Polarization Decay Research Articles

Spin polarization decay in spin-12and spin-32systems

We present a general unifying theory for spin polarization decay due to the interplay of spin precession and momentum scattering that is applicable to both spin-1/2 electrons and spin-3/2 holes. Our theory allows us to identify and characterize a wide range of qualitatively different regimes. For strong momentum scattering or slow spin precession we recover the D'yakonov-Perel result, according to which the spin relaxation time is inversely proportional to the momentum relaxation time. On the other hand, we find that, in the ballistic regime the carrier spin polarization shows a very different qualitative behavior. In systems with isotropic spin splitting the spin polarization can oscillate indefinitely, while in systems with anisotropic spin splitting the spin polarization is reduced by spin dephasing, which is non-exponential and may result in an incomplete decay of the spin polarization. For weak momentum scattering or fast spin precession, the oscillations or non-exponential spin dephasing are modulated by an exponential envelope proportional to the momentum relaxation time. Nevertheless, even in this case in certain systems a fraction of the spin polarization may survive at long times. Finally it is shown that, despite the qualitatively different nature of spin precession in the valence band, spin polarization decay in spin-3/2 hole systems has many similarities to its counterpart in spin-1/2 electron systems.

Depolarization in a photosensitive relaxor ferroelectric

This paper reports a study of depolarization of a photosensitive relaxor ferroelectric Sr0.61Ba0.39Nb2O6 with 0.44 at. % La and 0.023 at. % Ce impurities at room temperature and under illumination of up to 0.22 mW/cm2 in the absorption band of the crystal. The best fit to the experimental data was found with a function containing two time-dependent power-law terms. The approximation reflects two simultaneously operating polarization decay mechanisms, more specifically, polydomainization in the depolarizing field of a bounded crystal and screening of this field by photoinduced carriers. Exact double-humped distribution functions of nonexponential relaxation centers in the crystal have been derived from the relaxation times. Illumination shifts the distribution functions toward longer relaxation times.

Ligand protons in a frozen solution of copper histidine relax via a T1e-driven three-spin mechanism

Davies electron-nuclear double resonance spectra can exhibit strong asymmetries for long mixing times, short repetition times, and large thermal polarizations. These asymmetries can be used to determine nuclear relaxation rates in paramagnetic systems. Measurements of frozen solutions of copper(L-histidine)(2) reveal a strong field dependence of the relaxation rates of the protons in the histidine ligand, increasing from low (g( parallel)) to high (g( perpendicular)) field. It is shown that this can be attributed to a concentration-dependent T(1e)-driven relaxation process involving strongly mixed states of three spins: the histidine proton, the Cu(II) electron spin of the same complex, and another distant electron spin with a resonance frequency differing from the spectrometer frequency approximately by the proton Larmor frequency. The protons relax more efficiently in the g( perpendicular) region, since the number of distant electrons able to participate in this relaxation mechanism is higher than in the g( parallel) region. Analytical expressions for the associated nuclear polarization decay rate Tau(een) (-1) are developed and Monte Carlo simulations are carried out, reproducing both the field and the concentration dependences of the nuclear relaxation.

Optically detected coherent spin dynamics of a single electron in a quantum dot

The ability to sequentially initialize, manipulate and read out the state of a qubit, such as an electron spin in a quantum dot (QD), is a requirement in virtually any scheme for quantum information processing1,2,3. However, previous optical measurements of a single electron spin have focused on time-averaged detection, with the spin being initialized and read out continuously4,5,6,7,8. Here, we monitor the coherent evolution of an electron spin in a single QD. We use time-resolved Kerr rotation (KR) spectroscopy, an all-optical, non-destructive technique that enables us to monitor the precession of the spin in a superposition of Zeeman-split sublevels with nanosecond time resolution. The data show an exponential decay of the spin polarization with time, and directly reveal the g-factor and spin lifetime of the electron in the QD. Furthermore, the observed spin dynamics provide a sensitive probe of the local nuclear spin environment.

Exactly solvable spin dynamics of an electron coupled to a large number of nuclei; the electron-nuclear spin echo in a quantum dot

The model used to describe the spin dynamics in quantum dots after optical excitation is considered. Problems of the electron-spin polarization decay and the dependence of the steady-state polarization on magnetic field are solved on the basis of exact diagonalization of the model Hamiltonian. An important role of the nuclear state is shown and methods of its calculation for different regimes of optical excitation are proposed. The effect of spin echo generation after application of a π pulse of a magnetic field is predicted for the system under consideration.

Dynamics of Quantum Dot Nuclear Spin Polarization Controlled by a Single Electron

We present measurements of the buildup and decay of nuclear spin polarization in a single semiconductor quantum dot. Our experiment shows that we polarize the nuclei in a few milliseconds, while their decay dynamics depends drastically on external parameters. We show that a single electron can very efficiently depolarize nuclear spins in milliseconds whereas in the absence of the electron the nuclear spin lifetime is on the scale of seconds. This lifetime is further enhanced by 1-2 orders of magnitude by quenching the nonsecular nuclear dipole-dipole interactions with a magnetic field of 1 mT.

Magnetoresistance in Sr2FeMoO6:x glass composites

The effects of interfacial states on the temperature dependence of the magnetoresistance (MR) of Sr2FeMoO6-glass composites have been studied. X-ray diffraction analyses show that the glass is most likely located at the grain boundary without causing a change of the crystal structure of Sr2FeMoO6. The variation of the resistance with temperature and magnetic field indicates that the added glass layer has profound influence on the MR properties. At low temperature, the MR in low fields is enhanced notably because the insulating barrier for the intergranular tunneling is improved by adding the glass layer at the grain surface. However, at high temperature, the MR decreases rapidly with the increase of temperature due to, in addition to the enhancement of spin-independent hopping of electrons through the localized states, the fast decay of spin polarization at the surfaces of the grains. This decay is induced by the separation of the ferromagnetic grains with the nonmagnetic glass layer at the grain boundaries.

Semiclassical dynamics and long-time asymptotics of the central-spin problem in a quantum dot

The spin of an electron trapped in a quantum dot is a promising candidate implementation of a qubit for quantum information processing. We study the central-spin problem of the effect of the hyperfine interaction between such an electron and a large number of nuclear moments. Using a spin coherent path integral, we show that in this limit the electron spin evolution is well described by classical dynamics of both the nuclear and electron spins. We then introduce approximate yet systematic methods to analyze aspects of the classical dynamics, and discuss the importance of the exact integrability of the central-spin Hamiltonian. This is compared with numerical simulation. Finally, we obtain the asymptotic long-time decay of the electron spin polarization. We show that this is insensitive to integrability, and determined instead by the transfer of angular momentum to very weakly coupled spins far from the center of the quantum dot. The specific form of the decay is shown to depend sensitively on the form of the electronic wave function.

Induced polarization and surface electrochemistry

Induced-polarization (IP) responses arising from activation overvoltage are investigated, with the goal of relating established electrochemical properties and processes to IP time constants. In activation overvoltage, an electrical double layer with a well-defined areal capacitance forms at the mineral-electrolyte interface. This study shows that the known capacitance of the interface is compatible with observed initial time constants for the decay of electrode polarization. Furthermore, because the electrical double-layer capacitance is determined largely by the electrolyte and not the mineral, the conclusion is that, although the polarization is caused by the presence of the mineral, the time constants of the decay are determined primarily by the electrolyte and not the mineral. The dependence of the capacitance on the sign and magnitude of the potential implies that the effective decay time constant will be small at the start of depolarization and will progress to larger values as the decay proceeds.

Experimental study of induced polarization relaxation time spectra of shaly sands

The induced polarization decay curve contains much more information than chargeability which is often used in traditional induced polarization logging tool. The singular value decomposition algorithm makes it possible to transform induced polarization decay data into relaxation time spectrum. The spectrum logging tool can be used as an in-situ method to estimate pore structure, permeability, formation water resistivity and shaliness of reservoir with deep investigation depth and high signal-to-noise ratio. In order to use this new logging method in practice, effect of some factors on the relaxation time spectrum should be investigated. The induced polarization decay curve measurements have been performed on seven samples from Daqing Oil Field using a four-electrode technique. The effects of temperature, overburden pressure, electrolyte and salinity on the relaxation spectra of the core samples were analyzed qualitatively. When we discussed the effect of one parameter on the spectra, the others were fixed. The results show that the spectra shift to low time constants with temperature increasing, but the shape is almost the same. The temperature dependence of the spectrum can be corrected. When the overburden pressure increases, the relative amplitudes of low relaxation time constants are the same, and those of high constants decrease. When pressure is diminished again, the amplitudes of high time constants do not recover to the original distribution. The salinity has small influence on the spectra. The electrolyte has the influence on the spectra because of the diffusion coefficient different. This influence of electrolyte can also be corrected.

Anisotropic Static and Dynamic Nonlinear Response of Relaxor Ferroelectrics

It has recently been shown in the framework of the static random bond–random field model of relaxor ferroelectrics that the deformation of polar nanoregions by homogeneous lattice strains gives rise to anisotropy of the static nonlinear response. Here we investigate a similar mechanism for the dynamic nonlinear response using a phenomenological approach proposed by Glazounov and Tagantsev. Assuming that the characteristic relaxation time of each polarization decay mode obeys the Vogel-Fulcher (VF) law, the time dependent dielectric polarization is averaged over an ensemble with a suitable statistical distribution of VF temperatures T 0.

Coherent vector π pulse in optical amplifiers

We obtain an exact linearly polarized vector solitary solution for the amplification of an optical pulse with a time width short compared with both population and polarization decay time. This dissipative soliton results from the balance between the gain from inverted resonant two-level atoms and the linear loss of the host material. We suppose that the excited state of the active centers is degenerate on the projection of the angular momentum, with a symmetric state of inversion. Quite remarkably, we find that after the passing of the pulse, the atomic medium does not return to the ground state, and a finite Zeeman coherence is generated. Numerical simulations demonstrate the stability of vector π solitons with an arbitrary state of elliptical polarization in the presence of both linear birefringence and group-velocity dispersion of the host material.

Exciton spin dynamics in ZnO epilayers

AbstractWe used time‐resolved optical orientation experiments to study the low temperature spin dynamics of a ZnO epilayer. The sample shows a circular polarisation of the donor‐bound exciton of 11% with a decay time of 275 ps. A very narrow spectral dependence of the initial polarisation and a rapid decrease of the polarisation decay time with temperature are also observed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical spin orientation in (110) GaAs quantum wells at room temperature

AbstractWe used time‐resolved optical orientation experiments to study the spin dynamics in (110) oriented GaAs/Al0.4GaAs quantum wells at room temperature. We observe a clear dependence of the initial spin polarisation P(0) and of the polarisation decay time τs as a function of the excitation wavelength and power. As high values as P(0) ∼ 80% and τs ∼ 1.6 ns have been observed when exciting quasi‐resonantly with the heavy‐hole transition. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Environmental influence on point anodes performance in reinforced concrete

This work reports the most recent laboratory results from zinc-based point anodes performance in two different humidity environments (constant ∼95% and at outdoors with ∼77% relative humidities) in reinforced concrete small beams (10×15×60cm and 20×30×120cm), with and without chloride contamination (contaminated between 1% and 3.5wt% by cement weight). The commercially available point zinc-based anodes were placed at one end of the chloride-free or chloride-contaminated concrete beams to determine the polarization performance of these anodes along the reinforcing steel. Half cell potentials, current distribution, and polarization decay due to the anode interconnection with the reinforcing steel bar (rebar) were measured. A detailed discussion of the system efficiency at both humidities is included in this report.

Dielectric Properties and Ionic Conductivity of Anodic Oxide Films on Tantalum

We have investigated certain aspects of the dielectric properties of anodic oxide films on tantalum in connection with Dignam’s theory that a special current-driven polarization process is responsible for the transients shown by the ionic current on changing the electric field in the oxide. In his experiments, which were claimed to prove this theory, films were first held at potentials giving fields too low to produce significant ionic current. When voltage pulses were applied large enough to produce fields giving ionic current, extra current was observed to flow for a short time. This was taken as due to the decay of the dielectric polarization built up due to the ionic current, and partial numerical agreement with the model was found. In the present experiments the discharge current flowing after longer periods of voltage application or of ionic current flow was investigated with the aim of finding if more polarization was indeed produced by ionic current flow. The current was not found to be larger after discharge from fields giving ionic current, as required by Dignam’s model. It is argued that structural changes in the glassy oxide are responsible for the transients.

Polarization Switching Dynamics Governed by the Thermodynamic Nucleation Process in Ultrathin Ferroelectric Films

In most ferroelectrics, the domain nucleation barrier (U*) is thermally insurmountable; this is called "Landauer's paradox." However, we showed that, in ultrathin films, the large depolarization fields could lower U* to a level comparable to thermal energy (k(B)T), resulting in power-law decay of polarization. We empirically found a universal relation between the power-law decay exponent and U*/k(B)T. This relation will provide a practical but fundamental limit for capacitor-type ferroelectric devices, analogous to the superparamagnetic limit for magnetic memory devices.

Spin‐sensitive differential transmission experiments in quasi‐spherical CdS quantum dots

The properties of spin-polarized carriers in quasi-spherical CdS quantum dots in a strong confinement regime were investigated by the polarization-resolved pump-probe experiment with the focus on the initial stages of carrier dynamics. The observed sub-ps decay of the circular polarization of the signal was attributed to interlevel exciton transitions without electron spin flip. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

STUDIES ON THE RELAX BEHAVIOR OF SrBi2Ta2O9 THIN FILMS

ABSTRACT Relax behavior of SrBi2Ta2O9 thin film is investigated. In very short-time scale (less than 10 μs), the relaxation curve follows the characteristics of the voltage change, due to the signal and polarization delay. After 100 μs's relax time, the curve tends to be linear in logarithmic time axes, suggesting that the decay of retained polarization is logarithmic damping. Applied voltage and probe polarity can influence the relaxation curve. Higher applied voltage causes higher decay rate. This can be attributed to the depolarization field, which aggravates the retained polarization's decay. Different probe polarities cause different curves, probably due to the asymmetry of the top electrode and the bottom electrode.

DIELECTRIC AND FERROELECTRIC PROPERTIES OF Na0.42K0.08Bi0.5TiO3 AND Na0.25K0.25Bi0.5TiO3 RELAXOR CERAMICS

ABSTRACT Na0.42K0.08Bi0.5TiO3 (NKBT16) and Na0.25K0.25Bi0.5TiO3 (NKBT50) ceramics have been fabricated by the solid-state reaction. The dielectric responses show a tandem of relaxor-like anomalies, a broad maximum and a lower temperature hump. Temperature dependences of depolaration current of poled ceramics indicate that the lower temperature hump corresponds to the decay of spontaneous polarization. It is believed that at room temperature the ceramics would present polar clusters in a antiferroelectric matrix and electric field can suppress the relaxor behavior. NKBT50 exhibits a higher piezoelectric constant and a higher depolarization temperature. The use for device application has been indicated.