We report a peculiar tunneling phenomenon that occurs in lattices with nonreciprocal couplings. The nonreciprocity holds for an inner portion of the lattice, constituting a non-Hermitian interface between outer Hermitian sections. The couplings are mirrored about the interface center. As a standalone system that was widely studied in recent years, each section of the interface supports the non-Hermitian skin effect, in which modes are accumulated at one boundary. Here, we investigate what happens to a wave that propagates along the lattice and hits the interface. The skin mode accumulation, which effectively constitutes a barrier, forbids wave penetration into the interface, but surprisingly, under certain conditions, the wave is transmitted to the other side, keeping the interface dark as if the wave invisibly tunneled through it. Remarkably, the tunneling is independent of the interface length, and a perfect transmission can be achieved independently of frequency and nonreciprocity strength. We derive the phenomenon both for quantum and classical systems, and realize it experimentally in an active topoelectric metamaterial. Our study fosters the research of wave tunneling through other types of non-Hermitian interfaces, which may also include nonlinearities, time dependence, and more.

Sixfold-degenerate valleys in Si have attracted considerable attention for valleytronics application. Using optical pump-terahertz (THz) probe spectroscopy, we study the dynamics of valley polarization in bulk Si(001) at room temperature. Linearly polarized pump pulses excite electrons and holes with asymmetric distributions in momentum space, leading to in-plane anisotropic conductivity. By varying the polarization directions of the pump light relative to the in-plane crystalline axes, the valley polarization of electrons and the momentum asymmetry of holes are separately probed by THz pulses through light-induced linear birefringence and dichroism. We demonstrate that the valley relaxation time of electrons near the conduction band minimum exceeds 1.5 ps at room temperature, in good agreement with theoretically calculated intervalley phonon scattering with $f$ process. This work paves the way for Si-based room-temperature valleytronics.

The transport properties (electrical resistivity, thermal conductivity, and viscosity) of iron-hydrogen alloys are of great significance in the stability and evolution of planetary magnetic fields. Here, we investigate the thermal transport properties of iron doped with varying hydrogen content as functions of pressure ($P$) and temperature ($T$) for the top and bottom of Earth's outer core and beyond, corresponding to pressures of about 130 to 380 GPa and temperatures of 4000 to 7000 K. Using first-principles density functional theory molecular dynamic simulations (FPMD), we verify that crystalline ${\mathrm{FeH}}_x$ is superionic with H diffusing freely. We find a low-frequency viscosity of 10–11 $\text{mPa}\text{s}$ for liquid Fe-H alloys at Earth's outer core conditions. We find resistivity saturation with increasing temperatures in liquid Fe-H alloy at core pressures. The effect of H on electrical and thermal transport we find is small, so that the exact H content of the core is not needed. The primary effect of H is on the equation of state, decreasing the density at constant $P$ and $T$. We find the Lorenz number is smaller than the ideal value, and obtain for $X$(H)= 0.20 or 0.45 wt % H, thermal conductivity $\kappa$ of $\sim 105$ and $\sim 190{\text{Wm}}^{-1}{\text{K}}^{-1}$ at conditions near the core-mantle and inner-outer core boundary, respectively.

Point defects, commonly introduced by growth procedures or intentionally by doping, can strongly suppress the thermal conductivity κ of AlN, a key property that affects the performance and lifetime of its application in electronics and optoelectronics. However, the influence of point defects on the κ of AlN has not been rigorously evaluated. Particularly, point defects often exist in varying charge states, posing more challenges in understanding the scattering behavior of phonons by point defects. Here, we have comparatively investigated the effect of neutral and charged point defects on the κ of wurtzite AlN using an ab initio Green's function approach combined with the linearized Peierls-Boltzmann transport equation. Specifically, we consider typical point defects including oxygen, carbon, and silicon substitutions as well as aluminum and nitrogen vacancies. We find that the neutral defects result in stronger phonon scattering and lower κ than the charged ones, and the charge states more strongly affect the strength of phonon scattering by substitutions than by vacancies. Particularly, compared with the charged ${\mathrm{C}}_{\mathrm{N}}^{1-}$, the neutral ${\mathrm{C}}_{\mathrm{N}}^0$ induces orders-of-magnitude-larger phonon scattering featured by a strong phonon resonance ∼4.5 THz, which originates from the remarkably large perturbations of interatomic force constants caused by the asymmetric lattice distortion of the nearest neighbors around the defect site. As a result, ${\mathrm{C}}_{\mathrm{N}}^0$ results in strongly suppressed κ comparable with the aluminum vacancy case. These results highlight the strong effect of the charge states of point defects on the κ and phonon scattering landscape in AlN, which deepens the understanding of phonon transport in defective materials and will be helpful for the manipulation of thermal properties through defect engineering in relevant applications.

Stable structures of atomic phases of hydrogen are investigated up to about 9 TPa by using ab initio calculations, where the zero-point energy of protons is estimated with the use of the self-consistent harmonic approximation (SCHA). In addition to the structures so far proposed, random structure searches find a relatively isotropic structure of $Cmcm$ symmetry with two atoms in the primitive cell, which is called $Cmcm\text{−}2$ here. The analysis with the SCHA predicts the low-enthalpy structures as follows: Cs-IV $-\left(1.4\mathrm{TPa}\right)\to C{222}_1 -\left(3.7\mathrm{TPa}\right)\to I\overline{4}3d$. However, above $\sim 4$ TPa, several isotropic structures turn out to have quite close enthalpies owing to anharmonicity, which are the $I\overline{4}3d$, hcp, $Cmcm\text{−}2$, and fcc structures. It may also be possible that the bcc structure is added to this group above 5.8 TPa, for the bcc structure is stabilized dynamically above 5.8 TPa in the SCHA, while imaginary-frequency phonons exist up to about 233 TPa in the harmonic approximation.

Disordered lattice spin systems are crucial in both theoretical and applied physics. However, understanding their properties poses significant challenges to Monte Carlo simulations. In this work, we investigate the two-dimensional random-bond Ising model using the recently proposed tensor network Monte Carlo (TNMC) method. This method generates biased samples from conditional probabilities computed via tensor network contractions and corrects the bias using the Metropolis scheme. Consequently, the proposals provided by tensor networks function as block updates for Monte Carlo simulations. Through extensive numerical experiments, we demonstrate that TNMC simulations can be performed on lattices as large as $1024\times 1024$ spins with moderate computational resources, a substantial increase from the previous maximum size of $64\times 64$ in Markov-chain Monte Carlo. Notably, we observe an almost complete absence of critical slowing down, enabling the efficient collection of unbiased samples and averaging over a large number of random realizations of bond disorders. We successfully pinpoint the multicritical point along the Nishimori line with significant precision and accurately determined the bulk and surface critical exponents. Our findings suggest that TNMC is a highly efficient algorithm for exploring disordered and frustrated systems in two dimensions.

The recently discovered van der Waals material CeSiI exhibits both heavy fermion behavior and spiral order with strong magnetic anisotropy which makes it a potential host for topological spin textures such as skyrmions through electrical gating. A monolayer of CeSiI consists of two layers of Ce atoms on triangular lattices that sandwich a silicene layer. Motivated by the experiments, we explore the magnetic phase diagram in van der Waals heavy fermion materials as a function of anisotropy and applied magnetic field using an effective spin model. We demonstrate that the application of an external electric field can tune the Kondo coupling on each Ce layer differently, in turn allowing for controlling the intra- and interlayer magnetic couplings. Our analysis indicates that this fine tuning leads to the coexistence of different magnetic orders in a single monolayer. In particular, we show that a vortex phase can be stabilized only in the presence of an external electric field. Our results highlight the unique advantages and the tunability of van der Waals heavy fermion materials for the manipulation of chiral magnetic phases.

In this work, we studied the magnon-magnon coupling between ferromagnetic resonances in simple $\mathrm{Ni}/\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ (CFB) bilayers. The coupling efficiency can be enhanced to 0.214 by increasing the interlayer exchange through precise adjustment of the ferromagnetic (FM) layer thickness, enabling the system to reach the ultrastrong coupling (USC) regime. Moreover, the insertion of different thicknesses of a nonmagnetic Ta layer in the bilayer with the strongest coupling, the USC can also be modulated by changing interlayer exchange interaction through varying the thickness of the Ta interlayer in the Ni/Ta/CFB trilayers. Our work provides a path to achieve the ultrastrong magnon-magnon coupling in a very simple FM bilayer and exhibits its potential applications in magnonic devices.

Controlling the layer number of superconducting (SC) units can effectively modulate superconductivity of layered materials. Here, through first-principles calculations, the electronic and SC properties of beryllenes have been systematically investigated. The SC transition temperature $T_c$ shows nonmonotonic behavior, with maximum of 8.5 K for AB-stacking and 6.4 K for ABC-stacking trilayer beryllenes. Notably, these $T_c$ are two orders of magnitude higher than 0.026 K of bulk beryllium attributed to the enhanced electron-phonon coupling from phonon softening of surface Be and the increased density of states at the Fermi level. Additionally, several beryllenes exhibit nontrivial band topology, providing platforms for studying topological superconductivity. Moreover, all the beryllenes show the electride nature due to interstitial anionic electrons. These results highlight the importance of reduced dimensionality on physical properties and offer the first example of a single-element material with trilayer SC units achieving the highest $T_c$.

Using a state-of-the-art numerical scheme, we show that the Higgs mode under excitation exhibits chirped oscillations and exponential decay when fluctuations are included. This is in stark contrast to conventional BCS collisionless dynamics which predict power-law decay and the absence of chirping. The chirped amplitude mode enables us to determine the local modification of the effective potential even when the system is in a long-lived prethermal state. We then show that this chirped amplitude mode is an experimentally observable quantity since the photoinduced (super)current in pump-probe experiments serves as an efficient proxy for the order parameter dynamics, including the chirped dynamics. Our result is based on the attractive Hubbard model using dynamical mean-field theory within the symmetry-broken state after a excitation across the superconducting gap. Since the collective response involves long timescales, we extend the hierarchical low-rank compression method for nonequilibrium Green's functions to symmetry-broken states and show that it serves as an efficient representation despite long-lived memory kernels.

Classical percolation models predict the metal-insulator transition and the onset of the long-range ferromagnetic order at the same topological continuity threshold. We tested this prediction in thin films of ferromagnetic CoPd and found a dramatic difference between the conductance and magnetic thresholds. While the long-range ferromagnetic phase develops at or very close to the continuity threshold, the transition from the metal-like to insulator-like conductance develops in films several times thinner. We argue that atomically narrow low resistance gaps intersecting the fractal network of metallic clusters provide a consistent explanation of the effect. We identify the conduction threshold as the point in discontinuous films at which the resistance of intergranular junctions exceeds the quantum resistance mark.

A detailed study of the structural and magnetic properties of the spin-$\frac{3}{2}$ hyperkagome lattice compound ${\mathrm{Li}}_2{\mathrm{MgMn}}_3{\mathrm{O}}_8$ is reported. This material shows ferromagnetic response below $T_{\mathrm{C}}\simeq 20.6$ K, the temperature almost three times lower than the Curie-Weiss temperature ${\theta }_{\mathrm{CW}}\simeq 56.6$ K. Density-functional band-structure calculations suggest that this reduction in $T_{\mathrm{C}}$ may be caused by long-range antiferromagnetic couplings that frustrate nearest-neighbor ferromagnetic couplings on the hyperkagome lattice. Large magnetocaloric effect is observed around the $T_{\mathrm{C}}$ with a maximum value of isothermal entropy change $\mathrm{\Delta }{S}_{\mathrm{m}}\simeq 20$ J/kg K and a maximum relative cooling power of $RCP\simeq 840$ J/kg for the 7 T magnetic field change. This can be attributed to the presence of three ${\mathrm{Mn}}^{4+}$ ($S=\frac{3}{2}$) sites per formula unit as well as the frustrated nature of the spin system. The DC and AC magnetization measurements further reveal reentrant cluster-glass behavior below $T_{\mathrm{C}}$. A detailed study of the nonequilibrium dynamics via magnetic relaxation and memory effect measurements demonstrates that the system evolves through a large number of intermediate metastable states and manifests significant memory effect in the cluster-glass state.

In Weyl semimetals, the nonlinear planar Hall effect (NPHE) and spin-orbit torque (SOT) are prominent manifestations of nonlinear and linear charge-spin conversion, respectively. However, simultaneous investigations of these phenomena within a single material system are scarce, limiting our understanding of their intrinsic connection and underlying mechanisms. Here, we report the simultaneous observation of NPHE and SOT in a $\mathrm{TaIrT}{\mathrm{e}}_4/\mathrm{Py}$ heterostructure. By employing harmonic Hall measurements and developing a magnetic field-dependent method, we successfully separated the contributions from NPHE, fieldlike SOT, and dampinglike SOT, enabling accurate characterization of both linear and nonlinear charge-spin conversion properties. Our experiments revealed significant anisotropy along the [100] and [010] crystallographic directions of $\mathrm{TaIrT}{\mathrm{e}}_4$, with stronger nonlinear responses and fieldlike SOT along the [100] direction, and larger dampinglike SOT along the [010] direction. The distinct directional dependence of these phenomena provides insights into the interplay between surface and bulk contributions to charge-spin conversion in Weyl semimetals. These findings enhance our understanding of anisotropic charge-spin conversion mechanisms in Weyl semimetals, which may inform future research and development of spintronic devices based on topological materials.

The structural and electrical properties of hafnium oxide $\left({\mathrm{HfO}}_2\right)$ have attracted significant interest due to its ferroelectric compatibility with existing silicon technologies and its scalability. Pressure has shown excellent capability in modulating the crystal structure of various materials. However, earlier works have presented conflicting results regarding the structural phase transitions in ${\mathrm{HfO}}_2$ under high pressure when different experimental methods are employed. For instance, Raman spectroscopy suggested a phase transition from the monoclinic to the orthorhombic phase at approximately 2.7–4.3 GPa, while X-ray diffraction observed no structural phase transition until 10 GPa. To clarify the crystal structure of ${\mathrm{HfO}}_2$ under high pressure, this study conducted high-pressure charge transport investigations on both pure ${\mathrm{HfO}}_2$ and 5% yttrium (Y)-doped ${\mathrm{HfO}}_2$ polycrystals. The results reveal abrupt changes in the charge transport of ${\mathrm{HfO}}_2$ at 3.5 ± 0.5 GPa and 15.2 ± 0.6 GPa, respectively. Combined analysis using high-pressure Raman spectroscopy and first-principles calculations indicates that these two transitions correspond to structural phase transitions from the monoclinic phase to the orthorhombic-I (OI) phase, and then to the orthorhombic-II (OII) phase. The doping of Y in ${\mathrm{HfO}}_2$ leads to a reduction in the phase transition pressure. Moreover, in situ ferroelectric hysteresis loop measurements under pressure suggest that the two high-pressure orthorhombic phases exhibit paraelectric properties. Our findings resolve the debate regarding pressure-induced structural phase transitions in the low-pressure region and provide deep insights into the relationship between the structure and charge transport properties of ${\mathrm{HfO}}_2$.

We investigate the electronic structure of the kagome lattice model with first-, second-, and two kinds of third-nearest-neighbor hoppings. We reveal that by tuning the third-nearest-neighbor hoppings, not only single flat band but also degenerate flat band can be created on the $\mathrm{\Gamma }$-M line. We provide the detailed conditions to realize them. The coexistence of these bands can be almost realized near the fundamental band gap in graphene with triangular defects in a superhoneycomb arrangement. Furthermore, due to these flat bands, several sharp peaks appear in the optical conductivity. Our results strongly indicate that long-range electron hopping has a new possibility for designing electronic structures.

Manganese oxides or manganites, widely known for their colossal magnetoresistance (CMR) effect, are systems in which spin, charge, lattice, and orbital interactions are simultaneously active, resulting in a complex phase diagram. Considerable research done mostly at low doping suggested that the CMR effect is the manifestation of the interplay of these interactions that occur in the phase-separated state composed of coexisting clusters of ferromagnetic and paramagnetic and antiferromagnetic phases. In order to get deeper insight in the physics at high doping, we have undertaken magnetoresistance and magnetization measurements of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ thin-film samples with $x=0.5$, 0.52, 0.58, 0.62, 0.66, and 0.75, in the temperature range 1.5–300 K and in magnetic fields up to 16 T. We have observed similar characteristics of the phase diagram as for low doping: the CMR effect, ferromagnetism, and even metallic behavior for certain temperature, magnetic field, and doping values, demonstrating that the phase separation into ferromagnetic and antiferromagnetic clusters is here also in play. We find that the charge transport in the charge ordered phase takes place via spin-dependent variable-range hopping (VRH), which points towards Anderson-type localization of conducting electrons in a spin-disordered background of Mn magnetic moments. We also find a close relationship between the VRH activation energy, the CMR effect, the fraction of ferromagnetic clusters, and the charge ordering temperature, which implies that, aside from the spin disorder, the electron-electron and Jahn-Teller interactions also play an important role in the charge localization.

A recently proposed analytical solution for the equations of motion of the one-body Green's function of the double quantum dot is extended to the out-of-equilibrium situation. By solving a linear system for the density correlators, not only the local occupations but also charge and heat currents as well as transport coefficients and the figure of merit are analytically derived in terms of system parameters and external driving forces. The emerging regions of stable occupation and finite currents are explained in terms of addition and removal energies, corresponding to the poles of the Green's function. The analytical results are validated against the hierarchical equations of motion method, showing excellent agreement.

We present a numerically exact study of the Hubbard model with spin-dependent anisotropic hopping on the square lattice using auxiliary-field quantum Monte Carlo method. At half filling, the system undergoes Ising phase transitions on cooling, leading to the formation of Ising-type antiferromagnetic order for repulsive interactions and charge-density wave order for attractive interactions at finite temperatures. By elegantly implementing the sign-problem-free condition and Hubbard-Stratonovich transformations, we achieve significant improvements in precision control of the numerical calculations and obtain highly accurate results of the transition temperatures from weak to strong interactions across representative anisotropies. We further characterize the system by examining the temperature dependence of various thermodynamic properties, including the energy, double occupancy, specific heat, and charge susceptibility. Specifically, we provide unbiased numerical results of the entropy map on temperature-interaction plane, the critical entropy, and the spin, singlon and doublon correlations, all of which are directly measurable in optical lattice experiments. Away from half filling, we explore the behavior of the sign problem and investigate the possible emergence of stripe spin-density wave order in the system with repulsive interaction.