The study of heat transport in two-dimensional (2D) materials reveals novel behaviors due to quantum confinement effects, where in-plane and out-of-plane phonons play crucial roles. In 2D materials like graphene, it is widely recognized that the out-of-plane vibrational mode is the primary contributor to thermal conductivity owing to the mirror symmetry. Based on this perspective, the introduction of interlayer coupling, which breaks this symmetry, is expected to induce a significant reduction in thermal conductivity within 2D materials. Nevertheless, recent studies have presented unexpected findings, indicating that interlayer coupling can actually increase thermal conductivity of 2D materials. This controversial result suggests a nontrivial underlying mechanism governing the effects of interlayer coupling on thermal conductivity in 2D materials, necessitating further exploration. In our work, we investigate the modulation of thermal conductivity through interlayer coupling in a sandwich structure composed of hexagonal boron nitride (h-BN) and bilayer graphene (BG), specifically a h-BN/BG/h-BN system. Through molecular dynamics simulations, we find that the thermal conductivity from out-of-plane phonons can be significantly reduced, while that from in-plane phonons can be significantly increased, as the interlayer coupling strength increases. This results in a nontrivial, coupling-strength-dependent overall thermal conductivity. The phonon spectrum analysis conducted using our modified package reveals that the upshift and flattening of the out-of-plane (ZA and ZO) phonon modes are mainly responsible for these variations, and the extent of the upshift and flattening is proportional to the strength of interlayer coupling. This work offers new insights into manipulating the thermal conductivity of 2D materials.

As a typical nonlinear magnetoresistance, unidirectional magnetoresistance (UMR) has recently been observed in antiferromagnetic (AFM) systems with canted spin structure. However, a profound understanding of AFM UMR induced by interfacial spin-orbit coupling (SOC) is still lacking to date. In this paper, we choose van der Waals canted antiferromagnetic $\mathrm{CrP}{\mathrm{S}}_4$ due to the relatively low magnetic anisotropy and weak interlayer coupling, which enables a canted-AFM state between two adjacent layers under an in-plane magnetic field, and systematically investigate SOC-induced AFM UMR in $\mathrm{CrP}{\mathrm{S}}_4/\mathrm{Pt}$ heterostructure. We experimentally observe two characteristic UMR tendencies versus magnetic field in $\mathrm{CrP}{\mathrm{S}}_4/\mathrm{Pt}$ devices. The different UMR tendencies may be attributed to the interfacial defect-induced Fermi level change, which is supported by density-functional theory calculations and Boltzmann transport formalism. Our work provides a more profound understanding of the nontrivial AFM UMR and may facilitate the development of two-dimensional AFM spintronic devices.

We report the investigation of electronic collective modes in rare-earth- (RE) based magnets ${\left({\mathrm{Mo}}_{2/3}{\mathrm{RE}}_{1/3}\right)}_2\mathrm{AlC}$ (also known as RE-$i$-MAX phases), where $\mathrm{RE}=\mathrm{Gd}$, Yb, and Dy, using single crystal samples. In particular, a detailed investigation of the Raman spectra of Gd-$i$-MAX samples at low temperatures, with a focus on the phonon behavior in relation to the antiferromagnetic (AFM) phase transition at 26 K is presented. Significant shifts in the central frequencies of several low-frequency phonon modes were observed below 25 K, correlating with the Néel transition. Integrated Raman intensity measurements indicated a reduction in the electronic background below the AFM transition temperature, suggesting the opening of a magnetic gap. Our analysis showed no new phonon modes. Therefore, we do not see any indication of a Brillouin zone folding of phonon mode on cooling. However, the hardening of all phonon modes at low temperatures points to a strong spin-phonon coupling effect. Using a temperature-dependent model of phonon frequency, we determined the spin-phonon coupling constant $\lambda$ to be less than 0.1 ${\mathrm{cm}}^{-1}$ for all frequencies, which is of the same order of magnitude as found in other antiferromagnetic materials such as ${\mathrm{MnF}}_2$ and ${\mathrm{FeF}}_2$ with $T_N=68$ K and $T_N=78$ K, respectively, but significantly lower than that of CuO with $T_N=213$ K.

A quantum system governed by a non-Hermitian Hamiltonian may exhibit zero-temperature phase transitions that are driven by interactions, just as its Hermitian counterpart, raising the fundamental question of how non-Hermiticity affects quantum criticality. In this context, we consider a non-Hermitian system consisting of an $XY$ model with a complex-valued four-state clock interaction that may or may not have parity-time-reversal ($\mathcal{PT}$) symmetry. When the $\mathcal{PT}$ symmetry is broken, and time-evolution becomes nonunitary, a scaling behavior similar to the Berezinskii-Kosterlitz-Thouless phase transition ensues, but in a highly unconventional way, as the line of fixed points is absent. From the analysis of the $d$-dimensional renormalization group equations, we obtain that the unconventional behavior in the $\mathcal{PT}$ broken regime follows from the collision of two fixed points in the $d\to 2$ limit, leading to walking behavior or pseudocriticality. For $d=2+1$ the near critical behavior is characterized by a correlation length exponent $\nu =3/8$, a value smaller than the mean-field one. These results are in sharp contrast with the $\mathcal{PT}$-symmetric case where only one fixed point arises for $2<d<4$ and in $d=1+1$ three lines of fixed points occur with a continuously varying critical exponent $\nu$.

The candidate Kitaev magnet ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ exhibits a characteristic ferrimagnetic response at low temperatures, with a finite residual magnetization that changes sign at a compensation point located at around half the ordering temperature. We argue that the behavior can be naturally understood to arise in this material as a consequence of a noncollinear triple-$\mathbf{q}$ magnetic ground state. Using large-scale classical Monte Carlo simulations, we study the finite-temperature response of the pertinent honeycomb Heisenberg-Kitaev-$\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }$ model in weak training fields. Our model features all symmetry-allowed nearest-neighbor exchange interactions, as well as sublattice-dependent next-nearest-neighbor interactions, consistent with the reported crystal structure of the material. We also consider a six-spin ring exchange perturbation, which allows us to tune between the two different magnetic long-range orders that have been suggested for this material in the literature, namely, a collinear single-$\mathbf{q}$ zigzag state and a noncollinear triple-$\mathbf{q}$ state. We demonstrate that the experimentally observed ferrimagnetic response of ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ can be well described within our modeling if the magnetic ground state features noncollinear triple-$\mathbf{q}$ order. The observation of a compensation point, where the residual magnetization reverses sign, suggests a sublattice $g$-factor anisotropy, with a larger out-of-plane $g$ factor on the sublattice with stronger antiferromagnetic intrasublattice exchange. By contrast, a classical Heisenberg-Kitaev-$\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }$-type model with collinear zigzag ground state is insufficient even in principle to describe the observed behavior. Our results illustrate the unconventional physics of noncollinear magnetic long-range orders hosted by frustrated magnets with bond-dependent interactions.

High-$T_c$ superconductivity (SC) was recently found in the bilayer material ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ (La327) under high pressures. We study the bilayer two-orbital Hubbard model derived from the band structure of the La327. The model is solved by cluster dynamical mean-field theory with natural orbitals renormalization group as impurity solver at zero temperature, considering only normal states. With hole doping, we have observed sequentially the Mott insulator, pseudogap (PG), non-Fermi liquid (NFL), and Fermi liquid phases, with quantum correlations decreasing. The ground state of the La327 is in the NFL phase with Hund spin correlation, which transmits the Ni-$3d_{z^2}$ ($z$) orbital interlayer antiferromagnetic (AFM) correlation to the Ni-$3d_{x^2-y^2}$ orbitals. When the $\sigma$-bonding state of the $z$ orbitals ($z+$) is no longer fully filled, the interlayer AFM correlations weaken rapidly. At low pressures, the fully filled $z+$ band supports strong interlayer AFM correlations, potentially suppressing SC and favoring short-range spin density wave (SDW). Hole doping at low pressures may achieve a similar effect to high pressures, under which the $z+$ band intersects with the Fermi level, and consequently the spin correlations weaken remarkably, potentially suppressing the possible short-range SDW and favoring SC.

We compute the structure of flux $h/\left(2e\right)$ vortices in a $d$-wave superconductor, which emerges from a higher-temperature pseudogap metal. Such a transition is described by a continuum theory of the Higgs condensation of two flavors of charge $e$ bosons, which are fundamentals of an emergent SU(2) gauge field. Period-2 charge order is present near the vortex center. Upon coupling the electrons to the superconducting and charge order parameters, we find that the electronic local density of states does not have a zero-bias peak, in contrast to BCS theory. But there are subgap peaks at positive and negative bias, and these exhibit antiphase periodic spatial modulations, similar to observations in scanning tunneling microscopy experiments in the underdoped cuprates [K. Matsuba et al., J. Phys. Soc. Jpn. 76, 063704 (2007)].

First-principles approaches for phonon-limited electronic transport are typically based on many-body perturbation theory and transport equations. With that, they rely on the validity of the quasiparticle picture for electrons and phonons, which is known to fail in strongly anharmonic systems. In this work, we demonstrate the relevance of effects beyond the quasiparticle picture by combining ab initio molecular dynamics and the Kubo-Greenwood (KG) formalism to establish a nonperturbative, stochastic method to calculate carrier mobilities while accounting for all orders of anharmonic and electron-vibrational couplings. In particular, we propose and exploit several numerical strategies that overcome the notoriously slow convergence of the KG formalism for both electronic and nuclear degrees of freedom in crystalline solids. The capability of this method is demonstrated by calculating the temperature-dependent electron mobility of the strongly anharmonic oxide perovskites ${\mathrm{SrTiO}}_3$ and ${\mathrm{BaTiO}}_3$ across a wide range of temperatures. We show that the temperature dependence of the mobility is largely driven by anharmonic, higher-order coupling effects and rationalize these trends in terms of the nonperturbative electronic spectral functions.

We use density functional theory calculations to characterize the early stages of fluorination of silver's (100) and (110) surfaces. In the Ag(100) surface, the hollow site is the most favorable for F adatoms. In the Ag(110) surface, three adsorption sites, namely hollow, long bridge, and short bridge, exhibit similar energies. These locations are also more favorable than a F adatom occupying a vacancy site irrespectively of whether the vacancy was present in the pristine surface. The computed energy as a function of surface coverage is used to compute the equilibrium thermodynamics phase diagram. We argue that for the typical pressure and temperature of fluorination experiments, the state of the surface is not determined by thermodynamics but by kinetics. Combining these results with scanning tunneling microscopy (STM) topographic simulations we propose assignments to features observed experimentally. We present a minimal model of the apparent topography of adatoms in different locations in terms of hydrogenic orbitals, explaining the observed trends. The model links the STM apparent topography to structural information and the oxidation states of the Ag atoms near the adatom.

We report the successful growth of CeCuSi single crystals using a metallic flux method and the physical properties using structural, magnetic, electrical transport, optical, and heat capacity measurements. CeCuSi crystallizes in a hexagonal-bar shape, and single crystal x-ray diffraction confirms the ZrBeSi-type structure (space group $P{6}_3/mmc$). CeCuSi orders ferromagnetically below $T_{\text{C}}=15.5\mathrm{K}$ with easy magnetization direction within the basal plane. The ${\mathrm{Ce}}^{3+}$ ions are situated within a triangular lattice with a point group of $D_{3d}$. We perform a detailed crystalline electric field (CEF) analysis of the anisotropic magnetic susceptibility, the Schottky anomaly in heat capacity, and the Raman-active excitations. The results indicate a ground-state doublet with magnetic moment primarily in the basal plane, and a ferromagnetic interaction along both directions. The exponential behavior in resistivity and in heat capacity below $T_{\text{C}}$ can also be well explained by the ferromagnetic magnon model. We found that CeCuSi does not exhibit the CEF hard-axis ordering observed in many ferromagnetic Kondo lattice compounds. Our CEF analysis suggests that the exchange interactions along both axes are ferromagnetic, potentially explaining the absence of hard-axis ordering.

Recent experiments on multilayer rhombohedral graphene have unearthed a number of interesting phenomena in the regime where integer and fractional quantum anomalous Hall phenomena were previously reported. Specifically, at low temperature ($T$) and low applied currents, an “extended” integer quantum anomalous Hall (EIQAH) state is seen over a wide range of the phase diagram. As the current is increased, at low $T$, the EIQAH state undergoes a phase transition to a metallic state at generic fillings, and to the fractional quantum anomalous Hall (FQAH) state at the Jain fillings. Increasing temperature at the Jain fillings also leads to an evolution out of the EIQAH state to the Jain state. Here we provide an interpretation of many of these observations. We describe the EIQAH state as a crystalline state (either of holes doped into the $\nu =1$ state or an anomalous Hall crystal of electrons) that breaks moiré translation symmetry. At generic fillings, we show how an electric current-induced depinning transition of the crystalline order leads to peculiar nonlinear current-voltage curves consistent with the experiment. At Jain fillings, we propose that the depinning transition is preempted by an equilibrium transition between EIQAH and Jain FQAH states. This transition occurs due to the large polarizability of the Jain FQAH states, which enables them to effectively lower their energy in an applied electric field compared to the crystal states. We also discuss the finite-temperature evolution in terms of the relative entropies of the crystalline and FQAH states.

The magnetoelectric (ME) effects originating from exotic magnetic states provide opportunities to explore novel topics such as topology and intriguing excitation. Herein, we report the ME effect in a cubic garnet ${\mathrm{Mn}}_3{\mathrm{Al}}_2{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ with comprehensive magnetic and ferroelectric measurements. Our results reveal the dominant ME coefficient α with magnetic field perpendicular to electric field, in which electric polarization along the [110] axis is the strongest. We also observed a transition of coefficient α from negative to positive value upon switching of magnetic field. It is suggested that the ME effect of ${\mathrm{Mn}}_3{\mathrm{Al}}_2{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ originates from the combination of special hyperkagome lattice and 120 ° magnetic configuration, which leads to a $-3^{\prime }$ magnetic point group. The microscopic mechanism of the ME effect is discussed from viewpoints of toroidal moments and inverse Dzyaloshinskii-Moriya interaction. This work provides a scenario for understanding ME effect and further exploring the exotic state of ${\mathrm{Mn}}_3{\mathrm{Al}}_2{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ and other garnets.

Semiconductor quantum dots with confined electron or hole spins show promise for quantum information processing as they allow for efficient electric field-driven qubit manipulation. However, their susceptibility to electric noise poses a challenge that may hinder the effectiveness of these qubits. Here, we explore the impact of electric noise on a planar double quantum dot (DQD) spin qubit under the influence of AC gates applied to the dot levels, focusing on the flopping-mode spin qubit with spin-orbit interaction. We employ a rotating wave approximation within a time-dependent effective Hamiltonian to derive analytic expressions for the Rabi frequency of spin qubit oscillations with a single electron or hole in a DQD. We find that driving the qubit off-resonantly effectively mitigates the influence of charge noise, leading to a manifestation of a dynamic sweet spot. The proposed mode of operation notably improves the fidelity of quantum gates, particularly within specific ranges of drive parameters and detuning during qubit manipulation. Furthermore, our study unveils the potential of inducing a second-order dynamic sweet spot, a phenomenon tunable by drive and DQD parameters. Understanding the importance of driving qubits off-resonantly is essential for developing high-coherence planar DQD spin qubits, both for electrons in silicon and holes in germanium.

The combination of altermagnetism, twistronics, and valleytronics is of great significance for potential applications in advanced electronic devices. Twisted magnetic van der Waals bilayers have been identified as an ideal platform for altermagnetism of any type, such as $d$ wave, $g$ wave, and $i$ wave, by choosing the constituent monolayer with specific symmetry [Y. Liu et al., Phys. Rev. Lett. 133, 206702 (2024)]. Here, we propose a way for achieving valley polarization in twisted altermagnetism by applying an out-of-plane external electric field. Since the out-of-plane electric field creates a layer-dependent electrostatic potential, the valleys from different layers will stagger, producing valley polarization. We also demonstrate the effectiveness of our proposed way using the twisted tight-binding model. It is found that the applied electric field can also induce a valley/spin-gapless semiconductor and half metal besides valley polarization. Based on first-principles calculations, our proposed way to achieve valley polarization can be verified in twisted bilayer VOBr and monolayer $\mathrm{Ca}{\left(\mathrm{CoN}\right)}_2$ as a special twisted altermagnet. These findings provide different opportunities for innovative spintronics, twistronics, and valleytronics applications.

Motivated by recent experiments [Subero et al., Nat. Commun. 14, 7924 (2023)], we analyze photonic heat transport through a Josephson junction in a dissipative environment. For this purpose, we derive general expressions for the heat current in terms of nonequilibrium Green's functions for the junction coupled in series or in parallel with two environmental impedances at different temperatures. We show that even on the insulating side of the Schmid transition, the heat current is sensitive to the Josephson coupling exhibiting an opposite behavior for the series and parallel connection and in qualitative agreement with experiments. We also predict that this device should exhibit heat rectification properties and provide simple expressions to account for them in terms of the system parameters.

Dispersion equations are a common paradigm of collective excitation physics. However, in some systems, dispersion equations contain multivalued functions and their solutions are ambiguous. As an example, we consider graphene on a polar substrate where Dirac plasmons are coupled with surface optical phonons. The dispersion equation for this system contains square-root singularity. Using the initial value problem resolves this uncertainty and gives a unique solution. Particularly, we found that the lower plasmon-phonon mode, which in terms of dispersion can have a good quality factor, is almost absent in excitation spectra. The physical reason and experimental evidence of the mode collapse are discussed.

The unique dynamics of skyrmions, arising from their nontrivial topology, have garnered significant interest in spintronics due to their potential applications in advanced data storage and logic devices. However, research on synthetic ferrimagnetic (SFIM) skyrmions, which combine the advantages of ferromagnetic (FM) and synthetic antiferromagnetic (SAFM) skyrmions, remains limited. Here, we establish a theoretical framework for analyzing the dynamics of SFIM skyrmions, solving coupled Thiele equations for both FM layers to yield explicit expressions for the skyrmion velocity driven by both spin-orbit torques (SOTs) and spin-transfer torque. These analytical results are carefully compared with micromagnetic simulations, validating our theoretical approach. The framework developed can be extended to FM and SAFM skyrmions, which are special cases of SFIM skyrmions. Notably, we elucidate the reasons behind the low driving efficiency of SAFM skyrmions by SOT observed in recent experiments and propose methods to competently drive both SFIM and SAFM skyrmions. In this paper, we provide a comprehensive theoretical basis for the optimal design of spintronics applications based on SFIM and SAFM skyrmions, which can predict the related skyrmion dynamics accurately.