Breaking the diffraction limit and focusing laser beams to subwavelength scale are becoming possible with the help of recent developments in plasmonics. Such subwavelength focusing bridges different length scales of laser beams and matter. Here we consider optical vortex, or laser beam carrying orbital angular momentum (OAM), and discuss potential subwavelength magnetic phenomena induced by such laser. On the basis of numerical calculations using Landau-Lifshitz-Gilbert equation, we propose two OAM-dependent phenomena induced by optical vortices, generation of radially anisotropic spin waves and generation of topological defects in chiral magnets. The former could lead to the transient topological Hall effect through the laser-induced scalar spin chirality, and the latter reduces the time scale of generating skyrmionic defects by several orders compared to other known means.

We calculate the level compressibility $\chi (W,L)$ of the energy levels inside $[-L/2,L/2]$ for the Anderson model on infinitely large random regular graphs with on-site potentials distributed uniformly in $[-W/2,W/2]$. We show that $\chi (W,L)$ approaches the limit ${lim}_{L\to 0^{+}}\chi (W,L)=0$ for a broad interval of the disorder strength $W$ within the extended phase, including the region of $W$ close to the critical point for the Anderson transition. These results strongly suggest that the energy levels follow the Wigner-Dyson statistics in the extended phase, consistent with earlier analytical predictions for the Anderson model on an Erdös-Rényi random graph. Our results are obtained from the accurate numerical solution of an exact set of equations valid for infinitely large regular random graphs.

We investigated the magnetic structure of the heavy-fermion compound ${\mathrm{CePt}}_2{\mathrm{In}}_7$ below $T_N=5.34\left(2\right)$ K using magnetic resonant x-ray diffraction at ambient pressure. The magnetic order is characterized by a commensurate propagation vector ${\mathit{k}}_{1/2}=\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)$ with spins lying in the basal plane. Our measurements did not reveal the presence of an incommensurate order propagating along the high-symmetry directions in reciprocal space but cannot exclude other incommensurate modulations or weak scattering intensities. The observed commensurate order can be described equivalently by either a single-$\mathit{k}$ structure or by a multi-$\mathit{k}$ structure. Furthermore we explain how a commensurate-only ordering may explain the broad distribution of internal fields observed in nuclear quadrupolar resonance experiments [Sakai et al., Phys. Rev. B 83, 140408 (2011)] that was previously attributed to an incommensurate order. We also report powder x-ray diffraction showing that the crystallographic structure of ${\mathrm{CePt}}_2{\mathrm{In}}_7$ changes monotonically with pressure up to $P=7.3$ GPa at room temperature. The determined bulk modulus $B_0=81.1\left(3\right)$ GPa is similar to those of the Ce-115 family. Broad diffraction peaks confirm the presence of pronounced strain in polycrystalline samples of ${\mathrm{CePt}}_2{\mathrm{In}}_7$. We discuss how strain effects can lead to different electronic and magnetic properties between polycrystalline and single crystal samples.

We report time- and angle-resolved spectroscopic measurements in optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. The photoelectron intensity maps are monitored as a function of temperature, photoexcitation density, and delay time from the pump pulse. We evince that thermal fluctuations are effective only for temperatures near the critical value whereas photoinduced fluctuations scale linearly at low pumping fluence. The minimal energy to fully disrupt the superconducting gap slightly increases when moving off the nodal direction. No evidence of a pseudogap arising from other phenomena than pairing has been detected in the explored region of reciprocal space. On the other hand, a model accounting for the finite pair breaking explains the gap filling both in the near-nodal as well as in the off-nodal direction. Finally, we observed that nodal quasiparticles develop a faster dynamics when pumping the superconductor with fluence large enough to induce the total collapse of the gap.

To investigate the electronic structure of Weyl semimetals $\mathrm{Ta}Pn$ ($Pn=$ P, As), optical conductivity $\left[\sigma \right(\omega \left)\right]$ spectra are measured over a wide range of photon energies and temperatures, and these measured values are compared with band calculations. Two significant structures can be observed: a bending structure at $\hslash \omega \sim 85$ meV in TaAs, and peaks at $\hslash \omega \sim$ 50 meV (TaP) and $\sim 30$ meV (TaAs). The bending structure can be explained by the interband transition between saddle points connecting a set of $W_2$ Weyl points. The temperature dependence of the peak intensity can be fitted by assuming the interband transition between saddle points connecting a set of $W_1$ Weyl points. Owing to the different temperature dependence of the Drude weight in both materials, it is found that the Weyl points of TaAs are located near the Fermi level, whereas those of TaP are further away.

In the near field, Planck's law of blackbody radiation breaks down, and radiative heat transfer can be enhanced by orders of magnitude when surface polaritons are supported by interacting materials. However, such thermal radiation enhancement is strongly material dependent, thus difficult to control. Here, we propose a metamaterial-based structure consisting of patterned doped silicon nanorods that exhibits tunable narrow-band thermal emission. Direct numerical simulation based on the Wiener chaos expansion (WCE) method is performed to accurately investigate the heat transfer mechanism of metamaterials in the near field. The fundamental principle of the WCE method is elucidated, and an algorithm for symmetric and periodic structures is discussed. Implementation of the WCE method with the finite-difference time-domain method using the discrete dipole approximation (DDA) is also addressed in this paper.

The adsorption geometry, namely the height and the site, of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on the Cu(100) surface was determined by the normal incidence x-ray standing wave (NIXSW) technique including triangulation. The two PTCDA molecules in the superstructure unit cell, which have perpendicular azimuthal orientation, are both located at bridge sites, the long molecular axis being parallel to the bridge. Carboxylic oxygen atoms and several atoms of the carbon backbone are located close to on-top positions. The vertical distortion motif of PTCDA on Cu(100) differs from that on the three low-index Ag surfaces, because significant downward displacement of the carboxylic oxygen atoms is lacking. In particular, the carbon backbone of PTCDA adsorbs closer to the surface than extrapolated from Ag data. This suggests a relative increase of the attractive interactions between the carbon backbone of PTCDA and the Cu(100) surface versus the attractive interactions on the carboxylic oxygen atoms.

We have measured the reflectivity in the $ab$ plane and along the $c$ axis of a single crystal of the Dzyaloshinsky-Moriya multiferroic ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$, from 20 to 300 K and from 700 to 24 000 ${\mathrm{cm}}^{-1}$. The resulting optical conductivity $\sigma \left(\omega \right)$ has been compared with the results of calculations of its electronic band structure based on density functional theory (DFT). A broad midinfrared band, which in the $ab$ plane strongly hardens for decreasing temperature, is consistent with d-d transitions of the Cu atom. A further electronic absorption, whose edge falls in the near infrared and has a strong O-$p$ character, unusually softens with decreasing temperature by more than 0.15 eV, in both polarizations. Calculations show that the behavior with temperature of those electronic bands can be related to distortions of the ${\mathrm{CuO}}_4$ tetrahedra.

The organic charge-transfer salt $\kappa -\left(\text{BEDT-TTF}{)}_2\mathrm{Hg}\right(\mathrm{SCN}{)}_2\mathrm{Br}$ is a quasi-two-dimensional metal with a half-filled conduction band at ambient conditions. When cooled below $T=80\mathrm{K}$, it undergoes a pronounced transition to an insulating phase where the resistivity increases many orders of magnitude. In order to elucidate the nature of this metal-insulator transition, we have performed comprehensive transport, dielectric and optical investigations. The findings are compared with other dimerized $\kappa -\left(\text{BEDT-TTF}\right)$ salts, in particular the Cl analog, where a charge-order transition takes place at $T_{\mathrm{CO}}=30\mathrm{K}$.

We study the role of electron-phonon scattering for a pulse-triggered quantum dot single-photon source which utilizes a modified version of stimulated Raman adiabatic passage and cavity coupling. This on-demand source is coherently pumped with an optical pulse in the presence of a continuous-wave laser drive, allowing for efficient generation of indistinguishable single photons with polarizations orthogonal to the applied fields. In contrast to previous studies, we explore the role of electron-phonon scattering on this semiconductor system by using a polaron master equation approach to model the biexciton-exciton cascade and cavity mode coupling. In addition to background zero-phonon-line decoherence processes, microscopic electron–acoustic-phonon coupling, which usually degrades the indistinguishability and efficiency of semiconductor photon sources, is rigorously taken into account. We study how different system parameters (including cavity and laser detunings, cavity spectral width, temperature) affect the device performance and contrast the relative influence of intrinsic phonon coupling with other dephasing mechanisms. We describe how this biexciton-exciton cascade scheme allows for true single photons to be generated with over 90% quantum indistinguishability and efficiency simultaneously using realistic experimental parameters. We also show how the double-field dressing can be probed through the cavity-emitted spectrum.

We consider a strongly interacting quantum dot connected to two leads held at quite different temperatures. Our aim is to study the behavior of the Kondo effect in the presence of large thermal biases. We use three different approaches, namely, a perturbation formalism based on the Kondo Hamiltonian, a slave-boson mean-field theory for the Anderson model at large charging energies, and a truncated equation-of-motion approach beyond the Hartree-Fock approximation. The two former formalisms yield a suppression of the Kondo peak for thermal gradients above the Kondo temperature, showing a remarkably good agreement despite their different ranges of validity. The third technique allows us to analyze the full density of states within a wide range of energies. Additionally, we have investigated the quantum transport properties (electric current and thermocurrent) beyond linear response. In the voltage-driven case, we reproduce the split differential conductance due to the presence of different electrochemical potentials. In the temperature-driven case, we observe a strongly nonlinear thermocurrent as a function of the applied thermal gradient. Depending on the parameters, we can find nontrivial zeros in the electric current for finite values of the temperature bias. Importantly, these thermocurrent zeros yield direct access to the system's characteristic energy scales (Kondo temperature and charging energy).

We present a real-space formulation for calculating the electronic structure and optical conductivity of random alloys based on Kubo-Greenwood formalism interfaced with augmented space recursion technique [Mookerjee, J. Phys. C 6, 1340 (1973)] formulated with the tight-binding linear muffin-tin orbital basis with the van Leeuwen–Baerends corrected exchange potential [Singh, Harbola, Hemanadhan, Mookerjee, and Johnson, Phys. Rev. B 93, 085204 (2016)]. This approach has been used to quantitatively analyze the effect of chemical disorder on the configuration averaged electronic properties and optical response of two-dimensional honeycomb siliphene ${\mathrm{Si}}_x{\mathrm{C}}_{1\text{−}x}$ beyond the usual Dirac-cone approximation. We predicted the quantitative effect of disorder on both the electronic structure and optical response over a wide energy range, and the results are discussed in the light of the available experimental and other theoretical data. Our proposed formalism may open up a facile way for planned band-gap engineering in optoelectronic applications.

Understanding the collective dynamics in many-particle systems is of important significance in nonequilibrium physics. In this paper, we systematically study the long-time behavior in stochastically driven interacting quantum systems. We find that even though the stochastic forces will inevitably drive a system into a featureless steady state, the asymptotic behavior to approach the steady state can be highly nontrivial and exhibits rich universal dynamics due to the interplay between the stochastic driving and quantum many-body effects. Special attention is devoted to the effect of the mode-coupling perturbations, which may have important consequence for the long-time dynamics of the stochastically driven quantum many-body systems.

A scheme is presented for engineering momentum-space entanglement of fragmented magnon condensates. We consider easy-plane frustrated antiferromagnets in which the magnon dispersion has degenerate minima that represent “umbrella” chiral spin textures. We tune the Hamiltonian near a quantum critical point that is signaled by a singularity in the entanglement entropy. The ground state develops momentum-space entanglement of the chiral spin textures. The size of the entangled superposition is accessible experimentally through the magnetic structure factor. Our model is motivated by equilibrium magnon condensates in frustrated antiferromagnets such as ${\mathrm{CsCuCl}}_3$, and it can also be simulated in spin-orbit coupled Mott insulators in atomic optical lattices and circuit quantum electrodynamics.

The $J_1\text{−}{J}_2$ spin-chain model with nearest-neighbor $J_1$ and next-nearest-neighbor antiferromagnetic $J_2$ interaction is one of the most popular frustrated magnetic models. This model system has been extensively studied theoretically and applied to explain the magnetic properties of the real low-dimensional materials. However, the existence of different phases for the $J_1\text{−}{J}_2$ model in an axial magnetic field $h$ is either not understood or has been controversial. In this paper, we show the existence of higher order $p>4$ multipolar phase near the critical point ${(J_2/J_1)}_c=-0.25$. The criterion to detect the quadrupolar or spin nematic (SN)/spin density wave of type two $\left({\mathrm{SDW}}_2\right)$ phase using the inelastic neutron scattering (INS) experiment data is also discussed, and INS data of ${\mathrm{LiCuVO}}_4$ compound is modeled. We discuss the dimerized and degenerate ground state in the quadrupolar phase. The major contribution of binding energy in the spin-1/2 system comes from the longitudinal component of the nearest-neighbor bonds. We also study spin $\mathrm{nematic}/{\mathrm{SDW}}_2$ phase in spin-1 system in large $J_2/J_1$ limit.

Because of their universal nature, Fano fluctuations are expected to influence the response of superconducting-nanowire single-photon detectors (SNSPDs). We predict that photon counting rate ($PCR$) as a function of bias current ($I_B$) in SNSPDs is described by an integral over a transverse coordinate-dependent complementary error function. Fano fluctuations in the amount of energy deposited into the electronic system contribute to the finite width of this error function $\mathrm{\Delta }{I}_B$. The local response of an SNSPD can also affect this width: the location of the initial photon absorption site across the width of the wire can impact the probability of vortex-antivortex unbinding and vortex entry from the edges. In narrow-nanowire SNSPDs, the local responses are uniform, and Fano fluctuations dominate $\mathrm{\Delta }{I}_B$. We demonstrate good agreement between theory and experiments for a series of bath temperatures and photon energies in narrow-wire WSi SNSPDs. In a wide-nanowire device, the strong local dependence will introduce a finite width to the $PCR$ curve, but with sharp cusps. We show how Fano fluctuations can smooth these features to produce theoretical curves that better match experimental data. We also show that the time-resolved hotspot relaxation curves predicted by Fano fluctuations match the previously measured Lorentzian shapes (except for their tails) over the entire range of bias currents investigated experimentally.