Phonon Bottleneck Effect Research Articles

Optical spectroscopy of carrier relaxation processes in Si nanocrystals

We have investigated thermalization, trapping and recombination processes of carriers in Si nanocrystals embedded in a SiO 2 matrix. The study has been performed using time-resolved optical techniques of photoluminescence and induced absorption, allowing for 17 ps and 150 fs resolution, respectively. Based on these results, the possible relaxation processes are discussed. Special attention is paid to cooperative processes circumventing the effect of phonon bottleneck theoretically expected for Si nanocrystals.

Transverse spin relaxation time in organic molecules

We report a measurement of the ensemble-averaged transverse spin relaxation time $({T}_{2}^{\ensuremath{\ast}})$ in bulk and few molecules of the organic semiconductor tris-(8-hydroxyquinolinolato aluminum) or ${\text{Alq}}_{3}$. This system exhibits two characteristic ${T}_{2}^{\ensuremath{\ast}}$ times: the longer of which is temperature independent and the shorter is temperature dependent, indicating that the latter is most likely limited by spin-phonon interaction. Based on the measured data, we infer that the single-particle ${T}_{2}$ time is probably long enough to meet Knill's criterion for fault-tolerant quantum computing even at room temperature. ${\text{Alq}}_{3}$ is also an optically active organic, and we propose a simple optical scheme for spin qubit readout. Moreover, we found that the temperature-dependent ${T}_{2}^{\ensuremath{\ast}}$ time is considerably shorter in bulk ${\text{Alq}}_{3}$ powder than in few molecules confined in 1--2-nm-sized cavities. Because carriers in organic molecules are localized over individual molecules or atoms but the phonons are delocalized, we believe that this feature is caused by phonon bottleneck effect.

Relaxation rates of magnetization in LiYF4:Ho3+ crystals

Abstract Theory of magnetization relaxation in a diluted paramagnet based on calculations of relaxation rates due to electron–phonon coupling and magnetic dipole–dipole interactions is developed. Assuming fast establishment of equilibrium in the spin system and the Gaussian line shape for the spectral density of the dipole–dipole reservoir, we have derived a master equation for a paramagnetic ion. The developed approach is used to interpret dynamic magnetic measurements in LiYF 4 :Ho 3+ single crystals. We show specific variations of low temperature magnetization dynamics at the magnetic field values corresponding to (anti-)crossings of electron-nuclear sublevels of the Ho 3+ ground state. The measured dependences of the ac-susceptibility on frequency, holmium concentration and the applied magnetic field are described when taking into account the phonon bottleneck effect and the cross-relaxation. Evolution of the magnetization of the LiYF 4 :Ho 3+ in the reversed field is considered.

Temporal evolution of effects of ultrafast carrier dynamics in In0.33Ga0.67N: above and near the bandgap

Ultrafast carrier dynamics in the In0.33Ga0.67N epilayer were investigated in detail, using femtosecond transient differential non-degenerate optical absorption measurements. Following an excitation at 400 nm with fluence ranging from 25 µJ cm−2 to 3000 µJ cm−2, probing was carried out above and near the bandgap using different wavelengths generated from a super continuum source. We have found that bandgap renormalization plays a key role when probing at photon energies well above the bandgap and it is clearly distinct from other effects at the lowest fluence. The critical carrier density for the onset of noticeable bandgap renormalization effects in this material when probing well above the bandgap is approximately 5 × 1018 carriers cm−3. We have observed a decrease in the energy loss rate of this material as a function of photogenerated carrier density which is attributed to phonon bottleneck effect. For the lowest carrier density, we have extracted an optical phonon lifetime to be approximately 45 ± 9 fs.

Cross-relaxation and phonon bottleneck effects on magnetization dynamics inLiYF4:Ho3+

Frequency and dc magnetic field dependences of dynamic susceptibility in diluted paramagnets $\mathrm{Li}\mathrm{Y}{\mathrm{F}}_{4}:{\mathrm{Ho}}^{3+}$ have been measured at liquid helium temperatures in ac and dc magnetic fields parallel to the symmetry axis of a tetragonal crystal lattice. Experimental data are analyzed in the framework of the microscopic theory of relaxation rates in the manifold of 24 electron-nuclear sublevels of the lowest non-Kramers doublet and the first excited singlet in the ${\mathrm{Ho}}^{3+}$ ground multiplet $^{5}I_{8}$ split by the crystal field of ${S}_{4}$ symmetry. The one-phonon transition probabilities were computed using electron-phonon coupling constants calculated in the framework of the exchange charge model and were checked by optical piezospectroscopic measurements. The specific features observed in field dependences of the in- and out-of-phase susceptibilities (humps and dips, respectively) at the crossings (anticrossings) of the electron-nuclear sublevels are well reproduced by simulations when the phonon bottleneck effect and the cross-spin relaxation are taken into account.

Reduced Phonon Scattering in an Asymmetric Triple Barrier Resonant-Tunneling Diode at High Magnetic Fields

The relaxation of hot electrons injected by resonant tunneling into a wide undoped drift region is investigated by measuring the field dependence of the I – V characteristics. In the resonant tunneling region, an increase of the current up to 10% and a decrease in the peak voltage of the resonant tunneling characteristics are observed for a field parallel to the current. It is demonstrated that both phenomena result from a magnetic field induced phonon bottleneck effect in the drift region. In high-field regions, phonon scattering of hot electrons is reduced due to strong confinement into quasi-one-dimension states. However, at magneto-resonance fields, strong current minima are observed.

Master equations for pulsed magnetic fields: Application to magnetic molecules

We extend spin-lattice relaxation theory to incorporate the use of pulsed magnetic fields for probing the hysteresis effects and magnetization steps and plateaus exhibited, at low temperatures, by the dynamical magnetization of magnetic molecules. The main assumption made is that the lattice degrees of freedom equilibrate in times much shorter than both the experimental time scale (determined by the sweep rate) and the typical spin-lattice relaxation time. We first consider the isotropic case (a magnetic molecule with a ground state of spin S well separated from the excited levels and also the general isotropic Heisenberg-Hamiltonian where all energy levels are relevant) and then we include small off-diagonal terms in the spin Hamiltonian to take into account the Landau-Zener-Stuckelberg (LZS) effect. In the first case, and for an S=12 magnetic molecule we arrive at the generalized Bloch equation recently used for the magnetic molecule {V6} in [Phys. Rev. Lett. 94, 147204 (2005)]. An analogous equation is derived for the magnetization, at low temperatures, of antiferromagnetic ring systems. The LZS effect is discussed for magnetic molecules with a low spin ground state, for which we arrive at a very convenient set of equations that take into account the combined effects of LZS and thermal transitions. In particular, these equations explain the deviation from exact magnetization reversal at B0 observed in {V6}. They also account for the small magnetization plateaus (magnetic Foehn effect), following the LZS steps that have been observed in several magnetic molecules. Finally, we discuss the role of the phonon bottleneck effect at low temperatures and specifically we indicate how this can give rise to a pronounced Foehn effect. © 2005 The American Physical Society.

Photoinduced time-resolved electrodynamics of superconducting metals and alloys

The photoexcited state in superconducting metals and alloys was studied via pump-probe spectroscopy. A pulsed Ti:sapphire laser was used to create the non-equilibrium state and the far-infrared pulses of a synchrotron storage ring, to which the laser is synchronized, measured the changes in the material optical properties. Both the time- and frequency- dependent photoinduced spectra of Pb, Nb, NbN, Nb{0.5}Ti{0.5}N, and Pb{0.75}Bi{0.25} superconducting thin films were measured in the low-fluence regime. The time dependent data establish the regions where the relaxation rate is dominated either by the phonon escape time (phonon bottleneck effect) or by the intrinsic quasiparticle recombination time. The photoinduced spectra measure directly the reduction of the superconducting gap due to an excess number of quasiparticles created by the short laser pulses. This gap shift allows us to establish the temperature range over which the low fluence approximation is valid.

Acoustic Phonons and Phonon Bottleneck in Single Wall Nanotubes

Acoustic phonons are modeled by applying the elastic continuum model to single wall nanotubes (SWNTs) with the nanotube approximated as an elastic membrane with cylindrical symmetry. Donnell’s equations of motion for a cylindrical membrane are solved using the Rayleigh-Ritz method to give the acoustic vibrational modes of SWNTs. The dispersion relations are compared for various diameter nanotubes. These calculations provide the full set of phonon modes for the SWNTs; this is in contrast to most calculations that consider a subset of these modes. The allowed phonon assisted electronic transitions are considered for intravalley-intrasubband and intravalley-intersubband transitions. Finally, these results indicate that phonon-bottleneck effects should be important for the short nanotubes being considered for nanotube-based devices exhibiting quasi-ballistic transport.

Intensity noise characteristics in quantum-dot lasers: four-level rate equations analysis

In this paper, the intensity noise characteristics in the three regions of InGaAs QD structure is studied here. The four-level structure-dependent model introduced in this work enable us to study relaxation, recombination and emission processes in the QD region. Carrier recombination outside the dot (inside the quantum-well region), which is impossible to study with other models, is studied here. In this paper, it is shown that noise can be split into five sources. Phonon bottleneck effect is shown to increase the noise.

Continuum model for acoustic phonons in nanotubes: phonon bottleneck

AbstractElastic continuum model is used to model acoustic phonons in single wall nanotubes (SWNTs). Based on elastic continuum theory, acoustic vibrational modes are modeled for both zigzag and armchair nanotubes of finite length using a variational solution of Donnell's equation. The acoustic phonon modes in these calculations are determined for both even and odd modes of the acoustic displacement. The dispersion relations vary with the length of the tube, and the results indicate that phonon bottleneck effects occur in short SWNTs. The displacement field of the nanotube is used to calculate the deformation potential interaction Hamiltonian. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamic magnetization process of the diluted antiferromagnet RbMn cMg 1- cF 3 in pulse field up to [formula omitted

In the context of quantum spin dynamics of a two-level system, we study the dynamic magnetization process of dimerized Mn 2+-Mn 2+ pairs in the diluted antiferromagnet RbMn c Mg 1- c F 3. Measurements have been performed up to 55 T and lower temperature down to 0.1 K . Magnetization anomalies are associated with successive large and small magnetization steps at specific fields with regular interval. Main steps occur at equally spaced level-crossing fields B n (1⩽ n⩽5) with a separation of ΔB=6.4 T and subsidiary ones are classified into higher harmonics which fall just in between at the fields B n/2 (1⩽n⩽10) separated by ΔB/2,B n/3 (1⩽n⩽15) separated by ΔB/3,B n/4 (1⩽n⩽20) separated by Δ B/4 and so on when the energy levels have separation with a simple ratio such as (1:1), (1:2), (1:3) and so on. At the lowest temperature, the observed d M/d B curve becomes simpler, suggesting important roles of low-lying excited levels, together with a spin-phonon coupling and a phonon bottleneck effect.

Phonon bottleneck effects in InAs/GaInP quantum dots

The phonon bottleneck effect is studied in InAs/GaInP quantum dots (QDs). The time-resolved photoluminescence spectra from the lowest ( n=1) and second lowest ( n=2) exciton states in QDs are measured in the temperature range of 5– 200 K . We find that the rise time of the n=1 level is about 500 ps when the excitation energy is in resonance with the n=2 level. This is a long time compared to about 200 ps in non-resonant excitation, and therefore the bottleneck effect exists between the levels n=2 and 1. In order to understand the source of the bottleneck, the temperature dependence of carrier relaxation time between the exciton levels is investigated in the assigned temperature range. With increasing temperature, the relaxation time decreases to 200 ps at 200 K . The data are in good agreement with the theoretical fit of the relaxation probability for multi-phonon processes.

Experimental and theoretical investigation of electronic structure in colloidal indium phosphide quantum dots

We present detailed electronic structure calculations, based on an atomistic pseudopotential approach, for a 42 Å InP QD; theoretical results are correlated to results of experimental measurements. To better understand energy loss dynamics and mechanisms for hot carriers in QDs, we investigate relaxation between electronic levels in InP QDs with diameters significantly smaller than twice the Bohr exciton radius. Energy level spacings exceeding the optical phonon energy present the opportunity for slowed charge carrier cooling (phonon bottleneck effect). However, interaction between confined electrons and holes provides an alternate, efficient relaxation route. Therefore, we expect sufficiently fast charge separation to weaken the Coulomb interaction and decrease the electronic energy loss rate. We study electronic relaxation in InP QDs using variations of subpicosecond transient absorption spectroscopy.

High Field Magnetization Process in a Dodecanuclear Fe(III) Ring Cluster

A high field magnetization technique is applied to study the field-dependent behaviors of the molecular magnet, dodecanuclear Iron(III) ring cluster [Fe(OCH 3 ) 2 (dbm)] 12 , in which twelve numbers of Fe 3+ ( S =5/2) ions form a finite chain ring, where dbm = dibenzoylmethane. The low-lying states of the spin manifold are dramatically revealed by the observation of magnetization process using pulsed high magnetic field up to 55 T at temperature down to 0.1 K. We observed four distinct magnetization steps with nearly equal field separation of 10 T, arising from energy-level crossings for the discrete quantum states in the finite spin system. We discuss the structure of energy levels and the relaxation phenomena in the magnetization process around the level-crossing fields. It is found that the anomalous magnetization curve observed at 1.3 K, showing an unusual shape with characteristic hysteresis for increasing and decreasing field, is well reproduced by Phonon Bottleneck effect caused by the week heat exchange between sample and liquid-He bath during the fast field passage. Our results are compared and discussed with those obtained in the other iron magnets showing the step-like magnetization process due to quantum discrete energy levels.

Low growth rate InAs/GaAs quantum dots for room-temperature luminescence over 1.3 μm

Uncapped InAs/GaAs quantum dots with average height around 15 nm were grown under the regime of ultra-low growth rates (<0.01ML/s). When GaAs capped, such structures exhibited room-temperature luminescence in the 1.1–1.4 μm range. The effect of phonon bottleneck was not observed and the presence of multiple peaks in the photoluminescence spectra was caused by state filling, as observed in measurements as a function of the excitation-power density. A carrier dynamics that involved thermal emission of carriers into the GaAs barrier and recapture by the quantum dots was observed when the optical emissions were monitored at different temperatures.

Abnormal temperature dependence of photoluminescence from self-assembled InAs quantum dots covered by an InAlAs/InGaAs combination layer

In this report we have investigated the temperature dependence of photoluminescence (PL) from self-assembled InAs quantum dots (QDs) covered by an InAlAs/InGaAs combination layer. The ground state experiences an abnormal variation of PL linewidth from 15 K up to room temperature. Meanwhile, the PL integrated intensity ratio of the first excited state to the ground state for InAs QDs unexpectedly decreases with increasing temperature, which we attribute to the phonon bottleneck effect. We believe that these experimental results are closely related to the partially coupled quantum dots system and the large energy separation between the ground and the first excited states.

Blue Cathodoluminescence from Highly Er-Doped ZnO Thin FilmsInduced by the Phonon Bottleneck Effect

The room-temperature blue cathodoluminescence (CL) from highly Er-dopedZnO thin films has been studied by using different electron beamcurrents (EBCs). The ZnO:Er thin films used in our experiment wereprepared by simultaneous evaporation from two sources. The x-raydiffraction spectra show that the thin films have a strong preferentialc-axis (0002) orientation with a hexagonal crystallinestructure. The blue emission at 455 nm originating from theintra-4f shell transition (4F5/2·4I15/2)in Er3+ ions was observed at room temperature. The nonlineardependence of the CL intensity on the EBC shows a more intense blueemission above the threshold EBC of 0.6 µA, which is attributed to thephonon bottleneck effect.

Quantum coherence phenomena in semiconductor quantum dots: quantum interference, decoherence and Rabi oscillation

We performed extensive studies on quantum decoherence processes of excitons trapped in the various excited states of SAQDs. Energy level structure and dephasing times of excited states were first determined by conducting photoluminescence excitation spectroscopy and wave-packet interferometry on a large number of individual SAQDs. This large statistical basis allows us to extract the correlation between the energy level structure and dephasing times. The major decoherence mechanisms and their active regime were identified from this correlation. A significant suppression of decoherence was also observed in some of the energetically isolated excited states, providing an experimental evidence for the theoretical prediction, known as “phonon bottleneck effect”. Furthermore, we observed the direct experimental evidence of Rabi oscillation in these excited states with long decoherence times. In addition, a new type of quantum interference (QI) phenomenon was discovered in the wave-packet interferometry experiments performed in the strong excitation regime where the non-linear effects of Rabi oscillation become important. Detailed theoretical investigations attribute this phenomenon to the coherent dynamics resulting from the interplay of Rabi oscillation and QI.

Carrier relaxation of modulation-doped InAs/GaAs quantum dots

We present the carrier relaxation of modulation-doped InAs/GaAs quantum dots depending on the excitation wavelength and modulation-doping concentration by using the time-resolved spectroscopy. At the excitation below GaAs band gap, the relaxation processes become very slow, implying the observation of phonon bottleneck effects. On the other hand, at excitation far above GaAs band gap, phonon bottleneck effects are broken down due to Auger processes. Increasing modulation-doping concentration, Coulomb scattering between electrons in GaAs-doped layer and carriers in InAs quantum dots makes the relaxation times become fast.