Atomically thin materials are exceedingly susceptible to their dielectric environment. For transition metal dichalcogenides, sample placement on a substrate or encapsulation in hexagonal boron nitride (hBN) are frequently used. In this paper we show that the dielectric response due to optical phonons of adjacent materials influences excitons in two-dimensional (2D) crystals. We provide an analytic model for the coupling of 2D charge carriers to optical substrate phonons, which causes polaron effects similar to that of intrinsic 2D phonons. We apply the model to hBN-encapsulated ${\mathrm{WSe}}_2$, finding a significant reduction of the exciton binding energies due to dynamical screening effects.

I calculate the ferroelectric polarization dynamics induced by a femtosecond midinfrared pulse as measured in the recent experiment by R. Mankowsky et al., Phys. Rev. Lett. 118, 197601 (2017). It is due to the nonlinear coupling of the excited infrared-active phonon mode with the ferroelectric mode or to the excitation of the ferroelectric mode itself depending on the pulse frequency. To begin with, I write the ${\mathrm{LiNbO}}_3$ crystal symmetry invariant thermodynamic potential including electric field and nonlinear phonon coupling terms. I solve the equations of motion determined by this potential for phonon coordinates numerically. I explain the transient polarization reversal observed in the experiment by the action of the depolarizing electric field, which is due to bound charges at the polarization domain boundaries, and give a reasonable estimate for its value. I argue that the polarization can be reversed when this field is screened.

We demonstrate control of the magnetism of Pd(100) ultrathin films, which show $d$-electron quantum-well-induced ferromagnetism, via modulation of the interface electronic state using density functional calculation. From an analysis based on the phase model, forming the Au/Pd(100) interface induces hybridization of the wave function of $d$-electron quantum-well states, and modulates the term of the scattering phase shift as a function of the reciprocal lattice point. In contrast, forming the Al interface, which has only $s$ electrons at the Fermi energy, cannot modify the scattering phase shift. Our finding indicates the possibility of modifying the phase shift by tailoring the interface electronic states using hybridization of the wave function, and this efficiently changes the density of states near the Fermi energy of Pd films, and the switching between paramagnetism and ferromagnetism occurs based on the condition for ferromagnetism (Stoner criterion).

We demonstrate quantum computation of two-point correlation functions for a Heisenberg spin chain. Using the IBM Q 20-qubit quantum machines, we find that, for two sites, the correlation functions produce the exact results reliably. For four sites, results from the IBM Q 20-qubit Tokyo quantum computer are noisy due to read out errors and decoherence. Nevertheless, the correlation functions retain the correct spectral information. This is illustrated in the frequency domain by accurately extracting the magnon energies from peaks in the spectral function.

This work focuses on the generation of ferromagnetism at the surface of anatase ${\mathrm{TiO}}_2$ films by low-energy ion irradiation. Controlled ${\mathrm{Ar}}^{+}$-ion irradiation resulted in a thin $(\sim 10)$ nm ferromagnetic surface layer. The intrinsic origin and robustness of the magnetic order has been characterized by x-ray magnetic circular dichroism at room temperature revealing that a Ti band is spin-polarized. These results, together with density functional theory calculations, indicate that Ti vacancy-interstitial pairs are responsible for the magnetic order. Superconducting quantum interference device measurements show the existence of a perpendicular magnetic anisotropy and a low remanent magnetization. Magnetic force microscopy reveals that this low remanence is due to oppositely aligned magnetic domains with magnetization vectors normal to the main surface. The weak domain-wall pinning, the magnetic anisotropy, together with the simplicity of the preparation method, open up interesting possibilities for future applications. As an example, single domain patterns of $\sim 1\mu \mathrm{m}$ width and several $\mu \mathrm{m}$ length can be easily prepared.

Superfluidlike mass flow through 2 cm thick solid ${}^4\mathrm{He}$ samples sandwiched between two porous Vycor glass rods filled with superfluid was observed in 2008. The flow commences below 0.6 K, increases in magnitude with decreasing temperature, and shuts off abruptly below a temperature $T_d$ near 0.1 K. $T_d$ is found to increase with ${}^3\mathrm{He}$ impurities at the few parts per million level. The mass flow phenomenon is recently reproduced in $8\mu \mathrm{m}$ thick solid samples; however, the shutoff of mass flow at low temperature is not seen. Here, we report measurements on 2.5 mm thick solid samples. Mass flow rate reduction and extinction near 0.1 K is found only when the concentration of the helium gas, $X_3$, used to prepare the sample exceeds respectively $3.5\times {10}^{-4}$ and $2\times {10}^{-3}$. After the extinction, the mass flow shows a gradual but complete recovery with a characteristic time of many hours. The experimental evidence allows us to formulate a model that explains both the extinction and recovery phenomena. The extinction of the mass flow is due to the trapping of ${}^3\mathrm{He}$ atoms at the nodes or the intersections of the dislocation network which blocks the transport of ${}^4\mathrm{He}$ along the network. The slow recovery in the 2.5 mm samples is due to the migration of the trapped ${}^3\mathrm{He}$ atoms along the dislocation lines and drain into the superfluid inside the porous Vycor glass. Our model also explains naturally the absence of mass flow extinction in the $8\mu \mathrm{m}$ samples and the apparent absence of recovery in the 2 cm samples.

Several decades after the discovery of superconductivity in bismuthates, the strength of their electron-phonon coupling and its evolution with doping remain puzzling. To clarify these issues, polycrystalline hole-doped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{BiO}}_3$ ($0.1\le x\le 0.6$) samples were systematically synthesized and their bulk and microscopic superconducting properties were investigated by means of magnetic susceptibility and muon-spin rotation and relaxation ($\mu \mathrm{SR}$), respectively. The phase diagram of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{BiO}}_3$ was reliably extended up to $x=0.6$, which is still found to be a bulk superconductor. The lattice parameter $a$ increases linearly with K content, implying a homogeneous chemical doping. The low-temperature superfluid density, measured via transverse-field $\mu \mathrm{SR}$, indicates an isotropic fully gapped superconducting state with zero-temperature gaps ${\mathrm{\Delta }}_0/k_{\mathrm{B}}{T}_c=2.15$, 2.10, and 1.75, and magnetic penetration depths ${\lambda }_0=219$, 184, and 279 nm for $x=0.3$, 0.4, and 0.6, respectively. A change in the superconducting gap, from a nearly ideal BCS value ($1.76k_{\mathrm{B}}{T}_c$ in the weak-coupling case) in the overdoped $x=0.6$ region, to much higher values in the optimally doped case, implies a gradual decrease in electron-phonon coupling with doping.

We present a study of a simple model antiferromagnet consisting of a sum of nearest-neighbor $\mathrm{SO}\left(N\right)$ singlet projectors on the kagome lattice. Our model shares some features with the popular $S=1/2$ kagome antiferromagnet but is specifically designed to be free of the sign problem of quantum Monte Carlo. In our numerical analysis, we find as a function of $N$ a quadrupolar magnetic state and a wide range of a quantum spin liquid. A solvable large-$N$ generalization suggests that the quantum spin liquid in our original model is a gapped ${\mathbb{Z}}_2$ topological phase. Supporting this assertion, a numerical study of the entanglement entropy in the sign free model shows a quantized topological contribution.

Gilbert damping is a key property governing magnetization dynamics in ordered magnets. We present a theoretical study of intrinsic Gilbert damping induced by magnon decay in antiferromagnetic metals through $s-d$ exchange interaction. Our theory delineates the qualitative features of damping in metallic antiferromagnets owing to their bipartite nature. It provides analytic expressions for the damping parameters yielding values consistent with recent first-principles calculations. Magnon-induced intraband electron scattering is found to predominantly cause magnetization damping, whereas the Néel field is found to be damped via disorder. Depending on the conduction electron band structure, we predict that magnon-induced interband electron scattering around band crossings may be exploited to engineer a strong Néel field damping.

Rewritable optical storage is dominated by alloys of a small number of elements, overwhelmingly Ge, Sb, and Te. For over 30 years, Ge/Sb/Te alloys in the composition range ${\left(\mathrm{GeTe}\right)}_{1-x}{\left({\mathrm{Sb}}_2{\mathrm{Te}}_3\right)}_x(0\le x\le 1)$ have been the materials of choice in commercial devices: all have metastable rock-salt structures that change little over decades at archival temperatures, and all contain vacancies (cavities). The special status of Ge/Sb/Te alloys arises from the close similarity of their valence orbitals as measured by the orbital radial moments, so that bonds are stronger than in other combinations of elements of groups 14–16 with appropriate valences. The orbital similarity arises from the irregular changes in atomic orbitals and properties as the atomic number increases (“secondary periodicity”). Jones showed [P. R. Soc. London, Ser. A 147, 396 (1934)] that the simple cubic structure of (metallic) Bi (valence configuration $6s^{2}6p^3$) is unstable to a distortion to a (semimetallic) rhombohedral structure. This picture can be adapted to Ge/Sb/Te alloys to explain the metastable structure of the above family of compounds where vacancies almost always occur next to Te atoms, which form one sublattice of the rock-salt structure. The disorder in the Ge/Sb/vacancy sublattice is not random.

A severe limitation of current acoustic metasurfaces remains in their modest tunability to meet multifrequency requirements and alterable functionalities on demand. Here, a reconfigurable curved acoustic metasurface for acoustic cloaking and illusion is reported. The structure is composed of an array of tunable helical units to break this limitation and realize continuously versatile sound manipulations. We theoretically, numerically and experimentally investigate the channel length represented by the helical depth, which is used to achieve full 2π phase shift continuously over the frequency range from 2 to 7 kHz. As pragmatic examples, we present by full-wave numerical simulations the concept of a curved metasurface for the continuously tunable acoustic multifunction, including broadband carpet cloaking and ground illusion at a wide working band. Then, we experimentally demonstrate these functionalities by showing an excellent effect to restore the disturbed reflective field from a cloaked object or to mimic an arbitrary shaped ground.

We develop a theory to derive effective Floquet Hamiltonians in the weak-drive and low-frequency regime. We construct the theory in analogy with band theory for electrons in a spatially periodic and weak potential, such as occurs in some crystalline materials. As a prototypical example, we apply this theory to graphene driven by circularly polarized light of low intensity. We find an analytic expression for the effective Floquet Hamiltonian in the low-frequency regime which accurately predicts the quasienergy spectrum and the Floquet states. Furthermore, we identify self-consistency as the crucial feature effective Hamiltonians in this regime need to satisfy to achieve high accuracy. The method is useful in providing a realistic description of off-resonant drives for multiband solid-state systems where light-induced topological band structure changes are sought.

Coupling of spin waves with electromagnetic cavity field is demonstrated in an antiferromagnet, dysprosium ferrite (${\mathrm{DyFeO}}_3$). By measuring transmission at 0.2–0.35 THz and sweeping sample temperature, magnon-photon coupling signatures were found at crossings of spin-wave resonances with Fabry-Pérot cavity modes formed in samples. The obtained spectra are explained in terms of classical electrodynamics and a microscopic model.

We propose a density-based nonlocal correlation energy functional, corresponding to a revised form of the seamless van der Waals (vdW) density functional developed by Dobson and coauthors in the late nineties. The functional, termed as rDW99 in this work, has the same energy expression as the random phase approximation (RPA), but its key ingredient—the noninteracting density response function—is expressed entirely in terms of the electron density and density gradient. This is enabled by generalizing the Lindhard function from the homogeneous electron gas (HEG) to inhomogeneous electron systems. Compared to the original DW99 functional of Dobson and coauthors, we have introduced two revisions in rDW99: (1) A gap correction is incorporated into the generalized Lindhard function, making the functional more suitable for describing inhomogeneous, and in particular insulating, systems; (2) an approximate, yet fairly accurate, analytical form of the Lindhard function in real space is developed which facilitates the numerical evaluation of the rDW99 functional. In contrast with previously developed vdW density functionals, the nonlocal polarization effect is naturally captured in the rDW99 functional. The functional form, as it stands right now, is computationally still rather involving in a straightforward implementation. However, we anticipate that numerical techniques can be developed to significantly reduce the computational cost. As a first step, we assess the quality of rDW99 for describing the vdW interactions in the nonoverlapping asymptotic regime by computing the $C_6$ coefficients for a set of 43 atoms and molecules. With only one adjustable parameter, the mean absolute percentage error (MAPE) of the $C_6$ coefficients is estimated to be $8.7\%$. In this work, $C_6$ coefficients corresponding to the standard RPA method are also computed for the same test set, and the MAPE for the RPA $C_6$ coefficients is estimated to be $13.2\%$.

Antiferromagnetic quantum spin systems can exhibit a transition between collinear and spiral ground states, driven by frustration. Classically this is a smooth crossover and the crossover point is termed a Lifshitz point. Quantum fluctuations change the nature of the transition. In particular, it has been argued previously that in the two-dimensional (2D) case a spin liquid (SL) state is developed in the vicinity of the Lifshitz point, termed a Lifshitz SL. In the present work, using a field theory approach, we solve the Lifshitz quantum phase transition problem for the 2D frustrated XY model. Specifically, we show that, unlike the $SU\left(2\right)$ symmetric Lifshitz case, in the XY model, the SL exists only at the critical point. At zero temperature we calculate nonuniversal critical exponents in the Néel and in the spin spiral state and relate these to properties of the SL. We also solve the transition problem at a finite temperature and discuss the role of topological excitations.

The coherent spin dynamics of electrons in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells (QWs) is studied by time-resolved pump-probe Kerr rotation. The coupled QWs have different thicknesses; the narrow one is doped by ${\mathrm{Mn}}^{2+}$ magnetic ions. A short range proximity effect between them is observed: the Zeeman splitting of electrons in the wide QW is given in addition to the intrinsic electron $g$ factor by the exchange interaction with the ${\mathrm{Mn}}^{2+}$ ions mediated by electron tunneling into the narrow QW. The exchange interaction strength scales with the ${\mathrm{Cd}}_{0.88}{\mathrm{Mg}}_{0.12}\mathrm{Te}$ barrier thickness separating the QWs. The Kerr rotation signal measured on the wide QW shows two close frequencies of electron spin Larmor precession in a transverse magnetic field. These components have very different spin dephasing times, 50 ps and 1 ns. The two frequencies originate from electrons in the wide QW being either part of an exciton or being resident. The proximity effect of the exciton electron is smaller due to the binding by Coulomb interaction, which decreases the tunneling to the narrow well. The experimental data are in good agreement with model calculations.

Radiative heat transfer between bodies at the nanoscale can surpass blackbody limits on thermal radiation by orders of magnitude due to contributions from evanescent electromagnetic fields, which carry no energy to the far field. Thus far, principles guiding explorations of larger heat transfer beyond planar structures have assumed utility in surface nanostructuring, via enhancement of the density of states, and the possibility that such design paradigms can approach Landauer limits, in analogy to conduction. Here we derive fundamental shape-independent limits to radiative heat transfer, applicable in near- through far-field regimes, that incorporate material and geometric constraints such as intrinsic dissipation and finite object sizes, and show that these preclude reaching the Landauer limits in all but a few restrictive scenarios. Additionally, we show that the interplay of material response and electromagnetic scattering among proximate bodies means that bodies which maximize radiative heat transfer actually maximize scattering rather than absorption. Finally, we compare our new bounds to Landauer limits as well as limits that ignore the interplay between material and geometric constraints, and show that these prior limits lead to overly optimistic predictions. Our results have ramifications for the ultimate performance of thermophotovoltaics and nanoscale cooling, as well as incandescent and luminescent devices.

We investigate in a fully quantum-mechanical manner how the many-body excitation spectrum of topological insulators is affected by the presence of long-range Coulomb interactions. In the one-dimensional Su-Schrieffer-Heeger model and its mirror-symmetric variant strongly localized plasmonic excitations are observed which originate from topologically nontrivial single-particle states. These “topological plasmons” inherit some of the characteristics of their constituent topological single-particle states, but they are not equally well protected against disorder due to the admixture of nontopological bulk single-particle states in the polarization function. The strength of the effective Coulomb interactions is also shown to have strong effects on the plasmonic modes. Furthermore, we show how external modifications via dielectric screening and applied electric fields with distinct symmetries can be used to study topological plasmons, thus allowing for experimental verification of our atomistic predictions.

Selenium and tellurium are among the few elements that form $AB{\mathrm{O}}_3$ perovskite structures with a four valent ion in the $A$ site. This leads to highly distorted structures and unusual magnetic behavior. Here we investigate the Co and Ni selenite and tellurite compounds, ${\mathrm{CoSeO}}_3, {\mathrm{CoTeO}}_3, {\mathrm{NiSeO}}_3$, and ${\mathrm{NiTeO}}_3$, using first principles calculations. We find an interplay of crystal field and Jahn-Teller distortions that underpin the electronic and magnetic properties. While all compounds are predicted to show an insulating G-type antiferromagnetic ground state, there is a considerable difference in the anisotropy of the exchange interactions between the Ni and Co compounds. This is related to the Jahn-Teller distortion. Finally, we observe that these four compounds show characteristics generally associated with Mott insulators, even when described at the level of standard density functional theory. These are then dense bulk insulators that are correctly described as insulators in a standard band structure picture but that satisfy the common experimental criteria to classify them as Mott-type insulators.

We present a strategy to alleviate the sign problem in continuous-time quantum Monte Carlo (CTQMC) simulations of the dynamical-mean-field-theory (DMFT) equations for the spin-orbit-coupled multiorbital Hubbard model. We first identify the combinations of rotationally invariant Hund coupling terms present in the relativistic basis which lead to a severe sign problem. Exploiting the fact that the average sign in CTQMC depends on the choice of single-particle basis, we propose a bonding-antibonding basis $V_{j3/2\mathrm{BA}}$ which shows an improved average sign compared to the widely used relativistic basis for most parameter sets investigated. We then generalize this procedure by introducing a stochastic optimization algorithm that exploits the space of single-particle bases and show that $V_{j3/2\mathrm{BA}}$ is very close to optimal within the parameter space investigated. Our findings enable more efficient DMFT simulations of materials with strong spin-orbit coupling.