Developing energy-efficient magnetoresistive random-access memory based on spin-orbit torques (SOTs) faces challenges due to the limited effective SOTs exerted on neighboring ferromagnets (FMs) by spin polarizations from the heavy metals (HMs). Over the past decade, extensive research has sought new materials with heightened spin-orbit coupling (SOC) and robust intrinsic spin Hall angles. Here, we investigate the effect of metal insertion (MI) on spin-torque efficiency in the Pt/Co/MgO system. Notably, the Mg insertion between Co and MgO augments the effective dampinglike SOT efficiency to 0.487, while Mo insertion results in an efficiency of 0.26, compared to 0.10 without any insertion. This efficiency enhancement is achieved by increasing spin-orbital scattering in Co through more specular reflection at the MI/MgO interface and spin-transparent transmission at the Co/MI interface. We conducted an element-resolved spin-pumping experiment using the x-ray ferromagnetic resonance technique to observe spin transport behavior in the metal-inserted stack with a ferromagnetic detector layer, and we confirmed greater spin transparency in Co/Mg than in Co/Mo. Without altering the intrinsic spin Hall angle of HM or the interfacial spin transparency of the HM/FM interface, we significantly enhanced the spin-torque efficiency by inserting metal between FM and MgO. This approach of enhancing spin-torque efficiency holds great promise for advancing SOT technologies and facilitating efficient spin generation within spintronics research.

We investigate the magnetic properties of $S=1$ antiferromagnetic diamond-lattice, $\mathrm{Ni}{X}_2{\left(\mathrm{pyrimidine}\right)}_2$ $(X=\mathrm{Cl}, \mathrm{Br})$, hosting a single-ion anisotropy (SIA) orientation which alternates between neighboring sites. Through neutron diffraction measurements of the $X=\mathrm{Cl}$ compound, the ordered state spins are found to align collinearly along a pseudo-easy axis, a unique direction created by the intersection of two easy planes. Similarities in the magnetization, exhibiting spin-flop transitions, and the magnetic susceptibility in the two compounds imply that the same magnetic structure and a pseudo-easy axis is also present for $X=\mathrm{Br}$. We estimate the Hamiltonian parameters by combining analytical calculations and Monte Carlo (MC) simulations of the spin-flop and saturation field. The MC simulations also reveal that the spin-flop transition occurs when the applied field is parallel to the pseudo-easy axis. Contrary to conventional easy-axis systems, there exist field directions perpendicular to the pseudo-easy axis for which the magnetic saturation is approached asymptotically and no symmetry-breaking phase transition is observed at finite fields.

Point-contact Andreev reflection spectroscopy at low temperatures and high magnetic fields has been performed on a noncentrosymmetric $\mathrm{L}{\mathrm{a}}_3\mathrm{S}{\mathrm{e}}_4$ superconductor with a critical temperature $T_{\mathrm{c}}=8$ K. Two superconducting energy gaps ${\mathrm{\Delta }}_1$ and ${\mathrm{\Delta }}_2$, with $2{\mathrm{\Delta }}_1/k_{\mathrm{B}}{T}_{\mathrm{c}}\sim 5.8$ and $2{\mathrm{\Delta }}_2/k_{\mathrm{B}}{T}_{\mathrm{c}}\sim 2.3$, are directly observed in some of the spectra. The temperature and magnetic field effects help to resolve a two-gap structure even on the most frequent spectra where at low temperatures only a single gap is apparent, reflected in a pair of maxima around the zero bias. Two-gap superconductivity consistently with the point contact Andreev reflection spectroscopy is also supported by the heat capacity and the Hall probe magnetization measurements.

Brillouin light scattering (BLS) spectroscopy is an effective method for detecting spin waves in magnetic thin films and nanostructures. While it provides extensive insight into the properties of spin waves, BLS spectroscopy is impeded in many practical cases by the limited range of detectable spin wave wave numbers and its low sensitivity. Here we present a generalized theoretical model describing plasmon-enhanced BLS spectroscopy. Three types of plasmonic nanoparticles in the shape of an ellipsoid of rotation are considered: a single plasmon resonator, a sandwiched plasmonic structure in which two nanoparticles are separated by a dielectric spacer, and an ensemble of metallic nanoparticles on the surface of a magnetic film. The effective susceptibilities for the plasmonic systems at the surface of the magnetic film are calculated using the electrodynamic Green's function method, and the enhancement coefficient is defined. It is analytically shown that the ratio of the plasmon resonator height to its radius plays a key role in the development of plasmon-enhanced BLS spectroscopy. The developed model serves as a basis for the numerical engineering of the optimized plasmon nanoparticle morphology for BLS enhancement.

Magnetic monopole quasiparticles in spin ice materials hold the potential for exploring new frontiers of physics that extend beyond Maxwell's equations. We have previously presented a two-dimensional magnetic monopole gas (2DMG) with a net charge, confined at the interface between spin ice $(R_2{\mathrm{Ti}}_2{\mathrm{O}}_7,R=\mathrm{Dy},\mathrm{Ho})$ and antiferromagnetic iridate $(R_2{\mathrm{Ir}}_2{\mathrm{O}}_7,R=\mathrm{Dy},\mathrm{Ho})$, which is proposed to be driven by entropy. However, this mechanism needs to be verified and studied systematically. In this work, we quantitatively demonstrate that entropy is a key factor in the 2D confinement of the monopole gas. Starting from the nearest-neighbor interaction, we reveal that the competition between the entropy of spin ice, which favors the 2D confinement, and the entropy of the monopoles' random walks, which favors the deconfinement, dictates the distribution of the monopoles within a few layers close to the interface. Our entropy-based monopole model accurately reproduces the monopole distribution obtained from the energy-based spin model, affirming that 2D confinement is entropy driven. We further employ both models to show that the monopole distribution can be manipulated by an external magnetic field and temperature, holding promise for next-generation devices based on magnetic monopoles. In the presence of dipolar interactions, entropy continues to play a crucial role in controlling 2DMG behavior at finite temperatures. Our findings reveal the entropic mechanisms in 2DMG, enabling the manipulation of emergent quasiparticles at material interfaces.

We study classical spin ice under uniaxial strain along the [111] crystallographic axis. Remarkably, such strain preserves the extensive ice degeneracy and the corresponding classical Coulomb phase. The emergent monopole excitations remain thermodynamically deconfined exactly as in the isotropic case. However, their motion under local heat bath dynamics depends qualitatively on the sign of the strain. In the low-temperature limit for negative strain, the monopoles diffuse, while for positive strain, they localize. Introducing additional ring exchange dynamics into the ice background transforms the localized monopoles into subdimensional excitations whose motion is restricted to diffusion in the (111)-plane. The phenomena we identify are experimentally accessible in rare-earth pyrochlores under uniaxial stress as well as in tripod kagome materials. The diffusive versus localized nature of the monopoles manifests in characteristic magnetic noise spectra, which we compute.

We report multiple magnetic phase transitions in single-crystalline ${\mathrm{Ce}}_3{\mathrm{Pd}}_6{\mathrm{Sb}}_5$ using magnetic, transport, and thermodynamic measurements. ${\mathrm{Ce}}_3{\mathrm{Pd}}_6{\mathrm{Sb}}_5$ crystallizes in an orthorhombic structure (space group Pmmn), as a defect-induced variant of the well-known tetragonal ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$ structure. It undergoes two magnetic transitions: a second-order antiferromagnetic transition at $T_{\mathrm{N}1}$ = 6.6 K and a first-order one at $T_{\mathrm{N}2}=5.8\mathrm{K}$. The hybridization between $4f$ electrons and conduction electrons is relatively weak in ${\mathrm{Ce}}_3{\mathrm{Pd}}_6{\mathrm{Sb}}_5$, as revealed from the nearly full recovery of magnetic entropy $R\ln 2$ at $T_{\mathrm{N}1}$ and the low value of the electronic specific-heat coefficient of 30 mJ/(Ce-mol ${\mathrm{K}}^2$). We construct two field-temperature phase diagrams for magnetic fields applied parallel and perpendicular to the $c$ axis, which, for $H\parallel c$, shows multiple field-induced phases between $T_{\mathrm{N}1}\left(H\right)$ and $T_{\mathrm{N}2}\left(H\right)$, while Hall resistivity measurements point to the absence of a topological Hall effect. The crystalline electric field effect and modulated crystalline structure with two Ce sites are considered as the primary factors for the complex magnetic properties of ${\mathrm{Ce}}_3{\mathrm{Pd}}_6{\mathrm{Sb}}_5$.

${\mathrm{Pr}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_7{\mathrm{O}}_{15-\delta }$ is a unique member of cuprate superconductors where many studies suggest that CuO double chains are responsible for superconductivity. One characteristic and nontrivial feature of its electronic structure is a relatively large electron hopping $t$ between nearest-neighbor Cu sites with a Cu-O-Cu angle of around 90 degrees. In this study, we have theoretically pinned down the origin of a large $\left|t\right|$ in the double-chain structure of ${\mathrm{Pr}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_7{\mathrm{O}}_{15-\delta }$ using first-principles calculation and tight-binding-model analysis. We have found that, in the nearest-neighbor hopping $t$, $d\text{−}d$, and $d\text{−}p\text{−}p\text{−}d$ contributions roughly cancel each other out and the $d\text{−}p\text{−}d$ hopping path enhanced by the local distortion of the double chain is a key to get the large $\left|t\right|$. Double-well band dispersion arising from the relatively large $|t/t^{\prime }|$ allows the enhancement of spin-fluctuation-mediated superconductivity by the incipient-band mechanism, where the one band bottom plays a role of the incipient valley. Our study provides the important knowledge to understand the unique superconductivity in ${\mathrm{Pr}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_7{\mathrm{O}}_{15-\delta }$.

Using a computationally inexpensive frozen phonon approach we have developed a technique which can be used to screen large unit cell materials and systems for enhanced superconducting critical temperatures. The method requires only density functional theory calculated electronic band structures of phonon modes corresponding to atomic displacements for various materials. We have applied this method to well-known conventional superconductors including ${\mathrm{MgB}}_2$, ${\mathrm{H}}_3\mathrm{S}$ and other hydrides as examples.

Dual topological insulators (DTIs) are simultaneously protected by time-reversal and crystal symmetries, representing advantageous alternatives to conventional topological insulators. By combining ab initio calculations and the $\mathbf{k}·\mathbf{p}$ approach, here, we investigate the electronic band structure of a ${\mathrm{Na}}_2\mathrm{CdSn}$ triatomic layer and derive a low-energy $4\times 4$ effective model consistent with all the symmetries of this material class. We obtain the effective Hamiltonian using the Löwdin perturbation theory, the folding-down technique, and the theory of invariants and determine its parameters by fitting our analytical dispersion relations to those of ab initio calculations. We then calculate the bulk topological invariants of the system and show that the ${\mathrm{Na}}_2\mathrm{CdSn}$ triatomic layer is a giant-gap (hundreds of millielectronvolts) quasi-two-dimensional DTI characterized by both spin and mirror Chern numbers $-2$. In agreement with the bulk-boundary correspondence theorem, we find that a finite-width strip of ${\mathrm{Na}}_2\mathrm{CdSn}$ possesses two pairs of counterpropagating helical edge states per interface. We obtain analytical expressions for the edge state energy dispersions and wave functions, which are shown to agree with our numerical calculations. Our work opens an avenue for further studies of ${\mathrm{Na}}_2\mathrm{CdSn}$ as a potential DTI candidate with room-temperature applications in areas of technological interest, such as nanoelectronics and spintronics.

Quantum oscillation (QO) measurements constitute one of the most powerful methods for determining the Fermi surface (FS) of metals, exploiting the famous Onsager relation between the FS area and the QO frequency. The recent observation of non-Onsager QOs with a frequency set by the difference of two FS orbits in a bulk three-dimensional metal can be understood as the QO of quasiparticle lifetime (QPL) due to interorbital scattering [N. Huber et al., Nature (London) 621, 276 (2023)]. QPL oscillations (QPLOs) generalize magnetointersubband oscillations known from coupled two-dimensional metals. They may provide a novel tool for extracting otherwise hard-to-measure intraband versus interband scattering times of quasiparticles. Here, we provide a numerical lattice study of QPLOs comparing transport and thermodynamic observables. We explore the effect of different imperfections like general impurities, Hall-effect-induced electric fields, various forms of strain from bending, and magnetic field inhomogeneities. We confirm the basic phenomenology of QPLOs as predicted in analytical calculations and identify additional nonperturbative features. Remarkably, we find that some imperfections can stabilize, or even enhance, non-Onsager QPLOs in contrast to standard QO frequencies. We discuss various avenues for identifying QPLOs in experiments and how to use their dependence on imperfections to extract material properties.

Improving the performance of thermoelectric (TE) material necessitates a comprehension of its lattice dynamics and electron-phonon $\left(e\text{−}\mathrm{ph}\right)$ interactions. Herein, we examine the nonequilibrium photocarrier dynamics in SnSe, a representative TE compound, under pressure using optical pump-probe spectroscopy. Abrupt changes in $e$-ph couplings are revealed at approximately 0.8 GPa and 5.3 GPa, arising from the electronic Lifshitz transition and the Pnma-Cmcm phase change, respectively. Beyond 5.3 GPa, we observe the emergence of coherent phonon from the ${\mathrm{A}}_g^1$ mode in the terahertz range, with an enhancement at higher pressure. Furthermore, a significant reduction in lifetime and redshift in frequency highlight the strong anharmonicity of the ${\mathrm{A}}_g^1$ mode in the high-pressure $Cmcm$ phase. These findings are further supported by Raman spectroscopy and density functional theory calculations. This paper provides nontrivial insights into e-ph and lattice dynamics in the high-pressure phase of SnSe.

Temperature constraints are highly desirable in the experimental setup when seeking the synthesis of new carbon structures. Fluctuations of the Dirac field result in temperature-dependent corrections to the Helfrich-Canham formulation, which governs the classical elasticity of the graphene membrane at equilibrium. Here, we examine the emergent shapes allowed by the effective model up to quadratic order in Ricci curvature and discuss the constraints required to observe them. We determine the mechanical stability conditions and provide a phase diagram characterized by the appearance of a critical temperature $T_{\mathrm{c}}$ that distinguishes between carbon nanotubes and fullerene phases. The observation of minimal and developable surfaces is anticipated in the high- and low-temperature regimes, respectively. Additionally, a Beltrami trumpet surface is forecasted when the membrane is subjected to an external source balancing out internal Helfrich stresses.

The electronic transport properties of atomic-layer-deposited Ru thin films and nanowires were investigated in the temperature range between 0.1 and 30 K by means of magnetoresistance measurements. The nonlinear behavior of the longitudinal and Hall resistivities was used to extract information on carrier densities and mobilities. The analysis revealed that for thin films, holes exhibit lower mobility but have a much greater concentration than that of electrons, in contrast to bulk at room temperature where the electron and hole densities are expected to be equal, with holes having a larger mobility. For Ru nanowires, the saturation of magnetoresistance suggests a vanishing electron concentration possibly due to excess carrier concentration at the edge causing increased recombination supported by defects. For Ru thin films, weak antilocalization effects provided insight into phase coherence and spin-orbit scattering lengths, indicating that electron-phonon interactions are the dominant scattering mechanism.

We demonstrate the possibility of achieving an optical pulling force without complex beam-shaping techniques. By using optical pulses detuned from the resonant frequency of the structure, we create conditions in which the spectral maxima of the excited current and of the incident field do not match. Therefore, the phase shift between the two, which controls the optical force, becomes time dependent, thus enabling unexpected optical effects. Particularly, it is shown that tuning the central frequency of the pulse produces a temporary pulling force directed towards the illumination source.

Semiconductor nanoplatelets—the colloidal analog of quantum wells—exhibit excitonic transitions highly sensitive to their thickness and surface treatment. Here, we present a general theory that describes the underlying physical processes. In particular, we include a less-intuitive effect: how out-of-plane quantum confinement impacts in-plane effective masses. We combine first-principles calculations with models based on $\mathbf{k}·\mathbf{p}$ theory and the envelope function approximation to show how thickness and surface treatments (ligands or shells) influence excitonic properties. Our results reproduce experiments and provide intuition and quantitative insights on nanoplatelet behavior.

We present a modal analysis of a popular modeling framework for studying light-matter interaction between quantum emitters and general electromagnetic resonators, in which the active materials are modeled via Lorentzian material responses. The method relies on the Lippmann-Schwinger equation to calculate the complex resonance frequencies of the coupled system based only on a single calculation for the electromagnetic resonator without the active material and without fitting to calculated spectra. As an application example, we consider a recent dielectric cavity design featuring deep subwavelength confinement of light. We find the expected anticrossing of the emitter and cavity resonance frequencies, and comparing with independent reference calculations, we find an extraordinary quantitative agreement with a relative error below one part in ten thousand. To connect with the Jaynes-Cummings model, we relate the classical description of the emitter to the dipole moment in the corresponding quantum optical model. The combined framework therefore enables classical calculations to be used for evaluating the coupling strength entering quantum optical theories in a transparent way. The methodology is expected to be useful in the design and interpretation of ongoing and future experiments and devices involving single-photon sources for quantum information and quantum communication technology.

We explore phases of free fermions on arenas that do not tessellate a manifold. Specializing to arboreal arenas described by tree graphs which possess a notion of translational symmetry, we study possible fermionic phases in the BDI symmetry class on the $p$-coordinated Bethe lattice. We find that there are $p$ distinct obstructed atomic insulating phases that are characterized by distinct edge states, patterns of entanglement, and a winding characteristic that we define here. These distinct insulating phases are always separated by a metallic region in the parameter space rather than isolated quantum critical points. The metallic region itself comprises several distinct metallic phases that are distinguished by their winding characteristics and correlation functions. The correlation functions of distinct subsystems display nonanalytic behavior at distinct points in the metallic region, signaling a cascade of subsystem transitions. An intriguing feature of these arboreal metals is the presence of truncated subsystems with zero-energy boundary modes despite being gapless. This work suggests new opportunities for synthetic quantum systems to realize these novel phases.

We study the optical response of the alternating twisted trilayer graphene by making use of a unitary transformation for the trilayer Hamiltonian and the Kubo formulation of linear response theory. The layer-resolved optical conductivities are expressed in terms of contributions from effective twisted bilayer and single-layer systems along with their coupling. We show that the in-plane magnetic response is proportional to this coupling between the twisted bilayer and single-layer systems; and, due to the different energy scales, the in-plane magnetic response is negligibly small. We also formulate a local electromagnetic response that involves the vertical gradients of the magnetic field and moment.

We report on the observation and comprehensive study of the terahertz radiation induced magnetophotogalvanic effect (MPGE) in bulk CdHgTe crystals hosting Kane fermions. The MPGE has been detected in ${\mathrm{Cd}}_x{\mathrm{Hg}}_{1-x}\mathrm{Te}$ films with Cd contents $x=0.15$ and 0.22 subjected to an in-plane magnetic field. At liquid helium temperature, we observed multiple resonances in MPGE current upon variation of magnetic field. In the $x=0.22$ with noninverted band structure, the resonances are caused by cyclotron resonance (CR) and photoionization of an impurity level. In the $x=0.15$ films with an inverted band structure, they originate from the CR and interband optical transitions. The band structure calculated by the Kane model perfectly describes the positions of all resonances. In particular, the resonant MPGE caused by interband transitions excited by THz radiation is caused by the gapless energy spectrum of Kane fermions realized in materials with certain Cd contents and temperature ranges. In addition to the resonant MPGE current, we detected a nonresonant one due to indirect optical transitions (Drude-like). This contribution has a nonmonotonic magnetic field dependence increasing linearly at low magnetic field $B$, approaching a maximum at moderate field, and decreasing at high $B$. While the nonresonant MPGE decreases drastically with increasing temperature, it is well measurable up to room temperature. The developed theory demonstrates that the MPGE current arises due to cubic in momentum spin-dependent terms in the scattering probability. The asymmetry caused by these effects results in a pure spin current which is converted into an electric current due to the Zeeman effect.