The magnetic state and the Kondo effect in Mn phthalocyanine (MnPc) molecules on Pb(111) were investigated using low-temperature scanning tunneling microscopy (STM), density-functional theory, and numerical renormalization group calculations. We found that a unique spin state induced by the strong $\pi -d$ interaction between the Mn $d$-orbitals and ligand π-orbitals causes an underscreened Kondo effect observed both at Mn and the ligand. STM observations show that a Kondo resonance extends over the pyrrole ring in the Pc ligand rather than remaining localized at Mn. In gas-phase MnPc, we reveal that the $\pi -d$ interaction result in an $S=3/2$ collective spin state extended over the Pc ligand with a complex spin polarization inside the molecule. On Pb(111), charge transfer from the substrate reduces the total spin of MnPc from 3/2 to 1 while the intramolecular $\pi -d$ interaction is maintained. Partial screening of this collective $S=1$ spin state by the conduction electron in Pb(111) makes the Kondo resonance also observed at the ligand part, which is not directly hybridized with the substrate.

Ionization energy and electron affinity in organic solids are understood in terms of a single molecule perturbed by solid-state effects such as polarization energy, band dispersion, and molecular orientation as primary factors. However, no work has been done to determine the individual contributions experimentally. In this work, the electron affinities of thin films of pentacene and perfluoropentacene with different molecular orientations are determined to a precision of 0.1 eV using low-energy inverse photoemission spectroscopy. Based on the precisely determined electron affinities in the solid state together with the corresponding data of the ionization energies and other energy parameters, we quantitatively evaluate the contribution of these effects. It turns out that the bandwidth as well as the polarization energy contributes to the ionization energy and electron affinity in the solid state while the effect of the surface dipole is at most a few eV and does not vary with the molecular orientation. As a result, we conclude that the molecular orientation dependence of the ionization energy and electron affinity of organic solids originates from the orientation-dependent polarization energy in the film.

We report on the observation of multiple polariton modes, originating from an electronic coupling between quantum wells inside a planar microcavity. A series of excitonic transitions are measured for a bare quantum well stack and are precisely identified to electron-hole transitions originating from the coupling of the wells. When the quantum well stacks are placed at the antinode of a high $Q$-factor microcavity, a series of anticrossings is observed, which is characteristic of a multiplicity of polariton modes. This behavior is simulated using a coupled oscillator model accounting for the cavity mode and all allowed excitonic transitions.

Interference represents one of the most striking manifestations of quantum physics in low-dimensional systems. Despite evidence of quantum interference in charge transport having been known for a long time, signatures of interference-induced thermal properties have been reported only recently, paving the way for the phase-coherent manipulation of heat in mesoscopic devices. In this work we show that anomalous thermoelectric properties and efficient heat rectification can be achieved by exploiting the phase-coherent edge states of quantum Hall systems. By considering a tunneling geometry with multiple quantum point contacts, we demonstrate that the interference paths effectively break the electron-hole symmetry, allowing for a thermoelectric charge current flowing either from hot to cold or vice versa, depending on the details of the tunnel junction. Correspondingly, an interference-induced heat current is predicted, and we are able to explain these results in terms of an intuitive physical picture. Moreover, we show that heat rectification can be achieved by coupling two quantum Hall systems with different filling factors, and that this effect can be enhanced by exploiting the interference properties of the tunnel junction.

We study with angle-resolved photoelectron spectroscopy the evolution of the electronic structure of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, when holes or electrons are introduced, through Rh or La substitutions. At low dopings, the added carriers occupy the first available states, at the bottom or top of the gap, revealing an asymmetric gap of 0.7 eV, in good agreement with scanning tunnel microscope measurements. At further doping, we observe a reduction of the gap and a transfer of spectral weight across the gap, although the quasiparticle weight remains very small. We discuss the origin of the in-gap spectral weight as a local distribution of gap values.

The biexciton absorption spectrum of a pair of InAs/GaAs quantum dots is being studied by photoluminescence excitation spectroscopy. An absorption resonance with the characteristics of an instantaneous two-photon process reveals a coherent interdot two-photon transition. Pauli-selective tunneling is being used to demonstrate the transduction of the two-photon coherence into a nonlocal spin singlet state. The two-photon transition can be tuned spectrally by electric field, enabling amplification of its transition strength.

We describe charging a quantum dot induced electrostatically within a semiconducting graphene nanoribbon by electrons or holes. The applied model is based on a tight-binding approach with the electron-electron interaction introduced by a mean-field local-spin-density approximation. The numerical approach accounts for the charge of all the $p_z$ electrons and screening of external potentials by states near the charge-neutrality point. Both a homogeneous ribbon and a graphene flake embedded within the ribbon are discussed. The formation of transport gaps as functions of the external confinement potential (top-gate potential) and the Fermi energy (back-gate potential) are described in a qualitative agreement with the experimental data. For a fixed number of excess electrons, we find that the excess charge added to the system is, - depending on the voltages defining the work point of the device, (i) delocalized outside the quantum dot, - in the transport gap due to the top-gate potential; (ii) localized inside the quantum dot, - in the transport gap due to the back-gate potential; or (iii) extended over both the quantum dot area and the ribbon connections, - outside the transport gaps. The applicability of the frozen valence-band approximation to describe charging the quantum dot by excess electrons is also discussed.

We investigate the electronic and transport properties of gated bilayer graphene with one corrugated layer, which results in a stacking AB/BA boundary. When a gate voltage is applied to one layer, topologically protected gap states appear at the corrugation, which reveal as robust transport channels along the stacking boundary. With increasing size of the corrugation, more localized, quantum-well-like states emerge. These finite-size states are likewise conductive along the fold, but in contrast to the stacking boundary states, which are gapless, they present a gap. Additionally, we have studied periodic corrugations in bilayer graphene; our findings show that such corrugations between AB- and BA-stacked regions also behave as conducting channels in the direction of the folds. These topological states could be easily identified due to the spatial shape of the corrugations.

Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily sheared along the basal planes. Despite a growing attention towards their use as solid lubricants, so far no head-to-head comparison has been carried out. By means of ab initio modeling of a bilayer sliding motion, we show that graphene is characterized by a shallower potential energy landscape while more similarities are attained when considering the sliding forces; we propose that the calculated interfacial ideal shear strengths afford the most accurate information on the intrinsic sliding capability of layered materials. We also investigate the effect of an applied uniaxial load: in graphene, this introduces a limited increase in the sliding barrier while in TMDs it has a substantially different impact on the possible polytypes. The polytype presenting a parallel orientation of the layers ($R0$) bears more similarities to graphene while that with antiparallel orientation ($R180$) shows deep changes in the potential energy landscape and consequently a sharper increase of its sliding barrier.

We present results of a temperature-dependent high-resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in $\mathrm{N}{\mathrm{i}}_2\mathrm{MnGa}$. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval $(\sim 40\mathrm{K})$, discontinuous change in the unit cell volume as also in the modulation wave vector across the transition temperature, and considerable thermal hysteresis in the characteristic transition temperatures. The temperature variation of the modulation wave vector $\mathbf{q}$ shows smooth analytic behavior with no evidence for any devilish plateau corresponding to an intermediate or ground state commensurate lock-in phase. The existence of the incommensurate 7M-like modulated structure down to 5 K suggests that the incommensurate 7M-like modulation is the ground state of $\mathrm{N}{\mathrm{i}}_2\mathrm{MnGa}$ and not the Bain distorted tetragonal $\mathrm{L}{1}_0$ phase or any other lock-in phase with a commensurate modulation. These findings can be explained within the framework of the soft phonon model.

We systematically investigate the spin-pumping effect in a series of yttrium-iron-garnet (YIG)–platinum (Pt) bilayers, comparing both broadband ferromagnetic resonance and electrically detected spin-pumping experiments. We infer the effective spin-mixing conductance as a function of YIG and Pt layer thickness from ferromagnetic resonance measurements on pure YIG layers as well as on bilayer YIG/Pt samples in a frequency range from 2 to 20 GHz. In bare YIG films, we determine a Gilbert damping constant of approximately $\alpha =0.001$. With an additional platinum layer on top of the YIG samples, the damping is significantly enhanced. The effective spin-mixing conductance extracted from the ferromagnetic resonance experiments is in agreement with that determined from independent electrically detected spin-pumping experiments.

Based on ab initio calculations and Monte Carlo simulations, we present a systematic study of the magnetic ground state and finite temperature magnetism of ordered and disordered ${\mathrm{Ni}}_2\mathrm{MnAl}$ full Heusler compounds. By increasing the degree of the long-range chemical disorder between the Mn and Al sublattices, the magnetic order progressively changes from the ferromagnetic state in the ordered $L{2}_1$ phase toward a fully compensated antiferromagnetic state in the disordered $B2$ phase and we also conclude that the Ni atoms exhibit induced moments. We determine the Mn-Mn interactions by using the magnetic force theorem and find dominating, but rather weak ferromagnetic couplings in the ordered $L{2}_1$ phase. We used a recently proposed renormalization technique to include the weak Ni moments into the spin model, which indeed remarkably increased the nearest-neighbor Mn-Mn interaction. In accordance with the total energy calculations, in the disordered compounds, strong antiferromagnetic site-antisite Mn-Mn interactions appear. Determining the spin-spin correlation functions from Monte Carlo simulations, we conclude that above the transition temperature, short-range antiferromagnetic correlations prevail between the Mn atoms. In view of the potential application of disordered ${\mathrm{Ni}}_2\mathrm{MnAl}$ as a room temperature antiferromagnet, we calculate the magnetic anisotropy energies of tetragonally distorted samples in the $B2$ phase and find that they are smaller by two orders in magnitude than in the frustrated antiferromagnet ${\mathrm{IrMn}}_3$.

The dynamics of phonon modes in thin flakes of topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ has been studied using transient optical second-harmonic spectroscopy. Using the surface sensitivity of second-harmonic generation, the excitation and dynamics of a family of optical phonon modes are observed with a response dependent on the excitation polarization. Three coherent phonon modes, inaccessible in previous linear time-resolved optical measurements, were detected in the nonlinear dynamic response. A symmetry-allowed surface optical phonon mode in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ has been identified using surface and symmetry selectivity of the second-order up-conversion process. Possible second-harmonic generation mechanisms in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ as well as the enhanced sensitivity of the nonlinear processes for the phonon mode detection in topological insulators are discussed.

When a classical system is driven through a continuous phase transition, its nonequilibrium response is universal and exhibits Kibble-Zurek scaling. We explore this dynamical scaling in the context of a three-dimensional topological magnet with fractionalized excitations, namely, the liquid-gas transition of the emergent mobile magnetic monopoles in dipolar spin ice. Using field-mixing and finite-size scaling techniques, we place the critical point of the liquid-gas line in the three-dimensional Ising universality class. We then demonstrate Kibble-Zurek scaling for sweeps of the magnetic field through the critical point. Unusually slow microscopic time scales in spin ice offer a unique opportunity to detect this universal nonequilibrium physics in current experimental setups.

The nonlocal nature of the fermionic mode spanned by a pair of Majorana bound states in a one-dimensional topological superconductor has inspired many proposals aiming at demonstrating this property in transport. In particular, transport through the mode from a lead attached to the left bound state to a lead attached to the right will result in current cross correlations. For ideal zero modes on a grounded superconductor, the cross correlations are however completely suppressed in favor of purely local Andreev reflection. In order to obtain a nonvanishing cross correlation, previous studies have required the presence of an additional global charging energy. Adding nonlocal terms in the form of a global charging energy to the Hamiltonian when testing the intrinsic nonlocality of the Majorana modes seems to be conceptually troublesome. Here, we show that a floating superconductor allows observing nonlocal current correlations in the absence of charging energy. We show that the noninteracting and the Coulomb-blockade regime have the same peak conductance $e^2/h$ but different shot-noise power; whereas the shot noise is sub-Poissonian in the Coulomb-blockade regime in the large-bias limit, Poissonian shot noise is generically obtained in the noninteracting case.

Electron transfer into metal nanoparticles on oxide supports is associated with unusual morphological, electronic, and chemical properties of the charged system. Two fundamental charging routes have been identified so far, which are electron tunneling through ultrathin oxide films supported by a bulk metal and charge donation from single-ion impurities embedded in the oxide matrix. In this study, we have investigated whether both routes lead to the formation of metal deposits with identical properties. For this purpose, Au islands have been prepared on $1-2\mathrm{ML}$ thin $\mathrm{MgO}/\mathrm{Ag}\left(001\right)$ layers and on 25-ML-thick CaO films doped with Mo impurities. The morphological and electronic properties of the islands were analyzed with low-temperature scanning tunneling microscopy (STM) and spectroscopy. In both systems, pronounced electron confinement effects are observed in the nanostructures, arising from the quantization of one and the same Au electronic band. Moreover, clear experimental signatures for a charge transfer into the islands are found, such as a layer-by-layer growth of the ad-metal and a negative contrast of the oxide region around the deposits in STM images. Our data provide evidence that the charged nanostructures exhibit comparable properties independent of the origin of the extra electrons. This agreement suggests that ultrathin oxide films may be used as model systems for doped bulk oxides, as used in heterogeneous catalysis.

Excitons, composite electron-hole quasiparticles, are known to play an important role in optoelectronic phenomena in many semiconducting materials. Recent experiments and theory indicate that the band-gap optics of the newly discovered monolayer transition-metal dichalcogenides (TMDs) is dominated by tightly bound valley excitons. The strong interaction of excitons with long-range electromagnetic fields in these two-dimensional systems can significantly affect their intrinsic properties. Here, we develop a semiclassical framework for intrinsic exciton polaritons in monolayer TMDs that treats their dispersion and radiative decay on the same footing and can incorporate effects of the dielectric environment. It is demonstrated how both inter- and intravalley long-range interactions influence the dispersion and decay of the polaritonic eigenstates. We also show that exciton polaritons can be efficiently excited via resonance energy transfer from quantum emitters such as quantum dots, which may be useful for various applications.

To elucidate the pressure evolution of the electronic structure in an antiferromagnetic dimer-Mott (DM) insulator ${\beta }^{\prime }\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{ICl}}_2$, which exhibits superconductivity at 14.2 K under 8 GPa, we measured the polarized infrared (IR) optical spectra under high pressure. At ambient pressure, two characteristic bands due to intra- and interdimer charge transfers have been observed in the IR spectra, supporting that this salt is a typical half-filled DM insulator at ambient pressure. With increasing pressure, however, the intradimer charge transfer excitation shifts to much lower energies, indicating that the effective electronic state changes from half-filled to 3/4-filled as a result of weakening of dimerization. This implies that the system approaches a charge-ordered state under high pressure, in which charge degrees of freedom emerge as an important factor. The present results suggest that charge fluctuation inside of dimers plays an important role in the high-temperature superconductivity.

The resistance versus temperature across the metal-insulator transition (MIT) of ${\mathrm{V}}_2{\mathrm{O}}_3$ nanodevices exhibits multiple discontinuous jumps. The jump sizes range over three orders of magnitude in resistance and their distribution follows a power law, implying that the MIT of ${\mathrm{V}}_2{\mathrm{O}}_3$ occurs through avalanches. While the maximum jump size depends on the device size, the power law exponent for ${\mathrm{V}}_2{\mathrm{O}}_3$ is independent of device geometry and different than the one found earlier in $\mathrm{V}{\mathrm{O}}_2$. A two-dimensional random percolation model exhibits a power law distribution different from the one found in ${\mathrm{V}}_2{\mathrm{O}}_3$. Instead, the model gives a similar exponent found in another vanadium oxide, $\mathrm{V}{\mathrm{O}}_2$. Our results suggest that the MITs of $\mathrm{V}{\mathrm{O}}_2$ and ${\mathrm{V}}_2{\mathrm{O}}_3$ are produced by different mechanisms.