We have studied fluorescence in rubrene single crystals by use of fluorescence up-conversion, fluorescence anisotropy, and temperature-dependent time-resolved fluorescence techniques. Thermally activated singlet fission was demonstrated to play an important role in the quenching of two intrinsic fluorescence bands, 565 and 610 nm. At low temperatures, singlet fission is suppressed while another process, namely energy trapping, becomes pronounced. The 650 nm fluorescence originates from the hole trap states located 0.27 eV above the valence band.

We present a renormalization-group (RG) method which allows for an analytical study of the transient dynamics of open quantum systems on all time scales. Whereas oscillation frequencies and decay rates of exponential time evolution follow from the fixed point positions, the long-time behavior of preexponential functions is related to the scaling behavior around the fixed points. We show that certain terms of the RG flow are only cut off by inverse time, which leads to a difference between infrared and ultraviolet scaling. An evaluation for the Ohmic spin boson model at weak damping reveals significant deviations from previous predictions in the long-time regime. We propose that weak-coupling problems for stationary quantities can in principle turn into strong-coupling ones for the determination of the long-time behavior.

We study the transmission response of microwaves through two identical metallic plates machined with ultranarrow slit arrays. The measured and calculated spectra consistently display a striking transmission peak at wavelength much larger than any characteristic length of the structure (e.g., about 20-fold of the lattice period), which cannot be directly explained by the existing mechanisms. Both the LC-circuit-based microscopic picture and the effective-medium-based macroscopic model are established to capture the essential physics behind such unexpected resonance at the deep subwavelength scale. Prospective applications of this transmission property can be anticipated, considering the merits of compact and excellent immunity to structural imperfections.

Quantum spin liquids are highly entangled ground states of quantum systems with an emergent gauge structure, fractionalized spinon excitations, and other unusual properties. While these features clearly distinguish quantum spin liquids from conventional, mean-field-like states at zero temperature ($T$), their status at $T>0$ is less clear. Strictly speaking, it is known that most quantum spin liquids lose their identity at nonzero temperature, being then adiabatically transformable into a trivial paramagnet. This is the case for the $U\left(1\right)$ quantum spin liquid states recently proposed to occur in the quantum spin ice pyrochlores. Here we propose, however, that in practical terms, the latter quantum spin liquids can be regarded as phases distinct from the high-temperature paramagnet. Through a combination of gauge mean-field theory calculations and physical reasoning, we argue that these systems sustain both quantum spin liquid and thermal spin liquid phases, dominated by quantum fluctuations and entropy, respectively. These phases are separated by a first-order “thermal confinement” transition such that, for temperatures below the transition, spinons and emergent photons are coherently propagating excitations, and above it the dynamics is classical. Even for parameters for which the ground state is magnetically ordered and not a quantum spin liquid, this strong first-order transition occurs, preempting conventional Landau-type criticality. We argue that this picture explains the anomalously low-temperature phase transition observed in the quantum spin ice material Yb${}_2$Ti${}_2$O${}_7$.

We report a detailed investigation of the interplay between size quantization and local scattering centers in graphene nanoribbons, as seen in the local density of states. The spectral signatures, obtained after Fourier transformation of the local density of states, include characteristic peaks that can be related to the transverse modes of the nanoribbon. In armchair ribbons, the Fourier transformed density of states of one of the two inequivalent sublattices takes a form similar to that of a quantum channel in a two-dimensional electron gas, modified according to the differences in band structure. After addition of the second sublattice contribution, a characteristic modulation of the pattern due to superposition is obtained, similar to what has been obtained in spectra due to single impurity scattering in large-area graphene. We present analytic results for the electron propagator in armchair nanoribbons in the Dirac approximation, including a single scattering center within a $T$-matrix formulation. For comparison, we have extended the investigation with numerics obtained with an atomistic recursive Green's function approach. The spectral signatures of the atomistic approach include the effects of trigonal warping. The impurity induced oscillations in the local density of states are not decaying at large distance in few-mode nanoribbons.

Graphene is promising as a host material for electron spin qubits because of its predicted potential for long coherence times. In armchair graphene nanoribbons (aGNRs) a small band gap is opened, allowing for electrically gated quantum dots, and furthermore the valley degeneracy is lifted. The spin lifetime $T_1$ is limited by spin relaxation, where the Zeeman energy is absorbed by lattice vibrations, mediated by spin-orbit and electron-phonon coupling. We have calculated $T_1$ by treating all couplings analytically and find that $T_1$ can be in the range of seconds for several reasons: (i) low phonon density of states away from Van Hove singularities; (ii) destructive interference between two relaxation mechanisms; (iii) Van Vleck cancellation at low magnetic fields; (iv) vanishing coupling to out-of-plane modes in lowest order due to the electronic structure of aGNRs. Owing to the vanishing nuclear spin of ${}^{12}$C, $T_1$ may be a good measure for overall coherence. These results and recent advances in the controlled production of graphene nanoribbons make this system interesting for spintronics applications.

An extensive search for low-energy lithium defects in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method shows that the four-lithium-atom substitutional point defect is exceptionally stable. This defect consists of four lithium atoms with strong ionic bonds to the four under-coordinated atoms of a silicon vacancy defect, similar to the bonding of metal ions in Zintl phases. This complex is stable over a range of silicon environments, indicating that it may aid amorphization of crystalline silicon and form upon delithiation of the silicon anode of a Li-ion rechargeable battery.

We provide direct experimental evidence for the identical effect of the in-plane Fe${}_{1-x}$Co${}_x$ and of the out-of-plane O${}_{1-x}$F${}_x$ chemical dilution on the itinerant spin-density-wave (SDW) magnetic phase in CeFeAsO. Remarkably, the suppression of SDW is not sensitive at all to the different kinds of disorder introduced in the two cases. Still, it is clearly shown that the sizable in-plane disorder induced by the Fe${}_{1-x}$Co${}_x$ substitution is highly effective in suppressing $T_{\mathrm{c}}$. Differently from what is observed in CeFeAsO${}_{1-x}$F${}_x$, the ordered magnetic phase of the Ce sublattice is preserved throughout the whole phase diagram in CeFe${}_{1-x}$Co${}_x$AsO ($x⩽0.2$). An intriguing effect is encountered, whereby the magnetic coupling among Ce${}^{3+}$ ions is enhanced by the superconducting phase.

Element-specific phase-resolved x-ray ferromagnetic resonance (FMR) was used to study spin pumping within Co${}_{50}$Fe${}_{50}$(3)/Cu(6)/Ni${}_{80}$Fe${}_{20}$(5) (thicknesses in nanometers) spin valve structures with large areas, so that edge effects typical of nanopillars used in standard magnetotransport experiments could be neglected. The phase of precession of the Co${}_{50}$Fe${}_{50}$ fixed layer was recorded as FMR was induced in the Ni${}_{80}$Fe${}_{20}$ free layer. The field dependence of the fixed layer phase contains a clear signature of spin transfer torque (STT) coupling due to spin pumping. Fitting the phase delay yields the spin-mixing conductance, the quantity that controls all spin transfer phenomena. The STT coupling is destroyed by insertion of Ta into the middle of the Cu layer.

Ab initio molecular dynamics simulations were used to reveal the mechanism of the fivefold to sixfold transition in Si coordination numbers of CaSi${}_2$O${}_5$. The longest first-neighbor Si-O distance drops from 2.8 to 1.8 Å upon the triclinic to monoclinic transition. We find significant bulk modulus softening during the structure crossover, which is due to appearance of intermediate Si-O connections in the triclinic phase under slightly nonhydrostatic stress. Nonetheless, no soft phonon modes were found in either structure, indicating that both structures are dynamically stable. Across the transition, $c_{33}$ doubles and $c_{35}$ increases sixfold in magnitude due to the formation of new Si-O bonds. Chemical bonding analysis reveals distinctions in the electron localization function and bond ellipticity between the regular (1.8 Å) and the dangling Si-O bonds (2.8 Å), both of which suggest an impending disassociation of the dangling Si-O bond.

The effect of pressure on the acoustic modes and adiabatic elastic constants of the perovskite-type ($AB$O${}_3$) relaxor compound PbSc${}_{1/2}$Ta${}_{1/2}$O${}_3$ have been studied by Brillouin spectroscopy at room temperature up to 9.2 GPa. It is shown that the elastic constants are very sensitive to the pressure-induced structural transformations that were established by diffraction and Raman scattering analysis. Changes in the evolution of the pseudocubic elastic constants $c_{11}$ and $c_{44}$ occur at the two phase-transition pressures $p_c$${}_1$ $=$ 1.9 GPa and $p_{c2}$ ∼ 5.5 GPa. Changes in evolution of $c_{12}$ occur at the two characteristic pressures $p_1$${}^{*}$ $=$ 1.2 GPa and $p_2$${}^{*}$ ∼ 3.0 GPa, which mark changes in the local structure that precede the corresponding phase transitions. The elastic anisotropy and the width of the transverse acoustic mode increase in the pressure range between $p_c$${}_1$ and $p_c$${}_2$ with a maximum near $p_2$${}^{*}$. A strong decrease in the Cauchy parameter is observed at pressures above $p_2$${}^{*}$, which indicates an enhancement of the covalent character of chemical bonding. Increasing pressure gradually suppresses the central peak typical of the relaxor ergodic state, and the quasielastic scattering vanishes at a pressure slightly above $p_2$${}^{*}$.

We theoretically investigate the dielectric properties of dimer Mott insulators. We adopt the two-dimensional quarter-filled extended Hubbard model for $\kappa$-(BEDT-TTF)${}_2$$X$ that includes the coupling with the dimer lattice modes, and numerically calculate the ground state for the bond-length-alternated and disordered lattices. Electric dipole moments in dimers are induced by interdimer bond-order alternation, and electric dipole moments are aligned perpendicular to the direction of bond-order alternation. As a result, the electric polarizations arising from electric dipole moments in dimers are strongly coupled with the dimer lattice modes of interdimer bond-length alternation, and the more stable one of these two ferroelectric states is induced by disorder in dimer lattice. The ferroelectric states arise from the hybridization of in-gap charge excited states to the dimer Mott ground state for the equilibrium lattice, and one (the other) is strongly stabilized near the phase boundary with the metallic phase (in the spin-liquid phase). The present results are consistent with the experimentally observed dielectric anomaly in $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$.

We study ferromagnetism in the periodic Anderson model with and without a magnetic field by the Gutzwiller theory. We find three ferromagnetic phases: a weak ferromagnetic phase (FM0), a half-metallic phase without Fermi surface for the majority spin (FM1), and a ferromagnetic phase with almost completely polarized $f$ electrons (FM2). The Fermi surface changes from the large Fermi surface in the paramagnetic state to the small Fermi surface in FM2. We also find that the transitions between the ferromagnetic phases can be second-order phase transitions in spite of the absence of symmetry breaking. While we cannot define an order parameter for such transitions in an ordinary way, the topology of the Fermi surface characterizes the transitions, i.e., they are Lifshitz transitions.

We propose a model of the injection of spin-polarized current in systems with diffuse interfaces. The effect of the interfaces between a ferromagnet and a nonmagnet and between two ferromagnets on spin injection is investigated. We first generalize the formalism for calculating the spin accumulation by taking the spin accumulation as the difference of spin-up and spin-down density of states, which is necessary for treating the interface between different ferromagnets. Then, we include the effect of atomic species interdiffusion at the interface by using Fick's law. It is shown that the discontinuity of the spin accumulation at the interface depends strongly on the degree of interface mixing.

We study qubit-mediated energy transfer between two electron reservoirs by adopting a numerically exact influence functional path-integral method. This nonperturbative technique allows us to study the system's dynamics beyond the weak coupling limit. Our simulations for the energy current indicate that perturbative-Markovian master equation predictions significantly deviate from exact numerical results already at intermediate coupling $\pi \rho {\alpha }_{j,j^{\prime }}\gtrsim 0.4$, where $\rho$ is the metal (Fermi sea) density of states, taken as a constant, and ${\alpha }_{j,j^{\prime }}$ is the scattering potential energy of electrons, between the $j$ and $j^{\prime }$ states. Perturbative Markovian master equation techniques should be therefore used with caution beyond the strictly weak subsystem-bath coupling limit, especially when a quantitative knowledge of transport characteristics is desired.

Frozen modes arising in stacked subwavelength hole arrays are studied in detail. Their origin is proved to be connected with the interaction between the extraordinary transmission resonance and the Fabry–Perot cavity mode. The analysis is done for various situations that differ in metal plate thicknesses and sizes and shape of the holes. Dispersion results and finite-stack transmission spectra are in good agreement, both showing the features indicating hybridization. The boundaries of the hybridization are found in terms of the geometrical parameters. The effect of the number of stacked plates on the transmission has been demonstrated. Finally, it is shown that the group index of refraction $n_g$ in the considered finite structures can be larger than 200. The obtained estimates of $n_g$, which are based on dispersion and transmission results, well coincide with each other.

The conductance, the transmission, and the reflection probabilities through rectangular potential barriers and $p$-$n$ junctions are obtained for bilayer graphene taking into account the four bands of the energy spectrum. We have evaluated the importance of the skew hopping parameters ${\gamma }_3$ and ${\gamma }_4$ to these properties and show that for energies $E>{\gamma }_1/100$ their effect is negligible. For high energies two modes of propagation exist and we investigate scattering between these modes. For perpendicular incidence both propagation modes are decoupled, and scattering between them is forbidden. This extends the concept of pseudospin as defined within the two-band approximation to a four-band model and corresponds to the (anti)symmetry of the wave functions under in-plane mirroring. New transmission resonances are found that appear as sharp peaks in the conductance which are absent in the two-band approximation. The application of an interlayer bias to the system (1) breaks the pseudospin structure, (2) opens a band gap that results in a distinct feature of suppressed transmission in the conductance, and (3) breaks the angular symmetry with respect to normal incidence in the transmission and reflection.

A single graphene layer placed between two parallel Ni(111) surfaces screens the strong attractive force and results in a significant reduction of adhesion and sliding friction. When two graphene layers are inserted, each graphene is attached to one of the metal surfaces with a significant binding and reduces the adhesion further. In the sliding motion of these surfaces the transition from stick-slip to continuous sliding is attained, whereby nonequilibrium phonon generation through sudden processes is suppressed. The adhesion and corrugation strength continues to decrease upon insertion of the third graphene layer and eventually saturates at a constant value with increasing number of graphene layers. In the absence of Ni surfaces, the corrugation strength of multilayered graphene is relatively higher and practically independent of the number of layers. Present first-principles calculations reveal the superlubricant feature of graphene layers placed between pseudomorphic Ni(111) surfaces, which is achieved through the coupling of Ni-$3d$ and graphene-$\pi$ orbitals. The effect of graphene layers inserted between a pair of parallel Cu(111) and Al(111) surfaces is also discussed. The treatment of sliding friction under the constant loading force, by taking into account the deformations corresponding to any relative positions of sliding slabs, is the unique feature of our study.

We analyze the quantum Hall effect in single layer graphene with bilayer stripe defects. Such defects are often encountered at steps in the substrate of graphene grown on silicon carbide. We show that $\mathit{AB}$ or $\mathit{AA}$ stacked bilayer stripes result in large Hall conductivity fluctuations that destroy the quantum Hall plateaux. The fluctuations are a result of the coupling of edge states at opposite edges through currents traversing the stripe. Upon rotation of the second layer with respect to the continuous monolayer (a twisted-bilayer stripe defect), such currents decouple from the extended edge states and develop into long-lived discrete quasibound states circulating around the perimeter of the stripe. Backscattering of edge modes then occurs only at precise resonant energies, and hence the quantum Hall plateaux are recovered as twist angle grows.

We study the binding of chlorine atoms to carbon nanotubes and graphene at a Stone-Wales defect and to the sidewalls of pristine nanotubes. We show using ab initio thermodynamics that if the environment is chlorine-rich enough, the unzipping of carbon nanotubes into graphene nanoribbons with chlorinated edges may be thermodynamically feasible. By controlling the Cl chemical potential through temperature and pressure, opening selectively tubes below a threshold diameter might be possible. Additionally, we find increased binding energies for chlorine atoms bound to Stone-Wales defects as compared to defect-free graphene and nanotubes, indicating that defects might act as nucleation sites for tube unzipping. On graphene, no more than a single Cl atom may be bound to the defect in ambient conditions, limiting possible Cl-induced changes in the resulting nanoribbons.