Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons

We propose to use optical detection of magnetic resonance (ODMR) to measure the decoherence time T₂ of a single-electron spin in a semiconductor quantum dot. The electron is in one of the spin 1/2 states and a circularly polarized laser can only create an optical excitation for one of the electron spin states due to Pauli blocking. An applied electron spin resonance (ESR) field leads to Rabi spin flips and thus to a modulation of the photoluminescence or, alternatively, of the photocurrent. This allows one to measure the ESR linewidth and the coherent Rabi oscillations, from which the electron spin decoherence can be determined. We study different possible schemes for such an ODMR setup, including cw or pulsed laser excitation. An erratum to this article is available at .     

Theory of the Anomalous Nuclear Spin Relaxation in U2D2 Solid 3He

In pulsed NMR experiments on U2D2 solid ³ He it has been observed that in some cases free induction signals decay very quickly. It was also found that in such cases a large negative frequency shift from the Larmor frequency appears. We investigated the mechanism of this anomalous nuclear spin relaxation theoretically, using the Holstein-Primakoff method for the Hamiltonian including dipole and exchange interaction. It is shown that the Suhl Instability which had been observed only in the electron spin systems occurs also in the nuclear spin systems and these observed behaviors are attributed to the instability of the uniform precession of magnetization due to the excitation of spin waves.     

Decoherence of nuclear spin quantum memory in a quantum dot

Recently, an ensemble of nuclear spins in a quantum dot have been proposed as a long-lived quantum memory. A quantum state of an electron spin in the dot can be faithfully transfered into nuclear spins through controlled hyperfine coupling. Here we study the decoherence of this memory due to nuclear spin dipolar coupling and inhomogeneous hyperfine interaction during the storage period. We calculated the maximum fidelity of writing, storing, and reading operations. Our results show that nuclear spin dynamics can severely limit the performance of the proposed device for quantum information processing and storage based on nuclear spins.     

Suppression of π/2−π Spin Echo in Solid 3He in High Fields

High demagnetizing field in solid ³ He leads to rich interesting phenomena such as multiple spin echoes. On the other hand, it has a severe influence on the appearance of the ordinary spin echo using /2– RF pulse sequence. We have observed spin echoes in solid ³ He in 7.4 Tesla, which appeared to decay on a much shorter time scale than T ₂ estimated from theories. This can be explained as follows. The angle made by the magnetization with the field after the -pulse differs a little from that before the pulse, because the -pulse employed in the experiment is inevitably not ideal. Since due to the demagnetizing field, the frequency of the precession motion depends on the angle between the magnetization and the field, the frequencies before and after the -pulse are different, which smeared out the spin echo. We have computed the damping time in various conditions, and found good agreement with experimental results.     

Detailed Measurements of Nuclear Spin Polarizations in a Single InAlAs Quantum Dot Through Overhauser Shift of Photoluminescence

We investigated optical pumping of nuclear spin polarizations in a single self-assembled In_0.75Al_0.25As/Al_0.3Ga_0.7As quantum dot. The nuclear spin polarization exhibits the abrupt jump and hysteresis in the excitation power dependence at a particular excitation polarization. Measurement of circular polarization rate of the photoluminescence reveals that the abrupt change of the nuclear spin polarization is created mainly by the spin flip-flop process between nuclei and an electron of a positive charged exciton in this single quantum dot. Model calculation explains well the experimentally observed bistable behavior in InAlAs quantum dot. By using this abrupt change, the sign and magnitude of electron and hole g-factors in z-direction are verified.      

Electron Spin Coherence of Phosphorus Donors in Isotopically Purified 29Si

We investigate spin coherence time of electrons bound to phosphorus donors in silicon single crystals, employing a pulsed electron paramagnetic resonance technique. The samples were isotopically controlled so that they may possess different concentrations (about 5% and 100%) of ²⁹Si, which is the only non-zero-spin (spin-1/2) stable isotope of Si. Both ²⁹Si-concentration dependence and orientation dependence of the electron spin coherence time demonstrate that the decoherence is caused by spectral diffusion due to mutual flip-flops of the environmental nuclear spins. The detail analysis of spin echo decay curves enables the unique assignment of the host sites responsible for electron spin echo envelope modulation.     

Electron Spin Coherence of Phosphorus Donors in Isotopically Purified <Superscript>29</Superscript>Si

We investigate spin coherence time of electrons bound to phosphorus donors in silicon single crystals, employing a pulsed electron paramagnetic resonance technique. The samples were isotopically controlled so that they may possess different concentrations (about 5% and 100%) of ²⁹Si, which is the only non-zero-spin (spin-1/2) stable isotope of Si. Both ²⁹Si-concentration dependence and orientation dependence of the electron spin coherence time demonstrate that the decoherence is caused by spectral diffusion due to mutual flip-flops of the environmental nuclear spins. The detail analysis of spin echo decay curves enables the unique assignment of the host sites responsible for electron spin echo envelope modulation.     

Studies of the hyperfine enhanced nuclear antiferromagnet Cs2NaHoCl6

A neutron scattering study of the hyperfine enhanced nuclear spin system Cs₂NaHoCl₆ has been carried out following adiabatic demagnetization cooling. The nuclear spin system was cooled to about 2 mK, belowT _N =4.8 mK. We did not observe any antiferromagnetic diffraction peaks in the (1 ) reciprocal lattice plane. The anisotropy of the magnetic susceptibility above and below the Néel temperature was also measured. The magnetic structure is discussed.     

Nuclear orientation of recoil-implanted52Mn in Au down to 3 mK

Measurements of the -ray anisotropy of recoil-implanted⁵²Mn ions in pure Au down to 3 mK indicate marked deviations from free-ion behavior in low applied fields. The effective hyperfine field that explains the anisotropy is found to decrease below 10 mK. Although this behavior could be a signature of a bound Kondon state with a lower effective hyperfine coupling constant, it is better explained as arising from a combination of Kondo and relaxation effects. The data indicate that the Mn local moment relaxation timeT ₁ is comparable to or larger than the Larmor precession time of the Mn nuclei at 3 mK. Other possible reasons for an attenuated -ray anisotropy, such as nuclear quadrupole and second-order crystal field effects, are also considered.This work was supported by the Bundesministerium für Forschung und Technologie.     

Monte Carlo Study of the abBA Experiment: Detector Response and Physics Analysis

The abBA collaboration proposes to conduct a comprehensive program of precise measurements of neutron β-decay coefficients a (the correlation between the neutrino momentum and the decay electron momentum), b (the electron energy spectral distortion term), A (the correlation between the neutron spin and the decay electron momentum), and B (the correlation between the neutron spin and the decay neutrino momentum) at a cold neutron beam facility. We have used a GEANT4-based code to simulate the propagation of decay electrons and protons in the electromagnetic spectrometer and study the energy and timing response of a pair of Silicon detectors. We used these results to examine systematic effects and find the uncertainties with which the physics parameters a, b, A, and B can be extracted from an over-determined experimental data set.     

Nuclear alignment in a Kondo system as a very low temperature thermometer

Cu doped with less than 1 ppm Mn⁵⁴ appears to offer a number of advantages as a -ray anisotropy thermometer in the temperature range 1 to 20 mK. In order to use such a thermometer it is necessary to know how the hyperfine field at the Mn⁵⁴ nuclei varies with temperature and with applied field. We have studied this -ray anisotropy and find that the hyperfine field is independent of temperature in the range 4 mK <T<10 mK. Further, we have determined the hyperfine field to ±5% as a function of applied field in the range 60 Oe<H _a<1200 Oe. The latter data may be used to determine a Kondo temperature of 64±2 mK for this system.Work performed under the auspices of the U.S. Atomic Energy Commission.     

Spin Effects in Lateral Quantum Dots

We present the review of our work on spin effects in single lateral quantum dots with the emphasis on the results of Coulomb blockade spectroscopy studies. Realization of a spin-based quantum bit proposal in a lateral quantum dot is discussed. Described are the ways of isolating a single electron spin in a dot containing only one as well as many electrons. Demonstrated is a current readout of spin transitions in a dot by means of spin blockade spectroscopy due to spin polarized injection/detection mechanism in a lateral dot. Discussed are transitions induced both by changing a magnetic field and a number of electrons in a dot with the emphasis on the effects observed close to filling factor in a dot = 2.     

Gate Voltage Control of Nuclear Spin Relaxation in GaAs Quantum Well

Gate-voltage dependences of nuclear spin relaxation and decoherence times in a Schottky-gated n-GaAs/AlGaAs (110) quantum well (QW) are investigated by time-resolved Kerr-rotation measurements combined with pulsed-rf nuclear magnetic resonance (NMR). We show that the nuclear spin relaxation and decoherence times decrease with decreasing electron density, indicating that the hyperfine interaction is enhanced as the electronic states becomes localized in an impurity-doped QW.     

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.     

Quantum spin dynamics of the transverse Ising model in two dimensions

We use the method of recurrence relations to obtain the time-dependent spin correlation function of the Ising model in a transverse field in 2D. We find that the correlation function decays algebraically at long times as t_– where 2.2. This is to be contrasted with the 1D case where the decay is Gaussian. We expect that in 3D the dynamical correlation will also exhibit a power law decay. Our results can be used to understand the experimental shape functions for the induced moment in LiTb_pY _1–pF ₄ .     

Electron spin coherence exceeding seconds in high-purity silicon

Silicon is one of the most promising semiconductor materials for spin-based information processing devices. Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a (28)Si form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in pure (28)Si material (residual (29)Si concentration <50 ppm) with donor densities of 10(14)-10(15) cm(-3). We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T(2) up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ~200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T(2) of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers, or quantum memories for systems such as superconducting qubits.     

The Decoherence of GaAs Quantum Dot Qubit Due to Electron–Phonon Interactions

We investigate electron charge decoherence in a GaAs single-electron semiconductor quantum dot through electron–phonon interaction. We analytically and numerically evaluate decoherence time within the Lee–Low–Pines–Huybrecht variational calculation for all coupling strengths. The dependence of decoherence time on the electron-LO-phonon coupling strength and the size of quantum dot is investigated. Our results suggest that electron–phonon interaction has very important effects on charge decoherence.     

Effects of phonons and antiferromagnetic spin fluctuations on the pairing and density of states in high-T c superconductors

We solve the Eliashberg equations for a two-dimensional, tight-binding band and anisotropic interaction due to exchange of phonons and antiferromagnetic spin fluctuations. For small band fillings, a mixture of simple and extendeds-wave pairing is stable, while for band fillings closer to half-filling thed-wave pairing state becomes stable. The density of statesN() becomes highly asymmetric in for smaller band fillings, which is an effect of particle-hole asymmetry. For thed-wave stateN() is linear in for small and exhibits a logarithmic singularity at the gap amplitude. For the mixeds-wave stateN() shows the BCS singularity at the gap edge. Antiferromagnetic spin fluctuations give rise to a pseudogap inN() for the normal state.     

The cooldown process of superconducting tin films in contact with cold sapphire

Momentary current voltage characteristics of tin-tunnel junctions evaporated on an a-cut sapphire were measured during the cooldown process which starts immediately after stopping the steady-state heating from a constantan heater which was evaporated at the top of the tin diode with a SiO _x -layer in between. The measured momentary current-voltage characteristics cannot be distinguished within experimental error from thermal characteristics taken at a higher substrate temperature. Hence, at the tunnel junction, the electron system can be described at each time during the cooldown process by an electron temperatureT ₁ ^el (t). It is shown that the decay time of the electron system is far too long to be in accordance with a one-temperature model combined with the acoustic mismatch model, which has been verified in pulse-heating experiments during the heatup and steady-state time interval in a preceeding paper.¹ We observed a pure exponential decay of the electron temperature starting at the onset of superconductivity atT _C =3.72 K and ending at the bath temperature which is usually aboutT _C /2. The measured decay constant is within a 10% maximum deviation equal to the effective recombination time _eff of the injected quasiparticles, well-known from other investigations. We found the same dependence of on bath temperature, on the thickness of the tin film, and on oxygen content as was observed for _eff in earlier experiments. Simple models for the behavior of the electron and phonon system during the cooldown process are investigated to explain this unexpected decay behavior at temperatures far away from the bath temperature.     

Effect of phosphorus additions on the spacing between primary silicon particles in a Bridgman solidified hypereutectic Al-Si alloy

The effect of 100 ppm addition of phosphorus on primary silicon particle number density per unit area N _A and corresponding interparticle spacing is reported for a Bridgman solidified Al-20 wt%Si base alloy. The phosphorus (added as Al-Fe-P base or Al-Cu-P alloys) results in a factor of 3 increase in N _A and a factor of 2 reduction in for the range of conditions studied. In its absence the results conform to = 256 ± 24 m (K/s)_1/3 where is cooling rate during solidification in good agreement with earlier data. When published data on the effect of 0.02 to 0.2 wt%P are included the combined results are well represented by = 250 – 215 (wt%P)^0.17 ( in m, in K/s).