Using resonant soft x-ray techniques we follow the magnetic behavior of a strained epitaxial film of $\mathrm{CoC}{\mathrm{r}}_2{\mathrm{O}}_4$, a type-II multiferroic. The film is [110] oriented, such that both the ferroelectric and ferromagnetic moments can coexist in-plane. X-ray magnetic circular dichroism (XMCD) is used in scattering and in transmission modes to probe the magnetization of Co and Cr separately. The transmission measurements utilized x-ray excited optical luminescence from the substrate. Resonant soft x-ray diffraction (RXD) was used to study the magnetic order of the low temperature phase. The XMCD signals of Co and Cr appear at the same ordering temperature $T_C\approx 90\mathrm{K}$, and are always opposite in sign. The coercive field of the Co and of Cr moments is the same, and is approximately two orders of magnitude higher than in bulk. Through sum rules analysis an enlarged $\mathrm{C}{\mathrm{o}}^{2+}$ orbital moment ($m_L$) is found, which can explain this hardening. The RXD signal of the (q q 0) reflection appears below $T_S$, the same ordering temperature as the conical magnetic structure in bulk, indicating that this phase remains multiferroic under strain. To describe the azimuthal dependence of this reflection, a slight modification is required to the spin model proposed by the conventional Lyons-Kaplan-Dwight-Menyuk theory for magnetic spinels.

Paramagnetic ${\mathrm{Ce}}^{3+}$optical emitters have been studied by means of optically detected magnetic resonance (ODMR) via ${\mathrm{Ce}}^{3+}$ spin-dependent emission in cerium-doped garnet crystals which were both gadolinium free and contain gadolinium in a concentration from the lowest (0.1%) to 100%, i.e., to the superparamagnetic state. It has been shown that the intensity of photoluminescence excited by circularly polarized light into ${\mathrm{Ce}}^{3+}$ absorption bands can be used for selective monitoring the population of the ${\mathrm{Ce}}^{3+}$ ground-state spin sublevels. Direct evidence of the cross-relaxation effects in garnet crystals containing two electron spin systems, i.e., the simplest one of ${\mathrm{Ce}}^{3+}$ ions with the effective spin $S=\frac{1}{2}$ and the system of ${\mathrm{Gd}}^{3+}$ ions with the maximum spin $S=\frac{7}{2}$, has been demonstrated. Magnetic resonance of ${\mathrm{Gd}}^{3+}$ has been found by monitoring ${\mathrm{Ce}}^{3+}$ emission in cerium-doped garnet crystals with gadolinium concentrations of 0.1 at. %, 4%–8%, and 100%, which implies the impact of the ${\mathrm{Gd}}^{3+}$ spin polarization on the optical properties of ${\mathrm{Ce}}^{3+}$. Strong internal magnetic fields in superparamagnetic crystals were shown to modify the processes of recombination between UV-radiation-induced electron and hole centers that lead to the recombination-induced ${\mathrm{Ce}}^{3+}$ emission. Observation of spikes and subsequent decay in the cross-relaxation-induced ODMR signals under pulsed microwave excitation is suggested to be an informative method to investigate transient processes in the many-spin system of ${\mathrm{Ce}}^{3+}, {\mathrm{Gd}}^{3+}$, and electron and hole radiation-induced centers.

Single crystals of the ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$-Ru eutectic system are known to exhibit enhanced superconductivity at 3 K in addition to the bulk superconductivity of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ at 1.5 K. The 1.5 K phase is believed to be a spin-triplet, chiral $p$-wave state with a multicomponent order parameter, giving rise to chiral domain structure. In contrast, the 3 K phase is attributable to enhanced superconductivity of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ in the strained interface region between Ru inclusion of a few to tens of micrometers in size and the surrounding ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. We investigate the dynamic behavior of a topological junction, where a superconductor is surrounded by another superconductor. Specifically, we fabricated Nb/Ru/${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ topological superconducting junctions, in which the difference in phase winding between the $s$-wave superconductivity in Ru microislands induced from Nb and the superconductivity of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ mainly governs the junction behavior. Comparative results of the asymmetry, hysteresis, and noise in junctions with different sizes, shapes, and configurations of Ru inclusions are explained by the chiral domain-wall motion in these topological junctions. Furthermore, a striking difference between the 1.5 and 3 K phases is clearly revealed: the large noise in the 1.5 K phase sharply disappears in the 3 K phase. These results confirm the multicomponent order-parameter superconductivity of the bulk ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, consistent with the chiral $p$-wave state, and the proposed nonchiral single-component superconductivity of the 3 K phase.

Superconducting $2H-\mathrm{NbS}{\mathrm{e}}_2$ single crystals show intrinsic low pinning values. Therefore, they are ideal materials with which to explore fundamental properties of vortices. ($V, I$) characteristics are the experimental data we have used to investigate the dissipation mechanisms in a rectangular-shaped $2H-\mathrm{NbS}{\mathrm{e}}_2$ single crystal. Particularly, we have studied dissipation behavior with magnetic fields applied in the plane of the crystal and parallel to the injected currents, i.e., in the force-free state where the vortex helical instability governs the vortex dynamics. In this regime, the data follow the elliptic critical state model and the voltage dissipation shows an exponential dependence, $V\propto e^{\alpha \left(I-I_{C\parallel }\right)}, I_{C\parallel }$ being the critical current in the force-free configuration and α a linear temperature-dependent parameter. Moreover, this exponential dependence can be observed for in-plane applied magnetic fields up to 40° off the current direction, which implies that the vortex helical instability plays a role in dissipation even out of the force-free configuration.

The description of the molecular solid phase of ${\mathrm{O}}_2$, especially its ground-state antiferromagnetic insulating phase, is known to be quite unsatisfactory within the local and semilocal approximations conventionally used in the Kohn-Sham formalism of density functional theory (DFT). The recently developed van der Waals (vdW) density functionals, vdW-DF, that take into account nonlocal correlations have also shown subpar performance in this regard. The difficulty lies in the subtle balance between the vdW interactions and the exchange coupling between the spin-triplet state of molecules in the molecular crystal. Here, we report that the DFT+$U$ approach used in combination with the vdW-DF performs surprisingly well in this regard, and discuss the reasoning behind this behavior. We also apply this approach to study the recently reported magnetic-field-induced $\theta$ phase of solid ${\mathrm{O}}_2$.

Recent angle-resolved photoemission spectroscopy measurements of single-layer $1H-{\mathrm{TaS}}_2$ grown on Au(111) suggest strong electron doping from the substrate. In addition, scanning tunneling microscopy/spectroscopy measurements on this system show suppression of the charge-density-wave (CDW) instability that occurs in bulk $2H-{\mathrm{TaS}}_2$. We present results from ab initio density functional theory calculations of free-standing single-layer $1H-{\mathrm{TaS}}_2$ to explore the effects of doping on the CDW. In the harmonic approximation, we find that a lattice instability along the $\mathrm{\Gamma }M$ line occurs in the undoped monolayer, consistent with the bulk $3\times 3$ CDW ordering vector. Electron doping removes the CDW instability, in agreement with the experimental findings. The doping and momentum dependences of both the electron-phonon coupling and of the bare phonon energy (unscreened by metallic electrons) determine the stability of lattice vibrations. Electron doping also causes an expansion of the lattice, so strain is a secondary but also relevant effect.

The competition between antiferromagnetic (AF) order and singlet formation is a central phenomenon of the Kondo and periodic Anderson Hamiltonians and of the heavy fermion materials they describe. In this paper, we explore the effects of an additional conduction band on magnetism in these models, and, specifically, on changes in the AF-singlet quantum critical point (QCP) and the one particle and spin spectral functions. To understand the magnetic phase transition qualitatively, we first carry out a self-consistent mean field theory (MFT). The basic conclusion is that, at half filling, the coupling to the additional band stabilizes the AF phase to larger $fd$ hybridization $V$ in the PAM. We also explore the possibility of competing ferromagnetic phases when this conduction band is doped away from half filling. We next employ quantum Monte Carlo (QMC) which, in combination with finite size scaling, allows us to evaluate the position of the QCP using an exact treatment of the interactions. This approach confirms the stabilization of AF order, which occurs through an enhancement of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. QMC results for the spectral function $A(\mathbf{q},\omega )$ and dynamic spin structure factor $\chi (\mathbf{q},\omega )$ yield additional insight into the AF-singlet competition and the low temperature phases.

The Lifshitz-Zaremba-Kohn (LZK) theory is commonly considered as the correct large-distance limit for the van der Waals (vdW) interaction of adsorbates (atoms, molecules, or nanoparticles) with solid substrates. In the standard approximate form, implicitly based on local dielectric functions, the LZK approach predicts universal power laws for vdW interactions depending only on the dimensionality of the interacting objects. However, recent experimental findings are challenging the universality of this theoretical approach at finite distances of relevance for nanoscale assembly. Here, we present a combined analytical and numerical many-body study demonstrating that physical adsorption can be significantly enhanced at the nanoscale. Regardless of the band gap or the nature of the adsorbate specie, we find deviations from conventional LZK power laws that extend to separation distances of up to 10–20 nm. Comparison with recent experimental observations of ultra-long-ranged vdW interactions in the delamination of graphene from a silicon substrate reveals qualitative agreement with the present theory. The sensitivity of vdW interactions to the substrate response and to the adsorbate characteristic excitation frequency also suggests that adsorption strength can be effectively tuned in experiments, paving the way to an improved control of physical adsorption at the nanoscale.

We investigate the histograms of conductance values obtained during controlled electromigration thinning of Cu thin films. We focus on the question whether the most frequently observed conductance values, apparent as peaks in conductance histograms, can be attributed to the atomic structure of the wire. To this end we calculate the Fourier transform of the conductance histograms. We find all the frequencies matching the highly symmetric crystallographic directions of fcc-Cu. In addition, there are other frequencies explainable by oxidation and possibly formation of hcp-Cu. With these structures we can explain all peaks occurring in the Fourier transform within the relevant range. The results remain the same if only a third of the samples are included. By comparing our results to the ones available in the literature on work-hardened nanowires, we find indications that even at low temperatures of the environment, metallic nanocontacts could show enhanced electromigration at low current densities due to defects enhancing electron scattering.

The energy dispersion relation of massive relativistic particles is characterized by a gradual transition from parabolic to linear momentum dependence. A similar pseudorelativistic behavior is found in the energy bands of certain two-dimensional semiconductors. Focusing on phosphorene, we investigate the response of pseudorelativistic electrons to electromagnetic fields. It is demonstrated that a near-analytical expression for the induced current containing the field to all orders can be obtained. We analyze the harmonic content of the current and study the field-, frequency-, and band-gap dependence of the nonlinear response. A detailed analysis of third-harmonic generation and saturable absorption is made.

We present a theoretical analysis of Dirac magnetoplasmons in topological insulator nanowires. We discuss a cylindrical geometry where Berry phase effects induce the opening of a gap at the neutrality point. By taking into account surface electron wave functions introduced in previous papers and within the random phase approximation, we provide an analytical form of the dynamic structure factor. Dispersions and spectral weights of Dirac plasmons are studied with varying the radius of the cylinder, the surface doping, and the strength of an external magnetic field. We show that, at zero surface doping, interband damped plasmonlike excitations form at the surface and survive at low electron surface dopings $\left(\sim {10}^{10}{\text{cm}}^{-2}\right)$. Then, we point out that the plasmon excitations are sensitive to the Berry phase gap closure when an external magnetic field close to half quantum flux is introduced. Indeed, a well-defined magnetoplasmon peak is observed at lower energies upon the application of the magnetic field. Finally, the increase of the surface doping induces a crossover from damped interband to sharp intraband magnetoplasmons, which, as expected for large radii and dopings $\left(\sim {10}^{12}{\mathrm{cm}}^{-2}\right)$, approach the proper limit of a two-dimensional surface.

We present experimental evidence for a type-II noncentrosymmetric Weyl state in ${\mathrm{TaIrTe}}_4$ where it has been recently predicted theoretically. We find direct correspondence between angle-resolved photoemission spectroscopy data and calculated electronic structure both in the bulk and the surface, and clearly observe the exotic surface states which support the quasi-one-dimensional Fermi arcs connecting only four Weyl points. Remarkably, these electronic states are spin polarized in the direction along the arcs, thus highlighting ${\mathrm{TaIrTe}}_4$ as an interesting type-II Weyl semimetal with a promising application potential.

The recently observed $\left(\sqrt{3}\times \sqrt{3}\right)$ surface reconstruction in heteroepitaxial Si(111) thin films on metal substrates is widely considered as a promising platform to realize two-dimensional Dirac and topological states, yet its formation mechanism and structural stability remain poorly understood, leading to the controversial terminology of “multilayer silicene.” Based on valence bond and conjugation theory, we propose a $\pi$-conjugation plus charge-transfer model to elucidate such a unique “bamboo hat” surface geometry. The formation of planar ring-shaped $\pi$ conjugation and charge transfer from the rings to the upper buckled Si atoms greatly lowers the surface dangling-bond energy. We justify this unconventional Si structural model by analyzing from first-principles surface stress tensors and surface energies as a function of strain. Within the same formalism, additional metastable surface reconstructions with similar “bamboo hat” features are predicted, which opens possibilities to other exotic electronic states in Si.

We investigate the ground-state properties of the spin-$1/2$ XXZ model with power-law-decaying ($1/r^{\alpha }$) interactions, which describe spins interacting with long-range transverse (XX) ferromagnetic interactions and longitudinal (Z) antiferromagnetic interactions, or hard-core bosons with long-range repulsion and hopping. The long-range nature of the couplings allows us to quantitatively study the spectral, correlation, and entanglement properties of the system by making use of linear spin-wave theory, supplemented with density-matrix renormalization group in one-dimensional systems. Our most important prediction is the existence of three distinct coupling regimes, depending on the decay exponent $\alpha$ and number of dimensions $d$: (1) a short-range regime for $\alpha >d+{\sigma }_c$ (where ${\sigma }_c=1$ in the gapped Néel antiferromagnetic phase exhibited by the XXZ model, and ${\sigma }_c=2$ in the gapless XY ferromagnetic phase), sharing the same properties as those of finite-range interactions ($\alpha =\infty$); (2) a long-range regime $\alpha <d$, sharing the same properties as those of the infinite-range interactions ($\alpha =0$) in the thermodynamic limit; and (3) a most intriguing medium-range regime for $d<\alpha <d+{\sigma }_c$, continuously interpolating between the finite-range and the infinite-range behavior. The latter regime is characterized by elementary excitations with a long-wavelength dispersion relation $\omega \approx {\mathrm{\Delta }}_g+c{k}^z$ in the gapped phase, and $\omega \sim k^z$ in the gapless phase, exhibiting a continuously varying dynamical exponent $z=(\alpha -d)/{\sigma }_c$. In the gapless phase of the model the $z$ exponent is found to control the scaling of fluctuations, the decay of correlations, and a universal subdominant term in the entanglement entropy, leading to a very rich palette of behaviors for ground-state quantum correlations beyond what is known for finite-range interactions.

The photoluminescence (PL) lifetime and quantum yield (QY) of the two-photon cascade emission (TPCE) provide critical information for assessing the quality of optoelectronic devices. The QY of TPCE has been conventionally evaluated by second-order photon correlation, $g^{\left(2\right)}$, measurements under weak-excitation conditions in single nanocrystals (NCs). Here, we report that by measuring simultaneously both the $g^{\left(2\right)}$ and PL-decay curves, the QY of the TPCE can be determined even under strong-excitation conditions. With this technique, the effect of ionization on the QY of the TPCEs in single CdSe/ZnS NCs, which appears under strong-excitation condition, can be revealed. The obtained QYs were compared with the PL lifetime of the first and second photon emissions. We demonstrate that even for ionized NCs, the radiative recombination rates during TPCE are almost proportional to the product of the numbers of electrons and holes.

We study the influence of confinement on the atomic collapse due to a Coulomb impurity placed at the center of a graphene quantum dot of radius $R$. We apply the zigzag or infinite-mass boundary condition and consider both a point-size and a finite-size impurity. As a function of the impurity strength $Z\alpha$, the energy spectra are discrete. In the case of the zigzag boundary condition, the degenerate (with respect to the angular momentum $m$) zero-energy levels are pulled down in energy as $Z\alpha$ increases, and they remain below $\epsilon =-Z\alpha$. Our results show that the energy levels exhibit a $1/R$ dependence in the subcritical regime $[Z\alpha <|km+1/2|$, $k=1(-1)$ for the $K$ ($K^{\prime }$) valley]. In the supercritical regime ($Z\alpha >|km+1/2|$) we find a qualitatively very different behavior where the levels decrease as a function of $R$ in a nonmonotonic manner. While the valley symmetry is preserved in the presence of the impurity, we find that the impurity breaks electron-hole symmetry. We further study the energy spectrum of zigzag quantum dots in gapped graphene. Our results show that as the gap increases, the lowest electron states are pushed into the gap by the impurity.