Using a simplified microscopic model of coupled spin and lattice excitations in a ferromagnetic insulator we evaluate the magnetic-field dependence of the spin Seebeck effect at low temperatures. The model includes Heisenberg exchange coupling, a harmonic lattice potential, and a pseudodipolar exchange interaction. Our approach goes beyond previous work [Phys. Rev. B 98, 134421 (2018)] in that it does not rely on the a priori assumption of a fast equilibration of the magnon and phonon distributions. Our theory shows that singular features in the magnetic-field dependence of the spin Seebeck effect at low temperatures observed by Kikkawa et al. [Phys. Rev. Lett. 117, 207203 (2016)] are independent of the relative strength of magnon-impurity and phonon-impurity scattering.

We use vector Hamiltonian formalism (VHF) to study theoretically three-magnon parametric interaction (or three-wave splitting) in a magnetic disk existing in a magnetic vortex ground state. The three-wave splitting in a disk is found to obey two selection rules: (i) conservation of the total azimuthal number of the interacting spin-wave modes, and (ii) inequality of the radial numbers of the resultant modes, if the directly excited original mode is radially symmetric (i.e., if the azimuthal number of the directly excited mode is $m=0$). The selection rule (ii), however, is relaxed in sufficiently small magnetic disks, due to the influence of the vortex core. We also found that the efficiency of the three-wave splitting of the directly excited mode strongly depends on the azimuthal and radial mode numbers of the resultant modes. This property becomes qualitatively important in the case when several different splitting channels (several pairs of resultant modes) approximately satisfy the resonance condition for the splitting. The good agreement of the VHF analytic calculations with the experiment and micromagnetic simulations proves the capability of the VHF formalism to predict the actual experimentally realized splitting channels, and the magnitude of the driving field thresholds for the three-wave splitting.

Materials with nonsaturating, extremely large magnetoresistance (XMR) attract continuous research interest due to their great potential in the fields of spintronics and magnetic sensors/switches. Here, we report the discovery of pressure-driven superconductivity (SC) in trigonal ${\mathrm{PtBi}}_2$, a semimetal material with nontrivial triply degenerate point fermions exhibiting a nonsaturating XMR effect, above a critical pressure of $\sim 5–6$ GPa. The superconducting transition onset temperature is $\sim 2$ K and displays a very weak pressure dependence across a broad pressure range of $\sim 5–48$ GPa. The lattice structure of trigonal ${\mathrm{PtBi}}_2$ is stable against pressure at least to 12.9 GPa. Around the same critical pressure where the SC emerges, the Hall coefficient varies violently and shows a sign crossover from low pressure positive (hole dominated) to high pressure negative (electron dominated), probably indicating a close relationship between them.

B20-type compounds, such as MnSi and FeGe, host helimagnetic and skyrmion phases at the mesoscale, which are canonically explained by the combination of ferromagnetic isotropic interactions with weaker chiral Dzyaloshinskii-Moriya ones. Mysteriously, MnGe evades this paradigm as it displays a noncollinear magnetic state at a much shorter nanometer scale. Here we show that the length scale and volume-dependent magnetic properties of MnGe stem from purely isotropic exchange interactions, generally obtained in the paramagnetic state. Our approach is validated by comparing MnGe with the canonical B20-helimagnet FeGe. The free energy of MnGe is calculated, from which we show how triple-q magnetic states can stabilize by adding higher-order interactions.

In this paper, we investigate the magnetic, optical, and lattice responses of a $\mathrm{Pt}/\mathrm{Cu}/{\mathrm{Bi}}_1{\mathrm{Y}}_2{\mathrm{Fe}}_5{\mathrm{O}}_{12}/{\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ heterostructure to femtosecond laser excitation of the opaque Pt/Cu metallic bilayer. The electronic excitation generates coherent and incoherent phonons, which trigger high-frequency standing spin waves (SWs) in the dielectric ${\mathrm{Bi}}_1{\mathrm{Y}}_2{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ layer via a phonon-induced change of magnetic anisotropy. We find that the incoherent phonons (heat) can induce a fast $(<1\mathrm{ps})$ and slow $(>1000\mathrm{ps})$ decrease of the magnetic order by different spin-phonon interaction scenarios. These results open perspectives for generating high-frequency SWs in buried magnetic garnets.

The nonordinary surface transitions at the ($2+1$)-dimensional quantum critical point precluded in the corresponding classical critical point have been found recently. The mechanism of such behavior that is only found in dimerized Heisenberg models to date is still under debate. To illuminate the role of symmetry-protected topological (SPT) phases in inducing such nonordinary behaviors, we study a system on a two-dimensional square lattice consisting of interacting spin-1 Haldane chains, which has a genuine SPT phase—the Haldane phase—at weak interchain interactions and a quantum critical point, belonging to the classical three-dimensional (3D) O(3) universality class, to the Néel phase. Different from previously studied models, there is no dimerization in the current model. Cutting the system along the chain direction or perpendicular to the chain direction exposes two different surfaces. Using unbiased quantum Monte Carlo simulations, we find that the two different types of surfaces show completely different surface critical behaviors at the bulk critical point, resulting from different surface states in the SPT phase. For the system with surfaces along the chain direction, the surface critical behavior is of ordinary type of the bulk 3D O(3) critical point, while for the surfaces perpendicular to the chain direction, the surface critical behavior is nonordinary, consistent with nonordinary transitions found in dimerized Heisenberg models. Our numerical results demonstrate that the gapless surface state in the gapped SPT phase together with the gapless mode of the critical point is a pure quantum scenario that leads to the nonordinary transition.

The realization of quantum spin liquids on a triangular lattice magnet remains challenging due to quenched disorders present in real materials. Here, we report local-probe signatures of spin-liquid-like excitations in the structurally perfect triangular antiferromagnet ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$. Our zero-field and longitudinal-field muon spin relaxation measurements reveal both spatially and temporally correlated spins with no hint of static magnetism down to 80 mK, consistent with a quantum spin liquid. Comprehensive ${}^{23}\mathrm{Na}$ nuclear magnetic resonance experiments enable us to identify a field-induced crossover of magnetic excitations from a spinonlike to a gapped magnon at the critical field ${\mu }_0{H}_C=1.65$ T. Our findings uncover the temporal, thermal, and magnetic-field characteristics of the magnetic excitations in the triangular-lattice quantum spin liquid candidate ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$.

Two-dimensional van der Waals compounds with magnetic ions on a honeycomb lattice are hosts to a variety of exotic behavior. The magnetic interactions in one such compound, $\mathrm{Mn}\mathrm{P}{\mathrm{Se}}_3$, are investigated with elastic and inelastic neutron scattering. Magnetic excitations are observed in the magnetically ordered regime and persist to temperatures well above the ordering temperature, $T_{\text{N}}=74\mathrm{K}$, consistent with low dimensional magnetic interactions. The inelastic neutron scattering results allow a model spin Hamiltonian to be presented that includes dominant intralayer interactions of $J_{1ab}=0.45$ meV, $J_{2ab}=0.03$ meV, $J_{3ab}=0.19$ meV, consistent with theoretical predictions. Despite the quasi-2D behavior, appreciable interlayer interactions of $J_c=0.031\left(5\right)$ meV are required to model the data. No evidence for anisotropy in the form of a spin gap is observed in the data collected. The measurements on $\mathrm{Mn}\mathrm{P}{\mathrm{Se}}_3$ are contrasted with those on $\mathrm{Mn}\mathrm{P}{\mathrm{S}}_3$ and reveal a large increase in the interlayer exchange interaction in $\mathrm{Mn}\mathrm{P}{\mathrm{Se}}_3$ that may stabilize the similar ordering temperatures in the bulk compounds.

We have systematically studied the magnetoresistance effect in $\mathrm{Pt}/{\left(\mathrm{CoNi}\right)}_n$ multilayers featured with perpendicular magnetic anisotropy and fcc (111) texture. Angular dependence of the magnetoresistance, including high-order cosine terms, was found in the plane perpendicular to the electrical current, which can be attributed to the geometrical size effects and the anisotropic interface magnetoresistance effect. However, the contribution from spin Hall magnetoresistance (SMR) is absent. The spin Hall angle of Pt was evaluated by spin-torque ferromagnetic resonance measurements, and the theoretical magnitude of SMR in our samples was estimated to be one order higher than the accuracy of our experiments. The absence of SMR in our multilayers indicates that the Elliott-Yafet spin relaxation of itinerant electrons in the ferromagnetic metal is crucial when studying the spin transport in all-metallic heterostructures.

We propose an SU(4) spin-valley-fermion model to investigate the superconducting instabilities of twisted bilayer graphene (TBG). In this approach, bosonic fluctuations associated with an emergent SU(4) symmetry, corresponding to combined rotations in valley and spin spaces, couple to the low-energy fermions that comprise the flat bands. These fluctuations are peaked at zero wave vector, reflecting the “ferromagnetic-like” SU(4) ground state recently found in strong-coupling solutions of microscopic models for TBG. Focusing on electronic states related to symmetry-imposed points of the Fermi surface, dubbed here “valley hot-spots” and “van Hove hot-spots,” we find that the coupling to the itinerant electrons partially lifts the huge degeneracy of the ferro-SU(4) ground state manifold, favoring intervalley order, spin-valley coupled order, ferromagnetic order, spin-current order, and valley-polarized order, depending on details of the band structure. These fluctuations, in turn, promote attractive pairing interactions in a variety of closely competing channels, including a nodeless $f$-wave state, a nodal $i$-wave state, and topological $d+id$ and $p+ip$ states with unusual Chern numbers 2 and 4, respectively. Nematic superconductivity, although not realized as a primary instability of the system, appears as a consequence of the near-degeneracy of superconducting order parameters that transform as one-dimensional and two-dimensional irreducible representations of the point group $D_6$.

Time-reversal symmetry breaking (TRSB) with a Bardeen-Cooper-Schrieffer (BCS) -like superconductivity occurs in a small, but growing number of noncentrosymmetric (NCS) materials, although the mechanism is poorly understood. We present heat capacity, magnetization, resistivity, and muon spin resonance/relaxation ($\mu \mathrm{SR}$) measurements on polycrystalline samples of NCS ${\mathrm{La}}_7{\mathrm{Pd}}_3$. Transverse-field $\mu \mathrm{SR}$ and heat capacity data show ${\mathrm{La}}_7{\mathrm{Pd}}_3$ is a type-II superconductor with a BCS-like gap structure, while zero-field $\mu \mathrm{SR}$ results provide evidence of TRSB. We discuss the implications of these results for both the ${\mathrm{La}}_7{X}_3$ (where $X=$ Ni, Pd, Rh, Ir) group of superconductors and other CS and NCS superconductors for which TRSB has been observed.

We investigate charge pumping in the vicinity of order-obstructed topological phases, i.e., symmetry-protected topological phases masked by spontaneous symmetry breaking in the presence of strong correlations. To explore this, we study a prototypical Su-Schrieffer-Heeger model with finite-range interaction that gives rise to orbital charge density wave order and characterize the impact of this order on the model's topological properties. In the ordered phase, where the many-body topological invariant loses quantization, we find that not only is quantized charge pumping still possible, but it is even assisted by the collective nature of the orbital charge density wave order. Remarkably, we show that the Thouless pump scenario may be used to uncover the underlying topology of order-obstructed phases.

Optimal regulation of lattice thermal conductivity in low-dimensional materials is fundamental to obtain highly efficient miniaturized devices. To this aim, we use quantum-mechanical based analyses to understand how atomic type and structural geometry determine electron density and lattice dynamic features ruling the thermal conduction. As a case study, we consider layered van der Waals transition metal dichalcogenides with a finite number of layers. We find that a large thermal conductivity is realized when the atomic bonds display highly covalent character, promoting fast motions of the cations in correspondence of the low-frequency phonon band. Such an effect is the result of the entangled electronic and phonon features, which are captured by the covalency and cophonicity metric. The investigation protocol that we present has general applicability and can be used to design novel thermal low-dimensional materials irrespective of the kind of atomic topology and chemical composition.

We present an interpretable machine learning model to predict accurately the complex rippling deformations of multiwalled carbon nanotubes made of millions of atoms. Atomistic-physics-based models are accurate but computationally prohibitive for such large systems. To overcome this bottleneck, we have developed a machine learning model that comprises a novel dimensionality reduction technique and a deep neural network-based learning in the reduced dimension. The proposed nonlinear dimensionality reduction technique extends the functional principal component analysis to satisfy the constraint of deformation. Its novelty lies in designing a function space that satisfies the constraint exactly, which is crucial for efficient dimensionality reduction. Owing to the dimensionality reduction and several other strategies adopted in the present paper, learning through deep neural networks is remarkably accurate. The proposed model accurately matches an atomistic-physics-based model whereas being orders of magnitude faster. It extracts universally dominant patterns of deformation in an unsupervised manner. These patterns are comprehensible and explain how the model predicts yielding interpretability. The proposed model can form a basis for an exploration of machine learning toward the mechanics of one- and two-dimensional materials.

In surface-enhanced Raman-scattering experiments that use plasmonic nanostructures as substrates, the scattering spectrum contains a broad background usually associated with photoluminescence. This background exists above and below the frequency of the incident wave. The low-frequency part of this background is similar to the scattering spectrum of a plasmon nanoparticle, while the high-frequency part follows the Gibbs distribution. We develop a theory that explains experimentally observed features in both the high- and low-frequency parts of the photoluminescence spectrum from a unified point of view. We show that photoluminescence is attributed to the cascade Brillouin scattering of the incident wave by metal phonons under the plasmon resonance conditions. The theory is in good agreement with our measurements over the entire frequency range of the background.

The surfaces of rutile transition-metal oxides $\left({\mathrm{TMO}}_2\right)$ are widely investigated for catalysis, photoelectrochemical solar cells, memristors, and supercapacitors, but their structures have remained controversial. Here we employ density functional theory to predict that a universal behavior of metallic ${\mathrm{TMO}}_2$ surfaces, i.e., the stoichiometric ${\mathrm{TMO}}_2$ surfaces, exhibit a contrast reversal in simulated scanning tunneling microscopy (STM) images at different scanning biases. The predictions are verified by experimental STM imaging of ${\mathrm{RuO}}_2\left(110\right)$ surfaces and this feature is shown to enable accurate determinations of the ${\mathrm{TMO}}_2\left(110\right)$ surface structures under various conditions. This work provides different insights into the electronic properties of ${\mathrm{TMO}}_2\left(110\right)$ surfaces and offers an effective method to directly map the surface structure and point defects using bias-dependent STM.

${\mathrm{CeRhIn}}_5$ is a prototypical antiferromagnetic heavy-fermion compound, whose behavior in a magnetic field is unique. A magnetic field applied in the basal plane of the tetragonal crystal structure induces two additional phase transitions. When the magnetic field is applied along, or close to, the $c$ axis, a new phase characterized by a pronounced in-plane electronic anisotropy emerges at $B^{*}\approx$ 30 T, well below the critical field, $B_c\simeq$ 50 T, to suppress the antiferromagnetic order. The exact origin of this new phase, originally suggested to be an electronic-nematic state, remains elusive. Here we report low-temperature specific heat measurements in ${\mathrm{CeRhIn}}_5$ in high static magnetic fields up to 36 T applied along both the $a$ and $c$ axes. For fields applied along the $a$ axis, we confirmed the previously suggested phase diagram and extended it to higher fields. This allowed us to observe a triple point at $\sim 30$ T, where the first-order transition from an incommensurate to commensurate magnetic structure merges into the onset of the second-order antiferromagnetic transition. For fields applied along the $c$ axis, we observed a small but distinct anomaly at $B^{*}$, which we discuss in terms of a possible field-induced transition, probably weakly first-order. We further suggest that the transition corresponds to a change of magnetic structure. We revise magnetic phase diagrams of ${\mathrm{CeRhIn}}_5$ for both principal orientations of the magnetic field based entirely on thermodynamic anomalies.

A multichannel Kondo model, where two or more equivalent but independent channels of electrons compete to screen a spin-$\frac{1}{2}$ impurity, shows overcompensation of the impurity spin, leading to the non-Fermi-liquid behavior in various thermodynamic and transport properties. However, when the channel symmetry is broken, an impurity quantum phase transition can occur at zero temperature. Identification of an order parameter describing the impurity quantum phase transition is very difficult since it is beyond the conventional Landau-Ginzburg-Wilson theory. By employing the natural orbitals renormalization group method, we study both two-channel and three-channel Kondo models, from the perspective of spin correlation between the impurity and electrons in electronic channels. Here we demonstrate that by introducing the spin-correlation ratio as an order parameter we can characterize impurity quantum phase transitions driven by channel asymmetry. In particular, the universal critical exponents $\beta$ of the spin-correlation ratio and $\nu$ of the correlation length are explicitly determined by finite-size-scaling analysis, namely, $\beta =0.10\left(1\right)$ and $\nu =2.0\left(1\right)$, and $\beta =0.10\left(1\right)$ and $\nu =2.5\left(1\right)$ for the two-channel and three-channel Kondo models, respectively.

We performed ${}^{13}\mathrm{C}$ NMR measurements on an organic superconductor $\lambda \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S}\right)}_2\mathrm{Ga}{\mathrm{Cl}}_4$ [BETS: bis(ethylenedithio)tetraselenafulvalene] to investigate the pressure effect on the spin density wave (SDW) fluctuations at low temperatures, which were observed at ambient pressure. We found that the spin-lattice relaxation rate divided by temperature $(1/T_{1}T)$ is significantly enhanced at low temperatures at 3 kbar, implying enhancement of the SDW fluctuation. When further pressure is applied, increase in $1/T_{1}T$ at low temperatures is suppressed and $1/T_{1}T$ becomes temperature independent, indicating the Fermi liquid state. Moreover, we revealed that the Korringa factor under pressure becomes double compared to its value at ambient pressure, suggesting a significant change in electron correlation. We discuss the effect of the SDW fluctuation and the electron correlation on the unconventional superconductivity.

We present ab initio calculations of effective interaction parameters $U$ and $J$ for dioxides of actinides from uranium to curium. We first use a self-consistent scheme using $\mathrm{DFT}+U$ and constrained random phase approximation (cRPA). For ${\mathrm{UO}}_2$, and ${\mathrm{NpO}}_2$, we find self-consistent values of $U$ and $J$ leading to values of gap in agreement with experiments. For ${\mathrm{PuO}}_2$, the value of $U$ is underestimated. For ${\mathrm{AmO}}_2$ and ${\mathrm{CmO}}_2$, we find very low self-consistent values. We compare projected local orbital Wannier functions to maximally localized Wannier functions and find a weak effect of the localization on interaction parameters. We suggest that spin-orbit coupling, and antiferromagnetism, could improve these results partially. We also extend our calculations by treating the $p$ bands from oxygen as correlated, as in Seth et al. [Phys. Rev. Lett. 119, 056401 (2017)], and show that the results are rather independent of self-consistency in this approach. Comparing these calculations, our conclusion is that including electron interaction on oxygen $p$ orbitals is necessary both to improve the density of states and to compute more meaningful and predictive values of effective interaction parameters.