Lattice and charge inhomogeneities in transition metal oxides

Transition metal oxides show an extremely rich variety of ground states which frequently can be modified through doping, pressure, temperature. The conventional understanding of these compounds as e.g. typical ionic systems or typical metallic systems yields incorrect results concerning their dynamical or electronic structures. It is shown here that the intrinsic instability of the doubly negatively charged oxygen ion O²⁻ triggers many of the unusual properties.     

Interfacial electronic states in semiconductor heterostructures

It is shown that electronic states of a new type, with energy in the band gap can exist at a heterointerface. The interfacial states may be associated with Tamm surface states in the materials forming the heterointerface, but they can appear even if there are no surface states in the initial materials. In the plane of the heterojunction, the energy spectrum of interfacial states forms a two-dimensional band. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 393–398 (25 March 1998)     

Coulomb interactions and nanoscale electronic inhomogeneities in manganites

We study electronic inhomogeneities in manganites using simulations on a microscopic model with Coulomb interactions amongst two electronic fluids-one localized (polaronic), the other extended-and dopant ions. The long range Coulomb interactions frustrate phase separation induced by the large on site repulsion between the fluids. A single phase ensues which is inhomogeneous at a nanoscale, but homogeneous on mesoscales, with many features that agree with experiments. This, we argue, is the origin of nanoscale inhomogeneities in manganites, rather than phase competition or disorder effects.     

Backward electronic Tamm states in graphene-based heterostructures

We study the electronic surface waves (the so-called Tamm states) localized at the interface between a graphene-based superlattice and a homogeneous graphene by applying suitable electrodes on a graphene sheet. The magnitude as well as the sign of the slope of the Tamm dispersive curve can be flexibly tuned just by varying the external voltage. Particularly, in addition to the conventional forward Tamm states, backward Tamm states in which the wave vector of the electronic surface wave is antiparallel with the group velocity can be realized.     

Scanning tunneling microscopy and spectroscopy study of charge inhomogeneities in bilayer graphene

We report a room-temperature scanning tunneling microscopy and spectroscopy study of bilayer graphene prepared by mechanical exfoliation on a SiO₂/Si surface and electrically contacted with gold pads using a mechanical mask. The bulk conductivity shows contributions from regions of varying electron density, indicating significant charge inhomogeneity. Large-scale topographic images show ripple-like structures with a roughness of ∼1 nm, while the small-scale atomic resolution images show graphite-like triangular lattices. The local () tunnel spectra have an asymmetric V-shape with the minima location showing significant spatial variation, indicating inhomogeneity in electron density of order 10¹¹ cm⁻². The minimum in spectrum at a fixed location also shifts linearly with the gate voltage with a slope consistent with the field-induced carrier density.     

Mass and charge distributions with correlated exchange

The dependence of the inclusive variancesσ _A ² ,σ _Z/2 andσ _N ² in nuclear collisions is investigated as a function of the correlation coefficient ρ for isospin correlated nuclon exchange on the dinuclear potential energy surface (PES). Variances for neutron-proton cluster exchange (single step process with zero net momentum between the constituents) are also evaluated. This analysis is helpful to understand the nature of the exchange mechanism. There exist a few experimental data which are at variance with either uncorrelated neutron-proton exchange or with isospin correlated exchange. An explanation of this data may possibly lie in coexisting neutron-proton cluster exchange.     

Mapping between electronic structure and ac-Stark shift resonances in heterostructures

We have studied carrier dynamics in quantum wells subjected to strong ac-electric fields. The ac-Stark shifts were calculated within the Kane model. The inherent symmetries and couplings in Kane Hamiltonian lead to the carrier effective masses depending on the -vector. We have predicted a mapping between energy dispersion and ac-Stark effect. An anomalous behavior of light-hole states can be associated to the different dynamical responses of carrier to the driving ac-field.     

Spatially correlated charge transport in organic thin film transistors

Hole mobility in organic ultrathin film field-effect transistors is studied as a function of the coverage. For layered sexithienyl films, the charge carrier mobility rapidly increases with increasing coverage and saturates at a coverage of about two monolayers. This shows that the first two molecular layers next to the dielectric interface dominate the charge transport. A quantitative analysis of spatial correlations shows that the second layer is crucial, as it provides efficient percolation pathways for carriers generated in both the first and the second layers. The upper layers do not actively contribute either because their domains are smaller than the ones in the second layer or because the carrier density is negligible.     

Localized charge inhomogeneities and phase separation near a second-order phase transition

Localized charge inhomogeneities and phase separation are described in the framework of the phenomenological theory of phase transitions. It is shown that Coulomb interaction determines the charge distribution and the characteristic size of the emerging inhomogeneities. Phase separation associated with charge segregation becomes possible because of a high dielectric constant and a low excess charge density in the localization region. The phase diagram of the system is calculated, and estimates are obtained for the gain in energy associated with the emerging state. The role of Coulomb interaction is exposed, and corresponding estimates are given.     

Transverse transport in coupled strongly correlated electronic chains

One–particle interchain hopping in a system of coupled Luttinger liquids is investigated by use of exact diagonalizations techniques [1]. We give numerical evidence that inter-chain coherent hopping (defined by a nonvanishing splitting) can be totally suppressed for the Luttinger liquid exponent α～ 0:4 or even smaller α values. The transverse conductivity is shown to exhibit a strong incoherent part even when coherent inter-chain hopping is believed to occur. Implications for the optical experiments in quasi-1D organic or high-T _c superconductors is outlined.     

Surface and interface electronic properties of group III-nitride heterostructures

For group III-nitrides with wurtzite structure the presence of fixed polarization interface charges yields new challenges in order to understand and control Schottky barrier heights, band offsets and 2D confinement in heterostructure FETs. In this short review experimental results obtained by in situ photoemission spectroscopy on MBE AlGaN/GaN heterostructures grown on 6H–SiC are discussed, with emphasis on the presence and interplay of surface electronic states. Schrödinger–Poisson calculations are performed to get the complete band scheme at the selected heterojunctions. Results on the polarity dependence of Pt/GaN Schottky barrier values from the literature are also discussed.     

Acoustically driven charge separation in semiconductor heterostructures sensed by optical spectroscopy techniques

We demonstrate a method of using a two-layer sandwich structure, which includes a LiNbO₃ plate and a semiconductor heterostructure to create an inhomogeneous stress and piezoelectric harmonic potential in the semiconductor. Both the GaAs/AlGaAs quantum well (QW) structures and SiGe/Si heterostructures are attempted, working with and without using a piezoelectric field in the semiconductor layer. The standing-wave fields generated in the semiconductor and the electron and hole distributions driven by the piezoelectric field are computed by finite element method (FEM) techniques. It is experimentally shown that, in a GaAs/Al_xGa_1-x As asymmetric double quantum well structure, the resonance enhancement of the narrower QW photoluminescence band is observed, which may be explained by the resonant charge transfer between the wider and narrower QWs. It is also shown that the piezoelectric fields quench the pure LO-phonon lines in the Raman spectra, whereas the coupled LO-phonon-plasmon mode strengthens. Experimental results indicate that the charge separation occurs in the plane of the QWs due to the piezoelectric fields. The recombination of carriers in the SiGe/Si heterostructures can be effectively enhanced by the presence of ultrasonic stress, displaying features consistent with varying electrical activity at dislocations.     

Ultrafast charge transfer in dual graphene-WS_2 van der Waals quadrilayer heterostructures

Using dual graphene–WS_2 quadrilayer heterostructures as an example, we find that the ultrafast transfer of electrons from WS_2 to graphene takes place within 114 fs, and the Coulomb field of the charge can effectively affect the interlayer electron transfer. This effect illustrates that the charge transfer in such van der Waals heterostructures may be controlled by an externally applied electric field for promising applications in photoelectric devices.     

Momentum dependence of the dimensionality of the electronic states in heterostructures

Bound states of electrons (holes) in quantum wells and wires with asymmetric barriers can exist in bounded regions of two-and one-dimensional momentum space, respectively. As the corresponding momentum increases, both the disappearance (increase of dimensionality) and appearance (decrease of dimensionality) of bound states as well as the existence of a sequence of several such transformations of dimensionality are possible. In the case of anisotropic effective masses in the quantum wells and barriers, the forms of the lines of disappearance and appearance of bound states are different from the forms of the isoenergy lines. Therefore there is a finite energy interval (i.e., electron density interval) where bound states exist on only a part of an isoenergy line. The dimensionality of the states can be controlled with an electric field; this should be observable in a number of the experiments discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 188–193 (25 January 1997)     

Space charge effects and electronic bistability

Summary The excitation of metastable states in an atomic beam apparatus by means of electron collision is a widespread technique. We have observed a large bistable behaviour in our apparatus designed to provide an intense and collimated beam of metastable helium by excitation with orthogonally impinging electrons. This bistable behaviour largely affects the efficiency of the apparatus and is therefore worth of being carefully investigated. The apparatus has an electrode configuration equivalent to that of a tetrode valve with large intergrid distances. The bistability consists in a hysteresis cycle in the curve of the anode currentvs. grid voltage. Experimental measurements, supported by a simple theoretical model and by numerical simulation, stress out the crucial role played by space charge effects for the onset of bistability. A comparison with previous observation of this phenomenon is given. Spontaneous current oscillations with various shapes have been recorded in one of the two curves of the hysteresis cycle.     

Ground-state energy of strongly correlated electronic systems

We calculate the ground-state energy of strongly correlated electrons by using a twodimensional (CuO₂)_N system as an example. It constitutes the most important structural element of the high-T _c superconducting materials. The strong correlations are treated by projection technique. For that purpose a new form of the free energy is derived, which allows for applying that method. The ground-state energy for the half-filled band case is calculated by using a Padé approximation which agrees with series expansions up to order (t/( _p – _d ))⁶. Heret is thep-d hopping matrix element and _p , _d are the orbital energies of the O(2p _x(y)) and electrons.     

Interface recombination in P-type GaAs-(AlGa)As quantum well heterostructures

The process of non-radiative interface recombination is considered for quantum well heterostructures. Simple model calculations indicate that, contrary to previous suggestions, the quantum effects do not cause a large reduction in the effective interface recombination velocity, S_eff. In fact, for the same interface quality, quantum effects cause an increase in S_eff.