Graphynes represent an emerging family of carbon allotropes that recently attracted much interest due to the tunability of the Dirac cones in the band structure. Here, we show that the spin-orbit couplings in $\beta \text{-graphyne}$ could produce various effects related to the topological properties of its electronic bands. Intrinsic spin-orbit coupling yields high- and tunable Chern-number bands, which may host both topological and Chern insulators, in the presence and absence of time-reversal symmetry, respectively. Furthermore, Rashba spin-orbit coupling can be used to control the position and the number of Dirac cones in the Brillouin zone. These findings suggest that spin-orbit-related physics in $\beta \text{-graphyne}$ is very rich, and, in particular, that this system could provide a platform for the realization of a two-dimensional material with tunable topological properties.

We show that the use of momentum-space optical interferometry, which avoids any spatial overlap between two parts of a macroscopic quantum state, presents a unique way to study coherence phenomena in polariton condensates. In this way, we address the longstanding question in quantum mechanics: “Do two components of a condensate, which have never seen each other, possess a definitive phase?” [P. W. Anderson, Basic Notions of Condensed Matter Physics (Benjamin Cummings, Menlo Park, CA, 1984)]. A positive answer to this question is experimentally obtained here for light-matter condensates, created under precise symmetry conditions, in semiconductor microcavities, taking advantage of the direct relation between the angle of emission and the in-plane momentum of polaritons.

A one-dimensional electron model with parameters modulated adiabatically in closed cycles is proposed which can continuously pump spin to leads. By defining the spin-polarized Wannier functions, we reveal that the spin pumping is protected by the spin Chern numbers, so that it is stable to perturbations violating the time-reversal symmetry and spin conservation. Our work demonstrates the principle for realizing robust topological spin pumps independent of the time-reversal symmetry. A possible way to directly observe the bulk topological invariants is also suggested.

We employ semilocal density functionals [local spin-density approximation (LSDA), Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA), and meta-GGAs)], LSDA plus Hubbard $U$ (LSDA+$U$) theory, a nonlocal range-separated Heyd-Scuseria-Ernzerhof hybrid functional (HSE06), and the random-phase approximation (RPA) to assess their performances for the ground-state magnetism and electronic structure of a strongly correlated metal, rutile ${\mathrm{VO}}_2$. Using recent quantum Monte Carlo results as the benchmark, all tested semilocal and hybrid functionals as well as the RPA (with PBE inputs) predict the correct magnetic ground states for rutile ${\mathrm{VO}}_2$. The observed paramagnetism could arise from temperature-disordered local spin moments or from the thermal destruction of these moments. All semilocal functionals also give the correct ground-state metallicity for rutile ${\mathrm{VO}}_2$. However, in the ferromagnetic (FM) and antiferromagnetic (AFM) phases, LSDA+$U$ and HSE06 incorrectly predict rutile ${\mathrm{VO}}_2$ to be a Mott-Hubbard insulator. For the computed electronic structures of FM and AFM phases, we find that the Tao-Perdew-Staroverov-Scuseria (TPSS) and revised TPSS (revTPSS) meta-GGAs give strong $2p\text{−}3d$ hybridizations, resulting in a depopulation of the $2p$ bands of O atoms, in comparison with other tested meta-GGAs. The regularized TPSS (regTPSS) and meta-GGAs made simple, i.e., MGGA_MS0 and MGGA_MS2, which are free of the spurious order-of-limits problem of TPSS and revTPSS, give electronic states close to those of the PBE GGA and LSDA. In comparison to experiment, semilocal functionals predict better equilibrium cell volumes for rutile ${\mathrm{VO}}_2$ in FM and AFM states than in the spin-unpolarized state. For meta-GGAs, a monotonic decrease of the exchange enhancement factor $F_x(s,\alpha )$ with α for small $s$, as in the MGGA_MS functionals, leads to large (probably too large) local magnetic moments in spin-polarized states.

Near-field infrared spectroscopy by elastic scattering of light from a probe tip resolves optical contrasts in materials at dramatically subwavelength scales across a broad energy range, with the demonstrated capacity for chemical identification at the nanoscale. However, current models of probe-sample near-field interactions still cannot provide a sufficiently quantitatively interpretation of measured near-field contrasts, especially in the case of materials supporting strong surface phonons. We present a model of near-field spectroscopy derived from basic principles and verified by finite-element simulations, demonstrating superb predictive agreement both with tunable quantum cascade laser near-field spectroscopy of ${\mathrm{SiO}}_2$ thin films and with newly presented nanoscale Fourier transform infrared (nanoFTIR) spectroscopy of crystalline SiC. We discuss the role of probe geometry, field retardation, and surface mode dispersion in shaping the measured near-field response. This treatment enables a route to quantitatively determine nanoresolved optical constants, as we demonstrate by inverting newly presented nanoFTIR spectra of an ${\mathrm{SiO}}_2$ thin film into the frequency dependent dielectric function of its mid-infrared optical phonon. Our formalism further enables tip-enhanced spectroscopy as a potent diagnostic tool for quantitative nanoscale spectroscopy.

Many topologically nontrivial states of matter possess gapless degrees of freedom on the boundary, and when these boundary states delocalize into the bulk, a phase transition occurs, and the system becomes topologically trivial. We show that tensor networks provide a natural framework for analyzing such topological phase transitions in terms of the boundary degrees of freedom which mediate it. To do so, we make use of a correspondence between a topologically nontrivial ground state and its phase transition to a trivial phase established in T. Hsieh and L. Fu (arXiv:1305.1949). This involved computing the bulk entanglement spectrum (BES) of the ground state upon tracing out an extensive subsystem. This work implements BES via tensor network representations of ground states. In this framework, the universality class of the quantum critical entanglement Hamiltonian in $d$ spatial dimensions is either derived analytically or mapped to a classical statistical model in $d+1$ dimensions, which can be studied using Monte Carlo or tensor renormalization-group methods. As an example, we analytically derive the universality classes of topological phase transitions from the spin-1 chain Haldane phase and demonstrate that the Affleck-Kennedy-Lieb-Tasaki (AKLT) wave function (and its generalizations) remarkably contains critical six-vertex (and, in general, eight-vertex) models within it.

We calculate thermodynamic properties of soft-core lattice bosons with on-site $n$-body interactions using up to twelfth- and tenth-order strong coupling expansion in one- and two-dimensional cubic lattices at zero temperature. Using linked cluster techniques, we show that it is possible to exactly renormalize the two-body interactions for quasiparticle excitations and ground-state energy by resumming the three- and four-body terms in the system, which suggests that all higher-body on-site interactions may be exactly and perturbatively resummed into the two-body terms for similar system observables. The renormalization procedure that we develop is applicable to a broad range of systems analyzable by linked cluster expansions, ranging from perturbative quantum chromodynamics to spin models, giving either an exact or approximate resummation depending on the specific system and properties. Universality at various three-body interaction strengths for the two-dimensional boson Hubbard model is checked numerically.

Resonant cooling of different nuclear isotopes manifested in optically induced nuclear magnetic resonances (NMR) is observed in $n$-doped CdTe/(Cd,Mg)Te and ZnSe/(Zn,Mg)Se quantum wells and for donor-bound electrons in ZnSe:F and GaAs epilayers. By time-resolved Kerr rotation used in the regime of resonant spin amplification, we can expand the range of magnetic fields where the effect can be observed up to nuclear Larmor frequencies of 170 kHz. The mechanism of the resonant cooling of the nuclear spin system is analyzed theoretically. The developed approach allows us to model the resonant spin amplification signals with NMR features.

Quantum plasmonics is a field of research combining plasmonics with quantum optics and investigates interactions between photons and metallic nanostructures. So far, it has been proven that quantum properties of single photons to excite single surface plasmon polaritons (SPPs) are preserved in the process of photon-SPP-photon mode conversion in plasmonic nanostructures, which suggests the potential application of SPPs to the quantum information processing (QIP). Recently the Hong-Ou-Mandel (HOM) interference of single SPPs was observed in a plasmonic circuitry. However, the visibility was below the classical limit (50%) due to the simultaneous excitation of distinguishable SPP modes. We employed a directional coupler based on long-range surface-plasmon-polariton waveguides (LRSPP-DC) and superconducting photon-number-resolving detectors to directly observe the bosonic quantum interference of single SPPs beyond the classical limit. In addition, we demonstrated the indistinguishability of photons that excite single SPPs is well preserved in the process of photon-SPP mode conversion.

Using the tight-binding method in combination with first-principles calculations, we systematically derive a low-energy effective Hilbert subspace and Hamiltonian with spin-orbit coupling for two-dimensional hydrogenated and halogenated group-V monolayers. These materials are proposed to be giant-gap quantum spin Hall insulators with record huge bulk band gaps opened by the spin-orbit coupling at the Dirac points, e.g., from 0.74 to 1.08 eV in $\mathrm{Bi}X$ ($X=\mathrm{H}$, F, Cl, and Br) monolayers. We find that the low-energy Hilbert subspace mainly consists of $p_x$ and $p_y$ orbitals from the group-V elements, and the giant first-order effective intrinsic spin-orbit coupling is from the on-site spin-orbit interaction. These features are quite distinct from those of group-IV monolayers such as graphene and silicene. There, the relevant orbital is $p_z$ and the effective intrinsic spin-orbit coupling is from the next-nearest-neighbor spin-orbit interaction processes. These systems represent the first real 2D honeycomb lattice materials in which the low-energy physics is associated with $p_x$ and $p_y$ orbitals. A spinful lattice Hamiltonian with an on-site spin-orbit coupling term is also derived, which could facilitate further investigations of these intriguing topological materials.

The interplay between vacancies ($V$) and interstitial solutes $X$ ($X=\text{C}$, N, and O) and its impact on thermodynamic properties of $\alpha \text{−}\mathrm{Fe}$ solid solutions are studied, starting from first principles calculations. A systematic comparison between the three solutes is performed, investigating $X–\mathrm{Fe}$, $X$–$X$, and $V$–$X$ interactions. In the $\alpha \text{−}\mathrm{Fe}$ lattice, the strength of $X–\mathrm{Fe}$ interactions is found to govern the dissolution properties. Next to vacancies, the competition between solute volume effects and $X–\mathrm{Fe}$ interactions results in the preference of all the solutes to occupy off-centered sites. Low-energy configurations of small $V_n{X}_m$ clusters are calculated for $n$ and $m$ up to 4. They are used to parametrize lattice interaction models at the atomic scale. A detailed analysis of the cluster properties suggests the relevance of many-body terms in these models. The accuracy of the resulting models is verified through their satisfactory prediction of vacancy-solute cluster properties beyond the fitting database. From these models, an entire set of $V_n{X}_m$ clusters is generated with a new configurational space exploration method. Statistical treatment of the solid solution including these clusters is then achieved by means of low-temperature expansions, checked against Monte Carlo simulations in some specific conditions. Based on the calculation of equilibrium cluster distributions, it is shown that the solubility limit of oxygen in Fe, hardly measurable experimentally, is largely affected by the presence of small $V_n{\mathrm{O}}_m$ clusters.

We elaborate a novel and efficient method to obtain multiphase Helmholtz free energies from molecular dynamics (MD) simulations over wide ranges of volume and temperature in materials that can be described by temperature-independent ion forces, with both higher accuracy and order-of-magnitude cost savings compared to direct thermodynamic-integration techniques. Our method leverages and significantly extends the technique of reversible-scaling molecular dynamics (RSMD) proposed by de Koning et al. [Phys. Rev. Lett. 83, 3973 (1999)], which allows a free-energy difference in a given phase at constant volume to be calculated as a function of temperature from a single MD simulation. In mechanically stable solid phases, our approach carefully combines quasiharmonic lattice dynamics at low temperatures with an accurate and fully isolated RSMD simulation of the anharmonic vibrational free energy at high temperatures to produce a seamless free energy from zero temperature to above melt along constant-volume isochores. In the liquid, we combine a unique calculation of the free energy along a high-temperature reference isotherm with isochoric RSMD simulations from that temperature to below melt. In metastable solid phases that are mechanically unstable at low temperature, we use two-phase MD melt simulations together with the liquid free energy to obtain the solid free energy along the solidus melt line and then perform isochoric RSMD simulations to temperatures above and below that point. While our free-energy method is general, we have specifically adapted it here to the case of metals in which the ion forces are well described by model generalized pseudopotential theory (MGPT) multi-ion interatomic potentials, and additive electron-thermal free-energy contributions can be included. Then using refined Ta6.8$x$ MGPT potentials, we have converged total free energies and their components to very high and unprecedented sub-milli-Rydberg (mRy) numerical accuracy in the stable-bcc, liquid, and metastable-fcc phases of tantalum for volumes ranging from up to 26% expansion to nearly twofold compression and for temperatures to 25 000 K. In turn, we have successfully used the free energies so obtained to calculate physically accurate thermodynamic properties and gain new insight into their behavior, including sensitive thermodynamic derivatives, bcc and fcc melt curves, and a multiphase equation of state for tantalum (Ta) over the same temperature range and for pressures as high as 600 GPa. We show that the anharmonic free-energy component in the bcc solid, although only 1–5 mRy in magnitude for Ta, can have a significant (15%–20%) effect on thermal expansivity, the Grüneisen parameter, and melt temperatures. We further show that the electron-thermal free-energy component can similarly impact the specific heat and thermal expansivity in both the solid and the liquid, while only minimally affecting (to$\le 3$%) the bcc and fcc melt curves.

In this paper, we study how to preserve entanglement and nonlocality under dephasing produced by classical noise with large low-frequency components, such as $1/f$ noise, using dynamical decoupling techniques. We first show that quantifiers of entanglement and nonlocality satisfy a closed relation valid for two independent qubits locally coupled to a generic environment under pure dephasing and starting from a general class of initial states. This result allows us to assess the efficiency of pulse-based dynamical decoupling for protecting nonlocal quantum correlations between two qubits subject to pure-dephasing local random telegraph and $1/f$ noise. We investigate the efficiency of an “entanglement memory” element under two-pulse echo and under sequences of periodic, Carr-Purcell, and Uhrig dynamical decoupling. The Carr-Purcell sequence is shown to outperform the other sequences in preserving entanglement against both random telegraph and $1/f$ noise. For typical $1/f$ flux-noise figures in superconducting nanocircuits, we show that entanglement and its nonlocal features can be efficiently stored up to times one order of magnitude longer than natural entanglement disappearance times employing pulse timings of current experimental reach.

We find the relation between the energy of the absorbed photon and the threshold current at which the resistive state appears in the current-carrying superconducting film with probability about unity. In our calculations we use the modified hot-spot model, which assumes a different strength of suppression of the superconducting order parameter in the finite area of the film around the place where the photon is absorbed. To find the threshold current we solve the Ginzburg-Landau equation for the superconducting order parameter, which automatically includes the current continuity equation and it allows us to consider the back effect of current redistribution near the hot spot on the stability of the superconducting state. We find quantitative agreement with recent experiments, where we use the single fitting parameter which describes what part of the energy of the photon goes for the local destruction of the superconductivity in the film.

We measured the temperature dependence of the saturation magnetization $\left(M_s\right)$ of a $\left({\mathrm{La}}_{1-x}{\mathrm{Pr}}_x\right){}_{1-y}{\mathrm{Ca}}_y{\mathrm{MnO}}_3$ $(x\sim 0.60$, $y\sim \mathrm{0.33})$ film as a function of applied bending stress. Stress producing a compressive strain of −0.01% along the magnetic easy axis increased the Curie temperature by ∼6 K and the metal-insulator transition by ∼4 K. Regardless of whether or not stress is applied to the film, magnetic ordering occurs at temperatures significantly higher than the metal-insulator-transition temperature. The magnetization of the sample at the temperature of the metal-insulator transition is approximately the site percolation threshold for a two-dimensional spin lattice.

We study a neutron diffraction by A phase of MnSi using a dynamical theory of diffraction and three wave approximation. We show that the neutron diffraction is asymmetrical with respect to an incident plane. The asymmetry depends on a sign of an external magnetic field. This phenomenon can be considered as the Hall effect for neutrons.

The magnetothermal properties of monocrystalline yttrium iron garnet (YIG) are reported. The magnon contribution to both the thermal conductivity and specific heat at low temperatures has been determined by measuring these properties under an applied magnetic field, which allows us to freeze the magnon modes and isolate the phonon contribution relative to the zero-field behavior. These results are interpreted within the framework of a simple kinetic gas model for magnon heat conduction that allows us to estimate the magnon thermal mean free path, i.e., the inelastic scattering length scale for thermally driven bulk magnons. We observe this parameter to reach as high as approximately 100 μm at 2 K. It tracks the acoustic phonon thermal mean free path closely and decreases rapidly as the temperature is increased. This relatively short length scale suggests that magnon modes at thermal energies in YIG are not solely or directly responsible for coherent macroscale thermal spin transport (e.g., in the spin Seebeck effect) at high temperatures. Instead, these results support a growing consensus that subthermal magnons, i.e., those at energies below about 30 ± 10 K, are important for spin transport in YIG at all temperatures. These results also emphasize that magnon effects should be considered wavelength dependent, and that magnon-magnon interactions may be just as important for thermal spin transport as magnon-phonon scattering. This, in turn, has implications for understanding the characteristic temperature and length scales involved in spin caloritronic phenomena.