Nonzero Wave Vector Research Articles

Distortion of excitonic states by lattice defects and violation of the quasi-momentum conservation law in excitonic absorption

Absorption spectrum, related to creating excitons with nonzero wave vector inside a wide band, is considered for an insulator crystal with lattice defects. It is shown that the main contribution to absorption is determined not by the scattering section of defects but by the exciton state distortion within the core or a close vicinity of a defect. Its contribution to absorption differs from that of an impurity center by a great multiplier dependent on the size of the region of distortion. Due to this, lattice defects with a very low concentration ( ∼10 14 cm −3 ), present even in a perfect crystal, can manifest themselves in luminescence excitation spectrum. This can be used for the diagnostics of lattice defects in insulator crystals (e.g., scintillators) exposed to irradiation.

Anticrossing semiconducting band gap in nominally semimetallic InAs/GaSb superlattices

While $(\mathrm{InAs}{)}_{n}/(\mathrm{GaSb}{)}_{n}$ (001) superlattices are semiconducting for $n<{n}_{c}\ensuremath{\approx}28\mathrm{ML},$ for $n>{n}_{c}$ the InAs electron level ${e}_{\mathrm{InAs}}$ is below the GaSb hole level ${h}_{\mathrm{GaSb}},$ so the system is converted to a nominal semimetal. At nonzero in-plane wave vectors $({\mathbf{k}}_{\ensuremath{\Vert}}\ensuremath{\ne}0),$ however, the wave functions ${e}_{\mathrm{InAs}}$ and ${h}_{\mathrm{GaSb}}$ have the same symmetry, so they anticross. This opens up a ``hybridization gap'' at some ${\mathbf{k}}_{\ensuremath{\Vert}}={\mathbf{k}}_{\ensuremath{\Vert}}^{*}.$ Using a pseudopotential plane-wave approach as well as a (pseudopotential fit) eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ approach, we predict the hybridization gap and its properties such as wave-function localization and out-of-plane dispersion. We find that recent model calculations underestimate this gap severely.

Non-monotonic behaviour of the energy levels of quantum wells with a large mass mismatch in the presence of an in-plane magnetic field

We present a theoretical study of the electron states in a quantum well with finite barrier height in a magnetic field applied parallel to the quantum well. For large electron mass mismatch between the quantum well and the barrier, we found the surprising result that the electron energy for zero wave vector decreases with decreasing well width and the energy spectrum has a local minimum at a non-zero wave vector. The influence of well depth, mass ratio and magnetic field are investigated.

Magnetoelastic interaction in a spatially disordered ferromagnet containing a large number of defects

The description of a continuum with continuously distributed dislocations and disclinations is extended to the case of a spatially disordered ferromagnet. It is shown that the interaction of disclinations with spins can have the effect that the spin-wave frequency is equal to zero for a nonzero wave vector. The variation of the coupled-oscillation frequency is determined in the case of weak coupling between elastic waves and defects.

Interband tunnelling in semiconductor heterostructures

We describe an approach for the investigation of the interband tunnelling in semiconductor heterostructures made from materials based on the transfer Hamiltonian formalism and kp band model. Contrary to methods used earlier, this approach allows one to obtain not only the transmission coefficients but also resonant tunnelling times corresponding to the transitions through each barrier of a double-barrier structure taking into account the mixing of electron, light-hole (LH) and heavy-hole (HH) states. The equation for the tunnel matrix element is derived using the isotropic eight-band kp model for structures under bias. The analytical formulae for calculation of the tunnel matrix element, transmission coefficients and resonant tunnelling times are obtained for resonant tunnelling structures (RTSs) with type II heterojunctions under flatband conditions. The interband tunnelling in the InAs/AlSb/GaSb RTS is investigated using these formulae. It is shown that the resonant tunnelling times corresponding to transitions through the left and right barriers of a symmetrical RTS may differ by orders of magnitude owing to the spin-orbit interaction in the case of nonzero in-plane wavevector and depend significantly on the mixing of electron, LH and HH states.

Cathodoluminescence investigation of isotope effects in diamond

We report high-resolution cathodoluminescence (CL) measurements near the indirect gap of isotopically modified diamonds. From the energies of intrinsic phonon-assisted peaks, as well as the no-phonon recombination of boron-bound excitons, we determine the contribution of isotope disorder to the frequencies of various phonons with nonzero wave vector. The results confirm calculations of the phonon self-energy based on ab-initio lattice dynamics. For some modes we predict an asymmetric variation of the disorder-induced shift with the 13C concentration which is observed in second-order Raman spectra. The no-phonon emission from boron-bound excitons (D 0 and D 0 ′ lines) exhibits a fine structure which arises from a splitting of the neutral boron acceptor ground state (2 meV) and the initial bound excitons (3.3 meV). When varying the 13C content all components shift with a slope of (14.6 ± 0.5) meV/amu.

Polymerization-Induced Phase Separation: A Maximum in the Intensity of Scattered Light Associated with a Nucleation-Growth Mechanism

The appearance of a maximum in the intensity of scattered light at a nonzero wave vector for systems undergoing a polymerization-induced phase separation (PIPS) has been considered in the past as conclusive evidence of the presence of a spinodal demixing mechanism. However, recent results from light scattering studies of colloidal aggregation and phase separation in aqueous biopolymers systems and polymer blends prove that the maximum may also be generated by a nucleation-growth process (NG). The origin of this scattering behavior is the presence of a layer surrounding dispersed-phase particles that contains less solute concentration than the bulk (depletion layer). We apply this concept to a system undergoing PIPS through an NG mechanism. The analysis is constrained to the generation of a diluted dispersion of spherical particles where concentration profiles around particles may be analytically derived. Both Rayleigh−Gans and Mie scattering theories are used to describe the patterns of scattered light. I...

Surface Phonon of MgO Layer on TiC(100) Surface

The surface phonon of a hetero-epitaxial MgO layer on a TiC(100) surface has been investigated using high-resolution electron energy loss spectroscopy (HREELS). The Fuchs-Kliewer, Rayleigh and Wallis modes of MgO were detected. The vibration energy of the F-K wave in the MgO layer was higher than that of a MgO single crystal due to the two-dimensional character of the MgO layer. The Rayleigh mode having almost the same dispersion relationship with bulk MgO was measured at the non-zero wave vector. Among the microscopic surface phonons, the Wallis mode ( S2) has been detected, vibrational energy of which was the same as that of bulk.

Transient gelation by spinodal decomposition in colloid-polymer mixtures

We have investigated with small angle light scattering and optical microscopy transient gelation phenomena which occur in phase-separating colloid-polymer mixtures. The scattering intensity distribution shows a peak at non-zero wave vector and satisfies the asymptotic q−4 Porod behaviour. Consistent with these observations, optical micrographs show an alternating pattern of dark and bright domains. These findings suggest that the polymer-induced depletion forces lead to the formation of a bicontinuous network of colloid-rich and colloid-poor domains, via a spinodal decomposition process. This bicontinuous network rapidly attains a gel-like character as indicated by the arrest of speckle fluctuations. The occurrence of the gel is ascribed to polymer-induced aggregation between the colloids in the colloid-rich phase. Due to the reversible nature of the aggregation the network restructures and eventually the gel collapses, as is manifested by the rapid separation of the colloid-rich phase from the colloid-poor phase.

Theoretical study of magneto-excitons in the [111] GaAs quantum well

A Landau-level basis is used to calculate the magneto-excitons in a 110 Å [111] GaAs quantum well. The excitation spectra of heavy-hole and light-hole excitons are calculated with and without the axial approximation. It is found that the anisotropy of the valence-band structure can cause coupling between optically active and inactive heavy-hole excitons obtained in the axial approximation, and make the latter become optically active. We suggest that the experimentally observed doubly resonant Raman scattering peaked at 13 T magnetic field should take place for polarization, and that it is caused by the LO-phonon Fröhlich scattering with a nonzero in-plane wave vector of the phonon.

Resonant tunneling of holes in double-barrier structures in the presence of an in-plane magnetic field

Using the asymptotic transfer-matrix method, we investigate the resonant tunneling of holes in double-barrier semiconductor structures in the presence of an in-plane magnetic field. The transmission coefficients including ll (light to light hole), hl (light to heavy hole), hh (heavy to heavy hole), and lh (heavy to light hole) are calculated as a function of energy. As in the case of nonzero parallel wave vectors, the mixing of hole tunneling can also occur due to the in-plane magnetic field. Moreover, as has been observed by resonant magnetotunneling spectroscopy, we also find that the different resonances have quite different magnetic-field dependences.

Coupling of Excitons in Parallel Organic and Semiconductor Quantum Wires

Abstract We study the resonance energy transfer between parallel neighboring organic and semiconductor quantum wires which is caused by the resonance dipole-dipole interaction between the wires. Due to this interaction, a new hybrid excited state arises which is a mixture of Frenkel and Wannier-Mott excitons. For the transition dipole moments oriented perpendicular to the wires, the interwire hybridization strength is nonzero even for small exciton wavevectors along the wires and decays rather slowly with increasing inter-wire spacing. This is in contrast to two-dimensional quantum wells where the dipole-dipole coupling decays fast with the increasing interwell distance and takes place only for nonzero excitonic wavevectors. The hybrid wire excitons possess both a large radius and a relatively large oscillator strength which might be especially interesting for the nonlinear optics.

Role of excitons in double Raman resonances in GaAs quantum wells.

Raman scattering by longitudinal-optical phonons has been measured in GaAs-AlAs multiple quantum wells at high magnetic fields. Doubly resonant scattering processes are observed at photon energies corresponding to magneto-excitons with different principal quantum numbers for the incoming and outgoing channels. The existence of these initially forbidden scattering processes, their resonance energies, and their relative intensities are correctly reproduced by our theoretical description. The model takes into account the excitonic nature of the intermediate states, as well as scattering processes involving a nonzero in-plane phonon wave vector, which is required to allow inter-Landau level scattering of the exciton.

Fundamentals, overtones, and combinations in the Raman spectrum of graphite.

Raman spectra of highly oriented pyrolytic graphite (HOPG) and pyrolytic graphite (PG) have been investigated in the region between 200 and 7000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The high signal-to-noise ratio measurements reveal the existence of new bands in the fundamental and the higher-order regions. The excitation wavelength (\ensuremath{\lambda}) dependence of Raman spectra is also studied using 457.9-, 488.0-, and 514.5-nm excitation. It is found that the \ensuremath{\lambda} dependence of Raman bands is classified into three types, i.e., upward shifted, insensitive, and downward shifted bands; with the increase of the excitation wavelength. The ratios of the relative intensities of Raman bands against the ${\mathit{E}}_{2\mathit{g}}$ mode (1580 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) between the edge and the basal planes (I${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{edge}}$/I${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{basal}}$) are examined. The fundamental and higher-order modes which become Raman active by degradation of translational symmetry increase their relative intensities in the edge plane. On the other hand, the overtone and combination modes satisfying the wave-vector selection rule by the combination of nonzero wave vectors with opposite sign show similar relative intensities between the basal and the edge planes. The assignments of Raman bands is conducted using the \ensuremath{\lambda} dependence and I${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{edge}}$/I${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{basal}}$ for HOPG and PG. Most of the Raman bands in the higher-order region can be successfully assigned as the overtones and combinations between G, D, and D\ensuremath{'} modes.It is suggested that the additional fundamental modes exist at 810 and 1080 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which are due to features in the density of states other than the 1355- and 1620-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ modes. These fundamentals show \ensuremath{\lambda} dependence opposite to the 1355-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode. Although some of the observed fundamentals or higher-order modes for HOPG (\ensuremath{\sim}1480 and 1754 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and PG (1469 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) in this work cannot be completely explained by the existing density-of-states calculation, it is definitely denied that they are due to adsorbates, oxidized species, and other impurities.

Bound-free intersubband absorption in p-type doped semiconductor quantum wells.

A procedure is proposed for calculating the light absorption due to transitions from bound to free-hole states in semiconductor quantum wells. The continuum spectrum is described via the scattering states approach. In calculating the transition-matrix elements, account is taken of the position dependence of Luttinger parameters. Example calculations in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells indicate that bound-free absorption may dominate over bound-bound absorption in the wavelength range of interest for practical applications. It is also found that the position of the absorption peak is essentially influenced by states with nonzero transverse wave vector, due to the high nonparabolicity of the valence-band dispersion. This implies that energies of virtual states in the continuum cannot be simply deduced from the absorption profile. Transitions from the heavy-hole ground state to continuum give the principal contribution to absorption, the light-hole contribution being an order of magnitude lower. Calculated results show very good agreement with experimental measurements [K. M. S. V. Bandara et al., Phys. Rev. B 48, 7999 (1993)].

Exponential growth and possible condensation of the particle-hole excitations in moving hot Fermi liquids

The production rate of particle-antiparticle and particle-hole excitations in relativistically moving hot Fermi liquids is calculated. The kinetics of the damping of the excitations as well as of the exponential growth of the particle-hole excitations for some regions of space-like momenta is considered, in the center of mass coordinate projected along the quasi-particle current direction. It is shown that Cherenkov-like radiation of the excitations may form a Bose condensate with a non-zero wave vector. Explicit results are presented for pions and for various types of zero sounds in moving hot nuclear matter.

Hybrid excitons in parallel organic and inorganic semiconducting quantum wires.

We study excited states in parallel neighboring organic and inorganic semiconducting quantum wires. Due to the resonance dipole-dipole interaction between the wires, the hybrid exciton is a mixture of Frenkel and Wannier-Mott excitons. For the transition dipole moments oriented perpendicular to the wires, the interwire hybridization strength is nonzero even for small exciton wave vectors along the wires and decays rather slowly with increasing interwire spacing. This is in contrast to two-dimensional quantum wells where the dipole-dipole coupling decays fast with increasing interwell distance and is nonzero only for nonzero excitonic wave vectors. The hybrid wire excitons possess both a large radius and a relatively large oscillator strength, which could be especially interesting with respect to applications in nonlinear optics.

Mechanism of domain pattern coarsening in ferroelectric sodium nitrite

Abstract The mechanism of domain pattern coarsening in a nearly one-dimensional kink system in ferroelectric sodium nitrite, NaNO2, has been studied by means of an etching technique. Starting from a domain pattern of which the average domain width is 0.6 μm, the pattern coarsens initially through nucleation and growth of antistripples. However, nucleation of antistripples does no more occur between walls separated by more than 2 μm. In the late stage the pattern coarsens through the backward motion of the fronts of wedge-shaped domains and the average domain width increases in proportion to the square root of time. Throughout the whole process, the domain size distribution functions are well scaled and have a shape resembling the Poisson distribution with a cutoff for narrow widths. The structure function of the pattern shows a peak at a non-zero wavevector.

Variational and diffusion quantum Monte Carlo calculations at nonzero wave vectors: Theory and application to diamond-structure germanium.

A variational and diffusion quantum Monte Carlo study of germanium in the diamond structure is reported, in which local pseudopotentials are used to represent the ion cores. We calculate the energy of the free atom and the energy of the solid as a function of volume. The calculations for the solid are performed using a supercell method. We analyze the translational symmetry of the supercell Hamiltonian and show that the eigenstates can be labeled by two wave vectors ${\mathbf{k}}_{\mathit{s}}$ and ${\mathbf{k}}_{\mathit{p}}$. The wave vector ${\mathbf{k}}_{\mathit{s}}$ arises from the invariance of the Hamiltonian under the translation of any one electron coordinate by a supercell translation vector, while the wave vector ${\mathbf{k}}_{\mathit{p}}$, which is the crystal momentum of the wave function, arises from the invariance under the simultaneous translation of all electron coordinates by a translation vector of the crystal lattice. Our solid calculations are performed using wave functions with nonzero supercell wave vectors ${\mathbf{k}}_{\mathit{s}}$, which gives better convergence with the size of supercell than previous zero-wave-vector calculations. The relationship of this method to the special k-points techniques commonly used in band-structure calculations is discussed.

On the gluon plasmon self-energy at O(g)

The next-to-leading order contribution δβ μν ( ω,, q) to the polarization function of the hot gluon system is analysed at nonzero wave vectors q. Using Braaten-Pisarski resummation and general covariant gauges, δβ μν is found to be gauge-fixing independent and transverse on the longitudinal mass-shell. The real part of the longitudinal component δβ l is UV and IR stable (for real q). At imaginary q it is IR singular, and at the point ω = 0, q 2 = −3 m 2 it coincides with the result of Rebhan for next-to-leading order Debye screening. When q approaches the lightcone, δβ l diverges like 1 ω 2−q 2 , reflecting the breakdown of the Braaten-Pisarski decomposition scheme in this limit.