The recently synthesized two-dimensional electride ${\mathrm{Gd}}_2\mathrm{C}$ was proposed to be a ferromagnetic metal that possesses multiple pairs of Weyl points and may display a large anomalous Hall conductivity [Liu et al., Phys. Rev. Lett. 125, 187203 (2020)]. In view of its layered structure, here we carry out first-principles studies on the magnetic and electronic properties of ${\mathrm{Gd}}_2\mathrm{C}$ in the ultrathin monolayer limit. We find that monolayer ${\mathrm{Gd}}_2\mathrm{C}$ remains ferromagnetic like the bulk form and the hydrogenation can effectively tune its magnetism and electronic structure. With one-sided full coverage of hydrogen atoms, monolayer ${\mathrm{Gd}}_2\mathrm{C}$ becomes a half-metal with one spin channel around the Fermi level. For two-sided full hydrogenation, monolayer ${\mathrm{Gd}}_2\mathrm{C}$ transforms to an antiferromagnetic insulator with a band gap of 0.8 eV. Our studies show that monolayer electride ${\mathrm{Gd}}_2\mathrm{C}$ can perform multiple magnetic and electronic transitions with different levels of hydrogenation and may be also adopted to construct a planar heterojunction with selective area adsorption of hydrogen atoms, which has promising applications in future electronic and spintronic devices.

We propose to directly probe the chirality of Majorana edge states in 2D topological superconductors using polarization selective photon absorption. When shining circularly polarized light on a 2D topological superconductor in disk geometry, the photons can excite quasiparticles only when the polarization of the light matches the chirality of the Majorana edge states required by the angular momentum conservation. Hence, one can obtain the chirality of the Majorana edge states by measuring the photon absorption rate. We show that the polarization selective photon absorption can also serve as smoking gun evidence of the chiral Majorana edge mode.

The understanding of particle entanglement is an important goal in the studies of correlated quantum matter. The widely used method of scanning tunneling spectroscopy—which measures the local density of states (LDOS) of a many-body system by injecting or removing an electron from it—is expected to be sensitive to particle entanglement. In this paper, we systematically investigate the relation between the particle entanglement spectrum (PES) and the LDOS of fractional quantum Hall (FQH) states, the paradigmatic strongly correlated phases of electrons with topological order. Using exact diagonalization, we show that the counting of levels in both the LDOS and PES in the Jain sequence of FQH states can be predicted from the composite fermion theory. We point out the differences between LDOS and PES characterization of the bulk quasihole excitations, and we discuss the conditions under which the LDOS counting can be mapped to that of PES. Our results affirm that tunneling spectroscopy is a sensitive tool for identifying the nature of FQH states.

We perform magnetotransport studies of atomically thin InSe field-effect transistors with large Rashba spin-orbit coupling (SOC), and extract the Landau level (LL) gaps via thermal activation measurements. Surprisingly, the Landau level gaps at even and odd filling factors are extrapolated to have positive and negative intercepts at $B=0$, respectively, which result from the spin-split Landau spectrum. We also show that for a material with large spin-orbit coupling, its LL gaps may vary nonlinearly and nonmonotonically as a function of magnetic field. Thus, its effective mass and $g$-factor cannot be reliably extracted using conventional expressions, but depend rather sensitively on the SOC's strength and tunability.

We study the behavior of equilibrium spin currents near the magic angles of twisted bilayer and trilayer graphene in the presence of Rashba spin-orbit coupling. There is a substantial difference in the properties of local observables in twisted graphene layers, as compared with those in single and/or untwisted graphene layers. Remarkably, when plotted as a function of the twist angle $\theta$, the electronic charge density and the equilibrium spin currents are nonanalytic at angles that are close to within 1% of the magic angles. In addition to the occurrence of rich spin texture patterns, these findings enable the determination of magic angles within an accuracy of less than $0.{01}^{\circ }$ in terms of a scanning tunneling microscopy measurement of the local density and spin-resolved measuring devices for measuring equilibrium spin currents.

Motivated by the poor knowledge of the implication of planetary ice on the internal convection and anomalous magnetic field of ice giants, we perform extensive first-principles molecular dynamics simulation on ammonia ice to investigate its thermophysical behaviors at high temperature ($T$) and pressure ($P$) conditions relevant to the deep mantle of Neptune and Uranus. The melting curve, transport properties, and sound velocity of ammonia up to 350 GPa and 5500 K, spanning from fluid mixtures to a highly compressed superionic regime, are determined. A first-order phase transition of ammonia from the plastic state to the superionic state is observed explicitly, which is associated with the reduction of entropy. In contrast to previous predictions, the melting curve elucidates the existence of superionic ammonia in deep planetary interiors. Inspection of the reduction for transport properties and sound velocity along the isentrope of ice giants further evidences that superionic ammonia can sufficiently sustain the planetary dynamo mechanism and may contribute to the internal stratification which is responsible for the generation of the multipolar magnetic field. Finally, a comprehensive phase diagram of ammonia extended to a higher T-P regime is constructed, which provides a clear picture for studying the fundamental behavior and phase transition of ammonia in deep planets, and enhances our understanding of the interior structure and thermal convection of these planetary systems.

A class of Na-based antiperovskites ${\mathrm{Na}}_{3}XY$ $(X=\mathrm{F},\mathrm{H};Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ was recently reported with a remarkably high ionic conductivity ∼0.1 mS/cm near room temperature. Herein, we report comprehensive atomic dynamics investigations using ab initio and large-scale machine-learned molecular dynamics (MLMD) simulation on these sodium superionic conductors. Previous studies identified the role of soft phonons involving rotations of ${\mathrm{FNa}}_6$ octahedral units in Na diffusion. In contrast, our MLMD simulations show that the Na diffusion pathways are essentially uncorrelated and do not involve the collective dynamics of octahedral rotations or reorientations. Moreover, while the soft phonons do not involve any $Y$ atomic dynamics, the diffusion pathways are sterically hindered unless facilitated by $Y$ displacements and Na vacancies. However, we do find that the anharmonic low-energy phonon modes of wave vectors along the $M\text{−}R$ line at the Brillouin zone (BZ) boundary are important precursors of Na diffusion. These modes involve vibrations perpendicular to the F-Na ionic bond, along $\langle 100\rangle$ directions, which provide the initial pathways for the Na diffusion. We have calculated the branch-resolved phonon spectral energy density (SED) in the entire BZ using the MLMD simulations. The SED results as a function of temperature reveal the large anharmonicity of the soft phonon modes, which leads to floppy dynamics of Na atoms. The volume of cubic-${\mathrm{Na}}_3\mathrm{FS}$ is the smallest among the three antiperovskites and has the largest mean-phonon energy. Our calculated Na diffusion coefficient at 700 K in ${\mathrm{Na}}_3\mathrm{FS}$ of $\sim 0.63\times {10}^{-6}\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$ is significantly larger than $\sim 0.16\times {10}^{-6}\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$ in ${\mathrm{Na}}_3\mathrm{FTe}$, which clearly shows contrast with the hypothesis of a softer lattice leading to faster diffusion. The estimated diffusion barrier energy for ${\mathrm{Na}}_3\mathrm{FS}$ with 2% Na vacancy is ∼0.44 eV, which is in fair agreement with the reported value of 0.44 eV estimated from the total conductivity measurement with Na-vacant iodine-doped ${\mathrm{Na}}_3\mathrm{FS}$. Similarly, for ${\mathrm{Na}}_3\mathrm{FSe}$ and ${\mathrm{Na}}_3\mathrm{FTe}$, the barrier energies have been estimated to be 0.50 and 0.55 eV, respectively. Our large-scale MLMD-based theoretical study provides a comprehensive understanding of the role of soft phonons, host dynamics, and vacancies in Na diffusion in ${\mathrm{Na}}_3\mathrm{F}Y$ $(Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ and other materials of this class, which will be helpful in designing materials for application in solid-state batteries.

We study the magnetic susceptibility, magnetization, resistivity, and thermal conductivity of intermetallic HoAgGe single crystals at low temperatures and in magnetic fields along the $a$ and $c$ axis, while the electric and heat currents are along the $c$ axis. The magnetization curves show a series of metamagnetic transitions and small hysteresis at low field for $B\parallel a$, and a weak metamagnetic transition for $B\parallel c$, respectively. Both the magnetic susceptibility and $\rho \left(T\right)$ curve show anomalies at the antiferromagnetic transition ($T_{\mathrm{N}}\sim 11.3$ K) and spin reorientation transition ($\sim 7$ K). In zero field and at very low temperatures, the electrons are found to be the main heat carriers. For $B\parallel a$, the $\rho \left(B\right)$ curves display large and positive transverse magnetoresistance (MR) with extraordinary field dependence between $B^2$ and $B$ linear, accompanied with anomalies at the metamagnetic transitions and low-field hysteresis; meanwhile, the $\kappa \left(B\right)$ mainly decrease with increasing field and display some anomalies at the metamagnetic transitions. For $B\parallel c$, there is weak and negative longitudinal MR while the $\kappa \left(B\right)$ show rather strong field dependence, indicating the role of phonon heat transport.

In the absence of an external magnetic field and a spin-polarized charge current, an antiferromagnetic system supports two degenerate magnon modes. An applied thermal bias activates the magnetic dynamics, leading to a magnon flow from the hot to the cold edge (magnonic spin Seebeck current). Both degenerate bands contribute to the magnon current but the orientations of the magnetic moments underlying the magnons are opposite in different bands. Therefore, while the magnon current is nonzero, the net spin current is zero. To obtain a nonzero net spin current, one needs to apply either a magnetic field or a spin-polarized charge current that lifts the bands' degeneracy. Here, attaching a thermal contact to one edge of a helical nanowire, we study three different magnonic spin currents: (i) the exchange, and (ii) Dzyaloshinskii–Moriya spin currents flowing along the helical nanowire, and (iii) magnonic spin current pumped into the adjacent normal-metal layer. We find that the combination of Dzyaloshinskii–Moriya interaction and external magnetic field substantially enhances the spin current compared to the current generated solely through a magnetic field. Due to nonreciprocal magnons and magnon dichroism effect, the Dzyaloshinskii–Moriya and exchange spin currents show left-right propagation asymmetry, with $20\%$ of current rectification. The spin pumping current shows a slight asymmetry only in the case of a strong Dzyaloshinskii–Moriya interaction. The observed effects are explained in terms of the magnon dispersion relations and the magnon Doppler effect.

Magnetic, specific heat, and structural properties of the equiatomic Cantor alloy system are reported for temperatures between 5 and 300 K, and up to fields of 70 kOe. Magnetization measurements performed on as-cast, annealed, and cold-worked samples reveal a strong processing history dependence and that high-temperature annealing after cold working does not restore the alloy to a “pristine” state. Measurements on known precipitates show that the two transitions, detected at 43 and 85 K, are intrinsic to the Cantor alloy and not the result of an impurity phase. Experimental and ab initio density functional theory computational results suggest that these transitions are a weak ferrimagnetic transition and a spin-glass-like transition, respectively, and magnetic and specific heat measurements provide evidence of significant Stoner enhancement and electron-electron interactions within the material.

Ultracold atoms and molecules trapped in optical lattices are expected to serve as simulators of strongly correlated systems and topological states of matter. A fascinating example is to realize the Kitaev quantum spin liquid by using ultracold polar molecules. However, although experimental implementation of the Kitaev-type interaction was proposed, the stability of the Kitaev quantum spin liquid has not been fully investigated thus far. Here we study a quantum spin model with long-range angle-dependent Kitaev-type interactions proposed for the polar molecules, by the pseudofermion functional renormalization group method. We reveal that the ground state is magnetically ordered in both ferromagnetic and antiferromagnetic models regardless of the spatial anisotropy of the interactions, while the isotropic case is most frustrated and closest to the realization of the Kitaev quantum spin liquid. Furthermore, by introducing a cutoff in the interaction range, we clarify how the Kitaev quantum spin liquid is destroyed by the long-range interactions. The results urge us to reconsider the feasibility of the Kitaev quantum spin liquid in ultracold polar molecules.

Hexagonal manganites can exhibit intriguing ferroelectric (FE) and partially undistorted antiferroelectric (PUA) $Z_6$-vortex domain patterns, which are dual to each other. Due to the same origin, it can be expected that the two real-space topological structures will behave the same way in terms of nonelectric properties such as domain topology and scaling of vortex density. However, their electric properties would be very different from each other, while these differences are rarely understood due to the absence of investigation on PUA $Z_6$-vortex domain patterns. In this work, we study the response of the PUA $P\text{−}3c1$ structure to external electric fields and the switching dynamics of PUA $Z_6$-vortex domain patterns by combining Landau theory and phase-field simulations. We find that an electric field along the $c$ axis can tune the trimerizing tilting angle of the ${\mathrm{MnO}}_5$ trigonal bipyramids in PUA $P\text{−}3c1$ structure while no such effect is present in the FE $P{6}_{3}cm$ structure. As a result, a continuous phase transition sequence $P\text{−}3c1\to P3c1\to P{6}_{3}cm$ can be induced. For both the PUA $Z_6$-vortex domain pattern and the single PUA domain, the variation of the total polarization with a sinusoidal electric field manifests as a double-hysteresis loop. We find out that this is because the transition from $P{6}_{3}cm$ back to $P3c1$ is a first-order transition. The switching dynamics of PUA $Z_6$-vortex domain patterns is found to be very different from that observed on FE $Z_6$-vortex domain patterns. Our results advance the understanding of real-space topological structures and suggest that the PUA $P\text{−}3c1$ structure may exhibit excellent performance in dielectric energy storage.

We introduce ${\ell }_0{\ell }_2$-norm regularization and hierarchy constraints into linear regression for the construction of cluster expansions to describe configurational disorder in materials. The approach is implemented through mixed integer quadratic programming (MIQP). The ${\ell }_2$-norm regularization is used to suppress intrinsic data noise, while the ${\ell }_0$-norm is used to penalize the number of nonzero elements in the solution. The hierarchy relation between clusters imposes relevant physics and is naturally included by the MIQP paradigm. As such, sparseness and cluster hierarchy can be well optimized to obtain a robust, converged set of effective cluster interactions with improved physical meaning. We demonstrate the effectiveness of ${\ell }_0{\ell }_2$-norm regularization in two high-component disordered rocksalt cathode material systems, where we compare the cross-validation, convergence speed, and the reproduction of phase diagrams, voltage profiles, and Li-occupancy energies with those of the conventional ${\ell }_1$-norm regularized cluster expansion models.

The Uhlmann phase, which reflects the holonomy as the purified state of a density matrix traverses a loop in the parameter space, has been used to characterize topological properties of several systems at finite temperatures. We test the Uhlmann holonomy against various system-environment couplings in quantum dynamics described by the Lindblad equations of three topological systems, including the Su-Schrieffer-Heeger (SSH) model, Kitaev chain, and Bernevig-Hughes-Zhang (BHZ) model. The Uhlmann phase is shown to remain quantized in all the examples if the initial state is topological and only certain types of the Lindblad jump operators are present. Topological protection at finite temperatures against environmental effects in quantum dynamics is therefore demonstrated albeit only for a restricted class of system-environment couplings.

The combination of parametric pumping and spin-transfer torque is a powerful approach that enables high-level control over magnetic excitations in thin-film ferromagnets. The excitation parameters, such as pumping power and external field strength, affect the instabilities of individual magnon modes. We theoretically explore how the simultaneous effects of parametric pumping and spin transfer torque influence these magnetic instabilities in a thin-film ferromagnet. Within the Landau-Lifshitz-Gilbert framework, we perform micromagnetic simulations of magnon excitations in yttrium iron garnet by pumping, spin transfer torque, and a combination of the two. We find that consistent with experimental results, the magnitude and direction of the spin-transfer torque tune the parametric instability thresholds.

The discovery of room-temperature superconductors has aroused great attention in pressure-induced superhydrides. Herein we explore the doping effects on the newly proposed penta-graphene-like hydrides ${\mathrm{ZrH}}_{10}$ and ${\mathrm{HfH}}_{10}$ from first-principles calculations. Imaginary phonon frequencies of $P{6}_3/mmc{\mathrm{ZrH}}_{10}$ and ${\mathrm{HfH}}_{10}$ at lower pressures are found to be eliminated by hole doping, leading to more accessible superconductivity. At high pressures, phonons are prone to be softened by electron doping, which increases the electron-phonon coupling and the superconducting critical temperature ($T_c$). Doping is found to be an effective way to lower the stabilization pressure and increase the $T_c$ of $P{6}_3/mmc{\mathrm{ZrH}}_{10}$ and ${\mathrm{HfH}}_{10}$. This work paves an avenue for realizing low-pressure high-$T_c$ superconducting hydrides.

The interplay of superconductivity with electronic and structural instabilities on the kagome lattice provides a fertile ground for emergent phenomena. The vanadium-based kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=$ K, Rb, Cs) exhibit superconductivity on an almost ideal kagome lattice, with the superconducting transition temperature $T_{\mathrm{c}}$ forming two domes upon pressure tuning. The first dome arises from the competition between superconductivity and a charge-density wave, whereas the origin for the second dome remains unclear. Herein, we show that the appearance of the second superconducting dome in ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ is associated with transitions from hexagonal to monoclinic structures, evidenced by the splitting of structural peaks from synchrotron powder x-ray diffraction experiments and imaginary phonon frequencies in first-principles calculations. In ${\mathrm{KV}}_3{\mathrm{Sb}}_5$, the transition to an orthorhombic structure is further observed for pressure $p\gtrsim 20$ GPa, and is correlated with the strong suppression of $T_{\mathrm{c}}$ in the second superconducting dome. Our findings indicate that distortions of the crystal structure modulate superconductivity in $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ under pressure, providing a platform to study kagome lattice superconductivity in the presence of multiple electronic and structural instabilities.

${\mathrm{AgF}}_2$ has been proposed as a cuprate analog, which requires strong correlation and marked covalence. On the other hand, fluorides are usually quite ionic, and $4d$ transition metals tend to be less correlated than their $3d$ counterparts, which calls for further scrutiny. We combine valence band photoemission and Auger-Meitner spectroscopy of AgF and ${\mathrm{AgF}}_2$ together with computations in small clusters to estimate values of the Ag $4d$ Coulomb interaction $U_{4d}$ and charge-transfer energy ${\mathrm{\Delta }}_{pd}$. Based on these values, ${\mathrm{AgF}}_2$ can be classified as a charge-transfer correlated insulator according to the Zaanen-Sawatzky-Allen classification scheme. Thus, we confirm that the material is a cuprate analog from the point of view of correlations, suggesting that it should become a high-temperature superconductor if metallization is achieved by doping. We present also a computation of the Hubbard $U$ in density functional “$+U$” methods and discuss its relation to the Hubbard $U$ in spectroscopies.

We consider free fermion systems in arbitrary dimensions and represent the occupation pattern of each eigenstate as a classical binary string. We find that the Kolmogorov complexity of the string correctly captures the scaling behavior of its entanglement entropy (EE). In particular, the logarithmically enhanced area law for EE in the ground state and the volume law for EE in typical highly excited states are reproduced. Since our approach does not require bipartitioning the system, it allows us to distinguish typical and atypical eigenstates directly by their intrinsic complexity. We reveal that the fraction of atypical eigenstates which do not thermalize in the free fermion system vanishes exponentially in the thermodynamic limit. Our results illustrate explicitly the connection between complexity and EE of individual pure states in quantum systems.