We study the influence of Coulomb effects on conductance $\left(g\right)$ of 1,4-butanedithiol-linked gold nanoparticle (NP) films near a percolation insulator-to-metal transition. On the insulating side, $g\sim \exp [-{(T_{\circ }/T)}^{1/2}]$, where $T$ is absolute temperature, a behavior predicted by Efros-Shklovskii's theory for charges optimizing pathways that accommodate Coulomb charging barriers. On the metallic side below $\sim 20$ K, $g$ varies linearly with $T^{1/2}$. Such a correction to $g(T=0)$ is predicted by Altshuler-Aronov's theory for Fermi liquid metals when disorder mediates electron-electron $\left(e\text{−}e\right)$ Coulomb interactions. Remarkably, in the present system, the $T^{1/2}$ component of $g$ is significant compared to $g(T=0)$, and fitting to Boltzmann's transport theory yields elastic scattering lengths that are anomalously small—much smaller than the distance between atoms (Ioffe-Regel limit required for metals). Previous studies of materials such as fullerites, layered organic salts, and transition metal compounds have also reported such anomalously small scattering lengths and large $T^{1/2}$ components and attributed them to strong Coulomb mediated $e\text{−}e$ correlations, which we believe is likely the case in the present system as well. This study highlights a potential opportunity to use molecularly linked nanoparticle films as a platform to study strongly correlated electrons in a controlled fashion.

In spin-charge coupled systems, geometrical frustration of underlying lattice structures can give rise to nontrivial magnetic orders and electronic states. Here we explore such a possibility in the Kondo lattice model with classical localized spins on a triangular lattice by using a variational calculation and simulated annealing. We find that the system exhibits a four-sublattice collinear ferrimagnetic phase at 5/8 filling for a large Hund's-rule coupling. In this state, the system spontaneously differentiates into the up-spin kagome network and the isolated down-spin sites, which we call the kagome network formation. In the kagome network state, the system becomes Dirac half-semimetallic: The electronic structure shows a massless Dirac node at the Fermi level, and the Dirac electrons are almost fully spin polarized due to the large Hund's-rule coupling. We also study the effect of off-site Coulomb repulsion in the kagome network phase where the system is effectively regarded as a 1/3-filling spinless fermion system on the kagome lattice. We find that, at the level of the mean-field approximation, a $\sqrt{3}\times \sqrt{3}$-type charge order occurs in the kagome network state, implying the possibility of fractional charge excitations in this triangular lattice system. Moreover, we demonstrate that the kagome network formation with fully polarized Dirac electrons are controllable by an external magnetic field.

Vanadium sesquioxide ${\mathrm{V}}_2{\mathrm{O}}_3$ is considered a textbook example of Mott-Hubbard physics. In this paper, we present an extended optical study of its whole temperature/doping phase diagram as obtained by doping the pure material with $M=\text{Cr}$ or Ti atoms $\left({\mathrm{V}}_{1-x}{M}_x\right){}_2{\mathrm{O}}_3$. We reveal that its thermodynamically stable metallic and insulating phases, although macroscopically equivalent, show very different low-energy electrodynamics. The Cr and Ti doping drastically change both the antiferromagnetic gap and the paramagnetic metallic properties. A slight chromium content induces a mesoscopic electronic phase separation, while the pure compound is characterized by short-lived quasiparticles at high temperature. This study thus provides a new comprehensive scenario of the Mott-Hubbard physics in the prototype compound ${\mathrm{V}}_2{\mathrm{O}}_3$.

We investigate how imposing kinetic restrictions on quantum particles that would otherwise hop freely on a two-dimensional lattice can lead to topologically ordered states. The kinetically constrained models introduced here are derived as a generalization of strongly interacting particle systems in which hoppings are given by flux-lattice Hamiltonians and may be relevant to optically driven cold-atom systems. After introducing a broad class of models, we focus on particular realizations and show numerically that they exhibit topological order, as witnessed by topological ground-state degeneracies and the quantization of corresponding invariants. These results demonstrate that the correlations responsible for fractional quantum Hall states in lattices can arise in models involving terms other than density-density interactions.

The physics of spin-orbital entanglement in effective $j=\frac{1}{2}$ Mott insulators, which have been experimentally observed for various $5d$ transition-metal oxides, has sparked an interest in Heisenberg-Kitaev (HK) models thought to capture their essential microscopic interactions. Here, we argue that the recently synthesized ${\mathrm{Ba}}_3{\mathrm{IrTi}}_2{\mathrm{O}}_9$ is a prime candidate for a microscopic realization of the triangular HK model, a conceptually interesting model for its interplay of geometric and exchange frustration. We establish that an infinitesimal Kitaev exchange destabilizes the ${120}^{\circ }$ order of the quantum Heisenberg model. This results in the formation of an extended ${\mathbb{Z}}_2$-vortex crystal phase in the parameter regime most likely relevant to the real material, which can be experimentally identified with spherical neutron polarimetry. Moreover, using a combination of analytical and numerical techniques, we map out the entire phase diagram of the model, which further includes various ordered phases as well as an extended nematic phase around the antiferromagnetic Kitaev point.

Band structure is theoretically studied in partially flattened carbon nanotubes within an effective-mass scheme. Effects of interwall interactions are shown to be important in nonchiral nanotubes such as zigzag and armchair and can essentially be neglected in chiral nanotubes except in the close vicinity of nonchiral tubes. In fact, interwall interactions significantly modify states depending on relative displacement in the flattened region in nonchiral tubes and can convert semiconducting tubes into metallic and vice versa. They diminish rapidly when the chiral angle deviates from that of the zigzag or armchair tube, although the decay is slower in the vicinity of armchair tubes.

We present the results of our density functional theory study of ${\text{Co}}_n{\text{Mo}}_m$ nanoclusters with $n+m=x$ and $2\le \times \le 6$ atoms on the all-electron level using the generalized gradient approximation. The discussion of properties of the pure cobalt and molybdenum cluster is followed by an analysis of the respective mixed clusters of each cluster size $x$. We found that the magnetic moment of a given cluster is mainly determined by the Co content and increases with increasing $n$. The magnetic anisotropy on the other hand becomes smaller for larger magnetic moments. We observe an increase in binding energy, electron affinity, and average bond length with increasing cluster size as well as a decrease in ionization potential, chemical potential, molecular hardness, and the HOMO-LUMO gap.

Graphene irradiated by a circularly polarized laser has been predicted to be a Floquet topological insulator showing a laser-induced quantum Hall effect. A circularly polarized laser also drives the system out of equilibrium, resulting in nonthermal electron distribution functions that strongly affect transport properties. Results are presented for the Hall conductance for two different cases. One is for a closed system, such as a cold-atomic gas, where transverse drift due to nonzero Berry curvature can be measured in time-of-flight measurements. For this case the effect of a circularly polarized laser that has been suddenly switched on is studied. The second is for an open system coupled to an external reservoir of phonons. While for the former the Hall conductance is far from the quantized limit, for the latter, coupling to a sufficiently low temperature reservoir of phonons is found to produce effective cooling, and thus an approach to the quantum limit, provided the frequency of the laser is large as compared to the bandwidth. For laser frequencies comparable to the bandwidth, strong deviations from the quantum limit of conductance are found even for a very low temperature reservoir, with the precise value of the Hall conductance determined by a competition between reservoir-induced cooling and the excitation of photocarriers by the laser. For the closed system, the electron distribution function is determined by the overlap between the initial wave function and the Floquet states, which can result in a Hall conductance which is opposite in sign to that of the open system.

We argue that at finite carrier density and large displacement fields, bilayer graphene is prone to $\ell =0$ and $\ell =1$ Pomeranchuk Fermi-surface instabilities. The broken symmetries are driven by nonlocal exchange interactions which favor momentum-space condensation. We find that electron-electron interactions lead first to spontaneous valley polarization, which breaks time-reversal invariance and is associated with spontaneous orbital magnetism, and then under some circumstances to a nematic phase with reduced rotational symmetry. When present, nematic order is signaled by reduced symmetry in the dependence of optical absorption on light polarization.

We report hard x-ray photoemission spectroscopy measurements of the electronic structure of the prototypical correlated oxide ${\mathrm{Sr}}_x{\mathrm{Ca}}_{1-x}{\mathrm{VO}}_3$. By comparing spectra recorded at different excitation energies, we show that 2.2 keV photoelectrons contain a substantial surface component, whereas 4.2 keV photoelectrons originate essentially from the bulk of the sample. Bulk-sensitive measurements of the O $2p$ valence band are found to be in good agreement with ab initio calculations of the electronic structure, with some modest adjustments to the orbital-dependent photoionization cross sections. The evolution of the O $2p$ electronic structure as a function of the Sr content is dominated by $A$-site hybridization. Near the Fermi level, the correlated V $3d$ Hubbard bands are found to evolve in both binding energy and spectral weight as a function of distance from the vacuum interface, revealing higher correlation at the surface than in the bulk.

Excitons in semiconductors can have multiple lifetimes due to spin-dependent oscillator strengths and interference between different recombination pathways. In addition, strain and symmetry effects can further modify lifetimes via the removal of degeneracies. We present a convenient formalism for predicting the optical properties of $k=0$ excitons with an arbitrary number of charge carriers in different symmetry environments. Using this formalism, we predict three distinct lifetimes for the neutral acceptor bound exciton in GaAs, and confirm this prediction through polarization dependent and time-resolved photoluminescence experiments. We find the acceptor bound-exciton lifetimes to be $T_o\times (1,3,\frac{3}{4})$, where $T_o=(0.61\pm 0.12)\text{ns}$. Furthermore, we provide an estimate of the intralevel and interlevel exciton spin-relaxation rates.

We report a large $g$ factor tunability of a single hole spin in an InGaAs quantum dot via an electric field. The magnetic field lies in the in-plane direction $x$, the direction required for a coherent hole spin. The electrical field lies along the growth direction $z$ and is changed over a large range, 100 kV/cm. Both electron and hole $g$ factors are determined by high resolution laser spectroscopy with resonance fluorescence detection. This, along with the low electrical-noise environment, gives very high quality experimental results. The hole $g$ factor $g_h^x$ depends linearly on the electric field $F_z,d{g}_h^x/d{F}_z=(8.3\pm 1.2)\times {10}^{-4}$ cm/kV, whereas the electron $g$ factor $g_e^x$ is independent of electric field $d{g}_e^x/d{F}_z=(0.1\pm 0.3)\times {10}^{-4}$ cm/kV (results averaged over a number of quantum dots). The dependence of $g_h^x$ on $F_z$ is well reproduced by a $4\times 4 k·p$ model demonstrating that the electric field sensitivity arises from a combination of soft hole confining potential, an In concentration gradient, and a strong dependence of material parameters on In concentration. The electric field sensitivity of the hole spin can be exploited for electrically driven hole spin rotations via the $g$ tensor modulation technique and based on these results, a hole spin coupling as large as $\sim 1$ GHz can be envisaged.

The mean sputter yield produced by the impact of a single ion depends on the radii of curvature of the target surface at the point of impact. Using the Sigmund model of ion sputtering, we develop analytical formulas for this dependence for the case in which the radii of curvature are large compared to the size of the ion-induced collision cascade; both locally perpendicular and oblique ion impact are considered. The sputter yield is increased for impact on convex surfaces. The influence of surface curvature along the incident-ion azimuth and perpendicular to it are discussed separately. Our analytical results are in good agreement with Monte Carlo simulations for the specific case of 20 keV Ar ion impact on a cylindrical nanowire consisting of amorphous silicon. We also extend the results for this case to small radii of curvature using both Monte Carlo and molecular dynamics simulations.

Here we present a theoretical study of the thermoelectric transport through [2,2]paracyclophane-based single-molecule junctions. Combining electronic and vibrational structures, obtained from density functional theory (DFT), with nonequilibrium Green's function techniques allows us to treat both electronic and phononic transport properties at a first-principles level. For the electronic part, we include an approximate self-energy correction, based on the DFT+$\Sigma$ approach. This enables us to make a reliable prediction of all linear response transport coefficients entering the thermoelectric figure of merit $ZT$. Paracyclophane derivatives offer a great flexibility in tuning their chemical properties by attaching different functional groups. We show that, for the specific molecule, the functional groups mainly influence the thermopower, allowing us to tune its sign and absolute value. We predict that the functionalization of the bare paracyclophane leads to a largely enhanced electronic contribution $Z_{\mathrm{el}}T$ to the figure of merit. Nevertheless, the high phononic contribution to the thermal conductance strongly suppresses $ZT$. Our work demonstrates the importance to include the phonon thermal conductance for any realistic estimate of the $ZT$ for off-resonant molecular transport junctions. In addition, it shows the possibility of a chemical tuning of the thermoelectric properties for a series of available molecules, leading to equally performing hole- and electron-conducting junctions based on the same molecular framework.

We propose a new type of topological surface modes having cubic dispersion in three-dimensional topological superconductors. Lower order dispersions are prohibited by the threefold rotational symmetry and time-reversal symmetry. Cooper pairing in the bulk changes sign under improper rotations, akin to ${}^3\mathrm{He}$-B. The surface manifestations are a divergent surface density of states at the Fermi level and isospins that rotate three times as they circle the origin in momentum space. We propose that Heusler alloys with band inversion are candidate materials to harbor the novel topological superconductivity.

Anharmonic effects in an atomic monolayer thin crystal with honeycomb lattice structure are investigated by analytical and numerical lattice dynamical methods. Starting from a semiempirical model for anharmonic couplings of third and fourth orders, we study the in-plane and out-of-plane (flexural) mode components of the generalized wave vector dependent Grüneisen parameters, the thermal tension and the thermal expansion coefficients as a function of temperature and crystal size. From the resonances of the displacement-displacement correlation functions, we obtain the renormalization and decay rate of in-plane and flexural phonons as a function of temperature, wave vector, and crystal size in the classical and in the quantum regime. Quantitative results are presented for graphene. There, we find that the transition temperature $T_{\alpha }$ from negative to positive thermal expansion is lowered with smaller system size. Renormalization of the flexural mode has the opposite effect and leads to values of $T_{\alpha }\approx 300\mathrm{K}$ for systems of macroscopic size. Extensive numerical analysis throughout the Brillouin zone explores various decay and scattering channels. The relative importance of normal and umklapp processes is investigated. The work is complementary to crystalline membrane theory and computational studies of anharmonic effects in two-dimensional crystals.

The heat transport of the spin-gapped material $\left({\mathrm{CH}}_3\right){}_2{\mathrm{CHNH}}_3{\mathrm{CuCl}}_3$ $\left({\mathrm{IPA-CuCl}}_3\right)$, a candidate quantum magnet with Bose-Einstein condensation (BEC), is studied at ultralow temperatures and in high magnetic fields. Due to the presence of the spin gap, the zero-field thermal conductivity $\left(\kappa \right)$ is purely phononic and shows a ballistic behavior at $T<1$ K. When the gap is closed by magnetic field at $H=H_{c1}$, where a long-range antiferromanetic (AF) order of ${\mathrm{Cu}}^{2+}$ moments is developed, the magnons contribute significantly to heat transport and exhibit a ballistic $T^3$ behavior at $T<600$ mK. In addition, the low-$T\kappa \left(H\right)$ isotherms show sharp peaks at $H_{c1}$, which indicates a gap reopening in the AF state $\left(H>H_{c1}\right)$ and demonstrates limited applicability of the BEC model to ${\mathrm{IPA-CuCl}}_3$.

We study the dynamics of inorganic antiferroelectric nanoclusters formed during an order-disorder transition and demonstrate the coexistence of the two phases in a region of 2–3 K around the transition temperature $T_N\sim 215$ K. Single crystals of ${\mathrm{NH}}_4{\mathrm{H}}_2{\mathrm{AsO}}_4$, a model hydrogen-bonded compound, show an antiferroelectric-paraelectric transition studied by means of highly sensitive magic angle spinning ${}^{15}\mathrm{N}$ NMR at 21.1 T. The phase coexistence is demonstrated by a double-peak structure of the chemical shift. Two-dimensional chemical exchange spectroscopy and spin-lattice relaxation time $\left(T_1\right)$ measurements show that the clusters are dynamic with sizes $\sim 50$ nm and lifetimes approaching seconds as $T\to T_N$. Their occupancy increases rapidly to fill the crystal volume below $T_N$. This study provides evidence for the commonality of the phase transitions in systems with electric properties and provides an improved spectroscopic method for such studies.