Topological insulators are noninteracting, gapped fermionic systems which have gapless boundary excitations. They are characterized by topological invariants, which can be written in many different ways, including in terms of Green’s functions. Here we show that the existence of the edge states directly follows from the existence of the topological invariant written in terms of the Green’s functions, for all ten classes of topological insulators in all spatial dimensions. We also show that the resulting edge states are characterized by their own topological invariant, whose value is equal to the topological invariant of the bulk insulator. This can be used to test whether a given model Hamiltonian can describe an edge of a topological insulator. Finally, we observe that the results discussed here apply equally well to interacting topological insulators, with certain modifications.

Understanding the mechanisms limiting ambipolar transport in conjugated polymer field-effect transistors (FETs) is of both fundamental and practical interest. Here, we present a systematic study comparing hole and electron charge transport in an ambipolar conjugated polymer, semicrystalline poly(3,3${}^{\prime \prime }$-di-$n$-decylterselenophene) (PSSS). Starting from a detailed analysis of the device characteristics and temperature/charge-density dependence of the mobility, we interpret the difference between hole and electron transport through both the Vissenberg-Matters and the mobility-edge model. To obtain microscopic insight into the quantum mechanical wave function of the charges at a molecular level, we combine charge modulation spectroscopy (CMS) measuring the charge-induced absorption signatures from positive and negative polarons in these ambipolar FETs with corresponding density functional theory (DFT) calculations. We observe a significantly higher switch-on voltage for electrons than for holes due to deep electron trap states, but also a higher activation energy of the mobility for mobile electrons. The CMS spectra reveal that the electrons that remain mobile and contribute to the FET current have a wave function that is more localized onto a single polymer chain than that of holes, which is extended over several polymer chains. We interpret this as evidence that the transport properties of the mobile electrons in PSSS are still affected by the presence of deep electron traps. The more localized electron state could be due to the mobile electrons interacting with shallow trap states in the vicinity of a chemical, potentially water-related, impurity that might precede the capture of the electron into a deeply trapped state.

First-principles calculations reveal large zone-center spin-phonon coupling and magnetically-driven phonon anisotropy in cubic perovskites Sr$M$O${}_3$ ($M=\text{V}$,Cr,Mn,Fe,Co). In particular, the frequency and splitting of the polar Slater mode is found to depend strongly upon magnetic ordering. The coupling is parameterized in a crystal-structure-dependent Heisenberg model, and its main features seen to arise from the Goodenough-Kanamori rules. This coupling can be expected to produce distinct low-energy alternative phases, resulting in a rich variety of coupled magnetic, structural, and electronic phase transitions driven by temperature, stress, electric field, and cation substitution.

In elemental bismuth, emptying the low-index Landau levels is accompanied by giant Nernst quantum oscillations. The Nernst response sharply peaks each time a Landau level intersects the chemical potential. By studying the evolution of these peaks when the field rotates in three perpendicular planes defined by three high-symmetry axes, we have mapped the angle-resolved Landau spectrum of the system up to 12 T. A theoretical model treating electrons at the $L$ point with an extended Dirac Hamiltonian is confronted with the experimentally resolved spectrum. We obtain a set of theoretical parameters yielding a good but imperfect agreement between theory and experiment for all orientations of the magnetic field in space. The results confirm the relevance of the Dirac spectrum to the electron pockets and settle the long-standing uncertainty about the magnitude of the $g$ factor for holes. According to our analysis, a magnetic field exceeding 2.5 T applied along the bisectrix axis puts all carriers of the three electron pockets in their lowest ($j=0$) spin-polarized Landau level. On top of this complex angle-dependent spectrum, experiment detects additional and unexpected Nernst peaks of unidentified origin.

A technique for the structural and electronic characterization of the same individual single wall carbon nanotube is presented. Electron diffraction was performed on carbon nanotubes grown across a perforated silicon nitride film and the chiral indices of individual tubes were determined. By means of a scanning-probe-based nanomanipulator a selected tube was then placed in the desired location across prepatterned electrodes. After patterning further electrodes on-top of the tube electronic transport measurements showed room temperature conductances of up to 0.2 $G_0$. The application of a gate voltage allowed the tuning of the room temperature conductance of the device and measurements at liquid helium temperatures showed signatures of a Coulomb blockade, indicating the formation of a carbon nanotube quantum dot.

Frustrated magnets such as spin ice exhibit Coulomb phases, where correlations have power-law forms at long distances. Applied perturbations can cause ordering transitions which cannot be described by the usual Landau paradigm and are instead naturally viewed as Higgs transitions of an emergent gauge theory. Starting from a classical statistical model of spin ice, it is shown that a variety of possible phases and transitions can be described by this approach. Certain cases are identified where continuous transitions are argued to be likely; the predicted critical behavior may be tested in experiments or numerical simulations.

We discuss a scenario for interface-induced superconductivity involving pairing by dipolar excitations proximate to a two-dimensional electron system controlled by a transverse electric field. If the interface consists of transition-metal oxide materials, the repulsive on-site Coulomb interaction is typically strong and a superconducting state is formed via exchange of nonlocal dipolar excitations in the $d$-wave channel. Perspectives to enhance the superconducting transition temperature are discussed.

We construct a periodically time-dependent Hamiltonian with a phase transition in the quantum Hall universality class. One spatial dimension can be eliminated by introducing a second incommensurate driving frequency, so that we can study the quantum Hall effect in a one-dimensional (1D) system. This reduction to 1D is very efficient computationally and would make it possible to perform experiments on the 2D quantum Hall effect using cold atoms in a 1D optical lattice.

Electronic transport properties in the spin-orbit Mott insulator Ba${}_2$IrO${}_4$ were studied under high pressure ($P$), up to 15 GPa. Resistivity measurements revealed a metal-insulator (M-I) transition at 13.8 GPa in Ba${}_2$IrO${}_4$. In the insulating state ($P<13.8$ GPa), the resistivity well follows the variable-range-hopping regime, suggesting that the carrier conduction is dominated by Anderson localization. In the metallic state ($P\ge 13.8$ GPa), non-Fermi-liquid behavior was observed at low temperature. Ba${}_2$IrO${}_4$ exhibits an unconventional critical exponent (δ = 1 − 2) at the M-I transition. These results suggest that Ba${}_2$IrO${}_4$ has an unusual electronic state affected by the “marginal quantum critical point (MQCP).”

We investigate organic light-emitting diodes (OLEDs) comprising the singlet emitter system 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped into aluminium tris(8-hydro-xyquinoline) (Alq${}_3$) at high excitation densities. With the OLED active area reduced to $100\times 100\mu$m${}^2$, current densities up to 800 A/cm${}^2$ are achieved in pulsed operation. These devices exhibit an intense electroluminescence (EL) turn-on peak on the nanosecond time scale. With the help of streak camera measurements, we prove that the steady state EL of the fluorescent OLEDs is reduced due to singlet-triplet quenching. We demonstrate that short electrical pulses with a rise time of 10 ns make the separation of singlet emission and singlet-triplet quenching in time domain possible. By modeling the singlet and triplet population dynamics in the emission layer, we find that the triplet-triplet annihilation-rate coefficient in doped fluorescent materials is triplet-density dependent at high excitation density. The increased triplet lifetime usually observed in host:guest systems due to triplet trapping on guest molecules vanishes at high current densities. An increase in current density leads to an increased triplet-triplet annihilation rate, while the triplet density in the emission layer stays constant.

We have measured the spin splitting in single-layer and bilayer graphene by means of tilted magnetic-field experiments. By applying the Lifshitz-Kosevich formula for the spin-induced decrease of the Shubnikov-de Haas amplitudes with increasing tilt angle, we directly determine the product between the carrier cyclotron mass $m^{*}$ and the effective $g$ factor $g^{*}$ as a function of the charge-carrier concentration. By using the cyclotron mass for a single-layer and a bilayer graphene, we find an enhanced $g$ factor $g^{*}=2.7\pm 0.2$ for both systems.

The effect of disorder in polyhalide anions is investigated in terms of the degree of charge transfer and the Fermi surface in the incommensurate organic superconductor (MDT-TSF)I${}_{0.77}$Br${}_{0.52}$, where MDT-TSF is methylenedithio-tetraselenafulvalene. The Raman spectra suggest that the infinite polyhalide chains include the asymmetric trihalide anion I${}_2$Br${}^{-}$ and that the degree of charge transfer from the donor molecule to the anion is $0.429$, estimated from the lattice constants. The Shubnikov–de Haas (SdH) oscillation is in good agreement with this charge transfer degree. The beating behavior of the SdH oscillation gives a large interlayer transfer integral, which is consistent with a corrugated conducting sheet structure. The randomness of the disordered anions reduces the periodicity of the incommensurate anion potential; this destroys the Fermi surface reconstruction by the incommensurate anion potential. The factor dominating the superconducting transition temperature is not the randomness of the polyhalide anions but the effective cyclotron mass enhancement, i.e., the many-body effect.

The existence of a length scale ${\xi }_K\sim 1/T_K$ (with $T_K$ the Kondo temperature) has long been predicted in quantum impurity systems. At low temperatures $T\ll T_K$, the standard interpretation is that a spin-$\frac{1}{2}$ impurity is screened by a surrounding “Kondo cloud” of spatial extent ${\xi }_K$. We argue that renormalization group (RG) flow between any two fixed points (FPs) results in a characteristic length scale, observed in real space as a crossover between physical behavior typical of each FP. In the simplest example of the Anderson impurity model, three FPs arise, and we show that “free orbital,” “local moment,” and “strong coupling” regions of space can be identified at zero temperature. These regions are separated by two crossover length scales ${\xi }_{\text{LM}}$ and ${\xi }_K$, with the latter diverging as the Kondo effect is destroyed on increasing temperature through $T_K$. One implication is that moment formation occurs inside the “Kondo cloud”, while the screening process itself occurs on flowing to the strong coupling FP at distances $\sim {\xi }_K$. Generic aspects of the real-space physics are exemplified by the two-channel Kondo model, where ${\xi }_K$ now separates local moment and overscreening clouds.

Superfluorescence (SF) is the emission from a dense coherent system in population inversion formed from an initially incoherent ensemble. This is characterized by an induction time (${\tau }_{\mathrm{D}}$) for the spontaneous development of the macroscopic quantum coherence. Here we report detailed observation of SF on an ultrafast timescale from a quantum ensemble of coherent excitons in highly excited intrinsic-bulk ZnTe single crystal at 5 K, showing a characteristic ${\tau }_{\mathrm{D}}$ from 40 ps to 10 ps, quantum noise and fluctuations, and quantum beating and ringing. From this clear observation of SF from a spontaneous coherence of excitons bound to impurities and defect states, we infer that this is indicative of the formation of a spontaneous Bose-Einstein condensation (BEC) of excitons on an ultrafast timescale. The population density and dephasing time ($T_2$ or $T_2^{*}$) of exciton are two controlling factors in the generation of SF from the BEC.

We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The terahertz radiation power efficiency of these structures is ∼15 times higher than a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast terahertz spectroscopy, and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.

Tailoring the properties of single spins confined in self-assembled quantum dots (QDs) is critical to the development of new optoelectronic logic devices. However, the range of heterostructure engineering techniques that can be used to control spin properties is severely limited by the requirements of QD self-assembly. We demonstrate a new strategy for rationally engineering the spin properties of single confined electrons or holes by adjusting the composition of the barrier between a stacked pair of InAs QDs coupled by coherent tunneling to form a quantum dot molecule (QDM). We demonstrate this strategy by designing, fabricating, and characterizing a QDM in which the $g$-factor for a single confined electron can be tuned in situ by over 50$\%$ with a minimal change in applied voltage.

We study the field effect on the strain-induced Mott-insulating state of an organic superconductor $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br. The gate voltage dependences of the conductivity, Hall coefficient, and thermopower were investigated. While the conductivity obeyed an activation-type insulating formula, the Hall coefficient and thermopower exhibited continuous transition to a metal-like band structure under high gate voltage. This indicates the merger of Hubbard bands at finite electrostatic doping; that is, the field-induced Mott transition.

We describe a family of phase transitions connecting phases of differing nontrivial topological order by explicitly constructing Hamiltonians of the Levin-Wen [Levin and Wen, Phys. Rev. B 71, 045110 (2005)] type which can be tuned between two solvable points, each of which realizes a different topologically ordered phase. We show that the low-energy degrees of freedom near the phase transition can be mapped onto those of a Potts model, and we discuss the stability of the resulting phase diagram to small perturbations about the model. We further explain how the excitations in the condensed phase are formed from those in the original topological theory, some of which are split into multiple components by condensation, and we discuss the implications of our results for understanding the nature of general achiral topological phases in 2 $+$ 1 dimensions in terms of doubled Chern-Simons theories.

Ferrofluid-based liquid manipulation schemes typically actuate an immiscible liquid via a ferrofluid plug, using high magnetic flux (∼1 T) densities and strong field gradients created with bulky permanent magnets. They rely on surface tension effects to maintain the cohesion of the ferrofluid plug, necessitating miniature channels and slow (∼1 μl/min) flow speeds. Here, we demonstrate direct ferrohydrodynamic pumping using traveling magnetic fields at controllable speeds in a simple, closed-loop geometry without any mechanically actuated components. The pumping approach is compact, scalable, and practical. Using moderate field amplitudes (∼10 mT), we obtained a maximum volumetric flow rate of 0.69 ml/s using a readily available commercial ferrofluid. Our closed-loop pumping approach could lead to integrated and efficient liquid manipulation and cooling schemes based on ferrofluids.

Experiments and theoretical calculations of conservative forces measured by frequency modulation atomic force microscopy (FM-AFM) in vacuum are generally in reasonable agreement. This contrasts with dissipative forces, where experiment and theory often disagree by several orders of magnitude. These discrepancies have repeatedly been attributed to instrumental artifacts, the cause of which remains elusive. We demonstrate that the frequency response of the piezoacoustic cantilever excitation system, traditionally assumed flat, can actually lead to surprisingly large apparent damping by the coupling of the frequency shift to the drive-amplitude signal, typically referred to as the “dissipation” signal. Our theory predicts large quantitative and qualitative variability observed in dissipation spectroscopy experiments, contrast inversion at step edges and in atomic-scale dissipation imaging, as well as changes in the power-law relationship between the drive signal and bias voltage in dissipation spectroscopy. The magnitude of apparent damping can escalate by more than an order of magnitude at cryogenic temperatures. We present a simple nondestructive method for correcting this source of apparent damping, which will allow dissipation measurements to be reliably and quantitatively compared to theoretical models.