose solutions show that the superconducting critical temperature has a nonmonotonic behavior with respect to the phononic frequency in the Kramers-like point. This remarkable behavior is produced by a resonance that occurs when electrons and phonons on the topological surfaces have the same energy: this effectively increases the electron-phonon interaction and hence the Cooper pair binding energy, thus establishing an optimal condition for the superconducting phase. With this mechanism, the $T_c$ can be increased by well over a factor 2, and the maximum enhancement occurs in the degenerate phononic flat-band limit.

Motivated by recent progress in two-dimensional (2D) spintronics, we present a monolayer of Eu deposited on $1\mathrm{H}\text{−}\mathrm{W}{\mathrm{Se}}_2$ as a promising platform for engineering spin-orbit effects and Berry curvature. By first-principles calculations based on density functional theory, we show that $\mathrm{Eu}/\mathrm{W}{\mathrm{Se}}_2$ exhibits intriguing properties such as high magnetic anisotropy, valley-dependent polarization of spin and orbital angular momenta, and Rashba textures. These originate from magnetic and spin-orbit proximity effects at the interface and the interplay between localized $4f$ magnetic moments of Eu and mobile charge carriers of Eu and $\mathrm{W}{\mathrm{Se}}_2$. The analysis of the magnetic properties reveal a ferromagnetic configuration with an out-of-plane easy axis of the magnetization, which favor a pronounced anomalous Hall effect in the proposed system. Thus, we promote $4f$ rare-earth metals deposited on top of a transition-metal dichalcogenides as a promising platform for 2D spintronics.

${\mathrm{MnSb}}_2{\mathrm{O}}_6$ is based on the structural chiral $P321$ space group No. 150 where the magnetic ${\mathrm{Mn}}^{2+}$ moments $(S=5/2,$ $L\approx 0)$ order antiferromagnetically at $T_{\mathrm{N}}=12\mathrm{K}$. Unlike the related iron based langasite $\left({\mathrm{Ba}}_3{\mathrm{NbFe}}_3{\mathrm{Si}}_2{\mathrm{O}}_{14}\right)$ where the low-temperature magnetism is based on a proper helix characterized by a time-even pseudoscalar “magnetic” chirality, the ${\mathrm{Mn}}^{2+}$ ions in ${\mathrm{MnSb}}_2{\mathrm{O}}_6$ order with a cycloidal structure at low temperatures, described instead by a time-even vector “magnetic” polarity. A tilted cycloidal structure has been found [M. Kinoshita et al., Phys. Rev. Lett. 117, 047201 (2016)] to facilitate ferroelectric switching under an applied magnetic field. In this work, we apply polarized and unpolarized neutron diffraction analyzing the magnetic and nuclear structures in ${\mathrm{MnSb}}_2{\mathrm{O}}_6$ with the aim of understanding this magnetoelectric coupling. We find no evidence for a helicoidal magnetic structure with one of the spin envelope axes tilted away from the cycloidal $c$ axis. However, on the application of a magnetic field $\parallel \mathit{c}$ the spin rotation plane can be tilted, giving rise to a cycloid—helix admixture that evolves towards a distorted helix (zero cycloidal component) for fields great than $\approx 2$ T. We propose a mechanism for the previously reported ferroelectric switching based on coupled structural and magnetic chiralities requiring only an imbalance of structural chiral domains.

We derive the dynamical magnetic response functions in the Fulde-Ferrell (FF) state of a superconductor with inversion symmetry. The pair momentum $2q$ is obtained by minimization of the condensation energy and the resulting quasiparticle states and spectral functions exhibit the segmentation into paired and unpaired regions due to the finite $q$. The dynamical magnetic susceptibility is then calculated in linear response formalism in the FF state with finite-$q$ condensate resulting from $s$-wave or $d$-wave pairing. We show that quasiparticle excitations inside as well as between paired and unpaired segments contribute to the dynamical response. We discuss its dependence on frequency and momentum transfer which develops a characteristic symmetry breaking parallel to $q$. Furthermore, we investigate the possible influence on Knight shift and in the case of $d$-wave pairing on the spin resonance formation in the FF state.

We study fermions at quantum criticality with extremely retarded interactions of the form $V\left({\omega }_l\right)=(g/|{\omega }_l{\left|\right)}^{\gamma }$, where ${\omega }_l$ is the transferred Matsubara frequency. This system undergoes a normal-superconductor phase transition at a critical temperature $T=T_c$. The order parameter is the frequency-dependent gap function $\mathrm{\Delta }\left({\omega }_n\right)$ as in the Eliashberg theory. In general, the interaction is extremely retarded for $\gamma \gg 1$, except at low temperatures $\gamma >3$ is sufficient. We evaluate the normal state specific heat $T_c$, the jump in the specific heat $\mathrm{\Delta }\left({\omega }_n\right)$ near $T_c$, and the Landau free energy. Our answers are asymptotically exact in the limit $\gamma \to \infty$. At low temperatures, we prove that the global minimum of the free energy is nondegenerate and determine the order parameter, the free energy, and the specific heat. These answers are exact for $T\to 0$ and $\gamma >3$. We also uncover and investigate an instability of the $\gamma$ model: Negative specific heat at $T\to 0$ and just above $T_c$.

We have performed the bulk-sensitive spin-resolved hard x-ray photoelectron spectroscopy (HAXPES) to directly clarify the spin-dependent valence band electronic states and spin polarization of $L{2}_1$-ordered ${\mathrm{Co}}_2\mathrm{MnSi}$ as a predicted half-metal. The spin-resolved valence band HAXPES spectra clearly exhibited the difference in the majority and minority spin states in the bulk region of a 30-nm-thick ${\mathrm{Co}}_2\mathrm{MnSi}\left(001\right)$ thin film buried under a MgO(2 nm) layer at both temperatures of 21 and 300 K. As expected from the half-metallicity in ${\mathrm{Co}}_2\mathrm{MnSi}$, we found that the majority (minority) spin spectrum shows a metallic Fermi edge (a band gap), leading the high spin polarization of ∼90% at around the Fermi level at a temperature of 21 K. The spin-resolved HAXPES experiments revealed that the half-metallicity of ${\mathrm{Co}}_2\mathrm{MnSi}$ in the bulk region is independent on the temperature up to 300 K. The experiments also revealed the slight changes in the majority spin spectral shapes and the shift of the valence band maximum of the minority spin states with temperature. By comparing the experimental results with the first-principles calculations at a finite temperature obtained by the disordered local moment method, it is suggested that the slight changes in the majority and minority spin HAXPES spectra of ${\mathrm{Co}}_2\mathrm{MnSi}$ with temperature is mainly caused by the temperature-dependent Co $3d$ electronic states. It is also suggested that the conduction band minimum of the minority spin states is located sufficiently above the Fermi level at temperature up to 300 K.

The sine-Gordon field theory emerges as the low-energy description in a wealth of quantum many-body systems. Recent efforts have been directed towards realizing quantum simulators of the model, by interfering two weakly coupled one-dimensional cold atomic gases. The weak interactions within the atomic clouds provide a sine-Gordon realization in the semiclassical regime. Furthermore, the intricate microscopic dynamics prevents a quantitative understanding of the effective sine-Gordon validity realm. In this work, we focus on a spin-ladder realization and observe the emergent sine-Gordon dynamics deep in the quantum regime. We use matrix-product state techniques to numerically characterize the low-energy sector of the system and compare it with the exact field-theory predictions. From this comparison, we obtain quantitative boundaries for the validity of the sine-Gordon description. We provide encompassing evidence for the emergent field theory by probing its rich spectrum and by observing the signatures of integrable dynamics in scattering events.

The electromagnetic response of three-dimensional topological insulators can be described by an effective axion action with a quantized axion field. Once both time-reversal and inversion symmetries are broken, for example, in antiferromagnetic topological insulators, the axion field becomes dynamical along with magnetic fluctuations. The dynamical axion field, when coupled to electromagnetic fields, can lead to rich physical phenomena. Here, based on the modified Maxwell's equations, we reveal the existence of an exotic type of polariton excitation, termed a surface axion polariton, which propagates in the interface between a dynamical axion insulator and a dielectric. When doping the dynamical axion insulator to be metallic, the coexistence of axion-photon coupling and plasmon-photon coupling will further generate a mixed surface axion plasmon polariton. We also propose a Kretschmann-Raether configuration to experimentally detect the surface axion polariton. Our result provides an alternative way to study the axion electrodynamics in condensed matter physics.

${\mathrm{As}}_2{\mathrm{Te}}_3$ is a chalcogenide with many polymorphic phases. Of current interest are the $\beta$ and ${\beta }^{\prime }$ phases which have been investigated primarily in the context of their close relationship to the isostructural topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The $\beta$-to-${\beta }^{\prime }$ phase transition of ${\mathrm{As}}_2{\mathrm{Te}}_3$ exhibits a large anomaly in the resistivity that has remained unexplained. In this work we have measured the electronic transport properties of this phase transition in melt-quenched ${\mathrm{As}}_2{\mathrm{Te}}_3$ via dc resistivity and infrared reflectivity measurements. A sigmoidal fit has been used to describe the general behavior of the resistivity curves over a wide temperature range in order to characterize how hysteresis affects the phase transition anomaly. The optical measurements have identified the formation of a concomitant pseudogap of magnitude $\approx 100\mathrm{meV}$ affecting approximately $25\%$ of the carriers. Charge density wave order is compatible with the formation of a partial gap as well as the observation of thermal hysteresis, suggesting it could be responsible for the $\beta \text{−}{\beta }^{\prime }$ transition in melt-quenched ${\mathrm{As}}_2{\mathrm{Te}}_3$.

We explore the electronic and topological properties of single-crystal ${\mathrm{SrAl}}_2{\mathrm{Si}}_2$ using magnetotransport experiments in conjunction with first-principle calculations. We find that the temperature-dependent resistivity shows a pronounced peak $\sim 50$ K. We observe several remarkable features at low temperatures such as large nonsaturating magnetoresistance (MR), Shubnikov–de Haas oscillations and cusplike magnetoconductivity (MC) . The maximum value of MR turns out to be 459% at 2 K and 12 T. The analysis of the cusplike feature in MC indicates a clear signature of weak anti-localization. Our Hall resistivity measurements confirm the presence of two types of charge carriers in ${\mathrm{SrAl}}_2{\mathrm{Si}}_2$ with low carrier density.

We study the $S>1/2$ antiferromagnetic Heisenberg model on the 1/5-depleted square lattice as a function of the ratio of the intraplaquette coupling to the interplaquette coupling. Using stochastic series expansion quantum Monte Carlo simulations, we numerically identify three quantum phases, including the dimer phase, Néel phase, and plaquette valence-bond-solid phase. We also obtain the accurate quantum critical points that belong to the $O$(3) universality class using the large-scale finite-size scaling. Most importantly, we study the dynamic spin structure factors of different phases, which can be measured by inelastic neutron scattering experiments. The low-energy excitations can be explained as triplons in the dimer phase and plaquette valence-bond-solid phase, while in the Néel phase, the more prominent magnon mode can be found as the spin magnitude increases. Furthermore, we find a broader continuum at smaller $S$, which may be the dynamical signature of nearly deconfined spinon excitations.

Topological interpretations of phonons facilitate a platform for unique concepts in phonon physics. Though there are ubiquitous reports on topological electronic excitations, the same for phonons is extremely limited. Here, we propose a candidate material, ${\mathrm{KMgBO}}_3$, which showcases the coexistence of several multifold and multidimensional topological phonon excitations, which are protected by spatial and nonspatial symmetries. This includes zero-dimensional double, triple, and quadratic Weyl phonon nodes, one-dimensional nodal line/loops, and two-dimensional doubly degenerate nodal surface states. The nodal line/loop emerges from the spin-$\frac{1}{2}$ phonon nodes, while the two-dimensional doubly degenerate nodal surface arises from a combination of twofold screw-rotational and time-reversal symmetries. Application of strain breaks the $C_3$ rotational symmetry, which destroys the spin-1 double Weyl nodes but preserves other topological features. Interestingly, strain helps create two extra single Weyl nodes, which in turn preserve the total chirality. Alloying also breaks certain symmetries, destroying most of the topological phonon features in the present case. Thus, ${\mathrm{KMgBO}}_3$ is a promising candidate which hosts various Weyl points, large Fermi arcs with very clean phonon spectra, and tunable topological phonon excitations, and hence is certainly worth future theoretical/experimental investigation of topological phononics.

Tuning the electronic structure of materials, and thus their electronic, transport, and optical properties, is of fundamental importance for materials design and optimization. Although alloying is a well-established method for engineering the band gap of semiconductors, strain engineering has emerged as a promising approach to selective tuning of band-edge states. Using a combined density functional theory and $GW$ approach, we show that the highly directional intralayer and interlayer couplings, together with the unusual stereoelectronic effects of the Sn $5s$ lone pair in α-SnO, may be exploited to tune, in addition to the band gap, the valence and conduction band-edge states selectively using in-plane and/or out-of-plane strains. Whereas the uniaxial strain along the lattice $c$ direction primarily affects the position of the conduction band edge, the valence band edge is very sensitive to the biaxial $ab$ strain. We also establish a strain electronic phase diagram of α-SnO, including the insulator–metal phase transition boundary. It is predicted that a compressive biaxial strain of about 3% or an isotropic pressure of 5 GPa can trigger an insulator–metal transition. The quasiparticle band gap can be widely tuned from 0 to more than 2.0 eV with moderate strains.

A magnetotransport and quantum capacitance of the two-dimensional (2D) electron gas in $\mathrm{HgTe}/{\mathrm{Cd}}_x{\mathrm{Hg}}_{1-x}\mathrm{Te}$ quantum wells of a width $(20.2–46.0)$ nm are experimentally investigated. It is shown that the first energy subband of spatial quantization is split due to the spin-orbit interaction and the split branches are single spin, therewith the splitting strength increases with the increase of the quantum well width. The electron effective masses in the branches are close to each other within the actual density range. Magnetointersubband oscillations (MISO) observed in the structures under study exhibit the growing amplitude with the increasing electron density that contradicts to the expected decrease of wave-function overlap for the rectangular quantum well. To interpret the data obtained, we have used a self-consistent approach to calculate the electron energy spectrum and the wave function within framework of the $kP$ model. It has been, in particular, shown that the MISO amplitude increase results from the increasing overlap of the wave functions due to their shift from the gate electrode with the gate voltage increase known as a phenomenon of the negative electron polarizability. The results obtained from the transport experiments are supported by quantum capacitance measurements.