Sergey V. Yampolskii

Self-consistent analytical solution of a problem of charge-carrier injection at a conductor/insulator interface

We present a closed description of the charge-carrier injection process from a conductor into an insulator. Common injection models are based on single electron descriptions, being problematic especially once the amount of charge-carriers injected is large. Accordingly, we developed a model, which incorporates space-charge effects in the description of the injection process. The challenge of this task is the problem of self-consistency. The amount of charge carriers injected per unit time strongly depends on the energy barrier emerging at the contact, while at the same time the electrostatic potential generated by the injected charge carriers modifies the height of this injection barrier itself. In our model, self-consistency is obtained by assuming continuity of the electric displacement and the electrochemical potential all over the conductor/insulator system. The conductor and the insulator are properly taken into account by means of their respective density of state distributions. The electric-field distributions are obtained in a closed analytical form and the resulting current-voltage characteristics show that the theory embraces injection-limited as well as bulk-limited charge-carrier transports. Analytical approximations of these limits are given, revealing physical mechanisms responsible for the particular current-voltage behavior. In addition, the model exhibits the crossover between the two limiting cases and determines the validity of respective approximations. The consequences resulting from our exactly solvable model are discussed on the basis of a simplified indium tin oxide/organic semiconductor system.

Magnetic cloaking by a paramagnet/superconductor cylindrical tube in the critical state

Cloaking of static magnetic fields by a finite thickness type-II superconductor tube being in the full critical state and surrounded by a coaxial paramagnet shell is studied. On the basis of exact solutions to the Maxwell equations, it is shown that, in addition to previous studies assuming the Meissner state of the superconductor constituent, perfect cloaking is still realizable at fields higher than the field of full flux penetration into the superconductor and for arbitrary geometrical parameters of both constituents. It is also proven that simultaneously the structure is fully undetectable under the cloaking conditions. Different from the case of the Meissner state, the cloaking properties in the application relevant critical state are realized, however, only at a certain field magnitude.

Optical transmittance of photonic structures with linearly graded dielectric constituents

The optical transmittance of one-dimensional photonic structures generated from lossless dielectric slabs with linear, or piecewise linear, profiles of their electric permittivity for linearly polarized, normally incident electromagnetic radiation is studied both analytically and numerically. Resorting to an analysis of the respective photonic modes, the dependence on frequency of the optical property addressed is first established for the singly graded slab and the doubly graded slab, i.e. for the proper dielectric constituents, and subsequently for the photonic structures derived from them. Considering the singly graded periodic structure, the transmittance reveals a series of photonic bands and gaps, exhibiting maxima less than unity, as for the singly graded slab, and tending to saturate in a periodic sequence of centro-symmetric columnar peaks, separated by gaps of identical widths, at large frequencies; an asset that recommends this type of photonic structure as ideal. Looking at the doubly graded periodic structure, the transmittance reveals two series of photonic bands and gaps, exhibiting maxima of full transparency, as for the doubly graded slab, or maxima of reduced transparency and tending to saturate in essentially constant values with zero gaps at large frequencies; a trait known for all frequencies from an optically homogeneous, non-dispersive medium of semi-infinite extent. The properties revealed for both the singly graded periodic structure and the doubly graded periodic structure offer unique possibilities regarding practical applications of such novel photonic composites, optical filters exploiting their transmittance or optical mirrors utilizing their reflectance only being the most obvious.

Magnetic detectability of a finite size paramagnet/superconductor cylindrical cloak

Cloaking of static magnetic fields by a finite thickness type-II superconductor tube surrounded by a coaxial paramagnet shell is studied. On the basis of exact solutions to the London and Maxwell equations, it is shown that perfect cloaking is realizable for arbitrary geometrical parameters including the thin film case for both constituents. In contrast to previous approximate studies assuming perfect diamagnetism of the superconductor constituent, it is proven that cloaking provides simultaneously full undetectability, that is the magnetic moment of the structure completely vanishes as well as all high-order multipole moments as soon as the uniform field outside remains unaffected.

Entry of magnetic flux into a magnetically shielded type-II superconductor filament

In the framework of the London approximation the magnetic flux penetration into a type-II superconductor filament surrounded by a soft-magnet sheath and exposed to a transverse external magnetic field is studied. The lower transverse critical field as well as the critical field and the critical current of the first vortex nucleation at the superconductor/magnet interface are calculated on the basis of an exact solution for a vortex of arbitrary plane configuration. The Bean-Livingston barrier against the vortex nucleation is shown to strongly depend on the magnet sheath parameters.

Virgin magnetization of a magnetically shielded superconductor wire: Theory and experiment

On the basis of exact solutions to the London equation, the magnetic moment of a type II superconductor filament surrounded by a soft-magnet environment is calculated and the procedure of extracting the superconductor contribution from magnetic measurements is suggested. A comparison of theoretical results with experiments on MgB2/Fe wires allows the estimation of the value of critical current for the first magnetic flux penetration.

Superconducting wire subject to synchronous oscillating excitations: Power dissipation, magnetic response, and low-pass filtering

Numerical simulations of a type-II superconducting wire subject to an ac transport current and oscillating transverse magnetic field are performed using the theory of the critical state. Time-dependent distributions of the current and the density of magnetic flux, the local power dissipation, and cycles of the magnetic moment are displayed. Noticeable inhomogeneous dissipation and field distortions are exposed. Results for hysteretic ac losses are reported too, and significant differences to predictions of available approximate formulae identified. Finally, a distinct low-pass filtering effect intrinsic to the wire’s magnetic response is revealed.

Bipolar charge-carrier injection in semiconductor/insulator/conductor heterostructures: Self-consistent consideration

A self-consistent model of bipolar charge-carrier injection and transport processes in a semiconductor/insulator/conductor system is developed, which incorporates space-charge effects in the description of the injection process. The amount of charge carriers injected is strongly determined by the energy barrier emerging at the contact, but at the same time the electrostatic potential generated by the injected charge carriers modifies the height of this injection barrier itself. In our model, self-consistency is obtained by assuming continuity of the electric displacement and of the electrochemical potential all over the system. The constituents of the system are properly taken into account by means of their respective density of state distributions. The consequences resulting from our model are discussed on the basis of an indium tin oxide/organic semiconductor/conductor structure. The distributions of the charge carriers and the electric field through the electrodes and the organic layer are calculated. ...

Influence of electrical fatigue on hole transport in poly(p-phenylenevinylene)-based organic light-emitting diodes

The hole transport in poly(p-phenylenevinylene) (PPV) was investigated before and after bipolar electrical stress by the time-of-flight (TOF) method. Bipolar structures similar to organic light emitting diodes (OLEDs) were realized, yet with much thicker layers than usually prevailing in OLEDs. During fatigue, the hole mobility is reduced, the field dependence of the mobility is increased, and the hole transport becomes more and more dispersive. These results go along with the fatigue behavior of thin film OLEDs that were investigated by charge extraction via linearly increasing voltage (CELIV). Even though theoretical simulations could show that both thick- and thin-film PPV-based OLED structures are dominated by holes, the presented results indicate that the existence of electrons leads to degradation during hole transport. A possible reason for an enlarged electron density in the otherwise hole dominated device is suggested.

Self-consistent model of unipolar transport in organic semiconductor diodes: Accounting for a realistic density-of-states distribution

A self-consistent, mean-field model of charge-carrier injection and unipolar transport in an organic semiconductor diode is developed utilizing the effective transport energy concept and taking into account a realistic density-of-states distribution as well as the presence of trap states in an organic material. The consequences resulting from the model are exemplarily discussed on the basis of an indium tin oxide/organic semiconductor/metallic conductor structure. A comparison of the theory to experimental data of a unipolar indium tin oxide/poly-3-hexyl-thiophene/Al device is presented.