We have observed the characteristic temperature dependence of the intermolecular phonon spectrum in the organic dimer Mott insulator κ-(ET)${}_2$Cu${}_2$(CN)${}_3$ exhibiting a dielectric anomaly at ∼30 K. The anomalous spectral narrowing of the 55 cm${}^{-1}$ phonon peak at ∼30 K was analyzed in terms of motional narrowing within the framework of a stationary Gaussian process, i.e., the phonon frequency is modulated by the ultrafast charge fluctuation. The spectral narrowing occurs because the time constant of the correlation time ${\tau }_c$ and the amplitude $\Delta$ of the frequency modulation satisfy the relation ${\tau }_c<1/\Delta$ at 30 K. At temperatures below 30 K, the motional narrowing is disturbed by the increase of ${\tau }_c$, near the charge-glass or the short-range order at 6 K. On the other hand, for temperatures above 30 K, the motional narrowing is disturbed by the increase of $\Delta$ with increasing temperature.

A scenario for the formation of non-Fermi-liquid (NFL) Kondo effect (KE) with spin variable enumerating Kondo channels is suggested and worked out. In a doubly occupied symmetric triple quantum dot within parallel geometry, the NFL low-energy regime arises provided the device possesses both source-drain and left-right parity. Kondo screening follows a multistage renormalization group mechanism: reduction of the energy scale is accompanied by the change of the relevant symmetry group from $\text{SO}\left(8\right)$ to $\text{SO}\left(5\right)$. At low energy, three phases compete: (1) an underscreening spin-triplet (conventional) KE, (2) a spin-singlet potential scattering, and (3) a NFL phase where the roles of spin and orbital degrees of freedom are swapped.

Optical transitions in as-grown and annealed GaN${}_{0.02}$As${}_{0.98}$, Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$, and GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ bulklike layers have been studied with photoreflectance (PR), photoluminescence (PL), and microphotoluminescence (μPL) in a broad range of temperatures. The exciton binding energy and the temperature dependence of the energy gap have been determined from PR measurements. In addition, the alloy-related broadening of optical transitions has been analyzed and compared for the three alloys. The largest alloy inhomogeneities have been observed for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$. In μPL spectra of GaN${}_{0.02}$As${}_{0.98}$ and Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$ layers measured at low temperatures and low excitation conditions, sharp PL lines of 100–300-μeV widths have been clearly observed ∼10–20 meV below the energy gap of these alloys. Analyzing the temperature quenching of the sharp PL lines, they have been attributed to the recombination of localized excitons trapped at deep donor (acceptor)-like states. Such lines were not well resolved for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ layers. The analysis of thermal quenching of localized exciton emission suggests that for this alloy excitons are localized mostly on alloy content fluctuations. With the increase in the excitation power an additional PL band has been observed in μPL spectra at the higher-energy side. This band corresponds to the optical transitions between delocalized states which are observed in photoreflectance spectra. The emission of localized excitons without resolved sharp lines has been also observed with macro-PL measurements at low temperatures. In addition, the donor trap–valence band (DT-VB) recombination has been identified for the three alloys in macro-PL spectra. It has been clearly observed that the intensity of DT-VB recombination is the weakest for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ alloys whereas for GaN${}_{0.02}$As${}_{0.98}$ and Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$ alloys it is very similar. The observed differences between the three alloys and the role of postgrowth annealing have been analyzed and discussed in the context of the influence of III-V host on alloy inhomogeneities, formation of native point defects in III-V-N alloys, and their influence on the optical quality of III-V-N alloys.

We study both the intrinsic and extrinsic spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides. We find that whereas the skew-scattering contribution is suppressed by the large band gap, the side-jump contribution is comparable to the intrinsic one with opposite sign in the presence of scalar and magnetic scattering. Intervalley scattering tends to suppress the side-jump contribution due to the loss of coherence. By tuning the ratio of intra- to intervalley scattering, the spin Hall conductivity shows a sign change in hole-doped samples. The multiband effect in other doping regimes is considered, and it is found that the sign change exists in the heavily hole-doped regime, but not in the electron-doped regime.

Most recently, orbital-selective quantum well states of $d$ electrons have been observed in SrVO${}_3$ ultrathin films and SrTiO${}_3$ surfaces. We construct from first-principles simple tight-binding models for such perovskite oxide heterostructures and surfaces. We show that this model provides a simple intuitive physical picture and yields, already with only two parameters, quantitatively reliable results, consistent with experiment. For quantum wells confined to only a few atomic layers or a higher quantum number, a nearly free electron description, on the other hand, does not work.

We argue that the magnetic susceptibility data for the low-density two-dimensional (2D) silicon-based electron gas indicate that magnetically active electrons are localized in spin droplets. The droplets exist in both the insulating and metallic phases, and interact ferromagnetically, forming an effective 2D Heisenberg ferromagnet. Comparing the data with analytical and numerical results for a 2D Heisenberg ferromagnet, we determine parameters of the model and show that the value of the droplet spin is either $S=1/2$ or $S=1$ with very few electrons in the droplet. We discuss the dependence of the magnetic susceptibility and the specific heat on the external magnetic field, which follows from the model, and hence we suggest further experimental tests of the model.

We develop a theory of intervalley Coulomb scattering in semiconducting carbon-nanotube quantum dots, taking into account the effects of curvature and chirality. Starting from the effective mass description of single-particle states, we study the two-electron system by fully including Coulomb interaction, spin-orbit coupling, and short-range disorder. We find that the energy level splittings associated with intervalley scattering are nearly independent of the chiral angle and, while smaller than those due to spin-orbit interaction, large enough to be measurable.

We present a detailed analysis of the spin-flip excitations induced by a periodic time-dependent electric field in the Rashba prototype Au(111) noble metal surface. Our calculations incorporate the full spinor structure of the spin-split surface states and employ a Wannier-based scheme for the spin-flip matrix elements. We find that the spin-flip excitations associated with the surface states exhibit an strong dependence on the electron momentum magnitude, a feature that is absent in the standard Rashba model [E. I. Rashba, Sov. Phys. Solid State 2, 1109 (1960)]. Furthermore, we demonstrate that the maximum of the calculated spin-flip absorption rate is about twice the model prediction. These results show that, although the Rashba model accurately describes the spectrum and spin polarization, it does not fully account for the dynamical properties of the surface states.

We report on the crystal structure and electronic bands of LaAu${}_2$ and CeAu${}_2$ surface intermetallic compounds grown by high-temperature deposition on Au(111). By scanning-tunneling microscopy we study the formation of different alloy phases as a function of growth temperature and lanthanide coverage. We determine the specific growth conditions to achieve monolayers and bilayers of LaAu${}_2$ and CeAu${}_2$ with high crystalline quality. Due to lattice mismatch with the underlying Au substrate, both LaAu${}_2$ and CeAu${}_2$ exhibit long-range moiré patterns, which can serve as templates for further nanostructure growth. By angle-resolved photoemission we map the two-dimensional band structure of these surface alloys, discussing the nature of the different spectral features in the light of first-principles calculations.

We show the possibility of inducing an edge charge current by applying time-dependent strain in gapped graphene samples preserving time-reversal symmetry. We demonstrate that this edge current has the same origin as the valley Hall response known to exist in the system.

A phase-field theory of transformations between martensitic variants and multiple twinning within martensitic variants is developed for large strains and lattice rotations. It resolves numerous existing problems. The model, which involves just one order parameter for the description of each variant-variant transformation and multiple twinnings within each martensitic variant, allows one to prescribe the twin interface energy and width, and to introduce interface stresses consistent with the sharp interface limit. A finite-element approach is developed and applied to the solution of a number of examples of twinning and combined austenite-martensite and martensite-martensite phase transformations (PTs) and nanostructure evolution. A similar approach can be developed for reconstructive, electric, and magnetic PTs.

Crystal and domain structures, and ferroelectric properties of tensile-strained BiFeO${}_3$ epitaxial films grown on orthorhombic (110)${}_o$ PrScO${}_3$ substrates were investigated. All films possess a $M_B$-type monoclinic structure with 109° stripe domains oriented along the [$\overline{1}10$]${}_o$ direction. For films thicknesses less than ∼40 nm, the presence of well-ordered domains is proved by the detection of satellite peaks in synchrotron x-ray diffraction studies. For thicker films, only the Bragg reflections from tilted domains were detected. This is attributed to the broader domain size distribution in thicker films. Using planar electrodes, the in-plane polarization of the $M_B$ phase is determined to be ∼85 μC/cm${}^2$, which is much larger than that of compressive-strained BiFeO${}_3$ films. Our results further reveal that the substrate monoclinic distortion plays an important role in determining the stripe domain formation of the rhombohedral ferroic epitaxial thin films, which sheds light on the problem of understanding elastic domain structure evolution in many other functional oxide thin films as well.

We describe a fluctuating-surface current formulation of radiative heat transfer between bodies of arbitrary shape that exploits efficient and sophisticated techniques from the surface-integral-equation formulation of classical electromagnetic scattering. Unlike previous approaches to nonequilibrium fluctuations that involve scattering matrices—relating “incoming” and “outgoing” waves from each body—our approach is formulated in terms of “unknown” surface currents, laying at the surfaces of the bodies, that need not satisfy any wave equation. We show that our formulation can be applied as a spectral method to obtain fast-converging semianalytical formulas in high-symmetry geometries using specialized spectral bases that conform to the surfaces of the bodies (e.g., Fourier series for planar bodies or spherical harmonics for spherical bodies), and can also be employed as a numerical method by exploiting the generality of surface meshes/grids to obtain results in more complicated geometries (e.g., interleaved bodies as well as bodies with sharp corners). In particular, our formalism allows direct application of the boundary-element method, a robust and powerful numerical implementation of the surface-integral formulation of classical electromagnetism, which we use to obtain results in new geometries, such as the heat transfer between finite slabs, cylinders, and cones.

We investigate the magnetic structure of Mn${}_{3-x}$Fe${}_x$Sn${}_2$ using neutron powder diffraction experiments and electronic structure calculations. These alloys crystallize in the orthorhombic Ni${}_3$Sn${}_2$ type of structure ($Pnma$) and comprise two inequivalent sites for the transition metal atoms (4$c$ and 8$d$) and two Sn sites (4$c$ and 4$c$). The neutron data show that the substituting Fe atoms predominantly occupy the 4$c$ transition metal site and carry a lower magnetic moment than Mn atoms. Four kinds of magnetic structures are encountered as a function of temperature and composition: two simple ferromagnetic structures (with the magnetic moments pointing along the $b$ or $c$ axis) and two canted ferromagnetic arrangements (with the ferromagnetic component pointing along the $b$ or $c$ axis). Electronic structure calculations results agree well with the low-temperature experimental magnetic moments and canting angles throughout the series. Comparisons between collinear and noncollinear computations show that the canted state is stabilized by a band mechanism through the opening of a hybridization gap. Synchrotron powder diffraction experiments on Mn${}_3$Sn${}_2$ reveal a weak monoclinic distortion at low temperature ($\beta \sim 90.08$${}^{\circ }$ at 175 K). This lowering of symmetry could explain the stabilization of the $c$-axis canted ferromagnetic structure, which mixes two orthorhombic magnetic space groups, a circumstance that would otherwise require unusually large high-order terms in the spin Hamiltonian.

The structural (space group $P$6/mmtn) and magnetic parameters of TbCo${}_2$Ni${}_3$ are investigated using neutron powder diffraction and polarized neutron powder diffraction at several temperatures below the magnetic transition temperature. It is found that this compound orders ferrimagnetically at 315 K, with ferromagnetic Tb and Co/Ni sublattices. Below 100 K, the magnetic axis is perpendicular to the hexagonal axis. Above 100 K, the magnetic axis reorients slightly towards the hexagonal axis. The magnitudes and orientation of the ordered magnetic moments exhibit complex temperature dependences. The observed magnitude of the ordered Tb magnetic moment at $T$ $=$ 10 K, is found to be 8.1(2) ${\mu }_B$, smaller than the free Tb${}^{3+}$ ion. The significance of this property, and its relation to the mechanisms driving the magnetic order in the parent TbCo${}_5$ compound, are discussed.

We establish the low-temperature phase diagrams of the spin-$1/2$ and spin-1 Kondo lattice models as a function of the conduction-band filling $n$ and the exchange coupling strength $J$ in the regime of ferromagnetic effective exchange interactions ($n\lesssim 0.5$, $J/D\lesssim 2$). We show that both models have several distinct ferromagnetic phases separated by continuous Lifshitz transitions of the Fermi-pocket vanishing or emergence type: one of the phases has a true gap in the minority band (half metal with magnetization rigidity), the others only a pseudogap. The spin-$1/2$ model has the half-metal phase, two topologically different pseudogap phases, and a paramagnetic state for very large $J$. The spin-1 model has the half-metal phase and a single pseudogap phase with an electron pocket. We find that, quite generically, ferromagnetism and Kondo screening coexist rather than compete both in spin-$1/2$ and spin-1 models. We establish the hysteretic behavior of the systems in an external magnetic field: spin-flop transitions preempt further Lifshitz transitions at finite magnetic field. We establish a “ferromagnetic Doniach diagram”: in the spin-$1/2$ model the Curie temperature peaks near $J/D\approx 1.2$ and goes to zero, while in the spin-1 model it saturates.

We study the physical properties of a ballistic heterostructure made of a ferromagnet (FM) and a spin-triplet superconductor (TSC) with a layered structure stacking along the direction perpendicular to the planes where a chiral $p_x+i{p}_y$ pairing occurs and assuming spin-dependent processes at the interface. We use a self-consistent Bogoliubov–de Gennes approach on a three-dimensional lattice to obtain the spatial profiles of the pairing amplitude and the magnetization. We find that, depending on the strength of the ferromagnetic exchange field, the ground state of the system can have two distinct configurations, with a parallel or antiparallel collinearity between the magnetic moments in the bulk and at the interface. We demonstrate that a magnetic state having noncoplanar interface, bulk, and Cooper pairs spins may be stabilized if the bulk magnetization is assumed to be fixed along a given direction. The study of the density of states reveals that the modification of the electronic spectrum in the FM plays an important role in the setting of the optimal magnetic configuration. Finally, we find the existence of induced spin-polarized pair correlations in the FM-TSC system.

We use the approximation-free bold diagrammatic Monte Carlo technique to study the effects of a finite dispersion of the optical phonon mode on the properties of the Holstein polaron, especially its effective mass. For weak electron-phonon coupling the effect is very small, but it becomes significant for moderate and large electron-phonon coupling. The effective mass is found to increase (decrease) if the phonon dispersion has a negative (positive) curvature at the center of the Brillouin zone.

We studied the photoinduced dynamics of spinel MnV${}_{1.8}$Al${}_{0.2}$O${}_4$ and pyrochlore Yb${}_2$V${}_2$O${}_7$ as prototypical Mott-Hubbard insulators with magnetic ordering. We found that the Mott excitation spectrum is suppressed much sooner than 1 ps after photoirradiation, and subsequently it shows an energy shift over several or several tens of picoseconds, which is caused by the melting of magnetic ordering. This indicates that the Mott excitation spectrum is dominated not only by the Coulomb repulsion energy but also by magnetic interactions, and their different energy scales lead to the different time scales in the photoinduced dynamics.

Muon spin relaxation results on the pyrochlore iridate Nd${}_2$Ir${}_2$O${}_7$ are reported. Spontaneous coherent muon spin precession below the metal-insulator transition (MIT) temperature of about 33 K is observed, indicating the appearance of a long-range magnetic ordering of Ir${}^{4+}$ moments. With further decrease in temperature, the internal field at the muon site increases again below about 9 K. The second increase of internal field suggests the ordering of Nd${}^{3+}$ moments, which is consistent with a previous neutron experiment. Our results suggest that the MIT and magnetic ordering of Ir${}^{4+}$ moments have a close relationship and that the large spin-orbit coupling of Ir 5$d$ electrons plays a key role for both MIT and the mechanism of the magnetic ordering in pyrochlore iridates in the insulting ground state.