Under illumination at their plasmonic resonance wavelength, metal nanoparticles can turn into efficient nanosources of heat by light absorption. Heating a small volume makes it possible to achieve fast dynamics. In this paper, we investigate theoretically, numerically, and experimentally the temperature distribution of a plasmonic system generated by a modulated incoming light. In particular, we study the response in amplitude and phase of the temperature variations. The cases of single and multiple nanoparticles are both addressed. Many parameters are discussed such as the nature of the media (nanoparticle and surroundings), the size of the nanoparticle or of the plasmonic system, the nanoparticle interdistance, the frequency of the modulation, a possible finite surface thermal conductivity of the nanoparticles, and the dimensionality of the system. This work is also intended to determine how fast a plasmonic system is able to induce temperature variations in its surrounding medium.

Recent advances in experimental techniques emphasize the usefulness of multiple scanning probe techniques when analyzing nanoscale samples. Here, we analyze theoretically dual-probe setups with probe separations in the nanometer range, i.e., in a regime where quantum coherence effects can be observed at low temperatures. In a dual-probe setup the electrons are injected at one probe and collected at the other. The measured conductance reflects the local transport properties on the nanoscale, thereby yielding information complementary to that obtained with a standard one-probe setup (the local density of states). In this work we develop a real-space Green's function method to compute the conductance. This requires an extension of the standard calculation schemes, which typically address a finite sample between the probes. In contrast, the developed method makes no assumption of the sample size (e.g., an extended graphene sheet). Applying this method, we study the transport anisotropies in pristine graphene sheets, and analyze the spectroscopic fingerprints arising from quantum interference around single-site defects, such as vacancies and adatoms. Furthermore, we demonstrate that the dual-probe setup is a useful tool for characterizing the electronic transport properties of extended defects or designed nanostructures. In particular, we show that nanoscale perforations, or antidots, in a graphene sheet display Fano-type resonances with a strong dependence on the edge geometry of the perforation.

Time-reversal-invariant topological superconductors are a new state of matter which have a bulk superconducting gap and robust Majorana fermion surface states. These have not yet been realized in solid state systems. In this paper, we propose that this state can be realized in doped Weyl semimetals or Weyl metals. The Fermi surfaces of a Weyl metal carry Chern numbers, which is a required ingredient for such a topological superconductor. By applying the fluctuation-exchange approach to a generic model of time-reversal-invariant Dirac and Weyl semimetals, we investigate what microscopic interactions can supply the other ingredient, viz., a sign change of the superconducting gap function between Fermi surfaces with opposite Chern numbers. We find that if the normal state is inversion symmetric, on-site repulsive and exchange interactions induce various nodal phases as well as a small region of topological superconductivity on the phase diagram. Unlike in the ${}^3\text{He}$-B topological superconductor, the phase here does not rely on any special momentum dependence of the pairing amplitude. Breaking inversion symmetry precludes some of the nodal phases and the topological superconductor becomes much more prominent, especially at large ferromagnetic interaction. Our approach can be extended to generic Dirac or Weyl metals.

Birefringent fermions arise as massless fermionic low-energy excitations of a particular tight-binding model for spinless fermions on a square lattice which have two “speeds of light” [M. P. Kennett et al., Phys. Rev. A 83, 053636 (2011)]. We use mean-field theory to study phases that can arise when there are nearest-neighbor and next-nearest-neighbor repulsive interactions in this model and demonstrate robustness of the birefringent semimetal phase in the presence of weak interactions and identify transitions to staggered density and quantum anomalous Hall ordered phases. We consider the effect of coupling birefringent fermions to a magnetic field, and find analytic expressions for the corresponding Landau levels and demonstrate that their integer quantum Hall effect displays additional plateaus beyond those observed for regular Dirac fermions, such as in graphene. We briefly discuss a tight-binding construction that leads to three-dimensional birefringent fermions.

We report the electronic properties of the quadruple perovskite $\mathrm{CaCu}{}_3\mathrm{Ti}{}_4\mathrm{O}{}_{12}$ as obtained via several density-functional based methods and propose a new interpretation of optical experiments to the effect that four distinct transitions (centered around 0.7, 1.5, 2.5, and 3.5 eV) contribute to the spectrum. The comparison with experiment is satisfactory, especially after we account for the effects of spin disorder, which does not close the fundamental gap but suppresses the transition intensity. We find that some of the methods we employ tend to overestimate considerably the gaps for standard values of the respective adjustable parameters.

The electronic structure and properties of the orthorhombic phase of the $\mathrm{CH}{}_3\mathrm{NH}{}_3\mathrm{PbI}{}_3$ perovskite are computed with density functional theory. The structure, optimized using a van der Waals functional, reproduces closely the unit cell volume. The experimental band gap is reproduced accurately by combining spin-orbit effects and a hybrid functional in which the fraction of exact exchange is tuned self-consistently to the optical dielectric constant. Including spin-orbit coupling strongly reduces the anisotropy of the effective mass tensor, predicting a low electron effective mass in all crystal directions. The computed binding energy of the unrelaxed exciton agrees with experimental data, and the values found imply a fast exciton dissociation at ambient temperature. Also polaron masses for the separated carriers are estimated. The values of all these parameters agree with recent indications that fast dynamics and large carrier diffusion lengths are key in the high photovoltaic efficiencies shown by these materials.

A (single flavor) quadratic band crossing in two dimensions is known to have a generic instability towards a quantum anomalous Hall (QAH) ground state for infinitesimal repulsive interactions. Here we introduce a generalization of a quadratic band crossing which is protected only by rotational symmetry. By focusing on the representative case of a parabolic and flat band touching, which also allows for a straightforward lattice realization, the interaction induced nematic phase becomes the dominant instability in certain parts of the phase diagram already at weak coupling, by competing successfully with the QAH insulator. The full phase diagram of the model, together with its topological properties, is mapped out using a perturbative renormalization-group, strong-coupling analysis, the mean-field theory. Interestingly, the Berry flux varies continuously in the single flavor limit with various control parameters.

Intrinsic electron- and hole-phonon interactions are investigated in monolayer transition-metal dichalcogenides $M{X}_2$ (M=Mo, W; X=S, Se) based on a density functional theory formalism. Due to their structural similarities, all four materials exhibit qualitatively comparable scattering characteristics with the acoustic phonons playing a dominant role near the conduction and valence band extrema at the K point. However, substantial differences are observed quantitatively leading to disparate results in the transport properties. Of those considered, ${\mathrm{WS}}_2$ provides the best performance for both electrons and holes with high mobilities and saturation velocities in the full-band Monte Carlo analysis of the Boltzmann transport equation. It is also found that monolayer $M{X}_2$ crystals with an exception of ${\mathrm{MoSe}}_2$ generally show hole mobilities comparable to or even larger than the value for bulk silicon at room temperature, suggesting a potential opportunity in p-type devices. The analysis is extended to estimate the effective deformation potential constants for a simplified treatment as well.

${\mathrm{BaTiO}}_3$ ultrathin films (thickness $\approx 1.6$ nm) with in- and out-of-plane polarization are studied by first-principles calculations. Out-of-plane polarization is simulated using the method proposed by Shimada et al. [Phys. Rev. B 81, 144116 (2010)], which consists in building a supercell containing small domains with alternating up and down polarization. This allows one to investigate the properties of defect free ${\mathrm{BaTiO}}_3$ ultrathin films with polarization perpendicular to the surface, as a function of in-plane lattice constant, i.e., epitaxial strain. The configurations with polarization perpendicular to the surface $(c$ phase) are found stable under compressive strain, while under tensile strain, the polarization tends to lie in-plane ($aa$ phase), along [110]. In the $c$ phase, the most stable domain width is predicted to be 1 to 2 lattice constants, and the magnitude of the surface rumpling varies according to the direction of the polarization (upwards versus downwards), though its sign is unchanged, the oxygen anions pointing in all cases outwards. Finally, all the surfaces studied are found to be insulating. Analysis of the atom-projected electronic density of states gives insight into the surface contributions to the electronic structure. An important reduction of the Kohn-Sham band gap is predicted at ${\mathrm{TiO}}_2$ terminations in the $c$ phase ($\approx 1$ eV with respect to the $aa$ phase). The Madelung potential at the surface plays the dominant role in modifications of the surface electronic structure.

The lattice dynamics of ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{Ge}}_2{\mathrm{O}}_7$, a compound which develops Dzyaloshinsky-Moriya helical magnetism below $T_N=3.2$ K, has been studied by measuring the infrared reflectivity of a single crystal with the radiation polarized both in the $ab$ plane and along the $c$ axis of its tetragonal cell, from 7 to 300 K. In this compound, where the unit cell has no inversion symmetry, 14 $E$ phonon modes of the $ab$ plane, out of the 18 predicted, and nine $B_2$ modes of the $c$ axis, out of the ten predicted, have been observed. They have been assigned to the atomic motions by a comparison with shell-model calculations, which provided vibrational frequencies in good agreement with the experiment, while most calculated intensities turned out to be much lower than the experimental values. This discrepancy has been tentatively explained by assuming strong electron-phonon interactions, a hypothesis supported by the failure of the $f$-sum rule if restricted to the phonon region. Indeed, we observe a remarkable increase in the oscillator strengths between 300 and 200 K for the phonons of the $ab$ plane (between 300 and 100 K for the $c$ axis), which implies an increase in the dielectric constant of ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{Ge}}_2{\mathrm{O}}_7$ below room temperature.

By irradiating a thin metal foil with an intense short-pulse laser, we have created a uniform system far from equilibrium. The deposited energy is initially deposited only within the electronic subsystem, and the subsequent evolution of the system is determined by the details of the electron-phonon coupling. Here, we measure the time evolution of the lattice parameter through multilayer Bragg diffraction and compare the result to classical molecular dynamic simulations to determine the lattice temperature. The electron-ion coupling constant for gold is then determined by comparison with the evolution of a two-temperature electron-phonon system.

We present a parametric study of the diffuse magnetic scattering at $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ positions in reciprocal space, ascribed to a frozen antiferromagnetic spin ice state in single-crystalline ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Our high-resolution neutron scattering measurements show that the elastic $(-0.02$ meV $<E<0.02$ meV) $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ scattering develops strongly below $\approx$275 mK, and correlates with the opening of a spin gap of $\approx 0.06$ to 0.08 meV over most of the Brillouin zone. The nature of the transition at 275 mK has many characteristics of spin glass behavior, consistent with ac-susceptibility measurements. The application of a magnetic field of 0.075 T applied along the $\left[1\overline{1}0\right]$ direction destroys the $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ elastic scattering, revealing the fragility of this short-range ordered ground state. We construct a refined $H$-$T$ phase diagram for ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and $\left[1\overline{1}0\right]$ fields which incorporates this frozen spin ice regime and the antiferromagnetic long-range order previously known to be induced in relatively large fields. Specific heat measurements on the same crystal reveal a sharp anomaly at $T_{\mathrm{c}}\approx$ 450 mK and no indication of a transition near $\approx$275 mK. We conclude that the higher temperature specific heat peak is not related to the magnetic ordering but is likely a signal of other, nonmagnetic dipole correlations.

We discuss how phase coherence manifests in the heat transport through superconducting single and multichannel Josephson junctions in time-dependent situations. We consider the heat current driven through the junction by a temperature difference in dc voltage and ac phase biased situations. At low bias, the electromagnetic driving mainly modifies the form of the coherent resonance that transports a large part of the heat current, rather than simply dissipating energy in the junction. We find a description for the heat current in terms of quasiparticle $n$-photon absorption and emission rates, and discuss analytical and numerical results concerning them. In addition to the ensemble average heat transport, we describe also its fluctuations.

The results of heat capacity $C_{\mathrm{p}}(T,H)$ and electrical resistivity $\rho (T,H)$ measurements down to 0.35 K as well as muon spin relaxation and rotation $\left(\mu \mathrm{SR}\right)$ measurements on the noncentrosymmetric superconductor ${\mathrm{LaIrSi}}_3$ are presented. Powder neutron diffraction confirmed the reported noncentrosymmetric body-centered tetragonal ${\mathrm{BaNiSn}}_3$-type structure (space group $I4mm)$ of ${\mathrm{LaIrSi}}_3$. The bulk superconductivity is observed below $T_{\mathrm{c}}=0.72\left(1\right)$ K. The intrinsic $\Delta C_{\mathrm{e}}/{\gamma }_{\mathrm{n}}{T}_{\mathrm{c}}=1.09\left(3\right)$ is significantly smaller than the BCS value of 1.43, and this reduction is accounted for by the $\alpha$ model of BCS superconductivity. The analysis of the superconducting-state $C_{\mathrm{e}}\left(T\right)$ data by the single-band $\alpha$ model indicates a moderately anisotropic order parameter with the $s$-wave gap $\Delta \left(0\right)/k_{\mathrm{B}}{T}_{\mathrm{c}}=1.54\left(2\right)$, which is lower than the BCS value of 1.764. Our estimates of various normal- and superconducting-state parameters indicate a weakly coupled electron-phonon-driven type-I $s$-wave superconductivity in ${\mathrm{LaIrSi}}_3$. The $\mu$SR results also confirm the conventional type-I superconductivity in ${\mathrm{LaIrSi}}_3$ with a preserved time-reversal symmetry and hence a singlet pairing superconducting ground state.

We perform a first-principles study of the effect of strain on the migration of Si atoms in Ni. For that purpose, migration barriers are computed using the nudged elastic band method and attempt frequencies are computed using the direct force method. Good agreement is found with tracer diffusion experiments. We used the elastic dipole model to calculate effects of strain on migration barriers by performing calculations on unstrained cells, therefore reducing significantly the computing time. We validate this approach by comparing results with migration barriers calculated on strained cells and obtain an excellent agreement up to a strain of 1%. Computing all the jump frequencies in the neighborhood of Si solutes, the effect of strain is found to be nearly independent of the relative position of the solute atom. A simple elastic analysis models the changes in the vacancy jump with strain; this correlates with the changes in geometry for the “cage” of atoms surrounding the hopping atom at the saddle point.

Optical properties of hexagonal multiferroic oxides $R\mathrm{Mn}{\mathrm{O}}_3$, where $R=\mathrm{Ho}$, Er, Tm, Yb, and Lu, have been studied in the far-infrared spectral range between 100 and 2000 cm${}^{-1}$ and temperatures between 1.5 and 300 K by means of several experimental techniques: Mueller matrix spectroscopic ellipsometry, rotating analyzer ellipsometry, and optical transmission spectroscopy. Spectra of the optical phonons are described in terms of the temperature dependencies of their frequency, damping, and oscillator strength. For all studies, oxide materials’ clear signatures of the spin-phonon interaction have been found below the temperature of the antiferromagnetic phase transition $T_N$ due to magnetic ordering of ${\mathrm{Mn}}^{3+}$ spins. A decrease of the ionic radius for $R^{3+}$ ions between ${\mathrm{Ho}}^{3+}$ and ${\mathrm{Lu}}^{3+}$ in the corresponding $R\mathrm{Mn}{\mathrm{O}}_3$ compounds resulted in systematic variation of the frequency for several optical phonons. A magnetic excitation at ∼190 cm${}^{-1}$ was observed at low temperatures below $T_N$ and interpreted as resulting from two-magnon absorption.

The orthovanadate ${\mathrm{DyVO}}_3$ crystal, known to exhibit multiple structural, spin-, and orbital-ordering transitions, is presently investigated on the basis of magnetization, heat capacity, resistivity, dielectric, and polarization measurements. Our main result is experimental evidence for the existence of multiferroicity below a high $T_C$ of 108 K over a wide temperature range including different spin-orbital-ordered states. The onset of ferroelectricity is found to coincide with the antiferromagnetic $C$-type spin-ordering transition taking place at 108 K, which indicates that ${\mathrm{DyVO}}_3$ belongs to type-II multiferroics exhibiting a coupling between magnetism and ferroelectricity. Some anomalies detected on the temperature dependence of electric polarization are discussed with respect to the nature of the spin-orbital-ordered states of the V sublattice and the degree of spin alignment in the Dy sublattice. The orthovanadates $R$${\mathrm{VO}}_3$ ($R=$ rare earth or Y) form an important new category for searching for high$-T_C$ multiferroics.

Blount's theorem prohibits the existence of line nodes for odd-parity superconductors (SCs) in the presence of spin-orbit coupling (SOC). We studied the topological stability conditions of line nodes under inversion symmetry by generalizing the original statement and establishing a relation to surface zero-energy states. The topological instability of line nodes in odd-parity SCs implies the disappearance of corresponding flat zero-energy surface dispersions due to surface Rashba SOC, which provides an experimental means to distinguish line nodes in odd-parity SCs from those in other SCs.