Superfluid helium-4 (He II) is a widely adopted coolant in scientific and engineering applications owing to its exceptional heat transfer capabilities. However, boiling can spontaneously occur on a heating surface in He II when the heat flux exceeds a threshold value $q^{*}$, referred to as the peak heat flux. While the parameter $q^{*}$ holds paramount importance in the design of He II based cooling systems, extensive research has primarily focused on its behavior in steady homogeneous heat transfer from a flat heating surface. For inhomogeneous heat transfer from curved surfaces, $q^{*}$ exhibits intricate dependence on parameters such as the He II bath temperature $T_b$, the immersion depth $h$, and the curvature radius $R_0$ of the heating surface. A comprehensive understanding on how $q^{*}$ depends on these parameters remains elusive. In this paper, we report our systematic study on $q^{*}$ for steady heat transfer from cylindrical and spherical heaters in He II. We compute $q^{*}$ for a wide range of parameter combinations $(T_b,h,R_0)$ by solving the He II two-fluid equations of motion. The generated data have allowed us to develop a robust correlation that accurately reproduces $q^{*}$ for all the parameter combinations we explored. Our findings, particularly the establishment of the correlation, carry valuable implications for emergent applications that involve steady inhomogeneous heat transfer in He II systems.

Half semiconductors, capable of achieving 100% spin-polarized carriers under simple electrostatic gating, optical excitation, and thermal excitation conditions, have emerged as some of the most promising materials for spintronics. Thus, to find new half-semiconducting materials is highly desirable. Herein, we propose a two-dimensional half-semiconducting material, $\mathrm{Mn}{\left(\mathrm{BiTeS}\right)}_2$, and its various physical properties and potential applications are predicted. The energetic, dynamic, thermal, and mechanic stability is highly identified by variously related calculations, implying that $\mathrm{Mn}{\left(\mathrm{BiTeS}\right)}_2$ possibly exists in experiment. It holds a high magnetic anisotropy energy in the ferromagnetic ground state, with Curie temperature up to 54.3 K. It also possesses outstanding mechanical and optical properties as well as high carrier mobility. Besides, multiple kinds of sub-3 nm functional device prototypes are designed and their excellent transport behaviors are revealed. For example, the $pn$-junction diode exhibits a high spin filtering and strong rectification effect. While rectification for $A$-type pin-junction field-effect transistor is more advantageous, with rectification ratio reaching ${10}^{11}$. Besides, the nin-junction and pip-junction field-effect transistors manifest an intriguing negative differential resistance phenomenon. The pip-junction field-effect transistor can achieves 100% spin filtering, and the pin-junction phototransistor exhibits excellent optical response and can distinguish colors based on the spin-dependent photocurrents.

The magneto-optical property of magnets is used for detecting their magnetization and for information storage applications. The elastic interaction of light with magnetic materials is modulated by the spin-orbital coupling, which is manifested by the rotation of the polarization plane of scattered light with respect to the injected light. The magnetic order also participates in the inelastic Raman scattering of light, as experimentally demonstrated by an $\sim {40}^{\circ }$ rotation of the polarization plane by the two-dimensional ferromagnet ${\mathrm{CrI}}_3$. In this paper, we investigate the magneto-Raman properties of monolayer $\mathrm{Cr}{X}_3$ ($X=\mathrm{Cl}$, Br, and I) using first-principles methods. We calculate the Raman tensor and the Raman intensity due to the $A_{1g}$ phonon by taking the spin-orbit coupling into account. Experimental results on ${\mathrm{CrI}}_3$ are well reproduced. Both the rotation angle of the polarization plane of linearly polarized light and the Raman circular polarization strongly depend on the energy of the injected photons.

We investigate classical Heisenberg models on the distorted windmill lattice and discuss their applicability to the spin-$1/2$ spin liquid candidate $\text{Pb}\text{Cu}{\text{Te}}_2{\text{O}}_6$. We first consider a general Heisenberg model on this lattice with antiferromagnetic interactions $J_n$ ($n=1,2,3,4$) up to fourth neighbors. Setting $J_1=J_2$ (as approximately realized in $\text{Pb}\text{Cu}{\text{Te}}_2{\text{O}}_6$) we map out the classical ground-state phase diagram in the remaining parameter space and identify a competition between $J_3$ and $J_4$ that opens up interesting magnetic scenarios. Particularly, these couplings tune the ground states from coplanar commensurate or non-coplanar incommensurate magnetically ordered states to highly degenerate ground-state manifolds with subextensive or extensive degeneracies. In the latter case, we uncover an unusual classical spin liquid defined on a lattice of corner-sharing octahedra. We then focus on the particular set of interaction parameters $J_n$ that has previously been proposed for $\text{Pb}\text{Cu}{\text{Te}}_2{\text{O}}_6$ and investigate the system's incommensurate magnetic ground-state order and finite-temperature multistage ordering mechanism. We perform extensive finite-temperature simulations of the system's dynamical spin structure factor and compare it with published neutron scattering data for $\text{Pb}\text{Cu}{\text{Te}}_2{\text{O}}_6$ at low temperatures. Our results demonstrate that thermal fluctuations in the classical model can largely explain the signal distribution in the measured spin structure factor but we also identify distinct differences. Our investigations make use of a variety of different analytical and numerical approaches for classical spin systems, such as Luttinger-Tisza, classical Monte Carlo, iterative minimization, and molecular dynamics simulations.

We present experimental and theoretical evidence of novel bound state formation in the low transverse field ordered phase of the quasi-one-dimensional Ising-like material ${\text{CoNb}}_2{\text{O}}_6$. High-resolution single-crystal inelastic neutron scattering measurements observe that small transverse fields lead to a breakup of the spectrum into three parts, each evolving very differently upon increasing field. This can be naturally understood starting from the excitations of the ordered phase of the transverse field Ising model, domain wall quasiparticles (solitons). Here, the transverse field and a staggered off-diagonal exchange create one-soliton hopping terms with opposite signs. We show that this leads to a rich spectrum and a special field, when the strengths of the off-diagonal exchange and transverse field match, at which solitons become localized; the highest field investigated is very close to this special regime. We solve this case analytically and find three two-soliton continua, along with three novel bound states. Perturbing away from this novel localized limit, we find very good qualitative agreement with the experimental data. We also present calculations using exact diagonalization of a recently refined Hamiltonian model for ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ and using diagonalization of the two-soliton subspace, both of which provide a quantitative agreement with the observed spectrum. The theoretical models qualitatively and quantitatively capture a variety of nontrivial features in the observed spectrum, providing insight into the underlying physics of bound state formation.

We discover a variety of exact quantum many-body scars in higher-spin kinetically constrained models through the recently developed DMRG-S algorithm [Zhang et al., Phys. Rev. Lett. 131, 020402 (2023)]. Specifically, for the higher-spin PXP model on arbitrary bipartite lattices of any spatial dimension, we find exact many-body scars that are equidistantly spaced in the energy spectrum and exhibit similar structures to the ground state of the Affleck-Kennedy-Lieb-Tasaki model. For the one-dimensional Fermi-Hubbard model with a tilted potential in a certain parameter regime, whose effective model is equivalent to a kinetically constrained spin model with four degrees of freedom on each site, we find several many-body scars at energy $E=0$ and $E=\pm \sqrt{2}$ that can be exactly represented as matrix product states with finite bond dimensions. Our results demonstrate that larger local degrees of freedom in the kinetically constrained models provide a much broader space for the emergence of quantum many-body scars and weak ergodicity breaking.

We introduce a class of models, dubbed paired twist-defect networks, that generalize the structure of Kitaev's honeycomb model for which there is a direct equivalence between: (i) Floquet codes (FCs), (ii) adiabatic loops of gapped Hamiltonians, and (iii) unitary loops or Floquet-enriched topological orders (FETs) many-body localized phases. This formalism allows one to apply well-characterized topological index theorems for FETs to understand the dynamics of FCs, and to rapidly assess the code properties of many FC models. As an application, we show that the honeycomb Floquet code of Haah and Hastings is governed by an irrational value of the chiral Floquet index, which implies a topological obstruction to forming a simple, logical boundary with the same periodicity as the bulk measurement schedule. In addition, we construct generalizations of the honeycomb Floquet code exhibiting arbitrary anyon-automorphism dynamics for general types of Abelian topological order.

Higher-order topological insulators, which go beyond the conventional bulk-boundary correspondence, have been attracting extensive interest in past years. Very recently, it was pointed out that chiral-symmetric higher-order topological insulators can be characterized by a $\mathbb{Z}$ topological invariant (dubbed the multipole chiral number), which dictates the number of degenerate zero-energy states per corner. Here, we report an acoustic realization of the higher-order topological insulators protected by multipole chiral numbers. To do this, strong long-range couplings, which are not available in solid systems, are carefully designed in our acoustic system. Our acoustic measurements demonstrate unique spatial characteristics for such higher-order topological insulators protected by multipole chiral numbers. This study may enable possibilities for controlling sound, such as acoustic sensing and energy trapping.

We reported the quasiparticle relaxation dynamics of an optimally doped triclinic iron-based superconductor ${\left({\mathrm{Ca}}_{0.85}{\mathrm{La}}_{0.15}\right)}_{10}\left({\mathrm{Pt}}_3{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$ with bulk $T_c=30\mathrm{K}$ using polarized ultrafast optical pump-probe spectroscopy. Our results reveal anisotropic transient reflectivity induced by nematic fluctuations develops below $T_{\text{nem}}\approx 120$ K and persists in the superconducting states. Measurements under high pump fluence reveal three distinct, coherent phonon modes at frequencies of 1.6, 3.5, and 4.7 THz, corresponding to $A_{1g}\left(1\right), E_g$, and $A_{1g}\left(2\right)$ modes, respectively. The high-frequency $A_{1g}\left(2\right)$ mode corresponds to the $c$-axis polarized vibrations of FeAs planes with a nominal electron-phonon coupling constant ${\lambda }_{A_{1g}\left(2\right)}=0.139\pm 0.02$. Our results suggest that, at low temperatures, superconducting and nematic states coexist but compete with each other, and it is possible that $A_{1g}$ phonons are implicated in the formation of Cooper pairs in ${\left({\mathrm{Ca}}_{0.85}{\mathrm{La}}_{0.15}\right)}_{10}\left({\mathrm{Pt}}_3{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_2{\mathrm{As}}_2\right)}_5$.

Quantum dots are frequently used as charge-sensitive devices in low-temperature experiments to probe electric charge in mesoscopic conductors where the current running through the quantum dot is modulated by the nearby charge environment. Recent experiments have operated these detectors using reflectometry measurements up to gigahertz frequencies rather than probing the low-frequency current through the dot. In this work, we use an on-chip coplanar waveguide resonator to measure the source-drain transport response of two quantum dots at a frequency of 6 GHz, further increasing the bandwidth limit for charge detection. Similar to that in the low-frequency domain, the response is here predominantly dissipative. For large tunnel coupling, the response is still governed by the low-frequency conductance, in line with Landauer-Büttiker theory. For smaller couplings, our devices showcase two regimes where the high-frequency response deviates from the low-frequency limit and Landauer-Büttiker theory: When the photon energy exceeds the quantum dot resonance linewidth, degeneracy-dependent plateaus emerge. These are reproduced by sequential tunneling calculations. In the other case with large asymmetry in the tunnel couplings, the high-frequency response is two orders of magnitude larger than the low-frequency conductance $G$, favoring the high-frequency readout.

The cubic perovskite ${\text{SrFeO}}_{\text{3}}$ was recently reported to host hedgehog- and skyrmion-lattice phases in a highly symmetric crystal structure which does not support the Dzyaloshinskii-Moriya interactions commonly invoked to explain such magnetic order. Hints of a complex magnetic phase diagram have also recently been found in powder samples of the single-layer Ruddlesden-Popper analog ${\text{Sr}}_{\text{2}}{\text{FeO}}_{\text{4}}$, so a reinvestigation of the bilayer material ${\text{Sr}}_{\text{3}}{\text{Fe}}_{\text{2}}{\text{O}}_{\text{7}}$, believed to be a simple helimagnet, is called for. Our magnetization and dilatometry studies reveal a rich magnetic phase diagram with at least six distinct magnetically ordered phases and strong similarities to that of ${\text{SrFeO}}_{\text{3}}$. In particular, at least one phase is apparently multiple-$\mathbf{q}$, and the $\mathbf{q}\mathrm{s}$ are not observed to vary among the phases. Since ${\text{Sr}}_{\text{3}}{\text{Fe}}_{\text{2}}{\text{O}}_{\text{7}}$ has only two possible orientations for its propagation vector, some of the phases are likely exotic multiple-$\mathbf{q}$ order, and it is possible to fully detwin all phases and more readily access their exotic physics.

We investigate a long-ranged coupled and non-Hermitian two-dimensional array of nanomagnets, fabricated on a thin magnetic substrate and subjected to an in-plane magnetic field. We predict topology-driven edge and corner skin effects of magnetic eigenmodes with the localization position at boundaries precisely characterized by a topological winding tuple $({\mathcal{W}}_1,{\mathcal{W}}_2)$. By varying the direction of the in-plane field, all magnon states pile up either at different edges of the array with $({\mathcal{W}}_1=\pm 1,{\mathcal{W}}_2=0)$ or $({\mathcal{W}}_1=0,{\mathcal{W}}_2=\pm 1)$, or at different corners characterized by $({\mathcal{W}}_1=\pm 1,{\mathcal{W}}_2=\pm 1)$. Exploiting the non-Hermitian topology is potentially helpful for designing useful magnonic metasurfaces in the future.

Yttrium superhydrides have attracted much attention due to their multiple stoichiometries and excellent superconductivity under high pressure. Especially, ${\mathrm{YH}}_9$ have a superconducting critical temperature ($T_c$) of 243 K, which is only second to $\mathrm{La}{\mathrm{H}}_{10}$ among all superconductors. It exhibits a positive pressure dependence of $T_c$ below 200 GPa, contrary to the results of theoretical prediction. In order to explore the origin of $T_c$ at low pressure, we extensively investigated the crystal structures of ${\mathrm{YH}}_9$ at different pressure, and found a distorted cage structure with a symmetry of Pnma. This phase has the lowest enthalpy at pressure below 220 GPa, and its x-ray-diffraction patterns is consistent with experimental data. Most importantly, its pressure dependence of $T_c$ is in line with the experimental results indicating that the low $T_c$ at low pressure mainly comes from the low-symmetry Pnma phase. Further calculations show the structural distortion in the Pnma phase strongly affects the lattice vibration and electron-phonon coupling induced a positive pressure dependence of $T_c$. This work uncovers a remarkable correlation between superconductivity and structural distortion, and provides insight into clarifying the dome-shaped superconducting phase diagram ($T_c$-P) in yttrium hydrides.

Topological insulators and superconductors have recently attracted considerable attention, and many different theoretical tools have been used to gain insight into their properties. Here we investigate how perturbations can spread through exemplary one-dimensional topological insulators and superconductors using out-of-time ordered correlators. Out-of-time ordered correlators are often used to consider how information becomes scrambled during quantum dynamics. The wavefront of the out-of-time ordered correlator can be ballistic regardless of the underlying system dynamics, and here we confirm that for topological free fermion systems the wavefront spreads linearly at a characteristic butterfly velocity. We pay special attention to the topologically protected edge states, finding that information can become trapped in the edge states and essentially decoupled from the bulk, surviving for relatively long times. We generalise this to consider several different models with multiple possible edge states coexisting on a single edge.

The recent discovery of strong spin Hall effects (SHEs) in two-dimensional layered topological semimetals has attracted intensive attention due to their exotic electronic properties and potential applications in spintronic devices. In this paper, we systematically study the topological properties and intrinsic SHEs of layered transition metal carbides $M_2\mathrm{C}$ $(M=\mathrm{V},\mathrm{Nb},\mathrm{Ta})$. The results show that $d$ bands crossing near the Fermi level $\left(E_F\right)$ induce multiple nodal lines (NLs) and nodal points (NPs) in bulk and few-layered $M_2\mathrm{C}$, respectively. The inclusion of spin-orbit coupling breaks the degeneracy of NLs and NPs, contributing to large spin Hall conductivity (SHC) up to ∼1100 and $\sim 200\left(\hslash /e\right){\left(\mathrm{\Omega }\mathrm{cm}\right)}^{-1}$ for bulk and monolayer ${\mathrm{Ta}}_2\mathrm{C}$, respectively. Remarkably, we find that the magnitude of SHC exhibits a significant enhancement by increasing the layer thickness. For eight-layer ${\mathrm{Ta}}_2\mathrm{C}$, the maximum value of SHC can reach up to $\sim 600\left(\hslash /e\right){\left(\mathrm{\Omega }\mathrm{cm}\right)}^{-1}$, comparable to many reported three-dimensional topological materials. Analysis of spin Berry curvature reveals that the large SHC originates from layer number dependent nodal-point structure near the $E_F$, around where the repeated crossover between the valence and conduction bands creates large numbers of NPs in the Γ-$K$ and Γ-$M$ routes. Our findings not only provide a platform for experimental research of low-dimensional SHE, but also suggest an effective way of realizing giant SHE by controlling layer thickness.

We studied the topological features in Kondo insulator $\mathrm{Sm}{\mathrm{B}}_6$ by point-contact Andreev reflection spectroscopy with a Nb probe tip. Below the superconducting transition of Nb, the spectra exhibited a narrow dip at around zero bias superposed on a broad asymmetric background caused by the Kondo resonance. The contact-size dependence of the spectra revealed that the width of the resonance is suppressed near the surface. The spin polarization in the surface state was $\sim 0.56$, which is smaller than that in Bi-based topological insulators. These indicate that the Kondo breakdown plays a crucial role for the topological features, not only in the bulk state near the surface but also in the surface state on $\mathrm{Sm}{\mathrm{B}}_6$.

We present analytic expressions for the density of states and its consistent derivation for the two-dimensional Qi-Wu-Zhang (QWZ) Hamiltonian—a generic model for the Chern topological insulators of class A. This density of states is expressed in terms of elliptical integrals. We discuss and plot special cases of the dispersion relations and the corresponding densities of states. Spectral moments are also presented. The exact formulas ought to be useful in determining physical properties of the noninteracting Chern insulators and within the dynamical mean-field theory for interacting fermions with the QWZ Hamiltonian in the noninteracting limit.

We investigate classes of interacting systems that allow for a mapping to disordered noninteracting systems. As we show, such a mapping is possible for interacting systems with a suppressed density of states at the chemical potential, leading to suppressed screening, and systems near BCS-type instabilities. The mapping can also be applied qualitatively to other classes of systems that are approximately dual to each other. The established duality suggests a new approach to analytical and numerical studies of many-body and disorder-driven phenomena in a variety of systems and allows to predict, e.g., new phase transitions dual to the previously known ones. Using the established duality, we predict new disorder-driven transitions in nodal-line semimetals and systems with long-range hopping dual to, respectively, the BCS and BEC-vacuum transitions in interacting systems and new interaction-driven transitions dual to previously known non-Anderson disorder-driven transitions. The established principle can also be used to classify and describe phase transitions in dissipative systems described by non-Hermitian Hamiltonians.

Heusler compounds [XYZ for half-Heusler (HH) and $\mathrm{X}{\mathrm{Y}}_2\mathrm{Z}$ for full-Heusler (FH)] with semiconducting behaviors have been long suggested as promising high-performance thermoelectric (TE) materials. Recently, some off-stoichiometry Heusler compounds (such as the mixture of HH and FH compounds) were predicted as semiconductors as well, which disobey the traditional valence electron count rule and are named Slater-Pauling (S-P) semiconductors. However, the bonding behaviors in these S-P semiconductors and the relationship between the geometry and TE properties are still unclear. Therefore, focusing on the Ti-Fe-Sb and $M$-Co-Sn ($M=\mathrm{Ti}$, Zr, Hf) Heusler systems, we theoretically predict several thermodynamically stable off-stoichiometry Heusler compounds ($\mathrm{Ti}{\mathrm{Fe}}_{1.5}\mathrm{Sb}$ and $M{\mathrm{Co}}_{1.33}\mathrm{Sn}$) and clarify the bonding behaviors of those S-P semiconductors. From the geometry analysis of these compounds, in addition to the HH and FH local geometries, the disorder occupation of Y atoms (Fe or Co) in the lattice also induces the defective-HH and defective-FH substructures, and the stacking style forms second- and third-order Rubik's-cube-like structures. This unique stacking arrangement (or the Y occupations) not only plays an essential role in the electron redistribution in the lattice to form the band gap but also lowers the acoustic Debye temperature and strengthens the anharmonic to suppress the lattice thermal conductivity. Owing to the high power factor and low thermal conductivity, the calculated $zT$ value of $p$-type $\mathrm{Zr}{\mathrm{Co}}_{1.33}\mathrm{Sn}$ can reach 0.54 at 1000 K. This value could be further optimized using defect engineering. Thus, we not only elucidate the underlying mechanism behind the emergence of S-P semiconductors in Heusler systems but also offer valuable insight into experimental exploration and discovery of S-P semiconductors exhibiting superior TE performance.