A prominent feature of $d$-wave altermagnets is the pure spin current generated in the absence of spin-orbit interactions. In the context of symmetry, there are $s$-wave, $p$-wave, $d$-wave, $f$-wave, $g$-wave, and $i$-wave magnets. In this paper, making an analytic study of two-band Hamiltonian systems coupled with electrons, we demonstrate unexpectedly that only the $\ell \mathrm{th}$ order nonlinear transverse spin current proportional to $E^{\ell }$ is generated in higher-wave symmetric magnets when the number of nodes is $\ell +1$. Here $E$ is applied electric field. The nonlinear spin current is essential provided the linear spin current is absent. Indeed, only the third-order nonlinear spin current is generated in $g$-wave altermagnets, while only the fifth-order spin current is generated in $i$-wave altermagnets. In particular, only the second-order nonlinear spin current is generated in $f$-wave magnets, which leads to a perfect nonreciprocal spin current. On the other hand, there is no spin-current generation in $p$-wave magnets.

The process of spin-charge interconversion is critical in modern spintronics. Nonetheless, experiments conducted on a wide variety of magnetic heterostructures consistently report that charge-to-spin and spin-to-charge conversion efficiencies can be vastly different, especially in the case of topological insulators (TIs). This discrepancy between the two “reciprocal” effects remains unexplained, hampering the development of spin devices based on spin-charge conversion. In this study, we investigate both spin-charge and charge-spin interconversion processes in TI $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\text{/}\mathrm{Py}$ and Pt/Py bilayers experimentally using spin-torque ferromagnetic resonance and spin pumping techniques. We find that the measured charge-to-spin conversion efficiency $\left({\xi }_{\mathrm{C}\text{−}\mathrm{S}}\right)$ in TI/Py is $\sim 26$ times larger than the measured spin-to-charge conversion efficiency $\left({\xi }_{\mathrm{S}\text{−}\mathrm{C}}\right)$, whereas ${\xi }_{\mathrm{C}\text{−}\mathrm{S}}$ and ${\xi }_{\mathrm{S}\text{−}\mathrm{C}}$ are comparable in the case of Pt/Py. Using a theoretical model enforcing Onsager reciprocity, we show that spin-to-charge and charge-to-spin conversions in bilayers are genuinely inequivalent, and explain our results as arising from the distinct spin current leakage that takes place during the interconversion. This work clarifies previous conflicting reports on spin-charge interconversion processes and highlights the potential of interface engineering to achieve efficient spin transport in TI-based ferromagnetic heterostructures, paving the way for highly efficient spintronic devices.

Group IV elements exhibit diverse bonding and crystalline structures, including carbon-based configurations such as graphene and diamond, and silicon-based porous structures. This diversity gives rise to intriguing physical phenomena and applications, such as magic-angle superconductivity and superhard materials. However, the potential configurations formed by germanium and their corresponding physical properties remain largely unexplored. In this study, we employ a crystal structure prediction method to investigate calcium germanide under high pressures, identifying 12 exotic thermodynamically stable phases. Notably, the CaGe systems display a variety of bonding types, with germanium forming zigzag, square ring, armchair, and ladder configurations. Additionally, we found that the known $\mathrm{C}{\mathrm{a}}_2\mathrm{Ge}$ system undergoes a metal-insulator phase transition, transforming into a hexagonal structure with nontrivial band topology. For calcium-rich compounds, we discovered several electride phases with distinct cavity shapes. Among these, the $\mathrm{C}{\mathrm{a}}_4\mathrm{Ge}$ Imma phase possesses one-dimensional interstitial electrons. It exhibits a superconducting transition temperature of approximately 10 K at 60 GPa, which is higher than other calcium-based electrides at moderate pressures. Our findings demonstrate that introducing metal atoms can induce diverse bonding characteristics in group IV elements, resulting in a wide range of physical properties, including superconducting and topological behaviors.

We show that topological phases include disordered materials if the underlying invariant is interpreted as originating from coarse geometry. This coarse geometric framework, grounded in physical principles, offers a natural setting for the bulk-boundary correspondence, reproduces physical knowledge, and leads to an efficient and tractable numerical approach for calculating invariants. As a showcase, we give a detailed discussion of the framework for three-dimensional systems with time-reversal symmetry. We numerically reproduce the known disorder-free phase diagram of a tunable, effective tight-binding model and analyze the evolution of the topological phase under disorder.

The effect of the Rashba spin-orbit coupling (RSOC) on the topological properties of the one-dimensional (1D) extended $s$-wave superconducting Hamiltonian, in the presence of strong electron-electron correlation, is investigated. It is found that a nonzero RSOC increases the periodicity of the effective Hamiltonian, which results in the folding of the Brillouin zone (BZ) and consequently in the emergence of an energy gap at the boundary of the BZ. Essentially, the initial single band is divided into a number of subbands. If the chemical potential lies inside the energy gaps (subbands), then the phase is topologically trivial (nontrivial). This is the origin of the reentrant nature of the existent topological properties. The emergence of subbands allows us to drive the system in and out of the topological phase by the proper tuning of the chemical potential. A heterostructure involving van der Waals materials and a 1D moiré pattern for an investigation of the predicted effect has also been proposed. We also discuss how in-plane magnetic field can be used to control the RSOC coupling and induced periodicity in depleted InAs nanowire in which evidence of strong electron-electron correlation has been found.

Signatures of superconductivity under pressure have recently been reported in the bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ and trilayer ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ Ruddlesden-Popper (RP) nickelates with the general chemical formula ${\mathrm{La}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ ($n$ = number of perovskite layers along the $c$-axis). The emergence of superconductivity is always concomitant with a structural transition in which the octahedral tilts are suppressed, bringing the apical Ni-O-Ni angle to ${180}^{\circ }$ and causing an increase in the out-of-plane $d_{z^2}$ orbital overlap. Due to this strong interlayer coupling, a flat band of pure $d_{z^2}$ character crosses the Fermi level. Here, using first-principles calculations, we explore biaxial strain (both compressive and tensile) as a means to mimic the electronic structure characteristics of RP nickelates (up to $n=5$) under hydrostatic pressure. Our findings highlight that strain enables the decoupling of the structural and electronic structure effects obtained under hydrostatic pressure: While compressive strain brings the apical Ni-O-Ni angle closer to ${180}^{\circ }$, it shifts the $d_{z^2}$ flat bands away from the Fermi energy, giving rise to a more cupratelike electronic structure. In contrast, tensile strain reduces the apical Ni-O-Ni angle (to values of $\sim {160}^{\circ }$), but it recovers the flat $d_{z^2}$ band at the Fermi level appearing in the bilayer and trilayer RP nickelates under pressure. Overall, strain represents a promising way to tune the electronic structure of RP nickelates and could be an alternative route to achieve superconductivity at ambient pressure in this family of materials.

We propose a simple and efficient method to calculate the electronic self-energy in dynamical mean-field theory (DMFT), addressing a numerical instability often encountered when solving the Dyson equation. Our approach formulates the Dyson equation as a constrained optimization problem with a simple quadratic objective. The constraints on the self-energy are obtained via direct measurement of the leading order terms of its asymptotic expansion within a continuous time quantum Monte Carlo framework and the use of the compact discrete Lehmann representation of the self-energy yields an optimization problem in a modest number of unknowns. We benchmark our method for the noninteracting Bethe lattice, as well as DMFT calculations for both model systems and ab initio applications.

The crystal structure of ${\mathrm{KAgF}}_3$ was studied by powder neutron diffraction. ${\mathrm{KAgF}}_3$ exhibits at all temperatures an orthorhombic symmetry in space group $Pnma$ that allows for several distortions with respect to the ideal cubic perovskite structure. At all temperatures there is a strong splitting of Ag-F distances parallel to the $a,c$ planes that documents alternating occupation of holes in $x^2-y^2$ and $z^2-y^2$ orbitals. The orientation of the octahedron elongation and thereby the orbital order flips at a structural phase transition occurring around $T_s=240\mathrm{K}$ which is accompanied by a suppression of magnetic susceptibility. The orbital ordering is further enhanced in the low-temperature phase and the twisting of the ${\mathrm{AgF}}_4$ plaquettes forming the antiferromagnetic chains changes. Density functional theory (DFT) calculations show an enhancement of the magnetic interaction in the low temperature phase indicating that the transition and the orbital order are partially driven by the Kugel-Khomskii mechanism.

We explore a system comprising two oscillators that are coupled to an open channel at distinct locations. The coupling nature can be adjusted to be coherent, dissipative, or a combination of both, controlled by a tunable phase resulting from wave propagation between the oscillators. This setup allows us to observe characteristic energy level behaviors: Level repulsion occurs with coherent coupling, while level attraction is observed with dissipative coupling. In the regime of dissipative coupling, one of the eigenmodes becomes a dark mode, rendering it immune to external perturbations. By leveraging the tunable coupling through the open channel, we introduce a method for manipulating this dark mode, achieving both efficient excitation and an extended lifetime. Our results have broad applicability across various physical systems, including optical, acoustic, and magnetic configurations, underscoring the potential for innovative applications in mode storage and signal processing.

We introduce inelastic electron tunneling spectroscopy to distinguish between the weak and strong electronic coupling of a bistable molecule to a metal electrode. Ab initio electronic structure calculations demonstrate that the electronic coupling strength is related to the hybridization of molecular orbitals at the interface as evidenced by their energy broadening and shifting to the Fermi level of the metal electrode surface. The hybridization determines the lifetime of the electrons on the molecule, affecting the signal strength in inelastic electron tunneling spectroscopy. We establish this correlation by tuning the electronic coupling strength via an interaction with a scanning tunneling microscope tip, achieving intermediate states between weak and strong coupling.

We study a family of excitonic quantum phase transitions describing the evolution of a bilayer metallic state to an interlayer coherent state where excitons condense. We argue that such transitions can be continuous and exhibit a non-Fermi liquid counterflow response ${\rho }_{\mathrm{counterflow}}\left(\omega \right)\sim {\omega }^{2/z}$ that directly encodes the dynamical critical exponent $z$. Our calculations are performed within a controlled expansion around $z=2$. This physics is relevant to any system with spin, valley, or layer degrees of freedom. We consider two contexts for excitonic quantum criticality: (1) an interacting bilayer of strongly correlated Fermi liquids, and (2) a system of two narrow, half-filled Chern bands at zero external magnetic field, with total Chern number $C_{\mathrm{tot}}=0$, which may soon be realizable in moiré materials. The latter system hosts a time-reversed pair of composite Fermi liquid states, and the condensation of excitons of the composite fermions leads to an exotic exciton insulator* state with a charge neutral Fermi surface. Our work sheds new light on the physics of interlayer coherence transitions in two-dimensional materials.

In the self-consistent approximation for the two-dimensional mean-field model we found an analytic solution for the ground state with a coexisting $d$-wave symmetric bond ordered pair density wave (PDW) and spin (SDW) or charge (CDW) density waves, as observed in some high-temperature superconductors. In particular, the solution gives the same periodicity for CDW and PDW, and a pseudogap in the Fermi-excitation spectrum.

We analyze the competition between strong correlations and dissipation in quantum impurity systems from the Kondo regime to the valence fluctuation regime by developing a slave-boson theory for a non-Hermitian Anderson impurity model with one-body loss. Notably, in the non-Hermitian Kondo regime, strong correlations qualitatively change the nature of dissipation through renormalization effects, where the effective one-body loss is suppressed and emergent many-body dissipation characterized by the complex-valued hybridization is generated. We unveil the mechanism of a dissipative quantum phase transition of the Kondo state on the basis of this renormalization effect, which counterintuitively enhances the lifetime of the impurity against loss. We also find a crossover from the non-Hermitian Kondo regime to the valence fluctuation regime dominated by one-body dissipation. Our results can be tested in a wide variety of setups such as quantum dots coupled to electronic leads and quantum point contacts in ultracold Fermi gases.

The recent experimental observation of thermal rectification in a graphite tesla valve indicates the existence of nonlinear effects in the phonon hydrodynamic regime, which remains yet an open puzzle currently. In this work, we explore the effects of nonlinearity on phonon hydrodynamics in graphite based on a semianalytical solution of the nonlinear phonon Boltzmann transport equation with first-principles input. Our theoretical study predicts the saturation of macroscopic drift velocity of phonons to the minimal phase velocity with increasing temperature gradient, and the asymmetric heat flux distributions in reciprocal space that yield the nonlinearity. We also demonstrate an enhanced nonlinear effect with broadening graphite width, and an appreciable reduction of the thermal conductivity from the linear result. This work thus provides a foundation for future theoretical and experimental work on nonlinear phonon hydrodynamics and heat manipulation in graphitic structures.

The van der Waals interfaces play a crucial role in determining the physical properties and enhancing the device performances of two-dimensional materials. Here we report the investigation of the elastic coupling at the van der Waals interfaces between graphene layers and silicon oxide, via angstrom-indentation experiments combined with density functional theoretical calculations. By developing a theoretical model based on the Lennard-Jones potential, we demonstrate that the effective elastic modulus of the graphene/silicon oxide van der Waals interface is approximately 1.6 GPa, and the interlayer elastic modulus for graphene is 34.2 GPa. In addition, contact stiffness mapping of graphene with various numbers of layers on a silicon oxide substrate indicates that the very first two-dimensional (2D) layer could significantly screen the 2D–three-dimensional interface interactions.

Effects of a charge density wave (CDW) order on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction has been theoretically investigated. Assuming that the CDW with an incommensurate ordering vector ${\mathbf{Q}}_c$ is induced by a Fermi surface nesting, we show that the CDW order suppresses the conventional RKKY interaction and that it can induce a mode coupling between magnetic ordering vectors ${\mathbf{q}}_m$ and ${\mathbf{q}}_m\pm {\mathbf{Q}}_c$. This suggests that, below the CDW transition temperature, the correlation between localized spins may develop at both ${\mathbf{q}}_m$ and ${\mathbf{q}}_m\pm {\mathbf{Q}}_c$. Experimental implications of our result to van der Waals materials $R\mathrm{Te}{}_3$ are also discussed.

We report on a comprehensive thermodynamic study of a quasi-two-dimensional (quasi-2D) quantum magnet ${\mathrm{Cu}}_2{\left(\mathrm{OH}\right)}_3\mathrm{Br}$ which in the 2D layer can be viewed as strongly coupled alternating antiferromagnetic and ferromagnetic chains. In an applied magnetic field transverse to the ordered spins below $T_N=9.3$ K, a field-induced phase transition from the 3D ordered to a disordered phase occurs at $B_c=16.3$ T for the lowest temperature, which is featured by an onset of a one-half plateaulike magnetization. By performing quantum Monte Carlo simulations of the relevant 2D model, we find that the plateaulike magnetization corresponds to a partial symmetry restoration and the full polarization in the ferromagnetic chains. Our numerical simulations also show that the magnetization saturation occurs with full symmetry restoration at a much higher field of $B_s\simeq 95$ T, corresponding to a 1D quantum phase transition in the antiferromagnetic chains. We argue that the experimentally observed field-induced phase transition at $B_c$ follows from the partial symmetry restoration and the concomitant dimensional reduction.

We identify persistent oscillations in a nonintegrable quantum Ising chain. In the integrable chain with nearest-neighbor interactions, the nature, origin, and decay of post-transition oscillations are tied to the Kibble-Zurek mechanism. Remarkably, when coupling to the next-nearest neighbor is added, the resulting nonintegrable “zigzag” chain (still in the quantum Ising universality class) supports persistent oscillation: Topological defects (kinks) appear as a result of the quantum phase transition. However, in a “zigzag” Ising chain defects can form Cooper pairs. The oscillation of the Cooper-pair condensate has a frequency that depends on the binding energy gap between the paired and the unpaired defects, so it can be excited by resonant driving. While one might have expected that the integrability-breaking zigzag coupling causes relaxation, the oscillations we identify are persistent: Their longevity in our simulations is likely limited only by numerical accuracy. This oscillation of the Cooper-pair condensate of kinks is a manifestation of quantum coherence and should be experimentally accessible.