Nonequilibrium Acoustic Phonons Research Articles

Dynamics of a Cr spin in a semiconductor quantum dot: Hole-Cr flip-flops and spin-phonon coupling

A detailed analysis of the photoluminescence (PL) intensity distribution in singly Cr-doped CdTe/ZnTe quantum dots (QDs) is performed. First of all, we demonstrate that hole-Cr flip-flops induced by an interplay of the hole-Cr exchange interaction and the coupling with acoustic phonons are the main source of spin relaxation within the exciton-Cr complex. This spin flip mechanism appears in the excitation power dependence of the PL of the exciton as well as in the intensity distribution of the resonant PL. The resonant optical pumping of the Cr spin which was recently demonstrated can also be explained by these hole-Cr flip-flops. Despite the fast exciton-Cr spin dynamics, an analysis of the PL intensity under magnetic field shows that the hole-Cr exchange interaction in CdTe/ZnTe QDs is antiferromagnetic. In addition to the Cr spin dynamics induced by the interaction with carriers' spin, we finally demonstrate using time resolved optical pumping measurements that a Cr spin interacts with nonequilibrium acoustic phonons generated during the optical excitation inside or near the QD.

Controlling competing orders via nonequilibrium acoustic phonons: Emergence of anisotropic effective electronic temperature

Ultrafast perturbations offer a unique tool to manipulate correlated systems due to their ability to promote transient behaviors with no equilibrium counterpart. A widely employed strategy is the excitation of coherent optical phonons, as they can cause significant changes in the electronic structure and interactions on short time scales. Here, we explore a promising alternative route: the non-equilibrium excitation of acoustic phonons. We demonstrate that it leads to the remarkable phenomenon of a momentum-dependent temperature, by which electronic states at different regions of the Fermi surface are subject to distinct local temperatures. Such an anisotropic electronic temperature can have a profound effect on the delicate balance between competing ordered states in unconventional superconductors, opening a novel avenue to control correlated phases.

Observation of Size-Dependent Thermalization in CdSe Nanocrystals Using Time-Resolved Photoluminescence Spectroscopy

We report heat dissipation times in semiconductor nanocrystals of CdSe. Specifically, a previously unresolved, subnanosecond decay component in the low-temperature photoluminescence decay dynamics exhibits longer decay lifetimes (tens to hundreds of picoseconds) for larger nanocrystals as well as a size-independent, ~25-meV spectral shift. We attribute the fast relaxation to transient phonon-mediated relaxation arising from nonequilibrium acoustic phonons. Following acoustic phonon dissipation, the dark exciton state recombines more slowly via LO-phonon assistance resulting in the observed spectral shift. The measured relaxation time scales agree with classical calculations of thermal diffusion, indicating that interfacial thermal conductivity does not limit thermal transport in these semiconductor nanocrystal dispersions.

Probing of the multilayer Si/SiGe structure with a flux of nonequilibrium acoustic phonons

The Si/Si0.8Ge0.2/Si double quantum wells grown on a silicon substrate were probed in the reflection geometry with a flux of terahertz nonequilibrium acoustic phonons produced by the heat pulse technique. A comparison of the detected phonon arrival signals with those obtained under similar conditions from a silicon sample without a quantum well structure permits one to isolate the component due to the phonon reflection from quantum wells. This reflected signal was found to be strongly anisotropic.

Nonequilibrium interactions between two quantum circuits

We briefly overview our recent results on nonequilibrium interactions between neighbouringelectrically isolated nanostructures. One of the nanostructures is represented by anexternally biased quantum point contact (drive-QPC), which is used to supply energyquanta to the second nanostructure (detector). Absorption of these nonequilibrium quantaof energy generates a dc current in the detector, or changes its differential conductance. Wepresent results for a double quantum dot, a single quantum dot or a second QPC placed inthe detector circuit. In all three cases a detection of quanta with energies up to∼1 meV is possible for bias voltages across the drive-QPC in the mV range. The results arequalitatively consistent with an energy transfer mechanism based on nonequilibriumacoustic phonons.

Nonequilibrium phonons in SiGe quantum-well nanostructures upon excitation by picosecond laser pulses

The phonon pulses initiated by photoexcitation of structures containing Si0.8Ge0.2 double quantum wells under picosecond radiation of a MIRA 900P titanium-sapphire laser (λ = 760 nm) are studied. The propagation of nonequilibrium acoustic phonons is detected with a superconducting bolometer. The recorded bolometer response is found to differ substantially from that observed in photoexcitation of the same structure by nanosecond pulses of a nitrogen laser (λ = 337 nm). The generation of coherent acoustic phonons is suggested as an explanation.

Evolution of a system of nonequilibrium acoustic phonons in monocrystalline and pure coarse-grained ZnSe

Propagation of nonequilibrium acoustic phonons in ZnSe single crystals grown by the free-growth process in the hydrogen atmosphere as well as in coarse-grained polycrystalline ZnSe grown by chemical synthesis from the vapor phase was investigated. The latter material is characterized by a microtwinned structure inside chaotically oriented grains. The measurements were carried out by the heat-pulse technique in the "transmission" geometry both by optical generation of phonons and by metal-heater generation. The characteristics of basic phonon scattering processes are estimated.

Scattering of nonequilibrium acoustic phonons in coarse-grained pure ZnSe with random microtwin structure

Propagation of nonequilibrium acoustic phonons in a coarse-grained ZnSe produced through chemical synthesis from a vapor phase was studied for phonons generated optically or by a metallic heater. The material is characterized by a microtwin structure in randomly oriented grains. Phonon transport studies, in combination with low-temperature photoluminescence, optical and electron microscopy, and x-ray diffraction analysis, made it possible to establish the decisive role played by extended defects in scattering of high-frequency acoustic phonons in this matterial.

Nonequilibrium acoustic phonons in Y3Al5O12-based nanocrystalline ceramics

The phonon transport processes in high-transparency nanocrystalline laser ceramics based on cubic Y3Al5O12 garnet oxide were investigated by the heat-pulse technique. The propagation kinetics of nonequilibrium acoustic phonons was studied in the range of helium temperatures (1.7–3.8 K). The structural model is suggested for the intergrain layers, and their thickness is estimated.

Propagation of nonequilibrium acoustic phonons in high-purity coarse-grained ZnTe

Propagation of nonequilibrium acoustic phonons in coarse-grained ZnTe obtained through vacuum sublimation was studied using the heat pulse method under both optical and metallic-heater phonon generation. The phonon mean free paths in the samples studied were shown to be 14 µm and to be dominated by scattering from twin boundaries.

Propagation of nonequilibrium phonons in single-crystal ZnTe

Propagation of nonequilibrium acoustic phonons in ZnTe single crystals at helium temperature was studied with phonons being both optically generated and produced by a metal-film heater. Experiments with [110] ZnTe allowed for the detection of the arrival of longitudinal and transverse acoustic phonons, which are particularly well resolved when generated by a heated metal film. A bolometer also detected emission of ZnTe material with a time constant τ ∼ 90 ns, which can apparently be attributed to the luminescence of oxygen complexes, because the lifetimes of other complexes and bound excitons are substantially shorter.

Nonequilibrium phonon propagation in high-purity CdTe

Propagation of nonequilibrium acoustic phonons in samples of high-purity CdTe (impurity content ∼1016 cm−3) was studied using the heat pulse technique under pulsed photoexcitation. An analysis of nonequilibrium phonon propagation made by comparing the experimental response with Monte Carlo calculations assuming samples to be without twins provided an estimate for the spontaneous anharmonic phonon decay constant AL=2×10−52 s−1 Hz−5. The probability of free phonon transit through a twin boundary in a sample with twin structure was estimated as AC=0.96.

Acoustic phonon-assisted tunneling in GaAs/AlAs superlattices

Measurements have been made of the transient changes in tunnel current $\ensuremath{\Delta}I$ through a biassed 50 period Si doped GaAs/AlAs superlattice produced by pulses of nonequilibrium acoustic phonons. The bias ${V}_{\mathrm{max}}$ at which $\ensuremath{\Delta}I$ is greatest varies linearly with the heater temperature ${T}_{h}$ and hence with the dominant phonon frequency, the SL is acting as a phonon spectrometer with a linewidth (HWHM) $\ensuremath{\lesssim}{k}_{B}{T}_{h}/h\ensuremath{\sim}200\mathrm{GHz}$ (0.8 meV) for ${T}_{h}=10\mathrm{K}.$ The spectral response was confirmed using the phonons that are scattered and hence delayed in their passage across the substrate as a source of higher average frequency than those crossing ballistically. The measured values of $\ensuremath{\Delta}I(V)$ are compared with values calculated using a similar model to that of Glavin et al. [JETP Lett. 51, 191 (2000)].

The phonon-drag effect in low mobility gallium nitride epilayers

We have used phonon imaging to study the drag current induced by non-equilibrium acoustic phonons in a low-mobility n-type Gallium Nitride epilayer. Current densities up to j=4.7×10 3 A/m 2 were induced by transverse acoustic phonon drag of electrons in the structure. We also carried out numerical calculations of the phonon-drag current including the effects of phonon focussing in the sapphire substrate, acoustic anisotropy in the GaN and acoustic mismatch between the two materials. We show that the theory is able to account for the qualitative features of the drag image as well as the magnitude of the phonon-drag-induced current.

New method of detection of terahertz acoustic phonons in quantum well structures

The effect of nonequilibrium acoustic phonons upon the luminescence of a quasi-resonantly excited ultrathin CdTe/ZnTe quantum well has been studied. It was shown that changes in the quantum well luminescence spectrum manifest already for a low intensity of phonon generation. We suggest to use this effect for the terahertz acoustic phonon detection in low-dimensional heterostructures.

Interaction of excitons localized by a CdTe/ZnTe quantumwell fluctuation potential with nonequilibrium acousticphonons

It is found that the phonon flux generated by an external sourceinduces a change of the luminescence bandshape of an ultrathinquantum well CdTe/ZnTe excited quasi-resonantly by a He-Nelaser. The effect of nonequilibrium phonons manifests even atsmall lasing power and large distance between the phonongeneration zone and the quasi-resonant luminescence excitationzone. It is assumed that the phonon flux induces the tunneltransitions between localized exciton states; these transitionsare accompanied by stimulated phonon emission.This paper was presented at the 3rd RussianWorkshop on Nanophotonics, Nizhnii Novgorod, Russia, 26-29March 2001.

Generation of nonequilibrium phonons in semiconductors and dielectrics by a pulse-heated metallic film: Model and experiment

Characteristics of a phonon generator in the form of a pulse-heated metallic film, viz., the time dependences of the film temperature and the kinetics of phonon ejection from the film into a substrate, are considered. The time dependences of the film temperature are calculated for cadmium telluride, diamond, and silicon substrates. It is shown that the duration of film cooling substantially exceeds the heating pulse length and the film continues to generate phonons with lower frequencies at the end of heating pulse. The inference is drawn that the film cooling should be correctly taken into account in analysis of the propagation of nonequilibrium acoustic phonons, specifically for phonon processes occurring in nanostructures.

Absorption of nonequilibrium acoustic phonons by low-mobility electrons in GaN

We report direct phonon absorption experiments in n-type GaN epitaxial layers grown by molecular beam epitaxy. Nonequilibrium phonons with characteristic energies up to 15 meV (3.5 THz) are injected from a constantan heater. They propagate ballistically through the sapphire substrate, reach the GaN layer, and are absorbed by the degenerate three-dimensional electron gas. The phonon absorption is studied as a function of heater temperature through the effects of phonon induced changes of the device resistivity. The experimental results lead us to the conclusion that in low-mobility GaN, the momentum conservation cutoff for electron-phonon transitions is shifted to higher energy than predicted by standard theory.

The analysis of structural defects in piezo and ferroelectric materials by the “heat pulse” method

We consider the “heat pulse” technique for the analysis of structural imperfections. This method is based on the propagation of a pulse or packet of pulses of non-equilibrium acoustic phonons with 1011–1012 Hz frequencies. Analyse des défauts structuraux dans les matériaux piézo-ferroélectriques par la méthode “heat pulse”. Cette méthode, basée sur la propagation pulsée de phonons acoustiques hors équilibre vers 1011–1012 Hz, permet l'étude des défauts structuraux et autres imperfections présents dans les cristaux piézo-ferroélectriques.

Emission of acoustic phonons due to spin-flip electron transitions in nonsymmetric quantum wells

We have considered the emission of nonequilibrium acoustic phonons in the process of spin-flip transitions of electrons in nonsymmetric quantum wells. The spin splitting of the electron energy spectra causes the modifications of the energy and angular distributions as functions of the transverse voltage. The spin-flip contribution to the emission rate is substantial for the phonons being emitted at considerable angles to the normal direction. The intensity of phonon emission for such angles increases several times for InAs-based quantum wells, due to the contribution of spin-flip transitions.