We present an approach to describing fluctuational electrodynamic interactions, particularly van der Waals (vdW) interactions as well as radiative heat transfer (RHT), between material bodies of potentially vastly different length scales, allowing for going between atomistic and continuum treatments of the response of each of these bodies as desired. Any local continuum description of electromagnetic response is compatible with our approach, while atomistic descriptions in our approach are based on effective electronic and nuclear oscillator degrees of freedom, encapsulating dissipation, short-range electronic correlations, and collective nuclear vibrations (phonons). While our previous works using this approach have focused on presenting novel results, this work focuses on the derivations underlying these methods. First, we show how the distinction between “atomic” and “macroscopic” bodies is ultimately somewhat arbitrary, as formulas for vdW free energies and radiative heat transfer look very similar regardless of how the distinction is drawn. Next, we demonstrate that the atomistic description of material response in our approach yields electromagnetic interaction matrix elements which are expressed in terms of analytical formulas for compact bodies or semianalytical formulas based on Ewald summation for periodic media; we use this to compute vdW interaction free energies as well as RHT powers among small biological molecules in the presence of a metallic plate as well as between parallel graphene sheets in vacuum, showing strong deviations from conventional macroscopic theories due to the confluence of geometry, phonons, and electromagnetic retardation effects. Finally, we propose formulas for efficient computation of fluctuational electrodynamic interactions among material bodies in which those that are treated atomistically as well as those treated through continuum methods may have arbitrary shapes, extending previous surface-integral techniques.

We present a general nonequilibrium Green's function formalism for modeling heat transfer in systems characterized by linear response that establishes the formal algebraic relationships between phonon and radiative conduction, and reveals how upper bounds for the former can also be applied to the latter. We also propose an extension of this formalism to treat systems susceptible to the interplay of conductive and radiative heat transfer, which becomes relevant in atomic systems and at nanometric and smaller separations where theoretical descriptions which treat each phenomenon separately may be insufficient. We illustrate the need for such coupled descriptions by providing predictions for a low-dimensional system of carbyne wires in which the total heat transfer can differ from the sum of its radiative and conductive contributions. Our framework has ramifications for understanding heat transfer between large bodies that may approach direct contact with each other or that may be coupled by atomic, molecular, or interfacial film junctions.

Recent experimental advances probing coherent phonon and electron transport in nanoscale devices at contact have motivated theoretical channel-based analyses of conduction based on the nonequilibrium Green's function formalism. Transmission through each channel is known to be bounded above by unity, yet in practice usually falls far below this Landauer limit. Building upon recently derived radiative heat transfer limits and a unified formalism characterizing heat transport for arbitrary bosonic systems in the linear regime, we propose new bounds on conductive heat transfer. In particular, we demonstrate that our limits are typically far tighter than the Landauer limits per channel and are close to actual transmission eigenvalues by examining a model of phonon conduction in a one-dimensional chain. Our limits have ramifications for designing molecular junctions to optimize conduction.

The distribution of higher-order level spacings, i.e., the distribution of $\{s_i^{\left(n\right)}=E_{i+n}-E_i\}$ with $n\ge 1$ is derived analytically using a Wigner-like surmise for Gaussian ensembles of random matrix as well as Poisson ensemble. It is found $s^{\left(n\right)}$ in Gaussian ensembles follows a generalized Wigner-Dyson distribution with rescaled parameter $\alpha =\nu C_{n+1}^2+n-1$, whereas that in the Poisson ensemble follows a generalized semi-Poisson distribution with index $n$. Numerical evidences are provided through simulations of random spin systems as well as nontrivial zeros of the Riemann $\zeta$ function. The higher-order generalizations of gap ratios are also discussed.

We experimentally study the Mn substitution effect on thermoelectric properties in a full-Heusler alloy, ${\mathrm{Fe}}_2\mathrm{TiSi}$. By employing molecular beam epitaxy, homogeneous and $L{2}_1$-ordered ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers are achieved. The lattice constant, the saturation magnetic moment, and electrical resistivity are intentionally controlled with increasing Mn substitution. Notably, we find that the sign of the Seebeck coefficient is varied from positive to negative at around $x=0.2$. On the basis of the first-principles calculations, we qualitatively understand that the observed variation in the electrical and thermoelectric properties arises from the change in the electronic band structure near the Fermi level, dominated by Fe and Mn atoms in ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$.

Quantum tunneling of the magnetization is a major obstacle to the use of single-molecule magnets as basic constituents of next-generation storage devices. In this context, phonons are often only considered (perturbatively) as disturbances that promote the spin system to traverse the anisotropy barrier. Here, we demonstrate the ability of phonons to induce a tunnel splitting of the ground doublet which then reduces the required bistability due to Landau-Zener tunneling of the magnetization. Harmful are those phonons that modify the spin Hamiltonian so that its rotational symmetry about the field axis is destroyed. In our calculations we treat spins and phonons on the same footing by performing quantum calculations of a Hamiltonian where the single-ion anisotropy tensors are coupled to harmonic oscillators.

We use a combination of three computational methods to investigate the notoriously difficult frustrated three-dimensional pyrochlore $S=\frac{1}{2}$ quantum antiferromagnet, at finite temperature $T$: canonical typicality for a finite cluster of $2\times 2\times 2$ unit cells (i.e., 32 sites), a finite-$T$ matrix product state method on a larger cluster with 48 sites, and the numerical linked cluster expansion (NLCE) using clusters up to 25 lattice sites, including nontrivial hexagonal and octagonal loops. We calculate thermodynamic properties (energy, specific heat capacity, entropy, susceptibility, magnetization) and the static structure factor. We find a pronounced maximum in the specific heat at $T=0.57J$, which is stable across finite size clusters and converged in the series expansion. At $T\approx 0.25J$ (the limit of convergence of our method), the residual entropy per spin is $0.47k_{B}ln2$, which is relatively large compared to other frustrated models at this temperature. We also observe a nonmonotonic dependence on $T$ of the magnetization at low magnetic fields, reflecting the dominantly nonmagnetic character of the low-energy states. A detailed comparison of our results to measurements for the $S=1$ material ${\mathrm{NaCaNi}}_2{\mathrm{F}}_7$ yields a rough agreement of the functional form of the specific heat maximum, which in turn differs from the sharper maximum of the heat capacity of the spin ice material ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$.

Cooper pairs in chiral superfluids carry quantized units of relative orbital angular momentum (OAM). Various predictions of the intrinsic OAM density or the macroscopic OAM of a two-dimensional chiral superfluid differ by several orders of magnitude, which constitute the so-called angular momentum paradox. Following several previous studies, we substantiate the semiclassical Bogoliubov–de Gennes theory of the single-particle edge current and OAM in two-dimensional chiral superfluids in the BCS limit. The analysis provides a simple intuitive understanding for the vanishing of OAM for a non-$p$-wave chiral superfluid (such as $d+id$) confined in a rigid potential. When generalized to anisotropic chiral superconductors and three-dimensional chiral superfluids, the theory similarly returns an accurate description. We also present a detailed numerical study of the chiral phases in the BEC limit. Our study suggests that, in both BCS and BEC phases, the relative OAM of the individual Cooper pairs contribute to the total OAM additively, and that in both phases the corresponding macroscopic OAM density distribution is localized at the boundary.

Copper sulphide (${\mathrm{Cu}}_2\mathrm{S}$) and copper selenide (${\mathrm{Cu}}_2\mathrm{Se}$) are known to exist in solid (S/Se)-liquid (Cu) hybrid phases which exhibit favorable thermoelectrics properties. The diffusion characteristics and its mechanism in these systems are therefore of significant interest. In this paper, we analyze these properties through examining the atomic radial distributions, mean-square displacements, and velocity autocorrelations obtained from ab initio molecular dynamics simulations. Exceptionally high Cu diffusion coefficients with values over ${10}^{-5}{\mathrm{cm}}^2$/s are obtained, indicating the unexpected liquidlike behavior of Cu in these weakly ionic compounds. The diffusion mechanism obtained through analysis of the Cu atomic trajectories is found to be at variance with the previously proposed Chudley-Elliott jump diffusion model. In addition, tunability of these diffusion coefficients via small changes in the stoichiometry, namely, Cu deficiencies, is demonstrated. The higher number of low-frequency acoustic phonon modes associated with Se sublattices, compared to those with S sublattices, correlates well with the experimentally observed thermal conductivity difference between ${\mathrm{Cu}}_2\mathrm{S}$ and ${\mathrm{Cu}}_2\mathrm{Se}$.

Ni-Co-Mn-In Heusler-based compounds are interesting for their magnetocaloric properties and have been widely investigated for this purpose. For Co compositions more than 5 at% in (${\mathrm{Ni}}_{100-x}{\mathrm{Co}}_x{)}_{50}{\mathrm{Mn}}_{25+y}{\mathrm{In}}_{25-y}$ the material is no longer single phase, and for $y<25$, shell-ferromagnetic precipitation occurs. Our study is twofold: First we study here the shell-ferromagnetic properties of these systems and show that their ferromagnetic exchange can be strengthened by introducing Co into the precipitate. Second, we further show that both the multiphase character and shell-ferromagnetic precipitation have strong implications on the magnetocaloric properties.

The electronic structure and magnetism of individual Dy atoms adsorbed on the graphene/Cu(111) surface is investigated using the combination of the density functional theory with the Hubbard-I approximation to the Anderson impurity model ($\mathrm{DFT}+\mathrm{U}+\mathrm{HIA}$). We find that the results of the $\mathrm{DFT}+\mathrm{U}+\mathrm{HIA}$ depend on the choice of the double-counting term. For fully localized limit, the divalent ${\mathrm{Dy}}^{2+}$ adatom is found, with the total magnetic moment of $9.71{\mu }_B$. The spin and orbital magnetic moments are evaluated, and compared with the x-ray magnetic circular dichroism data. The calculated positive magnetic anisotropy energy determines the out-of-plane orientation of the Dy adatom magnetic moment, in agreement with available experimental data.

The coexistence of the type-II Weyl semimetal phase and superconductivity in the layered transition-metal dichalcogenide $\mathrm{Mo}{\mathrm{Te}}_2$ is of great interest as it is considered a promising candidate for an intrinsic topological superconductor. However, the ultralow superconducting critical temperature ($T_c$) of ∼0.1 K limits the exploration of the intrinsic superconducting properties. Here the superconductivity in $\mathrm{Mo}{\mathrm{Te}}_2$ is investigated by systematic ultralow temperature transport measurements with standard four-electrode configuration, together with the hard point-contact (PC) technique which can enhance the $T_c$ and detect the in situ spectroscopic information of the pairing symmetry. The temperature dependence of the superconducting critical field from transport measurements and the two-step double conductance peaks from the PC spectra measurements both suggest the two-band superconductivity in $\mathrm{Mo}{\mathrm{Te}}_2$. Together with the occasional observation of zero-bias conductance peaks in the spectra, our results suggest the unconventional two-band superconductivity with potential $s_{+-}$ pairing in $\mathrm{Mo}{\mathrm{Te}}_2$.

Proximity-effect-induced superconductivity is studied for high-quality ${\mathrm{Bi}}_2{\mathrm{Te}}_3/\mathrm{Fe}{\mathrm{Se}}_{0.5}{\mathrm{Te}}_{0.5}$ (FST) thin-film heterostructures with atomically sharp interfaces and for ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{TiO}}_2$/FST heterostructures where ${\mathrm{TiO}}_2$ is the ultrathin buffer layer. Adjusting the buffer layer thickness from zero to several nanometers is used to simulate different interface conditions: strongly, weakly, and zero-coupled interfaces. It is found that, for the strongly coupled interface, both the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin film and the FST have almost the same magnitude as the superconducting pair potential; for the weakly coupled interface, the pairing potential of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin film exhibits an approximately exponential decay with the ${\mathrm{TiO}}_2$ buffer layer thickness. Our experimental results can be explained qualitatively by several theories under different interface conditions.

Hybrid quantum-classical algorithms have been proposed as a potentially viable application of quantum computers. A particular example—the variational quantum eigensolver, or VQE—is designed to determine a global minimum in an energy landscape specified by a quantum Hamiltonian, which makes it appealing for the needs of quantum chemistry. Experimental realizations have been reported in recent years and theoretical estimates of its efficiency are a subject of intense effort. Here we consider the performance of the VQE technique for a Hubbard-like model describing a one-dimensional chain of fermions with competing nearest- and next-nearest-neighbor interactions. We find that recovering the VQE solution allows one to obtain the correlation function of the ground state consistent with the exact result. We also study the barren plateau phenomenon for the Hamiltonian in question and find that the severity of this effect depends on the encoding of fermions to qubits. Our results are consistent with the current knowledge about the barren plateaus in quantum optimization.

A defining feature of topologically ordered states of matter is the existence of locally indistinguishable states on spaces with nontrivial topology. These degenerate states form a representation of the mapping class group (MCG) of the space, which is generated by braids of defects or anyons, and by Dehn twists along noncontractible cycles. These operations can be viewed as fault-tolerant logical gates in the context of topological quantum error correcting codes and topological quantum computation. Here, we show that braids and Dehn twists can in general be implemented by a constant depth quantum circuit, with a depth that is independent of code distance $d$ and system size. The circuit consists of a constant depth local quantum circuit (LQC) implementing a local geometry deformation of the quantum state, followed by a permutation on (relabeling of) the qubits. The permutation requires permuting qubits that are separated by a distance of order $d$; it can be implemented by collective classical motion of mobile qubits or as a constant depth circuit using long-range SWAP operations (with a range set by $d$) on immobile qubits. We further show that (i) applying a given braid or Dehn twist $k$ times can be achieved with $\mathcal{O}(logk)$ time overhead, independent of code distance and system size, which implies an exponential speedup for certain logical gate sequences by trading space for time, and (ii) an arbitrary element of the MCG can be implemented by a constant depth (independent of $d$) LQC followed by a permutation, where in this case the range of interactions of the LQC grows with the number of generators in the presentation of the group element. Applying these results to certain non-Abelian quantum error correcting codes demonstrates how universal logical gate sets can be implemented on encoded qubits using only constant depth unitary circuits.

Periodic excitation of electron spin polarization by consecutive laser pulses in phase with Larmor spin precession about a magnetic field results in resonant spin amplification (RSA). We observe a drastic modification of RSA in $n$-doped bulk GaAs under the influence of external oscillating magnetic field. We find a double-peaked electron spin resonance instead of a single-peaked resonance expected without optical pumping. The frequency splitting increases linearly with amplitude of field oscillations, while the spin deviation increases quadratically. Moreover, we show that the oscillating field can both significantly suppress RSA and induce new conditions for resonance. Using quaternions to describe spin rotations, we develop a theory that simultaneously considers spin precession, decay, and amplification and reproduces the entire set of the experimental data. Using the radio-frequency field allows one to control the conditions of RSA and achieve fine tuning of average spin polarization without modifying the parameters of optical pumping.

Due to its highly corrugated moiré structure and the nanometer modulation of its electronic properties, graphene deposited on ruthenium substrates (G-Ru for short) is a versatile playground for molecular deposition and chemical reactivity. Under local ultrahigh pressure conditions, one can expect an even richer behavior, reinforced by the likely appearance of defects. We have theoretically investigated the formation of such defects on G-Ru by using density functional theory methods. We show that defects can be produced in the high areas of the moiré, either by generating reactive centers in graphene through the creation of vacancies or by modifying the relative carbon-ruthenium positions through conformational changes. The energetic cost to induce these defects is rather small, of the order of a few eV, so that defects are expected to appear when impacting a STM tip or applying high pressure by a diamond AFM tip in these regions. The different types of defects can be clearly distinguished from each other in our simulated STM images.

Bilayer Heisenberg ferromagnetic insulators hold degenerate terahertz Dirac magnon modes associated with two opposite valleys of the hexagonal Brillouin zone. We show that this energy degeneracy can be removed by breaking of the inversion symmetry ($\mathcal{I}$), leading to a topological magnon valley current. We show furthermore that this current leads to valley Seebeck effect for magnons and is thereby detectable. We perform calculations in the specific example of bilayer ${\mathrm{CrBr}}_3$, where $\mathcal{I}$ can be broken by electrostatic doping.

The adsorption of CO on ${\mathrm{Pt}}_4$ clusters supported on graphene with lattice vacancies is studied theoretically using the first-principles calculation. Our results show that the electronic structure of the graphene-supported ${\mathrm{Pt}}_4$ clusters is significantly modified by the interaction with carbon dangling bonds. As a result the adsorption energy of CO at a Pt site decreases almost linearly with the lowering of the Pt $d$-band center, in analogy with the linear law previously reported for CO adsorption on various Pt surfaces. An exceptional behavior is found for ${\mathrm{Pt}}_4$ supported on graphene with a tetravacancy, where CO adsorption is noticeably weaker than predicted by the shift in the $d$-band center. Detailed electronic structure analyses reveal that the deviation from the linear scaling can be attributed to lack of Pt $d$ states near the Fermi level that hybridize with CO molecular orbitals. The weakening of CO adsorption on the ${\mathrm{Pt}}_4$ clusters is considered as a manifestation of the support effect of graphene, and leads to the enhancement of CO poisoning tolerance that is crucial for developing high-performance Pt cluster catalysts.

We propose a theoretical study on the electromagnetic forces resulting from the superposition of TE and TM plane waves interacting with a sphere. Specifically, we first show that, under such an illumination condition, the sphere is subjected to a force transverse to the propagation direction of the waves. We then analyze the physical origin of this counterintuitive behavior using a multipolar decomposition of the electromagnetic modes involved in that scattering process. This analysis reveals that interference effects, due to the two-wave illumination, lead to a Kerker-like asymmetric scattering behavior resulting in this peculiar transverse force.