Materials possessing strong midinfrared responses are of current interest because of their potential application to long-wavelength metamaterials, photonic devices, molecular detection, and catalysis. Here, we utilize high-energy resolution (80 ${\mathrm{cm}}^{-1}$, 10 meV) electron-energy-loss spectroscopy (EELS) in a monochromated and aberration-corrected scanning transmission electron microscope (STEM) to resolve multipolar surface plasmon resonances (SPRs), sometimes called Fabry-Pérot (FP) resonances, in gold nanowires with mode energies spanning from $\sim 1000$ to $8000{\mathrm{cm}}^{-1}$. STEM-EELS provides access to these mid- to near-IR responses in a single acquisition, avoiding the difficulties inherent in obtaining the same data using near-field optical techniques. The experimentally measured FP resonance energies and linewidths, together with analytical modeling and full-wave numerical electrodynamics simulations, provide a comprehensive picture of the radiative and intrinsic contributions to the total damping rates. We find some FP modes with dephasing times $>60\mathrm{fs}$, which is almost twice the longest previously reported plasmon dephasing time for individual Au nanoparticles in the infrared. The long dephasing times and the broad tunability of the FP resonance energies throughout the infrared region suggest additional opportunities for harnessing infrared plasmonic energy before dephasing occurs.

We report a polarized Raman scattering study of the lattice dynamics of $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ under hydrostatic pressures up to 7.62 GPa. At ambient pressure, $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ exhibits the hyperhoneycomb crystal structure and a magnetically ordered state of spin-orbit entangled $J_{\mathrm{eff}}$ = 1/2 moments that are strongly influenced by bond-directional (Kitaev) exchange interactions. At a critical pressure of $\sim 4.1$ GPa, the phonon spectrum changes abruptly, consistent with the reported structural transition into a monoclinic, dimerized phase. A comparison to the phonon spectra obtained from density-functional calculations shows reasonable overall agreement. The calculations also indicate that the high-pressure phase is a nonmagnetic insulator driven by the formation of Ir–Ir dimer bonds. Our results thus indicate a strong sensitivity of the electronic properties of $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ to the pressure-induced structural transition.

The spin-Peierls transition at $T_{\mathrm{SP}}$ of spin-$\frac{1}{2}$ chains with isotropic exchange interactions has previously been modeled as correlated for $T>T_{\mathrm{SP}}$ and mean field for $T<T_{\mathrm{SP}}$. We use correlated states throughout in the $J_1\text{−}{J}_2$ model with antiferromagnetic exchange $J_1$ and $J_2=\alpha J_1$ between first and second neighbors, respectively, and variable frustration $0\le \alpha \le 0.50$. The thermodynamic limit is reached at high $T$ by exact diagonalization of short chains and at low $T$ by density matrix renormalization group calculations of progressively longer chains. In contrast to mean field results, correlated states of 1D models with linear spin-phonon coupling and a harmonic adiabatic lattice provide an internally consistent description in which the parameter $T_{\mathrm{SP}}$ yields both the stiffness and the lattice dimerization $\delta \left(T\right)$. The relation between $T_{\mathrm{SP}}$ and $\mathrm{\Delta }(\delta ,\alpha )$, the $T=0$ gap induced by dimerization, depends strongly on $\alpha$ and deviates from the BCS gap relation that holds in uncorrelated spin chains. Correlated states account quantitatively for the magnetic susceptibility of TTF-${\mathrm{CuS}}_4{\mathrm{C}}_4{\left({\mathrm{CF}}_3\right)}_4$ crystals ($J_1=79\mathrm{K}$, $\alpha =0$, $T_{\mathrm{SP}}=12\mathrm{K}$) and ${\mathrm{CuGeO}}_3$ crystals ($J_1=160\mathrm{K}$, $\alpha =0.35$, $T_{\mathrm{SP}}=14\mathrm{K}$). The same parameters describe the specific heat anomaly of ${\mathrm{CuGeO}}_3$ and inelastic neutron scattering. Modeling the spin-Peierls transition with correlated states exploits the fact that $\delta \left(0\right)$ limits the range of spin correlations at $T=0$ while $T>0$ limits the range at $\delta =0$.

We report a thorough theoretical investigation of magnon-assisted photon transitions in magnetic garnet micron-sized spheres, which operate as optomagnonic resonators. In this case, matching the intraband splitting of optical Mie modes, induced by particle magnetization, to the eigenfrequency of the uniform-precession spin wave, high-efficiency triply resonant optical transitions between these modes, through respective emission or absorption of a cavity magnon, are enabled. By means of rigorous full electrodynamic computations, supported by corresponding approximate analytical calculations, we provide compelling evidence of greatly increased optomagnonic interaction, compared to that in similar processes between whispering gallery modes of corresponding submillimeter spheres, due to the reduced magnon mode volume. We explain the underlying mechanisms to a degree that goes beyond existing interpretation, invoking group theory to derive general selection rules and highlighting the role of the photon spin as the key property for maximizing the respective coupling strength.

We investigate the transport properties of the van der Waals (vdW) junction between a layered superconductor $\mathrm{Nb}{\mathrm{Se}}_2$ and a graphene. A superconductor/graphene (S/G) vdW junction is fabricated by dry transferring a freshly exfoliated $\mathrm{Nb}{\mathrm{Se}}_2$ flake onto a graphene surface. This vdW junction provides a transparent superconductor/graphene contact as well as metal-induced doping in the graphene layer underneath, thereby facilitating vdW coupling-induced superconductivity in the graphene under the $\mathrm{Nb}{\mathrm{Se}}_2$ layer. The $\mathrm{Nb}{\mathrm{Se}}_2/\text{superconducting}$ graphene (Sc-graphene)/graphene structure causes the differential resistance of the vdW junction to exhibit zero-bias dip and multiple peaks at larger bias. All of these features are explained by the coexistence of two different S/G interfaces in the device; these are lateral Sc-graphene/graphene and vertical $\mathrm{Nb}{\mathrm{Se}}_2/\mathrm{Sc}$-graphene. A proximity-induced superconducting gap ${\mathrm{\Delta }}_{\mathrm{i}}$ in the Sc-graphene is detected by Andreev reflection at the lateral Sc-graphene/graphene junction and the determined ${\mathrm{\Delta }}_{\mathrm{i}}=0.05-0.06\mathrm{meV}$.

Oscillation of zero-energy density of states under the rotation of an in-plane magnetic field is used to identify node positions of the superconducting gap. Based on Eilenberger theory that can capture the vortex core contribution appropriately, we quantitatively study the effects of nonmagnetic impurity scattering on the oscillation in a $d_{x^2-y^2}$-wave superconductor, as a function of the magnetic field and the scattering rate in the Born and the unitary limits. We find that the sign of the oscillation can be changed by the impurity scattering at low fields, indicating that impurity effect is an important factor in the analysis of angle-resolved specific heat measurements.

We study coexisting phases of Co(Ti,Zr)(Sn,Sb) with the help of three coupled computational methods, which can capture the relevant physics at different length scales, namely, density functional theory, Monte Carlo simulations, and a mean field model. Our model material consists of three sublattices, one filled with Co, one with a Ti/Zr alloy, and one with a Sb/Sn alloy. The system forms phases in terms of spatial regions with specific Ti/Zr and Sb/Sn ratios. Depending on the overall stoichiometry and temperature, different spatial arrangements of coexisting phases are found. We found that the phase separation is controlled by two very different mechanisms. One is the mismatch of the lattices created by the smaller (larger) atomic radius of Ti (Zr); the second mechanism is given by the bond covalency whose strength is regulated by the change in the Sb/Sn valence electron number, which at the same time triggers magnetism. The simulations reveal up to three coexisting bulk phases, an optional interface phase, and the possible formation of magnetic domains. The occurrence of complex domain structures is of great importance for the reduction of the lattice thermal conductivity in this class of thermoelectric half-Heusler materials.

Spin-orbit coupling (SOC) plays a crucial role in determining the spin structure of an odd parity psedospin-triplet Cooper pairing state. Here, we present a thorough study of how SOC lifts the degeneracy among different $p$-wave pseudospin-triplet pairing states in a widely used microscopic model for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, combining a Ginzburg-Landau (GL) free energy expansion, a symmetry analysis of the model, and numerical weak-coupling renormalization group (RG) and random phase approximation (RPA) calculations. These analyses are then used to critically re-examine previous numerical results on the stability of chiral $p$-wave pairing. The symmetry analysis can serve as a guide for future studies, especially numerical calculations, on the pairing instability in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and can be useful for studying other multiband spin-triplet superconductors where SOC plays an important role.

Motivated by the recent discovery of superconductivity in the Sr-doped layered nickelate ${\mathrm{NdNiO}}_2$, we perform a systematic computational materials design of layered nickelates that are dynamically stable and whose electronic structure better mimics the electronic structure of high-$T_c$ cuprates than ${\mathrm{NdNiO}}_2$. While the Ni $3d$ orbitals are self-doped from the $d^9$ configuration in ${\mathrm{NdNiO}}_2$ and the Nd-layer states form Fermi pockets, we find more than ten promising compounds for which the self-doping is almost or even completely suppressed. We derive effective single-band models for those materials and find that they are in the strongly correlated regime. We also investigate the possibility of palladate analogs of high-$T_c$ cuprates. Once synthesized, these nickelates and palladates will provide a firm ground for studying superconductivity in the Mott-Hubbard regime of the Zaanen-Sawatzky-Allen classification.

Recently, in Kuhn and Richter [Phys. Rev. B 99, 241301(R) (2019)], tensor networks built on logical circuits were briefly introduced to retrieve exciton and biexciton states. Compared to a conventional approach the tensor network methods scales logarithmic instead of linear in the grid points of the Brioullin zone and linear instead of exponential in the number of electrons and holes. This enables calculations with higher precision on the full Brioullin zone than previously possible. In this paper extensive details for an efficient implementation and the corresponding mathematical background are presented. In particular, this includes applications and results for excitons, trions, and biexcitons (for monolayer ${\mathrm{MoS}}_2$ as an example), going beyond the initial brief introduction. Furthermore strategies for calculating selective excited bound states and tests of common approximations are discussed making use of the high-accuracy full Brioullin zone treatment of the tensor network method.

The hydrodynamic heat transport equation with appropriate boundary conditions and ab initio calculated coefficients is validated by comparing the corresponding analytical and numerical solutions with frequency-domain thermoreflectance experimental measurements in silicon. Special attention is devoted to identifying the resistive effects appearing at the interface between the metal transducer and the silicon substrate. We find that a Fourier model using frequency-dependent effective thermal conductivity cannot simultaneously explain the experimental phase shifts and the amplitude of the temperature oscillations, whereas the hydrodynamic model using intrinsic parameters provides good agreement across a wide temperature range. In addition, phenomenology appearing at reduced length and time scales in this kind of experiment at different temperatures is shown. Specifically, we find hydrodynamic modes of thermal transport that are analogous to pressure- and shear-wave propagation in viscoelastic media.

The influence of water on the out-of-plane polarization of ${\mathrm{BaTiO}}_3$ (BTO) ultrathin films is studied by means of density functional theory calculations. The adsorption is investigated for different coverages on both terminations of BTO with, for each case, all possible states of polarization, namely, paraelectric, polarized upward, and polarized downward. We thus demonstrate different behavior as a function of the termination. While ${\mathrm{H}}_2\mathrm{O}$ adsorbs only dissociatively on the BaO termination, with a reinforced interaction compared to BTO without out-of-plane polarization, only molecular adsorption is observed on the ${\mathrm{TiO}}_2$ termination. In addition, the presence of water is able to switch the polarization. Whatever the initial state of polarization is, water induces a downward state on the BaO termination and an upward polarization on the ${\mathrm{TiO}}_2$ one. A detailed analysis of this phenomenon is given.

The ability to control monochromatic and coherent phonons is crucial for many phonon-related applications but remains very challenging. Here, we demonstrate a promising phonon nanocapacitor for storing and emitting phonons, built by the carbon schwarzite based host-guest system (HGS). This phonon nanocapacitor takes advantage of the inherently strong phonon confinement of the hybridized modes in HGS. The monochromaticity and coherence of the stored phonons are well demonstrated via the ultralong phonon lifetime and coherent time. More interestingly, the frequency of the nanocapacitor is widely tunable from gigahertz to several terahertz by engineering the host-guest interaction. Finally, the stored phonons with different polarizations can be emitted separately with the application of uniaxial strain along a particular direction. This work may provide new opportunities for studying the coherent wave effect of phonons.

Transport properties of permalloy doped with V, Co, Pt, and Au are explored via ab initio calculations. For this purpose, the Kubo-Bastin formula is evaluated within the fully relativistic Korringa-Kohn-Rostoker Green function formalism. Finite-temperature effects are treated by means of the alloy analogy model. It is shown that the fact that the host is disordered and not crystalline has a profound effect on how the conductivities characterizing the anomalous Hall effect and the spin Hall effect depend on the dopant concentration. Several relationships between quantities characterizing charge and spin transport are highlighted. The rate of decrease of the longitudinal charge conductivity with increasing doping depends on the dopant type, following the sequence Co-Au-Pt-V. The dependence of the anomalous Hall and spin Hall conductivities on the dopant concentration is found to be nonmonotonic. Introducing a finite temperature changes the overall trends significantly. The theoretical results are compared with available experimental data.

We propose a theoretical scheme exploiting magnetic proximity effect to realize strong excitonic emission and large, nonvolatile valley polarization in monolayer ${\mathrm{MoS}}_2$ at room temperature. A moderate exchange field can lead to valley-selective switching of the exciton ground state from bright to dark and a large valley-exciton splitting at the same time. The duo work synergistically to achieve a temperature enhanced valley polarization: while a large valley splitting is the critical first step for creating a valley-exciton population imbalance, this imbalance is greatly enhanced by thermal excitation from the dark ground state, with orders of magnitude longer lifetime than that of the bright exciton. Realizing robust room-temperature emission and valley polarization is essential for controlling the valley degree of freedom for information processing.

We study the consequence of breaking reciprocity within the context of near-field radiative heat transfer between two planar bodies. Our findings introduce a thermodynamic constraint, which states that the heat transferred from one planar body to another at each frequency and in-plane wave vector is unchanged upon interchanging the two bodies, regardless of whether the materials are reciprocal or not. We further identify a unique signature of nonreciprocity, which is the breaking of the symmetry of the heat flux density between positive and negative in-plane wave vectors. We numerically demonstrate our findings in an example system consisting of magneto-optical materials. Our formalism applies to both near- and far-field regimes, opening opportunities for exploiting nonreciprocity in two-body radiative heat transfer systems.

In the heavy-fermion system ${\mathrm{Yb}}_2{\mathrm{Pd}}_2{\mathrm{In}}_{1-x}{\mathrm{Sn}}_x$, the interplay of crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida interactions leads to complex chemical-, pressure-, and magnetic-field phase diagrams still to be explored in full detail. By using a series of techniques, we show that even modest changes of parameters other than temperature are sufficient to induce multiple quantum-critical transitions in this highly susceptible heavy-fermion family. In particular, we show that, above $\sim 10$ kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low temperature, but it likely leads to a reorientation of the Yb magnetic moments and/or the competition among different antiferromagnetic configurations.

In the absence of dissipation a periodically driven BCS superconductor can enter a coherent nonlinear regime of collective Rabi oscillations which last for arbitrary long times [Ojeda Collado et al., Phys. Rev. B 98, 214519 (2018)]. Here we show that dissipation effects introduce dramatic changes: (i) The collective Rabi mode becomes a transient. (ii) At long times a steady state is reached showing strong nonlinear effects for large enough drive strength. We identify the physical parameters governing the various crossovers and present a detailed computation of time- and angle-resolved photoemission spectroscopy (tr-ARPES) and time-resolved tunneling spectra aiming at detecting the collective Rabi oscillations and the steady-state nonlinearities. We show also that second harmonic generation is allowed for a drive which acts on the BCS coupling constant.