We calculate the renormalized parameters for the quasiparticles and their interactions for the Hubbard model in the paramagnetic phase as deduced from the low-energy Fermi-liquid fixed point using the results of a numerical renormalization-group calculation (NRG) and dynamical mean-field theory (DMFT). Even in the low-density limit there is significant renormalization of the local quasiparticle interaction $\tilde{U}$, in agreement with estimates based on the two-particle scattering theory of J. Kanamori [Prog. Theor. Phys. 30, 275 (1963)]. On the approach to the Mott transition we find a finite ratio for $\tilde{U}/\tilde{D}$, where $2\tilde{D}$ is the renormalized bandwidth, which is independent of whether the transition is approached by increasing the on-site interaction $U$ or on increasing the density to half filling. The leading ${\omega }^2$ term in the self-energy and the local dynamical spin and charge susceptibilities are calculated within the renormalized perturbation theory (RPT) and compared with the results calculated directly from the NRG-DMFT. We also suggest, more generally from the DMFT, how an approximate expression for the $\mathbf{q},\omega$ spin susceptibility $\chi (\mathbf{q},\omega )$ can be derived from repeated quasiparticle scattering with a local renormalized scattering vertex.

We measured the optical transitions due to the internal energy levels of an exciton and biexciton in a CuCl single crystal using pump-probe spectroscopy. The transient absorption bands due to the transitions from the $1s$ to $2p$ and from the $1s$ to $3p$ exciton states were observed at 3 K, which is consistent with their reported energies. Simultaneously, the transient absorption peak due to the biexciton was observed, which corresponds to a transition from the lowest state (composed of two $1s$ excitons) to higher states (composed of $1s$ and $2p$ excitons). The value of the observed biexciton peak energy was reasonable considering the hydrogen molecule orbitals and the electron-to-hole effective mass ratio. In addition, the transient absorption peaks were broadened at 77 K, because of the increase in the homogeneous width of the $2p$ exciton state. The transient absorption spectrum was almost completely determined by this width. Our findings are of importance with regard to the optical phenomena in the infrared region related to the quantum coherence of excitons and biexcitons in semiconductors.

The properties of interstitial vanadium $\left({\mathrm{V}}_i\right)$ in Si and its interactions with the vacancy and the self-interstitial, as well as with hydrogen, are calculated using first-principles techniques. The stable configurations, gap levels, and binding energies agree well with the available experimental data. The nudged-elastic-band method is used to calculate the activation energies for diffusion of ${\mathrm{V}}_i$ in various charge states. They range from 1.46 (for ${{\mathrm{V}}_i}^{+}$) to 2.04 eV (for ${{\mathrm{V}}_i}^{-}$). The (trigonal) $\{{\mathrm{V}}_i,\mathrm{H}\}$ pair has a binding energy of 1.15 eV, a donor level at $E_c-0.61\mathrm{eV}$, and possibly an acceptor level $E_c-0.07\mathrm{eV}$. Substitutional vanadium $\left({\mathrm{V}}_s\right)$ can also trap H interstitials and form electrically active $\{{\mathrm{V}}_s,\mathrm{H}\}$ and $\{{\mathrm{V}}_s,\mathrm{H},\mathrm{H}\}$ complexes.

In pressure-induced reversible structural transitions, the term “reversible” refers to the recovery of the virgin structure in a material upon complete decompression. Pressure-induced amorphous-to-crystalline transitions have been claimed to be reversible, but evidence that amorphous material recovers its virgin amorphous structure upon complete depressurization has been lacking. In amorphous $\mathrm{A}{\mathrm{s}}_2\mathrm{S}{\mathrm{e}}_3 \left(\mathrm{a}\text{−}\mathrm{A}{\mathrm{s}}_2\mathrm{S}{\mathrm{e}}_3\right)$ chalcogenide, however, we report a novel reversible amorphous-to-crystalline transition that provides compelling experimental evidence that upon complete decompression, the recovered amorphous phase is structurally the same as that of the virgin (as-cast) amorphous phase. Combining the experimental results with ab initio molecular dynamics simulations, we elucidate that the amorphization is mediated by a surplus of total free energy in the high-pressure face-centered cubic phase as compared to the virgin amorphous phase and that the structural recovery to the virgin amorphous phase is a consequence of an enhancement in covalent bonding character over interlayer forces upon complete decompression. Furthermore, we observed a two-dimensional to three-dimensional network transition under compression and its reversibility upon decompression.

We studied time-resolved photocarrier transport through InAs/GaAs quantum dot superlattice (QDSL) solar cells (SCs) using time-of-flight spectroscopy with an optical probe QD structure beneath the QDSL. Carriers optically pumped in the top $p$-GaAs layer were transported through the intrinsic layer, including the QDSLs, before arriving at the probe QDs. The photoexcited carrier density significantly influenced the time-resolved photoluminescence (PL) of the QDSLs and probe QDs. The time-resolved PL profile of the probe QDs indicated that excitation densities in excess of $25\mathrm{nJ}/\mathrm{c}{\mathrm{m}}^2$ drastically decreased the rise time, suggesting rapid carrier transport through the QDSLs. This was also confirmed by QDSL carrier transport dynamics, for which the PL intensity of the excited states decayed rapidly above this excitation power density, $25\mathrm{nJ}/\mathrm{c}{\mathrm{m}}^2$, while the ground state remained constant. These results demonstrate that filling the ground states of QDSLs and starting to populate the excited state miniband accelerates carrier transport in QDSL SCs. Furthermore, according to two-step photon absorption measurements taken with a 1.3-μm infrared laser light source, electrons play a key role in the generation of extra photocurrent by sub-band-gap photon irradiation.

The realization of single molecular electronics is considered the next frontier to addressing and sustaining the storage needs of the future. In order to realize a single molecular device working at 300 K, two conditions must be satisfied: first, there must be no molecular diffusion, i.e., robust bonding between molecules and the contacting electrode, and second, stable electronic interface states. In this study, using a combination of 7-K and 300-K ultrahigh vacuum scanning tunneling microscopy/spectroscopy experiments and theoretical ab initio calculations, we investigated the adsorption of $\pi$-conjugated metal-free phthalocyanine (Pc) single molecules onto an Fe(001) whisker single crystal along with the resulting electronic interface structures. The Pc/Fe(001) system was found to prevent molecular diffusion even at 300 K, due to strong adsorption as well as the presence of a larger diffusion barrier than that of the Pc/Ag(001) system, in which molecules are known to diffuse at 300 K. The origin of such a robust bonding was studied by recovering the sample local density of states (LDOS) with the normalized $(dI/dV)/T$ curves, where the LDOS peaks are successfully explained by theoretical calculations.

We consider the existence of plasmons in a nonequilibrium situation where electrons from the valence band of graphene are pumped to states in the Brillouin zone around the M point by a high intensity UV electromagnetic field. The resulting out-of-equilibrium electron gas is later probed by a weak electromagnetic field of different frequency. We show that the optical properties of the system and the dispersion of the plasmons are strongly anisotropic, depending on the pumping radiation properties: its intensity, polarization, and frequency. This anisotropy has its roots in the saddlelike nature of the electronic dispersion relation around that particular point in the Brillouin zone. It is found that despite the strong anisotropy, the dispersion of the plasmons scales with the square root of the wave number but is characterized an effective Fermi energy, which depends on the properties of the pumping radiation. Our calculations go beyond the usual Dirac cone approximation taking the full band structure of graphene into account. This is a necessary condition for discussing plasmons at the M point in the Brillouin zone.

In a graphene $pn$ junction at high magnetic field, unidirectional “snake states” are formed at the $pn$ interface. In a clean $pn$ junction, each snake state exists in one of the valleys of the graphene band structure, and the conductance of the junction as a whole is determined by microscopic details of the coupling between the snake states at the $pn$ interface and quantum Hall edge states at the sample boundaries [Tworzydło et al., Phys. Rev. B 76, 035411 (2007)]. Disorder mixes and couples the snake states. We here report a calculation of the full conductance distribution in the crossover between the clean limit and the strong-disorder limit, in which the conductance distribution is given by random matrix theory [Abanin and Levitov, Science 317, 641 (2007)]. Our calculation involves an exact solution of the relevant scaling equation for the scattering matrix, and the results are formulated in terms of parameters describing the microscopic disorder potential in bulk graphene.

We present a microscopic theory of nonlinear damping and dephasing of low-frequency eigenmodes in nanomechanical and micromechanical systems. The mechanism of the both effects is scattering of thermally excited vibrational modes off the considered eigenmode. The scattering is accompanied by energy transfer of $2\hslash {\omega }_0$ for nonlinear damping and is quasielastic for dephasing. We develop a formalism that allows studying both spatially uniform systems and systems with a strong nonuniformity, which is smooth on the typical wavelength of thermal modes but is pronounced on their mean free path. The formalism accounts for the decay of thermal modes, which plays a major role in the nonlinear damping and dephasing. We identify the nonlinear analogs of the Landau-Rumer, thermoelastic, and Akhiezer mechanisms and find the dependence of the relaxation parameters on the temperature and the geometry of a system.

Solitons are peculiar excitations that appear in a wide range of nonlinear systems such as in fluids or optics. We show here that the collective transport of charges observed in charge density wave (CDW) systems can be explained by using a similar theory based on a traveling soliton lattice. A coherent x-ray diffraction experiment performed in the sliding state of a CDW material reveals peculiar diffraction patterns in good agreement with this assumption. Therefore, the collective transport of charges in CDW systems may be due to a nonlinear interaction leading to a self-localized excitation, carrying charges without deformation through the sample, on top of the CDW ground state. This single theory explains why charges remain spatially correlated over very long distances and reconciles the main features of sliding CDW systems observed by transport measurements and diffraction. This approach highlights a new type of charge transport in CDW systems and opens perspectives in controlling correlated charges without dispersion over macroscopic distances.

The electronic structure of a single Na monolayer on the surface of single-crystal ${\mathrm{HfSe}}_2$ is investigated using angle-resolved photoemission spectroscopy. We find that this system exhibits an almost perfect “nearly-free-electron” behavior with an extracted effective mass of $\sim 1m_e$, in contrast to heavier masses found previously for alkali-metal monolayers on other substrates. Our density-functional-theory calculations indicate that this is due to the large lattice constant, causing both exchange and correlation interactions to be suppressed, and to the weak hybridization between the overlayer and the substrate. This is therefore an ideal model system for understanding the properties of two-dimensional materials.

In polar semiconductors and oxides, the long-range nature of the electron-phonon $\left(e\text{−}\mathrm{ph}\right)$ interaction is a bottleneck to compute charge transport from first principles. Here, we develop an efficient ab initio scheme to compute and converge the $e\text{−}\mathrm{ph}$ relaxation times (RTs) and electron mobility in polar materials. We apply our approach to GaAs, where by using the Boltzmann equation with state-dependent RTs, we compute mobilities in excellent agreement with experiment at $250\text{–}500\mathrm{K}$. The $e\text{−}\mathrm{ph}$ RTs and the phonon contributions to intravalley and intervalley $e\text{−}\mathrm{ph}$ scattering are also analyzed. Our work enables efficient ab initio computations of transport and carrier dynamics in polar materials.

We present a method to microscopically derive a small-size $\mathbf{k}·\mathbf{p}$ Hamiltonian in a Hilbert space spanned by physically chosen ab initio spinor wave functions. Without imposing any complementary symmetry constraints, our formalism equally treats three- and two-dimensional systems and simultaneously yields the Hamiltonian parameters and the true ${\mathbb{Z}}_2$ topological invariant. We consider bulk crystals and thin films of ${\mathrm{Bi}}_2{\mathrm{Se}}_3, {\mathrm{Bi}}_2{\mathrm{Te}}_3$, and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. It turns out that the effective continuous $\mathbf{k}·\mathbf{p}$ models with open boundary conditions often incorrectly predict the topological character of thin films.

We study the Haldane-Hubbard model by exact renormalization group techniques. We analytically construct the topological phase diagram, for weak interactions. We predict that many-body interactions induce a shift of the transition line: in particular, repulsive interactions enlarge the topologically nontrivial region. The presence of new intermediate phases, absent in the noninteracting case, is rigorously excluded at weak coupling. Despite the nontrivial renormalization of the wave function and of the Fermi velocity, the conductivity is universal: at the renormalized critical line, both the discontinuity of the transverse conductivity and the longitudinal conductivity are independent of the interaction, thanks to remarkable cancellations due to lattice Ward identities. In contrast to the quantization of the transverse conductivity, the universality of the longitudinal conductivity cannot be explained via topological arguments.

MHz conductivity, torque magnetometer, and magnetization measurements are reported on single crystals of ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ and ${\mathrm{NdOs}}_4{\mathrm{Sb}}_{12}$ using temperatures down to 0.5 K and magnetic fields of up to 60 tesla. The field-orientation dependence of the de Haas-van Alphen and Shubnikov-de Haas oscillations is deduced by rotating the samples about the $\left[010\right]$ and $\left[0\overline{1}1\right]$ directions. The results indicate that ${\mathrm{NdOs}}_4{\mathrm{Sb}}_{12}$ has a similar Fermi surface topology to that of the unusual superconductor ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$, but with significantly smaller effective masses, supporting the importance of local phonon modes in contributing to the low-temperature heat capacity of ${\mathrm{NdOs}}_4{\mathrm{Sb}}_{12}$. By contrast, ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ undergoes a field-induced transition from an unusual semimetal into a high-field, high-temperature state characterized by a single, almost spherical Fermi-surface section. The behavior of the phase boundary and comparisons with models of the band structure lead us to propose that the field-induced phase transition in ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ is similar in origin to the well-known $\alpha \text{−}\gamma$ transition in Ce and its alloys.

This theoretical paper discusses microscopic models giving rise to special types of order in which conduction electrons are bound together with localized spins to create composite order parameters. It is shown that composite order is related to the formation of a spin liquid with gapped excitations carrying quantum numbers that are a fraction of those of an electron. These spin liquids are special in the sense that their formation necessarily involves spin degrees of freedom of both the conduction and the localized electrons and can be characterized by nonlocal order parameters. A detailed description of such spin-liquid states is presented with a special care given to a demonstration of their robustness against local perturbations preserving the Lie group symmetry and the translational invariance.

We analyze antiferromagnetism in $f$-electron superlattices. We show that the competition between the Kondo effect and the RKKY interaction in $f$-electron materials is modified by the superlattice structure. Thus, the quantum critical point which separates the magnetic phase and the Fermi-liquid phase depends on the structure of the $f$-electron superlattice. The competition between the Kondo effect and the RKKY interaction is also reflected in the magnetic interlayer coupling between different $f$-electron layers. We demonstrate that in the case of a weak Kondo effect the magnetic interlayer coupling behaves similarly to other magnetic heterostructures without the Kondo effect. However, close to the quantum phase transition, the dependence of the interlayer coupling on the distance between the $f$-electron layers is modified by the Kondo effect. Another remarkable effect, which is characteristic for an $f$-electron superlattice, is that the magnetic interlayer coupling does vanish stepwise depending on the distance between different $f$-electron layers. As a consequence, the quantum critical point depends also stepwise on this distance.

The magnetic phase diagram of $\mathrm{N}{\mathrm{i}}_{1-x}\mathrm{Z}{\mathrm{n}}_x\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$ demonstrates complex magnetic transitions for $x=0.05$ with two long-range magnetic-order transitions at ${T_C}_1=41\mathrm{K}$ and ${T_C}_2=17\mathrm{K}$. A glasslike feature emerges between ${T_C}_1$ and ${T_C}_2$ with the application of a direct current (dc) magnetic field in alternating current (ac) susceptibility measurements. Dielectric measurements show a change from the conventional dielectric to a quantum-paraelectric-like behavior without anomalies near the magnetic ordering temperature in the range $0.05<x\le 1$. The strong correlation among magnetic, dielectric, and ferroelectric properties near the structural relaxation temperatures 41 and 17 K confirms the emergence of multiferroic behavior in $\mathrm{N}{\mathrm{i}}_{0.95}\mathrm{Z}{\mathrm{n}}_{0.05}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$. A significant x-ray magnetic circular dichroism signal at the Ni site and ac susceptibility measurements indicate a spin-canting behavior between ${T_C}_1$ and ${T_C}_2$. The dilute doping of the non-Jahn-Teller Zn ion in the Jahn-Teller Ni site modulate the different magnetic exchange interactions of Ni and Cr, which could invoke spin-canting and multiferroic behaviors.