Phase memory effects in probe-field spectroscopy of two-level systems at low collision frequencies

The absorption (amplification) spectrum of a weak probe field by two-level atoms located in a strong resonant laser field and colliding with buffer-gas atoms is analyzed theoretically. The analysis is performed for low collision frequencies compared to the Doppler absorption linewidth (low gas pressure) and with allowance made for an arbitrary change in the phase of the radiation-induced dipole moment at elastic collisions between gas particles. The phase memory effects have been found to lead to a strong qualitative and quantitative transformation of the probe-field spectrum even at rare collisions, when the well-known Dicke manifestation mechanism of the phase memory effects (the removal of Doppler broadening due to the restriction of the spatial particle motion by collisions) is inoperative. The strong influence of the phase memory effects on the spectral resonances at low gas pressures stems from the fact that the phase-conserving collisions change the velocity dependence of the partial refractive index n(v) (the refractive index for particles moving with velocity v).     

Ferrimagnetic resonance spectra of magnetic bubble films at low microwave frequencies

Magnetic bubble films exhibit a number of ferrimagnetic resonance modes due to the spatial variation of the anisotropy. The resonance frequencies have been measured as a function of the applied bias fieldH ₀. In the lower field range the magnetization of the transient layer, which has negative anisotropy, is not yet parallel toH ₀. In this range the resonance frequencies are shifted to higher values due to pinning effects. In films grown by the vertical dipping method an additional layer on top of the transient layer is observed within which the magnetization rotates from the direction in the transient layer to that of the bulk of the film. In films grown by horizontal dipping no such layer could be detected. Each ferrimagnetic resonance mode excites transverse elastic waves in the film due to the magnetoelastic interaction and thus gives rise to elastic resonances of the whole crystal, film and substrate. These elastic resonances lead to a fine-structure of the ferrimagnetic resonances. The observed fine-structure vanishes periodically with frequency and from this behaviour the thickness of the magnetic film and of the transient layer has been determined.     

Nuclear magnetic resonance at microwave frequencies

Instrumental requirements for pulsed NMR studies of hyperfine splittings in rate-earth metals and compounds are considered, with particular attention to factors affecting the design of the specimen cavity. A spin-echo spectrometer for hyperfine studies in the 2 to 8 GHz frequency range is described.     

Electron paramagnetic resonance experiments at high frequencies and low temperatures in one dimensional TMMC

We report the existence of collective excitations of spins in TMMC at high magnetic fields and low temperatures, but higher than T_N at zero magnetic field. The main features of the resonances can be attributed to those of AFMR. Three kinds of crystallographic domains obtained from twining are established. Two magnon excitations may have also been observed.     

Heterodyne spectroscopy at very low frequencies

We have studied the spectrum of light scattered by capillary waves, thermally excited at the free surface of highly viscous liquids, by heterodyne spectroscopy techniques. We were able to measure spectral widths down to 0.2 Hz.     

Auditory filter shapes at low center frequencies

Auditory-filter shapes were estimated in normally hearing subjects for signal frequencies (fs) of 100, 200, 400, and 800 Hz using the notched-noise method [R. D. Patterson and I. Nimmo-Smith, J. Acoust. Soc. Am. 67, 229-245 (1980)]. Two noise bands, each 0.4fs wide, were used; they were placed both symmetrically and asymmetrically about the signal frequency to allow the measurement of filter shape and asymmetry. Two overall noise levels were used: 77 and 87 dB SPL. In deriving the shapes of the auditory filters, account was taken of the nonflat frequency response of the Sennheiser HD424 earphone, and also of the frequency-dependent attenuation produced by the middle ear. The auditory filters were asymmetric; the upper skirt was steeper than the lower skirt. The asymmetry tended to be greater at the higher noise level. The equivalent rectangular bandwidths (ERBs) of the filters at the lower noise level had average values of 36, 47, 87, and 147 Hz for values of fs of 100, 200, 400, and 800 Hz, respectively. The standard deviations of the ERBs across subjects were typically about 10% of the ERB values. The signal-to-masker ratio at the output of the auditory filter required to achieve threshold increased markedly with decreasing fs.     

Magnetic resonance imaging at frequencies below 1 kHz

Within the magnetic resonance imaging (MRI) community the trend is going to higher and higher magnetic fields, ranging from 1.5 T to 7 T, corresponding to Larmor frequencies of 63.8–298 MHz. Since for high-field MRI the magnetization increases with the applied magnetic field, the signal-to-noise-ratio increases as well, thus enabling higher image resolutions. On the other hand, MRI is possible also at ultra-low magnetic fields, as was shown by different groups. The goal of our development was to reach a Larmor frequency range of the low-field MRI system corresponding to the frequency range of human brain activities ranging from near zero-frequency (near-DC) to over 1 kHz. Here, first 2D MRI images of phantoms taken at Larmor frequencies of 100 Hz and 731 Hz will be shown and discussed. These frequencies are examples of brain activity triggered by electrostimulation of the median nerve. The method will allow the magnetic fields of the brain currents to influence the magnetic resonance image, and thus lead to a direct functional imaging modality of neuronal currents.     

Acoustic properties of glasses at low temperatures and low frequencies

We generalize the low temperature expressions for the variation of the sound velocity and sound absorption in glasses due to structural defects for the case of arbitrary frequencies. In case ωτ₂ ? 1 our expressions reduce to well known results.     

低频标准真空涨落的测量

采用自平衡零拍方案, 对低频段的标准量子真空涨落进行了测量. 实验确定了该系统的饱和光功率约为3.2 mW. 在10 Hz–400 kHz的频率范围内, 系统的共模抑制比平均为55 dB, 在100 Hz处高达63 dB, 对激光经典技术噪声具有很强的抑制作用. 当入射光功率为400 μ W 时, 真空涨落噪声达到11 dB. 此低频量子真空噪声探测系统可广泛应用于量子计量和量子光学等研究领域.     

Decay rates and wall absorption at low frequencies

This paper is concerned with evaluating the error of conventional estimates of the boundary absorption of rectangular enclosures, with particular reference to reverberation room sound power measurements. The reverberation process is examined theoretically; the relative contributions to the decay rate from different modes in a rectangular room are calculated from an ensemble average over rooms with nearly the same dimensions. It is shown that the traditional method of determining the absorption of the walls of reverberation rooms systematically underestimates the absorption at low frequencies; the error is computed from the ensemble average. Finally, an unbiased estimate of the sound power radiated by a source in a reverberation room is derived. This estimate involves measurement of the initial decay rates of the room and is, unlike the usual reverberation room sound power estimate, neither based on statistical diffuse field considerations nor on the normal mode theory.     

低分析频率压缩光的实验制备

基于PPKTP晶体的阈值以下光学参量振荡(OPO)过程,制备了共振于铷原子D1线795 nm的压缩真空态光场,研究了分析频率处于千赫兹范围的主要噪声来源,特别是795 nm激光及其二次谐波397.5 nm激光在晶体内吸收引起的非线性损耗增加和系统热不稳定的问题(397.5 nm激光处于PPKTP晶体透光范围边缘,具有高于其他波长数倍的吸收系数).以795 nm和1064 nm为例,分析了非线性损耗及晶体内热效应对压缩度的影响.受限于以上因素,795 nm压缩光很难得到1064 nm波段同样的压缩度.探测系统中的噪声耦合则限制了压缩频带.实验上对分析频率为千赫兹的经典噪声进行了有效控制,通过使用真空注入的OPO、垂直偏振及反向传输的腔长锁定光、低噪声的平衡零拍探测器、高稳定度的实验系统及量子噪声锁定等方法,最终在2.6—100 kHz的分析频段得到了约2.8 dB的795 nm压缩真空.该压缩光可用作磁场测量系统的探测光以提高测量灵敏度.     

Vortex mass in a superfluid at low frequencies

An inertial mass of a vortex can be calculated by driving it around in a circle with a steadily revolving pinning potential. We show that in the low-frequency limit this gives precisely the same formula that was used by Baym and Chandler, but find that the result is not unique and depends on the force field used to cause the acceleration. We apply this method to the Gross-Pitaevskii model, and derive a simple formula for the vortex mass. We study both the long-range and short-range properties of the solution. We agree with earlier results that the nonzero compressibility leads to a divergent mass. From the short-range behavior of the solution we find that the mass is sensitive to the form of the pinning potential, and diverges logarithmically when the radius of this potential tends to zero.     

Low-temperature elastic experiments on glasses at very low frequencies

The Grüneisen parameter and the low-frequency elastic loss of vitreous silica, PMMA, and of the metallic glasses PdSiCu, PdZr, and CuZr have been measured between 0.4 K and 6 K using an elasto-caloric technique. The absorption measurements between 5·10^–3 Hz and 30 Hz give further support for the tunneling model in a hitherto poorly investigated domain of relaxation times. New features of the low-temperature behaviour of glasses are a long-time creep and an absorption peak of the metallic glass PdZr nearT _c.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

Dielectric response of KCN crystal at ultra low frequencies

The dielectric response of pure KCN crystals (ε′, ε″ and tg σ) has been measured as a function of temperature in the frequency range 10⁻² Hz to 10⁴ Hz. In the antiferroelectric phase the width of the loss peak are found practically independent of temperature (1.4 decades) and close to a Debye behavior; the relaxation time of the CN⁻ dipoles is characterized by an Arrhenius behavior τ = τ₀ exp (U/KT) with τ₀ = 7.26 × 10⁻¹⁵s and U = 0, 147 eV confirming a classical temperature activated reorientation of the dipoles.