Measurements of physical properties, including transport and magnetic properties, specific heat, and thermal conductivity, have been performed on high-quality samples of ${\mathrm{LaNiO}}_3$ and ${\mathrm{LaCuO}}_3$ synthesized under high pressure. Some measurements, such as thermoelectric power and magnetic susceptibility, have been made under high pressure. The availability of a complete set of data enables a side-by-side comparison between these two narrowband systems. We have demonstrated unambiguously the mass enhancement due to electron-electron correlations in both systems relative to the recent density functional theory results. Correlations in these narrowband systems also enhance the magnetic susceptibility. Ferromagnetic spin fluctuations give rise to a strong Stoner enhancement in the magnetic susceptibility in the quarter-filled ${\mathrm{LaNiO}}_3$. Although we are able to tune the bandwidth by either chemical substitutions or by applying hydrostatic pressure on ${\mathrm{LaNiO}}_3$, the Stoner enhancement does not lead to the Stoner instability.

We report measurements of the attenuation of a beam of orthoexciton polaritons by a photoionizing optical probe. Excitons were prepared in a narrow resonance by two photon absorption of a 1.016 eV, 54 ps pulsed light source in cuprous oxide (${\mathrm{Cu}}_2\mathrm{O}$) at 1.4 K. A collinear, 1.165 eV, 54 ps probe delayed by 119 ps was used to measure the photoionization cross section of the excitons. Two photon absorption is quadratic with respect to the intensity of the pump and leads to polariton formation. Ionization is linear with respect to the intensity of the probe. Subsequent carrier recombination is quadratic with respect to the intensity of the probe, and is distinguished because it shifts the exciton momentum away from the polariton anticrossing; the photoionizing probe leads to a rise in phonon-linked luminescence in addition to the attenuation of polaritons. The evolution of the exciton density was determined by variably delaying the probe pulse. Using the probe irradiance and the reduction in the transmitted polariton light, a cross section of $(3.9\pm 0.2)\times {10}^{-22}$ ${\mathrm{m}}^2$ was deduced for the probe frequency.

A method of determining the temperature of the nonradiative reservoir in a microcavity exciton-polariton system is developed. A general relation for the homogeneous polariton linewidth is theoretically derived and experimentally used in the method. In experiments with a GaAs microcavity under nonresonant pulsed excitation, the reservoir temperature dynamics is extracted from the polariton linewidth. Within the first nanosecond the reservoir temperature greatly exceeds the lattice temperature and determines the dynamics of the major processes in the system. It is shown that, for nonresonant pulsed excitation of GaAs microcavities, the polariton Bose-Einstein condensation is typically governed by polariton-phonon scattering, while interparticle scattering leads to condensate depopulation.

Scattering scanning near-field microscopy recently provided spectroscopic access to the fundamentally distinct spatial, spectral, and coherence properties of the thermal near field. Using SiC as an example, we study its thermal surface phonon polariton (SPhP) response. In contrast to the strongly surface-confined thermal near field of localized vibrational modes, an extended exponential distance dependence is observed reflecting the spatial coherence of the SPhP thermal field. In addition, we observe pronounced spectral frustration with spectral shifts ranging from $\sim 5$ to $\sim 50$ ${\mathrm{cm}}^{-1}$ that cannot be explained by conventional tip-sample dipole coupling alone. We present an alternative model describing an effective medium change by the tip. The results highlight the possibility for local spectral and spatial tuning of the thermal SPhP resonance for control of the light-matter interaction using thermal near-field radiation.

The high carrier mobility in graphene promises its utility in electronics applications. Azobenzene is a widely studied organic molecule for switchable optoelectronic devices that can be synthesized with a wide variety of ligands and deposited on graphene. Using first-principles calculations, we investigate graphene doping by physisorbed azobenzene molecules with various electron-donating and -accepting ligands. We confirm previous experimental results that demonstrate greater $p$ doping of graphene for the $\mathit{trans}$ compared to $\mathit{cis}$ configuration when using a ${\mathrm{SO}}_3$ electron-accepting ligand; however, we find that ${\mathrm{NO}}_2$ ligands maximize the $p$-doping difference between isomers. We also examine how these doping effects change when the graphene monolayer is supported on a silica substrate. We then extend these findings by examining the doping effects of an applied electrical bias and mechanical strain to the graphene, which lead to changes in doping for both the $\mathit{trans}$ and $\mathit{cis}$ isomers. These results demonstrate a different type of multicontrol device combining light, electric field, and strain to change carrier concentration in graphene.

We show that Dirac fermions moving in two spatial dimensions with a generalized dispersion $E\sim p^N$, subject to an external magnetic field and coupled to a complex scalar field carrying a vortex defect with winding number $Q$ acquire $N\left|Q\right|$ zero modes. This is the same as in the absence of the magnetic field. Our proof is based on selection rules in the Landau level basis that dictate the existence and the number of the zero modes. We show that the result is insensitive to the choice of geometry and is naturally extended to general field profiles, where we also derive a generalization of the Aharonov-Casher theorem. Experimental consequences of our results are briefly discussed.

The surface second-harmonic generation from interacting spherical plasmonic nanoparticles building different clusters (symmetric and asymmetric dimers, trimers) is theoretically investigated. The plasmonic eigenmodes of the nanoparticle clusters are first determined using an ab initio approach based on the Green's functions method. This method provides the properties, such as the resonant wavelengths, of the modes sustained by a given cluster. The fundamental and second-harmonic responses of the corresponding clusters are then calculated using a surface integral method. The symmetry of both the linear and nonlinear responses is investigated, as well as their relationship. It is shown that the second-harmonic generation can be significantly enhanced when the fundamental field is such that its second harmonic matches modes with suitable symmetry. The role played by the nanogaps in second-harmonic generation is also underlined. The results presented in this article demonstrate that the properties of the second-harmonic generation from coupled metallic nanoparticles cannot be fully predicted from their linear response only, while, on the other hand, a detailed knowledge of the underlying modal structure can be used to optimize the generation of the second harmonic.

The inelastic mean free path ${\lambda }_{\mathrm{MFP}}$ is a critical parameter for electronic devices. Here, we demonstrate Kelvin probe force microscopy (KPFM) as a technique for studying ${\lambda }_{\mathrm{MFP}}$ in biased, semi-metallic single-walled carbon nanotubes (SWNTs). Having one of the longest room-temperature ${\lambda }_{\mathrm{MFP}}$ values of any known material, SWNTs provide a unique platform for probing mesoscopic transport. KPFM directly determined ${\lambda }_{\mathrm{MFP}}$ as a function of bias, quantitatively determined the contributions of different scattering mechanisms, and enabled comparative study of individual SWNTs with and without disorder. The room-temperature mean free paths for optical phonon and surface plasmon-polariton scattering were measured to be 62 ± 20 nm and 260 ± 50 nm, respectively. The optical phonon scattering length is significantly longer than inferred from previous measurements, and it resolves a longstanding discrepancy between SWNT theory and experiment, suggesting that KPFM could become a preferred quantitative technique for studying transport in nanoscale systems.

We show that the numerical strong disorder renormalization group algorithm of Hikihara et al. [Phys. Rev. B 60, 12116 (1999)] for the one-dimensional disordered Heisenberg model naturally describes a tree tensor network (TTN) with an irregular structure defined by the strength of the couplings. Employing the holographic interpretation of the TTN in Hilbert space, we compute expectation values, correlation functions, and the entanglement entropy using the geometrical properties of the TTN. We find that the disorder-averaged spin-spin correlation scales with the average path length through the tensor network while the entanglement entropy scales with the minimal surface connecting two regions. Furthermore, the entanglement entropy increases with both disorder and system size, resulting in an area-law violation. Our results demonstrate the usefulness of a self-assembling TTN approach to disordered systems and quantitatively validate the connection between holography and quantum many-body systems.

After excitation by femtosecond laser pulses, Gd and Tb exhibit ultrafast demagnetization in two steps, with the time constant of the second step linked to the coupling strength of the $4f$ magnetic moments to the lattice. In time-resolved magneto-optical Kerr effect measurements of ${\mathrm{Gd}}_{1-x}{\mathrm{Tb}}_x$ alloys, we observe a decrease in this time constant from 33 to 9 ps with Tb content $x$ increasing from 0 to 0.7. We explain this behavior by the stronger spin-lattice coupling of Tb compared to Gd, which increases the effective spin-lattice coupling in ${\mathrm{Gd}}_{1-x}{\mathrm{Tb}}_x$ with $x$. In contrast, the faster time constant of the first demagnetization step exhibits no dependence on $x$. Additional time- and element-resolved x-ray magnetic circular dichroism measurements show a two-step demagnetization of Gd and Tb in ${\mathrm{Gd}}_{0.6}{\mathrm{Tb}}_{0.4}$ with an equivalent time scale of the second step but a different magnitude of demagnetization which persists for 15 ps. We explain this by an increased coupling of the Gd $4f$ magnetic moments to the lattice compared to pure Gd, via interatomic exchange coupling to the neighboring Tb $4f$ moments mediated by $5d$ electrons, which has limited efficiency and allows an estimation of a characteristic time scale of the interatomic exchange coupling. We assign the first demagnetization step to the dynamics of the laser-excited $5d$ electrons, while the second demagnetization step is dominated by the strength of spin-lattice coupling of the $4f$ electrons.

Calculations of stable and metastable magnetic states as well as minimum energy paths for transitions between states are carried out using a noncollinear extension of the multiple-impurity Alexander-Anderson model and a magnetic force theorem which is derived and used to evaluate the total energy gradient with respect to orientation of magnetic moments—an important tool for efficient navigation on the energy surface. By using this force theorem, the search for stable and metastable magnetic states as well as minimum energy paths revealing the mechanism and activation energy of transitions can be carried out efficiently. For Fe monolayer on W(110) surface, the model gives magnetic moment as well as exchange coupling between nearest and next-nearest neighbors that are in good agreement with previous density functional theory calculations. When applied to nanoscale Fe islands on this surface, the magnetic moment is predicted to be 10% larger for atoms at the island rim, explaining in part an experimentally observed trend in the energy barrier for magnetization reversal in small islands. Surprisingly, the magnetic moment of the atoms does not change much along the minimum energy path for the transitions, which for islands containing more than 15 atom rows along either $\left[001\right]$ or $\left[1\overline{1}0\right]$ directions involves the formation of a thin, temporary domain wall. A noncollinear magnetic state is identified in a $7\times 7$ atomic row Fe island where the magnetic moments are arranged in an antivortex configuration with the central ones pointing out of the $\left(110\right)$ plane. This illustrates how the model can describe complicated exchange interactions even though it contains only a few parameters. The minimum energy path between this antivortex state and the collinear ground state is also calculated and the thermal stability of the antivortex state estimated.

The electronic states of surface adatoms in ${\mathrm{Na(Fe}}_{0.96}{\mathrm{Co}}_{0.03}{\mathrm{Mn}}_{0.01})\mathrm{As}$ have been studied by low-temperature scanning tunneling spectroscopy. The spectra recorded on the adatoms display both superconducting coherence peaks and an asymmetric resonance in a larger energy scale. The Fano-type line shape of the spectra points towards a possible Kondo effect at play. The apparent energy position of the resonance peak shifts about $5$meV to the Fermi level when measured across the critical temperature, supporting that the Bogoliubov quasiparticle is responsible for the Kondo screening in the superconducting state. The tunneling spectra do not show the subgap bound states, which is explained as the weak pair breaking effect given by the weak and broad scattering potential after the Kondo screening.

We study angular-dependent magnetoresistance in a low-$T_c$ layered cuprate ${\mathrm{Bi}}_{2.15}{\mathrm{Sr}}_{1.9}{\mathrm{CuO}}_{6+\delta }$. The low $T_c\sim 4$ K allows complete suppression of superconductivity by modest magnetic fields and facilitates accurate analysis of the upper critical field $H_{c2}$. We observe a universal exponential decay of fluctuation conductivity in a broad range of temperatures above $T_c$ and propose a method for extraction of $H_{c2}\left(T\right)$ from the scaling analysis of the fluctuation conductivity at $T>T_c$. Our main result is observation of a surprisingly low $H_{c2}$ anisotropy $\sim 2$, which is much smaller than the effective mass anisotropy of the material $\sim 300$. We show that the anisotropy is decreasing with increasing field and saturates at a small value when the field reaches the paramagnetic limit. We argue that the dramatic discrepancy of high-field and low-field anisotropies is clear evidence for paramagnetically limited superconductivity.

We present a study of the one-dimensional $S=1$ antiferromagnetic spin chain with large easy-plane anisotropy, with special emphasis on field-induced quantum phase transitions. Temperature and magnetic field dependence of magnetization, specific heat, and thermal conductivity is presented using a combination of numerical methods. In addition, the original $S=1$ model is mapped into the low-energy effective $S=1/2XXZ$ Heisenberg chain, a model which is exactly solvable using the Bethe ansatz technique. The effectiveness of the mapping is explored, and we show that all considered quantities are in qualitative, and in some cases quantitative, agreement. The thermal conductivity of the considered $S=1$ model is found to be strongly influenced by the underlying effective description. Furthermore, we elucidate the low-lying electron spin resonance spectrum based on a semianalytical Bethe ansatz calculation of the effective $S=1/2$ model.

We have performed a microscopic study of a straight quantized vortex line in three dimensions in condensed ${}^4\mathrm{He}$ at zero temperature using the shadow path integral ground state method and the fixed phase approximation. We have characterized the energy and the local density profile around the vortex axis in superfluid ${}^4\mathrm{He}$ at several densities, ranging from below the equilibrium density up to the overpressurized regime. For the Onsager-Feynman (OF) phase our results are exact and represent a benchmark for other theories. The inclusion of backflow correlations in the phase improves the description of the vortex with respect to the OF phase by a large reduction of the core energy of the topological excitation. At all densities the phase with backflow induces a partial filling of the vortex core and this filling slightly increases with density. The core size slightly decreases for increasing density and the density profile has well defined density dependent oscillations whose wave vector is closer to the wave vector of the main peak in the static density response function rather than to the roton wave vector. Our results can be applied to vortex rings of large radius $R$ and we find good agreement with the experimental value of the energy as a function of $R$ without any free parameter. We have studied also ${}^4\mathrm{He}$ above the melting density in the solid phase using the same functional form for the phase as in the liquid. We found that off-diagonal properties of the solid are not qualitatively affected by the velocity field induced by the vortex phase, both with and without backflow correlations. Therefore we find evidence that a perfect ${}^4\mathrm{He}$ crystal is not a marginally stable quantum solid in which rotation would be able to induce off-diagonal long-range coherence.

We report in-plane thermoelectric power measurements on single crystals of $\left({\mathrm{Ba}}_{1-x}{\mathrm{K}}_x\right){\mathrm{Fe}}_2{\mathrm{As}}_2\left(0.44\le x\le 1\right)$. We observe a minimum in the ${S|}_{T=\text{const}}$ versus $x$ at $x\sim 0.55$ that can be associated with the change in the topology of the Fermi surface, a Lifshitz transition, related to the electron pockets at the center of $M$ point crossing the Fermi level. This feature is clearly observable below $\sim 75$ K. Thermoelectric power also shows a change in the $x\sim 0.8–0.9$ range, where the maximum in the thermoelectric power collapses into a plateau. This Lifshitz transition is most likely related to the reconstruction of the Fermi surface associated with the transformation of the hole pockets at the $M$ point into four blades as observed by ARPES measurements.

The environment dependence of the on-site energy levels and intersite bond integrals within an orthogonal tight-binding (OTB) model for $s$-valent systems is determined by the competition between the attractive crystal-field and three-center terms and the repulsive overlap contribution. This is demonstrated explicitly for the case of symmetric ${\mathrm{H}}_N$ clusters, where the OTB parameters can be expressed analytically in terms of the nonorthogonal tight-binding (NOTB) on-site, intersite, and overlap matrix elements. The values of the OTB parameters are obtained directly from the density-functional theory eigenspectra of the ${\mathrm{H}}_N$ clusters with the on-site energy levels being found to be very sensitive to the local atomic environment but the intersite bond integrals much less so. This different behavior is shown to result from the large cancellations between the crystal-field, overlap, and three-center contributions. The common first-order expression for the OTB on-site energy level as the sum of the NOTB crystal field and a two-body overlap repulsion is found to be a poor approximation due to the sizable values of the overlap in these ${\mathrm{H}}_N$ clusters around equilibrium.

We consider theoretically the transport properties of a spinless resonant electronic level coupled to strongly dissipative leads, in the regime of circuit impedance near the resistance quantum. Using the Luttinger liquid analogy, one obtains an effective Hamiltonian expressed in terms of interacting Majorana fermions, in which all environmental degrees of freedom (leads and electromagnetic modes) are encapsulated in a single fermionic bath. General transport equations for this system are then derived in terms of the Majorana $T\text{-matrix}$. A perturbative treatment of the Majorana interaction term yields the appearance of a marginal, linear dependence of the conductance on temperature when the system is tuned to its quantum critical point, in agreement with recent experimental observations. We investigate in detail the different crossovers involved in the problem, and analyze the role of the interaction terms in the transport scaling functions. In particular, we show that single barrier scaling applies when the system is slightly tuned away from its Majorana critical point, strengthening the general picture of dynamical Coulomb blockade.