Bias driven coherent carrier dynamics in a two-dimensional aperiodic potential

We study the dynamics of an electron wave-packet in a two-dimensional square lattice with an aperiodic site potential in the presence of an external uniform electric field. The aperiodicity is described by at lattice sites (mx,my), with πα being a rational number, and νx and νy tunable parameters, controlling the aperiodicity. Using an exact diagonalization procedure and a finite-size scaling analysis, we show that in the weakly aperiodic regime (νx,νy<1), a phase of extended states emerges in the center of the band at zero field giving support to a macroscopic conductivity in the thermodynamic limit. Turning on the field gives rise to Bloch oscillations of the electron wave-packet. The spectral density of these oscillations may display a double peak structure signaling the spatial anisotropy of the potential landscape. The frequency of the oscillations can be understood using a semi-classical approach.     

Chirped Bloch oscillations in strain-balanced InGaAs/InGaAs superlattices

Bloch oscillations excited in a strain-balanced In_xGa_1 − xAs/In_yGa_1 − yAs superlattice by fs optical pulses at 1.55 μ m are investigated in time-resolved transmission spectroscopy. The transition from the coherent oscillatory motion to an incoherent drift transport of the electrons is observed via a transient frequency shift of the Bloch oscillations due to the associated screening of the applied electric field. These electric field changes are analyzed quantitatively as a function of the initial field strengths and excitation densities. The incoherent transport can be described by a drift-diffusion model. As a result, the carrier mobility in the superlattice is obtained on a picosecond timescale.     

The infrared and Raman spectrum of trans-polyacetylene: A self-consistent-field study

Using force constants which have been obtained for the first time from semi-empirical self-consistent-field calculations on various cluster models of periodic trans-polyacetylene and of neutral and charged soliton and polaron defects the infrared and Raman spectrum of undoped and doped trans-polyacetylene is analyzed and interpreted.     

Addendum to “Switching effect and the metal–insulator transition in electric field” by A.L. Pergament et al. [J. Phys. Chem. Solids 71 (2010) 874]

In the paper mentioned above we reported on the switching mechanism in vanadium dioxide which was shown to be based on the electronically-induced Mott insulator-to-metal transition occurring in conditions of the non-equilibrium carrier density excess in the applied electric field, and the proposed model involved the dependence of the carrier density n on electric field (the Poole–Frenkel effect), as well as the dependence of the critical electric field on n. The data on the n(T) dependence were obtained on the assumption of a temperature-independent carrier mobility μ, and the problem of n reduction at lower temperatures was not fully understood. In this Letter we revisit this problem in the light of some recent data on the μ(T) dependence for VO₂. It is shown that the adjusted values of n, taking into account this μ(T), correspond to the Mott critical density within an order of magnitude.     

Electron motion in a Holstein molecular chain in an electric field

A charge motion in an electric field in a Holstein molecular chain is modeled in the absence of dissipation. It is shown that in a weak electric field a Holstein polaron moves uniformly experiencing small oscillations of its shape. These oscillations are associated with the chain’s discreteness and caused by the presence of Peierls-Nabarro potential there. The critical value of the electric field intensity at which the moving polaron starts oscillating at Bloch frequency is found. It is shown that the polaron can demonstrate Bloch oscillations retaining its shape. It is also shown that a breathing mode of Bloch oscillations can arise. In all cases the polaron motion along the chain is infinite.     

Numerical method for hydrodynamic modulation equations describing Bloch oscillations in semiconductor superlattices

We present a finite difference method to solve a new type of nonlocal hydrodynamic equations that arise in the theory of spatially inhomogeneous Bloch oscillations in semiconductor superlattices. The hydrodynamic equations describe the evolution of the electron density, electric field and the complex amplitude of the Bloch oscillations for the electron current density and the mean energy density. These equations contain averages over the Bloch phase which are integrals of the unknown electric field and are derived by singular perturbation methods. Among the solutions of the hydrodynamic equations, at a 70 K lattice temperature, there are spatially inhomogeneous Bloch oscillations coexisting with moving electric field domains and Gunn-type oscillations of the current. At higher temperature (300 K) only Bloch oscillations remain. These novel solutions are found for restitution coefficients in a narrow interval below their critical values and disappear for larger values. We use an efficient numerical method based on an implicit second-order finite difference scheme for both the electric field equation (of drift-diffusion type) and the parabolic equation for the complex amplitude. Double integrals appearing in the nonlocal hydrodynamic equations are calculated by means of expansions in modified Bessel functions. We use numerical simulations to ascertain the convergence of the method. If the complex amplitude equation is solved using a first order scheme for restitution coefficients near their critical values, a spurious convection arises that annihilates the complex amplitude in the part of the superlattice that is closer to the cathode. This numerical artifact disappears if the space step is appropriately reduced or we use the second-order numerical scheme.     

Multiphonon Raman scattering and gap states in cis-polyacetylene

The different features of the Raman scattering in resonance with electronic absorption band in cis and trans-polyacetylene are discussed. The strong multiphonon scattering observed in cis-polyacetylene is accounted for in terms of a localized gap state, which dominates the scattering process near the absorption edge.     

Magnon gas and deviation from the Bloch law in a nanoscale Heisenberg ferromagnet

It is known that saturation magnetization of ferromagnetic nanoparticles and nanoclusters deviates strongly from the Bloch T^3/2 law. To describe the deviation one usually uses a modified power law T^α with a size dependent exponent α, which is motivated simply by its flexibility in fitting the observed behavior. By considering the Heisenberg spin model, we derive an explicit expression in terms of a magnon gas which generalizes the Bloch formula to a finite size system. Comparison to the experimental data shows a good agreement with the observed behavior and gives a better understanding of its physics.     

An alternative method for the exact calculation of Wannier–Stark localization in superlattices

We present an alternative method for the exact calculations of the Wannier–Stark (WS) localization in a long periodic potential corresponding to a (50 Å/30 Å) GaAs/Ga_0.7Al_0.3As superlattice. We show that the electric field dependence of the electron wavefunction has unique localization dynamics. One interesting prediction is a small effect involving the change of the dipole field with increasing WS field. It is argued that this may give rise to parasitic effects in Bloch oscillations and, therefore, to noise in coherent terahertz emission.     

Hofstadter spectrum in electric and magnetic fields

The problem of Bloch electrons in two dimensions subjected to magnetic and intense electric fields is investigated. Magnetic translations, electric evolution, and energy translation operators are used to specify the solutions of the Schrödinger equation. For rational values of the magnetic flux quanta per unit cell and commensurate orientations of the electric field relative to the original lattice, an extended superlattice can be defined and a complete set of mutually commuting space-time symmetry operators is obtained. Dynamics of the system is governed by a finite difference equation that exactly includes the effects of: an arbitrary periodic potential, an electric field orientated in a commensurable direction of the lattice, and coupling between Landau levels. A weak periodic potential broadens each Landau level in a series of minibands, separated by the corresponding minigaps. The addition of the electric field induces a series of avoided and exact crossing of the quasienergies, for sufficiently strong electric field the spectrum evolves into equally spaced discreet levels, in this “magnetic Stark ladder” the energy separation is an integer multiple of hE/aB, with a the lattice parameter.     

Transient Dynamics of Super Bloch Oscillations of a 1D Holstein Polaron under the Influence of an External AC Electric Field

Theoretical formalism for DC‐field polaron dynamics is extended to the dynamics of a 1D Holstein polaron in an external AC electric field using multiple Davydov trial states. Effects of carrier–phonon coupling on detuned and resonant scenarios are investigated for both phase and nonzero phase. For slightly off‐resonant or detuned cases, a beat between the usual Bloch oscillations and an AC driving force results in super Bloch oscillations, that is, rescaled Bloch oscillations in both the spatial and the temporal dimension. Super Bloch oscillations are damped by carrier–phonon coupling. For resonant cases, if the carrier is created on two nearest‐neighboring sites, the carrier wave packet spreads with small‐amplitude oscillations. Adding carrier–phonon coupling localizes the carrier wave packet. If an initial broad Gaussian wave packet is adopted, the centroid of the carrier wave packet moves with a certain velocity and with its shape unchanged. Adding carrier–phonon coupling broadens the carrier wave packet and slows down the carrier movement. Our findings may help provide guiding principles on how to manipulate the dynamics of the super Bloch oscillations of carriers in semiconductor superlattice and optical lattices by modifying DC and AC field strengths, AC phases, and detuning parameters.     

Anharmonic Bloch Oscillation of Electrons in Biased Superlattices

The oscillatory motion of electrons in a periodic potential under a constant applied electric field, known as Bloch oscillations (BO), is one of the most striking and intriguing quantum effects and was predicted more than eighty years ago. Oscillating electrons emit electromagnetic radiation and here we consider this BO effect for emission in the THz region. To date, it has been assumed that the Bloch oscillation of an electron is anharmonic oscillation, therefore with radiation emitted at the single Bloch frequency. We analyze scenarios when Bloch oscillations can be accompanied by the emission of radiation not only at the Bloch frequency but also with double and triple Bloch frequencies. The first scenario means that electrons could jump over neighboring Stark states. The second scenario of anharmonic emission is coupled to an opening of the minigap in the miniband.     

Influence of soliton distributions on the spin-dependent electronic transport through polyacetylene molecule

In this paper, a detailed numerical study of the role of selected soliton distributions on the spin-dependent transport through trans-polyacetylene (PA) molecule is presented. The molecule is attached symmetrically to magnetic semi-infinite three-dimensional electrodes. Based on Su– Schrieffer–Heeger (SSH) Hamiltonian and using a generalized Green’s function formalism, we calculate the spin-dependent currents, the electronic transmission and tunnelling magnetoresistance (TMR). We found that the presence of a uniform distribution of the soliton centres along the molecular chain reduced the size of the band gap of trans-PA molecule. Moreover, a sublattice of the correlated solitons as binary clusters, which are randomly distributed along the chain, can induce extended electronic states in the band gap of the molecule. In this case, the band gap of the molecule is suppressed and at lower voltages, the TMR bandwidth is narrowed. The current–voltage characteristic then shows an ohmic-like behaviour.     

Rabi oscillations in two-level semiconductor systems

Rabi oscillations in coherent optical excitations in bulk GaAs and quantum dot two-level systems may be converted into deterministic photocurrents, with the impurities or dots providing the tag for each qubit. Here we perform a theoretical analysis of the damping of Rabi oscillations in two-level semiconductor systems. Present calculations, through optical Bloch equations on excitonic two-level In_xGa_1−xAs quantum-dot systems, are found in good agreement with the corresponding experimental data. Calculated results indicate that the nature underlying the dephasing mechanism associated to the damping of the measured Rabi oscillations, which has previously remained as an open question, may be associated with a field-dependent recombination rate related to the inhomogeneous broadening of the excitonic lines in the In_xGa_1−xAs two-level QD system.     

ac Conductivity of semiconducting trans-polyacetylene

The ac conductivity of semiconducting trans-polyacetylene has been measured over the frequency range 10 Hz to 10 MHz at room temperature. Both ohmic and blocking contacts were used on samples prepared at several doping levels below the metallic limit. The complex conductivity was found to be frequency independent for all levels of doping and for both types of contacts. A discussion is given regarding soliton motion and inhomogeneous doping and their possible effects on ac conductivity in trans-polyacetylene.     

Bloch oscillations and Wannier-Stark ladder in the coupled LC circuits

We present a new scheme for realizing Bloch oscillations and Wannier-Stark ladder based on a lattice of coupled LC circuits. By converting the second order dynamical ODEs of the system into a first order Schrödinger-like equation, we propose an equivalent tight binding Hamiltonian to describe the circuit. We show that a synthesized electric field is produced by introducing a frequency mismatch into the resonant frequency of the adjacent LC resonators. The Wannier-Stark modes are the normal modes of the circuit and the Bloch oscillations can be observed in a coupled LC lattice. By addition of coupling capacitors between nodes of the circuit, we study the Bloch oscillation in the presence of long-range couplings. We also show that the circuit converts to a transmission line simulating synthetic electric fields in the continuum limit. The coupled LC circuit is, in some sense, amongst the simplest physical systems exhibiting Bloch oscillation and Wannier-Stark Ladder.     

PMR investigation of the structure of native and iodine doped polyacetylene films

The second moment of the proton magnetic resonance line of native cis- polyacetylene, thermally isomerized trans-polyacetylene, and iodine doped initially cis-polyacetylene has been measured by a pulse technique. The results differ qualitatively and quantitatively from previous reports. They are discussed in terms of the proposed crystalline structures of (CH)_x.     

Peierls distortion and vibrational spectra in undoped and doped trans-polyacetylene

We analyze the conditions under which the vibrational spectrum can provide information on the existence of the Peierls distortion in undoped trans-polyacetylene. If in the doped material sections of undimerized chains are generated, calculations predict a Raman active A_g band approximately between 1300-1250 cm^?1.     

Optical properties of GaNAs/GaAs triple quantum well structures

Optical properties of the GaNAs/GaAs triple quantum well structures were characterized by using photoreflectance and photoluminescence spectroscopy at different temperatures. The excitonic interband transitions of the triple quantum well systems were observed in the spectral range above hν=E_g(GaN_xAs_1−x). A matrix transfer algorithm was used to match the GaN_xAs_1−x/GaAs boundary conditions and calculate the triple quantum well subband energies numerically for theoretical comparison. The internal electric field in the system was extracted from Franz-Keldysh oscillations in the photoreflectance spectra. The influences of the annealing treatment on the transition energy and the internal electric field are also analyzed.     

Landau levels in deformed bilayer graphene at low magnetic fields

We review the effect of uniaxial strain on the low-energy electronic dispersion and Landau level structure of bilayer graphene. Based on the tight-binding approach, we derive a strain-induced term in the low-energy Hamiltonian and show how strain affects the low-energy electronic band structure. Depending on the magnitude and direction of applied strain, we identify three regimes of qualitatively different electronic dispersions. We also show that in a weak magnetic field, sufficient strain results in the filling factor ν=±4 being the most stable in the quantum Hall effect measurement, instead of ν=±8 in unperturbed bilayer at a weak magnetic field. To mention, in one of the strain regimes, the activation gap at ν=±4 is, down to very low fields, weakly dependent on the strength of the magnetic field.